Establishment of an enhanced wall function using the heat transfer analogy method for internal combustion engines.

This article focuses on the development and validation of an improved unsteady thermal wall function and an unsteady velocity wall function for modeling near-wall heat transfer and flow in internal combustion engines (ICEs) under various combustion modes, including conventional diesel combustion (CDC), homogeneous charge compression ignition (HCCI), and reactivity controlled compression ignition (RCCI). The study introduces corrections to the eddy-viscosity ratio and turbulent Prandtl number formulations across the entire boundary layer and incorporates unsteady terms accounting for pressure work and heat release from chemical reactions within a polytropic process framework. Validation against experimental data demonstrates that the unsteady heat transfer model better predicts local heat fluxes compared to steady models, particularly capturing peak heat fluxes influenced by combustion dynamics. The unsteady velocity wall function, derived via analogy with the thermal wall function, effectively represents near-wall velocity distributions affected by combustion-induced pressure and heat source variations, with notable differences observed among the combustion modes. The research highlights that while thermal and velocity boundary layer thicknesses increase during combustion, especially in CDC mode, these changes do not substantially diminish heat flux to the engine walls.