(299g) The Effect of Orthogonal Shear Flow On the Sedimentation of Particles in Viscoelastic Fluids

During the fracture simulation of oil and gas wells, suspensions of solids in polymeric solutions are pumped to help prop open the fracture. These high-density solids in suspension sediment due to gravity and also experience an orthogonal shear flow. Experimental data (van den Brule & Gheissary, J. Non-Newton. Fluid Mech., vol. 49, pp. 123-132, 1993; Tonmukayakul et al., 82^nd SoR Annual Meeting, 2010) has shown that both the shear thinning and the elasticity of the suspending polymeric solutions significantly affect the settling rate of the solids. The mechanism by which the elasticity of the carrying fluid affects the settling rate is not well understood. In the present work, we use simulations of viscoelastic flow past a single, torque-free sphere with a cross shear flow to study the effect of carrier fluid elasticity on the drag experienced by the sphere and thus on its settling rate. We also have completed experiments thus measuring the sedimentation rates of the solids in different viscoelastic fluids. In the numerical simulations, we use the FENE-P or the Giesekus constitutive model to represent the viscoelastic fluid with parameters obtained by fitting rheological data. For weakly shear thinning fluids, we obtain an increase in drag, i.e. a decrease in settling rate, as shear Weissenberg number is increased in both the numerical simulations and the experiments. The simulations are in quantitative agreement with the experiments at small Weissenberg number (Wi <2). At higher Weissenberg number, the numerical results are in qualitative agreement with settling experiments but, in general, the magnitude of the simulated decrease in settling rate is larger than that observed in the experiments. We present the detailed physical mechanism for the increase in the drag experienced by the sphere in the simulations and we show that polymer shear stress,Τ^P₁₃, (with 1, the sedimentation direction and 3, the shear direction) is the cause of the increase in sphere drag.