(158f) Numerical Simulations of Bubble Formation and Rise in Micro-Channels

The gas-liquid flow in micro-channels is of fundamental importance for many engineering applications like micro-reactors, monolith reactors, micro-mixers and heat exchangers and several other micro-fluidic devices. In this work, the bubble formation and their rise in circular capillaries in Taylor flow regime is investigated by using the Volume of Fluid (VOF) method. The dynamics of formation and rise of Taylor bubbles in glass capillaries of 1 mm, 0.5 mm, 0.75 mm and 0.3 mm diameter for air-water and air-octane systems was simulated. The effects of superficial gas and liquid velocities, channel geometry (nozzle wall thickness, nozzle diameter, capillary diameter), wall adhesion (contact angle), and fluid properties (surface tension, viscosity) on the dynamics of bubble formation was investigated. The predicted bubble shapes and bubble formation periods were validated using the experimental data reported by Salman et al. (2006) for a wide range of experimental parameters.

The key conclusions of our investigations are summarized here:

• The dynamics of bubble spreading and receding on the nozzle wall is important and therefore it is necessary to consider the nozzle geometry precisely in the simulations.

• At coarse grid resolutions, predictions of axi-symmetric simulations predictions were found to be in good agreement with experiments, but the predicted results were not grid independent and lead to unrealistic results as the grid was refined. Therefore, axi-symmetric simulations are not adequate to investigate the bubble formation mechanism in micro-channels and 3-D simulations are required for accurate prediction of bubble formation mechanisms in micro channels.

• The predicted bubble formation periods were in good agreement (within 10%) with the measurement for UL> 0.012 m/s and UG < 0.02 m/s. However, for UL < 0.0075 m/s and UG > 0.02 m/s, bubble modification due to bubble pairing and coalescence at the nozzle was observed and deviation of numerical predictions from experimental data increased, though the experimental data in itself contains large deviations (as large as 50 %) in such cases.

• The wall adhesion was found to strongly influence the bubble formation mechanism and must be considered in the simulations for accurate prediction of bubble formation dynamics. Bubble formation period was found to decrease with the increase in the capillary wall contact angle and to increase with the increase in the nozzle wall contact angle.

• The predicted bubble formation periods were found to strongly depend on the surface tension up to 0.072 N/m. With the increase in the surface tension, bubble tends to become more spherical, the bubble diameter and hence the bubble formation period was to found increase. For the range of liquid viscosities considered in the present work (0.0001 to 0.01 Pa.s), the effect of viscosity on bubble formation was found to be negligible which agreed well with the previous investigations reported in the literature.

• Bubble formation is sensitive to gas and liquid inlet geometry and channel dimensions. The bubble formation period was found to decrease with the decrease in the nozzle diameter and the capillary diameter.

The extensive validation of the computed bubble shapes and formation periods with the experimental results demonstrate that the Taylor flow regime in micro-channels can be successfully predicted for wide a range of operating parameters. The experimentally validated computational model will be very useful to simulate the transport processes and reactions in micro-capillaries/channels.

Reference:

Salman, W., Gavriilidis, A., Angeli, P. On the formation of Taylor bubbles in small tubes. Chemical Engineering Science. 2006, 61, 6653 – 6666.