CFD for Two-Phase Flows: Status, Recent Trends and · PDF fileSeite 3 Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics Need for multiphase

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


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


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!


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)


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


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


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!


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)


Baseline model strategy


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

0.02

0.04

0.06

G [-

]

MT_Loop-063L: 3.0 m

ExperimentCFX (total)dB<6 mmdB>6 mm


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


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


GENTOP – Plunging Jet

FC=1.0, FB=0.5


GENTOP – Churn turbulent flow

Churn-turbulent flow


flow

heat

0 0.002 0.004 0.006 0.008 0.01R [m]

0.0

0.2

0.4

0.6

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

0.2

0.4

0.6

0.8

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

200

300

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)


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.


Application – PTS

Simulations on TOPFLOW-PTS-Experiments (EU-project NURESAFE)


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)


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


Ultrafast X-ray tomography (1)

Working Principle

Source: Fischer et al., Meas. Sci. Technol. 19(9), 2008


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]


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


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!

Documents

CFD for Two-Phase Flows: Status, Recent Trends and · PDF fileSeite 3 Dr. Dirk Lucas | Head of Computational Fluid Dynamics division | Institute of Fluid Dynamics Need for multiphase