Numerical investigation of subcooled flow boiling in an inclined rectangular mini-channel at a low flow rate

Abstract

Under a low flow rate, gravity may become prominent for bubble behavior and heat transfer of flowing boiling because of the weakness of drag force from liquid, and its effect changes with the inclination angle of the mini-channel but without consensus. In this paper, based on a reasonable nucleus site density model and considering conjugate heat transfer, the coupled volume-of-fluid and level set method is adopted to study the subcooled flow boiling in an inclined three-dimensional rectangular mini-channel (0°–180°) with a characteristic size of 1.0 mm at a low flow rate of 88.8 kg m−2 s−1. The inclination angle is found to have a slight effect on the flow boiling, which is different from the conclusion drawn based on the traditional-macro channel. A bubbly flow appears when a heat flux of 300 kW/m2 is added. An unconventional impact force is proposed, which presses large bubbles to slip along the heating wall, with slight differences in the flow pattern under different inclination angles. When the inclination angle is close to 0°, the upstream small/medium bubbles leave the heating wall under gravitational effects, which is conducive to heat transfer. As the inclination angle approaches 90°, gravity pushes the large bubbles downstream to leave the channel, favoring the rewetting of the dry patches below. These two positive effects fail as the inclination angle approaches 180°, leading to slightly worse overall heat exchange efficiency. However, the maximum differences in the average and local wall superheating of the mini-channel are only 8.4% and 22.5%, respectively, across the range of inclination angles because the flow pattern remains similar under the effect of the impact force. In addition, the effect of inclination angle on flow boiling becomes weaker with the increase in heat flux because of happening of slug flow.