3D Numerical Analysis of the Unsteady Turbulent Swirling Flow in a Conical Diffuser using FLUENT and OpenFOAM

Transcription

1 3D Numerical Aalysis of the Usteady Turbulet Swirlig Flow i a Coical Diffuser usig FLUENT ad OpeFOAM Sebastia Mutea, Seior Researcher, Håka Nilsso, Associate Professor, Chalmers Techology, Gotheburg Romeo F. Susa-Resiga, Professor, Politehica Uiv.

2 Itroductio Decelerated swirlig flow i hydraulic turbie draft tube coe eds i vortex breakdow (with associated severe usteadiess ad pressure fluctuatios) whe operatig at partial discharge. The mai cause of VB is the icrease i swirl itesity dowstream a fixed pitch ruer as the discharge decreases, as a result of the mismatch betwee the swirl geerated by wicked gates ad the agular mometum extracted by the ruer. A certai level of swirl at draft tube ilet avoids the flow detachmet at coe wall, ad improves the coversio of the excess of kietic eergy ito static pressure. VB is associated with a cetral quasi-stagat regio; the vortex rope is a rolled-up vortex sheet which origiates betwee the cetral stalled regio ad the swirlig mai flow.

3 Helical vortex breakdow i decelerated swirlig flows The decelerated swirlig flow i Fracis turbie discharge coe evolves i helical vortex breakdow (precessig vortex rope) whe the swirl umber at ruer outlet icreases above a critical value. Nishi et al. (1988) suggest that the circumferetially averaged velocity field i the coe could be represeted as a dead (quasi-stagat) water regio surrouded by the swirlig mai flow. The spiral vortex is a rolled-up vortex sheet origiatig betwee the cetral stalled regio ad outer swirlig flow.

4 Cotet 3D computatioal domai ad BCs (computatioal domai correspods to the swirlig flow apparatus from test rig) Numerical set-up with FLUENT ad OpeFOAM 3D usteady turbulet flow simulatio of decelerated swirl i a straight draft tube Vortex rope visualizatio Pressure field aalysis compariso agaist experimetal data

5 Fracis turbie draft tube ad simplified straight coical diffuser

6 Pheomea i the draft tube flow Swirlig flow Flow ito a bed secodary flows Positive pressure gradiet i the diffuser - separatio

7 3D Computatioal domai & BCs Meridia cross sectio of the swirlig flow apparatus. 3D computatioal domai ad BCs

8 Boudary coditios Ilet velocity profiles as computed dowstream the swirl geerator (fixed ad rotatig blades) Fluet outlet radial equilibrium coditio for pressure (relatioship betwee pressure variatio i radial directio ad circumferetial velocity, obtaied from the radial mometum equatio) p r V r 2 OpeFOAM outlet uses zerogradiet for all variables ad sets the outlet average pressure to zero. Wall-fuctios at walls Axial ad circumferetial velocity profiles imposed o the ilet sectio of the 3D computatioal domai.

9 Numerical setup Structured mesh Velocity-pressure couplig FLUENT OpeFOAM 2 millios cells SIMPLE SIMPLE Turbulet model k-ε RNG Stadard k-ε Pressure discretizatio PRESTO Rhie&Chow Mometum discretizatio 2 d order 2 d order Time discretizatio 1 st order 2 d order Time step 1e-4 5e-5

10 Vortex rope visualizatio ; ; St= FLUENT (+6%) OpeFOAM (10%)

11 Fourier recostructio o sigal fuctio 2 1)/ ( )/ ( si cos ) ( N N t t B t t A A t g N s N N c A N c A 2 2 N c B T N ;

12 FLUENT ad OpeFOAM umerical results MG0 (base harmoics ad 2d harmoics) MG1 (base harmoics ad higher harmoics up to 4th)

13 FLUENT ad OpeFOAM umerical results MG2 (base harmoics ad higher harmoics up to 7th) MG3 (base harmoics ad higher harmoics up to 8th)

14 Pressure field aalysis ; MG0 ; MG1 MG2 MG3

15 Numerical results agaist Experimetal data

16 CONCLUSIONS 3D usteady turbulet (k-ε) simulatios were performed usig FLUENT ad OpeFOAM i order to uderstad the physics of the decelerated swirlig flow with vortex rope i coical diffuser The frequecy is reasoably evaluated Pressure fluctuatios associated with the vortex rope are computed i four sectios of the coical diffuser these are compared with experimetal data The usteady pressure field is quite well computed i the throat where the vortex rope is compact A sigificat discrepacy betwee umerical results ad experimetal data is obtaied i the dowstream part of the coical diffuser due to the vortex rope is too compact tha the real oe improved turbulet model (DES)

17 PERSPECTIVES vortex rope computed with k-ε model vortex rope computed with DES model