Recent experimental observations show that lifted diesel flames tend to propagate back towards the injector after the end of injection under conventional high-temperature combustion conditions. Earlier studies have referred to this phenomenon as “flashback,” but more recently the term “combustion recession” has been adopted to reflect findings that the process appears dominated by “auto-ignition” reactions upstream of the lifted flame after the end of injection. Since this process is only initiated after the end of injection, it is also closely linked to the end-of-injection entrainment wave and its impact on the transient mixture-chemistry evolution upstream of the lift-off length. A few recent studies have explored the physics of combustion recession with experimental and simplified modeling approaches, but the details of the chemical kinetics and convective-diffusive transport of reactive scalars in this phenomenon are still largely unexplored. There are also uncertainties in the capability of engine computational fluid dynamics (CFD) simulations to accurately capture entrainment wave and combustion recession phenomena.

In this study, highly-resolved numerical simulations have been employed to explore the mixing and combustion of a diesel spray after the end of injection and the influence of modeling choices on the prediction of these phenomena. The simulations are centered on a temperature sweep around the Engine Combustion Network (ECN) Spray-A conditions, from 800–1000 K, where different combustion recession behaviors are observed experimentally. Reacting spray simulations are performed in the open-source CFD software OpenFOAM, using a Reynolds-Averaged Navier-Stokes (RANS) approach with a traditional Lagrangian-Eulerian coupled formulation for two-phase mixture transport. Two reduced chemical kinetics models for n-dodecane by Yao et al. and Cai et al. are used to evaluate the impact of low-temperature chemistry and mechanism formulation on predictions of combustion recession behavior. Observations from the numerical simulations are consistent with recent findings that a two-stage auto-ignition sequence drives the combustion recession process; self-sustained reacting mixtures arise in distinct regions that are spatially separated from the lifted flame. Simulations with two different chemical mechanisms indicate that low-temperature chemistry reactions drive the likelihood for second-stage ignition and combustion recession that in turn strongly influence local entrainment in these mixtures and likelihood of combustion recession.

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