Internal combustion engine flows for motored operation

Application Area 2: Combustion

Application Challenge AC2-09

Abstract

This test case provides data on the in-cylinder flow for an IC engine under motored operation. The Technische Universität Darmstadt engine features a typically design of a modern spark-ignition direct injection engine. It is an optically accessible single cylinder engine especially designed to provide well characterized boundary conditions and reproducible engine operation; a prerequisite for any comparison of experiments and simulations. The in-cylinder flow is characterized by various particle image velocimetry (PIV) techniques to provide measurements at high spatial and temporal resolutions. The database for validation includes the first two statistical moments (mean and rms) of velocities, spatial flow structures and the temporal evolution of the flow field over the entire engine cycle in the central tumble plane. Information on the 3D flow is available within a volume up to 8 mm thick centered on the central tumble plane. Important boundary conditions as the in-cylinder, intake- and exhaust-port pressures as well as temperatures are given. Simulation results obtained from three investigations using LES (Large Eddy Simulation) and hybrid URANS (unsteady Reynolds-averaged Navier-Stokes)/LES are presented and compared with the experimental results.

This document contains the specifications of the Application Challenge proposed by the team of the Institute of Thermal Machinery, Częstochowa University of Technology. This team performed LES predictions of the Sandia Flame D within the EU-project MOLECULES FP5, Contract N° G4RD-CT-2000-00402. The computations were performed with the BOFFIN-LES code developed at Imperial College by the group of Professor W.P. Jones. The software for the Conditional Moment Closure model used in calculations was developed by Professor E. Mastorakos at Cambridge University and implemented in the BOFFIN-LES code by the team of the Institute of Thermal Machinery.

Sandia Flame D is a widely used test case for the validation of numerical models of non-premixed combustion. This flame is of the flamelet regime type in which a scale separation appears i.e. the smallest scales of the turbulent flow, the Kolmogorov scales, are significantly larger than the scales characteristic for the reaction zone. Such a flame facilitates the study of models of turbulence/chemistry interaction, allowing to separate the influence of turbulence and turbulence/chemistry interaction models from the influence of chemical kinetics models applied. Non-premixed combustion is limited by turbulent mixing and dominated by large scale structures. The quality of unsteady flow dynamics predictions seems to be crucial for the quality of the overall combustion process. Hence, within this document attention is restricted to LES calculations and neither RANS nor URANS predictions are included or analyzed.

To evaluate the sensitivity of the subgrid-scale-modeling quality on turbulent combustion predictions, two subgrid-scale models were tested: the classical Smagorinsky model and the dynamic version. Turbulent mixing features are then transmitted to the reaction front through turbulence/combustion interaction models that also influence the overall combustion process predictions. As turbulence/combustion interaction model, two different approaches were studied: the simple and efficient steady flamelet model and the more advanced simplified Conditional Moment Closure-CMC (In simplified CMC approach, the convective terms in physical space were neglected, making the model very close to the unsteady flamelet approach).

DOAPs for this type of reacting flow are velocity, mixture fraction, temperature and species concentration mean and fluctuating profiles quantified by their local maxima.