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 nitrogen 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 pressure drop. Flash evaporation occurs for the large pressure drop, and its effect on mass quality is significant and should be accounted. 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 separated 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 between liquid nitrogen and its vapor is also favorite to the homogeneous model prediction.