Wall Boiling Modeling and Model Validation in …

1 Wall Boiling Modeling and Model Validation in ANSYS CFD. Th. Frank, C. Lifante, Burns Head Funded CFD Development ANSYS Germany 2009 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary Outline Introduction Wall Boiling Modeling in ANSYS CFD. Bartolomej testcase (PWR). Lee testcase (BWR). Wall Boiling with conjugate heat transfer Wall Boiling in fuel assemblies Summary & Outlook Courtesy by E. Krepper (FZD). 2009 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary 1. Introduction Towards CFD for Flows through Nuclear Fuel Assemblies Prediction of Boiling flow through fuel assemblies Optimization of fuel assembly and spacer grid design Replacement/supplementation of very expensive experiments by knowledge obtained from CFD simulations Pressurizer Direct contact condenser Water steam To seperator con- enser High pressure 5 MW.

2 Coolers Circulation pump 10 MW. Void Fraction Feed Measurement water Device Control Pel. 15 MW valve BWR. Test Vessel Control PWR p 110 bar valve Pel. Test Vessel 9,5MW p 185 bar Downcomer Natural Circulation Loop 2009 ANSYS, Inc. All rights reserved. 3 ANSYS, Inc. Proprietary CFD Simulation for Fuel Assemblies in Nuclear Reactors Wall Boiling &. Material Properties Bulk Condensation Conjugate Heat Turbulence Transfer (CHT). Multiphase Flow FSI: Stresses &. Modeling Deformations Validation against Experiments 2009 ANSYS, Inc.

3 All rights reserved. 4 ANSYS, Inc. Proprietary 2. CFD Simulation for Fuel Assemblies in Nuclear Reactors Wall Boiling &. Material Properties Bulk Condensation Conjugate Heat Turbulence Transfer (CHT). Multiphase Flow FSI: Stresses &. Modeling Deformations Validation against Experiments 2009 ANSYS, Inc. All rights reserved. 5 ANSYS, Inc. Proprietary Wall Boiling in Fuel Assemblies The applications: PWR: nuclear reactor safety scenarios BWR: normal regime of operation Model history: Original mechanistic Model developed by Podowski et al.

4 @ RPI. 1st prototype for ANSYS CFX in EC project ECORA. Release of the Model in ANSYS CFX RPI wall Boiling Model available as UDF for ANSYS Fluent Courtesy by E. Krepper (FZD). 2009 ANSYS, Inc. All rights reserved. 6 ANSYS, Inc. Proprietary 3. The RPI Wall Boiling Model Why special Modeling for wall Boiling ? For subcooled flows with superheated walls, standard thermal phase change models for bulk Boiling /condensation will underpredict mass transfer rates Accounts for steam bubble growth on nucleation sites and bubble departure Mechanistic Model for wall driven Boiling Model outline: Mechanistic M h i ti wallll h heatt fl flux splitting litti convective heat transfer, evaporation, quenching Empirical submodels required for closure Available for different BC's.

5 Prescribed Twall or qwall, CHT walls Activated per boundary patch with individual Twall or qwall 2009 ANSYS, Inc. All rights reserved. 7 ANSYS, Inc. Proprietary Multiphase Flow Regimes for Boiling Water Flow subcooled bubbly annular spray flow flow slug flow flow flow ONB OSB. wall temperature T. Tsat mean fluid temperature x subcooled nucleate Boiling Boiling (saturated Boiling ). 2009 ANSYS, Inc. All rights reserved. 8 ANSYS, Inc. Proprietary 4. Flows with Subcooled Boiling (DNB) . RPI-Wall Boiling Model Mechanistic wall heat partioning Model : q Wall q F q Q q E.

6 Convective heat flux quenching heat flux evaporation heat flux q F A1 h F (TW T L ) q Q A2 hQ (TW T L ) q E m (h G h L ). y A2 A1 A2. Convective heat u Quenching heat flux flux t *. m *. m m *. 2009 ANSYS, Inc. All rights reserved. 9 ANSYS, Inc. Proprietary Grid dependent correlations Quenching heat flux q Q A2 hQ (TW T L ). y tW LCPL L. hQ 2 f TL 1-D approach . TL first grid node 1-D with refining grid TL TSat TW. CFD. 2009 ANSYS, Inc. All rights reserved. 10 ANSYS, Inc. Proprietary 5. Grid dependent correlations Evaporation heat flux q E m ((h G h L ) dW bubble departure diameter n nucleation site density per m.)

7 D 3. m W. G f n f bubble departure frequency 6. T K TL K . dW min m m , mm ex p S . 45 K .. small quenching & overestimated evaporation on fine grids wrong heat flux partitioning tends to film Boiling on fine grids (due to TL TW). 2009 ANSYS, Inc. All rights reserved. 11 ANSYS, Inc. Proprietary Revisited RPI Boiling Model Grid invariance of the Model required determine TL from temperature wall function (Kader, 1981). T Pr y e( ) ln( y ) e( 1/ ). L y u . y .. from definition: c PL u . T (TW TL ). q W. evaluating T+ at 2 different locations 2009 ANSYS, Inc.

8 All rights reserved. 12 ANSYS, Inc. Proprietary 6. Revisited RPI Boiling Model heat flux in boundary layer identical at both locations c PL u . q W, W y first cell (TW TL ) y first cell Ty first cell heat fluxes are equal c PL u . q W, y const (TW TL ) y const Ty const Ty const (TW TL ) y const (TW TL ) y first cell Ty first cell additional factor in correlations for dW , q F , q Q. assumption of y+const=250; Model parameter 2009 ANSYS, Inc. All rights reserved. 13 ANSYS, Inc. Proprietary RPI-Wall Boiling Model . Submodels for Model Closure Submodels for closure of RPI wall Boiling Model : Nucleation site density: Lemmert & Chawla , User Defined Bubble departure diameter: Tolubinski & Kostanchuk, Unal, Fritz, User Defined Bubble detachment frequency: Terminal rise velocity over Departure Diameter, User Defined Bubble waiting time: Proportional to Detachment Period, User Defined Quenching heat transfer.

9 Del Valle & Kenning, User Defined Turbulent Wall Function for liquid convective heat transfer coefficient Correlation for bulk flow mean bubble diameter required: Kurul & Podowski correlation via CCL. Supported combination of wall Boiling & CHT in the solid GGI & 1:1 solid-fluid interfaces 2009 ANSYS, Inc. All rights reserved. 14 ANSYS, Inc. Proprietary 7. RPI Wall Boiling Model in the ANSYS CFX-Pre GUI. 2009 ANSYS, Inc. All rights reserved. 15 ANSYS, Inc. Proprietary The Bartolomej et al. Testcase (1967,1982). 2009 ANSYS, Inc.

10 All rights reserved. 16 ANSYS, Inc. Proprietary 8. The Bartolomej Test Case R = mm Variable Value P q= R mm Z= 2 m Gin 900 kg/(s m2). q Subcooling K. Gin=900 kg/(s m2). 2009 ANSYS, Inc. All rights reserved. 17 ANSYS, Inc. Proprietary Numerical Grids Validation on mesh hierarchy with regular refinement factor of 4 (2d meshes). Grid Grid1 Grid2 Grid3. # Nodes 20x150 40x300 80x600. (uniform). Max y+ 264 133 69. t [s] 10-2 10-3 5x10-4. 2009 ANSYS, Inc. All rights reserved. 18 ANSYS, Inc. Proprietary 9. Multiphase Flow Model Steam-Water 2-phase flow: Water: continuous phase Water Steam: disperse bubbles (particle Model ).