Study on separation abilities of moisture separators based on droplet collision models

Abstract

Moisture separators are widely used for eliminating the droplets from the gas–water/steam mixture flows. The volume fraction of droplets is often not small in most separators, so the collision behavior of droplets is crucial in changing their velocity, size and number distributions to affect the separation abilities of these separators. In this paper, the collision models of droplets are established to simulate the droplet-laden flows in the moisture separators. First, a stochastic searching algorithm to find the collision droplet pairs is performed by utilizing the collision kernels and defining the mean free path of a droplet. Second, the criteria to distinguish the different regimes of binary droplet collision are carried out. The regime map drawn by these criteria is highly consistent with that from previous experiments. Third, a set of formulas to predict the velocity, size and number distributions of droplets after bouncing, coalescence, reflexive and stretching separation are proposed based on the conservation law and some empirical relations. Finally, the separation abilities of the cyclone and swirl-vane separators are investigated by simulating the droplet-laden flows coupled with the droplet collision models. As for the cyclone, the simulation separation efficiencies are in good agreement with the experimental data. It is further shown that the separation efficiency curve begins to climb up very quickly in the region with small droplet diameters, and then keep constant as the droplet diameter increasing. As for the swirl-vane separators, the simulated separation efficiencies are little larger than those of the experiment within 5% difference, which is mainly due to the fact that coalescence dominates the collision behaviors from the simulation results. The theoretical and simulation results show that these collision models present a new view to study the flowing behaviors of the droplets in the moisture separators.