Multiphase flows are flows of two or more thermodynamic phases moving together but not necessarily at the same velocity, and not necessarily at the same temperature. Each phase may contain several chemical species. In multiphase flows there is typically a carrier flow, and one or more dispersed phases carried by the flow. The volume of the dispersed phase divided by the total volume is called the dispersed phase volume fraction.

Multiphase flows naturally occur in many situations; examples are: tornadoes; volcanic plumes; avalanches and sand dunes. They are also important in many industrial and commercial systems; examples are: fuel sprays in liquid rocket, gas turbine and automotive engines; household sprays; medical dispenser sprays; agricultural sprays; ink jet printer sprays; paint sprays; fluidized beds; pipe flows in the context of underwater and other type of oil extraction, as well as sewage disposal; and centrifugal separators. Present understanding of these flows is insufficient for prediction or control. Specific aspects not currently understood include: (1) the behavior of multiphase flows at moderate and high dispersed phase volume fraction, (2) the dynamic and thermodynamic coupling between a moderate/high volume fraction dispersed phase and a high turbulence intensity carrier flow, and (3) the interactions among the condensed phase entities of a high volume fraction dispersed phase when carried by a high turbulence intensity flow. Some of the projects listed below address particular aspects of these unresolved issues.

Thermodynamics theory states that for each chemical species there is a pressure and a temperature above which there can be only a single phase, and therefore two-phase flows, for example, cannot exist. Single phase flows are a subset of multiphase flows in that there is a single thermodynamic phase, however, there also may be several chemical species each having its own velocity and temperature at the initial observation time. The pressure and temperature above which two-phase flows cannot exist are called critical, and conditions above either one of the critical pressure or temperature are called supercritical. Mixtures of specific chemical species have a critical locus, each point on this locus corresponding to a different mixture composition. The behavior of supercritical fluids is also a topic of contemporary research. Some of the projects described below address issues specific to supercritical fluid behavior.

Energy-producing systems are often based upon the gasification of one condensed phase and the oxidation of the emerging vapor. Oxidation chemical kinetics is complex, describing reactions among hundreds to thousands of species. The number of reactions is typically in the thousands. Because of the ensuing computational intensity, it is thus impossible to simulate turbulent, oxidative reactive flows using detailed chemical kinetic mechanisms. Instead, one strives to reduce the complex mechanisms to a much smaller set of representative species and also of relevant reactions. Most effort is devoted to reducing the number of species, since this is where the computational intensity stems from, as one must solve an equation for each species. One of the projects describes our approach to oxidative mechanism reduction for hydrocarbon kinetics. 

Personnel:

Harstad, Kenneth G.; Senior Research Engineer
Okong'o, Nora A.; Scientist
Radhakrishnan, Senthil; Caltech Postdoctoral Fellow
Enrica Masi; Caltech Postdoctoral Fellow

 

Projects:

1. Large Eddy Simulations of Particle Laden Mixing Layers
2. Direct Numerical Simulations of Particle Laden Jets
3. Modeling of Granular Flows
4. Large Eddy Simulations of Supercritical Mixing Layers
5. Modeling of Supercritical Fluid Drops
6. Properties of Supercritical Fluids
7. Properties of Supercritical Fluids
8. Multiphase Flow Modeling for Mars Planetary Protection
9. Hydrogen-Fuel Safety
10. Chemical mechanism reduction for oxidative hydrocarbon kinetics