Michael M. Micci

Molecular dynamics simulations of droplet evaporation

The complete evaporation of a three-dimensional submicron droplet under subcritical conditions has been modeled using molecular dynamics. The two-phase system consisted of 2048 argon atoms modeled using a Lennard-Jones 12-6 potential distributed between a single droplet and its surrounding vapor. The system was first allowed to relax to equilibrium, then the droplet was evaporated by increasing the temperature of the vapor phase atoms at the boundaries of the system until only the vapor phase remained. The computed evaporation rate agrees with that predicted by the Knudsen aerosol theory.

Direct simulation Monte Carlo model of Low Reynolds number nozzle flows

A numerical analysis of low Reynolds number nozzle flows is performed to investigate the loss mechanisms involved and to determine the nozzle wall contour that minimizes these losses. The direct simulation Monte Carlo method is used to simulate nitrogen flows through conical, trumpet-shaped, and bell-shaped nozzles at inlet stagnation temperatures of 300 and 1000 K. The Reynolds number of the flows based on throat diameter range from 90 to 125. The trumpet-shaped nozzle has the highest efficiency with the unheated flow. With the heated flow both the trumpet and bell-shaped nozzles have a 6.5% higher efficiency than the conical nozzle. The conical nozzle has the highest discharge coefficient, which is unaffected by the change in stagnation temperature; however, the increase in stagnation temperature increases the heat-transfer and viscous losses in the boundary layer. These results suggest that the trumpet-shaped wall contour performs most efficiently except near the throat region, where it incurs large viscous losses. However, the bell-shaped nozzle may increase its overall performance with an increase in stagnation temperature.

Investigation of stabilized resonant cavity microwave plasmas for propulsion

Results are presented for the performance of a microwave electrothermal thruster utilizing bluff-body and swirling flow stabilized plasmas with helium and nitrogen propellent. Stabilized plasmas have been produced in the TM011 electromagnetic cavity mode with coupling efficiencies approaching 100% and specific powers up to 27.3 MJ/kg. Fluid dynamic measurements indicate specific impulses of up to 543 s, efficiencies of 44-69%, and thrusts of 0.27—0.40 N with helium propellant. Spectroscopic results indicate plasma core electron temperatures of 11,840-12,170 K with extremely flat radial profiles. Nomenclature A = orifice area, m c = speed of light, m/s cp = specific heat, J/kg K g = gravitational acceleration, m/s2 7sp = specific impulse, s k = Boltzmanns constant, J/K m = particle mass, kg m = mass flow rate, kg/s P,- = incident power, W Po = chamber pressure, Pa Pr = reflected power, W R = gas constant, J/kg K 7} = thrust, N T(k, = stagnation temperature (cold), K Tl}l, = stagnation temperature (hot), K ue = exhaust velocity, m/s y = ratio of specific heats 77 = efficiency j]c = coupling efficiency A = wavelength, A

Ionic velocities in an ionic liquid under high electric fields using all-atom and coarse-grained force field molecular dynamics.

Molecular dynamics has been used to estimate ionic velocities and electrical conductivity in the ionic liquid 1-ethyl-3-methylimidazolium/tetraflouroborate (EMIM-BF(4)). Both an all-atom and coarse grained force fields were explored. The simulations were carried out at high electric fields where one might expect the Wien effect to become important in conventional electrolytes and that effect is observed. While the original Wilson theory used to explain the Wien effect in conventional electrolytes does not work well for ionic liquids, a minor modification of the theory allowed it to be used to qualitatively describe the data. The two coarse-graining methods were noisier as expected, but result in a significant savings in computational cost.

Direct measurement of high frequency, solid propellant, pressure-coupled admittances

This paper presents an experimental method that is capable of directly measuring solid propellant pressurecoupled responses at the high frequencies associated with tangential mode instabilities inside solid propellant rocket motors. The method utilizes a magnetic flowmeter to measure the velocity oscillation above a burning propellant surface simultaneously with a pressure oscillation measurement within an externally excited combustion chamber. A magnetic flowmeter burner was designed and constructed to evaluate this method of pressurecoupled response measurement. Response measurements were obtained for two formulations of AP/HTPB composite propellant at pressure oscillation frequencies of 4000 and 8000 Hz. The measurement data displayed repeatable trends in both the real and imaginary parts of the pressure-coupled response function.

Linear analysis of forced longitudinal waves in rocket motor chambers

A methodology centered around the forced longitudinal wave (FLW) motor is being developed to investigate dynamic responses of rocket motors. The FLW motor establishes periodic longitudinal pressure and velocity oscillations in solid propellant rocket chambers. A linear analysis was developed to study propellant pressureand velocity-coupled responses using dynamic pressure measurements at several locations in a motor. The analysis uses pressure amplitude and phase measurements. Variations in the propellant reponses are shown to produce measurable changes in the calculated oscillating pressures with velocity-coupled responses showing the greatest promise for determination from experimental data. Experimentally deduced velocity-coupled response functions are examined over a frequency range centered around the chamber fundamental mode for a range of interior flowfields and chamber pressures for 86%AP-14%HTPB propellants.

Investigation of free-floating resonant cavity microwave plasmas forpropulsion

Results of experiments with high-pressure helium and nitrogen discharges generated in a microwave resonant cavity for use in an electrothermal thruster are presented. The cavity, operating in the TMoii mode, generated the discharges within a quartz sphere, which allowed the discharge to be both free floating and away from solid surfaces. Input powers of up to 400 W were used with gas pressures up to 300 kPa (absolute) and mass flow rates up to 2.79 x 10~ 4 kg/s. Coupling efficiencies up to 19% have been demonstrated, and temperature measurements 200 mm downstream of the plasma indicate thermal efficiencies of up to 36.6% and total efficiencies of up to 25.2%, both increasing linearly with mass flow rate. The downstream temperature measurements also closely match the coupling efficiency behavior when plotted against pressure. Absolute measurements of the continuum radiation yield electron temperatures of between 10,200 and 10,900 K, which are insensitive to changes in the operating conditions.

Microwave electrothermal thruster chamber temperature measurements and performance calculations

The microwave electrothermal thruster (MET) uses microwave frequency energy to create and sustain a resonant cavity plasma to heat a propellant. A 2.45-GHz aluminum cylindrical thruster with converging copper-alloy nozzles was used for this study. A spectroscopic system was used to collect light emitted through a window in the plasma chamber. A Schumann-Runge oxygen-emission model was developed assuming an anharmonically vibrating, nonrigid rotating oxygen molecule. The commercially available LIFBASE software was used to model the ionized molecular nitrogen first negative system emission from the nitrogen plasmas. Experimental data were compared to the temperature-dependent models using least-squared difference summation schemes. Oxygen rotational temperatures of 2000 K and ionized nitrogen rotational temperatures of 5500 K were measured. These measurements were nearly constant for all chamber pressures and investigated absorbed specific powers. CEA2 code equilibrium thermochemical calculations show the relationship among enthalpy addition, temperature, and specific impulse for realistic operating conditions. Nitrogen was found to be an excellent choice as a propellant or propellant component, whereas oxygen was found to be a poor choice because of the temperatures achieved for the respective gases in the MET chamber.

Influence of thermodynamic state on nanojet break-up

Using non-equilibrium molecular dynamics, argon nanojet injection was simulated under vacuum conditions. A series of simulations with different shapes of solid platinum injectors was conducted. Observed droplet sizes and jet break-up characteristics resemble the Rayleigh break-up theory. However, the different injector shapes did not cause a significant change in the nanojet break-up behaviour. The liquid temperature inside the injector was found to be a controlling factor in determining the subsequent break-up characteristics. A higher liquid temperature is preferred for the faster nanojet break-up with the shorter break-up length.

Molecular Dynamics Studies of Thermophysical Properties of Supercritical Ethylene

This work involves the determination of transport coefficients and equation of state of supercritical fluids by molecular dynamics (MD) simulations on parallel computers using the Green-Kubo formulae and the virial equation of state, respectively. The MD program uses an effective Lennard-Jones potential, linked cell lists for efficient sorting of molecules, periodic boundary conditions, and a modified velocity Verlet algorithm for particle displacement. Previously, simulations had been carried out on pure argon, nitrogen and oxygen, and this has now been extended to ethylene, C^H^, at various supercritical conditions, with shear viscosity and thermal conductivity coefficients, and pressures computed for most of the conditions. The results compare well with experimental and National Institute of Standards and Technology (NIST) values.