M.Sc Thesis


M.Sc StudentLaso Miqueo Iker
SubjectOptimization of Energy Distribution in Nanosecond-Pulsed
High-Frequency Discharge Ignition
DepartmentDepartment of Aerospace Engineering
Supervisor DR. Joseph Lefkowitz
Full Thesis textFull thesis text - English Version


Abstract

Ignition becomes a challenge in gas turbines and high-speed engines when the residence time of the mixture in the combustor is very short. Traditional spark plugs tend to fail in these scenarios, and Nanosecond-Pulsed High-Frequency Discharge (NPHFD) Ignition has been pointed out as a probable optimal method. Previous works found three inter-pulse coupling regimes (fully-coupled, partially-coupled and decoupled) depending on the interaction between subsequent pulses, and each regime showed a different behavior and ignition probability. The fully-coupled regime (at short inter-pulse times) provided the highest ignition probability (almost 100%), whereas the partially-coupled regime (at intermediate inter-pulse times) resulted in the lowest ignition probability due to destructive interactions between the ignition kernels. No interactions between kernels existed in the decoupled regime (long inter-pulse times), where the ignition probability became a function of the number of pulses.

A new ignition tunnel was designed, manufactured and built in the Combustion and Diagnostics Laboratory at the Technion, improving some capabilities of previous designs. In order to validate the setup, hot-wire anemometry measurements were taken to characterize the flow profile, and previous results were matched qualitatively to ensure the repeatability of the experiments.

The present research continued with the parametric study of the NPHFD ignition in flowing mixtures and explored the effect a parameter that had not been varied before: the energy per pulse (E_pp). For a burst of pulses, the ignition probability and the inter-pulse coupling regimes showed new behaviors, suggesting that the partially-coupled regime can be avoided at a sufficiently high energy per pulse. In addition, low E_pp led to an ignition probability equal to 0% outside of the fully-coupled regime. The minimum ignition power - resulting in an ignition probability of 50% - was found to increase monotonically with the energy per pulse.

This research also sought energy distribution optimization for a set of conditions with fixed total energy deposition and fixed total discharge duration. A series of tests matching these two parameters were carried out while varying the number of pulses, E_pp and the inter-pulse time. It was concluded that the inter-pulse time is the main factor to determine the inter-pulse coupling regime and, consequently, the ignition probability.

Kernel growth rates were measured for all the conditions tested in the fully-coupled regime and no difference was found between the tested experimental conditions. It is believed that, at low flow velocities, the kernel growth is unaffected by the advection and only the energy deposited in the first discharge is useful to the kernel growth.