Study on gravity separation of air-water from the upper part of UTSG full-scale swirl separator in pressurized water reactor

•The effect of gravity heights on performance are studied by air-water experiment.•The calculation of the separation height is proposed for varied flow patterns.•Separation height for big droplet is only related with initial droplet velocity. The steam separator is one of the most critical components in the U-tube steam generator (UTSG), which qualifies moisture carryover less than 0.1% to minimize the damage to turbines. Take both high separation efficiency and low manufacture cost into account, the height of the gravity separation space should be designed appropriately. Air-water separation experiments are carried out to study the effect of several different gravity heights on the separation performance. A droplet mathematical model is proposed and the experimental results are discussed based on it. The analysis indicated that (1) For Stokes flow, weather initial droplet velocity ud0 is bigger or smaller than vapor velocity ug, the analytical solution of the maximum height h for the droplet to reach can be calculated by the same formula. (2) From the numerical solutions of droplet mechanics model, it is found that separation height is related with all three factors (ug,ud0,D) for the small droplet, but is only related with ud for the big droplet. (3) Experimental results show that there exists a minimum gravity separation height to obtain the maximum separation efficiency under the given gas–water conditions. (4) Initial droplet velocity is the key parameter required for the engineering design of separation space height.