effect of grooves on nucleate boiling heat transfer from downward facing hemispherical ... ·...

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Effect of grooves on nucleate boiling heat transfer from downward facing hemispherical surface

Dawen Zhong1, Jun Sun2, Ji’an Meng3 (), Zhixin Li3, Xiang Zhang4, Lin Chen5

1. Beijing Key Laboratory of Passive Nuclear Safety Technology, North China Electric Power University, Beijing 102206, China 2. Key Laboratory of Advanced Reactor Engineering and Safety, Ministry of Education, Tsinghua University, Beijing 100084, China 3. Key Laboratory of Thermal Science and Power Engineering, Ministry of Education, Tsinghua University, Beijing 100084, China 4. State Nuclear Power Technology Research & Development Center, Beijing 102209, China 5. School of Energy Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China Abstract The external reactor vessel cooling (ERVC) is the key method to ensuring the success of in-vessel retention (IVR), which is one strategy of the Generation III+ advanced light water nuclear reactor

to address severe accidents. The heat removal ability of ERVC is limited by the critical heat flux (CHF) on the outer surface of the lower head. In this paper, a heating system with the liquid metal as the intermediate heat medium in the scaled vessel was used to investigate the boiling heat

transfer. A three-dimensional (3D) hemispherical grooved surface on scaled vessel was proposed and its steady boiling performance was investigated in saturated water. Consequently, the liquid metal temperature exceeded 430 °C when the heating power was 185 kW and the boiling crisis

occurred, and the CHF increased from 967.4 to 2030.2 kW/m2 when the orientation increased from 5° to 85°. Compared with the 3D plain surface, the CHF enhancement was higher than 102%. The CHF enhancement could attribute to the increase of heat transfer area and the improvement of the

wettability. If the grooves on the reactor vessel could be manufactured by 3D printing in the future, the grooved surface will be a promising structure for ERVC.

Keywords downward facing boiling

critical heat flux (CHF)

liquid metal

grooved surface

Article History Received: 18 April 2019

Revised: 22 May 2019

Accepted: 23 May 2019

Research Article © Tsinghua University Press 2019

1 Introduction

In-vessel retention (IVR) is a key severe accident management strategy which was proposed for some advanced light water reactors. One method to achieve IVR is external reactor vessel cooling (ERVC). The key technology is to ensure that the critical heat flux (CHF) on the downward facing surface is greater than the heat flux from the core molten debris. Various methods have been proposed for enhancing ERVC performance, including using structured surface, nanofluids (Pham et al., 2012; Angayarkanni and Philip, 2015) and thermal insulation (Yang et al., 2005; Noh and Suh, 2013). The structured surfaces include porous coating (Yang et al., 2006; Jun et al., 2016; Tetreault-Friend et al., 2016; Sohag et al., 2017; Wang et al., 2018), pin fins (Chu et al., 2013; Zhong et al., 2015), micro channels (Bai et al., 2016; Hou et al., 2017; Zhong et al., 2018a, 2018b), honeycomb porous plates (Mt Aznam et al., 2016; Fogaça et al., 2018), and spherical porous bodies (Mori et al., 2018), and so on.

Yang et al. (2006) fabricated a downward facing hemispherical surface with micro-porous coatings by sintering, while Sohag et al. (2017) developed a Cold Spray technique to coat the same scale surface, both the coating surfaces were tested in the SBLB (Sub-scaled Boundary Layer Boiling) facility to investigate the CHF. The CHF on the micro-porous coating surface by sintering was higher than that by Cold Spray technique at larger orientation. Compared with the plain vessel, ~49%–112% CHF enhancement was achieved using the sintering coated vessel. Jun et al. (2016) investigated the nucleate boiling heat transfer coefficient and CHF on high-temperature thermally-conductive microporous coating (HTCMC) surfaces created by sintering copper powder. The CHF increased from ~1.4 MW/m2 at the inclination angle of 0° to ~2.0 MW/m2 at the inclination angle of 90°, which are 80%–350% higher than the CHF values of plain surface. Nanoporous hydrophilic surface (Tetreault-Friend et al., 2016) and micro nano bi-porous copper surface (Wang et al., 2018) could also enhance the CHF and boiling heat

Vol. 2, No. 1, 2020, 52–58Experimental and Computational Multiphase Flow https://doi.org/10.1007/s42757-019-0035-9


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transfer coefficient significantly, while the reliability is a huge challenge for ERVC. Therefore, the pin fin surfaces (Chu et al., 2013; Zhong et al., 2015) and micro channels surfaces (Bai et al., 2016; Hou et al., 2017; Zhong et al., 2018a, 2018b) were developed to improve the performance of ERVC. Hou et al. (2017) investigated the CHF of a hypervapotron structure surface under natural circulation conditions in TESEC facility. The minimum CHF of the hypervapotron structure surface is 0.61 MW/m2 at inclination angle of 10° while the maximum CHF is 2.15 MW/m2 at inclination angle of 70°; compared with the smooth surface, more than 80% CHF increase at 20°–30° inclination angle and approximate 120% CHF increase at higher inclination angles were obtained. Mt Aznam et al. (2016) investigated the CHF enhancement in combination with surface modification by attaching honeycomb porous plate (HPP) and nanoparticle deposition; the CHF of plain surface increases from 0.13 to 0.98 MW/m2 with the inclination angle changing from 10° to 180°, while the CHF of structured surface was 1.51 to 1.75 MW/m2 when the angle increased from 10° to 180°. Effect of HPP on CHF in saturated pool boiling of artificial seawater was studied by Fogaça et al. (2018). They concluded that the CHF enhancement by HPP and nanoparticle deposition was the role of capillary action.

Zhong et al. (2018a, 2018b) have developed a structured surface with interconnected-grooves surface and triangular array cavities (IGTAC) to enhance the CHF in saturated water, and the experiments on flat surface and 3D hemispherical surface were conducted. Zhong et al. (2018a) investigated the boiling performance of IGTAC surface by using the flat surface experimental apparatus, and more than 117% CHF enhancement was obtained on the optimized flat IGTAC surface. In order to obtain the boiling performance of IGTAC surface on hemispherical vessel, Zhong et al. (2018b)

investigated the boiling heat transfer of IGTAC surface on a scaled hemispherical vessel. However, compared with the 3D hemispherical plain surface, only more than 59% heat flux increase was obtained on 3D IGTAC surface. The reason is that the maximum temperature of liquid metal reached more than 400 °C, and the power was shut off to protect the heaters; therefore, the CHF on 3D IGTAC surface was not obtained. The maximum temperature of liquid metal could not be more than 400 °C, because the liquid metal flow was not fast enough to transfer more heat to the boiling surface, and the heaters would break at high temperature. In order to avoid the solid layer forming in the vessel, a new impeller was designed and manufactured to increase the flow rate of the liquid metal. Besides, compared with the IGTAC surface, the grooved surface is easier to manufacture and cheaper. Therefore, the pool boiling of water at saturated and atmospheric conditions on a 3D hemispherical grooved surface was investigated. The experimental results show that the CHF performance can be enhanced by the grooved surface. The findings of this study provide an alternative to improve the safety margin of IVR.

2 Experimental system

2.1 Experimental set-up

As shown in Fig. 1, a heating system with the liquid metal as the intermediate heat medium in the scaled vessel was used to investigate the boiling heat transfer and CHF on a 3D downward facing grooved surface. The experimental system was the same with Zhong et al. (2018b). The liquid metal acted as the intermediate heat medium, and was heated by the heaters and flowed in the internal vessel circularly. The heat was transferred from the heaters to the lower head.

Fig. 1 Schematic diagram of experimental apparatus (Zhong et al., 2018b; reproduced with permission © Elsevier Ltd. 2018).

D. Zhong, J. Sun, J. Meng, et al.

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Then, the outer surface of the lower head was cooled by boiling in the water pool. The heating power was still 226.5 kW at voltage of 380 V with the same distribution of 15 cartridge heaters; each piece of cartridge heaters is 1282.7 mm long and 19 mm in diameter. Tin (Sn) was selected as the intermediate heat transfer medium. The grooved hemispherical vessel was shown in Fig. 2 and Fig. 3. The wall heat flux on the outer surface of the vessel was controlled by adjusting the heating power of the heaters. The voltages and currents of the heaters were recorded by the data acquisition module.

The diameter of the AP1000 reactor vessel is about 4.4 m, and the material of vessel is SA508-Ⅲ, which is difficult to get for this experiment. Therefore, the heated vessel was made by 16MnR steel, which had a diameter of 273 mm with the thickness varying along the inclination angle. As shown in Fig. 2, the maximum thickness of the heated vessel was 10 mm at 0°, while the minimum thickness was 5 mm at the top of the hemispherical vessel. The thickness of the lower head has been demonstrated by numerical simulation and compared with the experimental data in foregoing experiment (Zhong et al., 2018b). As shown in Fig. 3, the parameters of the grooved surface are: groove width B = 1 mm, groove depth K = 2 mm, and spacing of grooves S = 2 mm.

As shown in Fig. 2, five K-type thermocouples, illustrated 1–5, were installed to take the wall temperature of outer surface at the orientation of 5°, 30°, 45°, 60°, 85°. Thermocouples were inserted into the grooves off the wetted

Fig. 2 Geometric parameters of the hemispherical vessel (Zhong et al., 2018b; reproduced with permission © Elsevier Ltd. 2018).

Fig. 3 Grooved hemispherical surface.

surface by 2 mm. The thermocouples were covered with the metal adhesive to prevent them to contact with water. Because it is difficult to place the thermocouple to take the inner surface temperature from the inside of the vessel. Five K-type thermocouples, illustrated 6–10, were inserted into the interior of the lower head at the orientation of 5°, 30°, 45°, 60°, 85° from the outside of the hemispherical vessel, which were used to take the temperature near the internal surface of the hemispherical vessel.

The liquid metal temperature in the vessel was taken by a thermocouple in a thermocouple tube as shown in Fig. 1. The liquid tin temperatures at two positions were taken by changing the length of the thermocouple tube. The green tube as shown in Fig. 1 was the thermocouple tube. One thermocouple was used to take the tin temperature after heat transfer with the water through the lower head, the position was labeled as “Bottom” in Fig. 1, and the tin tem-perature was named with Tbottom. Another thermocouple was used to take the tin temperature after heat transfer with the heaters, the position was labeled as “Top” in Fig. 1, and this temperature was named with Ttop. Except for the grooved surface, the other surfaces of the test vessel were surrounded by thermal insulation materials to reduce the heat loss. Thermal analysis was made using ANSYS Fluent 16.0 to investigate the temperature distribution and heat loss. The heat loss from the thermal insulation materials to the surrounding water was estimated to be less than 1%, and the uncertainty of heat flux q was calculated to be less than 4%. Additionally, the K-type thermocouples were calibrated for a maximum uncertainty of ±0.5 °C.

2.2 Experimental procedure

At the beginning of the experiment, the tin spheres were melted by the heaters in the molten pool. When the liquid metal temperature was high enough, about 400 °C, the nitrogen inlet valve was opened to pump the liquid metal from the molten pool into the vessel. The tube was heated during the liquid tin transport process, so that the liquid metal would not solidify in the tube. When the liquid tin is about to fill the vessel, the temperature at the top of the vessel increased rapidly which indicated that the tin filling process should be stopped.

During the tin filling process, the water tank was not filled with water so that the liquid tin would not solidify easily, and the temperature difference between vessel inside and outer surfaces was less than 10 °C at steady conditions, e.g., the temperature of Tbottom was 275 °C at a heating power of 7 kW, and the vessel outer surface temperature was about 272 °C. When the vessel was full of liquid tin, the vessel heaters were switched on with the heating power then slowly increased. The impeller inside the tin could not be


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used until all the tin was melted. When all the tin was heated to 300 °C water at saturation conditions was pumped into the water tank to submerge the vessel outer surface. Then, the impeller was started to pump the liquid tin inside the vessel and the steady pool boiling heat transfer experiments begun. In order to avoid the solid layer forming in the molten pool, a new impeller was designed and manufactured to increase the flow rate of the liquid metal, and hence the maximum tin temperature could exceed 400 °C. During the experiments, the heating power was increased in steps with steady boiling on the test surface. All the experimental data and operating conditions were recorded by the Advantech Adam software webAccess V 7.0 and displayed on the PC in real time. The wall heat flux on the grooved surface was controlled by adjusting the heating power. All the control signal and power supply were controlled by the power supply and control system.

3 Results and discussion

3.1 Results of grooved surface

The boiling performance on plain surface was tested when the heating power was 70, 75, 80, 85, 90, 95, 100 kW. The boiling crisis occurred at orientation of 85° when the heating power was 80 kW, and the CHF at the orientation of 85° was 857.3 kW/m2. The detail of plain surface could refer to Zhong et al. (2018b). During the grooved surface experiment, the boiling performance was tested at different heating power, and the variation of heating power with time is shown in Fig. 4. The heating power increased from 60 to 185 kW. Figure 5 shows the temperature variation of the liquid metal under and above the heaters. The tin temperature increased with the heating power, and Ttop was higher than Tbottom. The tin temperature variations at different heating power are showed in Fig. 6. The difference between Ttop

Fig. 4 Heating power vs. time for grooved surface.

Fig. 5 Temperature variation of the liquid metal.

and Tbottom was about 8–30 °C. Once the electrical heating power increases to another stable value, after about ten minutes, the tin temperature will be basically stable, which could be verified in Figs. 6(a)–6(e). That is to say, the boiling heat transfer on the grooved surface became steady. However, when the heating power was increased to 185 kW, the tin temperature kept increasing slowly; it could not keep steady state, which meant that the boiling crisis occurred on the grooved surface. The boiling state was controlled by the temperature of the liquid metal, which was adjusted by the heaters of this system. When the boiling changed from nucleate boiling to transition boiling, the boiling heat transfer coefficient will deteriorate. In the transition region, the water molecules near the heated surface convert into steam and form vapor blanket. The vapor blanket leads to low heat transfer coefficient, and hence it is difficult to take heat from the heating surface. Therefore, the tin temperature will increase. According to the boiling curve, as the wall temperature in the transition region increases, the heat transfer coefficient will decrease. Therefore, when the tin temperature increases, the boiling heat transfer coefficient will decrease, which will increase the tin temperature further. Ttop

exceeded 430 °C when heating power was 185 kW, which is higher than the tin temperature on plain surface and IGTAC surface (Zhong et al., 2018b).

The temperature variations on the grooved surface are shown in Fig. 7. As shown in Fig. 7, Tx is the grooved surface temperature at orientation of x°, e.g., T5 is the surface temperature at orientation of 5°. Ti85 is the temperature near the inside surface of the test vessel at orientation of 85° (the position of point 10 in Fig. 2). The temperature increased with the heating power, and the slope of temperature changed with the slope of heating power. The interval value of heating power was 5 kW when heating power changed from 60 to 120 kW, while it was 10 kW when heating power


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greater than 120 kW. As stated in Zhong et al. (2018b), for the plain surface, the surface temperature jumped and fell rapidly when the heating power changed, which meant that the boiling crisis occurred on the plain surface when heating power was 80 kW. However, for the grooved surface, the temperature jump–fall phenomena did not occur. When the heating power was 180 kW, Ti85 was 213.5 °C; as the wall thickness of thermocouple measuring hole was 1 mm, the temperature difference between Ti85 and tin temperature

near this thermocouple was at least 30 °C. That is to say, the tin temperature was higher than the melt point of tin (231.9 °C). Therefore, the temperature jump–fall phenomena would not happen. The CHF was obtained when the heating power was 185 kW, while the maximum heating power on IGTAC surface reached 125 kW and the boiling crisis did not occur on IGTAC surface. Therefore, it is difficult to distinguish which is better between the grooved surface and IGTAC surface.

Fig. 6 Tin temperature variation of the liquid metal at different heating power.


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Fig. 7 Temperature variations on grooved surface.

The heat flux of the grooved surface was calculated by the same method as using on the plain and IGTAC surfaces (Zhong et al., 2018b). Firstly, the average heat flux based on the bare external surface of the hemispherical vessel was heating power divided by outer surface area of the test vessel, and then the experimental data of flat grooved surface was used to predict the CHF. The heat flux on 3D grooved surface was then calculated by back stepping method. The CHF correlation of the flat grooved surface tested by the facility of Zhong et al. (2016) was listed in formula (1). Compared with the flat plain surface, the CHF increase on flat grooved surface was 65%–90%. The heat flux on 3D grooved surface was calculated by back stepping method and compared with the 3D plain surface and IGTAC surface in Table 1. Since the boiling crisis occurred on plain surface and grooved surface, the heat flux value of plain surface and grooved surface in Table 1 is CHF, while the heat flux value of IGTAC surface is not CHF. Besides, the CHF on grooved surface was compared with other surfaces in Fig. 8, and the plain surface (Yang et al., 2006), coated surface (Yang et al., 2006), and coated surface (Sohag et al., 2017) were tested in SBLB experimental apparatus. The coated surface shows significant enhancement of CHF comparing with the plain

Table 1 Heat flux or CHF variation with the orientation of different surfaces

Heat flux or CHF (kW/m2) Orientation

(°) Plain surface

(80 kW) (Zhong et al., 2018b)

IGTAC surface (125 kW)

(Zhong et al., 2018b)

Grooved surface

(185 kW)

5 454.8 585.1 967.4

30 544.8 776.2 1102.9

45 652.1 1012.3 1458.1

60 729.1 1145.3 1672.7

85 857.3 1366.9 2030.2

Fig. 8 Heat flux or CHF variations with orientation for different surfaces.

surface. The coated surface (Yang et al., 2006) was sintered with micro-porous aluminum coatings, while the coated surface (Sohag et al., 2017) was manufactured with stainless steel by Cold Spray technique. Sohag et al. (2017) indicated that the coated surface will be a viable technique for IVR of a commercial-sized reactor lower head. Compared with the 3D plain surface, the CHF increase of grooved surface was more than 102%, and the CHF enhancement could attribute to the increase of heat transfer area and the improvement of the wettability. The CHF on grooved surface was higher than other surfaces at most inclination angles, which also provide an alternative surface for the IVR.

929.2 2.680 , 0 37.5

CHF805.0 7.067 , 37.5 90

θ θθ θ

ì + < <ïï= íï + £ £ïî (1)

3.2 Discussion

The results of plain surface and IGTAC surface of Zhong et al. (2018b) showed that the heat flux on the outer surface of the lower head obtained by the experiments was lower than those of the simulation at a given temperature of the liquid metal. The main reason was that there was a crust that formed near the internal surface of the lower head. It was due to the pushing force of the impeller was not large enough to push the liquid metal flow quickly, and the liquid metal was cooled by the boiling heat transfer. The impeller needs to be redesigned to improve the pushing force. In the present study, the experimental results of grooved surface indicated that the experimental apparatus was improved comparing with that of Zhong et al. (2018b). The heating power could increase to 185 kW and the tin temperature could exceed 430 °C, which meant that the new impeller worked well. After the discussion with the engineer from factory, they indicated that the grooved surface was easier to manufacture


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comparing with the IGTAC surface. The boiling enhancement is significant, and it is a promising structure for ERVC. The structured surface adopted the method of machining, which is not suitable for the pressure vessel in engineering; it may affect the structural strength and stress distribution. However, the structured surface could be manufactured by 3D printing in the future, which will not affect the strength of pressure vessel. The grooved surface may be a viable technique for ERVC of a commercial-sized reactor vessel.

4 Conclusions

In this study, the steady state saturated boiling on a 3D grooved hemispherical surface was investigated by a novel heating system with the liquid metal as the intermediate heat transfer medium. The boiling crisis occurred when the heating power was 185 kW, and the CHF increased from 967.4 to 2030.2 kW/m2 when orientation increased from 5° to 85°. Compared with the 3D plain surface, the CHF increase was more than 102%. The CHF of the grooved surface was higher than coating surface by sintering and Cold Spray at most inclination angles, which meant that it was a promising structure surface for IVR. If the structural strength and stress concentration of the grooves could be solved, the grooves could be used in engineering.

Acknowledgements

The financial support extended by the National Natural Science Foundation of China (No. 51706068), Beijing Natural Science Foundation (No. 3192035), Beijing Key Research and Development Program (No. Z181100005118013), the China Advanced Light Water Reactors Major Projects (No. 2011ZX06004-008), and the Fundamental Research Funds for the Central Universities (No. 2017MS039) is gratefully acknowledgement.

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effect of grooves on nucleate boiling heat transfer from downward facing hemispherical ... ·...

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