No abstract

Detailed analysis of reactive multiphase flows is still challenging due to the complex linked transport processes. Until now, there is no reliable and reproducible experimental method for gas‐liquid flows, which enables the local measurement of dissolved species concentrations of educts, products, and side products at well‐defined and reproducible hydrodynamic conditions. A vertically arranged glass channel is used to capture gas bubbles with well‐defined wake structures. The hydrodynamic conditions in the wake can be predicted by dimensionless numbers and adapted for most industrially relevant systems. An experimental apparatus is presented for the investigation of the interdependency between hydrodynamics, mass transfer, and chemical reaction. The applicability of the setup is demonstrated with a chemical test system that shows a strong color shift from colorless to blue when the gas‐liquid reaction takes place.

Mass transfer from single CO2 Taylor bubbles in vertical minichannels was measured for various channel hydraulic diameters Dh. The effects of channel geometries on the mass transfer were also investigated by using square ducts and circular pipes. Bubble rising velocities, vB, in the ducts were much faster than those in the pipes due to large liquid flow areas in the corners of the ducts. The values of mass transfer coefficients in the pipes were almost the same as those in the ducts, in spite of a large difference in vB. Sherwood numbers, ShD, using Dh as a characteristic length, are well‐correlated in terms of the Eötvös number. The proposed ShD correlation can well predict a long‐term dissolution process of a Taylor bubble.

Microbubble aeration is utilized usefully for chemical and biological processes which consume large amount of dissolved gas in liquid because the microbubbles have long residence time in liquid, large specific gas-liquid interfacial area and fast mass transfer rate. To design the industrial process using microbubble aeration, however, the fundamental characteristic and behavior of microbubbles have to be investigated at first. In this study, therefore, both shrinking and rising behaviors of a single microbubble are simultaneously observed. A single microbubble was induced from a fine nozzle into the bottom of a tall transparent vessel filled with ion-exchanged water in which dissolved gas was reduced previously with vacuum degassing. The single rising microbubble was chased with a high-speed video camera along the vessels height. The mass transfer rate was measured from the shrinking behavior of the single microbubble which was captured and analyzed from the video image. The smaller the diameter of a microbubble, the more rapidly it decreased. Finally, the microbubble was vanished. With reducing concentrations of oxygen and nitrogen dissolved in water, the shrinking rate of a microbubble became faster. The mass transfer from shrinking microbubble of either air or pure oxygen can be evaluated by the sum of oxygen transfer and nitrogen transfer, in which each mass transfer coefficient was estimated by the equation of Ranz and Marshall. The estimated behavior of the single shrinking microbubble was agreed well with the observation. It is understood in this study that both oxygen transfer and nitrogen transfer from shrinking microbubble into water occur independently in relatively low dissolved gas concentration in water.

Mixing timescales and residence times in reactive multiphase flows can be 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 normalized residence times of passive particles advected in the wake velocity field show a peak for intermediate rise velocities. This fact 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 very coherent vortical pattern in the bubble wake which explains the long residence times. The increased residence times within the vortical structure and 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.