Study On Natural Convection Heat Transfer Of Liquid Lead-bismuth Eutectic Enhanced By Gas Lift

Lead-cooled fast reactors have become one of the promising types of Generation IV nuclear reactors,mainly because of the chemical inertness of the lead alloy coolant to air and water;The thermodynamic properties such as low vapour pressure and high boiling point in the relevant temperature range;The high atomic number and good neutronological properties in the microscopic cross-section of high scattering and small absorption,which make lead-cooled fast reactors inherently safe,proliferation-resistant,sustainable,radioactive waste management and economical.Liquid lead-bismuth eutectic have received a lot of attention as the main coolant for lead-cooled fast reactors,and the natural circulation capability can be significantly improved by using gas lift technology to inject inert gas into the rising tube of the natural circulation circuit of liquid lead-bismuth eutectic.This paper validates the mathematical model applicable to the natural circulation circuit of liquid lead-bismuth eutectic and the natural circulation circuit of gas-lifted reinforced liquid lead-bismuth eutectic based on the model of gas-lifted reinforced liquid lead-bismuth eutectic natural circulation circuit by comparing the calculated values of the numerical simulation with the experimental data;The transient numerical simulations of the initial stage of gas injection and the stop stage of gas injection in the natural circulation circuit of gas-lifted reinforced liquid lead-bismuth eutectic were used to analyse the spatial and temporal evolution of the flow heat transfer characteristics and the two-phase distribution characteristics in the initial stage and the stop stage of the gas lift circuit;By changing the working conditions of gas injection volume and heat source power,the changes of the flow heat transfer characteristics,two-phase distribution characteristics and gas lift efficiency in the circuit under different working conditions are compared and analysed.The main research contents and conclusions are as follows:(1)The accuracy of the numerical simulation method for the natural circulation loop of liquid lead-bismuth eutectic and the natural circulation loop of gas-lifted reinforced liquid lead-bismuth eutectic was verified,and the turbulence model and turbulent Prandtl number model with good applicability were obtained.By using different turbulence models and turbulent Prandtl number models for numerical simulations,it is concluded that the deviation between the calculated value and the experimental value is the smallest by using SST k-ω turbulence model and Cheng turbulent Prandtl number model.(2)The flow and heat transfer characteristics and two-phase distribution characteristics in the natural circulation loop of liquid lead-bismuth eutectic enhanced by gas lift during the two transient processes of gas injection initial stage and stop stage were investigated.The flow characteristics of the liquid lead-bismuth eutectic in the gas lift circuit were analysed in terms of mass flow rate and axial velocity in the riser and descender tubes during the initial stage and the stop stage;The volume fraction and phase distribution contours in the riser and descender tubes were analysed in terms of two-phase distribution characteristics;The heat transfer characteristics of heat source and cold source inlet temperature,outlet temperature and inlet and outlet temperature difference were analyzed.(3)The effects of different gas injection volume and heat source power on the flow and heat transfer characteristics,two-phase distribution characteristics and gas lift efficiency of gas lift enhanced natural circulation of liquid lead-bismuth eutectic were investigated.Increasing gas injection rate and heat source power can enhance the natural circulation ability of liquid lead-bismuth eutectic,but it does not change linearly with the change of working condition parameters.When changing the gas injection and heat source power conditions,the gas-liquid two-phase distribution characteristics are changed,and the air bubbles wrapped by the liquid lead-bismuth eutectic into the descending tube have an effect on the flow characteristics of the liquid lead-bismuth eutectic,and the air bubbles in the heat source and cold source sections have an effect on the heat transfer characteristics of the liquid lead-bismuth eutectic.Finally,the lifting efficiency formula of gas lift pump is introduced to evaluate the lifting efficiency of gas lift enhanced liquid lead-bismuth eutectic natural circulation loop under different working conditions.Based on the numerical simulation method,this paper explores the flow heat transfer law and two-phase motion distribution law in the natural circulation loop of gas lift enhanced liquid lead-bismuth eutectic,which provides a theoretical basis and reference for the numerical simulation of natural circulation loop of gas lift enhanced liquid lead-bismuth eutectic,and has certain practical value for the design of lead-cooled fast reactor cooling system.