High-speed measurements and conditional analysis of boundary-layer flows at engine speeds up to 2500 rpm in a motored IC engine

The importance of near-wall processes in internal combustion engines, such as near-wall heat transfer, will increase with the current trend toward engine downsizing. Engine boundary layers have been shown to differ from canonical steady-state turbulent boundary layers. A comprehensive understanding of the structure and behaviour of engine boundary layers is required, especially for predictive wall models used in numerical simulations.In the present study, the boundary layer flow over the piston surface in an optically accessible spark ignition engine was investigated using high-resolution, high-speed particle tracking velocimetry. To resolve the phase -averaged viscous sublayer for technically relevant engine speeds up to 2500 rpm, a vector resolution of up to 5 & mu;m perpendicular to the wall was implemented. During compression, the conventionally phase-averaged flow relative to the piston surface resembles canonical flows over a flat plate. However, the results show that no logarithmic layer is present even at higher engine speeds and Reynolds numbers, extending the findings of previous studies. At 2500 rpm and 0.95 bar intake load, the viscous sublayer ( y + = 5 ) gets as thin as 30 & mu;m. Additionally, the outer layer is small, as the bulk flow already starts at a wall-scaled distance of y + = 30 - 90 , which has to be considered when applying wall models in numerical simulations.A conditional analysis revealed flow modes that differ substantially from the conventional phase-average. Furthermore, a strong impact of temporal flow evolution and horizontal position on the boundary layer was found. Even in the case of wall-parallel flows, a strong acceleration and deceleration of the flow can be observed, which affects the adherence of boundary layer profiles to the law-of-the-wall. Another dominant flow mode is the impingement on the piston, where the degree of boundary layer development increases with distance from the stagnation point. These results show a striking similarity to classical channel flows with impinging wall jets. & COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.