垂直多孔表面管内高沸点工质强化流动沸腾换热的数值分析

Numerical analysis of enhanced flow boiling heattransfer in vertical porous tube

在对流动特性和换热机理进行分析的基础上, 建立了垂直多孔表面管内高沸点工质强化流动沸腾换热的数学模型和数值计算方法. 该模型认为环状流区域同时存在强制对流与核态沸腾两种换热方式. 液膜厚度、速度与温度等参数通过求解液膜的质量守恒、动量守恒、能量守恒方程获得. 核态沸腾换热则包括气泡脱离带走的热量及用于加热气化核心影响区流入液体的热量两部分. 提出了流动沸腾情况下的多孔层气泡脱离直径及气化核心密度计算方法. 对异丙苯在火焰喷涂型垂直多孔表面管内向上流动时的沸腾换热进行了数值预测, 大部分工况的计算值与实验结果符合良好. 在低干度区 (x=0. 1), 核态沸腾在总换热中的比例约为50%. 随着干度的增加, 核态沸腾受到越来越大的抑制, 当x为0 .5时, 这一比例降低到约15%.

A semi-analytical model was developed for the prediction of enhanced flow boiling heat transfer in vertical porous tubes. The model assumes that forced convection and nucleate boiling coexist in the annular flow regime. Conservations of mass, momentum, and energy are used to determine liquid film thickness and temperature. The heat flux due to nucleate boiling consists of those inside and outside micro-tunnels. A detailed analysis of various forces acting on the bubble is presented to predict its mean departure diameter. The active nucleation site density of porous layer is determined by correcting the pool boiling correlation with a suppression factor. The flow boiling heat transfer coefficients of high saturation temperature organic fluid (cumene) in a vertical plasma coated tube are studied numerically. It is shown that the present model can predict most of experimental values within ±20%. The numerical results also indicate that the nucleate boiling contribution to the overall heat transfer coefficient decreases from 50% to 15% with vapor quality increasing from 0.1 to 0.5.