Experimental and numerical studies on convective heat transfer from various urban canopy configurations

This paper investigates the Convective Heat Transfer Coefficient (CHTC) variation of urban canopy surfaces for different urban canopy configurations by experimental and numerical simulation. The Weather Research Forecasting (WRF) model coupled with the Urban Canopy Model (UCM) is considered as an effective tool for prediction of urban heat island phenomena in urban areas. In the Single layer UCM of WRF, the CHTC of urban canopy surfaces and its dependence on urban parameters such as building coverage ratio and building height variations are not explicitly modeled. In UCM, the CHTC of urban canopy surfaces are evaluated from Jürges relation. In this relation, the local CHTC from building walls and the ground depends only on the wind velocity inside the canopy. However, this cannot be justified since other urban parameters like building coverage ratio and building height variation also contribute to the CHTC of urban canopy surfaces. Moreover, Jürges relation cannot distinguish the difference between CHTC of different building wall surfaces, i.e., windward, leeward and side walls, but expresses the CHTC for walls generally. In our study, wind tunnel experiments were firstly conducted to roughly grasp the dependence of urban parameters on bulk heat transfer from an urban canopy in a thermally stratified wind tunnel. However, it is not an easy task in wind tunnel experiments to evaluate local CHTC, which vary for individual canyon surfaces such as building roofs, walls and the ground. Numerical simulation validated by wind tunnel experiments can be an alternative for prediction of CHTC from building surfaces in an urban area. The authors conducted CFD simulation with a low-Reynolds number model to evaluate the CHTC from canopy surfaces. The calculated CFD results for velocity and temperature profile and bulk heat transfer from an urban canopy showed good agreement with experimental results. After this validation, the effects of urban canopy parameters on local convective heat transfer on individual surfaces were investigated by CFD simulation. Based on these results, the CHTC will be modified with respect to urban parameters, and will be incorporated into the UCM of the WRF model in our future study.

► Convective heat transfer by CFD simulation satisfactorily models the experiment.
► Heat flux from canopy surfaces decreases with increase in building coverage ratio.
► CHTC from canopy surfaces also depends remarkably on the building coverage ratio.
► Jürges relation in UCM to evaluate the CHTC of wall surfaces is inappropriate.