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Application of heat transfer enhancement technology in ICRF antenna
Fusion Engineering and Design 139 (2019) 19–25
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Application of heat transfer enhancement technology in ICRF antenna a
a,⁎
a
a
a,c
T
a
Qingxi Yang , Wei Song , Labao Tao , Hao Xu , Zhaoxi Chen , Yongsheng Wang , Yanping Zhaoa, Qincheng Bib a b c
Institute of Plasma Physics, Chinese Academy of Sciences, 230031, China XI AN, Jiaotong, University, China CEA, IRFM, F-13108, Saint-Paul-Lez-Durance, France
A R T I C LE I N FO
A B S T R A C T
Keywords: Heat transfer enhancement Faraday shield ICRF antenna EAST
Plasma must be heated by external heating for ignition in the future fusion reactor, ICRF heating is a favourable high-density plasma heating method since the fast wave launched from ICRF antenna can be transmitted to plasma core even in high-density plasma. While heating power increasing, the exposed antenna surface endures higher heat load. Faraday Shield (FS), as one of the key components, faces plasma and undertakes high heat load during plasma discharge. High stress and deformation on the FS is produced without removing the heating, which drastically reduces the antenna performance, even damages FS structure to threat RF antenna safety. To explore high-efficiency heat transfer enhancement technology to remove heat on the FS, one finite element analysis was used to analyse heat and temperature distribution on FS by different heat transfer ways, optimizing the FS structure. Based on the analysis results, several novel heat transfer enhancement way are proposed. Some workpieces with proposed heat transfer enhancement structure were designed and manufactured for test. Afterwards, two mock-ups with one quarter dimension of EAST ICRF antenna were manufactured without and with optimum heat transfer enhancement technology based on former test results, they were tested under same high heat load, significant results are obtained and presented in the paper, results are shown that heat transfer enhancement technology are drastically improved heat transfer performance of FS, which lay the foundation to high-power, steady-state operation ICRF antenna design in the future.
1. Introduction
2. Motivation of exploring heat transfer enhancement method
As an auxiliary heating method, Ion Cyclotron Resonance Frequency (ICRF) heating is widely used in Tokamak devices, such as ITER, WEST and EAST. In EAST, ICRF heating system provides 12 MW power which is indispensable for plasma heating. Faraday Shield (FS) is a front component in ICRF system while it can prevent current straps from high thermal load caused by plasma and fast ions bombardment. [1] During the operation of EAST, heavy thermal load appears on FS of ICRF antenna which is a member of PFCs (Plasma Facing Components) due to dissipation of RF power and plasma radiation [2]. Since ICRF antenna design stepping into long pulse (1000s) operation phase, higher heating power and thermal load will be considered. As a result, current cooling channels of ICRF antenna will face great challenge especially the cooling channels for FS which are unable to serve the long-pulse steady state operation of EAST. In order to improve the heat transfer capability of ICRF antenna, heat transfer enhancement which is regarded as an effective cooling performance improving candidate with economic advantages has been proposed to be applied the cooling channels [3].
There are two ICRH antenna system in EAST and the FS studied here is located in the ICRH antenna system at EAST B port which is a twostrap antenna system. FS consists of a faraday shield box and 41 parallel u-shape cooling pipes as shown in Fig. 1. FS box is divided by middle plate so that cooling pipes can be welded in two rows. Deionized water flows from bottom pipe to top pipe. FS is one of the plasma facing components which sustains heavy thermal load from plasma, RF loss and bombardment of fast ion. FS in EAST ICRH antenna is made of 316 L stainless steel which is a kind of non-magnetic material. Thermal condition of FS in Tokamak is one-side heating condition. The total heating capability of EAST auxiliary heating system approaches 34 MW, while total area of PFC boundary is about 50 m2. So, the maximum heat flux density on FS is about 0.68 MW/m2. For EAST, there are several studies on the optimization of EAST ICRH antenna. These studies focused on the heat flux of the FS and propose a new cooling structure for ICRF antenna. Thermal hydraulic analysis is widely used as a verification and performed in most studies. [1,4] Most of the previous studies
⁎
Corresponding author.
https://doi.org/10.1016/j.fusengdes.2018.12.032 Received 2 September 2018; Received in revised form 28 October 2018; Accepted 12 December 2018 0920-3796/ © 2018 Published by Elsevier B.V.
Fusion Engineering and Design 139 (2019) 19–25
Q. Yang et al.
Re =
udi v
(2)
where v is the dynamic viscosity of fluid. The Fanning friction factor of material in pipe can be defined by:
f=
Δpdi 2LΔp ρu2
(3)
where Δp is the pressure drop of the pipe (Pa), LΔp is the length of pipe where pressure drop is obtained (m) and ρ is the fluid density kg / m3 . According to the Newton’s law of cooling, the average heat transfer factor of material in pipe is:
h=
q ¯ − T¯f TW
(4)
where q is the heat flux on heated wall h is HTC between ¯ is the average temperature of material in pipe and pipe (W / m2K ), TW inner wall of pipe (K), and T¯f is the average temperature of fluid in pipe (K). The average Nusselt number can be defined by [13]: (W / m2 ),
Fig. 1. Faraday shield of ICRF antenna in EAST.
Nu =
only focus on the dimension, shape, location and manufacturing process of cooling channels. However, we propose heat transfer enhancement to be applied in FS in this study. There are two widely-used heat transfer enhancement techniques which are respectively active method and passive method which was studied and applied for material and energy saving. [4] Passive method usually needs specified geometry or surface modifications but not external power input. Passive heat transfer enhancement has already studied to be applied on divertor of Tokamak and the results were positive [5,6]. There are two widely used advanced heat transfer methods in thermal engineering, which are vortex ring and twisted tape [7–10]. Twisted tape leads to a rotating liquid motion which will bring mixing of liquid layers. At the same time, secondary flow in tube will result in vortex disturbance and rib effect with close cooperation between tube and tape, so that the heat transfer performance is enhanced. [9] There are some previous studies on improving heat transfer coefficient (HTC) and Nusselt number induced by turbulence and vortex motion [11]. There are some numerical studies on heat transfer enhancement of subcooled boiling in which HTC under heat transfer enhancement was introduced to express the heat transfer capability [5,12]. In the previous study, screw tube and tube with twisted tape were found to have high HTC through computational fluid dynamics (CFD) analysis. FLUENT is a widely used ANSYS CFD (Computational Fluid Dynamics) simulation module in high temperature fluid study [2,5,12]. So, FLUENT was utilized to perform analysis for the tubes to assess their thermal hydraulic properties. To investigate the heat transfer capability enhancement in this study, some important parameters are introduced in the equations below. Velocity can be acquired by:
u = 14400
(5)
where λ is the thermal conductive coefficient of material in tube (W / m∙K ). However, heat transfer enhancement will not only increase the heat transfer coefficient and also bring some higher flow resistance. As the frictional coefficients of the tubes are known, the relationship of the heat transfer capability between heat enhanced tube and smooth tube can be explained by the following formula:
f Qr N = ( ur )Re /( r )kRei Qs Nus fs
(6)
where Qr is heat removed in tube with heat transfer enhancement and Qs is heat removed in smooth tube. Re in the lower right corner means that all other parameters in this formula should be calculated under the same Reynold’s number. i = P , Δp , qv , where P , Δp and qv are respectively constraints of pumping power, pressure drop and flow rate. In this study, the flow rate is the same for smooth tube and heat transfer enhanced tube which means k qv = 1, so
Qr Qs
=
( )
f Nur /( r ) . Nus Re fs Re
The formula
which gives the relation between heat removed by heat transfer enhanced tube and smooth tube has shown that heat transfer coefficient and pressure drop are both critical parameters for heat transfer capability. Heat transfer enhancement is imposed to increase the heat transfer coefficient while the pressure drop will increase as well. So that, pressure drop should also be investigated in this study. This study mainly gives a systematic study on the application of heat transfer enhancement in ICRF antenna. Wall temperature and pressure drop between inlet and outlet were investigated to assess the application of heat transfer enhancement in FS. In this experiment, screwed tube with twisted tape (Tube 4) is compared with traditional studied tubes which are respectively smooth tube (Tube 1), screwed tube (Tube 2) and tube with twisted tape (Tube 3). The structure of tubes used in this study are shown in Fig. 2. The experiment contains two parts which are respectively assessment of heat transfer capability of the individual tubes, and second step and investigation of heat transfer capability in the screwed tube with twisted tape mock-up comparing with smooth tube.
qv πdi2
hdi λ
(1)
where qv is volumetric flow rate (m3/ h ) and di is inner diameter of pipe (m). The Reynold number corresponding to velocity u can be expressed by:
Fig. 2. Structure view of (a) smooth tube, (b) screwed tube, (c) tube with twisted tape and (d) screwed tube with twisted tape.
20
Fusion Engineering and Design 139 (2019) 19–25
Q. Yang et al.
Fig. 3. Schematic view of (a) screw tube, (b) smooth tube with twisted tape and (c) screw tube with twisted tape.
Fig. 4. Mesh of tube with twisted tape.
flux density as variables. While heat transfer capability and pressure were taken into consideration, different types of tube structure were compared to find out the optimal cooling structure. Thermal hydraulic analysis was performed in ANSYS FLUENT. For the fluid mesh which is critical for the analysis, first layer thickness was 0.08 mm, growth rate was 1.2 with 6 boundary layers, and surface element size was 1 mm. The meshes in the four cases shared the mesh settings in boundary layer and surface element size to keep the similar y plus value in each case. In the hydraulic analyses, tube and fluid model were extended for 10 mm on both side for applying boundary conditions. The mesh of tube with twisted tape is presented as an example in Fig. 4, while k - ε turbulence model was applied for the analysis. Tube with heat transfer enhancement has evener temperature distribution along axial direction than smooth tube as shown in Fig. 5.
3. Thermal hydraulic analysis of FS with different cooling structure In the thermal hydraulic analyses, the four tubes have the same radial dimension as the schematic sketches shown in Fig. 3. The inner diameter and outer diameter of each tube were respectively 8 mm and 10 mm. The fin pinch was 1.25 mm. The twisted ratio of twisted tape was 2 and its thickness was 0.4 mm. The effective heating length of the tubes was 170 mm. In order to keep margin for design and operation, the heat flux density was assigned to 0.75 MW/m2 and 0.5 MW/m2. Heat loads were applied on the half of heated wall of tubes to supply one-side heating for the thermal hydraulic analysis. The alternative velocity of water are 1 m/s and 3 m/s. The inlet temperature is 30 ℃. The surface temperature was calculated taking flow velocity and heat 21
Fusion Engineering and Design 139 (2019) 19–25
Q. Yang et al.
Fig. 5. Temperature along axial direction of the four tubes under (a) 0.5 MW/m2 heat flux density, 1 m/s flow velocity, (b) 0.5 MW/m2 heat flux density, 3 m/s flow velocity, (c) 0.75 MW/m2 heat flux density, 1 m/s flow velocity, and (d) 0.75 MW/m2 heat flux density, 3 m/s flow velocity.
Fig. 6. Maximum temperature of the four tubes under different conditions: 0.5 MW/m2 heat flux density, 1 m/s flow velocity, 0.5 MW/m2 heat flux density, 3 m/s flow velocity, 0.75 MW/m2 heat flux density, 1 m/s flow velocity, and 0.75 MW/m2 heat flux density, 3 m/s flow velocity.
Fig. 7. Pressure drop between inlet and outlet of the four tubes at velocity of 1 m/s and 3 m/s.
s. Tubes in the thermal analysis was short, and the difference of pressure drop between the integrity structure of faraday shield mock-ups will be much smaller. Smooth tube with twisted tape, screw tube and screw tube with twisted tape were better than smooth tube in heat transfer capability. Although, screw tube with twisted tape has the highest pressure drop, its heat transfer capability is obviously better than other tubes. So, screw tube with twisted tape is recommended to be applied on FS of ICRF antenna.
Screw tube with twisted tape always has more heat transfer capability than other tubes. Its maximum temperature can be 62% less than smooth tube’s as shown in Fig. 6. Pressure drop increased about 10 times when the inlet velocity of tubes with heat transfer enhancement increase from 1 m/s to 3 m/s as shown in Fig. 7. However, the screw tube with twisted tape has the highest pressure drop which can approach 1000 times than that of smooth tube when inlet velocity is 3 m/ 22
Fusion Engineering and Design 139 (2019) 19–25
Q. Yang et al.
Fig. 8. Sketch of temperature measurement points.
Fig. 11. Pressure drop between inlet and outlet under mass flow rate in the mock-ups.
Fig. 9. Average temperature in each tube was related to flow velocity.
4. Experimental study of heat transfer performance
drop were investigated in the experiment. Flow rate was measured by E-mag C electromagnetic flow meter and pressure drop between inlet and outlet of pipe were measured by Rosemount pressure measure instrument. Measurement of tube temperature was one of key point in the experiment due to the thermocouple measuring point was exposed under the heat radiation caused by radiation heating source. Wall temperature was measured by the thermocouple wire which was made of Nickel-chromium alloy and Nickel-silicon alloy (K type) which was welded on the pipes as shown in Fig. 8(a). In order to improve the accuracy of the measurement, the measuring point was protected by glass fiber casing and high temperature structure adhesive. There were four cross sections for measurement distributed evenly between inlet and outlet as shown in Fig. 8(b). On each cross section, four thermocouples were set to measure temperature on top of pipe, side of pipe
For different types of tube structure, heat transfer capability and pressure were measured to find out the optimal cooling structure. Thermal load on experimental cases was one-side heating condition whose heat flux density was 0.75 MW/m2 in the experiment of the four tubes. The surface temperature was acquired under different flow velocity and heat flux density. The alternative velocity of water was 3 m/ s. The temperature of inlet was 30 ℃. The operating pressure for the experiment of mock-ups was 0.2 MPa. The parameters to be measured were temperature of outlet, pressure drop between inlet and outlet and surface temperature of pipes. Silicon carbon rod was for one-side heating in the experiment system. The diameter of silicon carbon rod was 20 mm and 700 mm in length. Heat flux density, flow velocity, wall temperature and pressure
Fig. 10. Details of (a) experiment system for mock-ups and (b) temperature measurement points (red points) on mock-up (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 23
Fusion Engineering and Design 139 (2019) 19–25
Q. Yang et al.
Fig. 12. (a) Average temperature, and (b) infrared image of smooth tube mock-up, and (c) Average temperature, and (d) infrared image of screw tube with twisted tape mock-up.
smooth tube is 17.4% higher than that of screw tube with twisted tape while has the lowest average temperature. Details of experiment system and 20 temperature measurement points are shown in Fig. 10. There were totally 15 silicon carbon rods were used for accurate heating flow control while the heating power varies from 14 kW to 40 kW. The relation between pressure drop and mass flow rate is shown in Fig. 11. The maximum pressure drop of screw tube with twisted tape is approximately 1.7 times more than that of smooth tube under the same mass flow rate which is acceptable for operation. Infrared thermometer was used to obtain the wall temperature of mock-ups and the flow rate distribution in every branch as well. Wall temperature was utilized to investigate heat transfer capability change with heat transfer enhancement tubes. The average temperature at two mock-ups’ measurement points under different heating power shown in Fig. 12. Average temperature under different heating power of each 10 branches of two mock-ups are respectively shown in Tables 1 and 2. The result shows that heat transfer capability wasn’t improved by heat transfer enhancement as expected at the first 10 branches, however improved heat transfer capability was obtained in the last 10 branches. It’s proved that heat transfer capability of screw tube with twisted tape mock-up didn’t decrease like that of smooth tube mock-up with increased fluid temperature due to the high wall temperature. The temperature line chart also shows that screwed tube with twisted tape mock-up can provide evener temperature distribution than the smooth tube.
Table 1 Average temperature at measurement points of smooth tube. Heating power (kW)
14
18.6
22
27
32.5
39.2
Average temperature at measurement point 1-10 Average temperature at measurement point 11-20
47.1
58.1
67.7
77.9
90.3
101.3
58.2
69
77.5
88.6
102.2
115.6
Table 2 Average temperature at measurement points of screw tube with twisted tape. Heating power (kW)
14
18.4
22.8
27
32.8
37
Average temperature at measurement point 1-10 Average temperature at measurement point 11-20
52.6
68.2
80.5
90
105.8
117.9
51.1
60.9
71.9
82.4
95.6
105.5
and bottom of pipe respectively. The thermocouple of inlet and outlet was made by copper-constantan and coated with lamination in water under one-side heating condition. Temperature measured at the measure points were utilized to calculate the average temperature which was expected to show the heat transfer performance difference between the tubes. Average temperature decreases with the flow velocity under 0.75 MW/m2 heat flux density for all tubes as shown in Fig. 9. However, the smooth tube has the highest average temperature among the tubes while the screw tube with twisted tape has the lowest one. The average temperature of 24
Fusion Engineering and Design 139 (2019) 19–25
Q. Yang et al.
5. Conclusion
comments and support with this study.
The heat transfer performance and flow property of smooth tube, screw tube, smooth tube with twisted tape and screw tube with twisted tape were studied under one-side heating condition by thermal hydraulic analysis. Application of heat transfer enhancement in FS will improved the heat transfer capability compared with current tube. Taking the requirement imposed by long-pulse and steady state operation of EAST into account, screw tube with twisted tape was recommended to be the best candidate among the thermal transfer enhancement proposals. Results of experiment match well with the result of the thermal analysis. Mock-up experiments were performed to investigate the temperature distribution and pressure drop, so that the heat transfer capability improvement after screw tube with twisted tape applied on FS can be verified. Screwed tube with twisted tape mock-up has better heat transfer capability than smooth tube one only at last 10 branches which means better heat transfer capability with increased fluid temperature. Taken together, application of screwed tube with twisted tape in FS has improvement in heat transfer capability against the increasement of fluid temperature and provides even temperature distribution as well.
References [1] Y.A.N.G. Qingxi, et al., Thermal analysis and optimization of the EAST ICRH antenna, Plasma Sci. Technol. 20 (2) (2018) 025603. [2] C.M. Qin, et al., Initial operation of high power ICRF system for long pulse in EAST, AIP Conference Proceedings Vol. 1689. No. 1. AIP Publishing, (2015). [3] Mohsen Sheikholeslami, Mofid Gorji-Bandpy, Davood Domiri Ganji, Review of heat transfer enhancement methods: focus on passive methods using swirl flow devices, Renew. Sustain. Energy Rev. 49 (2015) 444–469. [4] Zhaoxi Chen, et al., Design and optimization of the WEST ICRH antenna front face components based on thermal and hydraulic analysis, Fusion Eng. Des. 94 (2015) 82–89. [5] P. Liu, et al., Subcooled water flow boiling heat transfer in screw cooling tubes under one-sided heating conditions, Appl. Therm. Eng. 113 (2017) 621–631. [6] Masanori Araki, et al., Critical-heat-flux experiment on the screw tube under onesided-heating conditions, Fusion Technol. 29.4 (1996) 519–528. [7] A. Hasanpour, M. Farhadi, K. Sedighi, A review study on twisted tape inserts on turbulent flow heat exchangers: the overall enhancement ratio criteria, Int. Commun. Heat Mass Transfer 55 (2014) 53–62. [8] Cancan Zhang, et al., A comparative review of self-rotating and stationary twisted tape inserts in heat exchanger, Renew. Sustain. Energy Rev. 53 (2016) 433–449. [9] N. Piriyarungrod, et al., Heat transfer enhancement by tapered twisted tape inserts, Chem. Eng. Process.: Process Intensif. 96 (2015) 62–71. [10] Pongjet Promvonge, et al., Thermal performance enhancement in a heat exchanger tube fitted with inclined vortex rings, Appl. Therm. Eng. 62.1 (2014) 285–292. [11] K. Nanan, et al., Investigation of heat transfer enhancement by perforated helical twisted-tapes, Int. Commun. Heat Mass Transfer 52 (2014) 106–112. [12] Rui Zhang, et al., Effects of turbulence models on forced convection subcooled boiling in vertical pipe, Ann. Nucl. Energy 80 (2015) 293–302. [13] Qiang Jin, et al., Numerical investigation of heat transfer enhancement in ribbed channel for the first wall of DFLL-TBM in ITER, Fusion Eng. Des. 87 (7-8) (2012) 974–978.
Acknowledgments This work was carried out in Institute of Plasma Physics, Chinese Academy of Sciences and supported by the National Natural Science Foundation of China11375233. The authors wish to thank Qincheng Bi and his team from XI AN, Jiaotong, University for their discussion,
25
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Application of heat transfer enhancement technology in ICRF antenna a
a,⁎
a
a
a,c
T
a
Qingxi Yang , Wei Song , Labao Tao , Hao Xu , Zhaoxi Chen , Yongsheng Wang , Yanping Zhaoa, Qincheng Bib a b c
Institute of Plasma Physics, Chinese Academy of Sciences, 230031, China XI AN, Jiaotong, University, China CEA, IRFM, F-13108, Saint-Paul-Lez-Durance, France
A R T I C LE I N FO
A B S T R A C T
Keywords: Heat transfer enhancement Faraday shield ICRF antenna EAST
Plasma must be heated by external heating for ignition in the future fusion reactor, ICRF heating is a favourable high-density plasma heating method since the fast wave launched from ICRF antenna can be transmitted to plasma core even in high-density plasma. While heating power increasing, the exposed antenna surface endures higher heat load. Faraday Shield (FS), as one of the key components, faces plasma and undertakes high heat load during plasma discharge. High stress and deformation on the FS is produced without removing the heating, which drastically reduces the antenna performance, even damages FS structure to threat RF antenna safety. To explore high-efficiency heat transfer enhancement technology to remove heat on the FS, one finite element analysis was used to analyse heat and temperature distribution on FS by different heat transfer ways, optimizing the FS structure. Based on the analysis results, several novel heat transfer enhancement way are proposed. Some workpieces with proposed heat transfer enhancement structure were designed and manufactured for test. Afterwards, two mock-ups with one quarter dimension of EAST ICRF antenna were manufactured without and with optimum heat transfer enhancement technology based on former test results, they were tested under same high heat load, significant results are obtained and presented in the paper, results are shown that heat transfer enhancement technology are drastically improved heat transfer performance of FS, which lay the foundation to high-power, steady-state operation ICRF antenna design in the future.
1. Introduction
2. Motivation of exploring heat transfer enhancement method
As an auxiliary heating method, Ion Cyclotron Resonance Frequency (ICRF) heating is widely used in Tokamak devices, such as ITER, WEST and EAST. In EAST, ICRF heating system provides 12 MW power which is indispensable for plasma heating. Faraday Shield (FS) is a front component in ICRF system while it can prevent current straps from high thermal load caused by plasma and fast ions bombardment. [1] During the operation of EAST, heavy thermal load appears on FS of ICRF antenna which is a member of PFCs (Plasma Facing Components) due to dissipation of RF power and plasma radiation [2]. Since ICRF antenna design stepping into long pulse (1000s) operation phase, higher heating power and thermal load will be considered. As a result, current cooling channels of ICRF antenna will face great challenge especially the cooling channels for FS which are unable to serve the long-pulse steady state operation of EAST. In order to improve the heat transfer capability of ICRF antenna, heat transfer enhancement which is regarded as an effective cooling performance improving candidate with economic advantages has been proposed to be applied the cooling channels [3].
There are two ICRH antenna system in EAST and the FS studied here is located in the ICRH antenna system at EAST B port which is a twostrap antenna system. FS consists of a faraday shield box and 41 parallel u-shape cooling pipes as shown in Fig. 1. FS box is divided by middle plate so that cooling pipes can be welded in two rows. Deionized water flows from bottom pipe to top pipe. FS is one of the plasma facing components which sustains heavy thermal load from plasma, RF loss and bombardment of fast ion. FS in EAST ICRH antenna is made of 316 L stainless steel which is a kind of non-magnetic material. Thermal condition of FS in Tokamak is one-side heating condition. The total heating capability of EAST auxiliary heating system approaches 34 MW, while total area of PFC boundary is about 50 m2. So, the maximum heat flux density on FS is about 0.68 MW/m2. For EAST, there are several studies on the optimization of EAST ICRH antenna. These studies focused on the heat flux of the FS and propose a new cooling structure for ICRF antenna. Thermal hydraulic analysis is widely used as a verification and performed in most studies. [1,4] Most of the previous studies
⁎
Corresponding author.
https://doi.org/10.1016/j.fusengdes.2018.12.032 Received 2 September 2018; Received in revised form 28 October 2018; Accepted 12 December 2018 0920-3796/ © 2018 Published by Elsevier B.V.
Fusion Engineering and Design 139 (2019) 19–25
Q. Yang et al.
Re =
udi v
(2)
where v is the dynamic viscosity of fluid. The Fanning friction factor of material in pipe can be defined by:
f=
Δpdi 2LΔp ρu2
(3)
where Δp is the pressure drop of the pipe (Pa), LΔp is the length of pipe where pressure drop is obtained (m) and ρ is the fluid density kg / m3 . According to the Newton’s law of cooling, the average heat transfer factor of material in pipe is:
h=
q ¯ − T¯f TW
(4)
where q is the heat flux on heated wall h is HTC between ¯ is the average temperature of material in pipe and pipe (W / m2K ), TW inner wall of pipe (K), and T¯f is the average temperature of fluid in pipe (K). The average Nusselt number can be defined by [13]: (W / m2 ),
Fig. 1. Faraday shield of ICRF antenna in EAST.
Nu =
only focus on the dimension, shape, location and manufacturing process of cooling channels. However, we propose heat transfer enhancement to be applied in FS in this study. There are two widely-used heat transfer enhancement techniques which are respectively active method and passive method which was studied and applied for material and energy saving. [4] Passive method usually needs specified geometry or surface modifications but not external power input. Passive heat transfer enhancement has already studied to be applied on divertor of Tokamak and the results were positive [5,6]. There are two widely used advanced heat transfer methods in thermal engineering, which are vortex ring and twisted tape [7–10]. Twisted tape leads to a rotating liquid motion which will bring mixing of liquid layers. At the same time, secondary flow in tube will result in vortex disturbance and rib effect with close cooperation between tube and tape, so that the heat transfer performance is enhanced. [9] There are some previous studies on improving heat transfer coefficient (HTC) and Nusselt number induced by turbulence and vortex motion [11]. There are some numerical studies on heat transfer enhancement of subcooled boiling in which HTC under heat transfer enhancement was introduced to express the heat transfer capability [5,12]. In the previous study, screw tube and tube with twisted tape were found to have high HTC through computational fluid dynamics (CFD) analysis. FLUENT is a widely used ANSYS CFD (Computational Fluid Dynamics) simulation module in high temperature fluid study [2,5,12]. So, FLUENT was utilized to perform analysis for the tubes to assess their thermal hydraulic properties. To investigate the heat transfer capability enhancement in this study, some important parameters are introduced in the equations below. Velocity can be acquired by:
u = 14400
(5)
where λ is the thermal conductive coefficient of material in tube (W / m∙K ). However, heat transfer enhancement will not only increase the heat transfer coefficient and also bring some higher flow resistance. As the frictional coefficients of the tubes are known, the relationship of the heat transfer capability between heat enhanced tube and smooth tube can be explained by the following formula:
f Qr N = ( ur )Re /( r )kRei Qs Nus fs
(6)
where Qr is heat removed in tube with heat transfer enhancement and Qs is heat removed in smooth tube. Re in the lower right corner means that all other parameters in this formula should be calculated under the same Reynold’s number. i = P , Δp , qv , where P , Δp and qv are respectively constraints of pumping power, pressure drop and flow rate. In this study, the flow rate is the same for smooth tube and heat transfer enhanced tube which means k qv = 1, so
Qr Qs
=
( )
f Nur /( r ) . Nus Re fs Re
The formula
which gives the relation between heat removed by heat transfer enhanced tube and smooth tube has shown that heat transfer coefficient and pressure drop are both critical parameters for heat transfer capability. Heat transfer enhancement is imposed to increase the heat transfer coefficient while the pressure drop will increase as well. So that, pressure drop should also be investigated in this study. This study mainly gives a systematic study on the application of heat transfer enhancement in ICRF antenna. Wall temperature and pressure drop between inlet and outlet were investigated to assess the application of heat transfer enhancement in FS. In this experiment, screwed tube with twisted tape (Tube 4) is compared with traditional studied tubes which are respectively smooth tube (Tube 1), screwed tube (Tube 2) and tube with twisted tape (Tube 3). The structure of tubes used in this study are shown in Fig. 2. The experiment contains two parts which are respectively assessment of heat transfer capability of the individual tubes, and second step and investigation of heat transfer capability in the screwed tube with twisted tape mock-up comparing with smooth tube.
qv πdi2
hdi λ
(1)
where qv is volumetric flow rate (m3/ h ) and di is inner diameter of pipe (m). The Reynold number corresponding to velocity u can be expressed by:
Fig. 2. Structure view of (a) smooth tube, (b) screwed tube, (c) tube with twisted tape and (d) screwed tube with twisted tape.
20
Fusion Engineering and Design 139 (2019) 19–25
Q. Yang et al.
Fig. 3. Schematic view of (a) screw tube, (b) smooth tube with twisted tape and (c) screw tube with twisted tape.
Fig. 4. Mesh of tube with twisted tape.
flux density as variables. While heat transfer capability and pressure were taken into consideration, different types of tube structure were compared to find out the optimal cooling structure. Thermal hydraulic analysis was performed in ANSYS FLUENT. For the fluid mesh which is critical for the analysis, first layer thickness was 0.08 mm, growth rate was 1.2 with 6 boundary layers, and surface element size was 1 mm. The meshes in the four cases shared the mesh settings in boundary layer and surface element size to keep the similar y plus value in each case. In the hydraulic analyses, tube and fluid model were extended for 10 mm on both side for applying boundary conditions. The mesh of tube with twisted tape is presented as an example in Fig. 4, while k - ε turbulence model was applied for the analysis. Tube with heat transfer enhancement has evener temperature distribution along axial direction than smooth tube as shown in Fig. 5.
3. Thermal hydraulic analysis of FS with different cooling structure In the thermal hydraulic analyses, the four tubes have the same radial dimension as the schematic sketches shown in Fig. 3. The inner diameter and outer diameter of each tube were respectively 8 mm and 10 mm. The fin pinch was 1.25 mm. The twisted ratio of twisted tape was 2 and its thickness was 0.4 mm. The effective heating length of the tubes was 170 mm. In order to keep margin for design and operation, the heat flux density was assigned to 0.75 MW/m2 and 0.5 MW/m2. Heat loads were applied on the half of heated wall of tubes to supply one-side heating for the thermal hydraulic analysis. The alternative velocity of water are 1 m/s and 3 m/s. The inlet temperature is 30 ℃. The surface temperature was calculated taking flow velocity and heat 21
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Fig. 5. Temperature along axial direction of the four tubes under (a) 0.5 MW/m2 heat flux density, 1 m/s flow velocity, (b) 0.5 MW/m2 heat flux density, 3 m/s flow velocity, (c) 0.75 MW/m2 heat flux density, 1 m/s flow velocity, and (d) 0.75 MW/m2 heat flux density, 3 m/s flow velocity.
Fig. 6. Maximum temperature of the four tubes under different conditions: 0.5 MW/m2 heat flux density, 1 m/s flow velocity, 0.5 MW/m2 heat flux density, 3 m/s flow velocity, 0.75 MW/m2 heat flux density, 1 m/s flow velocity, and 0.75 MW/m2 heat flux density, 3 m/s flow velocity.
Fig. 7. Pressure drop between inlet and outlet of the four tubes at velocity of 1 m/s and 3 m/s.
s. Tubes in the thermal analysis was short, and the difference of pressure drop between the integrity structure of faraday shield mock-ups will be much smaller. Smooth tube with twisted tape, screw tube and screw tube with twisted tape were better than smooth tube in heat transfer capability. Although, screw tube with twisted tape has the highest pressure drop, its heat transfer capability is obviously better than other tubes. So, screw tube with twisted tape is recommended to be applied on FS of ICRF antenna.
Screw tube with twisted tape always has more heat transfer capability than other tubes. Its maximum temperature can be 62% less than smooth tube’s as shown in Fig. 6. Pressure drop increased about 10 times when the inlet velocity of tubes with heat transfer enhancement increase from 1 m/s to 3 m/s as shown in Fig. 7. However, the screw tube with twisted tape has the highest pressure drop which can approach 1000 times than that of smooth tube when inlet velocity is 3 m/ 22
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Fig. 8. Sketch of temperature measurement points.
Fig. 11. Pressure drop between inlet and outlet under mass flow rate in the mock-ups.
Fig. 9. Average temperature in each tube was related to flow velocity.
4. Experimental study of heat transfer performance
drop were investigated in the experiment. Flow rate was measured by E-mag C electromagnetic flow meter and pressure drop between inlet and outlet of pipe were measured by Rosemount pressure measure instrument. Measurement of tube temperature was one of key point in the experiment due to the thermocouple measuring point was exposed under the heat radiation caused by radiation heating source. Wall temperature was measured by the thermocouple wire which was made of Nickel-chromium alloy and Nickel-silicon alloy (K type) which was welded on the pipes as shown in Fig. 8(a). In order to improve the accuracy of the measurement, the measuring point was protected by glass fiber casing and high temperature structure adhesive. There were four cross sections for measurement distributed evenly between inlet and outlet as shown in Fig. 8(b). On each cross section, four thermocouples were set to measure temperature on top of pipe, side of pipe
For different types of tube structure, heat transfer capability and pressure were measured to find out the optimal cooling structure. Thermal load on experimental cases was one-side heating condition whose heat flux density was 0.75 MW/m2 in the experiment of the four tubes. The surface temperature was acquired under different flow velocity and heat flux density. The alternative velocity of water was 3 m/ s. The temperature of inlet was 30 ℃. The operating pressure for the experiment of mock-ups was 0.2 MPa. The parameters to be measured were temperature of outlet, pressure drop between inlet and outlet and surface temperature of pipes. Silicon carbon rod was for one-side heating in the experiment system. The diameter of silicon carbon rod was 20 mm and 700 mm in length. Heat flux density, flow velocity, wall temperature and pressure
Fig. 10. Details of (a) experiment system for mock-ups and (b) temperature measurement points (red points) on mock-up (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 23
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Fig. 12. (a) Average temperature, and (b) infrared image of smooth tube mock-up, and (c) Average temperature, and (d) infrared image of screw tube with twisted tape mock-up.
smooth tube is 17.4% higher than that of screw tube with twisted tape while has the lowest average temperature. Details of experiment system and 20 temperature measurement points are shown in Fig. 10. There were totally 15 silicon carbon rods were used for accurate heating flow control while the heating power varies from 14 kW to 40 kW. The relation between pressure drop and mass flow rate is shown in Fig. 11. The maximum pressure drop of screw tube with twisted tape is approximately 1.7 times more than that of smooth tube under the same mass flow rate which is acceptable for operation. Infrared thermometer was used to obtain the wall temperature of mock-ups and the flow rate distribution in every branch as well. Wall temperature was utilized to investigate heat transfer capability change with heat transfer enhancement tubes. The average temperature at two mock-ups’ measurement points under different heating power shown in Fig. 12. Average temperature under different heating power of each 10 branches of two mock-ups are respectively shown in Tables 1 and 2. The result shows that heat transfer capability wasn’t improved by heat transfer enhancement as expected at the first 10 branches, however improved heat transfer capability was obtained in the last 10 branches. It’s proved that heat transfer capability of screw tube with twisted tape mock-up didn’t decrease like that of smooth tube mock-up with increased fluid temperature due to the high wall temperature. The temperature line chart also shows that screwed tube with twisted tape mock-up can provide evener temperature distribution than the smooth tube.
Table 1 Average temperature at measurement points of smooth tube. Heating power (kW)
14
18.6
22
27
32.5
39.2
Average temperature at measurement point 1-10 Average temperature at measurement point 11-20
47.1
58.1
67.7
77.9
90.3
101.3
58.2
69
77.5
88.6
102.2
115.6
Table 2 Average temperature at measurement points of screw tube with twisted tape. Heating power (kW)
14
18.4
22.8
27
32.8
37
Average temperature at measurement point 1-10 Average temperature at measurement point 11-20
52.6
68.2
80.5
90
105.8
117.9
51.1
60.9
71.9
82.4
95.6
105.5
and bottom of pipe respectively. The thermocouple of inlet and outlet was made by copper-constantan and coated with lamination in water under one-side heating condition. Temperature measured at the measure points were utilized to calculate the average temperature which was expected to show the heat transfer performance difference between the tubes. Average temperature decreases with the flow velocity under 0.75 MW/m2 heat flux density for all tubes as shown in Fig. 9. However, the smooth tube has the highest average temperature among the tubes while the screw tube with twisted tape has the lowest one. The average temperature of 24
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5. Conclusion
comments and support with this study.
The heat transfer performance and flow property of smooth tube, screw tube, smooth tube with twisted tape and screw tube with twisted tape were studied under one-side heating condition by thermal hydraulic analysis. Application of heat transfer enhancement in FS will improved the heat transfer capability compared with current tube. Taking the requirement imposed by long-pulse and steady state operation of EAST into account, screw tube with twisted tape was recommended to be the best candidate among the thermal transfer enhancement proposals. Results of experiment match well with the result of the thermal analysis. Mock-up experiments were performed to investigate the temperature distribution and pressure drop, so that the heat transfer capability improvement after screw tube with twisted tape applied on FS can be verified. Screwed tube with twisted tape mock-up has better heat transfer capability than smooth tube one only at last 10 branches which means better heat transfer capability with increased fluid temperature. Taken together, application of screwed tube with twisted tape in FS has improvement in heat transfer capability against the increasement of fluid temperature and provides even temperature distribution as well.
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Acknowledgments This work was carried out in Institute of Plasma Physics, Chinese Academy of Sciences and supported by the National Natural Science Foundation of China11375233. The authors wish to thank Qincheng Bi and his team from XI AN, Jiaotong, University for their discussion,
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