扁平空冷翅片管内换热与流动特性数值研究

Numerical Study on Heat Transfer and Flow Inside Flat Finned Air-cooling Tube

扁平翅片管结构复杂,换热方式耦合多样,以往研究难以用常规实验与关联式方法,完整复现并分析管内热流特性。基于分布参数热力学理论,提出耦合换热模型描述翅片管两相流与空气间的换热,并将模型应用于1m长度工程管,通过数值求解,得到管内温度、热流密度、换热系数、液膜厚度、剪切力、压力降沿两相流和冷却空气流向全域内的变化规律:随着冷却空气横向流动,扁平管凝结性能急剧降低,热流密度从空气入口处20000W/m^(2)降至出口处5000W/m^(2);翅片管钎焊层热流密度高达14000W/m^(2),因此应注重提升钎焊层对翅片管换热的增强效用。管内两相流定性为环状流,随着两相流动,倾斜管能有效地将平板液膜汇聚到下半圆,平板液膜厚度沿管深增长缓慢,全管段液膜增长范围为16~25μm;平板热流密度和凝结空冷翅片管空气侧对流换热系数管深无下降趋势,这与工程管对应管段的实验结果相符。

Due to complicated structure and the coupling and variety of heat transfer in the flat finned tube,it is difficult to obtain and analyze the local characteristics of in-tube heat and flow with the traditional experimental and correlation methods.Based on the distributed parameter thermo-dynamic theory,a coupled heat transfer model is proposed to the heat transfer between the two-phase fluid and the cooling air.The model is applied to a 1 m length engineering tube,and the temperature,heat flux,heat transfer coefficient(HTC),film thickness,shear and pressure-drop are characterized with the computational fluid dynamics solution.It is found that the condensation heat flux decreased sharply with the lateral flow of the cooling air,from 20,000w/m^(2)at the air inlet to 5,000w/m^(2)at the outlet;meanwhile,the welder layer play a key role to export the condensation-released heat to cooling-air with the heat flux up to 14,000w/m^(2),more attention should be paid to the heat-enhanced effects on the finned tube from the material and configuration related to the welder layer.The pattern of two-phase fluid inside tube is verified as the annular flow,and the film thickness is found to be increasing in the range of 16-25μm with the flow of two-phase fluid,due to the inclined tube configuration that effectively converges the condensate from the plate to the lower semicircle.Consequently,both the heat flux and HTC do not decrease lengthwise,agreeing well with the result on the corresponding section of an experimental tube.