直接空冷岛顺流基管管内冷凝的传热传质分析

Analysis for Heat and Mass Transfer of Condensation inside Down-Flow Unit Tube of Air Cooled Heat Exchanger

顺流基管是火电厂直接空冷岛关键部件,分析基管管内传热传质机理,对优化基管传热和空冷岛安全运行有重要意义。考虑基管几何特征和安装倾角,建立模拟蒸汽在基管内壁上冷凝的三维坐标数学模型。求解模型数值解时,假设基管壁温未知,设计迭代算法估算壁温。通过对比蒸汽凝结率数值解与设计值,管内换热系数数值解与Nusselt经验公式值,验证了模型的有效性。对模型数值解进一步分析发现,膜厚偏微分方程能正确描述基管几何特征和倾角影响下的液膜特性,汽液两相分离位置的预测值与直接空冷岛运行实际相符,凝结水过冷度数值解不能完全匹配实际测量值。本文的研究成果,为设计高效换热基管和空冷岛冬季防冻,提供了理论依据。

The down-flow unit tube is a key component of air cooled heat exchanger（ACHE） in power plant. The study on the heat and mass transfer mechanism inside the unit tube is very important for heat-transfer efficiency optimization and safe operation of ACHE. In this paper, considering the geometry characteristics of tube, a 3D mathematical model was constructed to simulate the condensation of steam on the internal wall of unit tube. The temperature of tube wall was assumed to be unknown and an iterative algorithm was developed to evaluate this temperature. The numerical solution of steam condensate rate was compared with design value, and simultaneously, numerical solution of heat transfer coefficient was compared with Nusselt empirical formula value to verify the model validation. Further analysis on the model numerical solution shows that the partial differential equations of condensate film thickness can accurately describe the film characteristic which varies with geometry structure and declination of the tube. And the predicted value of vapour-liquid separation location agrees well with ACHE experimental operation. However, the numerical solution of the condensate sub-cooling temperature doesn＇t agree with actual measurement value very well. The result of this study is of great significance for optimization design of down-flow unit tubes, and also, it is meaningful for antifreezing protect of ACHE in winter.