Investigation of Surface Vortex Formation at Pump Intakes in PWR

Transcription

1 Investigation of Surface Vortex Formation at Pump Intakes in PWR P. Pandazis 1, A. Schaffrath 1, F. Blömeling 2 1 Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) ggmbh, Munich 2 TÜV NORD SysTec GmbH & Co. KG, Hamburg 46 th Annual Meeting on Nuclear Technology 7. May 2015, Berlin Nr.:

2 Outline Background Combined method to investigate surface vortices Applications for PWR Conclusions P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 2

3 Background Pump intakes Requires for an undisturbed long-term operation: avoidance of cavitation homogenous and non-rotational inflow avoidance of air entrainment unfavorable intake conditions lead to: fluctuating pump behavior vibrations, noise, mechanical damages decrease or collapse of flow rate Auckland et al Typical source of swirling or air entrainment surface vortices Wijdiek 1965 Surface vortices at pump intakes P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 3

4 Background Surface vortices Surface vortices are generated by the pressure drop resulting from pump suction and disturbances in the approaching flow. Surface vortices can be classified in 6 types air core grows with decreasing submergence type 1 type 2: critical submergence Structure of the flow field: vortex core: strong rotation, large gradients free vortex region: almost potential flow. 1. Coherent surface swirl 2. Surface dimple free vortex 3. Dye core to intake 4. Vortex pulling floating trash, but not air vortex core 5. Vortex pulling air bubbles to intake 6. Full air core to intake Type 3 P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 4

5 Background Surface vortices Avoidance of surface vortices sufficient submergence! Influence parameter on the critical submergence: suction velocity material properties circulation submergence no air suction critical submergence swirl in intake air inlet circulation Effect of vorticity on the submergence - Jain et al. Decreasing the critical submergence: homogenization of the flow field vortex breaker devices decrease the circulation vortex breaker devices -TU Budapest P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 5

6 Background Pressurized Water Reactor LOCA in a PWR containment, (e.g. break in the primary circuit) SCRAM isolation of containment coolant loss through break, refill by: high pressure systems low pressure, emergency cooling systems (ca. < 10 bar) - flooding tanks - containment sumps long term recirculation via the containment sump ( after ca. 20 min. in case of a large break) sump break steam generator reactor pressure vessel pressurizer sump in a PWR containment enough amount of coolant reliable pump operation requirement of a minimum sump level (submerge of pump intake) e.g. for avoiding of surface vortices P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 6

7 Background Pressurized Water Reactor Recommendations of the German Reactor Safety Commission (2005) concerning the determination of the minimum water level in the sump (critical submergence) large scale experiments (> 1: 20) application of the ANSI (American National Standard Institute) correlation new approach: Investigation of the critical submergence with numerical (CFD) method Results of ANSYS CFX simulations: efficient calculation of free vortex region high computational effort for the solution in the core region because of the strong gradients Analytical approaches: efficient calculation of the whole vortex region flow parameters are necessary from the free vortex region Combine the ANSYS CFX results with an analytical model to solve the complete flow field. P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 7

8 Outline Background Combined method to investigate surface vortices Applications for PWR Conclusions P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 8

9 Combined method Burgers & Rott vortex model Derived from conservation equations (mass & momentum) Stationary, axis-symmetrical vortices yields velocity field Extended by Ito et al. (2010) L g formula to calculate the gas-core length L g : vortex-core a ln ( 2 ) Γ 2 u t L g ν g 4 π r 0 r Definition of the critical submergence: critical gas-core length τ = 1 mm Two free parameters: Burgers-Rott model suction parameter a circulation Γ To be determined with CFD simulations! P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 9

10 Combined method Parameter determination Perform a two-phase ANSYS CFX simulation of the pump intake. suction parameter a: model reality u z deficiency of Burgers & Rott model: const. a const. z local velocity gradient is directly available from CFX results Circulation Γ : definition: a is the averaged velocity gradient along the vortex core edge Γ u ds, C is a closed curve around the vortex Integration is performed numerically by u z z using the velocity field from CFX C loc vortex core edge u z curve C u t Critical submergence can be interpolated by using two different simulations. P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 10

11 submergence (500 mm) Validation Experiment of Moriya cylindrical tank, vertical pump intake (outlet diameter 50 mm) tangential water inlet, width 40 mm vessel diameter (400 mm) inlet width (40 mm) water is pumped in a closed loop: constant water level (500 mm) stationary vortex flow inlet gradually increased mass flow vortical flow gas-core length increases with mass flow objectives of the experiment is the determination of the gas-core lengths outlet diameter (ø 50 mm) velocity distributions experimental facility P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 11

12 mass flow velocity Validation CFD model mesh sensitivity study horizontal mesh resolution 1.2 mm (1.8 Mio. elements) further refinement above the intake + wall air: 1 bar interface water mesh further sensitivity analyses: two-phase simulation with inhomogeneous phase model only momentum exchange at the interface SST-cc turbulence model CFD boundary conditions flow rates: 25, 50 and 100 l/min P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 12

13 Gas-core length [mm] Validation - Results Experiment L g combined method improves the results remarkably nearly no additional computational effort circulation and suction parameter are directly obtained from the CFD results CFX Combined method Volume flow [l/min] Next step: Investigation of the pump intake in a PWR sump P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 13

14 Outline Background Combined method to investigate surface vortices Applications for PWR Conclusions P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 14

15 Investigation of PWR sump Accident scenario: LOCA inside the containment PWR sump with 4 TH intake chambers, only 2 of the 4 TH-pumps are available by postulate concrete ceiling above the pump intake Injection of ECC water from the sump via the emergency core cooling system (TH) two cases (with different sump water level): case 1: 400 cm 2 break, water above of concrete ceiling case 2: water level below of the concrete ceiling (1 m) different modeling of the break-flows in the two cases fine & coarse grids of the sump TH - 2 TH - intake sump grids PWR sump (vertical cross section) break positions coarse sump grid TH - 3 Subdividing the CFD solution single-phase main model two-phase submodel TH - 1 TH - 4 fine sump grid PWR sump (horizontal cross section) P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 15

16 Case 1: 400 cm 2 break, water level in the sump is above the concrete ceiling CFD mesh of the containment sump unstructured tetra-mesh CFD model of the containment sump TH - 2 TH - 1 Main results: vortex core tends to inner wall concrete ceiling prevents the building of surface vortices no two-phase calculation performed swirling strength and flow above the intake concrete ceiling TH 1 intake fine-grid coarse-grid inflow P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 16

17 Case 2: water level in the sump is below the concrete ceiling ( 1 m submergence) free water surface above the intake Two step simulation: 1. step: main model complete PWR sump coarse (hybrid) mesh single phase (coolant) 2. step: submodel of the TH-1 sump fine mesh two-phase (coolant and air) boundary conditions taken from results of the main model Main model Submodel TH-1 sump air water CFD setup according of the validation fine sump grid (inlet) P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 17

18 Selected results for the PWR sump (main model) varied parameters in the analyses: different break positions TH-pump mass flows active TH-pumps significant vortex development near intakes in service in each case studied inhomogeneous flow field with low velocities in the other regions TH-1,2 operating evaluation lines for circulation vortex-core streamlines in the containment sump determination of the circulation for the combined method determination of initial and boundary conditions for the submodel P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 18

19 Selected results for the model of TH-1 sump (submodel) Goal: determination of the suction parameter for the combined method from the local velocity gradients: Application of the combined method to calculate the gas-core length. u z z loc vortex core phase interface TH intake surface vortex at the TH pump intake mass flow [kg/s] circulation [m 2 /s] a [1/s] Lg [m] both mass flows lead to air-entrainment to determinate the accurate critical submergence further calculations are required P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 19

20 Conclusions surface vortices have to be avoided to ensure the long-term cooling transport after a postulated LOCA scenario in a PWR containment an efficient combined method has been developed to investigate surface vortices in complex pump intakes: CFD: ANSYS CFX is used to determine the flow field in the free vortex region analytical model: Burgers & Rott model is used to compute the gas-core lengths the combined method has been successfully validated against the experiment of Moriya application of the combined method to investigate surface vortices at pump intakes of a PWR emergency cooling system due to the complex geometry: CFD model was subdivided into two parts sufficient determination of the place and the intensity of the vortices further simulations requires to determine the critical submergence P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 20

21 Thank you for your attention! Monticello dam, TwistedSiftler `05 This work is sponsored by the German Federal Ministry of Economics and Technology (BMWi) under the contract number The responsibility for the content of this publication lies with the author. P. Pandazis et al. Investigation of Surface Vortex Formation at Pump Intakes in PWR 21