cfd for two-phase flows: status, recent trends and · pdf fileseite 3 dr. dirk lucas | head of...
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Text optional: Institutsname Prof. Dr. Hans Mustermann www.fzd.de Mitglied der Leibniz-GemeinschaftDr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Dirk Lucasa and Eckart Laurienb
a Helmholtz-Zentrum Dresden - Rossendorf e. V.Institute of Fluid Dynamics
b Institute for Nuclear Technology and Energy Systems (IKE) Universität Stuttgart, Germany
CFD for Two-Phase Flows: Status, Recent Trends and Future Needs
46th Annual Meeting on Nuclear Technology, Estrel Convention Center Berlin, Germany, 5 - 7 May 2015
Seite 2Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Outline
MOTIVATION
STATUS OF CFD
GENERAL TRENDS & NEEDS
BASELINE MODELS & GENTOP
APPLICATIONS RELATED TO NRS
SUMMARY
continuous gas
continuous liquid
polydispersed gas
Seite 3Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Need for multiphase CFD in general
Two-phase flows occur in many industrial-relevant processes in• nuclear power plants,• chemical engineering,• oil and gas industries and others.
Reliable predictions of the flow characteristics are important for the design of the facilities, the optimization of processes and safetyanalyses.
Experimental results are often hardly transferable to modified geometries, flow condition or scales.
need for reliable numerical simulations In general fluid flow is 3D Computation Fluid Dynamics - CFD
Seite 4Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Need for CFD in NRS research
Steam-water flows occurs in the cooling circuits of Light Water Reactors (LWRs) under normal operational conditions as well as under accident scenarios, e.g. Loss Of Coolant (LOCA) scenarios.
CFD becomes more and more important for Nuclear Reactor Safety (NRS) research, since 3D effects determine the general flow characteristics especially in large components as e.g. the Reactor Pressure Vessel (RPV)
CFD will not replace system codes – the safe operation of nuclear power plants is guaranteed by present licensing procedures, but• the use of 1D-approaches requires additional conservatism,• with CFD more insights on special phenomena can be obtained and• NRS has always to reflect the actual state of the art of science and
technology.
Finally CFD has to be qualified to provide reliable predictions!
Seite 5Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
CFD is frequently used for industrial single phase flow problems, e.g. in• air conditioning and ventilation systems,• automotive industries,• aviation industries and others.
CFD is not mature for two-phase flows! General reasons:• complex gas-liquid interface,• complex interactions between the phases
Status of CFD
Source: www.tecchannel.de
()
Höhne, NED(2014)
Seite 6Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
CFD is still not mature for multiphase flows!
Presently CFD for multiphase flows is e.g. able to:• support the understanding of complex multiphase flows,• investigate the influence of geometrical modifications
or modified boundary conditions on the flow.
However CFD for multiphase flows has to be used with care:• phenomena at local scale are often not well understood• limited measuring techniques for such complex flows• real CFD-grade experiments are rare lack of knowledge and experimental data
This leads to shortcomings in many CMFD-simulations:• often not all relevant phenomena are considered in the simulations• many correlations with tuning parameters exist for single phenomena• “good or acceptable agreement with experimental data” is often
claimed for post-test simulations, but limited predictive capabilities!
Status of CFD for multiphase flows
Seite 7Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Two- or Multi-Fluid Model
Two different perspectives on multiphase flow
Resolved Interface Model:at each positioneither gas or liquid
Two Fluid Model:both gas and liquid everywhere with certain probability
averaging information on interface lostneed to add closure models
Seite 8Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Example: Two-phase Pressurized Thermal Shock (PTS)
Complexity of two-phase flows in NRS
Different flow regions and many single effects including their interaction have to be considered very complex system
Flow regions: Free liquid jet, Zone of the impinging jet, Zone of horizontal flow,Flow in the downcomer (liquid level at or below cold leg nozzle)
Different flow morphologies and transitions between them!
Seite 9Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Recent trends & Future needs
1 Consolidation of basic multiphase flow models• Baseline models for poly-dispersed bubbly and segregated flows. • At HZDR this is done in the modelling frames of the:
• inhomogeneous MUSIG (iMUSIG) approach for poly-dispersed flows and• Algebraic Interfacial Area Density (AIAD) model for segregated flows.
2 New methods to extent the range of applicability of CFDIn many relevant flow situations different morphologies occur in parallel• transitions between these morphologies may occur (bubble entrainment,
droplet generation in breaking jets,… )• The GENeralized TwO Phase Flow (GENTOP) model of HZDR
• considers the different fields in parallel and allows transitions between them• combines the iMUSIG and AIAD models. Gas (continuous)
Liquid (continuous)
Gas (dispersed) Liquid (dispersed)
Seite 10Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Baseline model strategy
Seite 11Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
The inhomogeneous MUSIG model
The inhomogeneous MUSIG-Modell allows to consider a small number of velocity groups and a larger number of size groups
Radial volume fraction profiles and bubble size distributions for air-water flow Rzehak et al., MMPE(2014)
Experimentstandardnew model
0.0 0.2 0.4 0.6 0.8 1.0r/R [-]
0.00
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G [-
]
MT_Loop-063L: 3.0 m
ExperimentCFX (total)dB<6 mmdB>6 mm
Seite 12Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Validation examples
Rectangular bubble column(Mohd Akbar et al., MST 2012)
Turbulent fluctuations
Round bubble column(Mudde et al., Ind.Eng.Chem.Res. 2009)
Gas volume fraction
Baseline model validation
Upwards pipe flow (Liu, ICMF 1998)
Gas volume fraction Liquid velocity
Counter-Current pipe flow (HZDR)
Gas volume fraction Gas and liquid velocity
Seite 13Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
GENTOP concept
Basic idea for implementation into ANSYS-CFXExtension of inhomogeneous MUSIG-framework (one or more dispersed gas phases) by a potentially continuous gas field.
velocity groupsJ=1…N V1 VcgV2
size fractionsK=1…∑M
…
d d d d d , d∑
coalescencebreakup
new models forcoalescenceand breakup
transfer into cg
breakup to dg
Seite 14Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
GENTOP – Plunging Jet
FC=1.0, FB=0.5
Seite 15Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
GENTOP – Churn turbulent flow
Churn-turbulent flow
Seite 16Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
flow
heat
0 0.002 0.004 0.006 0.008 0.01R [m]
0.0
0.2
0.4
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0.8
1.0
[-]
TSAT-TIN [K]13.8918.4323.1926.9429.58
measurements
0 0.002 0.004 0.006 0.008 0.01r [m]
0.0
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G [-
]
ExpTOT
dB<1.5 mmdB>1.5 mm
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5dB [mm]
0
100
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dG/d
d B [m
m-1]
x=3.5 mP1: R=0.0095 mP2: R=0.007 mP3: R=0.0045 mP4: R=0.001 m
DEBORA-TestsR12, 1.5 Mpa
applying a population balance model using different velocity fields gas profiles with core maximum can be described
simulations
0 0.002 0.004 0.006 0.008 0.01r [m]
0.0000
0.0005
0.0010
0.0015
0.0020
d B [m
]
ExpMUSIGmonodispersed approach
Krepper et al., NED(2013)
Application - wall boiling models
Simulations on wall boiling – now towards DNB (project BMWi)
Seite 17Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Application – Flashing flow in a nozzle
(a) Blinkov model only
(b) Blinkov model + coalescence
(c) Blinkov model + bulk nucleation
Void fraction
Flashing flows may occur in case of decreasing pressure (Project EKK)Nucleation models and population balance including bubble coalescence and breakup modelling are important for the simulation flashing flows.
Seite 18Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Application – PTS
Simulations on TOPFLOW-PTS-Experiments (EU-project NURESAFE)
Seite 19Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Application – Spent Fuel Pools
Loss of cooling/coolant scenario.
downward flow
upward flow
3D flow field Cooling of the individual fuel assemblies Dependence on building ventilation, rack spacing, fuel location, etc.
HVAC-Problem with Phase Change (SINABEL project, BMBF)
Seite 20Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
The TOPFLOW facility of HZDR
TOPFLOW: Transient Two Phase Flow Test FacilityTwo-phase flow in vertical pipe configurations: • Wire-mesh sensor• Fast X-ray tomography
Pressure tank: steam-water flow experiments at pressure equilibrium
Seite 21Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Ultrafast X-ray tomography (1)
Working Principle
Source: Fischer et al., Meas. Sci. Technol. 19(9), 2008
Seite 22Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Ultrafast X-ray tomography (2)
Upwards vertical air-water pipe flow is investigated – here: JL = 1.017 m/s
0.004 0.0368 0.0574 0.0898 0.140 0.219 0.342 0.534 0.835 1.305 2.038 3.185
JG [m/s]
Seite 23Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Conclusions• CFD for multiphase flows (CMFD) is not
yet mature • reasons: complex gas-liquid interface and
lack of knowledge on local phenomena• provides valuable insights on flow structures• is not yet predictive
• Main trends for CMFD basics• Consolidation (baseline models)• Extension of applicability
• Increasing number of applications of CMFD for NRS as result of the successful qualification process
• Need for CFD-grade experimental data
• DNS may also provide input to improve closure models for the multi-fluid approach
continuous gas
continuous liquid
polydispersed gas
Seite 24Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics
Acknowledgements Parts of this work is carried out in the frame of a current research project funded by the German Federal Ministry for Economic Affairs and Energy, project number 150 1411.
Announcement13th Multiphase Flow Conference & Short Course: Simulation, Experiment and Application24 – 26 November 2015, Dresden (HZDR)
Thank You!
Thank you for your attention!