Journal
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER
Volume 91, Issue -, Pages 948-960Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.ijheatmasstransfer.2015.08.031
Keywords
Internal combustion engine; Direct numerical simulation; Boundary layer; Compression stroke
Categories
Funding
- Swiss National Science Foundation (SNF) [200021 135514]
- Swiss Super Computational Center (CSCS) [s189]
- Swiss National Science Foundation (SNF) [200021_135514] Funding Source: Swiss National Science Foundation (SNF)
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The unsteady velocity and thermal boundary layers during the compression stroke under engine-relevant conditions are investigated using direct numerical simulation (DNS). The initial conditions are derived by a precursor DNS of the intake stroke, avoiding the use of any artificial initial and boundary conditions. The results show decreasing velocity and thermal boundary-layer thicknesses towards Top Dead Center (TDC) as a result of the decreasing kinematic viscosity. Compared to the fluctuating flow field, the azimuthally-averaged velocities are found to have a minor effect on the boundary-layer profiles, and as a result the boundary-layer structures at all engine walls during compression are similar. The averaged velocity and thermal boundary-layer profiles deviate strongly from the law of the wall, which is the basis for many wall heat transfer models in internal combustion engines. In density wall-normal units, the averaged as well as the rms boundary-layer profiles for velocity and temperature show a collapse onto a single curve during the whole compression stroke. This finding can serve as base for the development of novel wall heat transfer models. Finally, the DNS data showed that between 60% and 80% of the total wall heat losses during the compression stroke are attributed to convective transport due to wall-normal velocity fluctuations. (C) 2015 Elsevier Ltd. All rights reserved.
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