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On drops and turbulence in nuclear fuel assemblies of Boiling Water Reactors
KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The study aims to develop the understanding of the mechanistic-type approach to quantify drop deposition in nuclear fuel assemblies of Boiling Water Reactors. This includes the effect of spacers. Spacers have a complex geometry to serve their purposes, but optimization of them alone can improve the thermal limit parameters in nuclear fuel assemblies. Thus, a mechanistic model might prove useful to increase the safety of the reactor as well as economic competitiveness of the nuclear power plant.

In this thesis, measurement techniques, such as mobile pressure rod and Laser Doppler Velocimetry are developed and tested to provide local data of the flow around spacers. It is shown experimentally that the effect of spacer on the flow differs depending on the placement of the subchannel in the rod bundle. Partly, because the spacer part differs, but also due to a global velocity profile development. Very few studies in the literature indicate this effect. It is shown that single subchannel models using Computational Fluid Dynamics (CFD) can predict the average velocity increase downstream of the spacer; however, they are not capable of calculating the spacer effect on turbulence parameters. The single subchannel CFD model has limited capability to predict the pressure development inside the spacer part, mainly because cross-flows are not taken into consideration.

The deposition of drops in annular two-phase flow is still a scientific challenge. Only empirical correlations are used nowadays to quantify this process. Empirical coefficients are needed for each spacer type to calculate the deposition increase due to obstacle. The discussion about the deposition starts with the phenomenological description. The important input parameter, namely drop size, is carefully analysed, and a new correlation is proposed to calculate the mean drop diameter. The correlation is constructed on a larger experimental data base. Lagrangian Particle Tracking model is tested in its capability to calculate deposition. Additionally, a Eulerian-type model is developed and tested. Turbulent parameters of drops are tightly related to the turbulence of the gas phase and the inertia of the drops. Several approaches are discussed about how to calculate the root-mean-square fluctuating velocities of drops. Both, Lagrangian Particle Tracking and the Eulerian-type of models show good capability in calculating the obstacle effect on deposition, providing improvements are made in prediction of drop size. The effect of increased drop concentration plays a large role and it must be taken into consideration if good quantitative approaches are envisaged.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xi, 45 p.
Series
Trita-FYS, ISSN 0280-316X ; 2012:85
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-107115ISBN: 978-91-7501-572-9 (print)OAI: oai:DiVA.org:kth-107115DiVA: diva2:574603
Public defence
2012-12-14, FD5, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:30 (English)
Opponent
Supervisors
Funder
EU, European Research Council
Note

QC 20121207

Available from: 2012-12-07 Created: 2012-12-06 Last updated: 2012-12-07Bibliographically approved
List of papers
1. Experimental investigation of turbulent flow through spacer grids in fuel rod bundles
Open this publication in new window or tab >>Experimental investigation of turbulent flow through spacer grids in fuel rod bundles
2009 (English)In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 239, no 10, 2013-2021 p.Article in journal (Refereed) Published
Abstract [en]

This paper contains experimental data of pressure, velocity and turbulence intensity in a 24‐rod fuel bundle withspacer grids. Detailed pressure measurements inside the spacer grid have been obtained by use of a sliding pressuresensingrod. Laser Doppler Velocimetry technique was used to measure the local axial velocity and its fluctuatingcomponent upstream and downstream of the spacer grid in subchannels with different blockage ratios. Themeasurements show a changing pattern in function of radial position in the cross‐section of the fuel bundle. Forsubchannels close to the box wall, the turbulence intensity suddenly increases just downstream of the spacer andthen gradually decays. In inner subchannels, however, the turbulence intensity downstream of the spacer decreasesbelow its upstream value and then gradually increases until it reaches the maximum value at approximately twospacer heights. The present study reveals that spacer effects, such as local pressure distribution and turbulenceintensity enhancement, do not depend exclusively on the local geometry details, but also on the location in thecross‐section of the rod bundle.

Keyword
Axial velocity, Blockage ratio, Experimental data, Experimental investigations, Fluctuating components, Fuel bundle, Fuel rods, Laser Doppler Velocimetry, Local geometry, Maximum values, Radial position, Rod bundles, Sliding pressure, Spacer effects, Spacer grid, Subchannels, Turbulence intensity
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-9906 (URN)10.1016/j.nucengdes.2009.05.029 (DOI)000270411500032 ()2-s2.0-68449099150 (Scopus ID)
Note

QC 20101007. Uppdaterad från Submitted till Published (20101007).

Available from: 2009-02-18 Created: 2009-01-29 Last updated: 2012-12-07Bibliographically approved
2. Measurements and CFD Predictions of Velocity, Turbulence Intensity and Pressure Development in BWR Fuel Rod Assembly with Spacers
Open this publication in new window or tab >>Measurements and CFD Predictions of Velocity, Turbulence Intensity and Pressure Development in BWR Fuel Rod Assembly with Spacers
2007 (English)In: Proceedings of the 12th International Meeting on Nuclear Reactor Thermal Hydraulics (NURETH12), 2007Conference paper, Published paper (Refereed)
Abstract [en]

The current paper presents measurements of axial velocity and its fluctuating component across a spacer in a rod bundle, using LDV technique. The measurements were performed in single-phase water flow in three subchannels of an asymmetric 24-rod mock-up of SVEA-96 fuel bundle. The subchannels differ in the spacer part. CFD models of the three subchannels were developed using commercial CFX 10.0 code. The flow structure in the spacer region has been predicted with two different turbulence models available in the code. The predictions were compared to the current experiments and additionally to experiments of pressure distribution across the spacer reported previously. The comparison showed that CFX code could predict the pressure drop over the spacer with an accuracy of 20-30%. The axial velocity development in the middle point of the subchannel could be reasonably predicted. However, the turbulence intensity increase downstream the spacer observed in the experiments could not be adequately predicted by the chosen turbulence models.

Keyword
Rod bundle, spacer, experimental flow characteristics, CFD
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-9905 (URN)2-s2.0-44349123025 (Scopus ID)978-089448058-4 (ISBN)
Conference
The 12th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-12), Sheraton Station Square, Pittsburgh, Pennsylvania, U.S.A. September 30-October 4, 2007.
Note

QC 20101007

Available from: 2009-02-18 Created: 2009-01-29 Last updated: 2014-11-07Bibliographically approved
3. CFD modeling of boiling annular-mist flow for dryout investigations
Open this publication in new window or tab >>CFD modeling of boiling annular-mist flow for dryout investigations
2011 (English)In: Multiphase Science and Technology, ISSN 0276-1459, E-ISSN 1943-6181, Vol. 23, 223-251 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents applications of computational fluid dynamics (CFD) to modeling of two-phase annular-mist flows with evaporating liquid films. This type of two-phase flow exists in boiling channels prior to the onset of dryout. Annular-mist flows have a very complex structure since they contain liquid and vapor, both as the continuous and the dispersed phase. Due to this feature, both the Eulerian-Eulerian as well as the Eulerian-Lagrangian approaches are often used to model such flows. The two approaches, with pertinent conservation equations and closure relationships, are reviewed in this paper.

Place, publisher, year, edition, pages
Begell House, 2011
Keyword
liquid film, deposition, entrainment, droplet dynamics
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-107170 (URN)10.1615/MultScienTechn.v23.i2-4.50 (DOI)2-s2.0-84859402462 (Scopus ID)
Note

QC 20121207

Available from: 2012-12-07 Created: 2012-12-07 Last updated: 2017-12-07Bibliographically approved
4. Drop deposition in annular two-phase flow calculated with Lagrangian Particle Tracking
Open this publication in new window or tab >>Drop deposition in annular two-phase flow calculated with Lagrangian Particle Tracking
2013 (English)In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 265, 856-866 p.Article in journal (Refereed) Published
Abstract [en]

Lagrangian Particle Tracking is tested for its capability to predict deposition rates in pipes and pipes with obstacle. The drop size is one of the input parameters, which defines in its major part the deposition process. A new correlation is proposed to estimate the drop size, following a systematic analysis of the experimental drop sizes in annular twophase flow. The Lagrangian Particle Tracking model showed good capability of prediction in the cases where the drop size is known; however, when the drop size is estimated the inaccuracy in calculated deposition rate is high. If the drop size is known at the inlet of the channel, Lagrangian Particle Tracking shows good capability of predicting the deposition increase downstream of the obstacle for steam-water flows of 5, 10 and 15 bar pressure.

Place, publisher, year, edition, pages
Elsevier, 2013
Keyword
drops, deposition, Lagrangian Particle Tracking, annular two-phase flow, obstacle
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-106798 (URN)10.1016/j.nucengdes.2013.06.026 (DOI)000330085500086 ()2-s2.0-84887048282 (Scopus ID)
Note

QC 20140121.  Updated from submitted to published.

Available from: 2012-12-06 Created: 2012-12-04 Last updated: 2017-12-07Bibliographically approved
5. A model for inertial drop deposition suitable to predict obstacle effect
Open this publication in new window or tab >>A model for inertial drop deposition suitable to predict obstacle effect
2013 (English)In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 260, 121-133 p.Article in journal (Refereed) Published
Abstract [en]

The drop deposition increase due to flow obstruction is usually quantified solely by empirical coefficients. In this work we propose a new way to calculate the drop deposition with the capability to predict the obstacle effect. The model is based on the drop volume fraction, slip ratio and turbulence quantities of the continuous phase obtained from the two-fluid calculations. Additional relations are needed to calculate the fluctuating velocities of the drop phase. These relations are based on the fluid integral time scales. A number of relations are tested, which include the effect of drop inertia and drift parameter. The new model is tested in a number of flow combinations, including air-water and helium-water of 1.5 bar and steam-water at 70 bar pressure, for low and high drop concentration. The high concentration flow shows that further studies are needed to include drop size increase due to coalescence and reduction of velocity fluctuation due to drop collisions. The new model is tested for pipe flow containing an obstacle of steam-water flows of 5, 10 and 15 bar pressure. The new model shows the capability to qualify the obstacle effect. Further improvements are needed to increase the quantitative capability.

Keyword
inertial drops, deposition, Eulerian, annular two-phase flow, obstacle
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-106799 (URN)10.1016/j.nucengdes.2013.03.010 (DOI)000319645800011 ()2-s2.0-84876116417 (Scopus ID)
Note

QC 20140617

Available from: 2012-12-06 Created: 2012-12-04 Last updated: 2017-12-07Bibliographically approved

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