Channel aspect ratio effect on thermal performance of air-water slug flow through U-bend channels

This study investigates the effect of channel height-to-width ratio (Aspect Ratio, AR) on heat-transfer rates, pressure-drop coefficients (f) and thermal performances of air-water flows through horizontal and vertical U-bend rectangular channels at intermittent slug and slug-annular flow conditions. Interfacial two-phase flow structures, local and area-averaged Nusselt numbers (Nu), f coefficients, channel-wise averaged void fractions (α) and thermal performance factors (TPF) for three sets of horizontal and vertical U-bend channels of AR = 1, 0.83 and 0.33 are measured with liquid Reynolds numbers (Re_L) and air-to-water mass flow ratios (AW) in the range of 1500 ≤ Re ≤ 10000 and 0 ≤ AW≤0.024. Early transitions from slug flow to slug-annual flow along with the shortened air slug and the extended period of the trailing-edge bubbly flow over each intermittent cycle are promoted by decreasing AR to elevate both heat transfer rates and pressure drops for the U-bend channels with small AR. A set of selective Nu, f and TPF data illustrates the interdependency between Nu, f, TPF and the air-water flow structures in present test channels with different AR. The area averaged endwall Nu for each U-bend test channel (Nū) and the corresponding f and TPF are cross-examined to generate a set of heat-transfer and pressure-drop correlations, which permit the evaluations of isolated and interdependent Re_L, AW and AR effects on Nū and f, to assist various design applications.