STOCHASTIC MODELING OF CO AND NO IN PREMIXED METHANE COMBUSTION

The ability to use reduced CH4-air chemical mechanisms to predict CO a nd 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 me chanism. NO emissions from thermal, N2O-intermediate, and prompt pathw ays were included in the 5-, 9-, and 276-step mechanisms. Molecular mi xing was described with a deterministic, Interaction-by-Exchange-with- the-Mean (IEM) submodel. Random selection and replacement (without rep etition) of fluid particles were used to simulate through-flow. The ev olution of mean and root mean square (rms) temperature, CO, and NO in the PaSR was accurately described with the 9-step mechanism over a wid e range in mixing frequency and equivalence ratio. Also, the 9-step me chanism provided accurate instantaneous reaction rates and concentrati ons 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 overpre dicted instantaneous temperature reaction rates, most likely due to it s inferior parent mechanism, partial equilibrium assumption for OH, an d unallowed dissociation of neglected radical species. The detailed an d reduced mechanism predictions of the accessed composition space in t he PaSR covered only a small fraction of the allowable composition spa ce, thus facilitating the use of an efficient in situ chemical look-up table for multidimensional, pdf-method calculations. (C) 1998 by The Combustion Institute.