Assessing thermal non-equilibrium in hydrogen detonations with optical emission spectroscopy

This study employs optical emission spectroscopy to experimentally assess thermal non-equilibrium (NEQ) in stoichiometric H 2 detonations at 293 K under different initial pressures (30–100 kPa), diluents (Ar/N 2 ), dilution levels (40%–70%), and positions behind the shock. In total, more than 150 single-shot spectra are collected, and the study is conducted in three main parts. Firstly, the study focuses on collecting broadband emission spectra (200–700 nm) to identify the contributing species to each spectrum. The results indicate that only the hydroxyl radical (OH), near 310 nm, contributes significantly to all the spectra. Hence, this first result indicates that emission spectroscopy cannot be employed to measure the thermal non-equilibrium of the reactants (H 2 , O 2 , or N 2 ) in the induction zone for any studied condition. Secondly, the study specifically characterizes the OH ⋆ spectra, produced in the exothermic reaction zone, from which vibrational NEQ is observed. For most of the tested conditions, vibrational NEQ of OH(A 2 Σ + ) is detected, with a vibrational temperature 150–450 K above the gas temperature. In addition, our measurements indicate that the vibrational NEQ of OH (i) decreases with pressure, (ii) is stronger in N 2 -diluted than Ar-diluted mixtures, and (iii) increases with the dilution levels. Thirdly, the study is dedicated to the characterization of the relaxation of OH behind the detonation waves by measuring the evolution of the OH ⋆ spectra behind the shock. Our measurements evidence that OH relaxes to equilibrium faster at high pressure and low dilution, and that the relaxation is faster in Ar-diluted than N 2 -diluted mixtures. The experimental OH relaxation times agree with the predicted ones from the well-established Millikan and White correlations, within 5% or up to a factor of 3 on average, when including or neglecting the experimentally determined OH H 2 O contribution (i.e., determined from experimental fitting), respectively.