Stereoscopic Micro-PIV Measurement of Near-Wall Velocity Distribution in Strong Tumble Flow under Motored SI Engine Condition 2020-01-2019

In a state-of-the-art lean-burn spark ignition engine, a strong in-cylinder flow field with enhanced turbulence intensity is formed, and understanding the wall heat transfer mechanism of such a complex flow is required. The flow velocity and temperature profiles inside the wall boundary layer are strongly related to the heat transfer mechanism. In this study, two-dimensional three-component (2D3C) velocity distribution near the piston top surface was measured during the compression stroke in a strong tumble flow using a rapid compression and expansion machine (RCEM) and a stereoscopic micro-PIV system. The bore, stroke, compression ratio, and compression time were 75 mm, 128 mm, 15, and 30 ms (equivalent to 1000 rpm), respectively. A double-pulse Nd:YAG laser sheet was introduced from the intake side, and PIV images were captured near the piston surface around the TDC under motored conditions with a measurement region of 5.2 mm × 6.9 mm (1200 × 1600 pixel, 4.3 μm/pixel) using two CCD cameras. The 2D3C velocity fields show that the magnitude of the wall-tangential velocity (u) is generally larger than the wall-normal velocity (v) and comparable to the tumble axial (laser sheet-normal) velocity (w). As the measured time approaches TDC, u and v tend to decay, while w does not decay compared to u. As a result, the w component contributes more to the 3D vector magnitude than u and v at the closest observation time to TDC. In addition, the main flow is in the piston horizontal direction, and the direction fluctuates in the tumble axial direction because the w direction fluctuates in each cycle. Moreover, separation of the large and small-scale velocity components using a spatial filter enables calculation of turbulent kinetic energy (TKE) at the region near the piston top surface. The contribution of v’, u’, and w’ to TKE increases in this order. Additionally, a large cycle-to-cycle variation of TKE was observed.