Improving Near-Wall Combustion and Wall Heat Transfer Modeling in SI Engine Computations 972881
Recent direct numerical simulations of constant density isobaric turbulent flame-wall interaction have lead to the development of a wall model that can easily be implemented in turbulent combustion models used in conventional CFD codes (as e.g. flamelet models). An essential point of this model is the estimation of the mean heat loss of the turbulent flame brush to the cold combustion chamber walls, emphasizing the need for an accurate description of the boundary conditions on solid walls in terms of wall heat transfer and turbulence. With regard to mesh size and computing time, most industrial CFD codes use high-Reynolds number k - ϵ turbulence models in conjunction with a law-of-the-wall to describe near wall flow conditions. One important assumption for the validity of the law-of-the-wall is that the near wall flow is isothermal, the fluid properties in this region thus being regarded as constant. This assumption is obviously erronous in flows combining hot gases (generated by combustion and/or compression) with cold walls, as in IC engines.
We present a formulation of the law-of-the-wall that is equivalent to the classical one when the wall flow is isothermal and takes into account effects of variable fluid properties for non-isothermal conditions. A modification of the above cited turbulent combustion model is proposed to describe non-isobaric flame-wall interaction in SI engines. Both models are implemented in the KIVA-II code and are first validated on a simple axisymmetric pancake SI engine. The new models are shown to more accurately reproduce experimental local wall heat fluxes and pressure/time histories than the original ones. Finally, computations of intake and combustion in a 4-valve SI engine geometry show the ability of the new models to reproduce global engine characteristics quite satisfactorily over a range of operating parameters.