气液降膜流动中液相速度波动及其传质研究

为了研究降膜流动的动力学性质及其对气液传质过程的影响,在气液逆流的不同气液流动条件下采用激光多普勒(1aser Doppler anemometer,简称LDA)测量了降膜流动的液相速度分布和瞬时速度波动.和以往假定液膜外侧为自由表面,液膜表面处剪切力为零的Nusselt模型进行了比较,LDA测量结果表明气液逆流时降膜流动的最大液相速度出现在液膜表面之内,并且是以近界面区域的速度波动为特征的流动.在相同的降膜装置中进行了乙醇稀溶液的解吸实验,液相传质系数的实验测量值是渗透理论预测值的1～2倍.实验结果表明液相界面区域的速度波动加快了气液界面的表面更新速率,减小了传质阻力,强化了气液界面的传质过程.考虑液膜波动特征对气液接触情况的影响,从气液两相接触时间的角度出发,修正了渗透理论对液相平均传质系数的预测,预测结果和实验结果相吻合.

The Study on the Liquid Velocity Fluctuation and the Mass Transfer in the Gas-Liquid Falling Film

In order to study the hydrodynamics in the falling film, a laser Doppler anemometer (LDA) was used to measure the local liquid velocity and its instantaneous fluctuations in the falling film of a countercurrent gas-liquid flow. The experimental results show that the significant velocity fluctuation and the maximum velocity might occur in the vicinity of the liquid surface with relativity low liquid Reynolds number. To investigate the influence of the velocity fluctuation on the interfacial mass transfer behavior, the desorption processes of dilute alcohol solution were conducted in the same falling film contactor. The experimental liquid mass transfer coefficients are as much as 1~2 times higher than that predicted by the penetration theory. The experimental results demonstrate that the liquid velocity fluctuation increases the surface renewal and reduces the mass transfer resistance near the gas-liquid surface. Consequently, the mass transfer process is enhanced. Considering the velocity fluctuation effect on the gas-liquid flow, the penetration theory was modified and the predicted liquid mass transfer coefficients agree well with the experimental data.