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Experimental and numerical analysis of a new windowless target design of ADS
Annals of Nuclear Energy 80 (2015) 348–355
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Experimental and numerical analysis of a new windowless target design of ADS Hanyang Gu a,⇑, Xingliang Zhang a, Shenjie Gong a, Donghua Lu b, Xu Cheng a a b
School of Nuclear Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China China Nuclear Power Technology Research Institute, China Guangdong Nuclear Power Group, 518026 Shenzhen, China
a r t i c l e
i n f o
Article history: Received 17 September 2014 Received in revised form 1 February 2015 Accepted 4 February 2015 Available online 3 March 2015 Keywords: ADS Windowless target Free surface Upward swirling flow
a b s t r a c t The formation and control method of the coolant free surface is one of the key technologies for the design of windowless targets in the Accelerator Driven Systems (ADSs). In this work, a new design of windowless target for ADS based on upward swirling flow is proposed to form a stable free surface without recirculation zones. Experimental and numerical investigations on the free surface flow have been performed in a prototypical windowless target using water as working fluid. The experimental results show that a stable free surface is achieved when the mass flow rate is higher than 4.0 m3/h and the shape of free surface profile greatly depends on the inlet flow rate. CFD simulations have been carried out using VOF method with different turbulent models. The numerical results show that the prediction of BSL Reynolds stress model agrees qualitatively well with the experimental results. The free surface profile of Lead–Bismuth-Eutectic (LBE) in the windowless target is very similar to that of water according to the simulation results. The feasibility of the new windowless target for ADS based on upward swirling flow is preliminary validated. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction With more and more nuclear power plants being operated, management and disposal of high level nuclear wastes have been getting increased attention in recent years. Accelerator Driven System (ADS) is one of the most promising transmutation system which has been investigated worldwide to reduce minor actinides and long life fission products (Roelofs et al., 2008a). In center of the ADS subcritical core, a spallation target provides the primary neutron source which is then amplified by the subcritical core so as to achieve further fission and transmutation. At present, two types of spallation targets have been proposed and designed: solid window target and windowless target. During the design process of solid window target, two problems related with the solid window have to be concerned and resolved: (1) how to remove the intense heat produced during the operation. (2) How to endure the long-term irradiation by proton attacking and corrosion by heavy liquid metal at high temperature. Various designs of solid window targets had been developed and tested (e.g., Mansur et al., 2001; Tak et al., 2005; Fazio et al., 2008; Cheng et al., 2008; Futakawa et al., 2009).
⇑ Corresponding author. E-mail address: [email protected] (H. Gu). http://dx.doi.org/10.1016/j.anucene.2015.02.022 0306-4549/Ó 2015 Elsevier Ltd. All rights reserved.
In order to reduce the material requirements and increase the service life of target system, the second type spallation target, windowless target, is proposed and has attracted much attention. Within the EUROTRANS project, several windowless targets for the experimental accelerator driven facility XT-ADS were presented (Class, 2011). In windowless targets, by the influence of gravity, the heavy liquid metal flows downward in the circular nozzles surrounding the proton beam tube. The heavy metal free surface formed below the nozzle is desired to be stable and avoid splashing. In addition, recirculation zones should be avoided to prevent local overheating because the interaction of the proton beam with the heavy liquid metal will produce great heat. In order to establish criteria for windowless target designs, experimental and numerical studies of windowless targets were carried out within the EUROTRANS project. Tichelen et al. (2007) conducted water experiments using a Laser Doppler Anemometry (LDA) to present the velocity profile in the target. According to the experimental results with the working fluids of water and subsequent Hg, recirculation zones were observed clearly (Tichelen et al., 2007; Schuurmans et al., 2007; Batta and Class, 2007). To make the recirculation zones small and stable, three different designs were presented by Roelofs et al. (2008c). Furthermore, Roelofs et al. (2008b,c) proposed a proper swirl at the inlet of the target which could reduce the recirculation zone and stabilize the free surface (Batta and Class, 2008). Su et al. (2012) performed experimental and numerical investigations on
H. Gu et al. / Annals of Nuclear Energy 80 (2015) 348–355
the free surface flow in a scaled windowless target model referring to the design of XT-ADS (Batta and Class, 2007, 2008) using water as working fluid. The results showed that the recirculation zone is difficult to be eliminated in those designs. Liu et al. (2014) carried out numerical simulations to investigate the swirling affection on the flow pattern and the results proved that the added swirls make the recirculation zone small and stable. But a cavitation zone appears with the increase of swirling strength. In the present work, a new design of windowless target for ADS based on an upward swirling flow is presented. The free surface is stable and the recirculation zone is eliminated. Experimental and numerical studies are carried out to confirm the feasibility of the windowless target design. 2. A new windowless target design The windowless target design is as shown in Fig. 1. The windowless target is composed of the inside and outside guide tubes. An impeller with 10 twisted blades is fixed in the inner guide tube. The heavy metal liquid flows upward from the bottom of the inner guide tube with an impeller inside, and then flows downward at the top of the inner guide tube. Due to the existence of impeller,
a swirling flow with circumferential velocity is formed when the liquid passing the impeller. Under the action of centrifugal force, there forms a concave free interface at the outlet of inner guide tube. After that, the liquid flows downward through the annular space between the inner guide tube and outer guide tube under the gravity force. During such process, the deposited heat, which is produced by the spallation reaction as the proton beam passing through the free surface, is taken away from the target. 3. Experimental setup In order to validate the feasibility of the new designed target as shown in Fig. 1, a prototypical test section as shown in Fig. 2 is designed and manufactured. The diameters of the inner guide tube and outer guide tube are 70 mm and 203 mm, respectively. The impeller is fixed in the center of inner guide tube with a distance of 530 mm to the outlet of the upper end of the inner guide tube. Fig. 2(b) shows the structure of the impeller in detail. The center shaft of the impeller has a diameter of 22 mm and a total length of 92 mm. Ten blades with a spiral angle of 50° are evenly located on the outer surface of the center shaft. The test section is installed vertically, and water is used as the working fluid in this work. The
Proton beam
Free surface
Inner guide tube
Outer guide tube
Impeller
Outflow
Inflow
349
Outflow
(a) windowless target Fig. 1. A new windowless target design.
(b) Impeller
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(a)
(b)
Fig. 2. Test section. (a) Schematic of test section (b) Impeller
Fig. 3. Test loop.
water flows upward from the bottom of the inner guide tube into the test section. After flowing through the impeller, a swirling flow is formed and a concave free surface appears at the outlet of inner guide tube. At last, the water flows downward in the annular space between the inner tube and outer guide tube driven by the gravity force. The outlet of the inner guide tube is open to the environment
directly. The pressure drop of the impeller is measured by a pressure sensor installed at the inlet of inner guide tube as shown in Fig 2(a). The test is performed in a hydraulic test loop as shown in Fig. 3. Water is driven by a centrifugal pump. The flow rate is controlled by electric regulating valves in the main loop and bypass loop. The
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100 50 0 -50 H [mm]
3
4.0 m /h 3 5.0 m /h 3 6.0 m /h 3 8.0 m /h 3 10.0m /h
-100 -150 -200 -250
Inner guide tube
-300 -80
-60
-40
-20
0
20
40
60
80
R [mm] Fig. 6. Free surface profiles.
4. Experimental results Fig. 4. Free surface measuring system.
flow rates of the test conditions are measured by a mass flowmeter. The profile of free surface is measured by a conductivity probe as shown in Fig. 4. The conductivity probe is fixed on a vernier caliper vertically. A high electrical signal is obtained once the conductivity probe moves downward and reaches the free surface of the water, which indicating the location of local free surface. Thus, the free surface profiles at inner guide tube outlet can be obtained by moving the vernier calipers vertically and horizontally.
Fig. 5 gives the experimental visualization of free surfaces at the outlet of inner guide tube under different flow rates. When the flow rate is larger than 4 m3/h, a center concave stable free surface is formed at the outlet under the influence of centrifugal force. The concave depth increases with the increase of the flow rate. Fig. 6 gives the measured free surface profiles with different flow rates. The free surfaces are symmetrical and stable. With the increase of the flow rate, the centrifugal force produced by the tangential velocity increases, and the location of the center point of liquid separation inside the inner guide tube descends dramatically. When the flow rate increases from 5.0 m3/h to 10 m3/h, the location of the center point of liquid separation descends from
4 m3/h
7 m3/h
9 m3/h
11 m3/h Fig. 5. Free surface visualization.
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9
5. Numerical simulation
150
DP
8
120
7
90
K [-]
DP [kPa]
K
CFD analyses have been performed for the windowless target (Fig. 2) to get more detailed insight into flow configuration and free surface formation mechanism. Three-dimensional transient numerical simulations are performed using the commercial CFD code Fluent in this work. 5.1. Computational model
6
60
5
30 2
4
6
8
10
3
Q [m /h] Fig. 7. Pressure drop in the windowless target.
38.8 mm to 285.7 mm. Meanwhile, the outlet liquid thickness of free surface decreases due to the increase of the tangential velocity at the bell mouth of the inner guide tube. Fig. 7 shows that the pressure drop in the inner guide tube, measured by the pressure sensors installed at the inlet as shown in Fig 2(a), increases from 5.4 kPa to 8.1 kPa with the increase of the flow rate from 4.0 m3/h to 10 m3/h. Correspondingly, the resistance coefficient based on the inlet velocity of inner guide tube drops from 128.6 to 31.2. The pressure drop is mainly produced by the form drag when water flows through the impeller and forms circumferential flow.
The VOF method (Volume of Fluid) is adopted to treat the free surface flow, in which the free surface separating air and water is distinguished by a phase indicator function and a corresponding transport equation (Class, 2011; Liu et al., 2014). It is well known that one of the key factors affecting the accuracy of CFD predicting results is the turbulence modelling. Three two-equations eddy viscosity turbulent models (i.e. the standard k-e model (indicated as k-e in this paper), RNG k-e model (RNG) and SST model (SST)) and two Reynolds stress models Reynolds stress model of Speziale (SSG RSM) and BSL Reynolds stress model (BSL RSM)) are employed. More details about the turbulence models and the corresponding wall treatment can be found in ANSYS manual (2012). Structured mesh as shown in Fig. 8 is adopted in the analyses. For all turbulence models, special wall treatments are needed. The size of the first mesh close to the wall surface is important in the simulation. The dimensionless distance to the wall surface y+ < 2.1 is been used in the present study. Based on the grid independence test, 8,000,000 grids are employed, as shown in Fig. 9. The boundary conditions are defined as follows: (1) the walls of guide tube and impeller are defined as no slip wall condition. (2) The water inflow condition is a uniform velocity normal to the inlet
Fig. 8. Computation mesh.
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of the inner guide tube. (4) The upper air outflow boundary is imposed with atmosphere pressure. (5) The gravity force is considered.
results based on different turbulent models. It can be seen that the numerical results obtained by different turbulent models successfully simulate the concave type free surface at the outlet, while the predicting accuracies by various models comparing with
5.2. Numerical results 100
Fig. 10 shows the comparison of free surface profile under the flow rate of 6 m3/h between the experimental data and simulation 50
Experiment CFD by BSL RSM with different mesh number 2500000 3000000 6000000 8000000 10000000
0 3
H [mm]
H [mm]
0
50
Q= 5 m /h experiment CFD with BSL
-50
3
Q= 7m /h experiment CFD with BSL
-100
6 m3/h
3
Q= 9 m /h experiment CFD with BSL
-150 -50
-200 -250 -100 -80
-100 -80
-40
0
40
-60
-40
-20
80
0
20
40
60
80
100
R [mm]
R [mm] Fig. 12. Comparison between numerical and experimental results of free surface profile.
Fig. 9. Grid independence test.
9.0
60 8.5
Experiment CFD prediction
40 8.0
20
7.5
E xpe rim en t
-20
DP [kPa]
H [mm]
0
k -ε R N G k -ε S ST S SG B SL R SM
Q=6m3 /h
-40
7.0 6.5
-60 6.0
-80
5.5
-100 -120 -120
-90
-60
-30
0
30
60
90
5.0
120
2
3
4
5
6
8
9
10
11
12
Q [m /h]
Fig. 10. Prediction with different turbulent model.
5m3/h
7 3
R [mm]
6 m3/h
Fig. 13. Comparison between numerical and experimental results of pressure drop.
7 m3/h
8 m3/h
Fig. 11. Prediction by BSL RSM.
9 m3/h
10 m3/h
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H. Gu et al. / Annals of Nuclear Energy 80 (2015) 348–355
1.6
Axial velocity at H=-92mm
Axial Velocity [m/s]
1.4
6 m3/ h
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 Radius [mm]
(a) Fluid streamlines
(b) Fluid field below the free surface at the outlet Fig. 14. Flow field in the windowless target.
experimental data are different. The locations of free surface profiles simulated by the two-equations eddy viscosity models (i.e. k-e, RNG and SST) are obviously higher than that of the experimental result. Generally, the linear isotropic two-equation eddy viscosity models are not applicable to predict the swirling flow with strong anisotropic characteristics. The predictions by SSG and BSL RSM models approach the experimental result well, excepting the center region of the concave free surface. The bottom coordinate value of concave free surface at the center is around 107 mm while the corresponding predicted values by SSG and BSL RSM model are around 65 mm and 70 mm, respectively. In summary, BSL RSM model is the best one for predicting the free surface profile. Therefore, BSL RSM model is adopted in the following analyses. The free surfaces profiles simulated with BSL RSM model under different flow rates are as shown in Fig. 11. It can be found that a
concave stable free surface is formed at the outlet when the flow rates are within 5 m3/h–10 m3/h. The location of the lowest point of free surface moves downward with the increase of flow rate, which agrees well with the experimental results. The comparison between the experimental results and the numerical predictions on the free surface is given in Fig. 12. The predicted profile agrees well with the experimental result when the flow rate is 5 m3/h. With the increase of the flow rate, the center of the free surface obtained in the experiment moves downward faster than that in the simulation. But, in general, the simulated free surface profiles by BSL RSM model are quantitatively coincident with the experimental results except the central area. Fig. 13 shows the comparison of the pressure drops between the experimental results and the numerical predictions. It can be found that the numerical predictions agree well with the experimental results. For example, when the flow rate is 10 m3/h, the
H. Gu et al. / Annals of Nuclear Energy 80 (2015) 348–355
outlet of the inner guide tube. As a result, the centrifugal force produced by the tangential velocity is too weak to keep a stable free surface at the outlet. The CFD study using the VOF model is also performed. The numerical results show that the prediction of BSL Reynolds stress model agrees qualitatively well with the experimental data. No recirculation zone, which deteriorates the heat transfer, exists in the flow field of the windowless target based on the numerical analyses. The free surface profile of LBE is quite similar to that of water under the same inlet flow rate. The feasibility of the new windowless target for ADS based on upward swirling flow is preliminary validated. Detailed optimization design and corresponding experiments using LBE are carrying out.
50 25 3
H [mm]
0
Q=5 m /h water LBE
-25 -50
-75
Inner guide tube
-100 -120
-80
-40
0
40
80
355
Acknowledgement
120
R [mm]
The authors are grateful to National Natural Science Foundation of China (No. 91026020) for providing the financial support for this study.
Fig. 15. Free surface curves of liquid LBE and water.
Table 1 Physical properties of LBE liquid. Fluid 3
Density (kg/m ) Viscosity (Pa s) Surface tension (N/m)
References Water (20 °C)
LBE (400 °C)
998 1.005 10 0.0728
10,210 1.591 10 0.397
3
3
experimental pressure drop is 8.15 kPa, and the simulated result is 8.01 kPa. Fig. 14 shows the flow structure under the flow rate of 6 m3/h. A swirling flow with circumferential velocity is formed when the liquid passing the impeller as shown in the Fluid streamlines in Fig. 14(a). The axial velocity profile in the swirling flow shows an ‘‘M’’ type due to the centrifugal force, which leads to the concave free surface at the outlet of the inner guide tube. No recirculation zone is observed below the free surface as shown in the Fig. 14(b), which overcome the difficult in windowless targets based on downward flow due to gravity (Su et al., 2012).Therefore, the deposited heat due to spallation reaction can be taken away effectively for this new designed windowless target. The simulated free surface with LBE liquid under the flow rate of 5 m3/h is shown in Fig. 15. The physical properties of LBE liquid are tabulated in Table 1. It can be found that the shape of the free surface of LBE is close to that of water at the same flow rate except that its location is a little bit lower. Therefore, the current windowless target design is applicable for the real working liquid, LBE. 6. Conclusion This work proposes and validates a new windowless target design based on upward swirling flow. The experimental results show that the stable free surface is achieved when the inlet flow rate is higher than 4 m3/h. When the flow rate is less than 4 m3/ h, the tangential velocity driven by the impeller is too low at the
ANSYS, 2012. Fluent-12 Manuals. Batta, A., Class, A., 2007. Numerical investigations on geometrical designs of the windowless XT ADS spallation target. In: Proceeding of ICAPP 2007, Nice, France, May 13–18, paper 7479. Batta, A., Class, A., 2008. Free surface modeling and simulation of the water experiment for the XT ADS spallation target. In: Proceeding of ICAPP ’08, Anaheim, CA USA, June 8–12, paper 8049. Cheng, X., Pettan, C., Knebel, J.U., 2008. Experimental and numerical studies on thermal-hydraulics of spallation targets. Emerging Nuclear and Transmutation Systems: core Physics and Engineering Aspects. Argonne, Illinois, U.S.A., pp. 308–325, 28 Nov.-1 Dec.. Class, A.G., 2011. XT-ADS windowless spallation target thermo-hydraulic design & experimental setup. J. Nucl. Mater. 415, 378–384. Fazio, C., Gröschel, F., Wagner, W., et al., 2008. The MEGAPIE-TEST project: supporting research and lessons learned in first-of-a-kind spallation target technology. Nucl. Eng. Des. 238, 1471–1495. Futakawa, M., Haga, K., Wakui, T., et al., 2009. Development of the Hg target in the JPARC neutron source. Nucl. Instrum. Methods Phys. Res. A 600, 18–21. Liu, J., Gao, L., Yang, L., Lu, W.Q., 2014. Effect of adding a swirl on flow pattern and recirculation zone in ADS windowless spallation target. Nucl. Eng. Des. 276, 249–258. Mansur, L.K., Gabriel, T.A., Haines, J.R., et al., 2001. R&D for the spallation neutron source mercury target. J. Nucl. Mater. 296, 1–16. Roelofs, F., Jager, B.de, Class, A., et al., 2008a. European research on HLM thermal hydraulics for ADS applications. J. Nucl. Mater. 376, 401–404. Roelofs, F. et al., 2008. Application of a controlled swirl in the XT-ADS spallation target. In: Proceeding of ICAPP’08, Anaheim, CA, USA, June, 8–12. Roelofs, F. et al., 2008. Comparison of the hydraulics in concentric and three feeder XT-ADS windowless spallation target designs. In: Proceedings of ICAPP.08, Anaheim, CA, USA, June 8–12, paper 8029. Schuurmans, P., Guertin, A., Buhour, J.M., et al., 2007. Design and R&D support of the XT-ADS spallation target. In: Nuclear Science Utilisation and Reliability of High Power Proton Accelerators: WorkShop Proceedings, Mol, Belgium, 6–9 May, 245–252. Su, G.Y., Gu, H.Y., Cheng, X., 2012. Experimental and numerical studies on free surface flow of windowless target. Ann. Nucl. Energy 43, 142–149. Tak, N., Neitzel, H.J., Cheng, X., 2005. Computational fluid dynamics analysis of spallation target for experimental accelerator-driven transmutation system. Nucl. Eng. Des. 235, 761–772. Tichelen K.V., Kupschus P., Arien B., et al., 2007. Design and verification experiments for the windowless spallation target of the ADS prototype MYRRHA. In: Nuclear Science Utilisation and Reliability of High Power Proton Accelerators: Work Shop Proceedings, Mol, Belgium, 6–9 May, 311–325.
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Experimental and numerical analysis of a new windowless target design of ADS Hanyang Gu a,⇑, Xingliang Zhang a, Shenjie Gong a, Donghua Lu b, Xu Cheng a a b
School of Nuclear Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China China Nuclear Power Technology Research Institute, China Guangdong Nuclear Power Group, 518026 Shenzhen, China
a r t i c l e
i n f o
Article history: Received 17 September 2014 Received in revised form 1 February 2015 Accepted 4 February 2015 Available online 3 March 2015 Keywords: ADS Windowless target Free surface Upward swirling flow
a b s t r a c t The formation and control method of the coolant free surface is one of the key technologies for the design of windowless targets in the Accelerator Driven Systems (ADSs). In this work, a new design of windowless target for ADS based on upward swirling flow is proposed to form a stable free surface without recirculation zones. Experimental and numerical investigations on the free surface flow have been performed in a prototypical windowless target using water as working fluid. The experimental results show that a stable free surface is achieved when the mass flow rate is higher than 4.0 m3/h and the shape of free surface profile greatly depends on the inlet flow rate. CFD simulations have been carried out using VOF method with different turbulent models. The numerical results show that the prediction of BSL Reynolds stress model agrees qualitatively well with the experimental results. The free surface profile of Lead–Bismuth-Eutectic (LBE) in the windowless target is very similar to that of water according to the simulation results. The feasibility of the new windowless target for ADS based on upward swirling flow is preliminary validated. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction With more and more nuclear power plants being operated, management and disposal of high level nuclear wastes have been getting increased attention in recent years. Accelerator Driven System (ADS) is one of the most promising transmutation system which has been investigated worldwide to reduce minor actinides and long life fission products (Roelofs et al., 2008a). In center of the ADS subcritical core, a spallation target provides the primary neutron source which is then amplified by the subcritical core so as to achieve further fission and transmutation. At present, two types of spallation targets have been proposed and designed: solid window target and windowless target. During the design process of solid window target, two problems related with the solid window have to be concerned and resolved: (1) how to remove the intense heat produced during the operation. (2) How to endure the long-term irradiation by proton attacking and corrosion by heavy liquid metal at high temperature. Various designs of solid window targets had been developed and tested (e.g., Mansur et al., 2001; Tak et al., 2005; Fazio et al., 2008; Cheng et al., 2008; Futakawa et al., 2009).
⇑ Corresponding author. E-mail address: [email protected] (H. Gu). http://dx.doi.org/10.1016/j.anucene.2015.02.022 0306-4549/Ó 2015 Elsevier Ltd. All rights reserved.
In order to reduce the material requirements and increase the service life of target system, the second type spallation target, windowless target, is proposed and has attracted much attention. Within the EUROTRANS project, several windowless targets for the experimental accelerator driven facility XT-ADS were presented (Class, 2011). In windowless targets, by the influence of gravity, the heavy liquid metal flows downward in the circular nozzles surrounding the proton beam tube. The heavy metal free surface formed below the nozzle is desired to be stable and avoid splashing. In addition, recirculation zones should be avoided to prevent local overheating because the interaction of the proton beam with the heavy liquid metal will produce great heat. In order to establish criteria for windowless target designs, experimental and numerical studies of windowless targets were carried out within the EUROTRANS project. Tichelen et al. (2007) conducted water experiments using a Laser Doppler Anemometry (LDA) to present the velocity profile in the target. According to the experimental results with the working fluids of water and subsequent Hg, recirculation zones were observed clearly (Tichelen et al., 2007; Schuurmans et al., 2007; Batta and Class, 2007). To make the recirculation zones small and stable, three different designs were presented by Roelofs et al. (2008c). Furthermore, Roelofs et al. (2008b,c) proposed a proper swirl at the inlet of the target which could reduce the recirculation zone and stabilize the free surface (Batta and Class, 2008). Su et al. (2012) performed experimental and numerical investigations on
H. Gu et al. / Annals of Nuclear Energy 80 (2015) 348–355
the free surface flow in a scaled windowless target model referring to the design of XT-ADS (Batta and Class, 2007, 2008) using water as working fluid. The results showed that the recirculation zone is difficult to be eliminated in those designs. Liu et al. (2014) carried out numerical simulations to investigate the swirling affection on the flow pattern and the results proved that the added swirls make the recirculation zone small and stable. But a cavitation zone appears with the increase of swirling strength. In the present work, a new design of windowless target for ADS based on an upward swirling flow is presented. The free surface is stable and the recirculation zone is eliminated. Experimental and numerical studies are carried out to confirm the feasibility of the windowless target design. 2. A new windowless target design The windowless target design is as shown in Fig. 1. The windowless target is composed of the inside and outside guide tubes. An impeller with 10 twisted blades is fixed in the inner guide tube. The heavy metal liquid flows upward from the bottom of the inner guide tube with an impeller inside, and then flows downward at the top of the inner guide tube. Due to the existence of impeller,
a swirling flow with circumferential velocity is formed when the liquid passing the impeller. Under the action of centrifugal force, there forms a concave free interface at the outlet of inner guide tube. After that, the liquid flows downward through the annular space between the inner guide tube and outer guide tube under the gravity force. During such process, the deposited heat, which is produced by the spallation reaction as the proton beam passing through the free surface, is taken away from the target. 3. Experimental setup In order to validate the feasibility of the new designed target as shown in Fig. 1, a prototypical test section as shown in Fig. 2 is designed and manufactured. The diameters of the inner guide tube and outer guide tube are 70 mm and 203 mm, respectively. The impeller is fixed in the center of inner guide tube with a distance of 530 mm to the outlet of the upper end of the inner guide tube. Fig. 2(b) shows the structure of the impeller in detail. The center shaft of the impeller has a diameter of 22 mm and a total length of 92 mm. Ten blades with a spiral angle of 50° are evenly located on the outer surface of the center shaft. The test section is installed vertically, and water is used as the working fluid in this work. The
Proton beam
Free surface
Inner guide tube
Outer guide tube
Impeller
Outflow
Inflow
349
Outflow
(a) windowless target Fig. 1. A new windowless target design.
(b) Impeller
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(a)
(b)
Fig. 2. Test section. (a) Schematic of test section (b) Impeller
Fig. 3. Test loop.
water flows upward from the bottom of the inner guide tube into the test section. After flowing through the impeller, a swirling flow is formed and a concave free surface appears at the outlet of inner guide tube. At last, the water flows downward in the annular space between the inner tube and outer guide tube driven by the gravity force. The outlet of the inner guide tube is open to the environment
directly. The pressure drop of the impeller is measured by a pressure sensor installed at the inlet of inner guide tube as shown in Fig 2(a). The test is performed in a hydraulic test loop as shown in Fig. 3. Water is driven by a centrifugal pump. The flow rate is controlled by electric regulating valves in the main loop and bypass loop. The
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H. Gu et al. / Annals of Nuclear Energy 80 (2015) 348–355
100 50 0 -50 H [mm]
3
4.0 m /h 3 5.0 m /h 3 6.0 m /h 3 8.0 m /h 3 10.0m /h
-100 -150 -200 -250
Inner guide tube
-300 -80
-60
-40
-20
0
20
40
60
80
R [mm] Fig. 6. Free surface profiles.
4. Experimental results Fig. 4. Free surface measuring system.
flow rates of the test conditions are measured by a mass flowmeter. The profile of free surface is measured by a conductivity probe as shown in Fig. 4. The conductivity probe is fixed on a vernier caliper vertically. A high electrical signal is obtained once the conductivity probe moves downward and reaches the free surface of the water, which indicating the location of local free surface. Thus, the free surface profiles at inner guide tube outlet can be obtained by moving the vernier calipers vertically and horizontally.
Fig. 5 gives the experimental visualization of free surfaces at the outlet of inner guide tube under different flow rates. When the flow rate is larger than 4 m3/h, a center concave stable free surface is formed at the outlet under the influence of centrifugal force. The concave depth increases with the increase of the flow rate. Fig. 6 gives the measured free surface profiles with different flow rates. The free surfaces are symmetrical and stable. With the increase of the flow rate, the centrifugal force produced by the tangential velocity increases, and the location of the center point of liquid separation inside the inner guide tube descends dramatically. When the flow rate increases from 5.0 m3/h to 10 m3/h, the location of the center point of liquid separation descends from
4 m3/h
7 m3/h
9 m3/h
11 m3/h Fig. 5. Free surface visualization.
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5. Numerical simulation
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DP
8
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K [-]
DP [kPa]
K
CFD analyses have been performed for the windowless target (Fig. 2) to get more detailed insight into flow configuration and free surface formation mechanism. Three-dimensional transient numerical simulations are performed using the commercial CFD code Fluent in this work. 5.1. Computational model
6
60
5
30 2
4
6
8
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3
Q [m /h] Fig. 7. Pressure drop in the windowless target.
38.8 mm to 285.7 mm. Meanwhile, the outlet liquid thickness of free surface decreases due to the increase of the tangential velocity at the bell mouth of the inner guide tube. Fig. 7 shows that the pressure drop in the inner guide tube, measured by the pressure sensors installed at the inlet as shown in Fig 2(a), increases from 5.4 kPa to 8.1 kPa with the increase of the flow rate from 4.0 m3/h to 10 m3/h. Correspondingly, the resistance coefficient based on the inlet velocity of inner guide tube drops from 128.6 to 31.2. The pressure drop is mainly produced by the form drag when water flows through the impeller and forms circumferential flow.
The VOF method (Volume of Fluid) is adopted to treat the free surface flow, in which the free surface separating air and water is distinguished by a phase indicator function and a corresponding transport equation (Class, 2011; Liu et al., 2014). It is well known that one of the key factors affecting the accuracy of CFD predicting results is the turbulence modelling. Three two-equations eddy viscosity turbulent models (i.e. the standard k-e model (indicated as k-e in this paper), RNG k-e model (RNG) and SST model (SST)) and two Reynolds stress models Reynolds stress model of Speziale (SSG RSM) and BSL Reynolds stress model (BSL RSM)) are employed. More details about the turbulence models and the corresponding wall treatment can be found in ANSYS manual (2012). Structured mesh as shown in Fig. 8 is adopted in the analyses. For all turbulence models, special wall treatments are needed. The size of the first mesh close to the wall surface is important in the simulation. The dimensionless distance to the wall surface y+ < 2.1 is been used in the present study. Based on the grid independence test, 8,000,000 grids are employed, as shown in Fig. 9. The boundary conditions are defined as follows: (1) the walls of guide tube and impeller are defined as no slip wall condition. (2) The water inflow condition is a uniform velocity normal to the inlet
Fig. 8. Computation mesh.
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of the inner guide tube. (4) The upper air outflow boundary is imposed with atmosphere pressure. (5) The gravity force is considered.
results based on different turbulent models. It can be seen that the numerical results obtained by different turbulent models successfully simulate the concave type free surface at the outlet, while the predicting accuracies by various models comparing with
5.2. Numerical results 100
Fig. 10 shows the comparison of free surface profile under the flow rate of 6 m3/h between the experimental data and simulation 50
Experiment CFD by BSL RSM with different mesh number 2500000 3000000 6000000 8000000 10000000
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Fig. 9. Grid independence test.
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Experiment CFD prediction
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Fig. 10. Prediction with different turbulent model.
5m3/h
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Fig. 13. Comparison between numerical and experimental results of pressure drop.
7 m3/h
8 m3/h
Fig. 11. Prediction by BSL RSM.
9 m3/h
10 m3/h
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1.6
Axial velocity at H=-92mm
Axial Velocity [m/s]
1.4
6 m3/ h
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 Radius [mm]
(a) Fluid streamlines
(b) Fluid field below the free surface at the outlet Fig. 14. Flow field in the windowless target.
experimental data are different. The locations of free surface profiles simulated by the two-equations eddy viscosity models (i.e. k-e, RNG and SST) are obviously higher than that of the experimental result. Generally, the linear isotropic two-equation eddy viscosity models are not applicable to predict the swirling flow with strong anisotropic characteristics. The predictions by SSG and BSL RSM models approach the experimental result well, excepting the center region of the concave free surface. The bottom coordinate value of concave free surface at the center is around 107 mm while the corresponding predicted values by SSG and BSL RSM model are around 65 mm and 70 mm, respectively. In summary, BSL RSM model is the best one for predicting the free surface profile. Therefore, BSL RSM model is adopted in the following analyses. The free surfaces profiles simulated with BSL RSM model under different flow rates are as shown in Fig. 11. It can be found that a
concave stable free surface is formed at the outlet when the flow rates are within 5 m3/h–10 m3/h. The location of the lowest point of free surface moves downward with the increase of flow rate, which agrees well with the experimental results. The comparison between the experimental results and the numerical predictions on the free surface is given in Fig. 12. The predicted profile agrees well with the experimental result when the flow rate is 5 m3/h. With the increase of the flow rate, the center of the free surface obtained in the experiment moves downward faster than that in the simulation. But, in general, the simulated free surface profiles by BSL RSM model are quantitatively coincident with the experimental results except the central area. Fig. 13 shows the comparison of the pressure drops between the experimental results and the numerical predictions. It can be found that the numerical predictions agree well with the experimental results. For example, when the flow rate is 10 m3/h, the
H. Gu et al. / Annals of Nuclear Energy 80 (2015) 348–355
outlet of the inner guide tube. As a result, the centrifugal force produced by the tangential velocity is too weak to keep a stable free surface at the outlet. The CFD study using the VOF model is also performed. The numerical results show that the prediction of BSL Reynolds stress model agrees qualitatively well with the experimental data. No recirculation zone, which deteriorates the heat transfer, exists in the flow field of the windowless target based on the numerical analyses. The free surface profile of LBE is quite similar to that of water under the same inlet flow rate. The feasibility of the new windowless target for ADS based on upward swirling flow is preliminary validated. Detailed optimization design and corresponding experiments using LBE are carrying out.
50 25 3
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40
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Acknowledgement
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R [mm]
The authors are grateful to National Natural Science Foundation of China (No. 91026020) for providing the financial support for this study.
Fig. 15. Free surface curves of liquid LBE and water.
Table 1 Physical properties of LBE liquid. Fluid 3
Density (kg/m ) Viscosity (Pa s) Surface tension (N/m)
References Water (20 °C)
LBE (400 °C)
998 1.005 10 0.0728
10,210 1.591 10 0.397
3
3
experimental pressure drop is 8.15 kPa, and the simulated result is 8.01 kPa. Fig. 14 shows the flow structure under the flow rate of 6 m3/h. A swirling flow with circumferential velocity is formed when the liquid passing the impeller as shown in the Fluid streamlines in Fig. 14(a). The axial velocity profile in the swirling flow shows an ‘‘M’’ type due to the centrifugal force, which leads to the concave free surface at the outlet of the inner guide tube. No recirculation zone is observed below the free surface as shown in the Fig. 14(b), which overcome the difficult in windowless targets based on downward flow due to gravity (Su et al., 2012).Therefore, the deposited heat due to spallation reaction can be taken away effectively for this new designed windowless target. The simulated free surface with LBE liquid under the flow rate of 5 m3/h is shown in Fig. 15. The physical properties of LBE liquid are tabulated in Table 1. It can be found that the shape of the free surface of LBE is close to that of water at the same flow rate except that its location is a little bit lower. Therefore, the current windowless target design is applicable for the real working liquid, LBE. 6. Conclusion This work proposes and validates a new windowless target design based on upward swirling flow. The experimental results show that the stable free surface is achieved when the inlet flow rate is higher than 4 m3/h. When the flow rate is less than 4 m3/ h, the tangential velocity driven by the impeller is too low at the
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