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A study of the heat transfer performance of a pulsating heat pipe with ethanol-based mixtures
Applied Thermal Engineering 102 (2016) 1219–1227
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Research Paper
A study of the heat transfer performance of a pulsating heat pipe with ethanol-based mixtures Saiyan Shi a, Xiaoyu Cui a,⇑, Hua Han a, Jianhua Weng b, Zhihua Li a a b
Energy and Power Engineering College, University of Shanghai for Science & Technology, Shanghai 200093, China Energy and Machinery Engineering College, School of Energy and Power Engineering, Shanghai 200090, China
h i g h l i g h t s A certain working fluid added to ethanol can enhance the heat transfer of PHP. Ethanol-based working fluids behave differently at different filling ratios. Phase-change inhibition and mass transfer have opposite effects. The effect of phase-change depends on the degree of deviation from an ideal solution.
a r t i c l e
i n f o
Article history: Received 11 August 2015 Accepted 3 April 2016 Available online 18 April 2016 Keywords: Ethanol-based mixtures Pulsating heat pipe Thermal resistance Heat transfer performance
a b s t r a c t The heat transfer performances of a pulsating heat pipe (PHP) with ethanol–water, ethanol–methanol and ethanol–acetone are investigated experimentally. The mixing ratios (MRs) of the ethanol-based mixed working fluids are 2:1 and 4:1, the volume filling ratios (FRs) range from 45% to 90% and the heat input ranges from 10 W to 100 W. The experimental results are as follows: When the mixing ratio is 2:1, the heat transfer performance of PHP with ethanol–water is better than other working fluids at a filling ratio of 45% because of the phase-change inhibition in ethanol–water; at a filling ratio of 55%, PHP with ethanol–acetone shows better performance among those with mixed working fluids. Acetone with a relatively high value of (dp/dT)sat (saturation pressure gradient versus temperature) and relatively lower dynamic viscosity can lead to relatively high velocity in the PHP, which can decrease the temperature difference between the evaporation section and condensation section. When the mixing ratio is 4:1, the thermal resistance of PHP with ethanol–water that is close to being dried out under a filling ratio of 45% rises faster than that at a mixing ratio of 2:1 due to the presence of less deionized water (DI water); the heat transfer performance of PHP with ethanol–acetone is excellent at a filling ratio of 55% among the ethanol-based mixed working fluids because the thermal resistance is relatively small and the maximum heat input that PHP can endure is highest. When the volume filling ratio reaches 62%, 70% and 90%, the heat transfer performance of PHP with pure working fluids is better than that with ethanol-based mixed working fluids. This may be partially attributed to the offset of flow driving force caused by the mass transfer due to the concentration difference between the liquid and the vapour phase of mixtures. The filling ratio of 62% shows a marginal leading in terms of lower thermal resistance. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The pulsating heat pipe (oscillating heat pipe) was first presented by a Japanese researcher named Akachi [1,2] in the 1990s. As a new type of heat pipe, PHP showed great potential in the electronic cooler area. There are many factors that affect the heat transfer performance of PHPs, such as the orientation, filling ratio, ⇑ Corresponding author. E-mail address: [email protected] (X. Cui). http://dx.doi.org/10.1016/j.applthermaleng.2016.04.014 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.
heating patterns and working fluids, and so on, that have been studied actively in recent years. Mameli et al. [3] conducted an experimental investigation to study the effects of the orientation and filling ratio on PHP. It was found that PHP can work normally at an orientation ranging from 90° to 0° and an optimum filling ratio of 50%. Jiansheng et al. [4] experimentally investigated the effect of heating patterns on the heat transfer performance. It was found that a non-uniform heating pattern will increase the thermal resistance of PHPs but that it can also reduce the time of start-up. It is suggested that a non-uniform heating pattern could
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Nomenclature I U Q R Tc Te Ti
current (A) voltage (V) heat input (W) thermal resistance (°C/W) temperature of the condensation section (°C) temperature of the evaporation section (°C) temperature of the ith measuring point (°C)
be applied during start-up, while a uniform heating pattern could be applied during normal operation. The impact of the working fluid is becoming a hot research topic: several working fluids have been studied, including DI water [5], methanol [6], nanofluids [7–9], ethanol, and so on. Because of its suitable thermal–physical properties and because it is poisonless and cheap, many studies have been conducted regarding PHP with ethanol by scholars at home and abroad. Katpradit et al. [10] studied PHP with ethanol, R123 and water, respectively. The results showed that the critical heat flux increased with an increase in the latent heat of vaporization of the working fluid, but decreased when the section length increased. Charoensawan et al. [11] conducted an experiment regarding PHP with ethanol, water and R123 at a 50% filling ratio. The heat transfer performance of PHP charged with water is better than that of other working fluids under the same experimental conditions. Barua et al. [12] studied the heat transfer performance of PHP with ethanol and water at different filling ratios. PHP with water showed a better performance at a 35% filling ratio, while a suitable filling ratio for PHP with ethanol should be above 50%. Wang et al. [13] studied the heat transfer performance of PHP filled with ethanol, R141b and water at different diameters and found that water is the most promising working fluid because of its largest critical heat transfer rate and wide operating range. An experiment of the heat transfer performance of CLPHP charged with ethanol, water and FC72 was conducted by Zhang et al. [14]. It was concluded that the thermal oscillation amplitude might be due to surface tension, while the oscillation cycle period is possibly because the latent heat of evaporation, surface tension and latent heat of evaporation of ethanol are between water and FC72. Rittidech et al. [15] compared the heat transfer performance of an open-loop oscillating heat pipe with different working fluids, that is, ethanol, R123 and water. The result showed that a closed loop pulsating heat pipe with water performed better than ethanol in the experiment. Cui et al. [16] studied the operating mechanism of PHP charged with ethanol, water, methanol and acetone. The author showed that the heat transfer performance has nothing to do with the working fluid and filling ratio when the heat input is above 65 W. There is a limit of heat transfer performance that is probably related to the structure, material, size and inclination of the PHP. A similar experiment was carried out by Han et al. [17], who indicated that the thermal–physical properties of working fluids have a great impact on the heat transfer performance of PHP. However, because of the limit of the thermal–physical properties of pure working fluids, it is hard to improve the heat transfer performance of PHP filled with pure working fluids. Due to the difference of boiling points, mixed working fluids with temperature-slip and concentration-slip may contribute to special heat transfer performances of PHP that are different from pure working fluids. Nuntaphan et al. [18] studied thermosyphon heat pipe charged with a triethylene glycol (TEG)–water mixture. It was found that the use of a mixture can extend the heat transport limitation compared with pure water, showing a better heat transfer performance than that with pure TEG at high temperature
Subscript c e i max min
condensation section evaporation section index of thermocouples maximum minimum
applications. Zhu et al. [19] studied the heat transfer performance of PHP with water–acetone mixtures at different mixing ratios and found that mixtures can improve the start-up performance compared with pure water. It is predictable that the heat transfer performance of PHP may be different when charging with ethanol mixed with other working fluids. This paper studies the heat transfer performance of PHP with ethanol-based mixture working fluids compared to pure working fluids. 2. Experimental apparatus and data analysis 2.1. Experimental system The size of the channel has to be chosen not too large and not too small, as it will influence the working of the PHP. The best range for the channel diameter [20]:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r r 0:7 6 D 6 1:8 : ðql qv Þg ðql qv Þg where r, g, ql , and qv represent the surface tension, gravitational constant, density of liquid and density of vapour. For the pure fluids involved in our study, the ranges were calculated and listed in Table 1. Based on Table 1, the inner diameter of the sample PHP (Fig. 1) was selected at 2 mm and the outer diameter is 4 mm. The PHP has 9 bends, each with a semi-diameter of 10 mm, and has 10 parallel pipes in the vertical direction. The neighbouring pipes are 20 mm apart. The end of the pipe is connected to a valve that controls the vacuum and charges the liquid. From top to bottom, there is a condensation section, adiabatic section and evaporation section with corresponding lengths of 80 mm, 20 mm and 95 mm. Locations of the thermocouples are shown in Fig. 1(b). 20 thermocouples including 1 for the temperature of the outer wall of the dual-layer organic glass box and the rest for the temperature of fluid at different parts of the PHP are attached to the outer pipe walls (No. 1–6 for condensation section, No. 7–11 for evaporation section and No. 12–19 for pipes in-between). The experimental system is shown in Fig. 2. The heating device of the PHP is an electric heating wire with winding in the evaporation section. Both the evaporation section and the adiabatic section are put into a dual-layer organic glass box (vacuum in between) for thermal isolation and an aluminium foil-layer fixed on the inner wall to minimized thermal radiation loss, as shown in Fig. 3. The cooling device is a small axial flow fan and constant velocity (1.5 m/s) of forced air is measured by a hot-bulb anemometer. The condensation section, placed in the middle of the small air duct, is cooled by the flow fan. The data are displayed and recorded in real time by an Agilent via a LabVIEW program. The further information about experimental apparatus and instrumentation above is list in Table 2. The frequency for the collecting temperature (3 Hz) is larger than that for the pulsating temperature (0.333 Hz). For further information, see [19].
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S. Shi et al. / Applied Thermal Engineering 102 (2016) 1219–1227 Table 1 The range of diameter of PHP with different working fluids. Working fluids
Ethanol
Water
Methanol
Acetone
Range of diameter (mm)
1.09–2.85
1.75–4.85
1.13–2.94
1.15–3.02
2.2. Data reduction and error analysis The specific performance of a PHP is reflected by the pulsating temperature of each measuring point, while the heat transfer performance of a PHP is reflected by the mean temperature of the evaporation section and overall thermal resistance. The positions of the measuring points are shown in Fig. 1(b); the number of measuring points in the evaporation section ranges from 7 to 11 and in the condensation section from 1 to 6. Te (the mean temperature of the evaporation section) and Tc (the mean temperature of the condensation section) are determined after smooth running of the PHP:
Tc ¼
6 1X T ci 6 i¼1
ð1Þ
Te ¼
11 1X T ei 5 i¼7
ð2Þ
Fig. 2. Experiment system diagram of PHP.
The overall resistance of the PHP is determined by:
R¼
Te Tc Q
ð3Þ
Q is the heat load supplied by the heating wire. Because the heat leak of the heat preservation block is small, the heat absorbed in the evaporation section is equal to Q. Standard uncertainties can be expressed:
dQ ¼ Q
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 dU dI þ U I
dR ¼ R
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 dT e dT c dQ þ þ Q Te Tc Te Tc
Fig. 3. Insulation module.
ð4Þ
ð5Þ
The accuracy grade of the ammeter and voltmeter is 0.5. When the heating power is 10 W, 31.1 V is measured by the voltmeter, the range of which is 75 V, and 0.32 A is measured by the ammeter,
Fig. 1. The sample and temperature measuring point of PHP.
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Table 2 Experimental apparatus and instrumentation. Experimental apparatus and instrumentation
Type
Remarks
Heating wire Thermocouple
Ni–Cr T
Hot-bulb anemometer
QDF-3
Data acquisition
Agilent 34970A 34901A module
Specific resistance: 1.09 lX m Accuracy after calibration: ±0.1 °C response time: 0.3 s Range: 0.05–10 m/s measuring error: 65% Scan rate: 60 channels per second accuracy: 0.0256 °C (1 lV)
the range of which is 1 A. The accuracy of the thermocouple after calibration is 0.1 °C, and the accuracy of the instrument is 0.0256 °C. When the heating power is 10 W, dT e ¼ dT c ¼ dT ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 0:1 þ 0:0265 ¼ 0:103 °C and ðT e T c Þmin ¼ 12:6 °C (for pure water, FR = 62%). Then:
dQ ¼ Q
Fig. 4. Temperature/mole fraction (ethanol–water).
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 dU dI 0:5% 75 0:5% 1 ¼ þ þ U I 31:1 0:32
¼ 1:968% sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 dT e dT c dQ þ þ Q Te Tc Te Tc sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 0:103 þ 0:019682 ¼ 2:228% ¼ 2 12:6
dR ¼ R
Given a coverage factor (K) of 2 to cover those un-included elements, the expected maximum uncertainty (Umax) is
U max ¼
dR K ¼ 2:228% 2 4:5% R
Fig. 5. Temperature/mole fraction (ethanol–methanol).
3. Physical properties and phase diagrams of the working fluids 3.1. Physical properties of the working fluids The physical properties have a very important influence on the operation of the PHP. Previous research [21] showed that the working fluids applied in a self-excited PHP should have the following characteristics: high value of (dp/dT)sat, high latent heat, high specific heat, low surface tension, and low dynamic viscosity. The physical properties of the working fluids are shown in Table 3. All of ethanol’s properties are not optimal. DI water has a strong ability to resist dry-out and to carry energy because of its high specific heat and latent heat of vaporization; methanol and acetone, with relatively small dynamic viscosities, can lead to small motion resistance, while a relatively large value of (dp/dT)sat leads to better pulsation.
Fig. 6. Temperature/mole fraction (ethanol–acetone).
Table 3 Physical properties of different working fluids at standard atmospheric pressure [22]. Working fluids
Boiling point, TS °C
Liquid density, ql kg/m3 (20 °C)
LHV, Hfg kJ/kg
Liquid specific heat, Cpl kJ/(kg °C) (20 °C)
Dynamic viscosity, tl 103 Pa s (20 °C)
(dp/ dT)sat⁄ 103 Pa/°C (80 °C)
Thermal conductivity, kl W/(m °C) (20 °C)
Surface tension, r 103 N/m (20 °C)
Ethanol Deionized water Methanol Acetone
78.3 100.0
789 998
846 2257
2.39 4.18
1.15 1.01
4.23 1.92
0.172 0.599
22.8 72.8
64.7 56.2
791 792
1101 523
2.48 2.35
0.60 0.32
6.45 6.27
0.212 0.170
22.6 23.7
Note: (dp/dT)sat was calculated using the software Refprop.
S. Shi et al. / Applied Thermal Engineering 102 (2016) 1219–1227 Table 4 Volume fraction of the working fluid for different mixing ratios and the corresponding mole fraction ratios. Mixing ratio
Mixture
2:1 4:1
Ethanol–water
Ethanol–methanol
Ethanol–acetone
0.382 0.553
0.418 0.265
0.284 0.165
55% Test1
2.5
55% Test2 70% Test1
R/(ºC/W)
2
70% Test2 90% Test1
1.5
90% Test2
1 0.5 0
0
20
40
60
80
R/(ºC/W)
2.5
62% Test1
2
62% Test3
62% Test2
1.5 1 0.5
20
40
An experimental study on the heat transfer performance of PHP filled with ethanol-based mixtures, including ethanol–water, ethanol–methanol, and ethanol–acetone, is conducted in this paper, with mixing ratios of 2:1 and 4:1, volume filling ratios ranging from 45% to 90% and heat inputs ranging from 10 W to 100 W. It should be noted that when the heat input is less than 35 W, the slow motion and random distribution of the initial charging have great influence the results. Thus, the experimental results below 35 W are not analysed in this study because of the poor test repeatability as shown in Figs. 7 and 8, where: at each filling ratio, the PHP was first tested from low to high heating power step by step; the next test run started after it was cooled to the room temperature, to repeat the first test run. It can be observed that the results show better repeatability after 35 W. However, the overall trend can be used for reference. Besides, the error analysis has been taken into account when discussing the heat transfer performance of PHP and it will not have an influence on our conclusions.
4.1. Heat transfer performance of PHP at a mixing ratio of 2:1
Fig. 7. Comparison of two test runs charged with ethanol at different filling ratios.
0
4. Results and discussion
100
Q/W
0
1223
60
80
100
Q/W Fig. 8. Comparison of three test runs charged with ethanol (filling ratio of 62%).
3.2. Phase diagram analysis of the mixtures Phase diagrams of the ethanol-based mixtures, obtained from ASPEN PLUS 8.4, are shown in Figs. 4–6 (red1 line is the bubble line and blue line is the dew line), and the volume fractions of the mixtures for different mixing ratios and the corresponding mole fraction ratios are shown in Table 4. Ethanol–water is the maximum positive deviation solution, as it has the lowest constant boiling point and maximum phase range. When a phase-change occurs, the ethanol in the vapour phase is more than the water in the vapour phase; on the contrary, the DI water in the liquid phase is more than the ethanol in the liquid phase. The ethanol–methanol mixture can be regarded as an ideal mixture (a mixture that obeys Raoult’s Law) [23]; its phase-change range is very small. Ethanol–acetone is a positive-deviation solution; when a phase-change occurs, it leads to more acetone in the vapour phase and more ethanol in the liquid phase.
1 For interpretation of color in Figs. 4–6 and 15, the reader is referred to the web version of this article.
(1) Relatively small filling ratio (45%, 55%) For the heat transfer performance of PHP charged with pure working fluids (ethanol, water, methanol, acetone) can refer to [16,17]. This article mainly analyzes the heat transfer performance of PHP charged with mixtures. The overall thermal resistance of PHP charged with ethanol-based working fluids with volume mixing ratio of 2:1 and corresponding pure working fluids filled at 45% is shown in Fig. 9. The heat transfer performance of PHP charged with ethanol–water is excellent because the maximum heat input (heat input at an average temperature of 120 °C) is the highest at 65 W, while that of the others only arrives at 50 W. The thermal resistance of PHP charged with ethanol–water changes more gently and is always at a low level after 35 W, which indicates that the heat transfer performance of PHP charged with ethanol–water is better than that with pure ethanol and water at this filling ratio (45%). The reasons for this observation are as follows: when a phase-change occurs, ethanol transfers into the vapour phase more quickly than water and the pressure of the vapour phase is higher than the saturation pressure of DI water at the same temperature (shown in Fig. 4), which results in more water retention in the liquid phase and the relative restraint of the gasification of water. When the working fluids arrive at the condenser section, the opposite occurs, that is, water transfers into the liquid phase more quickly than ethanol and it is easier to store liquid water backflow in the evaporation section to wet the internal face. Thus, the thermal resistance of PHP charged with ethanol– water is relatively small (the thermal resistance gently increases in the dry-out condition, as shown in red circle). In conclusion, PHP charged with ethanol–water can operate at a relatively higher heat input and has better heat transfer performance. The thermal resistance of PHP charged with ethanol–acetone is higher than that of other mixtures at 35 W. The reasons are as follows: the energy carrying ability of ethanol–acetone is relatively small because of the minimum specific heat of acetone, which decreases sensible heat transfer and increases thermal resistance; however, the thermal resistance variation is gentler at a state of dry-out because of the phase change order in ethanol–acetone mixtures (shown in Fig. 6) and inhibition of ethanol gasification, as in ethanol–water. The variation trends of the thermal resistance of PHP charged with ethanol–methanol and pure ethanol are exactly similar, and the thermal resistance of PHP charged with the above two types of working fluids rises in parallel. The heat transfer performance of
S. Shi et al. / Applied Thermal Engineering 102 (2016) 1219–1227
ethanol/water 2:1 ethanol/methanol 2:1 ethanol/acetone 2:1 ethanol water methanol acetone
2.5
R/(ºC/W)
2 1.5 1
2 1.5 1 0.5
0.5 0
ethanol/water 2:1 ethanol/methanol 2:1 ethanol/acetone 2:1 ethanol water methanol acetone
2.5
R/(W/ºC)
1224
0 0
20
40
60
80
100
0
20
40
60
80
100
Q/W
Q/W
Fig. 10. Thermal resistance of PHP with a 55% filling ratio.
Fig. 9. Thermal resistance of PHP with a 45% filling ratio.
2.5
(2) Relatively high filling ratio (62%, 70%, 90%) The thermal resistances of PHPs charged with pure working fluids are generally below those with mixtures at a filling ratio of 62%, which can be observed in Fig. 11. The reason for this observation can be observed in combination with Fig. 12, which represents the temperature pulsating characteristics of PHP with ethanolbased mixtures (MR = 2:1) at a filling ratio of 62%. The increase in temperature fluctuations can be due to motion stops. If the working fluid stops or has a short stay in the evaporation section,
ethanol/water 2:1 ethanol/methanol 2:1 ethanol/acetone 2:1 ethanol water methanol acetone
2
R/(W/ºC)
PHP charged with ethanol–methanol is between pure ethanol and methanol and closer to ethanol because there is more ethanol in the ethanol–methanol mixture. The analysis is as below: the phase transition region of the ethanol–methanol mixture is very small (shown in Fig. 5), and the inhibition of the phase change does not have much effect; thus, ethanol mixed with methanol is not special. When the filling ratio increases to 55%, the thermal resistance curves of PHP charged with different working fluids are as shown in Fig. 10. The maximum heating power of the PHP increases compared with PHP at 45%. PHP charged with DI water does not appear to experience any dry-out and can be heated to 100 W. The thermal resistance is at a minimum after 35 W. The maximum heating power of PHP charged with ethanol–acetone increases to 80 W, which is higher than that with pure ethanol and acetone. The thermal resistance (before 65 W) of PHP charged with ethanol–acetone is much less than with ethanol–water. The reasons are as follows: acetone’s value of (dp/dT)sat is nearly 10 times larger than water, while the value of dynamic viscosity tl is only 1/3 of water, which can lead to small internal flow resistance when the PHP under normal operation; thus, the internal velocity is faster and temperature difference between the cold and hot ends decreases, and the phase transition is suppressed in the same way at a filling ratio of 55% as with 45%. The maximum heat input of ethanol–water PHP is higher than that with pure ethanol, but lower than with water. The reason is that the energy carrying ability is enhanced because of the relatively large specific heat and latent heat of vaporization of water, which is different from the 45% case. The thermal resistance of PHP with ethanol–methanol remains at the highest state, and the maximum heat input is only 65 W. The major reason for these observations might be that intermolecular interaction occurs in the mixtures: if there is more ethanol in the ethanol–methanol mixtures, it will form monomer and dimer structures [23]; hydrogen-bonding between the molecules of different components may occur in mixtures [24]. It is possible for such interaction to affect the thermo-physical properties or/and the motion state of the mixtures in PHP, which should be further researched.
1.5 1 0.5 0
0
20
40
60
80
100
Q/W Fig. 11. Thermal resistance of PHP with a 62% filling ratio.
the temperature there will rise due to persistent heating and that in the condensation section will decline correspondingly due to persistent cooling. The temperatures at the hot and cold ends have the following changes as the heating power increases: no oscillation, small frequency oscillation and large frequency oscillation. Definition of frequency is as follows: the number of temperature turning points, where the temperature experience decreasing after increasing, was defined as the number of pulsations. Higher frequency oscillation means more pulsations over a particular time period. A period of 200 s when the pulsation was relatively steady was selected to count the number of pulsations. Table 5 is shown as an example (at heating power of 50 W and filling ration of 62%). The larger the frequency of the pulsating temperature, the larger the velocity and the larger the frequency of the vapour–liquid slug scouring the internal wall, which can decrease the temperature difference between the hot and cold ends and the thermal resistance of the PHP. When the heating power is continuously increasing, the temperature with different working fluids rises by various degrees. PHP with ethanol–water starts oscillating at 20 W, and there is no oscillation before 20 W; PHP with ethanol– methanol has a small oscillation at 15 W and starts working normally at 20 W; PHP with ethanol–acetone starts pulsating at 10 W. The above phenomena are related to (dp/dT)sat and tl: acetone has a relatively high (dp/dT)sat and relatively low tl, which are benefits that help start the PHP. The thermal resistances of PHP (FR = 70%) with different working fluids approach each other as the heating power increases as shown in Fig. 13. The thermal resistances of PHPs charged with pure working fluids are generally below those of mixtures, which is similar to the 62% scenario. The analysis is as follows: the motion of the mixture in PHP is always accompanied by phase-change and the phase change of a zeotropic mixture is always accompanied by
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120 T8
100
T3
50W
15W
40
T3
80
35W
60
T8
100
20W
T/
T/
80
80W
65W
120
95W
10W 5W
50W
65W 80W 95W
20W
15W
10W
60
35W
5W
40
20 0
1200
2400
3600
4800
6000
20 0
7200
1800 3600 5400 7200 9000 10800 12600 14400 16200 18000
t/s
t/s
Ethanol-methanol
Ethanol-water 120 T8
100
T/
80
20W
60 40 20 0
65W
50W
T3
80W
35W
95W
15W
10W 5W
1200
2400
3600
4800
6000
7200
8400
t/s
Ethanol-acetone Fig. 12. Real-time temperature recordings with an increasing heating power (FR = 62%, ethanol-based mixtures, MR = 2:1).
Table 5 The pulsating times of PHPs with increasing heating power (FR = 62%). Ethanol–water
Ethanol–methanol
Ethanol–acetone
50 W
11waves
9 waves
6 waves
ethanol/water 2:1 ethanol/methanol 2:1 ethanol/acetone 2:1 ethanol water methanol acetone
R/(W/ºC)
2 1.5 1
2
R/(W/ºC)
FR = 62%, t = 200 s
2.5
ethanol/water 2:1 ethanol/methanol 2:1 ethanol/acetone 2:1 ethanol water methanol acetone
2.5
1.5 1 0.5 0
0
20
40
60
80
100
Q/W Fig. 14. Thermal resistance of PHP with a 90% filling ratio.
0.5 0
0
20
40
60
80
100
Q/W Fig. 13. Thermal resistance of PHP with a 70% filling ratio.
concentration shift, i.e. the concentration of each component in the liquid phase will differ from that in the vapour phase as previously described. Mass transfer caused by the concentration difference may offset part of the flow driving force, buffer the movement, retard the flow and hinder the heat transfer accordingly. As shown in Fig. 14, the thermal resistances of PHP with different working fluids (except ethanol–water) have little difference after 50 W. A high filling ratio is a restriction of vaporization; therefore, heat transfer relies more on sensible heat, and the influence of the phase-change decreases correspondingly. The thermal resistance of PHP with ethanol–water remains at the highest level, and the heat transfer performance is the worst. There are 3 main
and essential reasons for this: first, mass transfer is caused by the concentration difference; second, the dynamic viscosity of ethanol and water is larger than that of methanol and acetone; third: the density of water is larger than that of other fluids. All of the above reasons increase the flow resistance and cause the PHP to perform poorly. 4.2. Heat transfer performance of PHP at a mixing ratio of 4:1 (1) Relatively small filling ratio (45%, 55%) As shown in Fig. 15, the heat transfer performance of PHP charged with ethanol–water is outstanding because its heating power is larger than that with other working fluids, and the thermal resistance remains at a relatively low level. The reasons for these observations are the same as with the 2:1 mixing ratio at 45%. It is worth noting that the phenomenon of a rising thermal resistance is shown in the red circle. Compared with a mixing ratio of 2:1, thermal resistance obviously rises at 65 W (R = 0.654 °C/W
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2.5
ethanol/water 4:1 ethanol/methanol 4:1 ethanol/acetone 4:1 ethanol water methanol acetone
1.5 1 0.5 0
ethanol/methanol 4:1
0
20
40
60
80
100
R/(ºC/W)
acetone
1
0
ethanol/water 4:1 ethanol/methanol 4:1 ethanol/acetone 4:1 ethanol water methanol acetone
1.5
methanol
1.5
0.5
Fig. 15. Thermal resistance of PHP with a 45% filling ratio.
2
ethanol water
Q/W
2.5
ethanol/acetone 4:1
2
R/(ºC/W)
R/(ºC/W)
2
ethanol/water 4:1
2.5
0
20
40
60
100
Fig. 17. Thermal resistance of PHP with a 62% filling ratio.
ethanol/water 4:1
2.5
1
ethanol/methanol 4:1 ethanol/acetone 4:1
2
ethanol water
0
20
40
60
80
100
Q/W
R/(ºC/W)
0.5 0
80
Q/W
methanol
1.5
acetone
1
Fig. 16. Thermal resistance of PHP with a 55% filling ratio.
0.5
0
0
20
40
60
80
100
Q/W Fig. 18. Thermal resistance of PHP with a 70% filling ratio.
(2) Relatively high filling ratio (62%, 70%, 90%) The thermal resistance variation trend of PHP charged with a mixture with a mixing ratio of 4:1 is similar to that with a mixing ratio of 2:1 at a relatively high filling ratio. When the filling ratio is 62%, as shown in Fig. 17, the thermal resistance
ethanol/water 4:1 ethanol/methanol 4:1 ethanol/acetone 4:1 ethanol water methanol acetone
2.5 2
R/(ºC/W)
at 2:1, R = 0.710 °C/W at 4:1). The reason for this rise is that there is relatively less DI water in ethanol–water PHP at a mixing ratio of 4:1, which decreases the specific heat and latent heat of vaporization. The thermal resistance of PHP with ethanol–acetone is higher than that with pure acetone. It is shown that adding acetone to ethanol is not a benefit to the heat transfer performance of PHP. The thermal resistance of PHP with ethanol–methanol is between that with ethanol and that with methanol. As shown in Fig. 16, for mixtures, the best heat transfer performance of PHP with ethanol–acetone is at a filling ratio of 55% because the maximum heat power of PHP with ethanol– acetone is 80 W, which is larger than that with other working fluids (except water), and the thermal resistance of PHP with ethanol–acetone remains at a relatively low level. Combined with the graph of the thermal resistance of PHP with ethanol–acetone (MR = 2:1, FR = 55%), a conclusion can be drawn: adding acetone to ethanol can improve the heat transfer performance of PHP. The thermal resistance of PHP with ethanol–acetone rises more gently from 65 W to 80 W compared to the same heating power at a mixing ratio of 2:1. The reason for this rise is that there is less acetone in the mixture and the whole latent heat of vaporization increases. Although the heat transfer performance of PHP with water is excellent, adding water to ethanol cannot improve the heat transfer performance, and the thermal resistance of PHP with ethanol–water is relatively high compared with that with ethanol. The thermal resistance of PHP with an ethanol–methanol mixture is higher than that with pure ethanol and methanol, and the thermal resistance obviously rises at 50 W; thus, adding methanol to ethanol cannot improve the heat transfer performance of PHP.
1.5 1 0.5 0
0
20
40
60
80
100
Q/W Fig. 19. Thermal resistance of PHP with a 90% filling ratio.
S. Shi et al. / Applied Thermal Engineering 102 (2016) 1219–1227
of PHP with ethanol–acetone is slightly higher than that with other working fluids, and the thermal resistance shows a tendency to increase after 80 W. As shown in Fig. 18, when the filling ratio increases to 70%, the thermal resistances of the different working fluids approach each other as the heating power increases. Finally, the thermal resistances of PHP with mixed working fluids are higher than those with pure working fluids. When the filling ratio is 90%, as shown in Fig. 19, the thermal resistance of PHP with ethanol–water is higher than that with other working fluids and approaches the other values as the heating power increases.
5. Conclusion An experimental investigation is carried out to study the thermal resistance characteristics of PHP with ethanol-based working fluids at different filling ratios and mixing ratios. The results show that the heat transfer performance of PHP with a mixture is mainly related to the physical properties, phase-change characteristics and mass transfer resistance. The conclusions are as follows: 1. At a filling ratio of 45%, adding 50%v water to ethanol (ethanol– water mixture at 2:1) is favourable for the operation of PHP with the thermal resistance lower than that charged with pure ethanol or water; at a filling ratio of 55%, adding 25%v acetone to ethanol (ethanol–acetone mixture at 4:1) can promote the heat transfer performance of PHP. When the filling ratio increases to 62%, 70% and 90%, the heat transfer performance with pure fluids is better than that with mixtures and the filling ratio of 62% shows a marginal leading in terms of lower thermal resistance. 2. PHP with ethanol, when added to water, can delay dry-out and improve the heat transfer performance at a small filling ratio, while the opposite is true at a relatively high filling ratio; when added to acetone, it can improve the heat transfer performance only at a filling ratio of 55%; when added to methanol, it cannot improve the heat transfer performance. 3. When the filling ratio is relatively low (45%, 55%), the low boiling point component has a phase-change inhibition on the evaporation of the high boiling point component. As a result, dry-out can be delayed and the thermal resistance is decreased. When the filling ratio is relatively high (62%, 70%, 90%), the heat transfer performance of PHP with mixed working fluids is inferior to that with pure working fluids because the mass transfer caused by the concentration difference partly offsets the motion power, which affects the heat transfer performance.
Acknowledgements The authors would like to thank the National Natural Science Foundation of China (NSFC) for their financial support under Grant No. 51076104.
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References [1] H. Akachi, Structure of a Heat Pipe, US 4921041, 1990. [2] H. Akachi, F. Polasek, P. Stulc, Pulsating heat pipes, in: 5th International Heat Pipe Symposium, Melbourne, Australia, 1996, pp. 208–217. [3] Mauro Mameli, Vincenzo Manno, Sauro Filippeschi, Marco Marengo, Thermal instability of a closed loop pulsating heat pipe: combined effect of orientation and filling ratio, Exp. Therm. Fluid Sci. 59 (2014) 222–229. [4] Wang Jiansheng, Wang Zhenchuan, Li Meijun, Thermal performance of pulsating heat pipes with different heating patterns, Appl. Therm. Eng. 64 (2014) 209–212. [5] T. Nagasaki, T. Hokazono, Y. Ito, Heat transport characteristics of a pulsating heat pipe, in: ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference collocated with the ASME 2007 InterPACK Conference, American Society of Mechanical Engineers, 2007, pp. 763–770. [6] Kai-Shing Yang, Yu-Chi Cheng, Ming-Chung Liu, Jin-Cherng Shyu, Micro pulsating heat pipes with alternate microchannel widths, Appl. Therm. Eng. 83 (2015) 131–138. [7] R.R. Riehl, Performance Evaluation when using Nanofluids in Loop Heat Pipe and Pulsating Heat Pipe, SAE Technical Paper, 2007. [8] Yu-Hsing Lin, Shung-Wen Kang, Hui-Lun Chen, Effect of silver nano-fluid on pulsating heat pipe thermal performance, Appl. Therm. Eng. 28 (2008) 1312–1317. [9] Md. Riyad Tanshen, B. Munkhbayar, Md.J. Nine, Hanshik Chung, Hyomin Jeong, Effect of functionalized MWCNTs/water nanofluids on thermal resistance and pressure fluctuation characteristics in oscillating heat pipe, Int. Commun. Heat Mass Transfer 48 (2013) 93–98. [10] T. Katpradit, T. Wongratanaphisan, P. Terdtoon, P. Kamonpet, A. Polchai, Correlation to predict heat transfer characteristics of a closed end oscillating heat pipe at critical state, Appl. Therm. Eng. 25 (2005) 2138–2151. [11] Piyanun Charoensawan, Sameer Khandekar, Manfred Groll, et al., Closed loop pulsating heat pipes Part A: parametric experimental investigations, Appl. Therm. Eng. 23 (2003) 2009–2020. [12] Himel Barua, Mohammad Ali, Md. Nuruzzaman, Effect of filling ratio on heat transfer characteristics and performance of a closed loop pulsating heat pipe, Procedia Eng. 56 (2013) 88–95. [13] S. Wang, S. Nishio, Heat transport characteristics in closed loop oscillating heat pipes, in: 2005 ASME Summer Heat Transfer Conference July 17–22, 2005, San Francisco, California, USA, 2005. [14] X.M. Zhang, J.L. Xu, Z.Q. Zhou, Experimental study of a pulsating heat pipe using FC-72, ethanol, and water as working fluids, Exp. Heat Transfer 17 (2004) 47–67. [15] 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, in: 6th International Heat Pipe Symposium, ChiangMai, Thailand, 2000. [16] Xiaoyu Cui, Yue Zhu, Zhihua Li, Shende Shun, Combination study of operation characteristics and heat transfer mechanism for pulsating heat pipe, Appl. Therm. Eng. 65 (2014) 394–402. [17] Hua Han, Xiaoyu Cui, Yue Zhu, Shende Sun, 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. [18] A. Nuntaphan, J. Tiansuwan, T. Kiatsiriroat, Enhancement of heat transport in thermosyphon air preheater at high temperature with binary working fluid: a case study of TEG-water, Appl. Therm. Eng. 22 (3) (2002) 251–266. [19] Yue Zhu, Xiaoyu Cui, Hua Han, Shende Sun, The study on the difference of the start-up and heat-transfer performance of the pulsating heat pipe with water– acetone mixtures, Int. J. Heat Mass Transfer 77 (2014) 834–842. [20] Kuo-Hsiang Chien, Yur-Tsai Lin, Yi-Rong Chen, Kai-Shing Yang, Chi-Chuan Wang, A novel design of pulsating heat pipe with fewer turns applicable to all orientations, Int. J. Heat Mass Transfer 55 (2012) 5722–5728. [21] Sameer Khandekar, Nicolas Dollinger, Manfred Groll, Understanding operational regimes of closed loop pulsating heat pipes: an experimental study, Appl. Therm. Eng. 23 (2003) 707–719. [22] Fumin Shang, Dengying Liu, Haizhen Xian, Yongping Yang, Du Xiaoze, Flow and heat transfer characteristics of different forms of nanometer particles in oscillating heat pipe, J. Chem. Ind. Eng. China 58 (9) (2007) 2200. [23] B.G. Lone, P.B. Undre, S.S. Patil, P.W. Khirade, S.C. Mehrotra, Dielectric study of methanol–ethanol mixtures using TDR method, J. Mol. Liq. 141 (2008) 47–53. [24] I.M. Abdulagatov, F.Sh. Aliyev, M.A. Talibov, J.T. Safarov, A.N. Shahverdiyev, E.P. Hassel, High-pressure densities and derived volumetric properties (excess and partial molar volumes, vapor–pressures) of binary methanol + ethanol mixtures, Thermochim. Acta 476 (2008) 51–62.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
A study of the heat transfer performance of a pulsating heat pipe with ethanol-based mixtures Saiyan Shi a, Xiaoyu Cui a,⇑, Hua Han a, Jianhua Weng b, Zhihua Li a a b
Energy and Power Engineering College, University of Shanghai for Science & Technology, Shanghai 200093, China Energy and Machinery Engineering College, School of Energy and Power Engineering, Shanghai 200090, China
h i g h l i g h t s A certain working fluid added to ethanol can enhance the heat transfer of PHP. Ethanol-based working fluids behave differently at different filling ratios. Phase-change inhibition and mass transfer have opposite effects. The effect of phase-change depends on the degree of deviation from an ideal solution.
a r t i c l e
i n f o
Article history: Received 11 August 2015 Accepted 3 April 2016 Available online 18 April 2016 Keywords: Ethanol-based mixtures Pulsating heat pipe Thermal resistance Heat transfer performance
a b s t r a c t The heat transfer performances of a pulsating heat pipe (PHP) with ethanol–water, ethanol–methanol and ethanol–acetone are investigated experimentally. The mixing ratios (MRs) of the ethanol-based mixed working fluids are 2:1 and 4:1, the volume filling ratios (FRs) range from 45% to 90% and the heat input ranges from 10 W to 100 W. The experimental results are as follows: When the mixing ratio is 2:1, the heat transfer performance of PHP with ethanol–water is better than other working fluids at a filling ratio of 45% because of the phase-change inhibition in ethanol–water; at a filling ratio of 55%, PHP with ethanol–acetone shows better performance among those with mixed working fluids. Acetone with a relatively high value of (dp/dT)sat (saturation pressure gradient versus temperature) and relatively lower dynamic viscosity can lead to relatively high velocity in the PHP, which can decrease the temperature difference between the evaporation section and condensation section. When the mixing ratio is 4:1, the thermal resistance of PHP with ethanol–water that is close to being dried out under a filling ratio of 45% rises faster than that at a mixing ratio of 2:1 due to the presence of less deionized water (DI water); the heat transfer performance of PHP with ethanol–acetone is excellent at a filling ratio of 55% among the ethanol-based mixed working fluids because the thermal resistance is relatively small and the maximum heat input that PHP can endure is highest. When the volume filling ratio reaches 62%, 70% and 90%, the heat transfer performance of PHP with pure working fluids is better than that with ethanol-based mixed working fluids. This may be partially attributed to the offset of flow driving force caused by the mass transfer due to the concentration difference between the liquid and the vapour phase of mixtures. The filling ratio of 62% shows a marginal leading in terms of lower thermal resistance. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The pulsating heat pipe (oscillating heat pipe) was first presented by a Japanese researcher named Akachi [1,2] in the 1990s. As a new type of heat pipe, PHP showed great potential in the electronic cooler area. There are many factors that affect the heat transfer performance of PHPs, such as the orientation, filling ratio, ⇑ Corresponding author. E-mail address: [email protected] (X. Cui). http://dx.doi.org/10.1016/j.applthermaleng.2016.04.014 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.
heating patterns and working fluids, and so on, that have been studied actively in recent years. Mameli et al. [3] conducted an experimental investigation to study the effects of the orientation and filling ratio on PHP. It was found that PHP can work normally at an orientation ranging from 90° to 0° and an optimum filling ratio of 50%. Jiansheng et al. [4] experimentally investigated the effect of heating patterns on the heat transfer performance. It was found that a non-uniform heating pattern will increase the thermal resistance of PHPs but that it can also reduce the time of start-up. It is suggested that a non-uniform heating pattern could
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Nomenclature I U Q R Tc Te Ti
current (A) voltage (V) heat input (W) thermal resistance (°C/W) temperature of the condensation section (°C) temperature of the evaporation section (°C) temperature of the ith measuring point (°C)
be applied during start-up, while a uniform heating pattern could be applied during normal operation. The impact of the working fluid is becoming a hot research topic: several working fluids have been studied, including DI water [5], methanol [6], nanofluids [7–9], ethanol, and so on. Because of its suitable thermal–physical properties and because it is poisonless and cheap, many studies have been conducted regarding PHP with ethanol by scholars at home and abroad. Katpradit et al. [10] studied PHP with ethanol, R123 and water, respectively. The results showed that the critical heat flux increased with an increase in the latent heat of vaporization of the working fluid, but decreased when the section length increased. Charoensawan et al. [11] conducted an experiment regarding PHP with ethanol, water and R123 at a 50% filling ratio. The heat transfer performance of PHP charged with water is better than that of other working fluids under the same experimental conditions. Barua et al. [12] studied the heat transfer performance of PHP with ethanol and water at different filling ratios. PHP with water showed a better performance at a 35% filling ratio, while a suitable filling ratio for PHP with ethanol should be above 50%. Wang et al. [13] studied the heat transfer performance of PHP filled with ethanol, R141b and water at different diameters and found that water is the most promising working fluid because of its largest critical heat transfer rate and wide operating range. An experiment of the heat transfer performance of CLPHP charged with ethanol, water and FC72 was conducted by Zhang et al. [14]. It was concluded that the thermal oscillation amplitude might be due to surface tension, while the oscillation cycle period is possibly because the latent heat of evaporation, surface tension and latent heat of evaporation of ethanol are between water and FC72. Rittidech et al. [15] compared the heat transfer performance of an open-loop oscillating heat pipe with different working fluids, that is, ethanol, R123 and water. The result showed that a closed loop pulsating heat pipe with water performed better than ethanol in the experiment. Cui et al. [16] studied the operating mechanism of PHP charged with ethanol, water, methanol and acetone. The author showed that the heat transfer performance has nothing to do with the working fluid and filling ratio when the heat input is above 65 W. There is a limit of heat transfer performance that is probably related to the structure, material, size and inclination of the PHP. A similar experiment was carried out by Han et al. [17], who indicated that the thermal–physical properties of working fluids have a great impact on the heat transfer performance of PHP. However, because of the limit of the thermal–physical properties of pure working fluids, it is hard to improve the heat transfer performance of PHP filled with pure working fluids. Due to the difference of boiling points, mixed working fluids with temperature-slip and concentration-slip may contribute to special heat transfer performances of PHP that are different from pure working fluids. Nuntaphan et al. [18] studied thermosyphon heat pipe charged with a triethylene glycol (TEG)–water mixture. It was found that the use of a mixture can extend the heat transport limitation compared with pure water, showing a better heat transfer performance than that with pure TEG at high temperature
Subscript c e i max min
condensation section evaporation section index of thermocouples maximum minimum
applications. Zhu et al. [19] studied the heat transfer performance of PHP with water–acetone mixtures at different mixing ratios and found that mixtures can improve the start-up performance compared with pure water. It is predictable that the heat transfer performance of PHP may be different when charging with ethanol mixed with other working fluids. This paper studies the heat transfer performance of PHP with ethanol-based mixture working fluids compared to pure working fluids. 2. Experimental apparatus and data analysis 2.1. Experimental system The size of the channel has to be chosen not too large and not too small, as it will influence the working of the PHP. The best range for the channel diameter [20]:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r r 0:7 6 D 6 1:8 : ðql qv Þg ðql qv Þg where r, g, ql , and qv represent the surface tension, gravitational constant, density of liquid and density of vapour. For the pure fluids involved in our study, the ranges were calculated and listed in Table 1. Based on Table 1, the inner diameter of the sample PHP (Fig. 1) was selected at 2 mm and the outer diameter is 4 mm. The PHP has 9 bends, each with a semi-diameter of 10 mm, and has 10 parallel pipes in the vertical direction. The neighbouring pipes are 20 mm apart. The end of the pipe is connected to a valve that controls the vacuum and charges the liquid. From top to bottom, there is a condensation section, adiabatic section and evaporation section with corresponding lengths of 80 mm, 20 mm and 95 mm. Locations of the thermocouples are shown in Fig. 1(b). 20 thermocouples including 1 for the temperature of the outer wall of the dual-layer organic glass box and the rest for the temperature of fluid at different parts of the PHP are attached to the outer pipe walls (No. 1–6 for condensation section, No. 7–11 for evaporation section and No. 12–19 for pipes in-between). The experimental system is shown in Fig. 2. The heating device of the PHP is an electric heating wire with winding in the evaporation section. Both the evaporation section and the adiabatic section are put into a dual-layer organic glass box (vacuum in between) for thermal isolation and an aluminium foil-layer fixed on the inner wall to minimized thermal radiation loss, as shown in Fig. 3. The cooling device is a small axial flow fan and constant velocity (1.5 m/s) of forced air is measured by a hot-bulb anemometer. The condensation section, placed in the middle of the small air duct, is cooled by the flow fan. The data are displayed and recorded in real time by an Agilent via a LabVIEW program. The further information about experimental apparatus and instrumentation above is list in Table 2. The frequency for the collecting temperature (3 Hz) is larger than that for the pulsating temperature (0.333 Hz). For further information, see [19].
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S. Shi et al. / Applied Thermal Engineering 102 (2016) 1219–1227 Table 1 The range of diameter of PHP with different working fluids. Working fluids
Ethanol
Water
Methanol
Acetone
Range of diameter (mm)
1.09–2.85
1.75–4.85
1.13–2.94
1.15–3.02
2.2. Data reduction and error analysis The specific performance of a PHP is reflected by the pulsating temperature of each measuring point, while the heat transfer performance of a PHP is reflected by the mean temperature of the evaporation section and overall thermal resistance. The positions of the measuring points are shown in Fig. 1(b); the number of measuring points in the evaporation section ranges from 7 to 11 and in the condensation section from 1 to 6. Te (the mean temperature of the evaporation section) and Tc (the mean temperature of the condensation section) are determined after smooth running of the PHP:
Tc ¼
6 1X T ci 6 i¼1
ð1Þ
Te ¼
11 1X T ei 5 i¼7
ð2Þ
Fig. 2. Experiment system diagram of PHP.
The overall resistance of the PHP is determined by:
R¼
Te Tc Q
ð3Þ
Q is the heat load supplied by the heating wire. Because the heat leak of the heat preservation block is small, the heat absorbed in the evaporation section is equal to Q. Standard uncertainties can be expressed:
dQ ¼ Q
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 dU dI þ U I
dR ¼ R
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 dT e dT c dQ þ þ Q Te Tc Te Tc
Fig. 3. Insulation module.
ð4Þ
ð5Þ
The accuracy grade of the ammeter and voltmeter is 0.5. When the heating power is 10 W, 31.1 V is measured by the voltmeter, the range of which is 75 V, and 0.32 A is measured by the ammeter,
Fig. 1. The sample and temperature measuring point of PHP.
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Table 2 Experimental apparatus and instrumentation. Experimental apparatus and instrumentation
Type
Remarks
Heating wire Thermocouple
Ni–Cr T
Hot-bulb anemometer
QDF-3
Data acquisition
Agilent 34970A 34901A module
Specific resistance: 1.09 lX m Accuracy after calibration: ±0.1 °C response time: 0.3 s Range: 0.05–10 m/s measuring error: 65% Scan rate: 60 channels per second accuracy: 0.0256 °C (1 lV)
the range of which is 1 A. The accuracy of the thermocouple after calibration is 0.1 °C, and the accuracy of the instrument is 0.0256 °C. When the heating power is 10 W, dT e ¼ dT c ¼ dT ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 0:1 þ 0:0265 ¼ 0:103 °C and ðT e T c Þmin ¼ 12:6 °C (for pure water, FR = 62%). Then:
dQ ¼ Q
Fig. 4. Temperature/mole fraction (ethanol–water).
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 dU dI 0:5% 75 0:5% 1 ¼ þ þ U I 31:1 0:32
¼ 1:968% sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 dT e dT c dQ þ þ Q Te Tc Te Tc sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 0:103 þ 0:019682 ¼ 2:228% ¼ 2 12:6
dR ¼ R
Given a coverage factor (K) of 2 to cover those un-included elements, the expected maximum uncertainty (Umax) is
U max ¼
dR K ¼ 2:228% 2 4:5% R
Fig. 5. Temperature/mole fraction (ethanol–methanol).
3. Physical properties and phase diagrams of the working fluids 3.1. Physical properties of the working fluids The physical properties have a very important influence on the operation of the PHP. Previous research [21] showed that the working fluids applied in a self-excited PHP should have the following characteristics: high value of (dp/dT)sat, high latent heat, high specific heat, low surface tension, and low dynamic viscosity. The physical properties of the working fluids are shown in Table 3. All of ethanol’s properties are not optimal. DI water has a strong ability to resist dry-out and to carry energy because of its high specific heat and latent heat of vaporization; methanol and acetone, with relatively small dynamic viscosities, can lead to small motion resistance, while a relatively large value of (dp/dT)sat leads to better pulsation.
Fig. 6. Temperature/mole fraction (ethanol–acetone).
Table 3 Physical properties of different working fluids at standard atmospheric pressure [22]. Working fluids
Boiling point, TS °C
Liquid density, ql kg/m3 (20 °C)
LHV, Hfg kJ/kg
Liquid specific heat, Cpl kJ/(kg °C) (20 °C)
Dynamic viscosity, tl 103 Pa s (20 °C)
(dp/ dT)sat⁄ 103 Pa/°C (80 °C)
Thermal conductivity, kl W/(m °C) (20 °C)
Surface tension, r 103 N/m (20 °C)
Ethanol Deionized water Methanol Acetone
78.3 100.0
789 998
846 2257
2.39 4.18
1.15 1.01
4.23 1.92
0.172 0.599
22.8 72.8
64.7 56.2
791 792
1101 523
2.48 2.35
0.60 0.32
6.45 6.27
0.212 0.170
22.6 23.7
Note: (dp/dT)sat was calculated using the software Refprop.
S. Shi et al. / Applied Thermal Engineering 102 (2016) 1219–1227 Table 4 Volume fraction of the working fluid for different mixing ratios and the corresponding mole fraction ratios. Mixing ratio
Mixture
2:1 4:1
Ethanol–water
Ethanol–methanol
Ethanol–acetone
0.382 0.553
0.418 0.265
0.284 0.165
55% Test1
2.5
55% Test2 70% Test1
R/(ºC/W)
2
70% Test2 90% Test1
1.5
90% Test2
1 0.5 0
0
20
40
60
80
R/(ºC/W)
2.5
62% Test1
2
62% Test3
62% Test2
1.5 1 0.5
20
40
An experimental study on the heat transfer performance of PHP filled with ethanol-based mixtures, including ethanol–water, ethanol–methanol, and ethanol–acetone, is conducted in this paper, with mixing ratios of 2:1 and 4:1, volume filling ratios ranging from 45% to 90% and heat inputs ranging from 10 W to 100 W. It should be noted that when the heat input is less than 35 W, the slow motion and random distribution of the initial charging have great influence the results. Thus, the experimental results below 35 W are not analysed in this study because of the poor test repeatability as shown in Figs. 7 and 8, where: at each filling ratio, the PHP was first tested from low to high heating power step by step; the next test run started after it was cooled to the room temperature, to repeat the first test run. It can be observed that the results show better repeatability after 35 W. However, the overall trend can be used for reference. Besides, the error analysis has been taken into account when discussing the heat transfer performance of PHP and it will not have an influence on our conclusions.
4.1. Heat transfer performance of PHP at a mixing ratio of 2:1
Fig. 7. Comparison of two test runs charged with ethanol at different filling ratios.
0
4. Results and discussion
100
Q/W
0
1223
60
80
100
Q/W Fig. 8. Comparison of three test runs charged with ethanol (filling ratio of 62%).
3.2. Phase diagram analysis of the mixtures Phase diagrams of the ethanol-based mixtures, obtained from ASPEN PLUS 8.4, are shown in Figs. 4–6 (red1 line is the bubble line and blue line is the dew line), and the volume fractions of the mixtures for different mixing ratios and the corresponding mole fraction ratios are shown in Table 4. Ethanol–water is the maximum positive deviation solution, as it has the lowest constant boiling point and maximum phase range. When a phase-change occurs, the ethanol in the vapour phase is more than the water in the vapour phase; on the contrary, the DI water in the liquid phase is more than the ethanol in the liquid phase. The ethanol–methanol mixture can be regarded as an ideal mixture (a mixture that obeys Raoult’s Law) [23]; its phase-change range is very small. Ethanol–acetone is a positive-deviation solution; when a phase-change occurs, it leads to more acetone in the vapour phase and more ethanol in the liquid phase.
1 For interpretation of color in Figs. 4–6 and 15, the reader is referred to the web version of this article.
(1) Relatively small filling ratio (45%, 55%) For the heat transfer performance of PHP charged with pure working fluids (ethanol, water, methanol, acetone) can refer to [16,17]. This article mainly analyzes the heat transfer performance of PHP charged with mixtures. The overall thermal resistance of PHP charged with ethanol-based working fluids with volume mixing ratio of 2:1 and corresponding pure working fluids filled at 45% is shown in Fig. 9. The heat transfer performance of PHP charged with ethanol–water is excellent because the maximum heat input (heat input at an average temperature of 120 °C) is the highest at 65 W, while that of the others only arrives at 50 W. The thermal resistance of PHP charged with ethanol–water changes more gently and is always at a low level after 35 W, which indicates that the heat transfer performance of PHP charged with ethanol–water is better than that with pure ethanol and water at this filling ratio (45%). The reasons for this observation are as follows: when a phase-change occurs, ethanol transfers into the vapour phase more quickly than water and the pressure of the vapour phase is higher than the saturation pressure of DI water at the same temperature (shown in Fig. 4), which results in more water retention in the liquid phase and the relative restraint of the gasification of water. When the working fluids arrive at the condenser section, the opposite occurs, that is, water transfers into the liquid phase more quickly than ethanol and it is easier to store liquid water backflow in the evaporation section to wet the internal face. Thus, the thermal resistance of PHP charged with ethanol– water is relatively small (the thermal resistance gently increases in the dry-out condition, as shown in red circle). In conclusion, PHP charged with ethanol–water can operate at a relatively higher heat input and has better heat transfer performance. The thermal resistance of PHP charged with ethanol–acetone is higher than that of other mixtures at 35 W. The reasons are as follows: the energy carrying ability of ethanol–acetone is relatively small because of the minimum specific heat of acetone, which decreases sensible heat transfer and increases thermal resistance; however, the thermal resistance variation is gentler at a state of dry-out because of the phase change order in ethanol–acetone mixtures (shown in Fig. 6) and inhibition of ethanol gasification, as in ethanol–water. The variation trends of the thermal resistance of PHP charged with ethanol–methanol and pure ethanol are exactly similar, and the thermal resistance of PHP charged with the above two types of working fluids rises in parallel. The heat transfer performance of
S. Shi et al. / Applied Thermal Engineering 102 (2016) 1219–1227
ethanol/water 2:1 ethanol/methanol 2:1 ethanol/acetone 2:1 ethanol water methanol acetone
2.5
R/(ºC/W)
2 1.5 1
2 1.5 1 0.5
0.5 0
ethanol/water 2:1 ethanol/methanol 2:1 ethanol/acetone 2:1 ethanol water methanol acetone
2.5
R/(W/ºC)
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0 0
20
40
60
80
100
0
20
40
60
80
100
Q/W
Q/W
Fig. 10. Thermal resistance of PHP with a 55% filling ratio.
Fig. 9. Thermal resistance of PHP with a 45% filling ratio.
2.5
(2) Relatively high filling ratio (62%, 70%, 90%) The thermal resistances of PHPs charged with pure working fluids are generally below those with mixtures at a filling ratio of 62%, which can be observed in Fig. 11. The reason for this observation can be observed in combination with Fig. 12, which represents the temperature pulsating characteristics of PHP with ethanolbased mixtures (MR = 2:1) at a filling ratio of 62%. The increase in temperature fluctuations can be due to motion stops. If the working fluid stops or has a short stay in the evaporation section,
ethanol/water 2:1 ethanol/methanol 2:1 ethanol/acetone 2:1 ethanol water methanol acetone
2
R/(W/ºC)
PHP charged with ethanol–methanol is between pure ethanol and methanol and closer to ethanol because there is more ethanol in the ethanol–methanol mixture. The analysis is as below: the phase transition region of the ethanol–methanol mixture is very small (shown in Fig. 5), and the inhibition of the phase change does not have much effect; thus, ethanol mixed with methanol is not special. When the filling ratio increases to 55%, the thermal resistance curves of PHP charged with different working fluids are as shown in Fig. 10. The maximum heating power of the PHP increases compared with PHP at 45%. PHP charged with DI water does not appear to experience any dry-out and can be heated to 100 W. The thermal resistance is at a minimum after 35 W. The maximum heating power of PHP charged with ethanol–acetone increases to 80 W, which is higher than that with pure ethanol and acetone. The thermal resistance (before 65 W) of PHP charged with ethanol–acetone is much less than with ethanol–water. The reasons are as follows: acetone’s value of (dp/dT)sat is nearly 10 times larger than water, while the value of dynamic viscosity tl is only 1/3 of water, which can lead to small internal flow resistance when the PHP under normal operation; thus, the internal velocity is faster and temperature difference between the cold and hot ends decreases, and the phase transition is suppressed in the same way at a filling ratio of 55% as with 45%. The maximum heat input of ethanol–water PHP is higher than that with pure ethanol, but lower than with water. The reason is that the energy carrying ability is enhanced because of the relatively large specific heat and latent heat of vaporization of water, which is different from the 45% case. The thermal resistance of PHP with ethanol–methanol remains at the highest state, and the maximum heat input is only 65 W. The major reason for these observations might be that intermolecular interaction occurs in the mixtures: if there is more ethanol in the ethanol–methanol mixtures, it will form monomer and dimer structures [23]; hydrogen-bonding between the molecules of different components may occur in mixtures [24]. It is possible for such interaction to affect the thermo-physical properties or/and the motion state of the mixtures in PHP, which should be further researched.
1.5 1 0.5 0
0
20
40
60
80
100
Q/W Fig. 11. Thermal resistance of PHP with a 62% filling ratio.
the temperature there will rise due to persistent heating and that in the condensation section will decline correspondingly due to persistent cooling. The temperatures at the hot and cold ends have the following changes as the heating power increases: no oscillation, small frequency oscillation and large frequency oscillation. Definition of frequency is as follows: the number of temperature turning points, where the temperature experience decreasing after increasing, was defined as the number of pulsations. Higher frequency oscillation means more pulsations over a particular time period. A period of 200 s when the pulsation was relatively steady was selected to count the number of pulsations. Table 5 is shown as an example (at heating power of 50 W and filling ration of 62%). The larger the frequency of the pulsating temperature, the larger the velocity and the larger the frequency of the vapour–liquid slug scouring the internal wall, which can decrease the temperature difference between the hot and cold ends and the thermal resistance of the PHP. When the heating power is continuously increasing, the temperature with different working fluids rises by various degrees. PHP with ethanol–water starts oscillating at 20 W, and there is no oscillation before 20 W; PHP with ethanol– methanol has a small oscillation at 15 W and starts working normally at 20 W; PHP with ethanol–acetone starts pulsating at 10 W. The above phenomena are related to (dp/dT)sat and tl: acetone has a relatively high (dp/dT)sat and relatively low tl, which are benefits that help start the PHP. The thermal resistances of PHP (FR = 70%) with different working fluids approach each other as the heating power increases as shown in Fig. 13. The thermal resistances of PHPs charged with pure working fluids are generally below those of mixtures, which is similar to the 62% scenario. The analysis is as follows: the motion of the mixture in PHP is always accompanied by phase-change and the phase change of a zeotropic mixture is always accompanied by
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120 T8
100
T3
50W
15W
40
T3
80
35W
60
T8
100
20W
T/
T/
80
80W
65W
120
95W
10W 5W
50W
65W 80W 95W
20W
15W
10W
60
35W
5W
40
20 0
1200
2400
3600
4800
6000
20 0
7200
1800 3600 5400 7200 9000 10800 12600 14400 16200 18000
t/s
t/s
Ethanol-methanol
Ethanol-water 120 T8
100
T/
80
20W
60 40 20 0
65W
50W
T3
80W
35W
95W
15W
10W 5W
1200
2400
3600
4800
6000
7200
8400
t/s
Ethanol-acetone Fig. 12. Real-time temperature recordings with an increasing heating power (FR = 62%, ethanol-based mixtures, MR = 2:1).
Table 5 The pulsating times of PHPs with increasing heating power (FR = 62%). Ethanol–water
Ethanol–methanol
Ethanol–acetone
50 W
11waves
9 waves
6 waves
ethanol/water 2:1 ethanol/methanol 2:1 ethanol/acetone 2:1 ethanol water methanol acetone
R/(W/ºC)
2 1.5 1
2
R/(W/ºC)
FR = 62%, t = 200 s
2.5
ethanol/water 2:1 ethanol/methanol 2:1 ethanol/acetone 2:1 ethanol water methanol acetone
2.5
1.5 1 0.5 0
0
20
40
60
80
100
Q/W Fig. 14. Thermal resistance of PHP with a 90% filling ratio.
0.5 0
0
20
40
60
80
100
Q/W Fig. 13. Thermal resistance of PHP with a 70% filling ratio.
concentration shift, i.e. the concentration of each component in the liquid phase will differ from that in the vapour phase as previously described. Mass transfer caused by the concentration difference may offset part of the flow driving force, buffer the movement, retard the flow and hinder the heat transfer accordingly. As shown in Fig. 14, the thermal resistances of PHP with different working fluids (except ethanol–water) have little difference after 50 W. A high filling ratio is a restriction of vaporization; therefore, heat transfer relies more on sensible heat, and the influence of the phase-change decreases correspondingly. The thermal resistance of PHP with ethanol–water remains at the highest level, and the heat transfer performance is the worst. There are 3 main
and essential reasons for this: first, mass transfer is caused by the concentration difference; second, the dynamic viscosity of ethanol and water is larger than that of methanol and acetone; third: the density of water is larger than that of other fluids. All of the above reasons increase the flow resistance and cause the PHP to perform poorly. 4.2. Heat transfer performance of PHP at a mixing ratio of 4:1 (1) Relatively small filling ratio (45%, 55%) As shown in Fig. 15, the heat transfer performance of PHP charged with ethanol–water is outstanding because its heating power is larger than that with other working fluids, and the thermal resistance remains at a relatively low level. The reasons for these observations are the same as with the 2:1 mixing ratio at 45%. It is worth noting that the phenomenon of a rising thermal resistance is shown in the red circle. Compared with a mixing ratio of 2:1, thermal resistance obviously rises at 65 W (R = 0.654 °C/W
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2.5
ethanol/water 4:1 ethanol/methanol 4:1 ethanol/acetone 4:1 ethanol water methanol acetone
1.5 1 0.5 0
ethanol/methanol 4:1
0
20
40
60
80
100
R/(ºC/W)
acetone
1
0
ethanol/water 4:1 ethanol/methanol 4:1 ethanol/acetone 4:1 ethanol water methanol acetone
1.5
methanol
1.5
0.5
Fig. 15. Thermal resistance of PHP with a 45% filling ratio.
2
ethanol water
Q/W
2.5
ethanol/acetone 4:1
2
R/(ºC/W)
R/(ºC/W)
2
ethanol/water 4:1
2.5
0
20
40
60
100
Fig. 17. Thermal resistance of PHP with a 62% filling ratio.
ethanol/water 4:1
2.5
1
ethanol/methanol 4:1 ethanol/acetone 4:1
2
ethanol water
0
20
40
60
80
100
Q/W
R/(ºC/W)
0.5 0
80
Q/W
methanol
1.5
acetone
1
Fig. 16. Thermal resistance of PHP with a 55% filling ratio.
0.5
0
0
20
40
60
80
100
Q/W Fig. 18. Thermal resistance of PHP with a 70% filling ratio.
(2) Relatively high filling ratio (62%, 70%, 90%) The thermal resistance variation trend of PHP charged with a mixture with a mixing ratio of 4:1 is similar to that with a mixing ratio of 2:1 at a relatively high filling ratio. When the filling ratio is 62%, as shown in Fig. 17, the thermal resistance
ethanol/water 4:1 ethanol/methanol 4:1 ethanol/acetone 4:1 ethanol water methanol acetone
2.5 2
R/(ºC/W)
at 2:1, R = 0.710 °C/W at 4:1). The reason for this rise is that there is relatively less DI water in ethanol–water PHP at a mixing ratio of 4:1, which decreases the specific heat and latent heat of vaporization. The thermal resistance of PHP with ethanol–acetone is higher than that with pure acetone. It is shown that adding acetone to ethanol is not a benefit to the heat transfer performance of PHP. The thermal resistance of PHP with ethanol–methanol is between that with ethanol and that with methanol. As shown in Fig. 16, for mixtures, the best heat transfer performance of PHP with ethanol–acetone is at a filling ratio of 55% because the maximum heat power of PHP with ethanol– acetone is 80 W, which is larger than that with other working fluids (except water), and the thermal resistance of PHP with ethanol–acetone remains at a relatively low level. Combined with the graph of the thermal resistance of PHP with ethanol–acetone (MR = 2:1, FR = 55%), a conclusion can be drawn: adding acetone to ethanol can improve the heat transfer performance of PHP. The thermal resistance of PHP with ethanol–acetone rises more gently from 65 W to 80 W compared to the same heating power at a mixing ratio of 2:1. The reason for this rise is that there is less acetone in the mixture and the whole latent heat of vaporization increases. Although the heat transfer performance of PHP with water is excellent, adding water to ethanol cannot improve the heat transfer performance, and the thermal resistance of PHP with ethanol–water is relatively high compared with that with ethanol. The thermal resistance of PHP with an ethanol–methanol mixture is higher than that with pure ethanol and methanol, and the thermal resistance obviously rises at 50 W; thus, adding methanol to ethanol cannot improve the heat transfer performance of PHP.
1.5 1 0.5 0
0
20
40
60
80
100
Q/W Fig. 19. Thermal resistance of PHP with a 90% filling ratio.
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of PHP with ethanol–acetone is slightly higher than that with other working fluids, and the thermal resistance shows a tendency to increase after 80 W. As shown in Fig. 18, when the filling ratio increases to 70%, the thermal resistances of the different working fluids approach each other as the heating power increases. Finally, the thermal resistances of PHP with mixed working fluids are higher than those with pure working fluids. When the filling ratio is 90%, as shown in Fig. 19, the thermal resistance of PHP with ethanol–water is higher than that with other working fluids and approaches the other values as the heating power increases.
5. Conclusion An experimental investigation is carried out to study the thermal resistance characteristics of PHP with ethanol-based working fluids at different filling ratios and mixing ratios. The results show that the heat transfer performance of PHP with a mixture is mainly related to the physical properties, phase-change characteristics and mass transfer resistance. The conclusions are as follows: 1. At a filling ratio of 45%, adding 50%v water to ethanol (ethanol– water mixture at 2:1) is favourable for the operation of PHP with the thermal resistance lower than that charged with pure ethanol or water; at a filling ratio of 55%, adding 25%v acetone to ethanol (ethanol–acetone mixture at 4:1) can promote the heat transfer performance of PHP. When the filling ratio increases to 62%, 70% and 90%, the heat transfer performance with pure fluids is better than that with mixtures and the filling ratio of 62% shows a marginal leading in terms of lower thermal resistance. 2. PHP with ethanol, when added to water, can delay dry-out and improve the heat transfer performance at a small filling ratio, while the opposite is true at a relatively high filling ratio; when added to acetone, it can improve the heat transfer performance only at a filling ratio of 55%; when added to methanol, it cannot improve the heat transfer performance. 3. When the filling ratio is relatively low (45%, 55%), the low boiling point component has a phase-change inhibition on the evaporation of the high boiling point component. As a result, dry-out can be delayed and the thermal resistance is decreased. When the filling ratio is relatively high (62%, 70%, 90%), the heat transfer performance of PHP with mixed working fluids is inferior to that with pure working fluids because the mass transfer caused by the concentration difference partly offsets the motion power, which affects the heat transfer performance.
Acknowledgements The authors would like to thank the National Natural Science Foundation of China (NSFC) for their financial support under Grant No. 51076104.
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