Experimental study on the heat transfer characteristics of saturated liquid nitrogen flow boiling in small-diameter horizontal tubes

Experiments were performed to investigate the heat transfer characteristics of the saturated liquid nitrogen flow boiling in a horizontal tube with an internal diameter of 1.98 mm. The heat transfer coefficients were obtained within a wide range of operation conditions, i.e. inlet pressure from 230 to 420 kPa, mass flux from 200 to 690 kg·m⁻²·s⁻¹, heat flux from 1.5 to 67.1 kW·m⁻², and vapor quality from 0 to 0.81. In the low vapor quality region, heat transfer coefficients showed strong dependence on heat flux and pressure but varied little with mass flux or vapor quality, indicating the dominant role of the nucleate boiling over the convective evaporation. The influence of convective evaporation became noticeable in the intermediate-to-high vapor quality region. A modified correlation was proposed by including both the influences of the nucleate boiling and the convective evaporation, which could predict the experimental data with MAD of 16.4%.