Experimental study on bubble nucleation and departure for subcooled flow boiling in a narrow rectangular channel

In order to improve the prediction ability of nucleation site density and bubble departure diameter for subcooled flow boiling in a narrow rectangular channel, a series of experiments were conducted using visualization methods to analyze the effects of pressure, wall superheat, mass flow rate and fluid subcooling. The results show that nucleation site density is not only affected by wall superheat, increasing fluid subcooling or mass flow rate also suppresses bubble nucleation. The reason is that the temperature gradient is more likely affected by flow field and bulk temperature in narrow rectangular channels and is much steeper near the heating wall. Bubble departure diameter increases with the increasing wall superheat, decreasing fluid subcooling and decreasing mass flow rate. The influence of the operation pressure is not obvious in the experimental conditions. Increasing the wall superheat leads to larger bubble growth rates and larger bubble growth force to prevent bubble departure. The increase of the mass flow rate raises the drag force, which accelerates the bubble departure process. Finally, empirical correlations for nucleation site density and bubble departure diameter are proposed based on the experimental data. The average errors of the predicted nucleation site density and bubble departure diameter are 33.5% and 21.2%, respectively. (C) 2019 Elsevier Ltd. All rights reserved.