Ignition of uniform droplet-laden weakly turbulent flows following a laser spark

The forced ignition process has a stochastic nature, which can be intensified due to turbulence and mixture fluctuations. Although fuel droplets represent strong inhomogeneities which are generally detrimental to ignition, the presence of small droplets has been found to enhance flame speeds, decrease minimum ignition energy, and improve the ignitability of overall lean mixtures. In order to understand which factors are conducive to ignition of sprays, a spherically expanding flame is investigated, which is produced by a laser spark in a uniform dispersion of ethanol droplets in turbulent air. The flame is visualised by schlieren and OH*-chemiluminescence for overall equivalence ratios of 0.8-2, Sauter mean diameter of approximately 25 mu m, and u'/S-L ranging from 0.9 to 1.3, where u' and S-L denote the rms axial velocity and laminar burning velocity, respectively. The timescales of the spark's effects on the flame are measured, as well as quenching timescales and initial kernel sizes conditional on ignition or failure. Small kernels quenched faster than approximately 0.6 ms, that is, the duration of the flame overdrive, and a minimum kernel radius for ignition of 1 mm was observed. The short-mode of ignition failure was suppressed by increasing the laser energy and, consequently, the initial kernel size. Nevertheless, the ignitability of lean mixtures was only effectively improved through high-energy sparks and partial prevaporisation of the fuel. Virtually all kernels ignited once prevaporisation was increased, and the gas-phase equivalence ratio was approximately 75% of the lower flammability limit, with ignition being limited only by laser breakdown. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.