On the effects of fluid elasticity and gas holdup on Taylor bubble rising dynamics in viscoelastic media

In this work, the rise of Taylor bubbles in a vertical tube filled with viscoelastic media is investigated by means of volume-of-fluid-based direct numerical simulations. The rheological behaviors of the nonlinear viscoelastic liquids are described by the exponential Phan-Thien-Tanner constitutive model. The applicability of our simulations to capture the liquid film around a Taylor bubble has been validated by comparisons with numerical and experimental data in the literature. The effects of fluid elasticity [indicated by the Weissenberg (Wi) number] and gas holdup are mainly discussed in respect of Taylor bubble dynamics (e.g., rising velocity, flow field, stress field, liquid film, and so on). Our results show that the Taylor bubble steady rise velocity is logarithmically correlated with Wi and the slope is nearly constant at low Wi numbers. When the fluid elasticity is moderate, the viscoelastic stress in the liquid film is large, and the rising bubble is stretched to form a thin filament tail with a negative wake. Moreover, the fluid elasticity has negligible effects on the steady bubble rising velocities and the liquid film thickness for large Wi numbers. With increasing fluid elasticity and gas holdup, the long Taylor bubble fluctuates due to the surrounding liquid pulling deformation and gas pressure. The viscoelastic stress profiles exhibit a major non-monotonic dependence on the distance to the walls, which seems to be squeezed as the liquid film gets thinner under high elasticity. The dynamical Taylor bubble is found to be significantly affected by the surrounding fluid viscoelasticity and partially independent of the gas holdup, which will guide the design of microreactors in chemical applications.