Graduate Thesis Or Dissertation

 

Temperature Evolution of Spark Kernels in Quiescent and Cross-flow Conditions Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/dz010t373

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  • Numerous physical and chemical processes are required for successful ignition of a flammable mixture, many of which have been well characterized. However, one aspect of the ignition process that has received limited consideration is understanding the temperature of the spark kernel. A spark kernel is the volume of heated gas that develops after plasma formation and dissipation by an electrical discharge. Thermal measurements are critical for determining if reactions become self-sustaining and improving the validity of modeling efforts. The need for quantified kernel temperatures is extended to conditions that approach the flow fields within combustion applications, such as gas turbine engines. Based on the motivation, the focus of this work was to determine the temperature of kernels and investigate the influence a cross-flow has on the temporal temperature evolution. Representative results were compared against kernel temperatures in a quiescent environment to highlight differences between the two flow conditions. In this study, a sunken fire igniter was placed in an open loop wind tunnel and discharged into a non-reacting cross-flow. Kernel temperatures, after the plasma dissipated, were determined from radiation intensity measurements and by solving the radiation transfer equation. The temperature evolution was investigated in a quiescent environment and for a range of cross-flow velocities (5.8-15.6 m/s). For both quiescent and cross-flow conditions, kernels developed into a toroidal vortex. Surrounding air was entrained into the center of the kernel, resulting in relatively lower temperatures compared to the edges. Average peak kernel temperatures in quiescent conditions were 950 K, whereas kernels in a cross-flow approached 1250 K. The higher peak temperatures were attributed to a reduced relative velocity of the vortex caused by the interaction with the cross-flow. This resulted in decreased entrainment, particularly located at the upstream side of the kernel. Most of the temperature evolution of kernels was experienced within 1.3 ms after plasma was no longer detected; up to a 500 K difference was determined between 0.6 and 1.3 ms. Kernels beyond 1.3 ms reached a uniform temperature near 600 K and had little to no variation as radiation intensities dissipated beyond optical detection. Bifurcation of kernels was detected in one-third of all spark events for both quiescent and cross-flow conditions. The sensible energy of kernels was reported to decrease with time for all cases. Higher cross-flow velocities resulted in less sensible energy. This was attributed to a reduction in apparent kernel volume even with higher temperatures.
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