Effects of non-equilibrium plasma discharge on counterflow diffusion flame extinction

A non-equilibrium plasma assisted combustion system was developed by integrating a counterflow burner with a nano-second pulser to study the effects of atomic oxygen production on the extinction limits of methane diffusion flames at low pressure conditions. The production of atomic oxygen from the repetitive nano-second plasma discharge was measured by using two-photon absorption laser-induced fluorescence (TALIF). The results showed that both the atomic oxygen concentration production and the oxidizer stream temperature increased with the increase of the pulse repetition frequency for a constant plasma voltage. The experimental results revealed that the plasma activated oxidizer significantly magnified the reactivity of diffusion flames and resulted in an increase of extinction strain rates through the coupling between thermal and kinetic effects. Numerical computations showed that atomic oxygen quenching strongly depends on the oxidizer stream temperature. The kinetic effect of atomic oxygen production by a non-equilibrium plasma discharge on the enhancement of flame extinction limits was demonstrated, for the first time, at high repetition frequencies with elevated oxidizer temperatures. The reaction paths for radical production and consumption were analyzed. It was concluded that in order to achieve significant kinetic enhancement from atomic oxygen production on flame stabilization, the plasma discharge temperature needs to be above the critical crossover temperature which defines the transition point from radical termination to chain-branching.