Effects of fuel Lewis number on the minimum ignition energy and its transition for turbulent homogeneous fuel–air mixtures

The effects of fuel Lewis number on the minimum ignition energy (MIE) requirements for ensuring successful thermal runaway, and self-sustained flame propagation have been analysed for forced ignition of homogeneous fuel–air mixtures under decaying turbulence for a wide range of initial turbulence intensities using three-dimensional direct numerical simulations. The minimum energy demand for ensuring self-sustained flame propagation has been found to be greater than that for ensuring only thermal runaway irrespective of its outcome for large turbulence intensities, and the minimum ignition energy increases with increasing rms turbulent velocity irrespective of the fuel Lewis number. The MIE values have been found to increase more sharply with increasing turbulence intensity beyond a critical value for all fuel Lewis numbers considered here. The variations of the normalised MIE (MIE normalised by its laminar value) with increasing turbulence intensity beyond the critical point follow a power-law and the power-law exponent has been found to increase with an increase in fuel Lewis number. This behaviour has been explained using a scaling analysis. The stochasticity associated with forced ignition has been demonstrated by using different realisations of statistically similar turbulent flow fields for the energy inputs corresponding to the MIE and successful outcomes are obtained in most instances, justifying the evaluation of the MIE values in this analysis.