New Developments in Turbulent Combustion Modeling for Engine Design: ECFM-CLEH Combustion Submodel

Turbulent combustion in internal engines is known to cover a wide range of regimes and flame topology. Engine turbulent combustion modeling should account for these various regimes observed, as auto-ignition, premixed, partially premixed or diffusion flames. The corresponding reaction zones are controlled by spray evaporation, and vapor fuel turbulent mixing with air that is coupled with combustion. This paper discusses improved modeling for the non-premixed (or diffusion) combustion phase. A new closure is proposed that is called ECFM-CLEH, for Extended Coherent Flame Model (ECFM) with Combustion Limited by Equilibrium Enthalpy (CLEH). It simulates the different phases of Diesel combustion, auto-ignition, premixed and diffusion flame burning. In the premixed phase, combustion is mainly controlled by flame propagation, while fuel and air mixing plays a crucial role in diffusion flames. In ECFM-CLEH, auto-ignition is modeled from tabulated fully detailed chemistry of n-heptane. The tabulation strategy TKI (Tabulated Kinetics for Ignition) proposed by IFP is used. A balance equation for flame surface density is solved for the premixed turbulent flame part. Aside from these well-established closures, a novel expression for diffusion flame burning in Diesel engine is discussed. A lookup table for equilibrium fuel mass fraction is first constructed for a wide range of engine thermodynamic conditions. The equilibrium functions are calculated from the energy balance of the complex mixture including intermediate species and radicals dissociation. A presumed Beta-shape probability density function is adopted to average the equilibrium functions, and the fuel destruction rate by diffusion combustion is limited by the mean fuel concentration at equilibrium. This burning rate is expressed from the IEM (Interaction by the Exchange with the Mean) closure for small scale diffusion. A refined expression for the turbulent mixing time is also used in the modeling. These developments have been introduced in the commercial engine CFD software STAR-CD. Comparisons with measurements in Renault test engines are performed to estimate the prediction capabilities of the proposed modeling.