NTC-affected ignition in nonpremixed counterflow

Abstract

Using n-heptane as the model fuel, the role of the Negative Temperature Coefficient (NTC) chemistry on ignition is computationally studied for both homogeneous mixtures and nonpremixed counterflow. For homogeneous mixtures, results show that ignition occurs in two-stages within the NTC temperature regime, with the duration and temperature increment decreasing with increasing initial temperature for the first ignition stage, leading to increased duration of the second ignition stage and thereby the observed NTC-behavior. For the nonpremixed counterflow, results show that a secondary S-curve is developed on the lower branch of the conventional, primary S-curve for sufficiently low strain rates and/or sufficiently high pressures, with its own distinct ignition–extinction turning points. Sensitivity analysis and Chemical Explosive Mode Analysis (CEMA) show that this secondary S-curve is controlled by the low-temperature NTC-chemistry, with heat release negligible for states on its lower branch and discernable for states on its upper branch and hence assuming the characteristics of a weakly-burning flame. The possible existence of one- and two-staged ignition in the counterflow is observed, and its dependence on the flow strain rate and system pressure are identified.