Experimental Study On Flow Characteristics Of Non-newtonian Fluid And Single Bubble Mass Transfer In Chamber

Gas-liquid mass transfer in non-Newtonian fluid is common in industrial process.For example,in the biological stirred tank and bubble column,the liquid phase is often a non-Newtonian fluid,and the gas phase exists in the form of bubble flow.The previous studies on the mass transfer of multi-bubbles were mainly through the measurement of gas holdup,surficial gas velocity and liquid flow filed and investigated the influence of these factors on volume mass transfer coefficient kLa.However,many factors,such as bubble moving characteristics,liquid flow field and rheological properties have effects on volume mass transfer coefficient by influencing the mass transfer coefficient kL and specific surface area a.Therefore,to further understand gas-liquid mass transfer mechanism,it is significant to study the effects of bubble moving characteristics,liquid flow field and rheological characteristics on kL.In this work,the mass transfer experiment of single bubble in chamber was conducted.In the experiment,the gas phase was 99.995%oxygen,and the liquid phase was sodium polyacrylate NaPA solution(a shear thinning fluid)containing sodium sulfite Na2SO3.PIV(Particle Image Velocimetry)technology was used to obtain the flow information of liquid flow field in the chamber.The velocity fields,the distributions of turbulent kinetic energy and its dissipation rate,shear rate and apparent viscosity of the liquid in the chamber under different operating conditions were compared.It was found that the transfer ability of liquid velocity decreased with the increase of solution concentration.The high-speed camera technology was used to photograph the bubbles in the chamber,and the liquid-side mass transfer coefficient kL of single bubble was obtained by the rate of change of bubble volume with time.Effects of NaPA concentration,liquid flow rate and bubble diameter on bubble motion characteristics and liquid-side mass transfer coefficient kL were discussed.The mass transfer coefficient of bubbles was in the order of 10-5-10-4,and the mass transfer coefficient was not affected by the concentration of solution.However,the mass transfer coefficient decreased slightly with the increase of liquid flow rate and increased slightly with the increase of bubble diameter.The bubble slip velocity was obtained by the bubble velocity and liquid flow velocity.A dimensionless number I,the ratio of the turbulent kinetic energy of the liquid phase to the square of the velocity,was defined to characterize the turbulent intensity of the liquid phase,and the Sh mass transfer correlation about the bubble Reynolds number Reb and the turbulent intensity I was established.The mass transfer correlation was used to predict the change of bubble volume with time under different conditions,and it could accurately predict the change of the volume of bubble with diameter of 2-3 mm with time.Comparing the measured Sh with the predicted values of Higbie model and Fr?ssling model,we found that the measured Sh were between the two models and close to the predicted values of Frossling model.With the increase of Reb and I,the experimental values deviated from the predicted values of Fr?ssling model.