Chapter 6 - Gas–Liquid Flows

Abstract

Gas–liquid flows appear in natural and industrial processes in various forms and often feature complex inter-phase mass, momentum, and energy transfers. One example of naturally occurring gas–liquid flow is the dispersion of marine droplets. Gas–liquid flows are also found in abundance in industrial processes. One significant industrial application is venting of mixture vapors to liquid pools in chemical reactors. This chapter is concerned with gas–liquid flows. Within this flow system, the two phases that coexist simultaneously in the fluid flow often exhibit relative motion among the phases and heat and mass exchanges across the interface boundary. Owing to the complexities of interfaces and resultant discontinuities in fluid properties as well as from physical scaling issues, it is rather customary to apply a statistical, averaged approach in the form of a two-fluid model to resolve such a flow system. Separate transport equations governing the conservation of mass, momentum, and energy are solved for each phase and exchanges that take place at the interfaces between the two phases are explicitly accounted for in which the dynamics of the interaction between the two phases can be effectively described via suitable models of the inter-phase mass, momentum, and energy exchanges. Normally, the coupling between the two phases is very tight, which demands special numerical strategies and solution algorithms to be adopted. This particular flow system is also complicated considerably by the prevalence of particle–particle collisions. A number of population balance methods, along with suitable coalescence and break-up mechanisms, are discussed. In the context of computational fluid dynamics, the application of population balance models to describe the coalescence and break-up dynamics of these gas particles can be coupled with the two-fluid model to predict the wide range of particle sizes within the two-phase flow.