Automatic Reduction Procedures for Chemical Mechanisms in Reactive Systems

Sammanfattning: To deal with complex physical and chemical processes in reactive systems, such as combustion processes, it is necessary to find methods that simplify modelling in such a way that it becomes both more comprehensible and practically useful. In the work reported here a method for automatically reducing chemical reaction mechanisms has been developed for use in computer simulations of various combustion systems. The present method opens the way here for the application of adaptive kinetics, in which mechanisms are reduced at each operating point which allows a careful and efficient reduction to be carried out. The reduction in question is based on lifetime analysis and an assumption of quasi-steady state, abbreviated as QSSA. Species with short chemical lifetimes and/or with only a minor influence on the chemical system are selected to be in steady state, their concentrations being dealt with by means of simple algebraic equations. The concentrations of these species are calculated by being automatically implemented in FORTRAN subroutines. These routines calculate both the steady state concentrations and the remaining source terms for the non-steady state species by use of an internal iteration loop. Different procedures for selecting the steady state species are investigated for a range of different combustion systems. The simplest selection parameter is to use the chemical lifetime of the species only. By also introducing a maximum element mass fraction and the maximum enthalpy that a steady state species can occupy, violations of the required mass and energy conservations when applying QSSA could be avoided. This method was tested for a number of different physical setups or conditions, such as a chain-reactor sequence, use of a plug flow reactor (PFR) followed by a perfectly stirred reactor (PSR), a spark ignition engine (SI engine), a homogenous charge compression ignition engine (HCCI engine), and various flame configurations. For improved ranking a combined lifetime- and species sensitivity measure, termed here the level of importance (LOI), was developed as a selection parameter for automatically capturing species that although having short lifetimes, are of importance for the chemical system. It was shown to successfully identify the steady state species present in a mechanism. In order to enhance the efficiency of the reduction procedure, a method for the application of adaptive kinetics is developed and is applied to a case of ignition. At each operating point, a lifetime analysis is performed and species are moved in and out of steady state. In this manner, only true steady state species are considered throughout the computation. During ignition, the number of species treated in detail is larger than during afterburning, where most species are in steady state or contain small mass fractions. Emphasis has been placed on increasing the efficiency of the reduction method and presenting a thorough validation. A method is developed for validating the selection parameters and testing for self-consistency. The method's performance is also compared with that of another, well established reduction procedure, the computational singular perturbation method, CSP.