Experimental Investigation Of Near Wall Effect In Bubble Column And Validation Of Multiphase Models In Simulation

Bubble column reactor are widely used in chemical,petrochemical,biochemical and metallurgy industry,due to numerous advantages,such as,higher durability of catalyst,excellent heat-and mass-transfer characteristics,lack of moving parts,ease of operation and maintenance.Though geometry of bubble column reactor is simple,the local flow,turbulence and gas-water distribution interact in a very complex way.A side wall injected bubble column reactor was studied to enhance the understanding of the flow characteristics in bubble column.Visible experiment was conducted to analyze near wall effect,bubble shape and size and the height of free surface.Simulations was carried out to validate the near wall effect and multiphase model with the help of self-programming code.The main research contents and results are as follows:1)Mesh independence check for single phase simulation with polyhedral mesh in bubble column reactor.Comparing with the other meshing method,polyhedral mesh has more neighbors to achieve the velocity gradient with higher accuracy.Steady and unsteady simulation was conducted for water and air with STAR CCM+.Simulation result show that,the minimum grid needed to single phase simulation was 0.5 million.From the simulation results,one can conclude that,both root mean squared residuals and normalized residuals are capable to be convergence standard.2)Study on the flow characteristics in side wall injected bubble column by visible experiment.Experiment show that,the rising bubbles was close to wall at the air inlet of bubble column,the near wall effect last 6cm.Bubble shape and size was recorded by high speed camera.The height of liquid free surface(0.84m)was measured.3)Validate multiphase model by visible experiment.Multiphase simulation was conducted with polyhedral mesh,trimmed mesh and structured mesh.Simulation results show that,polyhedral mesh was enough to present the height of free surface and was not enough to present near wall effect and bubble shape.Near wall effect was discovered in the simulation with trimmed mesh and structured mesh,but the distance that near wall effect last was much longer than the experiment.Also,the simulation with trimmed mesh and structured mesh was not enough to present the bubble shape.Adaptive mesh refinement was employed in the simulation.Adaptive mesh refinement was generated based on the trimmed mesh and self programming code.The mesh around the bubbles was refined with the evolution of the flow.Comparison with the experiment show that,Adaptive mesh refinement was enough to capture the bubble shape and near wall effect.But as the flow evolve,the overflow of ligquid was occurred,the free surface of liquid was not presented.This was due to solution discontinuities across the free-surface interface(parasitic currents).Interface momentum dissipation model was added in the simulation with adaptive mesh refinement.Overflow was avoided and free surface of liquid was acquired,the height of free surface was 0.833 m.But the near wall effect last longer distance,this was due to the exchange of momentum between water and air was impaired by the interface momentum dissipation.The simulation also show that,The VOF model,which based on the Eulerian model,was enough to present the bubble shape and near wall effect with adaptive mesh refinement and interface momentum dissipation.The experiment and simulation result of side wall injected bubble column show that: adaptive mesh refinement and interface momentum dissipation model enable to present the near wall effect,bubble shape and liquid free and provide a new idea for further research and industrial application.