An optimized kinetics model for OH chemiluminescence at high temperatures and atmospheric pressures

Chemiluminescence from the OH(A -> X) transition near 307 nm is a commonly used diagnostic in combustion applications such as flame chemistry, shock-tube experiments, and reacting-flow visualization. Although absolute measurements of OH(X) concentrations are well defined, there is no elementary relation between emission from the electronically excited state (OH*) and its absolute concentration. Thus, to enable quantitative emission measurements, a kinetics model has been assembled and optimized to predict OH* formation and quenching at combustion conditions. Shock-tube experiments were conducted in mixtures of H(2)/O(2)/Ar, CH(4)/O(2)/Ar, and CH(4)/H(2)/O(2)/Ar with high levels of argon dilution (> 98%). Elementary reactions to model OH*, along with initial estimates of their rate coefficients, were taken from CH + O(2) reversible arrow OH* + CO (R0) H + O + M reversible arrow OH* + M (R1) Sensitivity analyses were performed to identify experimental conditions underwhich the shape of the measured OH* profiles and the magnitude of the OH* emission would be sensitive to the formation reactions. A fitting routine was developed to express the formation rate parameters as a function of a single rate, k(1) at the reference temperature (1490 K). With all rates so expressed, H(2)/CH(4) mixtures were designed to uniquely determine the value of k(1) at the reference temperature, from which the remaining rate parameters were calculated. Quenching rates were fixed at their literature values. The new model predicts the experimental data over the range of conditions studied and can be used to calibrate the emission diagnostic for other applications, such as measurements in real combustion environments, containing higher order hydrocarbon fuels and lower levels of dilution in air. (c) 2006 Wiley Periodicals, Inc.