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  • 1.
    Anglart, Henryk
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Caraghiaur, Diana
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    CFD modeling of boiling annular-mist flow for dryout investigations2011In: Multiphase Science and Technology, ISSN 0276-1459, E-ISSN 1943-6181, Vol. 23, p. 223-251Article in journal (Refereed)
    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.

  • 2. Bestion, Dominique
    et al.
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Caraghiaur, Diana
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Peturand, P.
    Smith, B.
    Andreani, M.
    Niceno, B.
    Krepper, E.
    Lucas, D.
    Moretti, F.
    Galassi, C.
    Macek, J.
    Vyskocil, L.
    Koncar, B.
    Hazi, G.
    Review of Available Data for Validation of Nuresim Two-Phase CFD Software Applied to CHF Investigations2009In: Science and Technology of Nuclear Installations, ISSN 1687-6075Article, review/survey (Refereed)
    Abstract [en]

    The NURESIM Project of the 6th European Framework Program initiated the development of a new-generation common European Standard Software Platform for nuclear reactor simulation. The thermal-hydraulic subproject aims at improving the understanding and the predictive capabilities of the simulation tools for key two-phase flow thermal-hydraulic processes such as the critical heat flux (CHF). As part of a multi-scale analysis of reactor thermal-hydraulics, a two-phase CFD tool is developed to allow zooming on local processes. Current industrial methods for CHF mainly use the sub-channel analysis and empirical CHF correlations based on large scale experiments having the real geometry of a reactor assembly. Two-phase CFD is used here for understanding some boiling flow processes, for helping new fuel assembly design, and for developing better CHF predictions in both PWR and BWR. This paper presents a review of experimental data which can be used for validation of the two-phase CFD application to CHF investigations. The phenomenology of DNB and Dry-Out are detailed identifying all basic flow processes which require a specific modeling in CFD tool. The resulting modeling program of work is given and the current state-of-the-art of the modeling within the NURESIM project is presented.

  • 3.
    Caraghiaur, Diana
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    On drops and turbulence in nuclear fuel assemblies of Boiling Water Reactors2012Doctoral 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.

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  • 4.
    Caraghiaur, Diana
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Adamsson, Carl
    Paul Scherrer Institute.
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    A model for inertial drop deposition suitable to predict obstacle effect2013In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 260, p. 121-133Article in journal (Refereed)
    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.

  • 5.
    Caraghiaur, Diana
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Adamsson, Carl
    Lab of Reactor Physics and System Behaviour (LRS), 5232 Villigen, Paul Scherrer Institute, Switzerland .
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Deposition of Inertial Drops in Eulerian Formulation2013In: Proceedings of NURETH-15, 2013Conference paper (Refereed)
    Abstract [en]

    The drop deposition increase due to flow obstacles is presently quantified by empirical coefficients. To avoid the empiricism, a new deposition model is proposed with the capability to predict the obstacle effect. By the proposed method the deposition is calculated from the local concentration of drops and the normal-to-wall fluctuating velocity. The model shows promising results in predicting the obstacle effect.

  • 6.
    Caraghiaur, Diana
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Adamsson, Carl
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Verification of the applicability of random walk models to calculate drop deposition in nuclear fuel assemblies2009In: Proceedings of the 13th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics, 2009Conference paper (Refereed)
    Abstract [en]

    The main objective of the paper is to verify the discrete random walk (DRW) and thecontinuous random walk (CRW) Lagrangian Particle Tracking models as a tool to calculatedeposition in nuclear fuel assemblies of Boiling Water Reactors, where the sophisticatedgeometry of the spacer does not permit at present a more computationally expensivealternative. The models have been applied for a wide range of flow conditions and for variousfluids including air-water, steam-water and helium-water flows. In general, the best agreementof predictions with measurements has been obtained for air-water flows. The majordiscrepancy of the models with experimental data has been noted for the high-pressure watervaporflows. Under such conditions the models over-predict the drop deposition rates by anorder of magnitude. It is concluded that the models require further development to beapplicable to BWR conditions. In particular, it is suggested the the drop-wall interactionmodel is improved. The comparison between the two models have shown a betterperformance of CRW model in cases of air-olive oil, air-water and helium-water flows. In case of steam-water at high pressure the behavior of the two models is very different. The major finding of the paper is that a favourable comparison of either model to air-water flowconditions does not prove a good performance at BWR conditions.

  • 7.
    Caraghiaur, Diana
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Annular Flow Deposition Model with Obstacle Effect2007In: Proceedings of the 6th International Conference on Multiphase Flow, ICMF 2007, July 9-13, Leipzig, Germany, 2007Conference paper (Refereed)
    Abstract [en]

    Lagrangian particle simulations were performed in round, square and subchannel-type ducts with obstacle in order to study the influence of the obstacle and the shape of the conduit on deposition. The deposition mass transfer coeffient calculated from these simulations was correlated to the averaged square root of turbulent kinetic energy in the channel upstream and downstream of the obstacle, thus to obtain a new deposition model to be included into a thermal-hydraulic subchannel code for the prediction of CHF phenomenon. The deposition results calculated by the new model were compared to Govan correlation, with the conclusion that round duct data compare good to the correlation, but not the other geometries. Since the correlation itself was developed from round duct experimental data, it can be stated that the new deposition model is able to predict the deposition rate fairly good for this case. The results from other geometries would have to be compared to experimental data to be able to conclude that the model is valid.

  • 8.
    Caraghiaur, Diana
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Drop deposition in annular two-phase flow calculated with Lagrangian Particle Tracking2013In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 265, p. 856-866Article in journal (Refereed)
    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.

  • 9.
    Caraghiaur, Diana
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Eulerian description model for deposition of drops in annular flow regime2010Conference paper (Refereed)
  • 10.
    Caraghiaur, Diana
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Measurements and CFD Predictions of Velocity, Turbulence Intensity and Pressure Development in BWR Fuel Rod Assembly with Spacers2007In: Proceedings of the 12th International Meeting on Nuclear Reactor Thermal Hydraulics (NURETH12), 2007Conference 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.

  • 11.
    Caraghiaur, Diana
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Verification of Discontinuous Random Walk Lagrangian Particle Tracking as a Tool to Model Deposition in Annular Flow2009In: Proceedings of the 17th International Conference on Nuclear Engineering ICONE17 July 12-16, 2009, Brussels, Belgium ICONE17-75297, Brussels, Belgium, 2009, p. 311-319Conference paper (Refereed)
    Abstract [en]

    A verification of the applicability of the discontinuous random walk (DWR) Lagrangian Particle Tracking (LPT) model to calculate deposition in annular two-phase flow has been conducted. The comparison of simulation results to experimental data of deposition of mono-sized droplets shows that the model follows the correct trend in inertiamoderated regime, but is un-reliable in diffusion-impaction deposition regime. The comparison to other experimental studies of annular flow of different density ratios between the two phases reveals that the density ratio is incorrectly incorporated into the model, since the experimental trend is reversed. It can be concluded that the applicability of DWR LPT model for deposition calculation in steam-water flow at BWR conditions cannot be validated by solely comparing the simulation results to air-water experimental data.

  • 12.
    Caraghiaur, Diana
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Frid, Wiktor
    Experimental investigation of turbulent flow through spacer grids in fuel rod bundles2009In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 239, no 10, p. 2013-2021Article in journal (Refereed)
    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.

  • 13.
    Caraghiaur, Diana
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Frid, Wiktor
    KTH, Superseded Departments (pre-2005), Energy Technology.
    Tillmark, Nils
    KTH, Superseded Departments (pre-2005), Mechanics.
     Detailed pressure drop measurements in single- and two-phase adiabatic air-water turbulent flows in realistic BWR fuel assembly geometry with spacer grids2004In: The 6th International Conference on Nuclear Thermal Hydraulics, Operations and Safety (NUTHOS-6) Nara, Japan, October 4-8, 2004, 2004Conference paper (Refereed)
    Abstract [en]

    In recent years, advanced numerical simulation tools based on CFD methods have been increasingly used in various multi-phase flow applications. One of these is two-phase flow in fuel assemblies of Boiling Water Reactors. The important and often missing aspect of this development is the validation of CFD codes against proper experimental data. The purpose of the current paper is to present detailed pressure measurements over a spacer grid in adiabatic single- and two-phase flow, which will be used to validate and further develop a CFD code for BWR fuel bundle analysis. The experiments have been carried out in an asymmetric 24-rod sub-bundle, representing ¼ of Westinghouse SVEA-96 nuclear reactor fuel assembly. Single-phase measurements have been performed at superficial velocities comprised between jliq: 0.90 – 4.50 m/s and in the two-phase, which was simulated by air-water mixture, measurements have been performed at void fractions ranging from 4 to 12% and liquid superficial velocity jliq : 4.50 m/s. In order to increase the number of the measured points, five pressure taps were drilled in one of the rods, which was easily moved vertically by a traverse system, covering most of the points in axial direction. The possibility to substitute any of the rods in the fuel bundle by the pressure sensing rod and the possibility to change the pressure taps facing-angle provides more measuring points inside the subchannels. A detailed pressure distribution comparison between single- and two-phase flows for different subchannel positions and different flow conditions was performed over one of the spacers.  In addition, single-phase pressure drop measurements on the upper part of the test section comprising two spacer grids has been carried out.

  • 14.
    Caraghiaur Garrido, Diana
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Experimental Study and Modelling of Spacer Grid Influence on Flow in Nuclear Fuel Assemblies2009Licentiate 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.

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