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2 m heat pipe-cryogenic targets for COSY-TOF experiment
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 551 (2005) 236–244 www.elsevier.com/locate/nima
2 m heat pipe-cryogenic targets for COSY-TOF experiment M. Abdel-Barya, S. Abdel-Samadb,, K. Kilianb a Atomic Energy Authority, NRC, Cyclotron Project, 13759 Cairo, Egypt Institut fu¨r Kernphysik, Forschungszentrum Ju¨lich, D-52425 Ju¨lich, Germany
b
Received 30 March 2005; received in revised form 30 May 2005; accepted 23 June 2005 Available online 18 July 2005
Abstract A liquid hydrogen/deuterium target is used in COSY external experiments. For the TOF and WASA applications at COSY, a 2.0 m long, 7 mm diameter heat pipe (HP) has been developed and tested under filling conditions of 9.45 l at 228 mbar with the four gases H2, D2, N2 and CH4. The cooling down times are 107, 200, 54 and 38 mins, respectively, from 300 K until working conditions have been achieved. The target liquid is produced at a cooled condenser and guided through a central tube assisted by gravitation into the target cell. The characteristics at steady-state operating conditions were measured for H2, N2 and CH4 as the working fluids. The effect of the operating temperature and the heat loads on the effective thermal conductivity and the liquid mass in the heat pipe was investigated. r 2005 Elsevier B.V. All rights reserved. PACS: 29.25.– t; 07.20.Mc; 07.20.Pe Keywords: Targets; Cryogenics; Low-temperature equipment; Heat pipes
1. Introduction At the COoler SYnchrotron (COSY) interactions of protons in the GeV energy range with target nuclei are studied. For experimental reasons, it is important that as many interactions as possible occur in a target volume as small as Corresponding author. Tel.: +49 2461 613917; fax: +49 2461 613930. E-mail address: [email protected] (S. Abdel-Samad).
possible. As a consequence, the COSY beam should be as small as possible and the density of the target material should be high. For hydrogen isotopes this means that liquid instead of gaseous hydrogen or deuterium must be used. A liquid hydrogen/deuterium target is therefore used at COSY Juelich in the external experiments TOF and BIG KARL. In an earlier version of the standard target [1], copper heat conductors were used between the cooling machine and the target region. Different heat conducting materials (aluminum, silver) were tested [2,3]. The shortest
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.06.059
ARTICLE IN PRESS M. Abdel-Bary et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 236–244
cooling down and heating up times were obtained with aluminum. It had the lowest heat capacity and high thermal conductivity at low temperature. A big improvement was achieved by changing from metallic conduction to heat transfer by convection in heat pipes [2,4,5]. Evaporation and condensation of a working fluid at different ends of a tube is used to transport a large amount of heat [6,7]. A 16 mm diameter heat pipe was tested, 32.25 cm and 2.0 m long [2]. The liquid hydrogen (LH2) cooling down times were 70 and 320 min, respectively. To further improve the performance of the target and to decrease the cooling down time with the new 2.0 m design, a 7 mm diameter heat pipe was developed instead of the 16 mm diameter one [8]. A copper heat conductor with this geometry would not work. The temperature difference along such a copper conductor would be 10 K. The available cooling machine is not able to provide the necessary cooling power and the required cold head temperature of less than 5 K. The mass of such a system is 3600 g. The cooling down time due to its heat capacity alone would be about 10 h. The heat pipe target system has to fulfill the following requirements:
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2. Heat pipe dimensions and design In order to decrease the cooling down/heating up time for the 2.0 m long liquid hydrogen (called LH2 in the following) targets, a heat pipe with a smaller diameter (7 mm) was developed. The material, the surface area and the heat capacity were reduced with this smaller diameter heat pipe compared with a 16 mm diameter heat pipe. The heat pipe-target system, the mechanical design and the test apparatus for a 32 cm system were described in detail by Abdel-Samad et al. [9]. The new aspect of the present work is an increased length of the heat pipe to 2.0 m compared with 32 cm, which opens up new applications. Fig. 1 shows a schematic diagram for the 2.0 m long and 7 mm diameter heat pipe with the target cell. The aluminum condenser has a surface area of condensation of 39 cm2. Table 1 summarizes the parameters of this long version. The tests were done at a 901 inclination angle to the horizontal beam direction. The cooling down times for H2, D2, N2 and CH4 were measured. The effective thermal conductivity, the mass flow rate and the pressure inside the heat pipe were measured.
3. Heat pipe-target system cooling down time (1) It has to stabilize the target temperature within 70.01 K in the range of liquid hydrogen/ deuterium. (2) The cooling system and all heat transferring connections around the target have to occupy minimum space and to have minimum mass in the target region in order to reduce background events. The shadow created by materials (all heat-transferring connections) in the target region reduces the solid angle seen by the detectors and creates background reactions. (3) The target thickness has to be well defined; therefore, the target liquid should be of homogeneous density without any bubbles which would change the effective target thickness drastically. (4) The cooling down time to working temperature or heating up to room temperature has to be as short as possible.
The bellows and the heat pipe-target system are filled with 9.54 l of hydrogen gas at 228 mbar pressure at room temperature and then the cooling machine is switched on. Fig. 2 shows details of the time dependences of the temperatures, the pressure, the bellows height and the liquid level for LH2. The hydrogen condensation starts after 39 min from switching on the cooling machine. The LH2 reaches the target cell after 107 min and a few seconds later the target is full and stable. The time difference of 68 min (10739 ¼ 68) is needed for the liquid to proceed down towards the evaporator section. During that time the liquid has to cool down the inner tube over its full length. Droplets of LH2 fall down into the inner tube towards the evaporator region. The liquid vaporizes taking latent heat from the inner tube and finally from the evaporator region. Finally, the system has steady liquefaction in the condenser
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and a downward flow of liquid in the inner tube, vaporization in the evaporator and flow of vapor upwards in the space between the inner tube and the heat pipe inner surface. The evaporator temperature drops and one starts to observe short pressure spikes during cooling down of the inner tube due to droplet evaporation (Fig. 2). As soon as the arriving liquid remains cooler than the boiling temperature, a liquid column appears in the target appendix, then the gas volume and pressure decrease, and the bellows height is reduced as shown in Fig. 2. The total integrated heat capacity of the heat pipe-target system in cooling down from 290 K to 15 K is 4.783 kJ (2.15 g plastic inner tube has C ¼ 683:0 J, 0.2 g H2 gas has C ¼ 640:0 J and both 41.75 g steel transport and 1.0 g copper evaporator sections have C ¼ 3:46 kJ). It takes 68 min for cooling with the LH2 flow starting at the condenser to reach 15 K at the evaporator. From that we calculate the rate of heat transfer Q_ with the liquid to be 1.17 W. The measured cooling down time and some characteristic values in the cooling down mode calculated from the thermal properties for H2, D2, N2 and CH4 are summarized in Table 2.
4. Effective thermal conductivity Fig. 1. Schematic drawing of the 2.0 m long heat pipe-target combination.
Table 1 Parameters of the 2.0 m long, 7 mm diameter heat pipe with aluminum condenser Total length Part with 16 mm diameter Part with 7 mm diameter Inner tube length Inner tube diameter Wall thickness Inner tube weight Weight of HP Weight of gas connector Weight of Al condenser Total weight
200 cm 7.5 cm 184.0 cm 185.5 cm 3.2 mm 0.1 mm 2.15 g 36.30 g 5.44 g 20.00 g 63.89 g
A high value of the thermal conductivity indicates a rapid heat flow rate which leads to stable liquid in the target cell at constant density without boiling. This is very important and absolutely necessary for a well-defined reaction between the proton beam and the LH2/LD2 target. The effective thermal conductivity Keff [10–12] for the heat pipe system is determined by the relation between the measured heat flow rate Q_ and the measured temperature difference DT between the external surfaces of the lower end (evaporator T2) and upper end (condenser T1): _ K eff ¼ ðLQÞ=ðDTAÞ
(1)
where L is the total length of the heat pipe between the measuring points T1 and T2 , and A is the cross-sectional area of the heat pipe.
ARTICLE IN PRESS M. Abdel-Bary et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 236–244
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300 Evaporator temp.
Bellows height Temp.(K), Pressure(mbar), Bellows(mm), LH2 (mm)
250 H2 gas pressure
200
150 Condenser temp. 100 LH2 Liquid 50
0 0
10
20
30
40
50
60 Time (min.)
70
80
90
100
110
Fig. 2. Time dependence of the temperature, pressure, bellows height and liquid level for LH2 of 2.0 m target with 7 mm heat pipe and 3 mm diameter inner tube.
Table 2 Characteristic values during cooling down mode calculated for H2, D2, N2 and CH4 2.0 m heat pipe-target H2
D2
N2
CH4
Cooling down time from 300 K to liquid working condition (min) _ Mass flow rate m(g/s) Vapor velocity Vv (cm/s) Reynolds number Re Gas density rg (g/cm3) at 220 mbar, 295 K Total mass in system (g) Evaporation energy of total mass (J)
107 2.6 103 26.6 600 0.019 103 0.175 77.9
200 1.7 103 10.5 184.8 0.033 103 0.355 107.9
54 18.9 103 65.3 820.2 0.253 103 2.389 473.9
38 14.9 103 114.9 792.9 0.145 103 1.379 702.4
Fig. 3 shows the effective thermal conductivity Keff of the hydrogen 200-cm heat pipe-target system as a function of the heat load for different condenser temperatures. The experimental results show that Keff increases with increasing heat load and with decreasing condenser temperature. The highest effective thermal conductivity is 1558 W/ cm K. Fig. 4 shows the comparison between the LH2 Keff and the experimentally determined Keff of copper, silver and aluminum heat conductors. The heat pipe conductivity is 100 times greater than the metallic conductors. The Keff is also measured for CH4 and N2. The maximum effective thermal conductivity for H2, CH4, N2 and the metallic conductors’ thermal conductivity are summarized in Table 3.
5. Liquid mass in the heat pipe-target system It is important for the target operation to have enough liquid in the system, firstly to fill the target cell with liquid for the nuclear reaction with the beam, and secondly to transfer the energy lost by the beam in the target to the cooling machine. The gas filling in the system is 2.15 l atm. For target and heat pipe operation, part of the available gas is converted into liquid. The total mass of the working fluid then consists of the mass of the gas in the bellows system, the cold gas inside the heat pipe, the mass of the liquid in the target and lower part of the heat pipe, and the mass of liquid flowing down inside the inner tube of the heat pipe. The bellows system is a purely
ARTICLE IN PRESS M. Abdel-Bary et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 236–244
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1600 1400
Keff (W/cm.K)
1200 1000 LH2 800 600 14.4 k 400
14.6 k 14.8 k
200
15.0 k 0 0.0
0.1
0.2
0.3 0.4 Heat load (W)
0.5
0.6
0.8
Fig. 3. Effect of heat load on the effective thermal conductivity Keff of the hydrogen 2.0 m heat pipe-target system at different condenser temperatures. 10000 LH2
1000
0.1W
Keff (W/cm.K)
0.2W 0.3W 0.4W
100
0.5W 0.7W Cu
10
Al Ag 1 14.4
14.6
14.8 Condenser temperature (K)
15.0
15.2
Fig. 4. Comparison of the effective thermal conductivity Keff of the hydrogen 2.0 m heat pipe-target system at different heat loads with that of the copper, aluminum and silver heat conductors in the liquid temperature range.
Table 3 Maximum effective thermal conductivities Keff values with the H2, N2 and CH4 2.0 m heat pipe-target system and with aluminum, copper and silver heat conductors Temperature range (K)
Working fluid
Heat pipe Keff (W/cm K)
KCopper (W/cm K)
KAluminum (W/cm K)
KSilver (W/cm K)
14.4–15.0 63.6–64.6 90.7–96.0
Hydrogen Nitrogen Methane
1558.4 592.2 352.20
13 5.0 4.78
38 8.5 3.0
85 4.5 4.2
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mechanical pressure stabilization system and is described in detail by Jeackle et al. [1]. Table 4 summarizes the total mass and the maximal liquid volume for each of the four working liquids (H2, D2, CH4 and N2) at 228 mbar filling pressure. The change in the gas volume in the bellows DV is determined from the bellows height. There is a strong relation between pressure and bellows height. This allows the remaining gas mass in the bellows to be determined at any bellows height (not a linear relation). This mass taken out of the bellows (mDv ¼ DV rgas at 295 K and at variable pressure) goes to a mass of cold gas in the heat pipe (mcold gas ¼ VHP rcold gas at 200 mbar, saturation) and the mass of real liquid in the system (ml ¼ mDv– mcold gas), where VHP is the volume of the heat pipe and ml is the mass of liquid in the heat pipe-target system. The subscripts are Table 4 Maximum mass of liquid in the 200 cm heat pipe-target system and target appendix Gas
H2 D2 CH4 N2
Filling (9.54 l at 228 mbar, 295 K) Liquid mass (mg)
Liquid volume (cm3)
174.8 315.6 1370.3 2388.9
2.32 1.95 3.08 2.78
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as follows: v indicates vapor, l indicates liquid, HP indicates heat pipe. The liquid mass and the cold gas mass together are the mass of the gas which has disappeared from room temperature filling because they have much higher densities. In the 200 cm target system some liquid is needed above the evaporator. It adheres to the surface of the condenser and in the inner tube (for H2, D2, N2 and CH4 are 1.65, 1.5, 1.3 and 1.3 cm3, respectively). The quantity required in the target cell and appendix is small compared to this (it is 0.5 cm3, which corresponds to 38 mg LH2, 86.5 mg LD2, 222.5 mg LCH4 and 429 mg LN2). Fig. 5 shows the amount of liquid H2 in grams inside the heat pipe-target system at various condenser temperatures and heat loads. Table 5 summarizes the amount of liquid CH4 and N2 (in bold italic) in grams at various condenser temperatures and heat loads. In all cases, the liquid mass decreases with increasing condenser temperature and increasing heat load on the evaporator due to the decrease in liquefaction rate in the condenser section and the increase in vaporization in the evaporator (target).
6. Transport of mass and heat The mass flows carry the heat load from the target cell to the cooling machine. This mass flows
0.18 LH2
0.16 0.14
Liquid mass (g)
0.1W 0.12
0.2W 0.3W
0.10
0.4W
0.08
0.5W
0.06 0.04 0.02 0.00 14.2
14.4
14.6
14.8
15.0 15.2 15.4 Condenser temp.( K)
15.6
15.8
16.0
16.2
Fig. 5. Mass of liquid hydrogen inside the 2.0 m heat pipe-target system versus the condenser temperature at different heat loads.
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Table 5 Amount of liquid CH4 and N2 at various condenser temperatures and heat loads Condenser 0.2 W 0.4 W 0.6 W 0.8 W 1.0 W 1.2 W temp. (K) LCH4 (g) LN2 (g) LCH4 (g) LN2 (g) LCH4 (g) LN2 (g) LCH4 (g) LN2 (g) LCH4 (g) LN2 (g) LCH4 (g) LN2 (g) 92.00/63.6 92.25/63.8 92.50/64.0 92.75/64.2 93.00/64.4 93.25/64.6 93.50/64.8
0.78 0.68 0.65 0.60 0.52 0.51 0.43
2.14 2.08 1.91 1.53 1.30 1.30 1.18
0.76 0.67 0.64 0.59 0.51 0.49 0.40
2.13 2.07 1.88 1.43 1.23 1.17 1.10
0.75 0.66 0.63 0.58 0.48 0.47 —
2.09 2.05 1.86 1.39 1.14 1.11 —
as liquid in the inner plastic tube from the condenser to the evaporator and back as vapor to the condenser in the space between the plastic tube and the stainless steel tube. _ in the heat pipeThe mass flow rate m target system is in dynamic equilibrium with the defined condenser temperature and fixed bellows pressure, and a constant heat load is given by _ fg , _ ¼ Q=h m
(2)
where Q_ is the heat transfer rate in W from the evaporator (target) to the condenser and hfg is the heat of vaporization in J/g. The mass flow rate of the working fluid increases with increasing heat load. Condensation of the vapor in the cooled section and evaporation of the condensate in the heated section makes a small pressure difference and drives mass flow in a closed cycle. At higher heat load than the maximum heat pipe can carry there is no liquid left in the target and the mass transport stops. The flow velocity of the vapor (cold gas) vv at a given heat load is calculated in the steady state from
0.73 0.64 0.60 0.53 0.45 — —
2.07 2.02 1.82 1.33 1.11 — —
0.71 0.62 0.56 0.48 — — —
2.02 1.98 1.76 1.25 — — —
0.67 0.59 0.53 — — — —
— — — — — — —
106 N s/m2 is the vapor viscosity and Av is the vapor cross-section area. For the hydrogen heat pipe in the operating mode at a flow velocity of the vapor hydrogen of 15 cm/s, Re ¼ 209 2000 indicating laminar flow. Also for D2, N2 and CH4 the Reynolds numbers are in the range of laminar flow. The axial mass flow upwards as vapor and downwards as liquid is constant and equal in equilibrium. Fig. 6 shows the mass flow rate and the flow velocity of the cold vapor inside the 200 cm heat pipe-target systems for hydrogen at different heat loads. An increase in the heat load leads to an increase in the mass circulation. The mass flow rate to transfer a certain heat load is nearly constant in the operating temperature range because the variation of the latent heat of vaporization hfg with boiling temperature is very small (o1%). Table 6 summarizes the vapor flow velocity and the mass flow rate of the cold vapor inside the 2.0 m heat pipe-target systems for N2 and CH4 at different heat loads.
7. Pressure in the heat pipe _ vv ¼ m=ðr v Av Þ.
(3)
The Reynolds number of the flow is Re ¼ rv vv d v =mv
(4)
where rv is the vapor density ¼ 0.2828 kg/m3, dv the diameter of the vapor flow, mv ¼ 0.801
The gas pressure in the systems is measured as 228 mbar with a filling of 9.54 l at room temperature. The bellows system [1] was designed to allow for thin windows of the target cell. It ensures a nearly constant pressure difference between the inside and outside of the target cell, independent of
ARTICLE IN PRESS M. Abdel-Bary et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 236–244 5
20
16
4
12
3
8
2
4
1
Mass flow rate (mg/s)
LH2
vapor flow velocity mass flow rate
Vapor flow velocity(cm/s)
243
0
0 0.0
0.1
0.2
0.3 0.4 Heat load (W)
0.5
0.6
0.7
Fig. 6. Mass flow rate and vapor flow velocity of hydrogen in the 2.0 m heat pipe-target system at different heat loads (7 mm diameter).
Table 6 Vapor flow velocity and mass flow rate of cold vapor inside the 2.0 cm heat pipe-target systems for CH4 and N2 at different heat loads Liquid methane
Liquid nitrogen
Heat load on evaporator (W)
Vapor flow velocity (cm/s)
Mass flow rate (mg/s)
Vapor flow velocity (cm/s)
Mass flow rate (mg/s)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.0 2.89 5.77 8.66 11.55 14.43 17.32 20.21
0.0 0.37 0.74 1.11 1.48 1.85 2.23 2.6
0.0 3.18 6.36 9.54 12.73 15.91 — —
0.0 0.93 1.86 2.79 3.73 4.66 — —
the liquid to gas transformation. When the target is in isolation vacuum there is approximately constant absolute pressure. The remaining gas in the system defines the bellows height, the gas pressure and the other related thermodynamic quantities like gas density, boiling and freezing temperature. At fixed pressure the working range for liquid is above the freezing temperature and below the boiling temperature. The gas flow inside the heat pipe is driven by a very small pressure difference.
This pressure difference DPv is estimated from Poiseuille’s law for the pressure gradient in a straight circular cylinder with laminar flow [6]: DPv ¼
rv v2v
8mv Q_ Le þ Lc þ La þ prv r4v hfg 2
(5)
where mv is the vapor viscosity (N s/m2), Le, Lc, La are the length of the evaporator, condenser and adiabatic sections, respectively, rv is the radius of
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the vapor flow and hfg is the heat of vaporization in J/g. The vapor pressure difference DPv is calculated from Eq. (5) for the hydrogen 2.0 m heat pipetarget system when the heat load Q_ is 0.5 W and the operating temperature 15.0 K to be DPv ¼ 0:159ðN=m2 Þ ¼ 1:59 103 mbar ¼ 15:9 Pa. This is indeed a small pressure difference. The hydrostatic pressure difference DPs for the liquid column in the target appendix in the gravitational field is given by
were successful. The LH2 cooling down time has been decreased to 33% compared to the 2 m long 16 mm diameter heat pipe (107 mins instead of 320 mins). Metallic conductors (copper, aluminum or silver) of 2.0 m length would not work with our cooling machines. The long heat pipe is a powerful and attractive development for TOF or WASA at COSY. One can even consider making curved heat pipes that follow more complicated tracks through detectors.
DPs ¼ rl gLl sin y
References
(6)
where rl is the liquid density, Ll is the liquid column length and y is the heat pipe inclination relative to the horizontal position. With y ¼ 901 to the horizontal, a liquid column height Ll of 4 cm and a liquid density rl of 0.0755 g/cm3 at 15 K, the hydrostatic pressure difference DPs is 0.296 mbar. This corresponds to a small increase in the boiling temperature of about 0.002 K in the target cell and it may help to eliminate bubbles in the lowest part of the liquid column (just the target region).
8. Conclusion In this study, new developments were implemented for the LH2/LD2 target at COSY. A 2.0 m long heat pipe with a 7 mm diameter adiabatic section was developed for future measurements in the middle of the 4 m diameter TOF tank. Tests with four working gases, H2, D2, N2 and CH4,
[1] V. Jaeckle, et al., Nucl. Instr. and Meth. A 349 (1994) 15. [2] S. Abdel-Samad, et al., Nucl. Instr. and Meth. A 495 (2002) 1. [3] S. Abdel-Samad, Ph.D. Thesis, Forschungszentrum Juelich/ RWTH Aachen University, Germany, 2001. [4] A. Faghri, Heat Pipe Science and Technology, Taylor & Francis, Washington, 1995. [5] S.W. Chi, Heat Pipe Theory and Practice, McGraw-Hill, Washington, 1976. [6] P.D. Dunn, D.A. Reay, Heat Pipes, fourth ed, Pergamon Press, New York, 1994. [7] J. Geue, Heat Pipes and Their Applications, Atomic Energy Commission, Lucas Heights, Australia, 1971. [8] M. Abdel-Bary, Ph.D. Thesis, Forschungszentrum Juelich/ RWTH Aachen University, Germany, 2004. [9] S. Abdel-Samad et al., ‘‘Deuterium heat pipe-cryogenic targets for COSY experiments’’ Nucl. Instr. and Meth. A, 2005, accepted paper for publication. [10] G.P. Peterson, An Introduction to Heat Pipes, Modeling, Testing and Applications, Wiley, New York, 1994. [11] C. Wongee, Y.H. Kang, H.Y. Kwak, Y.S. Lee, Applied Thermal Engineering 19 (1999) 807. [12] A. Abo El-Nasr, S.M. Haggar, J. Heat, Mass Transfer, vol. 32, Springer, Berlin, 1996, pp. 97–101.
Nuclear Instruments and Methods in Physics Research A 551 (2005) 236–244 www.elsevier.com/locate/nima
2 m heat pipe-cryogenic targets for COSY-TOF experiment M. Abdel-Barya, S. Abdel-Samadb,, K. Kilianb a Atomic Energy Authority, NRC, Cyclotron Project, 13759 Cairo, Egypt Institut fu¨r Kernphysik, Forschungszentrum Ju¨lich, D-52425 Ju¨lich, Germany
b
Received 30 March 2005; received in revised form 30 May 2005; accepted 23 June 2005 Available online 18 July 2005
Abstract A liquid hydrogen/deuterium target is used in COSY external experiments. For the TOF and WASA applications at COSY, a 2.0 m long, 7 mm diameter heat pipe (HP) has been developed and tested under filling conditions of 9.45 l at 228 mbar with the four gases H2, D2, N2 and CH4. The cooling down times are 107, 200, 54 and 38 mins, respectively, from 300 K until working conditions have been achieved. The target liquid is produced at a cooled condenser and guided through a central tube assisted by gravitation into the target cell. The characteristics at steady-state operating conditions were measured for H2, N2 and CH4 as the working fluids. The effect of the operating temperature and the heat loads on the effective thermal conductivity and the liquid mass in the heat pipe was investigated. r 2005 Elsevier B.V. All rights reserved. PACS: 29.25.– t; 07.20.Mc; 07.20.Pe Keywords: Targets; Cryogenics; Low-temperature equipment; Heat pipes
1. Introduction At the COoler SYnchrotron (COSY) interactions of protons in the GeV energy range with target nuclei are studied. For experimental reasons, it is important that as many interactions as possible occur in a target volume as small as Corresponding author. Tel.: +49 2461 613917; fax: +49 2461 613930. E-mail address: [email protected] (S. Abdel-Samad).
possible. As a consequence, the COSY beam should be as small as possible and the density of the target material should be high. For hydrogen isotopes this means that liquid instead of gaseous hydrogen or deuterium must be used. A liquid hydrogen/deuterium target is therefore used at COSY Juelich in the external experiments TOF and BIG KARL. In an earlier version of the standard target [1], copper heat conductors were used between the cooling machine and the target region. Different heat conducting materials (aluminum, silver) were tested [2,3]. The shortest
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.06.059
ARTICLE IN PRESS M. Abdel-Bary et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 236–244
cooling down and heating up times were obtained with aluminum. It had the lowest heat capacity and high thermal conductivity at low temperature. A big improvement was achieved by changing from metallic conduction to heat transfer by convection in heat pipes [2,4,5]. Evaporation and condensation of a working fluid at different ends of a tube is used to transport a large amount of heat [6,7]. A 16 mm diameter heat pipe was tested, 32.25 cm and 2.0 m long [2]. The liquid hydrogen (LH2) cooling down times were 70 and 320 min, respectively. To further improve the performance of the target and to decrease the cooling down time with the new 2.0 m design, a 7 mm diameter heat pipe was developed instead of the 16 mm diameter one [8]. A copper heat conductor with this geometry would not work. The temperature difference along such a copper conductor would be 10 K. The available cooling machine is not able to provide the necessary cooling power and the required cold head temperature of less than 5 K. The mass of such a system is 3600 g. The cooling down time due to its heat capacity alone would be about 10 h. The heat pipe target system has to fulfill the following requirements:
237
2. Heat pipe dimensions and design In order to decrease the cooling down/heating up time for the 2.0 m long liquid hydrogen (called LH2 in the following) targets, a heat pipe with a smaller diameter (7 mm) was developed. The material, the surface area and the heat capacity were reduced with this smaller diameter heat pipe compared with a 16 mm diameter heat pipe. The heat pipe-target system, the mechanical design and the test apparatus for a 32 cm system were described in detail by Abdel-Samad et al. [9]. The new aspect of the present work is an increased length of the heat pipe to 2.0 m compared with 32 cm, which opens up new applications. Fig. 1 shows a schematic diagram for the 2.0 m long and 7 mm diameter heat pipe with the target cell. The aluminum condenser has a surface area of condensation of 39 cm2. Table 1 summarizes the parameters of this long version. The tests were done at a 901 inclination angle to the horizontal beam direction. The cooling down times for H2, D2, N2 and CH4 were measured. The effective thermal conductivity, the mass flow rate and the pressure inside the heat pipe were measured.
3. Heat pipe-target system cooling down time (1) It has to stabilize the target temperature within 70.01 K in the range of liquid hydrogen/ deuterium. (2) The cooling system and all heat transferring connections around the target have to occupy minimum space and to have minimum mass in the target region in order to reduce background events. The shadow created by materials (all heat-transferring connections) in the target region reduces the solid angle seen by the detectors and creates background reactions. (3) The target thickness has to be well defined; therefore, the target liquid should be of homogeneous density without any bubbles which would change the effective target thickness drastically. (4) The cooling down time to working temperature or heating up to room temperature has to be as short as possible.
The bellows and the heat pipe-target system are filled with 9.54 l of hydrogen gas at 228 mbar pressure at room temperature and then the cooling machine is switched on. Fig. 2 shows details of the time dependences of the temperatures, the pressure, the bellows height and the liquid level for LH2. The hydrogen condensation starts after 39 min from switching on the cooling machine. The LH2 reaches the target cell after 107 min and a few seconds later the target is full and stable. The time difference of 68 min (10739 ¼ 68) is needed for the liquid to proceed down towards the evaporator section. During that time the liquid has to cool down the inner tube over its full length. Droplets of LH2 fall down into the inner tube towards the evaporator region. The liquid vaporizes taking latent heat from the inner tube and finally from the evaporator region. Finally, the system has steady liquefaction in the condenser
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and a downward flow of liquid in the inner tube, vaporization in the evaporator and flow of vapor upwards in the space between the inner tube and the heat pipe inner surface. The evaporator temperature drops and one starts to observe short pressure spikes during cooling down of the inner tube due to droplet evaporation (Fig. 2). As soon as the arriving liquid remains cooler than the boiling temperature, a liquid column appears in the target appendix, then the gas volume and pressure decrease, and the bellows height is reduced as shown in Fig. 2. The total integrated heat capacity of the heat pipe-target system in cooling down from 290 K to 15 K is 4.783 kJ (2.15 g plastic inner tube has C ¼ 683:0 J, 0.2 g H2 gas has C ¼ 640:0 J and both 41.75 g steel transport and 1.0 g copper evaporator sections have C ¼ 3:46 kJ). It takes 68 min for cooling with the LH2 flow starting at the condenser to reach 15 K at the evaporator. From that we calculate the rate of heat transfer Q_ with the liquid to be 1.17 W. The measured cooling down time and some characteristic values in the cooling down mode calculated from the thermal properties for H2, D2, N2 and CH4 are summarized in Table 2.
4. Effective thermal conductivity Fig. 1. Schematic drawing of the 2.0 m long heat pipe-target combination.
Table 1 Parameters of the 2.0 m long, 7 mm diameter heat pipe with aluminum condenser Total length Part with 16 mm diameter Part with 7 mm diameter Inner tube length Inner tube diameter Wall thickness Inner tube weight Weight of HP Weight of gas connector Weight of Al condenser Total weight
200 cm 7.5 cm 184.0 cm 185.5 cm 3.2 mm 0.1 mm 2.15 g 36.30 g 5.44 g 20.00 g 63.89 g
A high value of the thermal conductivity indicates a rapid heat flow rate which leads to stable liquid in the target cell at constant density without boiling. This is very important and absolutely necessary for a well-defined reaction between the proton beam and the LH2/LD2 target. The effective thermal conductivity Keff [10–12] for the heat pipe system is determined by the relation between the measured heat flow rate Q_ and the measured temperature difference DT between the external surfaces of the lower end (evaporator T2) and upper end (condenser T1): _ K eff ¼ ðLQÞ=ðDTAÞ
(1)
where L is the total length of the heat pipe between the measuring points T1 and T2 , and A is the cross-sectional area of the heat pipe.
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239
300 Evaporator temp.
Bellows height Temp.(K), Pressure(mbar), Bellows(mm), LH2 (mm)
250 H2 gas pressure
200
150 Condenser temp. 100 LH2 Liquid 50
0 0
10
20
30
40
50
60 Time (min.)
70
80
90
100
110
Fig. 2. Time dependence of the temperature, pressure, bellows height and liquid level for LH2 of 2.0 m target with 7 mm heat pipe and 3 mm diameter inner tube.
Table 2 Characteristic values during cooling down mode calculated for H2, D2, N2 and CH4 2.0 m heat pipe-target H2
D2
N2
CH4
Cooling down time from 300 K to liquid working condition (min) _ Mass flow rate m(g/s) Vapor velocity Vv (cm/s) Reynolds number Re Gas density rg (g/cm3) at 220 mbar, 295 K Total mass in system (g) Evaporation energy of total mass (J)
107 2.6 103 26.6 600 0.019 103 0.175 77.9
200 1.7 103 10.5 184.8 0.033 103 0.355 107.9
54 18.9 103 65.3 820.2 0.253 103 2.389 473.9
38 14.9 103 114.9 792.9 0.145 103 1.379 702.4
Fig. 3 shows the effective thermal conductivity Keff of the hydrogen 200-cm heat pipe-target system as a function of the heat load for different condenser temperatures. The experimental results show that Keff increases with increasing heat load and with decreasing condenser temperature. The highest effective thermal conductivity is 1558 W/ cm K. Fig. 4 shows the comparison between the LH2 Keff and the experimentally determined Keff of copper, silver and aluminum heat conductors. The heat pipe conductivity is 100 times greater than the metallic conductors. The Keff is also measured for CH4 and N2. The maximum effective thermal conductivity for H2, CH4, N2 and the metallic conductors’ thermal conductivity are summarized in Table 3.
5. Liquid mass in the heat pipe-target system It is important for the target operation to have enough liquid in the system, firstly to fill the target cell with liquid for the nuclear reaction with the beam, and secondly to transfer the energy lost by the beam in the target to the cooling machine. The gas filling in the system is 2.15 l atm. For target and heat pipe operation, part of the available gas is converted into liquid. The total mass of the working fluid then consists of the mass of the gas in the bellows system, the cold gas inside the heat pipe, the mass of the liquid in the target and lower part of the heat pipe, and the mass of liquid flowing down inside the inner tube of the heat pipe. The bellows system is a purely
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1600 1400
Keff (W/cm.K)
1200 1000 LH2 800 600 14.4 k 400
14.6 k 14.8 k
200
15.0 k 0 0.0
0.1
0.2
0.3 0.4 Heat load (W)
0.5
0.6
0.8
Fig. 3. Effect of heat load on the effective thermal conductivity Keff of the hydrogen 2.0 m heat pipe-target system at different condenser temperatures. 10000 LH2
1000
0.1W
Keff (W/cm.K)
0.2W 0.3W 0.4W
100
0.5W 0.7W Cu
10
Al Ag 1 14.4
14.6
14.8 Condenser temperature (K)
15.0
15.2
Fig. 4. Comparison of the effective thermal conductivity Keff of the hydrogen 2.0 m heat pipe-target system at different heat loads with that of the copper, aluminum and silver heat conductors in the liquid temperature range.
Table 3 Maximum effective thermal conductivities Keff values with the H2, N2 and CH4 2.0 m heat pipe-target system and with aluminum, copper and silver heat conductors Temperature range (K)
Working fluid
Heat pipe Keff (W/cm K)
KCopper (W/cm K)
KAluminum (W/cm K)
KSilver (W/cm K)
14.4–15.0 63.6–64.6 90.7–96.0
Hydrogen Nitrogen Methane
1558.4 592.2 352.20
13 5.0 4.78
38 8.5 3.0
85 4.5 4.2
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mechanical pressure stabilization system and is described in detail by Jeackle et al. [1]. Table 4 summarizes the total mass and the maximal liquid volume for each of the four working liquids (H2, D2, CH4 and N2) at 228 mbar filling pressure. The change in the gas volume in the bellows DV is determined from the bellows height. There is a strong relation between pressure and bellows height. This allows the remaining gas mass in the bellows to be determined at any bellows height (not a linear relation). This mass taken out of the bellows (mDv ¼ DV rgas at 295 K and at variable pressure) goes to a mass of cold gas in the heat pipe (mcold gas ¼ VHP rcold gas at 200 mbar, saturation) and the mass of real liquid in the system (ml ¼ mDv– mcold gas), where VHP is the volume of the heat pipe and ml is the mass of liquid in the heat pipe-target system. The subscripts are Table 4 Maximum mass of liquid in the 200 cm heat pipe-target system and target appendix Gas
H2 D2 CH4 N2
Filling (9.54 l at 228 mbar, 295 K) Liquid mass (mg)
Liquid volume (cm3)
174.8 315.6 1370.3 2388.9
2.32 1.95 3.08 2.78
241
as follows: v indicates vapor, l indicates liquid, HP indicates heat pipe. The liquid mass and the cold gas mass together are the mass of the gas which has disappeared from room temperature filling because they have much higher densities. In the 200 cm target system some liquid is needed above the evaporator. It adheres to the surface of the condenser and in the inner tube (for H2, D2, N2 and CH4 are 1.65, 1.5, 1.3 and 1.3 cm3, respectively). The quantity required in the target cell and appendix is small compared to this (it is 0.5 cm3, which corresponds to 38 mg LH2, 86.5 mg LD2, 222.5 mg LCH4 and 429 mg LN2). Fig. 5 shows the amount of liquid H2 in grams inside the heat pipe-target system at various condenser temperatures and heat loads. Table 5 summarizes the amount of liquid CH4 and N2 (in bold italic) in grams at various condenser temperatures and heat loads. In all cases, the liquid mass decreases with increasing condenser temperature and increasing heat load on the evaporator due to the decrease in liquefaction rate in the condenser section and the increase in vaporization in the evaporator (target).
6. Transport of mass and heat The mass flows carry the heat load from the target cell to the cooling machine. This mass flows
0.18 LH2
0.16 0.14
Liquid mass (g)
0.1W 0.12
0.2W 0.3W
0.10
0.4W
0.08
0.5W
0.06 0.04 0.02 0.00 14.2
14.4
14.6
14.8
15.0 15.2 15.4 Condenser temp.( K)
15.6
15.8
16.0
16.2
Fig. 5. Mass of liquid hydrogen inside the 2.0 m heat pipe-target system versus the condenser temperature at different heat loads.
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Table 5 Amount of liquid CH4 and N2 at various condenser temperatures and heat loads Condenser 0.2 W 0.4 W 0.6 W 0.8 W 1.0 W 1.2 W temp. (K) LCH4 (g) LN2 (g) LCH4 (g) LN2 (g) LCH4 (g) LN2 (g) LCH4 (g) LN2 (g) LCH4 (g) LN2 (g) LCH4 (g) LN2 (g) 92.00/63.6 92.25/63.8 92.50/64.0 92.75/64.2 93.00/64.4 93.25/64.6 93.50/64.8
0.78 0.68 0.65 0.60 0.52 0.51 0.43
2.14 2.08 1.91 1.53 1.30 1.30 1.18
0.76 0.67 0.64 0.59 0.51 0.49 0.40
2.13 2.07 1.88 1.43 1.23 1.17 1.10
0.75 0.66 0.63 0.58 0.48 0.47 —
2.09 2.05 1.86 1.39 1.14 1.11 —
as liquid in the inner plastic tube from the condenser to the evaporator and back as vapor to the condenser in the space between the plastic tube and the stainless steel tube. _ in the heat pipeThe mass flow rate m target system is in dynamic equilibrium with the defined condenser temperature and fixed bellows pressure, and a constant heat load is given by _ fg , _ ¼ Q=h m
(2)
where Q_ is the heat transfer rate in W from the evaporator (target) to the condenser and hfg is the heat of vaporization in J/g. The mass flow rate of the working fluid increases with increasing heat load. Condensation of the vapor in the cooled section and evaporation of the condensate in the heated section makes a small pressure difference and drives mass flow in a closed cycle. At higher heat load than the maximum heat pipe can carry there is no liquid left in the target and the mass transport stops. The flow velocity of the vapor (cold gas) vv at a given heat load is calculated in the steady state from
0.73 0.64 0.60 0.53 0.45 — —
2.07 2.02 1.82 1.33 1.11 — —
0.71 0.62 0.56 0.48 — — —
2.02 1.98 1.76 1.25 — — —
0.67 0.59 0.53 — — — —
— — — — — — —
106 N s/m2 is the vapor viscosity and Av is the vapor cross-section area. For the hydrogen heat pipe in the operating mode at a flow velocity of the vapor hydrogen of 15 cm/s, Re ¼ 209 2000 indicating laminar flow. Also for D2, N2 and CH4 the Reynolds numbers are in the range of laminar flow. The axial mass flow upwards as vapor and downwards as liquid is constant and equal in equilibrium. Fig. 6 shows the mass flow rate and the flow velocity of the cold vapor inside the 200 cm heat pipe-target systems for hydrogen at different heat loads. An increase in the heat load leads to an increase in the mass circulation. The mass flow rate to transfer a certain heat load is nearly constant in the operating temperature range because the variation of the latent heat of vaporization hfg with boiling temperature is very small (o1%). Table 6 summarizes the vapor flow velocity and the mass flow rate of the cold vapor inside the 2.0 m heat pipe-target systems for N2 and CH4 at different heat loads.
7. Pressure in the heat pipe _ vv ¼ m=ðr v Av Þ.
(3)
The Reynolds number of the flow is Re ¼ rv vv d v =mv
(4)
where rv is the vapor density ¼ 0.2828 kg/m3, dv the diameter of the vapor flow, mv ¼ 0.801
The gas pressure in the systems is measured as 228 mbar with a filling of 9.54 l at room temperature. The bellows system [1] was designed to allow for thin windows of the target cell. It ensures a nearly constant pressure difference between the inside and outside of the target cell, independent of
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20
16
4
12
3
8
2
4
1
Mass flow rate (mg/s)
LH2
vapor flow velocity mass flow rate
Vapor flow velocity(cm/s)
243
0
0 0.0
0.1
0.2
0.3 0.4 Heat load (W)
0.5
0.6
0.7
Fig. 6. Mass flow rate and vapor flow velocity of hydrogen in the 2.0 m heat pipe-target system at different heat loads (7 mm diameter).
Table 6 Vapor flow velocity and mass flow rate of cold vapor inside the 2.0 cm heat pipe-target systems for CH4 and N2 at different heat loads Liquid methane
Liquid nitrogen
Heat load on evaporator (W)
Vapor flow velocity (cm/s)
Mass flow rate (mg/s)
Vapor flow velocity (cm/s)
Mass flow rate (mg/s)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.0 2.89 5.77 8.66 11.55 14.43 17.32 20.21
0.0 0.37 0.74 1.11 1.48 1.85 2.23 2.6
0.0 3.18 6.36 9.54 12.73 15.91 — —
0.0 0.93 1.86 2.79 3.73 4.66 — —
the liquid to gas transformation. When the target is in isolation vacuum there is approximately constant absolute pressure. The remaining gas in the system defines the bellows height, the gas pressure and the other related thermodynamic quantities like gas density, boiling and freezing temperature. At fixed pressure the working range for liquid is above the freezing temperature and below the boiling temperature. The gas flow inside the heat pipe is driven by a very small pressure difference.
This pressure difference DPv is estimated from Poiseuille’s law for the pressure gradient in a straight circular cylinder with laminar flow [6]: DPv ¼
rv v2v
8mv Q_ Le þ Lc þ La þ prv r4v hfg 2
(5)
where mv is the vapor viscosity (N s/m2), Le, Lc, La are the length of the evaporator, condenser and adiabatic sections, respectively, rv is the radius of
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the vapor flow and hfg is the heat of vaporization in J/g. The vapor pressure difference DPv is calculated from Eq. (5) for the hydrogen 2.0 m heat pipetarget system when the heat load Q_ is 0.5 W and the operating temperature 15.0 K to be DPv ¼ 0:159ðN=m2 Þ ¼ 1:59 103 mbar ¼ 15:9 Pa. This is indeed a small pressure difference. The hydrostatic pressure difference DPs for the liquid column in the target appendix in the gravitational field is given by
were successful. The LH2 cooling down time has been decreased to 33% compared to the 2 m long 16 mm diameter heat pipe (107 mins instead of 320 mins). Metallic conductors (copper, aluminum or silver) of 2.0 m length would not work with our cooling machines. The long heat pipe is a powerful and attractive development for TOF or WASA at COSY. One can even consider making curved heat pipes that follow more complicated tracks through detectors.
DPs ¼ rl gLl sin y
References
(6)
where rl is the liquid density, Ll is the liquid column length and y is the heat pipe inclination relative to the horizontal position. With y ¼ 901 to the horizontal, a liquid column height Ll of 4 cm and a liquid density rl of 0.0755 g/cm3 at 15 K, the hydrostatic pressure difference DPs is 0.296 mbar. This corresponds to a small increase in the boiling temperature of about 0.002 K in the target cell and it may help to eliminate bubbles in the lowest part of the liquid column (just the target region).
8. Conclusion In this study, new developments were implemented for the LH2/LD2 target at COSY. A 2.0 m long heat pipe with a 7 mm diameter adiabatic section was developed for future measurements in the middle of the 4 m diameter TOF tank. Tests with four working gases, H2, D2, N2 and CH4,
[1] V. Jaeckle, et al., Nucl. Instr. and Meth. A 349 (1994) 15. [2] S. Abdel-Samad, et al., Nucl. Instr. and Meth. A 495 (2002) 1. [3] S. Abdel-Samad, Ph.D. Thesis, Forschungszentrum Juelich/ RWTH Aachen University, Germany, 2001. [4] A. Faghri, Heat Pipe Science and Technology, Taylor & Francis, Washington, 1995. [5] S.W. Chi, Heat Pipe Theory and Practice, McGraw-Hill, Washington, 1976. [6] P.D. Dunn, D.A. Reay, Heat Pipes, fourth ed, Pergamon Press, New York, 1994. [7] J. Geue, Heat Pipes and Their Applications, Atomic Energy Commission, Lucas Heights, Australia, 1971. [8] M. Abdel-Bary, Ph.D. Thesis, Forschungszentrum Juelich/ RWTH Aachen University, Germany, 2004. [9] S. Abdel-Samad et al., ‘‘Deuterium heat pipe-cryogenic targets for COSY experiments’’ Nucl. Instr. and Meth. A, 2005, accepted paper for publication. [10] G.P. Peterson, An Introduction to Heat Pipes, Modeling, Testing and Applications, Wiley, New York, 1994. [11] C. Wongee, Y.H. Kang, H.Y. Kwak, Y.S. Lee, Applied Thermal Engineering 19 (1999) 807. [12] A. Abo El-Nasr, S.M. Haggar, J. Heat, Mass Transfer, vol. 32, Springer, Berlin, 1996, pp. 97–101.