PhD defence

PhD defence: Investigations of the turbulent swirling flow in a two-stroke marine diesel engine

Thursday 09 Jan 14

Kristian Mark Ingvorsen defends his PhD "Investigations of the turbulent swirling flow in a two-stroke marine diesel engine".

The present thesis concerns experimental and numerical investigations of the turbulent swirling flow in the cylinder of a uniflow-scavenged low-speed twostroke marine diesel engine.

The swirl is generated during the scavenging process, when the scavenge air is blown into the cylinder through angled scavenge ports cut in the bottom of the liner. The swirling in-cylinder flow has a large
impact on the emission levels, engine performance, and engine reliability, why increased understanding of the flow is needed.
A scale model of a simplified engine cylinder is created for the experimental investigations.

The model has a transparent cylinder and a movable piston driven by a programmable linear motor. The flow in the cylinder is measured using stereoscopic particle image velocimetry (PIV) and laser Doppler anemometry (LDA). Investigation are performed under both steady-flow conditions and dynamic conditions, and a large database, suitable for validation of computational
fluid dynamics (CFD) models, is established.

For the steady-flow investigations, the piston is fixed at bottom dead center, corresponding to fully open ports, and the flow rate is kept constant. Four cases with port angles of α = 0◦, 10◦, 20◦, and 30◦ are considered, corresponding to different swirl strengths. In the α = 20◦ case, the flow in the cylinder is found to be dominated by a concentrated vortex with a well defined vortex core that extends the entire cylinder length. A central recirculation zone, or vortex breakdown, exist in the bottom of the cylinder and helical structures are found in the recirculation region.

The vortex core is found to precesses around the exhaust valve in the top of the cylinder, and the axial velocity distribution is characterized by a wake-like shape, created by a central velocity deficit. The increased swirl strength obtained with the α = 30◦ ports, does not lead to any considerable changes in the flow topology, whereas the vortex breakdown disappears in the case with α = 10◦

For the dynamic investigations, the scavenging process is simulated by opening and closing the scavenge ports using the moving piston. The results show that a vortex breakdown exists momentarily shortly after the piston passes bottom dead center. Large scale transient motion is observed in the axial velocity distribution in the period where the ports are closed and the piston is at standstill.
During the time when the ports are covered, the tangential velocity profiles are characterized by a small core region where the velocity increases proportional to the radial position, and a large annular region where the velocity is approximately constant. Comparison of the velocity profiles obtained under dynamic conditions with the profiles obtained under the steady-flow conditions show that the flow under dynamic conditions can not be assumed to be quasi-steady. The decay of swirl during the period where the ports are closed is shown to be accurately predicted by simple friction formulas developed for the flow over a flat plate.

In the numerical investigations, a CFD model is created for the flow in a fullscale fired engine. The simulations are based on the Reynolds-Averaged Navier-Stokes equations using a sector domain corresponding to a single scavenge port. Comparison with experimental data from a full scale fired engine shows good agreement on the in-cylinder pressure, mass of scavenged gas, and the charge i purity at exhaust valve closing. A qualitative comparison of the numerical predictions with the results from the dynamic model investigations shows a reasonable agreement on the axial velocity distribution.

However, the predicted tangential velocity profiles have a shape corresponding to a solid body rotation, which is qualitatively different from the measured profiles. Finally, work is carried out dealing with the development of two optical measurement techniques. The first technique is a planar concentration measurement technique based on Mie scattering from seeding particles suspended in the flow.
The technique uses the same basic setup as the PIV and is well suited for model investigations of the scavenging process.

The second technique is an ultra-highspeed method based on digital holography. The technique is developed for the investigation of supersonic particle-laden flows, but the principles may also be applicable for the supersonic flow in the exhaust duct.

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