Flow boiling of liquid nitrogen in a horizontal macro-tube at low pressure: Part Ⅱ – Heat transfer characteristics

Nitrogen is the most important cryogen in superconductivity. This study investigates the heat transfer characteristics of flow boiling in a horizontal macro-tube with an inner diameter of 10 mm, particularly focusing on flow boiling at negative gauge pressure. The experiments cover the following ranges: inlet pressure from −31.0 to 2.5 kPa, mass flux from 27.0 to 71.3 kg/(m²·s), and heat flux from 0 to 28.68 kW/m². The effects of operating parameters on the heat transfer coefficient (HTC) are examined, and the HTC data are compared with the predicted value from four correlations. At the bottom wall, the HTC distribution exhibits a “two-peak” shape. The primary peak is dominated by nucleate boiling in bubbly, plug, and slug flow with low vapor quality, while the secondary peak is dominated by convective evaporation with higher vapor quality. These two regions are roughly divided by the vapor quality of 0.3 to 0.4. In stratified flow, the HTC at the top wall is generally lower than that at the bottom. The HTC increases as the pressure decreases, particularly in the convective evaporation region. This is attributed to the high liquid–vapor density ratio, extensive thermal conductivity, and substantial surface tension at lower pressure. The effect of heat flux or mass flux on the HTC at the bottom wall differs between the nucleate boiling and convective evaporation regions. Increasing the heat flux improves the HTC where nucleate boiling dominates, while increasing the mass flux significantly improves convective evaporation, especially at higher heat flux. All selected correlations overestimate the HTC at the top wall. The Shah correlation demonstrates the best prediction accuracy for the HTC at the bottom wall, both at near atmospheric pressure (Mean Relative Error, MRE = 21 %) and at negative pressure (MRE = 19 %).