超临界流体水平管换热特性可视化研究

Visualization Experimental Investigations of the Heat Transfer Characteristics in a Horizontal Tube Under Supercritical Pressure

为研究超临界压力下水平管径向截面的二次流现象,分析其对流体换热能力的影响机理,优化相关换热器设计,本文采用PIV实验方法,首先分析了水与FC-72物性差别对径向截面二次流流场的影响;然后重点研究了热流密度、质量流速工况参数变化对超临界压力下FC-72的二次流流场的影响规律。实验结果表明:相同工况下低导热系数,高体积膨胀率的工质会增强截面的二次流强度;随着热流密度增加,截面二次流流速增加,二次流涡逐步向截面底部移动;质量流速的提高会削弱加热管顶部流体温度分层的影响,并促进了中心与边壁流体间的掺混,有利于流体换热能力的提高;此外,当截面顶部流体达到临界点,二次流涡与常温工况相比远离壁面并向截面中心靠拢。

A visualization experiment is performed to investigate the structure of secondary flow in the horizontal tube under supercritical pressure.In order to optimize the design of heat exchangers,the accurate secondary flow motion has been measured to get further insight into its influence on heat transfer.The Particle Image Velocimetry(PIV) technology has been used to study the influence of variations of thermophysical properties of water and FC-72,heat flux and mass flow rate on the secondary flow filed.The experimental results indicated that the intensity of the secondary flow motion increased with lower thermal conductivity and higher volume expansion under the same working condition.As the heat flux increased,the intensity of secondary flow induced by the buoyancy force got enhanced and vortices moved toward the lower part of tube cross section gradually.The raise of mass flow velocity would weaken the influence of temperature stratification in the upper part of the tube,and facilitate the mixing process between hot fluid near the tube wall and cold fluid in the center of tube cross section,which helps to improve the local heat transfer capability of the fluid.Furthermore,compared to ambient temperature conditions,when fluid temperature at the top of tube cross section approaches the critical point,two symmetrical counter-rotating vortices got closer to the tube center core.