Flow boiling of liquid nitrogen in micro-tube—two-phase heat transfer analysis

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 plus/minus 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.