Study Of The Structure And Performance Of Bubble Generator In Gas Removal System For MSR

During the operation of a molten salt reactor, the continuous removal of gaseous fission product xenon-135 from the circulating fuel has to be provided. As one of the key devices in the off-gas removal system, the bubble generator is for generating tiny bubbles in certain size and amount continuously. Firstly, certain amount of helium bubbles are added to the fuel, the bubbles absorb xenon-135 and krypton-88, then the bubbles containing poisonous gases are stripped out of the fuel for removing xenon-135 and krypton-85 in the liquid fuel. In the present study, a new type of bubble generator was designed for TMSR gas removal system referenced to the design of ORNL. A water test loop was fabricated to study the performance of experimental samples. The working principle, characteristics and key parameters of the venturi type bubble generator were also obtained.The bubble generator was designed based on a venturi tube structure. The generation mechanism of bubbles is made from turbulence and strong shear stress at the diffuser part.With the help of experiment results, the size of throat and air injected holes, along with the number of air holes, were improved to meet the perfect design requirements. Because the venturi tube is made of plexiglass, the movement and breakup of the bubbles were recorded by a high speed camera. Also the bubble distribution was studied, from which some parameters, such as slip ratio, void fraction and bubble diameter were obtained. The effect of gas and liquid flow rates on generated bubble diameter was also analyzed. The results show that the average bubble diameter is in the range of 0.5-0.6mm and the bubbles are uniformly dispersed in the main stream when the void fraction covers the range of 0.1%?0.2%,satisfying the requirement for a molten salt reactor.The CFD software FLUENT was used to simulate and analysis the flow field distribution in the venturi-type bubble generator under two assumptions. With the help of these simulated results, the mechanism of bubble breakup in the venturi-type bubble generator was studied.Because the void fraction in the bubble generator is very small, as a result, the gas phase has a very slight influence on the fluid flow, so it is feasible to study the mechanism of bubble breakup by calculating the velocity and pressure distribution of the turbulent flow in the generator without considering the effect of the gas flow. Then, in order to study the forces acting on the bubble in the generator, a rigid ball is placed in the position where the bubble breakup takes place intensively,to simulate pressure and velocity field around the bubble therein. The numerical results show that the pressure difference between the top and bottom of the ball is high, which could significantly slow down bubble. Finally, the combined action of pressure difference and unbalanced force in the bubble surface induce the bubble deformation and breakup near the entrance of diverging area.