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Experimental research on the heat performance of a flat copper-water loop heat pipe with different inventories
Experimental Thermal and Fluid Science 84 (2017) 110–119
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Experimental research on the heat performance of a flat copper-water loop heat pipe with different inventories Jiayin Xu a, Zhiyuan Wang a, Hong Xu b, Li Zhang b,⇑ a b
School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China State-Key Laboratory of Chemical Engineering, School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 25 July 2016 Received in revised form 25 January 2017 Accepted 2 February 2017 Available online 8 February 2017 Keywords: LHP Flat evaporator Charging ratio Visual observation Copper-water Heat leak
a b s t r a c t The heat performance of a flat copper-water LHP, including the start-up property, operation capacity and evaporator thermal resistance, with three inventories (charging ratio ranged from 40 to 60%) was experimentally investigated. In order to observe the flow in the compensation chamber and the evaporating zone, a flat evaporator with a transparent cover (56 mm in diameter and 30 mm in total thickness) was manufactured. The porosity of the sintered copper wick was 47.26%, and the averaged pore radius was 22.65 lm. Severe heat leak, namely bubbles attaching on the wick during the start-up and heat pipe effect during the operation at a high heat load of 120 W, was observed. In addition, intermittent backflow from the condenser, which resulted in strong temperature oscillations, was also shown at low heat loads. It was experimentally found that high inventory was an effective method to address heat leak in spite of strong temperature oscillations at low heat loads. The optimal inventory of the proposed LHP was 10.0 ml (charging ratio of 50%), it could start smoothly in no more than 250 s and operate steadily at least at a heat load of 120 W (heat flux of 16.97 W/cm2) with the allowable evaporator wall temperature of 90 °C. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction Loop heat pipes (LHPs) are robust, compact and passive two-phase heat-transfer devices without pump and they are firstly proposed by Russian scientist Maydanik in 1972 [1]. After several decades of development, mainly two types of LHP evaporator, the cylinder evaporator [2–4] and the flat evaporator [5–7], have been intensively investigated. Since LHPs can transport heat at a level of higher orders of magnitude than using highly conductive solid materials over long distance, they have been one of the most promising candidates in heat pump water heating system [8,9], waste heat utilization [10] and thermal control of satellites and spacecraft [1] as well as in the field of cooling electronics [11]. However, it is really difficult to figure out the complex characteristics of heat transfer and hydrodynamics processes in LHPs. Visual investigations were introduced to obtain additional information and a comprehensive understanding of the operation in LHPs, because traditional temperature measurement at different points of the device was insufficient [12]. The Ref. [12] focused on visual investigation of the condensation process of LHPs. Besides that, Entremond et al. [13] put a borescope into the liquid
⇑ Corresponding author. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.expthermflusci.2017.02.003 0894-1777/Ó 2017 Elsevier Inc. All rights reserved.
core to observe the start-up process. John et al. [14] used neutron radiography to observe the process of partial drying of the wick during operation. Vapor formation, nucleate boiling and evaporation were observed by Junwoo et al. [15] with a transparent evaporator made of Pyrex glass. The heat load imposed on the evaporator of a LHP can be divided into two parts: heat for evaporation and heat leak [16]. Generally, the heat that leaks through the thermal conduction of a wick will heat the working fluid in the compensation chamber, which results in a higher evaporator temperature and LHP performance degradation. Therefore, it is thought that a copper wick is not suitable for the LHP because of high thermal conductivity, and wicks made of low thermal conductivity materials, such as nickel [17], stainless steel [18] and titanium [19], are desirable. However, Maydanik et al. [20] proposed a LHP with a copper wick having a 1200 W heat transfer capacity, in that high thermal conductivity of the copper wick could promote efficient heat exchange in the evaporating zone. In order to alleviate heat leak resulted from a copper wick, Wang et al. [21] increased the thickness of copper wick to improve the start-up characteristics of the copper-water LHP at a low heat load. Zhang et al. [4] pointed out that heat leak due to the copper wick could also be restrained by high inventory of water because a saturated wick can avoid tremendous heat leak, meanwhile an extremely high charging ratio of 78.3% would cause strong temperature oscillations. Besides,
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Nomenclature
DP Q R T DT
pressure difference, Pa input power, W thermal resistance, °C/W temperature, °C temperature difference, °C
Greek symbols e porosity Subscripts bw bottom side of the wick cc compensation chamber ci inlet of the condenser co outlet of the condenser
other literatures focused on the effect of inventory on the heat transfer performance of LHPs with polytetrafluoroethylene (PTFE) wicks and metal foam wicks. The effect of ethanol inventory on the thermal performance of a miniature LHP with the PTFE wick was investigated by Nishikawa et al. [22]. It was found that the evaporator temperature increased with increasing inventory because of the increase in the thermal resistance of the condenser. A wick made from a multi-layer metal foam was introduced into LHP by Zhou [23], and it was concluded that 40 ml of ethanol was the optimized inventory. In this paper, the effect of inventories (charging ratio ranged from 40 to 60%) on a copper-water LHP is investigated, since heat leak resulted from a copper wick has always been a serious problem. Similar research has been carried out by Zhang et al. [4], but it is insufficient to figure out the complex characteristics of heat transfer and hydrodynamics processes in the cylindrical evaporator only in a traditional temperature measurement way. Thus, a flat evaporator with a transparent cover is manufactured, and a highspeed camera is applied to observe the flow in the compensation chamber and the evaporating zone. The temperature profiles at different heat loads will be analyzed together with the flow in record. Moreover, temperature difference between two surfaces of the wick is also measured and calculated to evaluate the heat leak under different inventories. 2. Experimental apparatus and procedures 2.1. Experimental set-up The schematic diagram of the flat LHP and the detail of the temperature measurement points were presented in Fig. 1. It was composed of an evaporator, two separated transport lines for liquid and vapor flows, a cross-flow condenser with an axial DC fan and a nickel-chrome serpentine heater. T-type thermocouples were applied for temperature measurement. Two were sheathed thermocouples (1 mm in diameter, Omega) for measuring wall temperature of the evaporator (Tew1 and Tew2), and the readings were averaged to consider as the evaporator wall temperature (Tew). Another four were fine wire duplex insulated thermocouples (0.38 mm in diameter, Omega) and inserted into the LHP. Two of them were placed at the inlet and outlet of the condenser for flow inlet and outlet temperature measurement (Tci and Tco), respectively. The other two were attached on the upper surface and the bottom surface of the capillary wick (Tuw and Tbw) to evaluate the heat leak during the experiments. The accuracies of all thermocouples in use were ±0.5 °C.
cont eo ev ew ez e,vap e,cc LHP sat uw
contact outlet of the evaporator evaporator evaporator wall evaporating zone (heat load imposed on) the evaporator to vaporize the working fluid (heat load imposed on) the evaporator to the compensation chamber loop heat pipe saturation upper side of the wick
Fig. 1. Schematic diagram of the flat LHP and the detail of temperature measurement points.
As shown in Fig. 2(a), the evaporator consisted of an evaporator cover, a capillary wick, a brass plate and two O-rings. It was designed as a flat disk shape to fit the electronic equipment with 56 mm in diameter and 30 mm in total thickness. In order to observe the flow in the evaporator, a polycarbonate plastic rod was machined, sanded and polished to serve as a transparent evaporator cover whose maximum permissible temperature of long term operation was 130 °C. Due to the low thermal conductivity of polycarbonate plastic, the heat leak conducting from the evaporator wall could be neglected. Six longitudinal grooves with an individual cross section of 2 mm width 3 mm depth was machined on the brass plate, served as vapor removal channels, and conducted heat from the nickel-chrome serpentine heater to the bottom face of the capillary wick. A capillary wick, which was 5 mm in thickness, was sandwiched between the evaporator cover and the brass plate. As presented as in Fig. 2(c), the space above the wick was 9 mm in thickness and acted as the compensation chamber (cc) to accommodate the excessive working fluid for keeping the wick wet. The space beneath the capillary wick was the evaporating zone (ez), where the evaporation was able to be observed from the transparent cover during experiments. An O-ring seal (36 mm in diameter) was positioned between the evaporator cover and the brass plate to prevent air infiltration from the ambient. Another small O-ring seal (28 mm in diameter) was also
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Fig. 2. Photos and schematic diagram of the evaporator.
placed between the evaporator cover and the upper surface of the wick to avoid any internal leaks of vapor from the evaporating zone to the compensation chamber driven by pressure difference. The transparent cover, the capillary wick, the brass plate and two O-rings were clamped together with a flange bolt connection to guarantee a leak-proof sealing. A nickel-chrome serpentine heater, which was shown in Fig. 3, was made by wire-cutting and bonded to the brass plate with high conductive Omega-bond 200 for electronic insulation. The input power was controlled by a DC power supply (Agilent N5769A). Electric current and voltage were measured by Agilent U3402A with accuracies of 0.2% and 0.012% for electric current and voltage respectively. The heating area of the evaporator was 7.07 cm2 (30 mm in diameter). A fiber glass layer clinging to the brass plate and the vapor line was employed to ensure thermal insulation to the ambient. In order to avoid the damage of the nickel-chrome serpentine heater, heat load imposed was limited to 120 W, and the evaporator wall temperature was limited to 90 °C. Two transport lines were 6 mm in diameter (4 mm in internal diameter), and the lengths were 140 mm for vapor flow and 70 mm for liquid flow separately. In order to dissipate the heat imposed on the evaporator, a fin-and-tube condenser made of copper was applied as shown in Fig. 4. The pipe in the fin-and-tube condenser was also 4 mm in internal diameter and the length was 260 mm. An axial fan was equipped on the condenser, which was controlled by the DC power supply with voltage of 6 V during experiments.
Fig. 4. Photo of a copper fin-and-tube condenser.
The capillary wick for the experiments was 30 mm in diameter, 5 mm in thickness. It was sintered from copper powder to study the effect of the inventory on the heat performance during the LHP start-up and operation. The particle diameter of copper powder in use was ranged in 132–146 lm (mesh size 200–250). Porosity and pore radius were measured by the mercury porosimeter (PoreMaster-60 GT, Quantachrome Instruments). The porosity (e) for the copper wick was 47.26% and the averaged pore radii were 22.65 lm. 2.2. Experimental procedure and data reduction
Fig. 3. Photo of a nickel-chrome serpentine heater.
Before the experiments, proper cleaning of all metal parts was essential to ensure the reproducibility of the experimental results. These parts were heated in a furnace under hydrogen atmosphere at a temperature of around 200 °C for 1 h to remove any corrosion on the surfaces. Similar procedure, such as acid cleaning, washing in the boiling water and drying, could be referred to the literature [24]. Deionized water was selected as the working fluid, because of its high latent heat, non-toxic nature, and easy availability. In order to avoid the effect of non-condensable gases on the evaporation, the deionized water should be degassed prior to charging into the LHP. The degassing and charging systems were composed of a reservoir with a heater, a vacuum pump, a graduated and sealed charging cylinder and three valves. During degassing, a heater in the reservoir was used to maintain the saturated condition of deionized water for at least 2 h. After that, the graduated and sealed
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charging cylinder connected with the LHP was evacuated by the vacuum pump. Then degassed water would flow into the graduated and sealed charging cylinder, then was cooled to the freezing point for removing limited amount of non-condensable gases left in the degassed water. Finally, specified volume of water was injected into the LHP. It should be noted that total volume of the LHP was 19.8 ml, which was measured by the charging cylinder, and the volume of the evaporator (including compensation chamber and the evaporating zone) was 7.5 ml. According to the literature [25], ideal inventory range for the present LHP was 7.83–12.09 ml, which must satisfy the equations for the cold and hot cases. Thus, three inventories (8.0, 10.0 and 12.0 ml) were selected to investigate the effect of inventory on the LHP performance. The corresponding charging ratio was ranged about from 40 to 60%. All tests were conducted with the above mentioned LHP under the ambient temperature of 22 ± 1 °C. Positive elevation, namely the condenser located above the evaporator, was selected to make the capillary wick flooded by water during the experiments. As a result, the evaporator was full of water before the experiments, even when the inventory was only 8 ml. During the start-up process, a constant heat load of 30 W was imposed to the brass plate. From temperature measurement and visualization, the evaporation in the evaporating zone and flow conditions in the compensation chamber could be monitored and recorded by the high-speed camera (Motion Xtra N4) to recognize whether the LHP started successfully or not. The recording frequency in this experiment was 30 fps with the exposure time of 331 ls. After a successful start-up, operation tests begun at a heat load of 40 W and continued at heat load changing with a stepwise increment of 20 W. For each heat load, a steady state was confirmed when the evaporator wall temperatures were maintaining stably within a maximum limit of ±0.5 °C. When the LHP reached a steady state, the flow condition in the evaporator was recorded by the high-speed camera. Due to the limitation of the nickel-chrome serpentine heater, the highest heat load during the operation test was limited to 120 W, and the evaporator wall temperature was also limited to 90 °C. Temperature difference (DT) between two surfaces of the wick was calculated by using Eq. (1)
DT ¼ ðT bw T uw Þ
ð1Þ
where DT was temperature difference across the wick; Tbw was the wick temperature of bottom surface; Tuw was the wick temperature of upper surface. A low temperature difference meant lower thermal resistance and thus high rate of heat leak from the evaporating zone to the compensation chamber through the wick. Moreover, it was necessary to create an enough pressure difference (DP) between two sides of the wick for pushing the working fluid to transport around the LHP. According to the second condition of LHP serviceability (Eq. (2)) [1], this pressure difference resulted from the characteristics of working fluid and the saturation temperature difference between the evaporating zone (Tez) and the compensation chamber (Tcc).
DP ¼
dP ðT ez T cc Þ dT Tsat
ð2Þ
where dP/dT was the derivative obtained by the slope of the saturation line of water at the temperature in the compensation chamber (Tcc). These two saturation temperatures (Tez and Tcc) were difficult to measure precisely, but the temperature difference (Tez-Tcc) was proportional to that across the wick (DT). Hence, the temperature difference across the wick (DT) was applied to replace the temperature difference (Tez-Tcc) in discussion.
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In addition, the evaporator thermal resistance was one of the most important parameters that reflect the heat performance of the evaporator during the operation.
Rev ¼
ðT ew T eo Þ ðT ew T ci Þ ¼ Q Q
ð3Þ
where Rev was evaporator thermal resistance; Q was heat load; Teo was the outlet temperature of the evaporator; Tci was the inlet temperature of the condenser. Because of good thermal insulation for the vapor line, Tci was close to Teo. The heat load Q applied to the evaporator can by divided into two parts:
Q ¼ Q e;v ap þ Q e;cc
ð4Þ
where Qe,vap is heat for evaporation; Qe,cc is heat leak. An uncertainty analysis was performed according to the method proposed by Kim et al. [26] and Patil et al. [27]. The uncertainty of temperature was ±0.5 K for all thermocouples. The maximum value of uncertainties of electric current, voltage, input power and evaporator thermal resistance were ±0.64%, ±0.50%, ±0.82% and ±4.93%, respectively. 3. Results and discussion 3.1. Effect of inventory on the start-up process The start-up processes of the LHPs using the same copper wick and charging with three inventories were presented in Fig. 5. Among these three temperature profiles, abrupt increases of Tci (the blue line) and Tco (the pink line) could be found. It should be noted that the condenser inlet and outlet temperatures (Tci and Tco) were employed to monitor the flow of working fluid in the LHP. The former increase of Tci meant that the evaporation had started in the evaporating zone, and vapor was about to enter into the condenser. While the later increase of Tco meant that the condensed water had passed the condenser, and working fluid circulation was established. This was a transition point from the transient operation to the steady operation or oscillating operation. With the help of these two temperature readings, time consuming as well as the flow condition could also be recognized. In addition, the temperature difference between two surfaces of the capillary wick during the start-up was shown in Fig. 6, and flow conditions in the evaporator taken by the high-speed camera were also shown in Figs. 7–9. As shown in Fig. 5(b), it was found that the LHP that employed a 10.0 ml inventory (50% charging ratio) could activate the evaporation in about 50 s, and the circulation of working fluid was finished in 250 s according to the condenser inlet (Tci) and (Tco) temperature readings. During the incipience of start-up (before the abrupt increase of Tco), the upper surface wick temperature (Tuw) was close to the bottom surface wick temperature (Tbw), which meant massive heat leak was found. Correspondingly, a lot of bubbles were also observed on the upper surface of the wick in Fig. 7(a). These bubbles were attaching on the wick without departure due to a low heat load of 30 W during the start-up. This phenomenon could be explained that heat could not dissipate in the form of vapor to ambient in time, and resulted in serve heat leak through the heat conduction across the copper wick according to Eq. (4). The situation did not change until the circulation of working fluid was achieved. When the subcooling water returned from the condenser (abrupt increase of Tco), namely the circulation of working fluid was realized, all temperature (except Tco) decrease rapidly. Once heat could be dissipated to ambient efficiently through the condenser, heat leak would be relieved. Besides that, continuous backflow (subcooling water) from the condenser could keep the
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(a) 8.0 ml (charging ratio 40%)
Fig. 6. Temperature difference between two surfaces of capillary wick during startup process with different charging ratios.
(b) 10.0 ml (charging ratio 50%)
(a) incipience
(b) steady state
Fig. 7. Flow condition of the evaporator with 10.0 ml inventory during the start-up (50%).
(c) 12.0 ml (charging ratio 60%) Fig. 5. Start-up processes of the LHP with three inventories at a constant heat load of 30 W.
water in the compensation chamber at a low temperature, for the temperature of subcooling water was always lower than that in the compensation chamber. As a result of that, Tuw was about 4.5 °C
lower than Tbw during the steady state in Fig. 5(b). The continuous backflow entered into the compensation chamber (the white frame on the upside of figure) was observed in Fig. 7(b) correspondingly, and the bubble on the wick disappeared. Thanks to the continuous backflow, no evident temperature oscillations were shown after the LHP was started, and a steady state was achieved. The evaporator wall temperature maintained at about 44 °C. By contrast, the temperature profile of the LHP with an 8.0 ml inventory (40% charging ratio) was quite different. As shown in Fig. 5 (a), the LHP could establish the circulation in 330 s, and evaporation in the evaporating zone was activated in about 60 s, according to Tci and Tco. Before the backflow of water from the condenser, similar massive heat leak and bubbles attaching on the wick were also found. After the circulation of working fluid achieved, strong temperature oscillations were shown in Fig. 5 (a). The flow condition in the evaporator was recorded by the high-speed camera and presented in Fig. 8. The reason for temperature oscillation was that the dry-out occurred periodically in the evaporating zone owing to low inventory. When there was no
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(a) dry-out
(b) backflow
(c) evaporation
(d) dry-out again
Fig. 8. Flow condition of the evaporator with 8.0 ml inventory during the start-up (40%).
water left in the evaporating zone, namely dry-out in the wick occurred (Fig. 8(a) and (d)), the heat imposed on the brass plate would make all the temperatures except Tco increase rapidly because no evaporation taking place in the evaporating zone would result in severe heat leak according to Eq. (4). The temperatures (except Tco) continued to increase until the subcooling water returned from the condenser to the compensation chamber. At this moment, obvious temperature decline could be found because of the backflow (Tco increased), and water level could be seen in Fig. 8(b). Unfortunately, the backflow water was not continuous, which could be testified by the fact that Tco decreased at once instead of maintaining. The temperature profile of Tco seemed like pulses. Owing to the enough superheat in the evaporating zone, water was evaporated into vapor immediately. Even so, the temperatures (except Tco) still increased, because intermittent backflow could not ensure that heat leak was compensated by the subcooling water from the condenser. On the other hand, water level in the compensation chamber would fall due to the intermittent backflow, and dry-out in the evaporating zone turned up again
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in Fig. 8(c) and (d). As a result, strong temperature oscillations were presented, and the peak evaporator wall temperature was up to 67.9 °C. Except the timing of backflow, the upper surface wick temperature (Tuw) was always close to the bottom surface wick temperature (Tbw). Unlike the LHP with a 10.0 ml inventory, the heat leak was severe throughout the start-up process. As the inventory was increased to 12.0 ml (60% charging ratio), a fast start but with temperature oscillations was shown in Fig. 5 (c). Owing to the large inventory, there was more water left in the vapor line and the compensation chamber than the former two inventories during the incipience of the start-up process. On one hand, more water in the vapor line meant the pressure in the evaporating zone was higher, and thus higher saturation temperature was necessary to activate evaporation. On the other hand, the benefit of redundant water in the compensation chamber was that heat leak could be compensated even though evaporation was not started in the evaporating zone. As a result of these two factors, it took more than 100 s to activate evaporation in the evaporating zone with a high Tci temperature of 52 °C, but the heat leak during this interval was not severe, for Tuw was not close to Tbw. No bubbles were found on the wick. In accordance with Tco, the working fluid circulation could be generated in about 240 s. After a successful start, obvious temperature oscillations were also found. Similar intermittent backflow also occurred in this case, which was also verified by the pulse-like Tco profile in Fig. 5(c). However, the frequency of temperature oscillation was higher, and the amplitude of temperature was smaller than ones of the LHP with an 8.0 ml inventory. The recorded flow condition in the evaporator during obvious temperature oscillation was shown in Fig. 9. In Fig. 9(a), there was some water in the compensation chamber, and evaporation was also found in the evaporating zone. As evaporation continued, the water level in the compensation chamber would descend because of the intermittent backflow. As shown in Fig. 9 (b), no water was seen in the compensation chamber with evaporation taking place in the evaporating zone, which meant that there was some water left in the wick and the evaporating zone. At this moment, heat leak in the wick could not be addressed by the water in the compensation chamber, which would result in a high saturation temperature in the wick. All temperature (except Tco) increased. Fortunately, a large amount of subcooling water entered into the compensation chamber (abrupt increase Tco) before the evaporating zone running out of water as presented in Fig. 9(c). All temperature (except Tco) decreased suddenly. However, evaporation in the evaporating zone was terminated by a large amount of subcooling water, since a large amount of subcooling water from the condenser would make the temperature of water in the wick too low to keep evaporation. Consequently, some time for heating the wick was needed to re-activate the evaporation in the evaporating zone (Fig. 9(d)). That was the reason why the increases of all temperature (except Tco) were shown after sudden decreases. Compared with the LHP with an 8.0 ml inventory, the LHP with a 12.0 ml inventory could keep the evaporator wall temperature (Tew) under control, since there was no dry-out in the wick as presented. During the temperature oscillations, the peak temperature was 60.3 °C, and Tew fluctuated around 57 °C, with a maximum limit of ±1.5 °C. In order to evaluate heat leak resulted from different inventories, temperature differences between Tbw and Tuw versus time during the start-up process were calculated and plotted in Fig. 6. As mentioned above, the pressure difference between two sides of the wick (DP), which played a vital role on the flow condition during the start-up and operation of LHP, depended on the temperature difference between two sides of the wick (DT). To establish a large enough DP for driving the circulation of water around the LHP, either a large value of DT or a high saturation temperature was compulsory. When heat leak was severe, namely a small value
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(a) water in cc evaporation in ez
(b) dry in the cc evaporation in ez
(d) water in cc evaporation in ez
(e) dry in cc evaporation in ez
(c) backflow in cc no evaporation in ez
Fig. 9. Flow condition of the evaporator with 12.0 ml inventory during the start-up.
of DT, a high saturation temperature was required which resulted in a high operation temperature. This characteristic was called the self-reliance of the LHP [1]. As seen in Fig. 6, at the beginning of initial 200 s, three temperature differences DT increased at first, then decreased. Three temperature differences DT started from 0 °C because there was almost no temperature difference between two sides of the wick before heat was imposed to the evaporator. When a heat load of 30 W was applied, DT began to increase, for the evaporating zone was heated by the brass plate at first to start evaporation, and resulted in an increasing Tbw. Then Tuw also increased to make DT decrease for the LHP with the inventory of 10.0 ml (50% charging ratio). The reason for the increase of Tuw
was that heat leak could conduct easily through the copper wick to heat the water in the compensation chamber especially when there was no evaporation in the evaporating zone. When evaporation was started in about 50 s, part of heat was used to evaporate water into vapor. The slope of increasing Tbw reduced, but this part of heat could not be dissipated to ambient efficiently. That made DT decrease to the value below 1 °C. Correspondingly, there were bubbles on the upper surface of the wick. When the circulation of water was finished, these bubble disappeared, because most part of heat was dissipated to ambient at this moment. The backflow of subcooling water also compensated the heat conducted by the wick. Hence, heat leak was suppressed, and DT suddenly
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increased to a value of 8 °C and finally maintained at a value of 5 °C when the steady state was accomplished. The LHP with the inventory of 8.0 ml had almost the same DT profile as that of 10.0 ml before the circulation was achieved, but temperature difference oscillation was shown due to the intermittent backflow. DT reduced to about 1 °C (massive heat leak) when dry-out occurred in the wick and the evaporating zone. DT increased to around 10 °C when backflow entered into the compensation chamber. For the LHP employed with the inventory of 12.0 ml, more water in the compensation chamber could relieve heat leak during the incipience of the start-up process. Thus, DT was around 5 °C during the initial 200 s, which was higher than that of the other two inventories. When a large amount of backflow water flowed into the compensation chamber, a sharp decrease of Tuw occurred, and DT increased up to 17.5 °C. The consequent decrease of Tbw made the water temperature in the evaporating zone so low that resulted in the termination of evaporation, therefore another interval was necessary to heat the wick and re-start evaporation. That was the reason why intermittent backflow also occurred in the LHP with a 12.0 ml inventory. Even evaporation was stopped, however, the minimum DT was still higher than that of the 8.0 ml inventory because there was no dry-out in the evaporating zone shown during temperature oscillation.
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(a) 8.0 ml (charging ratio 40%)
3.2. Effect of inventory on operation After the start-up process, the LHPs under different inventories operating with the increment of heat load were carried out. Averaged evaporator wall temperature (Tew) and averaged temperature differences between two sides of the wick (DT, also dT in Fig. 10) for comparison were shown in Fig. 10. However, temperature oscillations were also observed during the LHP operation, averaged Tew and DT could not tell us the flow condition in the evaporator. Therefore, temperature profiles of three LHPs at different heat loads were respectively shown in Fig. 11. As shown in Fig. 10, DT (also dT in Fig. 10) of the LHP with an 8.0 ml inventory was lower than ones of the other two inventories, which meant that the heat leak of LHP with 40% charging ratio was the most severe one among these three inventories. As a result, the LHP with 40% charging ratio had the highest averaged evaporator wall temperature (Tew) during the operations, while the heat capacity was only 80 W with the allowable evaporator wall temperature of 90 °C. Similar to the start-up process, strong temperature oscillation and dry-out in the wick were also shown in Fig. 11(a) at a heat load of 40 W owing to low inventory. However, steady operations were shown when heat load was
(b) 10.0 ml (charging ratio 50%)
(c) 12.0 ml (charging ratio 60%) Fig. 11. Temperature profiles of three LHPs at different heat loads during operation.
Fig. 10. LHPs operating at different heat loads.
added to 60 and 80 W. It could be explained that evaporation rate of the water in the evaporating zone was accelerated at a high heat load, which could speed up the flow circulating around the LHP, relieve or eliminate dry-out occurred in the wick and result in a steady operation of the LHP.
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For the LHP employed with a 10.0 ml inventory (charging ratio of 50%), no evident evaporator wall temperature oscillation was observed throughout the operation as shown in Fig. 11(b). The backflow into the evaporator was relatively stable. Heat leak was suppressed by the backflow from the condenser under the low heat load (40 and 60 W). As increased with heat load, Tuw became higher than Tco gradually, which meant that the heat leak became larger with the increase of heat load. At the heat load of 120 W, a heat pipe effect [16], namely bubble generated on the upper surface of the wick and departed for the compensation chamber owing to massive heat leak, was recorded with the help of highspeed camera in Fig. 12. These bubbles went up and condensed into liquid again in the compensation chamber when meeting with the subcooling water from the condenser, which was like inserting a heat pipe into the compensation chamber. In spite of that, the LHP could operate at a steady state and the evaporator wall temperature Tew could be maintained at 80.4 °C. For the LHP with a 12.0 ml inventory (Fig. 11(c)), evident temperature oscillation was also shown at the heat loads of 40 and 60 W (low heat load), and the steady states were accomplished at the heat loads from 80 to 120 W (high heat load). At low heat loads, the intermittent backflow, which was similar to the situation during the start-up process, was also shown during the LHP operation. The abrupt backflow from the condenser would weaken the evaporation in the evaporating zone, which resulted in the fluctuation of Tuw, Tci and Tew. On account of that, averaged evaporator wall temperature would be larger than that of the LHP with charging ration of 50% in Fig. 10. At high heat loads (80 W or above), the surperheat in the wick was high enough, the influence of a large amount of backflow on the evaporation would be negligible, so no apparent oscillations of Tuw and Tew were shown. The evaporator thermal resistance consisted of the evaporator wall thermal resistance Rwall (the brass plate), contact thermal resistance Rcont (between the wick and the brass plate) and the wick thermal resistance Rwick [28]. The former two thermal resistances had the same contribution to the evaporator thermal
Fig. 13. Evaporator thermal resistance.
resistance, and Rwick was the reflection of the wick saturation condition. The evaporator thermal resistances of these three inventories were shown in Fig. 13. All thermal resistances decreased with the increase of the heat load. Owing to the appearance of the dry-out in the wick, the LHP charging with 40% had highest value (0.24 °C/W) among three inventories at a heat load of 40 W. In addition, the evaporator thermal resistance of the LHP charging with 60% was larger than that of the LHP charging with 50%, since evaporation was interrupted by the backflow. As heat load increased, temperature oscillation became weak, steady states were accomplished and the evaporator thermal resistances of the LHPs charging with 50 and 60% reduced to 0.015 °C/W at the heat load of 120 W. 4. Conclusion In this paper, copper-water LHPs with charging ratio ranged from 40 to 60% were tested to investigate the effect of inventory on flow condition in the evaporator and heat-transfer performance. Based on the experimental results and the visual observation, the following conclusions could be drawn:
(a) no vapor
(b) vapor creation
(c) departure
(d) condensation in cc
Fig. 12. Photos of heat pipe effect in the LHP with a 10.0 ml inventory (50%) at the heat load of 120 W.
(1) The proposed copper-water LHPs with different inventories could start successfully in no more than 330 s at the heat load of 30 W (heat flux of 4.24 W/cm2). During the operation, two LHPs with the charging ratios of 50 and 60% were able to keep steady at least at the heat load of 120 W (heat flux of 16.97 W/cm2) under the allowable evaporator wall temperature of 90 °C. Hence, copper-water LHP could start fast and attain high heat transfer capacity in the temperature rang below 90 °C. (2) Fluid distribution and the flow condition in the evaporating zone and the compensation chamber were observed. Severe heat leak occurred in the copper-water LHP with low inventory (charging ratios of 40 and 50%) was confirmed: (1) during the incipience of start-up process, bubbles resulted from heat leak was attaching on the upper surface of the copper wick; (2) heat pipe effect was shown in the compensation chamber at a high heat load of 120 W (50%). (3) Intermittent backflow from the condenser was observed (charging ratios of 40 and 60%), which resulted in temperature oscillations at low heat loads (30 and 40 W): (1) because of inadequate inventory (40%), dry-out was found in the evaporating zone at low heat load; (2) a large amount of backflow (60%) would make the superheat in
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the evaporating zone too low to keep the evaporation. However, intermittent backflow and consequence temperature oscillation could be relieved at high heat loads. (4) Considered on the start-up process and the operation, the optimal inventory for the proposed flat LHP was 10.0 ml (charging ratio of 50%). Moreover, high inventory (60%) was an effective method for suppressing heat leak especially at high heat loads, but temperature oscillations with high frequency were shown during the start-up process and the operation at low heat loads (40 and 60 W).
Acknowledgements The authors are grateful for the support of the Natural Science Foundation of Shanghai under Grant No. 10ZR1408100. This paper is also supported by the Fundamental Research Funds for the Central Universities, and the National Natural Science Foundation of China (No. 51406120). References [1] Y. Maydanik, Loop heat pipes, Appl. Therm. Eng. 25 (2005) 635–657. [2] C.C. Yeh, C.N. Chen, Y.M. Chen, Heat transfer analysis of a loop heat pipe with biporous wicks, Int. J. Heat Mass Transf. 52 (2009) 4426–4434. [3] P.H.D. Santos, E. Bazzo, S. Becher, R. Kulenovic, R. Mertz, Development of LHPs with ceramic wick, Appl. Therm. Eng. 30 (2010) 1784–1789. [4] Li. Zhang, Xu. Jiayin, Xu. Hong, Effect of inventory on the heat performance of copper-water loop heat pipe, Exp. Therm. Fluid Sci. 44 (2013) 875–882. [5] Y. Maydanik, S. Vershinin, M. Chernysheva, S. Yushakova, Investigation of a compact copper-water loop heat pipe with a flat evaporator, Appl. Therm. Eng. 31 (2011) 3533–3541. [6] Dongdong Wang et al., Experimental study of the loop heat pipe with a flat disk-shaped evaporator, Exp. Therm. Fluid Sci. 57 (2014) 157–164. [7] Ji. Li, Lucang Lv, Performance investigation of a compact loop heat pipe with parallel condensers, Exp. Therm. Fluid Sci. 62 (2015) 40–51. [8] Z. Xingxing et al., The early design stage for building renovation with a novel loop-heat-pipe based solar thermal façade (LHP-STF) heat pump water heating system: techno-economic analysis in three European climates, Energy Convers. Manage. 106 (2015) 964–986. [9] X.X. Zhang, X.D. Zhao, J.H. Xu, X.T. Yu, Characterization of a solar photovoltaic/ loop-heat-pipe heat pump water heating system, Appl. Energy 102 (2013) 1229–1245.
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[10] Binjuine Huang, Pochien Hsu, Rungje Tsai, Muhammad Mustafa Hussain, A thermoelectric generator using loop heat pipe and design match for maximum-power generation, Appl. Therm. Eng. 91 (2015) 1082–1091. [11] Xianping Chen, Huaiyu Ye, Xuejun Fan, Tianling Ren, Guoqi Zhang, A review of small heat pipes for electronics, Appl. Therm. Eng. 96 (2016) 1–17. [12] E. Bartuli, S. Vershinin, Y. Maydanik, Visual and instrumental investigations of a copper-water loop heat pipe, Int. J. Heat Mass Transf. 61 (2013) 35–40. [13] B. Entremond, J. Ochterbeck, Investigation of loop heat pope startup using liquid core visualization, Proc. ASME Summer Heat Transfer Conf. 2 (2008) 387–393. [14] M. John, S. Jack, J. Brenizer, et al., Study of a loop heat pipe using neutron radiography, Appl. Radiat. Isot. 61 (2004) 701–705. [15] Junwoo Suh, Debra Cytrynowicz, Praveen Medis, Frank M. Gerner, H. Thurman Henderson, Flow visualization within the evaporator of planar loop heat pipe, Space Technol. Appl. Int. Forum (2005) 195–202. [16] J. Ku, Operating characteristics of loop heat pipes, in: 29th International Conference on Environmental System, Colorado, 1999. [17] Jeehoon Choi et al., Experimental investigation on sintered porous wicks for miniature loop heat pipe applications, Exp. Therm. Fluid Sci. 51 (2013) 271– 278. [18] G.P. Celata, M. Cumo, M. Furrer, Experimental tests of a stainless steel loop heat pipe with flat evaporator, Exp. Therm. Fluid Sci. 34 (2010) 866–878. [19] L. Vasiliev et al., Loop heat pipe for cooling of high-power electronic components, Int. J. Heat Mass Transf. 52 (2009) 301–308. [20] Y.F. Maydanik, S. Vershinin, M. Chernysheva, S. Yushakova, Investigation of a compact copper-water loop heat pipe with a flat evaporator, Appl. Therm. Eng. 31 (2011) 3533–3541. [21] S.F. Wang, W.B. Zhang, X.F. Zhang, J.J. Chen, Study on start-up characteristics of loop heat pipe under low-power, Int. J. Heat Mass Transf. 54 (2011) 1002– 1007. [22] M. Nishikawa, H. Nagano, Parametric experiments on a miniature loop heat pipe with PTFF wicks, Int. J. Therm. Sci. 85 (2014) 29–39. [23] W. Zhou et al., Development and tests of loop heat pipe with muti-layer metal foams as wick structure, Appl. Therm. Eng. 94 (2016) 324–330. [24] R. Singh, A. Akbarzadeh, M. Mochizuki, Operational characteristics of the miniature loop heat pipe with non-condensable gases, Int. J. Heat Mass Transf. 53 (2010) 3471–3482. [25] S. Launay, V. Sartre, J. Bonjour, Parametric analysis of loop heat pipe operation: a literature review, Int. J. Therm. Sci. 46 (2007) 621–636. [26] J.H. Kim, T.W. Simon, R. Viskanta, Journal of heat transfer policy on reporting uncertainties in experimental measurements and results, J. Heat Transf. 115 (1993) 5–6. [27] C.M. Patil, K.S.V. Santhanam, S.G. Kandlikar, Development of a two-step electrodeposition process for enhancing pool boiling, Int. J. Heat Mass Transf. 79 (2014) 989–1001. [28] M. Chernysheva, Y. Maydanik, Heat and mass transfer in evaporator of loop heat pipe, J. Thermo Phys. Heat Transf. 23 (2009) 725–731.
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Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Experimental research on the heat performance of a flat copper-water loop heat pipe with different inventories Jiayin Xu a, Zhiyuan Wang a, Hong Xu b, Li Zhang b,⇑ a b
School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China State-Key Laboratory of Chemical Engineering, School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 25 July 2016 Received in revised form 25 January 2017 Accepted 2 February 2017 Available online 8 February 2017 Keywords: LHP Flat evaporator Charging ratio Visual observation Copper-water Heat leak
a b s t r a c t The heat performance of a flat copper-water LHP, including the start-up property, operation capacity and evaporator thermal resistance, with three inventories (charging ratio ranged from 40 to 60%) was experimentally investigated. In order to observe the flow in the compensation chamber and the evaporating zone, a flat evaporator with a transparent cover (56 mm in diameter and 30 mm in total thickness) was manufactured. The porosity of the sintered copper wick was 47.26%, and the averaged pore radius was 22.65 lm. Severe heat leak, namely bubbles attaching on the wick during the start-up and heat pipe effect during the operation at a high heat load of 120 W, was observed. In addition, intermittent backflow from the condenser, which resulted in strong temperature oscillations, was also shown at low heat loads. It was experimentally found that high inventory was an effective method to address heat leak in spite of strong temperature oscillations at low heat loads. The optimal inventory of the proposed LHP was 10.0 ml (charging ratio of 50%), it could start smoothly in no more than 250 s and operate steadily at least at a heat load of 120 W (heat flux of 16.97 W/cm2) with the allowable evaporator wall temperature of 90 °C. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction Loop heat pipes (LHPs) are robust, compact and passive two-phase heat-transfer devices without pump and they are firstly proposed by Russian scientist Maydanik in 1972 [1]. After several decades of development, mainly two types of LHP evaporator, the cylinder evaporator [2–4] and the flat evaporator [5–7], have been intensively investigated. Since LHPs can transport heat at a level of higher orders of magnitude than using highly conductive solid materials over long distance, they have been one of the most promising candidates in heat pump water heating system [8,9], waste heat utilization [10] and thermal control of satellites and spacecraft [1] as well as in the field of cooling electronics [11]. However, it is really difficult to figure out the complex characteristics of heat transfer and hydrodynamics processes in LHPs. Visual investigations were introduced to obtain additional information and a comprehensive understanding of the operation in LHPs, because traditional temperature measurement at different points of the device was insufficient [12]. The Ref. [12] focused on visual investigation of the condensation process of LHPs. Besides that, Entremond et al. [13] put a borescope into the liquid
⇑ Corresponding author. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.expthermflusci.2017.02.003 0894-1777/Ó 2017 Elsevier Inc. All rights reserved.
core to observe the start-up process. John et al. [14] used neutron radiography to observe the process of partial drying of the wick during operation. Vapor formation, nucleate boiling and evaporation were observed by Junwoo et al. [15] with a transparent evaporator made of Pyrex glass. The heat load imposed on the evaporator of a LHP can be divided into two parts: heat for evaporation and heat leak [16]. Generally, the heat that leaks through the thermal conduction of a wick will heat the working fluid in the compensation chamber, which results in a higher evaporator temperature and LHP performance degradation. Therefore, it is thought that a copper wick is not suitable for the LHP because of high thermal conductivity, and wicks made of low thermal conductivity materials, such as nickel [17], stainless steel [18] and titanium [19], are desirable. However, Maydanik et al. [20] proposed a LHP with a copper wick having a 1200 W heat transfer capacity, in that high thermal conductivity of the copper wick could promote efficient heat exchange in the evaporating zone. In order to alleviate heat leak resulted from a copper wick, Wang et al. [21] increased the thickness of copper wick to improve the start-up characteristics of the copper-water LHP at a low heat load. Zhang et al. [4] pointed out that heat leak due to the copper wick could also be restrained by high inventory of water because a saturated wick can avoid tremendous heat leak, meanwhile an extremely high charging ratio of 78.3% would cause strong temperature oscillations. Besides,
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Nomenclature
DP Q R T DT
pressure difference, Pa input power, W thermal resistance, °C/W temperature, °C temperature difference, °C
Greek symbols e porosity Subscripts bw bottom side of the wick cc compensation chamber ci inlet of the condenser co outlet of the condenser
other literatures focused on the effect of inventory on the heat transfer performance of LHPs with polytetrafluoroethylene (PTFE) wicks and metal foam wicks. The effect of ethanol inventory on the thermal performance of a miniature LHP with the PTFE wick was investigated by Nishikawa et al. [22]. It was found that the evaporator temperature increased with increasing inventory because of the increase in the thermal resistance of the condenser. A wick made from a multi-layer metal foam was introduced into LHP by Zhou [23], and it was concluded that 40 ml of ethanol was the optimized inventory. In this paper, the effect of inventories (charging ratio ranged from 40 to 60%) on a copper-water LHP is investigated, since heat leak resulted from a copper wick has always been a serious problem. Similar research has been carried out by Zhang et al. [4], but it is insufficient to figure out the complex characteristics of heat transfer and hydrodynamics processes in the cylindrical evaporator only in a traditional temperature measurement way. Thus, a flat evaporator with a transparent cover is manufactured, and a highspeed camera is applied to observe the flow in the compensation chamber and the evaporating zone. The temperature profiles at different heat loads will be analyzed together with the flow in record. Moreover, temperature difference between two surfaces of the wick is also measured and calculated to evaluate the heat leak under different inventories. 2. Experimental apparatus and procedures 2.1. Experimental set-up The schematic diagram of the flat LHP and the detail of the temperature measurement points were presented in Fig. 1. It was composed of an evaporator, two separated transport lines for liquid and vapor flows, a cross-flow condenser with an axial DC fan and a nickel-chrome serpentine heater. T-type thermocouples were applied for temperature measurement. Two were sheathed thermocouples (1 mm in diameter, Omega) for measuring wall temperature of the evaporator (Tew1 and Tew2), and the readings were averaged to consider as the evaporator wall temperature (Tew). Another four were fine wire duplex insulated thermocouples (0.38 mm in diameter, Omega) and inserted into the LHP. Two of them were placed at the inlet and outlet of the condenser for flow inlet and outlet temperature measurement (Tci and Tco), respectively. The other two were attached on the upper surface and the bottom surface of the capillary wick (Tuw and Tbw) to evaluate the heat leak during the experiments. The accuracies of all thermocouples in use were ±0.5 °C.
cont eo ev ew ez e,vap e,cc LHP sat uw
contact outlet of the evaporator evaporator evaporator wall evaporating zone (heat load imposed on) the evaporator to vaporize the working fluid (heat load imposed on) the evaporator to the compensation chamber loop heat pipe saturation upper side of the wick
Fig. 1. Schematic diagram of the flat LHP and the detail of temperature measurement points.
As shown in Fig. 2(a), the evaporator consisted of an evaporator cover, a capillary wick, a brass plate and two O-rings. It was designed as a flat disk shape to fit the electronic equipment with 56 mm in diameter and 30 mm in total thickness. In order to observe the flow in the evaporator, a polycarbonate plastic rod was machined, sanded and polished to serve as a transparent evaporator cover whose maximum permissible temperature of long term operation was 130 °C. Due to the low thermal conductivity of polycarbonate plastic, the heat leak conducting from the evaporator wall could be neglected. Six longitudinal grooves with an individual cross section of 2 mm width 3 mm depth was machined on the brass plate, served as vapor removal channels, and conducted heat from the nickel-chrome serpentine heater to the bottom face of the capillary wick. A capillary wick, which was 5 mm in thickness, was sandwiched between the evaporator cover and the brass plate. As presented as in Fig. 2(c), the space above the wick was 9 mm in thickness and acted as the compensation chamber (cc) to accommodate the excessive working fluid for keeping the wick wet. The space beneath the capillary wick was the evaporating zone (ez), where the evaporation was able to be observed from the transparent cover during experiments. An O-ring seal (36 mm in diameter) was positioned between the evaporator cover and the brass plate to prevent air infiltration from the ambient. Another small O-ring seal (28 mm in diameter) was also
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Fig. 2. Photos and schematic diagram of the evaporator.
placed between the evaporator cover and the upper surface of the wick to avoid any internal leaks of vapor from the evaporating zone to the compensation chamber driven by pressure difference. The transparent cover, the capillary wick, the brass plate and two O-rings were clamped together with a flange bolt connection to guarantee a leak-proof sealing. A nickel-chrome serpentine heater, which was shown in Fig. 3, was made by wire-cutting and bonded to the brass plate with high conductive Omega-bond 200 for electronic insulation. The input power was controlled by a DC power supply (Agilent N5769A). Electric current and voltage were measured by Agilent U3402A with accuracies of 0.2% and 0.012% for electric current and voltage respectively. The heating area of the evaporator was 7.07 cm2 (30 mm in diameter). A fiber glass layer clinging to the brass plate and the vapor line was employed to ensure thermal insulation to the ambient. In order to avoid the damage of the nickel-chrome serpentine heater, heat load imposed was limited to 120 W, and the evaporator wall temperature was limited to 90 °C. Two transport lines were 6 mm in diameter (4 mm in internal diameter), and the lengths were 140 mm for vapor flow and 70 mm for liquid flow separately. In order to dissipate the heat imposed on the evaporator, a fin-and-tube condenser made of copper was applied as shown in Fig. 4. The pipe in the fin-and-tube condenser was also 4 mm in internal diameter and the length was 260 mm. An axial fan was equipped on the condenser, which was controlled by the DC power supply with voltage of 6 V during experiments.
Fig. 4. Photo of a copper fin-and-tube condenser.
The capillary wick for the experiments was 30 mm in diameter, 5 mm in thickness. It was sintered from copper powder to study the effect of the inventory on the heat performance during the LHP start-up and operation. The particle diameter of copper powder in use was ranged in 132–146 lm (mesh size 200–250). Porosity and pore radius were measured by the mercury porosimeter (PoreMaster-60 GT, Quantachrome Instruments). The porosity (e) for the copper wick was 47.26% and the averaged pore radii were 22.65 lm. 2.2. Experimental procedure and data reduction
Fig. 3. Photo of a nickel-chrome serpentine heater.
Before the experiments, proper cleaning of all metal parts was essential to ensure the reproducibility of the experimental results. These parts were heated in a furnace under hydrogen atmosphere at a temperature of around 200 °C for 1 h to remove any corrosion on the surfaces. Similar procedure, such as acid cleaning, washing in the boiling water and drying, could be referred to the literature [24]. Deionized water was selected as the working fluid, because of its high latent heat, non-toxic nature, and easy availability. In order to avoid the effect of non-condensable gases on the evaporation, the deionized water should be degassed prior to charging into the LHP. The degassing and charging systems were composed of a reservoir with a heater, a vacuum pump, a graduated and sealed charging cylinder and three valves. During degassing, a heater in the reservoir was used to maintain the saturated condition of deionized water for at least 2 h. After that, the graduated and sealed
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charging cylinder connected with the LHP was evacuated by the vacuum pump. Then degassed water would flow into the graduated and sealed charging cylinder, then was cooled to the freezing point for removing limited amount of non-condensable gases left in the degassed water. Finally, specified volume of water was injected into the LHP. It should be noted that total volume of the LHP was 19.8 ml, which was measured by the charging cylinder, and the volume of the evaporator (including compensation chamber and the evaporating zone) was 7.5 ml. According to the literature [25], ideal inventory range for the present LHP was 7.83–12.09 ml, which must satisfy the equations for the cold and hot cases. Thus, three inventories (8.0, 10.0 and 12.0 ml) were selected to investigate the effect of inventory on the LHP performance. The corresponding charging ratio was ranged about from 40 to 60%. All tests were conducted with the above mentioned LHP under the ambient temperature of 22 ± 1 °C. Positive elevation, namely the condenser located above the evaporator, was selected to make the capillary wick flooded by water during the experiments. As a result, the evaporator was full of water before the experiments, even when the inventory was only 8 ml. During the start-up process, a constant heat load of 30 W was imposed to the brass plate. From temperature measurement and visualization, the evaporation in the evaporating zone and flow conditions in the compensation chamber could be monitored and recorded by the high-speed camera (Motion Xtra N4) to recognize whether the LHP started successfully or not. The recording frequency in this experiment was 30 fps with the exposure time of 331 ls. After a successful start-up, operation tests begun at a heat load of 40 W and continued at heat load changing with a stepwise increment of 20 W. For each heat load, a steady state was confirmed when the evaporator wall temperatures were maintaining stably within a maximum limit of ±0.5 °C. When the LHP reached a steady state, the flow condition in the evaporator was recorded by the high-speed camera. Due to the limitation of the nickel-chrome serpentine heater, the highest heat load during the operation test was limited to 120 W, and the evaporator wall temperature was also limited to 90 °C. Temperature difference (DT) between two surfaces of the wick was calculated by using Eq. (1)
DT ¼ ðT bw T uw Þ
ð1Þ
where DT was temperature difference across the wick; Tbw was the wick temperature of bottom surface; Tuw was the wick temperature of upper surface. A low temperature difference meant lower thermal resistance and thus high rate of heat leak from the evaporating zone to the compensation chamber through the wick. Moreover, it was necessary to create an enough pressure difference (DP) between two sides of the wick for pushing the working fluid to transport around the LHP. According to the second condition of LHP serviceability (Eq. (2)) [1], this pressure difference resulted from the characteristics of working fluid and the saturation temperature difference between the evaporating zone (Tez) and the compensation chamber (Tcc).
DP ¼
dP ðT ez T cc Þ dT Tsat
ð2Þ
where dP/dT was the derivative obtained by the slope of the saturation line of water at the temperature in the compensation chamber (Tcc). These two saturation temperatures (Tez and Tcc) were difficult to measure precisely, but the temperature difference (Tez-Tcc) was proportional to that across the wick (DT). Hence, the temperature difference across the wick (DT) was applied to replace the temperature difference (Tez-Tcc) in discussion.
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In addition, the evaporator thermal resistance was one of the most important parameters that reflect the heat performance of the evaporator during the operation.
Rev ¼
ðT ew T eo Þ ðT ew T ci Þ ¼ Q Q
ð3Þ
where Rev was evaporator thermal resistance; Q was heat load; Teo was the outlet temperature of the evaporator; Tci was the inlet temperature of the condenser. Because of good thermal insulation for the vapor line, Tci was close to Teo. The heat load Q applied to the evaporator can by divided into two parts:
Q ¼ Q e;v ap þ Q e;cc
ð4Þ
where Qe,vap is heat for evaporation; Qe,cc is heat leak. An uncertainty analysis was performed according to the method proposed by Kim et al. [26] and Patil et al. [27]. The uncertainty of temperature was ±0.5 K for all thermocouples. The maximum value of uncertainties of electric current, voltage, input power and evaporator thermal resistance were ±0.64%, ±0.50%, ±0.82% and ±4.93%, respectively. 3. Results and discussion 3.1. Effect of inventory on the start-up process The start-up processes of the LHPs using the same copper wick and charging with three inventories were presented in Fig. 5. Among these three temperature profiles, abrupt increases of Tci (the blue line) and Tco (the pink line) could be found. It should be noted that the condenser inlet and outlet temperatures (Tci and Tco) were employed to monitor the flow of working fluid in the LHP. The former increase of Tci meant that the evaporation had started in the evaporating zone, and vapor was about to enter into the condenser. While the later increase of Tco meant that the condensed water had passed the condenser, and working fluid circulation was established. This was a transition point from the transient operation to the steady operation or oscillating operation. With the help of these two temperature readings, time consuming as well as the flow condition could also be recognized. In addition, the temperature difference between two surfaces of the capillary wick during the start-up was shown in Fig. 6, and flow conditions in the evaporator taken by the high-speed camera were also shown in Figs. 7–9. As shown in Fig. 5(b), it was found that the LHP that employed a 10.0 ml inventory (50% charging ratio) could activate the evaporation in about 50 s, and the circulation of working fluid was finished in 250 s according to the condenser inlet (Tci) and (Tco) temperature readings. During the incipience of start-up (before the abrupt increase of Tco), the upper surface wick temperature (Tuw) was close to the bottom surface wick temperature (Tbw), which meant massive heat leak was found. Correspondingly, a lot of bubbles were also observed on the upper surface of the wick in Fig. 7(a). These bubbles were attaching on the wick without departure due to a low heat load of 30 W during the start-up. This phenomenon could be explained that heat could not dissipate in the form of vapor to ambient in time, and resulted in serve heat leak through the heat conduction across the copper wick according to Eq. (4). The situation did not change until the circulation of working fluid was achieved. When the subcooling water returned from the condenser (abrupt increase of Tco), namely the circulation of working fluid was realized, all temperature (except Tco) decrease rapidly. Once heat could be dissipated to ambient efficiently through the condenser, heat leak would be relieved. Besides that, continuous backflow (subcooling water) from the condenser could keep the
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(a) 8.0 ml (charging ratio 40%)
Fig. 6. Temperature difference between two surfaces of capillary wick during startup process with different charging ratios.
(b) 10.0 ml (charging ratio 50%)
(a) incipience
(b) steady state
Fig. 7. Flow condition of the evaporator with 10.0 ml inventory during the start-up (50%).
(c) 12.0 ml (charging ratio 60%) Fig. 5. Start-up processes of the LHP with three inventories at a constant heat load of 30 W.
water in the compensation chamber at a low temperature, for the temperature of subcooling water was always lower than that in the compensation chamber. As a result of that, Tuw was about 4.5 °C
lower than Tbw during the steady state in Fig. 5(b). The continuous backflow entered into the compensation chamber (the white frame on the upside of figure) was observed in Fig. 7(b) correspondingly, and the bubble on the wick disappeared. Thanks to the continuous backflow, no evident temperature oscillations were shown after the LHP was started, and a steady state was achieved. The evaporator wall temperature maintained at about 44 °C. By contrast, the temperature profile of the LHP with an 8.0 ml inventory (40% charging ratio) was quite different. As shown in Fig. 5 (a), the LHP could establish the circulation in 330 s, and evaporation in the evaporating zone was activated in about 60 s, according to Tci and Tco. Before the backflow of water from the condenser, similar massive heat leak and bubbles attaching on the wick were also found. After the circulation of working fluid achieved, strong temperature oscillations were shown in Fig. 5 (a). The flow condition in the evaporator was recorded by the high-speed camera and presented in Fig. 8. The reason for temperature oscillation was that the dry-out occurred periodically in the evaporating zone owing to low inventory. When there was no
J. Xu et al. / Experimental Thermal and Fluid Science 84 (2017) 110–119
(a) dry-out
(b) backflow
(c) evaporation
(d) dry-out again
Fig. 8. Flow condition of the evaporator with 8.0 ml inventory during the start-up (40%).
water left in the evaporating zone, namely dry-out in the wick occurred (Fig. 8(a) and (d)), the heat imposed on the brass plate would make all the temperatures except Tco increase rapidly because no evaporation taking place in the evaporating zone would result in severe heat leak according to Eq. (4). The temperatures (except Tco) continued to increase until the subcooling water returned from the condenser to the compensation chamber. At this moment, obvious temperature decline could be found because of the backflow (Tco increased), and water level could be seen in Fig. 8(b). Unfortunately, the backflow water was not continuous, which could be testified by the fact that Tco decreased at once instead of maintaining. The temperature profile of Tco seemed like pulses. Owing to the enough superheat in the evaporating zone, water was evaporated into vapor immediately. Even so, the temperatures (except Tco) still increased, because intermittent backflow could not ensure that heat leak was compensated by the subcooling water from the condenser. On the other hand, water level in the compensation chamber would fall due to the intermittent backflow, and dry-out in the evaporating zone turned up again
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in Fig. 8(c) and (d). As a result, strong temperature oscillations were presented, and the peak evaporator wall temperature was up to 67.9 °C. Except the timing of backflow, the upper surface wick temperature (Tuw) was always close to the bottom surface wick temperature (Tbw). Unlike the LHP with a 10.0 ml inventory, the heat leak was severe throughout the start-up process. As the inventory was increased to 12.0 ml (60% charging ratio), a fast start but with temperature oscillations was shown in Fig. 5 (c). Owing to the large inventory, there was more water left in the vapor line and the compensation chamber than the former two inventories during the incipience of the start-up process. On one hand, more water in the vapor line meant the pressure in the evaporating zone was higher, and thus higher saturation temperature was necessary to activate evaporation. On the other hand, the benefit of redundant water in the compensation chamber was that heat leak could be compensated even though evaporation was not started in the evaporating zone. As a result of these two factors, it took more than 100 s to activate evaporation in the evaporating zone with a high Tci temperature of 52 °C, but the heat leak during this interval was not severe, for Tuw was not close to Tbw. No bubbles were found on the wick. In accordance with Tco, the working fluid circulation could be generated in about 240 s. After a successful start, obvious temperature oscillations were also found. Similar intermittent backflow also occurred in this case, which was also verified by the pulse-like Tco profile in Fig. 5(c). However, the frequency of temperature oscillation was higher, and the amplitude of temperature was smaller than ones of the LHP with an 8.0 ml inventory. The recorded flow condition in the evaporator during obvious temperature oscillation was shown in Fig. 9. In Fig. 9(a), there was some water in the compensation chamber, and evaporation was also found in the evaporating zone. As evaporation continued, the water level in the compensation chamber would descend because of the intermittent backflow. As shown in Fig. 9 (b), no water was seen in the compensation chamber with evaporation taking place in the evaporating zone, which meant that there was some water left in the wick and the evaporating zone. At this moment, heat leak in the wick could not be addressed by the water in the compensation chamber, which would result in a high saturation temperature in the wick. All temperature (except Tco) increased. Fortunately, a large amount of subcooling water entered into the compensation chamber (abrupt increase Tco) before the evaporating zone running out of water as presented in Fig. 9(c). All temperature (except Tco) decreased suddenly. However, evaporation in the evaporating zone was terminated by a large amount of subcooling water, since a large amount of subcooling water from the condenser would make the temperature of water in the wick too low to keep evaporation. Consequently, some time for heating the wick was needed to re-activate the evaporation in the evaporating zone (Fig. 9(d)). That was the reason why the increases of all temperature (except Tco) were shown after sudden decreases. Compared with the LHP with an 8.0 ml inventory, the LHP with a 12.0 ml inventory could keep the evaporator wall temperature (Tew) under control, since there was no dry-out in the wick as presented. During the temperature oscillations, the peak temperature was 60.3 °C, and Tew fluctuated around 57 °C, with a maximum limit of ±1.5 °C. In order to evaluate heat leak resulted from different inventories, temperature differences between Tbw and Tuw versus time during the start-up process were calculated and plotted in Fig. 6. As mentioned above, the pressure difference between two sides of the wick (DP), which played a vital role on the flow condition during the start-up and operation of LHP, depended on the temperature difference between two sides of the wick (DT). To establish a large enough DP for driving the circulation of water around the LHP, either a large value of DT or a high saturation temperature was compulsory. When heat leak was severe, namely a small value
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(a) water in cc evaporation in ez
(b) dry in the cc evaporation in ez
(d) water in cc evaporation in ez
(e) dry in cc evaporation in ez
(c) backflow in cc no evaporation in ez
Fig. 9. Flow condition of the evaporator with 12.0 ml inventory during the start-up.
of DT, a high saturation temperature was required which resulted in a high operation temperature. This characteristic was called the self-reliance of the LHP [1]. As seen in Fig. 6, at the beginning of initial 200 s, three temperature differences DT increased at first, then decreased. Three temperature differences DT started from 0 °C because there was almost no temperature difference between two sides of the wick before heat was imposed to the evaporator. When a heat load of 30 W was applied, DT began to increase, for the evaporating zone was heated by the brass plate at first to start evaporation, and resulted in an increasing Tbw. Then Tuw also increased to make DT decrease for the LHP with the inventory of 10.0 ml (50% charging ratio). The reason for the increase of Tuw
was that heat leak could conduct easily through the copper wick to heat the water in the compensation chamber especially when there was no evaporation in the evaporating zone. When evaporation was started in about 50 s, part of heat was used to evaporate water into vapor. The slope of increasing Tbw reduced, but this part of heat could not be dissipated to ambient efficiently. That made DT decrease to the value below 1 °C. Correspondingly, there were bubbles on the upper surface of the wick. When the circulation of water was finished, these bubble disappeared, because most part of heat was dissipated to ambient at this moment. The backflow of subcooling water also compensated the heat conducted by the wick. Hence, heat leak was suppressed, and DT suddenly
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increased to a value of 8 °C and finally maintained at a value of 5 °C when the steady state was accomplished. The LHP with the inventory of 8.0 ml had almost the same DT profile as that of 10.0 ml before the circulation was achieved, but temperature difference oscillation was shown due to the intermittent backflow. DT reduced to about 1 °C (massive heat leak) when dry-out occurred in the wick and the evaporating zone. DT increased to around 10 °C when backflow entered into the compensation chamber. For the LHP employed with the inventory of 12.0 ml, more water in the compensation chamber could relieve heat leak during the incipience of the start-up process. Thus, DT was around 5 °C during the initial 200 s, which was higher than that of the other two inventories. When a large amount of backflow water flowed into the compensation chamber, a sharp decrease of Tuw occurred, and DT increased up to 17.5 °C. The consequent decrease of Tbw made the water temperature in the evaporating zone so low that resulted in the termination of evaporation, therefore another interval was necessary to heat the wick and re-start evaporation. That was the reason why intermittent backflow also occurred in the LHP with a 12.0 ml inventory. Even evaporation was stopped, however, the minimum DT was still higher than that of the 8.0 ml inventory because there was no dry-out in the evaporating zone shown during temperature oscillation.
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(a) 8.0 ml (charging ratio 40%)
3.2. Effect of inventory on operation After the start-up process, the LHPs under different inventories operating with the increment of heat load were carried out. Averaged evaporator wall temperature (Tew) and averaged temperature differences between two sides of the wick (DT, also dT in Fig. 10) for comparison were shown in Fig. 10. However, temperature oscillations were also observed during the LHP operation, averaged Tew and DT could not tell us the flow condition in the evaporator. Therefore, temperature profiles of three LHPs at different heat loads were respectively shown in Fig. 11. As shown in Fig. 10, DT (also dT in Fig. 10) of the LHP with an 8.0 ml inventory was lower than ones of the other two inventories, which meant that the heat leak of LHP with 40% charging ratio was the most severe one among these three inventories. As a result, the LHP with 40% charging ratio had the highest averaged evaporator wall temperature (Tew) during the operations, while the heat capacity was only 80 W with the allowable evaporator wall temperature of 90 °C. Similar to the start-up process, strong temperature oscillation and dry-out in the wick were also shown in Fig. 11(a) at a heat load of 40 W owing to low inventory. However, steady operations were shown when heat load was
(b) 10.0 ml (charging ratio 50%)
(c) 12.0 ml (charging ratio 60%) Fig. 11. Temperature profiles of three LHPs at different heat loads during operation.
Fig. 10. LHPs operating at different heat loads.
added to 60 and 80 W. It could be explained that evaporation rate of the water in the evaporating zone was accelerated at a high heat load, which could speed up the flow circulating around the LHP, relieve or eliminate dry-out occurred in the wick and result in a steady operation of the LHP.
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For the LHP employed with a 10.0 ml inventory (charging ratio of 50%), no evident evaporator wall temperature oscillation was observed throughout the operation as shown in Fig. 11(b). The backflow into the evaporator was relatively stable. Heat leak was suppressed by the backflow from the condenser under the low heat load (40 and 60 W). As increased with heat load, Tuw became higher than Tco gradually, which meant that the heat leak became larger with the increase of heat load. At the heat load of 120 W, a heat pipe effect [16], namely bubble generated on the upper surface of the wick and departed for the compensation chamber owing to massive heat leak, was recorded with the help of highspeed camera in Fig. 12. These bubbles went up and condensed into liquid again in the compensation chamber when meeting with the subcooling water from the condenser, which was like inserting a heat pipe into the compensation chamber. In spite of that, the LHP could operate at a steady state and the evaporator wall temperature Tew could be maintained at 80.4 °C. For the LHP with a 12.0 ml inventory (Fig. 11(c)), evident temperature oscillation was also shown at the heat loads of 40 and 60 W (low heat load), and the steady states were accomplished at the heat loads from 80 to 120 W (high heat load). At low heat loads, the intermittent backflow, which was similar to the situation during the start-up process, was also shown during the LHP operation. The abrupt backflow from the condenser would weaken the evaporation in the evaporating zone, which resulted in the fluctuation of Tuw, Tci and Tew. On account of that, averaged evaporator wall temperature would be larger than that of the LHP with charging ration of 50% in Fig. 10. At high heat loads (80 W or above), the surperheat in the wick was high enough, the influence of a large amount of backflow on the evaporation would be negligible, so no apparent oscillations of Tuw and Tew were shown. The evaporator thermal resistance consisted of the evaporator wall thermal resistance Rwall (the brass plate), contact thermal resistance Rcont (between the wick and the brass plate) and the wick thermal resistance Rwick [28]. The former two thermal resistances had the same contribution to the evaporator thermal
Fig. 13. Evaporator thermal resistance.
resistance, and Rwick was the reflection of the wick saturation condition. The evaporator thermal resistances of these three inventories were shown in Fig. 13. All thermal resistances decreased with the increase of the heat load. Owing to the appearance of the dry-out in the wick, the LHP charging with 40% had highest value (0.24 °C/W) among three inventories at a heat load of 40 W. In addition, the evaporator thermal resistance of the LHP charging with 60% was larger than that of the LHP charging with 50%, since evaporation was interrupted by the backflow. As heat load increased, temperature oscillation became weak, steady states were accomplished and the evaporator thermal resistances of the LHPs charging with 50 and 60% reduced to 0.015 °C/W at the heat load of 120 W. 4. Conclusion In this paper, copper-water LHPs with charging ratio ranged from 40 to 60% were tested to investigate the effect of inventory on flow condition in the evaporator and heat-transfer performance. Based on the experimental results and the visual observation, the following conclusions could be drawn:
(a) no vapor
(b) vapor creation
(c) departure
(d) condensation in cc
Fig. 12. Photos of heat pipe effect in the LHP with a 10.0 ml inventory (50%) at the heat load of 120 W.
(1) The proposed copper-water LHPs with different inventories could start successfully in no more than 330 s at the heat load of 30 W (heat flux of 4.24 W/cm2). During the operation, two LHPs with the charging ratios of 50 and 60% were able to keep steady at least at the heat load of 120 W (heat flux of 16.97 W/cm2) under the allowable evaporator wall temperature of 90 °C. Hence, copper-water LHP could start fast and attain high heat transfer capacity in the temperature rang below 90 °C. (2) Fluid distribution and the flow condition in the evaporating zone and the compensation chamber were observed. Severe heat leak occurred in the copper-water LHP with low inventory (charging ratios of 40 and 50%) was confirmed: (1) during the incipience of start-up process, bubbles resulted from heat leak was attaching on the upper surface of the copper wick; (2) heat pipe effect was shown in the compensation chamber at a high heat load of 120 W (50%). (3) Intermittent backflow from the condenser was observed (charging ratios of 40 and 60%), which resulted in temperature oscillations at low heat loads (30 and 40 W): (1) because of inadequate inventory (40%), dry-out was found in the evaporating zone at low heat load; (2) a large amount of backflow (60%) would make the superheat in
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the evaporating zone too low to keep the evaporation. However, intermittent backflow and consequence temperature oscillation could be relieved at high heat loads. (4) Considered on the start-up process and the operation, the optimal inventory for the proposed flat LHP was 10.0 ml (charging ratio of 50%). Moreover, high inventory (60%) was an effective method for suppressing heat leak especially at high heat loads, but temperature oscillations with high frequency were shown during the start-up process and the operation at low heat loads (40 and 60 W).
Acknowledgements The authors are grateful for the support of the Natural Science Foundation of Shanghai under Grant No. 10ZR1408100. This paper is also supported by the Fundamental Research Funds for the Central Universities, and the National Natural Science Foundation of China (No. 51406120). References [1] Y. Maydanik, Loop heat pipes, Appl. Therm. Eng. 25 (2005) 635–657. [2] C.C. Yeh, C.N. Chen, Y.M. Chen, Heat transfer analysis of a loop heat pipe with biporous wicks, Int. J. Heat Mass Transf. 52 (2009) 4426–4434. [3] P.H.D. Santos, E. Bazzo, S. Becher, R. Kulenovic, R. Mertz, Development of LHPs with ceramic wick, Appl. Therm. Eng. 30 (2010) 1784–1789. [4] Li. Zhang, Xu. Jiayin, Xu. Hong, Effect of inventory on the heat performance of copper-water loop heat pipe, Exp. Therm. Fluid Sci. 44 (2013) 875–882. [5] Y. Maydanik, S. Vershinin, M. Chernysheva, S. Yushakova, Investigation of a compact copper-water loop heat pipe with a flat evaporator, Appl. Therm. Eng. 31 (2011) 3533–3541. [6] Dongdong Wang et al., Experimental study of the loop heat pipe with a flat disk-shaped evaporator, Exp. Therm. Fluid Sci. 57 (2014) 157–164. [7] Ji. Li, Lucang Lv, Performance investigation of a compact loop heat pipe with parallel condensers, Exp. Therm. Fluid Sci. 62 (2015) 40–51. [8] Z. Xingxing et al., The early design stage for building renovation with a novel loop-heat-pipe based solar thermal façade (LHP-STF) heat pump water heating system: techno-economic analysis in three European climates, Energy Convers. Manage. 106 (2015) 964–986. [9] X.X. Zhang, X.D. Zhao, J.H. Xu, X.T. Yu, Characterization of a solar photovoltaic/ loop-heat-pipe heat pump water heating system, Appl. Energy 102 (2013) 1229–1245.
119
[10] Binjuine Huang, Pochien Hsu, Rungje Tsai, Muhammad Mustafa Hussain, A thermoelectric generator using loop heat pipe and design match for maximum-power generation, Appl. Therm. Eng. 91 (2015) 1082–1091. [11] Xianping Chen, Huaiyu Ye, Xuejun Fan, Tianling Ren, Guoqi Zhang, A review of small heat pipes for electronics, Appl. Therm. Eng. 96 (2016) 1–17. [12] E. Bartuli, S. Vershinin, Y. Maydanik, Visual and instrumental investigations of a copper-water loop heat pipe, Int. J. Heat Mass Transf. 61 (2013) 35–40. [13] B. Entremond, J. Ochterbeck, Investigation of loop heat pope startup using liquid core visualization, Proc. ASME Summer Heat Transfer Conf. 2 (2008) 387–393. [14] M. John, S. Jack, J. Brenizer, et al., Study of a loop heat pipe using neutron radiography, Appl. Radiat. Isot. 61 (2004) 701–705. [15] Junwoo Suh, Debra Cytrynowicz, Praveen Medis, Frank M. Gerner, H. Thurman Henderson, Flow visualization within the evaporator of planar loop heat pipe, Space Technol. Appl. Int. Forum (2005) 195–202. [16] J. Ku, Operating characteristics of loop heat pipes, in: 29th International Conference on Environmental System, Colorado, 1999. [17] Jeehoon Choi et al., Experimental investigation on sintered porous wicks for miniature loop heat pipe applications, Exp. Therm. Fluid Sci. 51 (2013) 271– 278. [18] G.P. Celata, M. Cumo, M. Furrer, Experimental tests of a stainless steel loop heat pipe with flat evaporator, Exp. Therm. Fluid Sci. 34 (2010) 866–878. [19] L. Vasiliev et al., Loop heat pipe for cooling of high-power electronic components, Int. J. Heat Mass Transf. 52 (2009) 301–308. [20] Y.F. Maydanik, S. Vershinin, M. Chernysheva, S. Yushakova, Investigation of a compact copper-water loop heat pipe with a flat evaporator, Appl. Therm. Eng. 31 (2011) 3533–3541. [21] S.F. Wang, W.B. Zhang, X.F. Zhang, J.J. Chen, Study on start-up characteristics of loop heat pipe under low-power, Int. J. Heat Mass Transf. 54 (2011) 1002– 1007. [22] M. Nishikawa, H. Nagano, Parametric experiments on a miniature loop heat pipe with PTFF wicks, Int. J. Therm. Sci. 85 (2014) 29–39. [23] W. Zhou et al., Development and tests of loop heat pipe with muti-layer metal foams as wick structure, Appl. Therm. Eng. 94 (2016) 324–330. [24] R. Singh, A. Akbarzadeh, M. Mochizuki, Operational characteristics of the miniature loop heat pipe with non-condensable gases, Int. J. Heat Mass Transf. 53 (2010) 3471–3482. [25] S. Launay, V. Sartre, J. Bonjour, Parametric analysis of loop heat pipe operation: a literature review, Int. J. Therm. Sci. 46 (2007) 621–636. [26] J.H. Kim, T.W. Simon, R. Viskanta, Journal of heat transfer policy on reporting uncertainties in experimental measurements and results, J. Heat Transf. 115 (1993) 5–6. [27] C.M. Patil, K.S.V. Santhanam, S.G. Kandlikar, Development of a two-step electrodeposition process for enhancing pool boiling, Int. J. Heat Mass Transf. 79 (2014) 989–1001. [28] M. Chernysheva, Y. Maydanik, Heat and mass transfer in evaporator of loop heat pipe, J. Thermo Phys. Heat Transf. 23 (2009) 725–731.