Influence of External Heat Transfer by Convection on Time Lag and Decrement Factor of Building Walls

Abstract

In this study, the effect of external heat transfer convection on the time lag and on the decrement factor has been numerically investigated for different scenarios. For this purpose, a two-dimensional transient heat conduction equation was solved using the element finite method and COMSOL Multiphysics under convection boundary conditions. A periodic boundary condition was applied to the external surface of the wall where the convection is kept constant during the day until a time interval where it is forced. Three night time intervals were investigated: 18h to 00h, 00h to 06h and 18h to 06h. For all these cases, the computations were repeated for different values of air velocity (from 0.5 to 10m/s) in order to evaluate the time lag and the decrement factor. It was found that forcing the convection on the external face of the wall during night has a profound effect on the decrement factor and on the time lag. The best scenario has shown a cooling energy reduction by 20% compared to the reference case.