An experimental study on the influence of radial pressure gradient on bubbles dynamic behavior in subcooled flow boiling

Abstract

It is well aligned with the general consensus that the quality of boiling heat transfer is heavily dependent upon bubbles characteristics. Moreover, the accuracy of various numerical models developed to simulate flow boiling on a heated wall heavily depends on empirical data on bubbles dynamic behavior. In this paper, the influence of radial pressure gradient on bubbles dynamic behavior characteristics in upward subcooled flow boiling of water is experimentally investigated using high-speed photography. Thereby, a comparative study on bubbles characteristics is conducted between a vertical U-shaped channel, where flow experiences the radial pressure gradient, and a straight channel under the same geometrical and flow conditions. Since the bubbles of three different locations having wall inclination angles of roughly 0°, 45°, and 90° are captured in the U-shaped channel, the straight channel is tested in horizontal, 45-degree oriented, and vertical positions, and the results are compared. In this way, not only, is the effect of wall inclination on bubble characteristics eliminated, i.e. the comparisons merely distinguish the effect of radial pressure gradient, but also the influence of this phenomenon is examined in different wall inclinations. All the experiments are performed at atmospheric pressure with heat flux in the range of 36.3–61.5 kW.m-2, flow velocity in the range of 0.132–0.265 m.s-1, and inlet subcooling ranging from 1.7 to 5.7 °C. The comparisons reveal that departure diameter and nucleation frequency are two characteristics affected the most by the radial pressure gradient. Furthermore, bubble mean departure and lift-off diameters and nucleation frequency in the U-shaped channel are shown to be on average 127.05%, 66.80%, and 73.19% greater than those of the straight one. The results also indicate that on average, bubble mean departure, lift-off, and waiting times are, respectively, 41.52%, 59%, and 41.18% lower in the vertical U-shaped channel compared to the straight one. Additionally, it is found that excluding the lift-off time, bubble characteristics are more influenced by radial pressure gradient for the case of vertical channel (90° inclination angle).