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Measurements and CFD Predictions of Velocity, Turbulence Intensity and Pressure Development in BWR Fuel Rod Assembly with Spacers
KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.ORCID iD: 0000-0001-5595-1952
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.

Place, publisher, year, edition, pages
2007.
Keyword [en]
Rod bundle, spacer, experimental flow characteristics, CFD
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-9905Scopus ID: 2-s2.0-44349123025ISBN: 978-089448058-4 (print)OAI: oai:DiVA.org:kth-9905DiVA: diva2:158036
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
In thesis
1. Experimental Study and Modelling of Spacer Grid Influence on Flow in Nuclear Fuel Assemblies
Open this publication in new window or tab >>Experimental Study and Modelling of Spacer Grid Influence on Flow in Nuclear Fuel Assemblies
2009 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The work is focused on experimental study and modelling of spacer grid influence on single- and two-phase flow. In the experimental study a mock-up of a realistic fuel bundle with five spacer grids of thin plate spring construction was investigated. A special pressure measuring technique was used to measure pressure distribution inside the spacer. Five pressure taps were drilled in one of the rods, which could exchange position with other rods, in this way providing a large degree of freedom. Laser Doppler Velocimetry was used to measure mean local axial velocity and its fluctuating component upstream and downstream of the spacer in several subchannels with differing spacer part. The experimental study revealed an interesting behaviour. Subchannels from the interior part of the bundle display a different effect on the flow downstream of the spacer compared to subchannels close to the box wall, even if the spacer part is the same. This behaviour is not reflected in modern correlations. The modelling part, first, consisted in comparing the present experimental data to Computational Fluid Dynamics calculations. It was shown that stand-alone subchannel models could predict the local velocity, but are unreliable in prediction of turbulence enhancement due to spacer. The second part of the modelling consisted in developing a deposition model for increase due to spacer. In this study Lagrangian Particle Tracking (LPT) coupled to Discrete Random Walk (DRW) technique was used to model droplet movements through turbulent flow. The LPT technique has an advantage to model the influence of turbulence structure effect on droplet deposition, in this way presenting a generalized model in view of spacer geometry change. The verification of the applicability of LPT DRW method to model deposition in annular flow at Boiling Water Reactor conditions proved that the method is unreliable in its present state. The model calculations compare reasonably well to air-water deposition data, but display a wrong trend if the fluids have a different density ratio than air-water.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2009. 40 p.
Series
Trita-FYS, ISSN 0280-316X ; 2009:02
Keyword
spacer grid influence, annular flow, deposition, Lagrangian Particle Tracking
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-9983 (URN)978-91-7415-229-6 (ISBN)
Presentation
2009-02-26, FA31, Roslagstullsbacken 21, Albanova University Centre, 10:00 (English)
Opponent
Supervisors
Available from: 2009-02-18 Created: 2009-02-18 Last updated: 2012-11-26Bibliographically approved
2. On drops and turbulence in nuclear fuel assemblies of Boiling Water Reactors
Open this publication in new window or tab >>On drops and turbulence in nuclear fuel assemblies of Boiling Water Reactors
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:nbn:se:kth:diva-107115 (URN)978-91-7501-572-9 (ISBN)
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

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