From elementary kinetics in perfectly stirred reactors to reduced kinetics utilizable in turbulent reactive flow simulations for combustion devices

Abstract

This study addresses the protocol of tests for evaluating the performance of skeletal, reduced and highly-reduced chemical kinetics mechanisms for one-dimensional (1D) laminar flame simulations and the validity of these tests for turbulent flame applications. Taking n-heptane as an example fuel, it is first shown that there are apparent disagreements regarding the elementary-reaction chemical kinetics models which should be emulated by reduced kinetic mechanisms. Further it is also shown, expectably, that the functional relationship between mass fractions and temperature is different between 0D and 1D models, and it is different among 1D models having different molecular species-transport formulations. Having identified the retention of this functional relationship as pivotal in the success of highly-reduced reaction mechanisms to accurately simulate spatial configurations, it is demonstrated that the unity Lewis-number assumption biases this functional relationship such that the highly-reduced chemical kinetic mechanism LS2T 20-species mechanism (LS2T 20) used for 1D flame simulations fails to agree with the template but provides accurate predictions in conjunction with, for example, the mixture-average diffusion model at otherwise the same initial conditions as the unity-Lewis-number simulation. The LS2T 20 success extends to the cold-ignition regime. For rich conditions the mixture-average diffusion approximation is less acceptable, and if the transport model does not include the direct effect of the heavy species (which are all modeled in LS2T) on the light species (which are all computed in LS2T), moderate departures of the 1D predictions from the template are obtained around the peak OH mass fraction, whereas CO_2 is still excellently reproduced by LS2T 20. It is then conjectured that, independent of the reduction method, the more reduced is a kinetic mechanism, the more accurate the transport formulation should be to ensure the preservation of the species/temperature functional relationship; due to the extensive investigation necessary to evaluate this conjecture, this activity is relegated to future studies.

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