Hemmingsen, Casper Schytte

Ingvorsen, Kristian Mark

Mayer, Stefan

Walther, Jens Honore

Computational fluid dynamic simulations are performed for the turbulent swirling flow in a scale model of a low-speed two-stroke diesel engine with a moving piston. The purpose of the work is to investigate the accuracy of different turbulence models including two-equation Reynolds- Averaged Navier-Stokes models and large eddy simulations. The numerical model represent the full three-dimensional geometry and the piston motion is modeled by compressing cells in the axial direction. The CFD predictions are compared to experimental results and a reasonable agreement is found

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 20, 2017Swirling flow in a two-stroke marine diesel engineHemmingsen, Casper Schytte; Ingvorsen, Kristian Mark; Walther, Jens Honore; Mayer, StefanPublished in:Proceedings of the 26th Nordic Seminar on Computational MechanicsPublication date:2013Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Hemmingsen, C. S., Ingvorsen, K. M., Walther, J. H., & Mayer, S. (2013). Swirling flow in a two-stroke marinediesel engine. In A. Logg, & K. A. Mardal (Eds.), Proceedings of the 26th Nordic Seminar on ComputationalMechanicsProceedings of the 26th Nordic Seminar on Computational MechanicsA. Logg, K.A. Mardal (Eds.)c©Oslo, 2013Swirling flow in a two-stroke marine diesel engineCasper S. Hemmingsen1, Kristian M. Ingvorsen1, Jens H. Walther1,2, and Stefan Mayer3(1)Department of Mechanical Engineering, Technical University of Denmark, Denmark, jhw@mek.dtu.dk(2) Computational Science and Engineering Laboratory, ETH Zu¨rich, Switzerland, Walther@mavt.ethz.ch(3)MAN Diesel & Turbo, Denmark, Stefan.mayer@man.euSummary. Computational fluid dynamic simulations are performed for the turbulent swirling flowin a scale model of a low-speed two-stroke diesel engine with a moving piston. The purpose of thework is to investigate the accuracy of different turbulence models including two-equation Reynolds-Averaged Navier-Stokes models and large eddy simulations. The numerical model represent the fullthree-dimensional geometry and the piston motion is modeled by compressing cells in the axial direction.The CFD predictions are compared to experimental results and a reasonable agreement is found.Key words: CFD, RANS, LES, swirl, turbulence, two-stroke, marine dieselIntroductionLow-speed two-stroke (LSTS) diesel engines are used to power the worlds largest marine vessels,such as tankers and container ships. When the piston approaches the bottom dead center (BDC)of the cylinder, it uncovers a series of angled scavenge ports in the cylinder wall. Fresh air isblown into the cylinder through the scavenge ports, thereby flushing the old combustion gas outthrough the exhaust valve located in the cylinder head. This gas exchange process is known asuniflow-scavenging. The angled ports induce a rotational motion to the incoming scavenge air,thereby creating a swirling flow.Accurate computational fluid dynamics (CFD) simulations of the in-cylinder swirling flowis important for engine optimization and emission reduction. Recently, a database for CFDvalidation was established based on an experimental investigation of the flow in a dynamicscale model of a LSTS diesel engine [1]. The model has a moving piston but compression andcombustion are neglected.The purpose of the present work is to evaluate the accuracy of different turbulence modelsFigure 1. (a) Geometric of the main model, (b) details on the scavenge port geometry.0.00 0.25 0.50 0.75 1.00 z / D0.000 0.141 0.250 0.359 0.500 0.750 1.000 −80 −60 −40 −20 P/( 1 2ρV2 b)t / tcyctSPO tBDC tSPCFigure 2. Piston motion (black curve) and outlet pressure (red curve).0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.5 1.0 1.5 Vz / VscVθ / Vscr / DV i / Vsc  coarsemediumfine0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.5 1.0 1.5 r / DV i / VscVz / VscVθ / Vsc  cycle 1cycle 2cycle 3cycle 4Figure 3. Numerical sensitivity study showing radial velocity profiles. (left) Spatial convergence, (right)cyclic convergence.and to investigate the dynamics of the turbulent swirling flow. This is achieved by simulatingthe flow in the model using CFD and comparing the obtained predictions with the experimentalresults.MethodologyThe model geometry is shown in Figure 1 and described in detail in [1, 2]. The internal cylinderdiameter is D = 190mm and the Reynolds number of the flow is Re = VscD/ν = 50,000, whereν is the kinematic viscosity and Vsc is the characteristic scavenge velocity. In the present work,a port section is used with 30 equally spaced ports and a port angle of α = 20◦. The pistonmotion is presented in Figure 2 together with the pressure measured at the model outlet (P4 inFigure 1). The time is normalized with the total cycle time which is tcyc = 1.20 s.The simulations are performed with the commercial CFD code STAR-CCM+ version 8.02.008.A hybrid mesh is used with polyhedral cells in the main volume and prismatic cells on the wall.The mesh used for the simulations, referred to as the ’fine’ mesh, has approximately 5.2 millioncells. A second order linear upwind scheme is used for the spatial derivatives and a second orderimplicit scheme is used for the temporal derivatives. Both Reynolds-Averaged Navier-Stokesturbulence models and large eddy simulations are investigated.A sensitivity study is carried out in order to investigate the effect of the numerical parameters.Figure 3 shows the radial velocity profiles of tangential velocity Vθ and axial velocity Vz for0.000 0.141 0.250 0.359 0.500 0.750 1.000 −0.25 0.00 0.25 0.50 0.75 1.00 t / tcycWb / Vsc  tSPO tBDC tSPCLDACFDPIVFigure 4. Comparison of predicted and experimental bulk velocity.Figure 5. Comparison of radial velocity profiles. (top) tangential velocity Vθ, (bottom) axial velocity Vz.(blue curves) Spalart-Allmaras, (red curves) K-ω SST, (black curves) experiments.different mesh sizes and cycles after simulation start. It is seen that the results obtained onthe ’fine’ mesh can be considered mesh independent as no significant changes occur from the’medium’ to ’fine’ mesh. Furthermore, it is seen that the flow becomes approximately periodicalready after the second cycle.ResultsA comparison between the predicted and experiential bulk velocity Wb is presented in Figure 4.The numerical results are obtained with the Spalart-Allmaras turbulence model [4] and theexperimental results are measured with particle image velocimetry (PIV) and laser Doppleranemometry (LDA). After the scavenge port closing, the CFD predicts large amplitude oscilla-tions which do not exist in the experimental data. This is, however, a result of the experimentalphase-averaging and therefore not a ’true’ discrepancy. In general, it is concluded that thepredicted bulk velocity is in good agreement with the experiments.Figure 5 shows a comparison of the radial velocity profiles at t/tcyc = 0.30 which is shortlyafter the piston reaches BDC and corresponds to 25% port closure. Profiles are presentedFigure 6. Visualization of flow structures during the scavenging period using iso-surfaces of the λ2-criterion colored by the axial velocity.for simulations using both the Spalart-Allmaras and the k-ω SST [3] turbulence model. Thepredictions obtained with the two turbulence models are in general similar and in reasonableagreement with the experimental data. The tangential velocity is underestimated near theports and the axial velocity deficit is overpredicted in the top of the cylinder. Both models do,however, correctly predict the reversed axial flow at the centerline near the ports, which showsthe occurrence of a vortex breakdown.The numerical results are also used to investigate the complex flow dynamics of the turbulentswirling flow. In Figure 6 the unsteady flow structures are visualized using iso-surfaces of theλ2-criterion colored by the axial velocity. In the start of the scavenging process, multiple vortexrings are formed, and later in the process, a coherent vortex core is established. When the portsare fully open, the vortex core has a pronounced helical shape.References[1] K. M. Ingvorsen, K. E. Meyer, J. H. Walther, and S. Mayer. Turbulent swirling flow in adynamic model of a uniflow-scavenged two-stroke engine. Exp. Fluids, page submitted, 2013.[2] K. M. Ingvorsen, K. E. Meyer, J. H. Walther, and S. Mayer. Turbulent swirling flow in amodel of a uniflow scavenged two-stroke engine. Exp. Fluids, 54:1494, 2013.[3] Florian R. Menter. Zonal two equation k-ω turbulence models for aerodynamic flows. AIAAJ., 1993. Paper 93-2906.[4] P. R. Spalart and S. R. Allmaras. A one-equation turbulence model for aerodynamic flows.Rech. Aerospatiale, 1:5–21, 1994.

Swirling flow in a two-stroke marine diesel engine

http://orbit.dtu.dk/ws/files/58252361/Swirling_flow.pdf

Swirling flow in a two-stroke marine diesel engine

Abstract

Similar works

Full text

Available Versions

Online Research Database In Technology