The correlation of species concentration with heat release rate in an auto-igniting turbulent n-heptane spray flame

The quantification of the heat release process is important for engine combustion, for which species concentrations are often employed as the heat release rate indicators (HRRIs) considering the difficulty in the direct measurement of the local heat release rate (HRR). In this study, the correlation between HRR and species concentrations in auto-ignition and auto-ignition assisted flame propagation of n-heptane/air mixtures is first quantified to validate the commonly used formaldehyde-based HRRIs (i.e. [HCO] and [OH][CH2O]). Potential candidates for HRRIs are then proposed and further investigated in the auto-igniting n-heptane spray flame obtained with direct numerical simulations using a 44-species mechanism. It is found that [HCO] and [OH] [CH2O], as well as the potential candidates [C2H3], [CH2], [OH][HO2] and [H][HO2], in general all have good linear correlations with the HRR in the spray flame, and [H][HO2] has the best performance. Chemical explosive mode analysis is then performed to further analyze the performance of the HRRIs in the three combustion regimes: post-flame, auto-ignition and flame propagation. It is shown that the primary combustion mode in this spray flame is auto-ignition. Regime-dependent correlation has been observed and the above HRRIs yield the best linear correlation for the region with the Damkohler number being far more than unity in this turbulent auto-igniting n-heptane flame.