Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 1

CFD OF AIR FLOW IN HYDRO POWER GENERATORS

FOR CONVECTIVE COOLING, USING OPENFOAM

ECCOMAS CFD 2010

Pirooz Moradnia, Hakan Nilsson

Lisbon-Portugal

2010-06-16

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 2

Importance of cooling in generators

• Hydroelectric power generation stands for about half of the electricity generation in Sweden

• Modifications to the existing units would lead to significant contributions to the total energy

production

• An increased power output leads to more heat that needs to be removed

• The two large sources of energy losses in the generators: thermal and ventilation losses:

- Production of heat by the electric resistance in the generator coils (should be removed)

- The rotor and stator are cooled by air, which causes ventilation losses

• The stators should be cooled by air flowing through the stator air channels

• Focus of the present work: Axially cooled generators

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 3

Geometry

• A small generator at Uppsala University,

Sweden

• 4 cooling-channel rows

• 108 cooling channel in each row

• 12 poles

• Rotational speed: 500 rpm

• The flow is driven by the rotation of the ro-

tor, axially into the rotor and radially out

through the stator

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 4

Modelling in OpenFOAM

• A periodic 1/12 sector in the tangen-

tial direction since there are exactly

9 channels per pole

• Symmetry plane in the middle of the

generator (lower boundary in figure)

• No inlet and outlet boundaries, no

prescribed mass flow

• Recirculating flow without inlet and

outlet, thus no prescribed mass-flow

• The mass flow is given by the rota-

tion of the rotor

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 5

Stator cooling channels

The rotor rotates clockwise. The channel numbers will be shown again in the results section

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 6

Cases

• Frozen rotor concept: MRFSimpleFOAM (MRF = Multiple Reference Frames)

• Low-Re Launder-Sharma turbulence model

• Mesh generated with blockMesh (parameterized m4 script)

• 1 base case + 3 cases with one-at-a-time geometry modifications

- Case 1: The base case (7.2 M cells)

- Case 2: Case 1 with modified rotor body (9.1 M cells)

- Case 3: Case 2 with stator baffle (9.1 M cells)

- Case 4: Case 3 with radial fan blades (9.1 M cells)

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 7

Distribution of volume flows in the channels, (m3/s)

• The stator baffle and fan blades help making the distribution of the flow more uniform

• The volume flow decreases at the center of the pole (lower pressure)

2 4 6 8

0

5

10

15x 10

−4

channel

Vol

ume

flow

, [m

3 /s] −

top

row

Case1Case2Case3Case4

2 4 6 8

0

5

10

15x 10

−4

channel

Vol

ume

flow

, [m

3 /s] −

bot

tom

row

Case1Case2Case3Case4

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 8

Flow structure in the channels

• Contours of zero radial velocity seperate the recirculation area from the outgoing flow

• In the cases without the fan blade, the reversed flow covers the entire downstream side of

the stator windings

• The fan blades minimize the reversed flow region

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 9

Unit vectors of meridional flow

• Regions with upward velocity near the stator inner

wall

• Higher pressure make-up by the stator baffle and ro-

tor fan blades give more downward flow

• Separation just at the inlet in cases with stator baffle

(less powerful separation with fan blades)

• Purely inward flow at the inlet to the stator baffle

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 10

Further parametric studies

Rotor design C1 Rotor design C2 Rotor design C3 Rotor design C4 Rotor design C5

Base

Baffle

Blade

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 11

Distribution of volume flows, (m3/s), further studies

2 4 6 8

0

5

10

15x 10

−4

channel

Vol

ume

flow

, [m

3 /s]

C1

C1S

C1F

2 4 6 8

0

5

10

15x 10

−4

channel

C2

C2S

C2F

2 4 6 8

0

5

10

15x 10

−4

channel

C3

C3S

C3F

2 4 6 8

0

5

10

15x 10

−4

channel

C4

C4S

C4F

2 4 6 8

0

5

10

15x 10

−4

channel

C5

C5S

C5F

2 4 6 8

0

5

10

15x 10

−4

channel

Vol

ume

flow

, [m

3 /s]

C1

C1S

C1F

2 4 6 8

0

5

10

15x 10

−4

channel

C2

C2S

C2F

2 4 6 8

0

5

10

15x 10

−4

channel

C3

C3S

C3F

2 4 6 8

0

5

10

15x 10

−4

channel

C4

C4S

C4F

2 4 6 8

0

5

10

15x 10

−4

channel

C5

C5S

C5F

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 12

Validation cases

• Two well-known test cases - backward

facing step and Couette flow

• Comparisons with experiments and

theory

• Backward Facing step: A detailed

study of turbulence models in Open-

FOAM, led to the selection of the

Launder-Sharma turbulence model

• Laminar Couette flow, to verify the

pressure and velocity distributions

0 5 10 150

1

2

3

(x/H)

(y/H

)

U/Ucl

high−Re k−ε turbulence models

Experimentstandard k−εrealizable k−εRNG k−εNon Linear Shih k−εLien Cubic k−ε

0 5 10 150

1

2

3

(x/H)

(y/H

)

U/Ucl

low−Re k−ε turbulence models

ExperimentLam−Bremhorst k−εLaunder−Sharma k−ε

0 5 10 150

1

2

3

(x/H)

(y/H

)

U/Ucl

other turbulence models

ExperimentLRRLaunder−Gibson RSTMSpalart−Almarask−ω SST

0.37 0.375 0.38 0.385 0.39 0.395 0.4 0.4050

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

radius (m)

Pressure (Pa)

TheoryComputaions

0.37 0.375 0.38 0.385 0.39 0.395 0.4 0.4050

0.5

1

1.5

2

2.5

3

3.5

4

radius (m)

Velocity (m/s)

TheoryComputations

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 13

Conclusions

• Modification of the height of the rotor body did not

affect the results considerably

• Use of stator baffles to avoid outward flow at the inlet

• Higher and more even pressure distribution in the

machine, achieved by stator baffles

• Fan blades increase the pressure inside the machine

even more, leading to a higher pressure difference be-

tween the inside and outside

• Higher pressure difference between inside and out-

side the machine leads to a higher volume flow

• Higher pressure difference between inside and out-

side the machine leads to a decreased recirculation in

the stator channels

Pirooz Moradnia, Chalmers / Applied Mechanics / Fluid Dynamics 14

Thank you!

Acknowledgements

The work has been financed by SVC (www.svc.nu):

Swedish Energy Agency, ELFORSK, Svenska Kraftnat, 1

Chalmers, LTU, KTH, UU

SNIC (Swedish National Infrastructure for Computing) and C3SE (Chalmers Centre for Com-

putational Science and Engineering) have provided the computational resources.

1Companies involved: CarlBro, E.ON Vattenkraft Sverige, Fortum Generation, Jamtkraft, Jonkoping Energi, Malarenergi, Skelleftea Kraft, Sollefteaforsens,

Statoil Lubricants, Sweco VBB, Sweco Energuide, SweMin, Tekniska Verken i Linkoping, Vattenfall Research and Development, Vattenfall Vattenkraft, VG Power,

Oresundskraft, Waplans and Andritz Inepar Hydro