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Large eddy simulation techniques to simulate and c ontrol by design cyclic variability in otto cycle engines


The objective of CSIC under this contract was to develop a methodology to impose turbulent boundary conditions in the inlet of an engine simulation, as well as the necessary tools to implement the method in one of the large eddy simulation codes used by the partners (AVBP). The chosen method consists in generating an artificial homogeneous isotropic turbulent field and modulate the amplitude of fluctuations to adapt to the desired average inlet conditions as needed at run time. A non-reflecting boundary condition method is also used to avoid numerical or physical waves to be reflected in the inlet of the domain. This methodology represents a significant reduction in computational expenses (both in memory and time) as compared to the methods in which a large computational domain or an additional concurrent simulation is needed to include transition to turbulence. It is not limited to combustion systems, but can be of interest to the wide computational fluid dynamics community.
An algorithm is developed that joins automatic mechanism reduction with solution mapping methods. The joined method has been implemented into a simple stochastic reactor model for engine knock simulation using a kinetic mechanism for mixtures of n-heptane and iso-octane. The method was validated and showed a good level of speed-up.
The LES research simulation code AVBP has been updated during the project to yield a tool able to perform LES simulations of spark-ignited engines. The result is a high performance parallel simulation code able to treat moving, complex meshes with unstructured mesh logic. It includes specifically developed LES models for turbulence (subgrid kinetic energy model with wall laws), premixed combustion (the ECFM-LES model with submodels for spark-ignition and flame/wall interaction), as well as results achieved by associated project partners like automatic mesh management, a methodology for including kinetic schemes in a CPU time efficient manner, an Eulerian model for liquid jets as well as adapted inflow boundary conditions. This highly innovative tool is available to IFP for performing further research as well as commercial studies exploiting the potential of LES to predict unsteady engine phenomena like cold starts, cyclic variability etc. Its potential for exploitation goes beyond the sector of automobiles and includes industrial domains like aeronautics and energy generation. In this sense the envisaged tool can be expected to contribute towards making energy conversion by combustion more efficient and less polluting, which are major goals of EC funded research.
In the scope of this project, PSA is involved in the DB (DATABASE) workpackage and has provided a detailed experimental database to characterize spray behaviours and combustion process development. This database has been sent to interested partners to be used to validate different models. Two experiments have been realized: - Visualizations in a cold constant volume vessel configuration (DB1). - Visualizations in a PSA gasoline engine configuration (DB2).