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Transient thermodynamic behavior of cryogenic mixed fluid thermosiphon and its cool-down time estimation
Cryogenics 50 (2010) 352–358
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Transient thermodynamic behavior of cryogenic mixed fluid thermosiphon and its cool-down time estimation Jisung Lee *, Youngkwon Kim, Sangkwon Jeong 1 Cryogenic Engineering Laboratory, Division of Mechanical Engineering, School of Mechanical, Aerospace and Systems Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon, 305-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 September 2009 Received in revised form 15 January 2010 Accepted 9 February 2010
Keywords: Gas mixture (B) Nitrogen (B) Heat transfer (C) Phase transitions (C) Thermodynamics (C)
a b s t r a c t Thermosiphon is an efficient heat transfer device by utilizing latent heat of fluid at liquid–vapor phase change. One of the disadvantages of thermosiphon, however, is that the operational temperature range is fundamentally limited from the critical point to the triple point of the working fluid to maintain two phase state. Nitrogen (N2) and tetrafluoromethane (CF4) were selected as the mixed working fluid to widen their original operational temperature range. Thermodynamic behavior of mixture and its effect on the cool-down time were investigated. A simple calculation model was proposed to estimate the cooldown time of the thermosiphon evaporator prior to experiments. The calculated results agreed well with the experimental results within 5% error. The cool-down time reduction was not achieved by mixing two components at once due to the separation of mixture. One idea to avoid this problem was suggested in this paper where the estimated cool-down time was reduced 17.8% compared to pure N2. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
1. Introduction Thermosiphon is an efficient heat transfer device by utilizing latent heat of fluid at liquid–vapor phase change. Numerous thermosiphons have been employed in industries due to their peculiar simple structure, heat switch function, and also large effective thermal conductivity compared to metal conductors such as copper or aluminum [1,2]. One of the specific application in cryogenic field is a thermal shunt device such as a thermal linkage between the first and the second stages of cryocooler cold head. Cool-down performance of superconducting magnet is greatly improved by large cooling capacity of the first stage cold head [3]. One of the disadvantages of thermosiphon, however, is that the operational temperature range is fundamentally limited from the critical point to the triple point of the working fluid to maintain two phase state. The upper temperature limit becomes the start point of operation, and the lower temperature limit becomes the end point of operation during the transient temperature variation period as the thermal shunt. Fig. 1 shows the temperature range of various fluids between their critical and triple points [4]. Prenger et al. suggested multiple thermosiphons using fluids which have different operational temperature ranges. Nitrogen and ethane were used in separate thermosiphons and the cool-down time
* Corresponding author. Tel.: +82 42 350 3079; fax: +82 42 350 8207. E-mail addresses: [email protected] (J. Lee), [email protected] (Y. Kim), [email protected] (S. Jeong). 1 Tel.:+82 42 350 3039; fax: +82 42 350 8207.
was reduced compared to the single nitrogen [5,6]. In a different way, a binary mixture considering critical and triple points of each component has a potential to widen the operational temperature range. Research on the mixed working fluid has been conducted to improve a heat transport capability with Marangoni effect [7–9], and critical heat flux enhancement [10]. However, all these researches were conducted around or above room temperature and focused on the steady state operation. The feasibility of mixed fluid thermosiphon in a cool-down device has not been tested. The authors carefully investigated the behavior of mixed working fluid such as condensation, liquid accumulation, boiling, and freezing with the transparent thermosiphon [11]. In this paper, the thermodynamic behavior of mixed fluid is thoroughly examined by further consideration of mixture composition change in temperature and mole fraction diagram (T–x diagram). Additionally, a simple prediction model is proposed to estimate the cool-down time of the thermosiphon evaporator prior to experiment. 2. Understanding mixed fluid behavior 2.1. Description of experimental apparatus and method A thermosiphon is fabricated as shown in Fig. 2a, which is composed of a condenser, an adiabatic region, and an evaporator. The diameter and height of the condenser and evaporator are all 30 mm. The adiabatic region is 100 mm length with 12.7 mm outer diameter. Schematic diagram of the experimental apparatus is shown in Fig. 2b. A copper block (1.5 kg), mock up thermal load,
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Nomenclature Cp E m P Q Q_ R T t
specific heat (J/kg K) energy (J) mass (kg) pressure (MPa) heat to be removed (J) cooling capacity of cryocooler (W) temperature (K) time (s)
Subscript C c E o s sat tot
is cooled down by GM cryocooler (Model 350 CS, Austin Scientific) through the thermosiphon. The cold part of cryocooler, thermosiphon, and thermal load are placed in a vacuum chamber, and the outer surface is covered with multi layer insulation to minimize heat invasion by convection and radiation. Silicon diode sensors (DT-670-SD, Lake Shore Cryogenics) are attached on the outer surface of the cold head, condenser, evaporator, and the thermal load to measure the temperature variation during the cool-down process. The inside pressure of the thermosiphon and buffer tank is measured by the pressure transducer (Honeywell Sensotec Sensors) which is installed in the charge line. The accuracy of the temperature and pressure sensors are ±0.25 K and ±25 kPa, respectively. Nitrogen (N2) and tetrafluoromethane (CF4) are selected as the working fluid for this study because of their non-toxicity, nonflammability, and ease of handling. The two phase temperature range of each component overlaps as shown in Fig. 1. The valve located between the buffer tank and the thermosiphon (Fig. 2b) is closed and N2 is charged to the buffer tank, then CF4 is charged until the total pressure of mixture reaches 4.0 MPa. Then the valve is opened to charge the thermosiphon with the mixture, and the cool-down is started by the cryocooler. 2.2. Thermodynamic behavior of mixture The observation of fluid from the transparent thermosiphon experiments was helpful to analyze the mixed fluid behavior. The cool-down history from room temperature to cryogenic temperature of pure N2 and N2–CF4 (25/75 mol%) mixture is depicted respectively in Fig. 3. The condenser temperature decreases first
condenser area copper evaporator area overall stainless steel saturated state total
and then the evaporator temperature decreases. The constant temperature of condenser in Fig. 3a is the saturation temperature for single fluid, and the condenser and the evaporator temperatures merge when the evaporator temperature reaches this constant condenser temperature. Then the condenser and the evaporator are cooled down with negligible temperature difference. This temperature transition is a typical cool-down history with pure working fluid [12]. However, the result with mixed fluid in Fig. 3b is different from the case of single fluid. To understand the physical behavior of mixed fluid, it is helpful to divide the cool-down process into four sections according to the internal state of thermosiphon. Being contrary to pure fluids, the saturation temperature of mixture varies with composition even though the pressure is maintained at constant level [13]. The information about the saturation temperature is calculated by Peng–Robinson equation of state [4], and the dew points and bubble points of N2–CF4 (25/ 75 mol%) mixture at the measured pressure are plotted in Fig. 4. Only the vapor phase exists during the (a) period, so there is negligible heat transfer between the condenser and the evaporator by natural convection of fluid and conduction through the adiabatic wall. Hence, the evaporator temperature remains nearly constant while the condenser temperature decreases rapidly. The thermosiphon starts to operate at the (b) period when the condenser temperature reaches the dew point of the mixture. The evaporator cooling is accelerated in this period due to the continuous circulation of fluid by condensation and evaporation. A slight decrease of condenser temperature from the dew point is shown during the (b) period. One important fact is that the whole thermosiphon is not at thermal equilibrium during the transient
Critical point / Triple point 300
4.83 MPa 4.87 MPa
Temperature [K]
250
3.75 MPa 4.64 MPa
200
100
5.08 MPa
4.89 MPa
150
5.57 MPa 3.39 MPa
58 Pa 0.8 Pa
0.64 kPa 11.7 kPa 68.9 kPa 2.65 MPa
12.6 kPa
50
0.152 kPa 0.22 kPa
1.315 MPa 43.2 kPa 7 kPa
0 R23
Ethane
0.23 MPa
R14 Methane Argon Nitrogen Oxygen Flourine Neon Hydrogen Helium
Various fluids Fig. 1. Working fluids for thermosiphon in cryogenic temperature.
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(a)
pressure sensor
(b)
P valve buffer tank N 2
cryocooler
gas bombe
cold head
thermosiphon
thermal load
CF4
temperature sensors
radiation shield vacuum chamber
Fig. 2. (a) Configuration of thermosiphon and (b) schematic diagram of experimental apparatus.
operation. The condensation is started when the condenser temperature reaches the dew point of the mixture, while the evaporator temperature is still high near the room temperature. The composition of the condensed liquid is found by drawing a horizontal line in the T–x diagram in Fig. 5a. Intersecting point indicates that CF4 composition (0.92) is very high in the liquid. This condensed liquid is heated and evaporates while it is flowing down along the adiabatic wall or directly dropping down to the evaporator since the adiabatic wall and the evaporator temperature is higher than that of the liquid. As a result, CF4 composition increases in the liquid phase according to the equilibrium at the increased temperature. Finally, the liquid becomes pure CF4 right before the complete evaporation. A schematic diagram of the condensed liquid is in Fig. 5b. There is small amount of liquid on the adiabatic wall at the beginning of the (b) period, but the liquid quantity increases with the decrease of evaporator temperature. Therefore CF4 composition in the vapor phase starts to decrease from the initial mixing composition by the increase of CF4 rich liquid that is formed around the condenser and the adiabatic wall. Then the dew point of the mixture also decreases. This causes the decrease of condenser temperature during the (b) period.
Fig. 3. Cool-down history of thermosiphon with (a) pure N2 and (b) N2–CF4 (25/ 75 mol%) mixture.
The (c) period starts when the evaporator temperature reaches the saturation temperature of CF4. The condensed liquid starts to accumulate at the evaporator which is typified as the rapid decrease of internal pressure in Fig. 4. CF4 composition in the vapor phase becomes significantly low when pure liquid CF4 accumulates at the evaporator. Then, the dew point of mixture decreases accordingly. As a result, the condenser temperature decreases rapidly. The evaporator temperature follows the saturation temperature of CF4 which is cooled by weak boiling of the accumulated liquid CF4 and the natural convection of the N2 rich vapor. Briefly, heat transfer during the (c) period is considered as quiescent from the standpoint of overall cool-down process. As cooing continues, thermosiphon operation is resumed when the condenser temperature reaches saturation temperature of N2 at the (d) period. At the end of (c) period, pure liquid CF4 was accumulated at the evaporator, and N2 rich vapor existed at the condenser. However, N2 composition in the accumulated liquid gradually increases by liquid N2 droplets when the condenser temperature becomes low enough to condense N2. After then, the evaporator temperature follows the bubble point of mixture whose composition continuously changes by the condensed liquid N2 droplets. The evaporator temperature approaches to the bubble point of mixture at the initial mixing composition when the most of vapor is liquefied at the evaporator. This is confirmed in Fig. 4 at the final state of the (d) period. 3. Modeling transient cool-down process The transient cool-down process always exists in the cooling of cryogenic systems. When the load is connected to the cryocooler
J. Lee et al. / Cryogenics 50 (2010) 352–358
355
Fig. 4. T cool-down history of thermosiphon with N2–CF4 (25/75 mol%) mixture, saturation temperature of pure N2 and CF4, dew point and bubble point of the mixture.
through the thermosiphon, the cool-down time is mainly affected by following factors; – Cooling capacity of the cryocooler. – Thermal mass of the cryocooler cold head, thermosiphon, and the object to be cooled (load). – Thermodynamic properties of the working fluid.
Fig. 5. (a) T–x diagram of N2–CF4 mixture at constant pressure of 3 MPa. (b) Schematic diagram of condensed liquid quantity along the adiabatic wall of the thermosiphon.
A simple calculation method to estimate the cool-down time is developed in this paper. 3.1. Specifications and assumptions The modeling is based on the thermosiphon described in Section 2.1, and the schematic diagram of each part is shown in Fig. 6. The cooling capacity of the cryocooler (Q_ R ) is linearly dependent on the cold head temperature (T C ) as follows.
Q_ R ¼ 0:073T C þ 5:1ð50 6 T C 6 300Þ
ð1Þ
The thermal mass (mC p ) of the cryocooler cold head was estimated by no load cool-down test since the accurate information about the thermal mass is not known. Mass and material information of each component of the whole system is listed in Table 1. The basic equation is energy conservation law. The incoming energy is assumed as zero since the system is well insulated in a vacuum chamber, and the condenser is cooled by the cryocooler. Then the cool-down time is expressed as
dT Q_ R ¼ dt mC p
ð2Þ
For a simple approach, thermal mass of working fluid and adiabatic tube are neglected since the fraction of their thermal mass to the whole system is only 2.8%. In addition, the temperatures of the cryocooler cold head, the connector and the condenser are assumed to be identical (TC), and the evaporator and the load temperature is assumed to be same (TE) as well.
Fig. 6. Schematic diagram of the thermosiphon system for the cool-down time estimation model.
3.2. Cool-down time estimation with pure N2 The cool-down process is divided into three periods, and each temperature at the boundary condition is listed in Table 2. In the first period, TE is assumed to be constant, so the cool-down time is determined by the thermal mass of the cryocooler, the connector,
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Table 1 Specifications of cryocooler cold head, connector, thermosiphon, and load. Condenser part
Material Mass (kg)
Evaporator part
Cryocooler cold head
Connector
Condenser
Evaporator
Load
Stainless steel 0.45
Copper 0.2307
Copper 0.2618
Copper 0.2618
Copper 1.5
and the condenser. The first period is assumed to end when TC reaches the saturation temperature of N2 at initial charging pressure. TC is assumed to be constant during the second period and then the third period starts when TE reaches the saturation temperature at the initial charging pressure. In fact, there is a little pressure decrease due to the temperature decrease of the condenser (first period) and the condensation of vapor (second period), but it is not considered in the calculation. In the third period, TC and TE decrease together. The final temperature of the third period is set as 64 K when N2 starts to freeze. The cool-down time of each period is calculated by Eq. (2), and the result is in Table 3. A little fast cool-down of the condenser is predicted by the assumption of constant TE during the first period, while the amount of heat removed from the evaporator is increased during the second period. Therefore the overall cool-down time agrees well with the experimental result.
3.3. Cool-down time estimation with N2–CF4 mixture The cool-down process is divided into four periods, and each temperature at the boundary condition is listed in Table 2. The calculation methods of the first and the third periods are the same as that of the first period of pure N2, except for the final temperature of the first period. The first period with mixture finishes when TC reaches the dew point of the mixture. Additional assumptions are made for the second and the fourth periods since TC and TE simultaneously change at different rates. The total required cooling is fixed when the initial temperature, final temperature, and the thermal mass are known. The required cooling for the condenser part (QC) and the evaporator part (QE) are defined as follows.
Table 3 Cool-down time for pure N2, N2–CF4 (25/75 mol%), N2–CF4 (35/65 mol%), N2–CF4 (50/ 50 mol%). Periods
Calculation (minutes)
Experiment (minutes)
Pure N2 1st 2nd 3rd Total
50.2 121.8 54.9 226.9
63.8 109.8 52.8 226.4
N2–CF4 (25/75 mol%) 1st 2nd 3rd 4th Total
22.7 48.6 17.5 134.3 223.1
27.3 44.5 38.3 122.8 232.9
N2–CF4 (35/65 mol%) 1st 2nd 3rd 4th Total
24.5 54.3 12.1 136.3 227.2
29 51 26.6 127.2 233.8
N2–CF4 (50/50 mol%) 1st 2nd 3rd 4th Total
27.1 58.4 7.1 136 228.6
31.7 60.5 12 124.3 228.5
Q C ¼ mcondenser
Z
T C;1
cp;c ðT C ÞdT C
part T C;2
þ mcold
Z
T C;1
cp;s ðT C ÞdT C
head
ð3Þ
T C;2
Table 2 Boundary conditions of cool-down time calculation with pure N2 and N2–CF4 (35/ 65 mol%) mixture. Periods
Temperature
Initial value
Final value
Pure N2 1st
TC
290.7 K (initial T) 290.7 K (initial T) 125.4 K 290.7 K 125.4 K 125.4 K
125.4 K (Tsat of N2 at charging P)
203.2 K (dew point at charging P)
TC
289.8 K (initial T) 289.8 K (initial T) 203.2 K
TE TC TE TC TE
289.8 K 161.2 K 222.6 K 125.3 K 222.6 K
TE 2nd 3rd
TC TE TC TE
N2–CF4 (35/65 mol%) 1st TC TE 2nd
3rd 4th
Constant Constant 125.4 K (Tsat of N2 at charging P) 64 K 64 K
Constant 161.2 K (bubble point at charging P) 222.6 K (Tsat of CF4 at charging P) 125.3 K (Tsat of N2 at charging P) Constant 64 K 64 K
Q E ¼ mev aporator
Z
T E;1
part
cp;c ðT E ÞdT E
ð4Þ
T E;2
Q tot ¼ Q C þ Q E
ð5Þ
where T1 and T2 are the initial temperature and the final temperature respectively, mcondenser part is the mass of the connector and the condenser, mcold head is the mass of the cold head, mev aporator part is the mass of the evaporator and the load. TC is expressed by the following relation with the assumption of linear transition with respect to the cool-down time, t.
TC ¼
ðT C;1 T C;2 Þ t þ T C;1 to
ð6Þ
where to is the overall cool-down time which is the unknown. Substituting Eq. (6) to Eq. (1), relates the cooling capacity of cryocooler with the cool-down time.
ðT 1 T 2 Þ t þ 5:1 Q_ R ¼ 0:073 T 1 to
ð7Þ
The total required cooling is calculated by the following Eq. (8) which includes the unknown to.
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J. Lee et al. / Cryogenics 50 (2010) 352–358
Q tot ¼
Z
to
Q_ R dt
ð8Þ
0
As a result, the cool-down time, to, is calculated by combining Eqs. (5) and (8). T1 and T2 become the boundary temperature of the condenser and the evaporator during the second and the fourth periods. The second and the third periods are distinguished by the existence of liquid accumulation at the bottom of the thermosiphon, and the liquid starts to accumulate when TE reaches the saturation temperature of CF4. Final value of TC is assumed as bubble point because the vapor quality approaches to zero at the end of the second period. Therefore the boundary temperatures of the second period are set as follows; TE: From the initial temperature to the saturation temperature of CF4. TC: From the dew point to the bubble point of mixture. In the fourth period, the boundary temperatures are set as follows; TE: From the saturation temperature of CF4 to 64 K. TC: From the saturation temperature of N2 to 64 K. Although there is a pressure decrease in both of the second and the third periods, all the saturation temperature at each period is calculated at the initial charging pressure since the cooling capacity of cryocooler is not sensitive to the small variation of TC. The calculation results are summarized in Table 3. The cool-down time for N2–CF4 (25/75 mol%) and N2–CF4 (50/50 mol%) mixtures are also calculated. The final temperature of TC and TE in the fourth period are changed to 64 K and 70 K, respectively for the case of N2–CF4 (25/75 mol%) mixture. The lowest TE was 70 K in the experiment due to the freezing of CF4. The freezing point of liquid mixture varies with the composition of CF4 by the solubility of solid CF4 in the liquid N2 [14,15], but we observed that the solid CF4 silently subsided at the bottom of evaporator, and the thermosiphon continuously operated when the CF4 composition was less than 65 mol%. Therefore the final temperature at the fourth period does not affected by the mixing composition if the CF4 composition is less than 65 mol%. The calculated cool-down time for N2–CF4 (25/75 mol%) and N2–CF4 (50/ 50 mol%) mixtures are summarized in Table 3. Calculated cooldown time has 0.04–4.2% errors compared to the experimental results. 4. Discussion Additional cool-down time was calculated for the large mass of thermal load. The contribution of each period to the overall cooldown time varied, but the overall cool-down time was analogous in the case of pure N2 and that of N2–CF4 mixture. Cool-down time was also calculated with N2–Ar, N2–Kr, N2–NF3, N2–O2, and N2–CHF3 mixtures, but the cool-down time performance was not superior to that of pure one. Mixture containing high boiling point component was advantageous in the early stage, but the temperature glide (difference between the dew point and the bubble point) and the separation of mixture deteriorated the cool-down time performance and even stopped the normal operation of thermosiphon at the intermediate stage. We can operate thermosiphon by introducing the second fluid during the cooling sequence; charging the high boiling point component first and then the low boiling point component. One example is as follows. CHF3 is charged by 3.5 MPa then the final temperature is set as 158.9 K which is the saturation
Table 4 Boundary conditions of cool-down time calculation by introducing N2 during the cooling sequence with CHF3. Periods
Temperature
Initial Final Cool-down time value (K) value (K) (min)
1st
TC TE TC TE TC TE TC TE TC TE
290 290 286.2 290 286.2 286.2 158.9 158.9 126.2 158.9
2nd 3rd 4th 5th Total cool-down time
286.2 Constant Constant 286.2 158.9 158.9 126.2 158.9 64 64
1 95.1 3737.8 11.1 3513.4 7358.4
temperature of CHF3 at 10 kPa. The triple point of CHF3 is 118.02 K, but the thermosiphon does not operate effectively if the vapor pressure is too low. So, the limit vapor pressure is conservatively assumed as 10 kPa. In the next step, N2 is charged up to the critical pressure when the condenser and evaporator temperature reaches 158.9 K. Now, the thermosiphon is filled with liquid CHF3 and gaseous N2. The thermosiphon operates again when the condenser temperature reaches 126.2 K which is the critical temperature of N2. The thermosiphon will continuously operate down to 64 K. CHF3 is intentionally selected to have the triple point of high boiling component near the critical point of N2 which results in early separation of CHF3 in the low temperature range. For this purpose, the composition of the high boiling component should be less than 65%. Therefore CHF3 operates well alone in the high temperature range then freezes out and does not affect N2 operation at the low temperature range. For the cool-down time estimation, the single fluid calculation method is used when pure CHF3 is in the thermosiphon, and the mixed fluid calculation method is used when N2 is charged to the thermosiphon. In the case of 100 kg thermal load, each temperature at the boundary condition and the calculated cool-down time are listed in Table 4. The calculated cool-down time is reduced by 17.8% compared to the case of pure N2 which is 8952.5 min. The other interesting observation of mixed fluid thermosiphon is the extension of operational temperature range below 63.2 K the triple point of pure N2. It is known that there is no fluid except O2 and F which is able to stay as two phase state between the triple point of N2 (63.2 K) and the critical point of Ne (44.5 K) [16]. Both O2 and F are not only dangerous but also have very low saturated vapor pressure at the low temperature range. For example, the saturated vapor pressure of O2 at 63 K is only 1.5 kPa, so the thermosiphon with O2 actually does not operate effectively up to the freezing point of O2. In this aspect, it is noticeable that the eutectic temperature of N2–O2 mixture is 50.1 K [17], and the experimental observation confirmed that the lowest evaporator temperature was 61.1 K with N2–CF4 (50/50 mol%) which was 3 K lower than that with pure N2. These facts imply that some portion of the gap between Ne (Tcritical = 44.5 K) and N2 (Ttriple = 63.2 K) can be complemented by using fluid mixtures.
5. Conclusion N2–CF4 mixture as well as pure N2 was used as the working fluid for cryogenic thermosiphon during the transient cool-down process. The physical behavior of mixture over a wide temperature range was carefully investigated, and the thermodynamic behavior of the thermosiphon was revealed with experimental results and T–x diagram. The cool-down time estimation methodology was
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developed using only the properties of the working fluid and the initial conditions without employing complex heat transfer coefficient of the mixture. This model does not completely reproduce the experimental results, but the estimated cool-down time has less than 5% error. This scale of error is tolerated for preliminary calculation method to judge whether a mixture is advantageous or not before the actual experiment. The cool-down time reduction was not achieved by simply mixing two components at once due to the separation of mixture. One way to avoid this problem was suggested, and 17.8% of cool-down time reduction was estimated. This result indicates that the most efficient cooling is achieved when the condenser temperature is maintained as close as the evaporator temperature. This paper provides useful information about the behavior of mixed fluid thermosiphon in cryogenic cooling systems. Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Government (MOST) (No. R0A-2007-000-20062-0). References [1] Pioro LS, Pioro IL. Industrial two-phase thermosyphons. Begell House; 1997. [2] Faghri A. Heat pipe science and technology. Taylor & Francis; 1995. [3] Prenger FC, Hill DD, Daney DE, Daugherty MA, Green GF, Roth EW. Nitrogen heat pipe for cryocooler thermal shunt. Adv Cryo Eng 1996;41:147–54.
[4] REFPROP. NIST standard reference database 23 Version 8.0; 2007. [5] Prenger FC, Hill DD, Daney DE, Daugherty MA, Green GF, Chafe J, et al. Performance of cryocooler shunt heat pipes. Adv Cryo Eng 1998;43:1521–8. [6] Prenger FC, Hill DD, Daney DE, Daugherty MA, Green GF, Chafe J, et al. Heat pipes for enhanced cool down of cryogenic system. Cryocoolers 1997;9:831–9. [7] Raffaele S, Nicola DF, Raimondo F, Yoshiyuki A. Heat pipes with binary mixtures and inverse Marangoni effects for microgravity applications. Acta Astronaut 2007;61:16–26. [8] Raffaele S, Yoshiyuki A, Raimondo F. Comparative study of heat pipes with different working fluids under normal gravity and microgravity conditions. Acta Astronaut 2008;63:24–34. [9] Nicola DF, Raffaele S, Yoshiyuki A. New alcohol solutions for heat pipes: Marangoni effect and heat transfer enhancement. Int J Heat Mass Transfer 2008;51:6199–207. [10] Nuntaphan A, Tiansuwan J, Kiatsiriroat T. Enhancement of heat transport in thermosyphon air preheater at high temperature with binary working fluid: a case study of TEG–water. Appl Therm Eng 2002;22:251–66. [11] Lee J, Jeong S, Ko J, Kim Y, Jung S, Han Y. Transparent cryogenic thermosiphon using N2 and CF4 mixture as the working fluid. Supercond Cryogen 2009;11(2):37–40. [12] Jeong S, Kim Y, Noh C, Kim S, Jin H. Experimental investigation on the detachable thermosiphon for conduction-cooled superconducting magnets. Cryogenics 2006;46:705–10. [13] Radermacher R, Hwang Y. Vapor compression heat pumps with refrigerant mixtures. Taylor & Francis; 2005. [14] Prausnitz JM, Lichtenthaler RN, Azevedo EG. Molecular thermodynamics of fluid-phase equilibria. Prentice Hall PTR; 1999. [15] Hwang G, Lee J, Jeong S. Estimation of freezing point of hydrocarbon and hydrofluorocarbon mixtures for mixed refrigerant JT cryocooler. CEC/ICMC 2009, Tucson, Arizona, USA; June 28–July 2, 2009. [16] Prager RC, Basiulis A. The state-of-the-art of cryogenic heat pipes. In: AIAA 18th aerospace sciences meeting, January 14–16; 1980 [AIAA paper 80-0211]. [17] Ruhemann MB. Low temperature physics. Cambridge at the University Press; 1936.
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Transient thermodynamic behavior of cryogenic mixed fluid thermosiphon and its cool-down time estimation Jisung Lee *, Youngkwon Kim, Sangkwon Jeong 1 Cryogenic Engineering Laboratory, Division of Mechanical Engineering, School of Mechanical, Aerospace and Systems Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon, 305-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 September 2009 Received in revised form 15 January 2010 Accepted 9 February 2010
Keywords: Gas mixture (B) Nitrogen (B) Heat transfer (C) Phase transitions (C) Thermodynamics (C)
a b s t r a c t Thermosiphon is an efficient heat transfer device by utilizing latent heat of fluid at liquid–vapor phase change. One of the disadvantages of thermosiphon, however, is that the operational temperature range is fundamentally limited from the critical point to the triple point of the working fluid to maintain two phase state. Nitrogen (N2) and tetrafluoromethane (CF4) were selected as the mixed working fluid to widen their original operational temperature range. Thermodynamic behavior of mixture and its effect on the cool-down time were investigated. A simple calculation model was proposed to estimate the cooldown time of the thermosiphon evaporator prior to experiments. The calculated results agreed well with the experimental results within 5% error. The cool-down time reduction was not achieved by mixing two components at once due to the separation of mixture. One idea to avoid this problem was suggested in this paper where the estimated cool-down time was reduced 17.8% compared to pure N2. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
1. Introduction Thermosiphon is an efficient heat transfer device by utilizing latent heat of fluid at liquid–vapor phase change. Numerous thermosiphons have been employed in industries due to their peculiar simple structure, heat switch function, and also large effective thermal conductivity compared to metal conductors such as copper or aluminum [1,2]. One of the specific application in cryogenic field is a thermal shunt device such as a thermal linkage between the first and the second stages of cryocooler cold head. Cool-down performance of superconducting magnet is greatly improved by large cooling capacity of the first stage cold head [3]. One of the disadvantages of thermosiphon, however, is that the operational temperature range is fundamentally limited from the critical point to the triple point of the working fluid to maintain two phase state. The upper temperature limit becomes the start point of operation, and the lower temperature limit becomes the end point of operation during the transient temperature variation period as the thermal shunt. Fig. 1 shows the temperature range of various fluids between their critical and triple points [4]. Prenger et al. suggested multiple thermosiphons using fluids which have different operational temperature ranges. Nitrogen and ethane were used in separate thermosiphons and the cool-down time
* Corresponding author. Tel.: +82 42 350 3079; fax: +82 42 350 8207. E-mail addresses: [email protected] (J. Lee), [email protected] (Y. Kim), [email protected] (S. Jeong). 1 Tel.:+82 42 350 3039; fax: +82 42 350 8207.
was reduced compared to the single nitrogen [5,6]. In a different way, a binary mixture considering critical and triple points of each component has a potential to widen the operational temperature range. Research on the mixed working fluid has been conducted to improve a heat transport capability with Marangoni effect [7–9], and critical heat flux enhancement [10]. However, all these researches were conducted around or above room temperature and focused on the steady state operation. The feasibility of mixed fluid thermosiphon in a cool-down device has not been tested. The authors carefully investigated the behavior of mixed working fluid such as condensation, liquid accumulation, boiling, and freezing with the transparent thermosiphon [11]. In this paper, the thermodynamic behavior of mixed fluid is thoroughly examined by further consideration of mixture composition change in temperature and mole fraction diagram (T–x diagram). Additionally, a simple prediction model is proposed to estimate the cool-down time of the thermosiphon evaporator prior to experiment. 2. Understanding mixed fluid behavior 2.1. Description of experimental apparatus and method A thermosiphon is fabricated as shown in Fig. 2a, which is composed of a condenser, an adiabatic region, and an evaporator. The diameter and height of the condenser and evaporator are all 30 mm. The adiabatic region is 100 mm length with 12.7 mm outer diameter. Schematic diagram of the experimental apparatus is shown in Fig. 2b. A copper block (1.5 kg), mock up thermal load,
0011-2275/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2010.02.001
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Nomenclature Cp E m P Q Q_ R T t
specific heat (J/kg K) energy (J) mass (kg) pressure (MPa) heat to be removed (J) cooling capacity of cryocooler (W) temperature (K) time (s)
Subscript C c E o s sat tot
is cooled down by GM cryocooler (Model 350 CS, Austin Scientific) through the thermosiphon. The cold part of cryocooler, thermosiphon, and thermal load are placed in a vacuum chamber, and the outer surface is covered with multi layer insulation to minimize heat invasion by convection and radiation. Silicon diode sensors (DT-670-SD, Lake Shore Cryogenics) are attached on the outer surface of the cold head, condenser, evaporator, and the thermal load to measure the temperature variation during the cool-down process. The inside pressure of the thermosiphon and buffer tank is measured by the pressure transducer (Honeywell Sensotec Sensors) which is installed in the charge line. The accuracy of the temperature and pressure sensors are ±0.25 K and ±25 kPa, respectively. Nitrogen (N2) and tetrafluoromethane (CF4) are selected as the working fluid for this study because of their non-toxicity, nonflammability, and ease of handling. The two phase temperature range of each component overlaps as shown in Fig. 1. The valve located between the buffer tank and the thermosiphon (Fig. 2b) is closed and N2 is charged to the buffer tank, then CF4 is charged until the total pressure of mixture reaches 4.0 MPa. Then the valve is opened to charge the thermosiphon with the mixture, and the cool-down is started by the cryocooler. 2.2. Thermodynamic behavior of mixture The observation of fluid from the transparent thermosiphon experiments was helpful to analyze the mixed fluid behavior. The cool-down history from room temperature to cryogenic temperature of pure N2 and N2–CF4 (25/75 mol%) mixture is depicted respectively in Fig. 3. The condenser temperature decreases first
condenser area copper evaporator area overall stainless steel saturated state total
and then the evaporator temperature decreases. The constant temperature of condenser in Fig. 3a is the saturation temperature for single fluid, and the condenser and the evaporator temperatures merge when the evaporator temperature reaches this constant condenser temperature. Then the condenser and the evaporator are cooled down with negligible temperature difference. This temperature transition is a typical cool-down history with pure working fluid [12]. However, the result with mixed fluid in Fig. 3b is different from the case of single fluid. To understand the physical behavior of mixed fluid, it is helpful to divide the cool-down process into four sections according to the internal state of thermosiphon. Being contrary to pure fluids, the saturation temperature of mixture varies with composition even though the pressure is maintained at constant level [13]. The information about the saturation temperature is calculated by Peng–Robinson equation of state [4], and the dew points and bubble points of N2–CF4 (25/ 75 mol%) mixture at the measured pressure are plotted in Fig. 4. Only the vapor phase exists during the (a) period, so there is negligible heat transfer between the condenser and the evaporator by natural convection of fluid and conduction through the adiabatic wall. Hence, the evaporator temperature remains nearly constant while the condenser temperature decreases rapidly. The thermosiphon starts to operate at the (b) period when the condenser temperature reaches the dew point of the mixture. The evaporator cooling is accelerated in this period due to the continuous circulation of fluid by condensation and evaporation. A slight decrease of condenser temperature from the dew point is shown during the (b) period. One important fact is that the whole thermosiphon is not at thermal equilibrium during the transient
Critical point / Triple point 300
4.83 MPa 4.87 MPa
Temperature [K]
250
3.75 MPa 4.64 MPa
200
100
5.08 MPa
4.89 MPa
150
5.57 MPa 3.39 MPa
58 Pa 0.8 Pa
0.64 kPa 11.7 kPa 68.9 kPa 2.65 MPa
12.6 kPa
50
0.152 kPa 0.22 kPa
1.315 MPa 43.2 kPa 7 kPa
0 R23
Ethane
0.23 MPa
R14 Methane Argon Nitrogen Oxygen Flourine Neon Hydrogen Helium
Various fluids Fig. 1. Working fluids for thermosiphon in cryogenic temperature.
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(a)
pressure sensor
(b)
P valve buffer tank N 2
cryocooler
gas bombe
cold head
thermosiphon
thermal load
CF4
temperature sensors
radiation shield vacuum chamber
Fig. 2. (a) Configuration of thermosiphon and (b) schematic diagram of experimental apparatus.
operation. The condensation is started when the condenser temperature reaches the dew point of the mixture, while the evaporator temperature is still high near the room temperature. The composition of the condensed liquid is found by drawing a horizontal line in the T–x diagram in Fig. 5a. Intersecting point indicates that CF4 composition (0.92) is very high in the liquid. This condensed liquid is heated and evaporates while it is flowing down along the adiabatic wall or directly dropping down to the evaporator since the adiabatic wall and the evaporator temperature is higher than that of the liquid. As a result, CF4 composition increases in the liquid phase according to the equilibrium at the increased temperature. Finally, the liquid becomes pure CF4 right before the complete evaporation. A schematic diagram of the condensed liquid is in Fig. 5b. There is small amount of liquid on the adiabatic wall at the beginning of the (b) period, but the liquid quantity increases with the decrease of evaporator temperature. Therefore CF4 composition in the vapor phase starts to decrease from the initial mixing composition by the increase of CF4 rich liquid that is formed around the condenser and the adiabatic wall. Then the dew point of the mixture also decreases. This causes the decrease of condenser temperature during the (b) period.
Fig. 3. Cool-down history of thermosiphon with (a) pure N2 and (b) N2–CF4 (25/ 75 mol%) mixture.
The (c) period starts when the evaporator temperature reaches the saturation temperature of CF4. The condensed liquid starts to accumulate at the evaporator which is typified as the rapid decrease of internal pressure in Fig. 4. CF4 composition in the vapor phase becomes significantly low when pure liquid CF4 accumulates at the evaporator. Then, the dew point of mixture decreases accordingly. As a result, the condenser temperature decreases rapidly. The evaporator temperature follows the saturation temperature of CF4 which is cooled by weak boiling of the accumulated liquid CF4 and the natural convection of the N2 rich vapor. Briefly, heat transfer during the (c) period is considered as quiescent from the standpoint of overall cool-down process. As cooing continues, thermosiphon operation is resumed when the condenser temperature reaches saturation temperature of N2 at the (d) period. At the end of (c) period, pure liquid CF4 was accumulated at the evaporator, and N2 rich vapor existed at the condenser. However, N2 composition in the accumulated liquid gradually increases by liquid N2 droplets when the condenser temperature becomes low enough to condense N2. After then, the evaporator temperature follows the bubble point of mixture whose composition continuously changes by the condensed liquid N2 droplets. The evaporator temperature approaches to the bubble point of mixture at the initial mixing composition when the most of vapor is liquefied at the evaporator. This is confirmed in Fig. 4 at the final state of the (d) period. 3. Modeling transient cool-down process The transient cool-down process always exists in the cooling of cryogenic systems. When the load is connected to the cryocooler
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355
Fig. 4. T cool-down history of thermosiphon with N2–CF4 (25/75 mol%) mixture, saturation temperature of pure N2 and CF4, dew point and bubble point of the mixture.
through the thermosiphon, the cool-down time is mainly affected by following factors; – Cooling capacity of the cryocooler. – Thermal mass of the cryocooler cold head, thermosiphon, and the object to be cooled (load). – Thermodynamic properties of the working fluid.
Fig. 5. (a) T–x diagram of N2–CF4 mixture at constant pressure of 3 MPa. (b) Schematic diagram of condensed liquid quantity along the adiabatic wall of the thermosiphon.
A simple calculation method to estimate the cool-down time is developed in this paper. 3.1. Specifications and assumptions The modeling is based on the thermosiphon described in Section 2.1, and the schematic diagram of each part is shown in Fig. 6. The cooling capacity of the cryocooler (Q_ R ) is linearly dependent on the cold head temperature (T C ) as follows.
Q_ R ¼ 0:073T C þ 5:1ð50 6 T C 6 300Þ
ð1Þ
The thermal mass (mC p ) of the cryocooler cold head was estimated by no load cool-down test since the accurate information about the thermal mass is not known. Mass and material information of each component of the whole system is listed in Table 1. The basic equation is energy conservation law. The incoming energy is assumed as zero since the system is well insulated in a vacuum chamber, and the condenser is cooled by the cryocooler. Then the cool-down time is expressed as
dT Q_ R ¼ dt mC p
ð2Þ
For a simple approach, thermal mass of working fluid and adiabatic tube are neglected since the fraction of their thermal mass to the whole system is only 2.8%. In addition, the temperatures of the cryocooler cold head, the connector and the condenser are assumed to be identical (TC), and the evaporator and the load temperature is assumed to be same (TE) as well.
Fig. 6. Schematic diagram of the thermosiphon system for the cool-down time estimation model.
3.2. Cool-down time estimation with pure N2 The cool-down process is divided into three periods, and each temperature at the boundary condition is listed in Table 2. In the first period, TE is assumed to be constant, so the cool-down time is determined by the thermal mass of the cryocooler, the connector,
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Table 1 Specifications of cryocooler cold head, connector, thermosiphon, and load. Condenser part
Material Mass (kg)
Evaporator part
Cryocooler cold head
Connector
Condenser
Evaporator
Load
Stainless steel 0.45
Copper 0.2307
Copper 0.2618
Copper 0.2618
Copper 1.5
and the condenser. The first period is assumed to end when TC reaches the saturation temperature of N2 at initial charging pressure. TC is assumed to be constant during the second period and then the third period starts when TE reaches the saturation temperature at the initial charging pressure. In fact, there is a little pressure decrease due to the temperature decrease of the condenser (first period) and the condensation of vapor (second period), but it is not considered in the calculation. In the third period, TC and TE decrease together. The final temperature of the third period is set as 64 K when N2 starts to freeze. The cool-down time of each period is calculated by Eq. (2), and the result is in Table 3. A little fast cool-down of the condenser is predicted by the assumption of constant TE during the first period, while the amount of heat removed from the evaporator is increased during the second period. Therefore the overall cool-down time agrees well with the experimental result.
3.3. Cool-down time estimation with N2–CF4 mixture The cool-down process is divided into four periods, and each temperature at the boundary condition is listed in Table 2. The calculation methods of the first and the third periods are the same as that of the first period of pure N2, except for the final temperature of the first period. The first period with mixture finishes when TC reaches the dew point of the mixture. Additional assumptions are made for the second and the fourth periods since TC and TE simultaneously change at different rates. The total required cooling is fixed when the initial temperature, final temperature, and the thermal mass are known. The required cooling for the condenser part (QC) and the evaporator part (QE) are defined as follows.
Table 3 Cool-down time for pure N2, N2–CF4 (25/75 mol%), N2–CF4 (35/65 mol%), N2–CF4 (50/ 50 mol%). Periods
Calculation (minutes)
Experiment (minutes)
Pure N2 1st 2nd 3rd Total
50.2 121.8 54.9 226.9
63.8 109.8 52.8 226.4
N2–CF4 (25/75 mol%) 1st 2nd 3rd 4th Total
22.7 48.6 17.5 134.3 223.1
27.3 44.5 38.3 122.8 232.9
N2–CF4 (35/65 mol%) 1st 2nd 3rd 4th Total
24.5 54.3 12.1 136.3 227.2
29 51 26.6 127.2 233.8
N2–CF4 (50/50 mol%) 1st 2nd 3rd 4th Total
27.1 58.4 7.1 136 228.6
31.7 60.5 12 124.3 228.5
Q C ¼ mcondenser
Z
T C;1
cp;c ðT C ÞdT C
part T C;2
þ mcold
Z
T C;1
cp;s ðT C ÞdT C
head
ð3Þ
T C;2
Table 2 Boundary conditions of cool-down time calculation with pure N2 and N2–CF4 (35/ 65 mol%) mixture. Periods
Temperature
Initial value
Final value
Pure N2 1st
TC
290.7 K (initial T) 290.7 K (initial T) 125.4 K 290.7 K 125.4 K 125.4 K
125.4 K (Tsat of N2 at charging P)
203.2 K (dew point at charging P)
TC
289.8 K (initial T) 289.8 K (initial T) 203.2 K
TE TC TE TC TE
289.8 K 161.2 K 222.6 K 125.3 K 222.6 K
TE 2nd 3rd
TC TE TC TE
N2–CF4 (35/65 mol%) 1st TC TE 2nd
3rd 4th
Constant Constant 125.4 K (Tsat of N2 at charging P) 64 K 64 K
Constant 161.2 K (bubble point at charging P) 222.6 K (Tsat of CF4 at charging P) 125.3 K (Tsat of N2 at charging P) Constant 64 K 64 K
Q E ¼ mev aporator
Z
T E;1
part
cp;c ðT E ÞdT E
ð4Þ
T E;2
Q tot ¼ Q C þ Q E
ð5Þ
where T1 and T2 are the initial temperature and the final temperature respectively, mcondenser part is the mass of the connector and the condenser, mcold head is the mass of the cold head, mev aporator part is the mass of the evaporator and the load. TC is expressed by the following relation with the assumption of linear transition with respect to the cool-down time, t.
TC ¼
ðT C;1 T C;2 Þ t þ T C;1 to
ð6Þ
where to is the overall cool-down time which is the unknown. Substituting Eq. (6) to Eq. (1), relates the cooling capacity of cryocooler with the cool-down time.
ðT 1 T 2 Þ t þ 5:1 Q_ R ¼ 0:073 T 1 to
ð7Þ
The total required cooling is calculated by the following Eq. (8) which includes the unknown to.
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Q tot ¼
Z
to
Q_ R dt
ð8Þ
0
As a result, the cool-down time, to, is calculated by combining Eqs. (5) and (8). T1 and T2 become the boundary temperature of the condenser and the evaporator during the second and the fourth periods. The second and the third periods are distinguished by the existence of liquid accumulation at the bottom of the thermosiphon, and the liquid starts to accumulate when TE reaches the saturation temperature of CF4. Final value of TC is assumed as bubble point because the vapor quality approaches to zero at the end of the second period. Therefore the boundary temperatures of the second period are set as follows; TE: From the initial temperature to the saturation temperature of CF4. TC: From the dew point to the bubble point of mixture. In the fourth period, the boundary temperatures are set as follows; TE: From the saturation temperature of CF4 to 64 K. TC: From the saturation temperature of N2 to 64 K. Although there is a pressure decrease in both of the second and the third periods, all the saturation temperature at each period is calculated at the initial charging pressure since the cooling capacity of cryocooler is not sensitive to the small variation of TC. The calculation results are summarized in Table 3. The cool-down time for N2–CF4 (25/75 mol%) and N2–CF4 (50/50 mol%) mixtures are also calculated. The final temperature of TC and TE in the fourth period are changed to 64 K and 70 K, respectively for the case of N2–CF4 (25/75 mol%) mixture. The lowest TE was 70 K in the experiment due to the freezing of CF4. The freezing point of liquid mixture varies with the composition of CF4 by the solubility of solid CF4 in the liquid N2 [14,15], but we observed that the solid CF4 silently subsided at the bottom of evaporator, and the thermosiphon continuously operated when the CF4 composition was less than 65 mol%. Therefore the final temperature at the fourth period does not affected by the mixing composition if the CF4 composition is less than 65 mol%. The calculated cool-down time for N2–CF4 (25/75 mol%) and N2–CF4 (50/ 50 mol%) mixtures are summarized in Table 3. Calculated cooldown time has 0.04–4.2% errors compared to the experimental results. 4. Discussion Additional cool-down time was calculated for the large mass of thermal load. The contribution of each period to the overall cooldown time varied, but the overall cool-down time was analogous in the case of pure N2 and that of N2–CF4 mixture. Cool-down time was also calculated with N2–Ar, N2–Kr, N2–NF3, N2–O2, and N2–CHF3 mixtures, but the cool-down time performance was not superior to that of pure one. Mixture containing high boiling point component was advantageous in the early stage, but the temperature glide (difference between the dew point and the bubble point) and the separation of mixture deteriorated the cool-down time performance and even stopped the normal operation of thermosiphon at the intermediate stage. We can operate thermosiphon by introducing the second fluid during the cooling sequence; charging the high boiling point component first and then the low boiling point component. One example is as follows. CHF3 is charged by 3.5 MPa then the final temperature is set as 158.9 K which is the saturation
Table 4 Boundary conditions of cool-down time calculation by introducing N2 during the cooling sequence with CHF3. Periods
Temperature
Initial Final Cool-down time value (K) value (K) (min)
1st
TC TE TC TE TC TE TC TE TC TE
290 290 286.2 290 286.2 286.2 158.9 158.9 126.2 158.9
2nd 3rd 4th 5th Total cool-down time
286.2 Constant Constant 286.2 158.9 158.9 126.2 158.9 64 64
1 95.1 3737.8 11.1 3513.4 7358.4
temperature of CHF3 at 10 kPa. The triple point of CHF3 is 118.02 K, but the thermosiphon does not operate effectively if the vapor pressure is too low. So, the limit vapor pressure is conservatively assumed as 10 kPa. In the next step, N2 is charged up to the critical pressure when the condenser and evaporator temperature reaches 158.9 K. Now, the thermosiphon is filled with liquid CHF3 and gaseous N2. The thermosiphon operates again when the condenser temperature reaches 126.2 K which is the critical temperature of N2. The thermosiphon will continuously operate down to 64 K. CHF3 is intentionally selected to have the triple point of high boiling component near the critical point of N2 which results in early separation of CHF3 in the low temperature range. For this purpose, the composition of the high boiling component should be less than 65%. Therefore CHF3 operates well alone in the high temperature range then freezes out and does not affect N2 operation at the low temperature range. For the cool-down time estimation, the single fluid calculation method is used when pure CHF3 is in the thermosiphon, and the mixed fluid calculation method is used when N2 is charged to the thermosiphon. In the case of 100 kg thermal load, each temperature at the boundary condition and the calculated cool-down time are listed in Table 4. The calculated cool-down time is reduced by 17.8% compared to the case of pure N2 which is 8952.5 min. The other interesting observation of mixed fluid thermosiphon is the extension of operational temperature range below 63.2 K the triple point of pure N2. It is known that there is no fluid except O2 and F which is able to stay as two phase state between the triple point of N2 (63.2 K) and the critical point of Ne (44.5 K) [16]. Both O2 and F are not only dangerous but also have very low saturated vapor pressure at the low temperature range. For example, the saturated vapor pressure of O2 at 63 K is only 1.5 kPa, so the thermosiphon with O2 actually does not operate effectively up to the freezing point of O2. In this aspect, it is noticeable that the eutectic temperature of N2–O2 mixture is 50.1 K [17], and the experimental observation confirmed that the lowest evaporator temperature was 61.1 K with N2–CF4 (50/50 mol%) which was 3 K lower than that with pure N2. These facts imply that some portion of the gap between Ne (Tcritical = 44.5 K) and N2 (Ttriple = 63.2 K) can be complemented by using fluid mixtures.
5. Conclusion N2–CF4 mixture as well as pure N2 was used as the working fluid for cryogenic thermosiphon during the transient cool-down process. The physical behavior of mixture over a wide temperature range was carefully investigated, and the thermodynamic behavior of the thermosiphon was revealed with experimental results and T–x diagram. The cool-down time estimation methodology was
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developed using only the properties of the working fluid and the initial conditions without employing complex heat transfer coefficient of the mixture. This model does not completely reproduce the experimental results, but the estimated cool-down time has less than 5% error. This scale of error is tolerated for preliminary calculation method to judge whether a mixture is advantageous or not before the actual experiment. The cool-down time reduction was not achieved by simply mixing two components at once due to the separation of mixture. One way to avoid this problem was suggested, and 17.8% of cool-down time reduction was estimated. This result indicates that the most efficient cooling is achieved when the condenser temperature is maintained as close as the evaporator temperature. This paper provides useful information about the behavior of mixed fluid thermosiphon in cryogenic cooling systems. Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Government (MOST) (No. R0A-2007-000-20062-0). References [1] Pioro LS, Pioro IL. Industrial two-phase thermosyphons. Begell House; 1997. [2] Faghri A. Heat pipe science and technology. Taylor & Francis; 1995. [3] Prenger FC, Hill DD, Daney DE, Daugherty MA, Green GF, Roth EW. Nitrogen heat pipe for cryocooler thermal shunt. Adv Cryo Eng 1996;41:147–54.
[4] REFPROP. NIST standard reference database 23 Version 8.0; 2007. [5] Prenger FC, Hill DD, Daney DE, Daugherty MA, Green GF, Chafe J, et al. Performance of cryocooler shunt heat pipes. Adv Cryo Eng 1998;43:1521–8. [6] Prenger FC, Hill DD, Daney DE, Daugherty MA, Green GF, Chafe J, et al. Heat pipes for enhanced cool down of cryogenic system. Cryocoolers 1997;9:831–9. [7] Raffaele S, Nicola DF, Raimondo F, Yoshiyuki A. Heat pipes with binary mixtures and inverse Marangoni effects for microgravity applications. Acta Astronaut 2007;61:16–26. [8] Raffaele S, Yoshiyuki A, Raimondo F. Comparative study of heat pipes with different working fluids under normal gravity and microgravity conditions. Acta Astronaut 2008;63:24–34. [9] Nicola DF, Raffaele S, Yoshiyuki A. New alcohol solutions for heat pipes: Marangoni effect and heat transfer enhancement. Int J Heat Mass Transfer 2008;51:6199–207. [10] Nuntaphan A, Tiansuwan J, Kiatsiriroat T. Enhancement of heat transport in thermosyphon air preheater at high temperature with binary working fluid: a case study of TEG–water. Appl Therm Eng 2002;22:251–66. [11] Lee J, Jeong S, Ko J, Kim Y, Jung S, Han Y. Transparent cryogenic thermosiphon using N2 and CF4 mixture as the working fluid. Supercond Cryogen 2009;11(2):37–40. [12] Jeong S, Kim Y, Noh C, Kim S, Jin H. Experimental investigation on the detachable thermosiphon for conduction-cooled superconducting magnets. Cryogenics 2006;46:705–10. [13] Radermacher R, Hwang Y. Vapor compression heat pumps with refrigerant mixtures. Taylor & Francis; 2005. [14] Prausnitz JM, Lichtenthaler RN, Azevedo EG. Molecular thermodynamics of fluid-phase equilibria. Prentice Hall PTR; 1999. [15] Hwang G, Lee J, Jeong S. Estimation of freezing point of hydrocarbon and hydrofluorocarbon mixtures for mixed refrigerant JT cryocooler. CEC/ICMC 2009, Tucson, Arizona, USA; June 28–July 2, 2009. [16] Prager RC, Basiulis A. The state-of-the-art of cryogenic heat pipes. In: AIAA 18th aerospace sciences meeting, January 14–16; 1980 [AIAA paper 80-0211]. [17] Ruhemann MB. Low temperature physics. Cambridge at the University Press; 1936.