Performance Of A Bubble Generator And The Principle Of Bubble Breakup Involved

Molten salt reactor is one of the candidates of the Generation IV advanced nuclear reactors with closed fuel cycle.It has an advantage in preventing the nuclear proliferation.To ensure that the reactor can safely run continuously,fission gases such as Xenon etc.,has to be removed from the liquid fuel on line.As one of the key devices in the gas removal system,the bubble generator is for generating tiny bubbles with certain size and amount continuously.Its performance has great influence on the safety and economy of the reactor.As a result,it would be helpful for developing the gas removal system for MSR to study the performance of the bubble generator as well as the mechanism of bubble breakup involved.Based on the experiments in a water test loop simulating the gas removal system,the distributions of velocity,pressure and turbulent kinetic energy along the flow direction of the venturi bubble generator were analyzed with the software FLUENT.In the simulation,the multiphase model(Mixture),standard k-? turbulent model and the SIMPLEC method were chosen.The velocity distribution along the flow direction showed that the air was entrained in the water flow near the wall of the generator.The peak of the pressure gradient in the diverging portion of the bubble generator indicated that the quick recovery of pressure might accelerate the bubble breakup.The turbulent kinetic energy is large at the entrance of the diverging portion,suggesting that a strong energy exchange therein which is an another important factor to break up the bubbles.In addition,the motions and breakup processes of bubbles in the bubble generator were analyzed with the high-speed video data.In the simulation,a rigid ball was placed in the position where the bubble breakup took place intensively for simulating the bubble therein with the Software FLUENT.The shear force on the ball,the velocity and pressure fields around the ball were obtained by numerical calculation.The numerical result showed that the shear force acting on the ball was much larger than the surface tension,and increased with the water flow rate.Therefore,the mechanism of the bubble breakup could be interpreted as follows: the sudden deceleration of a bubble in the diverging portion of the bubble generator created large velocity gradient in the vicinity of the bubble surface,consequently greater shear force resulted,exceeding the surface tension of the bubble and tearing up the bubble quickly.After that,further fragmentations took place due to great turbulence in the diverging portion of the bubble generator.Based on the analysis of the bubble breakup mechanism,the structural parameters of the bubble generator were optimized with the CFD.The final result can provide design reference for the bubble generator in molten salt reactor.