CFD simulations of the effect of inlet conditions on Taylor flow formation

The mechanism of Taylor bubble formation and the resulting bubble size in capillaries at low superficial gas velocity (UGS < 0.04 m/s) were investigated using Computational Fluid Dynamics (CFD). A co-flow inlet configuration in a 1 mm ID capillary with two gas nozzle sizes of 0.11 mm and 0.34 mm ID, respectively, was studied. Air and three liquids – water, octane and “semi-octane” – were used as test fluids. Bubble formation followed a multi-stage mechanism while the bubble shape during formation deviated from the spherical one assumed in the literature. The three-phase contact line was also found to move along the top wall of the nozzle for the small size nozzle, which had an effect on the bubble size formed. Simulated bubble sizes compared favourably with experimental data in a similar system. Bubble sizes were found to increase with increasing gas and decreasing liquid velocities and increasing nozzle size and nozzle wall thickness. From the fluid properties, surface tension was found to have a strong effect on bubble size but not density or viscosity. An increase in contact angle also increased bubble size. From the available literature correlations those that included phase fraction or ratios of superficial phase velocities were found to predict better the observed bubble sizes.