Study on heat transfer characteristics of ethane pulsating heat pipe in middle-low temperature region

Applied Thermal Engineering 152 (2019) 697–705

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Research Paper

Study on heat transfer characteristics of ethane pulsating heat pipe in middle-low temperature region

T

Xi Chen , Yi Lin, Shuai Shao, Weidong Wu ⁎

School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

HIGHLIGHTS

effects of HIs, IAs and operating temperatures on the EPHP are studied. • The performance of EPHP in middle-low temperature region is tested. • Thermal temperature fluctuations at different temperatures are analyzed at 0° IA. • The • Theoretical analysis of heat transfer characteristics of EPHP is conducted. ARTICLE INFO

ABSTRACT

Keywords: Ethane Pulsating heat pipe Thermal performance Filling ratio Heat input

A series of experiments were conducted to identify the thermal performance of an ethane pulsating heat pipe (EPHP) in middle-low temperature region (−90 °C to 0 °C). The copper EPHP with an inner diameter of 2 mm was bent to 10 turns, and the lengths of the evaporation section, condensation section, and adiabatic section were 150 mm, 150 mm, and 100 mm, respectively. Tests of the EPHP were performed with different heat inputs (HIs), inclination angles (IAs), filling ratios (FRs) and operating temperatures, so as to understand the effect of various operational parameters on the heat transfer performance. The experimental results showed that the optimal FR for the best EPHP performance was immune to the operating temperature and HI, and always remained about 30%. As the temperature of condensation section decreased from 0 °C to −90 °C, there existed the optimal temperature of −80 °C corresponding to the lowest thermal resistance. At different operating temperatures and HIs, the best performance of the EPHP could be achieved at about 30° IA. The lower the working temperature was, the easier it was to start the EPHP at 0° IA. Moreover, it was found that at relatively high HI (30–50 W), the latent heat of vaporization (LHV) was the dominant property that determined the thermal performance of the EPHP. When the HI was less than 20 W, the effect of the LHV on heat transfer dwindled, and the heat transfer had a close relationship with the liquid specific heat.

1. Introduction Pulsating heat pipe (PHP), first proposed by Akachi [1], is an efficient passive heat transfer device. Due to simple construction, high heat transfer performance and gravity independence [2–4], the PHP has been widely applied to space and electronic cooling, waste heat recovery, solar cell and other fields [5–7]. The thermal performance of the PHP is primarily influenced by the nature and response of working fluid when the set-up is done. At present, the most common working fluids for the PHP include water, alcohols, acetone etc., and they are mainly applied to investigate the influence of various operational parameters on the PHP in room temperature region. The filling ratio (FR) is one of the most important physical parameters affecting the

⁎

thermal performance of the PHP. According to the studies [8–10], an optimal FR should exist at which the PHP has a minimum thermal resistance. So far, it is widely believed that the optimal FR for the PHP filled with water is 50%. As references, Afrose et al. [11] performed an experiment on vertical PHP with acetone and deionized water as the working fluids. The results revealed that the optimal FR for deionized water was 50% and for acetone it was 70%. Another set of experiment was carried out with ethanol at various FRs and inclination angles (IAs) by Rahman et al. [12]. The FRs of ethanol for the experiment were 40%, 50%, 60% and 70%. The IAs were 0° (vertical), 30°, 45° and 60° with variation in heat input (HI). They concluded from the study that the better performance of the PHP could be achieved when the FR was 40% for low HI, and the best condition was for 50–60% FR at 0° IA. Wang

Corresponding author. E-mail addresses: [email protected], [email protected] (X. Chen).

https://doi.org/10.1016/j.applthermaleng.2019.02.125 Received 8 December 2018; Received in revised form 25 February 2019; Accepted 25 February 2019 Available online 26 February 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.

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Vacuuming the EPHP

Changing the FR

Charging the EPHP Cooling down the EPHP

Changing operating temperature

Setting the IA

Changing the IA Changing the HI

Setting the HI Reaching steady state Recording test data Changing working conditions

Fig. 1. Variation of the inner diameter with operating temperature.

Fig. 4. Flowchart of experimental procedure.

ethanol and deionized water as the working fluids with 50% FR. The experimental data showed that deionized water needed more HI to startup than ethanol, and the heat transfer thermal resistance of PHP decreased as the increasing HI. The tests conducted by Xian et al. [17] were focused on establishing the influence of pulse heating on the thermal performance of the PHP with water. The study claimed that pulse heating could enhance the heat transfer of PHP, and the heat transfer temperature difference under pulse heating mode was highly dependent on the pulse duration and HI. It is widely believed that PHP's orientation will affect its thermal performance, and this was further verified by the works of Xue et al. [18]. They concluded that with the increment of IA, the thermal resistance of the PHP decreased. In addition, Paudel and Michna [19] found that the thermal resistance of the PHP was minimum at 90° IA. Rahman et al. [20] presented a study of a closed loop PHP. The PHP was charged with methanol, and the tests were carried out at 0°, 30° and 45°. It was found that there was a strong influence of IA, and the maximum heat transfer was at 45° IA. Besides the above working media, many cryogenic fluids, such as nitrogen, helium, hydrogen etc. are also adopted to investigate the operating characteristic of the PHP in cryogenic temperature region. Fonseca et al. [8] carried out a series of experiments to test a helium-filled PHP. The FR ranged from 20% to 90% in the study. They reported that the best FR for helium PHP was 69.5%. Another test of Fonseca et al. [21] was performed using nitrogen as the working fluid with different FRs, HIs and IAs. The experiment outcome showed that the effective thermal conductivity of the PHP could reach 70,000 W/m·K at 20% FR, and the gravity had a great impact on the heat transfer performance of the PHP even with a lot of turns. Experiments performed by Jiao et al. [22] on the PHP with nitrogen were with the FR of 48%. The results demonstrated that when the HI increased from 22.5 W to 321.8 W, the thermal resistance of the PHP decreased from 0.256 K/W to 0.112 K/W. Deng

Fig. 2. Schematic of the experimental system.

et al. [13] chose 35%, 53%, and 70% FR of acetone and ethanol as the working fluids to investigate the heat transfer performance of the PHP. It was observed that in the stable operation stage, the better thermal performance achieved at a higher FR in the PHP with ethanol, while the better thermal performance achieved at a lower FR in the PHP with acetone. In addition, the HI, IA etc. are also the important parameters that determine the heat transfer performance of the PHP. The study results [14,15] pointed out that there existed a minimum HI to make the PHP operate and the PHP could only operate successfully when the HI was greater than this minimum value. Kim et al. [16] experimentally investigated the thermal performance of parallel-connected PHP using

T6

T1 T2 T3 T4 T5

T8

T9

Evaporation section

Condensation section Lc=100mm

T7

La=150mm

Le=150mm

Fig. 3. Layout of the temperature measuring points and dimensions of the EPHP. 698

T10

D=90mm

Adiabatic section

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the thermal performance of the ethane pulsating heat pipe (EPHP) under anti-gravity conditions and the cooling process and temperature fluctuation characteristics of the EPHP under different IAs and HIs. In order to better guide the applications of the EPHP in middle-low temperature field, a series of new experiments of the EPHP in middle-low temperature region (−90 to 0 °C) were implemented in this paper to investigate the influence of FRs and operating temperatures on thermal performance of the EPHP. Then the relationship of working fluids’ thermo-physical properties with the thermal performance of the EPHP was explored to find dominant thermo-physical property for the heat transfer under different working situations.

0.10 0.09

R(°C/W)

0.08 0.07 0.06

20W 30W 40W 50W 60W 70W operating temperature =-30°C IA=45°

0.05 0.04 0.03

2. Experimental system and procedure

0.02 20

25

30

35

40 45 FR(%)

50

55

60

65

2.1. Experimental system

(a) Operating temperature of -30 0.10 0.09

R(°C/W)

0.08 0.07 0.06

The proper inner diameter of capillary tube is the basis for the successful start-up and operation of PHP. The inner diameter of the capillary tube can be expressed from Eq. (1) [25]:

20W 30W 40W 50W 60W operating temperature=-50°C IA=45°

0.7

0.04 0.03 25

30

35

40 45 FR(%)

50

55

60

65

(b) Operating temperature of -50 0.32 0.28

R(°C/W)

0.24 0.20 0.16

5W 10W 15W 20W 25W operating temperature=-90°C IA=45°

0.12 0.08 0.04 0.00 20

25

30

35

40 45 FR(%)

50

55

60

l

v) g

< d < 1.8

(

l

v)g

(1)

where d is the inner diameter of capillary tube; is the surface tension of working fluid; l and v are the densities of working fluid at liquid phase and gas phase; g is the gravity. For ethane as the working fluid, when the PHP runs in the temperature region of 0 °C to −90 °C, the variation of the inner diameter with operating temperature was shown in Fig. 1. In this work, the inner diameter of the capillary tube was selected at 2 mm and the outer diameter was 3 mm. The capillary tube was made of copper and bent into a closed loop serpentine made of 20 parallel channels and 10 U-turns. 20 round holes of 2 mm were fabricated in the two cylindrical copper blocks, forming the evaporation section and condensation section. Therefore, the lengths of the evaporation section, condensation section, and adiabatic section were 150 mm, 150 mm, and 100 mm, respectively. Fig. 2 gives the schematic of the experimental system, which included an EPHP sample, a Stirling cooler, charging system, thermal insulation system, electric heating and a data acquisition device. The copper block of the condensation section was connected to the Stirling cooler, and contact grease was applied between them to reduce the thermal contact resistance. The electric heating film was tightly glued on the copper block of the evaporation section. Temperature measurements of condensation section and evaporation section were performed with ten standard platinum resistance thermometers (PT-100). Among the ten resistance thermometers, five were evenly glued at the copper block of the condensation section, with their average value representing the temperature of the condensation section. The other five were evenly glued at the copper block of the evaporation section, with their average value representing the temperature of the evaporation section. In addition, the ten resistance thermometers were on the same tube. Fig. 3 gives the layout of the temperature measuring points and dimensions of the EPHP. The temperature data were acquired by Agilent 34970A. In the experiment, the EPHP was placed in a box, and the inner wall of the box was affixed with fiberglass. To reduce the heat loss of the evaporation section and the condensation section, two cylindrical copper blocks were firstly wrapped using aluminum foil, then the insulated cotton was used to wrap the two copper blocks. Finally, the box was filled with the insulation material pearlite, so that the entire EPHP could be completely submerged.

0.05

0.02 20

(

65

(c) Operating temperature of -90 Fig. 5. Thermal resistance of the EPHP under different FRs and operating temperatures.

et al. [23] chose 35%, 51%, and 70% FRs of hydrogen to investigate the thermal performance of the PHP. It was found that the heat transfer performance of the PHP was the best when the FR was 35%, but the maximum heat transfer limit occurred when the FR was 70%. In summary, the researches of the PHP are mainly concentrated in norm temperature and cryogenic temperature versions. Nowadays, the applications of the PHP in middle-low temperature field such as food freezing and cell cooling have gradually increased [24], but the operating characteristics of the PHP in middle-low temperature region have been barely investigated so far. Our previous work [4] demonstrated

2.2. Data processing and uncertainty analysis In the experiment, the temperature of the condensation section fluctuated around a certain temperature value by adjusting the HI and the input power of the Stirling cooler, and this temperature value was 699

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0.070

0.060 0.055 0.050

0.060 0.055

0.045

0.035

0.035

0.030

0.030

0.025

0.025

0

10

20

30

40 Q(W)

50

60

0.020

70

0.055 0.050

30 40 Q(W)

50

60

30° 45° 60° 90° FR=30% operating temperature=-70°C

0.055 0.050

0.045 0.040

0.045 0.040 0.035 0.030

0.035

0.025

0.030

0.020

0.025

0.015 0

10

20

30

40 Q(W)

50

60

70

0.060 0.055 0.050

10

15

20 25 Q(W)

0.200

30

35

40

45

0° 30° 45° 60° 90° FR=30% operating temperature=-80°C

0.175 0.150

0.045 0.040

0.125 0.100 0.075

0.035

0.050

0.030

0.025

0.025 0.020

5

0.225

30° 45° 60° 90° FR=30% operating temperature=-40°C

0.065

0

(e) Operating temperature of -70

(b) Operating temperature of -20

0.070

R(°C/W)

0.020

20

0.060

R(°C/W)

0.060

10

0.065

30° 45° 60° 90° FR=30% operating temperature=-20°C

0.065

0

(d) Operating temperature of -60

0.070

R(°C/W)

0.045 0.040

(a) Operating temperature of -10

R(°C/W)

0.050

0.040

0.020

30° 45° 60° 90° FR=30% operating temperature=-60°C

0.065

R(°C/W)

0.065

R(°C/W)

0.070

30° 45° 60° 90° FR=30% operating temperature=-10°C

0.000

0

10

20

30 40 50 Q(W) (c) Operating temperature of -40

60

70

0

5

10

15 20 Q(W)

25

30

35

(f) Operating temperature of -80

Fig. 6. Thermal resistance of the EPHP in different operating temperatures.

regarded as the operating temperature of the EPHP. Namely, the operating temperature of the EPHP meant the temperature of the condensation section. The heat transfer thermal resistance was selected to evaluate the thermal performance of the EPHP. The heat transfer thermal resistance R (°C/W) could be obtained from the following equation:

R=

T T Tcon = eva Q Q

end of the EPHP.

(2)

Teva =

1 (T6 + T7 + T8 + T9 + T10) 5

(3)

Tcon =

1 (T1 + T2 + T3 + T4 + T5) 5

(4)

where T1–T10 are the average temperatures of platinum resistance thermometers at steady state (30-min average for positive angles case, 60-min average for horizontal case). The uncertainties in the experimental results were estimated by the following equations:

where Q is the HI added to evaporation section by the heater. Teva and Tcon are the averaging temperatures of evaporation section and condensation section, respectively, which can be calculated by the corresponding measuring points. ΔT is the temperature difference of the two 700

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Fig. 7. Temperature fluctuations of the EPHP in different operating temperatures.

Q = Q

(

R = R

(

dU 2 dI ) + ( )2 U I Teva 2 T Q ) + ( con )2 + ( )2 T T Q

of 5 W and a new equilibrium state would be reached. The same procedure was repeated for the different IAs, FRs and operating temperatures.

(5) (6)

3. Experimental results and discussion

The accuracies of the PT-100 resistance thermometers were calibrated to be ± 0.1 K. The voltage regulation accuracy of power supply for HI was ± 0.1 V. The range of voltage (U) and current (I) was 0–30 V and 0–10A, respectively. According to the equations, the maximum experimental uncertainty for thermal resistance was less than 8.6%.

3.1. The influence of FR on thermal performance of the EPHP The thermal performance of the EPHP at 45° IA with FRs of 22%, 30%, 40%, 50% and 60% was experimentally investigated in this work firstly. Fig. 5 gives the variation of the thermal resistance under different FRs and operating temperature. It can be clearly seen that the optimum FR corresponding to the lowest thermal resistance did not change with the HI, and remained at about 30%. Then, as the operating temperature of the EPHP declined from −30 °C to −90 °C, the optimum FR also maintained at about 30%. That is to say, for this EPHP the optimum FR was not affected by the operating temperature and HI, and was always 30%. Therefore, for the results and analysis performed later, the FR of the EPHP was selected at 30%. According to the researches [4,26], the following explanations may be applied to the results provided in this paper. It is well known that an optimal FR should exist for different work fluid at which the PHP has a minimum thermal resistance. For low FRs, the fluid approached to the vapor single phase. Therefore, there was very few amounts of liquid for sensible heat transfer. Moreover, the sufficient distinct plugs cannot be formed, which resulted in a tendency towards dry-out or partial multiple dry-

2.3. Experimental procedure The flowchart of experimental procedure was shown in Fig. 4. To begin with, the EPHP was purified with the vacuum pump, the ethane tank and three valves as shown in Fig. 2. Then a certain amount of the ethane was charged into the EPHP and the reservoir. After that, the Stirling cooler was started to cool down the EPHP to the operating temperature. When the EPHP had been cooled down to the operating temperature, the EPHP was adjusted to a certain IA, and the evaporation section was heated by the heater. By adjusting the HI of the electric heating film and the input power of the Stirling cooler, the temperature of the condensation section fluctuated around the operating temperature. After the desired steady-state was reached at the given HI, The temperatures of the evaporation section and the condensation section were measured and recorded. Then the HI was increased in increment 701

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560

450 400

540

Liquid density LHV

3.8 3.6

520

3.4

500

3.2

480

3.0

460

2.8

440

2.6

420

2.4

400

2.2

ρl ( kg/m 3 )

LHV (kJ/kg)

500

350 300 250

3.2. Operating characteristic of the EPHP under different operating temperatures and IAs

4.0

Liquid specific heat

The operating characteristic of the EPHP with FR of 30% at 0°, 30°, 45°, 60° and 90° IA was tested. The operating temperature ranged from 0 °C to −90 °C.Fig. 6 gives the thermal resistance variation under different operating temperatures and IAs. It can be clearly seen that in the temperature regions of 0 °C to −80 °C, the variation trends of the thermal resistance were very similar and displayed a typical “V” shape. The optimal IA corresponding to the lowest value was independent of the HI, and always was obtained at 30°. Moreover, as the operating temperature declined, the optimal IA kept independent, i.e. the optimum IA of EPHP was not significantly affected by the thermal-physics properties of the working fluid. The explanation for this phenomenon was as follows. The pressure fluctuations caused by the continuous generation and rupture of bubbles at the evaporator and the condenser, in conjunction with the uneven void fraction distribution in respective tubes, were the main driving force for heat transport. When the EPHP operated at certain IA, the gravity assisted in the return of the working fluid to the evaporation section, avoiding the dry-out the working fluid. According to the researches of Tong [30] and Qu [31], the oscillation of the working fluid in the PHP was similar to the spring-mass damping system. Based on this, we can speculate that the driving force and the restoring force were well balanced at 30° IA. However, it should be pointed out that in the temperature region of 0 °C to −70 °C, the EPHP could operate stably at the IAs of 30° to 90°, but cannot keep steady state at 0° IA. Fig. 7 gives the temperature fluctuations at the IA of 0° (horizontal mode) under different operating temperatures. As shown in Fig. 7(a), in the temperature of −50 °C, the temperature difference of the two ends gradually increased, i.e. there was no flow behavior in the tubes. It was because that the component of the gravity force on the flow direction of working fluid was zero. The EPHP lacked sufficient driving force for steady operation. Therefore, the flow fell into a state of stagnation, and cannot return to its normal state. When the operating temperature dropped to −70 °C, as shown in Fig. 7(b), the flow of working fluid would occasionally fall into stagnation state for a long time, then started to move again. Strangely, if the operating temperature declined to −80 °C and −90 °C, as shown in Fig. 7(c) and (d), the EPHP began to operate stably in horizontal mode. It can be determined that the thermo-physical properties of ethane were the main reason behind this phenomenon. There existed a suitable temperature region to obtain the best performance of the EPHP.

C pl ( kJ/ ( kg ° C ))

580

550

380 -110 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10

0

2.0 10

T(°C)

(a) Liquid specific heat, liquid density and LHV

σ ×10 3(N/m )

16 14 12 10 8

180 160

70

Dynamic viscosity Surface tension

60

(dp/dT)sat 50

140

40

120 30

100

6

80

4

60

2

40 -110 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 T(°C)

(b) Dynamic viscosity, surface tension and dp dT

(dp/dT)sat (kPa/°C )

18

200

nl ×10 6(Pa ·s)

20

20 10 0

0 10

sat

Fig. 8. Thermo-physical properties of ethane at different temperatures.

outs of the evaporation section. The operation of the EPHP was unstable and undesirable. For high FRs, the formation of bubbles tended to be restricted. Hence, the limited bubbles were not enough to provide the required perturbations to drive the liquid and overcome resistance, causing lower the performance of the EPHP. Bastakoti et al. [27] reported that the influence of FR on the thermal resistance of the PHP varied with other operating parameters like HI, IA and working fluid. It was a good explanation for that in the temperature range of −50 °C and −90 °C, when the FR was over 50%, the thermal resistance decreased with the further increase of the FR. The effect of operating temperature should be the main cause of this phenomenon. The influence of operating temperature on the PHP was mainly represented by the influence of the physical properties of the working fluid on the PHP. The physical properties of the working fluid, such as the surface tension, latent heat, specific heat, viscosity etc., have profound effects on the heat transfer performance of PHP. According to the researches of Rittidech [28] and Qu [29], the heat transfer in PHP was mainly carried out by sensible heat and the role of LHV was concentrated on the driving force, the circulation flow velocity and the oscillation of liquid slugs and vapor plugs. Therefore, although the increase of FR was not conducive to the oscillation and circulation flow velocity, it would significantly increase the amount of sensible heat transfer. As the operating temperature dropped, the thermos-physical properties of the working fluid showed different changes. Therefore, due to the combined influence of various factors, PHP exhibited different performance at −30 °C, −50 °C, −90 °C when the FR was higher than 50%. The influence of the physical properties of the working fluid on the heat transfer performance of the EPHP will be discussed in detail in Section 3.3.

3.3. Analysis of the influence of operating temperature on the thermal performance of the EPHP 3.3.1. The variations of thermos-physical parameters of ethane under different temperatures The temperature of the working fluid inside a working PHP affects certain temperature dependent properties, which affects the thermal performance of the PHP. For better analyzing and understanding, it is necessary to know the main thermo-physical properties of ethane that most likely affect the EPHPs’ heat transfer performance. Fig. 8 shows the variations of six thermos-physical parameters of ethane at different temperatures. These parameters include liquid density ( l ), liquid specific heat (CPl ), latent heat of vaporization (LHV), surface tension ( ), dynamic viscosity ( l ) and(dp / dT )sat , respectively. It can be seen that except for the liquid specific heat and (dp / dT )sat increased with the temperature increasing, the remaining four parameters gradually decreased with the temperature increasing. According to the conclusions of the earlier studies [29,32,33], the working fluids of PHP ought to have such properties as high (dp / dT )sat value, low dynamic viscosity, appropriate LHV and surface tension etc. Lower dynamic viscosity ( l ) results in lower shear forces near the wall surface, which helps to reduce the flow resistance and contributes to the smooth start-up of PHP. Higher LHV requires more energy for the 702

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Fig. 9. Variations of temperature difference and thermal resistance in different operating temperatures and IAs.

working fluid to realize phase change (vaporization or condensation) and generate bubbles for the start-up and oscillating operation [34]. However, it could lead to better latent heat transfer because more energy could be absorbed or released during phase change process, especially when the HI is relatively high. Higher (dp / dT )sat means the saturation pressure is more sensitive to temperature variation. Therefore, the working fluid can produce larger pressure fluctuation under small temperature variation, which results in the greater driving force for fluid flow. The successful operation of PHP lies in the vapor bubble

and liquid slug system formed inside the tube due to the surface tension. However, higher surface tension, together with dynamic contact angle hysteresis, may create additional pressure drop and increase the flow resistance [33]. Therefore, the working fluid of PHP ought to have suitable surface tension. The higher liquid specific heat leads to the better heat transfer capacity of working fluid because it can bring better sensible heat transfer.

703

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3.3.2. Results and discussions Fig. 9 gives the variations of temperature difference and thermal resistance under different operating temperatures at the IAs of 30°, 45° and 60°, respectively. If one of the temperature difference and the thermal resistance is known, the other one can be calculated. Therefore, one of them can be used to represent the heat transfer performance of the PHP. The following analysis was aimed at the thermal resistance. When the HI ranged from 10 W to 50 W, it can be clearly seen that the EPHP showed different heat transfer performance in different operating temperatures. The variation trend of thermal resistance under different IAs was somewhat similar. From Fig. 9, it can be concluded that there existed an optimal operating temperature of −80 °C corresponding to the lowest thermal resistance, which meant the EPHP had the best heat transfer capability in the temperature of −80 °C. It was noted that at relatively high HI (from 30 W to 50 W), when the temperature ranged from 0 °C to −80 °C, the thermal resistance curves generally showed a decreasing trend with the operating temperature decreasing. This phenomenon was especially obvious at the IA of 60°. As shown in Fig. 8, when the operating temperature changed from 0 °C to −80 °C, the values of dynamic viscosity and surface tension increased accordingly, the values of (dp / dT )sat and liquid specific heat declined gradually. It is well known that higher dynamic viscosity and surface tension would increase the flow resistance, so they are unfavorable properties for PHP’s operation. Still, the thermal resistance of the EPHP decreased with the temperature decreasing. Therefore, the dynamic viscosity and surface tension were not the critical properties affecting the heat transfer capacity of the EPHP. The higher (dp / dT )sat and liquid specific heat would cause better sensible heat transfer and greater driving force for fluid motion, so the decreasing of (dp / dT )sat and liquid specific heat is disadvantageous for the heat transfer of the PHP. Nevertheless, lower operating temperature led to better heat transfer performance of the EPHP, they may not be the dominant factors either. The liquid density was mainly reflected in the influence of gravity, i.e. the influence of IA on the EPHP’s operation. However, when the IA raised from 30° to 60°, the variation trends of the thermal resistance were not obvious (in Fig. 6), which meant the liquid density was also not the critical property. From Fig. 8(a), it can be observed that the value of LHV increased gradually as the temperature declined, so that more energy could be absorbed or released during phase change, which will enhance the heat transfer capability of the EPHP. Therefore, the LHV should be the dominant property that most likely determines the performance of the EPHP when the HI was relatively high. When the HI was relatively low (less than 20 W), the working fluid performed a slower flow inside the tubes, less bubbles were generated in the evaporation section. In this case, the role of LHV was weakened. The heat was mainly transferred as sensible heat, which indicated that the influence of liquid specific heat could not be neglected. Hence, the trend of thermal resistance curves were different from the case of relatively high HI. When the operating temperature dropped to −90 °C, the thermal resistance began to rise. For this phenomenon, the explanation was that, the various thermal properties of the working fluid achieved a good balance at this temperature. For this reason, we can boldly speculate that for a given structure of the PHP, the working fluid employed in tubes should have its proper temperature region to ensure that the PHP can work efficiently.

about 30%. Namely, the optimum FR was not affected by the operating temperature and HI. (2) In the middle-low temperature region(−90 to 0 °C), the EPHP achieved optimal performance at −80 °C. The optimal IA did not change with the operating temperature of the EPHP and remained at 30° IA. (3) In the temperature region of 0°C to −70°C, the EPHP cannot keep a steady state at 0° IA. When the operating temperature declined to −80°C and −90°C, the EPHP could operate steadily in horizontal mode, and the horizontal operation of the EPHP was more steady at −90 °C. (4) When the HI was relatively high, the LHV of ethane was the most important factor affecting the thermal performance of the EPHP. When the HI was low, the influence of the specific heat of the ethane liquid cannot be neglected. Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 50906054 and No. 51676129). References [1] H. Akachi, Structure of a heat pipe, US Patent 4921041, 1990. [2] Y. Zhang, A. Faghri, Advances and unsolved issues in pulsating heat pipes, Heat Transf. Eng. 29 (2008) 20–44. [3] R. Bruce, M. Barba, A. Bonelli, et al., Thermal performance of a meter-scale horizontal nitrogen Pulsating Heat Pipe, Cryogenics 93 (2018) 66–74. [4] X. Chen, S. Shao, J. Xiang, et al., Experimental investigation on ethane pulsating heat pipe based on Stirling cooler, Int. J. Refrig. 88 (2018) 506–515. [5] J. Qu, H. Wu, Q. Wang, Experimental investigation of silicon-based micro pulsating heat pipe for cooling electronics, Nanoscale Microscale Thermophys. Eng. 16 (2012) 37–49. [6] P. Meena, S. Rittidech, N. Poomsa-ad, Application of closed-loop oscillating heatpipe with check valves (CLOHP/CV) air-preheater for reduced relative-humidity in drying systems, Appl. Energy 84 (2007) 553–564. [7] L. Lu, Z. Liu, H. Xiao, Thermal performance of an open thermosyphon using nanofluids for high-temperature evacuated tubular solar collectors Part 1: indoor experiment, Sol. Energy 85 (2011) 379–387. [8] L.D. Fonseca, J. Pfotenhauer, F. Miller, Results of a three evaporator cryogenic helium pulsating heat pipe, Int. J. Heat Mass Transf. 120 (2018) 1275–1286. [9] D.A. Baitule, P.R. Pachghare, Experimental analysis of closed loop pulsating heat pipe with variable filling ratio, Int. J. Mech. Eng. Robot. Res. 2 (2013) 113–121. [10] H. Barua, M. Ali, M. Nuruzzaman, et al., Effect of filling ratio on heat transfer characteristics and performance of a closed loop pulsating heat pipe, Proc. Eng. 56 (2013) 88–95. [11] M.L. Rahman, T. Afrose, H.K. Tahmina, et al., Effect of using acetone and distilled water on the performance of open loop pulsating heat pipe (OLPHP) with different filling ratios, AIP Conf. Proc. 1754 (2016) 1–8. [12] M.L. Rahman, M. Chowdhury, N.A. Islam, et al., Effect of filling ratio and orientation on the thermal performance of closed loop pulsating heat pipe using ethanol, AIP Conf. Proc. 1754 (2016) 1–7. [13] Y. Wang, W.Y. Li, Experimental investigations on thermal performance of a multiturn closed loop pulsating heat pipe, Adv. Mater. Res. 433–440 (2012) 5854–5860. [14] P. Charoensawan, P. Terdtoon, Thermal performance of horizontal closed-loop oscillating heat pipes, Appl. Therm. Eng. 28 (2008) 460–466. [15] R.K. Sarangi, M.V. Rane, Experimental investigations for start up and maximum heat load of closed loop pulsating heat pipe, Procedia Eng. 51 (2013) 683–687. [16] B. Kim, L. Li, J. Kim, et al., A study on thermal performance of parallel connected pulsating heat pipe, Appl. Therm. Eng. 126 (2017) 1063–1068. [17] H. Xian, W. Xu, Y. Zhang, et al., Thermal characteristics and flow patterns of oscillating heat pipe with pulse heating, Int. J. Heat Mass Transf. 79 (2014) 332–341. [18] Z. Xue, W. Qu, Experimental study on effect of inclination angles to ammonia pulsating heat pipe, Chin. J. Aeronaut. 27 (2014) 1122–1127. [19] S.B. Paudel, G.J. Michna, Effect of inclination angle on pulsating heat pipe performance, 12th International Conference on Nanochannels, Microchannels and Minichannels, ASME Proceedings, Electronics Cooling, (2014). [20] M.L. Rahman, R.A. Sultan, T. Islam, N.M. Hasan, M. Ali, An experimental investigation on the effect of fin in the performance of closed loop pulsating heat pipe (CLPHP), Procedia Eng. 105 (2015) 137–144. [21] L. Fonseca, F. Miller, J. Pfotenhauer, Experimental heat transfer analysis of a cryogenic nitrogen pulsating heat pipe at various liquid fill ratios, Appl. Therm. Eng. 130 (2018) 343–353. [22] A.J. Jiao, H.B. Ma, J.K. Critser, Experimental investigation of cryogenic oscillating heat pipes, Int. J. Heat Mass Transf. 52 (2009) 3504–3509. [23] H.R. Deng, Y.M. Liu, R.F. Ma, et al., Experimental investigation on a pulsating heat pipe with hydrogen, IOP Conf. Ser.: Mater. Sci. Eng. 101 (2015) 012065. [24] M.A. Nazari, M.H. Ahmadi, R. Ghasempour, A review on pulsating heat pipes: from solar to cryogenic applications, Appl. Energy 222 (2018) 475–484.

4. Conclusions An experiment bench of PHP using ethane as working fluid was designed and built in this paper. The heat transfer characteristics of EPHP in the middle-low temperature region were experimentally investigated. The conclusions were as follows: (1) As the operating temperature of the EPHP declined from −30 °C to −90 °C, the optimum FR did not change with HI, and remained at 704

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X. Chen, et al. [25] Y.H. Lin, S.W. Kang, T.Y. Wu, Fabrication of polydimethylsiloxane (PDMS) pulsating heat pipe, Appl. Therm. Eng. 29 (2009) 573–580. [26] S. Khandekar, Thermo-hydrodynamics of closed loop pulsating heat pipes, 2004. [27] D. Bastakoti, H.N. Zhang, D. Li, et al., An overview on the developing trend of pulsating heat pipe and its performance, Appl. Therm. Eng. 141 (2018) 305–332. [28] S. Rittidech, P. Terdtoon, P. Tantakom, et al., Effect of inclination angles, evaporator section lengths and working fluid properties on heat transfer characteristics of a closed-end oscillating heat pipe, 6th International Heat Pipe Symposium, Chiang Mai, Thailand, (2000). [29] W. Qu, Contact angle hysteresis and capillary resistance of pulsating heat pipe, J. Eng. Thermophys. 24 (2003) 301–303. [30] B.Y. Tong, T.N. Wong, K.T. Ooi, Closed-loop pulsating heat pipe, Appl. Therm. Eng.

21 (2001) 1845–1862. [31] W. Qu, H.B. Ma, Theoretical analysis of starup of a pulsating heat pipe, Int. J. Heat Mass Transf. 50 (2007) 2309–2316. [32] M.B. Shafii, A. Faghri, Y. Zhang, Thermal modeling of unlooped and looped pulsating heat pipes, ASME J. Heat. Transf. 123 (2001) 1159–1172. [33] S. Khandekar, M. Schneider, P. Schafer, et al., Thermofluid dynamic study of flatplate closed-loop pulsating heat pipes, Microscale Thermophys. Eng. 6 (2002) 303–317. [34] H. Han, X. Cui, Y. Zhu, et al., A comparative study of the behavior of working fluids and their properties on the performance of pulsating heat pipes (PHP), Int. J. Therm. Sci. 82 (2014) 138–147.

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