竖管降膜结构变量对液膜流动特性的影响

Study on Structural Effect of Falling Film Flow Characteristics in Vertical Pipe

液膜蒸发冷却过程是一个传热、传质相互耦合的复杂过程,为研究竖管降膜蒸发冷却器中竖管结构对降膜分布的影响,采用流体模拟软件对影响竖直管内气液两相逆流的环缝隙宽度、空气进口宽度和管长进行数值模拟,分析了液膜厚度、x方向分速度和努塞尔数的变化规律。研究结果表明,液膜的波峰与波谷分别对应着x方向分速度的波谷与波峰。在一定的喷淋量下,环缝隙宽度增大时,液膜分布先稳定后趋于波动;空气进口增大时,液膜的波动程度是增大的;就管长而言,液膜的波动程度是逐渐增大的,在管长大于等于0.7 m后,液膜无法完整润湿壁面,管壁出现干斑。这3个结构因素中,管长对液膜的传热性能和分布性影响最大。

Film evaporation is a cooling process during heat and mass transferring complex mutually coupled. In order to study the influence of vertical pipe structure on falling film distribution in vertical tube falling film evaporative cooler, CFD software was used to simulate the annular gap width, air inlet width and tube length which affect gas-liquid two-phase counter-flow in vertical pipe. The variation of liquid film thickness, x-direction sub-speed and Nu-number was analyzed. The results show that the peaks and troughs of liquid film correspond to the troughs and peaks of x-direction velocity. Under a certain amount of spraying, when the annular gap width increases, the liquid film distribution first stabilizes and then tends to fluctuate, when the air inlet increases, the fluctuation degree of the liquid film increases. As far as the pipe length is concerned, the fluctuation degree of the liquid film increases gradually. After the tube length is greater than or equal to 0.7 m, the liquid film cannot completely wet the wall surface and dry spots appear on the tube wall. Among these three structural factors, the tube length has the greatest influence on the heat transfer performance and distribution of the liquid film.