Stochastic modeling of CO and NO in premixed methane combustion

The ability to use reduced CH4-air chemical mechanisms to predict CO and NO emissions in premixed turbulent combustion has been evaluated in a Partially Stirred Reactor (PaSR) model. CO emissions were described with reduced 4-, 5-, and 9-step mechanisms and a detailed 276-step mechanism. NO emissions from thermal, N2O-intermediate, and prompt pathways were included in the 5-, 9-, and 276-step mechanisms. Molecular mixing was described with a deterministic, Interaction-by-Exchange-with-the-Mean (IEM) submodel. Random selection and replacement (without repetition) of fluid particles were used to simulate through-flow. The evolution of mean and root mean square (rms) temperature, CO, and NO in the PaSR was accurately described with the 9-step mechanism over a wide range in mixing frequency and equivalence ratio. Also, the 9-step mechanism provided accurate instantaneous reaction rates and concentrations for a broad region of the accessed composition space in the PaSR. The 5-step mechanism performed less reliably than the 9-step mechanism at phi = 1.0 but performed similarly to the 9-step mechanism at phi = 0.65. The Li-step mechanism underpredicted mean CO values and overpredicted instantaneous temperature reaction rates, most likely due to its inferior parent mechanism, partial equilibrium assumption for OH, and unallowed dissociation of neglected radical species. The detailed and reduced mechanism predictions of the accessed composition space in the PaSR covered only a small fraction of the allowable composition space, thus facilitating the use of an efficient in situ chemical look-up table for multidimensional, pdf-method calculations. (C) 1998 by The Combustion Institute.