Measurements of HO₂ Radical in a Preheated Plasma Flow Reactor

Absolute, time-resolved HO2 number density is measured by Cavity Ringdown Spectroscopy (CRDS) in dilute fuel-O2-Ar mixtures excited by a repetitive ns pulse discharge in a heated plasma flow reactor. Number density of HO2 radical generated during the plasma-assisted oxidation of H2, CH4, and C2H4 is measured at relatively low temperatures in the reactor, ranging from 300 to 600 K. The measurements are made at the discharge pulse repetition rate of 10 kHz, for different discharge burst durations, 5-100 pulses (0.5-10 ms), 100 µs after the last pulse. Additional measurements are made in the afterglow after the discharge burst, for different delays between the end of the burst and the laser pulse, ranging from 100 µs to 89 ms. The results show that for most experimental conditions, HO2 is generated only during the discharge burst, as expected. However, in the H2-O2-Ar mixture at elevated temperatures, 400-600 K, HO2 persists in the afterglow during the entire flow residence time in the reactor, up to 10 ms. This indicates increased reactivity and a more significant effect of the chain propagation and chain branching processes, which dominate the radical recombination reactions even at these relatively low temperatures. Comparison of the experimental results with the kinetic modeling predictions provides quantitative insight into the processes controlling generation and decay of HO2 radical in fuel-air mixtures excited by the plasma, mapping the conditions at which the generation of the primary radical pool by the plasma results in significant fuel oxidation and subsequent ignition.