A Simulation of the Effects of Varying Repetition Rate and Pulse Width of Nanosecond Discharges on Premixed Lean Methane-Air Combustion

Two-dimensional kinetic simulation has been carried out to investigate the effects of repetition rate and pulse width of nanosecond repetitively pulsed discharges on stabilizing premixed lean methane-air combustion. The repetition rate and pulse width are varied from 10？kHz to 50？kHz and from 9？ns to 2？ns while the total power is kept constant. The lower repetition rates provide larger amounts of radicals such as O, H, and OH. However, the effect on stabilization is found to be the same for all of the tested repetition rates. The shorter pulse width is found to favor the production of species in higher electronic states, but the varying effects on stabilization are also found to be small. Our results indicate that the total deposited power is the critical element that determines the extent of stabilization over this range of discharge properties studied. 1. Introduction Nanosecond repetitively pulsed discharges have been studied as possible sources to stabilize combustion at fuel-lean and blow-off conditions [1]. The stabilization has been attributed to the significant radical production and gas heating within the discharges [1, 2]. The relative importance of these two outcomes on the stabilization has been debated. However, the effects of these two discharge outcomes on stabilization are difficult to separate because the two mechanisms stem from the same process of collisional quenching of electronically excited species (produced by direct electron impacts) [2, 3]. Also, the influence of radicals such as O and H is more pronounced at high gas temperature where the branching and propagation reactions can compete with the termination reactions. It is notable that Pilla et al. [4] reported that they were able to stabilize combustion when the discharge is at filamentary mode (e.g., streamers) rather than at glow mode. Deminsky et al. [5] also reported that an acceleration of the ignition process is seen in premixed supersonic flows with filamentary discharge modes. This suggests that the gas heating that is often more pronounced in the filamentary mode is necessary for significant radical production and that the elevated gas temperature results in the persistence of radicals during the time between pulses. A few tens of microjoule of energy per pulse, tens of kilohertz repetition rates, and a few kilovolt peak voltages are typical operation conditions of nanosecond repetitive discharges used in combustion stabilization. Under these conditions, the average electron number density that can be sustained is about 1011？cm？3 peaking as high as 1015？cm？3 with a