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

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Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines C.1 Article Manuscript Appendix C Appendix C C-1 Appendix C
Manuscript J Mar Sci Technol manuscript No. (will be inserted by the editor) PIV Study of the Effect of Piston Position on the In-Cylinder Swirling Flow During the Scavenging Process in Large Two-Stroke Marine Diesel Engines S. Haider 1 · T. Schnipper 1 · A. Obeidat 1 · K. E. Meyer 1 · V. L. Okulov 1 · S. Mayer 2 · J. H Walther 1,3 1 Department of Mechanical Engineering, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark 2 MAN Diesel & Turbo, Teglholmsgade 41, DK-2450 Copenhagen SV, Denmark 3 Chair of Computational Science, ETH Zürich, CH-8092 Zürich, Switzerland Received: date / Accepted: date Abstract A simplified model of a low speed large two-stroke marine diesel engine cylinder is developed. The effect of piston position on the in-cylinder swirling flow during the scavenging process is studied using the stereoscopic particle image velocimetry technique. The measurements are conducted at different cross-sectional planes along the cylinder length and at piston positions covering the air intake port by 0 %, 25 %, 50 % and 75 %. When the intake port is fully open, the tangential velocity profile is similar to a Burgers vortex whereas the axial velocity has a wake-like profile. Due to internal wall friction, the swirl decays downstream and the size of the vortex core increases. For increasing port closures the tangential velocity profile changes from a Burgers vortex to a forced vortex, and the axial velocity changes correspondingly from a wake-like profile to a jet-like profile. For piston position with 75 % intake port closure, the jet-like axial velocity profile at a cross-sectional plane close to the intake port changes back to a wake-like profile at the adjacent downstream cross-sectional plane. This is characteristic of a vortex breakdown. The non-dimensional velocity profiles show no significant variation with the variation in Reynolds number. Keywords swirl · scavenging · two-stroke engine · PIV 1 Introduction Low Speed Engine (LSE) are large two-stroke marine diesel engines that use air swirling inside the engine cylinder for scavenging process. The air intake ports are located near the bottom-dead-center (BDC). The exhaust port is located in the cylinder head. An exhaust valve is used to open and close the exhaust port whereas the intake ports are controlled J. H. Walther. E-mail: jhw@mek.dtu.dk by piston motion. The fresh air enters the cylinder near the bottom-dead-center and flows upward and removes the exhaust gases though the exhaust port at the cylinder head. Since the scavenging air flows from the bottom to the top of the cylinder, this type of scavenging is called uniflow scavenging. Uniflow scavenging accounts for higher engine thermal efficiency due to better air/gas exchange (Pevzner, 1998). Efficient scavenging improves the combustion efficiency of the two-stroke diesel engines (Nakagawa et al, 1990). The scavenging ports, depending on different designs, areatanangleof15 ◦ –25 ◦ with the cylinder radius to impart tangential velocity producing a swirling air column (Litke, 1999). The resulting in-cylinder confined swirling flow removes the exhaust gases from the cylinder, provides fresh air charge for the next cycle and introduces swirl to enhance mixing of injected fuel and its consequent combustion. Moreover, it also increases cooling of the cylinder liner and in case of a non-axis symmetric swirl, can result in an uneven temperature distribution at the walls. Thus investigation and optimization of the scavenging process is key parameter for the performance and development of fuel efficient and low emission marine engines. Experimental results available in scientific literature, focused on studying the uniflow scavenging process in large low speed marine diesel engines, are very few compared to scavenging in other (smaller and high speed) two stroke diesel engines. Previous studies include the experiments by Nishimoto and Takeyuki (1984), who obtained the shape of the front surface of the scavenging air using thermocouple in a uniflow model engine. The model engine used hot air for scavenging the cylinder filled with air at room temperature. It was observed that with the increase in the engine RPM (rotations per minute) and port angle, the scavenging air front surface profile changes from jet like to a wake like profile analogous to vortex breakdown in an axial flow vor-
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