LES of a spark ignition engine using artificial thickening and flamelet generated manifolds
Introduction and Motivation
Due to the limited fossil fuel reserves and the climate change, there is an urgent need to improve the fuel efficiency of Internal Combustion Engines (ICE) as well as to reduce the pollutant emissions (nitrogen oxide, carbon monoxide, unburned hydrocarbons, particulate matter …). For this purpose, it is important to carry out a comprehensive study of all of the physical phenomena taking place in ICE, such as fuel and air mixed two-phase flow, ignition, flame propagation as well as pollutant formation. In this context, Computational Fluid Dynamics (CFD) is a powerful tool for researchers, as a supplement to experiments. The three dimensional field of the computed variables can be analyzed and used for better understanding of the complex phenomena, and subsequent engine analysis can be quickly realized to improve engine design.
Method and Theory
he combustion process in IC Engines strongly depends on the turbulence and cyclic fluctuation of in-cylinder flow. In comparison with Reynolds Averaged Navier Stokes (RANS), Large Eddy Simulation (LES) offers in principle the possibility to predict the Cycle-to-Cycle Variations (CCV) in ICE. In this work, the flamelet generated manifolds (FGM) technique coupled with artificial flame thickening (ATF) approach is applied in the context of LES to model the combustion process in a spark ignited ICE.
- In this work the optical spark ignition (SI) engine built and measured at TU Darmstadt has been simulated. The engine operates with homogeneous mixtures from port fuelled injection of isooctane. As CFD tool, the open source code KIVA-4mpi is used. The implementation of LES turbulence model as well as FGM & ATF approach in this code has been validated.
- Cold flow simulations in this TU Darmstadt SI Engine have been carried out for 50 cycles. Firstly, since a new engine configuration is considered, it was necessary to verify that the numerical grid correctly and accurately represented the engine’s geometry and that the experimental pressure curve could also be reproduced in the simulations. Secondly, the numerical setup was assessed by comparing the predictions with a comprehensive set of velocity measurements for different spatial and crank-angle positions. Statistics were also evaluated over 50 engine cycles.
- Simulations of the ignition and flame propagation in TU Darmstadt SI Engine have been carried out for many cycles. The chemistry of iso-octane has been represented using the FGM technique which spans over the relevant temperature and pressure range of the engine. LES of combustion modelling has been performed adopting the thickened flame approachwhere the thickening factor automatically adapts to the local conditions of pressure and grid size to ensure a proper resolution of the chemical source term. In addition to the adaptive thickening, a flame sensor has been used to detect the flame.