- Email: [email protected]
Experimental study on effect of inclination angles to ammonia pulsating heat pipe
Chinese Journal of Aeronautics, (2014),27(5): 1122–1127
Chinese Society of Aeronautics and Astronautics & Beihang University
Chinese Journal of Aeronautics [email protected] www.sciencedirect.com
Experimental study on effect of inclination angles to ammonia pulsating heat pipe Xue Zhihu, Qu Wei
*
China Academy of Aerospace Aerodynamics (CAAA), Beijing 100074, China Received 30 October 2013; revised 25 March 2014; accepted 4 May 2014 Available online 19 August 2014
KEYWORDS Ammonia; Inclination angles; Pulsating heat pipe; Thermal performance; Thermal resistance
Abstract In this paper, a novel study on performance of closed loop pulsating heat pipe (CLPHP) using ammonia as working fluid is experimented. The tested CLPHP, consisting of six turns, is fully made of quartz glass tubes with 6 mm outer diameter and 2 mm inner diameter. The filling ratio is 50%. The visualization investigation is conducted to observe the oscillation and circulation flow in the CLPHP. In order to investigate the effects of inclination angles to thermal performance in the ammonia CLPHP, four case tests are studied. The trends of temperature fluctuation and thermal resistance as the input power increases at different inclination angles are highlighted. The results show that it is very easy to start up and circulate for the ammonia CLPHP at an inclining angle. The thermal resistance is low to 0.02 K/W, presenting that heat fluxes can be transferred from heating section to cooling section very quickly. It is found that the thermal resistance decreases as the inclination angle increases. At the horizontal operation, the ammonia CLPHP can be easy to start up at low input power, but hard to circulate. In this case, once the input power is high, the capillary tube in heating section will be burnt out, leading to worse thermal performance with high thermal resistance. ª 2014 Production and hosting by Elsevier Ltd. on behalf of CSAA & BUAA.
1. Introduction The closed loop pulsating heat pipe (CLPHP), is considered as one of the promising technologies for a high heat transfer device on small space. It has been widely utilized in electronics * Corresponding author. Tel.: +86 10 68376749. E-mail addresses: [email protected] (Z. Xue), weiqucaaa@ hotmail.com (W. Qu). Peer review under responsibility of Editorial Committee of CJA.
Production and hosting by Elsevier
and computer components due to its potential heat transport capability, simple structure, compact sizes and low cost of manufacturing.1–4 Although simple in its construction, CLPHP seems very complicated when we try to understand its operation for phase change and interactions between the vapor plugs and liquid slugs. The CLPHP has been studied in experiment by many researches,5–10 focusing on either visual observation of flow patterns in CLPHP or various operation parameters related to thermo-hydrodynamics of CLPHP. Gi et al.11 performed the flow visualization of an R142b CLPHP with an 8 mm camera to record the flow, but little information of detailed description of flow pattern was observed. Tong et al.12 firstly visualized the flow patterns of a glass CLPHP filled 60% with methanol. It was found that
http://dx.doi.org/10.1016/j.cja.2014.08.004 1000-9361 ª 2014 Production and hosting by Elsevier Ltd. on behalf of CSAA & BUAA.
Experimental study on effect of inclination angles to ammonia pulsating heat pipe
Fig. 1
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Schematic of full visualization setup.
large amplitude oscillations from evaporator to the condenser during the startup period; however, in steady operating state, the working fluid circulated, coupled with complex processes such as nucleation boiling and coalescence of bubbles. The direction of circulation was consistent once circulation was attained, but it could be different for the same experimental run. Qu and Ma13 also conducted an experimental investigation on a glass CLPHP with water, and two kinds of vapor bubbles, i.e., small round (Globe) bubbles and long column (Taylor) bubbles, were formed and circulated in the whole CLPHP. It can be concluded that the startup of CLPHP and steady circulation were primarily due to the boiling heat transfer associating with the temperature difference and pressure difference between heating section and cooling section. Charoensawan et al.14 studied the parameter effect on thermal performance of the copper CLPHP, including the variation of internal diameter, number of turns, working fluid and inclination angle. The working fluids employed were water, ethanol and R-123. The results indicated a strong influence of gravity on the performance that the highest heat flux of CLPHP in horizontal mode decreased obviously compared to vertical mode if the turns were less than a certain number. Besides, thermo-physical properties of working fluids affected the performance strongly. Khandekar et al.15 indicated that the operating mechanism of CLPHP was not well understood and the present state of the art could not predict required design parameters for a given task. And he summarized the choice of working fluids which had high value of (dp/dT)sat, low dynamic viscosity, low latent heat and low surface tension. It is found that the working fluids of CLPHP employed as mentioned earlier usually are usually water, ethanol, methanol, acetone, R-134a,16 etc. But for ammonia, there is rarely used, especially in the whole glass CLPHP. Nevertheless, ammonia has much more advantages than these working fluids. It has much higher value of (dp/dT)sat, much lower superheat required,17 little lower dynamic viscosity and surface tension than those fluids, and much lower latent heat than water. So we infer that ammonia could be regarded as one kind of excellent fluid for CLPHP.
In our recent work, a novel study on full visualization and startup performance of glass CLPHP using ammonia as working fluid was experimented.18 It was found that the ammonia CLPHP was quite easier to startup than other working fluids, even under the conditions that the temperature of evaporator is only 4 K higher than condenser. This paper will observe the thermal performance of the ammonia CLPHP at different inclination angles. Firstly, the experimental setup and procedures are introduced. Then, the temperature fluctuations of evaporator and condenser are presented, and operation characteristics at different inclination angles are detailed. Lastly, the thermal resistance is analyzed, on behalf of the performance of ammonia CLPHP. 2. Experiment introduction 2.1. Experimental setup The schematic of the tested CLPHP constituting six meandering turns is shown in Fig. 1. The prototype is fully made of quartz glass capillary tubes with the total length of 320 mm from the top to the bottom. The inner and outer diameters of the glass tube are 2 mm and 6 mm, respectively. The evaporator section of the CLPHP is heated by the 0.2 mm electrical wires of 4.86 X/m resistance. The heat wires are wrapped at intervals of 1.5 mm on the outer wall surface of the CLPHP. The input heat power can be adjusted by step of 40 W. The total length of heating section shown as Le in Fig. 1 is 100 mm. In order to prevent heat radiation losses from heat wire, there will cover some insulation materials in the evaporator. The condenser part of the CLPHP is cooled by cooling water in cooler box which is circulated by a cold bath appliance. The inlet temperature of cooling water is maintained at 28 C with ±2 C accuracy. The length of cooling section named as Lc is 100 mm. The length of adiabatic section named as La is 120 mm. The motions of vapor bubbles and liquid slugs are photographed and recorded by a high speed CCD (Charge Coupled Device) camera. There are eight thermocouples (T1 to T8) located as shown in Fig. 1 to
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Z. Xue, W. Qu Table 1
Fig. 2
Sketch mapping of experimental platform.
measure the temperature fluctuations of the evaporator, adiabatic and condenser sections of the CLPHP and the inlet and outlet of cooler box. Four thermocouples are located in different tubes to give an average value of evaporator temperature. Two thermocouples are placed in the corresponding tubes to provide an average temperature of condenser. All temperature data are recorded by the highly sensitive temperature logger (Agilent 34970A with resolution 0.1 C, accuracy ±1 C) and connected to a PC for scanning the data every one second. The whole CLPHP system including glass ammonia CLPHP, heat wires, cooler box, and CCD recorder is fixed on one 500 mm · 600 mm aluminum plate. When the experiment is conducted, as one of the important factors, operation orientation angles of CLPHP should be taken into consideration. Therefore, a rotating component is used in Fig. 2, and an angle controller bolt is coupled to realize the variation of angle from 90 to +90 by step 30. 2.2. Experimental procedure The experimental procedure is as follows: Step 1: Clean the glass CLPHP using the de-ionized water injected by a micro pump. Other components are cleaned with some alcohol using ultrasonic waves. Cleaning is absolutely an indispensable process to remove any surface impurities, if not removed, it could disturb the pattern in later stages of the fabrication process. Step 2: Put the glass CLPHP in a constant temperature oven for the dehydration bake about 20 min at 120 C to ensure that the water or alcohol dries out completely. Step 3: For CLPHP filling, it is the most difficult and suffering thing. Since the saturation pressure of ammonia at 30 C reaches in 12 · 105 Pa, clearly the pressure is too high to have sealing easily, especially for the glass tube with smooth outer surface. However, these difficulties are solved well, and the vacuum of pulsating heat pipe (PHP) can reach 104 Pa. The ammonia CLPHP is charged with 50%. Step 4: Install the cooler box and get the condenser of CLPHP entry into the box with good sealing.
Design of experiment program.
Case
Inclination angle ()
Input power (W)
I II III IV
90 60 30 0
40 40 40 40
80 80 80 80
120 120 120 120
160 160 160 160
200 200 200 200
240 240 240
280 280 280
Step 5: Wrap the capillary with heat wires, and make the heating section garnished with a layer of heat insulations. Step 6: Fix the thermocouples in suitable positions. Set up the digital video camera, to record the operations and flow patterns of working fluid. Step 7: Switch the power supply to 40 W, simultaneously, turn on the scanning software to collect the temperature. After the temperature curve is balanced, adjust the heating power to 80 W (Plus 40 W per step). The experiment program is designed as illustrated in Table 1. The overall thermal resistance is defined in the following formula: R¼
Te Tc Q
ð1Þ
where R, Te, Tc, Q are the average thermal resistance, average temperature of the evaporator sector, average temperature of the condenser sector and heat power load of the ammonia CLPHP, respectively. 3. Analysis of results 3.1. Startup For ammonia CLPHP, its oscillations are quite easy to start up, varying with the normal liquid working fluid, owing to the particular identities of the ammonia fluid. As shown in Figs. 3 and 4, the ammonia has the largest value of the ratio of saturation pressure to temperature and the lower value of superheated requirement.17 In a result, the ammonia PHP has the highest power from the saturation pressure to drive, and much shorter time for startup compared to other working fluids. In this experiment, the ambient atmosphere temperature is 29 C. At this temperature, ammonia becomes gas already in a standard atmosphere pressure. But in capillary, there are liquid slugs and gas plugs occurring at the same time; there-
Fig. 3
Saturation pressure as a function of temperature.
Experimental study on effect of inclination angles to ammonia pulsating heat pipe
1125
temperature of evaporator is only 4 C higher than condenser. In horizontal heating mode, the startup performance also behaved very conveniently and quickly compared to other working fluids. The same phenomena are observed in this experiment. It is found that it is not easy for ammonia fluid to observe the nucleation boiling at the whole period of operations. Once the temperature satisfies the condition, as the latent heat of the ammonia is overcome and the pressure is in excess of the surface tension, the liquid slug will transform to the gas slug quickly along the liquid film and liquid-gas interface, and shift the position as soon as possible. Fig. 4
Superheat required for PHP startup.
3.2. Effect of inclination angle fore, the ammonia should be almost in a saturation state. As the liquid ammonia changes to gas, there is no need of time to accumulate enough power for phase changes. On the other hand, for saturated ammonia, there will be large variation in pressure, when there are a few changes in its temperature. So the ammonia CLPHP is very sensitive to the variations of temperature compared with the normal working fluid. In our earlier work,18 the startup specifics of ammonia CLPHP with filling ratio of 70% had been studied. The results showed that the startup power required was very small. When in bottom heating mode, the PHP can start when the
Fig. 5
The heat transport performance of the ammonia CLPHP with a filling ratio FR of 50% at different inclination angles b is illustrated in Fig. 5(a)–(d). The heat input power is changed from 40 W to 280 W by step 40 W. It is seen that the temperatures of evaporator and condenser behave fluctuation continually. This phenomenon can be attributed to the oscillations of vapor bubbles and liquid slugs in capillary, even in the state of circulation for PHP. When heat power of 40 W is loaded, the oscillations are started as soon as possible; after a few seconds, the circulations are formulated. But the direction of bulk
Temperature response to heat load at different inclination angles.
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Fig. 6
Z. Xue, W. Qu
Effect of inclination angle on the thermal resistance.
circulation is always changed, sometimes going as clockwise, then suddenly running towards anti-clockwise. It is noted that this phenomenon is maintained in the whole period, and has a kind of periodicity, just as a spring. It’s an interesting thing if you pay attentions to the frequency relationship between the fluctuation of slug-train and change of bulk circulation. From Fig. 5(a)–(c), there are no difference at low heat power load when the PHP plate was placed in inclining condition, no matter what the inclination (orientation) angle is. When the ammonia CLPHP works at high heat load, for example, heat power in excess of 200 W, the amplitude of temperature in evaporator will increase much quicker than at low heat load; however, it is not arisen as the same speed in condenser, as shown in Fig. 5(a)–(c). Therefore, it is obvious that the thermal performance for CLPHP is getting a little worse. At this time, the heat flux for PHP exceeds 1 W/cm2, of course, far away from the extreme heat flux of CLPHP. Now, if the working fluid is not enough, there will maybe occur to the lack of liquid phase in evaporator, and the temperature in the capillary will be explicit to get higher. As shown in Fig. 5(a)–(c), the curves in black, red and blue have higher value of temperature than the green curve. Fig. 5(d) gives the temperature response to the heat load in inclination angle of 0. We define the inclination angle of 0 as the horizontal mode for PHP. It is a fact that the PHP is not easy to start up and has circulation in horizontal mode. But for ammonia CLPHP, it can start up normally, and intermittent circulations are also formulated at heat load of 80 W. As the input heat power increases, shown in the figure, the temperature acquired by Thermocouples 3 and 4 in evaporator increases crazily, much higher than the Thermocouple 1 and 2. From the visualization of operations, it is found that the heating sections of these two capillary tubes, where the Thermocouples 3 and 4 are positioned, are in short of liquid film. It is observed that the liquid slugs are likely to assemble at the cool sections, where there is low pressure. As the PHP operates in horizontal mode, for the liquid slugs in condenser, in left side having a high pressure, but in right side also having a high pressure, it is balanced without other additional forces to break out, for example, the gravity. So there will not be circulation moving. In this case, the heat cannot transfer from heating section to cool section. Consequently, some capillaries of the CLPHP are in the state of ‘‘burnout’’. This is a terrible thing, happening when the evaporator falls short of liquid without replenishment from the condenser. One reason of this is that the filling ratio is only 50%; if increased, such as 70% or 80%, there will be better enough.
Fig. 6 shows the effect of inclination angle on the thermal resistance of the ammonia CLPHP with 50% charging. When the orientation angles are varied from 30 to 90, the thermal resistance is decreased from 0.04 K/W to 0.03 K/W, 0.03 K/W to 0.028 K/W, 0.025 K/W to 0.021 K/W, respectively, as the input power is increased from 40 W to 80 W. If the input power is increased, the thermal resistance will be arisen a little continually, until 0.07 K/W at the input power of 280 W. Thermal resistance growing can be explained in terms of three aspects. Firstly, there is not sufficient time for the heat balance in experiment. Secondly, the temperature of cooler water has a fluctuation over 3 K, leading to the temperature fluctuation of condenser. The last one is the filling ratio, which influences the heat transport at high input power. When the CLPHP operates at the state of high input power, its flow pattern will be varied from the state of low input power. If it is not rich in working fluid, or low filling ratio, the thermal resistance will increase. From the Fig. 6, it is obviously seen that the thermal resistance is increasing sharply at the orientation angle of 0. The thermal performance of CLPHP is getting worse and worse, which has manifested that the phenomenon of temperature increase largely. 4. Conclusions It can be summarized from all the above-mentioned results that (1) The ammonia CLPHP has good startup performance, even if the temperature of evaporator is not higher than condenser, no matter what the inclination angles have being, owing to the ammonia working fluid having many particular identities. (2) For ammonia CLPHP, the thermal resistance is as low as 0.02 K/W, indicating that the heat fluxes can be transferred from heating section to cooling section very quickly. The results show that the thermal resistance decreases as the inclination angle increases. (3) The proper filling ratio is a critical parameter for the thermal performance of the ammonia CLPHP. To avoid the circumstance of ‘‘burnout’’, the best filling ratio is recommended to 70% or 80%, especially at the horizontal operation. (4) The ammonia CLPHP is an ideal candidate for electrical cooling with very low thermal resistance. So the aluminum-ammonia CLPHP for industry application will studied in future.
Acknowledgement This study was supported by the National Natural Science Foundation of China (No. 51176190). The authors would like to express their sincere appreciation for all of the supports provided. References 1. Zhang Y, Faghri A. Advances and unsolved issues in pulsating heat pipes. Heat Transfer Eng 2008;29(1):20–44. 2. Dmitrin VI, Maidanik YF. Experimental investigation of a closedloop oscillating heat pipe. High Temp 2007;45(5):703–7.
Experimental study on effect of inclination angles to ammonia pulsating heat pipe 3. Lin Y, Kang S, Wu T. Fabrication of polydimethylsiloxane (PDMS) pulsating heat pipe. Appl Therm Eng 2009;29(2–3):573–80. 4. Lin ZL, Wang SF, Chen JJ. Experimental study on effective range of miniature oscillating heat pipes. Appl Therm Eng 2011;31(5): 880–6. 5. Xu JL, Li YX, Wong TN. High speed flow visualization of a closed loop pulsating heat pipe. Int J Heat Mass Transfer 2005;48(16):3338–51. 6. Lin L, Ponnappan R, Leland J. Experimental investigation of oscillating heat pipes. J Thermophys Heat Transfer 2001;15(4): 395–400. 7. Ma HB, Choi US, Tirumala Murli, Wilson C, Yu Q, Park K. An experimental investigation of heat transport capability in a nanofluid oscillating heat pipe. J Heat Transfer 2006;128(11):1213–6. 8. Wilson C, Borgmeyer B, Jacobson D, Hussey D, Winholtz RA, Ma HB. Thermal and visual observation of water and acetone oscillating heat pipes. ASME J Heat Transfer 2011;133(6): 061502:1–5. 9. Rittidech S, Pipatpaiboon N, Terdtoon P. Heat-transfer characteristics of a closed-loop oscillating heat-pipe with check valves. Appl Energy 2007;84(5):565–77. 10. Chen P, Lee Y, Chang T. Predicting thermal instability in a closed loop pulsating heat pipe system. Appl Therm Eng 2009;29(8–9): 1566–76. 11. Gi K, Sato F, Maezawa S. Flow visualization experiment on oscillating heat pipe. Proceedings of the eleventh international heat pipe conference. Tokyo, Japan; 1999. p. 149–53. 12. Tong BY, Wong TN, Ooi KT. Closed-loop pulsating heat pipe. Appl Therm Eng 2001;21:1845–62. 13. Qu W, Ma HB. Theoretical analysis of startup of a pulsating heat pipe. Int J Heat Mass Transfer 2007;50(11–12):2309–16.
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14. Charoensawan P, Khandekar S, Groll M. Closed looped pulsating heat pipes: part A: parametric experimental investigation. Appl Therm Eng 2003;23(16):2009–20. 15. Khandekar S, Charoensawan P, Groll M. Closed looped pulsating heat pipes: part B: visualization and semi-empirical modeling. Appl Therm Eng 2003;23(16):2021–33. 16. Pachghare PR, Mahalle AM. Effect of inclination angle on the closed loop heat pipe thermal performance. Proceeding of the ASME summer heat transfer conference. Minneapolis, MN, USA; 2013. p. V002T07A027. 17. Taft BS, Williams AD, Drolen BL. Review of pulsating heat pipe working fluid selection. J Thermophys Heat Transfer 2012;26(4):651–6. 18. Xue ZH, Xie MH, Qu W, Li W. Ammonia pulsating heat pipe: full visualization and thermal performance. Proceedings of the eleventh international heat pipe symposium. Beijing, China; 2013. p. 98–103. Xue Zhihu is currently an engineer at China Academy of Aerospace Aerodynamics. He received his master degree from Beihang University, China, in 2011. His research interests include thermal control, electronics cooling and heat pipes. Qu Wei is currently a professor at China Academy of Aerospace Aerodynamics. He received his Ph.D. degree from Harbin Institute of Technology, China, in 1998. He is the Editorial Board Member of the two Journals of Heat Pipe Science and Technology, and Frontiers in Heat Pipe. His main research interests include thermal control and advanced thermal management, fundamental mechanism of high temperature heat pipe, pulsating heat pipe, rotating heat pipe, and thermosyphon, and their engineering applications.
Chinese Society of Aeronautics and Astronautics & Beihang University
Chinese Journal of Aeronautics [email protected] www.sciencedirect.com
Experimental study on effect of inclination angles to ammonia pulsating heat pipe Xue Zhihu, Qu Wei
*
China Academy of Aerospace Aerodynamics (CAAA), Beijing 100074, China Received 30 October 2013; revised 25 March 2014; accepted 4 May 2014 Available online 19 August 2014
KEYWORDS Ammonia; Inclination angles; Pulsating heat pipe; Thermal performance; Thermal resistance
Abstract In this paper, a novel study on performance of closed loop pulsating heat pipe (CLPHP) using ammonia as working fluid is experimented. The tested CLPHP, consisting of six turns, is fully made of quartz glass tubes with 6 mm outer diameter and 2 mm inner diameter. The filling ratio is 50%. The visualization investigation is conducted to observe the oscillation and circulation flow in the CLPHP. In order to investigate the effects of inclination angles to thermal performance in the ammonia CLPHP, four case tests are studied. The trends of temperature fluctuation and thermal resistance as the input power increases at different inclination angles are highlighted. The results show that it is very easy to start up and circulate for the ammonia CLPHP at an inclining angle. The thermal resistance is low to 0.02 K/W, presenting that heat fluxes can be transferred from heating section to cooling section very quickly. It is found that the thermal resistance decreases as the inclination angle increases. At the horizontal operation, the ammonia CLPHP can be easy to start up at low input power, but hard to circulate. In this case, once the input power is high, the capillary tube in heating section will be burnt out, leading to worse thermal performance with high thermal resistance. ª 2014 Production and hosting by Elsevier Ltd. on behalf of CSAA & BUAA.
1. Introduction The closed loop pulsating heat pipe (CLPHP), is considered as one of the promising technologies for a high heat transfer device on small space. It has been widely utilized in electronics * Corresponding author. Tel.: +86 10 68376749. E-mail addresses: [email protected] (Z. Xue), weiqucaaa@ hotmail.com (W. Qu). Peer review under responsibility of Editorial Committee of CJA.
Production and hosting by Elsevier
and computer components due to its potential heat transport capability, simple structure, compact sizes and low cost of manufacturing.1–4 Although simple in its construction, CLPHP seems very complicated when we try to understand its operation for phase change and interactions between the vapor plugs and liquid slugs. The CLPHP has been studied in experiment by many researches,5–10 focusing on either visual observation of flow patterns in CLPHP or various operation parameters related to thermo-hydrodynamics of CLPHP. Gi et al.11 performed the flow visualization of an R142b CLPHP with an 8 mm camera to record the flow, but little information of detailed description of flow pattern was observed. Tong et al.12 firstly visualized the flow patterns of a glass CLPHP filled 60% with methanol. It was found that
http://dx.doi.org/10.1016/j.cja.2014.08.004 1000-9361 ª 2014 Production and hosting by Elsevier Ltd. on behalf of CSAA & BUAA.
Experimental study on effect of inclination angles to ammonia pulsating heat pipe
Fig. 1
1123
Schematic of full visualization setup.
large amplitude oscillations from evaporator to the condenser during the startup period; however, in steady operating state, the working fluid circulated, coupled with complex processes such as nucleation boiling and coalescence of bubbles. The direction of circulation was consistent once circulation was attained, but it could be different for the same experimental run. Qu and Ma13 also conducted an experimental investigation on a glass CLPHP with water, and two kinds of vapor bubbles, i.e., small round (Globe) bubbles and long column (Taylor) bubbles, were formed and circulated in the whole CLPHP. It can be concluded that the startup of CLPHP and steady circulation were primarily due to the boiling heat transfer associating with the temperature difference and pressure difference between heating section and cooling section. Charoensawan et al.14 studied the parameter effect on thermal performance of the copper CLPHP, including the variation of internal diameter, number of turns, working fluid and inclination angle. The working fluids employed were water, ethanol and R-123. The results indicated a strong influence of gravity on the performance that the highest heat flux of CLPHP in horizontal mode decreased obviously compared to vertical mode if the turns were less than a certain number. Besides, thermo-physical properties of working fluids affected the performance strongly. Khandekar et al.15 indicated that the operating mechanism of CLPHP was not well understood and the present state of the art could not predict required design parameters for a given task. And he summarized the choice of working fluids which had high value of (dp/dT)sat, low dynamic viscosity, low latent heat and low surface tension. It is found that the working fluids of CLPHP employed as mentioned earlier usually are usually water, ethanol, methanol, acetone, R-134a,16 etc. But for ammonia, there is rarely used, especially in the whole glass CLPHP. Nevertheless, ammonia has much more advantages than these working fluids. It has much higher value of (dp/dT)sat, much lower superheat required,17 little lower dynamic viscosity and surface tension than those fluids, and much lower latent heat than water. So we infer that ammonia could be regarded as one kind of excellent fluid for CLPHP.
In our recent work, a novel study on full visualization and startup performance of glass CLPHP using ammonia as working fluid was experimented.18 It was found that the ammonia CLPHP was quite easier to startup than other working fluids, even under the conditions that the temperature of evaporator is only 4 K higher than condenser. This paper will observe the thermal performance of the ammonia CLPHP at different inclination angles. Firstly, the experimental setup and procedures are introduced. Then, the temperature fluctuations of evaporator and condenser are presented, and operation characteristics at different inclination angles are detailed. Lastly, the thermal resistance is analyzed, on behalf of the performance of ammonia CLPHP. 2. Experiment introduction 2.1. Experimental setup The schematic of the tested CLPHP constituting six meandering turns is shown in Fig. 1. The prototype is fully made of quartz glass capillary tubes with the total length of 320 mm from the top to the bottom. The inner and outer diameters of the glass tube are 2 mm and 6 mm, respectively. The evaporator section of the CLPHP is heated by the 0.2 mm electrical wires of 4.86 X/m resistance. The heat wires are wrapped at intervals of 1.5 mm on the outer wall surface of the CLPHP. The input heat power can be adjusted by step of 40 W. The total length of heating section shown as Le in Fig. 1 is 100 mm. In order to prevent heat radiation losses from heat wire, there will cover some insulation materials in the evaporator. The condenser part of the CLPHP is cooled by cooling water in cooler box which is circulated by a cold bath appliance. The inlet temperature of cooling water is maintained at 28 C with ±2 C accuracy. The length of cooling section named as Lc is 100 mm. The length of adiabatic section named as La is 120 mm. The motions of vapor bubbles and liquid slugs are photographed and recorded by a high speed CCD (Charge Coupled Device) camera. There are eight thermocouples (T1 to T8) located as shown in Fig. 1 to
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Z. Xue, W. Qu Table 1
Fig. 2
Sketch mapping of experimental platform.
measure the temperature fluctuations of the evaporator, adiabatic and condenser sections of the CLPHP and the inlet and outlet of cooler box. Four thermocouples are located in different tubes to give an average value of evaporator temperature. Two thermocouples are placed in the corresponding tubes to provide an average temperature of condenser. All temperature data are recorded by the highly sensitive temperature logger (Agilent 34970A with resolution 0.1 C, accuracy ±1 C) and connected to a PC for scanning the data every one second. The whole CLPHP system including glass ammonia CLPHP, heat wires, cooler box, and CCD recorder is fixed on one 500 mm · 600 mm aluminum plate. When the experiment is conducted, as one of the important factors, operation orientation angles of CLPHP should be taken into consideration. Therefore, a rotating component is used in Fig. 2, and an angle controller bolt is coupled to realize the variation of angle from 90 to +90 by step 30. 2.2. Experimental procedure The experimental procedure is as follows: Step 1: Clean the glass CLPHP using the de-ionized water injected by a micro pump. Other components are cleaned with some alcohol using ultrasonic waves. Cleaning is absolutely an indispensable process to remove any surface impurities, if not removed, it could disturb the pattern in later stages of the fabrication process. Step 2: Put the glass CLPHP in a constant temperature oven for the dehydration bake about 20 min at 120 C to ensure that the water or alcohol dries out completely. Step 3: For CLPHP filling, it is the most difficult and suffering thing. Since the saturation pressure of ammonia at 30 C reaches in 12 · 105 Pa, clearly the pressure is too high to have sealing easily, especially for the glass tube with smooth outer surface. However, these difficulties are solved well, and the vacuum of pulsating heat pipe (PHP) can reach 104 Pa. The ammonia CLPHP is charged with 50%. Step 4: Install the cooler box and get the condenser of CLPHP entry into the box with good sealing.
Design of experiment program.
Case
Inclination angle ()
Input power (W)
I II III IV
90 60 30 0
40 40 40 40
80 80 80 80
120 120 120 120
160 160 160 160
200 200 200 200
240 240 240
280 280 280
Step 5: Wrap the capillary with heat wires, and make the heating section garnished with a layer of heat insulations. Step 6: Fix the thermocouples in suitable positions. Set up the digital video camera, to record the operations and flow patterns of working fluid. Step 7: Switch the power supply to 40 W, simultaneously, turn on the scanning software to collect the temperature. After the temperature curve is balanced, adjust the heating power to 80 W (Plus 40 W per step). The experiment program is designed as illustrated in Table 1. The overall thermal resistance is defined in the following formula: R¼
Te Tc Q
ð1Þ
where R, Te, Tc, Q are the average thermal resistance, average temperature of the evaporator sector, average temperature of the condenser sector and heat power load of the ammonia CLPHP, respectively. 3. Analysis of results 3.1. Startup For ammonia CLPHP, its oscillations are quite easy to start up, varying with the normal liquid working fluid, owing to the particular identities of the ammonia fluid. As shown in Figs. 3 and 4, the ammonia has the largest value of the ratio of saturation pressure to temperature and the lower value of superheated requirement.17 In a result, the ammonia PHP has the highest power from the saturation pressure to drive, and much shorter time for startup compared to other working fluids. In this experiment, the ambient atmosphere temperature is 29 C. At this temperature, ammonia becomes gas already in a standard atmosphere pressure. But in capillary, there are liquid slugs and gas plugs occurring at the same time; there-
Fig. 3
Saturation pressure as a function of temperature.
Experimental study on effect of inclination angles to ammonia pulsating heat pipe
1125
temperature of evaporator is only 4 C higher than condenser. In horizontal heating mode, the startup performance also behaved very conveniently and quickly compared to other working fluids. The same phenomena are observed in this experiment. It is found that it is not easy for ammonia fluid to observe the nucleation boiling at the whole period of operations. Once the temperature satisfies the condition, as the latent heat of the ammonia is overcome and the pressure is in excess of the surface tension, the liquid slug will transform to the gas slug quickly along the liquid film and liquid-gas interface, and shift the position as soon as possible. Fig. 4
Superheat required for PHP startup.
3.2. Effect of inclination angle fore, the ammonia should be almost in a saturation state. As the liquid ammonia changes to gas, there is no need of time to accumulate enough power for phase changes. On the other hand, for saturated ammonia, there will be large variation in pressure, when there are a few changes in its temperature. So the ammonia CLPHP is very sensitive to the variations of temperature compared with the normal working fluid. In our earlier work,18 the startup specifics of ammonia CLPHP with filling ratio of 70% had been studied. The results showed that the startup power required was very small. When in bottom heating mode, the PHP can start when the
Fig. 5
The heat transport performance of the ammonia CLPHP with a filling ratio FR of 50% at different inclination angles b is illustrated in Fig. 5(a)–(d). The heat input power is changed from 40 W to 280 W by step 40 W. It is seen that the temperatures of evaporator and condenser behave fluctuation continually. This phenomenon can be attributed to the oscillations of vapor bubbles and liquid slugs in capillary, even in the state of circulation for PHP. When heat power of 40 W is loaded, the oscillations are started as soon as possible; after a few seconds, the circulations are formulated. But the direction of bulk
Temperature response to heat load at different inclination angles.
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Fig. 6
Z. Xue, W. Qu
Effect of inclination angle on the thermal resistance.
circulation is always changed, sometimes going as clockwise, then suddenly running towards anti-clockwise. It is noted that this phenomenon is maintained in the whole period, and has a kind of periodicity, just as a spring. It’s an interesting thing if you pay attentions to the frequency relationship between the fluctuation of slug-train and change of bulk circulation. From Fig. 5(a)–(c), there are no difference at low heat power load when the PHP plate was placed in inclining condition, no matter what the inclination (orientation) angle is. When the ammonia CLPHP works at high heat load, for example, heat power in excess of 200 W, the amplitude of temperature in evaporator will increase much quicker than at low heat load; however, it is not arisen as the same speed in condenser, as shown in Fig. 5(a)–(c). Therefore, it is obvious that the thermal performance for CLPHP is getting a little worse. At this time, the heat flux for PHP exceeds 1 W/cm2, of course, far away from the extreme heat flux of CLPHP. Now, if the working fluid is not enough, there will maybe occur to the lack of liquid phase in evaporator, and the temperature in the capillary will be explicit to get higher. As shown in Fig. 5(a)–(c), the curves in black, red and blue have higher value of temperature than the green curve. Fig. 5(d) gives the temperature response to the heat load in inclination angle of 0. We define the inclination angle of 0 as the horizontal mode for PHP. It is a fact that the PHP is not easy to start up and has circulation in horizontal mode. But for ammonia CLPHP, it can start up normally, and intermittent circulations are also formulated at heat load of 80 W. As the input heat power increases, shown in the figure, the temperature acquired by Thermocouples 3 and 4 in evaporator increases crazily, much higher than the Thermocouple 1 and 2. From the visualization of operations, it is found that the heating sections of these two capillary tubes, where the Thermocouples 3 and 4 are positioned, are in short of liquid film. It is observed that the liquid slugs are likely to assemble at the cool sections, where there is low pressure. As the PHP operates in horizontal mode, for the liquid slugs in condenser, in left side having a high pressure, but in right side also having a high pressure, it is balanced without other additional forces to break out, for example, the gravity. So there will not be circulation moving. In this case, the heat cannot transfer from heating section to cool section. Consequently, some capillaries of the CLPHP are in the state of ‘‘burnout’’. This is a terrible thing, happening when the evaporator falls short of liquid without replenishment from the condenser. One reason of this is that the filling ratio is only 50%; if increased, such as 70% or 80%, there will be better enough.
Fig. 6 shows the effect of inclination angle on the thermal resistance of the ammonia CLPHP with 50% charging. When the orientation angles are varied from 30 to 90, the thermal resistance is decreased from 0.04 K/W to 0.03 K/W, 0.03 K/W to 0.028 K/W, 0.025 K/W to 0.021 K/W, respectively, as the input power is increased from 40 W to 80 W. If the input power is increased, the thermal resistance will be arisen a little continually, until 0.07 K/W at the input power of 280 W. Thermal resistance growing can be explained in terms of three aspects. Firstly, there is not sufficient time for the heat balance in experiment. Secondly, the temperature of cooler water has a fluctuation over 3 K, leading to the temperature fluctuation of condenser. The last one is the filling ratio, which influences the heat transport at high input power. When the CLPHP operates at the state of high input power, its flow pattern will be varied from the state of low input power. If it is not rich in working fluid, or low filling ratio, the thermal resistance will increase. From the Fig. 6, it is obviously seen that the thermal resistance is increasing sharply at the orientation angle of 0. The thermal performance of CLPHP is getting worse and worse, which has manifested that the phenomenon of temperature increase largely. 4. Conclusions It can be summarized from all the above-mentioned results that (1) The ammonia CLPHP has good startup performance, even if the temperature of evaporator is not higher than condenser, no matter what the inclination angles have being, owing to the ammonia working fluid having many particular identities. (2) For ammonia CLPHP, the thermal resistance is as low as 0.02 K/W, indicating that the heat fluxes can be transferred from heating section to cooling section very quickly. The results show that the thermal resistance decreases as the inclination angle increases. (3) The proper filling ratio is a critical parameter for the thermal performance of the ammonia CLPHP. To avoid the circumstance of ‘‘burnout’’, the best filling ratio is recommended to 70% or 80%, especially at the horizontal operation. (4) The ammonia CLPHP is an ideal candidate for electrical cooling with very low thermal resistance. So the aluminum-ammonia CLPHP for industry application will studied in future.
Acknowledgement This study was supported by the National Natural Science Foundation of China (No. 51176190). The authors would like to express their sincere appreciation for all of the supports provided. References 1. Zhang Y, Faghri A. Advances and unsolved issues in pulsating heat pipes. Heat Transfer Eng 2008;29(1):20–44. 2. Dmitrin VI, Maidanik YF. Experimental investigation of a closedloop oscillating heat pipe. High Temp 2007;45(5):703–7.
Experimental study on effect of inclination angles to ammonia pulsating heat pipe 3. Lin Y, Kang S, Wu T. Fabrication of polydimethylsiloxane (PDMS) pulsating heat pipe. Appl Therm Eng 2009;29(2–3):573–80. 4. Lin ZL, Wang SF, Chen JJ. Experimental study on effective range of miniature oscillating heat pipes. Appl Therm Eng 2011;31(5): 880–6. 5. Xu JL, Li YX, Wong TN. High speed flow visualization of a closed loop pulsating heat pipe. Int J Heat Mass Transfer 2005;48(16):3338–51. 6. Lin L, Ponnappan R, Leland J. Experimental investigation of oscillating heat pipes. J Thermophys Heat Transfer 2001;15(4): 395–400. 7. Ma HB, Choi US, Tirumala Murli, Wilson C, Yu Q, Park K. An experimental investigation of heat transport capability in a nanofluid oscillating heat pipe. J Heat Transfer 2006;128(11):1213–6. 8. Wilson C, Borgmeyer B, Jacobson D, Hussey D, Winholtz RA, Ma HB. Thermal and visual observation of water and acetone oscillating heat pipes. ASME J Heat Transfer 2011;133(6): 061502:1–5. 9. Rittidech S, Pipatpaiboon N, Terdtoon P. Heat-transfer characteristics of a closed-loop oscillating heat-pipe with check valves. Appl Energy 2007;84(5):565–77. 10. Chen P, Lee Y, Chang T. Predicting thermal instability in a closed loop pulsating heat pipe system. Appl Therm Eng 2009;29(8–9): 1566–76. 11. Gi K, Sato F, Maezawa S. Flow visualization experiment on oscillating heat pipe. Proceedings of the eleventh international heat pipe conference. Tokyo, Japan; 1999. p. 149–53. 12. Tong BY, Wong TN, Ooi KT. Closed-loop pulsating heat pipe. Appl Therm Eng 2001;21:1845–62. 13. Qu W, Ma HB. Theoretical analysis of startup of a pulsating heat pipe. Int J Heat Mass Transfer 2007;50(11–12):2309–16.
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14. Charoensawan P, Khandekar S, Groll M. Closed looped pulsating heat pipes: part A: parametric experimental investigation. Appl Therm Eng 2003;23(16):2009–20. 15. Khandekar S, Charoensawan P, Groll M. Closed looped pulsating heat pipes: part B: visualization and semi-empirical modeling. Appl Therm Eng 2003;23(16):2021–33. 16. Pachghare PR, Mahalle AM. Effect of inclination angle on the closed loop heat pipe thermal performance. Proceeding of the ASME summer heat transfer conference. Minneapolis, MN, USA; 2013. p. V002T07A027. 17. Taft BS, Williams AD, Drolen BL. Review of pulsating heat pipe working fluid selection. J Thermophys Heat Transfer 2012;26(4):651–6. 18. Xue ZH, Xie MH, Qu W, Li W. Ammonia pulsating heat pipe: full visualization and thermal performance. Proceedings of the eleventh international heat pipe symposium. Beijing, China; 2013. p. 98–103. Xue Zhihu is currently an engineer at China Academy of Aerospace Aerodynamics. He received his master degree from Beihang University, China, in 2011. His research interests include thermal control, electronics cooling and heat pipes. Qu Wei is currently a professor at China Academy of Aerospace Aerodynamics. He received his Ph.D. degree from Harbin Institute of Technology, China, in 1998. He is the Editorial Board Member of the two Journals of Heat Pipe Science and Technology, and Frontiers in Heat Pipe. His main research interests include thermal control and advanced thermal management, fundamental mechanism of high temperature heat pipe, pulsating heat pipe, rotating heat pipe, and thermosyphon, and their engineering applications.