强化管表面结构对竖管降膜流动特性的影响

Effects of Surface Structure Outside the Enhanced Tube on Flow Characteristics of Vertical Falling Film

为了研究强化管表面结构对竖管降膜流动特性的影响,搭建了竖管降膜流动冷态实验台,利用光谱共焦位移传感器和高速摄像仪对强化管和光管进行了对比研究。实验中,降膜流量通过调节高位水箱内静液柱高度H和布液器环隙间距Δd控制,使降膜雷诺数Re在0~4500变化。测试结果表明:当0<Re≤2000时,强化管与光管液膜厚度一致,但当Re>2000时,强化管的液膜厚度逐渐高于光管,且随着Re的增大,二者差值越趋增大;相对于光管,强化管润湿管壁的最小流量约为光管的1/3,明显小于光管,表明强化管有利于液膜的铺展;当降膜流动处于层流时,在入口段强化管与光管的降膜波动特性无明显差别,但光管较先进入充分发展区,随后波动强度逐渐小于强化管;当降膜流动呈湍流时,强化管的波动强度明显高于光管,当Re达到4200时,在测试范围内强化管的平均波动强度较光管高出约8%,说明强化管的表面结构增强了液膜的扰动,更利于降膜蒸发过程传热传质。以上结论可为强化管用于竖管降膜蒸发器的研究提供有益参考。

An experimental platform is established to study the effects of surface structure outside the enhanced tube on flow characteristics of vertical falling film.A comparative study of an enhanced tube and a smooth tube is experimentally carried out by using a spectral confocal displacement sensor and a high-speed camera.Falling film flow rate is controlled by adjusting the height level H of a hydrostatic column in the higher water tank and the annular gapΔd of distributor.The Reynolds number of working fluid varies from 0 to 4500 in the experiments.Test results show that the liquid film thickness of the enhanced tube is the same as that of the smooth tube when the Reynolds number ranges from 0 to 2000.However,when the Reynolds number is greater than 2000,the liquid film thickness of the enhanced tube is gradually larger than that of the smooth tube,and the difference gradually increases with the increase of the Reynolds number.Besides,it is found that the minimum flow rate for wetting the wall of the enhanced tube is about 1/3 of that of the smooth tube,indicating that the enhanced tube is beneficial to the spread of liquid film.Furthermore,when falling film is in laminar flow,there is no significant difference on falling film fluctuation characteristics between the enhanced tube and the smooth tube in the entrance region.But the falling film of the smooth tube enters to the full development region earlier,and then the fluctuation intensity of the smooth tube is gradually lower than that of the enhanced tube.When the falling film is in turbulent flow,the fluctuation of the enhanced tube is obviously higher than that of the smooth tube.When Reynolds number of the falling film reaches 4200,the average fluctuation intensity of the enhanced tube is about 8%higher than that of the smooth tube,which shows that the surface structure of the enhanced tube raises the disturbance of the liquid film and is more conducive to heat and mass transfer in falling film evaporation.These results will provide a useful reference for the study of the enhanced tube used in vertical falling film evaporator.