LES of n-Dodecane Spray Combustion Using a Multiple Representative Interactive Flamelets Model

Abstract

-A single-hole n-dodecane spray flame is studied in a Large-Eddy Simulation (LES) framework under Diesel-relevant conditions using a Multiple Representative Interactive Flamelets (MRIF) combustion model. Diesel spray combustion is strongly affected by the mixture formation process, which is dominated by several physical processes such as the flow within the injector, break-up of the liquid fuel jet, evaporation and turbulent mixing with the surrounding gas. While the effects of nozzle-internal flow and primary breakup are captured within tuned model parameters in traditional Lagrangian spray models, an alternative approach is applied in this study, where the initial droplet conditions and primary fuel jet breakup are modeled based on results from highly resolved multiphase simulations with resolved interface. A highly reduced chemical mechanism consisting of 57 species and 217 reactions has been developed for n-dodecane achiving a good computational performance at solving the chemical reactions. The MRIF model, which has demonstrated its capability of capturing combustion and pollutant formation under typical Diesel conditions in Reynolds-Averaged Navier-Stokes (RANS) simulations is extended for the application in LES. In the standard RIF combustion model, representative chemistry conditioned on mixture fraction is solved interactively with the flow. Subfilter-scale mixing is modeled by the scalar dissipation rate. While the standard RIF model only includes temporal changes of the scalar dissipation rate, the spatial distribution can be accounted for by extending the model to multiple flamelets, which also enables the possibility of capturing different fuel residence times. Overall, the model shows good agreement with experimental data regarding both, low and high temperature combustion characteristics. It is shown that the ignition process and pollutant formation are affected by turbulent mixing. First, a cool flame is initiated at approximately stoichiometric mixture and propagates towards the rich side. Hence, heat and radicals are transported away from the most reactive mixture and thus the ignition is delayed. At the same time, the transported heat and radicals increase the reactivity of rich mixtures, which strongly affects the CO formation. NO was found to increase compared to the no transport case due to enhanced mixing, which is related to a broader high-temperature zone and the additional transport of oxygen from lean into high-temperature regions.

Heavy-Duty Diesel Engine Spray Combustion Processes: Experiments and Numerical Simulations

“…liquid length is sustained in the reactive case, this illustrates how formaldehyde may surround the rich core where liquid fuel is still present. In several modeling results for the 0.09-mm Spray A conditions, formaldehyde is also predicted to surround the liquid core, reaching much further upstream compared to experimental observations [9,34,35,37,60]. Potentially, low number densities in combination with turbulent fluctuations of the liquid core, and elastic scattering of the laser light off the liquid fuel obscure these observations for a smaller injector orifice.…”

A Reduced-order Model for Multiphase Simulation of Transient Inert Sprays

“…In addition, C vap was set to unity in this case to reduce the evaporation rate. and the CAS model with Rosin-Rammler PDF as the initial droplet size distribution, which is obtained from the DNS of primary breakup of Spray A in Davidovic et al (2017). The results of lognormal and gamma PDFs are not shown here, as they do not significantly differ from the Rosin-Rammler PDF.…”

“…The combined Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) (Patterson and Reitz, 1998) breakup model and Miller and Bellan (1999) evaporation models were used in the simulation. For details on methods, models, and numerical setup, the reader is referred to Davidovic et al (2017).…”

“…At high injection pressures and high ambient densities, the liquid fuel jet breaks into fine droplets very close to the nozzle, ≈ 10D noz , and therefore the secondary breakup processes may be negligible. For this reason, the KH-RT breakup model can be switched off, and only drag and evaporation govern the droplet dynamics, as shown by Davidovic et al (2017) in their LES results. For comparability with these results, the breakup model was switched off in the CAS model and the URANS simulation for the case discussed in this section.…”

IFP Energies nouvelles International Conference

“…Davidovic et al [3] propose a LES formulation of a Multiple Representative Interactive Flamelet model, and apply it to the LES of a reactive Diesel-like liquid spray in a well mastered constant volume vessel for which detailed experimental results are available via the Engine Combustion Network.…”