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

TL;DR

This study uses detailed simulations to analyze spark ignition in ethanol sprays, revealing how droplet size, mixture composition, and ignition energy influence flame development and minimum ignition energy.

Contribution

It introduces a new framework for understanding spark-ignited ethanol spray flames, including the concepts of ECF, flame ER, and the effects of droplet size and mixture composition on ignition.

Findings

Re-ignition occurs for fuel droplets of 15 μm in rich and lean mixtures.

Two categories of spray flames are identified: homogeneous and heterogeneous.

Minimum ignition energy varies with gas composition and equivalence ratio.

Abstract

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 {\mu}m 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.