- Email: [email protected]
Development of modelling tools for thermo-hydraulic analyses and design of JT-60SA TF coils
Fusion Engineering and Design 86 (2011) 1483–1487
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Development of modelling tools for thermo-hydraulic analyses and design of JT-60SA TF coils Benoit Lacroix a,∗ , Christophe Portafaix a , Pietro Barabaschi b , Jean-Luc Duchateau a , Patrick Hertout a , Valerie Lamaison a , Sylvie Nicollet a , Pascal Reynaud a , Rosaria Villari c , Louis Zani b a
CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France Fusion For Energy, D-85748 Garching, Germany c Euratom-ENEA Association, IT-00044 Frascati, Italy b
a r t i c l e
i n f o
Article history: Available online 17 May 2011 Keywords: Thermo-hydraulics JT-60SA TF coil Cable in conduit conductor Modelling
a b s t r a c t In the framework of the JT-60SA project, the Toroidal Field (TF) coils design has required to address reliably the choice between multiple design options and to calculate the temperature margin criterion for the superconductor. For this purpose, a tool was developed in two stages, interfacing the ANSYS code, used to model the thermal diffusion between the casing and the winding pack, with the GANDALF code which solves the 1D thermo-hydraulics inside each conductor. The first version of this Thermo-hydraulic EXtended TOol (TEXTO) was developed for producing conservative results and has allowed to simulate the fast discharge of the magnet, providing valuable results such as the mass flow expelled from each pancake. In the second stage, the ANSYS code was configured for modelling the helium transport in the casing and in the winding pack, thus computing more realistic transverse heat fluxes to be injected into the GANDALF code for an accurate calculation of the temperature margin. This second version of TEXTO, which integrates the TACOS (Thermo-hydraulic Ansys COmputation Semi 3D) module, has been used for studying the feasibility of positioning the helium inlets in the electrical connections. The temperature margin has then been found close but below the criterion of 1 K. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the framework of the broader approach agreement between Europe and Japan, the JT-60U tokamak, in Naka, Japan, will be upgraded to JT-60SA, a fully superconducting tokamak. Among the components to be provided by Europe, the Toroidal Field (TF) system, which includes 18 coils with NbTi Cable In Conduit Conductors (CICC), will be supplied by France and Italy (coils) and by Germany (superconducting current leads). The main characteristics of the TF coil are presented in Fig. 1. According to the design proposed in [1], the winding pack (WP) is composed of 12 conductor pancakes wound in 6 turns each. Both casing and conductors are cooled by forced circulation of supercritical helium. The conductors He inlets are located in the coil inner side, in order to provide fresh coolant to the inner turns which face both the plasma and the maximal magnetic field (in the straight area of the internal leg). Each He inlet feeds two adjacent pancakes, thus leading to two opposite flow directions, clockwise (CW) and anti-clockwise (ACW).
∗ Corresponding author. Tel.: +33 4 42 25 76 80; fax: +33 4 42 25 26 61. E-mail address: [email protected] (B. Lacroix). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.03.103
Design activities have been performed in order to optimize the TF coils performance, cost and feasibility. Analyses have covered normal operation, including plasma burn and a disruption occurring conservatively at the end of burn. During such a scenario, heat loads in the TF coils are mainly due to energy deposited by neutrons and secondary gammas (nuclear heating), and to eddy current losses associated with the plasma disruption. In order to ensure the safety of the magnet operation, the temperature margin Tma = Tcs–Tcond, with Tcs = current sharing temperature and Tcond = conductor temperature, must remain above 1 K in every conductor. The calculation of the Tma criterion thus requires a thermo-hydraulic analysis of the helium flow along each CICC, and a thermal analysis of the transverse heat diffusion between the stainless steel casing and the winding pack, which leads to a 3D problem. In addition, the fast safety discharge (FSD) of the TF current must ensure the protection of the magnet, in case of a quench occurrence. The conductor design fulfills the hot spot criterion, which specifies a maximal conductor temperature of 150 K or 250 K, according to whether the helium in the cable is taken into account or not during the current decay. Besides, the FSD induces eddy currents and thus heat generation in the casing and the vacuum vessel. The heat deposition in the casing is transferred to the CICCs by a diffusion
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B. Lacroix et al. / Fusion Engineering and Design 86 (2011) 1483–1487
Fig. 3. Schematic view of codes involved in the TACOS/TEXTO tools.
Fig. 1. Main features of JT-60SA TF coil.
process through the insulation and the conductors jackets, resulting in a significant expulsion of He out of the winding pack, which has to be assessed for the design of the cryogenic system. 2. Development of the TACOS/TEXTO tools 2.1. First version of the TEXTO tool For the JT-60SA TF coil design activities, the choice has been made to adopt a pseudo 3D approach based on a 1D thermohydraulic model for the CICCs and on a 2D finite elements model for the transverse thermal diffusion. A first version of the TEXTO tool [2] was thus developed to interface ANSYS (2D thermal) and GANDALF (1D thermo-hydraulics) [3], while taking into account the nuclear losses computed with MCNP5 and the magnetic field computed with TRAPS. The development of this Thermo-hydraulic EXtended TOol (TEXTO) around GANDALF and ANSYS allows to take advantage of two commercial and widely used codes, easy to customize for taking into account different kinds of input data. This first version of TEXTO relies on a conservative approach for the ANSYS 2D thermal model. The calculation of the transient heat flux transferred from the casing to the winding pack is performed in two typical cross sections of the TF coil, one in the internal leg and the other in the external leg. The inner surface of the jackets is submitted to a boundary condition of forced convection, with a constant heat transfer coefficient and the assumption that the He temperature in the winding pack stays constant at 4.4 K. In addition, a penalizing heat balance in the casing cooling channels leads to maximize the casing temperature. The thermal fluxes computed with these penalizing hypotheses, assumed to be uniform along each conductor turn in each leg, are
then injected into GANDALF, which provides directly a conservative calculation of the temperature margin as a function of abscissa and time. 2.2. Implementation of the TACOS module In order to compute the temperature margin in a less conservative but more accurate way, a simplified thermo-hydraulic approach was presented in [1]. The ANSYS thermal model is extended to 10 coil cross sections, 3 in the internal leg and 7 in the external leg, as shown in Fig. 2. The He flow is simulated by transferring the temperature maps inside the cables and the cooling channels, from one section to the next, at each time interval t = x/VHe , with x the distance between 2 sections and VHe the He velocity. This process takes into account the He inlets, the jumps from turn to turn and the He outlets. An equivalent solid material corresponding to He + strands in the cables or only He in the casing channels provides the adequate heat capacity. The main assumption is the invariance of He velocity and pressure along each channel. In comparison with the first version of TEXTO, the implementation of this process in the ANSYS thermal calculation provides much more realistic transverse heat fluxes, but also He temperature along each conductor. This ANSYS adaptation for thermo-hydraulic purpose has thus led to a pseudo 3D model of a whole TF coil, which supports the independent feature of the module named TACOS, for Thermo-hydraulic Ansys COmputation Semi 3D. A schematic view of the codes involved in the TACOS/TEXTO tools is presented in Fig. 3. Several validation calculations were performed in order to quantify the impact of the simplified 1D thermo-hydraulic approach [4]. Results of comparisons between TACOS/TEXTO and GANDALF and VINCENTA codes showed a satisfying consistency, notably regarding the dispersion of conductor temperatures provided by TACOS
Fig. 2. ANSYS TF coil mesh for the pseudo 3D thermo-hydraulic and thermal model. Each coil segment is modelled by its corresponding 2D cross section.
B. Lacroix et al. / Fusion Engineering and Design 86 (2011) 1483–1487 Table 1 Main characteristics of FLOWER downscaled He collecting circuit.
Quench line
Pure He storage tank
Discharge valve Gandalf / Flower
From cryoplant
Line to valve Towards cryoplant
TF CICC
1485
Element
Length
Hydraulic diameter
Section or Volume
Initial conditions
Line to valve
3m
26.6 mm
46.3 mm2
Valve Quench line
– 100 m
– 100 mm
36.4 mm2 36.4 mm2
–
4.63 m3
4.04 bar 4.51 K – 1 bar 300 K 1 bar 300 K
He storage tank–
Fig. 4. GANDALF/FLOWER model of one TF CICC with the He collecting circuit.
and GANDALF, which remains lower than ± 0.05 K at minimum Tma location. This can be explained by the specificity of CICCs thermo-hydraulics: the approach based on the displacement of temperature maps is well adapted for a supercritical He flow under normal operating conditions, characterized by weak compressible effects (moderate velocity ∼ 0.2 m/s and Re ∼ 3000) and by the large thermal inertia of the fluid. The reliability of TACOS/TEXTO being ensured, this tool was then used to evaluate the impact of several TF coil design options, in the framework of the cost and feasibility optimization [5]. 3. Simulation of the fast safety discharge with TEXTO first version As previously mentioned, the fast safety discharge of the magnet induces eddy currents, and thus heat deposition in the casing and in the vacuum vessel. The heat generated in the casing is transferred to the conductors by thermal diffusion through the resin, the insulation layers and the CICCs jackets (2D thermal). This leads to a significant expulsion of He out of the winding pack, which has to be assessed for the design of the cryogenic system (1D thermohydraulics). In order to study such a scenario, the latest version of TACOS/TEXTO is unsuitable, due to the inconsistency between the simplified 1D thermo-hydraulic approach in the conductors and the expected high flow variations. On the opposite, the first version of TEXTO is more appropriate, insofar as the penalizing hypothesis for the heat diffusion calculation contributes to provide conservative results, especially for the He expulsion out of the conductors during the beginning of the discharge. In order to limit the disturbance of the FSD on the cryogenic system operation, it is foreseen to isolate hydraulically the TF windings from the cryoplant (by closing dedicated valves) and to collect the He expelled at conductors outlets, via one controlled or safety valve per coil. This valve should be located as close as possible to the TF coil (a few meters) and linked to He storage tanks by a warm quench line of about 100 m length.
For modelling this circuit, the GANDALF code can be coupled to FLOWER [6], a code dedicated to the simulation of the cryogenic environment of a superconducting magnet. As GANDALF models a single CICC, the collecting circuit described with FLOWER has to be downscaled to the level of one conductor: the volumes and the pipes cross sections are thus reduced accordingly, whereas the hydraulic diameters are kept the same, so that Reynolds numbers are not affected. The model is presented in Fig. 4, and the main features of the downscaled He collecting circuit are listed in Table 1 . As it is limited to a single pancake, the model doesn’t take into account thermal nor hydraulic coupling between the most critical pancakes and the other ones. The heat generation due to the eddy currents induced in the casing has been assessed analytically. Assuming that the TF current (ITF ) exponential decay is not affected by the electromagnetic coupling with the casing and that no energy is generated into the vacuum vessel, the casing heat power (Pcasing ) for one coil can be expressed as follows. M dITF /dt + Rcasing Icasing = 0
with ITF = I0 e−t/
2 ⇒ Icasing = (M I0 /Rcasing ) e−t/ Pcasing = Rcasing Icasing = (M 2 I02 /Rcasing 2 ) e−2t/
Considering a FSD time constant = 10 s, a casing resistance Rcasing = 10.5 , and a winding pack and casing mutual inductance M = 2.42 mH, the casing power in W is Pcasing = 3.684. 106 e−t/5 . This power has to be applied in the casing mass of one coil, neither including EF coils gravity supports nor outer inter coil structures, so into about 8500 kg of stainless steel. The simulation has been performed considering that the discharge valve is opened 1 s after the beginning of the FSD. Conservatively, a perfect contact has been assumed between the casing and the winding pack, and no mass flow has been taken into account in the casing cooling channels. The ANSYS thermal calculation has shown that the casing maximal temperature reaches 56 K at t = 45 s, in both internal and external legs. Due to the materials surrounding the winding pack 30
Lateralpancake
Q (g/s)
30
Lateral pancake
25
Median pancake
20
T He (K)
Valve opening
20
Medianpancake No heatflux on intermediateturns of medianpancake
t=40s
15 t=80s
10 10
t=80s
t=40s
t=40s
5
0
t=80s t=0s
t=0s
t=0s
0 0
20
40
60
80
Time (s) Fig. 5. Mass flow expelled at conductors outlets.
100
0
20
40
60
80
x (m) (abscissa from inlet) Fig. 6. Temperature distributions at times 0, 40 and 80 s.
100
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Fig. 7. Schematic view of He inlets reference and alternative layouts.
Table 2 Results of Tma calculations with both He inlets layouts (the abscissa x is counted from the joggle). He inlets layout
CICC Median
Reference Lateral Median Alternative Lateral
ACW CW ACW CW ACW CW ACW CW
Tma (K)
x (m)
t (s)
Tcond (K)
B (T)
1.31 1.19 1.37 1.20 0.97 1.22 0.99 1.25
8.2 14.8 8.2 14.5 8.2 14.8 8.2 14.5
136 162 136 163 212 307 210 305
4.87 5.00 4.89 5.12 5.22 4.98 5.26 5.08
5.59 5.58 5.44 5.28 5.59 5.58 5.44 5.28
(4 mm of glass-epoxy insulation, plus 5 to 10 mm of epoxy resin), transverse heat fluxes from the casing to the peripheral conductors reach a maximum value within 1 to 4 minutes, and then decay more or less linearly in about one hour. The thermo-hydraulic results have been examined for two pancakes, the lateral one and the median one. As the lateral pancake is in contact with the casing all along its length, it is more critical than the median one which is in contact only on the inner and outer turns. Mass flows expelled at conductors outlets are presented in Fig. 5. For both pancakes, at t = 2.4 s, a peak of about 33 g/s follows the valve opening, this value depending mainly on the conductor initial pressure. Then, 15 s later, as expected, the expelled mass flow is significantly higher in the lateral CICC than in the median one. The temperature profiles along each conductor are linked to the heat deposition, as shown in Fig. 6. As the lateral pancake experiences more heat flux (up to 30 W/m) than the median one (up to 15 W/m), the maximal pressure, reached at conductor inlet, is of 16.2 bar (at t = 64 s) in the lateral pancake, versus 6.2 bar (at t = 84 s) in the median one. 4. Study of an alternative He inlets location with TACOS/TEXTO The present design of JT-60SA TF coils comprises He inlets located in the coil inner side, at high field. Each He inlet is integrated to one double pancake joggle, so about 3.3 m above the median plane, in order to inject fresh He in both CW and ACW conductors as close as possible to the critical zone. Due to the delicate design and integration of the He inlets in the joggles, it could be beneficial to position them at the electrical connections level, at low field on the external side of the TF coil (see Fig. 7). But this alternative layout is expected to decrease the temperature margin, due to the longer flow path between the He inlet and the critical zone. In order to quantify the impact of this alternative He inlet design, the TACOS/TEXTO tool has been used for computing the tempera-
ture margin as reliably as possible for both layouts. The scenario taken into account is the same as in [4], a 100 s burn (from t = 15 to 115 s) followed by a disruption. The main simulation hypotheses are also the same as in [4], excepted static thermal loads which have been here neglected in order to shorten the calculations, considering their limited impact on Tma (less than 0.05 K). Besides, with the alternative layout, due to the half joint electrical resistance of 1 n, the He temperature downstream of the joint at each double pancake inlet is now 4.44 K (Tjoint = 0.04 K with ITF = 25.7 kA and QHe = 4 g/s). The study has focused on the median and lateral double pancakes, so on 4 conductors. With the alternative layout, due to the inversion of the He flow, a CW pancake becomes hydraulically an ACW one. Nevertheless, for an easier understanding, the corresponding pancakes have been still referred as CW hereafter. The minimal temperatures margins have been calculated for both layouts, taking into account the corresponding flow paths in the pseudo 3D model. Results are presented in Table 2. The impact of positioning He inlets at joints levels differs according to the flow direction of the pancake: whereas the margin is slightly increased (by 0.03 and 0.05 K) for the CW pancakes, it is decreased by 0.34 and 0.38 K for the ACW ones. These effects result from the combination of the flow direction with the power distribution (mainly the nuclear heating), which drives the conductor temperature and impacts the margin accordingly. Finally, with outer He inlets, the margin is decreased by 0.3 to 0.4 K for respectively ACW median and lateral pancakes, leading to values of 0.97 and 0.99 K, thus very close but below the criterion of 1 K. 5. Conclusion In the framework of JT-60SA TF coils design, a dedicated thermohydraulic tool was developed in two stages. The first version of TEXTO was based on a conservative approach for computing transverse heat fluxes from casing to conductors. It has been applied to
B. Lacroix et al. / Fusion Engineering and Design 86 (2011) 1483–1487
the TF fast discharge simulation, providing the assessment of the He expulsion out of the winding pack. A similar calculation could be performed for the coil fast discharge in the cold test facility. This TEXTO first version was then upgraded by the implementation of a simplified He flow model in the thermal calculation, namely the TACOS module. The TACOS/TEXTO tool thus provides more accurate results, especially for the Tma criterion calculation. It has allowed to quantify the impact of an alternative location of He inlets at joints level. In the future detailed design phase, these tools could be used for validating possible TF coil design evolutions and for taking into account updated reference scenarios.
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References [1] P. Barabaschi, “JT-60SA Device Design Revision Aimed to Reduction of TF Magnet Cost V6.0”, F4E report BA STP PC 02-7.2 (2008). [2] S. Nicollet, et al., Cryogenics 50 (2010) 18–27. [3] L. Bottura, Journal of Computational Physics 125 (1996) 26–41. [4] B. Lacroix et al., Advances in Cryogenic Engineering AIP Conference Proceedings 1218, pp. 471-479 (2010). [5] C. Portafaix, et al., IEEE Transactions on Applied Superconductivity 20 (2010) 1794–1797. [6] L. Bottura, C. Rosso, Cryogenics 43 (2003) 215–223.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Development of modelling tools for thermo-hydraulic analyses and design of JT-60SA TF coils Benoit Lacroix a,∗ , Christophe Portafaix a , Pietro Barabaschi b , Jean-Luc Duchateau a , Patrick Hertout a , Valerie Lamaison a , Sylvie Nicollet a , Pascal Reynaud a , Rosaria Villari c , Louis Zani b a
CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France Fusion For Energy, D-85748 Garching, Germany c Euratom-ENEA Association, IT-00044 Frascati, Italy b
a r t i c l e
i n f o
Article history: Available online 17 May 2011 Keywords: Thermo-hydraulics JT-60SA TF coil Cable in conduit conductor Modelling
a b s t r a c t In the framework of the JT-60SA project, the Toroidal Field (TF) coils design has required to address reliably the choice between multiple design options and to calculate the temperature margin criterion for the superconductor. For this purpose, a tool was developed in two stages, interfacing the ANSYS code, used to model the thermal diffusion between the casing and the winding pack, with the GANDALF code which solves the 1D thermo-hydraulics inside each conductor. The first version of this Thermo-hydraulic EXtended TOol (TEXTO) was developed for producing conservative results and has allowed to simulate the fast discharge of the magnet, providing valuable results such as the mass flow expelled from each pancake. In the second stage, the ANSYS code was configured for modelling the helium transport in the casing and in the winding pack, thus computing more realistic transverse heat fluxes to be injected into the GANDALF code for an accurate calculation of the temperature margin. This second version of TEXTO, which integrates the TACOS (Thermo-hydraulic Ansys COmputation Semi 3D) module, has been used for studying the feasibility of positioning the helium inlets in the electrical connections. The temperature margin has then been found close but below the criterion of 1 K. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the framework of the broader approach agreement between Europe and Japan, the JT-60U tokamak, in Naka, Japan, will be upgraded to JT-60SA, a fully superconducting tokamak. Among the components to be provided by Europe, the Toroidal Field (TF) system, which includes 18 coils with NbTi Cable In Conduit Conductors (CICC), will be supplied by France and Italy (coils) and by Germany (superconducting current leads). The main characteristics of the TF coil are presented in Fig. 1. According to the design proposed in [1], the winding pack (WP) is composed of 12 conductor pancakes wound in 6 turns each. Both casing and conductors are cooled by forced circulation of supercritical helium. The conductors He inlets are located in the coil inner side, in order to provide fresh coolant to the inner turns which face both the plasma and the maximal magnetic field (in the straight area of the internal leg). Each He inlet feeds two adjacent pancakes, thus leading to two opposite flow directions, clockwise (CW) and anti-clockwise (ACW).
∗ Corresponding author. Tel.: +33 4 42 25 76 80; fax: +33 4 42 25 26 61. E-mail address: [email protected] (B. Lacroix). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.03.103
Design activities have been performed in order to optimize the TF coils performance, cost and feasibility. Analyses have covered normal operation, including plasma burn and a disruption occurring conservatively at the end of burn. During such a scenario, heat loads in the TF coils are mainly due to energy deposited by neutrons and secondary gammas (nuclear heating), and to eddy current losses associated with the plasma disruption. In order to ensure the safety of the magnet operation, the temperature margin Tma = Tcs–Tcond, with Tcs = current sharing temperature and Tcond = conductor temperature, must remain above 1 K in every conductor. The calculation of the Tma criterion thus requires a thermo-hydraulic analysis of the helium flow along each CICC, and a thermal analysis of the transverse heat diffusion between the stainless steel casing and the winding pack, which leads to a 3D problem. In addition, the fast safety discharge (FSD) of the TF current must ensure the protection of the magnet, in case of a quench occurrence. The conductor design fulfills the hot spot criterion, which specifies a maximal conductor temperature of 150 K or 250 K, according to whether the helium in the cable is taken into account or not during the current decay. Besides, the FSD induces eddy currents and thus heat generation in the casing and the vacuum vessel. The heat deposition in the casing is transferred to the CICCs by a diffusion
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B. Lacroix et al. / Fusion Engineering and Design 86 (2011) 1483–1487
Fig. 3. Schematic view of codes involved in the TACOS/TEXTO tools.
Fig. 1. Main features of JT-60SA TF coil.
process through the insulation and the conductors jackets, resulting in a significant expulsion of He out of the winding pack, which has to be assessed for the design of the cryogenic system. 2. Development of the TACOS/TEXTO tools 2.1. First version of the TEXTO tool For the JT-60SA TF coil design activities, the choice has been made to adopt a pseudo 3D approach based on a 1D thermohydraulic model for the CICCs and on a 2D finite elements model for the transverse thermal diffusion. A first version of the TEXTO tool [2] was thus developed to interface ANSYS (2D thermal) and GANDALF (1D thermo-hydraulics) [3], while taking into account the nuclear losses computed with MCNP5 and the magnetic field computed with TRAPS. The development of this Thermo-hydraulic EXtended TOol (TEXTO) around GANDALF and ANSYS allows to take advantage of two commercial and widely used codes, easy to customize for taking into account different kinds of input data. This first version of TEXTO relies on a conservative approach for the ANSYS 2D thermal model. The calculation of the transient heat flux transferred from the casing to the winding pack is performed in two typical cross sections of the TF coil, one in the internal leg and the other in the external leg. The inner surface of the jackets is submitted to a boundary condition of forced convection, with a constant heat transfer coefficient and the assumption that the He temperature in the winding pack stays constant at 4.4 K. In addition, a penalizing heat balance in the casing cooling channels leads to maximize the casing temperature. The thermal fluxes computed with these penalizing hypotheses, assumed to be uniform along each conductor turn in each leg, are
then injected into GANDALF, which provides directly a conservative calculation of the temperature margin as a function of abscissa and time. 2.2. Implementation of the TACOS module In order to compute the temperature margin in a less conservative but more accurate way, a simplified thermo-hydraulic approach was presented in [1]. The ANSYS thermal model is extended to 10 coil cross sections, 3 in the internal leg and 7 in the external leg, as shown in Fig. 2. The He flow is simulated by transferring the temperature maps inside the cables and the cooling channels, from one section to the next, at each time interval t = x/VHe , with x the distance between 2 sections and VHe the He velocity. This process takes into account the He inlets, the jumps from turn to turn and the He outlets. An equivalent solid material corresponding to He + strands in the cables or only He in the casing channels provides the adequate heat capacity. The main assumption is the invariance of He velocity and pressure along each channel. In comparison with the first version of TEXTO, the implementation of this process in the ANSYS thermal calculation provides much more realistic transverse heat fluxes, but also He temperature along each conductor. This ANSYS adaptation for thermo-hydraulic purpose has thus led to a pseudo 3D model of a whole TF coil, which supports the independent feature of the module named TACOS, for Thermo-hydraulic Ansys COmputation Semi 3D. A schematic view of the codes involved in the TACOS/TEXTO tools is presented in Fig. 3. Several validation calculations were performed in order to quantify the impact of the simplified 1D thermo-hydraulic approach [4]. Results of comparisons between TACOS/TEXTO and GANDALF and VINCENTA codes showed a satisfying consistency, notably regarding the dispersion of conductor temperatures provided by TACOS
Fig. 2. ANSYS TF coil mesh for the pseudo 3D thermo-hydraulic and thermal model. Each coil segment is modelled by its corresponding 2D cross section.
B. Lacroix et al. / Fusion Engineering and Design 86 (2011) 1483–1487 Table 1 Main characteristics of FLOWER downscaled He collecting circuit.
Quench line
Pure He storage tank
Discharge valve Gandalf / Flower
From cryoplant
Line to valve Towards cryoplant
TF CICC
1485
Element
Length
Hydraulic diameter
Section or Volume
Initial conditions
Line to valve
3m
26.6 mm
46.3 mm2
Valve Quench line
– 100 m
– 100 mm
36.4 mm2 36.4 mm2
–
4.63 m3
4.04 bar 4.51 K – 1 bar 300 K 1 bar 300 K
He storage tank–
Fig. 4. GANDALF/FLOWER model of one TF CICC with the He collecting circuit.
and GANDALF, which remains lower than ± 0.05 K at minimum Tma location. This can be explained by the specificity of CICCs thermo-hydraulics: the approach based on the displacement of temperature maps is well adapted for a supercritical He flow under normal operating conditions, characterized by weak compressible effects (moderate velocity ∼ 0.2 m/s and Re ∼ 3000) and by the large thermal inertia of the fluid. The reliability of TACOS/TEXTO being ensured, this tool was then used to evaluate the impact of several TF coil design options, in the framework of the cost and feasibility optimization [5]. 3. Simulation of the fast safety discharge with TEXTO first version As previously mentioned, the fast safety discharge of the magnet induces eddy currents, and thus heat deposition in the casing and in the vacuum vessel. The heat generated in the casing is transferred to the conductors by thermal diffusion through the resin, the insulation layers and the CICCs jackets (2D thermal). This leads to a significant expulsion of He out of the winding pack, which has to be assessed for the design of the cryogenic system (1D thermohydraulics). In order to study such a scenario, the latest version of TACOS/TEXTO is unsuitable, due to the inconsistency between the simplified 1D thermo-hydraulic approach in the conductors and the expected high flow variations. On the opposite, the first version of TEXTO is more appropriate, insofar as the penalizing hypothesis for the heat diffusion calculation contributes to provide conservative results, especially for the He expulsion out of the conductors during the beginning of the discharge. In order to limit the disturbance of the FSD on the cryogenic system operation, it is foreseen to isolate hydraulically the TF windings from the cryoplant (by closing dedicated valves) and to collect the He expelled at conductors outlets, via one controlled or safety valve per coil. This valve should be located as close as possible to the TF coil (a few meters) and linked to He storage tanks by a warm quench line of about 100 m length.
For modelling this circuit, the GANDALF code can be coupled to FLOWER [6], a code dedicated to the simulation of the cryogenic environment of a superconducting magnet. As GANDALF models a single CICC, the collecting circuit described with FLOWER has to be downscaled to the level of one conductor: the volumes and the pipes cross sections are thus reduced accordingly, whereas the hydraulic diameters are kept the same, so that Reynolds numbers are not affected. The model is presented in Fig. 4, and the main features of the downscaled He collecting circuit are listed in Table 1 . As it is limited to a single pancake, the model doesn’t take into account thermal nor hydraulic coupling between the most critical pancakes and the other ones. The heat generation due to the eddy currents induced in the casing has been assessed analytically. Assuming that the TF current (ITF ) exponential decay is not affected by the electromagnetic coupling with the casing and that no energy is generated into the vacuum vessel, the casing heat power (Pcasing ) for one coil can be expressed as follows. M dITF /dt + Rcasing Icasing = 0
with ITF = I0 e−t/
2 ⇒ Icasing = (M I0 /Rcasing ) e−t/ Pcasing = Rcasing Icasing = (M 2 I02 /Rcasing 2 ) e−2t/
Considering a FSD time constant = 10 s, a casing resistance Rcasing = 10.5 , and a winding pack and casing mutual inductance M = 2.42 mH, the casing power in W is Pcasing = 3.684. 106 e−t/5 . This power has to be applied in the casing mass of one coil, neither including EF coils gravity supports nor outer inter coil structures, so into about 8500 kg of stainless steel. The simulation has been performed considering that the discharge valve is opened 1 s after the beginning of the FSD. Conservatively, a perfect contact has been assumed between the casing and the winding pack, and no mass flow has been taken into account in the casing cooling channels. The ANSYS thermal calculation has shown that the casing maximal temperature reaches 56 K at t = 45 s, in both internal and external legs. Due to the materials surrounding the winding pack 30
Lateralpancake
Q (g/s)
30
Lateral pancake
25
Median pancake
20
T He (K)
Valve opening
20
Medianpancake No heatflux on intermediateturns of medianpancake
t=40s
15 t=80s
10 10
t=80s
t=40s
t=40s
5
0
t=80s t=0s
t=0s
t=0s
0 0
20
40
60
80
Time (s) Fig. 5. Mass flow expelled at conductors outlets.
100
0
20
40
60
80
x (m) (abscissa from inlet) Fig. 6. Temperature distributions at times 0, 40 and 80 s.
100
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Fig. 7. Schematic view of He inlets reference and alternative layouts.
Table 2 Results of Tma calculations with both He inlets layouts (the abscissa x is counted from the joggle). He inlets layout
CICC Median
Reference Lateral Median Alternative Lateral
ACW CW ACW CW ACW CW ACW CW
Tma (K)
x (m)
t (s)
Tcond (K)
B (T)
1.31 1.19 1.37 1.20 0.97 1.22 0.99 1.25
8.2 14.8 8.2 14.5 8.2 14.8 8.2 14.5
136 162 136 163 212 307 210 305
4.87 5.00 4.89 5.12 5.22 4.98 5.26 5.08
5.59 5.58 5.44 5.28 5.59 5.58 5.44 5.28
(4 mm of glass-epoxy insulation, plus 5 to 10 mm of epoxy resin), transverse heat fluxes from the casing to the peripheral conductors reach a maximum value within 1 to 4 minutes, and then decay more or less linearly in about one hour. The thermo-hydraulic results have been examined for two pancakes, the lateral one and the median one. As the lateral pancake is in contact with the casing all along its length, it is more critical than the median one which is in contact only on the inner and outer turns. Mass flows expelled at conductors outlets are presented in Fig. 5. For both pancakes, at t = 2.4 s, a peak of about 33 g/s follows the valve opening, this value depending mainly on the conductor initial pressure. Then, 15 s later, as expected, the expelled mass flow is significantly higher in the lateral CICC than in the median one. The temperature profiles along each conductor are linked to the heat deposition, as shown in Fig. 6. As the lateral pancake experiences more heat flux (up to 30 W/m) than the median one (up to 15 W/m), the maximal pressure, reached at conductor inlet, is of 16.2 bar (at t = 64 s) in the lateral pancake, versus 6.2 bar (at t = 84 s) in the median one. 4. Study of an alternative He inlets location with TACOS/TEXTO The present design of JT-60SA TF coils comprises He inlets located in the coil inner side, at high field. Each He inlet is integrated to one double pancake joggle, so about 3.3 m above the median plane, in order to inject fresh He in both CW and ACW conductors as close as possible to the critical zone. Due to the delicate design and integration of the He inlets in the joggles, it could be beneficial to position them at the electrical connections level, at low field on the external side of the TF coil (see Fig. 7). But this alternative layout is expected to decrease the temperature margin, due to the longer flow path between the He inlet and the critical zone. In order to quantify the impact of this alternative He inlet design, the TACOS/TEXTO tool has been used for computing the tempera-
ture margin as reliably as possible for both layouts. The scenario taken into account is the same as in [4], a 100 s burn (from t = 15 to 115 s) followed by a disruption. The main simulation hypotheses are also the same as in [4], excepted static thermal loads which have been here neglected in order to shorten the calculations, considering their limited impact on Tma (less than 0.05 K). Besides, with the alternative layout, due to the half joint electrical resistance of 1 n, the He temperature downstream of the joint at each double pancake inlet is now 4.44 K (Tjoint = 0.04 K with ITF = 25.7 kA and QHe = 4 g/s). The study has focused on the median and lateral double pancakes, so on 4 conductors. With the alternative layout, due to the inversion of the He flow, a CW pancake becomes hydraulically an ACW one. Nevertheless, for an easier understanding, the corresponding pancakes have been still referred as CW hereafter. The minimal temperatures margins have been calculated for both layouts, taking into account the corresponding flow paths in the pseudo 3D model. Results are presented in Table 2. The impact of positioning He inlets at joints levels differs according to the flow direction of the pancake: whereas the margin is slightly increased (by 0.03 and 0.05 K) for the CW pancakes, it is decreased by 0.34 and 0.38 K for the ACW ones. These effects result from the combination of the flow direction with the power distribution (mainly the nuclear heating), which drives the conductor temperature and impacts the margin accordingly. Finally, with outer He inlets, the margin is decreased by 0.3 to 0.4 K for respectively ACW median and lateral pancakes, leading to values of 0.97 and 0.99 K, thus very close but below the criterion of 1 K. 5. Conclusion In the framework of JT-60SA TF coils design, a dedicated thermohydraulic tool was developed in two stages. The first version of TEXTO was based on a conservative approach for computing transverse heat fluxes from casing to conductors. It has been applied to
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the TF fast discharge simulation, providing the assessment of the He expulsion out of the winding pack. A similar calculation could be performed for the coil fast discharge in the cold test facility. This TEXTO first version was then upgraded by the implementation of a simplified He flow model in the thermal calculation, namely the TACOS module. The TACOS/TEXTO tool thus provides more accurate results, especially for the Tma criterion calculation. It has allowed to quantify the impact of an alternative location of He inlets at joints level. In the future detailed design phase, these tools could be used for validating possible TF coil design evolutions and for taking into account updated reference scenarios.
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References [1] P. Barabaschi, “JT-60SA Device Design Revision Aimed to Reduction of TF Magnet Cost V6.0”, F4E report BA STP PC 02-7.2 (2008). [2] S. Nicollet, et al., Cryogenics 50 (2010) 18–27. [3] L. Bottura, Journal of Computational Physics 125 (1996) 26–41. [4] B. Lacroix et al., Advances in Cryogenic Engineering AIP Conference Proceedings 1218, pp. 471-479 (2010). [5] C. Portafaix, et al., IEEE Transactions on Applied Superconductivity 20 (2010) 1794–1797. [6] L. Bottura, C. Rosso, Cryogenics 43 (2003) 215–223.