Olivier Colin

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.

Combustion and soot modelling of a high-pressure and high-temperature Dodecane spray

Diesel engines are known to be one of the main sources of soot particle emission in the atmosphere. The norms to restrain these emissions have created a need of accurate soot models for piston engine emissions prediction in the automotive industry. This article addresses this question by coupling a sectional soot model with a tabulated combustion model for Reynolds-averaged Navier–Stokes simulations of the engine combustion network Spray A, a high-pressure Dodecane spray with conditions very similar to diesel engine sprays. The sectional soot model, which has already been used in diesel engines Reynolds-averaged Navier–Stokes simulations with a simpler combustion model, is implemented in the IFP-C3D Reynolds-averaged Navier–Stokes computational fluid dynamics code. At each time and location, transport equations are solved for several soot sections, including source terms for collisional and chemical processes. The soot model is here coupled to the approximated diffusion flame – presumed conditional moment model, which is a tabulated combustion model. It allows to represent the minor species required by the soot model with a much lower computational cost than a kinetic solver and includes complex turbulence–chemistry interactions. The predictions of these models agree with the experimental measurements for most of the Spray A cases for flame structure and soot production. These results show that detailed soot models can be coupled to tabulated combustion models with good results in turbulent flames.