Experimental investigation of boundary layer flow near the cylinder wall in a direct-injection spark-ignition engine using particle image velocimetry

Abstract

In a direct-injection spark-ignition (DISI) optical engine with a constant motored speed of 800 rpm, particle image velocimetry measurements on the in-cylinder bulk flow and the boundary layer flow near the cylinder wall were performed during the intake and compression strokes. Two different tumble flow levels were explored using a flap inserted in the intake port. The near-wall flow pattern under the effect of the bulk flow is more stable when the flap is fully closed with a higher intensity tumble. The serious optical distortion in the near-cylinder wall region under high magnification was addressed, and a high resolution of 76 μm × 76 μm within a field of view of 5 mm × 5 mm was achieved in six different measurement regions. Using inner scaling parameters (friction velocity u_τ and viscous length-scale δ_υ), the dimensionless ensemble-averaged wall-tangential velocity profiles exhibit good consistency with the law-of-the-wall in the viscous sublayer but considerably departure in the logarithmic region. According to the low friction Reynolds number, the absence of the logarithmic layer was mainly attributed to the complete overlap between the viscosity-affected inner region and the outer region of the boundary layer. The near-wall velocity profiles were also normalized by three different outer scaling parameters. The collapse is significantly improved for y/δ >0.07 when the velocity profiles are normalized by δ/δ^*(1−u/u_∞). The fluctuation RMS velocity in the wall-normal direction is greater, particularly for the high tumble flow level condition.