On the minimum ignition energy and its transition in the localised forced ignition of turbulent homogeneous mixtures

Abstract

The minimum energy requirements for ensuring (i) just successful ignition and (ii) successful self-sustained flame propagation without the assistance of an external energy source following a successful ignition event have been analysed for the forced ignition of a homogeneous stoichiometric methane-air mixture under a wide range of turbulence intensities using three-dimensional Direct Numerical Simulation (DNS) data. It has been found that the minimum energy needed for successful ignition is also sufficient to ensure self-sustained flame propagation for small turbulence intensities. However, for large turbulence intensities, the minimum energy for ensuring self-sustained flame propagation can be considerably greater than the minimum energy needed just to successfully ignite the mixture. At low turbulence intensities, the thermal runaway has been obtained for the minimum ignition energy after the end of the energy deposition indicating an autoignition. For larger energy inputs and turbulence intensity, the thermal runaway was obtained during the energy deposition period. It has been found that the minimum energy requirements for ignition and self-sustained flame propagation increase with increasing turbulence intensity but a transition in this behaviour has been observed. There is a critical turbulence intensity such that the increase in the energy demand is significantly more rapid above the critical value than that for turbulence intensities smaller than the critical value. This has been found to be qualitatively consistent with previous experimental findings. The stochastic nature of the ignition event has been demonstrated by considering different realisations of statistically similar turbulent flow fields. The conditions giving rise to a successful ignition have been identified by a detailed analysis of the energy budget. A scaling analysis has been performed for the critical condition for ensuring self-sustained flame propagation and the insights gained from this analysis have been utilised to explain the physical mechanisms behind the transition of the minimum ignition energy.