Study on bubble growth interphase heat transfer of rapid decompression

Abstract

In a loss of coolant accident in a nuclear reactor, steam bubbles undergo superheated growth, where the interphase heat transfer coefficient significantly influences both the bubble growth rate and the void fraction of the two-phase flow. The bubbles should be grown within a homogeneous temperature field in order to facilitate a more comprehensive investigation into the characteristics of interphase heat transfer. Most existing studies are based on vapor bubbles generated within vaporization holes on walls. However, due to temperature gradients near the wall, these studies have limitations regarding heat transfer conditions and bubble shape. To investigate the realistic growth behavior characteristics of vapor bubbles during reactor loss-of-coolant accidents, the experiment was conducted on bubble growth in superheated liquid phases using a visual pressure vessel capable of creating instantaneous pressure drops. The system pressure ranged from 0.20 to 2.93 MPa. By employing high-speed cameras, pressure sensors, and thermocouples, the study investigated the impact of varying degrees of superheat, bubble Reynolds numbers, and Prandtl numbers on bubble growth rate. In this paper, a novel approach is proposed to investigate the interphase heat transfer coefficient under pressure drop, and its results are compared with previous methods that neglected the influence of vapor expansion. The correlation function incorporates Reynolds number, Jacob number, and Prandtl number, indicating the equation reveals the quantitative correlation between the interphase heat transfer coefficient and convection as well as heat conduction. Average heat transfer coefficients are determined for various operating conditions, leading to the development of an empirical correlation equation for the interphase heat transfer coefficient with an average relative error of 16.6%. Comparisons between the new correlation and other experimental data demonstrate its effectiveness in predicting the growth radius of the bubble.