Structural analysis and regime diagrams of laminar counterflow spray flames with low-temperature chemistry

The understanding of spray combustion is a complex scientific problem, as it involves the interaction between evaporation, convection, diffusion, and chemistry. The focus of this study is to examine the effects of chemistry, in particular the low-temperature chemistry, on the structure of canonical counterflow spray flames at relatively low strain rate conditions. Parametric studies using a Eulerian formulation are carried out for monodisperse n-dodecane counterflow spray flames, in order to analyze the sensitivity of the flame structure to strain rate and droplet diameter over a wide range of equivalence ratios. These results reveal complex and multi-modal spray-flame structures that exhibit collocated, distributed, and cool flames. For the collocated flame, the presence of single and multiple low-temperature reaction zones is identified, which results from the evaporation and low-temperature chemistry interaction. The impact of low-temperature chemistry on the distributed flame is not pronounced. For low strain-rate conditions, the presence of a weak cool flame without high-temperature reaction zones is reported. Regime diagrams with respect to Stokes number and equivalence ratio are constructed, identifying distinctly different flame structures. By independently varying the strain rate and changing the droplet diameter, qualitatively similar but quantitatively different regime diagrams are obtained due to the lack of intrinsic self-similarity of the spray flamelet solution. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.