蒸汽在扁平管内逆流流动凝结实验与理论分析

针对火电机组直接空冷凝汽器扁平管内蒸汽凝结换热过程,设计并搭建了水冷扁平管内可视化流动凝结实验台,开展管内凝结换热研究。建立涵盖壁面薄液膜和圆弧段液池的流动凝结分析模型,得到凝结液膜发展相关方程。特别对于壁面液膜双曲型发展方程,考虑其对流特征和物理守恒特性,提出共轭梯度法-牛顿法-模拟退火相结合的稳态求解算法,避免传统非稳态方法的复杂性。可视化实验结果表明,管内汽液两相分层流动,扁平管圆弧底部形成了液池。对应的理论分析结果更加细致地表明,管内凝结换热为分层流型,液池液膜厚度为1 mm量级,壁面液膜相比液池液膜薄很多,因而具有较高的凝结传热系数。分析结果为判断冬季低温环境运行时管内冻结起始位置提供了依据。

Experimental and theoretical analysis on reflux flow condensation of vapor inside flat tube

For investigating condensation heat transfer characteristic inside flat tubes widely applied in air-cooled condenser in large power plant, a water cooled visualization condensation test system is designed and built for an indirect study on the flow regime of condensation process in a condenser. Base on the observed stratified flow regime, a theoretical model is built for condensation flow and heat transfer process including condensate water bath development on the arc wall and condensate film development on the other wall. Especially, for condensate film on the wall, a hyperbolic equation is proposed. Considering the convection and conservation characteristic in the equation, a solution algorithm for steady conservation equation is produced by combination of the conjugate gradient method, the Newton method and simulated annealing method, which can avoid the complex of traditional non-steady method. The visualization results indicate that the flow regime of condensation process is stratified flow regime and the corresponding theoretical calculation results indicate that the condensate film thickness on the wall is far less than condensate bath height, which is in the 1 mm order of magnitude, and leads to high condensation heat transfer coefficient. The results supply grounds for judging the onset position of freezing in flat tube under air cooling condition in power plant in winter.