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Measurement of fluid temperature with an arrangement of three thermocouples
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MODELING OF THERMALLYDRIVEN TRANSPORT IN INTERIOR SPACES FOR APPLICATIONS TO ZONAL MIXING MODELSW. Winters DOI: 10.1615/IHTC13.p6.250 SinopsisZonal 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 pressuredriven 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 thermallydriven flow situations for which this pressuredriven 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 thermallydriven flow will be established between the two rooms even when an HVAC system is not present. Although under steadystate 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 thermallydriven flow. In the present work we demonstrate how multidimensional CFD calculations of turbulent flow and heat transfer can be used to motivate new thermallydriven flow models for zonal codes. We consider two thermallydriven flow situations in twodimensional geometries. The transport is calculated with the ν^{2} f turbulence model. In the first thermallydriven flow we consider a tall airfilled 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 thermallydriven 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 4.6×10^{6} to 1.6×10^{10}. 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 (A_{h}=0.1). For large apertures such as a doorway (e.g., A_{h}=0.7) and large Grashof numbers (Gr_{h}>10^{7}) 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 thermallydriven contaminant transport. Results from our zonal model calculations show that temperature differences as small as 1 K can induce thermallydriven mixing rates similar to the pressuredriven mixing rates of a typical HVAC system. 
