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/(m·s), and heat flux from 0 to 28.68 kW/m. 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 %).

中文翻译：

---

液氮在水平粗管中的低压流动沸腾：第二部分——传热特性

氮是超导中最重要的冷冻剂。本研究研究了内径为 10 mm 的水平粗管中流动沸腾的传热特性，特别关注负表压下的流动沸腾。实验涵盖以下范围：入口压力为-31.0至2.5 kPa，质量通量为27.0至71.3 kg/(m·s)，热通量为0至28.68 kW/m。研究了操作参数对传热系数 (HTC) 的影响，并将 HTC 数据与四个相关性的预测值进行比较。在底壁处，HTC 分布呈现“双峰”形状。主峰以低蒸汽品质的泡状流、塞流和段塞流中的核沸腾为主，而副峰以高蒸汽品质的对流蒸发为主。这两个区域大致以0.3至0.4的蒸气质量划分。在分层流中，顶壁的 HTC 一般低于底部的 HTC。 HTC 随着压力降低而增加，特别是在对流蒸发区域。这是由于较高的液汽密度比、广泛的导热性以及较低压力下的较大表面张力。热通量或质量通量对底壁 HTC 的影响在核态沸腾和对流蒸发区域之间有所不同。增加热通量可改善核沸腾占主导地位的 HTC，同时增加质量通量可显着改善对流蒸发，尤其是在较高热通量下。所有选定的相关性都高估了顶壁的 HTC。 Shah 相关性表明，在接近大气压（平均相对误差，MRE = 21 %）和负压（MRE = 19 %）下，底壁 HTC 的预测精度最高。