CFDOFAIRFLOWINHYDROPOWERGENERATORS FOR CONVECTIVE COOLING, USING OPENFOAM

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1 CFDOFAIRFLOWINHYDROPOWERGENERATORS FOR CONVECTIVE COOLING, USING OPENFOAM ECCOMAS CFD 21 Pirooz Moradnia, Håkan Nilsson Lisbon-Portugal Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 1

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 Anincreasedpoweroutputleadstomoreheatthatneedstoberemoved 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 Thestatorsshouldbecooledbyairflowingthroughthestatorairs Focus of the present work: Axially cooled generators Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 2

3 Geometry A small generator at Uppsala University, Sweden 4 cooling- rows 18coolingineachrow 12poles Rotational speed: rpm Theflowisdrivenbytherotationoftherotor, axially into the rotor and radially out through the stator Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 3

4 Modelling in OpenFOAM Aperiodic1/12sectorinthetangential direction since there are exactly 9 s per pole Symmetryplaneinthemiddleofthe generator(lower boundary in figure) Noinletandoutletboundaries, no prescribed mass flow Recirculating flow without inlet and outlet, thus no prescribed mass-flow Themassflowisgivenbytherotationoftherotor Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 4

5 Stator cooling s The rotor rotates clockwise. The 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 -Case1:Thebasecase(7.2Mcells) -Case2:Case1withmodifiedrotorbody(9.1Mcells) -Case3:Case2withstatorbaffle(9.1Mcells) -Case4:Case3withradialfanblades(9.1Mcells) Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 6

7 Distributionofvolumeflowsinthes, (m 3 /s) Thestatorbaffleandfanbladeshelpmakingthedistributionoftheflowmoreuniform Thevolumeflowdecreasesatthecenterofthepole(lowerpressure) Volume flow, [m 3 /s] top row 1 x Case1 Case2 Case3 Case4 Volume flow, [m 3 /s] bottom row 1 x Case1 Case2 Case3 Case4 Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 7

8 Flow structure in the s Contours of zero radial velocity seperate the recirculation area from the outgoing flow Inthecaseswithoutthefanblade,thereversedflowcoverstheentiredownstreamsideof the stator windings The fan blades minimize the reversed flow region Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 8

9 Unit vectors of meridional flow Regions with upward velocity near the stator inner wall Higherpressuremake-upbythestatorbaffleandrotor fan blades give more downward flow Separationjustattheinletincaseswithstatorbaffle (less powerful separation with fan blades) Purelyinwardflowattheinlettothestatorbaffle Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 9

10 Further parametric studies Rotor design C1 Rotor design C2 Rotor design C3 Rotor design C4 Rotor design C Base Baffle Blade Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 1

11 Distributionofvolumeflows, (m 3 /s),furtherstudies 1 x 1 4 C1 C1S C1F 1 x 1 4 C2 C2S C2F 1 x 1 4 C3 C3S C3F 1 x 1 4 C4 C4S C4F 1 x 1 4 C CS CF Volume flow, [m 3 /s] x 1 4 C1 C1S C1F 1 x 1 4 C2 C2S C2F 1 x 1 4 C3 C3S C3F 1 x 1 4 C4 C4S C4F 1 x 1 4 C CS CF Volume flow, [m 3 /s] Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 11

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 Pressure (Pa) (y/h) (y/h) (y/h) Experiment standard k ε 2 realizable k ε 1 RNG k ε Non Linear Shih k ε (x/h) 1 Lien Cubic k ε 1 U/U high Re k ε turbulence models cl Experiment Lam Bremhorst k ε Launder Sharma k ε (x/h) 1 1 U/U cl low Re k ε turbulence models 3 Experiment 2 LRR Launder Gibson RSTM 1 Spalart Almaras k ω SST 1 1 (x/h) U/U cl other turbulence models Velocity (m/s) Theory Computations. Theory Computaions radius (m) radius (m) Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 12

13 Conclusions Modificationoftheheightoftherotorbodydidnot affect the results considerably Useofstatorbafflestoavoidoutwardflowattheinlet 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 between the inside and outside Higher pressure difference between inside and outsidethemachineleadstoahighervolumeflow Higher pressure difference between inside and outside the machine leads to a decreased recirculation in the stator s Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 13

14 Thank you! Acknowledgements The work has been financed by SVC( SwedishEnergyAgency,ELFORSK,SvenskaKraftnät, 1 Chalmers, LTU, KTH, UU SNIC(Swedish National Infrastructure for Computing) and C3SE(Chalmers Centre for Computational Science and Engineering) have provided the computational resources. 1 Companiesinvolved: CarlBro,E.ONVattenkraftSverige,FortumGeneration,Jämtkraft,JönköpingEnergi,Mälarenergi,SkellefteåKraft,Sollefteåforsens, Statoil Lubricants, Sweco VBB, Sweco Energuide, SweMin, Tekniska Verken i Linköping, Vattenfall Research and Development, Vattenfall Vattenkraft, VG Power, Öresundskraft, Waplans and Andritz Inepar Hydro Pirooz Moradnia, Chalmers/ Applied Mechanics/ Fluid Dynamics 14