Qualification of Thermal hydraulic codes within NURESIM D. Bestion (CEA, France)

Transcription

1 Qualification of Thermal hydraulic codes within NURESIM D. Bestion (CEA, France) The thermalhydraulic codes used for nuclear safety applications Validation and Verification of codes Validation of system codes Validation of CFD codes Experience from NURESIM project Perspectives for future applications of two-phase CFD to nuclear reactor TH 1

2 The thermalhydraulic codes used for nuclear safety applications: different scales and models A multi-scale analysis of accidental transients 2

3 The thermalhydraulic codes used for nuclear safety applications: different scales and models System Scale System codes 0D, 1D 3D porous (coarse) 2-Fluid 6 eq. Multi-field TIA, Turbulence Component Scale Component codes Local Scale CFD codes 3D porous (finer) 3D CFD open medium 2-Fluid 4 eq. 2-Fluid 6 eq. Multi-field TIA, Turbulence 2-Fluid 6 eq. Micro Scale DNS+ITM 1-Fluid 3 eq. 3

4 Validation & Verification Terminology used by Writing groups of OECD-CSNI- GAMA WG1: Best Practice Guidelines for CFD Codes used for Nuclear Reactor Safety WG2: Assessment CFD Codes used for Nuclear Reactor Safety WG3: Extension of CFD Codes to Two-Phase Flow Safety Problems ASSESSMENT = Validation+ Verification Validation: How good the equations are Verification: How good the equations are solved Validation measures differences between the physical model (system of equations) and the real physics (calculation of experiments) Verification measures differences between what you want to solve and what you solve: numerical errors (benchmarks, comparison to analytical solutions) Sensitivity analysis: measures impact of initial and boundary conditions, of geometrical simplifications, of every basic model, Uncertainty analysis: what is the uncertainty of code predictions 4

5 System codes System Code : model the whole circuits All flow regimes All heat transfer regimes (about 40 different regimes) 0 < P < 250 bars ;T up to 2000 C; 0 < α < 1; V zero to supersonic various geometries Safety Code : Validated in the simulation domain Reducing the «User s effect» Best-estimate Code : requires uncertainty evaluation Industrial Code : robustness, CPU time, QA 5

6 Validation of system codes Closure laws : 14 terms of balance equations 100 à 200 correlations CATHARE Validation on Separate Effect Tests : SET tests from 40 test facilities several tens of measured values compared CATHARE Validation on Integral Effect Tests : + 20 IET from 8 test facilities 100 to 300 of measured values compared VALIDATION + Basic Uncertainty evaluation of a CATHARE version = 30 men.year work OECD-NEA extended SET & IET matrices cover the domain of simulation and all basic models 6

7 3D Single-Phase and two-phase CFD Single -Phase 1 phase for porous body Two -Phase Two-fluid for porous body 1 phase RANS models 1 phase LES models C C F M D F D?? Two-fluid for open medium Two-fluid + LES + LIS + ITM?? LES + ITM 1 phase DNS models Two-phase pseudo-dns with ITM 7

8 Methodology for using two-phase CFD Identification of all important flow processes of the appliaction Selecting a Basic model Single fluid model two-fluid 6 eq model multi-field model Filtering turbulent scales and two-phase intermittency scales: RANS models Two-phase LES DNS Identification of Local Interface structure simply based on local void fraction based on Ai and α Interface Tracking Method? Additional transport equations: Transport or turbulent quantities: k-ε, Rij-ε, Transport of interfacial area or particle number density, Modelling Interfacial transfers & validation Modelling Turbulent transfers & validation Modelling Wall transfers & validation 8

9 Two-phase CFD tools used for nuclear TH A few 2-phase CFD tools from nuclear R&D NEPTUNE (CEA + EDF) NURESIM Platform NPHASE (RPI + PSU) Many R&D tools (JRC-ATFM, GRS-FLUBOX,JAERI- ACE3D ) Commercial codes with 2-phase capabilities CFX, FLUENT, STAR-CD + codes & modules devoted to containment applications (TONUS, GOTHIC, GASFLOW ) or to other applications + codes & modules for two-phase in porous medium (COBRA, NASCA, FLICA, ) 9

10 Application of two-phase CFD to boiling bubbly flows for DNB investigations A rather simple interface structure: many small bubbles Simple identification of Local Interface structure No need of a ITM A steady or quasi-steady flow RANS approach is OK Selection of a basic model Two-fluid Multi-group of bubbles Turbulence modeling K- ε, then Rij-ε, Instrumentation available for α, δ, Ai, Tl, Vb, Vl Validation is possible DNS+ITM become mature for giving information on small scale processes and for closure relations: 10

11 Modelling boiling flow for CHF investigations Macro-Scale (order of about 1 cm) Mixing between sub-channels, cross-flows, turbulence Grid spacers effects on averaged flow parameters P, G, Xth Predicting of averaged flow parameters P, G, Xth Correlating of DNB with P, G, Xth Meso-scale (order of about 1 mm) Bubble transport and dispersion Bubble growing and collapse Coalescence and break up Turbulents transfers of heat and momentum Local grid spacers effects Predicting of local flow parameters P, Vl, VV, Tl, α, d b Correlating of DNB with local values of P, Vl, VV, Tl, α, d b Micro scale (order of 1 µm or less) activation of nucleation sites growing of attached bubbles sliding of attached bubbles along wall coalescence of attached bubbles, bubble detachement wall rewetting after detachement Prediction of P, V, T, & positions of all interfaces DNB is no more correlated but predicted 3D porous Subchannel analysis DNS+ITM type 1 type 2 type 3 type 4 type 5 type 6 CFD open medium 11

12 Validation matrix for CFD application to boiling bubbly flows for DNB investigations Air-water bubbly flows with measurements of α, δ, Ai, Vb, Vl (DEDALE) Heated boiling flow in pipes with measurements of α, δ, Ai, Tl, Vb (DEBORA, ASU, Purdue, KAERI) Heated boiling flow in pipes with turbulence promoter with measurements of α, δ, Ai, Tl, Vb (DEBORA) Micro-visualisation of pool boiling (QLOVICE) Turbulence measurements in rod bundles with spacer grids (AGATE) BFBT data in rod bundles with measurements of α LWL (NRI) data: VVER rod bundle with CHF tests + DNS +ITM simulations (KFKI, CEA) + identification of further experimental needs Ex: separate effect validation of bubble condensation 12

13 PTS in two-phase conditions HP injection steam suplly cold leg "break" Zone A recirculation Zone B high turbulent mixing COSI ECCS injection tests Zone C thermal stratification weir "downcomer" Important phenomena have to be simulated At system scale At local scale (RANS) Up to microscopic scale?(dns?) 13

14 Identification of important flow processes in PTS investigations ECCS jet area: Instabilities of the jet from ECC injection, Condensation on the jet itself before mixing, Entrainment and migration of steam bubbles below the water level Turbulence production below the jet Stratified flow in cold leg Interfacial transfer of momentum at free surface Interfacial transfer of heat & mass at free surface Turbulence production in wall shear & in interfacial shear layers Heat transfers with cold leg and RPV walls Effects of turbulent diffusion upon condensation Interactions between interfacial waves, interfacial turbulence production and condensation Effects of temperature stratification upon turbulent diffusion Influence of non-condensable gases on condensation Interface configuration in top of downcomer Flow separation or not in dowcomer at cold leg nozzle 14

15 Application of two-phase CFD to separate-phase flows for PTS investigations A simple interface structure : a flat or wavy interface Is a ITM necessary? A steady or quasi-steady flow Filtering all turbulent scales should be OK but how does it affect the interfacial wave pattern? Selection of a basic model Single fluid +ITM? Two-fluid? Instrumentation available for α, δ, Ai, Tl, Vb, Vl Validation is possible DNS & LES results are available to help modeling interfacial and turbulent transfers (e.g. D Lakehal et al.) 15

16 Validation matrix for PTS Separate effect tests Entrainment of bubbles (Bonetto-Lahey) Turbulence below jet (Iguchi) Air-water stratified flow with turbulence measurements (Fabre et al.) Steam water stratified flow with V and T (LAOKOON, Kim et al.) + need of future TOPFLOW experiments Mixed effect tests COSI tests: condensation at ECCS injection UPTF TRAM tests Integral effect tests ROSA IV OECD tests 16

17 Verification Numerical benchmarks are necessary: CHF Demonstrating the capability to simulate boiling flow Measuring numerical diffusion Coupling wall with fluid Defining mesh convergence criteria PTS Demonstrating the capability to simulate free surface flows with waves, friction and mass stransfers Selecting the best treatment of free surface (ITM?) Measuring numerical diffusion Defining mesh convergence criteria 17

18 Perspectives for future applications of twophase CFD to nuclear reactor TH Using two-phase CFD still requires a preliminary identification and analysis of the main basic flow processes which need to be properly modeled for any application. Selecting a modeling approach should be consistent with the experimental validation possibilities which depend on the available instrumentation techniques. Further developments of instrumentation techniques and new experimental programs are needed including complex flows with droplets, liquid films and free surface An extensive physical benchmarking covering a wide variety of flow situations is recommended in view of confronting several modeling approach depending on the application. 18

19 Perspectives for future applications of twophase CFD to nuclear reactor TH Best Practice Guidelines (already addressed in the ECORA project) are necessary for selecting a modeling approach, a nodalisation, to control the numerical errors This should be regularly revisited complemented and updated, depending on future progress of these methods for each twophase application. Sensitivity and uncertainty methodologies for two-phase CFD have to be considered on a longer term period. 19