Experimental Study And Simulation On Mass Transfer Of Single Bubbles In A Turbulent Flow Chamber

Gas-liquid multi-phase flow is widely applied in chemical processes such as distillation,fermentation,waste water treatment and bubble column reactors.The bubbly flow is often present in the enhancement process of heat transfer and mass transfer.The bubble dispersion and bubble velocity are directly related to the performance of the reactors.The previous studies on gas-liquid mass transfer were carried out in the bubbly flow reactors.The researches were focused on the relationships between the volumetric mass transfer coefficient kLa and the flow field characteristics,gas superficial velocity and gas holdup.However,many physical and mechanical factors have impacts on the liquid-phase ruass transfer coefficient kL and the specific surface area a.In order to explain the gas-liquid mass transfer mechanism more clearly,it is especially important to explore the effects of flow field characteristics and bubble velocity on liquid-side mass transfer coefficient kL.In this paper,the mass transfer coefficient between oxygen and sodium sulfite solution was carried out in a turbulent chamber.The bubble motion was recorded by a high-speed camera.The mass transfer coefficient of oxygen in the turbulent chamber was calculated by the shrinkage of the bubble volume with time.The effects of temperature and flow rate on the mass transfer coefficient were elaborated.We found that the bubble mass transfer coefficient is of the order of 10-4 m s-1 in turbulent flow.The mass transfer coefficient increases with the temperature and flow rate.The enhancement factor caused by the catalyzed reaction is between 2.0 to 3.5,and it changes with the liquid Reynolds number in the chamber.The liquid flow field and the gas-liquid flow field in the chamber were measured by 2D particle image velocimetry(PIV).We found that the single bubble hindered the liquid flow.When the bubble velocity and liquid velocity were obtained,the bubble slip velocity can be calculated.The results show that the bubble slip velocity plays a vital role in the mass transfer,while the influence of bubble size is relatively weak.By comparing the experimental values with the predictions of the classical models,we found that the mass transfer coefficients were between the predicted values of the Frossling model and the Higbie model.With the increase of bubble Reynolds number,the mass transfer coefficients were closer to the predicted values by Higbie model and Clift model.The bubble Reynolds number and liquid Reynolds number both contributes to the mass transfer coefficient,the bubble Reynolds number affects the mass transfer coefficient of the bubble more than the liquid Reynolds number does.In order to obtain the 3D flow field in the chamber,Fluent was used to simulate the single-phase flow field and gas-liquid multi-phase flow in chamber.The predictions of three different turbulence models were compared and we found that the fluid velocity,root mean square velocity and turbulent kinetic energy predicted by large eddy simulation(LES)were consistent with the experimental values,indicating that LES could predict the turbulent vortex flow field in the chamber.In the multi-phase simulation,the LES and VOF models were used to predict the bubble motion characteristics in the chamber,including the bubble velocity and bubble slip velocity.We found that the models could simulate the single bubble motion in the turbulent flow field accurately,including the bubble trajectories,bubble velocity and bubble slip velocity.The bubble mass transfer coefficients were predicted by the different mass transfer models.We found that the coefficient predicted by the semi-empirical model was close to the experimental value.