Numerical modelling of unburned hydrocarbon emissions in gas engines with varied fuels

Abstract

A numerical model is developed for the prediction of unburned hydrocarbons (UHC) emissions in gas engines. Local UHC sources in the engine combustion chamber are simulated and the unburned fuel leaving the engine is calculated and compared with experimental data from a single cylinder research engine for different operating conditions. The calculation procedure is applicable for complex three dimensional geometries and regards the local flow and turbulence conditions. A hybrid model is proposed to integrate the detailed reaction kinetics via separate sub models coupled with Computational Fluid Dynamics (CFD). A high pressure turbulent flame propagation model is applied, being based on detailed reaction kinetic simulations of the laminar flame speeds for the engine conditions. Flame wall quenching, crevices and post-oxidation of UHC are treated as separate submodels. The UHC emissions are solved as scalars to account for the convection and diffusion transport in the engine, incorporating a one-step post-oxidation model with detailed chemistry. The 3D-CFD results are validated with experiments, showing on one hand side the fidelity of the proposed numerical approach and on the other hand side the possibility to differentiate the different sources of UHC. Additionally a numerical study is conducted to investigate the influence of stoichiometric gas engines with exhaust gas recirculation. The model is applicable also for biogas engines with reduced reactivity.