特征时间对剪切稀化流体气泡上浮特性的影响

Effects of characteristic time on bubble dynamic in shear thinning fluids

非牛顿气液两相流广泛存在于工农业生产中,气泡和液相间的接触非常复杂,对相间的传递效率具有重要的影响。为了理解气泡在非牛顿流体中的运动特性,基于连续表面张力模型与Carreau本构模型,本文运用volume of fluid (VOF)法研究了单气泡在剪切稀化流体内自由上浮的运动特性。研究发现:气泡的动力学特性与液相的特征时间λ密切相关,液相的剪切稀化程度越强(流变指数n越小)或表面张力越小(Eo数越大)时,特征时间λ对气泡变形和尾涡的影响越大。在给定的剪切稀化程度和表面张力下,λ越大,气泡终端速度越大,其尾涡强度和尺度也越大,导致气泡周围液相高剪切速率区和低表观黏度区的范围越宽。此外,当液相的表面张力较小时,在气泡尾部出现了黏度盲区;并且随着λ的增大,黏度盲区逐渐脱离气泡尾部并破碎;黏度盲区的出现减小了气泡周围液相低表观黏度区的面积,增大了气泡上浮过程的摩擦阻力,降低了气泡的终端速度。

Non-Newtonian gas-liquid two-phase flows exist widely in industrial and agricultural processes.The interaction between bubble and liquid phases is very complicated,which has the important influence on the transfer efficiency between phases.To understand hydrodynamic properties of bubbles in shear-thinning fluids,based on the continuous surface tension model and the Carreau constitutive model,the volume of fluid(VOF)method was used to numerically investigate the single bubble dynamics in the shear-thinning fluid.The present results showed that,the hydrodynamic characteristics of bubbles are closely related to the characteristic timeλof the liquid phase.The stronger the shear thinning degree of the liquid phase(the smaller rheological index n)or the smaller the surface tension(the larger Eötvös number Eo)is,the more obvious the effect ofλon bubble deformation and wake vortexes is.Under the same shear-thinning degree and the surface tension,the largerλis,the higher the terminal velocity of the bubble is,and the size and strength of the wake vortex is,resulting in the wider ranges of the high shear rate region and the low apparent viscosity region around the bubble.In addition,for different degrees of shear thinning,at the conditions of low surface tension,there is a viscosity blind region at the tail of the bubble,and with the increase of theλ,the viscosity blind region gradually breaks away from the bubble and breaks up;the viscosity blind region reduces the region of the low apparent viscosity around the bubble,increases the friction drag during the floating process of the bubble,and also reduces the bubble terminal velocity.