Project
Flame- and spray-wall interactions influence on unburned hydrocarbons and carbon monoxide emissions within “Fuel Science Center”
The project is aiming for deep understanding of the in-cylinder processes regarding hydrocarbon and carbon monoxide emissions. Therefore, significant numerical effort is necessary to develop a methodology to investigate mixture composition and temperature, turbulent boundary layer near the cylinder walls, spatial and temporal evolution of the flame quenching distance and of the species composition inside the wall-boundary layer. Accordingly, highly resolved grids near the wall need to be applied. Further, a methodology which allows a measurement of the wall-near gas composition needs to be investigated to fully understand the processes and enable a deep evaluation of the results gained at the test bench. The research project is part of the “Fuel Science Center” (EXC 2186) and is funded by the DFG.
Project Details
Project term
January 9, 2022–September 9, 2023
Affiliations
RWTH Aachen University
Institute
Chair of Thermodynamics of Mobile Energy Conversion Systems
Principal Investigator
Methods
The CFD software used at TME is Converge CFD from Convergent Science. It allows a direct coupling of flow simulations, a detailed chemical reaction mechanism and CHT coupling. The modeling approach of turbulence will be RANS for the most simulations. A few simulations will use the LES approach in order to investigate the cycle-to-cycle variations inside the cylinder which cannot be computed with the RANS methodology. To assess the data scripting in Python and Ensight will be used which is distributed with 20.1 version of ANSYS.
Results
In the last year, e.g. computing period, multiple insights on processes during the gas exchange and combustion of thermal coatings could be derived. It could be observed that an application of a thermal coating leads to a reduced mean wall temperature also during the gas exchange of an internal combustion engine when applied to a piston top land. Nevertheless, this results only from a temperature reduction by a least 100 K where the fuel spray impinges on the piston top land. The locally significantly reduced wall temperature leads to a lower fraction of the fuel vaporized upon impingement of the fuel spray. Thus, this leads to a notable increased wall film application by a factor of 4. Since, the porous structure of the coating and the surface roughness was not yet modeled these influences can not be derived. Although, it is questionable whether these can be modeled accurately due to the low wall resolution of current engine simulations. Further, the influence of such coatings on the post-oxidation of partially oxidized hydrocarbons and carbon monoxide can be derived due to a globally higher wall temperature by at least 20 K and locally near the wall by 80 K. Directly over the wall, an area which cannot be resolved without extensive computational effort, the temperature of the gas equals the wall temperature, which achieves mean thermal swings of 200 K achieving up to 800 K. Locally the maximum piston temperature can achieve up to 1000 K for several crank angles. The inhomogeneous piston surface temperature is caused by the contact time with the hot gas and therefore the flame propagation. These significant higher wall temperatures lead to oxidation of hydrocarbons and carbon monoxide near the wall. Therefore, hydrocarbon and carbon monoxide emissions should be lower for a thermal coating application. Here, test bench results show nearly the same or even higher hydrocarbon emissions. Those may result from the additional crevice volume introduced by the thermal coating. Approaches to model these are part of the next computational campaign.
Additional Project Information
DFG classification: 404-02 Technical Thermodynamics
Software: CONVERGE CFD, Python3, Ensight
Cluster: CLAIX