Experimental investigation on natural convection and thermal stratification of IRWST using PIV measurement

International Journal of Heat and Mass Transfer 136 (2019) 128–145

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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt

Experimental investigation on natural convection and thermal stratification of IRWST using PIV measurement Weian Du a, Yusheng Liu b, Hongsheng Yuan c, Shouxu Qiao a,⇑, Sichao Tan a,⇑ a

Fundamental Science on Nuclear Safety and Simulation Technology Laboratory, Harbin Engineering University, Harbin 150001, China Nuclear and Radiation Safety Center, Ministry of Environmental Protection, Beijing 100084, China c Science and Technology on Rector System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu 610213, China b

a r t i c l e

i n f o

Article history: Received 21 August 2018 Received in revised form 4 December 2018 Accepted 16 January 2019

Keywords: IRWST PRHR HX Natural convection Thermal stratification PIV measurement

a b s t r a c t Passive Residual Heat Removal Heat Exchanger (PRHR HX) immerged in the In-containment Refueling Water Storage Tank (IRWST) plays an important role in removing the core decay heat under non-Loss of Coolant Accident (LOCA) accident. In the initial stage of the Passive Residual Heat Removal System (PRHRS) activating, the natural convection is the main way to exchange heat between the secondary side of the PRHR HX with IRWST. However, it is almost impossible to acquire the thermohydraulic performance in IRWST since the influence of the natural convection and thermal stratification in IRWST happens simultaneously. In this paper, Hierarchical Two-Tiered Scaling (H2TS) method is employed to get scaling criteria based on control equations. Anda visualized water tank with five C-shape electrical heating tubes was scaled-down and applied to investigate the thermohydraulic performance for IRWST of CAP1400 Nuclear Power Plants (NPPs). Particle Image Velocimetry (PIV) measurement was used to investigate the evolution of the flow field of total twenty-five surfaces in three axial directions under three steady- and two variable-heating conditions. Meanwhile, three thermocouple bundles were adopted to acquire temperature data in three regions. The experimental results show that the temperature deviation in the same height of the IRWST is flattened by plenty of vortexes. As a result, local circulation and the temperature difference is less than 1 °C. While the thermal stratification along the vertical direction is obvious, the distribution of flow field and temperature demonstrate that a ‘‘dead zone” exists in bottom of the IRWST. Besides, a ‘‘thermal interface” region forms in the lower middle region as hot and cold fluid mixing. An obvious upwelling flow is observed near the PRHR HX heating rod bundle from flow fields of three axial directions. At the same time, the maximum height of the upwelling flow decreases with time increasing. On the one hand, the natural convection is induced by the thermal stratification. What is more, the thermal stratification will impair the scale and strength of natural convection, which will intensify the thermal stratification conversely. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The Passive Residual Heat Removal System has been applied widely in third generation advanced passive safety NPPs. PRHRS plays an important role in removing the decay heat and ensuring the core cooling. The C-shape PRHR HX in IRWST is an essential equipment of the PRHRS. It will be initiated under the condition of Station Black-out accident (SBO) or LOCAs, and it is designed to cooperate with other passive systems to mitigate the severe accident. When the non-LOCA accident happens, the heat transfer mechanism between secondary side of PRHR HX and IRWST will ⇑ Corresponding authors. Tel.: +86 451 82569655; fax: +86 451 82569655. E-mail addresses: [email protected] (S. Qiao), [email protected] (S. Tan). https://doi.org/10.1016/j.ijheatmasstransfer.2019.01.067 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.

change from natural convection to nucleate boiling and pool boiling eventually. Certain researchers have investigated the heat transfer coefficient between the secondary side of PRHR HX and IRWST under different conditions. Men [1] and Zhou et al. [2] investigated the heat transfer mechanism under different stages experimentally. Their results substantiate that the Dittus-Boelter correlation [3] is more suitable for natural convection inside PRHR HX tubes, and the McAdams correlation [4] agrees well with the experimental results for the natural convection outside the tubes. The boiling heat transfer calculation outside tubes can be evaluated more precisely by corrected Rohsenow correlation [5]. At the initial stage of the PRHRS operation, natural convection takes the dominant role in removing the residual heat transported by natural circulation between the PRHR HX and the reactor core. The fluid around the secondary of PRHR HX bundles will be heated,

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Nomenclature General symbols a thermal diffusivity, m2/s A area, m2 c Specific heat capacity, J /(kg
°C) d heating rod diameter, m D IRWST diameter, m H height, m l characteristic length, m M magnification factor, mm/pixels Q Heat, kJ m0 reference velocity, m/s X displacement of particle, mm

then hot fluid raises and accumulates at the top of the tank due to buoyancy force. The water along the height direction in IRWST mixes with each other weakly, since the high density of cold fluid impedes an efficient mixture with the upward fluid. The temperature gradient along the radial direction is small, so that a stable thermal stratification forms in IRWST [6]. The hot fluid cannot flow downwards under buoyancy effects. In such case, the natural convection will be constrained by the thermal stratification. The thermal stratification is intensified in turn. When the temperature difference between the wall of the PRHR HX rod bundle and the mainstream fluid is large enough, nucleate boiling happens. Plenty of small generating bubbles attach to the wall of C-shape tubes, and then weaken heat transfer capability severely. Therefore, it is significant to investigate the thermal hydraulic performance of IRWST for nuclear reactor safety. Many researchers have investigated the thermal hydraulic characteristic of PRHR HX in IRWST through experiment or numerical simulation. As for numerical study, many researchers adopt computational fluid dynamics (CFD) to simulate the flow pattern and thermal stratification in IRWST. Some researchers have coupled system codes such as RELAP5 to acquire heat exchange power of PRHR HX versus time, and applied three-dimensional fluid computing software like ANSYS to observe the flow field or the temperature distribution in IRWST [7]. Specially, full size PRHR HX and IRWST model is preferred to adopt by researchers in studying the flow pattern and temperature distribution since the numerical simulation is more efficient and economic as compared to experiments. Ge et al. [8] studied the three-dimensional thermalhydraulic transient performance of a full-size AP1000 IRWST and PRHR HX with FLUENT, using the porous media approach and the distributed resistance method. Three dimensional distributions of the fluid velocity and temperature in the IRWST were obtained and thermal stratification was observed. Particularly, ‘‘dead zone” near the bottom of the IRWST is identified and named to consider the low temperature and low velocity in that region. Xue et al. [9] studied the temperature distribution and flow field under nonsteady-state by numerical simulation. The study focuses on mutual influence between temperature difference and velocity field. However, bent tubes were used instead of the C-shape heat transfer tubes in his study, therefore the horizontal sections effect of PRHR HX were ignored. Kim [10] paid more attention to thermal stratification when considering the arrangement of the sparger. The results show that the geometry and distance of the sparger have little influence on thermal stratification. Some suggestions were proposed for optimization of the IRWST and PRHR HX. Jia et al. [11] numerically studied the thermal stratification and natural circulation of IRWST

Greek letters b Swelling coefficient, 1/°C m kinematic viscosity, m2/s q density, kg/m3 subscripts R ratio between the test facility and prototype rod heating rod irw IRWST P pressure PRH PRHR sat-nor from normal condition to saturation condition

using FLUENT. Different number of C-shape tubes and coolant inlet temperature of PRHR HX were considered in the simulation. However, a regular rectangular tank was used to simulate the IRWST and only eight and sixteen tubes were adopted respectively, which did not agree well with the prototype. Xia et al. [12] studied the detailed information of the temperature distribution and the flow fields by three-dimensional numerical simulation. The complex spiral movement existing near the bundle area was found. Besides, many generating vortexes intensify the heat transfer effect. The discussion above shows that many empirical correlations were embedded in general three-dimensional CFD software to match the natural convection and thermal stratification. However, some correlations which these three-dimensional softwares are based on are acquired from one-dimensional experiment model mostly, such that there may be some derivation between the simulated result and the experimental data. Therefore, it is essential to performed detailed experimental study to investigate the thermohydraulic of PRHR HX and IRWST. What is more, the simulative calculation models can be improved and optimized based on the detailed experimental data-base. The experimental researches can be divided into two types, integrated effect experiment and the mechanism experiment. The mechanism experiment paid more attention to local thermal hydraulics performance of PRHR HX bundle. The Westinghouse Electric Corporation [13] that first proposed the concept of the passive residual heat removal in 1990s applied it in advanced third-generation nuclear power plant – AP600 and carried out the famous ‘‘three tubes experiment” to validate the correlations and the heat transfer capability of PRHR HX in IRWST. However, the experiment neglected the impact of the horizontal sections which is important for heat transfer of natural convection. Lu and Zhang et al. [14,15] built a scaled-down PRHR HX and IRWST model to investigate the velocity field and thermal stratification phenomenon, using the local PIV visual measurement and thermocouples. Li et al. [16] studied the single-phase natural convection characteristic in a visual experiment, and fitted the Churchill-Chu empirical correlation to the experimental data. Zhou et al. [17] conducted the single heating tube experiment and analyzed the thermal stratification and its evolutional process. A model for heat transfers between the wall of the heating tube and the tank is proposed. The integrated effect experiments are usually conducted on integral test facilities. Ellis et al. [18] performed an overall scaled-down integrated effect experiment, APEX-600, to demonstrate the natural convection phenomenon, the mechanism of boiling heat transfer of PRHR HX, and the thermal stratification of IRWST. Meanwhile, JAERI [5] and Park et al. [19] developed an inte-

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Table 1 Dimensionless numbers of different positions in IRWST. Position of the fluid

Grashof number

Reynolds number

Prandtl number

Mainstream area Vertical section of C-shape tube Horizontal section 1 of C-shape tube Horizontal section 2 of C-shape tube

1.00 1.00 1.00

6.27E08 2.96E04 2.96E04

2.73E08 1.29E04 1.29E04

Near wall area Vertical section of C-shape tube Horizontal section 1 of C-shape tube Horizontal section 2 of C-shape tube

3.94E15 8.77E08 6.00E12

3.94E15 8.77E08 6.00E12

1.72E15 3.82E08 2.61E12

Table 2 Design ratio of different parameters of the test facility. Parameter

Design ratio

Diameter ratio of C-shape tube Ratio of height and length of C-shape tubes Scaled-down diameter of IRWST Ratio of working temperature Ratio of Heat transfer power of PRHR Ratio of Heat transfer power of C-shape tubes Number of C-shape tubes Tube form Tube spacing

1:1 1:5 1:5 1:1 1:125 1:5 1:25 Consistent Consistent

grated effect experimental test bench separately, to simulate the transient response of PRHR HX and IRWST in different prototype reactors. However, it is difficult to build a full-size experimental test bench due to the huge size of the prototype. According to the discussion above, CFD simulation or coupled CFD and system code simulation are widely used to study the temperature and flow fields in the full-size model or scaled-down model of IRWST and PRHR HX. Numerical simulation could demonstrate the most intuitive performance of the thermal stratification and fluid flow, but the reliability and accuracy of numerical simulation results are not guaranteed due to the lack of sufficient experimental data. Even though some integrated effect experiments have been carried out, attention was mainly focused on the heat removal capability or efficiency of the PRHR HX and. The mechanism experiments concentrate more on local temperature distributions and fluid flow. However, the temperature distribution and the fluid flow field are global performance.

In the present paper, a visual scaled-down IRWST with PRHR HX test facility is constructed based on the CAP1400 NPP prototype to investigate the thermal stratification and fluid flow field. Thermocouples and PIV are utilized to measure the temperature and velocity distribution in the test section. 2. Experimental set-up 2.1. Scaling analysis As mentioned above, what this paper focuses on is natural convection between the secondary of PRHR HX and IRWST. However, the IRWST prototype is an irregular shape tank with C-shape PRHR HX located at one side. It is hard to adopt an authentic IRWST and PRHR HX prototype for the experimental research, and it is also impossible to obtain overall velocity field by means of the PIV measurement. In such case, an overall scaled-down IRWST and PRHR HX experimental test facility based on the Hierarchical TwoTiered Scaling (H2TS) methodology [20,21] was conducted. According to the literature [22], the temperature difference is the driving force for thermal stratification and natural convection, so thermal stratification would agree well with scaling criteria when the natural convection similarity is considered as the primary scaling criteria. In this paper, only main steps for getting the scaling criteria are discussed. The dimensionless control equations (mass, momentum and energy conservation equations, respectively) for natural convection are as follows:

@uþ @ v þ þ ¼0 @xþ @yþ

ð1Þ

Fig. 1. Three-dimensional diagram of the experimental test bench. ① Simulative tank; ➁ C-shape electrical heating rods; ➂ Laser; ④ High-speed camera; ⑤ Electric regulator; ⑥ Data acquisition and computer.

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"

uþ

þ @uþ @ 2 uþ @ 2 uþ þ þ @u þ v ¼ P T þ P þ Gr Re @xþ @yþ @ðxþ Þ2 @ðyþ Þ2

uþ

" # @h @2h @2h þ @h þ v ¼ P þ Pr @xþ @yþ @ðxþ Þ2 @ðyþ Þ2

# ð2Þ

ð3Þ

According to the definition of the Grashof number, Reynolds number and Prandtl number, the dimensionless numbers in Eqs. (2) and (3) can be defined as following: The similar Reynolds number:

PRe ¼

m

lv 0

¼

1 Re

cosity is similar with that of natural convection, similar Grashof number, Reynolds number and Prandtl number need to be equal to each other, as shown in Eq. (8).

PGr;R  1

ð7Þ

PPr;R ¼ PGr;R ¼ PRe;R ¼

1 3

DT R lR

¼1

ð8Þ

ð4Þ

The similar Grashof number:

PGr ¼

bg DTl

v 20

¼

Gr Re2

ð5Þ

The similar Prandtl number:

PPr ¼

a 1 1 ¼ lv 0 Pr Re

ð6Þ

In order to guarantee similarity between the experimental test facility and prototype, the three similarity criterion numbers, which represent the ratio of fluid buoyancy, viscosity and thermophysical properties in fluid flow state under natural convection condition, should be the same individually. In different region of the tank, the fluid viscosity effect is different. Table 1 shows that the Grashof number is much larger than the Reynolds number and the Prandtl number in the tank where the fluid viscosity is small and the convection is dominant, and the Grashof number is constant. For the region where natural convection dominates the flow, the fluid viscosity is relatively small and similar Grashof number shown in Eq. (7) became the dominant scaling criterion that needs to be satisfied. For the region where the contribution of fluid visFig. 3. shooting planes of PIV measurement.

Fig. 2. C-shape tubes’ dimensions of three different shapes.

Table 3 Experimental conditions. Condition

Heating power

Initial temperature

Power change

Transition time

IRWST-1 IRWST-2 IRWST-3 IRWST-4 IRWST-5

8.53 kW 10.51 kW 12 kW 14.36–8.55 kW 10.218–8.55 kW

37 °C 31 °C 27 °C 32.5 °C 32.5 °C

Constant Constant Constant Variable Variable

– – – 9003 s 9003 s

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In this experiment, several C-shape electrical heated tubes are used to simulate the prototype PRHR HX, and the heating power of each tube needs to be regulated in order to agree well with the prototype. In such case, the numbers and the heat flux can be calculated by Eqs. (9) and (10).

Q_ rod;R ¼ drod;R Hrod;R nR ¼

D2irw;R dR

ð9Þ ð10Þ

At the same time, the water volume of IRWST model needs to agree with the consistency of time as given by the following equation:

tR ¼

Airw;R Hirwprh;R qR cp;R DT satnor;R ¼1 Q_

NPPs, the density and the specific heat capacity of the fluid is consistent. Let DR and HR represent the diameter of the IRWST and the ratio of height to length of the C-shape tube, the total heat flux can be determined by Eqs. (12) and (13).

Q_ PRH;R ¼ Airw;R Hirwprh;R ¼ 1

ð12Þ

Q_ PRH;R ¼ D2R HR

ð13Þ

According to the scaling criteria and taking consider in the prototype parameters and operation conditions, the flow state of the tank and the C-shape tubes can be determined at different positions. Table 2 shows all design ratio that should be satisfied in the experimental test facility.

ð11Þ

PRH;R

2.2. Experimental facilities and test conditions

In the above equations, Hirwprh;R represents the water level ratio of IRWST and PRHR between the test facility and prototype. DT satnor;R represents the temperature rise ration from the normal temperature to saturation temperature between the test facility and prototype. Since the working temperature and pressure of the fluid are the same between the experimental test bench and the prototype

As illustrated in Section 2.1, dimensions of the tank can be defined based on the scaling criteria listed in Table 2. Meanwhile, the number and heating power of the C-shape tubes and the initial experimental conditions can be determined. Based on the scaling criteria listed in Table 2, the size of the tank is approximately 700 * 700 * 800 mm with a visual area of

Fig. 4. Key monitoring lines in tank.

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W. Du et al. / International Journal of Heat and Mass Transfer 136 (2019) 128–145 Table 4 Main parameters of the PIV system. Target Flow of Measurement Target flow Measurement area Uniform flow speed

2-D water flow 600 mm * 600 mm 0.04 m/s

Tracer particle Tracer particle Average diameter Average specific gravity

Polyamide resin 10 lm 1.04 g/cm3

Calibration Distance of reference point lr Distance of reference image Lr Magnification factor M Laser parameter Light source Laser power Thickness of laser light sheet Laser power instability

600 mm 2048pixels 0.293 mm/pixels Continuous 10 W 1.5 mm 3%

Camera parameter Camera model Spatial resolution Sampling frequency Distance from the target

FASTCAM WX100 2048
2048 125 Hz 3000 mm

Data process Pixel unit analysis Data type Search area size

Cross correlation method Time-summarize 16
16pixels

500 * 600 mm on the left, front and right sides. The tank is constructed by 15 mm thick Polycarbonate. As shown in Fig. 1, the experimental test facility consists of four major sub-systems: I. The test section (PRHR HX and IRWST models), II. The PIV measurement system (laser and HD camera), III. The temperature data acquisition system, and IV. the heating and cooling control system. Five C-shape tubes of three different sizes (as shows in Fig. 2) immerged in one-side of the IRWST in cross shape. The electrical heating power of tubes are the same and they are regularized by the same electrical regulator. The heating power of each C-shape electrical tube is 3000 W and the maximum heating power of PRHR HX heating rod bundle is 15 kW. According to the scaling criteria and the actual operating conditions, three constant and two variable heating power conditions as listed in Table 3 are conducted. Prior to the test, the IRWST is filled with purified water at atmospheric pressure. Then the PRHR HX operates at the full heating power and heats the water in tank to saturated temperature. During the heating process, more water should be fed to the tank to compensate the evaporation and maintain the water level. When water temperature reaches saturation temperature, the PRHR HX sustains at full heating power for half hour to further distill the water and exhaust the dissolved gas and non-condensable gas in the tank. Then the formal experiments can be conducted. 2.3. Measurements Continuous high-power laser (HPL) with wavelength of 532 nm and high-speed camera are used to capture the fluid flow field in IRWST. Since the angle of divergence is 13°for HPL and the overall

Fig. 5. Upwelling flow near the PRHR HX bundle.

Fig. 6. Fluid velocity field in Z-1 surface.

visual region of the tank is 500 * 500 mm at least, the HPL is about 3 m far away from the IRWST. The resolution of the camera 2048 * 2048 pixels is chosen in the formal experiment to capture the wide visual area in the present experiment. Meanwhile, the brightness of capture camera decreases as the capture frequency increases. In the present condition, the capture rate of 125 frames per second (FPS) with the maximum resolution could fully meet

Table 5 Summary of uncertainties for velocity u. Parameter

Error sources

Parameter error

Sensitive coefficient

Error contribution

M X Dt du

Magnification factor Image displacement Image interval Experiment

0.0005967 mm/pixel 0.2 pixel 10 ns 1.011 mm/s Combined uncertainty

136.53 pixel/s 12.21 mm/(pixel
s) 6000 mm/s2 1 uu

0.0815 mm/s 2.441 mm/s 0.00006 mm/s 1.011 mm/s 2.64 mm/s

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Fig. 9. Velocity field in the region of dead zone. Fig. 7. The overall circulation in X-1 surface and its evolution.

(a) Transient temperature variation along Line 1

(c) Transient temperature variation along Line 3

(b) Transient temperature variation along Line 2

(d) Thermal stratification along Line 1

Fig. 8. Transient temperature variation in the benchmark test condition.

W. Du et al. / International Journal of Heat and Mass Transfer 136 (2019) 128–145

the experimental requirement. The purpose of this experiment is to acquire the flow filed in various planes from three directions, and explore the evolution of the natural convection of IRWST, so that the capture planes in the formal experiment could represent the overall fluid flow pattern. Fig. 3 shows the whole capture planes along the X/Y/Z planes at each location. The flow field analyzed in this paper is mostly near the PRHR HX and the wall of IRWST. The tracking behaviors of particles are particularly crucial for PIV measurement accuracy. In the present PIV measurements, polyamide resin particles were used as tracer particles to minimize the particle lag. The particles had a nominal diameter of 10 mm and the equivalent density of 1.04 g/cc. The particle lag, which cannot faithfully track the motion of working fluid, is the main source of uncertainty. Li et al. [23] have researched the tracing behaviors of polyamide resin particles using Stokes number. Poggie et al. [24] have demonstrated the detailed process to calculate the Stokes number for each PIV experiment. And the result shows that the Stoke number is 8.2 * 108, far less than 1 which indicates that the tracing particles is suitable in these experimental conditions. Photron FASTCAM Viewer (PFV) software is used in the PIV measurement to record the images. Besides the DaVis postprocessing software is used to compute the velocity vector and contour plots of flow field. As for the temperature measurement, 29 calibrated T-type thermocouples with 1 mm in diameter are utilized to measure the fluid temperature in different tank location. The thermocouple locations are precisely set along three thin stainless-steel tubes with the probes outside the tube and wires inside the tube to ensure that the existence of the thermocouples has little influence on the flow patterns. As shows in Fig. 4, several thermocouples are fixed on three thin stainless-steel rods called as Line 1–3. Each rod locates in disparate region of IRWST in order to monitor the thermal stratification. Most thermocouples are disposed along the vertical section of PRHR HX where the natural convection occurs. All thermal signal parameters are recorded at the data-sampling rate of 10 Hz by the input modules (NI 9212), then analyzed by the Data Acquisition System (NI cADQ-9178).

At the same time, according to the error propagation equation, Eq. (14) can be transformed to the particle differential equation form shown in Eq. (15).

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2  2  2  @u @u @u du ¼ ðDuÞ2 þ Dt þ M þ X @ Dt @M @X

u¼M

ðX  X 0 Þ DX þ Du ¼ M þ Du Dt Dt

ð15Þ

In the above equations, DX is the displacement of particle in the two successive images. Dt is the time interval of the two successive images. M is the magnification factor, which needs to be identified through calibration.du is usually categorized as an uncertainty factor rather than a measurement parameter, which needs to consider the difference between the flow filed acceleration and the projection procedure from the 3-D physical space to the 2-D image plane. Table 4 shows the main parameters in this experiment for PIV measurement, including the tracer particle, parameters of highspeed camera and laser, and the calibration between laser plane and image plane. There are a series of calculated process to get uncertainty of M, DX, Dt and duwith the respective sensitive coefficient. Plugging the

2.4. Measurements uncertainty analysis The experimental uncertainty analysis mainly includes two aspects, namely the direct and indirect measurement of parameter. The uncertainty of the PIV measurement can bring in the direct measurement uncertainty. The uncertainty of diameter and length of three different C-shape tubes, heating power, thermocouples and the acquisition system will cause measurement uncertainty, which refer to indirect measurement uncertainty. According to the calibration data of thermocouple and the maximum uncertainty of data acquisition system (DAS), the absolute error of above two parameters are 0.5% and 0.25% in the range of 0–200 °C, respectively. The combined temperature measurement uncertainty is 0.56%. At the same time, the maximum uncertainty of length and diameter of the C-shape tubes are 0.214% and 6%, respectively. And the heating power is regulated by the electrical regulator, since the uncertainty of voltage and current are 0.91% and 1.47%, respectively. For the indirectly measurement parameter, error transmission equation is used to evaluate the measurement error. The heating power is determined by the voltage and current, so that the uncertainty of the heating power is 1.73%. Huang et al. [25,26] deuced a typical propagation of measurement uncertainty in PIV measurement. The uncertainty of the whole PIV system given in Eq. (14) considers the coupling between the subsystems and the estimation principle.

135

Fig. 10. Transient temperature variation of TC-1-2.

ð14Þ Fig. 11. Thermal stratification characteristic zones in the IRWST.

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(a) 360s

(b) 720s

(c) 1260s

(d) 1800s Fig. 12. Evolution of the fluid flow field in X-3 surface.

value listed in Table 5 into Eq. (15), the combined uncertainty of the PIV measurement is about 2.64 mm/s in the entire measurement surfaces.

3. Typical phenomenon in IRWST When the PRHR HX activates, the fluid around the secondary side of the PRHR HX is heated and flows upward under the force of buoyancy. Fig. 5 records the velocity field near the PRHR HX rod bundle. It shows that the strong upward velocity field exists in the annular region of the PRHR HX with small velocity in other regions. As the hot fluid arrives the free surface, the fluid flows

transversely to the remote side of the IRWST and reaches the boundary wall as shown in Fig. 6. From this figure, the velocity field of the fluid near the top free surface diverges from the PRHR HX bundle to the surrounding walls. Portion of the hot fluid accumulates at the top region of the IRWST under gravity. Certain hot fluid is refrigerated by indirect heating fluid. After reaching the boundary wall, it flows downwards. At the same time, there exists a big circulation at the upper and middle region of the IRWST as shown in Fig. 7. This surface is parallel with the PRHR HX rod bundle and near one side of the IRWST. Finally, the fluid goes back to the PRHR HX rod bundle. The experimental condition of images from Figs. 5–7 is the same—the initial condition IRWST-3. The heating time of Figs. 5 and 6 is 180 s, and Fig. 7 is 360 s. When

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(a) 360s

(b) 1080s

(c) 1440s

(d) 1800s

137

Fig. 13. Evolution of the fluid flow field in Y-1 surface.

compared with the stream lines presented in the literature [8] and [14], the tendency of the stream lines is the same nearly, even though the initial condition and the design dimension of the IRWST and PRHR HX are different. Fig. 8(a)–(c) plot the transient temperature curves along Line 1, Line 2 and Line 3 with the heating power of 12 kW and the initial temperature of 27 °C. As can be seen, the thermal stratification phenomenon is obvious as the temperature for each monitoring point increases smoothly and steadily. As the thermal stratification reaches the zenith, the gradient of the temperature difference decreases. The temperature variation characteristics reflect the fluid flow performance. Fig. 8(d) further shows the thermal stratification phenomenon along Line 1 at different time. The temperature gradient becomes more and more obvious in the heating process.

And the temperature gradient at the top region of the IRWST is bigger than the other regions at the beginning of the experiment since the hot fluid will accumulate in this region firstly. From Fig. 4, it shows that the monitoring points TC-2-7 and TC2-8 located above the upper horizonal section of PRHR HX. The fluid temperature variation curves of these two thermocouples are displayed in Fig. 8(b), and it demonstrates that the fluid temperature in this region is higher than others. As has been mentioned above, the upwelling flow will arrive and accumulate at the top of the IRWST. The hot fluid cannot flow downwards under the buoyancy force, which leads to the high fluid temperature in this region. The temperature is almost the same at the monitoring points of TC-2-2 to TC-2-6, located at the same height along with the vertical section of PRHR HX. Even though the Line 2 thermocouple bundle

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is located near the PRHR HX rod bundle, the temperature difference in the Line 1 and Line 3 thermocouple bundles in the same region is small. In other words, the temperature gradient in this region is small, since the length of the vertical section of PRHR HX is nearly 500 mm. After researching the velocity field in this region, a strong natural convection happens. Firstly, there is a violate flow around and inner chamber of the C-shape tubes as has been shown in Fig. 5, and plenty of vortexes exists around the upwelling flow which will flatten the temperature around the upwelling fluid. Then a large circulation is formed in this region even though the strength and location changed with time. Taking consideration of all the accounts in this region, it can be concluded that at the same height along the vertical section of C-shape tubes, there exists a violent convection zone and the temperature distribution is almost uniform. The temperature variation curves of the thermocouple monitoring points TC-1-1 and TC-3-1 record the fluid temperature at the bottom of the IRWST. They remain the initial temperature during the entire heating time. Fig. 9 shows the evolution of the flow field with time in the Z-4 plane in the initial experimental condition of IRWST-3 and heating time of 1720 s. The area located under the lower horizonal section is called as ‘‘dead zone” by Ge et al. [8] for the low temperature and low velocity. This is because the fluid in this region cannot be heated by the PRHR HX directly, and thus it is barely influenced by the recirculation flow and thermal mixing effect than that in the upper region. The average velocity in the ‘‘dead zone” is less than 5 mm/s because the fluid in this region is only influenced by thermal conduction but not natural circulation. When the pool-boiling occurs in the tank, the fluid in the upper region reaches the saturation temperature, and the temperature will arise violently and vibrationally. Fig. 10 records the transient temperature curves under different initial conditions. It is seen that the temperature at the monitoring point TC-1-2 (same height with the horizontal section of the PRHR HX) demonstrates a different variation tendency. Fluid temperature in this region increases slowly and remains the initial temperature during 1200 s, when compared to the other monitoring points located in the upper region. Lu et al. [14] concluded that there was a ‘‘thermal interface” between the hot and cold fluid. As has been mentioned earlier, there exists a large circulation in the tank. With change of time, the location of this circulation moves down and locates in the lower and middle region of the tank. In such case, the fluid is heated by PRHR HX in the initial heating stage, then flows upwards under buoyancy. In consequence, the fluid in the lower region cannot participate in the convection directly. So ‘‘dead zone” exists in lower region of the tank, and the heat transfer between ‘‘dead zone” and upper hot region is

Fig. 15. The change time of temperature curve’s slope.

only through thermal conduction. As the fluid in the upper and middle region accumulates, the heat and circulation cannot break through the hot region. The circulation is compressed in the middle and lower region of the tank, then the temperature of ‘‘thermal interface” increases slowly since only part of the hot fluid is carried by the circulation. Once the ‘‘thermal interface” reaches the IRWST ‘‘dead zone”, the temperature in the lower region will jump to high values with unsteady oscillations. At this time, the overall temperature in the tank approaches the saturation points, the stratification phenomenon is then reduced. In summary, according to the observation of the temperature variation law along the vertical height of the PRHR HX, the following characteristic zone are observed and show in the Fig. 11: (1) High temperature zone – High temperature fluid region in the upper horizonal section of the PRHR HX due to the accumulation of upwelling plumes; (2) Natural convection zone – Cold fluid comes from the walls mixes with the hot fluid diffuses from the center of the PRHR HX along the axis height of the C-shape tubes; (3) Dead zone – The bottom of the horizontal section of C-shape heating tubes with no internal heat source and no heat convection, but only the heat conduction occurs. 4. Results and discussion 4.1. The evolution of upwelling flow

Fig. 14. The maximum height and velocity of the upwelling plumes.

As has been mentioned above, once the PRHR HX activates, the fluid around the PRHR HX bundle is heated and flows upwards under the buoyancy driving force. Fig. 12 shows the evolution of the upwelling flow field with time in the X-3 surface, which is near the PRHR HX bundle. And the heating power is 12 kW and the initial temperature is 27 °C. At the initial heating stage, such as 360 s in Fig. 12(a), the fluid heated by the lower and vertical section of the PRHR HX flows upwards continuously and sharply and reaches the top free surface. The overall temperature in the tank maintains in the original temperature and the density difference between the hot fluid and unheated fluid is large enough for the heated fluid reaching the free surface under buoyancy driving force. As time goes on, part of the hot fluid is not carried by the circulation to flow down and accumulates at the top region of tank. In such case, the temperature at the top region increases immediately and results in a reduced density difference between the upwelling fluid and the top fluid. As shown in Fig. 12(b)–(d), the maximum height that

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D V

E V

F V

G V

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Fig. 16. Evolution of the fluid flow field in Z-3 surface.

the upwelling flow can reach decreases gradually in the annular region of C-shape tubes. In this region, the fluid is heated mostly by the lower section of the PRHR HX and the heat transfer is limited. Once the hot fluid aggregates at the top region, heated upwelling cannot break through the ‘‘rampart” due to lacking of enough density difference. However, fluid around vertical section of PRHR HX can be heated steadily so as to the heated fluid can break through the ‘‘rampart” and reach the top free surface. When the upwelling flow is not able to break through the ‘‘rampart” formed by the hot fluid at the top of the IRWST, the flow direction of upwelling flow will be induced by the large circulation, or the combination of two flows produces the three-dimensional flow field in the IRWST. At this time, hot fluid flows to cold fluid and new natural convection is formed. However, due to the effect of density difference, the upwelling flows cannot flow downward

so that it will turn to both sides of the tank, as shown in Fig. 13. Fig. 13 records the evolution of velocity field in the Y-1 surface when the heating power is 12 kW and the initial temperature is 27 °C. From four images, it is seen that there exists a diffusion zone from the center region of the PRHR HX bundle to sides of the tank. With change of time, the maximum height of the diffusion zone decreases gradually. As has been mentioned above, the height of the upwelling velocity field decreases with time. The maximum height in different time for the X-3 and Y-1 planes is determined and the maximum height at the same time in the X and Y planes are the same. The comparison shows that the hypothesis about the flow direction of the upwelling fluid is valid. Meanwhile, there are plenty of vortexes around diffusion region, such as Fig. 13(a). Vortexes are generated by the viscosity difference between the hot fluid and the cold fluid, and they can flatten the temperature

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(a)Transient temperature variation in 265mm

(b)Transient temperature variation in 370mm

(c)Transient temperature variation in 725mm Fig. 17. Temperature distribution variation in different height along axis direction.

Fig. 18. The average strength of vortex along Y-1 plane.

distribution in the radial direction. Under the influence of the vortex, the angle between main fluid of diffusion flow and the vertical section of the PRHR HX becomes to be right gradually with decreasing vortex intensity. Fig. 14 shows the maximum height and the maximum velocity in the X-3 plane when the initial experimental condition is IRWST3. It is seen that the maximum height and the maximum velocity of upwelling flow decreases sharply at the initial stage. This is because the temperature difference between the heated fluid in the lower section of the PRHR HX bundle and the fluid at the top region of the IRWST is so huge. It results in a large density difference and great buoyancy driving force for the upwelling flow. From Fig. 14, the maximum height of the upwelling flow decreases with time. But the maximum velocity is fluctuant. The natural convection is aroused by the temperature difference, and the existence of natural convection weakens the temperature difference along the height which reduces the strength of natural convection in turn. Then temperature difference rises and a new speed peak appears in the upwelling flow. During the whole heating process, new speed peak will appear continuously.

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As Fig. 8(a)–(c) have shown, the time of slope change of transient temperature curves along the axis directions is different for monitoring points at different heights. The corresponding time of the slope change is determined and shown in Fig. 15. The corresponding time decreases with the increasing height of monitoring points. This is because the hot fluid accumulates at the top region and then develops downwards, leading to the enlargement of the hot fluid region. It shows that the corresponding time interval of two adjacent thermocouples decreases with the increasing the location of the monitoring point. This is because the overall temperature maintains at the original value at the initial heating stage. When the PRHR HX activities, the heated fluid can diffuse in the tank and the natural convection between the hot fluid and the cold

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fluid is violent. When the hot fluid region develops to the bottom of the tank, the ‘‘thermal interface” and the ‘‘dead zone” need to be heated, and more heat is acquired with more time. Similarly, the corresponding time interval decreases when the distance between the thermocouple bundle and the PRHR HX rod bundle decreases. Fig. 16 shows the evolution of the velocity field in the Z-3 surface when the heating power is 12 kW and the initial temperature is 27 °C. From the evolution process in X-3 and Y-1 surfaces and considering the location of the Z-3 surface, the maximum height of the upwelling flow will arrive at this surface in 540720 s. And the upwelling flow turns to both side of the wall. At the initial time, Fig. 16(a) displays the uniform flow field from three sides of the IRWST to the PRHR HX bundle to supple the water, carried by

᧤a᧥ 360s (IRWST-1)

᧤b᧥ 1800s (IRWST-1)

᧤c᧥ 360s (IRWST-2)

᧤d᧥ 1800s (IRWST-2) Fig. 19. Influence of different initial heating conditions.

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᧤e᧥ 360s (IRWST-3)

᧤f᧥ 1800s (IRWST-3) Fig. 19 (continued)

along the radial direction overlaps with each other for the same heating conditions. In this paper, the cause of such temperature distribution along the radial direction can be analyzed directly by the PIV measurement results. There exists a large circulation in the IRWST during the whole heating process as mentioned in Section 3. The circulation will mix the hot and cold fluid sufficiently. Meanwhile, when combined the temperature distribution with the flow field in Fig. 13, the flow pattern of diffusion flow will be flattened due to the existence of plenty vortexes around the diffusion region. On the other hand, the temperature distribution in the radial direction of the IRWST will become uniform due to the effect of the vortexes. And as the temperature difference along the radial direction reduction, the strength of the vortexes around the diffusion region decreases in the later stage as shown in the following Fig. 18. Fig. 18 shows the average strength of vertex along Y-1 plane under the initial condition IRWST-3. In the late stage, the flow pattern in the IRWST tends to be gentle. In turn, a stable thermal stratification forms along the height. Fig. 20. The maximum height of upwelling flow under different operation conditions.

the upwelling flow. However, Fig. 16(b) shows a different flow field distribution. The region near the PRHR HX bundle exists a reversed flow, which is believed to be the diffuse flow. With change of time, the region of reversed region increases gradually. The phenomenon in Z-3 surface agrees well with that of in the X and Y surfaces. 4.2. Temperature distribution along the radial direction It has been demonstrated by the previous study that the temperature distribution along the radial direction is almost the same [6]. In the current experiment, some thermocouples are set in the same height and they can be used to monitor the fluid temperature along the radial direction, such as TC-1-5 and TC-3-3 at the natural convection zone, and TC-3-9 and TC-2-8 at the high temperature zone. Fig. 17 compares the temperature data of different thermocouples bundles at the same height under different initial heating conditions. It is observed that the fluid temperature distribution

4.3. Influence of different initial condition Fig. 19 records the influence of different initial heating condition on the flow field in the X-1 plane under different initial operation conditions, respectively. It is seen clearly that an overall circulation exists in these six figures and the location of the circulation varies with time as the average velocity does. The circulation mainly lies in the convection region. At the initial heating stage, the large circulation locates at the middle and top region, and the average velocity is big. The buoyancy driving force in this stage is bigger as the density difference between the lower section and upper section is large. Under this circulation inertia force, the heated fluid at the top region will be carried downwards, and heats the cold fluid at the bottom region. Comparing the evolution of the fluid flow field under the same plane in different heating conditions, the overall tendency of the flow field is approximately the same although the location of the circulation and the velocity magnitude are a little different at the same time. Fig. 20 shows the maximum height of the upwelling

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flow under three different initial operation conditions. The maximum height which the upwelling flow can be reached decreases with time. From the Fig. 20, it shows that there is little difference of the maximum height at the same heating time under different initial conditions. The flow pattern of the upwelling flow exists a little difference at the same heating time under different operational conditions. And the difference is bigger at the initial stage when compared with the later stage about the upwelling flow pattern in the annual region of the C-shape tubes. But the main shape of the upwelling flow maintains the same under different experimental conditions. In other words, different initial heating conditions have limited influence on the flow field under the condition of natural convection.

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4.4. Velocity field and thermal stratification under variable condition As has been analyzed above, the change of initial heating conditional will not change the main fluid flow field. Fig. 21 displays the evolution of three different shooting planes in the X/Y/Z directions as the heating power varies from 10.218 kW to 8.55 kW, the sustained time is 9003 s and the initial temperature is 32 °C. Meanwhile, when observing the flow field, they are similar to that of the steady heating power conditions. Comparing the temperature distribution under variable heating condition (Fig. 22(a)) to the steady heating power condition (Fig. 22(b)), they are almost the same. In conclusion, when natural convection dominates role the heat transfer between the PRHR HX and the IRWST, the

Fig. 21. The evolution of the fluid flow field under variable heating power in different surfaces.

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Fig. 21 (continued)

(a) Temperature curves along Line 1 in the condition of variable heating power

(b) Temperature curves along Line 1 in the condition of steady heating power

Fig. 22. Temperature variation along Line 1 in different heating conditions.

distribution of the velocity field and thermal stratification are almost the same. 5. Conclusion In this paper, the H2TS method is used to get an overall scaleddown IRWST with PRHR HX. A rectangular tank with five C-shape heating rods bundle is built to simulate the IRWST and PRHR HX for the advanced third-generation NPPs. The PIV measurement is applied in the experiments to study the velocity field and thermocouples are adopted to research the thermal stratification. The overall hydraulic performance combined with the natural convection and thermal stratification of the IRWST and the PRHR HX is analyzed. The main conclusions are summarized as follows. (1) At the initial stage of the PRHRS operation, natural convection dominates a main role in removing the residual heat between the PRHR HX and IRWST. The natural convection is generated by the thermal stratification, but, thermal stratification impedes the natural convection.

(2) When the PRHR HX and IRWST activates, heated fluid flows upwards under buoyancy force and reaches the top free surface. The fluid flows transversely to the boundary wall. After that, it flows downwards and finally goes back to the PRHR HX, which forms an overall circulation in the IRWST. (3) With the help of the vortexes and lager circulation, the temperature distribution along the radial direction is uniform approximately. Meanwhile, based on the temperature distribution along the vertical direction, the tank is divided into three regions, the high temperature zone, the natural convection zone and dead zone. Between the dead zone and the natural convection zone, there exists a ‘‘thermal interface” region, which has a great influence on the thermal stratification phenomenon. (4) An obvious upwelling flow is observed near the PRHR HX bundle directly. When the hot fluid accumulates at the top region of the IRWST, the heated fluid cannot break through the ‘‘rampart” and flows transversely to both sides of the tank. The maximum height of the upwelling flow decreases gradually with time.

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(5) Different initial heating conditions have a limited influence on the velocity field. When comparing the variable heating condition with the steady heating condition, the tendency of velocity field and thermal stratification are almost the same. Conflict of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, ‘‘experimental investigation on natural convection and thermal stratification of IRWST using PIV measurement”. Acknowledgement This work is financially supported by National Key R&D Program of China (2017YFE0106200), the Youth Leading Scholar Supporting Program in General Colleges and Universities of Heilongjiang Province (1254G017) and the Fundamental Research Funds for Key Projects for the Central Universities (HEUCFD1512, HEUCFX1513, 350021500201). References [1] Q.M. Men, Study on Heat Transfer Mechanism and Calculation Method of Passive Residual Heat Removal Heat Exchanger, PhD thesis, East China University of Science and Technology, Shanghai, 2015 (in Chinese). [2] X. Zhou, X.S. Wang, Q.M. Men, X.Y. Meng, Heat transfer analysis of passive residual heat removal heat exchanger under natural convection condition in tank, J. East China Univ. Sci. Technol. 2014 (1) (2015) 1–8 (in Chinese). [3] J.H. Lienhard, A Heat Transfer Textbook, fourth ed., Philogiston Press, Phlogiston, Cambridge, 2008, pp. 356–369. [4] C. Popiel, Free convection heat transfer from vertical slender cylinders: a review, Heat Transf. Eng. 29 (6) (2008) 521–536. [5] T. Yonomoto, Y. Kukita, R.R. Schultz, Heat transfer analysis of the passive residual heat removal system in ROSA/AP600 experiments, Nucl. Technol. 124 (1) (1998) 18–30. [6] Y.H. Zhang, Research on Thermal Hydraulic Characteristic of In-containment Refueling Water Storage Tank used in AP1000, PhD thesis, North China Electric Power University, Beijing, 2017 (in Chinese). [7] W.W. Zhang, T.L. Cong, W.X. Tian, S.Z. Qiu, G.H. Su, Y.C. Xie, X. Jiang, Numerical analysis on heat removal capacity of passive residual heat removal heat exchanger, Yuanzineng Kexue Jishu/Atomic Energy Sci. Technol. 49 (6) (2014) 1032–1038 (in Chinese).

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