Numerical investigation of absorber's roughness effect on heat transfer in upward solar air heaters

摘要

The aim of this study is to investigate numerically the effect of absorber's surface roughness on heat transfer in upward solar air heaters.For this purpose,we used Law-of-the-Wall Modified for Roughness(LMWR)available in ANSYS Fluent code.This law of the wall helped us to simulate air flow over roughened absorber in the case of solar air heaters without having to built ribs geometry on the surface in the pre-processing step.We tested two cases,the first with transverse ribs and the second with uniformly distributed ribs;;For both,the nondimensional roughness heights are respectively of 66 and 50 which are in the transitional regime.The results for these different configurations(2D,3D,transverse and uniformly distributed ribs)are compared to experimental and numerical results available in the literature.In the first case,the overall behavior seems to be correctly represented by the LWMR but with a considerable gap.Our results showed that Nusselt number obtained by LWMR is correctly predicted for circular ribs at low Reynolds numbers but for high values the inaccuracy becomes more important.However,for semi-circular ribs,the gap remains constant with Reynolds number.In the second case,the results obtained by LWMR are very accurate for small ribs diameters at low to moderate Reynolds numbers(Re<12000).For higher ribs diameters,the threshold Reynolds value is lower(about Re=7000).The case of uniformly distributed ribs can be relatively well described by LWMR for low relative distance L/e,at Reynolds numbers less than 10000.Our results showed the interest and limits of LWMR.