|Title: Mass Transfer and Chemical Reactions in Reactive Deformable Bubble Swarms|
|Written by: Koynov, A.; Tryggvason, G.; Schlüter, M.; Khinast, J.G.|
|in: Applied Physics Letters 88 March 2006|
|Volume: 88 Number: 13|
|on pages: 134102-1 - 134102-3|
Abstract: A hybrid numerical/experimental technique was developed for the study of the impact of the multiphase hydrodynamics on mass transfer and chemical reactions at deformable interfaces. Different material properties and flow conditions can yield flows with qualitatively different mass transfer and transport characteristics. As many (bio-) reaction systems exhibit sensitivity to mass transport in general, and mixing specifically, it is possible to control their product distribution by tailoring the system parameters. Bubbly flows are frequently found in a large number of systems—both naturally occurring and man made. For example, bubbles supply oxygen to many biological systems in aquatic environments, such as cell cultures in bioreactors,1,2 bacteria in remediation plants,3 and fish in aquariums. Transfer of gas into a liquid is of importance for many other processes on a large scale, such as the aeration of lakes and deep-ocean bubble plumes,4 on a medium scale, such as chemical and biochemical reactors,5,6 as well as for small-scale systems, such as flavor release during the consumption of champagne and other carbonated beverages. While the primary function of bubbly flows is the delivery of a gaseous reactant to liquid-phase reactive systems, in many processes they also serve the secondary role of continuous-phase agitation. As the bubbles rise, their buoyancy-driven motion engenders liquid flow, thus effecting continuous-phase transport and mixing. The relatively low liquid-phase shear rates, typical of bubbly flows, make them the preferred mixing tool for shear-sensitive environments such as animal cell suspensions in aerated bioreactors. Although bubbly flow systems are, in principle, easy to design, detailed analysis is still an open problem, as the combination of interfacial mass transfer, continuous phase chemistry and fluid flow, through and near deformable interfaces, constitutes a physical system of formidable complexity. This letter reports on a collaborative numerical and experimental effort, which provides a previously unattainable insight into the behavior of such systems. Since the hydrodynamics of multiphase flows have long been a matter of interest for scientists, a wide variety of methods (both numerical and experimental) have been developed for their study. Optical techniques, such as particle image velocimetry and particle tracking velocimetry,7,8 in which high definition photographs are taken of laser-illuminated, buoyant particles, dispersed in the liquid phase, can yield detailed information of the flow fields in the bubbly flows. However, at higher gas-volume fractions these techniques fail. Methods that simultaneously examine chemical reactions, mass transfer and bubble hydrodynamics have not been developed. On the numerical front, several techniques, such as those based on the volume of fluid,9–11 front tracking12 and lattice Boltzmann methods,13,14 have recently been successfully used to simulate the buoyancy-driven motion of deformable bubbles in viscous liquids. Yet computational methods that are capable of describing the combined effects of flow and chemical species transport/reaction have not been reported so far. In this letter we report a critical breakthrough in this field and present a new and powerful method to study this class of reactive flows, by combining multiphase reactive direct numerical simulations (DNS) with state-of-the-art noninvasive experimental techniques. The first step in our study towards understanding the phenomena observed in gas-liquid flows was the investigation of the motion of single bubbles.15 Both experimental and numerical studies have confirmed that the dynamics of rising bubbles can change significantly, depending on several parameters controlling the inertial, viscous, gravitational and surface tension forces in the system. For different values of these parameters, the bubble’s shape and wake can exhibit significant differences, which may impact the mass transfer, mixing and multiphase chemical reactions.