Numerical modeling of deforming bubble transport related to cavitating hydraulic turbines

Abstract

Cavitation is a problem in many hydroelectric power plants since it can cause adverse effects on performance and damage to nearby solid surfaces. The concerns of this thesis are the numerical aspects of flow simulations in cavitating hydraulic turbines which contain several difficulties: turbulent and complex flows, steady and moving parts of the geometry, bubble transport and cavitation development. The focus is on the accuracy and reliability of several different aspects of these difficulties, namely the study of bubble transport without phase change.



Two main strategies are chosen. Firstly the investigation of the turbulent bubble-flow interaction in a turbine geometry and secondly the investigation of the bubble deformation and the bubble-flow interaction. Consequently, different methods in order to handling these types of three dimensional multi-phase flows are presented. Volume of Fluid (VOF) is used for immersed fluid-fluid flows and improved methods are presented and evaluated for the phase transport and the interface treatment. This includes the Direction Averaged Normal model (DAN) and the Direction Averaged Curvature model (DAC). The Volume of Solid (VOS) method is also presented and evaluated. VOS is built on VOF and is a second order accurate boundary treatment method in Cartesian grids for both stationary and moving geometries of complex shape.



All the methods are tested using different three dimensional cases which leads to a confirmation of the high accuracy. The high accuracy of the VOF model is verified by comparing it with the experimental data for both rising wobbling bubbles and the bubble formation for air injection in the bottom of a water channel. The real advantage of the VOS method is demonstrated for a turbulent flow past a rotating propeller placed in a square channel, where the turbulence and the bubble transport are simulated using Large Eddy Simulation (LES) and Lagrangian Particle Tracking (LPT) respectively.

Abstract (Swedish)

Popular Abstract in Swedish

Kavitation uppstår när trycket är så lågt så att vätska övergår till gas och det är ett problem i många vattenkraftverk på grund av att det kan försämra turbinverkingsgraden samt skada närliggande fasta ytor. Denna avhandling behandlar de numeriska svårigheter som man måste komma tillrätta med för att kunna datorsimulera kavitation i vattenturbiner. Dessa beror på den turbulenta och komplexa strukturen på vattenflödet, att geometrin har både fasta och rörliga delar, gasbubblornas rörelse samt kavitationens beteende. Avhandlingen fokuserar på nogrannheten och tillförlitligheten av flera av dessa svårigheter genom studier av bubblors rörelse i vatten.



Två strategier har använts. Den första behandlar små bubblor som transporteras i det turbulenta flödet i en vattenturbin och den andra behandlar stora bubblor som deformeras och interagerar med omgivande vätska. Flera olika modeller har presenterats när det gäller numerisk behandling av ytor mellan gas-vätska samt vätska-fast kropp.



Alla metoder har testats för en mängd olika fall för att påvisa den höga nogrannheten i de numeriska modellerna genom jämförelser med analytiska resultat och/eller experimentella data. Bland annat så har luftbubblor i vatten undersökts genom studier av (1) fritt stigande, deformerade bubblor, (2) formationen av bubblor vid luftinsprut i botten av en kanal samt (3) transporten av små sfäriska bubblor i det turbulenta flödet i en vattenturbin.

• author: Lörstad, Daniel
• supervisor: [unknown] [unknown]
• opponent: Prof Sommerfeld, Martin, Germany
• organization: Department of Energy Sciences
• publishing date: 2003
• type: Thesis
• publication status: published
• subject: Fluid Mechanics and Acoustics
• keywords: Gases, fluid dynamics, plasmas, Gaser, fluiddynamik, plasma, Energy research, Energiforskning, Direction Averaged Curvature (DAC), Direction Averaged Normal (DAN), Volume of Solid (VOS), Volume of Fluid (VOF), Lagrangian Particle Tracking (LPT), Large Eddy Simulation (LES), bubble, multiphase, turbulent flows, CFD, hydro-turbine
• pages: 232 pages
• defense location: Room M:B, M-building, Lund Institute of Technology, Lund
• defense date: 2003-09-12 10:15:00
• ISBN: 91-628-5755-X
• language: English
• LU publication?: yes
• additional info: Article: 1. Lörstad D & Fuchs L. LES of turbulent flow in a draft tube of a water turbine, In Gebart R, Gustavsson H and Karlsson R (eds). Proc. of Turbine-99 - Workshop on a draft tube flow in Porjus, Sweden, 1999. Universitetstryckeriet Luleå. Article: 2. Lörstad D & Fuchs L. A Volume of Fluid (VOF) method for handling solid objects using fixed Cartesian grids. In Sarler B and Brebbia C A (eds). Moving Boundaries VI - Computational Modelling of Free and Moving Boundary Problems, pages 143-152, UK, 2001. Wessex Institute of Technology, WIT Press. Article: 3. Lörstad D, Fuchs L & Lindsjö H. Bubble transport in a turbulent rotating flow. Proceedings of the Hydraulic Machinery and Systems 21st IAHR Symposium, Lausanne, 2002. Article: 4. Lörstad D, Francois M, Shyy W & Fuchs L. Volume of Fluid and Immersed Boundary investigations of a single rising droplet. AIAA paper 2003-1282. 41st Aerospace Science Meeting and Exhibit, Reno, Nevada, USA, 6-9 January 2003. Article: 5. Lörstad D, Francois M, Shyy W & Fuchs L. Assessment of Volume of Fluid and Immersed Boundary methods for droplet calculations. Submitted to International Journal for Numerical Methods in Fluids, March, 2003. Article: 6. Lörstad D & Fuchs L. High order surface tension VOF model for 3D bubble flows of high density ratio. Submitted to Journal of Computational Physics, August, 2003. Article: 7. Blomgren C-H, Andree G, Lörstad D & Fuchs L. Rising air bubbles in a square channel - experiments and numerical simulations. Submitted to International workshop on multiphase and complex flow simulation for industry. Carg`{e}se, Corsica, France, July, 2003.
• id: 28ac6b07-7235-449f-8174-aa17d0a598b5 (old id 466083)
• date added to LUP: 2016-04-01 16:01:52
• date last changed: 2018-11-21 20:38:13