A computational assessment of flame speed correlation in an ultra-leanpre-chamber engine

Predictive modeling of pre-chamber combustion engines relies primarily on the correct description of laminar and turbulent flame speeds. For engineering applications, the correlations of the flame speeds with physical variables involve empirical constants that are valid for a limited range of operating conditions. The current work aims at assessing the significance of laminar flame speed prediction in the simulation of ultra-lean pre-chamber engine combustion operated with methane. Gulder's empirical correlation for laminar flame speed was chosen as a reference and was further modified for equivalence ratio, pressure and temperature ranges beyond what it was originally derived for, in order to confirm the original hypothesis; the pressure and temperature dependence were adopted as a power-law correlation. Based on the computational results using the skeletal reaction mechanism, the correlation was modified better represent the flame speeds at ultra-lean engine conditions, using GRI 3.0 as a reference. The modified correlation for methane was implemented in CONVERGE, a three-dimensional computational fluid dynamics (CFD) solver, and the results were validated against the experimental data. In all cases, the original formulation of Peters's turbulent flame speed correlation was used and was found to have insignificant effect on the conditions under study, confirming the importance of the accurate determination of the laminar flame speed that dominates over any turbulence corrections for high Karlovitz number effects. The flame topology was also investigated to provide insights into the observed pressure behavior among the tested cases. Finally, the relevant turbulent combustion regimes encountered in the pre-chamber combustion engine conditions were examined in the Borghi-Peters diagram, further confirming the findings of the study.