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This study investigates n-dodecane split injections of “Spray A” from the Engine Combustion Network (ECN) using two different turbulence treatments (RANS and LES) in conjunction with a Conditional Moment Closure combustion model (CMC). The two modeling approaches are first assessed in terms of vapor spray penetration evolutions of non-reacting split injections showing a clearly superior performance of the LES compared to RANS: while the former successfully reproduces the experimental results for both first and second injection events, the slipstream effect in the wake of the first injection jet is not accurately captured by RANS leading to an over-predicted spray tip penetration of the second pulse. In a second step, two reactive operating conditions with the same ambient density were investigated, namely one at a diesel-like condition (900K, 60bar) and one at a lower temperature (750K, 50bar).

The 4th Workshop of the Engine Combustion Network (ECN) was held September 5-6, 2015 in Kyoto, Japan. This manuscript presents a summary of the progress in experiments and modeling among ECN contributors leading to a better understanding of soot formation under the ECN “Spray A” configuration and some parametric variants. Relevant published and unpublished work from prior ECN workshops is reviewed. Experiments measuring soot particle size and morphology, soot volume fraction (fv), and transient soot mass have been conducted at various international institutions providing target data for improvements to computational models. Multiple modeling contributions using both the Reynolds Averaged Navier-Stokes (RANS) Equations approach and the Large-Eddy Simulation (LES) approach have been submitted. Among these, various chemical mechanisms, soot models, and turbulence-chemistry interaction (TCI) methodologies have been considered.

Numerical simulations of diesel sprays in a constant-volume vessel have been performed with the conditional moment closure (CMC) combustion model for a broad range of conditions. On the oxidizer side these include variations in ambient temperature (800-1100 K), oxygen volume fraction (15-21%) and density (7.3-58.5 kg/m₃) and on the fuel side variation in injector orifice diameter (50-363 μm) and fuel pressure (600-1900 bar); in total 22 conditions. Results are compared to experimental data by means of ignition delay and flame lift-off length (LOL). Good agreement for both quantities is reported for the vast majority of conditions without any changes to model constants: the variations relating to the air side are quantitatively accurately predicted; for the fuel side (viz. orifice diameter and injection pressure) the trends are qualitatively well reproduced.

Large Eddy Simulation (LES) of premixed turbulent combustion in a confined cylinder setup at engine relevant conditions has been carried out for three different initial turbulence intensities, mimicking different flame propagation regimes. Direct Numerical Simulation (DNS) of the setup under investigation provides the reference data to be compared against. The DNS fields have been filtered on the LES grid and are used as initial conditions for the LES at onset of combustion, guaranteeing a direct comparability of the single realizations between the modeled and reference data. Two different combustion models, the G-Equation and CMC-premixed (Conditional Moment Closure) are compared with respect to their predictive capabilities as well as their usability and computational cost. While the G-Equation is a widely adopted approach for industrial applications and usually relies on a tunable turbulent flame speed closure, the novel LES-CMC comes as a tuning parameter free model.

In this study, a novel 3D-CFD combustion model employing Flamelet Generated Manifolds (FGM) for dual fuel combustion was developed. Validation of the platform was carried out using recent experimental results from an optically accessible Rapid Compression Expansion Machine (RCEM). Methane and n-dodecane were used as model fuels to remove any uncertainties in terms of fuel composition. The model used a tabulated chemistry approach employing a reaction mechanism of 130 species and 2399 reactions and was able to capture non-premixed auto ignition of the pilot fuel as well as premixed flame propagation of the background mixture. The CFD model was found to predict well all phases of the dual fuel combustion process: I) the pilot fuel ignition delay, II) the Heat Release Rate of the partially premixed conversion of the micro pilot spray with entrained methane/air and III) the sustained background mixture combustion following the consumption of the spray plume.

Optical soot pyrometry is a mature experimental technique that has been applied to a broad range of combustion systems for measuring soot temperature and concentration. Even though the method is widely used and well documented, the line of sight nature of the technique makes the interpretation of its results challenging. Notably, gradients in temperature and soot concentration along the line of sight or across the field of view can introduce significant levels of uncertainty in the results. This paper presents a numerical study where the signal from the experimental two-color pyrometry technique in a marine diesel engine reference experiment is reconstructed employing computational fluid dynamics (CFD) and a detailed Line-by-Line (LBL) spectral radiation model. The analysis is aimed at qualitatively supporting interpretability of experimental observations.