微通道中液氮的流动沸腾——换热特性分析

FLOW BOILING OF LIQUID NITROGEN IN MICRO-TUBE-TWO-PHASE HEAT TRANSFER ANALYSIS

对微通道中液氮流动沸腾换热特性进行试验研究和分析。给出典型的沸腾曲线,分析壁温、干度和换热系数沿微通道管程的变化规律,考察热流密度、质量流量和压力对流动沸腾换热的影响。将126个试验数据点与四个换热关联式比较,并对微通道中流动沸腾换热机理进行分析。结果表明,在多数情况下干度和热流密度对沸腾换热系数的影响较小,换热系数主要决定于质量流量和压力,随两者增加而增加,换热以对流蒸发为主导机理。KLIMENKO关联式预测效果最好,TRAN微通道关联式次之,对常规管道得到广泛使用的CHEN关联式和SHAH关联式都远远高估了试验值。基于两相流压降和换热特性分析,推知微通道中的两相流流型不同于常规管道:在低干度情况下,流型以弥散泡状流为主;而在高干度情况下,流型以由雾状汽芯和不规则液膜组成的环状流为主。

The pressure drop and heat transfer characteristics of liquid nitrogen flow boiling in a circular micro-tube, the heat transfer characteristics are investigated. The typical flow boiling curve is presented, and the effect of heat flux, pressure and mass flux on the flow boiling heat transfer coefficients are clarified. The experimental data are compared with four correlations, and the heat transfer mechanism of liquid nitrogen flow boiling in micro-tube is analyzed. The results show that heat transfer coefficients are independent of the mass quality and heat flux, while increase with the pressure and mass flux. This fact suggests that the dominated heat transfer mechanism should not be the nucleate boiling, but the convective evaporation. Among the four correlations, KLIMENKO correlation can give the best prediction with an error _ 35%, and is followed by TRAN model. The classical CHEN and SHAH correlations for the conventional channels overestimate the experimental data significantly. According to the pressure drop and heat transfer characteristics obtained, the flow pattern of liquid nitrogen flow boiling in micro-tube is reasonably postulated to be different from that in the conventional channels. For low mass quality, it is the dispersed bubble flow; for high mass quality, it is the annular flow with the irregular liquid layer and the ＂mist＂ flow of liquid droplet in the vapor core.