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Numerical modeling of deforming bubble transport related to cavitating hydraulic turbines

Lörstad, Daniel LU (2003)
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.... (More)
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. (Less)
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... (More)
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. (Less)
Please use this url to cite or link to this publication:
author
opponent
  • Prof Sommerfeld, Martin, Germany
organization
publishing date
type
Thesis
publication status
published
subject
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
publisher
Daniel Lörstad, Lagerbrings väg 7A, 224 60 Lund,
defense location
Room M:B, M-building, Lund Institute of Technology, Lund
defense date
2003-09-12 10:15
ISSN
0282-1990
ISBN
91-628-5755-X
language
English
LU publication?
yes
id
28ac6b07-7235-449f-8174-aa17d0a598b5 (old id 466083)
date added to LUP
2007-09-10 12:53:44
date last changed
2016-09-19 08:44:54
@phdthesis{28ac6b07-7235-449f-8174-aa17d0a598b5,
  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.<br/><br>
<br/><br>
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.<br/><br>
<br/><br>
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.},
  author       = {Lörstad, Daniel},
  isbn         = {91-628-5755-X},
  issn         = {0282-1990},
  keyword      = {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},
  language     = {eng},
  pages        = {232},
  publisher    = {Daniel Lörstad, Lagerbrings väg 7A, 224 60 Lund,},
  school       = {Lund University},
  title        = {Numerical modeling of deforming bubble transport related to cavitating hydraulic turbines},
  year         = {2003},
}