Swirling flow in model of large two-stroke diesel engine

A scale model of a simplified cylinder in a uniflow scavenged large two-stroke marine diesel engine is constructed to investigate the scavenging process. Angled ports near the bottom of the cylinder liner are uncovered as the piston reaches the bottom dead center. Fresh air enters through the ports forcing the gas in the cylinder to leave through an exhaust valve located in the cylinder head. The scavenging flow is a transient (opening/closing ports) confined port-generated turbulent swirl flow, with complex phenomena such as central recirculation zones, vortex breakdown and vortex precession. The model has a transparent cylinder five diameters long and is fitted with a static valve with a simplified geometry. The piston motion is controlled by a linear motor. The flow in the experiment has a Reynolds number of 50000 based on cylinder diameter and bulk velocity. Stereoscopic Particle Image Velocimetry (PIV) is used to investigate the scavenging flow for cases with both static and moving piston. Measurements are carried out for several cross-sectional planes covering the majority of the cylinder length. The effect of swirl intensity is investigated using four different port angles going from 0 – 30 degree. Although the flow has a relatively low swirl number of around 0.4, a central recirculation zone is observed indicating a vortex breakdown. The steady flow is also analyzed with Proper Orthogonal Decomposition (POD). The analysis reveals systematic variations in the shape and location of the vortex core. Transient measurements using phase-locked PIV are carried out with moving piston. The transient measurements reveal a violent change in flow topology as a central recirculation zone is rapidly formed, resulting in a change from large positive to negative velocities of the axial component.