Mika Nuutinen

Imbalance wall functions with density and material property variation effects applied to engine heat transfer computational fluid dynamics simulations

Heat transfer is a significant factor affecting internal combustion engine efficiency, emissions and performance. This study concentrates on model development for convective heat transfer and near-wall turbulent flow. The solution of complete fluid dynamics equations within the very thin-wall boundary layers is, and will be in the near future, very impractical in engineering scale flows with any present day method. An advanced numerical near-wall treatment method within the Reynolds averaged Navier–Stokes framework has been developed. The method solves simplified boundary layer equations for enthalpy, momentum, turbulent kinetic energy and dissipation in wall adjacent cells on cellwise high-resolution subgrids, adaptive to local conditions. The boundary layer equations include temperature gradient–induced density/multicomponent material property variations and complete imbalance contributions, for example, convection, transients, pressure gradient and external sources, in compact forms. The resulting numerical wall functions are valid with near-wall grid resolution ranging from the viscous sublayer to the fully turbulent region, thus avoiding the conflicting near wall resolution requirements of common low–Reynolds and high–Reynolds number turbulence models. The advanced near wall treatment method, comprising the numerical imbalance wall functions and accordingly modified low–Reynolds number turbulence model, is implemented in STAR-CD 4.12, extensively utilized in engine simulations. The near wall treatment method is validated against available measurements and direct numerical simulation data of strongly heated pipe flow. Performance of the near-wall treatment method in engine conjugate heat transfer simulations is also demonstrated. Local and average effects of variable properties and imbalance contributions on piston surface heat transfer, friction and turbulent sources are elaborated and contrasted to the standard high–Reynolds number near-wall treatment.