investigation of surface vortex formation at pump · pdf file ·...
TRANSCRIPT
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.
Monticello dam, TwistedSiftler `05