- Email: [email protected]
High heat flux test of tungsten brazed mock-ups developed for KSTAR divertor
G Model
ARTICLE IN PRESS
FUSION-8674; No. of Pages 4
Fusion Engineering and Design xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
High heat flux test of tungsten brazed mock-ups developed for KSTAR divertor J.H. Song a , K.M. Kim a,∗ , S.H. Hong a , H.T. Kim a , S.H. Park a , H.K. Park a , H.J. Ahn a , S.K. Kim b , D.W. Lee b a b
National Fusion Research Institute, Daejeon, Republic of Korea Korea Atomic Energy Research Institute, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 31 August 2015 Received in revised form 22 February 2016 Accepted 10 March 2016 Available online xxx Keywords: Tungsten brazed flat type KSTAR divertor High heat flux test Finite element analysis (FEA)
a b s t r a c t The tungsten (W) brazed flat type mock-up which consists of W, OFHC-Cu (oxygen-free high conductive copper) and CuCrZr alloy has been designed for KSTAR divertor in preparation for KSTAR upgrade with 17 MW heating power. For verification of the W brazed mock-up, the high heat flux test is performed at KoHLT-EB (Korea High Heat Load Test Facility-Electron Beam) in KAERI (Korea Atomic Energy Research Institute). Three mock-ups are tested for several thousand thermal cycles with absorbed heat flux up to 5 MW/m2 for 20 s duration. There is no evidence of the failure at the bonding joints of all mock-ups after HHF test. Finite element analysis (FEA) is performed to interpret the result of the test. As a result, it is considered that the local area in the water is in the subcooled boiling regime. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The divertor will be exposed to heat and particle loads during plasma operation in fusion devices. One of the most important functionality of the divertor is to withstand such high heat and particle loads. In KSTAR, graphite tiles have been used as Plasma Facing Component (PFC) since the first plasma operation in 2008. As the graphite tiles are mechanically bolted on heat sink plates, the cooling efficiency between tiles and heat sink plates are not good. In order to enhance the thermal contact between them, thin carbon sheets have been inserted. For the plan, so called base mode, of KSTAR upgrade corresponds to a total heating power of 17 MW (a heat flux on the divertor, 4.3 MW/m2 ) and a pulse duration of 20 s, the KSTAR team has been planning to develop the divertor based on metals [1]. Divertor technologies involved in the development of high heat components are tungsten (W) bonding technology, active cooling, and shaping of the castellation structure. For the development of W bonding technology for the KSTAR divertor launched in early 2013, the W brazed mock-up has been designed; the plasma facing tile using pure W, the interlayer using oxygen-free high conductive copper
∗ Corresponding author. E-mail address: [email protected] (K.M. Kim).
(OFHC-Cu), the block and tube using CuCrZr alloy. To confirm the integrity of the W brazed mock-up, the high heat flux (HHF) test is performed with absorbed heat flux up to 5 MW/m2 . Also, finite element analysis (FEA) is performed to interpret the result of HHF test.
2. Experimental setup and analysis method 2.1. Fabrication of test mock-up The flat type mock-ups are fabricated by vacumm brazing process. The optimized brazing conditions are found to be a surface roughness on W of 6 m Rs and a loading of 20 kPa at 980 ◦ C for 30 min [2]. Total six mock-ups are fabricated, and then the three mock-ups (mock-up #2, #3, and #4) are selected for HHF test by the quality control conducted using ultrasonic test. Fig. 1 shows the schematic and photograph of the test mock-ups. The mock-up consists of eight W tiles. The two slots are machined for thermocouples (TCs) through blocks to reach the tile. The size of the mock-up is 28 mm × 50 mm area and 40 mm height (W is 5 mm, OFHC-Cu is 2 mm, and CuCrZr is 33 mm in height) with a cooling tube of 12 mm inner and 15 mm outer diameter. Total length of the tube is 150 mm. In Table 1, the details of the material specification used for fabrication is listed.
http://dx.doi.org/10.1016/j.fusengdes.2016.03.050 0920-3796/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: J.H. Song, et al., High heat flux test of tungsten brazed mock-ups developed for KSTAR divertor, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.03.050
G Model FUSION-8674; No. of Pages 4
ARTICLE IN PRESS
2
J.H. Song et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
Fig. 1. Schematic and photograph of test mock-ups for HHF test.
2.2. Test facility and conditions The Korea High Heat Load Test facility-Electron Beam (KoHLTEB) in Korea Atomic Energy Research Institute (KAERI) is used for the HHF test on W brazed mock-ups. The specification of KoHLTEB is as follows: a maximum allowable beam power is 300 kW with electron gun (a total beam power is 800 kW), and maximum accelerating voltage is 60 kV where the allowable target dimension is 70 cm × 50 cm in a vacuum chamber [3]. The temperature of the mock-up are measured by the calorimetry, thermocouple, and pyrometer: the coolant temperature is measured by the calorimetry. The surface temperature of the mock-up is measured by the one-color pyrometer to monitor the abnormal event. The local temperature of the mock-up is measured by two K-type TCs which are inserted in 3 mm below W surface (TC1) and 2 mm below Cu/CuCrZr interlayer (TC2). The high heat flux test is performed under water cooling conditions corresponding to an inlet temperature of 16.8 ◦ C, a pressure of 0.354 MPa, and a flow rate 0.35 kg/s. The incident heat flux to the mock-up is calculated based on the beam power and surface area. The absorbed heat flux to the mock-up is estimated from the coolant water calorimetry.
Fig. 2. Mesh used for FEM analysis.
2.4. Finite element analysis
2.3. Thermal cyclic test Screening tests are preliminarily performed to evaluate the thermal fatigue quality of mock-up. The applied power on the mock-up is increased stepwise from shot to shot [4]. The three mock-ups are exposed to absorbed heat fluxes ranging from 0.18 to 4.7 MW/m2 for 30 s pulse duration. Thermal cyclic tests are conducted to study the thermal fatigue behavior of the mock-ups and to validate the manufacturing performance. The eight W tiles with total area 1323 mm2 are loaded with absorbed heat flux 5 MW/m2 for 20 s heat-on and 20 s heatoff. Because of the thermal shield installed in the test facility to protect the mock-up excepting for eight W tiles, the real heated area is decreased from 1400 mm2 (28 mm × 50 mm) to 1323 mm2 (27 mm × 49 mm).
Transient thermal hydraulic analysis is performed on a model with the finite element method (FEM) by using ANSYS-CFX. The ITER specifications data is considered for all these material properties [5]. As shown in Fig. 2, the finite element mesh is composed of tetra- and hexa-hedral elements for 954,761 nodes and 1,479,964 elements. Individual mesh size of 0.5–1 mm is used in all parts. To describe the effects at the boundary between tube and fluid more accurately, an inflation of 5 layers is employed to the fluid surface. For turbulence modeling, shear stress transport (SST) viscosity model is used. 3. Results and discussion 3.1. Experimental results Table 2 summarizes the HHF test results of mock-ups. All mockups are successfully survived under the high heat flux of about 5 MW/m2 with 20 s heating and 20 s cooling time. The mock-up
Table 1 Material specification. No.
Parts
Information
1 2 3 4
Plasma facing material (tile) Heat sink (block and tube) Interlayer Brazing alloy
W: ITER specifications, 99.94% pure W CuCrZr alloy: ITER specifications, Cu-base, Cr (0.6–0.9%), Zr (0.07–0.15%) OFHC-Cu: C10200 (O < 10 ppm) NiCuMn filler: Plate type, Ni (9.5%), Cu (52.5%), Mn (38%)
Please cite this article in press as: J.H. Song, et al., High heat flux test of tungsten brazed mock-ups developed for KSTAR divertor, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.03.050
G Model
ARTICLE IN PRESS
FUSION-8674; No. of Pages 4
J.H. Song et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
3
Table 2 HHF test results of test mock-ups. Mock-up no.
Absorbed heat flux [MW/m2 ]
No. of cycle [cycle]
On/off time [s]
Max. temp. of surface [◦ C]
Max. temp. of W (3 mm depth) [◦ C]
Max. temp. of CuCrZr (2 mm depth) [◦ C]
Remark
#2 #3 #4
∼5 ∼5 ∼5
2000 1000 1000
20/20 20/20 20/20
520 – 471
386 415 369
244 243 242
No failure No failure No failure
Fig. 5. Local temperature distributions of mock-up #2 during from 42nd to 44th cycle.
Fig. 3. Photographs of mock-ups after HHF test.
areas are observed, it is not considered as cracks or delamination. It is expected that large holes originally appear during the brazing process, because these shapes are rounded and smooth. Fig. 5 shows the local temperature distributions of mock-up #2 under the absorbed heat flux of 5 MW/m2 with 20 s duration time for three consecutive cycles (42nd–44th cycle). The maximum local temperatures of W (TC1) and CuCrZr (TC2) are 385.90 ◦ C and 244.13 ◦ C, respectively. The time evolution of the temperature observed by TC1 and TC2 are consistent showing cyclic pattern. For all mock-ups, the similar thermal response have been observed. As a fixed emissivity of W surface is set for the one-color pyrometer during the experiment, the surface temperature is monitored for abnormal event of sudden temperature increase and no such event has occurred. The maximum surface temperature of the mock-up #2 is measured at about 506.71 ◦ C. 3.2. Thermal hydraulic analysis results
Fig. 4. SEM images of mock-ups after HHF test.
#2 is tested for 2000 cycles, and the mock-up #3 and #4 are tested for 1000 cycles without any sudden increase in temperature. As shown in Fig. 3, there is no evidence of delamination and failure at the bonding joints of all mock-ups after HHF test. Before HHF tests, the two mock-ups (#2 and dummy) are tested for the screening test. After the screening test, some black spots are observed on the W surface in the mock-up #2. So, the black spots are considered some of the initial dust in the vacuum chamber for the screening test. To evaluate the conditions of brazed bonding interlayer after HHF tests, Scanning Electron Microscope (SEM) images of mockups are analyzed as shown in Fig. 4. Within W/OFHC-Cu interface, there are no crack and delamination. But, the cross section morphology of W layer of mock-up #4 is quite different. It is supposed that there is the difference of a polishing process in mock-up #4 for a SEM. Within OFHC-Cu/CuCrZr interface, although some porous
To predict the temperature profile of the tested W mock-up, the thermal hydraulic analysis is performed. Also, the thermal cyclic analysis is done to compare results with the experimental observation of 43rd cycle data. The same coolant conditions and heat load are used as follows: a water flow rate of 0.35 kg/s at 16.8 ◦ C and 0.354 MPa and a heat flux of 5.10 MW/m2 . With regard to the critical heat flux (CHF), Tong-75CHF correlation for a smooth tube is used [6]. Based on the coolant conditions, the incident critical heat flux (ICHF) is obtained as 19.64 MW/m2 . Two types of transient thermal hydraulic analysis are performed for 20 s heat-on: the one considers in the single-phase flow (SPF) and the other considers in the subcooled boiling regime (SBR). The first thermal hydraulic analysis considered in the SPF is performed. Since the wall temperature calculated as 330.42 ◦ C exceeds the water saturation temperature of 139.2 ◦ C (at 0.35 MPa), the prediction indicates that the local area in the water is in the SBR is considered. To verify this prediction, the second thermal hydraulic analysis to describe the SBR is performed by including the wall boiling model [7]. Fig. 6 shows the comparison of the local temperature obtained by experiment and simulation considering the SPF of mock-up #2
Please cite this article in press as: J.H. Song, et al., High heat flux test of tungsten brazed mock-ups developed for KSTAR divertor, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.03.050
G Model FUSION-8674; No. of Pages 4
ARTICLE IN PRESS
4
J.H. Song et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
Fig. 6. Comparison of the local temperature obtained by experiment and simulation considering the single-phase flow (SPF) of mock-up #2 (43rd cycle).
Fig. 8. Temperature profiles of mock-up and tube at 20 s heat-on considering subcooled boiling regime (SBR).
Fig. 7. Comparison of the local temperature obtained by experiment and simulation considering the subcooled boiling regime (SBR) of mock-up #2 (43rd cycle).
temperature profile of the tube, the effect of subcooled boiling is found by the continuous cooling area behind the mock-up. 4. Conclusions
at 43rd cycle. Fig. 7 shows the comparison of the local temperature obtained by experiment and simulation considering the SBR of mock-up #2 at 43rd cycle. At 20 s, maximum temperatures are summarized as shown in Table 3. These values show that the temperature difference considering the SBR (Fig. 7) is smaller than that considering the SPF (Fig. 6). Because the SBR has an increase of the heat removal capacity from the wall. So, the heat transfer coefficient (HTC) of the water is increased. During the heat-off period (20–40 s), there are the slope differences of temperature between the SBR analysis and the experiment as shown in Fig. 7. Those are supposed that the W mock-up is not brazed perfectly, so thermal contact resistance in W/OFHC and OFHC/CuCrZr interface are expected to occur. And those seems that the thermal contact resistance is more likely to affect the heat-off than heat-on period. As a result, the simulated local temperatures considering the SBR show good agreement with the experimentally observed local temperatures than that considering the SPF. So, the local area of the water is considered in the SBR. Fig. 8 shows the temperature profiles for mock-up and tube at 20 s heat-on considering the SBR. The maximum temperature corresponds to a mock-up of 509.49 ◦ C and a tube of 232.16 ◦ C. In the
Table 3 Summary of results at 20 s heat-on. Position
Max. temperature [◦ C]
TC1
Test
385.9
TC2
Test
244.13
SPF SBR SPF SBR
T [◦ C] 507 412.7 361.64 269.24
121.1 26.8 117.51 25.11
Three W brazed mock-ups are tested at KoHLT-EB in KAERI. The high heat flux test is performed under high heat flux of about 5 MW/m2 for 20 s duration up to 2,000 cycles. There is no evidence of the failure at the bonding joints of all mock-ups after HHF test. The FEA of mock-ups is also performed to interpret the results of the experiment. As a result, the local area in the water is considered in the subcooled boiling regime. Acknowledgments This work is supported by the Ministry of Science, ICT and Future Planning, the Republic of Korea. References [1] Korea Superconducting Tokamak Advanced Research Project: Development of Tokamak Structure and Vacuum System, Report of KBSI, 1998, RP 17-2-1. [2] K.M. Kim, et al., Manufacturing & high heat flux testing of tungsten brazed mockups in KSTAR, in: Proceeding of the 26th IEEE Symposium on Fusion Engineering (SOFE), Texas, USA, 2015. [3] S.K. Kim, et al., Manufacturing and examination for ITER blanket first wall small-scale mockups with KoHLT-EB in Korea, IEEE Trans. Plasma Sci. 143 (2014) 8–1442. [4] J.H. You, H. Bolt, R. Duwe, J. Linke, H. Nickel, Thermomechanical behavior of actively cooled, brazed divertor components under cyclic high heat flux loads, J. Nucl. Mater. 250 (1997) 184–191. [5] ITER, Materials Properties Handbook, 2007. [6] A.R. Raffray, et al., Critical heat flux analysis and R&D for the design of the ITER divertor, Fusion Eng. Des. 45 (1999) 377–407. [7] F. Crescenzi, S. Roccella, E. Visca, A. Moriani, Comparison between FEM and high heat flux thermal fatigue testing results of ITER divertor plasma facing mock-ups, Fusion Eng. Des. 89 (2014) 985–990.
Please cite this article in press as: J.H. Song, et al., High heat flux test of tungsten brazed mock-ups developed for KSTAR divertor, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.03.050
ARTICLE IN PRESS
FUSION-8674; No. of Pages 4
Fusion Engineering and Design xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
High heat flux test of tungsten brazed mock-ups developed for KSTAR divertor J.H. Song a , K.M. Kim a,∗ , S.H. Hong a , H.T. Kim a , S.H. Park a , H.K. Park a , H.J. Ahn a , S.K. Kim b , D.W. Lee b a b
National Fusion Research Institute, Daejeon, Republic of Korea Korea Atomic Energy Research Institute, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 31 August 2015 Received in revised form 22 February 2016 Accepted 10 March 2016 Available online xxx Keywords: Tungsten brazed flat type KSTAR divertor High heat flux test Finite element analysis (FEA)
a b s t r a c t The tungsten (W) brazed flat type mock-up which consists of W, OFHC-Cu (oxygen-free high conductive copper) and CuCrZr alloy has been designed for KSTAR divertor in preparation for KSTAR upgrade with 17 MW heating power. For verification of the W brazed mock-up, the high heat flux test is performed at KoHLT-EB (Korea High Heat Load Test Facility-Electron Beam) in KAERI (Korea Atomic Energy Research Institute). Three mock-ups are tested for several thousand thermal cycles with absorbed heat flux up to 5 MW/m2 for 20 s duration. There is no evidence of the failure at the bonding joints of all mock-ups after HHF test. Finite element analysis (FEA) is performed to interpret the result of the test. As a result, it is considered that the local area in the water is in the subcooled boiling regime. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The divertor will be exposed to heat and particle loads during plasma operation in fusion devices. One of the most important functionality of the divertor is to withstand such high heat and particle loads. In KSTAR, graphite tiles have been used as Plasma Facing Component (PFC) since the first plasma operation in 2008. As the graphite tiles are mechanically bolted on heat sink plates, the cooling efficiency between tiles and heat sink plates are not good. In order to enhance the thermal contact between them, thin carbon sheets have been inserted. For the plan, so called base mode, of KSTAR upgrade corresponds to a total heating power of 17 MW (a heat flux on the divertor, 4.3 MW/m2 ) and a pulse duration of 20 s, the KSTAR team has been planning to develop the divertor based on metals [1]. Divertor technologies involved in the development of high heat components are tungsten (W) bonding technology, active cooling, and shaping of the castellation structure. For the development of W bonding technology for the KSTAR divertor launched in early 2013, the W brazed mock-up has been designed; the plasma facing tile using pure W, the interlayer using oxygen-free high conductive copper
∗ Corresponding author. E-mail address: [email protected] (K.M. Kim).
(OFHC-Cu), the block and tube using CuCrZr alloy. To confirm the integrity of the W brazed mock-up, the high heat flux (HHF) test is performed with absorbed heat flux up to 5 MW/m2 . Also, finite element analysis (FEA) is performed to interpret the result of HHF test.
2. Experimental setup and analysis method 2.1. Fabrication of test mock-up The flat type mock-ups are fabricated by vacumm brazing process. The optimized brazing conditions are found to be a surface roughness on W of 6 m Rs and a loading of 20 kPa at 980 ◦ C for 30 min [2]. Total six mock-ups are fabricated, and then the three mock-ups (mock-up #2, #3, and #4) are selected for HHF test by the quality control conducted using ultrasonic test. Fig. 1 shows the schematic and photograph of the test mock-ups. The mock-up consists of eight W tiles. The two slots are machined for thermocouples (TCs) through blocks to reach the tile. The size of the mock-up is 28 mm × 50 mm area and 40 mm height (W is 5 mm, OFHC-Cu is 2 mm, and CuCrZr is 33 mm in height) with a cooling tube of 12 mm inner and 15 mm outer diameter. Total length of the tube is 150 mm. In Table 1, the details of the material specification used for fabrication is listed.
http://dx.doi.org/10.1016/j.fusengdes.2016.03.050 0920-3796/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: J.H. Song, et al., High heat flux test of tungsten brazed mock-ups developed for KSTAR divertor, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.03.050
G Model FUSION-8674; No. of Pages 4
ARTICLE IN PRESS
2
J.H. Song et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
Fig. 1. Schematic and photograph of test mock-ups for HHF test.
2.2. Test facility and conditions The Korea High Heat Load Test facility-Electron Beam (KoHLTEB) in Korea Atomic Energy Research Institute (KAERI) is used for the HHF test on W brazed mock-ups. The specification of KoHLTEB is as follows: a maximum allowable beam power is 300 kW with electron gun (a total beam power is 800 kW), and maximum accelerating voltage is 60 kV where the allowable target dimension is 70 cm × 50 cm in a vacuum chamber [3]. The temperature of the mock-up are measured by the calorimetry, thermocouple, and pyrometer: the coolant temperature is measured by the calorimetry. The surface temperature of the mock-up is measured by the one-color pyrometer to monitor the abnormal event. The local temperature of the mock-up is measured by two K-type TCs which are inserted in 3 mm below W surface (TC1) and 2 mm below Cu/CuCrZr interlayer (TC2). The high heat flux test is performed under water cooling conditions corresponding to an inlet temperature of 16.8 ◦ C, a pressure of 0.354 MPa, and a flow rate 0.35 kg/s. The incident heat flux to the mock-up is calculated based on the beam power and surface area. The absorbed heat flux to the mock-up is estimated from the coolant water calorimetry.
Fig. 2. Mesh used for FEM analysis.
2.4. Finite element analysis
2.3. Thermal cyclic test Screening tests are preliminarily performed to evaluate the thermal fatigue quality of mock-up. The applied power on the mock-up is increased stepwise from shot to shot [4]. The three mock-ups are exposed to absorbed heat fluxes ranging from 0.18 to 4.7 MW/m2 for 30 s pulse duration. Thermal cyclic tests are conducted to study the thermal fatigue behavior of the mock-ups and to validate the manufacturing performance. The eight W tiles with total area 1323 mm2 are loaded with absorbed heat flux 5 MW/m2 for 20 s heat-on and 20 s heatoff. Because of the thermal shield installed in the test facility to protect the mock-up excepting for eight W tiles, the real heated area is decreased from 1400 mm2 (28 mm × 50 mm) to 1323 mm2 (27 mm × 49 mm).
Transient thermal hydraulic analysis is performed on a model with the finite element method (FEM) by using ANSYS-CFX. The ITER specifications data is considered for all these material properties [5]. As shown in Fig. 2, the finite element mesh is composed of tetra- and hexa-hedral elements for 954,761 nodes and 1,479,964 elements. Individual mesh size of 0.5–1 mm is used in all parts. To describe the effects at the boundary between tube and fluid more accurately, an inflation of 5 layers is employed to the fluid surface. For turbulence modeling, shear stress transport (SST) viscosity model is used. 3. Results and discussion 3.1. Experimental results Table 2 summarizes the HHF test results of mock-ups. All mockups are successfully survived under the high heat flux of about 5 MW/m2 with 20 s heating and 20 s cooling time. The mock-up
Table 1 Material specification. No.
Parts
Information
1 2 3 4
Plasma facing material (tile) Heat sink (block and tube) Interlayer Brazing alloy
W: ITER specifications, 99.94% pure W CuCrZr alloy: ITER specifications, Cu-base, Cr (0.6–0.9%), Zr (0.07–0.15%) OFHC-Cu: C10200 (O < 10 ppm) NiCuMn filler: Plate type, Ni (9.5%), Cu (52.5%), Mn (38%)
Please cite this article in press as: J.H. Song, et al., High heat flux test of tungsten brazed mock-ups developed for KSTAR divertor, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.03.050
G Model
ARTICLE IN PRESS
FUSION-8674; No. of Pages 4
J.H. Song et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
3
Table 2 HHF test results of test mock-ups. Mock-up no.
Absorbed heat flux [MW/m2 ]
No. of cycle [cycle]
On/off time [s]
Max. temp. of surface [◦ C]
Max. temp. of W (3 mm depth) [◦ C]
Max. temp. of CuCrZr (2 mm depth) [◦ C]
Remark
#2 #3 #4
∼5 ∼5 ∼5
2000 1000 1000
20/20 20/20 20/20
520 – 471
386 415 369
244 243 242
No failure No failure No failure
Fig. 5. Local temperature distributions of mock-up #2 during from 42nd to 44th cycle.
Fig. 3. Photographs of mock-ups after HHF test.
areas are observed, it is not considered as cracks or delamination. It is expected that large holes originally appear during the brazing process, because these shapes are rounded and smooth. Fig. 5 shows the local temperature distributions of mock-up #2 under the absorbed heat flux of 5 MW/m2 with 20 s duration time for three consecutive cycles (42nd–44th cycle). The maximum local temperatures of W (TC1) and CuCrZr (TC2) are 385.90 ◦ C and 244.13 ◦ C, respectively. The time evolution of the temperature observed by TC1 and TC2 are consistent showing cyclic pattern. For all mock-ups, the similar thermal response have been observed. As a fixed emissivity of W surface is set for the one-color pyrometer during the experiment, the surface temperature is monitored for abnormal event of sudden temperature increase and no such event has occurred. The maximum surface temperature of the mock-up #2 is measured at about 506.71 ◦ C. 3.2. Thermal hydraulic analysis results
Fig. 4. SEM images of mock-ups after HHF test.
#2 is tested for 2000 cycles, and the mock-up #3 and #4 are tested for 1000 cycles without any sudden increase in temperature. As shown in Fig. 3, there is no evidence of delamination and failure at the bonding joints of all mock-ups after HHF test. Before HHF tests, the two mock-ups (#2 and dummy) are tested for the screening test. After the screening test, some black spots are observed on the W surface in the mock-up #2. So, the black spots are considered some of the initial dust in the vacuum chamber for the screening test. To evaluate the conditions of brazed bonding interlayer after HHF tests, Scanning Electron Microscope (SEM) images of mockups are analyzed as shown in Fig. 4. Within W/OFHC-Cu interface, there are no crack and delamination. But, the cross section morphology of W layer of mock-up #4 is quite different. It is supposed that there is the difference of a polishing process in mock-up #4 for a SEM. Within OFHC-Cu/CuCrZr interface, although some porous
To predict the temperature profile of the tested W mock-up, the thermal hydraulic analysis is performed. Also, the thermal cyclic analysis is done to compare results with the experimental observation of 43rd cycle data. The same coolant conditions and heat load are used as follows: a water flow rate of 0.35 kg/s at 16.8 ◦ C and 0.354 MPa and a heat flux of 5.10 MW/m2 . With regard to the critical heat flux (CHF), Tong-75CHF correlation for a smooth tube is used [6]. Based on the coolant conditions, the incident critical heat flux (ICHF) is obtained as 19.64 MW/m2 . Two types of transient thermal hydraulic analysis are performed for 20 s heat-on: the one considers in the single-phase flow (SPF) and the other considers in the subcooled boiling regime (SBR). The first thermal hydraulic analysis considered in the SPF is performed. Since the wall temperature calculated as 330.42 ◦ C exceeds the water saturation temperature of 139.2 ◦ C (at 0.35 MPa), the prediction indicates that the local area in the water is in the SBR is considered. To verify this prediction, the second thermal hydraulic analysis to describe the SBR is performed by including the wall boiling model [7]. Fig. 6 shows the comparison of the local temperature obtained by experiment and simulation considering the SPF of mock-up #2
Please cite this article in press as: J.H. Song, et al., High heat flux test of tungsten brazed mock-ups developed for KSTAR divertor, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.03.050
G Model FUSION-8674; No. of Pages 4
ARTICLE IN PRESS
4
J.H. Song et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
Fig. 6. Comparison of the local temperature obtained by experiment and simulation considering the single-phase flow (SPF) of mock-up #2 (43rd cycle).
Fig. 8. Temperature profiles of mock-up and tube at 20 s heat-on considering subcooled boiling regime (SBR).
Fig. 7. Comparison of the local temperature obtained by experiment and simulation considering the subcooled boiling regime (SBR) of mock-up #2 (43rd cycle).
temperature profile of the tube, the effect of subcooled boiling is found by the continuous cooling area behind the mock-up. 4. Conclusions
at 43rd cycle. Fig. 7 shows the comparison of the local temperature obtained by experiment and simulation considering the SBR of mock-up #2 at 43rd cycle. At 20 s, maximum temperatures are summarized as shown in Table 3. These values show that the temperature difference considering the SBR (Fig. 7) is smaller than that considering the SPF (Fig. 6). Because the SBR has an increase of the heat removal capacity from the wall. So, the heat transfer coefficient (HTC) of the water is increased. During the heat-off period (20–40 s), there are the slope differences of temperature between the SBR analysis and the experiment as shown in Fig. 7. Those are supposed that the W mock-up is not brazed perfectly, so thermal contact resistance in W/OFHC and OFHC/CuCrZr interface are expected to occur. And those seems that the thermal contact resistance is more likely to affect the heat-off than heat-on period. As a result, the simulated local temperatures considering the SBR show good agreement with the experimentally observed local temperatures than that considering the SPF. So, the local area of the water is considered in the SBR. Fig. 8 shows the temperature profiles for mock-up and tube at 20 s heat-on considering the SBR. The maximum temperature corresponds to a mock-up of 509.49 ◦ C and a tube of 232.16 ◦ C. In the
Table 3 Summary of results at 20 s heat-on. Position
Max. temperature [◦ C]
TC1
Test
385.9
TC2
Test
244.13
SPF SBR SPF SBR
T [◦ C] 507 412.7 361.64 269.24
121.1 26.8 117.51 25.11
Three W brazed mock-ups are tested at KoHLT-EB in KAERI. The high heat flux test is performed under high heat flux of about 5 MW/m2 for 20 s duration up to 2,000 cycles. There is no evidence of the failure at the bonding joints of all mock-ups after HHF test. The FEA of mock-ups is also performed to interpret the results of the experiment. As a result, the local area in the water is considered in the subcooled boiling regime. Acknowledgments This work is supported by the Ministry of Science, ICT and Future Planning, the Republic of Korea. References [1] Korea Superconducting Tokamak Advanced Research Project: Development of Tokamak Structure and Vacuum System, Report of KBSI, 1998, RP 17-2-1. [2] K.M. Kim, et al., Manufacturing & high heat flux testing of tungsten brazed mockups in KSTAR, in: Proceeding of the 26th IEEE Symposium on Fusion Engineering (SOFE), Texas, USA, 2015. [3] S.K. Kim, et al., Manufacturing and examination for ITER blanket first wall small-scale mockups with KoHLT-EB in Korea, IEEE Trans. Plasma Sci. 143 (2014) 8–1442. [4] J.H. You, H. Bolt, R. Duwe, J. Linke, H. Nickel, Thermomechanical behavior of actively cooled, brazed divertor components under cyclic high heat flux loads, J. Nucl. Mater. 250 (1997) 184–191. [5] ITER, Materials Properties Handbook, 2007. [6] A.R. Raffray, et al., Critical heat flux analysis and R&D for the design of the ITER divertor, Fusion Eng. Des. 45 (1999) 377–407. [7] F. Crescenzi, S. Roccella, E. Visca, A. Moriani, Comparison between FEM and high heat flux thermal fatigue testing results of ITER divertor plasma facing mock-ups, Fusion Eng. Des. 89 (2014) 985–990.
Please cite this article in press as: J.H. Song, et al., High heat flux test of tungsten brazed mock-ups developed for KSTAR divertor, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.03.050