Rate of droplet deposition in steam–water annular flow and effect of a flow obstacle

Deposition rate of droplets in steam–water annular two-phase flow was measured using a 5 mm diameter vertical round tube as a test section. In the experimental conditions tested in this work, the droplet mass transfer coefficient decreased with an increase in the droplet concentration in the gas core flow and with an increase in the length of a deposition section. The dependence on these two parameters agreed fairly well with predictions by available correlations. Placing a small cylindrical tube concentrically in the test section round tube, the effect of a flow obstacle on the deposition rate of droplets was also experimentally investigated. It was found that the obstacle effect was significant and the deposition rate of droplets increased approximately three times in average. The obstacle effect measured in this work was compared with an empirical correlation and a simple mechanistic model that were developed using experimental results of air–water annular flows. Fairly good agreement was achieved in both cases, which would indicate that the mechanism of deposition enhancement induced by the flow obstacle is similar between air–water and steam–water flows.