Numerical and Semi-Analytical Estimation of Convective Heat Transfer Coefficient for Buildings in an Urban-Like Setting
Doctor of Philosophy
Civil and Environmental Engineering
Bitsuamlak, Girma T.
British Colombia Institute of Technology
Urban building arrangements such as packing density, orientation and size are known to influence the microclimate surrounding each building. Studies on the impact of urban microclimatic changes on convective heat transfer coefficient (CHTC) from a stock of buildings, however, have been rare in surveyed literature. The present study focuses on numerical and analytical investigation of CHTC from building-like models with homogeneous set of equal and unequal planar and frontal densities. Consequently, the study discusses the CHTC response in relation to broader changes in the urban surface form. Part of the process involves the development of a simplified one-dimensional semi-analytical CHTC model based on a simplified analytical mean wind velocity flow profile for urban canopies. The remaining portion consists of performing sets of CFD simulations to obtain CHTC values for a broader range of packing densities. Analysis and results of the study reveal that CHTC is affected both by changes in planar as well as frontal densities. These changes might lead to up to 2.5 times higher or approximately an order lower CHTC compared to the conventional u10 formulations (based on upstream reference conditions at 10 m from the ground), which are mostly done without considering packing density effects. It is observed that the least CHTC values lie at higher planar densities, whereas the highest CHTC corresponds to the combination of the lowest planar and highest frontal densities for the windward and leeward surfaces. An increase in planar area density increases the CHTC at smaller frontal densities for top and lateral surfaces. The study reveals the CHTC estimate from conventional models may have mostly been overestimated compared with the values for buildings in city neighbourhoods. New correlations for estimating CHTC for three canopy flow regimes are proposed. A relationship is also recognized between CHTC and the land-use class assignment of an urban neighbourhood. The simplified analytical model, proposed correlations and the semi-analytical model are expected to enable estimation of CHTC for buildings located in urban neighbourhoods based on the built area density or land use class.
Summary for Lay Audience
Building heating/cooling energy consumption saving measures help decrease the operating cost and tackle the issue of climate change through reducing greenhouse gas emission. An aspect of this saving strategy is the proper measurement of the various energy fluxes. One way a building loses energy is through convection. Convection is the energy the building loses when the wind blows over its surface. As such, a high-intensity wind, thus, takes more heat than a low-intensity wind. However, wind effect on a building that is standing alone (or near few buildings) is different from a building that is surrounded by many nearby buildings. The strength of convective heat loss is often represented by a parameter known as convective heat transfer coefficient (CHTC). This parameter is usually only related to wind intensity (velocity). The current research attempts to develop an accurate estimate of this CHTC based on how buildings are packed (packing density) in a neighborhood, besides the wind speed. Through the findings of this research and the proposed correlations, one can now estimate CHTC (and hence convective heat loss) for buildings with various packing densities (neighborhoods such as residential, downtown, industrial, etc.).
Awol, Anwar Demsis, "Numerical and Semi-Analytical Estimation of Convective Heat Transfer Coefficient for Buildings in an Urban-Like Setting" (2019). Electronic Thesis and Dissertation Repository. 6750.
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