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Gas entrainment by one single French PWR spray, SARNET-2 spray benchmark
Nuclear Engineering and Design 282 (2015) 44–53
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Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Gas entrainment by one single French PWR spray, SARNET-2 spray benchmark J. Malet a,∗ , S. Mimouni b , G. Manzini c , J. Xiao d , L. Vyskocil e , N.B. Siccama f , R. Huhtanen g a
Institut de Radioprotection et de Sûreté Nucléaire, Saclay, France Electricité de France, EDF MF2E, Chatou, France RSE, Milano, Italy d IKET, KIT, Karlsruhe, Germany e UJV Rez, Czech Republic f NRG, Safety & Power, The Netherlands g VTT, PO Box 1000, FI-02044 VTT, Finland b c
h i g h l i g h t s • • • • •
This paper presents a benchmark performed in the frame of the SARNET-2 EU project. It concerns momentum transfer between a PWR spray and the surrounding gas. The entrained gas velocities can vary up to 100% from one code to another. Simplified boundary conditions for sprays are generally used by the code users. It is shown how these simplified conditions impact the gas entrainment.
a r t i c l e
i n f o
Article history: Received 19 March 2013 Received in revised form 24 November 2014 Accepted 12 December 2014
a b s t r a c t This paper presents a benchmark performed in the frame of the SARNET-2 EU project, dealing with momentum transfer between a real-scale PWR spray and the surrounding gas. It presents a description of the IRSN tests on the CALIST facility, the participating codes (8 contributions), code-experiment and code-to-code comparisons. It is found that droplet velocities are almost well calculated one meter below the spray nozzle, even if the spread of the spray is not recovered and the values of the entrained gas velocity vary up to 100% from one code to another. Concerning sensitivity analysis, several ‘simplifications’ have been made by the contributors, especially based on the boundary conditions applied at the location where droplets are injected. It is shown here that such simplifications influence droplet and entrained gas characteristics. The next step will be to translate these conclusions in terms of variables representative of interesting parameters for nuclear safety. © 2014 Elsevier B.V. All rights reserved.
1. Introduction During the course of a severe accident in a pressurized water reactor (PWR), spray systems are used in the containment in order to prevent overpressure and to enhance the gas mixing in case of the presence of hydrogen. Spray modelings are thus part of thermalhydraulic containment codes. The two major phenomena involved
∗ Corresponding author. Tel.: +33 169088740. E-mail addresses: [email protected] (J. Malet), [email protected] (S. Mimouni), [email protected] (G. Manzini), [email protected] (J. Xiao), [email protected] (L. Vyskocil), [email protected] (N.B. Siccama), risto.huhtanen@vtt.fi (R. Huhtanen). http://dx.doi.org/10.1016/j.nucengdes.2014.12.008 0029-5493/© 2014 Elsevier B.V. All rights reserved.
in spray behavior in such applications are the thermodynamical effect of a spray (steam condensation on droplets, evaporation, etc.) and the dynamical effect (entrainment and mixing of gases). This paper is proposed in the frame of the EU network SARNET2, within the Sub-Work Package WP7-2, Task 1 (spray activities) in charge of IRSN. In the past, validation of spray modelings has been performed on large-scale facilities (CVTR, NUPEC, CSE) using several spray nozzles (Malet, 2003). More specific studies have been proposed in the frame of SARNET (Malet et al., 2011a), as a synthesis report of spray models in containment applications (Métier and Malet, 2007), spray benchmark on TOSQAN test 101 and MISTRA MASPn tests on heat and mass exchanges between drops and the gaseous atmosphere (Malet and Métier, 2007) and spray benchmark on TOSQAN test 113 and MISTRA MARC2 tests on momentum
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transfer between drops and the gaseous atmosphere (Malet and Vizet, 2008). The conclusion of the SARNET activities was that the level of spray model validation obtained was encouraging for the use of spray modeling for risk analysis. However, it was not sufficient and further activities were thus well encouraged in order to evaluate the influence of different parameters in the modeling, such as benchmarks based on separate-effect tests. Separate-effect tests were thus proposed for SARNET-2 project. The first benchmark deals with heat and mass transfer (HMT) tests on single droplet (Malet, 2011; Malet et al., 2011b). The second benchmark concerns momentum transfer on one single PWR spray. The third one concerns the momentum transfer on two interacting PWR sprays. This paper presents the results of the second elementary benchmark. Experimental results are obtained on the CALIST (Characterization and Application of Large Industrial Spray Transfer, 160 m3 ) facility at the IRSN. Benchmark specifications are given in (Foissac and Malet, 2011). In this benchmark, code-experiment and code-to-code comparisons are performed at different distances from the nozzle outlet, inside and outside the spray. Sensitivity studies are also performed by the participants, and some of them are presented here. Beyond the common objective of codes validation, the other objective of this benchmark is to show the potentialities of CFD calculations of the flow induced by small droplets (300 m) in a large volume (160 m3 ) and to evaluate the influence, on droplet and entrained gas velocities, of usual simplifications generally made for safety analysis, such as the use of a single droplet size instead of a droplet size distribution. 2. Description of the benchmark 2.1. Description of the French PWR spray systems Spray systems in nuclear power plants are composed of over 500 interacting water droplet sprays with a range of droplet diameters from 100 m to 1000 m (Foissac et al., 2011). They are activated under gaseous mixture composed of steam, hydrogen and air at a total pressure of around 2–3 bars and under gas temperature around 100–120 ◦ C. The French PWR containments (Fig. 1) have generally two series of nozzles placed in circular rows. A schematic view of these spray rings and the associated theoretical spray envelopes are given in Fig. 1. The nozzle type used in many PWRs, in particularly French 900 MWe PWRs, is the so-called SPRACO 1713A, distributed by Lechler under reference 373.084.17.BN (Fig. 1). This nozzle is generally used with water at a relative pressure of 3.5 bars, inducing a flow rate of approximately 1 l/s. The outlet orifice diameter is 9.5 mm. Droplet injection temperature is between 20 ◦ C and 60 ◦ C. 2.2. Description of the CALIST facility The experiments are carried out in the CALIST facility sketched in Fig. 2. The set-up is composed of a hydraulic circuit supplying and, for those experiments, a single spray nozzle with a flow-rate of 1 l/s at a relative pressure of 3.5 bars. The pulverized water is collected in a 5 m3 pool. The axial position of the spray nozzle can be changed using a monitored carriage. The measurement of the spray characteristics is performed using phase-Doppler interferometry (PDI). All measurements are performed under air conditions at ambient pressure of 1 bar and gas ambient temperature of 20 ◦ C. More information on the measurements of the spray can be found in Foissac et al. (2011). Measurements of entrained gas around the spray is performed using very small water droplets obtained with fog nozzle activated in the roof of the facility, so that their velocity at the location of the PWR spray is negligible compared to the entrained gas velocity: the Stokes number of these small droplets
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have been evaluated to be around 0.07, much lower than 1, indicating that these droplets are good tracers (Foissac, 2011). Uncertainty on the droplet characteristics measurements are considered from the three following sources (more details in Foissac, 2011): - Uncertainty on the repeatability (all measurements are repeated three times, leading to an average value of the considered variable and its associated standard deviation); - Uncertainty linked to the angular position: the spray is not perfectly symmetric so that measurement points are considered at 4 angular positions; - Uncertainty linked to spray pulsation, which has been estimated to be of an angle of 1. 2.3. Participants and codes The list of participating institutions, the participant’s names, code versions, code types and droplet type of modeling are presented in Table 1. Seven institutions have participated with seven different code versions. Most of the codes are CFD codes, and based on a Lagrangian approach, considering the gravity and drag forces. The participants have also performed sensitivity studies. Here, only one calculation is considered per institution in the codeexperiment comparisons in order to have clear graphs. This exercise was proposed as a blind exercise even if some data were already partially published. Most of the codes use a Schiller and Naumann (1935) or the Morsi and Alexander (1972) correlation for the drag coefficient. A description of the size of the calculated domain, the type of mesh, the number of cells for the mesh is presented in Table 2. Boundary conditions of the fluid domain, as well as the one of the droplet phase are described in Tables 3 and 4. It should be noticed that the difference in the injection height in the EDF calculations does not lead to any difference in the results: these ones are always given at a relative position to the spray nozzle outlet. 3. Discussion on the results 3.1. Boundary conditions Droplet sizes and velocity profiles measured 20 cm below the nozzle outlet are considered for the boundary conditions of the spray. This is the common way to handle spray in CFD when the atomization zone is not simulated. These boundary conditions are given in the benchmark specification report (Foissac and Malet, 2011) as well as in Foissac et al. (2011). It was verified that these conditions were well used by the participants and these input data are given in Fig. 3. RSE contributions considered simplified velocity and droplet size profiles at the injection (droplet size distribution not freely settable), whereas other participants used the exact experimental profile. On this figure and subsequent ones, velocity equal to zero implies a very low volume fraction of droplets, negative values of the vertical respectively radial component of the gas/droplet velocities represent flow/droplets directed downwards respectively toward the spray axis. 3.2. Comparison results Droplet velocity results at 40 cm from the nozzle outlet are presented in Fig. 4 for all codes except CFX, for which the data were not distributed at the time of the benchmark. It can be observed that most of the numerical calculations provide the same results for the droplet velocities except FDS simulation by RSE, which is probably explained by the different boundary conditions used for these calculations. What is also interesting to see is that the spread of the
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Fig. 1. Spray rings and envelopes in a French PWR (not at scale) and PWR spray nozzle.
Fig. 2. CALIST water-spray experimental facility. Table 1 Participants for the blind and open calculations. Institution
Code name
Type of code
Type of calculation
Droplet modeling
Drag law – reference
EDF (IRSN) IRSN KIT NRG RSE RSE UJV VTT
NEPTUNE ANSYS/CFX v.13 GASFLOW ANSYS/FLUENT v.6.3 ECART vs. 4W0T FDS vs. 5.5.3 Serial ANSYS/FLUENT v.13 ANSYS/FLUENT v.13
CFD CFD CFD CFD LP CFD CFD CFD
Transient Steady-state Steady-state Transient Transient Transient Transient Transient
Eulerian Lagrangian Lagrangian Lagrangian Lagrangian Lagrangian Lagrangian Lagrangian
Schiller and Naumann (1935) Morsi and Alexander (1972) Schiller and Naumann (1935) Morsi and Alexander (1972) – – Morsi and Alexander (1972) Morsi and Alexander (1972)
Table 2 Description of the computational domain and associated mesh. Institution
EDF (IRSN) IRSN KIT NRG RSE RSE RSE UJV VTT a b c
Code name
NEPTUNE CFX GASFLOW FLUENT ECART FDS FDS FLUENT FLUENT
Size of the calculated domain
Type of calculation symmetry
L × l × h (m × m × m)
Volume (m3 )
3 × 3.5 × 3.5 3 × 3.5 × 3.5 H = 3.7 m, R = 3 m 7.0 × 6.2 × 3.7 7.0 × 6.2 × 3.7 7.0 × 3.7 7.0 × 6.2 × 3.7 H = 3.7 m, R = 3.5 m 7.0 × 6.2 × 3.7 (pool edges included)
147 147 105 161 161 26 m2 161 142 161
1/4th geometry is meshed. 2D slice of 3.6◦ . Single values correspond to an average size.
Mesh Type of mesh
3D – 2 planes symmetrya 3D – 2 planes symmetrya 2D, revolution symmetryb 3D 3D 2D (plane symmetry) 3D 2D (axi-symmetry) 3D
Hexahedral Tetrahedral Cartesian Tetrahedral LP code Cartesian Cartesian Hexahedral (quad) Tetrahedral
Number of cells 790,000 450,000 9800 730,000 11 2590 160,580 32,000 730,000
Cell sizec (m) 0.02–0.035 0.05 0.04 0.005–0.1 0.3–0.75 0.1 0.1 0.02 0.005–0.1
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Table 3 Wall boundary conditions of the calculations. Institution
Code name
Fluid
Droplets
EDF (IRSN)
NEPTUNE
No slip wall, standard wall functions
IRSN
CFX
No slip wall, standard wall functions
Velocity component normal to walls = 0 m/s Particles disappear when touching the walls
KIT NRG
GASFLOW FLUENT
No slip wall, standard wall functions
RSE RSE UJV
ECART FDS FLUENT
VTT
FLUENT
Rigid free-slip Particles disappear when touching the walls Not applicable (Lumped-parameter code) Simplified law of the walls No slip wall, standard wall functions Particles disappear when touching the walls but HMT on walls active (no effect here) No slip wall, standard wall functions Particles disappear when touching the walls
Table 4 Injection boundary conditions of the calculations. Institution
EDF (IRSN) IRSN KIT NRG RSE RSE UJV VTT
Code name
NEPTUNE CFX GASFLOW FLUENT ECART FDS FLUENT FLUENT
Fluid
Droplets
Injection meshed (yes/no)
Injection location (if applicable)
Velocity
Height of injection (m)
Velocity
Size
Yes Yes (for sensitivity studies) No No No No No No
2m 2.3 m No No No No No No
Profile 0 m/s No No No No No No
2m 2.3 m 2.3 m 2.3 m 2.3 m 2.3 m 2.3 m 2.3 m
Profile Profile Profile Profile Constant 2 values Profile Profile
Profile of PDFs Profile of PDFs Profile (+sensitivity with PDF) PDF Constant 2 values PDF Profile (+sensitivity with PDF)
spray droplets (indicated by the location of the numerical results: if no results are given on a given position of the graph, this means that there are no droplets anymore on this location) are almost the same for all calculations at this distance from the nozzle. The calculated spreads are almost the same for all calculations, except for
the ECART results, which is a lumped-parameter code (only one significant value for large regions of space). Droplet size profiles at 40 cm from the spray nozzle are presented in Fig. 5. It can be seen that CFD contributions, which have used the detailed experimental boundary conditions (at 20 cm) for
Fig. 3. Droplet vertical velocity, radial velocity and size at 20 cm from the nozzle outlet (boundary conditions).
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Fig. 4. Droplet vertical and radial velocity at 40 cm from the nozzle outlet.
Fig. 5. Droplet size at 40 cm from the nozzle outlet.
the droplet size profile, obtain good results: droplet sizes are then rather well recovered at 40 cm down from the nozzle outlet. It is also found by most of the codes that the largest droplets go on the side of the spray envelope, whereas the smallest droplets are more entrained in the internal zone of the spray. NRG and UJV contributions show the same tendency as in the experiments. However, the higher values for the NRG droplet sizes are not explained at this stage, but could be due to post-processing problems. The other contributions found the experimental range of order for the values of the droplet sizes, i.e. between 200 and 400 m. Entrained gas velocities at 40 cm from the nozzle outlet are presented in Fig. 6. From such results, three zones may be considered in a qualitative way.
- The one outside the spray envelope, around the spray, which is approximately 0.3 m from the axis: for this zone, the vertical velocity component is almost constant and most of the codes find a value between 1 and 2 m/s, which is not far away from the experimental data. Almost the same observation can be made for the radial velocity component outside the spray. - The second zone is the zone where droplets are present, i.e. for most of the codes, between 10 and 30 cm: this is the zone where the spatial variation of the gas velocity is important; most of the codes follow the same variation, with different levels for the absolute values of the velocities (almost a factor 2 between the minimum and maximum value). - The last zone is the zone inside this hollow-cone spray, where differences are observed; most of the calculations show that the flow is mainly directed downward, the radial component being almost equal to zero; the flow is unsteady and the flow is not perfectly axi-symmetrical (Foissac and Malet, 2011). The value of the downward vertical entrained gas velocity can vary by a factor 5, from 2 to 10 m/s, which is rather a large difference for this zone. At 60 cm from the nozzle outlet (Figs. 7 and 8), numerical results on the droplet velocity and size profiles are not as close to the experimental data as at Zo = −40 cm. The ‘small’ errors already observed on droplet sizes or velocities at Zo = −40 cm accumulates along the path of the droplet and become larger the longer distance the droplet moves from the boundary condition. This is clearly observed for GASFLOW, FLUENT by VTT, NEPTUNE codes. Most of the codes do not recover the experimental values of droplet size on the spray envelope, except the FLUENT contributions of NRG and UJV. This spreading of the numerical results is almost higher for the gas entrainment (Fig. 9), especially inside the spray where differences between codes are larger (a maximum for the absolute value of the axial gas velocity between 5 and 9 m/s depending
Fig. 6. Gas vertical and radial velocity at 40 cm from the nozzle outlet.
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Fig. 7. Droplet vertical (left) and radial (right) velocity at 60 cm from the nozzle outlet.
Fig. 8. Droplet size at 60 cm from the nozzle outlet.
on the participants). Results of gas entrainment outside the spray are good for most of the contributions, even if a spread of 50% is observed (for the absolute value of the axial gas velocity between 1 to 2 m/s depending on the participants). These observations on droplet velocities and size, as well as on gas entrainment, are almost the same, even enhanced, at 95 cm from the nozzle outlet (Fig. 10). A small comment can be made on the CFX calculations at 95 cm: The sharpness on the peak of the droplet velocity is linked to the post-processing of trajectories: it is difficult in Lagrangian calculations to extract a “smooth” profile from packets of trajectories. If the trajectories are expanded, the post-processed profiles result in some strong radial variation of the velocity. However, the tendency of a local increase of the droplet velocity is linked with the local increase of the entrained gas.
As a result, it can be said that for the contributions using the detailed boundary conditions (polydisperse size distribution and velocity profiles), the shape of the droplet velocity and size profiles are well recovered (CFX, FLUENT by NRG and UJV). For the contributions with monodisperse droplet size (FLUENT by VTT, GASFLOW), differences are observed with the experiments (Fig. 8): the displacement of the larger droplets is not well recovered (droplets of 300 m on the spray envelope for NEPTUNE, FLUENT by VTT, GASFLOW instead of 500 m expected from the experiments). This is associated to differences in terms of velocity, and can have an effect on the residence time of the particles and on the length of the zone where the droplet velocities reach their equilibrium. Concerning the gas velocities, the velocity in the outer region (between 0.6 m and 1 m) is almost well calculated (up to 50% error), but this is not the case in the zone where droplets are present, as well as inside the spray. Most of the codes found an axial flow inside the spray, but the value of the entrained gas velocity can be different up to a factor 3 just 1 m below the spray nozzle. This should have an impact on the overall entrainment rate inside the spray. It should be also noticed that the code-experiment discrepancies on the droplet velocities profiles increase with the distance from the nozzle. One explanation comes from cumulative error on gas entrainment with the distance to the nozzle. It should also be added that the gas initial velocity was not a specified boundary condition of the benchmark. Most of the participants have used a zero value for the gas initial velocity. The maximum value of this gas initial velocity at 40 cm, i.e. close to the nozzle is between 4 and 8 m/s depending on the participants (Fig. 6). At this location, the droplet injection velocity is higher, around 14 m/s (Fig. 4): the relative velocity is rather high and the initial value of the gas velocity does not impact so much the droplet velocity. That is one reason why the participants have almost the same droplet velocities at 40 cm. On the contrary, at 95 cm (i.e. far from nozzle outlet), the
Fig. 9. Gas vertical (left) and radial (right) velocity at 60 cm from the nozzle outlet.
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Fig. 10. Gas and droplet vertical velocity at 95 cm from the nozzle outlet.
droplet velocity is reduced (6–7 m/s) and can be closer to the value of the gas velocity (7–8 m/s), as presented in Fig. 10: the relative velocity is reduced, so that differences between participants on the droplet velocities at 95 cm can be partially explained by the differences on the gas velocities, i.e. on the gaseous entrainment. This observation is rather logical, since droplet measurements used for boundary conditions are performed 20 cm below the spray nozzle, i.e. in a zone where droplets are formed, after the atomization zone, so that gas entrainment already occurs in this zone. As a result, gas velocity should be also a variable measured with details for the boundary conditions. An experimental technique that could measure simultaneously in the same zone (here at 20 cm) the droplet and the gas velocities would be interesting to develop. One interesting zone to compare is the zone at the spray nozzle height, i.e. the Zo = 0 m zone. In this zone, it is considered that there is no droplet, since the boundary conditions of the calculations are given at Zo = −20 cm, i.e. 20 cm below the spray nozzle. These boundary conditions have a rather important vertical component of the droplet velocity. It should be emphasized that what is called here a boundary conditions for droplet (the injection zone) is a zone inside the domain that is not on the walls of the domain. As a result, the flow is calculated above this zone up to the upper wall and gas velocity at 0 m represents the induced gas entrainment by the imposed boundary conditions on droplets 20 cm below. Results are presented in Fig. 11. It can be observed that the experimental uncertainty is much lower here, because the uncertainty due the spray pulsation is not of concern here. At this location, the flow is found, in the experiments as well as in the code contributions, to be directed downward (no radial component of the gas velocity), except for the FLUENT contribution submitted by VTT, for which an initial gas velocity has been initialized to a non-zero value in order to represent the gas entrainment at the end of the atomization zone. This boundary condition by the VTT calculation seems
to have an overestimated influence (the maximum absolute value of the vertical gas velocity reached almost 6 m/s instead of 3 m/s in the experiment). For all contributions, the experimental spatial evolution of the horizontal profile of the gas vertical velocity is found by none of the codes (Fig. 11). This is explained by the fact that all participants were free to consider the gas velocity boundary conditions as they wanted. Since no data on gas velocities were available inside the spray at 20 cm, recommendation was to perform calculations by fitting the gas vertical velocity to the gas velocity profile measured outside the spray 20 cm below the spray nozzle (given in the specifications). However, the gas velocity outside the spray is found to be not very relevant (almost constant with the height), so that recommendation resulting from this benchmark would be to use the exact gas velocity profile (but measurements are needed).
3.3. Sensitivity analysis 3.3.1. Impact of the use of a mean diameter instead of a droplet size distribution at the injection Because of the large size of the reactor containment, studies using spray systems at the real scale are generally performed using a spray characterized by one mean droplet size instead of a droplet size distribution. Fig. 12 shows the impact of such assumption on droplet velocities obtained with ANSYS/CFX. It can be seen that under conditions with a single droplet size (data called ‘MONO’), all droplets have almost the same trajectories, leading to a different spray envelope than in a case considering an initial size distribution (‘POLY’). In the case presented here, the spray envelope is reduced (the largest droplets of a size distribution define the spray envelope) and there is no droplet at all in the center of the spray (no small droplets to be entrained). This has an effect on the entrained
Fig. 11. Gas vertical and radial velocity at 0 cm, i.e. at nozzle outlet.
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Fig. 12. IRSN results – comparison on droplet and gas velocities between ANSYS results obtained with single mean diameter (MONO) and size distribution (POLY) at the inlet spray boundary conditions.
gas velocity, since the maximal axial velocity is changed of around 25%. The radial velocity is also impacted. KIT has also performed calculations (Fig. 13) with one droplet size class and 8 droplet size classes. They found also that the use of one class of droplet size changes the spray spread: the ring of the spray nozzle is much narrow for a so-called “monodispersed” case. However, the maximum value of the vertical gas velocity is not changed a lot.
3.3.2. Impact of the use of a constant velocity profile at the injection In most of the results presented in the main part, the exact experimental droplet size distribution and velocity profiles have been used as boundary conditions for the droplets (see curves at 20 cm). In this section, the influence of ‘simplified’ initial droplet velocity conditions is studied. Two calculations were performed by IRSN using ANSYS/CFX: with a meshed surface as a boundary, where the exact velocity profiles can be imposed (calculation called EVP DSD) and a ‘particle injection’ which can be applied on
a defined location that does not exactly corresponds to a mesh surface (interpolation is done by the code), with a constant velocity profiles (calculations called CVP DSD). The initial conditions thus obtained have been already described in Section 3.1. The consequences of these different boundary conditions are shown in Fig. 14 at 40 cm and 95 cm from the nozzle outlet: the droplet dispersion and the droplet gas velocities are not the same. The entrained gas velocities are also clearly impacted: for example, the maximum value of the axial velocity is changed of 50%.
3.3.3. Impact of the choice of combined simplifications on droplet size and velocities Combining the two sensitivity studies described above, i.e. imposing a monodispersed size instead of a size distribution, as well as a constant mean velocity instead of a velocity profile, has also been investigated. Results are presented here for the NEPTUNE-CFD code in Fig. 15. The droplet velocity profiles for the ‘MONO CVP case’ show values generally higher in the spray center and lower on the spray
Fig. 13. KIT results – entrained gas velocity for a calculation with one class of droplet size, and 8 classes of droplet size – results at 95 cm from the nozzle outlet.
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Fig. 14. IRSN results – droplet and gas velocity radial profiles using an exact meshed surface as injection boundary (EVP DSD) or a ‘particle injection’ with a constant velocity profiles (CVP DSD) – results at 40 cm and 95 cm from the nozzle outlet.
Fig. 15. EDF-IRSN results – comparison on droplet velocities between results obtained with single mean diameter and a constant velocity profile (right) and a size distribution and a real velocity profile (left) applied at the inlet spray boundary conditions – results at 40, 60 and 95 cm from the nozzle outlet.
envelope, which can be explained easily since no droplet ‘separation’ by inertia occurs. ANSYS results by IRSN for these conditions lead to the same observations. 4. General conclusion Benchmark calculations on a real PWR spray nozzle have been investigated here with different codes, mainly CFD codes, from seven different institutions.
It is found that droplet velocities are almost well calculated one meter below the spray nozzle, even if the spread of the spray is not recovered, especially for calculations that do not consider a size distribution for the droplet inputs. Concerning the entrainment by the spray, the gas velocities vary from one code to another from 50 to 100% differences on the external entrainment whereas for the internal entrainment, the maximal value of the entrained gas vertical velocity is found to be up to 30% different from one code to
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another. Results on the internal entrainment are also different if gas entrainment is taken into account in the spray boundary conditions. However, none of the codes is able to reproduce the shape and values of the gas velocity profile above the droplet injection zone indicating that no calculations reproduce the initial entrainment rate. Concerning sensitivity analysis, several ‘simplifications’ have been made in the contributions, especially based on the boundary conditions applied at the location where droplets and gas are injected. Such simplifications are realistic for nuclear containment calculations that do not allow, for the today computer power, to use exact boundary conditions (which are also not always known since no PWR spray droplet characteristics have been measured up to now under typical conditions of a severe accident). It is found that these simplifications can have a significant impact on the results. The use of a constant droplet velocity instead of an experimental velocity profile at the injection mainly influences the entrained gas axial velocity. The use of a ‘single droplet size’ instead of a ‘droplet size distribution’ plays a significant role on the spread of the spray, i.e. on the thickness of the annulus where droplets can be found. Finally, the gas conditions applied at the droplet injection location influences the values of the axial entrained gas velocity. We have shown here that such simplifications influence droplet and entrained gas characteristics. The next step will be to translate these conclusions in terms of variables representative of interesting parameters for nuclear safety. One important concern is the competition between depressurization (induced by the cold droplets injected in the air–steam mixture typical of a nuclear reactor accident), and gas mixing induced by spray entrainment. This is of particular importance in the presence of hydrogen in the gas mixture. It is known that droplet condensation will mainly occur in a reduced region compared to the size of the reactor containment, since droplet will rapidly reach the thermodynamical equilibrium (in less than one or two meters below spray nozzle). As a result, this competition between depressurization and mixing should be addressed very locally in the vicinity of the spray nozzle. This competition is also probably different ‘inside’ the spray, where droplets are tiny and entrained gas velocities higher, and on the spray envelope, where droplets are bigger and entrained gas velocities lower.
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To investigate analytically the competition of these phenomena is rather difficult since the evolution of the droplet characteristics depends on strongly coupled phenomena. Since this benchmark has shown the potentialities of CFD calculations for simulation of the flow induced by small droplets (300 m) in a large volume (160 m3 ), it is recommended to perform CFD calculations focused on the reduced zone of the containment, where most of the heat, mass and momentum transfers occur, in order to bring some insights for different configurations typical for nuclear safety concerns. References Foissac, A., (Ph.D. thesis)2011. Modélisation entre gouttes en environnement hostile. Université Pierre & Marie Curie – Paris 6, ISRN/IRSN-2011/ 147. Foissac, A., Malet, J., 2011. Gas entrainment by one single PWR spray – SARNET-2: elementary benchmark. In: Specification Report IRSN/DSU/SERAC/LEMAC/1119. Foissac, A., Malet, J., Vetrano, R.M., Buchlin, J.M., Mimouni, S., Feuillebois, F., Simonin, O., 2011. Droplet size and velocity measurements at the outlet of a hollow cone spray nozzle. Atomization Sprays 21, 893–905. Malet, J., 2003. Presentation of the tests matrix for the TOSQAN facility Spray Program. In: Technical Report DSU/SERAC/LEMAC/03-06. Malet, J., 2011. SARNET-2, droplet heat and mass transfer studies. In: Final Comparison Report IRSN/DSU/SERAC/LEMAC/11-04. Malet, J., Métier, P., 2007. SARNET spray benchmark: thermal hydraulic part, TOSQAN test 101, code-experiment comparison report. In: Technical Report IRSN/DSU/SERAC/LEMAC/07-03. Malet, J., Vizet, J., 2008. SARNET spray benchmark dynamic part: TOSQAN test 113, code-experiment comparison. In: Technical Report IRSN/DSU/SERAC/LEMAC/08-04. Malet, J., Blumenfeld, L., Arndt, S., Babic, M., Bentaib, A., Dabbene, F., Kostka, P., Mimouni, S., Movahed, M., Paci, S., Parduba, Z., Travis, J., Urbonavicius, E., 2011a. Sprays in containment: final results of the SARNET spray benchmark. Nucl. Eng. Des. 241, 2162–2217. Malet, J., Gelain, T., Mimouni, S., Manzini, G., Arndt, S., Klein-Hessling, W., Xu, Z., Povilaitis, M., Kubisova, L., Parduba, Z., Paci, S., Siccama, N.B., Stempniewicz, M.H., 2011b. Spray model validation on single droplet heat and mass transfers for containment applications – SARNET 2 benchmark. In: The 14th International Topical Meeting on Nuclear Reactor Thermal hydraulics, NURETH-14, Toronto, Canada, September 25–30. Métier, P., Malet, J., 2007. SARNET spray benchmark spray models. In: Technical Report IRSN/DSU/SERAC/LEMAC/07-20. Morsi, S.A., Alexander, A.J., 1972. An investigation of particle trajectories in twophase flow systems. J. Fluid Mech. 55 (2), 193–208. Schiller, L., Naumann, A., 1935. A drag coefficient correlation. VDI Zeitung 77, 318–320.
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Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Gas entrainment by one single French PWR spray, SARNET-2 spray benchmark J. Malet a,∗ , S. Mimouni b , G. Manzini c , J. Xiao d , L. Vyskocil e , N.B. Siccama f , R. Huhtanen g a
Institut de Radioprotection et de Sûreté Nucléaire, Saclay, France Electricité de France, EDF MF2E, Chatou, France RSE, Milano, Italy d IKET, KIT, Karlsruhe, Germany e UJV Rez, Czech Republic f NRG, Safety & Power, The Netherlands g VTT, PO Box 1000, FI-02044 VTT, Finland b c
h i g h l i g h t s • • • • •
This paper presents a benchmark performed in the frame of the SARNET-2 EU project. It concerns momentum transfer between a PWR spray and the surrounding gas. The entrained gas velocities can vary up to 100% from one code to another. Simplified boundary conditions for sprays are generally used by the code users. It is shown how these simplified conditions impact the gas entrainment.
a r t i c l e
i n f o
Article history: Received 19 March 2013 Received in revised form 24 November 2014 Accepted 12 December 2014
a b s t r a c t This paper presents a benchmark performed in the frame of the SARNET-2 EU project, dealing with momentum transfer between a real-scale PWR spray and the surrounding gas. It presents a description of the IRSN tests on the CALIST facility, the participating codes (8 contributions), code-experiment and code-to-code comparisons. It is found that droplet velocities are almost well calculated one meter below the spray nozzle, even if the spread of the spray is not recovered and the values of the entrained gas velocity vary up to 100% from one code to another. Concerning sensitivity analysis, several ‘simplifications’ have been made by the contributors, especially based on the boundary conditions applied at the location where droplets are injected. It is shown here that such simplifications influence droplet and entrained gas characteristics. The next step will be to translate these conclusions in terms of variables representative of interesting parameters for nuclear safety. © 2014 Elsevier B.V. All rights reserved.
1. Introduction During the course of a severe accident in a pressurized water reactor (PWR), spray systems are used in the containment in order to prevent overpressure and to enhance the gas mixing in case of the presence of hydrogen. Spray modelings are thus part of thermalhydraulic containment codes. The two major phenomena involved
∗ Corresponding author. Tel.: +33 169088740. E-mail addresses: [email protected] (J. Malet), [email protected] (S. Mimouni), [email protected] (G. Manzini), [email protected] (J. Xiao), [email protected] (L. Vyskocil), [email protected] (N.B. Siccama), risto.huhtanen@vtt.fi (R. Huhtanen). http://dx.doi.org/10.1016/j.nucengdes.2014.12.008 0029-5493/© 2014 Elsevier B.V. All rights reserved.
in spray behavior in such applications are the thermodynamical effect of a spray (steam condensation on droplets, evaporation, etc.) and the dynamical effect (entrainment and mixing of gases). This paper is proposed in the frame of the EU network SARNET2, within the Sub-Work Package WP7-2, Task 1 (spray activities) in charge of IRSN. In the past, validation of spray modelings has been performed on large-scale facilities (CVTR, NUPEC, CSE) using several spray nozzles (Malet, 2003). More specific studies have been proposed in the frame of SARNET (Malet et al., 2011a), as a synthesis report of spray models in containment applications (Métier and Malet, 2007), spray benchmark on TOSQAN test 101 and MISTRA MASPn tests on heat and mass exchanges between drops and the gaseous atmosphere (Malet and Métier, 2007) and spray benchmark on TOSQAN test 113 and MISTRA MARC2 tests on momentum
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transfer between drops and the gaseous atmosphere (Malet and Vizet, 2008). The conclusion of the SARNET activities was that the level of spray model validation obtained was encouraging for the use of spray modeling for risk analysis. However, it was not sufficient and further activities were thus well encouraged in order to evaluate the influence of different parameters in the modeling, such as benchmarks based on separate-effect tests. Separate-effect tests were thus proposed for SARNET-2 project. The first benchmark deals with heat and mass transfer (HMT) tests on single droplet (Malet, 2011; Malet et al., 2011b). The second benchmark concerns momentum transfer on one single PWR spray. The third one concerns the momentum transfer on two interacting PWR sprays. This paper presents the results of the second elementary benchmark. Experimental results are obtained on the CALIST (Characterization and Application of Large Industrial Spray Transfer, 160 m3 ) facility at the IRSN. Benchmark specifications are given in (Foissac and Malet, 2011). In this benchmark, code-experiment and code-to-code comparisons are performed at different distances from the nozzle outlet, inside and outside the spray. Sensitivity studies are also performed by the participants, and some of them are presented here. Beyond the common objective of codes validation, the other objective of this benchmark is to show the potentialities of CFD calculations of the flow induced by small droplets (300 m) in a large volume (160 m3 ) and to evaluate the influence, on droplet and entrained gas velocities, of usual simplifications generally made for safety analysis, such as the use of a single droplet size instead of a droplet size distribution. 2. Description of the benchmark 2.1. Description of the French PWR spray systems Spray systems in nuclear power plants are composed of over 500 interacting water droplet sprays with a range of droplet diameters from 100 m to 1000 m (Foissac et al., 2011). They are activated under gaseous mixture composed of steam, hydrogen and air at a total pressure of around 2–3 bars and under gas temperature around 100–120 ◦ C. The French PWR containments (Fig. 1) have generally two series of nozzles placed in circular rows. A schematic view of these spray rings and the associated theoretical spray envelopes are given in Fig. 1. The nozzle type used in many PWRs, in particularly French 900 MWe PWRs, is the so-called SPRACO 1713A, distributed by Lechler under reference 373.084.17.BN (Fig. 1). This nozzle is generally used with water at a relative pressure of 3.5 bars, inducing a flow rate of approximately 1 l/s. The outlet orifice diameter is 9.5 mm. Droplet injection temperature is between 20 ◦ C and 60 ◦ C. 2.2. Description of the CALIST facility The experiments are carried out in the CALIST facility sketched in Fig. 2. The set-up is composed of a hydraulic circuit supplying and, for those experiments, a single spray nozzle with a flow-rate of 1 l/s at a relative pressure of 3.5 bars. The pulverized water is collected in a 5 m3 pool. The axial position of the spray nozzle can be changed using a monitored carriage. The measurement of the spray characteristics is performed using phase-Doppler interferometry (PDI). All measurements are performed under air conditions at ambient pressure of 1 bar and gas ambient temperature of 20 ◦ C. More information on the measurements of the spray can be found in Foissac et al. (2011). Measurements of entrained gas around the spray is performed using very small water droplets obtained with fog nozzle activated in the roof of the facility, so that their velocity at the location of the PWR spray is negligible compared to the entrained gas velocity: the Stokes number of these small droplets
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have been evaluated to be around 0.07, much lower than 1, indicating that these droplets are good tracers (Foissac, 2011). Uncertainty on the droplet characteristics measurements are considered from the three following sources (more details in Foissac, 2011): - Uncertainty on the repeatability (all measurements are repeated three times, leading to an average value of the considered variable and its associated standard deviation); - Uncertainty linked to the angular position: the spray is not perfectly symmetric so that measurement points are considered at 4 angular positions; - Uncertainty linked to spray pulsation, which has been estimated to be of an angle of 1. 2.3. Participants and codes The list of participating institutions, the participant’s names, code versions, code types and droplet type of modeling are presented in Table 1. Seven institutions have participated with seven different code versions. Most of the codes are CFD codes, and based on a Lagrangian approach, considering the gravity and drag forces. The participants have also performed sensitivity studies. Here, only one calculation is considered per institution in the codeexperiment comparisons in order to have clear graphs. This exercise was proposed as a blind exercise even if some data were already partially published. Most of the codes use a Schiller and Naumann (1935) or the Morsi and Alexander (1972) correlation for the drag coefficient. A description of the size of the calculated domain, the type of mesh, the number of cells for the mesh is presented in Table 2. Boundary conditions of the fluid domain, as well as the one of the droplet phase are described in Tables 3 and 4. It should be noticed that the difference in the injection height in the EDF calculations does not lead to any difference in the results: these ones are always given at a relative position to the spray nozzle outlet. 3. Discussion on the results 3.1. Boundary conditions Droplet sizes and velocity profiles measured 20 cm below the nozzle outlet are considered for the boundary conditions of the spray. This is the common way to handle spray in CFD when the atomization zone is not simulated. These boundary conditions are given in the benchmark specification report (Foissac and Malet, 2011) as well as in Foissac et al. (2011). It was verified that these conditions were well used by the participants and these input data are given in Fig. 3. RSE contributions considered simplified velocity and droplet size profiles at the injection (droplet size distribution not freely settable), whereas other participants used the exact experimental profile. On this figure and subsequent ones, velocity equal to zero implies a very low volume fraction of droplets, negative values of the vertical respectively radial component of the gas/droplet velocities represent flow/droplets directed downwards respectively toward the spray axis. 3.2. Comparison results Droplet velocity results at 40 cm from the nozzle outlet are presented in Fig. 4 for all codes except CFX, for which the data were not distributed at the time of the benchmark. It can be observed that most of the numerical calculations provide the same results for the droplet velocities except FDS simulation by RSE, which is probably explained by the different boundary conditions used for these calculations. What is also interesting to see is that the spread of the
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Fig. 1. Spray rings and envelopes in a French PWR (not at scale) and PWR spray nozzle.
Fig. 2. CALIST water-spray experimental facility. Table 1 Participants for the blind and open calculations. Institution
Code name
Type of code
Type of calculation
Droplet modeling
Drag law – reference
EDF (IRSN) IRSN KIT NRG RSE RSE UJV VTT
NEPTUNE ANSYS/CFX v.13 GASFLOW ANSYS/FLUENT v.6.3 ECART vs. 4W0T FDS vs. 5.5.3 Serial ANSYS/FLUENT v.13 ANSYS/FLUENT v.13
CFD CFD CFD CFD LP CFD CFD CFD
Transient Steady-state Steady-state Transient Transient Transient Transient Transient
Eulerian Lagrangian Lagrangian Lagrangian Lagrangian Lagrangian Lagrangian Lagrangian
Schiller and Naumann (1935) Morsi and Alexander (1972) Schiller and Naumann (1935) Morsi and Alexander (1972) – – Morsi and Alexander (1972) Morsi and Alexander (1972)
Table 2 Description of the computational domain and associated mesh. Institution
EDF (IRSN) IRSN KIT NRG RSE RSE RSE UJV VTT a b c
Code name
NEPTUNE CFX GASFLOW FLUENT ECART FDS FDS FLUENT FLUENT
Size of the calculated domain
Type of calculation symmetry
L × l × h (m × m × m)
Volume (m3 )
3 × 3.5 × 3.5 3 × 3.5 × 3.5 H = 3.7 m, R = 3 m 7.0 × 6.2 × 3.7 7.0 × 6.2 × 3.7 7.0 × 3.7 7.0 × 6.2 × 3.7 H = 3.7 m, R = 3.5 m 7.0 × 6.2 × 3.7 (pool edges included)
147 147 105 161 161 26 m2 161 142 161
1/4th geometry is meshed. 2D slice of 3.6◦ . Single values correspond to an average size.
Mesh Type of mesh
3D – 2 planes symmetrya 3D – 2 planes symmetrya 2D, revolution symmetryb 3D 3D 2D (plane symmetry) 3D 2D (axi-symmetry) 3D
Hexahedral Tetrahedral Cartesian Tetrahedral LP code Cartesian Cartesian Hexahedral (quad) Tetrahedral
Number of cells 790,000 450,000 9800 730,000 11 2590 160,580 32,000 730,000
Cell sizec (m) 0.02–0.035 0.05 0.04 0.005–0.1 0.3–0.75 0.1 0.1 0.02 0.005–0.1
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Table 3 Wall boundary conditions of the calculations. Institution
Code name
Fluid
Droplets
EDF (IRSN)
NEPTUNE
No slip wall, standard wall functions
IRSN
CFX
No slip wall, standard wall functions
Velocity component normal to walls = 0 m/s Particles disappear when touching the walls
KIT NRG
GASFLOW FLUENT
No slip wall, standard wall functions
RSE RSE UJV
ECART FDS FLUENT
VTT
FLUENT
Rigid free-slip Particles disappear when touching the walls Not applicable (Lumped-parameter code) Simplified law of the walls No slip wall, standard wall functions Particles disappear when touching the walls but HMT on walls active (no effect here) No slip wall, standard wall functions Particles disappear when touching the walls
Table 4 Injection boundary conditions of the calculations. Institution
EDF (IRSN) IRSN KIT NRG RSE RSE UJV VTT
Code name
NEPTUNE CFX GASFLOW FLUENT ECART FDS FLUENT FLUENT
Fluid
Droplets
Injection meshed (yes/no)
Injection location (if applicable)
Velocity
Height of injection (m)
Velocity
Size
Yes Yes (for sensitivity studies) No No No No No No
2m 2.3 m No No No No No No
Profile 0 m/s No No No No No No
2m 2.3 m 2.3 m 2.3 m 2.3 m 2.3 m 2.3 m 2.3 m
Profile Profile Profile Profile Constant 2 values Profile Profile
Profile of PDFs Profile of PDFs Profile (+sensitivity with PDF) PDF Constant 2 values PDF Profile (+sensitivity with PDF)
spray droplets (indicated by the location of the numerical results: if no results are given on a given position of the graph, this means that there are no droplets anymore on this location) are almost the same for all calculations at this distance from the nozzle. The calculated spreads are almost the same for all calculations, except for
the ECART results, which is a lumped-parameter code (only one significant value for large regions of space). Droplet size profiles at 40 cm from the spray nozzle are presented in Fig. 5. It can be seen that CFD contributions, which have used the detailed experimental boundary conditions (at 20 cm) for
Fig. 3. Droplet vertical velocity, radial velocity and size at 20 cm from the nozzle outlet (boundary conditions).
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Fig. 4. Droplet vertical and radial velocity at 40 cm from the nozzle outlet.
Fig. 5. Droplet size at 40 cm from the nozzle outlet.
the droplet size profile, obtain good results: droplet sizes are then rather well recovered at 40 cm down from the nozzle outlet. It is also found by most of the codes that the largest droplets go on the side of the spray envelope, whereas the smallest droplets are more entrained in the internal zone of the spray. NRG and UJV contributions show the same tendency as in the experiments. However, the higher values for the NRG droplet sizes are not explained at this stage, but could be due to post-processing problems. The other contributions found the experimental range of order for the values of the droplet sizes, i.e. between 200 and 400 m. Entrained gas velocities at 40 cm from the nozzle outlet are presented in Fig. 6. From such results, three zones may be considered in a qualitative way.
- The one outside the spray envelope, around the spray, which is approximately 0.3 m from the axis: for this zone, the vertical velocity component is almost constant and most of the codes find a value between 1 and 2 m/s, which is not far away from the experimental data. Almost the same observation can be made for the radial velocity component outside the spray. - The second zone is the zone where droplets are present, i.e. for most of the codes, between 10 and 30 cm: this is the zone where the spatial variation of the gas velocity is important; most of the codes follow the same variation, with different levels for the absolute values of the velocities (almost a factor 2 between the minimum and maximum value). - The last zone is the zone inside this hollow-cone spray, where differences are observed; most of the calculations show that the flow is mainly directed downward, the radial component being almost equal to zero; the flow is unsteady and the flow is not perfectly axi-symmetrical (Foissac and Malet, 2011). The value of the downward vertical entrained gas velocity can vary by a factor 5, from 2 to 10 m/s, which is rather a large difference for this zone. At 60 cm from the nozzle outlet (Figs. 7 and 8), numerical results on the droplet velocity and size profiles are not as close to the experimental data as at Zo = −40 cm. The ‘small’ errors already observed on droplet sizes or velocities at Zo = −40 cm accumulates along the path of the droplet and become larger the longer distance the droplet moves from the boundary condition. This is clearly observed for GASFLOW, FLUENT by VTT, NEPTUNE codes. Most of the codes do not recover the experimental values of droplet size on the spray envelope, except the FLUENT contributions of NRG and UJV. This spreading of the numerical results is almost higher for the gas entrainment (Fig. 9), especially inside the spray where differences between codes are larger (a maximum for the absolute value of the axial gas velocity between 5 and 9 m/s depending
Fig. 6. Gas vertical and radial velocity at 40 cm from the nozzle outlet.
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Fig. 7. Droplet vertical (left) and radial (right) velocity at 60 cm from the nozzle outlet.
Fig. 8. Droplet size at 60 cm from the nozzle outlet.
on the participants). Results of gas entrainment outside the spray are good for most of the contributions, even if a spread of 50% is observed (for the absolute value of the axial gas velocity between 1 to 2 m/s depending on the participants). These observations on droplet velocities and size, as well as on gas entrainment, are almost the same, even enhanced, at 95 cm from the nozzle outlet (Fig. 10). A small comment can be made on the CFX calculations at 95 cm: The sharpness on the peak of the droplet velocity is linked to the post-processing of trajectories: it is difficult in Lagrangian calculations to extract a “smooth” profile from packets of trajectories. If the trajectories are expanded, the post-processed profiles result in some strong radial variation of the velocity. However, the tendency of a local increase of the droplet velocity is linked with the local increase of the entrained gas.
As a result, it can be said that for the contributions using the detailed boundary conditions (polydisperse size distribution and velocity profiles), the shape of the droplet velocity and size profiles are well recovered (CFX, FLUENT by NRG and UJV). For the contributions with monodisperse droplet size (FLUENT by VTT, GASFLOW), differences are observed with the experiments (Fig. 8): the displacement of the larger droplets is not well recovered (droplets of 300 m on the spray envelope for NEPTUNE, FLUENT by VTT, GASFLOW instead of 500 m expected from the experiments). This is associated to differences in terms of velocity, and can have an effect on the residence time of the particles and on the length of the zone where the droplet velocities reach their equilibrium. Concerning the gas velocities, the velocity in the outer region (between 0.6 m and 1 m) is almost well calculated (up to 50% error), but this is not the case in the zone where droplets are present, as well as inside the spray. Most of the codes found an axial flow inside the spray, but the value of the entrained gas velocity can be different up to a factor 3 just 1 m below the spray nozzle. This should have an impact on the overall entrainment rate inside the spray. It should be also noticed that the code-experiment discrepancies on the droplet velocities profiles increase with the distance from the nozzle. One explanation comes from cumulative error on gas entrainment with the distance to the nozzle. It should also be added that the gas initial velocity was not a specified boundary condition of the benchmark. Most of the participants have used a zero value for the gas initial velocity. The maximum value of this gas initial velocity at 40 cm, i.e. close to the nozzle is between 4 and 8 m/s depending on the participants (Fig. 6). At this location, the droplet injection velocity is higher, around 14 m/s (Fig. 4): the relative velocity is rather high and the initial value of the gas velocity does not impact so much the droplet velocity. That is one reason why the participants have almost the same droplet velocities at 40 cm. On the contrary, at 95 cm (i.e. far from nozzle outlet), the
Fig. 9. Gas vertical (left) and radial (right) velocity at 60 cm from the nozzle outlet.
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Fig. 10. Gas and droplet vertical velocity at 95 cm from the nozzle outlet.
droplet velocity is reduced (6–7 m/s) and can be closer to the value of the gas velocity (7–8 m/s), as presented in Fig. 10: the relative velocity is reduced, so that differences between participants on the droplet velocities at 95 cm can be partially explained by the differences on the gas velocities, i.e. on the gaseous entrainment. This observation is rather logical, since droplet measurements used for boundary conditions are performed 20 cm below the spray nozzle, i.e. in a zone where droplets are formed, after the atomization zone, so that gas entrainment already occurs in this zone. As a result, gas velocity should be also a variable measured with details for the boundary conditions. An experimental technique that could measure simultaneously in the same zone (here at 20 cm) the droplet and the gas velocities would be interesting to develop. One interesting zone to compare is the zone at the spray nozzle height, i.e. the Zo = 0 m zone. In this zone, it is considered that there is no droplet, since the boundary conditions of the calculations are given at Zo = −20 cm, i.e. 20 cm below the spray nozzle. These boundary conditions have a rather important vertical component of the droplet velocity. It should be emphasized that what is called here a boundary conditions for droplet (the injection zone) is a zone inside the domain that is not on the walls of the domain. As a result, the flow is calculated above this zone up to the upper wall and gas velocity at 0 m represents the induced gas entrainment by the imposed boundary conditions on droplets 20 cm below. Results are presented in Fig. 11. It can be observed that the experimental uncertainty is much lower here, because the uncertainty due the spray pulsation is not of concern here. At this location, the flow is found, in the experiments as well as in the code contributions, to be directed downward (no radial component of the gas velocity), except for the FLUENT contribution submitted by VTT, for which an initial gas velocity has been initialized to a non-zero value in order to represent the gas entrainment at the end of the atomization zone. This boundary condition by the VTT calculation seems
to have an overestimated influence (the maximum absolute value of the vertical gas velocity reached almost 6 m/s instead of 3 m/s in the experiment). For all contributions, the experimental spatial evolution of the horizontal profile of the gas vertical velocity is found by none of the codes (Fig. 11). This is explained by the fact that all participants were free to consider the gas velocity boundary conditions as they wanted. Since no data on gas velocities were available inside the spray at 20 cm, recommendation was to perform calculations by fitting the gas vertical velocity to the gas velocity profile measured outside the spray 20 cm below the spray nozzle (given in the specifications). However, the gas velocity outside the spray is found to be not very relevant (almost constant with the height), so that recommendation resulting from this benchmark would be to use the exact gas velocity profile (but measurements are needed).
3.3. Sensitivity analysis 3.3.1. Impact of the use of a mean diameter instead of a droplet size distribution at the injection Because of the large size of the reactor containment, studies using spray systems at the real scale are generally performed using a spray characterized by one mean droplet size instead of a droplet size distribution. Fig. 12 shows the impact of such assumption on droplet velocities obtained with ANSYS/CFX. It can be seen that under conditions with a single droplet size (data called ‘MONO’), all droplets have almost the same trajectories, leading to a different spray envelope than in a case considering an initial size distribution (‘POLY’). In the case presented here, the spray envelope is reduced (the largest droplets of a size distribution define the spray envelope) and there is no droplet at all in the center of the spray (no small droplets to be entrained). This has an effect on the entrained
Fig. 11. Gas vertical and radial velocity at 0 cm, i.e. at nozzle outlet.
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Fig. 12. IRSN results – comparison on droplet and gas velocities between ANSYS results obtained with single mean diameter (MONO) and size distribution (POLY) at the inlet spray boundary conditions.
gas velocity, since the maximal axial velocity is changed of around 25%. The radial velocity is also impacted. KIT has also performed calculations (Fig. 13) with one droplet size class and 8 droplet size classes. They found also that the use of one class of droplet size changes the spray spread: the ring of the spray nozzle is much narrow for a so-called “monodispersed” case. However, the maximum value of the vertical gas velocity is not changed a lot.
3.3.2. Impact of the use of a constant velocity profile at the injection In most of the results presented in the main part, the exact experimental droplet size distribution and velocity profiles have been used as boundary conditions for the droplets (see curves at 20 cm). In this section, the influence of ‘simplified’ initial droplet velocity conditions is studied. Two calculations were performed by IRSN using ANSYS/CFX: with a meshed surface as a boundary, where the exact velocity profiles can be imposed (calculation called EVP DSD) and a ‘particle injection’ which can be applied on
a defined location that does not exactly corresponds to a mesh surface (interpolation is done by the code), with a constant velocity profiles (calculations called CVP DSD). The initial conditions thus obtained have been already described in Section 3.1. The consequences of these different boundary conditions are shown in Fig. 14 at 40 cm and 95 cm from the nozzle outlet: the droplet dispersion and the droplet gas velocities are not the same. The entrained gas velocities are also clearly impacted: for example, the maximum value of the axial velocity is changed of 50%.
3.3.3. Impact of the choice of combined simplifications on droplet size and velocities Combining the two sensitivity studies described above, i.e. imposing a monodispersed size instead of a size distribution, as well as a constant mean velocity instead of a velocity profile, has also been investigated. Results are presented here for the NEPTUNE-CFD code in Fig. 15. The droplet velocity profiles for the ‘MONO CVP case’ show values generally higher in the spray center and lower on the spray
Fig. 13. KIT results – entrained gas velocity for a calculation with one class of droplet size, and 8 classes of droplet size – results at 95 cm from the nozzle outlet.
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Fig. 14. IRSN results – droplet and gas velocity radial profiles using an exact meshed surface as injection boundary (EVP DSD) or a ‘particle injection’ with a constant velocity profiles (CVP DSD) – results at 40 cm and 95 cm from the nozzle outlet.
Fig. 15. EDF-IRSN results – comparison on droplet velocities between results obtained with single mean diameter and a constant velocity profile (right) and a size distribution and a real velocity profile (left) applied at the inlet spray boundary conditions – results at 40, 60 and 95 cm from the nozzle outlet.
envelope, which can be explained easily since no droplet ‘separation’ by inertia occurs. ANSYS results by IRSN for these conditions lead to the same observations. 4. General conclusion Benchmark calculations on a real PWR spray nozzle have been investigated here with different codes, mainly CFD codes, from seven different institutions.
It is found that droplet velocities are almost well calculated one meter below the spray nozzle, even if the spread of the spray is not recovered, especially for calculations that do not consider a size distribution for the droplet inputs. Concerning the entrainment by the spray, the gas velocities vary from one code to another from 50 to 100% differences on the external entrainment whereas for the internal entrainment, the maximal value of the entrained gas vertical velocity is found to be up to 30% different from one code to
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another. Results on the internal entrainment are also different if gas entrainment is taken into account in the spray boundary conditions. However, none of the codes is able to reproduce the shape and values of the gas velocity profile above the droplet injection zone indicating that no calculations reproduce the initial entrainment rate. Concerning sensitivity analysis, several ‘simplifications’ have been made in the contributions, especially based on the boundary conditions applied at the location where droplets and gas are injected. Such simplifications are realistic for nuclear containment calculations that do not allow, for the today computer power, to use exact boundary conditions (which are also not always known since no PWR spray droplet characteristics have been measured up to now under typical conditions of a severe accident). It is found that these simplifications can have a significant impact on the results. The use of a constant droplet velocity instead of an experimental velocity profile at the injection mainly influences the entrained gas axial velocity. The use of a ‘single droplet size’ instead of a ‘droplet size distribution’ plays a significant role on the spread of the spray, i.e. on the thickness of the annulus where droplets can be found. Finally, the gas conditions applied at the droplet injection location influences the values of the axial entrained gas velocity. We have shown here that such simplifications influence droplet and entrained gas characteristics. The next step will be to translate these conclusions in terms of variables representative of interesting parameters for nuclear safety. One important concern is the competition between depressurization (induced by the cold droplets injected in the air–steam mixture typical of a nuclear reactor accident), and gas mixing induced by spray entrainment. This is of particular importance in the presence of hydrogen in the gas mixture. It is known that droplet condensation will mainly occur in a reduced region compared to the size of the reactor containment, since droplet will rapidly reach the thermodynamical equilibrium (in less than one or two meters below spray nozzle). As a result, this competition between depressurization and mixing should be addressed very locally in the vicinity of the spray nozzle. This competition is also probably different ‘inside’ the spray, where droplets are tiny and entrained gas velocities higher, and on the spray envelope, where droplets are bigger and entrained gas velocities lower.
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To investigate analytically the competition of these phenomena is rather difficult since the evolution of the droplet characteristics depends on strongly coupled phenomena. Since this benchmark has shown the potentialities of CFD calculations for simulation of the flow induced by small droplets (300 m) in a large volume (160 m3 ), it is recommended to perform CFD calculations focused on the reduced zone of the containment, where most of the heat, mass and momentum transfers occur, in order to bring some insights for different configurations typical for nuclear safety concerns. References Foissac, A., (Ph.D. thesis)2011. Modélisation entre gouttes en environnement hostile. Université Pierre & Marie Curie – Paris 6, ISRN/IRSN-2011/ 147. Foissac, A., Malet, J., 2011. Gas entrainment by one single PWR spray – SARNET-2: elementary benchmark. In: Specification Report IRSN/DSU/SERAC/LEMAC/1119. Foissac, A., Malet, J., Vetrano, R.M., Buchlin, J.M., Mimouni, S., Feuillebois, F., Simonin, O., 2011. Droplet size and velocity measurements at the outlet of a hollow cone spray nozzle. Atomization Sprays 21, 893–905. Malet, J., 2003. Presentation of the tests matrix for the TOSQAN facility Spray Program. In: Technical Report DSU/SERAC/LEMAC/03-06. Malet, J., 2011. SARNET-2, droplet heat and mass transfer studies. In: Final Comparison Report IRSN/DSU/SERAC/LEMAC/11-04. Malet, J., Métier, P., 2007. SARNET spray benchmark: thermal hydraulic part, TOSQAN test 101, code-experiment comparison report. In: Technical Report IRSN/DSU/SERAC/LEMAC/07-03. Malet, J., Vizet, J., 2008. SARNET spray benchmark dynamic part: TOSQAN test 113, code-experiment comparison. In: Technical Report IRSN/DSU/SERAC/LEMAC/08-04. Malet, J., Blumenfeld, L., Arndt, S., Babic, M., Bentaib, A., Dabbene, F., Kostka, P., Mimouni, S., Movahed, M., Paci, S., Parduba, Z., Travis, J., Urbonavicius, E., 2011a. Sprays in containment: final results of the SARNET spray benchmark. Nucl. Eng. Des. 241, 2162–2217. Malet, J., Gelain, T., Mimouni, S., Manzini, G., Arndt, S., Klein-Hessling, W., Xu, Z., Povilaitis, M., Kubisova, L., Parduba, Z., Paci, S., Siccama, N.B., Stempniewicz, M.H., 2011b. Spray model validation on single droplet heat and mass transfers for containment applications – SARNET 2 benchmark. In: The 14th International Topical Meeting on Nuclear Reactor Thermal hydraulics, NURETH-14, Toronto, Canada, September 25–30. Métier, P., Malet, J., 2007. SARNET spray benchmark spray models. In: Technical Report IRSN/DSU/SERAC/LEMAC/07-20. Morsi, S.A., Alexander, A.J., 1972. An investigation of particle trajectories in twophase flow systems. J. Fluid Mech. 55 (2), 193–208. Schiller, L., Naumann, A., 1935. A drag coefficient correlation. VDI Zeitung 77, 318–320.