(371b) CFD Modeling of Viscoelastic Droplet Breakup

When polymers having different rheologies are mixed together, there is often a desire to break up large polymer droplets into smaller ones, creating a more homogeneous blend with more desirable physical properties.  It would be beneficial to have a model to help understand and predict how rheological characteristics will affect droplet breakup and blending.  Building an empirical model requires the generation of a large number of polymer samples, followed by complex blending experiments in the melt phase.   Another approach, which minimizes the need for extensive experimentation, is to utilize computational fluid dynamics (CFD) to explore the effects of the polymer rheology, equipment design, and operating conditions on droplet break-up efficiency.  This is challenging since most polymeric materials are viscoelastic; in the melt state, they exhibit memory or stress relaxation responses and normal stress effects.  In addition, a multiphase code is required to explore droplet deformation and breakup phenomena.  Unfortunately, there are presently no commercial CFD codes that can handle both viscoelasticity and multiphase phenomena. This need prompted the development of a model using OpenFOAM® (open source CFD code) to simulate two-phase flow of viscoelastic materials.  The numerical algorithm has been validated using two well-known viscoelastic free-surface effects, namely the die swell effect and the Weissenberg (rod climbing) effect.  Additional effects such as transient and steady-state deformation of single drops subject to shear and planar elongational flows were also used for validation. 

In this paper, we will examine some of the results obtained when using the OpenFOAM model to simulate viscoelastic droplet deformation and break-up characteristics in pure shear and elongational flow fields.  The effects of viscosity and elasticity will be discussed.  Additionally, the ability to simulate the observed phenomenon of droplet transverse stretching in a shear field will be discussed.