The Role of Turbulent-Chemistry Interaction in Simulating End-of-Injection Combustion Transients in Diesel Sprays
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Copyright 2017 SAE International. This study investigates the role of turbulent-chemistry interaction in simulations of diesel spray combustion phenomena after end-of-injection (EOI), using the commercially-available CFD code CONVERGE. Recent experimental and computational studies have shown that the spray flame dynamics and mixture formation after EOI are governed by turbulent entrainment, coupled with rapid evolution of the thermo-chemical state of the mixture field. A few studies have shown that after EOI, mixtures between the injector nozzle and the lifted diffusion flame can ignite and appear to propagate back towards the injector nozzle via an auto-ignition reaction sequence; referred to as "combustion recession". Because combustion recession occurs in the near-nozzle region, where characteristic fuel jet scales are on the order of the injector nozzle diameter, typical engine CFD simulations with relatively large grid scales may not accurately capture sub-grid scale turbulent mixing and mixing-chemistry interactions in this phenomenon. In this study, CFD simulations of combustion recession in diesel spray flames are executed to explore this topic. The Representative Interactive Flamelets (RIF) model with a multiple flamelets approach is employed to account for the non-uniformity of reactive scalars at the sub-grid scale. The results are compared with a laminar chemistry based combustion model, i.e. Well-Stirred Reactor (WSR) model. Both simulations are performed using a Reynolds-Averaged Navier-Stokes (RANS) framework, such that all the resolved quantities are characterized by ensemble average variables. Both chemistry modeling methods utilize the same chemical kinetics, so that the effect of turbulent-chemistry interaction can be assessed independent of kinetics. The results from both approaches are validated against experimental measurements of liquid and vapor penetration lengths under non-reacting conditions, and ignition delay time and flame lift-off length under reacting conditions. Results show that the combustion model choice plays a significant role in the prediction of both initial spray flame stabilization and combustion recession. The introduction of turbulent mixing in combustion modeling significantly changes the turbulent mean scalar fields, and may better represent unsteady flame flapping motions. This study also reveals that there are needed improvements to the RIF model to better account for fast changes in flamelet history associated with the local scalar dissipation rate, which is prevalent in diesel spray combustion problems.
