Heat Transfer Augmentation with Pin Fins in the Entry Region of Circular Channels

Abstract

Entry region heat transfer in a circular channel is known to be significantly higher than at fully developed conditions. This presents an additional heat sink opportunity for turbine engines in aircraft, which could utilize the short entry length before the fan to reject heat. This study focuses on pin fin arrays, which have been widely studied in rectangular channels but not in circular channels. Pin fin arrays with pin heights of Hf/D=1/24 and 1/48 are investigated at two spanwise spacings S1/d=2 and 4, and two streamwise spacings S2/d=1.73 and 3.46. The arrays are mounted in the first diameter of a circular channel, with velocity boundary layer measurements at x/D=1 and x/D=4. Experiments are performed at Reynolds numbers varying from 1*10⁵ to 4.9*10⁵. Heat transfer augmentation of pin fin arrays is significantly higher than a smooth array, especially due to the increase in surface area near the inlet. Because of this, narrow streamwise spaced arrays provide higher heat transfer compared to wider streamwise spaced arrays. All pin fin arrays reduced near wall velocity flow and increased core flow. The shape of the velocity profile at x/D=1 for the Hf/D=1/24 arrays were dependent on the spacing of the array. Shorter Hf/D=1/48 arrays produced the same velocity profile at x/D=1 for y/D>0.04. However, by x/D=4, all arrays produced a constant velocity profile that only varied due to fin height. Narrow streamwise spacing with wide spanwise spacing pin arrays provide the highest thermal performance and heat transfer performance overall. Thus, from these results, entry region heat transfer can provide significant heat rejection capabilities.