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International Heat Transfer Conference 13

ISSN: 2377-424X (online)
ISSN: 2377-4371 (flashdrive)

MODELING OF THERMALLY-DRIVEN TRANSPORT IN INTERIOR SPACES FOR APPLICATIONS TO ZONAL MIXING MODELS

W. Winters
Computational Reactive Processes Department Sandia National Laboratories P.O. Box 969 Livermore, California 94551-0969 USA

Gregory H. Evans
Sandia National Laboratories, Livermore, USA

Ralph Greif
Department of Mechanical Engineering, University of California, Berkeley, USA

DOI: 10.1615/IHTC13.p6.250
13 pages

Resumo

Zonal codes are often used to model air flow and contaminant transport in buildings. These codes treat rooms and other large volumes as perfectly mixed zones (zero dimensional) connected by pathways along which pressure-driven flow takes place. Zonal models are often an adequate approximation for buildings that are actively ventilated by HVAC systems since significant pressure gradients are available to drive the flow and facilitate mixing. However, there is a large class of thermally-driven flow situations for which this pressure-driven model fails to predict the proper flow and contaminant transport between connected rooms. Consider for example two unventilated rooms at different temperatures connected by a single doorway. A thermally-driven flow will be established between the two rooms even when an HVAC system is not present. Although under steady-state conditions the net mass flow across the doorway will be zero, a contaminant introduced into one of the rooms will be transported to the second room via the thermally-driven flow. In the present work we demonstrate how multi-dimensional CFD calculations of turbulent flow and heat transfer can be used to motivate new thermally-driven flow models for zonal codes. We consider two thermally-driven flow situations in two-dimensional geometries. The transport is calculated with the ν2 -f turbulence model. In the first thermally-driven flow we consider a tall air-filled room with temperature differences on opposing walls as large as 10 K. Room aspect ratios (height/width) are varied from 1 to 3. The predicted rates of heat transfer and mass flow of air are characterized as a function of Rayleigh number based on room height. Our heat transfer results are compared to computational results from the literature and to analytical predictions. In the second thermally-driven flow configuration we consider two unventilated rooms at different temperatures separated by a partition containing an aperture (or doorway). Results for mass flow rate of air are presented for aperture aspect ratios (aperture to room height ratio) from 0.1 to 0.7 and Grashof numbers (based on aperture height) from 4.6×106 to 1.6×1010. Our results are compared to the analytical result of Brown and Solvason which is based on an idealized Bernoulli flow. The results for air mass flow rate are in reasonable agreement with the analytical results for small aperture aspect ratios (Ah=0.1). For large apertures such as a doorway (e.g., Ah=0.7) and large Grashof numbers (Grh>107) the predicted mass flow rate of air between the rooms is 2 to 3 times smaller than the analytical result indicating the analysis may have limitations. We describe the process for incorporating multidimensional air flow predictions into zonal models of thermally-driven contaminant transport. Results from our zonal model calculations show that temperature differences as small as 1 K can induce thermally-driven mixing rates similar to the pressure-driven mixing rates of a typical HVAC system.

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Measurement of fluid temperature with an arrangement of three thermocouples