Image-processing of time-averaged interface distributions representing CCFL characteristics in a large scale model of a PWR hot-leg pipe geometry

Annals of Nuclear Energy 103 (2017) 282–293

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Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene

Image-processing of time-averaged interface distributions representing CCFL characteristics in a large scale model of a PWR hot-leg pipe geometry Suleiman Al Issa ⇑, Rafael Macián-Juan Technische Universität München (TUM), Department of Nuclear Engineering, Boltzmannstrasse 15, 85748 Garching, Germany

a r t i c l e

i n f o

Article history: Received 28 October 2016 Accepted 18 January 2017

Keywords: PWR Hot-leg CCFL characteristics Time-averaged interface distribution CFD validation data Two-phase flows Complex geometry SBLOCA

a b s t r a c t Countercurrent Flow Limitation (CCFL) was experimentally investigated in a 1/3.9 downscaled COLLIDER facility with a 190 mm pipe’s diameter using air/water at 1 atmospheric pressure. Previous investigations provided knowledge over the onset of CCFL mechanisms. In current article, CCFL characteristics at the COLLIDER facility are measured and discussed along with time-averaged distributions of the air/water interface for a selected matrix of liquid/gas velocities. The article demonstrates the time-averaged interface as a useful method to identify CCFL characteristics at quasi-stationary flow conditions eliminating variations that appears in single images, and showing essential comparative flow features such as: the degree of restriction at the bend, the extension and the intensity of the two-phase mixing zones, and the average water level within the horizontal part and the steam generator. Consequently, making it possible to compare interface distributions obtained at different investigations. The distributions are also beneficial for CFD validations of CCFL as the instant chaotic gas/liquid interface is impossible to reproduce in CFD simulations. The current study shows that final CCFL characteristics curve (and the corresponding CCFL correlation) depends upon the covered measuring range of water delivery. It also shows that a hydraulic diameter should be sufficiently larger than 50 mm in order to obtain CCFL characteristics comparable to the 1:1 scale data (namely the UPTF data). Finally, the study shows that the change of the flow condition inside the hot-leg is not only related to the water and air inlet velocities, but is also dependent upon the existent interface distribution within the hot-leg, and that several CCFL cases of identical inlet flow conditions can exist with different interface distribution and pressure difference. The last result is of a special importance to the investigation of this phenomenon during SBLOCA accidents, since the entire phenomenon is driven by pressure difference between the steam generator and reactor vessel, as well as by gravity. This result show also that CCFL characteristics cannot be investigated using 1D codes, as the interface distribution within the hot-leg during a SBLOCA accident will depend upon flow history or previous interface distribution. Current investigations support the effort to provide more knowledge over CCFL in order to extrapolate results obtained in downscaled models into the 1:1 scale. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Countercurrent Flow Limitation (CCFL) is one of the important two-phase flows phenomenon related to the safety of pressurized water reactors (PWRs) during SBLOCA accidents. The phenomenon plays an important role in several equipment’s of the chemical and mechanical industries. however CCFL of water and steam is of great importance for the safety analysis of nuclear reactors (Wintterle et al., 2008; Wang and Kondo, 1990; Navarro, 2005; Wongwises, ⇑ Corresponding author. E-mail addresses: [email protected] (S. Al Issa), [email protected]. de (R. Macián-Juan). http://dx.doi.org/10.1016/j.anucene.2017.01.021 0306-4549/Ó 2017 Elsevier Ltd. All rights reserved.

1996; Ohnuki, 1986; Jeong, 2002; Minami et al., 2010a; Kim and No, 2002) CCFL can occur in the hot-leg of a pressurized water reactor PWR during SBLOCA accidents (Small-Break Loss of Coolant Accidents), and in the event of loss of residual heat removal system (loss of RHR). During Three Mile Island (TMI) accident in 1979, no coolant flowed from the pressurizer to the primary circuit during the accident at Unit 2 due to CCFL. During CCFL occurrence inside the hot-leg, the supply of cooling water into the reactor core by reflux condensation is limited partially or totally (Navarro, 2005; Wongwises, 1996; Jeong, 2002; Gargallo et al., 2005; Deendarlianto et al., 2011; Ohnuki et al., 1988). Few investigations were held at large pipe geometry with a detailed identification of the flow pattern and the liquid interface

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structure during the occurrence of CCFL. Most experimental investigations were carried out either at small channel diameter ðD < 80 mmÞ, or in a large dimensions but with a rectangular cross section and narrow channel width (non-pipe geometry, see Table 1). Therefore, COLLIDER test facility was built with a hotleg pipe geometry and a large diameter of 190 mm, yielding a scale of 1/3.9 of a PWR geometry. In our previous studies (Al Issa and Macian, 2011, 2014), the investigations emphasized on the mechanisms and flow transitions that lead to the onset of CCFL and some CFD validations were conducted (Al Issa and Macian, 2016) during these transients. Two main mechanisms of CCFL were identified: Bend-CCFL and ADE-CCFL (ADE = Active Droplets Entrainment). Bend-CCFL occurs at the bend and is characterized by large rollwaves, high gas/liquid mixing, and a sporadic two-phase flow plugging at medium water inlet velocities. ADE-CCFL occurs at the entrance of the steam generator, is mainly caused by intensive droplets entrainment, and it initiates ahead of the Bend-CCFL (For detailed description see (Al Issa and Macian, 2014, 2016). However an important aspect of the CCFL phenomenon is to determine the relation between the water delivery and the gas velocity after the onset of Bend-CCFL, or what is known as CCFL characteristics, or partial delivery curve (for a graphical illustration see Fig. 6). Table 1 shows available studies that provided CCFL characteristics data in a hot-leg geometry with close geometrical parameters ðL=D; I=D; hÞ to the 1:1 scale. Most studies emphasized upon measurement of the CCFL characteristics curve, and some provided high-quality visualization of the gas/liquid interface during the CCFL occurrence. Most of the provided visualizations were selected individual images at some moment during CCFL occurrence. CCFL is known to have a very complex interface comprising a lot of details: large roll-waves that breaks into the steam generator, bubbles within the recirculation zone at the bend, gas/liquid foamy mixture, and droplets blown into the steam generator. With the increasing interest for CFD(Computational Fluid Dynamics) simulations of the CCFL phenomenon, there is a need for a reliable and well-defined validation data. Most of the CFD simulations are steady-state calculations considering that points along the CCFL curves occurs at constant inlet flow parameters (Minami et al., 2010b; Höhne and Deendarlianto, 2011; Utanohara et al., 2012, 2011; Kinoshita et al., 2014). This raises the necessity for a representative interface distribution at numerous experimental points along the CCFL characteristics curve. Individual images can be less representative due to the chaotic and fast changing behavior of the interface. Additionally, it is well known that most CFD steady state simulations suffer from pressure difference fluctuations, due to the fact that the actual interface cannot be reproduced precisely which is, along with the associated interfacial area concentration, the main factor that affect the resulting pressure drop along the hot-leg.

This led to the idea of performing extensive measurements of the CCFL characteristics at the COLLIDER test facility along with the calculation of time-averaged interface distributions that can be more representative of the gas/liquid interface and the occurring restriction at the bend. The aim is to enrich the available data base over this phenomenon with a matrix of validation data at a facility that has a hot-leg geometry and a relatively large scale (See Table 1).

2. Experimental set-up 2.1. COOLIDER test facility Experimental investigations were carried out at the COLLIDER test facility (Fig. 1) under atmospheric pressure and room temperature using distilled water and filtered air. For a precise measurement of the physical parameters (and especially the air density), air and water temperatures were measured in each case. Wallis parameters were applied for air and water velocities, and these are defined as follows:

J 0:5 G;L

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u qG;L ¼ tJ G;L gDðqL  qG Þ

ð1Þ

where: Jk = superficial velocity of phase k, qk = Density of phase k, D = Channel’s diameter, g = Gravity’s acceleration, and k:G = Gas, L = Liquid. The geometrical parameters of the hot-leg were similar to those of a PWR’s hot-leg: L/D = 9.47, I/D = 1.87, and h = 50° with D = 190 mm. Based on previous investigations, Five inlet water velocities were selected for the investigation of CCFL characteristics. These values cover a range of J0:5 w;in ¼ 0:085—0:305, which is the most important range for LOCA-related CCFL studies. These values represent different regions of the onset of CCFL as recognized in previous articles (Al Issa and Macian, 2014). A detailed description of these regions and mechanisms of the onset of CCFL can be found in Al Issa and Macian (2014, 2016). Fig. 2 shows a cross section of the COLLIDER test facility along with the applied measuring devices. A centrifugal pump provides the water flow and a radial blower provides the air flow. The pump’s and the blower’s rotation speed were electrically controlled using frequency inverters that allow to change the frequency in steps as low as 0.1 Hz. Table 2 shows the measuring devices, and their measuring error. For low water inlet velocities ðV_ < 0:7l=sÞ the measurements of the flow meter were calibrated using the slope of the rising water level in the reactor vessel at a constant opening of the control valve for a sufficient period of time.

Table 1 CCFL investigations in PWR hot-leg geometry. Authors

Diameter D [mm]

L/D**

I/D**

Riser Angle h

System Pressure [bar]

Ohnuki et al. (1988) UPTF (Mayinger et al., 1993) MHYRESA (Geffraye et al., 1995) Navarro (2005) Deendarlianto et al. (2011) Minami et al. (2008) COLLIDER (Al Issa and Macian, 2014) Minami et al. (2010)

25.4 750 351 36|54 250  50* 150  10* 190 50

10.24 9.60 7.53 9.26|9.72 8.48 8.27 9.47 8.4

2.31 1.69 3.02 1.85|2.78 0.92 1.17 1.87 1.2

50 50 50 50 50 50 50 50

1 3|15 1 1 3|1.5 1 1 1

L = Length of the horizontal part, I = Length of the riser, D = channels diameter. * Rectangular channel dimensions: height  width. ** Values may be different in several references due to the starting measuring point of I and L weather at the bend’s edge or center.

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Fig. 1. Cross section of COLLIDER showing the water and air flow directions in addition to the applied measuring devices (Al Issa and Macian, 2014).

Fig. 2. Illustration of the measured: water inlet velocity Jw;in , The water delivery velocity J D , and the backflow velocity J B during the occurrence of a Bend-CCFL or ADE-CCFL.

Fig. 3. A variety of raw images during the onset of bend-CCFL at J0:5 ¼ 0:305 at a constant inlet air velocity. w

Fig. 4. Raw and processed image during the onset of bend-CCFL at J0:5 w;in ¼ 0:305. The arrow shows the recognized interface through the junction glass, the detail is not recognizable in raw image (left).

2.2. Experimental procedure Fig. 2 illustrates the measured flow velocities after the onset of CCFL (Bend-CCFL, or ADE-CCFL). The water inlet flow rate  J w;in is measured directly via the flow meter. The water level within the reactor vessel simulator represents the balance between the water inlet velocity J w;in and the delivered velocity into the reactor vessel

through the hot-leg J D and is equal to the backflow water velocity J B as follows:

J D  J w;in ¼ J B

ð2Þ

The water level is constant when no CCFL occurs ðJB ¼ 0Þ, and decreases when CCFL starts to occur ðJD  Jw;in < 0Þ. The experi-

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285

Fig. 5. Middle: time-averaged interface distribution obtained during the onset of Bend-CCFL at J0:5 ¼ 0:305. Left + right: raw images for comparison. w

Fig. 6. Illustration of the course of the CCFL characteristics experiments. Curves of the onset of Bend-CCFL and ADE-CCFL are also shown. The black vertical line show the inlet J0:5 w;in in ADE-CCFL cases as a reference.

Table 2 Applied measuring devices. Parameter

Device

Error

Notes

Temperature Water velocity

Pt 100 Ultrasonic

±0.5 °C ±0.5%

–

Air velocity

Pitot Tube

±1.1%

Water level in RVs Pressure difference between RVsSGs

Capacitive ceramic absolute pressure transducers Capacitive metallic differential pressure transducer

±0.36 mm ±(0.5– 7.15%)

mental procedure for obtaining the CCFL characteristics curve went according to the following procedure:  The desired water inlet value J0:5 w;in was set using the control system of COLLIDER.  The blower rotation speed was increased gradually by small steps. This changes the driving pressure DP between the reactor vessel simulator and the steam generator simulator.  After a new rotation speed is set, a sufficient time is given to allow all flow parameters to reach a constant level (or quasiconstant), this includes: o The water inlet velocity Jw;in . o The air inlet velocity Ja . o The pressure difference DP. o The slope of the water decrease in the reactor vessel simulator.  If no Bend-CCFL occurs, the blower rotation speed is increased by an additional step.  After the onset of CCFL, the blower rotation speed will be further increased to obtain measuring points as close to the zero liquid penetration point (ZLP) as possible (Measurements at the ZLP itself are avoided).  After the measurement at the maximum rotation speed is carried out, the rotation speed is decreased in a reverse order back

With calibration at low flow rates (J0:5 < 0:135) L A high-precision and pre-calibrated transducers are implemented Two transducers are applied, one in air and one in water Error dependent upon pressure difference value

to the lowest value and measurements along the CCFLcharacteristics curve are obtained in the decreasing direction of the air velocity. As a result of the experimental observations during CCFLcharacteristics at COLLIDER, it was found that it is not recommended to approach the zero liquid penetration limit (ZLP) very closely. The reason is that as the ZLP is closely approached (or reached), the entire water flow into the hot-leg is blown back to the steam generator. If a separation wall is adopted within the steam generator, as in COLLIDER set-up and most downscaled facilities, the water will be drained out of the hot-leg. This will lead over time into a continuous decrease of the water inventory within the hot-leg itself, and consequently into a decrease of the restriction caused by the water accumulation and the complex interface within the hot-leg. The air velocity will start to increase yielding an inaccurate (Overestimated) measurement of the air velocity at the ZLP. This can be noticed in some previous experimental investigations from the large scattering in the measurements of the CCFLcharacteristics curve at the ZLP limit. As such, and for a precise measurement of the ZLP value, it is recommended to approach the ZLP, but without reaching a very low J D  0 or J D ¼ 0. A more precise value of the ZLP can be deduced by the extrapolation of the CCFL-characteristics near the ZLP limit.

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3. Time-averaged air/water interface distributions The motivation to produce time-averaged plots of the air/water interface for the CCFL characteristics experiments at different cases originates out of different reasons: 1. The individual acquisitions by the high speed camera recordings are usually obtained at several hundred frames per second. The individual images introduce different possible distributions of the same case during quasi steady state flow conditions that can take place after the onset of CCFL. Despite that this is natural, as the mechanism of CCFL includes the breaking of large roll-waves into the steam generator and a very complex interface, this situation leads to a difficulty in the determination of: the degree of restriction among different cases at different flow conditions, a representative distribution of the quasi steady state case, and the relative importance of different flow details that emerge during each case. Fig. 3 shows different arbitrarily selected images during the onset of CCFL at J 0:5 w;in ¼ 0:305 over 1 s time. As it can be clearly seen, the wavy water level within the horizontal part fluctuates considerably. Also, the mixing degree, the formation/distribution of the air/ water foamy mixture, and the amount of blown mixture into the steam generator are quite different and cannot be used for a comparison against another case or another similar result in a different study/test facility. 2. There is an increased interest in performing CFD simulations and validation of CCFL. Most calculations are performed as steady state calculations at fixed inlet flow parameters. Although the comparison with individual images taken at relatively high frame rate can be used for a general quantitative comparison, but it cannot be ideal specially when comparing different cases. It is also impossible to reproduce the instantaneous gas/liquid distribution through the calculations (Despite that matches can occur during transient calculations) due the chaotic nature of the interface. The time-averaged distributions can provide an alternative to the individual images along with the averaged values of the inlet gas/liquid velocities and the pressure difference. The latter was found to be problematic in CFD calculations as it fluctuates a lot even in steady state simulations. 3. In experiments with a circular geometry cross section, the air/ water interface is usually acquired by a side-view projection of the existing interface illuminated by a backlight. Unlike narrow rectangular channels, this leads into an increased degree of complexity of the obtained distribution due to the additional distribution over the cross sectional area. The time-averaging helps to improve the resolution of this issue. The identification of the actual air/water interface is necessary ahead of performing the averaging process. The raw images processing includes the removing of the background and the facility structure. Several image-processing techniques should be applied in order to achieve this goal:  Recognition of the actual interface in reference to an image of the empty hot-leg: this may eliminate large percentage of facility structure and background, however since the flow conditions during CCFL characteristics are quite turbulent, the structure vibrates within the acquired images and complicate subtraction of the background at critical positions (Such as the junction between the hot-leg and the steam generator) due to the existence of many small parts such as nuts, screws, flanges etc.  Elimination of the background by applying a dynamic intensity limit: this will remove the remaining background intensity that

does not represent the interface, the limit should be optimized for each set of acquisitions (As lightening effects may differ). The dynamic limit consider the changing minimum and maximum in each single image and thus can be constant for a whole set of experiments.  According to the result in each case, a low-pass filter is additionally applied for enhancing the removal of the structure from the raw images: this step solves the issue of vibrating structure which may complicate step number 1. By applying such a filter vibrations in the range of 2  3 pixels can be well resolved. Fig. 4 shows an example of this treatment. The background was well removed, and the applied treatment allowed the recognition of the interface even to a some extent through the glass wall of the steam generator (Which was made as free from screws in COLLIDER design for this purpose). The continuity of the obtained interface through this area will be more recognizable in the time-averaged images shown in next sections. After the determination of the suitable processing parameters in each case, a time average of all the images is performed. The images for obtaining the time-average are selected so that all repetitive flow details that are observed in each case are included. In principle, the selected images cover the approach and the breakup of one large wave into the steam generator starting from its formation within the horizontal part of the hot-leg. As the water inlet velocity increases, and due to the increased restriction and air/water mixing at the bend, the high air velocity occurring there leads to a secondary blockage caused by the suction of the air/ water mixture formed during the primary blockage (rather than a growing wave along the horizontal pipe’s axis, for details see (Al Issa and Macian, 2014). In these cases, the selected time period includes these secondary waves/blows as it is a part of the CCFL repetitive characteristics. Fig. 5 shows the calculated average for J 0:5 ¼ 0:305 at the maxw imum rotational speed. Detailed flow parameters are shown in next sections. Two raw images are shown to the left and the right for comparison. The resulting grey scale is proportional to the mixing degree of the air/water. A darker intensity is observed within the mixing regions, while a light intensity is observed in the ‘‘pure liquid” and ‘‘pure air” regions. Consequently, the grey scale gives an indication -rather quantitative- of the air/water interface concentration. A more detailed discussion of the averaging results are found in next sections. 4. Results 4.1. CCFL characteristics curve Fig. 6 shows the course of all conducted experiments in 0:5 ðJ0:5 Þ plot. At low J0:5 values ðJ0:5 0:45Þ the J0:5 is equal to D ; Ja a a d

¼ J0:5 the water inlet value J0:5 D w;in (point A). The curve of the onset of Bend-CCFL, and the onset of ADE-CCFL are also shown. An example of the experiment run at J0:5 w;in ¼ 0:185 is highlighted. After the falls into a decreased value at onset of Bend-CCFL (point B), J0:5 d the CCFL-characteristics curve (point C). Several measurements are then made along the curve in the increase/decrease directions (C ? D ? C). The plot clearly shows why the experiments were carried out using the control of the blower rotation speed instead of the input air velocity. Using a stepwise increase of the air velocity, a large portion of the CCFL characteristics curve will be overpassed after the onset of Bend-CCFL, and J0:5 will jump directly D into a low water delivery value (point D). Measurements along the (D ? C) section will be then obtained only during the decrease of the air velocity. Additionally, the phenomenon is driven by pressure difference during accidental conditions and no control of the

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flow velocity is imposed. The onset of a hydraulic jump(HJ) that

0.7

occurs ahead of the Bend-CCFL at low J0:5 w;in values is indicated by

0.65

a temporarily decrease of J0:5 D . The decrease of the water delivery cause by the ADE-CCFL mechanism is also noticeable after the onset of ADE-CCFL (Example E ? F). ZLP point was not measured ¼ 0:62  0:63 directly. It was extrapolated and estimated at J 0:5 a in COLLIDER.

● ● __ __

0.6 0.55 0.5 0.45 0.4

4.1.1. COLLIDER results Fig. 7 shows the CCFL characteristics curve obtained at COLLIDER test facility out of several experiments performed at different 0:5 water inlet velocities with J 0:5 w;in ¼ 0:105  0:305 and DJ w;in ¼ 0:01. The plot shows the relation between the exiting air velocity and the water delivery into the reactor vessel (see Fig. 2). The test procedure went according to the description given in Section 2.2. Again, all measurements were performed after the onset of BendCCFL (ADE-CCFL results are not included since they correspond to a totally different mechanism and follow a different curve), and after all flow parameters (including the decrease rate of the water level in the reactor vessel simulator) reached a stable quasiconstant value. No differences were observed between data obtained during the increase/decrease of the air velocity at the same flow parameters. Measurements which are very close to

the zero liquid penetration ðJ 0:5 ¼ 0Þ are avoided as explained in D Section 2.2. The CCFL characteristics curve fits a second order curve as Fig. 7 shows It deviates from being a linear correlation between 0:5 , especially near low J 0:5 values ðJ 0:5 < 0:1Þ and near medJ 0:5 D ; Ja D D

ðJ 0:5 D

> 0:25Þ. In order to fit the entire curve’s range ium values using linear correlations, the curve should be divided into low and medium ranges as Fig. 8 shows (in red and blue colors). The resulting range-adapted linear curves differ from the entire-range linear curve in Fig. 7, and specially the slope of the line. This shows that the experimentally obtained linear CCFL-correlations fits are strongly affected by the range of covered J 0:5 values in experiD ments. Consequently, the presented correlation in this section can be written as follows: circular For L=D ¼ 9:47; I=D ¼ 1:87; h ¼ 50 ; D ¼ 190 ½mm, cross-section, and smooth-rounded water entrance (Fig. 7): Second-order fit for full range of J 0:5 D : 2

0:5 J 0:5 ¼ 0:75ðJ 0:5 þ 0:62 0:02 < J 0:5 < 0:3 a D Þ  0:47J D D

Linear fit for full range of

J 0:5 a

¼

0:71J 0:5 D

ð3Þ

J 0:5 D :

þ 0:63 0:02 <

J 0:5 D

< 0:3

ð4Þ

0.35

COLLIDER CCFL characteristics line

0.3 0

0.05

0.1

0.15

0.2

0.25

0.3

Fig. 8. CCFL characteristics curve fit by linear correlations at low (Blue), and medium values (Red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

J 0:5 ¼ 0:59J 0:5 þ 0:63 Low J 0:5 : 0:02 < J 0:5 < 0:15 a D D D

ð5Þ

¼ 0:79J 0:5 þ 0:65 Medium J 0:5 : 0:15 < J 0:5 < 0:3 J 0:5 a D D D

ð6Þ

Fig. 9 shows the relative prediction error of the above correlations. If a range-adapted linear curves are adopted, the maximum prediction error remains similar to the second order correlation at < 1%. 4.1.2. A comparison with previous studies Fig. 10 shows a comparison among different data of the CCFL characteristics that represent a hot-leg geometry with a bend angle of 50°. Table 3 shows the references for used data. Data from a full

Prediction Error of CCFL correlations [%]

2 1 0 -1 -2 -3 -4 -5 -6 -7

Quadratic fit error Entire-range linear fit

Range-adapted linear fit

0

0.1

0.2

0.3

0.4

Fig. 9. Prediction error of suggested CCFL characteristics correlations against experimental data.

Or range-adapted linear fits (Fig. 8): Two linear fits for low ð0:02 < J 0:5 < 0:15Þ and medium ð0:15 < J 0:5 < 0:3Þ ranges: D D

0.7

UPTF 0.3MPa Dh0.65 UPTF 0.3MPa D0.75 COLLIDER D0.19 Minami D0.05 HZDR 0.3MPa H0.25 Ohnuki D0.0254 Navarro D0.054

0.7

0.65 0.6

UPTF 1.5MPa Dh0.65 UPTF 1.5MPa D0.75 Geffray D0.351 Minami H0.15 HZDR 0.15MPa H0.25 Ohnuki D0.0254+Hutze Navarro D0.036

0.6

0.55 0.5

0.5 0.45

0.4

0.4 0.35

COLLIDER CCFL characteriscs line

0.3

0.3

0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.1

0.2

0.3

0.4

0.35

Fig. 7. CCFL characteristics curve obtained from COLLIDER: Black line: linear fit, red line: second-order fit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 10. A comparison of CCFL characteristics in a hot-leg geometry from different authors. Legend: D denotes a pipe geometry followed by the diameter [m], H: denotes a rectangular channel followed by its height [m]. Dh: the hydraulic diameter was applied.

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scale geometry (UPTF data, Mayinger et al. (1993)) are shown as a reference, and these are plotted as continuous lines for 1.5 MPa and 0.3 Mpa pressures. Data at each pressure are presented twice due to the fact that UPTF facility has a Hutze pipe inside the hotleg. Consequently, the applied channel diameter was once considered to be the hot-leg pipe without the Hutze ðD ¼ 0:75 ½mÞ, and another the hydraulic diameter in the Hutze region ðDh ¼ 0:65 ½mÞ. As it can be seen, most data in downscaled experiments agree better with the UPTF data at 1.5 MPa, and are closer to the data with D ¼ 0:75 ½m (Not Dh ¼ 0:65 ½m). There exist no clear explanation of this notation until the Hutze effect is experimentally clarified. UPTF data near ZLP tends to leave the general trend and jump into higher air velocities values (A possible explanation of this behavior is given in Section 2.2). Data from Minami et al. (2008a) with a rectangular channel of 150 [mm] height and 10 [mm] width are remarkably lower than other data. This indicates the implication of a very narrow channel upon results. The same applies for data from Ohnuki et al. (1988) with a channel of 25.4 [mm] diameter (with and without a Hutze). The influence of a very small diameter upon CCFL characteristics data was mentioned by Murase et al. (2012), who indicates that results for D > 0.05 [m] become more consistent. Data of Minami et al. (2010a) using a 50 [mm] channel are within the range of data from higher diameters, however the trend is not clear near low liquid delivery values as the number of the available data points do not allow to make a reliable extrapolation back to J 0:5 ¼ 0. Data from D HZDR (At 0.15 MPa and 0.3 MPa in a 250  50[mm] rectangular channel, Vallee’ et al. (2011)) show a lot of scatter near low < 0:05 values. Several different measurements at the ZLP J 0:5 D exists, especially from the 0.15 MPa results. The data do not cover

transfer will still be affected by the rectangular non-pipe geometry). A substitution of the channel height- as a characteristic length- by another parameter should be considered. The hydraulic diameter based on the area and perimeter of the channel can be a good substitution (Such a consideration will place Minami H0.15 data at an equivalent Dh ¼ 0:019 ½mm, which agrees well with Minami H0.15 data proximity to the Ohnuki D0.0254 ones and the deviation of both from other data). 2. The measuring range should cover a wide range of J 0:5 and speD < 0:1, and cially where it deviates from linearity: J 0:5 D J 0:5 > 0:25. Sufficient number of measuring points should exist. D 3. Measurements at the ZLP should be avoided (See Section 2.2), however more measuring points are required in its proximity to obtain a reliable extrapolation estimate of the ZLP value.

4.2. Time averaged interface distributions 4.2.1. Insufficiency of inlet flow parameters to identify CCFL characteristics A number of CCFL-characteristics measurements were selected to calculate the corresponding time-averaged interface distributions. Since a limited number of results can be shown, only cases in the decrease direction of the air velocity are presented starting from the maximum rotation speed and downwards. Fig. 11, Left shows the selected cases in (Rotation speed, J 0:5 ) domain. They a are grouped according to the inlet water velocity J 0:5 w;in . As it can be noticed, there exist a set of cases (Framed by a rectangular

> 0:15 (One point exist though at J 0:5 ¼ 0:2). The trend of most J 0:5 D D data follows the second order trend. An overall judgement of the

0.7

range and trend requires dense measurements over a wide J 0:5 D specially in the regions where the deviation from linearity occurs

0.6

(J 0:5 D

0.155

J 0:5 D

< 0:1, and > 0:25, see previous section). Data from Geffraye et al. (1995) as well as UPTF data does not seem to follow a second-order trend over the entire data range. there is no enough UPTF data points to make a solid judgement of the overall trend. The comparison in Fig. 10 shows that: 1. Regarding the CCFL characteristics curve after the onset of Bend-CCFL: The adoption of a hot-leg geometry with D 50 [mm] or a rectangular channel with a sufficiently large width is necessary (other CCFL characteristics like the distribution of the interface and its effect upon the interfacial momentum

0.105

0.205 0.255

0.5

0.305 0.4 0.3 0

0.1

0.2

0.3

0.4

Fig. 12. Measuring points used to show the time-averaged interface distributions in J0:5 ; J0:5 domain. Legend: Value indicates the inlet water velocity in Wallis d a parameters J0:5 w;in .

Table 3 References for data in Fig. 9. Data Legend References

Geffray D0.351 Geffraye et al. (1995)

UPTF Dh0.65/D0.75 Mayinger et al. (1993)

Minami D0.05 Minami et al. (2010a)

0.7

0.105 0.155 0.205 0.255 0.305

0.6 0.5 0.4

Minami H0.15 Minami et al. (2008a)

HZDR H0.25 Vallee’ et al. (2011)

0.7

0.105

0.6

0.155

Ohnuki D0.0254 Ohnuki et al. (1988)

0.205 0.5

0.255

0.4

0.305

0.3

0.3

30

40

50

60

70

80

90

0

5

10

15

20

Fig. 11. Measuring points used to show the time-averaged interface distributions in this work. Left: a plot against the blower percentage rotation speed, Right a plot against the pressure difference. Legend: Value indicates the inlet water velocity in Wallis parameters J 0:5 w;in .

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red box) where the rotation speed is decreased but the inlet flow 0:5 (and even J 0:5 ) values. parameters are almost constant: J 0:5 w;in ; J a d However, these cases are in fact different and this can be seen from

Fig. 11, right which shows a plot of the same cases in ðDP; J 0:5 Þ a domain. These cases have apparently different pressure difference values and next section will show that different interface distributions exist for each of them. These are cases that show a very interesting feature of CCFL characteristics: The CCFL characteristics cannot be determined by picking up the inlet flow parameters and a CCFL-correlation as in 1D codes, regardless of how sophisticated the CCFL-correlation is, and how broad the coverage of the drag coefficient is. The central point about understanding the CCFL characteristics is that only two-phase flows which: 1. Consider the transitional change of the spatial distribution of the gas/liquid interface. 2. Combine a very well-formulated and validated drag (momentum-transfer) coefficient. 3. Consider the flow history (the exact preceding interface distribution). Can accurately predict the CCFL status (interface distribution, water delivery flow rate, and pressure difference) at a specific set 0:5 ðJ0:5 Þ. w;in ; Ja

of inlet parameters 1D codes are thus insufficient to predict CCFL characteristics, even if the results of the CCFL curves (Onset of CCFL, CCFL-characteristics curve) are somehow well pre-

289

dicted. The point here is that the course of CCFL occurrence and the change of characteristics during an accident/transient will be different from the validation against systematic CCFL experiments that provide CCFL description according to a fixed set of input flow parameters. As such, the validation of CCFL during transitions using CFD codes becomes of a special importance.

4.2.2. Time-averaged interface plots Fig. 12 shows the selected points for the calculation of the timeaveraged interface distributions along the CCFL-Characteristics line in ðJ 0:5 ; J 0:5 Þ domain. The points that have equal input parameters d a but different interface distribution and pressure difference are encircled. As it can be seen, it is difficult to identify these points when all measured pints are merged into one CCFLcharacteristics curve. The selected points are arranged into five groups according to the inlet water velocity and they cover the entire CCFL characteristics curve shown in Fig. 7. The CCFL characteristics curve moves from the lowest water inlet velocity/highest air velocities (Left) at J 0:5 w;in ¼ 0:105 down to the highest water inlet velocity/lowest air velocity (Right) at J 0:5 w;in ¼ 0:305. Fig. 13 shows a matrix of the processed gas/liquid interface. Each row corresponds to an inlet water velocity ðJ 0:5 w;in Þ. Within each row, the images from left to right move along the corresponding CCFL characteristics curve shown in Fig. 12 from the highest air velocity down to the lowest one (in the same order shown in

Fig. 13. CCFL characteristics shown through a processed images picked up at the moment a large roll-wave reach its maximum restriction at the bend.

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Fig. 14. CCFL characteristics shown through time-averaged interface distribution over a period of a repetitive pattern of the flow.

Fig. 11). To provide a comparative view within this Figure, the processed images are selected at the point where the large roll-wave reach its maximum restriction at the bend. The value of the corresponding air superficial velocity, the pressure difference, and the water delivery flow rate (given in l/s for a more convenient display and easy comparison) are displayed within each image. Fig. 14 shows the time-averaged interface distributions that correspond to the same matrix in Fig. 13. The time period that was used to calculate the time-average in Fig. 14 is selected so that it covers the observed repetitive behavior of the interface in each case. The total number of the images used for the calculation varied as such from 100 to 700 images (all cases were acquired at 500 frame/s). The comparison between Figs. 13 and 14 shows the following:  Individual images of the interface cannot be used as an accurate representative for: the degree of restriction, distribution of the interface, the average water surface in the horizontal part, the amount of mixing at the bend, and the degree of entrainment at the entrance of the steam generator. This incapability becomes more emphasized if a side projection of the interface in a pipe geometry was applied (It remains also true for a rectangular cross section regarding the distribution’s instantaneous variation along the channel’s axial direction).  The calculated grey scale within the hot-leg represents the limit between the air and water and the degree of mixing rather than a void fraction. The lower intensity regions indicate a region

where higher mixing occurs (Higher formation of foam, waves’ crest or droplets/bubbles).  Within the horizontal part: It provides a good representation of the average water level, the restriction degree at the bend, and a plausible quantification of the two-phase mixing degree within the hot-leg.  Within the steam generator. The grey scale indicates the average water level at the inlet, and the degree of the droplets entrainment despite being less accurate than representation of mixing degree in the hot-leg. The reason is that droplets give a wide variety of the resulting grey scale depending on the shape/light reflection off the droplets’ surface. Additionally, the size of the steam generator is remarkably larger than the hot-leg in the direction perpendicular to the images, this may result in an overestimation/underestimation of the degree of entrainment in some cases. However the trend of the calculations in most obtained distributions agree well with the trend of entrainment degree observed in experiments.  The plots along with provided flow parameters offer a better material for the validation of steady-state CFD calculations compared to selected individual images, where the observed pattern can vary a lot from one image to another within the repetition cycle. Most important application is the usage as a comparative reference among different cases at different J 0:5 w;in values. The cases that have similar inlet flow parameters, but different interface distribution and pressure difference can be recognized in

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the last two columns of the matrix of Fig. 14. The air velocity and the water delivery rate in these cases are sometimes identical. The difference occurs between the interface distribution and the pressure difference and thus transitions that start from the flow condition in column 3 will be quite different from those starting from column 4 in either a decreased/increased air flow rate and they lead into a different result. Experimentally, the transition between cases in columns 3 and 4 was achieved through the change of the driving pressure (rotation speed) rather than the air input velocity, and thus we expect this case to occur in real application as the pressure difference is the driving force for the air flow within the hot-leg. The grey scale in Fig. 14 is difficult to be recognized by eye due to the fine gradient of the intensity, and the dependency upon monitor handling of dark tones. In order to show a better representation of this value, the grey scale in Fig. 14 was converted into a rainbow color scale. The red represents the darkest grey value and the blue the white one. The results are shown in Fig. 15. The color scale of the interface provides additional insight into the obtained time-averages:  The continuity of the time-average through the structure and the plexi glass wall at the junction between the hot-leg and the steam generator can be easily noticed. This gives a good credibility to the applied processing algorithm.

 The red color (Darkest value of the grey scale) occurs in regions of a high mixing. This is more concentrated exactly in the region where the large roll-waves break and a lot of air/water mixture foam is formed.  The upper yellow color zone (closer to the upper channel wall) is close to the average air/water surface in the horizontal part of the channel as well as in the steam generator.  The upper green color zone coincides with the maximum reach of the air/water two-phase mixture. Within the hot-leg it represents the edge of waves crests, and at the bend it represents the maximum restriction that occurs in each case. As an example one

can

compare

the

first

case

of

J0:5 w;in ¼ 0:255

row

ðJ a ¼ 11:1 m s1 Þ and the first case of J0:5 w;in ¼ 0:305 row ðJ a ¼ 9:9 m s1 ). The red color zone seems to be similar, however we know from optical observations (video) of both cases, that the CCFL is more intense in the second case of J0:5 w;in ¼ 0:305. Looking at the green color one can see that the restriction in the second case is much higher and it covers almost the entire bend region starting from its junction to the horizontal part. It touches the upper limit of the channel all over the bend, meanwhile it does not reach the upper edge of the channel in the J0:5 w;in ¼ 0:255 case, and it does not start at the entrance of the bend (from the hot-leg side).

Fig. 15. Colored representation of the grey-scale in the time-averaged interface distributions shown in Fig. 14.

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 The difference between column 3 and column 4 (where the 0:5 ) becomes more proinput flow parameters are equal: J 0:5 w;in ; J a

exists in both nounced and clear. Despite the fact that equal J0:5 D cases, they are quite different: The increase of the air velocity starting from the column-3-case can lead into an intense CCFL, meanwhile the increase starting from the column-4-case can lead into the column-3-case where no difference of the water delivery exist. Despite that only 2 cases of this phenomenon are shown for each J0:5 w;in value in this study, this behavior of increasing/decreasing the driving pressure difference while maintaining identical inlet parameters can occur over a wider range in reality. This behavior also explains why different experiments with different blowers/air source can deliver different interface distributions at the same inlet parameters. For instance, if two blowers with different characteristics (volume flow rate against pressure difference) were used, two results may occur at the same inlet flow rates if the two blowers provides different pressure differences at the tested air flow rate. This is also an important point of understanding CCFL characteristics among different experimental works.

5. Conclusions The CCFL characteristics were experimentally measured at COLLIDER test facility within a PWR hot-leg pipe geometry and a relatively large diameter. Correlations that fit the experimental data were introduced and discussed. The data obtained in this work covered a range of 0:105 < J0:5 w;in < 0:305, and a water delivery range < 0:3. The comparison against previous experimenof 0:02 < J0:5 D tal data including the UPTF (1/1 scale) showed the dependency of the final correlation upon the investigated range of water inlet velocity, the necessity to obtain enough data at low and medium delivery flow rates, and to avoid measurements at the zero liquid penetration limit. The comparison confirmed previous observations that CCFL characteristics after the occurrence of Bend-CCFL are similar for channels of a hydraulic diameter 50 mm and similar geometrical parameters to the 1:1 scale geometry (L/D, I/D, and an inclination angle of 50°). The characteristic length for rectangular channels should be changed from the channel’s height (as widely used) into the channel hydraulic diameter. Time-averaged plots of the interface distribution were calculated from processed images representing the repetitive behavior of the interface during CCFL occurrence for a matrix of selected cases. The obtained time-averages proved to be a useful method of presenting the CCFL-characteristics that: bypass the individuality of single images, shows the average repetitive behavior of the chaotic interface, and emphasize the most important details as: the average water surface in horizontal part, the degree of restriction at the high mixing regions at the bend, and the degree of entrainment in the steam generator. These plots are useful to compare among cases from different experiments. The article showed that the frequently used inlet parameters 0:5 ðJ0:5 Þ are not sufficient to identify a CCFL case along the CCFL w;in ; Ja characteristics line and the interfacial distribution for each point is required. The history of the flow including: the interface spatial distribution of the gas/liquid interface, and the flow parameters must be known in order to predict the next status of CCFL when moving along the CCFL characteristics curve. The applied method of controlling the rotation speed instead of a stepwise increase of the input air velocity was crucial to bypass the effect of blower’s characteristics and understand this important aspect of CCFL characteristics. This shows also the complex nature of this phenomenon, the need for a continuous application of advanced data

processing methods, and the importance of the spatial resolution of the gas/liquid interface within the hot-leg for the investigation of CCFL in general. Finally, this article provides an additional evidence that CCFL phenomenon, which can be encountered in SBLOCA accidents, cannot be handled accurately using 1D codes regardless of how sophisticated and wide-ranged the applied drag correlations are. Only a two-phase flow model that consider the spatial distribution of the interface is a reliable method of predicting CCFL characteristics. A practical solution can be seen through the application of a coupled 1D-3D/2D simulation in which the primary loop is modeled via 1D system code, while the hot-leg is modeled via a 2D/3D model.

Acknowledgments The Authors would like to thank assistants Dipl.-Ing. Andreas Hibler and Dipl.-Ing. Tyll Bodden for their technical support. The two-phase laboratory at technical university Munich was funded by a grant from E.ON. Kernkraft, Germany.

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