Numerical Simulation Studies On Bubbles Rising Behaviors In Liquid Metal

The accelerator driven subcritical system(ADS)device with advanced design concept uses a bubble lift pump instead of a traditional mechanical pump for a primary cooling cycle to enhance the natural circulation of lead-bismuth eutectic alloy(LBE)by injecting inert gas bubbles into LBE.It is common to find that circular bubbles and elastic bubbles(also known as Taylor bubbles)in LBE.The two-phase flow characteristics of bubbles and LBE will greatly affect the cooling cycle capability and safety of the system.Therefore,it is significant conduct deep studies on bubbles rising behaviors in LBE.First,the numerical simulation of a single nitrogen bubble in liquid lead-bismuth alloy(LBE)from static to the full development of dynamic behavior based on diffuse-interface method was done to get the relationship of rising velocity and deformation of the bubble by time.Comparing results of simulation of the flow characteristics about inert gas and liquid LBE with Grace's graphical correlation to verify the feasibility and the accuracy of diffuse-interface method on simulation of rising bubble's behaviors in liquid LBE.By comparing rising behaviors of five different initial diameters of bubble in liquid LBE based on diffuse-interface method,we found that the smaller bubble initial diameter was,the disturbance would be more intense in the rising process and splitting phenomenon occured more easily in the rising process when the bubble initial diameter was larger.The simulation results were analyzed to explain the rising behaviors of Taylor bubble in LBE.Then,numerical studies on double bubble rising behaviors in LBE were carried out based on the diffuse-interface method.The interaction of of two bubbles arranged coaxial vertically and with different initial diameters and different initial distances was studied.The deformation of bubbles during the rising process,the bubbles coalescence,the velocity field around the coalesced bubbles,the fracture of the trailing bubble and the velocity field around the traling bubble when fracture happened were analyzed.Both different initial diameters and different initial distances have great influence on the rising behaviors of double bubbles.Finally,CFD studies were carried out on a single Taylor bubble rising behaviors in liquid LBE in a vertical tube by using diffuse-interface method.The velocity obtained by numerical simulation was compared with the velocity model and the calculation results of the falling liquid film were compared with the experimental datas,which further verified the feasibility and accuracy of diffuse-interface method in capturing the macroscopic interface motion.Deformation of the bubble and velocity changed during the bubble rising process with different initial liquid velocities by numerical simulation.Further investigations into simulative results were conducted,finding that shape of the bubble tail changed a lot,sub-bubbles would be split out at both ends of the tail,vortexes were produced in the wake region and flow in the wake region was strongly disturbed.Influences of the rising bubble on the wake region extended as far as to 80 mm.