Experimental investigation of nitrogen flow boiling heat transfer in a single mini-channel

Abstract

Flow boiling heat transfer of nitrogen at high subcritical pressure conditions in a single vertical mini-channel with the diameter of 2.0 mm was experimentally investigated. The tested mass flux varied from 530 to 830 kg/(m²·s), the inlet pressure ranged from 630 to 1080 kPa, and the heat flux ranged from 0 to 223.2 kW/m². Effects of the mass flux and the inlet pressure on the nitrogen boiling curve were examined. Results showed that within the limited test conditions, the merging of three boiling curves indicates the dominance of nucleate boiling and the inlet pressure has a positive enhancement on heat transfer performance. Three heat transfer trends were identified with increasing heat flux. At low heat fluxes, the heat transfer coefficient increases first and then decreases with vapour quality. At intermediate heat fluxes, the heat transfer coefficient versus the vapour quality presents an inverted “U” shape. At high heat fluxes, a double valley shape was observed and the partial dry-out in intermittent flow and annular flow helps to interpret the phenomenon. The increasing inlet pressure increases the heat transfer coefficient over a wide range of vapour quality until the partial dry-out inception. The lower surface tension and lower latent heat of evaporation enhance the nucleate boiling for higher inlet pressure. A modified experimental correlation (mean absolute error (MAE)=19.3%) was proposed on the basis of the Tran correlation considering both the nucleate boiling and the partial dry-out heat transfer mechanism.

概要

目 的

面向液体火箭发动机再生冷却, 针对较高亚临界压力下单个垂直微小通道中液氮的流动沸腾传热特性开展实验研究, 讨论并分析热流密度、 密流和入口压力对沸腾曲线和局部换热系数的影响, 以获得液氮在微小通道中较高亚临界压力下的流动沸腾传热机理及实验关系式.

创新点

• 1. 通过工况参数对沸腾曲线和局部换热系数的影响分析, 得到液氮在微小通道中较高亚临界压力下的流动沸腾传热机理;

• 2. 提出微小通道中较高亚临界压力下的流动沸腾传热修正关系式.

方 法

• 1. 通过实验方法, 得到液氮在微小通道中较高亚临界压力下的沸腾曲线和局部换热系数;

• 2. 通过实验与理论分析相结合, 得到液氮在微小通道中较高亚临界压力下的流动沸腾传热机理;

• 3. 通过理论分析, 将实验结果与六种预测关系式进行比较, 并根据实验数据提出一种改进的实验关系式 (表 7).

结 论

• 1. 热流密度对换热系数有较大影响, 随着热流密度的增大, 出现了三种变化趋势;

• 2. 在实验范围内, 密流的增大抑制了核态沸腾, 并且降低了环状流的局部换热系数;

• 3. -入口压力的增大在较大干度范围内增大了局部换热系数, 直到局部蒸干的出现;

• 4. 综合考虑核态沸腾和局部蒸干两种主导传热机理, 在 Tran 关系式的基础上提出了一种适用于较高亚临界压力条件下微小通道中液氮流动沸腾的修正实验关系式 (平均绝对误差为 19.3%).