The design of the VPI plasma torch, which has been used as an ignitor and flameholder in supersonic combustion studies, has been modified in order to decrease the electrode wear and to increase stability. The plasma torch can be used as a source of hydrogen or nitrogen radicals which initiate and stabilize combustion. During previous testing of the unmodified torch, electrode erosion limited operation of the torch to about two hours. The improved torch features a flow swirler in the gas inlet, which adds vortex stabilization to the arc. The vortex stabilization causes the anode attachment point of the arc to be anchored in the low pressure region, downstream of the constrictor. This lowers the heat flux to the anode, so that erosion is decreased. The torch body was redesigned with an emphasis on the alignment of the electrodes. Also, the electrode gap in the improved torch was made continuously adjustable, allowing fine adjustment of the electrode gap during operation of the torch. The operational characteristics of the improved torch were monitored by a microcomputer-based data acquisition system. Stable operation of the improved torch with pure nitrogen was demonstrated, thus eliminating the requirement for argon to stabilize the arc. Operational characteristics of the improved torch running on argon, nitrogen, argon/hydrogen and argon/nitrogen mixtures as feedstocks, are reported. The electrode wear was studied between tests by observation with a microscope, and by measuring the mass change of the electrodes. The electrode erosion of the improved torch was reduced significantly. Anode lifetimes of greater than 20 hours have been demonstrated with operation on mixtures of nitrogen and argon. / Master of Science

An investigation has been conducted to determine the effects of jet-to-free-stream dynamic pressure ratio on the penetration and mixing of a sonic hydrogen jet injected normal to a Mach 4 airstream. The hydrogen gas was injected from a circular nozzle flush mounted in a flat plate with a turbulent boundary-layer thickness of 2.70 injector exit diameters at the injector station. The investigation was conducted for values of the dynamic pressure ratio ranging from 0.5 to 1.5. At five downstream stations between 7 and 200 injector diameters the mixing region was surveyed to obtain hydrogen volume concentration and pressure profiles. Results of the investigation indicate that the thick boundary layer had significant effects on the penetration and maximum concentration trajectories when compared to data correlations from other sources. The penetration trajectory was found to be proportional to the dynamic pressure ratio raised to the 0.3 power. The decay of the maximum concentration was very rapid in the near field and inversely proportional to (x/dj)^0.8 at downstream distances greater than 30 jet diameters. At a particular value of x/d the maximum concentration was proportional to q_r^½. Nondimensional concentration profiles, represented by Gaussian-type functions, on the vertical centerline showed similarity at values of x/d equal to or greater than 60. / Master of Science

Experiments were performed to determine the effects of impinging oscillating shocks of different frequencies on a 15° downstream angled, underexpanded, sonic helium jet injected into a supersonic airflow. Information on mixing, penetration, total pressure loss and turbulence structure from these experiments was used to estimate mixing control achieved by adding an oscillating shock to the helium injection flow field. Tests were conducted at Mach 3.0, with a total pressure of 6.5 atm, a total temperature of 290 K and a Reynolds number of 51.0 x 10⁶ per meter. Oscillating shocks of three different frequencies were studied. The frequencies selected were designed to allow tuning of the shock frequency to the estimated frequency, about 100 - 150 kHz, of the largest eddies in the approach boundary layer. Visualization using nanoshadowgraph photography showed large turbulent structures in all cases. In addition, there were clear changes in eddy size with changing shock frequency visible on the nanoshadowgraphs. The primary measurement made for the mixing studies was the molar concentration of helium. Concentration data, as well as mean flow data, was collected at nine lateral positions at each of three axial stations downstream of the helium injector. The resulting data produced contours of helium concentration, total pressure, Mach number, velocity, mass flux and static flow properties. Additional tests were conducted to determine the shock oscillation frequency, the correlation between the oscillating shock and the turbulence in the shear layer and the angle of large-scale structures in the flow. Mixing and penetration rates were determined from the helium concentration data. The major result of this study was that impingement of an oscillating shock on a high-speed shear layer can be used to control the rate of mixing. Depending on the shock oscillation frequency, mixing enhancement or inhibition can be produced. It was found that increasing shock oscillation frequency resulted in more rapid injectant concentration decay and increased freestream air entrainment leading to a stoichiometric H₂-air mixture ratio while also reducing penetration of the helium injectant. A strong correlation was found between the highest frequency shock and changes in the mixing flow field. The maximum oscillation frequency was approximately 140 kHz, which was consistent with numerical estimates for the frequency necessary for mixing augmentation under these test conditions. It was concluded that oscillating shock impingement has promise as a means of controlling gaseous mixing in a high-speed cross-flow. / Ph. D.

A method is developed to solve the conical flow equations in spherical coordinates using a rectangular finite volume approach. The only mapping done is the mapping of the spherical solution surface to that of a flat plane using a stereographic projection. The mapped plane is then discretised into rectangular finite volumes. The rectangular volumes are allowed to intersect the body surface in an arbitrary manner. A full potential formulation is used to represent the flow-field velocities. The full potential formulation prevents the formation of vortices in the flow-field but all other essential features of the supersonic conical flow are resolved. An upwind density shift is used to introduce an artificial viscosity in a conservative manner to eliminate non-physical expansion shocks and add numerical damping. The rectangular finite volume method is then extended to deal with infinitely thin conical fins. Numerical tests of cones, elliptical cones, conical wing-bodies and waveriders (with very thin winglets) have been done. Very good agreement with experimental results is found. / M.S.

Thesis: M.S., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 1977 / Includes bibliographical references. / M.S. / M.S. Massachusetts Institute of Technology, Department of Aeronautics and Astronautics

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Thesis (Ph. D.)--California Institute of Technology, 1995. / "UMI number: 9539115"--P. [i]. Includes bibliographical references. Available on microfilm and online via the World Wide Web.

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