(673b) Pressure Effects IN Combustion KINETICS

Progress in Computational Fluid Dynamics (CFD) has led to the capability of describing reacting flows with increasingly detailed chemistry. It offers the advantage of simulations as a supplement to direct physical testing This is a progress report of efforts in developing a fundamentally based chemical kinetics data base for such activities. Real fuels are complex mixtures of hydrocarbons. With current interest in alternative fuels such mixtures will even more complex. .Most of the existing databases are based on single component fuels. Adjustments are necessary in order to match global targets. For mixtures, there is no guarantee that the individual adjustments are consistent. Databases of any fuel mixture is not the sum of that for the individual components. With a fundamentally based database, mixing rules are transparent and adjustments can be made properly.

Reactions in kinetics databases can be classified as bimolecular and unimolecular. The situation with respect the former is satisfactory. There are extensive measurements and correlations have been developed. Such effects are easily assimilated in modern databases. Unimolecular reactions in the most general sense are more complex and is the subject of the present work. A convenient format for the representation of chemical kinetic data in combustion databases has been in the Arrhenius or modified Arrhenius forms. They imply that molecules have a Boltzmann distribution. In combustion systems the high temperature and large concentrations of reactive intermediates can lead to distortions of the Boltzmann distributions. Collisions will change these distributions. This results in pressure dependencies in the rate expressions. .

There are two types of pressure dependent unimolecular reactions. The first begins with a molecule in a Boltzmann distribution. Reactions in the high energy end are so fast that the tail of the distribution becomes truncated. The consequence is a reduction from the high pressure value. The second category involves a molecule that is formed by the combination of two reactive species. This molecule will contain the excess energy from the formation of the bond. Collisions with the bath will de-excite the molecule and remove sufficient energy so that at the highest pressure thermal rate constants will be obtained. In the intermediate pressure region rate constants will be higher than thermal. Both phenomena are well understood. The actual procedures to produce the required pressure dependent rate constants will be discussed. The types of reaction system where these processes are operative will be reviewed