Implementation of simplified chemical kinetics based on intrinsic low-dimensional manifolds

Abstract

A general procedure for simplifying chemical kinetics and its use in reacting flow models is developed, which is based on the dynamical systems approach. In contrast to conventional reduced mechanisms no information is required concerning which reactions are to be assumed to be in partial equilibrium nor which species are assumed to be in steady state. Based on a local eigenvector analysis, the method identifies the fast time scales of the chemical reaction systems, which differ typically by orders of magnitude. Assuming that the fastest relaxation processes in chemical reactions proceed infinitely fast (i.e., are in local equilibrium), it is then possible to reduce the state space globally, such that it can be described by means of only a small number of reaction progress variables. The only “inputs” to the procedure are the detailed kinetics mechanism and the number of degrees of freedom required in the simplified scheme. Then the state properties given by the simplified scheme are automatically determined as functions of the coordinates associated with the degrees of freedom. A tabulation procedure allows an efficient use of the results in CFD codes. Furthermore a general procedure for coupling the reduced mechanism with other than chemical processes like flow and molecular transport is discussed. Results are presented for the CO/H 2 /air system both for a simple homogeneous closed system and a flow reactor.