Automatic reduction procedure for chemical mechanisms applied to premixed methane/air flames

Abstract

An existing skeletal mechanism for laminar premixed methane/air flames has been used as a starting point for further automatic reduction by quasi-steady-state approximation (QSSA) for species with short chemical lifetimes and/or minor influence on the chemical system. Individual species are ranked with respect to static and dynamic characteristics according to a level of importance (LOI) measure obtained from their chemical lifetimes, diffusion velocities and flame-zone residence times in combination with a species sensitivity measure. The maximum element mass fraction and the maximum enthalpy occupied by a certain molecular species are constrained in order to limit the mass and energy deficiency caused by QSSA. Maximum values of lifetime and LOI are accumulated over the entire flame length for a range of fuel/air equivalence ratios . Species with low LOI are selected for QSSA, and their concentrations are calculated iteratively by solving the coupled algebraic system. Kinetic models with a varying degree of reduction are then automatically generated and implemented as FORTRAN source code by setting different lower LOI and element mass fraction limits. It is found that the lifetime and LOI measure differ due to the inclusion of sensitivity counteracting the rise in lifetime at low temperatures. The species ranking by the LOI disfavors reasonably stable species, which are removed from the system. The laminar burning velocities as predicted by the most strongly reduced mechanism with five global reaction steps show very good agreement with detailed calculations. The profiles of steady-state species also agree well if, the corresponding species lifetime is short.