Many chemical operations involve dispersed gas-liquid / gas-liquid-solid flows. Last decade witnessed the development and application of the continuum (Euler/Euler) and the discrete particle (Euler/Lagrange) models to simulate gas-liquid / gas-liquid-solid flows in multiphase reactors. Though the above mentioned models could predict the time-averaged and dynamic characteristics of gas-liquid/gas-liquid-solid flows quantitatively well for simple systems with low gas hold up, the quantitative of predictions of gas-liquid/ gas-liquid-solid systems with higher gas hold up are still out of reach. This is primarily because of the lack of adequate closure models that can account for bubble shape/ size on different forces acting on bubbles (drag, lift and virtual mass) and more importantly the influence of neighboring bubbles (or gas hold up) on the magnitude of the above-mentioned forces. In the present work, we report experimental and numerical investigations of rise behavior of single/multiple bubbles in quiescent and sheared liquids.

A high-speed digital camera was used to measure the rise velocities single/multiple bubbles of different diameters in quiescent liquids. The Volume-of-Fluid method implemented in a commercial solver FLUENT v6.2 was used to simulate the rise behavior of single/multiple bubbles in both quiescent and sheared liquids. Simulations of rise of a single bubble of different diameters (1-12 mm) and of simultaneous rise of multiple bubbles of different diameters with volume fractions up to 0.1256 in quiescent liquid were performed and the predictions were compared with the experiments. Simulations of a single bubble of different diameter in linear shear flow of liquids with different viscosities were performed under different shear rates (0 ≤ |dV_L/dY| ≤ 10 s^-1). The simulated bubble trajectory was compared with the experimental results of Tomiyama et al. (2001). Further details of the comparison of lift coefficient for various system parameters e.g. bubble diameter, shear rate and liquid phase properties and the effect of neighboring bubbles on the lift force will be discussed in detail.

The experimental and numerical investigations of the rise behavior of the single/multiple bubbles in both quiescent and sheared liquid will be useful to understand the effect of neighboring bubbles on the magnitude of the inter-phase coupling forces and can be used to improve the closures for the continuum models.

Reference:

Tomiyama, A., H. Tamai, I. Zun, S. Hosokawa, Chem. Eng. Sci., 57, 1849-1858 (2001).