Behaviour in Large Pools of Liquid

1. Behaviour in Large Pools of Liquid E. Krepper 4th NC IAEA Research Coordination Meeting Vienna, Sept. 10th-13th 2007

2. Outline • Relevance and problems • Theoretical basics • Experimental basics

3. 1. Relevance and problems Natural circulation in large pools • Large pools • heat sink for heat removal from the reactor or the containment by natural circulation • source of water for core cooling • Natural circulation  Nearly homogeneous temperature distribution • But: 3D convection flow can cause temperature stratifications • Can compromise the heat transfer process • Generation of steam: • influence on the pressure increase in the containment • might lead to an annihilation of the stratification process

4. 1. Relevance and problems: Examples for large pools AP600/AP1000 core cooling system: from IAEA CRP Systems Description Report (draft)

5. 1. Relevance and problems: Examples for large pools Decay Heat Removal System of the AHWR: from IAEA CRP Systems Description Report (draft)

6. 1. Relevance and problems: Examples for large pools SWR-1000 safety concept:

7. 2. Theoretical basis Gravity driven flows - Open problems • With higher Rayleigh numbers conditions of applicability of the Boussinesq approximation are exceeded • Validity of commonly applied turbulence models is exceeded: • Anisotropy because of temperature stratification • Decoupling of the transport of turbulent energy and turbulent momentum • Heat transfer from hot walls into the fluid: • Lower heat flux: Single phase convection • With higher heat fluxes: Boiling at the surface, condensation in the bulk

8. 2. Theoretical basis Boussinesq Approximation: • Density r independent on pressure • Density r linearly dependent on temperature: • Precondition: bDT << 1

9. 2. Theoretical basis : Modelling of turbulence Turbulence • Rayleigh No.: • Ra > 1010 the flow has to be considered turbulent

10. 2. Theoretical basis : Modelling of turbulence Navier Stokes Equations • Mass • Momentum • Energy

11. 2. Theoretical basis : Modelling of turbulence Common successful : Reynolds averaging • Decomposition of all flow quantities in mean and fluctuating part: • Applying it into the basic equation: • set of equations similar to laminar flow, but now with the mean parts • fluctuating parts either averaged to zero or their products yield new terms: • Reynolds stress: • Reynolds flux:

12. 2. Theoretical basis : Modelling of turbulence Eddy hypothesis: • Reynolds Stresses are calculated as a linear function of the deformation tensor: • Reynolds Fluxes as linearly dependent on the mean total enthalpy gradient: • turbulent heat diffusity • K-Epsilon Turbulence model:

13. 2. Theoretical basis : Modelling of turbulence Possible solutions • Extension of the common used turbulence models by additional components considering the gravity • additional production terms in the turbulence equations for k and e • Applying a Reynolds Stress Turbulence model • resolving the turbulence values in all three directions • Applying the Large Eddy Simulation: Direct simulation of large turbulent scales • modelling only small turbulence scales • direct simulation of large turbulent scales • Applying Direct Numerical Simulation • applicability limited to flow with small Reynolds No. caused by the strong increased numerical effort • used for the generation of benchmark solution for more complex models

14. 2. Theoretical basis Two Phase Flow • Euler/Euler approach: For each phase the full set of equations • Exchange terms between the phases: • Momentum: Drag, Non drag • Mass: Condensation, Evaporation • Heat transfer • Exchange terms depend on the morphology of the flow, e.g. on the bubble size • Momentum: non drag lift-force dependent on bubble size • mass transfer dependent on interfacial area • population balance models (bubble coalescence, bubble fragmentation) • momentum methods • Heat transfer via a wall • wall boiling model

15. 3. Experimental basis NOKO • Parameters: • Primary pressure 1 MPa • transferred power ca. 0.6 MW • Operated in FZ Jülich 1995 – 2000 • Main subject: • Investigation of the heat transfer capability of an emergency condenser by condensation in horizontal tubes • Additional tests 1998 – 2000 • Investigation of the heating up processes on the secondary side

16. 3. Experimental basis TOPFLOW • Injecting steam in the bundle at a certain pressure (1 – 6.5 MPa) • Heating up the tank by removing of maximum condensate • Secondary side: at normal conditions (0.1 Mpa) • Determination of the characteristic curve of the bundle by removing determined amounts of condensate Steam inlet Condensate outlet

17. 3. Experimental basis Side wall heated tank • Cylindrical water tank • D = 0.25 m, • H = 0.25 m • M = 10 kg • Side walls electrical heated: P = 4 kW • Measurement of the transient temperature field by thermocouples

18. 3. Experimental basis Experiments in the Reactor engineering division of BARC • investigation of heating up processes at a heater plate • temperature measurement by thermocouples • flow visualization by Aluminum particles see presentation given at the 2nd RCM on the IAEA CRP, Corvallis Sept. 2005 by N.V. Satish Kumar, A.G. Patel, N.K. Maheshwari, P.K. Vijayan and D. Saha: “Thermal Stratification Studies related to the Passive Decay Heat Removal System of Advanced Heavy Water Reactor”