Automated Chemical Mechanism Reduction

Over the past several years, REI has gained extensive experience with the generation and utilization of reduced chemical kinetic mechanisms. This work has been facilitated by the use of a software package developed by Prof. J.-Y. Chen of U.C. Berkeley. The software, CARM (Computer Assisted Reduction Method), automatically creates reduced chemical kinetic mechanisms starting with a detailed mechanism and a set of input problems representing the conditions under which the mechanism is to be used.

The CARM software reduces mechanisms using the quasi-steady-state (QSS) assumption for a number of species from a detailed chemical kinetic mechanism. The QSS assumption consists of assuming that the net rate of production of a species is zero. This assumption is valid under combustion conditions for a large number of minor and intermediate species. Assuming the QSS approximation for a species is not equivalent to

  • Removing its effect from the chemical system,
  • Assuming its concentration does not change, or
  • Assuming it is unimportant to the combustion process.

Concentrations of the QSS species are calculated in the subroutine produced by CARM. These values are functions of the non-QSS species, temperature and pressure and will change during a time-dependent integration or iterative solution as the inputs change. The QSS species help determine the reaction rates of the non-QSS species and thus play an important role in the chemical process. A species may be critical to the reaction dynamics yet still be well-approximated by a QSS relation. In this work we use an automated technique to apply this method to larger mechanisms and larger hydrocarbons than in previous studies.

There are four basic steps in the formulation of a reduced chemical kinetic mechanism:

  1. Identification of a short or “skeletal” mechanism containing only the most essential species and reaction steps of the detailed mechanism.
  2. Identification of appropriate quasi-steady-state approximations.
  3. Elimination of reactions using the algebraic relations obtained in Step 2.
  4. Solution of the coupled and nonlinear set of algebraic equations obtained in the previous steps to find the QSS species concentrations reaction rates of the non-QSS species.

CARM automates this procedure, producing source code for the calculation of the chemical source terms defined by the reduced mechanism. As inputs, CARM uses a set of perfectly stirred reactor (PSR) solutions as test problem results representing conditions of interest. CARM ranks species by the error, ?i, introduced by assuming they are in quasi-steady state using the expression

where Wip and Wic are respectively the rates of production and consumption for species i, and Xi is the mole fraction. The subroutine produced by CARM contains code that iteratively solves the coupled, nonlinear set of algebraic equations giving the concentrations of the quasi-steady-state species. The CARM-produced subroutines typically consist of about 3000 or more lines of FORTRAN code.

REI has created with proprietary CARM software to provide seamless generation and testing of reduced kinetic mechinisms within a variety of computational environments.