Towards a fundamental understanding of key physical aspects of narrow-throat pre-chamber combustion, the current work utilizes three-dimensional computational fluid dynamics (CFD) simulations using the CONVERGETM CFD solver, to analyze the effect of pre-chamber geometry, piston design, combustion models, and flame speed correlations in an engine operated with methane. The simulations were performed at lean operating conditions whilst the pre-chamber was fuel enriched with a direct fuel supply. The modeling work was performed in conjunction with metal engine testing at identical conditions, which provided validation data for the model. The particular pre-chamber utilized in this work fits in the diesel injector pocket of the cylinder head, thus features a narrow throat, which requires marginal engine modification, hence lowering the technical and economic barriers to deployment of this technology in production vehicles. The combustion process is simulated with the G-Equation model for flame propagation and/or with the multi-zone well-stirred reactor (MZ-WSR) model to determine the post-flame composition and to predict possible auto-ignition of reactant mixture; MZ-WSR and G-Equation were also compared separately and showed the potential to match experimental data upon appropriate calibration. When used, the laminar flame speed was tabulated from a methane oxidation mechanism; the turbulent flame speed was computed using Peters' relation. While the narrow throat was found to have a major impact on the pre-chamber combustion, the jet-piston interaction was also identified as crucial if additional improvements on engine emissions and performance are desired. On the fundamental modeling aspect, the significance of laminar flame speed prediction in the simulation of ultra-lean engine combustion was assessed. For engineering applications, the correlations of flame speeds with physical variables involve empirical constants that are valid for a limited range of operating conditions. In all cases, the original formulation of Peters' turbulent flame speed correlation was used; the results confirm the importance of the accurate determination of the laminar flame speed, which overrules any ad hoc constant corrections for high Karlovitz regimes. Finally, the relevant turbulent combustion regimes encountered in pre-chamber combustion engine conditions were examined using the Borghi-Peters diagram, further confirming the findings.
|Date of Award||Oct 20 2022|
|Original language||English (US)|
- Physical Sciences and Engineering
|Supervisor||Hong G. Im (Supervisor)|