Simulation Of Nanosecond Repetitively Pulsed Plasma Assisted Counter-Flow Diffusion Flame

Abstract

Plasma-assisted combustion (PAC) is a promising technology for improving combustion efficiency and extending operating conditions, such as enabling lean combustion at lower equivalence ratios 1 . Nanosecond repetitive pulsed (NRP) plasma is especially effective because it generates high concentrations of excited molecules and radicals with low energy consumption 1 . This study proposes a computationally efficient model for investigating PAC of diffusion flames using nanosecond repetitively pulsed (NRP) plasma. In the model, NRP plasma discharges are generated in the oxidizer stream of a counter-flow diffusion flame. We examine numerically the effect of changing the oxidizer flow rate into the plasma discharge region (hence increasing gas residence time in the plasma) and the pulse repetition frequency (PRF) of the NRP plasma discharges on the extinction curves of the counter-flow diffusion flame. We show that plasmas with higher PRFs produce higher oxygen atom concentrations and gas temperature, which are both beneficial for combustion, can extend the extinction strain rate limit more than decreasing the oxidizer flow velocity. Increasing the PRF from 1 to 2 kHz increases the extinction strain rate limit by approximately 153%; decreasing the oxidizer flow velocity from 4 m/s to 0.25 m/s (1:16 residence time) increases the extinction strain rate limit by only 78%. Using the model to simulate the ignition of the flame with NRP plasma discharges shows that using a higher PRF requires applying more pulses to ignite the flame. For example, a PRF of 4 kHz requires 9 pulses for ignition, whereas a PRF of 8 kHz requires 12 pulses. Possible extensions of the model to a wider range of NRP-based PAC applications will be discussed.