| Coal gasification is a key technology for high-efficiency and environmental friendly utilization of coal resources.The entrained-flow gasifier with quench type has been most commonly used in industry in China.The scrubbing-cooling chamber is an important part of a coal gasifier to wash and cool the generated high-temperature syngas.Research on the complex multiphase flow in scrubbing-cooling chambers is of practical significance for scrubbing and separation of syngas.In this paper,computational fluid dynamics(CFD)was used to study the gas-liquid flow in the bubbling area of scrubbing-cooling chambers.Different closure models were used to verify the influence on the simulated results and numerical simulation was used to optimize the structure of the scrubbing-cooling chamber.Chapter 1 summarizes the current problems of the scrubbing-cooling chamber and the experimental theoretical study results of each component in the scrubbing-cooling chamber.And the CFD methods and models used in this paper were introduced.In Chapter 2,numerical simulation was implemented to study the effects of different models of drag,lift and turbulence on gas holdup,axial liquid velocity and liquid streamline in scrubbing-cooling chambers.We found that axial gas holdup distribution calculated by the Schiller-Naumann drag model and the DBS drag model fit well with the experiments.When the lift coefficient is greater than 0,the gas radial diffusion was significantly enhanced,resulting in a lower predicted value of gas holdup than the experiment.But the difference of turbulence models has little influence on the simulation.This effect is much smaller than the effect of different drag models and lift models.In Chapter 3,five different structures of the scrubbing-cooling chambers were compared about the gas-liquid flow characteristics by using the specific drag and turbulence model.We found that the gas rises along the wall of central pipe in the scrubbing-cooling chamber with no internals,and there is almost no radial diffusion,leading to gas accumulation,which lowers device space utilization and interfacial area fluctuations.Four branch pipes divert a portion of gas to the central region of the chambers,which may otherwise be entrained along the wall of central pipe.The branch pipes promote radial dispersion and is helpful to improve the space utilization and maintain stable operation.However,further increase in the number of branch pipes makes the gas all flow out of the branch pipes.The gas holdup near the wall of central pipe is almost zero,which reduces the global gas holdup under the liquid surface and the space utilization rate of the equipment.Therefore,it is necessary to select a suitable number of branch pipes.Further installation of a draft tube enhances the amount of gas transported by the branch pipes,and increases the global gas holdup under the liquid surface of the scrubber,which can effectively intensify gas-liquid circulation and interfacial contact.The reduction of the diameter of draft tube reduces the proportion of the rising zone,thus promoting gas accumulation and changing the axial height of gas-liquid circulation.Based on the simulation of gas-liquid flow in scrubbing-cooling chambers,the influence of interfacial forces and turbulence models on the hydrodynamics was discussed first,and we found that Schiller-Naumann and DBS drag model can accurately predict the gas holdup distribution.Then,a specific drag model was used to simulate scrubbers of different structures.Simulated results indicate that the added branch pipes divert a portion of gas to the central region of the chambers and promote the radial dispersion.Further installation of a draft tube enhances the amount of gas transported by the branch pipes,and increases the global gas holdup under the liquid surface of the scrubber.It can effectively intensify gas-liquid circulation and washing efficiency. |
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