Forced ignition and oscillating flame propagation in fine ethanol sprays

Abstract

The present work investigates forced ignition and oscillating propagation of spray flame in a mixture of fine ethanol droplets and air. The Eulerian-Eulerian method with two-way coupling is used and a detailed chemical mechanism is considered. Different droplet diameters and liquid fuel equivalence ratios (ER) are studied. The evaporation completion front (ECF) is defined to study the interactions between the evaporation zone and flame front. The gas composition at the flame front is quantified through an effective ER. The results show that the kernel trajectory is considerably affected by droplet size and liquid ER. Generally, the flame ER reaches the maximum when the ECF starts to move from the spherical center. It gradually decreases and reaches a constant value when the flame freely propagates. Quasi-stationary spherical flame is observed when the liquid ER is low, whilst kernel extinction/re-ignition appears when liquid ER is high. These flame behaviors are essentially affected by the heat conduction and species diffusion timescales between the droplet evaporation zone and flame front. The dependence of the minimum ignition energy (MIE) on liquid ER is U-shaped and there is an optimal liquid equivalence ratio range (ERo) with the smallest MIE. Long and short ignition failure modes are observed, respectively for small and large liquid ERs. When liquid ER is less than ERo (long failure mode), larger energy is required to initiate the kernel due to very lean composition near the spark. For liquid ER larger than ERo (short mode), ignition failure is caused by strong evaporative heat loss and rich gas composition due to heavy droplet loading. Flame speed oscillation is seen when the initial droplet size is above 10 μm. The oscillation frequency linearly increases with the liquid ER. Meanwhile, larger droplets lead to smaller oscillation frequencies.