Effects of the Prandtl number on local heat transfer from a circular cylinder in crossflow.

摘要

The effects of the Prandtl number on local heat transfer from a circular cylinder in crossflow is investigated. The Prandtl number is varied from 0.7 to 180 using air, water and six mixtures of ethylene glycol and water as the working fluids. The Reynolds number is adjusted in the range between 2 × 103 and 1 × 105. The relationship between the local Nusselt number and both the Reynolds number and the Prandtl number are determined. Additionally, flow visualization studies are carried out to help in understanding the interaction between flow characteristics and heat transfer mechanisms.; A uniformly heated cylinder is employed in this study. Generally, the Prandtl number significantly affects local heat transfer around a circular cylinder in which the Nusselt number increases with an increasing Prandtl number. For Reynolds number greater than 5 × 103, the distribution of local Nusselt number can be divided into three regions. In the first region (0° &thetas; 85°) where &thetas; is angle around cylinder from front stagnation line, the Nusselt number monotonically decreases from the front stagnation point due to growth of the boundary layer and reaches a minimum near 85° where the boundary layer separates.; The Nusselt number in the second region (85° &thetas; 135°) increases with &thetas; due to the reattachment flow of the shear layer. In the third region (135° &thetas; 180°), the Nusselt number increases with &thetas; as a result of periodic vortices which alternatively shed from both sides of the cylinder.; When Reynolds number is less than 5 × 103, the effect of Prandtl number on heat transfer downstream of the separation point is small. The Nusselt number is approximately constant after the separation point. However, the Prandtl number still strongly influences heat transfer on the front part of the cylinder.