Investigation of wall heat transfer and thermal stratification under engine-relevant conditions using DNS

Unsteady wall heat transfer and thermal stratification during the compression stroke under engine relevant conditions are investigated using direct numerical simulations (DNS). In order to avoid artificial initial and boundary conditions the initial conditions are obtained from a separate DNS of the intake stroke involving thermal and composition mixing. The dynamically changing thermodynamic properties were found to strongly affect turbulence and wall heat transfer during the compression stroke. The increasing pressure results in a strongly reduced kinematic viscosity, and thus in significantly reduced length scales in the flow and temperature fields towards the top dead center (TDC). This has a direct impact on wall heat transfer, since reduced length scales lead to increased temperature gradients at the walls. Hence the heat transfer coefficient, which expresses the hydrodynamic influence on the heat transfer, increases by a factor of approximately five during compression. For the simulated conditions, the heat transfer coefficient extracted from the DNS data is found to agree reasonably well with the global correlation by Hohenberg but deviates strongly from the Woschni correlation. The influence of the boundary layers is not limited to the region close to walls, since close to TDC it affects the temperature distribution in the cylinder core. Vortical structures are identified, which transport cold gases from the boundary layer into the inner cylinder indicating that the assumption of an isentropic core temperature in the inner cylinder is not valid.