DOCSLIB.ORG

Heat Transfer in Flat-Plate Solar Air-Heating Collectors

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text

Load more

Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-818-X Heat transfer in flat-plate solar air-heating collectors Y. Nassar & E. Sergievsky Abstract All energy systems involve processes of heat transfer. Solar energy is obviously a field where heat transfer plays crucial role. Solar collector represents a heat exchanger, in which receive solar radiation, transform it to heat and transfer this heat to the working fluid in the collector's channel The radiation and convection heat transfer processes inside the collectors depend on the temperatures of the collector components and on the hydrodynamic characteristics of the working fluid. Economically, solar energy systems are at best marginal in most cases. In order to realize the potential of solar energy, a combination of better design and performance and of environmental considerations would be necessary. This paper describes the thermal behavior of several types of flat-plate solar air-heating collectors. 1 Introduction Nowadays the problem of utilization of solar energy is very important. By economic estimations, for the regions of annual incident solar radiation not less than 4300 MJ/m^ pear a year (i.e. lower 60 latitude), it will be possible to cover - by using an effective flat-plate solar collector- up to 25% energy demand in hot water supply systems and up to 75% in space heating systems. Solar collector is the main element of any thermal solar system. Besides of large number of scientific publications, on the problem of solar energy utilization, for today there is no common satisfactory technique to evaluate the thermal behaviors of solar systems, especially the local characteristics of the solar collector. By calculating the effective of solar collectors, considering all characteristics constant Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-818-X 576 Advanced Computational Methods in Heat Transfer VI by the length direction (x), such as, over all heating loss UL, connective and radiative heat transfer coefficients a?, a,,... etc., which resulting to get unexactly information and to complicate the evaluation of economic effect of using solar energy. 2 Heat transfer in flat-plate solar air-heating collectors Very little works considered the thermal characteristics of solar collectors as a function of the length direction [10], the variation of results obtained by itegration method Hottel and Whillier [3] and by our offered model plotted in figure 1. *t \J \J r/r o o A p*n PQ J O U 0) pmmM! jwj Jo O/: U A % !^w fc 340 cx __ ~ \ § 320 % v If 'Vi. i nn tr ;r Tg Taout Tarn Tp Figure 1: Comparison between the results obtained by integration method and by finite difference method, for a flat-plate solar air-heating collector first type with one glass cover Where Tp, Tg, Tarn are the average temperatures of absorber plate Glass cover and air and Taout is the outlet air temperature [K]. The classical approach of treating the heat transfer processes in flat-plate solar collectors is documented in all modern textbooks of solar energy, eg. Duffie & Beckman [2], Kreider & Krieth [6], Meinel & Meinel [7], Klein [5] and Zvirin & Aronov [11]. The theory is based on calculating the heat losses through the various components of the collectors, and representing the heat fluxes between two nodes j, k as (Xj-k(Tj-Tk), where a is an effective heat transfer coefficient (by convective, conduction or radiation). Figure 2 is a schematic of heat transfer balance for a single-glazed collector type Jf° 1 (the extension to multi-glazed and to other types is straightforward). The radiation coefficients are written as: (i) i i -i Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-818-X Advanced Computational Methods in Heat Transfer VI 577 (2) where: 7 is the temperature and £ is the emissivity; the subscribes p, g, <%> and sky indicated to the absorber plate, glass cover, ambient and effective sky, given by empirical correlation [11], as: 7^ = 0,0552.7^ or 7^ = 7^-6 (K) (3) L_ \_ULJ^i I glass covet Inlet air ir pass channe absorbr pbte Useful Ene^y qu Figure 2. Schema of the heat balance in a solar collector. The conduction coefficient in the case, of the backside insulation (of thickness Axb and thermal conductivity A,b) is expressed as A (4) Ax, Another convection correlation is used for determining the heat losses to the ambient by free and forced convection under the effects of wind with velocity V* [4]: <%<% =2,8 + 3,0.^(m/j) (5) Where the heat convection coefficients are taken from appropriate correlation for the Nusselt number Nu, for example: Nu% = 0,0233. Re ^ . Pr^ for turbulent flow; Nuy, = 0,332. Re*P . Pr^'^ for laminar flow (6) where Re% = ; u/is the local velocity of the working fluid However, the above maintained eqn (6) only for fully hydrodynamic developed turbulent or laminar flow, without accounting the entrance length, which presented Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-818-X 578 Advanced Computational Methods in Heat Transfer VI approximately 40-50% of the channel length [1], in where Mussel number is higher than the fully developed region, not accounting the effect of temperatures variations of the upper and lower plates and not accounting the effect of the turbulence intensity, which increase the convective coefficient by 50% in the intervals of 8-9% turbulence intensity [8]. In the case of solar collectors formula for Nusselt number must be taking into account of variation of temperatures of walls and the relation of turbulence intensity by the length of the channel of solar collector. So the task of this paper is, to obtain a relationship between all these factors and Nusselt number. To achieve this aim the following tasks were setting up: 1. Realization of experimental and numerical investigations, to obtain the local characteristics fields of velocity, temperature and turbulence intensity through the length of the channel flat-plate solar air-heating collector. 2. Development of a technique to calculate flow and heat exchange parameters in the channel 3. Evaluation of thermal behavior of flat-plate solar air-heating collectors. 3 Experimental approach Experiment was setup on aerodynamic open type installation (figure 3), to obtain the fields of speed, turbulence intensity, temperature of air flowing through the channel model of solar collector, and temperatures of solar collector's elements The measurements was collected using hot-wire and thermonanometer and block measurement from DISA. The obtained results used, furthermore to prove the reliability of the offered mathematical models, used to evaluate Nusselt's number and as boundary conditions for solving k-e turbulent model. 2 glass co' air flow Co 5 absorber plate £:insulation be Figure 3: Experimental installation, where 1 radiation source; 2 fan; 3 model flat- plate air-heating solar collector; 4 sensor of measurement; 5 measurement block; 6 IBM computer, 7 aerodynamic channel Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpreAdvancess.com,d ISBNComputationa 1-85312-818-Xl Method s in Heat Transfer VI 579 4 Numerical approach Experimental results shown, that the specialty of heat transfer in solar collectors are: 1. The upper and lower walls of the solar collector' s channel are different. 2. There is a large turbulence intensity at the inlet of the channel, and its damped not only by the thickness of the sublayer, but also by the length direction of the channel. 3. Inlet Reynold's number Ren approximately 10*. The problem of the calculation the temperatures of solar collector's elements, that there are not enough information about the mechanism heat exchange in the air channel. To understanding the heat transfer processes in solar collectors, we used complex PHOENICS, a working sheet file Ql was written down under our conditions, the obtained results plotted in figure 4, 5. 40 60 80 100 1 20 Air te m perature, [c ] Figure 4: Two dimensional temperature behavior of air flowing through the channel of flat-plate solar air heating collector first type obtained via PHOENICS. Figure 5: Two dimensional turbulence intensity evaluation of air flowing through the channel of flat-plate solar air heating collector first type obtained via PHOENICS. Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-818-X 580 Advanced Computational Methods in Heat Transfer VI Nassar [9] developed a model for Nusselt's number as functions of variation of the temperatures of die upper and lower walls and turbulence intensity (Tu) by the length direction the channel, which found as: (7) (8) Tux%(x) = 7w/«%-exp (9) J U where NupX is the local Nusselt number of the absorber plate; NugX is the local Nusselt number of the glass cover; Ntt^x = 0,0233. Re ^ .Pr^; L channel length; Tuin% , Tux% is the inlet ant local turbulence intensity receptively; \Ke Tu = .J ; where 7T, 7 and V are the impulsion velocities in x,y and z coordinates, Ke is the kinetic energy and U« is velocity We investigated the reliability of these equations in the regions of and 0. !%<Tu%<14%. 34567 Channel length x = 0,081 .X Figure 6: Comparison of convective heat transfer coefficient by using: 1, 2. our offered model; 3. experiment; 4. Nu^ =0,0233Re£* .Pr°'* and 5.
Recommended publications
  • Heat Transfer Model of a Rotary Compressor S

    Heat Transfer Model of a Rotary Compressor S

    Purdue University Purdue e-Pubs International Compressor Engineering Conference School of Mechanical Engineering 1992 Heat Transfer Model of a Rotary Compressor S. K. Padhy General Electric Company Follow this and additional works at: https://docs.lib.purdue.edu/icec Padhy, S. K., "Heat Transfer Model of a Rotary Compressor" (1992). International Compressor Engineering Conference. Paper 935. https://docs.lib.purdue.edu/icec/935 This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/ Herrick/Events/orderlit.html HEAT TRANSFER MODEL OF A ROTARY COMPRESSOR Sisir K. Padhy General Electric C_ompany Appliance Park 5-2North, Louisville, KY 40225 ABSTRACT Energy improvements for a rotary compressor can be achieved in several ways such as: reduction of various electrical and mechanical losses, reduction of gas leakage, better lubrication, better surface cooling, reduction of suction gas heating and by improving other parameters. To have a ' better understanding analytical/numerical analysis is needed. Although various mechanical models are presented to understand the mechanical losses, dynamics, thermodynamics etc.; little work has been done to understand the compressor from a heat uansfer stand point In this paper a lumped heat transfer model for the rotary compressor is described. Various heat sources and heat sinks are analyzed and the temperature profile of the compressor is generated. A good agreement is found between theoretical and experimental results. NOMENCLATURE D, inner diameter D.
  • Investigation of Compressor Heat Dispersion Model Da Shi Shanghai Hitachi Electronic Appliances, China, People's Republic Of, Shid@Shec.Com.Cn

    Investigation of Compressor Heat Dispersion Model Da Shi Shanghai Hitachi Electronic Appliances, China, People's Republic Of, [email protected]

    Purdue University Purdue e-Pubs International Compressor Engineering Conference School of Mechanical Engineering 2014 Investigation Of Compressor Heat Dispersion Model Da Shi Shanghai Hitachi Electronic Appliances, China, People's Republic of, [email protected] Hong Tao Shanghai Hitachi Electronic Appliances, China, People's Republic of, [email protected] Min Yang Shanghai Hitachi Electronic Appliances, China, People's Republic of, [email protected] Follow this and additional works at: https://docs.lib.purdue.edu/icec Shi, Da; Tao, Hong; and Yang, Min, "Investigation Of Compressor Heat Dispersion Model" (2014). International Compressor Engineering Conference. Paper 2314. https://docs.lib.purdue.edu/icec/2314 This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/ Herrick/Events/orderlit.html 1336, Page 1 Investigation of Rotary Compressor Heat Dissipation Model Da SHI 1, Hong TAO2, Min YANG 3* 1SHEC, R&D Centre, Shanghai, China Contact Information (+8621-13564483789, [email protected]) 2SHEC, R&D Centre, Shanghai, China Contact Information (+8621-13501728323,[email protected]) 3SHEC, R&D Centre, Shanghai, China Contact Information (+8621-15900963244, [email protected]) * Corresponding Author ABSTRACT This paper presented a model of rotary compressor heat dissipation, which can be used to calculate heat dissipation under forced-convective/natural-convective and heat radiation mode respectively. The comparison, between calculated and experimental result for both constant speed compressor and variable speed one, shows that the average heat dissipation error is below 20% and discharge temperature deflection is less than 4 ℃。 1.
  • Convective Heat Transfer Coefficient for Indoor Forced Convection Drying of Corn Kernels

    Convective Heat Transfer Coefficient for Indoor Forced Convection Drying of Corn Kernels

    Int. J. Mech. Eng. & Rob. Res. 2013 Ravinder Kumar Sahdev et al., 2013 ISSN 2278 – 0149 www.ijmerr.com Vol. 2, No. 4, October 2013 © 2013 IJMERR. All Rights Reserved Research Paper CONVECTIVE HEAT TRANSFER COEFFICIENT FOR INDOOR FORCED CONVECTION DRYING OF CORN KERNELS Ravinder Kumar Sahdev1*, Chinu Rathi Saroha1 and Mahesh Kumar2 *Corresponding Author: Ravinder Kumar Sahdev, [email protected] In this present research paper, an attempt has been made to determine the convective heat transfer coefficient of corn kernels under indoor forced convection drying mode. The experiments were conducted in the month of May 2013 in the climatic conditions of Rohtak (28° 40': 29 05'N 76° 13': 76° 51'E). Corn kernels were dried from initial moisture content 43% dry-basis. Experimental data was used to evaluate the values of constants (C and n) in Nusselt number expression by using linear regression analysis and consequently convective heat transfer coefficient was determined. The convective heat transfer coefficient for corn kernels was found to be 1.04 W/m2 °C. The experimental error in terms of percent uncertainty has also been calculated. Keywords: Corn kernels, Convective heat transfer coefficient, Indoor forced convection drying INTRODUCTION the interior of the corn kernels takes place due Corn is one of the main agricultural products to induced vapor pressure difference between in many countries. It is an important industrial the corn kernels and surrounding medium. raw material in the starch industry (Haros and The convective heat transfer coefficient is Surez, 1997; and Soponronnarit et al., 1997a). an important parameter in drying rate It is also a grain that can be eaten raw off the simulation since the temperature difference cob.
  • Heat Transfer (Heat and Thermodynamics)

    Heat Transfer (Heat and Thermodynamics)

    Heat Transfer (Heat and Thermodynamics) Wasif Zia, Sohaib Shamim and Muhammad Sabieh Anwar LUMS School of Science and Engineering August 7, 2011 If you put one end of a spoon on the stove and wait for a while, your nger tips start feeling the burn. So how do you explain this simple observation in terms of physics? Heat is generally considered to be thermal energy in transit, owing between two objects that are kept at di erent temperatures. Thermodynamics is mainly con- cerned with objects in a state of equilibrium, while the subject of heat transfer probes matter in a state of disequilibrium. Heat transfer is a beautiful and astound- ingly rich subject. For example, heat transfer is inextricably linked with atomic and molecular vibrations; marrying thermal physics with solid state physics|the study of the structure of matter. We all know that owing matter (such as air) in contact with a heated object can help `carry the heat away'. The motion of the uid, its turbulence, the ow pattern and the shape, size and surface of the object can have a pronounced e ect on how heat is transferred. These heat ow mechanisms are also an essential part of our ventilation and air conditioning mechanisms, adding comfort to our lives. Importantly, without heat exchange in power plants it is impossible to think of any power generation, without heat transfer the internal combustion engine could not drive our automobiles and without it, we would not be able to use our computer for long and do lengthy experiments (like this one!), without overheating and frying our electronics.
  • (CFD) Modelling and Application for Sterilization of Foods: a Review

    (CFD) Modelling and Application for Sterilization of Foods: a Review

    processes Review Computational Fluid Dynamics (CFD) Modelling and Application for Sterilization of Foods: A Review Hyeon Woo Park and Won Byong Yoon * ID Department of Food Science and Biotechnology, College of Agricultural and Life Science, Kangwon National University, Chuncheon 24341, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-33-250-6459 Received: 30 March 2018; Accepted: 23 May 2018; Published: 24 May 2018 Abstract: Computational fluid dynamics (CFD) is a powerful tool to model fluid flow motions for momentum, mass and energy transfer. CFD has been widely used to simulate the flow pattern and temperature distribution during the thermal processing of foods. This paper discusses the background of the thermal processing of food, and the fundamentals in developing CFD models. The constitution of simulation models is provided to enable the design of effective and efficient CFD modeling. An overview of the current CFD modeling studies of thermal processing in solid, liquid, and liquid-solid mixtures is also provided. Some limitations and unrealistic assumptions faced by CFD modelers are also discussed. Keywords: computational fluid dynamics; CFD; thermal processing; sterilization; computer simulation 1. Introduction In the food industry, thermal processing, including sterilization and pasteurization, is defined as the process by which there is the application of heat to a food product in a container, in an effort to guarantee food safety, and extend the shelf-life of processed foods [1]. Thermal processing is the most widely used preservation technology to safely produce long shelf-lives in many kinds of food, such as fruits, vegetables, milk, fish, meat and poultry, which would otherwise be quite perishable.
  • Recent Developments in Heat Transfer Fluids Used for Solar

    Recent Developments in Heat Transfer Fluids Used for Solar

    enewa f R bl o e ls E a n t e n r e g Journal of y m a a n d d n u A Srivastva et al., J Fundam Renewable Energy Appl 2015, 5:6 F p f p Fundamentals of Renewable Energy o l i l ISSN: 2090-4541c a a n t r i DOI: 10.4172/2090-4541.1000189 o u n o s J and Applications Review Article Open Access Recent Developments in Heat Transfer Fluids Used for Solar Thermal Energy Applications Umish Srivastva1*, RK Malhotra2 and SC Kaushik3 1Indian Oil Corporation Limited, RandD Centre, Faridabad, Haryana, India 2MREI, Faridabad, Haryana, India 3Indian Institute of Technology Delhi, New Delhi, India Abstract Solar thermal collectors are emerging as a prime mode of harnessing the solar radiations for generation of alternate energy. Heat transfer fluids (HTFs) are employed for transferring and utilizing the solar heat collected via solar thermal energy collectors. Solar thermal collectors are commonly categorized into low temperature collectors, medium temperature collectors and high temperature collectors. Low temperature solar collectors use phase changing refrigerants and water as heat transfer fluids. Degrading water quality in certain geographic locations and high freezing point is hampering its suitability and hence use of water-glycol mixtures as well as water-based nano fluids are gaining momentum in low temperature solar collector applications. Hydrocarbons like propane, pentane and butane are also used as refrigerants in many cases. HTFs used in medium temperature solar collectors include water, water- glycol mixtures – the emerging “green glycol” i.e., trimethylene glycol and also a whole range of naturally occurring hydrocarbon oils in various compositions such as aromatic oils, naphthenic oils and paraffinic oils in their increasing order of operating temperatures.
  • Forced and Natural Convection

    Forced and Natural Convection

    FORCED AND NATURAL CONVECTION Forced and natural convection ...................................................................................................................... 1 Curved boundary layers, and flow detachment ......................................................................................... 1 Forced flow around bodies .................................................................................................................... 3 Forced flow around a cylinder .............................................................................................................. 3 Forced flow around tube banks ............................................................................................................. 5 Forced flow around a sphere ................................................................................................................. 6 Pipe flow ................................................................................................................................................... 7 Entrance region ..................................................................................................................................... 7 Fully developed laminar flow ............................................................................................................... 8 Fully developed turbulent flow ........................................................................................................... 10 Reynolds analogy and Colburn-Chilton's analogy between friction and heat
  • Convectional Heat Transfer from Heated Wires Sigurds Arajs Iowa State College

    Convectional Heat Transfer from Heated Wires Sigurds Arajs Iowa State College

    Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1957 Convectional heat transfer from heated wires Sigurds Arajs Iowa State College Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Physics Commons Recommended Citation Arajs, Sigurds, "Convectional heat transfer from heated wires " (1957). Retrospective Theses and Dissertations. 1934. https://lib.dr.iastate.edu/rtd/1934 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. CONVECTIONAL BEAT TRANSFER FROM HEATED WIRES by Sigurds Arajs A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR 01'' PHILOSOPHY Major Subject: Physics Signature was redacted for privacy. In Chapge of K or Work Signature was redacted for privacy. Head of Major Department Signature was redacted for privacy. Dean of Graduate College Iowa State College 1957 ii TABLE OF CONTENTS Page I. INTRODUCTION 1 II. THEORETICAL CONSIDERATIONS 3 A. Basic Principles of Heat Transfer in Fluids 3 B. Heat Transfer by Convection from Horizontal Cylinders 10 C. Influence of Electric Field on Heat Transfer from Horizontal Cylinders 17 D. Senftleben's Method for Determination of X , cp and ^ 22 III. APPARATUS 28 IV. PROCEDURE 36 V. RESULTS 4.0 A. Heat Transfer by Free Convection I4.0 B. Heat Transfer by Electrostrictive Convection 50 C.
  • THERMODYNAMICS, HEAT TRANSFER, and FLUID FLOW, Module 3 Fluid Flow Blank Fluid Flow TABLE of CONTENTS

    THERMODYNAMICS, HEAT TRANSFER, and FLUID FLOW, Module 3 Fluid Flow Blank Fluid Flow TABLE of CONTENTS

    Department of Energy Fundamentals Handbook THERMODYNAMICS, HEAT TRANSFER, AND FLUID FLOW, Module 3 Fluid Flow blank Fluid Flow TABLE OF CONTENTS TABLE OF CONTENTS LIST OF FIGURES .................................................. iv LIST OF TABLES ................................................... v REFERENCES ..................................................... vi OBJECTIVES ..................................................... vii CONTINUITY EQUATION ............................................ 1 Introduction .................................................. 1 Properties of Fluids ............................................. 2 Buoyancy .................................................... 2 Compressibility ................................................ 3 Relationship Between Depth and Pressure ............................. 3 Pascal’s Law .................................................. 7 Control Volume ............................................... 8 Volumetric Flow Rate ........................................... 9 Mass Flow Rate ............................................... 9 Conservation of Mass ........................................... 10 Steady-State Flow ............................................. 10 Continuity Equation ............................................ 11 Summary ................................................... 16 LAMINAR AND TURBULENT FLOW ................................... 17 Flow Regimes ................................................ 17 Laminar Flow ...............................................
  • Forced Convection Heat Transfer Convection Is the Mechanism of Heat Transfer Through a Fluid in the Presence of Bulk Fluid Motion

    Forced Convection Heat Transfer Convection Is the Mechanism of Heat Transfer Through a Fluid in the Presence of Bulk Fluid Motion

    Forced Convection Heat Transfer Convection is the mechanism of heat transfer through a fluid in the presence of bulk fluid motion. Convection is classified as natural (or free) and forced convection depending on how the fluid motion is initiated. In natural convection, any fluid motion is caused by natural means such as the buoyancy effect, i.e. the rise of warmer fluid and fall the cooler fluid. Whereas in forced convection, the fluid is forced to flow over a surface or in a tube by external means such as a pump or fan. Mechanism of Forced Convection Convection heat transfer is complicated since it involves fluid motion as well as heat conduction. The fluid motion enhances heat transfer (the higher the velocity the higher the heat transfer rate). The rate of convection heat transfer is expressed by Newton’s law of cooling: q hT T W / m 2 conv s Qconv hATs T W The convective heat transfer coefficient h strongly depends on the fluid properties and roughness of the solid surface, and the type of the fluid flow (laminar or turbulent). V∞ V∞ T∞ Zero velocity Qconv at the surface. Qcond Solid hot surface, Ts Fig. 1: Forced convection. It is assumed that the velocity of the fluid is zero at the wall, this assumption is called no‐ slip condition. As a result, the heat transfer from the solid surface to the fluid layer adjacent to the surface is by pure conduction, since the fluid is motionless. Thus, M. Bahrami ENSC 388 (F09) Forced Convection Heat Transfer 1 T T k fluid y qconv qcond k fluid y0 2 y h W / m .K y0 T T s qconv hTs T The convection heat transfer coefficient, in general, varies along the flow direction.
  • 4. Forced Convection Heat Transfer

    4. Forced Convection Heat Transfer

    Page 4.1 4. Forced Convection Heat Transfer 4.1 Fundamental Aspects of Viscous Fluid Motion and Boundary Layer Motion 4.1.1 Viscosity 4.1.2 Fluid Conservation Equations - Laminar Flow 4.1.3 Fluid Conservation Equations - Turbulent Flow 4.2 The Concept pf Boundary Layer 4.2.1 Laminar Boundary Layer 4.2.1.1 Conservation Equations - Local Formulation 4.2.1.2 Conservation Equations -Integral Formulation 4.2.2 Turbulent Boundary Layer 4.3 Forced Convection Over a Flat Plate 4.3.1 laminar Boundary Layer 4.3.1.1 Velocity Boundary Layer - Friction Coefficient 4.3.1.2 Thermal Boundary Layer - Heat Transfer Coefficient 4.3.2 Turbulent Flow 4.3.2.1 Velocity Boundary Layer - Friction Coefficient 4.3.2.2 Heat Transfer in the Turbulent Boundary Layer 4.4 Forced Convection In Ducts 4.4.1 laminar Flow 4.4.1.1 Velocity Distribution and Friction Factor In Laminar Flow 4.4.1.2 Bulk Temperature 4.4.1.3 Heat Transfer in Fully Developed Laminar Flow 4.4.2 Turbulent 4.4.2.1 Velocity Distribution and Friction Factor 4.4.2.2 Heat Transfer In Fully Developed Turbulent Flow 4.4.2.3 Non- Circular Tubes Page 4.2 4. Forced Convection Heat Transfer In Chapter 3, we have discussed the problems of heat conduction and used the convection as one of the boundary conditions that can be applied to the surface of a conducting solid. We also assumed that the heat transfer rate from the solid surface was given by Newton's law of cooling: =h,A~w-tJ q" 4.1 In the above application, he' the convection heat transfer coefficient has been supposed known.
  • Low Outgassing of Silicon-Based Coatings on Stainless Steel Surfaces for Vacuum Applications

    Low Outgassing of Silicon-Based Coatings on Stainless Steel Surfaces for Vacuum Applications

    Low Outgassing of Silicon-Based Coatings on Stainless Steel Surfaces for Vacuum Applications David A. Smith, Martin E. Higgins Restek Corporation, 110 Benner Circle Bellefonte, PA 16823, www.restekcoatings.com Bruce R.F. Kendall Elvac Laboratories 100 Rolling Ridge Drive, Bellefonte, PA 16823 Performance Coatings This presentation was developed through the collaborative efforts of Restek Performance Coatings and Bruce Kendall of Elvac Labs. Restek applied the coatings to vacuum components and Dr. Kendall developed, performed and interpreted the experiments to evaluate the coating performance. lit. cat.# RPC-uhv3 1 Objective Evaluate comparative outgassing properties of vacuum components with and without amorphous silicon coatings Performance Coatings lit. cat.# RPC-uhv3 2 Theoretical Basis – Heat Induced Outgassing • Outgassing rate (F) in monolayers per sec: F = [exp (-E/RT)] / t’ t’ = period of oscillation of molecule perp. to surface, ca. 10-13 sec E = energy of desorption (Kcal/g mol) R = gas constant source: Roth, A. Vacuum Technology, Elsevier Science Publishers, Amsterdam, 2nd ed., p. 177. • Slight elevation of sample temperature accelerates outgassing rate exponentially Performance Coatings The design of the first set of experiments allowed the isolation and direct comparison of outgassing rates with increasing temperature. By applying heat, the outgassing rates are exponentially increased for the purpose of timely data collection. These comparisons with experimental controls will directly illustrate the differences incurred by the applied coatings. lit. cat.# RPC-uhv3 3 Experimental Design – Heated Samples • Turbo pump for base pressures to 10-8 Torr – pumping rate between gauge and pump: 12.5 l/sec (pump alone: 360 l/sec) – system vent with dry N2 between thermal cycles • Comparative evaluation parts – a-silicon coated via CVD (Silcosteel®-UHV); 3D deposition – equally treated controls without deposition Performance Coatings The blue sample (left) was a standard coating commercially available through Restek.