LES4ICE 2014 – LES for Internal Combustion Engine Flows 4-5 December 2014, IFP Energies nouvelles (France) PREFERRED TOPIC *: Oral (only) Oral and full paper Poster (*) The final decision belongs to the Scientific Committee. ASSESSMENT OF LES-CMC SIMULATIONS FOR SPRAY A COMBUSTION Daniele FARRACE, Yuri M. WRIGHT, Konstantinos BOULOUCHOS Aerothermochemistry and Combustion Systems Laboratory (LAV), ETH Zurich Background In the last decades, engine combustion research has focused on the understanding of the in- cylinder emissions mechanisms with the aim of their reduction. Supporting a variety of experimental diagnostics, extensive studies have been conducted using numerical tools in the RANS context, allowing for deeper insight into in-cylinder mean phenomena. However, due to the non-linear temperature dependence of chemical reactions and to the unsteady turbulent fluctuating nature of the flow field in spray applications, more complex numerical tools are needed for further advances in engine development. In this context, LES constitutes the current state-of-the-art, since it is able to resolve the large energy containing turbulent scales of motion and to predict a detailed instantaneous description of the mixing field. As a consequence it has seen successful application to study cycle-to-cycle variations as reviewed in [1]. Despite recent progress, considerable challenges persist in accurately predicting turbulent reacting flows, since for many practical turbulent diffusion combustion applications with high Reynolds and Damkoehler numbers, molecular mixing and chemical reaction occur at the smallest dissipative scales [2], which are not resolved. Similar to RANS, LES therefore also requires advanced combustion models to account for turbulence- chemistry interaction; the latter indeed has been observed to significantly affect combustion and emission formation [3-5]. For non-premixed combustion, most modelling efforts are mixture fraction based, where the fluctuations of reacting scalars and temperature are correlated with those of the mixture fraction. It becomes clear that an accurate description of the flow field is of supreme importance, supporting even more the LES strategy. Motivated by the successful results obtained at diesel engine conditions in the RANS framework [6-11] in this work LES is coupled with the Conditional Moment Closure (CMC) combustion model to simulate the Sandia Spray A [12]. This work concentrates on the validation of the numerical framework to ensure accurate and reliable predictions for future investigations. The computed results are compared to experimental data for non-reacting cases in terms of spray tip penetration length and fuel spatial distribution, as well as for reacting cases by means of ignition delay and lift-off length. Method and setup The commercial CFD solver STAR-CD [13] has been used for the calculation of the turbulent instantaneous two-phase flow field with a Lagrangian-Eulerian multi-phase approach. Resolved scales are separated from sub-grid scales with a filtering technique, the filter width is considered equal to the mesh size. The resulting stress tensor is closed with the in-built Dynamic Structure [14] sub-grid scale model, allowing for more tractable grid sizes as discussed in more detail in [1]. Atomization is treated with the well-established ‘blob’ model while for droplet break-up the Kelvin-Helmholtz and Rayleigh-Taylor (KHRT) model has been adopted. The droplet contribution to the sub-grid turbulent kinetic energy is considered according to [15]. Momentum, energy, mixture fraction (MF) and mixture fraction variance (MFV) equations are solved by STAR-CD whereas species mass fractions are obtained by