EFFECT OF TIP CLEARANCE ON THE THERMAL AND HYDRODYNAMIC PERFORMANCE OF SHROUDED PIN FIN ARRAYS

Abstract

The use of shrouded pin-fin arrays to enhance heat transfer is a common occurrence in many engineering disciplines, from compact heat exchangers in the process industry, to turbine blade cooling in the aerospace industry, to system thermal management in the electronics industry. In this study, experimental methods are employed to study the effect of introducing a small clearance gap between the tips of a pin-fin array and its shroud. Three arrays consisting of tapered cylindrical pin-fins were employed for this purpose. They exhibited pin height-to-diameter ratios (H/D) of 0.5, 0.8 and 1.1, and pin pitch ratios (SL/D and ST/D) of 1.15 and 1.3 respectively. The arrays were heated uniformly and cooled with distilled water in a closed-loop test facility. Heat transfer visualization studies were conducted using thermochromic liquid crystals to estimate convection coefficients as a function of Reynolds number (10^2 < Re < 2x10^4), through a range of clearances (0 to 25 percent of pin height). Adiabatic pressure drop across the pin-fin array was also measured. In terms of pressure drop performance, the impact of tip clearance was found to be greatest at low Re, (< 5x10^3), with the effect being significantly diminished by Re = 1.5x10^4. While the dimensionless pressure drop (f) eventually decreases with the addition of clearance, there appears to be an initial increase in f relative to the non-clearance case for clearances on the order of 5-10 percent of pin height. This increase was seen to be as much as 50 percent at Re on the order of 10^2. Observed heat transfer trends were somewhat mixed. In some cases, a modest increase in overall heat transfer was experienced, apparently due to the exposure of the pin tips and their associated area to the cooling fluid. Highest rates of overall heat transfer tended to occur within a clearance range of 10 to 18 percent of pin height. Local heat transfer however was generally seen to decrease. On a constant pumping power basis, the inclusion of clearance was found to generally produce higher heat transfer per unit pumping power for a dimensionless clearance (Cg/D) less than 0.2, when compared to the non-clearance case.