A tabulated diffusion flame model applied to diesel engine simulations

The tabulated diffusion flamelet model approximated diffusion flame-presumed conditional moment is here adapted to the Reynolds-averaged Navier–Stokes simulation of diesel engines. The first model modification concerns the effects of variable pressure, which are necessary to retrieve the chemical species concentrations during the expansion stroke. They are accounted for following an approach similar to the variable volume tabulated homogeneous chemistry approach. The second model modification concerns the local fresh gases temperature stratification modeling that needs to be included due to the liquid injection and is based on the transport equation for the fresh gases enthalpy conditioned in the air. The resulting model is called engine approximated diffusion flames and is able to account for the auto-ignition of the diffusion flame, the local mixture fraction heterogeneity through a presumed probability density function, complex chemistry effects, variable pressure, and temperature stratification. As a first validation, an ideal homogeneous adiabatic engine is computed and successfully compared with the reference solution of the same case obtained with a kinetic solver. Then, six diesel engine operating points at various loads, engine speeds, and dilutions are simulated and compared with experimental measurements. It is shown that the proposed model correctly reproduces the mean pressure evolution and gives a correct estimation of the CO mass fraction. Furthermore, coupled to the NO relaxation approach model, relatively accurate NO predictions are obtained. Finally, different simplified formulations of engine approximated diffusion flames are evaluated, showing that all model components are necessary to correctly estimate the pressure evolutions and pollutant emissions.