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.
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Prediction of combustion instability is essential in reducing development costs of large scale rocket engines. Recently, high fidelity CFD models have shown some ability to represent the stability characteristics of sub-scale experimental combustors. A challenge remains in acquiring experimental measurements of critical combusting flow properties, especially measurements of the unsteady heat addition field, for comparison to models. Previous studies have shown that OH* and CH* emission intensity is a function of pressure, strain rate, equivalence ratio, and turbulence level. Since these flame properties have large variations at all times in an unstable rocket combustor it is difficult to decouple the chemiluminescent emission from all variables except heat release. Emission due to chemiluminescence can, however, be isolated from other flame property variables in a computational model. A detailed chemical kinetics model is used to investigate the relationship between chemiluminescent species and heat release rate of the flame. This predicted relationship is then compared to experimental spectral measurements captured with a fiber optic probe both to check the validity of the model and to provide insight on the ability of the chemiluminescent light emission to indicate heat release in the unstable CVRC flame.
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