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

Abstract

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.

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

“…They reported a Nusselt number augmentation factor of 3 and conductance augmentation factor of 5. They found that a clearance at one end of the fin reduced the end wall effects which negatively impact heat transfer [33]. Rozati et al also found that at Reo < 400, that clearances of f = 0.1 H and 0.4 H provided the greatest augmentation of the convective heat transfer coefficient and conductance by factors of 3.2 and 4.6, respectively.…”

“…This paper reports work that follows on the work by Rozati et al [33] by examining pin fins with a clearance at both ends and extending the Reynolds number range to 10 < Reo < 600. A constant heat flux is specified on the fins and plate walls.…”

“…Rozati et al [33] reported that, for a range of 5 < Reo < 400, that single clearances augment the Nusselt number by (1) reducing the viscous dominated region on the pins and (2) reducing the pin wake shadow on the plate walls. They also showed that a single clearance augments the conductance in the same way but with the added benefit of the additional exposed surface area.…”

“…The work by Rozati et al [33], coupled with a heat and mass transfer analogy, yielded an expectation that pin fins with dual clearances may be beneficial for surface coated, catalytic reactor applications, where surface mass transfer is important and conduction from the plate walls to the pins is unnecessary.…”

“…Rozati et al [33] conducted a computational study with a constant heat flux applied to the plate walls and the cylinder wall and used staggered pins with single tip clearances, ranging from / = 0.0 H to 0.4 H for H/D = 1. Rozati et al [33] reported that, for a range of 5 < Reo < 400, that single clearances augment the Nusselt number by (1) reducing the viscous dominated region on the pins and (2) reducing the pin wake shadow on the plate walls.…”