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Numerical study for optimization of the air cooling system for the Fast Discharge Resistors protecting the ITER magnets
Fusion Engineering and Design 98–99 (2015) 1169–1175
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Numerical study for optimization of the air cooling system for the Fast Discharge Resistors protecting the ITER magnets Victor Tanchuk a,∗ , Sergey Grigoriev a , Vladimir Lokiev a , Alexander Roshal a , Dmitry Mikhaluk b , Ilya Kapranov b a b
JSC “D.V. Efremov Institute of Electrophysical Apparatus”, RUS-196641 St. Petersburg, Russia JSC “CADFEM CIS”, RUS-195197 St. Petersburg, Russia
h i g h l i g h t s • The essential non-uniformity of air flow distribution in parallel channels, which considerably increases the cooling time, is observed when performing the numerical analysis of the FDR cooling system.
• The potential stagnant areas with the low or even missing cooling air circulation specify the significant time delays in cooling-down of far-away (relative to the chimney) resistors.
• The measures are proposed to improve the FDR cooling efficiency, namely: (ɑ) to change the height of chimneys from 30 to 50 m, (b) to increase the flow cross-sections of air inlet and outlet pipes and (c) to increase number of independent circuits.
a r t i c l e
i n f o
Article history: Received 5 September 2014 Received in revised form 18 June 2015 Accepted 26 June 2015 Available online 29 July 2015 Keywords: ITER magnets Coil quench Fast Discharge Resistors Cooling system Air natural circulation
a b s t r a c t The superconducting magnets of the ITER are capable of accumulating up to 50 GJ. In case of coil quench the energy stored in the coils must be extracted rapidly. The problem can be solved by the Fast Discharge Resistors (FDR) under development at the Efremov Institute. The fast discharge of the coils results in practically adiabatic heating of the resistive elements up to 250 ◦ C. The resistors should be cooled to their initial temperature within a reasonable time (4–6 h). With this in mind, the authors have designed the cooling system based on natural air circulation. When performing the numerical analysis of the cooling process, the authors have faced the problem of the essential non-uniformity of air flow distribution in parallel channels, which considerably increases the cooling time. Thus, while the cooling time does not exceed 3 h for a single FDR module, it exceeds 10 h for several modules. The numerical studies performed over the last few years have allowed the authors to propose a number of measures to optimize the air cooling system so as to mitigate the negative effect of the air flow non-uniformity in the FDR cooling system. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The Fast Discharge Resistors (FDRs) are supposed to absorb up to 50 GJ in case of coil quench of the ITER superconducting magnets. As a result the FDR resistive elements are heated up to 250 ◦ C practically adiabatically. The problem is to cool the resistors down to their initial temperature within 4–6 h. The authors designed the air cooling system based on natural air circulation developing in the system of channels formed by the
∗ Corresponding author. E-mail address: [email protected] (V. Tanchuk). http://dx.doi.org/10.1016/j.fusengdes.2015.06.140 0920-3796/© 2015 Elsevier B.V. All rights reserved.
supply/return pipes, vertical modules and chimneys (see Fig. 1) and proceeded with research of the issues examined before, i.e. analysis of FDR thermal conditions during free-convective cooling [1]. When performing the numerical analysis of the cooling process, the authors have faced the problem of the essential non-uniformity of air flow distribution in parallel channels, which considerably increases the cooling time. While the cooling time does not exceed three hours for a single FDR module, it exceeds ten hours for several modules. It will be shown below in this paper that the main reason for cooling time delay is the non-optimal, as regards hydrodynamics, conditions of cooling air circulation in the channels of far-away (relative to the chimney) resistors. The potential stagnant areas with
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natural convection developed by the action of temperature difference gradually expands over the whole piping and causes cooling air to flow through the resistor modules. Then, within several hours the resistor temperature decreases and the natural air circulation in the system channels decays. The purpose of the study is to analyze the changes made in the design of the FDR air cooling system to reduce the cooling-down time. 3. Calculation model development
Fig. 1. General view of the Fast Discharge Resistors and air cooling system: 6 central solenoid (CS) FDRs, 6 poloidal field (PF) FDRs and 9 toroidal field (TF) FDRs.
the low or even missing cooling air circulation specify the significant time delays in cooling-down of the corresponding resistor modules. 2. Design description of FDR air cooling system The FDRs design and their cooling system are described in detail in [1]. Therefore, only the main geometric parameters and their changes to optimize the cooling system are given here. The discharge resistors [2] are based on unified resistor sections made from St 08 steel tapes in a tight serpentine pattern to minimize inductance. Each module consists of a few sections (from 2 to 4) electrically connected in parallel. The sections in the modules (0.76 m × 0.72 m in flow section) are placed vertically, one above another. The modules which form one resistor are arranged in 2–4 rows as shown in Fig. 1. All modules within each row are mechanically connected and form one branch of the air cooling (Fig. 2). The supply channels below the floor level have a uniform crosssection of 0.6 m × 0.71 m and terminate in an inlet branch 0.54 m in diameter. The outlet branches 0.64 m in diameter are brought to the main collector 1.6 m in diameter. Hence, the analyzed cooling system network consists of resistor modules, supply channels with shafts for intake of atmospheric air and exhaust piping terminating in two chimneys 30 m in height and 2.5 m in diameter (Fig. 1). The cooling process begins with heat release on the resistive elements resulting in their heating to a temperature of 250 ◦ C. The
Fig. 2. Calculation model of a single row of TF FDR.
To analyze the heat exchange processes in the FDR cooling system due to the natural air circulation, the calculation model of the cooling system is used which was developed at the previous stage of the study and described in detail in [1]. It is to be recalled that for the developed model the flow in the cooling system was calculated by numerical solution of gas dynamics equations (Navier–Stokes equations) with Reynolds averaging of turbulent pulsation (RANS) [3] together with the thermal diffusion problem using the ANSYS FLUENT package [4]. Account was taken of the gravitation effect. The –ε turbulence model was used. Air was considered as a viscous thermal-conducting compressible gas. Areas filled with resistive elements were simulated as porous homogeneous volumes with distributed hydraulic resistance and heat supply. All walls were considered to be technically smooth, impermeable and non-thermal-conducting. The total pressure at the intake system inlets was maintained equal to the atmospheric one (101,325 Pa) and the static temperature equal to 30 ◦ C. The turbulence parameters corresponded to a weakly disturbed flow. The static atmospheric pressure was set at the chimney outlet. The three-dimensional geometric model of the piping and resistors network was developed in the ANSYS Design Modeller package [5]. The computational mesh developed by the block structure in the ANSYS ICEM CFD package [6] contains about 8 million hexahedrons. 4. Numerical investigations of air flow in FDRs cooling system 4.1. Simulation of the cooling-down process in a single resistor module row When designing the FDR cooling system [1], it has been found that the cooling processes in the parallel channels are accompanied by a considerable nonuniformity. The observable cooling nonuniformity results in significant retardation of the cooling process for all resistors. To specify the reasons for such a nonuniform cooling air flow distribution in the parallel channels and to eliminate the defects of the developed cooling system, the features of freeconvective air circulation in the parallel channels were thoroughly examined. The analysis was performed using the model of the resistor row for the TF coils (Fig. 2). Fig. 3 shows the obtained cooling air temperature distribution. According to the calculation results module 5 (see Fig. 2) which is nearest to the outlet of the cooling network cools down faster than other modules. Fig. 4 shows time distributions of cooling air mass flows corresponding to five resistor modules connected to a single cooling system. The obtained non-uniform distribution of air flowing through the parallel branches of the cooling network is caused by the character of the air flow which can be observed using the absolute air flow rate distribution (see Fig. 5). Pressure reduction in the supply manifold due to the lower density of hot air in the resistor modules as compared with the cold air
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Fig. 3. Air temperature distribution (◦ C) in a single row of TF FDR in 1 h of cooling.
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density leads to acceleration of the flow in the supply manifold. As a result of inertial motion, the larger air volume reaches the farthest module while the module nearest to the inlet cross-section receives the minimum air volume. The larger flow rate of cooling air passing through module 5 results in enhanced heat transfer and faster cooling of resistive elements (see the maximum temperature distribution, Fig. 3). The initial period of the cooling process is characterized by absolute prevalence of module 5 with respect to the flow rate passing through it as compared with the other modules (Fig. 4). The other modules reach the maximum flow rates only about 1–2 h after the cooling process starts, i.e. after module 5 becomes cool to a considerable extent. Such a considerable nonuniformity may be explained not only by the above-mentioned effect of the inertial motion of the flow in the supply manifold but also by the effect of ejection of the outlet flow from module 5. Ejection is performed owing to the flow of a high velocity (∼6 m/s) in the outlet manifold. As the temperature of the resistive elements of module 5 goes down, the aerostatic force generated in this module and, consequently, the flow passing through it is reduced. The same effect, though to a lesser degree, is observed in the neighbouring modules. Thus, the flow rates for all modules are equalized 3 h after the cooling process starts and then have similar values (Fig. 4). 4.2. Investigations to improve FDR cooling system efficiency by means of minor design changes
Fig. 4. Evolution of air flow rate in each FDR module cooled in parallel.
Fig. 5. Air flow rate (m/s) distribution in a single row of TF FDR in 1 h of cooling.
Based on the results given above the measures were proposed to improve the cooling air flow conditions in the whole cooling system and to improve the FDR cooling efficiency. Among the proposed measures: (a) to change the height of chimneys from 30 to 50 m, and (b) to increase the flow crosssections of air inlet and outlet pipes. The extended chimney enables the aerostatic thrust in the cooling system circuit to grow. This, in turn, results in the increased volumes of cooling air flowing through the resistor modules and in the better characteristics of cooling-down of the resistive elements. The increased cross-section of the inlet pipes and horizontal outlet pipe leads to the higher flow rate of cooling air as well due to the reduced velocity in the corresponding pipe sections and reduced local hydraulic resistance. To analyze the cooling efficiency at such changes the average values of maximum temperatures for the TF FDR groups were calculated. When averaging, the resistors were grouped depending on their distances from the chimney (Fig. 6). Thus, in the left branch three groups were examined: group “1–2” including resistors 1 and 2, group “3–4” including resistors 3 and 4, and group “5” including resistor 5. In the right branch only two groups were examined, i.e. groups “1–2” and “3–4”. The height of the chimneys of the initial cooling system design (Fig. 1) is extended (from 30 to 50 m) and the cross-section of the inlet and outlet pipes is increased. Meanwhile, the cooling system circuit layout remains unchanged. Fig. 7 shows the actual and average distribution of maximum air velocity for the version of the cooling system with the above-stated changes. At the very beginning of the FDR cooling process the maximum velocity averages about 26 m/s which is 6–7 m/s higher than the velocity for the initial design. The maximum velocity curve (Fig. 7) has changed as compared with that of the initial design: the downward bias of the velocity curve is evident. As a result, in approximately 1.5 h of cooling time the level of the maximum flow velocity in the pipe becomes lower than that of the initial design. Thus, to the end of the time interval under examination the velocity drops to the value just under 4 m/s which is almost one half as much as the values recorded for the initial design.
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Fig. 8. Temperature curves obtained when averaging the maximum temperature values for resistors 1–2, 3–4 and 5 and for the whole left branch (LB) of the cooling system.
The given distributions clearly confirm the decreased cooling nonuniformity. The temperature curves are much closer to each other. The maximum time spread is reduced to 1.5–2 h. It can be concluded that the increased aerostatic thrust in the chimney and the grown capacity of the outlet pipe provide passage of the large volumes of cooling air and reduction in the negative effect of resistor groups 3–4 and 5 on resistor group 1–2. The hot air flow passing through resistor groups 3–4 and 5 stops blocking the air flow from resistor groups 1–2 in the both cooling system branches. Fig. 9 shows the air temperature distribution in the TF FDR cooling system for the design with the increased height of the chimney and the enlarged cross-sections of the inlet and outlet pipes. It is evident that in 6 h the main part of the FDRs reaches the required temperature conditions (30–40 ◦ C). Fig. 6. General view of the TF FDR cooling system including separate resistor groups.
Fig. 8 shows the time distributions of the maximum temperatures for the module rows. Nonuniform cooling of the resistors of different groups observed in the previous case is significantly reduced. In 6 h the majority of the modules is able to cool down to 50–55 ◦ C.
4.3. Optimization of the TF and PF/CS FDR cooling system by increasing the number of independent parallel circuits It is proposed to increase the number of the independent circuits of the cooling system so as to reduce the mutual influence of the resistor groups. 4.3.1. TF FDR cooling system Fig. 10 shows the general view of TF FDR cooling system with the separate circuits formed by each resistor group. Two separate
Fig. 7. Maximum air velocity in the TF FDR cooling system when increasing the height of the chimney and the cross-section of the outlet pipe.
Fig. 9. Air temperature (◦ C) distribution in the TF FDR cooling system for the design with the increased height of the chimney and enlarged cross-sections of the outlet pipes, t = 6 h.
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Fig. 10. General view of TF FDR cooling system with five independent circuits.
Fig. 11. Temperature curves obtained when averaging the maximum temperature values for resistors 1–2, 3–4 and 5 and for the whole left branch (LB) of the cooling system.
circuits are made for groups 1–2 and 3–4 in the right branch and three circuits are made for groups 1–2, 3–4 and 5 in the left branch. Fig. 11 shows the time distributions of the maximum temperatures for the module rows. It is evident that the temperature curve spread is reduced considerably as compared with the previous design version. The most appreciable difference in the averaged temperature distributions is observed for resistor groups 1–2. The module rows of these groups were subjected to the strongest negative influence of the resistors located closer to the chimney. The multiple circuits made it possible to eliminate to the utmost the negative effect which consisted in blocking the cooling air flow passing from the module rows of groups 1–2.
Fig. 12. Air temperature (◦ C) distribution in the TF FDR multicircuit cooling system, t = 6 h.
Fig. 13. General view of the PF/CS FDR cooling system including separate resistor groups.
The temperature distribution in the cooling system is presented in Fig. 12. The temperature distribution changes are noticeable only in the upper part of module rows 1-1, 1-2, 2-1, 2-2, 3-1 and 4-1. Due to introduction of the multiple circuits into the TF FDR cooling system in addition to the changes made at the first stage, the hot air exhaust conditions have been significantly improved. The negative effect of some resistor groups on the others has been almost completely eliminated. As a result, for 90% of the resistors the modified design of the cooling system provides the temperature conditions which are close to the required ones and differ from them by about 10–15 ◦ C only. 4.3.2. PF/CS FDR cooling system Fig. 13 (as per Fig. 1) shows the general view of the PF/CS FDR cooling system. By analogy with the TF FDRs, the multicircuit configuration was examined for the cooling system of the PF/CS FDRs. The general view of the analyzed cooling system is presented in Fig. 14. Fig. 15 shows the maximum air velocity distribution over the whole calculated area. It can be seen that the changes made in the cooling system have led to an increase in the maximum velocity by 2–3 m/s. However, the averaged velocity distribution for the PF/CS FDR cooling system differs from the similar distribution obtained for the TF FDR cooling system (see Fig. 7). As it has been already specified, there are two main signs that the multiple circuits ensure considerable improvement of the
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Fig. 14. General view of the PF/CS FDR cooling system with four independent circuits.
cooling air flow conditions. The first is exceeding of the maximum velocity at the beginning of the cooling process as compared with the non-multicircuit design. The second is a rate of velocity drop which is much faster for the multicircuit system than for the initial design. At the end of the cooling process it causes the smaller values of the maximum velocity for the multicircuit system. The increased flow rate of air passing through the hot resistors provides their better cooling and faster aerostatic thrust drop. This drop, in turn, immediately results in the reduced cooling air velocities. The obtained maximum velocity distribution (Fig. 15) does not demonstrate any significant drop to the end of the cooling process. The obtained result indicates that for the PF/CS FDR cooling system the problem consists not in the negative effect of some resistor groups on the others (it would otherwise be eliminated by means of the multiple circuits) but in some other design features. Fig. 16 shows the maximum temperature distribution for each module row. The wide cooling rate spread is observed. The first resistor group cools down faster than others despite its farthest location from the chimney. The cooling rate of the second and third groups (especially module row 2-2) is rather low. Such an obvious divergence seems to be caused by a large number of the modules in the rows of the second and third groups. Row 2-2 contains the largest number of the modules (i.e., 9). The other rows have 7–8 modules. As a result, in four hours of the
Fig. 16. Maximum temperatures of the resistors for the module rows of the PF/CS FDR multicircuit cooling system.
Fig. 17. Air temperature (◦ C) distribution in the PF/CS FDR multicircuit cooling system, t = 4 h.
cooling process the maximum temperature in row 2-2 decreases only to 230 ◦ C (see Fig. 16). Thus, the cooling efficiency is much lower in the rows containing more than five modules. The obtained results show that this problem cannot be solved by improvement of the cooling air supply and exhaust conditions. Fig. 17 shows the temperature distributions throughout the PF/CS FDR cooling system. It should be noted that the degree of heating throughout the height for several modules may be considerably different despite their identical maximum temperatures. 5. Conclusion The developed calculation model makes it possible to simulate the cooling process for the Fast Discharge Resistors in the natural convection mode taking into account the features of the piping for supply of cooling air and exhaust of heated air. The proposed design changes of the FDR cooling system, i.e. the introduced multiple circuits and increased capacity of the outlet circuits, have resulted in the improved characteristics of TF FDR cooling. Contrary, the cooling time for PF/CS FDRs exceeds the target of four hours due to the design features of the PF/CS FDR cooling system, even after the introduced changes. It is possible to reduce the PF/CS FDRs cooling time to the required four hours by decreasing the number of the resistor modules in a single row to four-five. Disclaimer
Fig. 15. Maximum air velocity in the PF/CS FDR multicircuit cooling system.
The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.
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References [1] V. Tanchuk, S. Grigoriev, V. Lokiev, A. Roshal, I. Song, O. Buzykin, Air cooled Fast Discharge Resistors for ITER magnets, Fusion Eng. Des. 86 (6) (2011) 1445–1449. [2] B. Bareyt, et al., The switching network and discharge circuit of ITER, in: Proceeding of 19th SOFT, August, Portugal, 1996.
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[3] T.J. Chung, Computational Fluid Dynamics, Cambridge University Press, 2002, pp. 1012. [4] ANSYS FLUENT Theory Guide. Release 14.0. November 2011. [5] Documentation for ANSYS Workbench, SAS IP Inc., release 11.0, 2007. [6] Documentation for ANSYS ICEM CFD/AI* Environment 11.0, SP1, SAS IP Inc., 2007.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Numerical study for optimization of the air cooling system for the Fast Discharge Resistors protecting the ITER magnets Victor Tanchuk a,∗ , Sergey Grigoriev a , Vladimir Lokiev a , Alexander Roshal a , Dmitry Mikhaluk b , Ilya Kapranov b a b
JSC “D.V. Efremov Institute of Electrophysical Apparatus”, RUS-196641 St. Petersburg, Russia JSC “CADFEM CIS”, RUS-195197 St. Petersburg, Russia
h i g h l i g h t s • The essential non-uniformity of air flow distribution in parallel channels, which considerably increases the cooling time, is observed when performing the numerical analysis of the FDR cooling system.
• The potential stagnant areas with the low or even missing cooling air circulation specify the significant time delays in cooling-down of far-away (relative to the chimney) resistors.
• The measures are proposed to improve the FDR cooling efficiency, namely: (ɑ) to change the height of chimneys from 30 to 50 m, (b) to increase the flow cross-sections of air inlet and outlet pipes and (c) to increase number of independent circuits.
a r t i c l e
i n f o
Article history: Received 5 September 2014 Received in revised form 18 June 2015 Accepted 26 June 2015 Available online 29 July 2015 Keywords: ITER magnets Coil quench Fast Discharge Resistors Cooling system Air natural circulation
a b s t r a c t The superconducting magnets of the ITER are capable of accumulating up to 50 GJ. In case of coil quench the energy stored in the coils must be extracted rapidly. The problem can be solved by the Fast Discharge Resistors (FDR) under development at the Efremov Institute. The fast discharge of the coils results in practically adiabatic heating of the resistive elements up to 250 ◦ C. The resistors should be cooled to their initial temperature within a reasonable time (4–6 h). With this in mind, the authors have designed the cooling system based on natural air circulation. When performing the numerical analysis of the cooling process, the authors have faced the problem of the essential non-uniformity of air flow distribution in parallel channels, which considerably increases the cooling time. Thus, while the cooling time does not exceed 3 h for a single FDR module, it exceeds 10 h for several modules. The numerical studies performed over the last few years have allowed the authors to propose a number of measures to optimize the air cooling system so as to mitigate the negative effect of the air flow non-uniformity in the FDR cooling system. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The Fast Discharge Resistors (FDRs) are supposed to absorb up to 50 GJ in case of coil quench of the ITER superconducting magnets. As a result the FDR resistive elements are heated up to 250 ◦ C practically adiabatically. The problem is to cool the resistors down to their initial temperature within 4–6 h. The authors designed the air cooling system based on natural air circulation developing in the system of channels formed by the
∗ Corresponding author. E-mail address: [email protected] (V. Tanchuk). http://dx.doi.org/10.1016/j.fusengdes.2015.06.140 0920-3796/© 2015 Elsevier B.V. All rights reserved.
supply/return pipes, vertical modules and chimneys (see Fig. 1) and proceeded with research of the issues examined before, i.e. analysis of FDR thermal conditions during free-convective cooling [1]. When performing the numerical analysis of the cooling process, the authors have faced the problem of the essential non-uniformity of air flow distribution in parallel channels, which considerably increases the cooling time. While the cooling time does not exceed three hours for a single FDR module, it exceeds ten hours for several modules. It will be shown below in this paper that the main reason for cooling time delay is the non-optimal, as regards hydrodynamics, conditions of cooling air circulation in the channels of far-away (relative to the chimney) resistors. The potential stagnant areas with
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natural convection developed by the action of temperature difference gradually expands over the whole piping and causes cooling air to flow through the resistor modules. Then, within several hours the resistor temperature decreases and the natural air circulation in the system channels decays. The purpose of the study is to analyze the changes made in the design of the FDR air cooling system to reduce the cooling-down time. 3. Calculation model development
Fig. 1. General view of the Fast Discharge Resistors and air cooling system: 6 central solenoid (CS) FDRs, 6 poloidal field (PF) FDRs and 9 toroidal field (TF) FDRs.
the low or even missing cooling air circulation specify the significant time delays in cooling-down of the corresponding resistor modules. 2. Design description of FDR air cooling system The FDRs design and their cooling system are described in detail in [1]. Therefore, only the main geometric parameters and their changes to optimize the cooling system are given here. The discharge resistors [2] are based on unified resistor sections made from St 08 steel tapes in a tight serpentine pattern to minimize inductance. Each module consists of a few sections (from 2 to 4) electrically connected in parallel. The sections in the modules (0.76 m × 0.72 m in flow section) are placed vertically, one above another. The modules which form one resistor are arranged in 2–4 rows as shown in Fig. 1. All modules within each row are mechanically connected and form one branch of the air cooling (Fig. 2). The supply channels below the floor level have a uniform crosssection of 0.6 m × 0.71 m and terminate in an inlet branch 0.54 m in diameter. The outlet branches 0.64 m in diameter are brought to the main collector 1.6 m in diameter. Hence, the analyzed cooling system network consists of resistor modules, supply channels with shafts for intake of atmospheric air and exhaust piping terminating in two chimneys 30 m in height and 2.5 m in diameter (Fig. 1). The cooling process begins with heat release on the resistive elements resulting in their heating to a temperature of 250 ◦ C. The
Fig. 2. Calculation model of a single row of TF FDR.
To analyze the heat exchange processes in the FDR cooling system due to the natural air circulation, the calculation model of the cooling system is used which was developed at the previous stage of the study and described in detail in [1]. It is to be recalled that for the developed model the flow in the cooling system was calculated by numerical solution of gas dynamics equations (Navier–Stokes equations) with Reynolds averaging of turbulent pulsation (RANS) [3] together with the thermal diffusion problem using the ANSYS FLUENT package [4]. Account was taken of the gravitation effect. The –ε turbulence model was used. Air was considered as a viscous thermal-conducting compressible gas. Areas filled with resistive elements were simulated as porous homogeneous volumes with distributed hydraulic resistance and heat supply. All walls were considered to be technically smooth, impermeable and non-thermal-conducting. The total pressure at the intake system inlets was maintained equal to the atmospheric one (101,325 Pa) and the static temperature equal to 30 ◦ C. The turbulence parameters corresponded to a weakly disturbed flow. The static atmospheric pressure was set at the chimney outlet. The three-dimensional geometric model of the piping and resistors network was developed in the ANSYS Design Modeller package [5]. The computational mesh developed by the block structure in the ANSYS ICEM CFD package [6] contains about 8 million hexahedrons. 4. Numerical investigations of air flow in FDRs cooling system 4.1. Simulation of the cooling-down process in a single resistor module row When designing the FDR cooling system [1], it has been found that the cooling processes in the parallel channels are accompanied by a considerable nonuniformity. The observable cooling nonuniformity results in significant retardation of the cooling process for all resistors. To specify the reasons for such a nonuniform cooling air flow distribution in the parallel channels and to eliminate the defects of the developed cooling system, the features of freeconvective air circulation in the parallel channels were thoroughly examined. The analysis was performed using the model of the resistor row for the TF coils (Fig. 2). Fig. 3 shows the obtained cooling air temperature distribution. According to the calculation results module 5 (see Fig. 2) which is nearest to the outlet of the cooling network cools down faster than other modules. Fig. 4 shows time distributions of cooling air mass flows corresponding to five resistor modules connected to a single cooling system. The obtained non-uniform distribution of air flowing through the parallel branches of the cooling network is caused by the character of the air flow which can be observed using the absolute air flow rate distribution (see Fig. 5). Pressure reduction in the supply manifold due to the lower density of hot air in the resistor modules as compared with the cold air
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Fig. 3. Air temperature distribution (◦ C) in a single row of TF FDR in 1 h of cooling.
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density leads to acceleration of the flow in the supply manifold. As a result of inertial motion, the larger air volume reaches the farthest module while the module nearest to the inlet cross-section receives the minimum air volume. The larger flow rate of cooling air passing through module 5 results in enhanced heat transfer and faster cooling of resistive elements (see the maximum temperature distribution, Fig. 3). The initial period of the cooling process is characterized by absolute prevalence of module 5 with respect to the flow rate passing through it as compared with the other modules (Fig. 4). The other modules reach the maximum flow rates only about 1–2 h after the cooling process starts, i.e. after module 5 becomes cool to a considerable extent. Such a considerable nonuniformity may be explained not only by the above-mentioned effect of the inertial motion of the flow in the supply manifold but also by the effect of ejection of the outlet flow from module 5. Ejection is performed owing to the flow of a high velocity (∼6 m/s) in the outlet manifold. As the temperature of the resistive elements of module 5 goes down, the aerostatic force generated in this module and, consequently, the flow passing through it is reduced. The same effect, though to a lesser degree, is observed in the neighbouring modules. Thus, the flow rates for all modules are equalized 3 h after the cooling process starts and then have similar values (Fig. 4). 4.2. Investigations to improve FDR cooling system efficiency by means of minor design changes
Fig. 4. Evolution of air flow rate in each FDR module cooled in parallel.
Fig. 5. Air flow rate (m/s) distribution in a single row of TF FDR in 1 h of cooling.
Based on the results given above the measures were proposed to improve the cooling air flow conditions in the whole cooling system and to improve the FDR cooling efficiency. Among the proposed measures: (a) to change the height of chimneys from 30 to 50 m, and (b) to increase the flow crosssections of air inlet and outlet pipes. The extended chimney enables the aerostatic thrust in the cooling system circuit to grow. This, in turn, results in the increased volumes of cooling air flowing through the resistor modules and in the better characteristics of cooling-down of the resistive elements. The increased cross-section of the inlet pipes and horizontal outlet pipe leads to the higher flow rate of cooling air as well due to the reduced velocity in the corresponding pipe sections and reduced local hydraulic resistance. To analyze the cooling efficiency at such changes the average values of maximum temperatures for the TF FDR groups were calculated. When averaging, the resistors were grouped depending on their distances from the chimney (Fig. 6). Thus, in the left branch three groups were examined: group “1–2” including resistors 1 and 2, group “3–4” including resistors 3 and 4, and group “5” including resistor 5. In the right branch only two groups were examined, i.e. groups “1–2” and “3–4”. The height of the chimneys of the initial cooling system design (Fig. 1) is extended (from 30 to 50 m) and the cross-section of the inlet and outlet pipes is increased. Meanwhile, the cooling system circuit layout remains unchanged. Fig. 7 shows the actual and average distribution of maximum air velocity for the version of the cooling system with the above-stated changes. At the very beginning of the FDR cooling process the maximum velocity averages about 26 m/s which is 6–7 m/s higher than the velocity for the initial design. The maximum velocity curve (Fig. 7) has changed as compared with that of the initial design: the downward bias of the velocity curve is evident. As a result, in approximately 1.5 h of cooling time the level of the maximum flow velocity in the pipe becomes lower than that of the initial design. Thus, to the end of the time interval under examination the velocity drops to the value just under 4 m/s which is almost one half as much as the values recorded for the initial design.
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Fig. 8. Temperature curves obtained when averaging the maximum temperature values for resistors 1–2, 3–4 and 5 and for the whole left branch (LB) of the cooling system.
The given distributions clearly confirm the decreased cooling nonuniformity. The temperature curves are much closer to each other. The maximum time spread is reduced to 1.5–2 h. It can be concluded that the increased aerostatic thrust in the chimney and the grown capacity of the outlet pipe provide passage of the large volumes of cooling air and reduction in the negative effect of resistor groups 3–4 and 5 on resistor group 1–2. The hot air flow passing through resistor groups 3–4 and 5 stops blocking the air flow from resistor groups 1–2 in the both cooling system branches. Fig. 9 shows the air temperature distribution in the TF FDR cooling system for the design with the increased height of the chimney and the enlarged cross-sections of the inlet and outlet pipes. It is evident that in 6 h the main part of the FDRs reaches the required temperature conditions (30–40 ◦ C). Fig. 6. General view of the TF FDR cooling system including separate resistor groups.
Fig. 8 shows the time distributions of the maximum temperatures for the module rows. Nonuniform cooling of the resistors of different groups observed in the previous case is significantly reduced. In 6 h the majority of the modules is able to cool down to 50–55 ◦ C.
4.3. Optimization of the TF and PF/CS FDR cooling system by increasing the number of independent parallel circuits It is proposed to increase the number of the independent circuits of the cooling system so as to reduce the mutual influence of the resistor groups. 4.3.1. TF FDR cooling system Fig. 10 shows the general view of TF FDR cooling system with the separate circuits formed by each resistor group. Two separate
Fig. 7. Maximum air velocity in the TF FDR cooling system when increasing the height of the chimney and the cross-section of the outlet pipe.
Fig. 9. Air temperature (◦ C) distribution in the TF FDR cooling system for the design with the increased height of the chimney and enlarged cross-sections of the outlet pipes, t = 6 h.
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Fig. 10. General view of TF FDR cooling system with five independent circuits.
Fig. 11. Temperature curves obtained when averaging the maximum temperature values for resistors 1–2, 3–4 and 5 and for the whole left branch (LB) of the cooling system.
circuits are made for groups 1–2 and 3–4 in the right branch and three circuits are made for groups 1–2, 3–4 and 5 in the left branch. Fig. 11 shows the time distributions of the maximum temperatures for the module rows. It is evident that the temperature curve spread is reduced considerably as compared with the previous design version. The most appreciable difference in the averaged temperature distributions is observed for resistor groups 1–2. The module rows of these groups were subjected to the strongest negative influence of the resistors located closer to the chimney. The multiple circuits made it possible to eliminate to the utmost the negative effect which consisted in blocking the cooling air flow passing from the module rows of groups 1–2.
Fig. 12. Air temperature (◦ C) distribution in the TF FDR multicircuit cooling system, t = 6 h.
Fig. 13. General view of the PF/CS FDR cooling system including separate resistor groups.
The temperature distribution in the cooling system is presented in Fig. 12. The temperature distribution changes are noticeable only in the upper part of module rows 1-1, 1-2, 2-1, 2-2, 3-1 and 4-1. Due to introduction of the multiple circuits into the TF FDR cooling system in addition to the changes made at the first stage, the hot air exhaust conditions have been significantly improved. The negative effect of some resistor groups on the others has been almost completely eliminated. As a result, for 90% of the resistors the modified design of the cooling system provides the temperature conditions which are close to the required ones and differ from them by about 10–15 ◦ C only. 4.3.2. PF/CS FDR cooling system Fig. 13 (as per Fig. 1) shows the general view of the PF/CS FDR cooling system. By analogy with the TF FDRs, the multicircuit configuration was examined for the cooling system of the PF/CS FDRs. The general view of the analyzed cooling system is presented in Fig. 14. Fig. 15 shows the maximum air velocity distribution over the whole calculated area. It can be seen that the changes made in the cooling system have led to an increase in the maximum velocity by 2–3 m/s. However, the averaged velocity distribution for the PF/CS FDR cooling system differs from the similar distribution obtained for the TF FDR cooling system (see Fig. 7). As it has been already specified, there are two main signs that the multiple circuits ensure considerable improvement of the
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Fig. 14. General view of the PF/CS FDR cooling system with four independent circuits.
cooling air flow conditions. The first is exceeding of the maximum velocity at the beginning of the cooling process as compared with the non-multicircuit design. The second is a rate of velocity drop which is much faster for the multicircuit system than for the initial design. At the end of the cooling process it causes the smaller values of the maximum velocity for the multicircuit system. The increased flow rate of air passing through the hot resistors provides their better cooling and faster aerostatic thrust drop. This drop, in turn, immediately results in the reduced cooling air velocities. The obtained maximum velocity distribution (Fig. 15) does not demonstrate any significant drop to the end of the cooling process. The obtained result indicates that for the PF/CS FDR cooling system the problem consists not in the negative effect of some resistor groups on the others (it would otherwise be eliminated by means of the multiple circuits) but in some other design features. Fig. 16 shows the maximum temperature distribution for each module row. The wide cooling rate spread is observed. The first resistor group cools down faster than others despite its farthest location from the chimney. The cooling rate of the second and third groups (especially module row 2-2) is rather low. Such an obvious divergence seems to be caused by a large number of the modules in the rows of the second and third groups. Row 2-2 contains the largest number of the modules (i.e., 9). The other rows have 7–8 modules. As a result, in four hours of the
Fig. 16. Maximum temperatures of the resistors for the module rows of the PF/CS FDR multicircuit cooling system.
Fig. 17. Air temperature (◦ C) distribution in the PF/CS FDR multicircuit cooling system, t = 4 h.
cooling process the maximum temperature in row 2-2 decreases only to 230 ◦ C (see Fig. 16). Thus, the cooling efficiency is much lower in the rows containing more than five modules. The obtained results show that this problem cannot be solved by improvement of the cooling air supply and exhaust conditions. Fig. 17 shows the temperature distributions throughout the PF/CS FDR cooling system. It should be noted that the degree of heating throughout the height for several modules may be considerably different despite their identical maximum temperatures. 5. Conclusion The developed calculation model makes it possible to simulate the cooling process for the Fast Discharge Resistors in the natural convection mode taking into account the features of the piping for supply of cooling air and exhaust of heated air. The proposed design changes of the FDR cooling system, i.e. the introduced multiple circuits and increased capacity of the outlet circuits, have resulted in the improved characteristics of TF FDR cooling. Contrary, the cooling time for PF/CS FDRs exceeds the target of four hours due to the design features of the PF/CS FDR cooling system, even after the introduced changes. It is possible to reduce the PF/CS FDRs cooling time to the required four hours by decreasing the number of the resistor modules in a single row to four-five. Disclaimer
Fig. 15. Maximum air velocity in the PF/CS FDR multicircuit cooling system.
The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.
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References [1] V. Tanchuk, S. Grigoriev, V. Lokiev, A. Roshal, I. Song, O. Buzykin, Air cooled Fast Discharge Resistors for ITER magnets, Fusion Eng. Des. 86 (6) (2011) 1445–1449. [2] B. Bareyt, et al., The switching network and discharge circuit of ITER, in: Proceeding of 19th SOFT, August, Portugal, 1996.
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[3] T.J. Chung, Computational Fluid Dynamics, Cambridge University Press, 2002, pp. 1012. [4] ANSYS FLUENT Theory Guide. Release 14.0. November 2011. [5] Documentation for ANSYS Workbench, SAS IP Inc., release 11.0, 2007. [6] Documentation for ANSYS ICEM CFD/AI* Environment 11.0, SP1, SAS IP Inc., 2007.