低压低流速条件下的过冷沸腾换热特性

为探究低压低流速条件下的过冷沸腾换热特性，开展本实验研究。通过分析实验中采集的热工参数和可视化图像，探究了沸腾滞后现象、沸腾失稳现象以及沸腾换热特性。实验发现沸腾起始点壁面过热度较高，而沸腾的发生大幅提高了换热系数，因此出现了显著的沸腾滞后现象。实验中较为光滑的加热面可达到较高的过热度，而低压下快速产生的气泡尺寸较大，在较低的热流密度下气液界面发生剧烈变化，使气泡破裂为多个小气泡并成为核化点。在过冷沸腾换热系数的预测中，Dittus-Boelter对流换热关系式不再适用，采用Hallman关系式和Gnielinski关系式计算对流换热系数，并引入壁面过热度对池式沸腾换热系数进行修正，可使过冷沸腾换热系数的预测精度大幅提高。

Subcooled Flow Boiling Heat Transfer under Low Pressure and Low Flow Velocity

An experimental study was conducted to investigate the characteristics of subcooled flow boiling heat transfer under low pressure and low flow velocity. By analyzing the thermal parameters and visual images acquired during the experiment, boiling hysteresis, boiling instability, and boiling heat transfer coefficient were explored. The experiment shows that the wall superheat at the boiling incipience is high, and the occurrence of boiling greatly increases the heat transfer coefficient, so there is a remarkable boiling hysteresis phenomenon. The relatively smooth heating surface can achieve a higher degree of superheat, while bubbles generated rapidly at low pressure is large in size, and the gas-liquid interface shows a significant disturbance at low heat flux, breaking the bubble into multiple small bubbles which serve as nucleation sites. In the prediction of subcooled boiling heat transfer coefficient, Dittus-Boelter convection heat transfer correlation is no longer applicable. The Hallman correlation and Gnielinski correlation were used to calculate the convection heat transfer coefficient, and the wall superheat was introduced to modify the pool heat transfer coefficient. The above procedure can greatly improve the prediction accuracy of the subcooled boiling heat transfer coefficient.