Research On The Rising Motion Behavior And Wake Characteristics Of Bottom-blown Bubble

The bottom blowing gas disperses into bubbles of different shapes and sizes at the nozzle and rises in the liquid pool.The movement of bubbles carrying liquid promotes momentum transfer between gas and liquid,and also affects heat and mass transfer between gas and liquid.At the same time,when bubbles carry the liquid phase to move,they will generate vortices of varying strength and size,and the generation and dissipation of these vortices play a crucial role in strengthening the momentum transfer between gas and liquid.Therefore,conducting research on the motion behavior of bottom blown bubbles,revealing the deformation,velocity change,motion trajectory,and wake characteristics of the bubble rising process,is of great significance for optimizing injection parameters.This article first constructs a gas-liquid two-phase flow calculation model for the rising process of single bubble/coaxial double bubble,and verifies the calculation model.Then,numerical simulations were conducted to investigate the motion behavior and wake characteristics of a single bubble during its ascent process.The main conclusions are as follows:(1)In terms of velocity variation characteristics.The velocity gradually increases first and then decreases;The higher the liquid viscosity,the slower the change in velocity,and the larger the initial diameter of the bubble,the greater the change in bubble velocity.(2)In terms of bubble shape change characteristics.The bubble changes from a circular shape to an elliptical shape,and finally becomes a flat circular or cap shaped shape;The larger the liquid viscosity and the smaller the initial diameter of the bubble,the smaller the amplitude of the aspect ratio change.(3)The single bubble wake structure was reconstructed using the Ω vortex recognition method,and the results showed that the single bubble wake was symmetrical in a "lamellar" shape.As the liquid viscosity increased,the bubble wake and bottom small vortex structure decreased,and the separation time of the wake vortex core prolonged;The larger the initial diameter of the bubble,the larger the wake and small vortex structure at the bottom of the bubble,and the easier it is for vortex core separation to occur.To reveal the motion behavior and wake characteristics of continuous bubbles formed during the actual injection process under the mutual influence,the motion behavior and wake characteristics of coaxial double bubble rising process were studied.The main conclusions are as follows:(1)Coaxial double bubbles undergo three stages:independent rise of bubbles,accelerated approaching of double bubbles,and coalescence.(2)In terms of speed variation characteristics.The upward behavior of the upper bubble is relatively similar to that of a single bubble;In the initial stage of rising,the lower bubble first shows a single upward trend,and then accelerates to rise,causing the double bubbles to accelerate and approach,and the acceleration of the lower bubble increases exponentially.(3)In terms of shape characteristics.The upper bubble changes from a circular shape to a hat shape with a concave lower boundary,and the aspect ratio is always less than 1.The lower bubble first changes from a circular shape to an elliptical shape,gradually returning to a circular shape,and finally becoming a pearshaped shape.The minimum aspect ratio is 0.85 and the maximum is 1.2,with a large span of aspect ratio changes.(4)Research based on vortex recognition reconstruction shows that changes in surface curvature of bubbles affect the magnitude of surface vorticity,with the upper bubble surface vorticity being greater than the lower bubble.During the rise of the double bubble,the wake of the upper bubble gradually moves downwards.When the wake rotating centers of the upper and lower bubbles merge with each other and form a whole circulation flow,the instantaneous acceleration of the lower bubbles reaches the maximum,and the wake acceleration effect is the strongest.The larger the initial spacing between two bubbles and the smaller the initial diameter,the weaker the interaction between two bubbles.