微细通道内液氮流动沸腾的流型特性

Flow Patterns and Flow Regime Maps of Liquid Nitrogen Flow Boiling in Micro/mini-channels

采用高速摄像,得到内径为1.931mm、1.042mm、0.531mm的竖直上升圆管内液氮流动沸腾的主要流型为泡状流、弹状流、搅拌流和环状流;并且在1.042mm、0.531mm管内发现受限气泡流。并绘制流型图,分析表面张力,压力和管径对流型转变的影响。表面张力是影响流型转变的重要物性参数,相对于空气—水的流型图,对应的弹状流/搅拌流,搅拌流/环状流流型转变线向较低的气体表观速度方向移动;而泡状流/弹状流的转变线向较高的气体表观速度方向移动。压力越高,相应的流型转变曲线向较低的气体表观速度方向移动。管径对流型转变有重要影响,随着管径的减小,相应的流型转变线向较低的气体表观速度方向移动。试验结果与通用的流型转变理论模型作比较,发现理论模型的预测结果与试验结果相差较大。

High speed digital camera is employed to acquire the typical flow patterns of liquid nitrogen flow boiling in vertical upward tubes with inner diameters of 1.931 mm, 1.042 mm and 0.531 mm. The flow patterns are bubbly flow, slug flow, churn flow and annular flow. Confined bubbles are observed in 1.042 mm and 0.531 mm tubes. Flow regime maps are depicted according to experimental results and the surface tension and diameter are considered to be the major factors affecting the flow pattern transitions. It is found that the slug/churn and chum/annular transition lines of the present experiment shift to lower superficial vapor velocity; while bubbly/slug transition lines to higher superficial vapor velocity compared to gas-water flow regime maps with the similar hydraulic diameters. The corresponding transition lines move to lower superficial velocity along with the reduction of tube inner diameter. The results indicate that no existing theoretical model can explain the flow regime maps in the present experiment.