Effects of evaporation on chemical reactions in counterflow spray flames

Abstract

The role of evaporation on chemical reactions in counterflow spray flames remains a key issue due to the non-linear inter-phase interactions through mass, momentum, and energy conservation. An extended chemical explosive mode analysis to illustrate the role of evaporation (ECEMA) was previously developed by separating the evaporation source term from non-chemical terms and projecting them onto the chemical explosive mode (CEM). Evaporation was found to promote chemical reactions when the fuel supply effect dominated the evaporative cooling and to inhibit reactions otherwise. In this work, ECEMA is applied to one-dimensional laminar and three-dimensional turbulent counterflow spray flames. For laminar cases, ECEMA is applied to multi-modal spray flame solutions including the distributed, collocated, and cool flame. The analysis for the collocated and cool flame shows similar behavior, namely, that evaporation inhibits chemical reactions due to heat absorption near the fuel injection region and enhances chemistry around the zero-crossing CEM. For the distributed flame, the promotion effect is observed for most of the domain. For the turbulent counterflow flame, three CEM regions are identified, namely, a hybrid region, an inhibition region, and a promotion region. In particular, for the hybrid region near the spray injection side, the contribution of evaporation to chemical reactions changes from inhibition to promotion. For the inhibition region in the middle, significant suppressing of chemical reactions by evaporation is observed. In the promotion region near the oxidizer side, evaporation remains promoting chemical reactions. For the case investigated, the dominant combustion modes are the assisted ignition and local extinction.