Study On The Interaction Mechanism Between Liquid Micro-Element And Wall Under Different Field Forces By CFD Simulation

The phenomenon of droplets impacting the solid wall is widely found in common chemical equipment such as stirred tanks,packed towers,and higge reactors.The flow pattern and dispersion degree of the liquid in the reactor will significantly affect mass and heat transfer.The interaction law between the packing walls can provide theoretical basis and technical support for the structural optimization of the higge device.In this paper,the VOF method and dynamic contact angle model are used to construct 2D axisymmetric and 3D CFD models of droplets impacting solid flat walls.Then,the spreading and return of Newtonian droplets impacting moving walls,spherical walls and non-Newtonian fluids impacting flat walls are numerically simulated.Finally,a CFD model of a small 2D representative basic unit in a rotating packed bed(RPB)was established to study the hydrodynamic characteristics of the packing layer.The main findings are as follows:(1)The variation of the spreading factor of the droplet impacting the solid flat wall is within±10%of the experimental result,which verifies the rationality of the model and can accurately represent the change of the dynamic contact angleθ_din the process of impacting the wall.Reducing the contact angle,surface tension and liquid viscosity can reduce the energy consumption of the droplet spreading process;while increasing the initial velocity of the droplet can significantly increase the initial kinetic energy,thereby increasing the gas-liquid interface area.By analyzing the energy conversion of the droplet hitting the solid surface,the correlation formula of the maximum gas-liquid interface area of the droplet hitting the flat wall is obtained.Compared with the CFD simulation results,the error of the calculated value is within±15%.(2)The simulated values of spreading factors of Newtonian droplets impacting moving wall,spherical wall and non-Newtonian fluid impacting flat wall are within±15%of the experimental results.The droplet hits the moving wall,and the liquid near the wall will be accelerated to move with the wall.A smaller contact angle can make the liquid stay on the wall for a longer time,thus increasing the gas-liquid interface area.Under the action of gravity,the spherical wall is conducive to the spreading of droplets on the spherical surface.When the ratio of spherical wall to droplet diameterβ_r<10,the smallerβ_ris,the more significant the promotion effect is.The spreading factor and gas-liquid interface area of non-Newtonian fluid droplets hitting a flat wall are mainly affected by their infinite shear viscosity.(3)The simulation results of the 2D representative basic unit in the RPB show that the error between the simulated value of the liquid holdup and the calculated value of the empirical correlation formula obtained through experiments is within 20%;increasing the contact angle,the liquid holdupε_Land the effective interface area A_edecreases significantly;increasing the rotational speed will reduce the liquid holdup and increase the liquid specific surface area A_s,but if the rotational speed is too high,the effective interface area will be reduced;increasing the liquid volume or viscosity will increase the liquid holdup,but the increase in the effective interfacial area will be limited.Based on the CFD simulation results,the correlation formula of the RPB hydrodynamic characteristic parameters is obtained,and the error between the calculated value and the CFD simulation value is within±20%.At high rotational speed,the shear rate in REU is large,and the apparent viscosity of the CMC solution is close to its corresponding infinite shear viscosity,which makes it easier to disperse into fine droplets.