Assessment of Current Chemiluminescence Kinetics Models at Engine Conditions

Chemiluminescence continues to be of interest as a cost-effective optical diagnostic for gas turbine combustor health monitoring. However, most chemical kinetics mechanisms of the chemiluminescence of target species such as OH*, CH*, and CO₂* were developed from atmospheric-pressure data. The present paper presents a study wherein the ability of current kinetics models to predict the chemiluminescence trends at engine pressures was assessed. Shock-tube experiments were performed in highly diluted mixtures of H₂/O₂/Ar at a wide range of pressures to evaluate the ability of a current kinetics model to predict the measured trends. At elevated pressures up to 15 atm, the currently used reaction rate of H + O + M = OH* + M (i.e., without any pressure dependence) significantly over predicts the amount of OH* formed. Other important chemiluminescence species include CH* and CO₂*, and separate experiments were performed to assess the validity of existing chemical kinetics mechanisms for both of these species at elevated pressures. A pressure excursion using methane-oxygen mixtures highly diluted in argon was performed up to about 15 atm, and the time histories of CH* and CO₂* were measured over a range of temperatures from about 1700 to 2300 K. It was found that the existing CH* mechanism captured the T and P trends rather well, but the CO₂* mechanism did a poor job of capturing both the temperature and pressure behavior. With respect to the modeling of collider species, it was found that the current OH* model performs well for N₂, but some improvements can be made for CO₂.