Investigation of Surface Vortex Formation at

Pump Intakes in PWR

P. Pandazis1, A. Schaffrath1, F. Blömeling2

1Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, Munich

2TÜV NORD SysTec GmbH & Co. KG, Hamburg

46th Annual Meeting on Nuclear Technology

7. May 2015, BerlinNr.: 1501410

Outline

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 2

• Background

• Combined method to investigate surface vortices

• Applications for PWR

• Conclusions

Background – Pump intakes

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 3

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

Typical source of swirling or air entrainment

surface vortices

Surface vortices at pump intakes

Wijdiek 1965

Auckland

et al. 2009

Background – Surface vortices

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 4

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

Structure of the flow field:

• vortex core: strong rotation, large gradients

• free vortex region: almost potential flow.

Type 3

vortex core

free vortex

1. Coherent surface swirl 2. Surface dimple

3. Dye core to intake4. Vortex pulling floating

trash, but not air

5. Vortex pulling airbubbles to intake 6. Full air core to intake

type 1 → type 2: critical submergence

Background – Surface vortices

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 5

Decreasing the critical submergence:

• homogenization of the flow field

• vortex breaker devices

vortex breaker devices -TU Budapest

Avoidance of surface vortices sufficient submergence!

decrease the circulation

submergence

swirl in intake circulation

air inlet

no air suction

critical submergence

Effect of vorticity on the submergence - Jain et al.

Influence parameter on the critical submergence:

• suction velocity

• material properties

• circulation

Background – Pressurized Water Reactor

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 6

• 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)

LOCA in a PWR containment, (e.g. break in the primary circuit)

sump in a PWR containment

break

sump

reactor pressure vessel

pressurizer

steam generator

• enough amount of coolant

• reliable pump operation

requirement of a minimum sump level

(submerge of pump intake) e.g. for

avoiding of surface vortices

Background – Pressurized Water Reactor

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 7

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

Combine the ANSYS CFX results with an analytical model

to solve the complete flow field.

Analytical approaches:

• efficient calculation of the whole vortex

region

• flow parameters are necessary from the

free vortex region

Outline

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 8

• Background

• Combined method to investigate surface vortices

• Applications for PWR

• Conclusions

Combined method – Burgers & Rott vortex model

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 9

• Derived from conservation equations (mass & momentum)

• Stationary, axis-symmetrical vortices

yields velocity field

• Extended by Ito et al. (2010)

formula to calculate the gas-core length Lg:

• Definition of the critical submergence:

critical gas-core length τ = 1 mm

2

4

2ln

π

Γ

gν

)(aL

g

ut

r0

r

vortex-core

Lg

Burgers-Rott model

Two free parameters:

• suction parameter a

• circulation Γ∞To be determined with CFD simulations!

Perform a two-phase ANSYS CFX simulation of the pump intake.

• suction parameter a:

deficiency of Burgers & Rott model:

local velocity gradient is directly available from CFX results

a is the averaged velocity gradient along the vortex core edge

• Circulation Γ∞:

definition:

C is a closed curve around the vortex

Integration is performed numerically by

using the velocity field from CFX

Critical submergence can be interpolated by using two different simulations.

Combined method – Parameter determination

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 10

const.z

uaconst. z

loc

z

z

u

,dsuΓ

C

curve C

vortex core edge utuz

model reality

Validation – Experiment of Moriya

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 11

• cylindrical tank, vertical pump intake

(outlet diameter 50 mm)

• tangential water inlet, width 40 mm

• water is pumped in a closed loop:

constant water level (500 mm)

stationary vortex

• gradually increased mass flow

• gas-core length increases with mass flow

• objectives of the experiment is the deter-

mination of the

gas-core lengths

velocity distributions experimental facility

vortical flow

vessel diameter (400 mm)

outlet diameter

(ø 50 mm)

flow inlet

su

bm

erg

en

ce

(50

0 m

m)

inlet width

(40 mm)

• further sensitivity analyses:

two-phase simulation with inhomogeneous phase model

only momentum exchange at the interface

SST-cc turbulence model

flow rates: 25, 50 and 100 l/min

Validation – CFD model

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 12

• mesh sensitivity study

horizontal mesh resolution 1.2 mm (1.8 Mio. elements)

further refinement above the intake + wall

mass

flow

velo

city

water

air: 1 bar

interface

CFD boundary conditions

mesh

Validation - Results

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 13

• combined method

improves the results

remarkably

• nearly no additional

computational effort

• circulation and suction

parameter are directly

obtained from the CFD

results

0

50

100

150

200

250

300

350

400

450

500

0 10 20 30 40 50 60 70 80 90 100

Ga

s-co

re le

ngt

h [

mm

]

Volume flow [l/min]

Experiment

CFX

Combined method

Lg

Next step:

Investigation of the pump intake in a PWR sump

Outline

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 14

• Background

• Combined method to investigate surface vortices

• Applications for PWR

• Conclusions

Investigation of PWR sump

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 15

PWR sump (vertical cross section)

TH - intake sump grids

coarse sump grid

TH - 1

TH - 2

TH - 3

TH - 4fine sump grid

break positions

PWR sump (horizontal cross section)

Subdividing the CFD solution

• single-phase main model

• two-phase submodel

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 cm2 break, water above of concrete

ceiling

case 2: water level below of the concrete ceil-

ing (1 m)

• different modeling of the break-flows in

the two cases

• fine & coarse grids of the sump

Case 1: 400 cm2 break, water level in the sump is above the concrete

ceiling

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 16

Main results:

• vortex core tends to inner wall

• concrete ceiling prevents the buil-

ding of surface vortices

• no two-phase calculation per-

formed

TH - 1

CFD mesh of the containment sump

• unstructured tetra-mesh

CFD model of the containment sump

inflow

TH - 2

TH – 1

intake

concrete ceiling

swirling strength and flow above the intake

fine-gridcoarse-grid

• free water surface above

the intake

Two step simulation:

Case 2: water level in the sump is below the concrete ceiling

( 1 m submergence)

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 17

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

Submodel

Main model

TH-1 sump

fine sump grid (inlet)

water

air

CFD setup according

of the validation

Selected results for the PWR sump (main model)

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 18

• 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

streamlines in the containment sump

TH-1,2 operating

• determination of the circulation for

the combined method

• determination of initial and boundary

conditions for the submodel

vortex-core

evaluation lines for circulation

• both mass flows lead to air-entrainment

• to determinate the accurate critical submergence further calculations are required

Selected results for the model of TH-1 sump (submodel)

P. Pandazis et al. – Investigation of Surface Vortex Formation at Pump Intakes in PWR 19

Application of the combined method to

calculate the gas-core length. surface vortex at the TH pump intake

vortex core

TH intake

phase interface

mass flow [kg/s] circulation [m2/s] a [1/s] Lg [m]

100 0.43 0.12 11

150 0.41 0.23 20

Goal:

determination of the suction parameter for

the combined method

• from the local velocity gradients:loc

z

z

u

Conclusions

P. Pandazis et al.– Investigation of Surface Vortex Formation at Pump Intakes in PWR 20

• 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 21

Thank you for your attention!

This work is sponsored by the German Federal Ministry of Economics and Technology (BMWi) under the

contract number 1501410.

The responsibility for the content of this publication lies with the author.

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