The use of OH* and CH* as heat release markers in combustion dynamics

The study examines chemiluminescence measurements and computations as a method of validation. Because chemiluminescent radicals are present in small concentrations and associated timescales are also small, a common assumption is that these are quasi-steady-state species and their transport can be ignored. This paper examines the above assumption using available data and simulation results, specifically aiming at a single element rocket combustor under strong pressure fluctuations. Variations in the kinetics rate constants are considered for assessing sensitivity of the results. Two additional aspects are explored: the ability of the excited species to represent the chemical heat release and the optical thickness of the medium. For the conditions of the study, the quasi-steady-state assumption in the case of OH* is found to be marginally insufficient while in the case of CH*, it is found to be acceptable. Modeled heat release is qualitatively better captured by CH* than OH* because of higher difference in the peak to asymptotic concentrations and lower ground state concentration.