Integrated analysis of the scavenging process in marine two-stroke diesel engines

Transcription

1 Integrated analysis of the scavenging process in marine two-stroke diesel engines Gothenburg regional OpenFOAM user group meeting. 13 november 2013 Fredrik H. Andersen, Industrial Ph.D. student Process Development, LDF, BU Low speed Department of Mechanical Engineering, Technical University of Denmark < 1 >

2 Outline of presentation 1. Project description and motivation 2. Two-stroke engine cycle for uniflow scavenged marine cross-head engines 3. CFD model of the scavenging process i. LDFengineClass (engine time, shape and topology changes) ii. Top level solver 4. Results i. Normal liner design (30 scavenge ports) ii. High swirl liner design (15 scavenge ports) iii. Comparison / validation with PIV measurements 5. Concluding remarks < 2 >

3 Project description CFD code / model that is able to give a qualitatively description/prediction of the scavenging process in two-stroke cross-head engines Generic geometry (All MAN two-stroke engines?) Integrated with existing in-house code and design tools Technical spesifications 4T50ME-X < 3 >

4 Two-stroke engine cycle The two-stroke engine cycle can be described of two phases with 5 sub processes: Closed volume phase: 1. Combustion/Expansion 2. Compression Open Volume phase: 3. Blow down 4. Scavenging 5. Push out < 4 >

5 OpenFOAM 1.6-ext simulations of the scavenging process Topological changes LDFengine class (twostrokeengine) Exhaust PIMPLE (PISO/SIMPLE) algorithm Transport of passive scalar Sampling planes / probes Mass fluxes (intake, ports, exhaust) Exhaust Valve Pseudo LES Smagorinsky Mesh ~ 1 mill cells Parallel by manual decomposition (30 proc) Computational time: ~1-2 weeks Hexahedral and polyhedral cells snappyhexmesh and blockmesh Scavenge ports intake < 5 >

6 LDF-engine Mesh Class Wrapper class to simulate the engine cycle the mesh has to be subjected to shape changes and topological changes to account for the motion of piston and valve Primitive topology modifiers: Sliding interface: Connecting the scavenge ports / box with the cylinder mesh Add / remove layers: Motion of exhaust valve Generalized Grid Interface: Handle the interface between valve and cylinder cells < 6 >

7 Add / remove layers + GGI Valve motion is achieved with adding and removing layers of cells. Requires a plane surface of faces to operate on. Maximum and minimum thickness of layer is specified in constant/enginegeometry Generalized Grid Interface handles the communication between the moving cells and the stationary cells in the cylinder < 7 >

8 Piston motion Piston motion in LDFengineClass: Harmonic motion of pointzone with weighting function (Heywood, 1988) < 8 >

9 Sliding interface Operates on patches corresponding patch names, stationary patch is master and moving patch is slave. Liner Slave patch ports Master patch < 9 >

10 Top level solver scalarpimpleenginefoam.c Standard files: CreateEngineDynamicMesh.H CreateFields.H heqn.h peqn.h Ueqn.H readenginetimecontrols.h Additional files: portfluxes.h sampledict.h scalartransport.h startsummary.h massflow.h < 10 >

11 scalartransport.h LDFengine class provides acces to cylindercellzone Run time calculation of useful parameters: Mass of fresh air in cylinder Total mass in cylinder Volume average temperature in cylinder Volume average density in cylinder Volume average pressure in cylinder Angular momentum in cylinder Axial momentum in cylinder Volume fraction iso-volume at BDC < 11 >

12 Results Influence of geometric swirl number Mass flow rates over scavenge ports show oscillations that correspond with the fundamental frequency of a cylinder with one open end. Calculated frequency: Theoretical frequency: < 12 >

13 Influence of cylinder liner design Normal swirl number (30 scavenge ports) Geometric Swirl number: Scavenging efficiency as f(cad): < 13 >

14 Influence of cylinder liner design High swirl number (15 scavenge ports) Geometric Swirl number: Scavenging efficiency as f(cad): < 14 >

15 Comparison with PIV measurements Comparing CFD with in-cylinder PIV velocity measurements Good agreement in timing of tangential velocity increase Standard port layout (30 ports, α = 20) Ω = 0.26 High swirl port layout (15 ports, α = 20) Ω = 0.52 < 15 >

16 Concluding remarks Numerical model capable of reproducing and predicting trends in marine twostroke diesel engines Allows interpolation between experimental results and in-cylinder flow Complex and large geometry makes computational expences and mesh quality challenging High swirl increase mixing of burned gases and the scavenge air which in turn increase the requrement of volume displacement in the cylinder. < 16 >

17 Thank you for your attention! All data provided in this document is non-binding. This data serves informational purposes only and is especially not guaranteed in any way. Depending on the subsequent specific individual projects, the relevant data may be subject to changes and will be assessed and determined individually for each project. This will depend on the particular characteristics of each individual project, especially specific site and operational conditions. < 17 >