CH* chemiluminescence modeling for combustion diagnostics

CH* chemiluminescence is often employed in combustion diagnostics, for example as a measure of heat release rate and for equivalence ratio sensing. However, most interpretations of CH* chemiluminescence rely either on heuristic arguments or empirical data gathered under limited conditions. More rigorous analysis is required to understand the effects of combustion conditions, e.g., pressure, reactant composition and preheat, strain and exhaust gas recirculation, on CH* chemiluminescence. Chemiluminescence modeling holds promise in this regard. The predictive accuracy of four proposed CH* chemiluminescence formation models were experimentally tested in premixed, methane-air and prevaporized Jet-A-air flames. Two of the models, based on CH* formation via reactions between C2H and O or O-2, are able to predict the experimental data within the experimental uncertainty, in both room temperature methane and preheated Jet-A flames. The utility of CH* chemiluminescence for sensing heat release rate and equivalence ratio (phi), when combined with OH* chemiluminescence, is then analyzed in methane flames for varying pressure, preheat temperature and strain. The CH*/OH* chemiluminescence ratio is found to be useful for sensing equivalence ratio in lean methane systems, but only at certain pressure and reactant temperature conditions. The relationship between CH* chemiluminescence and heat release varies with phi, pressure, temperature and strain. At high pressures, however, the dependence on phi and strain are small, making CH* attractive for heat release sensing applications in gas turbine combustors. (c) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.