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 Relevant published and unpublished work from prior ECN workshops is reviewed.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

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In this study, numerical simulations of an n-dodecane spray flame-known as Spray A-with multiple injections (0.5 ms injection/0.5 ms dwell/0.5 ms injection) have been carried out using the transported probability density function method in the Reynolds-averaged Navier-Stokes framework. In terms of the methodology employed, the transported probability density function method can handle the multiple-injections case without any modification because the model does not assume that thermodynamical states lie on a low-dimensional manifold such as the mixture fraction manifold, as is the case for many other turbulent combustion models, for example, the representative interactive flamelet model and the conditional moment closure model. Simulation results have been compared with recent experimental data in terms of inert and reactive jet tip penetration and vapor boundary (from schlieren imaging), ignition delay and flame base location of the first and second fuel injection, spatial distribution of formaldehyde (CH 2 O) and polyaromatic hydrocarbon (from 355-nm planar-laser-induced fluorescence). Particular attention has been paid to the ignition behavior of the second fuel injection. The timing and progression of the first-and second-stage ignition events are qualitatively well reproduced by the model. Simulation results have been further analyzed to assess the validity of the beta-function as the presumed shape of the mixture fraction probability density function, which is typically employed in mixture fraction-based models. The beta-function probability density function was found to provide a good approximation throughout the jet region apart from a brief period of around 100 ms when the second fuel stream encounters the pre-existing fuel-air mixture from the first fuel injection. Overall, it is shown that the transported probability density function model is able to capture the main features related to auto-ignition involved with multiple injections, and simulation results can be used to assess some of the underlying assumptions invoked by other models.

The importance of radiative heat transfer on the combustion and soot formation characteristics under nominal ECN Spray A conditions has been studied numerically. The liquid n-dodecane fuel is injected with 1500 bar fuel pressure into the constant volume chamber at different ambient conditions. Radiation from both gas-phase as well as soot particles has been included and assumed as gray. Three different solvers for the radiative transfer equation have been employed: the discrete ordinate method, the spherical-harmonics method and the optically thin assumption. The radiation models have been coupled with the transported probability density function method for turbulent reactive flows and soot, where unresolved turbulent fluctuations in temperature and composition are included and therefore capturing turbulence-chemistry-soot-radiation interactions.Results show that the gas-phase (mostly CO 2 ad H 2 O species) has a higher contribution to the net radiation heat transfer compared to soot. The effect of radiation absorption was found to be important and the typical radiation time scale is observed to overlap with the long injection duration, leading to a moderate influence on the temperature distribution. The flame lift-off length is not affected by radiation and differences in soot formation are perceivable but only minor. The performance of the DOM and P1 models is comparable, whereas the optically thin assumption leads to a higher cooling effect. It is anticipated that NO x formation rates are expected to be influenced by radiative heat transfer in a more pronounced manner.