HEAT-TRANSFER ENHANCEMENT WITH AN INCLINE D SEMICYLINDRICAL TURBULENCE PROMOTER - THE RELATIONSHIP BETWEEN OPTIMUM CLEARANCE AND FILM THICKNESS

Heat transfer enhancement in a rectangular duct with a semicylindrical turbulence promoter was experimentally studied for a high-Prandtl num ber fluid. The measurements of mass transfer coefficient and wall shea r stress as well as flow visualization are performed in order to inves tigate how the performance of a promoter of heat transfer enhancement is influenced by its inclination angle and by the clearance between th e promoter and the wall surface. When the inclination angle is 0 degre es, 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 al most equal to the film thickness for a smoothed duct without a promote r. As the inclination angle increases, what influences the variation o f performance with clearance gradually shifts to the extent of heat tr ansfer 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 inclin ation angles examined here. It is, however, desirable to set the incli nation angle to 30 degrees, since the distribution of the local mass t ransfer coefficient for this angle shows a less winding curve than tho se for other angles.