Heat Transfer and Flow Field Characteristics of Variable Aspect Ratio, Serpentine Passages

This study investigates the effect of acceleration and deceleration in the turn region of a two-pass, serpentine channel. Channel aspect ratios of 2:1 and 4:1 are considered in three combinations to investigate the effect of acceleration and deceleration through the turn region in a smooth, two-pass channel. For each aspect ratio combination, the experiments include Reynolds numbers ranging from 15,000 to 45,000 in the first passage and from 9,000 to 75,000 in the second pass. Surface heat transfer coefficient (h) distributions are measured by transient thermochromic liquid crystal technique, and the flow field characteristics are measured by tomographic PIV. Enhanced heat transfer on the side wall and downstream in the second pass is observed for all cases. Based on the first-pass Nusselt number (Nu) ratio, the accelerating case provides the best heat transfer enhancement. However, based on the second-pass Nu ratio, the decelerating case is more promising. If the Reynolds number is controlled, the constant cross-sectional case has the highest Nu and h in the turn. If the mass flowrate is controlled, the accelerating case exhibits the lowest overall average Nu and h. The flow field results suggest that the secondary flow induced by the turn is weakened when the flow accelerates after the turn for the accelerating channel. The opposite is observed for the decelerating channel. The strength of the counterrotating vortex pair, turbulence level, and local heat transfer enhancements are closely correlated in a serpentine channel.