Study On Flow Field And Concentration Profile Around Bubbles In Non-newtonian Fluid

Study on momentum and mass transfer from bubbles is very important to many industrially processes, such as fixed and fluidized bed reactors. Indeed, based on analytical and/or numerical and experimental results, it is now possible to predict the process of mass transfer from a bubble to Newtonian fluids over wide ranges of conditions. However, much less is known about the flow and mass transfer in non-Newtonian fluids encountered extensively in chemical, food and polymer process engineering applications. Therefore, it is essential to study momentum and mass transfer around bubbles in non-Newtonian fluids.Concentration distributions on liquid side around fixed bubbles in water and non-Newtonian fluids (PAM aqueous solutions) were studied using a micro holographic interferometer. Concentration distribution and concentration near the interface on liquid side were determined when mass transfer reached stable in terms of the shift of the interference stripes recorded by an image acquisition system and processed by a digital image processing system, respectively. Experimental results show that as the mass concentration of PAM increases, the visco-elasticity of liquid increases, leading to the decrease of interfacial concentration, but the declining extent becomes mild gradually. Interfacial concentration in PAM aqueous solution is less than that in water. However, the reduction trend of interfacial concentration in PAM aqueous solution weakens with the increase of the liquid velocity due to the shear thinning effect of solution.The flow fields around fixed or moving bubbles in Newtonian fluids (glycerol solution) and non-Newtonian fluids (PAM or CMC aqueous solution) were measured by a particle image velocimetry (PIV). Experimental results show that in glycerin solution, viscosity restrains the formation of eddy in the vicinity of bubble, and component of the velocity in the y-direction decreases away from bubble in the tail-vortex area of the bubble. However, in PAM or CMC aqueous solution, there exist eddy structure in the vicinity of bubble, and there appears a region under low velocity in the tail-vortex area of the bubble.On the basis of Navier-Stockes equation and convection-diffusion equation, combining surface tension model and penetration model, the model of moment and mass transfer between bubble and non-Newtonian fluid was proposed. The formation of a single bubble and mass transfer for a fixed bubble were simulated by VOF method, respectively. The agreement between simulation results and experimental data confirmed the validity of the present model. Furthermore, the flow field and concentration distribution around bubbles in power-law fluid (n<1) were simulated. Area of concentration distribution around fixed bubble decreases with the increase of power-law index. During the rising process of a single ellipsoidal bubble, concentration distribution nephogram in the tail-vortex area of the bubble appears long-tail as deformation rate of bubble is less than 2, but it is fractal when deformation rate is bigger than 2. For in-line rising bubbles, within the interactional area, due to the influence of former bubble, the concentration boundary layer in the frontal part of latter bubble is elongated and connected together with the concentration distribution of former bubble. When the two bubbles are closing to each other, the eddy structure of former bubble disappears gradually, and the area of concentration distribution between two bubbles becomes broadened. The bifurcation of concentration distribution area in the rear of bubble occurs due to the deformation of bubble after coalescence. With the rise of bubble coalesced, the concentration and velocity distributions around the bubble become gradually similar to those around a single bubble rising in fluid without any disturbance from other bubbles.