Pure Rotational CARS Measurements of Thermal Energy Release and Ignition in Nanosecond Pulse Burst Air and Hydrogen-Air Plasmas

Abstract

Pure rotational Coherent Anti-Stokes Raman Spectroscopy (CARS), complemented by UV emission and ICCD imaging, is used to study low-temperature plasma assisted fuel oxidation kinetics and ignition in a repetitive nanosecond pulse discharge in hydrogen-air at stoichiometric and fuel lean conditions at 40 Torr pressure. Air and premixed fuel-air mixtures are excited by a burst of high-voltage nanosecond pulses (peak voltage 20 kV, pulse duration ~25 nanoseconds) at a 40 kHz pulse repetition rate and burst repetition rate of 10 Hz. The number of pulses in the burst is varied from a few pulses to a few hundred pulses. The results are compared to a new hydrogen-air plasma chemistry model which incorporates nonequilibrium plasma processes, low temperature H2 – air chemistry, non-empirical scaling of nanosecond discharge pulse energy coupled to the plasma with the pulse waveform and the number density, and quasi-one-dimensional conduction heat transfer. Experimental centerline time-resolved temperature and O2 mole fraction, determined as a function of number of pulses in a burst, are found to agree well with model predictions. The results demonstrate that the heating rate in low temperature hydrogen-air plasmas is much faster than in pure air plasmas, primarily due to energy release from the exothermic reactions of fuel with O and H atoms generated in non-equilibrium quantities in the plasma. Specifically, it is found that the initial heating rate at room temperature is controlled by the low temperature processes, O + HO2 → OH + O2 and OH + H2 → H2O + H, where HO2 is formed by three-body recombination of O and H2. At intermediate temperatures, 500 – 600 K, significant chain branching, with associated additional energy release, occurs in reactions of O with HO2, as well as in O + H2 → OH + H reaction. Both chain branching and net exothermic heat release in plasma chemical reactions becomes more pronounced at higher temperatures, eventually resulting in ignition. Sensitivity analysis also shows that generation of radicals in the plasma is key to low-temperature plasma chemical fuel oxidation and associated heat release, while ignition is primarily controlled by the well known chain branching sequence O + H2 → OH + H and H + O2 → OH + O. Rapid plasma chemical hydrogen oxidation, in ϕ = 0.5 and ϕ = 1.0 mixtures, leads to a distinct maximum in temperature which is both predicted, and observed, after approximately 700 discharge pulses (17.5 msec), concurrent with a predicted and observed rapid loss in O2. UV emission and ICCD imaging provides further evidence that plasma chemical reactions lead to volumetric ignition at pressures in the approximate range 40 - 100 Torr, and equivalence ratios in the approximate range φ = 0.3 to φ = 1. Experimental ignition delay times are found to be a strong function of pressure, but a weak function of equivalence ration, general trends which are consistent with modeling predictions.