Effect of Pin Tip Clearance on Flow and Heat Transfer at Low Reynolds Numbers

Cylindrical pin fins with tip clearances are investigated in the low Reynolds number range $5<ReD<400$ in a plane minichannel. Five tip gaps are investigated ranging from a full pin fin $(t*=0.0)$ to a clearance of $t*=0.4D*$⁠, where $D*$ is the pin diameter. It is established that unlike high Reynolds number flows, the flow and heat transfer are quite sensitive to tip clearance. A number of unique flow effects, which increase the heat transfer performance, are identified. The tip gap affects the heat transfer coefficient by eliminating viscosity dominated end wall effects on the pin, by eliminating the pin wake shadow on the end walls, by inducing accelerated flow in the clearance, by reducing or impeding the development of recirculating wakes, and by redistributing the flow along the height of the channel. In addition, tip gaps also reduce form losses and friction factor. A clearance of $t*=0.3D*$ was found to provide the best performance at $ReD<100$⁠; however, for $ReD>100$⁠, both $t*=0.2D*$ and $0.3D*$ were comparable in performance.