Study On Law And Mechanism Of Bubble Formation In Gas Removal Technology Of Molten Salt Reactor

As one type of Generation?reactors,molten salt reactor has a very good future prospect.During the operation of the molten salt reactor,radioactive fission gases such as krypton and xenon will be produced by fission reaction.Krypton and xenon are so-called neutron poison due to their large neutron absorption cross section,the existence of which will influence the economics of the reactor.Therefore,the removal technology of fission gas is one of the technical bottlenecks in the development of molten salt reactor.Aiming at the special properties such as high viscosity,high density and high surface tension of molten salt,the technical scheme of bubbling-degassing technology for the removal of fission gas in molten salt reactor is proposed internationally.The basic principle of bubbling-degassing technology is introduced as follows:a venturi bubble generator is adopted to generate micro bubbles in the molten salt coolant,then the fission gas enters the bubble through mass transfer effect,and finally the fission gas-containing bubbles are removed from the coolant by gas-liquid separator.According to mass transfer theory,the smaller the bubble diameter,the higher the mass transfer efficiency,but the difficulty of separating small bubbles increases,so how to generate bubbles of appropriate size and how to control the bubble size becomes the key.To this end,the mechanism of bubble generation in venturi bubble generator is studied in this paper.From macroscopical view,the dependence of bubble size on fluid properties,flow parameters and geometry parameters is obtained by visualization experiment.From microscopic viewpoint,the detachment characteristics of venturi orifices was studied via theoretical modeling,experimental visualization and numerical calculation.Also,through visualization and particle image velocimetry PIV test,the critical criterion of bubble breakup is established.The specific content of this paper mainly includes the following aspects:?1?Study on the macroscopic performance of the bubble generator.Firstly,the bubble performance in terms of bubble Sauter diameter under different flow parameters and different fluid properties is investigated.The flow parameters are changed by the flow rate of the liquid?or the liquid phase Reynolds number?and the gas content,and by changing the concentration of the CaCl₂ solution,the fluid properties such as density,viscosity,surface tension,etc.are varied.It is found that the average Sauter diameter has a-1power dependence on the liquid phase Reynolds number,0.6 power dependence on the liquid surface tension,the-0.1 power dependence on the liquid viscosity,-0.5 power dependence on the liquid phase density and is proportional to the gas content.Secondly,the size and distribution of bubbles under different geometric parameters were studied experimentally.Three key parameters including the number of injection hole,the diameter of the injection hole and the divergent angle were explored.It is found that the average diameter of Sauter does not depend on the diameter and the number of the injection hole.The diffusion angle is the most critical parameter for controlling the final bubble size.The bubble size distribution is a classical log-normal distribution.Study on the flow mechanism of bubble generation?including the bubble shedding characteristics of the injection hole and bubble breakup in the divergent section of the venturi tube?.Firstly,by using the mechanical equilibrium equation,a theoretical simplified model to discribe bubble shedding is proposed.Secondly,the variation of bubble shedding mode,shedding volume and frequency with the gas flow rate,liquid flow rate,the concentration of CaCl2 solution are obtained by means of visualization.In the meantime,the evolution of bubble morphology during the process of inception,growth and detachment is analyzed.In addition,numerical simulation was used to reproduce the growth and shedding of the bubble.The results show that there are three typical modes of bubble shedding:single bubble mode,multi-pulse bubble mode and jet mode;the frequency of bubble shedding is generally in the range of 500-1000 Hz;the volume of bubble shedding mainly depends on the stay time when the bubble is attached to the injection hole and the gas flow rate;fluctuations in pressure and mass flow are important factors influencing bubble growth and bubble detachment.To discover the bubble breakup mechanism in the divergent section,PIV technique is used to obtain the information of the velocity field,the Reynolds stress and the turbulent kinetic energy.The visualization experiment by using the high-speed camera is also used to obtain the deformation of a single bubble.The crushing process is analyzed,and the fracture characteristics of single bubbles under different Reynolds numbers are analyzed.Finally,combining the bubble deformation evolution and the turbulence field,the criterion of bubble breakage is finally obtained,i.e.when the critical Weber number is greater than 7,the bubble is likely to rupture.In summary,focusing on the bubble generation method applied in the bubbling-degassing technology of molten salt reactor,this paper obtains the macroscopic relationship between the bubble size and the flow parameters,geometric parameters and the fluid properties,which provides a solid foundation for the design of the bubble generator.On the microscopic level,the mechanism of bubble shedding and breakup is revealed,and the criterion of bubble rupture is proposed,which provides an elaborated data support for numerical modeling of bubble breakup.