Supervisor: Lars Davidson ^{lada@chalmers.se}

Cooperation:
UNICC, Chalmers,
GE Energy (Sweden) AB,
LMH-IMHEF-EPFL, Lausanne, Switzerland,
Chalmers Medialab,
CERCA, Montreal, Canada,
Vattenfall Utveckling AB

Financing:
The Swedish National Energy Admininstration,
GE Energy (Sweden) AB and
the Swedish electrical utilities R&D company

Publications: [1-17], see references below

Start of project : Spring 1997
End of project : August 2002

Keywords:
CFD, Finite Volume, Numerical, Turbulence, Parallel, Multiblock, Kaplan, Francis, Turbine, Turbomachinery, Tip Clearance, Validation, Verification, Visualization

INTRODUCTION
This work is part of a Swedish water turbine program financed by a collaboration between the Swedish power industry via ELFORSK (Swedish Electrical Utilities Research and Development Company), the Swedish National Energy Administration and GE Energy (Sweden) AB. The purpose of the Swedish water turbine program is to increase Swedish water power competence in order to meet the growing water power demand in Sweden and demands on preservation of the environment and efficiency.

A parallel multiblock finite volume CFD (Computational Fluid Dynamics) code CALC-PMB (Parallel MultiBlock) for computations of turbulent flow in complex domains has been developed and used for the computations of the flow through water turbines. The main features of the CALC-PMB CFD code are the use of conformal block structured boundary fitted coordinates, a pressure correction scheme (SIMPLEC), cartesian velocity components as the principal unknowns, and collocated grid arrangement together with Rhie and Chow interpolation. The computational blocks are solved in parallel with Dirichlet-Dirichlet coupling using PVM (Parallel Virtual Machine) or MPI (Message Passing Interface). During the computations, the computational blocks are assigned to separate PVM or MPI processes. The level of parallelization is thus determined by the block size distribution and the distribution of the processes on the available processors. The parallel efficiency is excellent, with super scalar speedup for load balanced applications. The ICEM CFD/CAE grid generator is used for grid generation and Ensight, Tecplot and Matlab are used for post-processing.

To be able to investigate the results from computational fluid dynamics in a Cave Automatic Virtual Environment (CAVE), a close collaboration with people at Chalmers Medialab and people at CERCA was established during 2000. The people at CERCA have developed a configurable visualization software tool for the display and analysis of numerical solutions. The software is called Vu. A short description of 3D visualization of water turbine flow using Vu can be found here.

Two different types of water turbines are investigated:

THE KAPLAN WATER TURBINE

This work is focused on tip clearance losses in Kaplan water turbines (figure 1), which reduces the efficiency of the turbine by about 0.5%. The investigated turbine is a test rig, at GE Energy (Sweden) AB, with a runner diameter of 0.5m (figure 2). It has four runner blades and 24 guide vanes. The tip clearance between the runner blades and the shroud is 0.25mm.

Figure 1. The complicated geometry of a Kaplan turbine.

Figure 2. The geometry of the Kaplan turbine studied in this work.

The Hölleforsen Kaplan wicket gate and runner geometry.

The Hölleforsen Kaplan runner and draft tube geometry.

THE GAMM FRANCIS WATER TURBINE

This work investigates the turbulent flow in a Francis water turbine (figure 3). The GAMM Francis turbine (figure 4) is used for validation of the CALC-PMB CFD code in the area of hydraulic machinery. The investigated Francis runner has 13 runner blades. Detailed measurements of the flow in this turbine was performed in conjunction with the GAMM Workshop in 1989. This validation is performed in a collaboration with LMH - IMHEF - EPFL in Lausanne, Switzerland, where three months of the work was carried out.

Figure 3. The complicated geometry of a Francis turbine.

Figure 4. The geometry of the GAMM Francis turbine studied in this work.

NUMERICAL CONSIDERATIONS

To resolve the turbulent flow in the tip clearance and the boundary layers, a low Reynolds number turbulence model is used. Because of computational restrictions, complete turbine simulations found in the literature usually use wall functions instead of resolving the boundary layers, which makes tip clearance investigations impossible. Since part of the computational domain is rotating, Coriolis and centripetal effects are included in the momentum equations. At this stage, the k-\omega model of Wilcox, which can be integrated all the way to the wall, is used without terms for rotational effects. This is common in turbomachinery computations for reasons of numerical stability and the small impact of such terms in these kinds of industrial applications. To resolve the turbulent boundary layers and the tip clearance at the same time that the grid size should be kept as low as possible and the control volumes as orthogonal as possible, the computational grid is created in a multiblock topology using ICEM CFD/CAE. The computations of both the guide vanes and the runner are confined to a single guide vane or runner blade. This includes no extra restrictions since the boundary conditions are assumed to be stationary axisymmetric and the stationary Reynolds averaged solution is thus periodic.

Since we are developing the code ourselves, we are in full control of numerical errors, assumed approximations, etc. We can easily implement new turbulence models or exchange the numerical solver so that we can do Large Eddy Simulations (LES) using different subgrid models.

RESULTS

The computational results are in accordance with observations made by GE Energy (Sweden) AB and the GAMM measurements. Some results are presented below as figures with captions. Download the references presented below for a more thorough investigation and discussion of the results.

My Ph.D. Thesis. Click on it to download it or to view the slides of my presentation.

Multiblock grid of four (of 24) guide vanes of the Kaplan water turbine investigated in this work. Only the grid planes attached to surfaces are shown.

Multiblock grid of the runner of the Kaplan water turbine investigated in this work. Only the grid planes attached to surfaces are shown.

The Kaplan runner static pressure distribution at radius 0.16m. Static pressure in kPa. The level is not adjusted to a real case.

The "reduced" pressure distribution on the Kaplan runnerblades. The "reduction" of the pressure is from the centripetal force of the rotating coordinate system. Static pressure in kPa. The level is not adjusted to a real case.

The "reduced" pressure distribution on the Kaplan geometric surfaces. The "reduction" of the pressure is from the centripetal force of the rotating coordinate system.

Tip vortex core and shroud boundary layer stream ribbons, colored by the magnitude of the relative velocity (red is high and blue is low). The shroud boundary layer stream ribbon is scraped off the shroud boundary layer.

Multiblock grid of the GAMM Francis runner. Only the grid planes attached to surfaces are shown.

The outlet velocity profiles of the GAMM Francis runner. Discrete points are measurements and continous lines are computations. Preliminary results.

The GAMM Francis runner. Off-design surface static pressure and recirculating streamlines colored by the magnitude of the velocity. The picture shows a large recirculation region with flow both upwards and downwards and swirl in two directions.

The same picture with particles.

The flow in the Hölleforsen spiral casing. A collaboration project with Mr. Eduardo Oliveira, LEA - Laboratório de Energia e Ambiente Engenharia Mecânica ENM-UnB, University of Brazil

DISCUSSION

The introduction of CFD in the area of hydraulic machine research is believed to increase a detailed knowledge of the flow inside the machines and to speed up the design procedure. This requires that the experience from CFD in this area is increased, which cannot be achieved without detailed experimental investigations to be used for comparisons. With sufficient experience of CFD in the area of hydraulic machine research, CFD will definitely be used in future hydraulic machine development.

Academic cooperation:

Visualization of the computational results in a Cave Automatic Virtual Environment (CAVE) is investigated in cooperation with Chalmers Medialab and people at CERCA.

Industrial cooperation:

The work is carried out in cooperation with GE Energy (Sweden) AB. They have supplied the geometry of a part of a Kaplan water turbine including guide vanes and runner blades.

International cooperation:

Part of the work is carried out in cooperation with LMH - IMHEF - EPFL in Lausanne, Switzerland, where three months of the work was carried out. They have supplied the geometry and performed the measurements of the GAMM Francis turbine.

Status:

A thesis for the degree of Doctor of Philosophy was presented on August 30, 2002.

REFERENCES

- click to view postscript file