Microbubble formation at a nozzle in liquid mercury

A liquid mercury target for MW class pulsed neutron sources is being developed in the Japan Atomic Energy Agency (JAEA). Cavitation will be induced by pressure waves that are caused by highly intense proton beam injection into the mercury target. Microbubbles 50 to 200 mu m in diameter injected into the mercury target are plausibly effective for mitigating cavitation. The mitigation is dependent on the conditions of the injected bubble size and population. It is, therefore, important to understand bubble formation behavior in mercury in order to develop a microbubble injection method. Computational fluid dynamics (CFD) simulations were carried out under various mercury and gas flow rates to investigate the bubble formation behavior in mercury. Moreover, bubbles in stagnant mercury were visualized with X-ray to observe the formation behavior of bubbles at a micro-gas nozzle and compared with the simulation results. It was found that high surface tension makes the bubble grow around the outer surface of the nozzle under the stagnant condition and makes it larger until its effect decreases in the flow. In addition, the bubble diameter under the stagnant condition increases with increasing contact angle.