Implementation Of An Improved Bubble Coalescence And Breakup Model For TFM-PBM Simulations Of Gas-Liquid Flows In Bubble Columns

China is the largest consumer of coal in the world,and the excess utilization of coal has brought severe pollutant emissions such as SOx,NOx and the particulates.As a clean alternative energy,coal indirect liquefaction can make full use of the heat and chemical energy,and reduce the air pollutions caused by direct coal combustion.The Fischer-Tropsch(F-T)synthesis is a key step of coal indirect liquefaction technique.Bubble column reactors are used in the F-T process due to numerous advantages of simple structures,easy operation,and good mass and heat transfer characteristics.A systematic study of the multiphase hydrodynamics of bubble column is of great importance for an optimal design and operation of bubble column reactors.In this thesis,the bubble column that is applied in indirect coal liquefaction is considered;an improved bubble coalescence and breakup model is proposed and incorporated with the gas-liquid Euler-Euler Two Fluid simulation.The computation model is validated against literature experimental data.Major contributions are outlined as follows.Firstly,an extensive literature review was conducted on bubble column hydrodynamics in the perspectives of experimental and numerical studies.The review of the experimental work was focused on flow pattern description,local parameters measurement of the bubble columns,and that of the numerical simulations was mainly on two fluid model(TFM)and its related sub-models(interphase forces,turbulence and bubble size).The population balance model(PBM)with vivid bubbles size distributions was identified as a key sub-model for the TFM.Secondly,the population balance equation and the discretization were described,and the bubble coalescence and breakup and its reallocation mechanisms were elaborated.Three classical mechanisms were chosen as a typical model for the reference of comparison.An improved bubble coalescence and breakup rate model was proposed.The bubble coalescence was modeled by considering bubble collisions induced by turbulent fluctuations,buoyancy driven,wake entrainment,and viscous shear,and the liquid film drainage model was used for the description of the coalescence efficiency of collisions.The bubble breakup was analyzed in terms of bubble interactions with turbulent eddies which coupled the restriction of surface energy with the capillary pressure.Subsequently,to validate the improved bubble rate model,the bubbles number density and bubbles volume density were chosen as the parameters in the comparison with the three typical bubble coalescence and breakup rate models.The improved model showed superior performance and agreed with the experiment data better than those of other bubble rate models.Furthermore,a generic TFM-PBM simulation scheme was developed in combination of the improved bubble coalescence and breakup models,and applied in the simulations of bubble columns operated in different flow regimes.The simulation results were validated against the literature experimental data,including the gas holdup,liquid axial velocity,bubble size distributions.Finally,the impacts of vertical internals on buoyancy-driven gas-liquid hydrodynamic of bubble columns were simulated using TFM-PBM coupled model.The numerical results showed the radial wall lubrication force exerted on bubbles greatly impacts the radial distribution of time-averaged gas holdup,especially in the internals affecting region.The predicted results agree with experimental data,confirming that the present model was suitable for the descriptions of gas-liquid hydrodynamics of the bubble column with internals.Significant decreases in turbulent kinetic energy were predicted,suggesting that turbulence was obviously affected by the presence of walls,but the turbulent dissipation rates enhanced significantly in the gaps between the internal walls.The mean bubble sizes decreased and the gas holdup increased,confirming that bubbles motions were largely restricted by internals.The internals positions influenced the liquid circulations,particularly in churn-turbulent flow regime,the large scale liquid circulations were always present in the column regardless of inserting amount internals.