Chemical Lumping of detailed Mechanisms

Obtained by Computer Aided Designed

At the moment, complete mechanisms for modelling the combustion of organic compounds include several thousands of elementary reactions and thus, due to the present limitations of computer hardware, they cannot be embedded in three dimensionnal computationnal fluid dynamic codes in order to be employed to model the reactive flows found in real combustion devices. This leads to an urgent need of techniques to reduce the size of complex chemical mechanisms. Our laboratory has developed an automatic technique of reduction based on the chemical lumping of species involved in the primary mechanism . The basis of this technique was proposed by Ranzi et .al. (Ranzi E., Faravelli T., Gaffuri P., Pennati G. and Sogaro A., Comb. Flame, 102:179 (1995)). This permits to obtained reduced primary mechanisms from the complete primary mechanisms generated by EXGAS in files compatible with CHEMKIN II (Sandia National Laboratories).

The first step of the procedure consists to lump the free radicals which have the same molecular formula and the same functional group and to generate automatically the corresponding lumped mechanism. The concentration of a lumped species is equal to the sum of the concentrations of the real isomers, so that there is no mass balance problem with this kind of lumping. At this stage, the problem is to estimate the rate constants of the lumped reactions which permit to obtain results in a good agreement with the detailed mechanism. A reliable starting point for this estimation is obtained for the lumped reaction as a weighted average of the rate coefficients of the relevant elementary steps in the complete mechanism. More exactly, by assuming that the propagation chains are long, it is possible to apply the aproximation of quasi-stationary state for each free radicals involved in the detailed mechanism. Thus, the relative concentration of free radicals can be calculated by solving a linear system of equations and relative rates can be obtained for each reaction and used for this weighting.

Validation
This approach has permitted to strongly reduce primary mechanisms. Lumped primary mechanisms for the oxidation of iso-octane and n-heptane, for which the number of species has decreased respectively from 370 to 92 and from 475 to 100, give results in a good agreement with those obtained with the complete mechanisms.

In conclusion, this chemical lumping process can be automatically operated, leads to the minimun loss of chemical and kinetic informations and provides a good starting point for further mathematical reduction techniques.

Relevant publications

Lumping of the primary mechanism for oxidation of n-heptane

	T = 950K, P = 1 atm, Perfectly Stirred Reactor Molar mixture ratio (n-heptane : O2 : N2 ) of (0.15 : 8.25: 91.6). The points refer to experimental observations of Chakir et al. (1992)

Reduction rate obtained in the primary mechanism by the lumping procedure

Lumping of the primary mechanism for oxidation of iso-octane

	T = 923K, P = 0.29atm, Perfectly Stirred Reactor Molar mixture ratio (Iso-Octane : O2 : N2 ) of (9 : 18 : 73). The points refer to experimental observations of Simon et al. (1995).

Reduction rate obtained in the primary mechanism by the lumping procedure