Browse Publications Technical Papers 2017-01-2200
2017-10-08

Prediction of the Combustion and Emission Processes in Diesel Engines Based on a Tabulated Chemistry Approach 2017-01-2200

Turbulent combustion modeling in a RANS or LES context imposes the challenge of closing the chemical reaction rate on the sub-grid level. Such turbulent models have as their two main ingredients sources from chemical reactions and turbulence-chemistry interaction. The various combustion models then differ mainly by how the chemistry is calculated (level of detail, canonical flame model) and on the other hand how turbulence is assumed to affect the reaction rate on the sub-grid level (TCI - turbulence-chemistry interaction).
In this work, an advanced combustion model based on tabulated chemistry is applied for 3D CFD (computational fluid dynamics) modeling of Diesel engine cases. The combustion model is based on the FGM (Flamelet Generated Manifold) chemistry reduction technique. The underlying chemistry tabulation process uses auto-ignition trajectories of homogeneous fuel/air mixtures, which are computed with detailed chemical reaction mechanisms. The TCI modeling is based on a presumed PDF approach. The look-up tables have up to six dimensions: pressure, temperature, mixture fraction, mixture fraction variance, progress variable and its variance.
By applying such type of model, it is possible to account for a very large number of chemical reactions within the CFD simulation. The results of the reactions are pre-tabulated and picked up from the data bases during the CFD runtime. This data fetching process is very efficient so that the 3D simulation only requires the calculation time of much simpler combustion models. The runtime for CFD simulations, including chemistry pre-processing, only mildly increases with the number of species used in the reaction mechanism. This leads to the fact, that simulations with 1000+ species have been realized within about 20 hours on 8 CPU cores.
In the current framework also emission models have been introduced. The overall results, which are discussed in this paper, show very good response of the modeling approach compared to measurements in production engines.

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