Dilute Particle Suspensions

Abstract

Natural and industrial two-phase flows, i.e., particle suspensions, are all around us, ranging from dust-storms in arid regions to bubbly flows in pipes or air-fuel injection in ICEs. Traditionally, two-phase flow theory/application was the domain of applied mathematicians, as well as chemical, environmental and nuclear engineers. However, for pipe-network design, pump sizing and applied force evaluation, mechanical engineers have to know basic two-phase flow modeling techniques. Furthermore, biomedical engineers encounter fluidparticle dynamics problems in both the cardiovascular and the pulmonary systems.

By definition, two-phase flow is the interactive flow of two distinct phases with common interfaces in, say, a conduit. Each phase, representing a volume fraction (or mass fraction) of solid, liquid or gaseous matter, has its own properties, velocity, and temperature. Typical dilute (or dense) particle suspension flows include droplets in gas flow, liquid–vapor, i.e., bubbly, flow as well as liquid or gas flow with solid particles. In addition to predicting the flow phases, it is also important to know the flow regimes, i.e., characteristic flow patterns based on the interfaces formed between the phases (see Fig. 6.1). Two-phase systems can be grouped into flows of separated phases, mixed phases, and dispersed phases. Examples of separated flows include liquid layers on a wall in gas flow, e.g., the mucus layer in lung airways, and liquid jets in gas flow (or vice versa). Mixed-phase flows are encountered in phase-change proc- esses, such as boiling nuclear reactor channels and steam pipes with vapor core and annular liquid wall film as well as heat pipes with large vapor bubbles and evaporating liquid layers on heated surfaces. Most frequently, two-phase flows appear as dispersed phases, such as dilute particle suspensions in gas or liquid flows, droplets in gas flow (e.g., sprays) or bubbles in liquid flows (e.g., chemical reactors). Clearly, alternative two-phase flow classifications exist. For example, solid particles, droplets or bubbles form the dispersed (or particle) phase while the carrier fluid is the continuous (or fluid) phase. The degree of phase coupling, i.e., from one-way for very dilute suspensions to four-way in dense suspensions. In the latter case, not only fluid flow affects particle motion and vice versa but particle–particle interactions due to collision are expected and particle- induced flow fields affect other particles, as in drafting.