A Study on Kinetic Mechanisms of Diesel Fuel Surrogate n-Dodecane for the Simulation of Combustion Recession

Abstract

Combustion recession, an end of injection (EOI) diesel spray phenomenon, has been found to be a robust correlation parameter for UHC in diesel LTC strategies. Previous studies have shown that the likelihood of capturing combustion recession in numerical simulations is highly dependent on the details of the low-temperature chemistry reaction mechanisms employed. This study aims to further the understanding of the effects of different chemical mechanisms in the prediction of a reactive diesel spray and its EOI process: combustion recession. Studies were performed under the Engine Combustion Network's (ECN) "Spray A" conditions using the Reynolds-Averaged Navier-Stokes simulation (RANS) and the Flamelet Generated Manifold (FGM) combustion model with four different chemical mechanisms for n-dodecane that are commonly used in the engine simulation communitiesincluding recently developed reduced chemistry mechanisms. The flamelet database for each of the chemical mechanism is generated using two methods: 0D homogeneous reactor (HR) ignition flamelets and 1D igniting counterflow diffusion (ICDF) flamelets. The effect of different tabulation approaches is investigated first following by the discussion of the impact of chemical mechanisms on the prediction of combustion recession. Further discussions include an evaluation of the performance of chemical mechanisms in predicting the most relevant reacting spray characteristics compared to the ECN experimental database: ignition delay time (IDT), flame lift-off length (LOL) and flame reactive region. Results show that the choice of both tabulation method and chemical mechanism play a significant role in initial flame stabilization and end of injection (EOI) transient processes. In general, both tabulation techniques were able to qualitatively capture the flame characteristics before EOI, however ICDF tabulation is better suited for the FGM approach in order to capture combustion recession. Furthermore, the chemical mechanisms studied indicate that mechanisms with stronger low temperature chemistry predictions are more likely to promote combustion recession under an FGM framework.

Simulation of ECN diesel spray A using conditional source-term estimation

“…For all grid sizes, the liquid penetration shown in Figure 6 is seen to be more sensitive during the transient period of the spray. A similar trend is observed for the RANS simulations previously shown in [49] where, for the coarse grid, the liquid penetration spikes around 0.2 ms, although significant differences were observed for the RANS non-reacting simulations at steady state for the coarse grid. The LES simulation results are compared to experimental data over the injection duration where the liquid penetration is shown to be insensitive to grid size after the transient period.…”

Effect of Ethanol Addition on the Laminar Burning Velocity of Gasoline Surrogates With Toluene

“…The accuracy of the burning velocity is dependent on the realism of the chemical kinetic reactions scheme and the accuracy of its respective rate constants and molecular transport coefficients. While large chemical mechanisms can be used for numerical studies, [20,21], reduced mechanisms are often adopted for high-fidelity engine simulations [22]. Therefore, in this study, the mechanism used is a reduced four-component, toluene/n-heptane/ethanol/iso-octane (THEO), gasoline surrogate kinetics mechanism developed by Li et al [23] which originated from a detailed chemical kinetic model developed at King Abdullah University of Science and Technology (KAUST) [24].…”