How coherent structures dominate the residence time in a bubble wake: An experimental example

Timescales and residence times in reactive multiphase flows are essential for product selectivity. For instance when a gas species is consumed, e.g., by a competitive consecutive reaction with moderate reaction kinetics where reaction timescales are comparable to relevant mixing timescales. To point out the importance of the details of the fluid flow, we analyze experimental velocity data from a Taylor bubble wake by means of Lagrangian methods. By adjusting the channel diameter in which the Taylor bubble rises, and thus the rise velocity, we obtain three different wake regimes. Remarkably, the residence times of passive particles advected in the bubble wake's velocity field show a peak for intermediate rise velocities. This observation seems unintuitive at first glance because one expects a faster removal of passive tracers for a faster overall flow rate. However, the details of the flow topology analyzed using Finite Time Lyapunov Exponent (FTLE) fields and Lagrangian Coherent Structures (LCS) reveal the existence of a coherent vortical pattern in the bubble wake which explains the long residence times. The increased residence times within the vortical structure in combination with the close bubble interface acting as a constant gas species source could enhance side product generation of a hypothetical competitive consecutive reaction, where the first reaction with the gas species forms the desired product and the second the side product. (C) 2019 Elsevier Ltd. All rights reserved.