Heat Transfer Enhancement in Variable Aspect Ratio Serpentine Passages With Hemispherical Dimples

Abstract The current study focuses on the effect of acceleration and deceleration in a two-pass channel with hemispherical dimples. Acceleration is produced by reducing the cross-section of the channel from 4:1 in the first pass to 2:1 in the second pass. To investigate the effect of deceleration, the channel aspect ratio increases from 2:1 to 4:1. The Reynolds number based on the smooth, rectangular channel in the first passage, ranges from 15,000 to 45,000. In the second passage, the Reynolds number ranges from 9,000 to 75,000. The staggered hemispherical dimples share a similar geometry to open literature, with a depth-to-imprint ratio of δ/d = 0.29 and a spacing of SX/d = SY/d = 1.15 in the streamwise and spanwise directions. The heat transfer characteristics are investigated by a transient, narrow-band thermochromic liquid crystal technique, and the flow field is recorded via tomographic PIV. The heat transfer results in the first passage show the dimples enhance heat transfer approximately two times that of the smooth channel. For the accelerating channel, the combined effect of dimples and the turn produced higher heat transfer enhancement than the dimples alone in a straight channel. However, for the decelerating channel, the heat transfer enhancement becomes even lower than a dimpled, straight channel without a turn. The time-averaged flow field measurement results show the flow has higher turbulence and vorticity after acceleration. With deceleration, the separation region is even larger than the smooth channel, and with a lower turbulence intensity in the second passage than the smooth channel.