Analysis of convective heat transfer coefficient using shear stress-based velocity scale

Abstract

When modeling the thermal performance of a building, it is very important to consider the heat transport from the wall of the building to the interior environment. In energy simulations (ES), the convective heat transfer coefficient (hc) determines this heat transfer. Therefore, hc is a significant factor in the calculation of room temperature. In the field of architecture, hc utilizes the temperature of the turbulent zone as a reference temperature. However, the definition of this reference temperature is ambiguous. The preferred definition of the reference temperature is y+, and this is based on Prandtl's wall law. The reference temperature has a large impact on the calculation of hc, but it is difficult to evaluate the convective component. Nevertheless, it is possible to extract the velocity component near the wall using a shear stress-based velocity scale, but previous studies have not clarified how the latter and the reference temperature contribute to hc. Until now, the method used to calculate hc has been based on a combination of experimental and analytical methods, such as the use of inflow air temperature as the reference temperature in wind tunnel experiments. In this study, we analyze the method used for calculating heat flow and introduce a new method for calculating hc. In an actual environment, the turbulent flow field is expected to be strongly affected by the nearby flow velocity and reference temperature. To model such a turbulent flow field, we conducted a basic study in which a unidirectional flow was blown across a floor mat. Specifically, the influences of the shear stress-based velocity scale and the reference temperature were investigated by varying the inflow velocity temperature from 10 °C to 40 °C. The analysis first showed that hc and the shear stress-based velocity scale were highly correlated (R2 = 0.8073). However, an analysis of the outliers revealed a significant contribution from the reference temperature. In conclusion, the value of hc cannot be determined solely by the shear stress-based velocity scale, even in cases involving forced convection. Our findings provide very important information for ES settings in the field of architecture and will significantly contribute to the field of thermal environment simulation.