微通道中液氮的流动沸腾——两相流动压降分析

对液氮在直径为0.531 mm,加热长度为250mm的圆管中的流动沸腾压降和传热特性进行研究.作为第一部分,主要对微通道中液氮的两相流动压降进行试验研究与分析.结果表明:在核态沸腾起始时,质量流量迅速降低,而压降突然增大,并伴随着明显的温度滞后,幅度约为4.0～5.0 K.由于压降很大,在微通道内液氮的两相流动中会出现闪蒸,从而对质量干度产生重要影响.最后,利用均相模型和三个两相流动模型(L-M模型,Chisholm B系数模型和Friedel模型)对微通道沿程压降进行分析和比较.不同于常规通道的是,均相模型可以很好地预测压降试验结果,而三个两相流动模型的预测偏差较大,这是由于在微小通道中的高速流动情况下,汽相和液相混合比较均匀;同时液氮的液汽密度比很小,这也有利于均相模型的预测.

FLOW BOILING OF LIQUID NITROGEN IN MICRO-TUBES: TWO-PHASE FLOW PRESSURE DROP

The pressure drop and heat transfer characteristics are investigated experimentally for flow boiling of liquid nitro- gen in a circular micro-tube with a diameter of 0.531 mm, and a heated length of 250 mm. As the first part of the study, the pressure drop characteristics of two-phase nitrogen flow in the micro-tube are analyzed. Results indicate that mass flux drops suddenly, while pressure drop increases at the onset of nucleate boiling (ONB), and apparent temperature hysteresis is observed. Heat flux and mass flux have important influence on the pres-sure drop. Flash evaporation occurs for the large pressure drop, and its effect on mass quality is significant and should be ac- counted. Homogeneous model and three separated flow models, i.e., L-M model, Chisholm B coefficient model and Friedel model, are employed to analyze the measured pressure drop data. Contrary to conventional channel, homogeneous model enables to predict the experimental data well, and three sepa- rated flow models yield poor predictions. The reason is that the liquid and vapor phases of nitrogen are uniformly mixed in the micro-tube for the high velocity, and the low density ratio be- tween liquid nitrogen and its vapor is also favorite to the ho- mogeneous model prediction.