Abstract
Non-intrusive flow measurements are useful for the understanding of physical processes central to engine combustion and emission performance, such as fuel-air mixing. In the present work, a test rig with maximum optical access is designed and fabricated to study the incylinder flow due to reciprocating piston during compression using particle image velocimetry (PIV) and proper orthogonal decomposition (POD), a technique for investigating turbulent flows. Very few investigations of the incylinder flow using the PIV-POD combination are reported in literature. The effects of the piston configuration and the piston position on the incylinder flow are investigated, for two piston configurations, viz., flat piston, and spherical bowl, six piston positions, from the bottom dead centre (BDC) to 150° after BDC, in steps of 30°. PIV measurements are carried out in the vertical plane passing through ports at 600 rpm crank speed. The ensemble-averaged velocity fields obtained from PIV reveal a large scale rotation particularly in the early part of the compression stroke, i.e., from BDC to 90° after BDC (aBDC). In the later part of compression, the flow is observed to be directed towards the cylinder head. POD is applied to the measured instantaneous velocity to study the incylinder turbulent flow. The POD modes show the flow fluctuations due to the effect of turbulence and cycle-to-cycle variations. For the spherical bowl case at 150° aBDC, the share of the fluctuation kinetic energy in the second and higher POD modes is higher than that (a) at any other piston position (b) in the case of flat piston. Thus, the POD analysis of instantaneous velocity measured using PIV, indicates that a spherical bowl is a better choice in terms of turbulence production compared to a flat piston, for use in engines.
