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On the Natural Displacement Ventilation Flow Through a Full Scale Enclosure, Driven by a Source of Buoyancy at Floor Level S

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On the Natural Displacement Ventilation Flow Through a Full Scale Enclosure, Driven by a Source of Buoyancy at Floor Level S Seventh International IBPSA Conference Rio de Janeiro, Brazil August 13-15, 2001 ON THE NATURAL DISPLACEMENT VENTILATION FLOW THROUGH A FULL SCALE ENCLOSURE, DRIVEN BY A SOURCE OF BUOYANCY AT FLOOR LEVEL S. A. Howell and I. Potts University of Newcastle upon Tyne Newcastle upon Tyne, NE1 7RU, England technique, which uses salt water to generate density ABSTRACT differences and motivate the displacement flow. This paper presents experimental data for the This paper reports independent validation work, temperature stratification established within a where a natural displacement ventilation flow, driven full-scale enclosure, for the natural displacement by a source of buoyancy at floor level, is established ventilation flow driven by a source of buoyancy at within a full-sized enclosure with air as the working floor level within the space with air as the working fluid. This is an extension of previously published fluid. work (Howell and Potts, 1998), where a smaller scale A range of predictive techniques is also enclosure was studied. The current investigation is, investigated for the flow, and for each technique a however, truly representative of the ventilation flows critical comparison with the experimental data is observed in real buildings. presented. It is found that there are differences between It is confirmed that the salt-bath modelling the ventilation flow through the full-scale test room technique and related mathematical model of Linden and that predicted by the simple mathematical et, al. (1990) is not appropriate for the prediction of models for the steady-state case. The causes of the this type of ventilation flow in full-scale buildings. differences are identified and discussed. Computational fluid dynamics is a powerful Computational fluid dynamics is a powerful technique that can yield realistic predictions for this predictive technique that can realistically describe type of ventilation flow. Even this approach, natural ventilation flows within buildings. Even this however, requires further investigation before it can approach, however, requires further investigation to be used routinely. In particular, this paper illustrates ensure that useful predictions are obtained. A major the effect of applying different models of turbulence, problem is the validity of the turbulence models that and highlights the requirement to employ a complete must be employed for ventilation simulations. A thermal radiation model when using the CFD wide variety of flow regimes are observed in many technique to predict this class of flow. buildings, and it is apparent that as yet, no single model of turbulence can accurately describe the INTRODUCTION range of features generally found in room turbulence. Almost half of the total energy consumption in In addition, a simulation should incorporate a model the UK is used in buildings, and this is dominated by to account for the effects of heat transfer due to electricity use for heating, ventilation, air- thermal radiation. conditioning and lighting systems (CIBSE, 1998). Numerical predictions of the ventilation flow Clearly, if more environmentally friendly strategies through the test room have been performed using a could by employed for the design of the building CFD computer code, and are compared with the flow services, such as the provision for ventilation by through the enclosure. A variety of turbulence natural means, then this energy consumption, models are employed for the computer simulations, together with the related emissions of carbon dioxide together with a model for thermal radiation, and the which account for three-quarters of all greenhouse predictions obtained are compared and discussed. gas emissions, could be significantly reduced. The experimental results obtained from the test One of the major barriers to the natural room form a useful set of benchmark data for the ventilation approach is the lack of a reliable design validation of CFD codes that are intended to be used tool for such flows. Linden and his co-workers for the investigation of buoyancy-driven ventilation (University of California, previously at the flows through full-scale enclosures. University of Cambridge) have developed some simple mathematical models for natural displacement EXPERIMENTAL SETUP ventilation of an enclosure over the past decade. This The experimental test-room was representative work has recently been reviewed (Linden, 1999). of a full-sized office space. This was a natural The models are validated using the salt-bath - 627 - extension to the previous experimental investigation Each resistance thermometer formed one leg of of Howell and Potts (1998). a bridge circuit, and had a resistance of 100Ω. The The test-room was constructed within a large supply voltage to the bridge was maintained at chamber at the University of Newcastle. The 100mV, so that resistive heating of the element was chamber provided two essential features for the insignificant. experimental work. Firstly, the envelope of the The imbalance voltage across the bridge chamber was well sealed, so that any interference due provided a measure of the change in resistance of the to external wind effects was minimised. resistive thermometer element and therefore of Secondly, the physical dimensions of the temperature. The imbalance voltage was monitored chamber were large compared with those of the by a programmable amplifier, with a gain of 1000, test-room, so that the walls of the chamber did not and was then converted to a digital signal, which was affect the ventilation flow through the test-room. subsequently transferred to a PC for further analysis. Indeed, the height of the chamber was about two and The thermometers were calibrated using a a half times that of the test-room, and there was water-bath and a mercury-in-glass thermometer, sufficient room at the end walls of the test-room so which was accurate to within 0.1K. Although the that the flow through the openings was unaffected by platinum temperature-resistance relationship for the the presence of the boundaries of the chamber. thermometer elements was quadratic in nature, for The test-room was assembled from standard the small changes in temperature which occurred 2440x1220mm sheet materials attached to an within the test-room, it was acceptable to assume a aluminium frame. The frame was constructed from linear relationship without introducing significant ‘Dexion’ slotted angle, and sheets of chipboard error. Calibration was performed at two bolted to the frame form the walls, ceiling and floor temperatures, one at each end of the range of of the enclosure. Any gaps at joints between the temperatures expected during the experiments. panels were sealed, so that the test-room was airtight. Assuming a linear variation of temperature with The internal surfaces of the test-room were painted resistance, a third temperature in the middle of the matt white. The floor of the test-room was raised expected range was measured. The range of 100mm above the existing concrete floor of the wind temperatures measured by the thermometers was less tunnel chamber in order to prevent the thermally than one-tenth of a degree Kelvin. massive floor from significantly affecting the airflow within the test-room. SIMULATIONS The end walls were manufactured from Computer simulations of the flow through the polycarbonate sheet, so that the inside of the test-room were performed using the Computational enclosure was visible from outside. It is these walls Fluid Dynamics package Fluent, version 5.4. This is which incorporated the openings to the enclosure. an unstructured code, which allows the engineer The size of each opening could be adjusted by sliding much more freedom in terms of grid density. a polycarbonate panel to the required position, the Fluent 5 employs a collocated grid panel being secured in place by a channel machined arrangement, together with Rhie-Chow interpolation into the structural batten used to keep the (Rhie and Chow, 1983) to prevent non-realistic polycarbonate sheet rigid. oscillations developing in the pressure field as the A 225W plate heater measuring 0.4×0.2m was simulation proceeds. Pressure-velocity coupling was positioned in the centre of the floor to provide the achieved using the SIMPLEC algorithm heat source. (van Doormaal and Raithby, 1984). Measurements of air temperature were The third-order QUICK differencing scheme performed extensively throughout the test-room (Leonard, 1979) was used to evaluate the convection using twelve Platinum resistance thermometers. One terms for the Navier-Stokes equations, the energy of the thermometers was positioned in the inlet equation and the turbulent transport equation, in region to record the temperature of the air entering order to reduce numerical diffusion which is the test-room. The remaining thermometers were generally more significant on an unstructured grid. mounted on a vertical mast, in order to obtain the Linear interpolation was used to determine the face instantaneous vertical temperature profile at any pressures required for the source terms in the particular position. The mast was supported by a discretised Navier-Stokes equations. frame, which itself was supported by rollers engaged The effects of turbulence were modelled in this on guide rails, so that it could be accurately traversed paper using a variety of two-equation turbulence along the x-axis of the test-room. The frame was models. Each model is based upon the eddy viscosity positioned from outside the enclosure by means of a concept and requires additional transport equations to pulley and rope system, and the guide rails had be solved for two new flow variables; the turbulence periodic notches machined into the upper edge for kinetic energy k and the rate of dissipation of accurate and repeatable location of the frame. turbulence kinetic energy ε. The models of - 628 - turbulence investigated were the standard k-ε model grid. A wall boundary condition with a specified (Launder and Spalding, 1974), the k-ε RNG model constant heat flux was used to model the 225W heat (Yakhot and Orzsag, 1986), and the realisable k-ε source.
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