Graduate Thesis Or Dissertation
 

Temperature Evolution of Nonreacting Spark Kernels at Sub-Atmospheric Pressures

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  • In the event of a mid-flight flameout of a gas turbine engine, reignition is paramount to occupant safety. Ignition at high altitude can be challenging, due in part to the potentially lower ambient pressures. An understanding of the effects of sub-atmospheric pressures on the temperature evolution of spark kernels is needed to better understand the ignition process at high altitudes. Ultimately such information can be used to maximize ignition probability at high altitudes. The objective of this work was to determine the effects of decreasing pressure on the spatial and temperature evolution of spark kernels. Spark kernels were produced inside a vacuum chamber using a sunken fire igniter. An infrared camera was used to collect radiation intensity measurements from the spark kernels. An inverse deconvolution technique was employed to determine path-averaged temperatures from the radiation measurements. The technique determined temperatures using a narrow-band radiation emissions model (RADCAL). Temperatures determined using the technique agreed within 3% to temperature measurements using a thermocouple over a flat flame calibration burner. Spark kernel temperatures were determined in quiescent air at absolute pressures from 300 to 1000 mbar. Kernel temperatures were measured from 0.67 to 4 ms after plasma was detected. Decreasing the ambient pressure caused an increase in average kernel temperature, and conversely a decrease in peak temperatures. For example, average kernel temperatures after 0.67 ms were 1270 K at 300 mbar, and 1125 K at 1000 mbar. Peak temperatures (i.e. the 90th percentile temperatures) were 2070 and 2360 K at 380 and 1000 mbar, respectively. Peak temperatures were hottest in a small region 0.6 igniter diameters from the igniter tip. Kernel temperatures decreased until the kernels became undetectable after 4 ms. Electrical energy deposition and conversion efficiency to sensible energy decreased with decreasing pressure. For example, energy deposition decreased from 1.02 J at 1000 mbar to 0.85 J at 300 mbar, while conversion efficiency decreased from 80% to 30% at the same pressures. This pressure dependence of energy deposition and conversion efficiency is attributed to increased heat loss to the electrodes at decreased pressures due to lower breakdown voltage of air causing higher electrical current through the electrodes. The decrease in energy deposition and efficiency at lower pressure helps explain part of the challenge with achieving ignition at high altitude. Kernel volume was independent of pressure. Kernels formed into a toroidal shape with an apparent volume of approximately 2.1 cubic igniter diameters (i.e., 3.7 cm3) after 0.67 ms. The apparent kernel volume decreased approximately linearly over the kernels’ lifetimes. Kernels penetrated faster and further from the igniter at lower pressures. For example, radiation from kernels at 1000 mbar was detected as far as 3 igniter diameters from the igniter tip, while at 300 mbar kernels were detected at least 3.4 diameters from the igniter tip. This observation is significant because the fuel-air mixture within combustors is not homogenous, hence the placement of the spark kernel within the flow is an important design consideration. The increased speed and penetration depth at lower pressure is attributed to an increased ratio of energy deposition to kernel mass due to the decreased gas density at lower pressure. The energy to mass ratio also explains the relationship between pressure and average kernel temperature.
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  • Pending Publication
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  • 2020-04-10 to 2021-05-10

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