Spark-ignited kernel dynamics in fine ethanol sprays and their relations with minimum ignition energy

Spark ignition of ethanol droplet/vapor/air mixture is studied with a Eulerian-Eulerian method and detailed chemical mechanism. The flame kernel-droplet interaction is quantified with an evaporation completion front (ECF). Two categories of spray flames can hence be defined based on the relative location between the ECF and flame front, i.e., homogeneous and heterogeneous spray flames. An element-based equivalence ratio (ER) at the flame front (flame ER for short) is introduced to measure the gas composition in evaporating sprays. For overall fuel-lean mixtures, quasi-stationary spherical flame (QSSF) occurs due to lean flame ER and the composition at the QSSF front is homogeneous. For overall fuel-rich two-phase mixtures, re-ignition, after the spark-ignited kernel fails, is observed when the droplet diameter is 15 μm for fuel sprays with both fuel-lean and fuel-rich background gas. This is due to rich flame ER and/or strong evaporative heat loss. Meanwhile, the kernel is born in a heterogeneous mixture and transition into homogeneous state is found. For both overall lean and rich two-phase mixtures, fuel droplets affect the ignitability and flame trajectories. Moreover, ignition energy affects the flame ER and front distance at the early stage of kernel development. Lastly, the minimum ignition energies (MIE) with different gas and overall ERs are investigated. Three regimes (A, B and C) are identified from the MIE variations with overall ER and the corresponding flame kernel dynamics behind them are summarized. Regime A is characterized by the QSSF phenomenon, whilst regime C embodies the ignition failure and re-ignition transients when the droplet size is relatively large. Furthermore, regime B only appears with a narrow range of overall ER when initial background gas ER is above unity. These regimes are further generalized in parameter space of overall ER versus initial background gas ER.