Large eddy simulation of a reacting spray flame with multiple realizations under compression ignition engine conditions

Abstract

An n-dodecane spray flame (Spray A from Engine Combustion Network) was simulated using a δ function combustion model along with a dynamic structure large eddy simulation (LES) model to evaluate its performance at engine-relevant conditions and to understand the transient behavior of this turbulent flame. The liquid spray was treated with a traditional Lagrangian method and the gas-phase reaction was modeled using a δ function combustion model. A 103-species skeletal mechanism was used for the n-dodecane chemical kinetic model. Significantly different flame structures and ignition processes are observed for the LES compared to those of Reynolds-averaged Navier-Stokes (RANS) predictions. The LES data suggests that the first ignition initiates in a lean mixture and propagates to a rich mixture, and the main ignition happens in the rich mixture, preferably less than 0.

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

“…Those detached flame spots are mainly causing the underprediction of the flame lift-off length, while the main flame seems to be in better agreement with the experimental value. It should be noted that only a single realization was performed in the present work and ignition delay time scattered up to 0.05 ms in LES studies of the Spray A case with multiple realizations [10,12]. Overall, the simulation seems to capture the underlying physical processes well, since all quantities are in good agreement with the experimental findings.…”

“…Even though the ignition delay was only marginally affected by turbulence effects, the laminar chemistry model failed to predict other flame characteristics like flame lift-off and heat-release, while the MRIF model showed overall good agreement with experimental results. Recently, a few LES studies of the Spray A case have been published [10][11][12].…”

“…Pei et al [10] performed simulations with laminar chemistry showing that LES capture the experimentally observed behavior more accurately compared to RANS. However, even though the ignition delay was predicted fairly well by the LES, the flame lift-off length was significantly overpredicted.…”

A Progress Review on Soot Experiments and Modeling in the Engine Combustion Network (ECN)

“…The ANL lift-off length trend is nearly linear, with a shorter lift-off length than the experiment at 800 K and a longer value at 900 K and above; however, this might be due to the insufficient number of LES realizations for temperatures other than the Spray A case. Although the standard error of the lift-off length for the 15 LES realizations is quite small, fluctuations in lift-off length extend down to 17 mm and up to 24 mm during the transient simulation [23]. More realizations are necessary to build up sufficient statistics for a representative trend at this and other conditions; however, the high computational cost of these simulations is prohibitive.…”

“…Note that the ANL ignition delay time at 900 K is the average of five LES realizations and the standard error from these simulations is included in the table as well. Pei et al [23] provides more detail regarding the variation among different LES realizations. The ANL, UNSW, and POL simulations best reproduce the experimental trends and quantities for ignition delay, with only a slight difference between the tPDF and well-mixed models of UNSW; however, it should be noted that the chemical mechanism implemented in the POL and UNSW simulations was specifically tuned by Yao et al [25] to ECN spray combustion data.…”