Flow Boiling Characteristics In Micro/mini Channels With High Heat Flux Surface

In order to fully understand the flow boiling characteristics in micro/mini gap channels with high heat flux surfaces,experimental studies was carried out.The length and width of the heat transfer surface are 54 mm and 10 mm,respectively.The gap heights are 0.5 mm,1 mm,and 2 mm.Atmospheric pressure deionized water is the work fluid and the experimental conditions are as follows:the mass flux range is 200-400 kg m^-2 s^-1,the heat flux range is 0-280 W cm^-2,and the fluid inlet temperatures are60℃and 80℃.Finally,the following conclusions are obtained.（1）Two-phase flow visualizations.Four two-phase flow patterns:bubbly,sweeping,churn flow,and annular flow were observed.The single flow pattern continues to develop with the increase of heat flux.The unsteady flow occurred early in the sweeping flow and ended in the middle of the sweeping flow.The condensation of upstream bubbles and the rapid growth of many downstream bubbles are the main reasons for the unsteady flow.Through establishing a flow pattern diagram,the decrease of mass flux promotes the transition of different flow patterns to the direction of lower heat flux.The decrease of gap height increases the restriction effect of height on bubble growth,accelerating the flow pattern transition.In addition,the quality of different flow patterns in the channel hardly changes under different mass flux and gap heights.（2）The effect of experimental factors on the boiling heat transfer characteristics.The heat transfer coefficient increases with the heat flux increase,and there are three heat transfer stages:single-phase,subcooled flow boiling,and saturated flow boiling.The heat flux,mass flux,and gap height have a more significant influence on the heat transfer coefficient of subcooled flow boiling than saturated flow boiling.The mass flow has a proportional effect on the subcooled boiling heat transfer coefficient and an inverse proportional effect on the saturated boiling heat transfer coefficient.The reduction of the gap height promotes the heat transfer coefficient of subcooling boiling and has a non-monotonic effect on the heat transfer coefficient of saturated boiling.The critical heat flux increases with the increase of the mass flux,and the influence of the mass flux decreases with the decrease of the gap height.The critical heat flux decreases with decreasing gap height and increases with decreasing inlet temperature.（3）Evaluation of existing correlation and establishment of new correlation.The Shah Correlation and Sun-Mishima Correlation are the correlations with higher prediction accuracy for the subcooled and saturated boiling heat transfer coefficient in the micro/minigap channel.The mean absolute errors are 12.94%and 13.30%,respectively.The Qu Correlation and Kureta correlation have higher prediction accuracies for the critical heat flux of saturated flow boiling and subcooled flow boiling.The mean absolute errors are 7.5%and 8.5%,respectively.The prediction accuracy of the existing pressure drop correlation was poor.A pressure drop correlation was proposed by updating the two-phase length calculation with a mean absolute error of 10.8%.