A CSP-BASED SKELETAL MECHANISM GENERATION PROCEDURE: AUTO-IGNITION AND PREMIXED LAMINAR FLAMES IN N-HEPTANE/AIR MIXTURES

We use a procedure based on the decomposition into fast and slow dynamical components offered by the Computational Singular Perturbation (CSP) method to generate automatically skeletal kinetic mechanisms for the simplification of the kinetics of n-heptane oxidation. The detailed mechanism of the n-heptane oxidation here considered has been proposed by Curran et al. [H.J. Curran, P. Gaffuri, W.J. Pitz, and C.K. Westbrook, n-Heptane, detailed mechanism, Version 2, www-cms.llnl.gov/combustion/combustion2.html, 2002] and involves 561 species and 2538 reactions. This work achieved three main goals. First, we carried out a thorough error analysis aimed at verifying which of the two algorithmic options involving or not the scaling of the CSP indices is the most suited for achieving accurate skeletal mechanisms. The comparative analysis showed that although both options produce valid mechanisms, scaling the indices seems to offer a smoother dependence of the accuracy with respect to the number of species retained in the mechanism, and for this reason seems to be the most preferable. Second, by using the scaled index option, we generated two series of accurate skeletal mechanisms for n-heptane oxidation, one series valid for both a wide range of initial temperatures and equivalence ratios, and the other only for the high temperatures regime (and for different equivalence ratios). Finally, we verified that it is possible to use skeletal mechanisms generated with respect to auto-ignition phenomena for computing premixed laminar flames with high accuracy, especially with respect to macroscopic parameters such as laminar flame speed, equilibrium temperature, and velocity, temperature and major species fields across the flame. This test showed that for a premixed laminar flame it is not essential to include the low temperature kinetics of n-heptane. This allowed us to obtain a satisfactory approximation of the flame structure with a rather small mechanism, which includes only 66 species out of the original 561. These findings empirically demonstrate that the reduction can be performed in a simple configuration, like the homogeneous auto-ignition considered in this paper and the resulting reduced mechanism applied successfully to a more complex configuration such as a premixed or counterflow flame, or, even, a fully multidimensional CFD reactive flow simulation. It is noteworthy to stress, in closing, that the 66-species mechanism seems a good and affordable candidate to tackle the direct simulation of n-heptane combustion.

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