Passive Design Options to Improve Solar Chimney Performance
Mr. Kurt Lewis
A Thesis Submitted to Fulfil the Requirements for the Degree of Master of Philosophy
Academic Supervisor:
Prof. Chengwang Lei
Centre for Wind, Waves and Water
School of Civil Engineering
Faculty of Engineering
The University of Sydney
September 2020 Declaration
I hereby declare that the work presented in this thesis is solely produced by me. And to the best of my knowledge, the work is original except where otherwise indicated by reference to other authors. No part of this work has been submitted for any other degrees or diploma.
Kurt Lewis
September 2020
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Acknowledgments
Many people have supported me throughout the duration of my degree.
First, I would like to thank Professor Chengwang Lei for giving me the opportunity, I have stumbled many times over the duration and his help did much to help me keep focus.
Next, I would like to thank all my research group members who helped me learn all I needed as well as giving me much-needed support. Gary, Haoyu, Fanxing, Duy, Jing and Yuchen have all been very helpful with CFD and natural convection with Xiamei helping with the more obscure coding stuff now and then.
Zachary Benitez helped me with learning how to code in python and expanding my knowledge of MATLAB, while Theo Gresley-Daines helped me to understand how the experiments for solar chimneys were conducted and some of the physical limitations that would be encountered.
The staff at Sydney Informatics Hub were patient with my incessant questions as to why my code would not work on ARTEMIS. The use of the HPC also meant I could expedite the considerable number of simulations I had to do.
Thanks to my Dad, Stephen Lewis, who has never doubted that I could do anything and supported me through all the things that went awry. Gaby for putting up with me being frustrated, Paola for understanding the process and introducing me to Empanadas and Mum for everything.
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Abstract
Due to the increasing need for HVAC systems and concerns with global climate change, solar chimney for building ventilation is considered as a promising technique to reduce carbon footprint and to achieve thermal comfort. Although research into solar chimney has been conducted for many decades, there is little research into the effects of the surface geometry and optical and radiative properties of the glazing and absorber walls on the performance of the solar chimney.
Two CFD studies have been conducted in this project using the software package ANSYS Fluent: The first to determine the effect that tinting on the glazing may have on the solar chimney performance, and the second to evaluate whether the introduction of a single horizontal baffle would achieve a performance enhancement. All simulations are conducted on standard vertical solar chimneys.
Two-dimensional RANS simulation in an expanded computational domain is conducted to determine the effect of tinting on glazing. A combination of the level of tinting (i.e. how much irradiance would be retained by the tint) and the location of the tinting on the glazing has been tested. Additionally, the implementation of a radiation model is compared with that without radiation model. It is found that, when no radiation model is enabled, the implementation of tinting on the glazing has a considerable impact on the performance of the solar chimney, with the peak performance occurring when 50% of the incoming irradiance is retained by the full height of the glazing. This improvement is attributed to preventing a reverse flow and generating a second thermal plume along the glazing. The improvement depends on the severity of the reverse flow. When the radiation model is enabled, there is no noticeable improvement as there is no reverse flow observed in an unaltered chimney. Accordingly, if reverse flow is observed in a solar chimney, it is recommended that tinting be applied to the glazing to preserve performance.
Similarly, two-dimensional RANS simulation has been conducted over an expanded computational domain to determine the effect of a horizontal baffle on the mass flow rate through the solar chimney. To account for heating the absorber and baffle in a consistent manner, the wall thicknesses of the absorber and glazing are both modelled explicitly in the simulation. Due to the large number of parameters to test, the Taguchi method is adopted to
iii reduce the required number of simulations, resulting in a reduction of required tests from 243 down to 27. Following a Taguchi Optimisation analysis, a baffle placed at 80% of the chimney height and of 30% of the chimney air-gap width leads to an average 6% performance improvement when radiation transfer is considered. However, when the radiation model is disabled, the above baffle configuration does not give improved performance. Following a similar Taguchi Optimisation analysis, it is determined that a horizontal baffle placed at 90% of the chimney height and of 10% of the chimney air-gap width provides a performance improvement when radiation transfer is not considered. It is also found that such a baffle configuration also provides an average 6% performance enhancement when the radiation model is enabled and for all other tested parameters and modelling scenarios. The addition of a horizontal baffle to a solar chimney could be an effective method to achieve an increase in performance with minimal cost and construction time and can be retrofitted to existing solar chimneys.
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Table of Contents
Declaration ...... i
Acknowledgments ...... ii
Abstract ...... iii
Table of Contents ...... v
List of Figures ...... viii
List of Tables ...... xii
Nomenclature ...... xiii
1 Introduction ...... 1
The Basis of Solar Chimney...... 2
Buoyancy Driven Flow ...... 3
Laminar vs Turbulent ...... 4
Aims and Objectives ...... 6
Outcomes and Contributions ...... 6
2 Literature Review ...... 8
Review Articles of the Solar Chimney Literature ...... 8
Design Considerations...... 8
2.2.1 Chimney Height ...... 9
2.2.2 Inclination Angle and Orientation ...... 10
2.2.3 Ideal Air-Gap Width and Aspect Ratio ...... 14
2.2.4 Other Attributes and Materials...... 16
2.2.5 Alternative Designs ...... 17
CFD for Solar Chimney ...... 22
LES studies in natural convection ...... 23
Combination with Other Technologies ...... 24
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Solar Heaters and Solar Dryers ...... 25
Summary ...... 26
3 Computational Fluid Dynamics - Methodology and Validation ...... 28
Introduction ...... 28
Geometry and Boundaries ...... 28
Meshing ...... 30
Operating Conditions ...... 30
Boundary Conditions...... 31
Turbulence Modelling ...... 32
Radiation Modelling ...... 32
Numerical Schemes ...... 32
Dependency Tests ...... 33
Validation with Experimental Data ...... 34
4 The Effect of Tinted Glazing ...... 38
Design Concept ...... 38
Chimney Dimensions ...... 38
Design Methodology ...... 38
Results ...... 40
4.4.1 No Radiation Model ...... 40
4.4.2 With Radiation Model Enabled ...... 42
Conclusion ...... 44
5 The Effect of a Horizontal Baffle ...... 45
Design Concept ...... 45
Design Methodology ...... 45
Chimney Dimensions ...... 45
Taguchi Method – With Radiation Model Enabled ...... 46
Taguchi Results – With Radiation Model Enabled ...... 47
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Follow-up Simulations ...... 51
Results with Radiation Model Disabled ...... 55
Additional Baffle Configurations with Radiation Model Enabled...... 61
Conclusions ...... 62
6 Summary and Future Research Options ...... 64
Summary of the Present Investigations ...... 64
Possible Experiment ...... 64
LES Simulation Options...... 65
Validation and Use of Solar Load Model...... 65
New Solar Chimney Designs ...... 65
Recommended Future Designs...... 66
References ...... 67
Appendix A – Automation Methods ...... 71
Parametric Study using ANSYS Workbench ...... 71
Parametric Study Using Python, ICEM and the HPC ...... 71
Appendix B – Simulation Results for the Taguchi Method with Radiation Model Disabled . 73
Appendix C – Stream function for Baffle Chimney with Radiation Model Enabled ...... 74
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List of Figures
Figure 1 – Sirocco Room from Southern Italy (left) and Windcatchers in Yazd, Iran (right). Source: Saeli & Saeli (2015) and Jomehzadeh, Nejat, Calautit et al. (2017)...... 1 Figure 2 - Schematic of a solar chimney showing the absorber height (H), the air gap width (W) and the inclination angle (Φ)...... 2 Figure 3 - The mechanism of wind driven ventilation (left) compared to buoyancy driven flow (right)...... 3 Figure 4 - The temperature and velocity profiles in thermal and viscous boundary layers. Source: Mohaghegh & Abolfazli (2016)...... 4 Figure 5 – Model used for determining solar position in Saleem et al. (2016)...... 11 Figure 6 - Optimum angle for each month of the year. Source: Prasad & Chandra (1990). .. 12 Figure 7 - Simple geometry of the Silvestrini Bell showing (1) Absorber surface and (2) window/outlet. Source: Benedittini et al. (1981) ...... 18 Figure 8 - Multi-channel chimney. Source: Zhao et al. (2015) ...... 19 Figure 9 - Innovative design for solar chimney generator. Source: Shahreza & Imani (2015)...... 20 Figure 10 – Midline perforated chimney. Source: Lei et al. (2016) ...... 20 Figure 11 - Discontinuous fin geometry from the design. Source: Hosseini et al. (2017). .... 21 Figure 12 - Windcatcher used in conjunction with solar chimney. Source:Bansal et al. (1994)...... 24 Figure 13 - The tested design of Flat (left), groove (centre) and chevron (right). Source: El- Sawi et al. (2010)...... 26 Figure 14 - Multi V design. Source: Patil (2012)...... 26 Figure 15 - SpaceClaim geometry of tinted glazing chimney with various regions and boundaries labelled...... 29 Figure 16 – ICEM geometry of a solar chimney with a horizontal baffle. The right image shows a section around the baffle, where the wall thicknesses of the baffle, absorber and glazing can be seen...... 29 Figure 17 - Section showing mesh near chimney outlet in ICEM CFD...... 30 Figure 18 - Comparison with the experimental data for a 1m Chimney across a range of air- gap widths with an input heat flux of (a) 400 Wm-2, (b) 600 Wm-2 and (c) 800 Wm-2...... 35 Figure 19 - Comparison of the temperature profiles across the chimney outlet...... 36 Figure 20 - Comparison of the velocity profiles at the chimney outlet...... 36
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Figure 21- Comparison of the temperature profiles at 95% of the chimney height between the present CFD study and the experiment for a 700mm height chimney for Rayleigh numbers of a) 4.19 × 1011 b) 2.79 × 1011 and c) 8.05 × 1010...... 37 Figure 22 - Diagram showing labelling of each section of the chimney. The 3D model is a mock-up only to give a better visual representation. Each section receives the same total heat flux, for example: the heat flux of glazing A added to the heat flux on absorber A is the same as the sum of the heat flux on glazing B plus the heat flux on absorber B...... 39 Figure 23 - Effect of tinting on the predicted mass flow rate without a radiation model. (a) Combinations 1-4; (b) Combinations 4-7; (c) Combinations 4 and 8-11. Combinations are taken from Table 4 ...... 41 Figure 24 – Stream function at the chimney outlet, showing reverse flow for the unaltered chimney (left) and no reverse flow for the full height tinted chimney (Combination 4) with 50% of irradiance retained by the glazing (right)...... 42 Figure 25 – Predicted ventilation performance of solar chimney when radiation model is enabled, showing no significant benefit from the addition of tinting. All the four combinations have nearly identical performance...... 43 Figure 26 – Stream function at the outlet of the unaltered solar chimney with radiation model enabled, showing no reverse flow...... 43 Figure 27 – Solar chimney with a horizontal baffle on the absorber wall...... 46 Figure 28 - Signal to Noise plots for the five variables tested. Showing increasing height, lower aspect ratio (a wider chimney) and heat flux leading to increased performance...... 48 Figure 29 - Performance of solar chimney when the baffle is placed at 60% of the chimney height, showing the same performance across all the tested baffle widths. The results demonstrate that a single variable Taguchi analysis does not fully resolve the scenario...... 49 Figure 30 - Interaction plot of the baffle height and baffle width, showing peak performance with a baffle of 30% of the air-gap width placed at 80% of the baffle height...... 50 Figure 31 - Performance of solar chimney with the baffle placed at 80% of the chimney height, which peaks with a baffle of 30% of the air-gap width...... 51 Figure 32 – Contours of stream function for (a) the unaltered chimney; (b) chimney with a baffle at 40% of the chimney height and of 30% of the chimney air-gap width; and (c) ‘ideal’ chimney with a baffle at 80% of the chimney height and of 30% of the air-gap width. Results are obtained with 2.0m chimney height, 0.4m chimney air-gap width and 600 Wm-2 heat flux...... 53
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Figure 33 - Density profile at the chimney outlet showing lower average density for the ideal chimney. Results are obtained with 2.0m chimney height, 0.4m chimney air-gap width and 600 Wm-2 heat flux...... 54 Figure 34 - Temperature contours for the unaltered chimney (left) compared to the ideal chimney with a baffle at 80% of the height and of 30% of the air-gap width (right). Results are obtained with 2.0m chimney height, 0.4m chimney air-gap width and 600 Wm-2 heat flux. . 54 Figure 35 – Stream function at the chimney outlet for a 2m high and 0.4m air gap width chimney subject to a 600 Wm-2 heat flux. When a baffle is present, it is placed at 80% of the chimney height. (a) no baffle, (b)-(f) with a baffle of (b) 10%, (c) 20%, (d) 30%, (e) 40% and (f) 60% of the air-gap width...... 55 Figure 36 - Single variable analysis plot for no radiation model simulations...... 56 Figure 37 - Interaction between the baffle height and baffle width when radiation model is disabled...... 56 Figure 38 – Contours of stream function showing the recirculation zone in an unaltered chimney (left) and the chimney with a horizontal baffle at 80% of the chimney height and 20% of the chimney air-gap width (right). Results are obtained with 2m chimney height, 0.4m air- gap width and 600 Wm-2 heat flux...... 57 Figure 39 – Contours of stream function for an unaltered chimney (left), a chimney with baffle at 90% of the chimney height and of 10% of the air-gap width (middle) and a chimney at 90% of the chimney height and of 20% of the air-gap width (right). Results are obtained with radiation model disabled with a 1.5m chimney height, 0.225m chimney air-gap width and 600 Wm-2 heat flux...... 59 Figure 40 – Contours of stream function for an unaltered chimney (left), a chimney with baffle at 90% of the chimney height and of 10% of the air-gap width (middle) and a chimney with baffle at 90% of the chimney height and of 20% of the air-gap width (right). Results are obtained with 1m chimney height, 0.2m chimney air-gap width and 600 Wm-2 heat flux. .... 60 Figure 41 – Contours of stream function obtained with the baffle at 90% of the chimney height and of 20% of the chimney air-gap width with solar irradiance at 400 Wm-2 (left), 600 Wm-2 (middle) and 800 Wm-2 (right) respectively. Results are obtained with 2m chimney height and 0.3m chimney air-gap width...... 60 Figure 42 - Comparison of the contours of stream function for unaltered chimney (left), a chimney with a baffle at 80% of the chimney height and of 30% of the air-gap width (middle), and a chimney with a baffle at 90% of the chimney height and of 10% of the air-gap width
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(right). All results are obtained with radiation model enabled for a 2m chimney height, 0.3m chimney air-gap width and 600 Wm-2 heat flux...... 62 Figure 43 - Stepped Horizontal Baffle (left) and wedge-shaped baffle (right)...... 66 Figure 44 - Snippet of SpaceClaim script written in iron python ...... 71 Figure 45 – Section of code used on HPC...... 72 Figure 46 - The stream function for a 2m high chimney, a 0.4m air-gap width, for a baffle of 30% of the chimney air-gap width and 600 Wm-2 heat flux. From left to right: no baffle, baffle at 20% of the height, baffle at 40% of the height, baffle at 60% of the height, and baffle at 80% of the height...... 74
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List of Tables
Table 1 - Recommended inclination angle by latitude...... 13 Table 2 - Materials properties of Aluminium and Glass Glazing ...... 31 Table 3 - Mesh dependency test results – predicted air mass flow rate using different meshes...... 33 Table 4 - Combinations of Tinting application for solar chimney showing when a portion of the glazing receives tinting – a red square shows when there is application of tint to that portion of the glazing. The letters correspond to the letters in Figure 22...... 39 Table 5 - Results for the range of simulations from the orthogonal array shown in ...... 49 Table 6 – Follow-up simulation results with comparisons between the ideal baffle configuration (at 80% of the chimney height and of 30% of the chimney air-gap width) and the unaltered solar chimney...... 52 Table 7 - Expanded simulation results comparisons between the ideal baffle configuration (at 80% of the chimney height and of 20% of the chimney air-gap width) and the unaltered solar chimney with the radiation model disabled...... 57 Table 8 - Performance of solar chimney with a baffle at 90% of the chimney height with radiation model disabled...... 59 Table 9 - Results with baffle at 90% of the chimney height and with radiation model enabled...... 61 Table 10 - Taguchi array results when radiation model is disabled...... 73
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Nomenclature
ACH air change per hour AR aspect ratio BH baffle height BW baffle width Cp specific heat capacity g gravity H chimney height k thermal conductivity LES large eddy simulation ṁ mass flow rate (kg/s) n simulation number for Taguchi analysis q'' surface heat flux Ra Rayleigh number S/N signal to noise ratio
T∞ ambient stream temperature
Tw wall temperature W chimney air-gap width Y output value for Taguchi analysis α thermal diffusivity β thermal expansion co-efficient μ dynamic viscosity ν kinematic viscosity ρ density Φ inclination angle
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1 Introduction
Until the modern era, thermal comfort was mostly achieved using natural ventilation methods. The techniques for natural ventilation were widespread and relatively easy to achieve on a large scale, with notable examples like the Sirocco room from Southern Italy, where cooling was driven by the Mediterranean Sirocco winds (and which also took advantage of evaporative cooling) and the Windcatcher which was ubiquitous around the Middle East and Persia. Examples of both solutions are shown in Figure 1. Both primarily took advantage of strong winds to achieve thermal comfort in buildings. Other passive techniques such as painting building exteriors white to reduce the amount of heat entering buildings were also used.
Figure 1 – Sirocco Room from Southern Italy (left) and Windcatchers in Yazd, Iran (right). Source: Saeli & Saeli (2015) and Jomehzadeh, Nejat, Calautit et al. (2017). With the advent of mechanical heating and cooling methods and the exacerbating effects of a changing climate, there has been an increase in global power demand to achieve thermal comfort. In 2010, heating, ventilation and air-conditioning (HVAC) was estimated to account for 32% of the energy sector according to Ürge-Vorsatz, Cabeza, Serrano et al. (2015), and the trends indicate that this number would have only increased since the time of the study. With an overwhelming majority of electricity generated using non-renewable and climate polluting fossil fuels, passive thermal comfort strategies would reduce the required power and carbon footprint. These technologies can also be propagated more readily throughout the developing world where there is often no dependable power infrastructure to allow for improved health outcomes and increased productivity.
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Natural Ventilation techniques include passive technologies such as cross-wind ventilation, solar heaters, solar dryers, Trombe walls, and solar chimneys. With the global trend of rising temperatures, the techniques which tend to increase the temperatures inside buildings may not be effective in reducing power consumption in buildings. This leads to the solar chimney being of particular interest to researchers as it is primarily used to reduce the temperature in buildings. When combined with other passive technologies, temperatures lower than ambient may be achieved. The Basis of Solar Chimney
Solar chimney uses solar irradiance to heat up an absorber, and the absorber then transfers that energy into the air in a glazing enclosed channel. The difference in the density of the warmer air relative to the cool ambient then causes a buoyancy driven flow. Figure 2 shows the general design of a solar chimney and the various components and the direction of air flow. Several important geometric properties of the solar chimney are shown such as the height (H), the air- gap width (W) and the inclination angle (Φ) as well as the absorber surface and the glazing. Solar chimneys can be used to both reduce building temperatures to the ambient temperature or heat up a room when the external temperature is low.
Figure 2 - Schematic of a solar chimney showing the absorber height (H), the air gap width (W) and the inclination angle (Φ).
Research into solar chimneys has been conducted since the 1980’s with the research by Bouchair & Fitzgerald (1988) cited by many researchers as being one of the earliest. There are
2 some more obscure references to similar cooling methods, but they are not considered as being traditional solar chimneys, one example being the Silvestrini Bell Benedittini, Mustacchi, Vagliera et al. (1981).
The interest in solar chimneys benefits from several positive features. Primarily it is a passive ventilation technique, requiring no electricity for operation and being more effective with higher solar irradiance. Apart from the initial capital costs, it is also carbon neutral, and therefore reducing the carbon footprint of the building which it helps to ventilate. Solar chimneys can also be integrated into the building façade, allowing for more aesthetic design options. Buoyancy Driven Flow The mechanism of buoyancy driven flow is what drives natural ventilation in the solar chimney. As mentioned previously, air is heated up in an enclosed channel to initiate a buoyant flow. This flow then pulls air from the interior of the building, which is either recirculated as warmer air for heating or ejected from the building to reduce the indoor temperature to the ambient temperature. The difference between the buoyancy driven flow and wind driven cross- ventilation is illustrated in Figure 3, which shows wind forcing air through a building, in contrast to hot air rising and leaving the building under the effect of buoyancy.
Figure 3 - The mechanism of wind driven ventilation (left) compared to buoyancy driven flow (right). There are several areas of interest when considering buoyancy driven flows and the design of a solar chimney. These include the thermal boundary layer and the viscous boundary layer. When air is heated by contact with a vertical hot surface (as is considered in the studies
3 described below) it develops both thermal and viscous boundary layers, profiles of which are shown in Figure 4. The mechanics of these boundary layers must be considered when developing effective designs for solar chimneys.
Figure 4 - The temperature and velocity profiles in thermal and viscous boundary layers. Source: Mohaghegh & Abolfazli (2016).
The temperature is the highest at the wall (Tw) and then diminishes to the ambient temperature
(T∞), while the velocity is zero at the wall, increasing to a maximum within the thermal boundary layer and then diminishing to zero away from the wall. The region from the hot wall to the location where the temperature reaches the ambient temperature is referred to as the thermal boundary layer, the thickness of which is denoted by δt; and the region from the wall to the location where the velocity reaches zero is referred to as the velocity boundary layer, the thickness of which is denoted by δ. Both the thermal and velocity boundary layers are illustrated in Figure 4. The behaviour of these two boundary layers as a result of the geometry and other modification to a solar chimney is considered in this study. Laminar vs Turbulent In pressure driven flows, the Reynolds number is used to characterise the properties of the fluid flows. If the Reynolds number is below a critical value, the flow is laminar. As the Reynolds number increases beyond a critical value, the flow transitions to turbulent. However, In the case of buoyancy driven flows, the Reynolds number is not relevant due to the lack of an externally imposed velocity. Instead, the non-dimensional Rayleigh number, as defined in Equation (1), is used to characterise buoyancy driven flows. For the convective air flow adjacent to an isothermally heated plate, the Rayleigh number is defined as
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휌훽(푇 −푇 )퐻3푔 푅푎 = 푤 ∞ (1) µ훼
where:
휌 – the density of air
훽 – the thermal expansion co-efficient of air
푇푤 – the temperature at the wall surface
푇∞ - the ambient temperature
H – a characteristic length scale (in this case the height of the absorber wall)
푔 – the gravitational acceleration
µ - the dynamic viscosity of air
훼 – the thermal diffusivity of air
However, in solar chimney applications, the temperature of the absorber wall is usually unknown, non-uniform and not constant across the entire absorber surface. It is more appropriate to treat the absorber wall as an isoflux surface due to the absorption of the incident solar radiation. Accordingly, an alternative definition of the Rayleigh number is adopted in the present investigation:
훽퐻4푞′′푔 푅푎 = (2) 훼휈푘 where:
푞′′ - the input heat flux
휈 – the kinematic viscosity of air
푘 – the thermal conductivity of air
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The Rayleigh number can give an indication of the flow regime inside a solar chimney. Transition to a turbulent flow inside the chimney begins around the Rayleigh number Ra = 2 × 1013 and the flow becomes fully turbulent around Ra = 1014 according to Chen, Bandopadhayay, Halldorsson et al. (2003). Most simulations performed in this study are in the laminar flow regime, with the largest chimney sizes simulated approaching a fully turbulent flow. Aims and Objectives The aim of this project is to determine, if any, the effects of various design parameters and features on the performance of a solar chimney. All simulations are conducted for a vertical solar chimney with a uniform input heat flux. The effects of the chimney height, air-gap width and incident solar irradiance have all been studied in considerable depth in the literature. Further, the azimuth, orientation and inclination angles of roof-top solar chimney have also seen considerable research. The findings of these studies are presented briefly in the literature review of interest. Further discussion of these areas is not relevant to this thesis as it is concerned with the effects of passive design options for a wall mounted vertical chimney. While a range of chimney heights, air-gap widths and incident solar irradiance are tested, the chimney designs considered in this study are in a vertical configuration only. The primary foci of the study are:
• To determine the validity of the adopted computational fluid dynamic (CFD) models through comparison with previous experimental data and numerical analysis; • To determine the effect of tinting applied to the glazing and whether it improves the performance of vertically oriented solar chimney or not; • To determine the effect of the introduction of a horizontal baffle on a vertical absorber wall and the optimal baffle configuration using the Taguchi method. Outcomes and Contributions
The outcomes and contributions of the present study include:
• Determine the optimum horizontal baffle configurations for maximum performance across a range of parameters; • Increase solar chimney performance through relatively minor changes and additions; • Further promote the adoption of the Taguchi method for optimum designs to allow for more extensive ranges of parameters for study;
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• Further promote the use of solar chimney as a valid ventilation technique to reduce carbon footprint; • Combine the Taguchi method with automation techniques and high-performance computing to allow for extensive parametric analysis in CFD studies.
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2 Literature Review Review Articles of the Solar Chimney Literature
Multiple reviews of the existing literature have been conducted on solar chimney to provide consolidated accounts and more clarity as to the various design recommendations. Two such reviews are of considerable depths.
Khanal & Lei (2011) performed a comprehensive review of the solar chimney research available at the time. It discusses in a reasonable depth a breakdown of the existing solar chimney research and the findings of ideal geometry at the time. It shows some of the values that are associated with ideal performance but also mentions how they can be in contradiction of one another. The conclusion given by the authors is that, at the time, further studies into solar chimney and associated phenomena are needed.
Shi, Zhang, Yang et al. (2018) presented an extensive review to determine the primary design considerations for solar chimneys, covering approximately fifty studies that attempted to conduct parametric analysis, both numerically and experimentally. Several conclusions are given about the effects of the chimney height, air-gap width, inclination angles and glazing etc. The ideal inclination is commonly reported to be 45°, the optimum air gap width of 0.2 to 0.3m, and a height to air-gap width ratio of around 10. Double glazing is also recommended. These configurations are not universally claimed in the literature as being ideal, but the numbers given are relatively common, particularly the claimed best performing air-gap width. There is also discussion of surface roughness features and other novel design concepts, but there has not been significant research in that area, and no recommendation is made in the review. Design Considerations
There have been considerable studies into the various design parameters of the solar chimney, with particular interest in the air-gap width, height, aspect ratio (height to air-gap width), absorptive surface shape and surface orientation. A portion of the studies validate various sized chimneys as being able to achieve adequate ventilation performance or thermal comfort. Most studies identify and test multiple parameters. In this review, existing studies are grouped by the main parameters under investigation.
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2.2.1 Chimney Height In general, a taller solar chimney gives a better performance as there is more absorber surface to catch the incoming solar irradiance.
Although not specifically conducted to find an ideal height, Bansal, Mathur, & Bhandari (1992) carried out a numerical study of a solar chimney of 1.5m height with a collector area of 2.25m2 to demonstrate its viability as a ventilation strategy and identify critical design parameters. The chimney height, cross-sectional area (and therefore the air gap width) and temperatures at inlet and outlet are listed as the critical parameters. The numerically derived results show that a solar chimney of the tested size can achieve a ventilation rate of 100-350 m3/hr, depending upon the incident radiation. Though limited by a lack of experimental data to verify the claim, this study demonstrates that the solar chimney ventilation technique can produce a high air change rate with a relatively small chimney.
There have been some comparative studies between vertical solar chimneys which are integrated into the wall and roof mounted solar chimneys. M. M. AboulNaga & Abdrabboh (2000) combined these designs and numerically calculated the performance of a combined rooftop and wall mounted solar chimney, showing that the combined system achieved a flow rate three times higher than that of the roof chimney alone. Calculations were carried out using a spreadsheet program to determine the effect of the various parameters on the combined design. From this study, a larger chimney height was shown to provide a higher flow rate, as expected. The chimney considered in this analysis was quite large, with a maximum height of 3.45m.
To facilitate the introduction of solar chimney into existing buildings, Mathur, Bansal, Mathur et al. (2006) studied “small size” solar chimneys, with a chimney height of 1m and a width of 1m. The small size of the solar chimney was synonymous to the dimensions of windows, and thus could be fitted to an existing window frame. A range of chimney air-gap sizes, heights and inlet sizes were tested for a series of input heat fluxes. They reported that the chimney performance, when measured in air change per hour (ACH), ranged from 2.4 to 5.8 ACH for a 27m3 room.
A large scale study of the effect of chimney height was conducted by Ryan & Burek (2010), who tested a range of heights, air-gap widths and heat fluxes. For a 1m width, these values varied from 0.5m to 2m for the chimney height and 20mm to 150mm for the air-gap width, and the heat fluxes ranged from 200-W/m2 to 1000-W/m2. The results indicated that an increase of
9 the chimney height increases the overall performance, as does the increase of the air-gap size. As expected, there is an increased performance as the input heat flux is increased. For these chimney sizes, no reverse flow is observed.
Some studies conducted on solar chimneys are very specific in the dwelling in which they are installed, as opposed to the more general free standing studies. Chung, Ahmad, Ossen et al. (2015) tested a series of dimensions for solar chimneys for ventilation of a Malaysia terrace house. Five air gap widths were tested with a 2m chimney height, and six chimney heights were tested with a 1m air gap width. In their study, even though chimneys with larger air-gap widths achieved higher flow rates, a reverse flow was observed, which resulted in hot air heated by the chimney flowing into the ventilated room, causing discomfort to occupants.
Parametric studies are common for assessing solar chimney performance. For instance, Saleem, Bady, Ookawara et al. (2016) did a very extensive study of 1,586 design combinations using MATLAB to find an ideal chimney geometry. The study indicated that a chimney of 1.4m height, 0.6m width, 0.2m air gap width and 45° inclination angle gave the best ventilation throughout the course of the day when simulated with local weather data for Alexandria, Egypt. The calculated values from the MATLAB program were validated against simulations and experimental data.
2.2.2 Inclination Angle and Orientation The solar chimney designs considered in this thesis are for a vertical wall mounted chimney only. Therefore, the inclination angle and orientation are not relevant to this thesis. The literature review on these parameters of the solar chimney described here is to establish a thorough understanding of the solar chimney as a whole and to investigate if there were any novel designs for possible future research given the viability of proposed designs.
To optimise the solar chimney performance at a given location, the incoming energy, in the form of captured solar irradiance, needs to be maximised. Several criteria can be adopted to determine the ideal orientation and inclination, such as the highest instantaneous performance, the highest overall performance, or ideal time of operation. Determining the ideal inclination angle is the subject of a considerable number of studies. Figure 5 shows a popular method of determining ideal orientation, adopted by Saleem et al. (2016) and several others, where the incident solar radiation is determined based on the position of the sun and the local solar irradiance data for a given latitude and longitude. The method shown in Figure 5 is used to determine the location of the sun relative to the solar chimney and calculate the amount of solar
10 irradiance reaching the surface of the absorber using plane projections, so that an appropriate heat flux can be applied to simulate the incident radiation. More details about the use of Figure 5 can be found in Saleem et al. (2016).
Figure 5 – Model used for determining solar position in Saleem et al. (2016). To determine which direction to face a vertical solar chimney, Bouchair & Fitzgerald (1988) carried out research on the directional orientation (Azimuth) of the active wall. The testing was conducted for a chimney located at 33 degrees north. With a range of directions being tested, facing the chimney to the West gave the best overall performance. The reason given for this is that, in the afternoon, the sun is more directly in the west. It was discussed that, as one moves closer to the equator, an east or west facing chimney would give a better overall performance during the day.
A mathematical analyses to determine the optimum inclination angle was conducted by Prasad & Chandra (1990). Two criteria were adopted for optimisation, maximum collected irradiance and maximum flow rate. The ideal value for each criterion is considered year-round, with an ideal angle for each month of the year, shown in Figure 6. For use as a ventilation strategy during the hotter months (during summer), approximately 52° is ideal for achieving the maximum flow rate, while for use as a room heater (maximum collected irradiance), approximately 54° is ideal. This overlap of angles is fortuitous as a solar chimney constructed between these angles will give good performance as both a heater and a ventilation device. It
11 is worth noting that this study was conducted for the Mediterranean and middle eastern latitudes. The results will differ closer to the equator or the south and north poles.
Figure 6 - Optimum angle for each month of the year. Source: Prasad & Chandra (1990). Solar panels may overheat and have reduced performance if not well ventilated. To bring ambient air to reduce the temperature of solar panels, a solar chimney design has been used. Sandberg & Moshfegh (1998) tested several variables to determine which angle and panel location would be the best. The solar panel location is not of particular interest for this study, but the ideal angle is something which could be applied to building ventilation. For the range of values tested, it was found that an inclination angle of 50° gave the highest flow rate. The numerical values were validated with experiment.
During the course of their investigation of solar chimney for building ventilation, AboulNaga (1998) calculated the best performing inclination angle to be 35°. The inclination angle differs from those found by later researchers, but this is most likely due to the limitation in the number of angles tested as the full range of values up to the vertical position was not compared.
The effect of the inclination angle was also tested for a fixed height and air-gap by Chen et al. (2003), for angles starting at 90° and decreasing to 30° from the horizontal. The result showed that an angle of 45° resulted in significant performance enhancement, giving a 45% increase in the air-flow rate compared to the corresponding vertical system. This study did not consider the location of the sun in the sky, giving variable irradiance dependent upon the angle. 12
Zhai, Dai, & Wang (2005b) experimentally tested an inclined solar chimney to determine the ideal inclination angle, which was done by using an electric heating mat to simulate the input heat flux. With this experimental setup, 45° provided the maximum flow rate. A deficiency of their study is that it did not consider the solar orientation when claiming that 45° was the ideal angle. Comparing with other investigations, there is a difference in the reported ideal inclination angles and how the ideal inclination angle is determined, as this study does not take into account the difference in the incident radiation due to different orientation.
Mathur, Mathur, & Anupma (2006) performed an intensive study of the effects of the inclination angle, chimney air-gap width and inlet sizes during the progression of a day, comparing both theoretical and experimental values. For the tested samples, it was found that 45° inclination achieved the best performance compared to 30° and 60° inclinations. The recommendation for the ideal inclination angle came with the caveat of adjusting the angle based upon latitude. The recommended ideal angle is shown in Table 1. For the effects of the inlet and air gap sizes, they reported that larger sizes generally performed better.
Table 1 - Recommended inclination angle by latitude. Source: Mathur et al., (2006b).
To confirm the ideal inclination angle of solar chimney for the location of Edinburgh, Scotland, which lies at a latitude of 55.9° north, Harris & Helwig (2007) tested a range of angles using the solar irradiance data for the local area. In their study, it was found that 62.5° inclination
13 angle provided the maximum ventilation rate. However, the 90° inclination (i.e. a vertical chimney) provided a performance very close to that of 62.5°, while the 45° inclination was within 15% of the best performance. These results align reasonably well with the results obtained by Mathur, Bansal, et al. (2006).
Sakonidou, Karapantsios, Balouktsis et al. (2008) developed numerical models to determine the optimum inclination angle by investigating the amount of energy that a given angle would receive from the same solar irradiance. The ideal angle was determined in a similar method to Prasad & Chandra (1990) by comparing which angle would give the maximum flow and which angle would give the maximum absorbed solar irradiance. Given the study was conducted for the solar conditions in Greece rather than the solar irradiance received by Iran, there was no overlap in the ideal angles for heating application and the ideal ventilation angle. It was reported that the ideal angle for heating in winter was 45°, while the ideal ventilation angle for summer was 63°. The angle differs from the optimum angle identified by Mathur, Mathur, et al. (2006)
Mahdavinejad, Fakhari, & Alipoor (2013) tested for the ideal angle across a series of latitudes ranging from 27° North to 38° North. The solar chimney tested for their study was quite large, with a total absorber length of 8m and chimney air-gap width of 0.15m, leading to an aspect ratio of over 50, which is quite high. The results of their study are consistent with those of Mathur, Bansal, et al. (2006)but differ from those of Sakonidou (2008).
A Mathematical analysis was conducted by Imran, Jalil, & Ahmed (2015) to test the effects of the air-gap, chimney width, input heat flux and inclination angle. A reasonable range of values was tested, with the numerical results validated by experiment. It was found that an increase of the air-gap width or solar irradiance resulted in increased exit velocities. The optimum angle was found to be 60°, with the performance increasing as the inclination angle was increased. Since 60° was the maximum angle they tested, it is not shown if further increasing the inclination angle until the vertical position would give better performance.
2.2.3 Ideal Air-Gap Width and Aspect Ratio AboulNaga (1998) studied the performance of solar chimneys for providing cooling in Al-Ain, located in the United Arab Emirates. In that study, with a chimney height of 1.5m, the ideal air-gap width was determined as 0.2m, giving an aspect ratio (AR) of 7.5. A chimney air-gap width of less than 0.1m was discouraged by the author.
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Balocco (2002) simulated the performance of a chimney with different air-gap widths. The study evidenced that increasing the air-gap width could increase the flow rate through a solar chimney. Testing was conducted for a chimney height of 6m and air-gap widths of 7cm to 35cm. The 35cm chimney reduced summer over-heating by 27.5%. An Aspect ratio between 20 and 30 was recommended.
Ong & Chow (2003) developed mathematical models to predict the performance of solar chimneys. An experimental setup was constructed to test the validity of these models and to obtain data. The chimney had a height of 1.875m, a width of 0.45m and air-gap widths ranging from 0.1m to 0.3m. Their experimental results showed that a 0.3m air gap had 56% better performance compared to a 0.1m air gap when subjected to 650 W/m2 incident radiation. It was concluded that the mathematical model shows a reasonable agreement with the experimental data, and that for a chimney width up to 0.3m, no reverse flow was observed. This observation is worth noting as reverse flow was observed in much smaller chimneys.
In a large experimental study, Chen et al. (2003) attempted to identify the ideal air-gap ratio and inclination angle for a range of incident solar radiation values. The results of their study are used extensively in the literature for CFD validation. This study is notable for the very large chimney air-gap widths tested, with the aspect ratio down to 2.5. It was reported that with a chimney height of 1.5m, a chimney air-gap width of 0.6m gave the maximum flow rate. Reverse flow was observed during the experiment when the air-gap width was increased to 0.3m. However, according to their study, the effect of the reverse flow was not significant. Further, the improvement in the performance by increasing the chimney air-gap width to values larger than 0.2m was not significant.
Guohui Gan (2006) conducted a CFD study to determine the optimum air-gap width for a solar chimney. For a 6m high chimney, the optimum width was reported to be between 0.55m and 0.6m, giving an aspect ratio of approximately 10. With the 0.6m air-gap width, some reverse flow was observed. It was also mentioned that even at air-gap widths as low as 0.2m, reverse flow may occur due to heat losses to the environment via the external glazing. The study also investigated the effect on performance if the entire glazing was replaced with a photovoltaic cell, which showed a similar performance to the standard glazing chimneys.
The roof wall integrated chimney, which was shown by M. M. AboulNaga & Abdrabboh (2000) to be better than a single roof-mounted or wall-integrated chimney was further studied by Wei, Qirong, & Jincui (2011). Their investigations found the optimal height to air-gap ratio
15 to be 12:1. Interestingly, their study identified an ideal inclination angle of 14° from the horizontal. This was because of the unconventional geometry of the studied chimney and likely due to the small range and limited resolution of the inclination angles (between 12° and 14°) over which the study was conducted. Due to the same reasons, the conclusion regarding this inclination angle as being ideal cannot confidently be transferred to other chimney designs.
Jing, Chen, & Li (2015) experimentally tested a series of aspect ratios, from 5 to 1.667. The purpose of their study was to develop experimental models to accurately characterise chimney flow behaviour as the existing models often overpredicted the airflow for solar chimneys. Their results showed that the optimum air flow rate was achieved at an aspect ratio of 2. The issue of reverse flow was discussed in depth, particularly when in comparison to the experiments of Ryan & Burek (2010), which showed that, as the air-gap was increased, reverse flow began to form. An empirical expression for calculating the flow rate was developed and showed reasonable agreement with the available experimental data.
2.2.4 Other Attributes and Materials To test the effect of changes to the glazing configurations on the performance of a solar chimney, Guohui. Gan & Riffat (1998) conducted a CFD analysis of single, double and triple glazing configurations for winter operation. Triple glazing was shown to be the best performer for the design considerations, which would make it ideal for use as a heating strategy in winter months. However, it was not discussed whether the increased chimney internal temperatures help to improve summer performance or not.
Although not strictly focused on solar chimney, the study for room ventilation conducted by Ziskind, Dubrovsky, & Letan (2002) investigated several technologies of achieving thermal comfort. Of particular interest in this study is the identification of room stratification as being significant and that the inlet for a solar chimney should be as high as possible in a room to remove the hottest air and thereby reduce the overall room temperature.
To determine how much effect the construction materials would have on the performance of a solar chimney, Pavlou, Vasilakopoulou, & Santamouris (2009) tested many optical and material properties of the glazing, such as double glazing low emissivity glass, which improved performance by 18%. The study indicated that the effect of materials choice, even something as innocuous as the emissivity of the glazing, could have a significant effect on the performance of the chimney. This gives credence to the claims made by Guohui. Gan & Riffat (1998).
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Amori & Mohammed (2012) numerically and experimentally studied the effects of certain design parameters on solar chimney performance. The results of their study identified an optimum side inlet position, located underneath the absorber. However, the variation of the results did not appear to be significant with only a small variance between the different inlet configurations. A chimney with a 75° inclination angle was also tested, which gave higher flow rates and temperatures than the vertical chimney. They further reported that the introduction of a PCM increased the working time of the chimney after sunset by about 5 hours but reduced the maximum flow rate.
Shi & Zhang (2016) tested the effect of the geometry of a room on the performance of a solar chimney. The study concluded that the geometry of the room has a negligible effect on the chimney performance. This is an important finding as it means that CFD studies that include either no room or simplified room geometry may retain a reasonable degree of accuracy.
2.2.5 Alternative Designs Many “non-standard” and innovative chimney designs and configurations have been identified in the literature. Additionally, some designs that are not strictly for ventilation are shown for their possible application to solar chimney strategies.
An incredibly innovative design, first proposed by G. V. Silvestrini and then later investigated by Benedittini et al. (1981) led to the concept and then the development of the Silvestrini bell, which is a conical roof mounted solar chimney design. Figure 7 shows the simplified design of a Silvestrini Bell. Benedittini et al. (1981) presented the equations for deriving the heat transfer and flow rates. The Silvestrini bell was described as a cone of heat storage material with a glass cover for insulation, with a window for the admission of natural light or the removal of hot air. Four experiments were conducted for Silvestrini Bell designs, which showed a good agreement with the derived mathematical models. Surprisingly, further research into this design has not been reported, given that it is a relatively simple addition to a building that allows for reasonable ventilation rates.
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Figure 7 - Simple geometry of the Silvestrini Bell showing (1) Absorber surface and (2) window/outlet. Source: Benedittini et al. (1981) To increase the effectiveness of having air on both sides of the absorber, Zhai, Dai, & Wang (2005a) compared two roof solar chimney designs - one with a single flow channel (single pass) and the other has two flow channels (double pass, one on top of the other). It was found that a double pass roof solar chimney could achieve an ACH of 20, while a single pass chimney only achieved an ACH of 12, representing a 60% difference in the performance. It was also recommended that two shorter chimneys should be used instead of one longer chimney, if possible, due to the increased thermal efficiency of smaller chimneys.
Following the review of solar chimney technologies, Khanal & Lei (2012) conducted numerical investigations to develop a better understanding of reverse flow in solar chimneys. The study concluded that the reverse flow had a significant negative effect on the flow rate through a solar chimney. A new design was proposed to resolve this issue, that is, the inclined passive wall solar chimney (IPWSC), in which the air channel narrows towards the outlet. This chimney design was researched further, both numerically and experimentally (Khanal & Lei (2014, (2015)) across a range of Rayleigh numbers. A solar chimney with a height of 0.7m and a base width of 0.1m was tested across a range of passive wall inclination angles subject to different heat fluxes. The studies showed significant enhancement of the performance over the standard vertical passive wall solar chimney, and the enhancement depends on the heat flux, and thus the extent of the reverse flow. It was reported that the inclined passive wall chimney with an angle of 6° and a heat flux of 500 W/m2 was able to provide sufficient air change per hour to ventilate a room of 27 m3 to satisfy the ASHREA standards. The study showed that there was an ideal angle of inclination corresponding to each heat flux condition. Manca, Nardini, Romano et al. (2014) also conducted research on the effect of the inclined passive wall. Although not as rigorous with the range of values as those tested in the previous study, the results were just promising, showing a scaling effect to the introduction of the IPWSC as
18 the chimney used in the study was more than 5 times the size of those used by Khanal & Lei (2014). For the series of heat fluxes tested the inclined chimney performed better than the standard vertical chimney.
Zhao, Lei, & Wang (2015) studied a multi-channel chimney design. This design involved the introduction of an “endothermic triangle wall” (shown in Figure 8). The endothermic triangle replaced the active wall as the heat providing surface in the solar chimney system. The endothermic triangle wall design showed a performance increase over the conventional design by 18% to 25.4% depending on the dimensions. Although not achieving the same performance increase as the chimney developed by Zhai et al. (2005a), it further proves that, if the absorber is exposed to two or more channels of air, it increases chimney performance.
Figure 8 - Multi-channel chimney. Source: Zhao et al. (2015) Shahreza & Imani (2015) developed an innovative chimney design to test a concept. Using intensifier mirrors and a conical heat surface they were able to achieve very high volumetric flow rates. By focusing light from outside the chimney onto the absorber, it was clear that it would achieve high flow rates simply due to the large amount of solar irradiance. When tested, the chimney shown in Figure 9 achieved a maximum flow velocity of 5.12 m/s. Although intended for solar chimney power plants, it could be applied to ventilation systems. If this were applied as a rooftop chimney, it is possible to compete with traditional roof ventilators. However, it needs to be compared with a solar chimney of an equivalent size to the mirrors to see if the new design is also more efficient.
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Figure 9 - Innovative design for solar chimney generator. Source: Shahreza & Imani (2015). As a variant of the multi-channel designs, Lei, Zhang, Wang et al. (2016) introduced a perforated plate into the midline of a conventional inclined solar chimney, as shown in Figure 10, which shows a cutaway image of the chimney design, where the secondary absorber can be seen in front of the original absorber plate. The holes in the secondary plate are to allow sunlight to reach the rear absorber, increasing the amount of warm surface area in contact with the air. The effect of this introduced plate was tested across a range of chimney widths and inclination angles. At 60° inclination and 1m length, with an air-gap width of 0.5m, the perforated plate chimney outperformed the conventional chimney by 35.0-39.7%. The performance of this chimney design seems superior to that developed by Zhao et al. (2015), but further testing needs to be done to compare the two designs.
Figure 10 – Midline perforated chimney. Source: Lei et al. (2016) Mokheimer, Shakeel, & Al-Sadah (2017) investigated a solar chimney design with specifications aimed at preventing direct solar gain on buildings rather than direct ventilation,
20 particularly in arid or desert regions. The study compared several absorber configurations such as glass absorber, opaque white or black absorber and a glass absorber attached to a porous metal foam. With chimney dimensions of 3m height, 1m width and a 0.3m air-gap width, the porous metal absorber gave both the best building temperature reduction and the highest flow rate. The use of a 15cm porous metal foam increased the amount of heat that is transferred into the air in the chimney, increasing the buoyant effect and reducing the convective transfer into the building. The proposed uses of such a design were interesting, such as using the high temperature air to desalinate water.
Hosseini, Ramiar, & Ranjbar (2017) studied the effect of an altered absorber wall geometry on the performance of solar chimney. Rectangular ducts of different sizes were put on the absorber wall that ran vertically from inlet to outlet. A range of design parameters were tested such as the depth and width of the horizontal channels and the introduction of horizontal breaks at half the height of the solar chimney, which are shown in Figure 7. These ducts were shown to have a considerable effect on increasing the mass flow rate. The design is similar in geometry to heat sinks used in electronics and in the power transmission industry. This study concluded that changing the surface geometry of a solar chimney can provide an increase of performance by approximately 10% compared to a conventional chimney.
Figure 11 - Discontinuous fin geometry from the design. Source: Hosseini et al. (2017). Zavala-Guillén, Xamán, Hernández-Pérez et al. (2018) studied a double sided vertical solar chimney for several air-gap widths and heights. The chimney had a central absorber and glazing walls on both sides. The ideal orientation was facing east-west to receive morning and evening
21 solar irradiance while receiving a minimum flux at noon due to orientation. The maximum height of 2m was identified as the best option for the vertical span of the double-sided chimney. A chimney air-gap width of 0.125m was determined as the ideal width, which was considered a slightly lower ratio than that was considered ideal for single channel chimneys. This could be in part due to that the same energy is given to heating double the amount of air. When both air-gaps are considered, the aspect ratio is approaching the recommended ideal value of 10. With the introduction of reflectors as used by Shahreza & Imani (2015), the east-west facing chimney may have good performance throughout the day. CFD for Solar Chimney CFD methods are constantly being developed to accurately simulate the performance of a solar chimney, to varying degrees of success. Many changes can be made to mathematical models and CFD methods to replicate real world conditions. Attempts were made by many researchers to show which models come closest to experimental data obtained. For instance, Bacharoudis, Vrachopoulos, Koukou et al. (2007) and Raghib Shakeel, Al-Sadah, & Mokheimer (2017) tested many CFD models for accuracy, with the former considering the k-ε model to be more accurate while the latter giving the k-ω as the best for conducting CFD simulations. Guohui Gan (2010) tested the effect of the computational domain on the results of CFD studies. It was reported that, by enlarging the computational domain to several times larger than the chimney dimensions, more accurate simulation of flow at the inlet and outlet locations can be achieved. This was applicable to most design simulations of both complex and simple systems. The difference between the small and large domain simulations was significant. The estimated difference in the CFD predictions was a minimum of 4% for the studies mentioned above.
Ugurlubilek. (2016) tested a number of turbulence models for an extensive range of Rayleigh numbers for a differentially heated model. The standard k-ε, RNG k-ε, Realizable k-ε (RKE), Standard k-ω, SST k-ω and Reynolds Stress Model are compared with each other and with the previous CFD studies for Rayleigh numbers ranging from 108 to 1013. It was reported that for low Rayleigh numbers, the results of the various turbulence models were similar, and as the Rayleigh number approached and extended beyond 1013, a 3D RANS model would produce better results.
Ji (2014) compared turbulence modelling for three scenarios, which include two sizes for 2D differentially heated cavities and a 3D open ended cavity heated on only one wall with the other walls modelled as adiabatic. Radiation is also modelled in these CFD studies. The four
22 turbulence models compared were the standard k-ε, RNG k-ε, standard k-ω and SST k-ω. When compared to the experimental data from the literature, the standard k-ω model gave values that aligned the closest with experimental values, suggesting that the k-ω model being better than the other turbulence models. The finding derived from their study is consistent with that recommended by the ANSYS user manual for natural convection CFD scenarios.
When one considers the inherent issues of experimental data and that many researchers give error margins of 10% to 15% for the ubiquitous hot-wire anemometer, it is worth questioning how much fitting and tailoring should be done in the CFD. For example, the experiment conducted by Chen et al. (2003) is often used for validating CFD models. If the measurements taken in this experiment have a considerable error, then that same error is carried over into the simulation. Fortunately, temperature measurements are done using thermocouples, which have low margins of error. LES studies in natural convection
Although not conducted in this study, Large Eddy Simulations (LES) would be the next logical step to ensure the results of CFD analysis are reliable. Since studies focused exclusively on solar chimney conducted using LES cannot be found, the studies conducted by Lau, Yeoh, Timchenko et al. (2011), which involves a differentially heated open-ended cavity (which could be considered as a solar chimney), is described here. In this study, the temperature and velocity results obtained for various numerical models are compared to an experimental study from the literature. Two LES models, the Vreman and Smagorinsky models, and one 3D RANS study were evaluated for the accuracy of the predicted wall temperature and stream velocities. Among the three CFD models, the Vreman LES model provided values that were closest to the experimental data. Unfortunately, as the code adopted in their study was developed in house, their study is not easily replicable.
The LES model was also adopted to research natural convection in a PV panel array. The study conducted by Tkachenko, Timchenko, Giroux-Julien et al. (2016) compared the LES simulations to experiment. The LES model used for their study was the Vreman model, and the numerical result was comparable to the experimental results. This gives further credence to the use of the Vreman LES model for solar chimney simulations. Their study also included PIV (Particle Image Velocimetry) measurement of the velocity field inside the chimney, which is a non-invasive method of taking measurements. Provided that the PIV is conducted correctly, it is more reliable and accurate compared to spot measurements using anemometers.
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Combination with Other Technologies
As the goal of the relevant research and development is to achieve as much ventilation, cooling or heating as possible, solar chimney has been combined with all the other myriad methods and techniques.
One of the earliest designs was to combine the solar chimney with the wind catcher ventilation technique, which was studied by Bansal, Mathur, & Bhandari (1994). The study showed that the combination of the two ventilation methods could increase the air change rate significantly when compared to either technology operating individually. In the combined system, the solar chimney is used to pull air from individual rooms, while the wind catcher is used to provide fresh air into the rooms. This is illustrated in Figure 12.
Figure 12 - Windcatcher used in conjunction with solar chimney. Source:Bansal et al. (1994). Maerefat & Haghighi (2010a, (2010b) investigated the solar chimney paired with various heat exchange techniques, including the earth to air heat exchanger and evaporative cooling. They reported that the solar chimney in isolation is only able to bring the room down to the outside ambient temperature. However, when combined with the mentioned techniques, temperatures several degrees below the ambient could be achieved. This combined technique is particularly useful in areas of hot arid climates. When the humidity was low and the incident radiation was
24 high, up to 10°C cooling could be achieved. Cooling was even viable when the incident solar radiation was low.
Solar chimneys are frequently used as a method of cooling photovoltaic panels while simultaneously providing natural ventilation. DeBlois, Bilec, & Schaefer (2013) used this combined approach for home cooling. This combination was also mentioned as an option in the CFD study of Guohui Gan (2006).
In two separate studies, Li & Liu (2014) and Liu & Li (2015) tested the combination of solar chimneys with phase change materials (PCM). PCM have a high heat capacity and remain at relatively low temperatures, ideal for heat storage. In their investigations, the PCM was placed behind the absorber plate to increase the operational period of the solar chimney by storing thermal energy through phase change. It was reported that the introduction of PCM to the solar chimney reduced the instantaneous flow rate of the chimney but extended the working time after the incident solar radiation disappeared. Amori & Mohammed (2012) also included PCM in their designs. The thoughts behind the PCM used in solar chimney can be divided, as some researchers and designers are more concerned with instantaneous performance which, as mentioned, is reduced by the introduction of the PCM, while others are more concerned with maintaining thermal comfort and ventilation into the evening. The choice to add PCM to a solar chimney design is determined by the desired outcome. Solar Heaters and Solar Dryers Solar heaters and solar dryers have also attracted significant research attention. The major difference between solar heater and dryer designs and solar chimneys is that the solar heaters and dryers usually have an externally driven flow, with a fan or pump pushing fluid over the absorber surfaces. This consideration means that there is less focus on the effects of drag and a more focus on increasing the heat transfer rate. Cost-performance increase analysis needs to be conducted if the complicated designs from solar heaters and dryers shows positive performance increases.
El-Sawi, Wifi, Younan et al. (2010) tested the effect of chevron and groove shaped absorbers and reported that a chevron design had improved heat transfer capacity over a flat absorber surface. Figure 13 shows the different designs of the absorber surface. The Chevron design was the best performer for getting the highest air temperature, but the drag may be more significant.
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Figure 13 - The tested design of Flat (left), groove (centre) and chevron (right). Source: El-Sawi et al. (2010). An extensive review of solar heater surfaces was conducted by Patil (2012) which was an attempt to determine which surface topologies and configurations would provide the best heat transfer. Many surface designs were considered in that study, including objects like helical wires. It was found that broken inclined rib surfaces and wedge geometries, such as those illustrated in Figure 14, showed a significant performance improvement over flat surfaces. Further study of the flow phenomena associated with the different surface designs is recommended. Since these surface geometries were primarily tested for performance in pressure driven flows, their performance in buoyancy driven flows has not been studied.
Figure 14 - Multi V design. Source: Patil (2012). Summary From the above literature review, the following conclusions can be made:
• If appropriately designed, solar chimney can provide reasonable natural ventilation and thermal comfort, both in cooling and heating applications. • The ideal height to air-gap width ratio (also called the aspect ratio) is in the range from 5 to 10, but the evidence is not unanimous, and thus further research is required. It appears to be dependent upon the incident radiation levels. • An increase of the chimney height generally leads to an increase of the ventilation performance but a decrease of its efficiency. • The ideal inclination of the roof-top solar chimney is dependent upon the azimuth and latitude.
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• For simulating the solar chimney performance in 2D, an expanded computational domain and a k-ω turbulence model are recommended. The k-ω turbulence model is also advised as one of the appropriate models in the ANSYS User Manuals. • The Vreman LES model for 3D simulations provides reasonable agreement with experiment. • Numerical simulation generally over-predicts the flow from solar chimneys but are reasonably close to experimental values. • Reverse flow is a phenomenon that affects chimneys of various sizes. Solar chimney designs that suppress the reverse flow can perform better than the standard chimney designs. • The instances when reverse flow occurs needs to be investigated in further detail. • Increasing the heat transfer area by the introduction of surface roughness features (such as rectangular ducts or chevrons) can enhance the chimney performance. • Novel designs show considerable promise, and research into these areas should be continued. • The choice of appropriate building materials can have a significant effect on desired chimney performance.
Several research gaps can be identified in the literature:
• Apart from the solar heater and dryer applications, there is little research into diversifying the design of the absorber or glazing walls. • Whether specific materials or optical properties of the absorber wall or glazing would be of benefit for enhancing the solar chimney performance or not requires more research attention. • Reverse flow as a phenomenon needs to be investigated further. • Three-dimensional designs and the corresponding CFD modelling need to be investigated more extensively.
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3 Computational Fluid Dynamics - Methodology and Validation Introduction
The numerical simulations described in this study are conducted and analysed using the commercial product suite ANSYS Workbench, including Fluent, SpaceClaim, ICEM CFD and CFD-Post. Following is an outline of how each step was conducted in the process of simulation, followed by the validation of the CFD model against experimental data. The simulations are run to a steady-state solution, and if a steady-state simulation does not achieve the desired accuracy, a quasi-steady solution is sought. Since the present simulations are carried out in a two-dimensional reference frame, a comparison to a one-dimensional analytical model is not relevant. Further, the comparison between analytical models and two-dimensional numerical simulations has been reported extensively in the literature, and thus including the same comparison would not be of significant academic value. Geometry and Boundaries
ICEM CFD and SpaceClaim is used to generate the geometries for this investigation. Scripting is utilised to facilitate parametric changes to the designs whenever appropriate. Following the recommendations from Guohui Gan (2010), the computational domain is expanded to properly resolve the flow around the chimney inlet and outlet. Figure 15 shows the geometry created for the tinted glazing study (refer to Chapter 4), showing the relevant boundaries including the Pressure Inlet, Pressure Outlet, Absorber wall, Glazing wall, Interior region, Exterior Region, Chimney Inlet and Chimney outlet etc. The boundary conditions are described in Section 3.5 below.
For the study of solar chimney with a horizontal baffle described in Chapter 5, the thicknesses of the absorber and glazing are considered in the geometry construction to more accurately model the heat transfer and losses via the glazing. Figure 16 shows the geometry of the horizontal baffle. In this figure, the absorber wall and glazing wall can also be seen.
28
Figure 15 - SpaceClaim geometry of tinted glazing chimney with various regions and boundaries labelled.
Figure 16 – ICEM geometry of a solar chimney with a horizontal baffle. The right image shows a section around the baffle, where the wall thicknesses of the baffle, absorber and glazing can be seen. 29
Meshing
ICEM CFD and ANSYS Meshing programs are used to generate meshes. For this study the internal areas of a chimney such as the interior region shown in Figure 15, or the walls (in the horizontal baffle study, as shown in Figure 16) have uniform mesh sizing, while the external regions start as the same size as the interior region but inflate in size as they move towards the outer boundaries. Figure 17 presents a section of the meshing for the chimney with a horizontal baffle, showing the inflation starting from the chimney outlet, with mesh size increasing with the distance from the chimney internals.
Figure 17 - Section showing mesh near chimney outlet in ICEM CFD.
Operating Conditions
The operating conditions are set to properly account for the thermal expansion of air, as well as other bulk properties such as gravity and ambient temperature. The heat transfer properties of aluminium and glass are used to determine the heat transfer through the absorber and glazing walls respectively. The ambient air temperature is set to 300K, while the gravity is set to 9.81 ms-2, and the reference density is set to 1.177 kg/m3, which is the density of air at 300K. A linear interpolation is adopted to calculate the air density for any given temperature value based on the two adjacent data points from the properties table. The properties for the Aluminium and Glazing are summarised in Table 2.
30
Table 2 - Materials properties of Aluminium and Glass Glazing
Property Value (Aluminium) Value (Glazing) Density - ρ [kg/m3] 2719 2500
Specific heat Cp 871 870 [J/(kg.K)] Thermal Conductivity - k 202.4 0.96 [W/(m.K)]
Boundary Conditions Boundary conditions are specified in the ANSYS Fluent solver and are established as follows (refer to Figure 15 for the name conventions of relevant boundaries):
The Pressure Inlet and Pressure Outlet conditions are set to allow air entering the domain at the ambient pressure and temperature.
When testing the effect of tint, the walls are set to zero thickness, adiabatic uncoupled walls, with no heat transfer occurring via the chimney walls to the exterior regions. The internal facing side of the absorber is set to have an input heat flux and emit radiation, while the internal facing region of the glazing is set to absorb the radiation emitted by the absorber, which is then transferred into the fluid via convection.
The interior and exterior regions are set to be air, which is initially at the ambient temperature and pressure with a density of 1.177 kg/m3.
When testing the effect of a horizontal baffle on the solar chimney performance, the thicknesses of all rigid walls are also modelled explicitly. Referring to Figure 16, the wall of the absorber which faces the exterior is modelled similarly to that in the tinting model, with the heat flux applied to the interior of the wall only. The interior of the wall is solid, which has the materials properties of aluminium. Baffle walls and Absorber walls facing towards the interior of the air channel are not given a heat flux and are left as coupled, allowing heat transfer through these walls to happen.
The glazing walls are coupled, with the interior of the glazing wall defined as solid and having the properties of glass.
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Turbulence Modelling
In accordance with the recommendations from the literature and the ANSYS Fluent manual, the k-ω turbulence model is adopted. Although the air flow in the chimney channel is mostly within the laminar flow regime according to the calculated Rayleigh numbers, the plume arising from the solar chimney channel becomes turbulent, and thus the turbulence model is necessary to resolve the flow in the expanded computational domain. Radiation Modelling
When the radiation model is enabled, the Discrete Ordinance model is adopted, which calculates the amount of radiation emitted at a discrete number of angles. The radiation model is computed every 10 iterations. The Theta and Phi divisions are set to 2, and the Theta and Phi pixels are set to 1, with Non-Gray Model bands disabled. The above settings are the default settings of the solver, and no reason to alter these settings was determined.
The radiation model determines the energy exchange based upon the surface temperature and black body emission. With the temperature ranges expected for these simulations, the emitted radiation will be in the infra-red wavelength band. Testing is conducted both with and without a radiation model.
The effect of having a radiation model enabled is the introduction of radiative heat transfer from the absorber surface. In this case, heat is radiated from one surface to another, which is quantified by a radiative transfer equation (i.e. the Discrete Ordinance model) with the amount of energy transferred dependent upon the temperature of the surface. For the present simulations, this means that heat is transferred from the absorber to the glazing via radiation, allowing a secondary plume to form along the glazing wall. With the inclusion of wall thickness in these simulations, the heat losses to the environment via the glazing wall are also accounted for. Numerical Schemes
In the present numerical simulations, a pressure-based solver is adopted. The coupled numerical scheme is used, with the PRESTO! scheme for the pressure component of the spatial discretisation. The Gradient is set to Least Squares Cell Based, and the first-order upwind scheme is adopted for advection terms. The Pseudo Transient option is enabled for the k-ω model when solving for a steady state solution.
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Dependency Tests
Three dependency tests are conducted: mesh, residual and time-step dependency tests respectively. They are conducted on a chimney design for a height of 1m and an air-gap width of 100mm with an input heat flux of 800 Wm-2, the highest heat flux calculated in this study.
As with all CFD investigations, the increased accuracy comes at the cost of increased computational time. A compromise between the relative numerical accuracy and the computational time needs to be made. A large range of mesh sizes are tested. Table 3 shows the results of the mesh dependency study. The sensitivity of the predicted air mass flow rate leaving the solar chimney to the mesh resolution is determined with reference to the mass flow rate obtained using the finest mesh. Based on the mesh dependency test result shown in Table 3, the mesh with 3 × 105 cell counts is adopted in further simulations since an order of magnitude increase of the mesh density only changes the predicted value by 2.3%.
Table 3 - Mesh dependency test results – predicted air mass flow rate using different meshes.
Percent change - Cell Count Mass flow rate (kg/s) relative to the finest mesh 3645901 0.0346 0 2465987 0.0345 -0.19 790159 0.0343 -0.98 294245 0.0338 -2.30 131788 0.0332 -4.09 71258 0.0326 -5.76
A residual dependency test is also conducted, the results of which show no variation of the resultant mass flow rate among the three specified residuals of 1.0E-05, 1.0E-06 and 1.0E-07. Further, the difference in the number of iterations required to achieve convergence to the residuals of 1.0E-05 and 1.0E-06 is relatively small, while to achieve convergence to the residual of 1.0E-07 requires significantly more iterations. Therefore, 1.0E-06 is selected as the residual for convergence in further simulations.
In addition, a time-step dependency test is also conducted, and the results indicate less than 0.5% variation when the time-step is changed from 0.5s to 0.25s and 0.125s. Accordingly, the time-step of 0.5s is chosen for transient simulations.
33
Validation with Experimental Data
The results of the present simulations are compared with the experimental data and other numerical analysis from the literature. This has been tested across a range of the chimney heights, widths and heat fluxes. The three experimental studies to which the present simulations are compared were conducted by Ryan & Burek (2010), Chen et al. (2003) and Khanal & Lei (2014) respectively.
The study conducted by Ryan & Burek (2010) considered the effect of the chimney height, air- gap width and input heat flux on the performance of a solar chimney, with the output flow rate calculated by measuring the velocity using anemometers at a series of locations across the domain of the chimney. Heat flux was supplied by an electric heat mat.
Chen et al. (2003) conducted an experiment to investigate the relationship between the air-gap width and the phenomenon of reverse flow for relatively large air-gap width chimneys. During the course of their experimentation, extensive measurements of the temperature and velocity were taken across the air-gap width of the various chimneys. Again, heat flux was supplied by an electric heat mat.
Khanal & Lei (2014) conducted an experimental study on a solar chimney to determine the validity of the effect of the inclined passive wall that was first proposed and investigated in Khanal & Lei (2012). Similar to the above two studies heat mat was used to simulate incident heat flux. In the experiment, thermocouples were used to measure the temperature of the solar chimney across the air-gap width. The experimental temperature profile was compared against CFD prediction.
Figure 18 shows a comparison with the experimental study of a 1m high chimney conducted by Ryan & Burek (2010) for a range of heat fluxes. It can be seen in this figure that the simulation provides a reasonable agreement with the experimental results, with an average variation of approximately 10%. This discrepancy can be attributed to many factors, such as an overestimation of the radiation absorbed by the glazing, an underestimate of the heat loss to the environment, no consideration of the drag effects of the sidewall, system errors in the experimental equipment and an uneven heat distribution on the absorber during the experiment etc.
The present numerical simulation is also compared with the solar chimney experiment performed by Chen et al. (2003), as shown in Figure 19. The chimney dimensions used in this
34 comparison are a chimney height of 1500mm, an air-gap width of 200mm and an incident heat flux of 400 Wm-2. For this comparison, the temperature profiles across the simulated chimney and the experimental chimney are shown. Wall thickness is considered in the simulation to give a better account of the heat losses to the external environment via the glazing. This numerical treatment of the wall thickness however comes at the costs of increased computational time and resource requirements, taking several times longer to finish simulation when compared to the simple adiabatic wall model.
200 )
180 αρ 160 Present Simulation ṁ/ 140 120 100 80 60 Experiment - Ryan 40 and Burek (2010)
Mass Mass Flow ( Rate 20 0 20 40 60 80 100 (a) Chimney Air-Gap Width (mm)
250
) αρ 200
ṁ/ Present Simulation
150
100 Experiment - Ryan
50 and Burek (2010) Mass Mass Flow ( Rate 0 20 40 60 80 100 (b) Chimney Air-Gap Width (mm)
) 300
αρ 250 Present Simulation ṁ/ 200 150
100 Experiment - Ryan 50 and Burek (2010)
Mass Mass Flow ( Rate 0 20 40 60 80 100 Chimney Air-Gap Width (mm) (c)
Figure 18 - Comparison with the experimental data for a 1m Chimney across a range of air-gap widths with an input heat flux of (a) 400 Wm-2, (b) 600 Wm-2 and (c) 800 Wm-2.
35
1.2 )
1 amb
T Present Simulation - 0.8
wall Experiment (Chen et. al., 2003)
0.6 )/ (T )/
0.4
amb T
- 0.2 (T 0 0 20 40 60 80 100 120 140 160 180 200 Distance from absorber wall (mm)
Figure 19 - Comparison of the temperature profiles across the chimney outlet. Additionally, a comparison of the velocity profile at the outlet also shows a good agreement with the experiment, as shown in Figure 20. The difference in the profiles at the near-wall regions is mainly due to the lack of data points in the experiment as the experiment did not measure up to the wall due to the constraints of the anemometer and other probes used.
0.6
0.5 Present Simulation 0.4 Experiment (Chen et. al., 2003) 0.3
0.2 Velocity (m/s) 0.1
0 0 20 40 60 80 100 120 140 160 180 200 Distance from absorber wall (mm)
Figure 20 - Comparison of the velocity profiles at the chimney outlet. In the experiment conducted by Khanal & Lei (2014), the chimney has a height of 700mm with an air gap width of 100mm, giving an aspect ratio of 7. Three input heat fluxes are compared, 100, 300 and 500 Wm-2, giving Rayleigh numbers of 8.05 × 1010, 2.76 × 1011 and 4.19 × 1011 respectively. Figure 21 compares the results of the present CFD simulation with the experimentally recorded temperatures.
36
1.0 ) Ra = 4.19 × 1011
amb 0.8 T
- Current CFD Study
0.6 wall Experiment (Khanal & Lei - 2014)
)/ (T )/ 0.4
amb T
- 0.2 (T (T 0.0 (a) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/W
1.0 ) Ra = 2.79 × 1011
amb 0.8 T
- Current CFD Study 0.6 wall Experiment (Khanal & Lei - 2014)
)/ (T )/ 0.4
amb T
- 0.2 (T (T 0.0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 (b) x/W
1.0 ) Ra = 8.05 × 1010
amb 0.8 T
- Current CFD Study 0.6 wall Experiment (Khanal & Lei - 2014)
)/ (T )/ 0.4
amb T
- 0.2 (T (T 0.0 (c) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/W Figure 21- Comparison of the temperature profiles at 95% of the chimney height between the present CFD study and the experiment for a 700mm height chimney for Rayleigh numbers of a) 4.19 × 1011 b) 2.79 × 1011 and c) 8.05 × 1010. Based on the comparisons with the available data, it can be concluded that the present simulations give a good prediction of real thermal flow behaviours in solar chimney. Many factors may affect the alignment of the simulation with the experiment, such as the experimental method, the measuring techniques adopted, the materials used for both the absorber and glazing, the size of the room in which the experiment is taken place and the inherent instabilities and properties of the ambient air including its density and pressure. As most of these details are either incomplete or not reported in the literature, some level of variation between the present simulations and the previous experiments are expected.
37
4 The Effect of Tinted Glazing Design Concept
The standard design of a solar chimney has a single absorber surface to be heated by incident solar irradiance. This leads to large portions of the air in the solar chimney channel remaining at the ambient temperature and is not contributing to the performance of the solar chimney directly. There is also the possibility of reverse flow occurring at the outlet, which will stagnate the flow in the chimney. In this study, tinting is applied on the glazing surface, aiming to achieve two outcomes: to attain a higher average temperature inside the chimney, and, if it occurs, to counter any reverse flow. Chimney Dimensions The vertical height of the chimney is set to 500mm, with an aspect ratio of 5, giving an air-gap width of 100mm. These dimensions combined with an input heat flux of 500 Wm-2 give a Rayleigh number of 1.2 × 1011, which is in the laminar flow regime. Each segment of the glazing for which the tinting is considered is ¼ of the overall height, that is, 125mm, as shown in Figure 22. Design Methodology The solar chimney walls (i.e. the glazing and absorber walls) are split into eight equal sized segments, four on the glazing wall and four on the absorber wall, as shown in Figure 22. These segments are paired with each other based on the height, with the top absorber portion paired with the upper glazing portion. Together these pairs will receive a sum total heat flux that represents the irradiance landing on the surfaces. Each pair receives the same total heat flux. The distribution of the heat flux between the absorber and glazing is then varied to determine which configuration of the tinting on the glazing may provide the highest performance. This sectioning of the absorber and glazing wall leads to 45 combinations of tinting and irradiance. Simulations were conducted using ANSYS automation methods on the ARTEMIS supercomputing facility, with further details of this automation presented in Appendix A.
The number of combinations to be tested is shown in Table 4, for each combination, the glazing with applied tint is tested to retain 25%, 50%, 75% and 100% of solar irradiance. The base case has no tinting and is represented by 0% heat flux retained by the glazing. Further, Combination 4, which has full height tinting, is equivalent to the base case when the amount of irradiance
38 retained by the glazing is 100%. As an example, in combination 2, the top half of the glazing is tinted, which in the simulation is represented by distributing a portion of the total heat flux to the tinted section, leading to a reduction of the heat flux on the absorber wall. The heat retaining capacity of the tinting on the glazing is varied, starting at 25% heat flux retained on the glazing to full heat flux retained on the glazing.
Figure 22 - Diagram showing labelling of each section of the chimney. The 3D model is a mock-up only to give a better visual representation. Each section receives the same total heat flux, for example: the heat flux of glazing A added to the heat flux on absorber A is the same as the sum of the heat flux on glazing B plus the heat flux on absorber B. Table 4 - Combinations of Tinting application for solar chimney showing when a portion of the glazing receives tinting – a red square shows when there is application of tint to that portion of the glazing. The letters correspond to the letters in Figure 22.
Segment 1 2 3 4 5 6 7 8 9 10 11
A B C D
39
Results
Two sets of simulations were conducted to test the effectiveness of tinted glazing, one without a radiation model and the other with a radiation model enabled. We first examine the predicted mass flow rates when radiation transfer is not considered.
4.4.1 No Radiation Model It is found that, when radiative transfer is not considered, tinting on the glazing provides a significant improvement to the performance of the solar chimney. This increased performance can be attributed to the reduction or suppression of the reverse flow at the chimney outlet, which is illustrated in Figure 24. The reverse flow is observed in the chimney with no tinting. However, if half of the irradiance is retained by the glazing, the reverse flow does not occur. The reduction of the reverse flow is observed for each case in which tinting is applied to the glazing. It is found that more significant tinting leads to a stronger buoyant plume adjacent to the glazing, which weakens the reverse flow. From the results shown in Figure 23 we can see that the best performance enhancement of 55% is achieved when 50% of the input irradiance is retained by the full height of the glazing (Combination 4), which results in symmetric heating in the air channel. The general trend of the findings is that as tinting is applied to the various segments of the glazing, there is a general enhancement of the ventilation performance compared to the base case. For most configurations the enhancement of the performance increases as the amount of solar irradiance retained by the glazing increases towards 100%. The only exceptions are Combinations 4 and 5, which have peak performance when the energy from the solar irradiance is equally distributed between the glazing and absorber walls. It is further observed in Figure 23 that Combinations 8 to 11 all provide significant performance enhancement, approaching peak performance when 100% of the energy from solar irradiance is retained by the glazing, which is equivalent to the case with the total heat flux equally distributed between the glazing and absorber. The maximum performance enhancement achieved with Combinations 8 to 11 is very close to the peak performance of Combination 4, which can be seen in Figure 23(c).
40
110 105
) 100
αρ 95 90 Combination 1 85 Combination 2 80 Combination 3 75 Combination 4 Mass Mass flow rate(ṁ/ 70 65 60 0 25 50 75 100 (a) Heat flux distribution to tint (%)
110 105
) 100 αρ 95 90 Combination 4 85 Combination 5 80 75 Combination 6
Mass Mass flow rate(ṁ/ 70 Combination 7 65 60 0 20 40 60 80 100 (b) Heat flux distribution to tint (%)
110
) 105 αρ 100 95 90 Combination 4 85 Combination 8 80 Combination 9 Mass Mass flow rate(ṁ/ 75 Combination 10 70 Combination 11 65 60 0 20 40 60 80 100 (c) Heat flux distribution to tint (%)
Figure 23 - Effect of tinting on the predicted mass flow rate without a radiation model. (a) Combinations 1-4; (b) Combinations 4-7; (c) Combinations 4 and 8-11. Combinations are taken from Table 4
41
Figure 24 – Stream function at the chimney outlet, showing reverse flow for the unaltered chimney (left) and no reverse flow for the full height tinted chimney (Combination 4) with 50% of irradiance retained by the glazing (right). 4.4.2 With Radiation Model Enabled When the effect of radiation transfer is considered, the behaviour of the tinted solar chimney is very different from that observed without radiation transfer, and tinting shows no significant effect on the solar chimney performance. Figure 25 shows the results of Combinations 1 to 4 when a radiation model is enabled, showing that there is little to no performance difference when tinting is applied to the glazing. The difference in the outcomes between the simulations with and without radiation transfer can be attributed to the absence of the reverse flow with the radiation model enabled. The contours of the stream function obtained with the base case without tinting but with the radiation model enabled are plotted in Figure 26, which show no reverse flow. Combinations 5 to 11 show identical results to those of Combinations 1 to 4. Regardless of where the tinting is applied, there is no significant change in the predicted mass flow rate.
42
120
) 110 αρ 100 Combination 1 90 Combination 2 80 Combination 3 70
Mass Mass flow rate(ṁ/ Combination 4 60 0 20 40 60 80 100
Heat flux distribution to tint (%)
Figure 25 – Predicted ventilation performance of solar chimney when radiation model is enabled, showing no significant benefit from the addition of tinting. All the four combinations have nearly identical performance.
Figure 26 – Stream function at the outlet of the unaltered solar chimney with radiation model enabled, showing no reverse flow. It is concluded from the above observation that the radiation model has a significant effect on the simulations, causing a considerable variation of the predicted flow rates. The elevated temperature at the glazing causes a strong buoyant plume to form next to the glazing, which suppresses the reverse flow. This may be due to an overestimation of the amount of radiation transferring energy to the glazing, as well as the possible lack of heat losses to the external environment.
In the literature, the reverse flow was not observed in some studies, whereas other studies with a similar chimney configuration reported a considerable effect of the reverse flow. Further examination of the experimental scenarios and how numerical simulations are conducted reveals that the effect of the material properties on the occurrence of the reverse flow in the solar chimney can be quite significant. The radiation model used in this study, as mentioned
43 above, assumes that the glazing absorbs radiation in the infrared spectrum. This is true for glazing used in most construction. However, if the solar chimney is constructed using materials such as Perspex or Acrylic panels, which are common in laboratory experimentation due to its robustness and low cost, there would be little to no absorption of radiation from the hot surface. This is because Perspex or Acrylic materials are transparent to infrared wavelengths, and thus the secondary plume occurring on the glazing would not exist and the reverse flow would be more significant. Conclusion
The present numerical investigation reveals that, when the radiation model is enabled, tinting the glazing has no significant effect on the performance of the solar chimney as there is already considerable heat transferred to the glazing by radiation, which leads to the unaltered chimney not experiencing reverse flow. However, when the radiation model is not enabled, there is considerable improvement to the solar chimney performance with the introduction of tinting, with the peak performance achieved when the glazing has the full height tint that retains 50% of the incoming irradiance. Accordingly, if reverse flow is observed in solar chimney, tinting of the glazing is recommended.
These simulations highlight that the appropriate choice of the glazing materials can change the performance of a solar chimney by retaining a certain amount of the infrared radiation emitted from the absorber. Materials such as glass are opaque to infrared radiation, while other common glazing materials such as Perspex and Acrylic are not. The application of tinting can help to even out the difference in the performance between the various materials.
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5 The Effect of a Horizontal Baffle Design Concept
Previous research into similar technologies, such as solar heaters, has given credence to the introduction of a horizontal baffle on the absorber as being beneficial to the performance of a solar chimney. The introduction of the baffle is posited to cause disruption to the buoyant plume and increased turbulent mixing, which would in turn lead to an increased average temperature of the air in the solar chimney, resulting in a more significant buoyant effect due to the reduced average density. The efficacy of the horizontal baffle is tested in two sets of simulations, first with a radiation model enabled, and then with the radiation model disabled. Design Methodology A series of parameters are considered to test the effects of the baffle for a range of chimney dimensions. The relationships between the solar chimney performance and the chimney height, the aspect ratio and the input heat flux are well known and firmly established. The chimney sizes covered in this investigation are those representing the sizes that are commonly reported in the literature.
To test each of the combinations of the various parameters is an exhaustive and time- consuming process. Although it is possible with the current computing resources and automation methods, it is not ideal nor necessary. To this end, the Taguchi method is used. The Taguchi Method is combined with the automation methods as discussed in Appendix A to allow for relatively fast simulation times.
Further, in the present investigation the thicknesses of the various wall materials and the radiation transfer are present in the simulations in lieu of the zero-thickness adiabatic surfaces considered in the previous investigation to better simulate the effect of heat losses from the chimney through the glazing. Chimney Dimensions
Figure 27 shows the geometry of the chimney design. A parametric analysis of this design is conducted for a range of chimney heights, air-gap widths and baffle locations, and they are tested over a range of heat flux (HF). The three heights of the solar chimney that are tested are 1m, 1.5m and 2m respectively, and the three aspect ratios (AR) to be tested are 10, 6.667 and 5 respectively. The three tested heat fluxes are 400, 600 and 800 W/m2. Initially, three baffle
45 heights (BH) at 40%, 60% and 80% of the chimney height with three baffle widths (BW) of 20%, 30% and 40% of the air-gap width are simulated. Additional simulations outside the above-mentioned parameter ranges have also been conducted to gain a deeper understanding of certain flow phenomenon.
Figure 27 – Solar chimney with a horizontal baffle on the absorber wall. Taguchi Method – With Radiation Model Enabled The Taguchi method is a statistical analysis method that helps to reduce the number of required tests to quantify the effect of each tested parameter and therefore determine an optimum outcome or design.
Starting in the 1950’s, Genichi Taguchi developed many mathematical techniques to improve manufacturing methods and determine ideal designs (Dehnad (1989)). An example of the ideal design method used in heat transfer research is conducted by Gunes, Manay, Senyigit et al. (2011).
An orthogonal array, shown in Table 5, is established using the range of parameters being tested. This orthogonal array helps to compare the various interactions between parameters with each parameter being tested with every other parameter at least once. This method reduces the required number of simulations in this study from 35 down to 27 simulations, after which follow-up simulations are conducted.
46
The performance of a Taguchi design is measured via the signal to noise ratio, S/N. The calculation of the S/N depends upon the desired outcome. If one is trying to maximise the desired performance, then the signal to noise ratio is calculated using:
푆 1 = −10 × log(∑( )/푛) 푁 푌2 (3) where:
푌 – the quantity to be optimised (e.g. the mass flow rate)
푛 – the number of experiments or simulations
Similarly, if one is trying to minimise the output performance then the signal to noise ratio is calculated as:
푆 = −10 × log(∑ 푌2/푛) 푁 (4)
For both scenarios, we look for the maximum signal to noise ratio. In this study the mass flow rate is to be maximised, and thus Equation (3) is used.
Although these calculations can be done manually, the program Minitab1 is used to perform the Taguchi analysis, which is similar to the method adopted by Gunes et al. (2011). The Taguchi method is not a panacea that will just give the best outcome or design: knowledge must be applied to the scenario, and the relationships between the various variables must be considered. Depending on the tool used to conduct the study, a relationship between the different variables can be visualised. More information about the particulars of the Taguchi Method can be found in the Minitab Software manual which is available online. Taguchi Results – With Radiation Model Enabled
The simulations for the Taguchi analysis have been conducted, and the results are shown in Table 5. The results of a Taguchi analysis can be presented in a variety of ways. Figure 28 shows the outcome of single variable analysis, in which the relationships between each pair of the variables are not shown. The trends of the solar chimney performance that have been
1https://support.minitab.com/en-us/minitab/18/help-and-how-to/modeling-statistics/doe/supporting- topics/taguchi-designs/taguchi-designs/ (accessed on 6/04/2020) 47 confirmed in the literature can be clearly seen from this analysis for the ranges of the parameter values tested. In general, a larger chimney height or a higher heat flux (i.e. a higher Rayleigh number) will both lead to a higher air mass flow rate, and a lower aspect ratio (a wider chimney) will lead to a higher mass flow rate.
Figure 28 - Signal to Noise plots for the five variables tested. Showing increasing height, lower aspect ratio (a wider chimney) and heat flux leading to increased performance.
According to the single variable analysis shown in Figure 28Error! Reference source not found., a baffle placed at 60% of the chimney height and of 30% of the chimney width provides the best performance. To check if this is indeed the case, Figure 29 plots the predicted ventilation rate versus the normalised baffle width with the baffle placed at 60% of the chimney height for three combinations of the chimney height, aspect ratio and heat flux. Combination 1 has a height of 1m, an aspect ratio of 6.667 and an input heat flux of 600 Wm-2; Combination 2 has a height of 1.5m, an aspect ratio of 5 and an input heat flux of 400 Wm-2; and Combination 3 has a height of 2m, an aspect ratio of 10 and an input heat flux of 800 Wm-2. These combinations are defined in Table 5. It is clear in the figure that no significant variation of the ventilation rate with the baffle width can be observed in any of the combinations. This highlights the limitation of the single variable analysis.
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Table 5 – Results of the Taguchi Analysis.
Height AR q’’ Rayleigh Combination BH (H /H) BW (W /W) Results (ṁ/αρ) (m) (H/W) (Wm-2) Number b b Number 0.2 152.0 10 400 1.5 × 1012 0.4 0.3 152.0 n/a 0.4 152.0 0.2 214.6 1 6.6 600 2.2 × 1012 0.6 0.3 219.1 1 0.4 219.1 0.2 290.7 5 800 3.0 × 1012 0.8 0.3 308.5 5 0.4 254.9 0.2 286.2 10 600 1.1 × 1013 0.8 0.3 295.1 4 0.4 232.5 0.2 402.5 1.5 6.6 800 1.5 × 1013 0.4 0.3 398.0 n/a 0.4 411.4 0.2 420.3 5 400 7.5 × 1012 0.6 0.3 424.8 2 0.4 424.8 0.2 465.1 10 800 4.7 × 1013 0.6 0.3 469.5 3 0.4 460.6 0.2 523.2 2 6.6 400 2.4 × 1013 0.8 0.3 532.1 6 0.4 424.8 0.2 693.1 5 600 3.5 × 1013 0.4 0.3 702.1 n/a 0.4 711.0
500 )
αρ 450
ṁ/ 400 350 300 250 200 Mass Mass Flow ( Rate 0.2 0.3 0.4 BW (WB/W) Combination 1 Combination 2 Combination 3
Figure 29 - Performance of solar chimney when the baffle is placed at 60% of the chimney height, showing the same performance across all the tested baffle widths. The results demonstrate that a single variable Taguchi analysis does not fully resolve the scenario.
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Accordingly, multiple variables analysis is carried out. Figure 30 shows the relationship between the baffle width and baffle height, which indicates that the best configuration of the baffle has a width of 30% of the air-gap width and is placed at 80% of the chimney height. To confirm this, Figure 31 presents the ventilation rate versus the baffle width with the baffle located at 80% of the chimney height for three combinations of the chimney height, aspect ratio and heat flux. Combination 4 has a height of 1m, an aspect ratio of 5 and an input heat flux of 800 Wm-2; Combination 5 has a height of 1.5m, an aspect ratio of 10 and an input heat flux of 600 Wm-2; and Combination 6 has a height of 2m, an aspect ratio of 6.6 and an input heat flux of 400 Wm-2. These combinations are defined in Table 5. It is seen in Figure 31 that, when the baffle is placed at 80% of the chimney height, the baffle of 30% of the air-gap width indeed gives the best ventilation performance among the baffles of all widths under consideration,
Figure 30 - Interaction plot of the baffle height and baffle width, showing peak performance with a baffle of 30% of the air-gap width placed at 80% of the baffle height.
50
550 )
αρ 500
ṁ/ 450 400 350 300
250 Mass Mass Flow ( Rate 200 0.2 0.3 0.4
BW (WB/W) Combination 4 Combination 5 Combination 6
Figure 31 - Performance of solar chimney with the baffle placed at 80% of the chimney height, which peaks with a baffle of 30% of the air-gap width. Follow-up Simulations
For a complete comparison of the performance, the “ideal” case derived from the Taguchi method must be tested over a range of parameter settings. Accordingly, the ideal baffle position from the above Taguchi analysis is tested for each chimney height and aspect ratio combination, and the result is compared with the corresponding unaltered solar chimney. This required several additional simulations.
Table 6 shows the results of the follow-up simulations to give a full assessment of the suggested parameters from the Taguchi analysis. It is seen from the table that the introduction of the baffle is more beneficial if the aspect ratio of the solar chimney is small (i.e. for wider solar chimney) or if the chimney height is small. It is also seen in the table that the performance enhancement is only weakly dependent upon the incident heat flux.
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Table 6 – Follow-up simulation results with comparisons between the ideal baffle configuration (at 80% of the chimney height and of 30% of the chimney air-gap width) and the unaltered solar chimney.
Result Result with Height AR HF Rayleigh without ‘Ideal’ Percent (m) (H/W) (Wm-2) Number baffle Baffle improvement (ṁ/αρ) (ṁ/αρ) 10 163.8 166.7 3.7 1 6.6 2.2 × 1012 213.7 227.4 8.2 5 261.4 290.6 10.4 10 283.2 289.8 3.6 1.5 6.6 600 1.5 × 1013 371.6 393.2 7.5 5 455.5 514.0 9.8 10 428.9 436.7 2.5 6.6 3.5 × 1013 559.3 609.4 7.0 2 5 686.8 766.8 7.4 400 2.4 × 1013 507.8 547.2 5.0 6.6 800 4.7 × 1013 612.1 646.9 6.0
The increased performance may be attributed to several flow processes observable in the simulations, which lead to different structures and behaviours of the thermal plume above the chimney (seen in Figure 32). Figure 32 also shows the stream function obtained for an unaltered chimney, a chimney with a baffle placed at 40% of the chimney height and of 30% of chimney air-gap width, and a chimney with a baffle placed at 80% of the chimney height and of 30% of the chimney air-gap width. The configuration shown in Figure 32(c) is the ideal configuration identified for best performance from the Taguchi analysis. It is seen in Figure 32(b) that with the baffle at 40% of the chimney height, the thermal flow separating from the baffle reattaches to the absorber wall behind the baffle. Further downstream, the reattached flow develops normally. This is compared to the scenario with the baffle placed at 80% of the chimney height, in which the separating flow from the baffle does not reattach to the absorber wall. Instead, it exits the chimney at an angle, resulting in the larger plume structure at the chimney exit. An expanded range of contour plots of the stream function can be seen in Appendix C.
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(a) (b) (c)
Figure 32 – Contours of stream function for (a) the unaltered chimney; (b) chimney with a baffle at 40% of the chimney height and of 30% of the chimney air-gap width; and (c) ‘ideal’ chimney with a baffle at 80% of the chimney height and of 30% of the air-gap width. Results are obtained with 2.0m chimney height, 0.4m chimney air-gap width and 600 Wm-2 heat flux. It is speculated that the increased performance may be attributed to the relatively lower air density at the chimney outlet. Figure 33 compares the air density values across the outlets of the unaltered and ‘ideal’ chimneys, which shows a much lower air density for the ideal baffle chimney, especially in the region near the absorber wall. A lower air density leads to a larger buoyant force, thus increasing the velocity of the air flow. The positioning of the baffle at 80% of the chimney height likely leads to minimal friction on the low-density air. In contrast, when the baffle is placed at lower heights, any improvement associated with the presence of the baffle is lost to viscous effects on the walls, leading to little or no improvement of the solar chimney performance compared to the unaltered solar chimney.
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1.2 )
3 1.15
1.1 Unaltered 1.05 Ideal Chimney
Density Density (kg/m 1
0.95 0 50 100 150 200 250 300 350 400 Distance from Absorber (mm)
Figure 33 - Density profile at the chimney outlet showing lower average density for the ideal chimney. Results are obtained with 2.0m chimney height, 0.4m chimney air-gap width and 600 Wm-2 heat flux. Figure 34 compares the temperature contours between the ideal chimney configuration and the unaltered one. It is seen in this figure that a large amount of warm air is present behind the baffle, where air is recirculating. This large volume of warm air helps to achieve a higher average temperature near the outlet and hence the lower air density and stronger buoyancy effect.
Figure 34 - Temperature contours for the unaltered chimney (left) compared to the ideal chimney with a baffle at 80% of the height and of 30% of the air-gap width (right). Results are obtained with 2.0m chimney height, 0.4m chimney air- gap width and 600 Wm-2 heat flux. Figure 35 shows the stream function obtained with different baffle widths with the baffle placed at 80% of the chimney height. Two additional baffle widths are tested to show the trend, starting
54 with no baffle and gradually increasing to a baffle width that is 60% of the chimney air-gap width. If the baffle is too small (refer to Figure 35(b) and (c)), the recirculation zone behind the baffle causes too much friction, and hence little to no improvement of the performance is achieved. Further, if the baffle is too large (refer to Figure 35(e) and (f)), it causes reverse flow at the chimney outlet. At 30% of the chimney air-gap width, the separated flow from the baffle is not reattaching to the absorber wall but is directed out from the chimney, leading to the increased performance due to the formation of a stronger buoyant plume.
(a) (b) (c)
(d) (e) (f)
Figure 35 – Stream function at the chimney outlet for a 2m high and 0.4m air gap width chimney subject to a 600 Wm-2 heat flux. When a baffle is present, it is placed at 80% of the chimney height. (a) no baffle, (b)-(f) with a baffle of (b) 10%, (c) 20%, (d) 30%, (e) 40% and (f) 60% of the air-gap width. Results with Radiation Model Disabled
Testing has also been carried out without the radiation model for completeness, and the detailed results of the Taguchi analysis are shown in Appendix B. A Taguchi analysis across the same parameter ranges is conducted, and the result with the single variable analysis is shown in
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Figure 36, which confirms the well-established trends of increased height and heat flux and reduced aspect ratio increasing the chimney performance. The interaction between the baffle height and baffle width is shown in Figure 37, which is used to identify the ideal baffle configuration.
Figure 36 - Single variable analysis plot for no radiation model simulations.
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Figure 37 - Interaction between the baffle height and baffle width when radiation model is disabled. The results of this analysis show that a baffle placed at 80% of the chimney height and of 20% of the chimney width is the best baffle configuration across the ranges of values tested. The performance of such baffle configuration is compared to an unaltered chimney across the ranges of the chimney heights, aspect ratios and heat fluxes. The results are presented in Table 7, which shows no significant improvement among all the cases. The small variations of the performance are mostly within the simulation uncertainties, and thus the different configurations are considered to have the same performance. It is observed from the numerical data that, when the radiation model is disabled, a reverse flow occurs at the outlet for both the
56 unaltered chimney and the chimney with a baffle, which reduces the effectiveness of the baffle compared to that with the radiation model enabled.
Table 7 - Expanded simulation results comparisons between the ideal baffle configuration (at 80% of the chimney height and of 20% of the chimney air-gap width) and the unaltered solar chimney with the radiation model disabled.
Result Height AR HF Rayleigh ‘Ideal’ Baffle Percent without baffle -2 Number (ṁ/αρ) improvement (m) (H/W) (Wm ) (ṁ/αρ) 10 124.0 125.4 0.5 1 6.6 2.2 × 1012 160.9 166.9 -0.2 5 199.6 212.9 -0.4 10 225.2 228.3 0.3 1.5 6.6 600 1.5 × 1013 287.3 296.8 0.6 5 353.8 377.0 0.3 10 341.3 346.3 -0.5 6.6 3.5 × 1013 431.7 447.9 -0.6 2 5 533.2 566.4 0.0 400 2.4 × 1013 408.2 423.3 -3.0 6.6 800 4.7 × 1013 452.8 469.1 0.8
Figure 38 shows the recirculation zones in the unaltered chimney and the chimney with the best baffle configuration for the no radiation model case. The re-attachment length of the separated flow from the baffle is shorter for the no radiation model case compared to the radiation model enabled scenario.
Figure 38 – Contours of stream function showing the recirculation zone in an unaltered chimney (left) and the chimney with a horizontal baffle at 80% of the chimney height and 20% of the chimney air-gap width (right). Results are obtained with 2m chimney height, 0.4m air-gap width and 600 Wm-2 heat flux.
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Based on the re-attachment length and the air movement away from the absorber shown in Figure 38, it is hypothesised that moving the baffle to a higher position may lead to performance benefits. To verify this hypothesis, testing has been done with the baffle placed at 90% of the chimney height to determine if the baffle at that location can prevent the reverse flow and possibly provide the increased buoyant effect, which is seen in the radiation model enabled simulations. Table 8 shows the results of the tests with the baffle placed at 90% of the chimney height and of 10% and 20% of the chimney air-gap width respectively.
It is seen in Table 8 that, with the baffle width of 20% of the air gap width, the performance variation is inconsistent and is very dependent upon the incident heat flux, chimney width and height. For some cases, a better performance than that with a baffle of 10% of the air gap width is observed, whereas for other cases, the performance of the solar chimney with a baffle of 20% of the air gap width is significantly worse than that of an unaltered chimney.
When the baffle is of 10% of the chimney width, there is a consistent but smaller performance improvement. The performance increases as the aspect ratio decreases (equivalent to having a larger air-gap width), but the heat flux has little effect on the performance enhancement when the baffle is placed at this location.
When no radiation model is enabled, the configuration with a baffle placed at 90% of the chimney height and of 10% of the chimney air-gap width consistently improves the chimney performance, while the configuration with a baffle of 20% of the chimney air-gap width is beneficial only in some specific situations. The reason for the inconsistent performance with the baffle of 20% of the chimney width is further investigated.
Figure 39 illustrates the different flow behaviours between an unaltered chimney and the chimneys with a baffle placed at 90% of the chimney height and of 10% and 20% of the chimney air-gap width respectively. The baffle of 20% of the air-gap width leads to a positive performance improvement. While both baffle widths weaken the reverse flow, the reduction is more significant with the baffle of 20% of the air-gap width.
When comparing the stream function plot for the baffle at 90% of the chimney height and of 20% of the chimney width (refer to Figure 39 (c)) with that for the ideal configuration of the baffle for the radiation model enabled scenarios (see Figure 35(d)), similar flow structures between these two cases can be observed, which lead to similar buoyant plume behaviours. In both cases, there is a significant reduction of the reverse flow.
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Table 8 - Performance of solar chimney with a baffle at 90% of the chimney height with radiation model disabled.
Unaltered 10% width baffle 20% width baffle Height AR HF Performance (m) (H/W) (Wm-2) Performance Improvement Performance Improvement (ṁ/αρ) (ṁ/αρ) (%) (ṁ/αρ) (%) 10 124.0 125.4 1.1 130.6 5.1 1 6.6 160.9 166.9 3.6 161.7 0.5 5 199.6 212.9 6.2 188.1 -6.1 10 225.2 228.3 1.4 235.2 4.3 1.5 6.6 600 287.3 296.8 3.2 314.1 8.6 5 353.8 377.0 6.1 339.4 -4.2 10 341.3 346.3 1.4 355.3 3.9 6.6 431.7 447.9 3.6 429.7 -0.5 2 5 533.2 566.4 5.9 504.6 -5.7 6.6 400 408.2 423.3 3.6 395.6 -3.2 6.6 800 452.8 469.1 3.5 490.1 7.6
Figure 39 – Contours of stream function for an unaltered chimney (left), a chimney with baffle at 90% of the chimney height and of 10% of the air-gap width (middle) and a chimney at 90% of the chimney height and of 20% of the air-gap width (right). Results are obtained with radiation model disabled with a 1.5m chimney height, 0.225m chimney air-gap width and 600 Wm-2 heat flux. Figure 40 shows a scenario in which the baffle of 20% of the chimney width gives a negative performance improvement. While there is a complete suppression of the reverse flow near the glazing, there is a strong recirculation behind the baffle next to the absorber.
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Figure 40 – Contours of stream function for an unaltered chimney (left), a chimney with baffle at 90% of the chimney height and of 10% of the air-gap width (middle) and a chimney with baffle at 90% of the chimney height and of 20% of the air-gap width (right). Results are obtained with 1m chimney height, 0.2m chimney air-gap width and 600 Wm-2 heat flux. Figure 41 shows the effects of the incident heat flux on the thermal flow behaviour for a chimney with a baffle located at 90% of the chimney height and of 20% of the air-gap width. It is seen in the figure that at the relatively low heat fluxes (i.e. 400 and 600 W/m-2) reverse flow penetrates into the chimney along the absorber wall and above the baffle, thereby negating the performance enhancement associated with the reduction of the reverse flow near the glazing. At an incident heat flux of 800 Wm-2, reverse flow is still present on the glazing side, but it is much weaker compared to the reverse flow observed in an unaltered chimney. Further, there is no reverse flow observed along the absorber under this heat flux. This explains why there is a distinct performance enhancement (7.6% compared to the unaltered chimney) in this case.
Figure 41 – Contours of stream function obtained with the baffle at 90% of the chimney height and of 20% of the chimney air-gap width with solar irradiance at 400 Wm-2 (left), 600 Wm-2 (middle) and 800 Wm-2 (right) respectively. Results are obtained with 2m chimney height and 0.3m chimney air-gap width.
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Additional Baffle Configurations with Radiation Model Enabled To see if a baffle would enhance the solar chimney performance regardless of the absorptivity of the glazing, the range of parameters leading to performance improvement without a radiation model are recalculated with the radiation model enabled. The test results with the baffle located at 90% of the chimney height and of 10% and 20% of the chimney width with the radiation model enabled are shown in Table 9. It is found in this table that the configuration with the baffle at 90% of the chimney height and of 20% of the air-gap width does not lead to a performance improvement with the radiation model enabled. However, the configuration with the baffle of 10% of the air-gap width does lead to an improved performance that is comparable to the performance achievable with the baffle at 80% of the chimney height and of 30% of the air-gap width. The average performance increase across the range of the tested cases with the former configuration is 6%, which is comparable to the average performance enhancement with the latter case.
Table 9 - Results with baffle at 90% of the chimney height and with radiation model enabled.
Unaltered 10% width baffle 20% width baffle Height AR HF Performance (m) (H/W) (Wm-2) Performance Improvement Performance Improvement (ṁ/αρ) (ṁ/αρ) (%) (ṁ/αρ) (%) 10 163.8 166.7 1.8 142.2 -13.2 1 6.6 213.7 227.4 6.4 179.8 -15.9 5 261.4 290.6 11.2 211.7 -19.0 10 283.2 289.8 2.3 245.1 -13.4 1.5 6.6 600 371.6 393.2 5.8 308.5 -17.0 5 455.5 514.0 12.8 363.0 -20.3 10 428.9 436.7 1.8 361.3 -15.8 6.6 559.3 609.4 9.0 453.7 -18.9 2 5 686.8 766.8 11.7 534.0 -22.2 6.6 400 507.8 547.2 7.8 406.8 -19.9 6.6 800 612.1 646.9 5.7 491.1 -19.8
Figure 42 shows the stream function obtained for a chimney with a baffle at 90% of the chimney height and of 10% of the air-gap width compared against those of an unaltered
61 chimney and a chimney with a baffle at 80% of the chimney height and of 30% of the air-gap width when the radiation model is enabled. It is seen in the figure that the behaviours of both baffled chimneys are similar. In both cases, the re-attachment of the separated flow from the baffle occurs at the chimney outlet, resulting in the region of re-circulation occupying the full distance from the baffle to the chimney outlet.
Figure 42 - Comparison of the contours of stream function for unaltered chimney (left), a chimney with a baffle at 80% of the chimney height and of 30% of the air- gap width (middle), and a chimney with a baffle at 90% of the chimney height and of 10% of the air-gap width (right). All results are obtained with radiation model enabled for a 2m chimney height, 0.3m chimney air-gap width and 600 Wm-2 heat flux. Conclusions
Based upon the simulations conducted in this investigation, when no radiation model is enabled, a baffle located at 90% of the chimney height with a width of 10% of the chimney air- gap width can increase the chimney performance by about 6%. This is due to the reduction of the reverse flow along the glazing, increased mixing due to the re-circulation behind the horizontal baffle and the consequent lower air density and stronger buoyancy effect at the chimney outlet. A baffle at 90% of the chimney height and of 20% of the air-gap width outperforms the baffle of 10% of the chimney air-gap width under certain conditions. However, at certain heights and solar irradiance, this configuration may give a negative performance improvement, and thus is not recommended.
When the radiation model is enabled, a horizontal baffle located at 90% of the chimney height and of 10% of the chimney air-gap width leads to a performance improvement of up to 12%, with an average improvement of about 6%. A baffle located at 80% of the chimney height and of 30% of the chimney air-gap width leads to an improvement of up to 10% with an average
62 improvement of 6%. The improvement is relatively larger when the solar chimney has a lower aspect ratio. The primary improvement when radiation model is enabled is associated with the lowered air density and increased average temperature in the solar chimney due to the recirculation behind the horizontal baffle, causing increased mixing.
Experimental verification is recommended to validate the present simulations. If the simulations are indeed indicative of the real-world performance, then the horizontal baffle is a low-cost solution to improve the performance of a solar chimney.
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6 Summary and Future Research Options Summary of the Present Investigations
Validation of the present numerical models has been conducted by comparing with the experimental data available in the literature. There is a good agreement between the present CFD simulations and the previous experimental results.
When tinting is applied to a chimney, it can result in considerable improvement to the chimney performance by countering the effect of reverse flow if the radiation transfer is not considered in the numerical model. Application of tint such that 50% of the incoming solar irradiance is retained by the glazing provides the best performance enhancement. However, when the radiation model is enabled, the performance variation between an unaltered chimney and chimneys with tinted glazing is negligible.
A horizontal baffle placed at 90% of the chimney height and of 10% of the chimney width provides performance enhancement consistently over the entire range of parameters tested when radiation model is either enabled or disabled. When the radiation model is enabled, a baffle placed at 80% of the chimney height and of 30% of the chimney width also provides significant performance enhancement.
Both the simulations of the adjusted tinting and the introduction of a horizontal baffle show the importance of the modelling methods and how they should align with the real-world situations. The radiation models assume that a surface can absorb infrared radiation and may overestimate the amount of energy transferred from the absorber surface to the glazing surface, leading to an overestimate of the predicted performance of a solar chimney. Possible Experiment As the present studies are limited to numerical simulations only due to time constraints, it is recommended that an experimental verification is conducted. The predicted performance enhancement due to the introduction of a baffle can be considerable, and an experimental verification of the enhancement would be ideal. There are challenges to conducting such an experimental verification as the values of the heat flux, the chimney height and width are relatively large, which makes the model construction and experimental measurements difficult. At the largest chimney heights and widths, the volume of the air channel is approximately a cubic metre (if constructed at 1m wide). This requires a large enough space for the experimental
64 tests, and the required heating may cause significant variation of the room temperature, which in turn may affect the measurements. LES Simulation Options More comprehensive three-dimensional CFD studies using Large Eddy Simulation (LES) approach may better reveal the thermal flow behaviour in solar chimney. A single in-depth LES study would be able to validate the RANS simulations. The downside of the LES study is that the mesh requirements for the chimney domain alone (not including the expanded domain) to better resolve the flows would be several orders of magnitude larger than that adopted in the current two-dimensional studies. This along with the inherent transient requirements of a LES study means that any conducted simulation would require considerable computational resources and be conducted over a long timeframe. The LES studies reported in the literature give confidence on producing accurate results in the CFD simulations. Validation and Use of Solar Load Model
The solar load model is a method of applying heat-flux on a surface, which will consider many materials factors, such as the absorptivity, transmission, and reflectance of a surface, allowing for the modelling of glazing and other transparent materials. An advantage of this method is that it can be set to represent the location of the sun in the sky (as an incoming vector), allowing for detailed investigations of the effect of orientation of a solar chimney. In addition, surfaces which do not easily allow the application of a heat flux such as nonlinear curves or complicated 3D geometry can be more readily simulated using the solar load model. This can be combined with the Discrete Ordinance or Surface to Surface radiation model to more accurately model heating due to solar irradiance.
This method has not been explored in a great depth in the literature and could be of great use to the natural ventilation research field. The limitation of this method is that a full three- dimensional model and thus more time and computing resources are required. New Solar Chimney Designs
As mentioned in the previous literature review, many technologies and design options can be adopted to improve the performance of a solar chimney. A study into the combined effect of some of these technologies is recommended. For example, the present simulations show that the horizontal baffle has a positive effect on the chimney performance, as does the inclusion of
65 an inclined passive wall. The combined effect of both these implementations would be of interest, particularly if the performance increases synchronise well. Recommended Future Designs Based upon the research conducted in this study and the information available from the literature, the horizontal baffle is likely to be effective in enhancing the solar chimney performance. Some minor adjustments, such as changing it to a wedge-shaped baffle or introducing stepping to the baffle may lead to further improvements upon the current design. Figure 43 shows these two possible design options. In addition to combining with other designs, the effect of the baffle angle has not been studied. Angling the baffle upstream or downstream may either increase the performance or reduce the required size of the baffle.
Figure 43 - Stepped Horizontal Baffle (left) and wedge-shaped baffle (right).
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Appendix A – Automation Methods Parametric Study using ANSYS Workbench
A range of studies have been conducted using the parametric feature of ANSYS Fluent and Workbench, which allows one to automate some of the more menial tasks for CFD studies. Changing the distribution of heat flux is automated in ANSYS workbench. This is done using the IRON Python coding language, which is built upon open source python. The added features allow for the use of meta-data in the program, enabling the study of the relationships between various geometric components. Figure 44 shows a screenshot of a portion of the code used in the tinting study. Tutorials and documentation are available on ANSYS forums and through the local ANSYS distributor.
Figure 44 - Snippet of SpaceClaim script written in iron python This method of automation is useful if the geometric design is relatively simple as more complex designs may lead to the code creating unexpected outcomes. Nevertheless, this method allows rapid building of case files and simulations. Parametric Study Using Python, ICEM and the HPC
ICEM CFD is used in this CFD study due to the better scripting compatibility with the high- performance cluster. Python is used to write script and instruction files. The values for the
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Taguchi orthogonal array are input into a python script which prepared the various scripts for ICEM, Fluent and the HPC, Figure 45 show snippets of the code used.
Figure 45 – Section of code used on HPC.
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Appendix B – Simulation Results for the Taguchi Method with Radiation Model Disabled
Table 10 - Taguchi array results when radiation model is disabled.
Height AR HF Rayleigh BH BW Results -2 (m) (H/W) (Wm ) Number (Hb/H) (Wb/W) (ṁ/αρ) 0.2 106.2 1.5 × 10 400 0.4 0.3 100.6 1012 0.4 94.9 0.2 154.8 2.2 × 1 6.6 600 0.6 0.3 148.9 1012 0.4 138.9 0.2 209.1 3.0 × 5 800 0.8 0.3 206.0 1012 0.4 196.6 0.2 224.4 1.1 × 10 600 0.8 0.3 216.6 1013 0.4 203.3 0.2 294.3 1.5 × 1.5 6.6 800 0.4 0.3 279.5 1013 0.4 263.2 0.2 329.9 7.5 × 5 400 0.6 0.3 304.7 1012 0.4 278.5 0.2 359.8 4.7 × 10 800 0.6 0.3 346.9 1013 0.4 326.7 0.2 396.3 2.4 × 2 6.6 400 0.8 0.3 380.8 1013 0.4 355.3 0.2 537.2 3.5 × 5 600 0.4 0.3 499.5 1013 0.4 448.7
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Appendix C – Stream function for Baffle Chimney with Radiation Model Enabled
Figure 46 - The stream function for a 2m high chimney, a 0.4m air-gap width, for a baffle of 30% of the chimney air-gap width and 600 Wm-2 heat flux. From left to right: no baffle, baffle at 20% of the height, baffle at 40% of the height, baffle at 60% of the height, and baffle at 80% of the height.
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