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 achieved by reducing the aspect ratio of the serpentine passages from 4:1 at the inlet to 2:1 at the exit. For deceleration, the flow transitions from 4:1 to 2:1 passage. 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 0.29 and a spacing of 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 particle imaging velocimetry (PIV). The heat transfer results in the first passage show that 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 in a straight channel. However, for the decelerating channel, the heat transfer enhancement becomes lower than a dimpled, straight channel without a turn. The flow field measurement results show the flow has higher turbulence and vorticity after acceleration. With deceleration, the separation region in the dimpled channel is larger than the smooth channel, and the turbulence intensity in the second passage is also lower.