Gawloski, Nicholas Adam (2017-05). Experimental Investigation of Pulsed Electrical Discharge Energy Deposition in Supersonic Flows. Master's Thesis.
Thesis
Energy deposition in the form of a direct current spark discharge in Mach 2.2 flow is experimentally investigated, to measure the conversion efficiency of electrical energy into thermal energy. The energy is impulsively deposited into to the flow. This impulsive deposition occurs much faster than the flow can react, creating a strong blast wave and a high temperature, high pressure region in the flow. The plasma discharge is created between two sharpened tungsten electrodes inclined into the flow, powered by a pulsing high voltage RC circuit. Schlieren imagery was utilized to track the expansion of the blast wave and the expansion of the high temperature, high pressure region created by the spark discharge into a low density region. To estimate the conversion efficiency of electrical energy into the thermal energy in the flow, a high temperature thermodynamic heating and expansion model was developed to simulate the deposition process. The model consisted of two portions. First, the thermodynamic properties of N2, O2, N, O, Ar, were calculated from the NASA PAC database, valid for temperatures 200-20,000K. A high temperature air mixture was then constructed from the five species assuming chemical equilibrium for temperature from 200-20,000K. Second, a heating and expansion model was developed utilizing the high temperature air properties. The energy deposition was modeled as a constant volume heating process to simulate its impulsive nature. The initial volume of the plasma is calculated from direct imaging of the discharge at delays ranging from 80-120ns. The resulting high temperature, high pressure region is then mixed with varying amounts of ambient air to simulate heat transfer. The high temperature region is then expanded isentropically back to ambient pressure, where the final volume was compared to the experimental Schlieren images to estimate the conversion efficiency. Three different capacitors were used during experimentation to vary the input electrical energy three orders of magnitude; specifically, the energies were calculated to be: 3.4, 20.8 and 203mJ per pulse. The range of possible efficiencies for the three cases was found to be: 70-90%, 14-30%, and 5-12%. The lower energy case was found to be the most efficient with regards to energy conversion. However, the largest energy case was found to be more effective because it resulted in the region with the highest expanded temperature (4150K) and lowest expanded density (0.017 [kg m^-3]) in its final expanded state.
