Experimental Investigations on Laminar Burning Velocities of n-Heptane + Air Mixtures at Higher Mixture Temperatures Using Externally Heated Diverging Channel Method

Abstract

Experimental measurements of laminar burning velocity are presented for mixture temperatures above the autoignition temperature of premixed n-heptane + air mixtures using an externally heated mesoscale diverging channel method. Direct measurements of laminar burning velocity are carried out at 1 atm of pressure for an unburnt mixture temperature range (350–600 K) with the mixture equivalence ratio varying from ϕ = 0.6 to 1.5. Nonlinear power-law correlation is proposed to delineate the effect of change in the mixture temperature on the burning velocity variation at various equivalence ratios. A minimum value for the temperature exponent is observed for stoichiometric n-heptane + air mixtures. The maximum value of laminar burning velocity is measured for ϕ ≈ 1.1 at all mixture temperatures. The reported data sets are compared with the available experimental results and the predictions of various detailed kinetic models, i.e., LLNL V3.1 (2011), JetSurF 2.0 (2010), and Poli Mi (2014). Good agreement of the present data is observed with the predictions of the LLNL V3.1 reaction mechanism at all unburnt mixture temperatures. The predictions of other kinetic models show slight under-prediction at higher mixture temperatures. Sensitivity analysis using the LLNL V3.1 mechanism is reported to highlight the contribution of key reactions enhancing/reducing the laminar burning velocity at 470 and 600 K mixture temperatures. With an increase in the mixture temperature from 470 to 600 K, the influence of the chain branching reaction (H + O₂ ↔ O + OH) on laminar burning velocity increases nearly 63.5%. Formation of a vinyloxy radical (CH₂CHO) from the oxidation of a vinyl radical (C₂H₃) enhances the burning velocity at a 470 K temperature. Oxidation of C₂H₃ (at 600 K) becomes crucial in governing engine knocking in low-temperature oxidation of n-heptane. It is deduced from the reaction pathway diagram that the production of a highly reactive compound, C₂H₂, at 600 K plays a significant role and governs the overall reaction rate of the mixture.