Standard Article International J of Engine Research 1–13 Ó IMechE 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1468087415588710 jer.sagepub.com Investigation of wall heat transfer and thermal stratification under engine-relevant conditions using DNS Martin Schmitt 1 , Christos E Frouzakis 1 , Yuri M Wright 1 , Ananias G Tomboulides 2 and Konstantinos Boulouchos 1 Abstract 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 approxi- mately 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 cor- relation. 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 cylin- der is not valid. Keywords Direct numerical simulation, unsteady wall heat transfer, thermal stratification, compression stroke Date received: 13 February 2015; accepted: 17 April 2015 Introduction Wall heat transfer in internal combustion engines (ICEs) is a very complex process involving very short time and length scales. Within 10 ms the flux can vary from close to zero to up to 10 MW/m 2 and local peaks of up to 5 MW/m 2 within 1 cm can be observed 1 . During the engine cycle the strong variations of the thermodynamic and chemical properties have a signifi- cant impact on the boundary layer structure. In addi- tion, the valve and piston motions induce a complex, unsteady, turbulent and cycle specific flow field, which strongly influences the heat transfer to and from the walls. Due to this complexity the development of mod- eling approaches is a challenging task. However, the ICE wall heat transfer is far too important to be neglected, since the boundary layer contains a substan- tial fraction of the in-cylinder mass (10–20%). In par- ticular for Otto and HCCI engine concepts this can result in a substantial impact on the efficiency and unburnt hydrocarbon emissions. In addition, the local heat flux defines the maximum thermal stresses on the engine structure, which is very important for the struc- tural stability of the engine. Hence, an improved under- standing of the ICE wall heat transfer that can assist the development of better predictive models is crucial for future engine design. 1 Aerothermochemistry and Combustion Systems Laboratory, Swiss Federal Institute of Technology, Zurich, Switzerland 2 Department of Mechanical Engineering, University of Western Macedonia, Kozani, Greece Corresponding author: Martin Schmitt, ETH Zu ¨rich Institut f. Energietechnik, ML L 15 Sonneggstrasse 3, 8092 Zuerich, Switzerland. Email: martin.schmitt@lav.mavt.ethz.ch at ETH Zurich on June 30, 2015 jer.sagepub.com Downloaded from