A numerical investigation of NH3/O2/He ignition limits in a non-thermal plasma

Abstract

This numerical study of the ignition characteristics of ammonia in a pulsed plasma discharge includes the assembly of a kinetic model for the oxidation of ammonia/oxygen/helium mixtures under a plasma discharge. The model was used to perform a series of simulations under varying pulsed discharge frequencies and pulse numbers, at atmospheric pressure and moderate to high temperatures (600–1500 K). A zero-dimensional solver which combines the ZDPlasKin and CHEMKIN software is used to explore the effect of pulse number and frequency on ignition delay time. For a moderate amount of pulses, a reduction of 40–60% in ignition delay time is achieved, with higher pulse repetition frequencies (PRFs) yielding shorter ignition delay times. Analysis of OH radical time evolution reveals that high PRFs support an increasing radical pool at low temperatures, whereas at lower PRFs radicals recombine in between pulses. In the thermal runaway phase, the radicals formed in conventional chain branching events are prevalent, so that OH formed in later pulses has little effect. When looking low temperatures and high PRFs, higher pulse frequencies allow for lower initial temperatures which will result in ignition. At a high enough frequency, the hysteresis of ignition and extinction is altered due to a high amount of radicals supplied and sustained by the plasma, so that there is a smooth transition and reactions at all temperatures.