Research On Flow Structure In Bubble Column Based On Compromise Of Gas-dominated And Liquid-dominated Mechanisms

As a typical gas-liquid device,bubble column reactor has a wide range of applications in chemical,environmental protection,biology,energy and other fields.As the typical mesoscale structure of bubble column reactor,large and small bubbles have very important research value.In this dissertation,the mesoscale structure of large and small bubbles in bubble column reactor is studied through three methods:experimental,modeling and simulation.In experimental study,we use dynamic gas disengagement（DGD）and dual conductivity probe（DCP）methods to demarcate large and small bubbles.While the number-based BSD only presents a single peak,the dual peaks can only be found from the volume-based BSD.The demarcation method based on the holdup ratio of the two bubble groups proves more reasonable to reflect the intrinsic nature of bubble structure.A sharp change of gas holdup is detected for the uniform distributor.The difference in gas holdup structure of coarse and uniform distributors is contributed mainly by small-bubbles at homogeneous regime,yet by large-bubbles at transition and heterogeneous regimes.However,the coexistence of small and large-bubbles is the intrinsic nature,and the effect of gas distributor is marginal in heterogeneous regime.In modeling study,we analyze and propose the dominant mechanisms of gas-dominated and liquid-dominated of gas-liquid EMMS model,namely f=min and N_surf=min.Based on this,the stability condition of the gas-liquid EMMS model is modified as N_surf×f=min.Compared with the original gas-liquid EMMS model,the change of gas holdup with superficial gas velocity in transition regime is from abrupt to gradual.The modified gas-liquid EMMS model is used to analyze the influence of superficial liquid velocity on gas holdup.The results show that the modified gas-liquid EMMS model can predict the influence of superficial liquid velocity on flow regime transition.In addition,by introducing compromising factor p,the stability condition is modified as N_surf×f ^p=min.With changing value of p,the modified gas-liquid EMMS model can predict both abrupt and gradual changes of gas holdup,which means that the principle compromise in competition（CIC）between different dominant mechanisms is the key to predict flow regime transition.In addition,we extends the gas-liquid EMMS model to a radial model based on EMMS principle,which is further compared with the two-fluid model.The computing load and cost of gas-liquid EMMS radial model are smaller than that of two-fluid simulation.The radial gas holdup and liquid velocity calculated by this model are more consistent with experimental results.In simulation study,based on the gas-liquid EMMS gradual model,a new gas-liquid drag model is proposed.By using Open FOAM,both gas-liquid EMMS gradual drag model and Schiller-Naumann drag model are coupled with two-fluid model to simulate bubble columns.When only drag force is considered,radial liquid velocity distribution simulation results show that gas-liquid EMMS gradual drag model is better than that of Schiller-Naumann drag model.While for radial gas holdup distribution,simulation results of the two drag models are almost equivalent.