Experimental and Numerical Study of Swirling ... - Solid Mechanics

Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke
Marine Diesel Engines
Abstract
In low speed large two-stroke marine diesel engines, uniflow scavenging is
used to remove the exhaust gases from the cylinder and fill the cylinder with
fresh air charge for the next cycle. The swirl enhances the mixing of fuel with
air and improves combustion efficiency. The thesis focuses on characterizing
the confined swirling flow during the scavenging process. A simplified
experimental model of an engine cylinder is developed. Smoke visualization
results show that at fully open intake port there is a well-defined vortex core.
The core size increases in a hollow conical shape along the flow downstream.
As the port closes, the mixing of smoke particles in the core with
surrounding regions is enhanced. The hollow conical smoke pattern
disappears and resembles to a jet. Laser Doppler Anemometry measurements
are conducted in the swirl generator and at the entrance to the test cylinder.
The results show that the incylinder swirling flow has a precessing vortex
core. The precession frequency is found to be linearly dependent on the
volumetric flow rate at a given swirl number.
The stereoscopic particle image velocimetry (SPIV) measurements are
conducted for two sets of experiments. In the first experiment, the intake
port is kept fully open and three different cylinder lengths are investigated.
The results indicate that the incylinder flow is a concentrated vortex decaying
downstream due to wall friction. The mean axial velocity has a wake-like
profile. The radial velocity is very small compared to tangential and axial
components. No reverse flow is observed in the vortex core. The initially
confined vorticity in the vortex core region is distributed to outer regions
along the flow. Turbulent kinetic energy is high in the vortex core and near
wall regions. The incylinder flow is majorly governed by the flow conditions
at the cylinder inlet and the increased length of cylinder provides further
decay of the swirl. The profiles of velocity components remain the same for a
given cross-sectional plane common in different cylinder lengths. The mean
position of the vortex center is not aligned with the cylinder axis at all
measuring position and follows a helical path along the cylinder length. For
cylinder length of eight diameters, the mean vortex path does not complete
one revolution and instead re-twists at one side of the cylinder axis.
In the second SPIV experiment, the measurements are conducted to
characterize the effect of piston position on the in-cylinder swirling flow. The
piston is positioned to cover the cylinder intake port by 0%, 25%, 50% and
75%. For increasing port closures the tangential velocity profile changes to a