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Heat Mass Transfer (2017) 53:3329–3340DOI 10.1007/s00231-017-2051-2

ORIGINAL

Experimental studies of surface modified oscillating heat pipes

Tzong‑Shyng Leu1 · Cheng‑Han Wu1

Received: 29 October 2016 / Accepted: 19 April 2017 / Published online: 26 April 2017 © Springer-Verlag Berlin Heidelberg 2017

OHP system, hybrid OHP system shows more stable and energetic circulation flow. It is found that instead of strati-fied flow, vapor slug flows are identified within the evapo-rator section of the hybrid OHP system that can effectively generate higher pressure force for two phase interfacial flow. This effect is attributed to be the main mechanism for better performance of the hybrid OHP system.

List of symbolsIh OHP heater input current (A)Pin OHP input power (W)Pout OHP output power (W)Q OHP input heat flux (W)Rth Thermal resistance (°C/W)Ta Temperature of adiabatic section (°C)Te Average temperature of evaporator (°C)Tw,in Temperature of the inlet cooling water (°C)Tw,out Temperature of the outlet cooling water (°C)Vh OHP heater input voltage (V)θc Contact angle (°)

1 Introduction

Compare with traditional heat pipes, oscillating heat pipe (OHP) is a new type of two-phase heat transfer device which has the characteristics of simple construction, high heat flux capability and no need of wicking structures for liquid transport. Many research works have been found about OHP systems. Khandekar and Charoensawan study [1] studied various system design parameters such as the diameter, number of loops, and the inclination angle of the working fluid, etc. and discussed how these design parame-ters affect the performances of OHP systems in their study. In 2007, Khandekar and Yang [2] investigated the operating

Abstract Oscillating heat pipe (OHP) is a two-phase heat transfer device which has the characteristics of sim-ple construction, high heat flux capability and no need of wicking structures for liquid transport. There are many studies in finding the ways how to improve the system performance OHP. In this paper, studies of the effects of contact angle (θc) on the inner wall of OHP system have been conducted first. Glass OHP systems with unmodified (θc = 26.74°), superhydrophobic (θc = 156.2°), superhy-drophilic (θc < 10°) and hybrid (superhydrophilic within evaporator region and superhydrophobic within condensa-tion region) surfaces, are studied. The research results indi-cated that thermal resistance of these four OHP systems can be significantly affected by different surface modifica-tion approaches. Although superhydrophobic OHP system can still work, the thermal resistance (Rth) is the highest one of the four OHP systems, Rth = 0.36 °C/W at 200 W. Unmodified pure glass and superhydrophilic OHP systems have similar performance. Thermal resistances are 0.28 and 0.27 °C/W at 200 W respectively. The hybrid OHP achieves the lowest thermal resistance, Rth = 0.23 °C/W at 200 W in this study. The exact mechanism and effects of contact angle on OHP systems are investigated with the help of flow visualization. By comparing the flow visu-alization results of OHP systems before and after surface modification, one tries to find the mechanism how the sur-face modified inner wall surface affects the OHP system performance. In additional to the reason that the superhy-drophobic dropwise condensation surface inside the hybrid

* Tzong-Shyng Leu tsleu@mail.ncku.edu.tw

1 Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan

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limitation of OHP systems. OHP systems, relative to tradi-tional heat pipes, are more complicated. Therefore, many OHP researchers use flow visualization techniques to study the flow behaviors of OHP system. Khandekar et al. pub-lished two papers [3, 4] in 2003 and 2008 which reported the observation of OHP internal flow patterns by using multi-loop or single-loop system. In their studies, bubble, slug and annual flow patterns are found in OHP system. The proportion of slug and annular flow patterns increase with the increase of input power. In addition to some research studies on OHP operating parameters, there are also many studies in finding the ways how to improve the system per-formance OHP. Yulong et al. [5] mixed different mixing ratios of nanoparticles of boehmite alumina of different shapes (platelet, blade, cylinder, and brick) with working fluids (deionized water or ethylene glycol) within OHP sys-tems. The experimental results found that maximum heat transfer performance of OHP system can be improved for the system mixed with cylindrical-shaped nanoparticles. The primary reason why the nanoparticles charged into an OHP can improve the heat transfer performance is sus-pected that the oscillated cylindrical-shaped nanoparticles in the OHP will effectively affect the thermal and veloc-ity boundary layers. Haizhen et al. [6] proposed two meth-ods for improving the performance of OHP, the first one is the unsteady energy input which heated the evaporator with intermittent pulsed heating instead of the usual con-tinuous heating; The second method is to use non-uniform diameter, instead of uniform diameter, OHP systems. The

results showed the heat transfer rate can be increased by 15–38% in an intermittent pulse heating method, and the heat transfer rate is higher by use of non-uniform diameter when the input power greater than 100 W. It is found that the unsteady pulse heating can enhance OHP performance by interacting with oscillating flow or pressure oscillat-ing inside the OHP. The combined effect of external and internal thermal fields is a critical area that requires further investigation. Also Kammuang Lue et al. [7] used the non-uniform power input on the evaporator to affect the system. The results showed that low thermal resistance is achieved when the evaporator is heated at upper, middle and lower locations with high, middle and low power inputs. Such heating arrangement also promotes internal circulation flow pattern in one direction. Yulong [8] investigated the supe-rhydrophobic surface effect on the oscillating motion and heat transfer performance in an OHP. In their OHP, the inner surface of the heat pipe was chemically coated with a superhydrophobic self-assembled monolayer (SAM) of n-octadecyl mercaptan. The heat pipe was charged with water at a filling ratio of 70%. The OHP was tested verti-cally, i.e., the evaporator was on the bottom and the con-denser was on the top. As their results show the oscillating motion in the hydrophobic OHP can start. In other words, the OHP with a hydrophobic inner surface can function. It is very different from the capillary-force-driven conven-tional heat pipe, which cannot function if the inner sur-face is hydrophobic. For an OHP, the gas spring constant of the vapor bubble plays a key role toward initiating and

Fig. 1 Sketch of a unmodified, superhydrophilic or superhydro-phobic surface OHP systems, b hybrid OHP systems

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sustaining the oscillating motion. This indicates that the functionality of an OHP is not too sensitive to the surface wetting condition. However, the heat transport capability occurring in the hydrophobic OHP is not as good as that with a hydrophilic inner surface [9]. The thermal resistance of the OHP with the hydrophobic surface is larger than that with the hydrophilic inner surface.

Although all research works proved wettability can affect OHP heat transfer performance, the exact mecha-nism and effects of contact angle on OHP system is still unclear. They only attributed to the reason that the con-densation inside the OHP system changed its behavior from film-wise condensation to drop-wise condensation

without further detailed discussion. In this paper, four different surface modified OHP systems are studied. Glass OHP systems with unmodified (θc = 26.7°), supe-rhydrophobic (θc = 156.2°), superhydrophilic (θc < 10°) and hybrid (superhydrophilic within evaporator region and superhydrophobic within condensation region) sur-faces are fabricated, as shown in Fig. 1. The objectives is to study the system performance and mechanism of four different treatment OHP system. By comparing the testing results of OHP systems before and after surface modification, one tries to find the mechanism how the wettability of inner wall surface affects the OHP system performance.

Fig. 2 Sketch of OHP experi-mental setup

Fig. 3 DI water droplets on the a unmodified b superhydrophilic and c superhydrophobic glass surfaces

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2 Experimental setup

The experimental setup shown in Fig. 2 consists of an OHP system, electrical wire heater, cooling bath and thermal couple temperature measuring systems. In order to form liquid plugs, a glass tube with an inner diameter of 2.2 mm and outer diameter of 4.0 mm was used to construct OHP system. As shown in Fig. 2, the OHP has three turns and three sections: evaporator, adiabatic, and condenser sec-tions with the length of 80 mm each. The condenser section was directly cooled by a constant-temperature cooling bath. The data acquisition system controlled by a computer was used to record the experimental data. Total of 20 K-type thermocouples were used in the current experiment.

Sixteen thermocouples on the outer surface of the tube at evaporator, adiabatic and condenser sections measure the wall temperatures of the OHP and two thermocouples at cooling bath to measure the cooling water inlet tempera-ture (Tw,in) and outlet temperature (Tw,out). The temperature measurement accuracy of the system is ±1.0 °C. The maxi-mum total uncertainty is about ±1.8 °C. The response time of thermal couple test between 0 and 100 °C is less than 0.5 s.

Figure 2 shows the locations of these thermocouples. The six temperature measurement points within evapora-tor section were presented as the average temperature Te. Thermal couples at adiabatic section in Fig. 2 were used to measure temperature of OHP system and denoted as Ta1

Fig. 4 Temperature measure-ment of a unmodified OHP, b superhydrophilic, c superhydro-phobic, d hybrid OHP systems at power inputs from 70 to 200 W

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to Ta6 from left to right. The whole test section including the OHP, cooling block, and heater were well insulated to minimize the heat loss.

From the temperature measurements, one can calculate thermal resistance (Rth) and efficiency of system perfor-mance (η) to present OHP system performance. OHP ther-mal resistance (Rth) and Coefficient of Performance (η) are defined as:

(1a)Rth =Te − (Tw,in + Tw,out)/2

Q

(1b)η =Pout

Pin

where Te is the average temperature of evaporator section, and Tw,in and Tw,out are the cooling water inlet and outlet temperature at the cooling bath. Pin is power input of OHP system which equals heat flux (Q) of OHP in watt and Pout is output power can be defined as:

where Ih, Vh are the input current and voltage of the heat-ing plate. m is the mass flow rate of cooling water, and C (~4.18 J/KgK) is specific heat of water.

(2a)Pin = Q = Ih · Vh

(2b)Pout = Q = mC(Tw,out − Tw,in)

Fig. 4 continued

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3 Procedures of the surface modification on inner wall of copper tubing

The glass oscillating heat pipe is designed by using outer diameter (OD) 4 mm and inner diameter (ID) 2.2 mm glass tubes. In superhydrophilic OHP surface modification, glass tube is immersed in Onid superhydrophilic modifier for 10 min and then heated in high heat furnace at 300 °C for 40 min to fabricate superhydrophilic inner surface of glass tube. On the other hand, in superhydrophobic OHP surface modification, glass tube is immersed in 3 M Novec EGC-1720 electronic liquid for 10 min and then heated in a fur-nace at 110 °C for 30 min to make inner surface of glass tube with the nanostructures which can enhance Teflon

conglutinate on glass surface. After EGC-1720 coated, glass tube is immersed in Teflon liquid for 15 min and then heated in a furnace at 135 °C for 30 min. The inner surface of glass tube becomes superhydrophobic. Hybrid OHP is combined superhydrophilic evaporator and superhydropho-bic condenser by using the above-mentioned modification methods.

After the inner surface was treated with the method described above, the contact angle was measured using the contact angle instrument (FTÅ125, First Ten Ångstroms, U.S.A). However, it is very hard to accurately measure the contact angle of the droplet formed on the inner curve surface of the tubing with a diameter of 2.2 mm. In order to accurately determine the contact angle, contact angle

Fig. 4 continued

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measurement on flat glass plate was conducted. Following the same procedure described for the glass tubing of the OHP, one flat glass plate was chemically treated. Figure 3 shows the static contact angle of a water droplet was about θc = 26.74° for unmodified glass surface, θc < 10° for supe-rhydrophilic surface and θc = 156.19° for the superhydro-phobic surface.

4 Experimental results and discussion

The working fluid (water in this study) at a filling ratio of 50% was charged into the OHP. During the test, the inlet temperature of cooling bath was fixed at 15 °C. The power supply was switched on and the input power at 70 W was applied on the evaporator section of the OHP initially. The power was gradually increased in a stepwise

mode with a power increment of 10 W from 70 to 200 W. When the input power was applied or increased, the sys-tem needed time to reach a new steady state. The experi-mental data show that a few minutes at least was needed for the system to reach a new steady state. The input power and temperature data were recorded by a personal computer. The OHP was tested at vertical position (the evaporator on the bottom heated by a uniform electrical flat heater).

Figure 4 shows the temperature measurement of unmodified OHP (Fig. 4a) and superhydrophilic OHP (Fig. 4b), superhydrophobic surface-modified OHP (Fig. 4c) and hybrid OHP (Fig. 4d) at power inputs from 70 to 200 W. As shown in Fig. 4, evaporator temperature (Te) is the average temperature of the six temperature measurement points within evaporator section (Fig. 2). Ta1 to Ta6 is the surface temperature measurement at adiabatic

Fig. 4 continued

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section of OHP from left to right (Fig. 2). For unmodified and superhydrophilic OHP systems in Fig. 4a b, adiabatic section temperature measurements Ta1 to Ta6 show oscil-lating behaviors for 70–100 W. Unlike oscillating behav-iors for unmodified OHP system in 70–100 W of Fig. 4a, less oscillating intermittent circulation behaviors for 110–200 W are found. The temperature measurements separate into high temperature for surface temperature of adiabatic section on the even tubing (Ta2, Ta4 & Ta6) and low tem-perature for surface temperature of adiabatic section on the odd tubing (Ta1, Ta3 & Ta5) from time to time for input power Pin = 110–200 W.

For superhydrophobic OHP system in Fig. 4c, evapo-rator section temperature measurements Te show large amplitude oscillating behaviors for 70–130 W and small amplitude oscillating behaviors for 140–200 W. Adiabatic section temperature measurements Ta1 to Ta6 show oscil-lating behaviors for 70–150 W and intermittent circula-tion behaviors for 160–200 W. For hybrid OHP systems in Fig. 4d, adiabatic section temperature measurements Ta1 to Ta6 show oscillating behaviors for 70–140 W and stable cir-culation behaviors for 150–200 W.

Thermal resistance (Rth) and Coefficient of Perfor-mance (η) of four different surface modified OHP sys-tems are shown in Fig. 5 and Fig. 6. The thermal resist-ance decreases with the increase of input power for most of four surface modified OHP systems, except superhy-drophobic OHP system from 70 to 90 W. Compared ther-mal resistance and Coefficient of Performance between the OHP system before and after modification, it can be

seen from Fig. 5 and Fig. 6 that although the the thermal resistance (Rth) and Coefficient of Performance (η) of the hybrid OHP are inferior to those of the modified OHP and the super-hydrophilic OHP at the low input power of 70–100 W, the thermal resistance (Rth) and Coef-ficient of Performance (η) of the hybrid OHP are better with the increase of the input power after the input power of 110 W. Thermal resistance reaches the lowest about 0.23 °C/W and Coefficient of Performance reaches the highest about 90.09% for hybrid OHP system with input power Pin = 200 W.

The average temperature measurement of the evaporator section shows similar results at 200 W, but there is a huge difference for the temperature measurement of the adiaba-tic section for this lowest thermal resistance case. Working fluid has the trend to stably circulate inside hybrid OHP system in one direction which explain why the temperature difference between Ta2, Ta4 & Ta6 (the even tubing) and Ta1, Ta3 & Ta5 (the odd tubing). But the mechanism for this huge change for hybrid OHP system is still unclear. Flow visu-alization inside hybrid OHP system is performed to study the mechanism of surface-modified OHP system behavior change.

Figures 7, 8, 9 and 10 show the image sequence of two phase flow visualization of four different surface modified OHP systems at power input 200 W, includ-ing unmodified (Fig. 7) and superhydrophilic (Fig. 8),

Fig. 5 Thermal resistance for four different surface modified OHP systems

Fig. 6 Coefficient of performance of four different types of OHP systems

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superhydrophobic surface-modified (Fig. 9) and hybrid (Fig. 10) OHP systems. As shown in Figs. 7a, 8a, 9a and 10a, two phase flow within adjacent tubes shows bub-ble or slug flow pattern with low vapor quality in one tube. The direction of bubbles or vapor slugs in the tube is moving downward from condensate to evapo-ration section (see the marked box regions in Figs. 7a, 8a, 9a and 10a), while the higher vapor quality annual, mist or churn flow patterns moving up from the evapo-ration section to the condensate section is found in its adjacent tube. At this moment in time, flow visualiza-tion results do not show too much difference among two phase flow patterns within four different surface modi-fied OHP systems at power input 200 W. Flow direction

can be inferred throughout the OHP system that internal working fluid flow behavior showing a circulation phe-nomenon. However, flow visualization shows change if one looks more carefully by zooming in only one loop of evaporator regions within four different surface modi-fied OHP systems. For unmodified (Fig. 7b), superhy-drophilic (Fig. 8b) and hybrid (Fig. 10b) OHP systems, the vapor blocks or bubbles expand their volume rap-idly once they enter the evaporator regions. Since the evaporator surfaces of unmodified, superhydrophilic and hybrid OHP systems are either hydrophilic (Fig. 7b) or superhydrophilic (Figs. 8b and 10b), the working fluid can spread over the heating surfaces easily. It enhances the evaporation process and generate a huge pressure

Fig. 7 Image sequence of two phase flow visualization for unmodified pure glass OHP at 200 W a complete OHP view b single loop zoom in view of evaporator and adiabatic sections

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force within the OHP systems. On the superhydrophobic OHP system (Fig. 9b), Leidenfrost phenomena are found in the evaporator section. Among unmodified, superhy-drophilic and hybrid OHP systems, one major differ-ence is that stratified flow (the expanding vapor slug above a liquid slug around the tubes turning) is found for unmodified (Fig. 7b), and superhydrophilic (Fig. 8b) OHP systems, but no stratified flow is found within hybrid OHP system. Instead of stratified flow, a continu-ous expanding vapor plug is identified within the hybrid (Fig. 10b) OHP system. From the image sequence of two phase flow visualization for hybrid OHP at 200 W Fig. 10a, b), one can see flow patterns in Fig. 10a that

even tubes are different from the odd tubes. The even tubes show that the continuous vapor plugs (Fig. 10b) originated from the evaporator section become the mist and churn flow patterns (Fig. 10a) in the adiabatic sec-tion. These continuous expanding vapor plugs generate higher pressure force than the stratified flow found in the unmodified and superhydrophilic OHP systems. One possible speculation is that this effect promotes a sta-ble and stronger circulation for hybrid OHP system that causes the temperature at the evaporator of the hybrid OHP system (Fig. 4d) is lower than the unmodified (Fig. 4a), and superhydrophilic (Fig. 4b) OHP systems at 200 W.

Fig. 8 Image sequence of two phase flow visualization for superhydrophilic OHP at 200 W a complete OHP view b single loop zoom in view of evaporator and adiabatic sections

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5 Conclusions

For hybrid OHP system, the internal flow behavior has changed dramatically. Hybrid OHP system shows that working fluid has the prefer trend to circulate inside OHP in one direction energetically and stably, which is simi-lar effect achieved by using one-way valve within OHP system, that can enhance OHP system performance. The thermal resistance of hybrid OHP system decreases

to 0.23 °C/W. Compared with the thermal resistance of the unmodified OHP system 0.28 °C/W, it is about 18% decrease. The mechanism for this enhancement for hybrid OHP system is speculated two phase flow pattern changes to promote the strong and stable circulation for hybrid OHP system. Instead of stratified flow found within unmodi-fied, and superhydrophilic OHP systems, continuous vapor plug flows are discovered in the evaporator sections of the hybrid OHP system. They generate stronger pressure force

Fig. 9 Image sequence of two phase flow visualization for superhydrophobic OHP at 200 W a complete OHP view b single loop zoom in view of evaporator and adiabatic sections

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to support strong and stable circulation and lower the ther-mal resistance of the hybrid OHP system.

Acknowledgements The study was supported by the funding from Ministry of Science and Technology, Taiwan under the contract of MOST 105-2221-E-006-122 and NSC 102-2221-E-006-091-MY3.

Compliance with ethical standards

Conflict of interest On behalf of all authors, the corresponding author states that there is no conflict of interest.

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Fig. 10 Image sequence of two phase flow visualization for hybrid OHP at 200 W a complete OHP view b single loop zoom in view of evapo-rator and adiabatic sections