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Two cases of turbulent mixing

Julia Boschan July 2013. Two cases of turbulent mixing. Part I: Stratified Helium layer Break-Up by a Vertical Jet Motivation Experimental setup Background Numerical Set-Up Results and Discussion Part II: Mixing of gas streams at high density ratios Motivation Experimental Set-Up

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Two cases of turbulent mixing

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  1. Julia Boschan July 2013 Two cases of turbulent mixing

  2. Part I: Stratified Helium layer Break-Up by a Vertical Jet Motivation Experimental setup Background Numerical Set-Up Results and Discussion Part II: Mixing of gas streams at high density ratios Motivation Experimental Set-Up Numerical Set-Up Results and Discussion Outline • Part I: Stratified Helium layer Break-Up by a Vertical Jet • Motivation • Experimental setup • Background • Numerical Set-Up • Results and Discussion • Part II: Mixing of gas streams at high density ratios • Motivation • Experimental Set-Up • Numerical Set-Up • Results and Discussion

  3. Motivation Studies are made to: Observe behavior of Hydrogen in NPP Study the possibility of the formation of a Hydrogen layer in the containment The destruction of such a layer by natural diffusion or Other influencing factors. Aim of this study: Observe the destruction of such a layer by a turbulent jet perpendicular to the layer Using numerical methods for simulation Compare results with experiments

  4. The Experiment MiniPanda facility: 4:1 scale down of the Panda facility two vessels : 2 m height, 1 m diameter Bend Interconnecting pipe Helium injected into the second vessel Air injection 1 m from the bottom of the vessel Supporting Rods in second vessel Open vent in first vessel

  5. Theoretical Background Main effect: Jet interaction with stratified light gas layer Interaction Froude number:

  6. Conducted Experiments and cases chosen for the Benchmark

  7. Simulation Set Up – Mesh Creation Mesh was created in ANSYS –ICEMCFD Number of elements: 220.000 Structured mesh with hexahedral elements O-grids

  8. Simulation Set Up – Fluent RANS k-ε model Standard Wall Function Species transport Adaptive time step

  9. Initial and boundary conditions Constant inlet gas temperature Constant inlet velocity flat velocity profile Temperature dependent gas properties Temperature dependent diffusion coefficient Constant wall temperature with infinite heat capacity of walls

  10. Results Katherometer Thermocouples Center axis Vent outlet Pipe outlet • 5 thermal wire mesh sensors • 3 horizontal and 1 vertical in Vessel 2

  11. The Jet Transports gas from the bottom to the top Height is determiend by the density ratio of jet and surroundings (stagnation point)

  12. The Jet - 2 Jet is slowed down by high density difference Density differnce decreases with time Jet reaches higher

  13. MPII_1 – Fr = 0.7 Stagnation point moves upward Jet reaches to at 2300 s • Experiments faster • Delay of appox. 200s (toward end)

  14. MPII_1 – Fr = 0.7 Helium decreases towards a stable value: ̴0.15 Further decreases • Experimental curvesfaster • Higher stablevalue

  15. MPII_2 – Fr = 1.3 Better reproduction of experiments Less numerical effort (inertia dominated) • Higher Fr number, fastererosion • Jet reaches top after 400 s

  16. MPII_6 – Fr = 0.8 Less helium : 0.35 at top Jet reaches top at 1900 s Closer to base case • Good agreement with experiment • Numerical effort reduced due to less helium

  17. Comparison 200 s 100 s 300 s 400 s 500 s 600 s 700 s 800 s 900 s 1000 s 1100 s 1200 s 1300 s 1400 s 1500 s 1600 s 1800 s 1700 s 1900 s 2000 s 2100 s 2200 s 2300 s

  18. Pure diffusion Diffusion coefficeint is temperture dependent Good agreement at the top and bottom Over predicted reults in the middle region

  19. Pure Diffusion Influence of the jet visible Top detectors follow pure diffusion Bottom detector show opposite behaviour

  20. Sensitivity studies 6 extra studies using the base case MPII_1: • Considering the time dependence of the inlet temperature • Considering a developed velocity profile jet inlet • Considering the supporting rods • Variation on the turbulent Schmidt number • Changing to SST k-ω turbulence model • Analysis of the turbulence production due to buoyancy

  21. Time dependence of the inlet temperature Conclusion on considering the heat up phase the jet is less diffusive but shows only minor difference later • The difference are visible in the temperature profile the jet is less diffusive • But is again comparable in the after the heat-up phase Little influence on helium distribution compared to the base case

  22. Inlet velocity profile Conclusion on inlet velocity profile only minor changes can be seen, thus the inlet velocity profile is not considered an influencing factor Only very minor differences can be seen

  23. Supporting Rods • Rods damp the diffusion • Layer break up delayed • Higher helium concnetration and temperatures • Difference of simulation and experiment higher Conclusion on supporting rods Supporting rods damp the bulk flow and cause delay in the layer break up Cause high numerical diffusion, but indicate higher differences between simulation and experimental results • Very fine mesh required around the rods • High Courant number • Smaller time step New mesh Considering the supporting rods Diameter: 5 mm Pipe flow 400.000 cells Julia Boschan PSI, 08 July 2013

  24. Turbulent Schmidt number Conclusion variation of the turbulent Schmidt number Varying the turbulent Schmidt number in the range suggested by literature does not improve the simulation significantly • Ratio tubulent viscosity and turbulent diffusion • Sc=0.5 or Sc=0.9 compared to Sc=0.7 • Variation in the range suggested by literature • Minor differences to the base case • No correction to the experiment

  25. SST k-ω turbulence model Conclusion using SST k-ω turbulence model Evolution of the helium layer shows no difference in the two cases the temperature field is less diffusive resulting in less diffusive jet and lower bulk temperature • Turbulence model shows no difference in the He layer evolution 500 s • the temperture contour plot is less diffusive

  26. Turbulence production due to bouyancy • Term in k and ω equation • When negative: turbulence damped (e.g. stable stratification) • When positive: tubulence aided (e.g. unstable stratification) • Using the GGD –model • He – layer still shows faster errosion • Fluent default term gives best result • When the term is no present • Faster errosion of the He layer • Damping by bouyancy

  27. Conclusion on turbulence production due to buoyancy term The additional term must be added to the equation as otherwise the result is unphysical. The default value accounts well for this quantity. Default 100 s GGDM Default 500 s GGDM Default 1000 s GGDM Default 1500 s GGDM

  28. Outline • Part I: Stratified Helium layer Break-Up by a Vertical Jet • Motivation • Experimental setup • Background • Numerical Set-Up • Results and Discussion • Part II: Mixing of gas streams at high density ratios • Motivation • Experimental Set-Up • Numerical Set-Up • Results and Discussion

  29. Numerical Set-Up • Conditioning section: • Two inlet legs, • quadratic cross section (30 x 60 mm) • Mesuarmanet section: • One channel • Square cross section (60 x 60 mm) Communication from B. Krohn • CFD domain: • 100 mm before the splitter plate • 850 mm long • Mesh: • 1 million cells • Wall refinement: y+= 0.3

  30. Numerical Set-up 2 • Wall treatment • SST k-ω: • Bulk: Standard k-ε • Wall: Standard k-ω • function • Low Re Model: • Integrate equation to the wall • Damping functions • Other: Enhanced Wall treatment • One equation model near the wall • Fine mesh near the wall • Five RANS – based models: • Standard k-ε • RNG k-ε • SST k-ω • Reynolds Stress Model • Low Reynolds number model

  31. Cases • 3 cases with increasing density differences • 2 cases with high density difference and velocity difference Δu= u1-u2/u1+u2

  32. Results • Contourplots in centerline • Data extraction along y-direction at fixed location on the centerline • Comparison of: • Velocity • Turbulent kinetic energy • N2 mol fraction

  33. T1_0 base case - Velocity • No density differences: N2 - N2 in both legs • 8 m/s average velocity at both inlets • Symmetric profile • Low velocity region in the middle • Development of the boundary layer

  34. T1_0 base case 300 mm

  35. T1_0 base case 300 mm • Shape is well reproduced • High difference in the magnitude ( ̴1 m/s) • Less developed boundary layer (thickness δ ) in the simulation

  36. T1_0 base case - Concentration

  37. T1_1 and T1_2 case - Velocity T1_1 T1_2 • Loss of symmetry • Lower density  higher velocity • T1_2 case helium laminar (constant δ) • Velocity difference to the experiment (flat inlet profile)

  38. T1_1 and T1_2 case – Concentration T1_1 • Higher density difference  thicker mixing layer • Inflection pointis pushed to the right T1_2 T1_1 T1_2

  39. T2_1 and T2_2 case - Velocity T2_1 T2_2 • Velocity difference visible • Tilting towards the nitrogen • helium laminar (constant δ) • Velocity difference to the experiment (flat inlet profile)

  40. T2_1 and T2_2 case – Concentration T2_1 T2_2 T2_1 T2_2

  41. Performance of different turbulence models T1_0 T1_2 600 mm

  42. Performance of different turbulence models T1_0 T1_2 600 mm

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