William A. Sirignano

The nitridizing of aluminum in microgravity using spray combustion technology

This is an analytical/computational study to establish the feasibility of synthesizing ceramic AlN and AlN/Al metal matrix composites using spray combustion techniques. A numerical model is being developed to gain a fundamental understanding of the nitridization and/or oxidation processes of Al and other metals, particularly with regard to the flame location and condensation radius. Results of this model will be used for the design of a small-scale combustion facility to examine the combustion of metal droplets in both nitrogen and oxygen environments under normal gravity and microgravity condition.

Flame spread above liquid fuel pools

In this project, the researchers are studying the important fundamental mechanisms in ignition and flame spread phenomena in reduced gravity facilities, such as drop towers, space shuttles, and the space station. Heat and mass transports, which are vital to the combustion process, are enhanced by buoyancy effects and by surface tension variations. This theoretical and computational study concerns two-phase natural convective and thermocapillary flows in enclosures with nonuniform heating from above. It relates to fire safety in various situations where liquid fuel and an ignition source can come into contact. An associated experiment to be flown in space is being developed at NASA Lewis Research Center.

High energy fuel vaporization and combustion

Advanced hydrocarbon liquid fuels can have such complex chemical bonding that a larger energy release can occur in combustion. Decomposition can occur in the liquid-phase upon heating, complicating the spray vaporization and burning. A gasification occurs in the liquid phase upon decomposition causing a foamy zone. This study involves analyses of this new phenomenon. Transport and chemical reaction are studied in the liquid droplet interior, in the surrounding gas-phase, and in the interfacial foamy zone.

Liquid oxygen droplet vaporization and mixing

Liquid propellant rocket motors often have injection of both liquid oxygen and liquid fuel streams into the combustion chamber. Analysis of the droplet heating and vaporization and the mixing of the oxygen and fuel vapors is required for the predictions of the performance and stability of the motor. This research involves detailed studies of the droplet interior and surface, and of the surrounding gases.

Turbulence - droplet interactions

In a practical spray, droplet trajectories and vaporization rates are significantly modified by interactions with the turbulent gas field. In this computational research, a three-dimensional, unsteady Navier-Stokes equations solution is sought to describe the interactions of vortical structures with droplets.

Nonlinear distortion of liquid sheets and jets

In the creation of practical sprays, the liquid injection process is critical. The nonlinear distortion of injected liquid sheets and jets is studied analytically and computationally resulting in the prediction of the wave phenomenon that causes the interface distortion and ultimate atomization of the liquid stream.

Transcritical droplet behavior

When liquid is sprayed into a high pressure multicomponent gas, the critical pressure depends upon the local mixture composition. Therefore, subcritical and supercritical domains can exist in the same chamber. Vaporization, transport, and chemical reaction are studied in such a situation. An important fluid dynamic feature is the break-up of liquid droplets due to reduced surface tension and large shear forces.

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