Publication: Effect of terrain on the forced convective heat transfer from the inclined windward roof of a low-rise building

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Title Effect of terrain on the forced convective heat transfer from the inclined windward roof of a low-rise building
Authors/Editors* P Karava, C M Jubayer, E Savory
Where published* 13th Int Conf on Wind Engineering, Amsterdam, June
How published* Proceedings
Year* 2011
Keywords Photovoltaic systems, convective heat transfer, atmospheric boundary layer, terrain effects
The motivation behind the present research is the development of accurate thermal analysis models for the design and control of building-integrated Photovoltaic/Thermal (PV/T) systems. Since the convective heat transfer between the roof and the external airflow has a significant impact on the electrical and thermal efficiency of such systems, the objective of this paper is to evaluate the exterior wind-induced convective heat transfer coefficient (CHTC) for the inclined roof surfaces of low-rise buildings. Specifically, the aim is to develop, for the first time, dimensionless correlations for the CHTC that take into account the effects, on the roof velocity and thermal boundary layers, of the building / roof geometry and the incident atmospheric boundary layer, notably the terrain roughness length (z0) and the turbulence intensity at eaves height. High resolution, 3-D, steady, Reynolds-Averaged Navier-Stokes (RANS) CFD simulations of the wind flow field near the roof of a building with plan dimensions of 4.2 m by 6 m, a 3 m eaves height and a 30 degree roof slope, have been conducted, with the results validated by experimental data from a 1:50 scale model tested at Boundary Layer Wind Tunnel (BLWT) II at the University of Western Ontario. The heat transfer model was validated using the boundary layer correlation for an isothermal horizontal flat plate in uniform flow. This paper presents full-scale RANS simulations (for the same building geometry) of forced convective heat transfer at the windward roof. The building is placed in two terrain categories (open and suburban) characterized by four different aerodynamic roughness lengths and different eaves height ReL (from 2.2x10^5 to 7.7x10^5). Since the focus of the present study is on forced convection, buoyancy forces arising from density changes are not included in the analysis. The building geometry was chosen to resemble that of a full-scale outdoor test-building, located at Concordia University, Montreal, Canada, with a roof-mounted PV/T system to allow for future comparisons with full-scale data. Preliminary CFD simulations showed that exterior CHTC on the system is independent of the airflow through the BIPV/T channel and so the channel underneath the panels is not included in the work presented here. Most of the simulations are performed for a building with 30 degree roof slope as this is known to result in near optimal electrical and thermal efficiency for BIPV/T systems. It is assumed that the BIPV/T system is installed on the windward side of the roof and the approaching wind is normal to the eaves, such that the flow is either attached or the leading edge separated flow region is short.
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