Heat transfer enhancement with an inclined semicylindrical turbulence promoter?The relationship between optimum clearance and film thickness

Abstract

Heat transfer enhancement in a rectangular duct with a semicylindrical turbulence promoter was experimentally studied for a high Prandtl number fluid. The measurements of mass transfer coefficient and wall shear stress as well as flow visualization were performed in order to investigate how the performance of a heat transfer enhancement promoter is influenced by its inclination angle and by the clearance between the promoter and the wall surface. When the inclination angle is 0°, the variation of performance with clearance is closely related to the extent of heat transfer enhancement in the region of slipping flow. In this case, the clearance exhibiting the maximum performance is almost equal to the film thickness for a smooth duct without a promoter. As the inclination angle increases, what influences the variation of performance with clearance gradually shifts to the extent of heat transfer enhancement in the region of circulating and reattaching flows, and the clearance exhibiting the maximum performance decreases. There is no remarkable difference in the maximum performance between inclination angles examined here. It is, however, desirable to set the inclination angle to 30°, since the distribution of the local mass transfer coefficient for this angle is less curved than those for other angles.