Flow boiling of saturated liquid nitrogen in a horizontal macro-tube under negative gauge pressure

This study explores the flow boiling of saturated liquid nitrogen within a 10 mm inner diameter horizontal macro-tube, focusing on the impact of negative gauge pressure. The experiments cover a range of inlet pressure from -79.9 to -50.2 kPa, mass flux from 29.7 to 108.8 kg/(m²·s), and heat flux from 0 to 28.64 kW/m². The investigation examines two-phase flow patterns, flow boiling instabilities, and heat transfer characteristics. Heat transfer coefficient (HTC) data are compared with predictions from four existing correlations, and a novel correlation is proposed. A uniform temperature distribution across the top and bottom walls suggests a prevalence of annular flow. Decreased pressure supports the formation of stable annular flow due to increased velocity difference between vapor and liquid phases resulting from the heightened liquid-vapor density ratio. Thermal oscillations are observed in the unstable annular flow near the inlet and in the intermittent dry-out region where the annular flow transitions to mist and vapor flow. Both pressure reduction and mass flux increase reduce thermal oscillations by minimizing gravitational effects and enhancing flow inertia. A decrease in pressure results in a reduced dry-out type critical heat flux (CHF) due to increased droplet entrainment flow rate. Convective evaporation is identified as the primary heat transfer mechanism, with nucleate boiling becoming apparent at high mass flux conditions. Decrease in pressure and the increase in mass flux both facilitate flow boiling heat transfer by suppressing thermal oscillations and enhancing evaporation at the liquid-vapor interface. Among the selected correlations, the Shah correlation demonstrates the highest prediction accuracy with a mean relative error (MRE) of 28.87 %. The newly proposed enhancement factor type model correlation shows even higher prediction accuracy with an MRE of 14.18 %.