High-speed micro particle image velocimetry studies of boundary-layer flows in a direct-injection engine

摘要

Mass and energy exchange processes near the in-cylinder walls have become increasingly important for internal combustion engine research and development. Heat-transfer and fuel-deposition processes are not well enough understood to fully predict their magnitude and temporal and spatial variation. To a large extent, improvements in modeling are limited because of a lack of experimental guidance on the dynamics of boundary-layer flows in engines. High-speed particle image velocimetry and micro particle image velocimetry were used to study the boundary-layer flow field at the cylinder head of a motored internal combustion engine for three different engine speeds (400, 800 and 1100 r/min). Velocity measurements were taken throughout the compression stroke and the beginning of the expansion stroke, and comparisons to the frequently used law-of-the-wall were made. The low-resolution particle image velocimetry images showed that the average bulk flow maintained a tumble flow throughout the compression and expansion strokes and velocities scaled with engine speed. Adequate spatial resolution of the boundary-layer flow required the use of high-resolution (micro particle image velocimetry) measurements which showed that the log-law does not accurately describe the experimental boundary-layer structure in the outer layer (30 < y~+ < 50). Using the log-law, the velocity for earlier crank angles (180-260 crank angle degrees) was over predicted while the velocity for later crank angles (300-330 crank angle degrees) was under predicted, as engine speed increased. However, the experimental velocity profiles near the wall (y~+ < 4) matched the description of the viscous sublayer (u~+ = y~+). Furthermore, it was observed that the thickness of the viscous sublayer decreased as the engine speed increased. The viscous sublayer could not be adequately resolved at 1100 r/min because of the limitations in adequate seeding density near the surface and the resulting loss of data points closest to the wall. The capability of high-speed particle image velocimetry measurements is demonstrated by the ability to identify and track vortical structures that move through the outer and buffer layers. Future work must address the evolution of such flow structures, along with measurements of temperature, to enable development and validation of boundary-layer models that are not restricted by the assumptions necessary for the law-of-the-wall such as steady bulk flow conditions and properties.