关键词: 表面活性剂, 流体动力学, 悬浮系, 非牛顿流体, 剪切稀化, 颗粒沉降, 模拟

Abstract:

The suspended particle flow of viscoelastic surfactant solution widely exists in nature and industrial production. The nonlinear rheological properties and stress relaxation effect of viscoelastic surfactant solution have a significant impact on the particle settlement. The FENE-P and Giesekus constitutive models were used to study the sedimentation characteristics of particles in surfactant solutions. The hydrodynamics of the viscoelastic fluid is solved regarding the settling spherical particle as the reference system. Firstly, the rheological properties of the FENE-P and Giesekus fluids are obtained from the numerical solution of constitutive equations under the planar shear flow and the uniaxial tension conditions. It shows that both constitutive models exhibit not only the shear thinning but also the tension hardening, and the shear band appears when the mobility factor is larger than 0.8 for Giesekus fluids. The numerical results show that the elasticity of the fluid causes the instability of the settling velocity of the particle at the beginning. The stronger the elasticity of the fluid is, the stronger the instability of the settling velocity of the particle is. While the shear thinning weakens the instability of the settling velocity of the particle. The shear thinning and tension hardening affect the flow simultaneously and a negative wake is formed behind the particle. The negative wake region enlarges when the shear thinning and tension hardening are enhanced, and the negative wake is also enhanced with the increase in the fluid elasticity. However, the simulated settling velocity of the sphere using the FENE-P and Giesekus models becomes steady after a while, and the sphere reaches a constant terminal velocity eventually. It shows some difference from the unusual experimental phenomenon, e.g. the continuous velocity fluctuation of the settling sphere in actual surfactant wormlike micellar fluids. Thus, the FENE-P and Giesekus models are incapable when they are used to describe the particle behaviors in these surfactant wormlike micellar fluids. The breakage and reformation of the micro-scale wormlike micelles in surfactant solutions caused by the reversed velocity in the negative wake region may be the reason for the fluctuation of the settling velocity of a particle in this kind of viscoelastic fluids. Therefore, an advanced constitutive model is needed to clarify the phenomenon in the future study.

Key words: surfactants, hydrodynamics, suspensions, non-Newtonian fluids, shear thinning, particle settling, simulation