水平管降膜蒸发器中液膜厚度周向分布研究

STUDY OF CIRCUMFERENTIAL THICKNESS DISTRIBUTION OF LIQUIDE FILM WITHIN HORIZONTAL TUBE FALLING FILM EVAPORATOR

采用VOF模型对水平管表面液膜厚度分布进行数值研究,并将结果与实验数据进行比较,二者吻合较好。结果表明:液膜沿水平管周向的流动可分为瞬态过程及之后的稳态过程,瞬态过程包括液体自由下降、冲击管壁、液膜发展、充分发展以及脱离管壁5个阶段。稳态过程中,液膜厚度沿圆管周向分布为先减小,再增大。水平管上、下半部液膜厚度分布不对称,顶部和底部存在两个切向速度滞点,下部滞点处可能会出现无液区。随着液膜雷诺数的增大,液膜厚增大,下部无液区范围也增大。较大的管径会导致液膜在同一角位置处流速增加,液膜厚度减小。

The numerical investigation was performed to analyze the liquid film thickness distribution on the horizontal tube surface by using VOF model, and the result was compared with experimental data, getting good agreement between two data. The results show that the whole process of liquid film flowing along the horizontal tube surface includes the transient sub-process and steady-state sub-process, and the former can be divided into five stages, including the free fall of liquid, impacting the wall, liquid film development, full development and separation from the wall. In the steady state process, the thickness of liquid film along the circumference of the tube decreases firstly and then increases. The distribution of film thickness in the upper and lower area of horizontal tube is asymmetric, there are two tangential velocity dips in the top and bottom of the horizontal tube, and the liquid free zone can appear in the lower stagnation point. With the increase of Reynolds number of liquid film, the liquid film thickness increases and the lower liquid free zone also increases. The bigger tube diameter will cause higher film flow speed and thinner film thickness at the same angular position.