Towards a kinetics model of CH chemiluminescence
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Visible emission from CH(A2Δ→-X2∏) chemiluminescence, CH*, near 431 nm has been measured in a shock tube and its kinetics studied at high temperatures (1200-2300 K) and atmospheric pressures. Constant optical settings were maintained throughout a series of experiments from which the functional variation of maximum intensity with temperature and stoichiometry was measured. Experimental mixtures consisted of CH4 + H2 as the fuel with varying initial concentrations of O2, all highly diluted in Ar. From these data, the variation of maximum intensity with equivalence ratio was determined for various temperatures. Chemical kinetics simulations indicated that the shape of the maximumintensity curve as a function of equivalence ratio (for T = constant) is most strongly sensitive to the key CH* formation reaction, C2 + OH = CH* + CO (R3) so that its rate coefficient could be measured directly by fitting the experimental curves. The formation path C2H + O = CH* + CO (R1) was of secondary importance and a new estimate of its rate is offered as well. Calculations were performed with the well-known methane mechanism GRI-Mech 3.0 with CH* reactions and thermodynamic data added. Additional formation reactions are considered, and quenching rates are fixed at their literature values. It was found that the addition of excess O2 to the experimental mixtures significantly reduced the CH* yield and that the extent of this reduction, relative to the stiochiometric point, was most sensitive to the formation rate measured (i.e. k3). These data constitute strong evidence in favor of the classical formation path through C2. The recommended rate expressions are: k3 = 2×1014 cm3 mol-1 s-1 k 1 = 5.2×1011exp(-2600/RT) with order-of-magnitude uncertainties possible. Uncertainties in the C2H kinetics are considered, and results are compared to previously reported values. The strong reduction in concentration with O2 addition was sufficient to establish the dominant formation path (R1 and R3) for the conditions herein, setting the direction for future studies. Suggestions are made for the further optimization of a CH* kinetics model with applications in combustion diagnostics, propulsion, and other such aerospace sciences.
author list (cited authors)
Hall, J. M., De Vries, J., Amadio, A. R., & Petersen, E. L.