Thermal performance of the flat micro-heat pipe with

International Journal of Heat and Mass Transfer 125 (2018) 658–669

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Thermal performance of the flat micro-heat pipe with the wettability gradient surface by laser fabrication Xiaozhu Xie ⇑, Qing Weng, Zhiqiang Luo, Jiangyou Long, Xin Wei School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou, Guangdong 510006, China

a r t i c l e

i n f o

Article history: Received 13 December 2017 Received in revised form 19 March 2018 Accepted 23 April 2018

Keywords: Gradient wettability surface Flat micro-heat pipe (FMHP) Capillary grooved structure Heat transfer Laser fabrication

a b s t r a c t The flat micro-heat pipe (FMHP), a high-efficiency heat conducting device, mainly depends on the phase change backflow in the internal micro-groove to enhance the heat transfer. Thus, the smaller capillary structure and higher capillary flow capability are the key factors to enhance the thermal performance. The grooved structure is processed by pulsed fiber laser to achieve a larger capillary force. Combined with the surface properties modification of laser interaction with metal and the theory of gradient wettability surface driving force, different capillary structure and wettability surface are prepared. Then gradient wettability surface are regulated through immersing the samples into hydrogen peroxide solution. The thermal performance of the FMHP with different capillary structure as well as gradient wettability surface is carried out and the gradient structure contrast is investigated. Results indicate that the capillary structure of FMHP with a wettability gradient distribution of the contact angle varied from 0° to 45° possesses a higher thermal power of 50 W and lower thermal resistance of 0.002 °C/W. The minimum thermal resistance of the heat pipe with gradient wettability surface is tenfold lower than that without gradient wettability surface. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Thermal conductivity of flat micro-heat pipe (FMHP) has already exceeded any known metal at present [1]. As a kind of heat-conducting device suitable for high heat flux, FMHP has been widely applied in electronic components, precision instrument power supply and the electronic device of aerospace with its excellent isothermality and high thermal conductivity. Because of the features and applications, there are many researchers carried on the experiment to analyze the influence factors of wick structure, working fluids, evaporation and condensing to enhance the heat transfer performance. The wick structure is an important factor in limiting the heat transfer of the FMHP. The thermal conductivity of the wick is mainly affected by the capillary backflow of the working liquid inside the pipe. Therefore, increasing the ability of capillary flow by fabricating a capillary structure with a larger capillary force can enhance the thermal transfer efficiency. In the preparation of internal capillary structure of FMHP, the wick used as internal reflux has two types including the grooved and sintered. The sintered wick has a strong capillary pressure, but the backflow resis⇑ Corresponding author. E-mail address: [email protected] (X. Xie). https://doi.org/10.1016/j.ijheatmasstransfer.2018.04.110 0017-9310/Ó 2018 Elsevier Ltd. All rights reserved.

tance between the wick and inner wall of the tube is large, resulting in a strong contact thermal resistance. Meanwhile, the sintering process is complex and the structure of the wick is easily damaged. However, different from the sintered wick, the grooved wick mainly depends on the performance of the capillary grooved structure. When the amount of capillary force backflow cannot satisfy the evaporation, the evaporative section easily dries up and the tube wall temperature will rise sharply, causing the tube to burn out. Thus the strength of capillary capacity is restricted by the groove heat pipe working ability. The microstructure is mainly expressed in the improvement of the capillary force driving the liquid backflow, and the smaller capillary structure is, the larger capillary force can be achieved. There are also many methods for the preparation of capillary grooved wick. Chen et al. [2] processed hetero-groove structure on the silicon substrate by the method of Bosch deep reactive ion etching (DRIE). And then the effect of liquid flows was analyzed with different shape of the grooves structures. The result indicated that the straight channel with different channel width could raise the capillary flow. Cao et al. [3] used the electrical discharge machines (EDM) to process the groove with the structural parameter of 100 lm width and 250 lm depth on the copper micro heat pipe. The effective thermal conductance of the heat pipe was on the order of 40 times that

X. Xie et al. / International Journal of Heat and Mass Transfer 125 (2018) 658–669

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Nomenclature a Av b h L 4T R q Pc Pa Qmax k

groove interval, lm steam chamber cross-sectional area, m2 groove width, lm groove depth, lm length of micro channel, m temperature difference, °C thermal resistance, °C/W heat flux, W/m2 capillary driven force, N/m gradient driven force, N/m maximum amount of thermal power, W heat conductivity co-efficiency, W/(m °C)

of copper based on the external cross-sectional area of the miniature heat pipe. In order to further enhance the capillary force of the grooved wick and promote the liquid working medium circulation flow, the inner wall of the FMHP processed to gradient wettability surface is a kind of effective method to improve the capillary flow and thermal performance. Heat transfer enhancement [4] is one of the potential applications among the various applications of the gradient wettability surface. The gradient wettability surface causes the imbalance of young’s force for working liquid and the formation of the Marangoni effect, which facilitating the circulation of liquid flow [5], and increasing the heat transfer in heat pipe. The chemical composition modification and the surface microstructure fabrication are the main two kinds of methods to regulate the wettability on the metal surface. The chemical composition modification including the vapor deposition [6], the chemical modification [7] and the illumination method [8], can cause the changes of the active molecules on the surface and form the gradient properties. Youngsuk Nam et al. [9,10] applied the electroforming and chemical deposition to fabricate the copper postwicks on the silicon substrate. Then the copper post-wick structure was treated by the chemical oxidation, which can improve the wetting property and the capillary force. The performance of the micro post-wick provided both high effective transfer coefficients and high critical heat flux. Sun et al. [11] used the laser processing to derive a series of different interval of parallel grooves on the smooth silicon surface, and prepared a rough surface with a gradient surface morphology. Yoshihiro et al. [8] used the wavelength of 172 nm UV light to irradiate the SiO2 surface, making the high surface energy group moving to the low surface energy group. Then the intensity of illumination time was controlled to cause the contact angle (CA) changing from 100° to 25° gradually gradient surface. The functional structure on the surface can further enhance the heat transfer performance of micro heat pipe. In the aspect of theory of the gradient surface enhancing the heat transfer performance, Suman [12] investigated effects of surface-tension gradients on the performance of a micro-grooved heat pipe. They found that with a favorable surface-tension gradient, the liquid pressure dropping across the heat pipe can be decreased by 90%, and the maximum heat throughput can be increased by 20%. By contrasting with an unfavorable surfacetension gradient, the liquid pressure dropping increases by 150%, and the maximum heat throughput decreases by 15%. Qu et al. [13] studied a functional surface with the axial ladder contact angle distribution. The thermal performance of a triangular micro heat pipe was analyzed based on a one-dimensional steady-state model. Compared with the traditional micro heat pipe with a uniform contact angle distribution on its surface, the simulation results showed that a micro heat pipe with a functional

Greek symbols p density, kg/m3 r surface tension of liquid, N/m h contact angle, ° g absolute viscosity of liquid, Pas hmax maximum contact angle, ° condensing section contact angle, ° h0 hx contact angle of x section, ° he evaporative contact angle, °

surface can remove a greater amount of heat under the same condition. The EDM could machine a deep groove, but when the width of the groove is less than 100 lm, the capillary structure may be difficult to obtain. Electrochemical machining method can fabricate all kinds of complex capillary structure directly in the micro heat pipe inner surface without any damage to the wall. But the processing time is too long and the process is impolite to the environment. Laser processing method can process the high aspect ratio and suitable capillary structure, and with the characteristics of the flexibility and non-contact processing, it won’t cause damage to the pipe wall and is able to fabricate all kinds of complicated capillary structure. Also laser processing method used on the copper substrate can fabricate complex microstructure and change the molecular activity on the surface of copper substrate by high energy laser beam irradiation, so that both stability and superior performance of gradient surface wetting can be achieved [14]. In order to strengthen the FMHP heat transfer performance, the pulse fiber laser is used to fabricate different interval microgrooves on copper substrates, and the different gradient surfaces are regulated. The gradient driving force model is established to analyze the change of driving force on different gradient wettability surface. The thermal performances of the FMHP with different capillary structure as well as different gradient wettability surface are carried out. Combined with theoretical analysis, optimal gradient distribution on the micro-groove surface can be achieved. 2. Experimental setup Fig. 1 shows the structure diagram of temperature test system for FMHP. The experimental device is made of temperature information acquisition unit, the replaceable flat-plate heat pipe, heating system, condensation system, vacuum pump and liquid injection. The copper substrate with different grooved structures can be replaced easily and quickly. The upper cover plate, copper substrate and the lower cover plate are sealed together and fastened by the screw and silica gel to prevent liquid leakage and ensure the tightness well. In this device, the evacuation and the liquid injection can be carried out simultaneously in the replaceable flat-plate heat pipe. At one side of the heat pipe, the heater is cling to the shell of pipe, which supplies the thermal energy through the power source. And on the other side, the condenser provides a cold source to chilling and dissipates the heat from the heat source. Both of them form a cycle system and make the liquid moving in the evaporative and condensing section of the heat pipe. The temperature signal is measured by the four thermocouples and transmitted to the data acquisition card. K-type thermocouples with diameter of 1 mm are bonded on copper substrate surface, two points on the evaporator section (T1, T2), two on the condenser

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Fig. 1. Structure diagram of temperature test system for FMHP.

section (T3, T4). The temperature profile and difference will be processed through computer. Therefore, the heat transfer performance data of different grooved structure and different gradient surface can be acquired. The advantage of the replaceable flat micro-heat pipe test device is that a number of experimental groups only need to replace the copper substrate with capillary structure. In order to reduce the effect of the viscous limit and sound velocity limit, the replaceable flat micro-heat pipe steam cavity size is set to 100 mm 10 mm 5mm (length width thickness). The upper PMMA cover plate is used to observe the liquid movement with different capillary structure under different working conditions. Table 1 shows the detailed parameters of the heat pipe. In the steps of working liquid charging, the vacuum pump is firstly used to bring the FMHP to vacuum condition and the working liquid is injected into the pipe steam cavity by operating appropriate valves. Secondly, the valve connects the vacuum pump and it is closed before opening the liquid charging valve. Thirdly, when liquid charging unit valve is opened, the liquid gets sucked into FMHP due to the low pressure prevailing in it. In the pipe, there exists a possibility for a part of the water injected to get evaporated. After injecting distilled water, the heat pipe is vacuumized again. Fig. 2 shows the laser fabricating system including fiber pulse laser source, scanning and X/Y/Z mobile platform. A focal beam

diameter of 20 lm can be achieved. The micro-grooves structures on the copper substrate are fabricated by the scanning through the control of PC. The copper slag can be taken away by the dust exhauster. The copper substrate sample in the experiment is 99% purity with the dimension of 100 mm 20 mm 1 mm (length width thickness). Before laser ablation, it is polished to no scratches by SiC (500–3000 mesh) in sandpaper and ultrasonic cleaned about 15 min in deionized water to remove the dusts and impurities. Both SEM (HITACHI-S-3400 N-II, Hitacchi, Japan) and laser focusing microscope (OLYMPUS-OLS4000, Olympus, Japan) are used to detect the morphology features. The testing equipment is used to compare and analyze the capillary groove morphology characteristics of micro heat pipe under different processing parameters. The surface contact angle with different groove structures is measured by the contact angle tester (OCA15Pro, Dataphysics, German).

Table 1 The parameters of the FMHP and test conditions.

@u @v þ ¼0 @x @y

Overall length (mm) Length of the evaporator section (mm) Length of the condenser section (mm) Heat pipe wall material Wall thickness (mm) Working fluid Working fluid filling ratio Heat input to the evaporator (kW) Heat flux at the evaporator section (kW/m2) Cooling water inlet temperature (°C) The vacuum degree (Pa)

100 10 10 Copper 1 Distilled water 0.3–0.5 0.2–0.6 30–60 20 0.9 Pa

3. Theory analyses 3.1. Gradient driving model According to the working liquid flow diagram shown in Fig. 3, the velocity distribution of the inner micro heat pipe capillary flow is given as [15]:

ð1Þ

The Navier-Stokes equation describes the liquid working medium flow characteristics as [16]:

! @u @u @u @P @2u @2u þu þv ¼ þ qg x þ g p þ @t @x @y @x @x2 @y2

! @v @v @v @P @2v @2v p þ qg y þ g þ þu þv ¼ @y @t @x @y @x2 @y2

ð2Þ

ð3Þ


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Fig. 2. Schematic diagram of pulse fiber processing system.

Fig. 3. Schema of micro heat pipe working.

where p is the liquid pressure, u and v is the flow velocity of x, y direction respectively, g is the acceleration of gravity, g is the liquid viscosity. The speed of the liquid in inner pipe moving along the section orientation (non-axial) is small, so y = ±h/2 and v = 0 are considered as the boundary conditions. Navier-Stokes equation is simplified as follows: 2

@ u 1 @P ¼ @y2 g @x

ð4Þ

The velocity of the working liquid moving to different distance x of the micro heat pipe is expressed as:

2 dx h @p ¼ dt 12g @x

ð5Þ

Because the liquid film thickness in micro heat pipe condensing section is higher, when the liquid working medium flows, the gravity drive force can be expressed as:

Pg ¼ qgH

ð6Þ

The capillary force is the main driven force neglecting the liquid gravity effect. According to the Young – Laplace formula [17], it is expressed as:

Pc ¼

2r cos h h

ð7Þ

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Table 2 The contact angle and morphology regulation at different grooved interval. Groove interval (lm)

Contact angle

Morphology

Groove morphology

50

5μm

50μm

80

50μm

5μm

100

50μm

5μm

150

50μm

5μm

200

5μm

50μm

Fig. 4. The cross section of the micro groove at the interval of 80 lm.

Fig. 5. The variation of the CA with time stored in atmosphere and vacuum.

cos hx ¼ cos h0 þ ðcos he cos h0 Þ From Eq. (7), the backflow of capillary force is related to capillary structure surface contact angle h, surface tension r and the groove width of h. The capillary force is increased with the decrease of the contact angle h and groove width h. If the contact angle of surface is continuous and homogeneous, the contact angle hx can be defined as:

x L

ð8Þ

where the hx is the contact angle, x is the distance away from the condensing section, he is the contact angle of evaporative section, h0 is the contact angle of condensing section. The driving force produced by gradient wettability surface is expressed as:


@Pa 2rðcos he cos h0 Þ ¼ hL @x

ð9Þ

From Eq. (9), the driving force of the gradient wetting surface is not only related to the surface tension r and the groove width h, but also associated with the changes of surface contact angle. From Eq. (9), it can be derived that the great contact angle range from he to h0 , the greater driving force is. The driving force of the working liquid at x position is expressed as:

@p qgH 2r cos hx 2rðcos he cos h0 Þ ¼ þ þ @x x hL xh

ð10Þ

From Eq. (10), if the distance x is increased, the components gravity and capillary force of the driving force are decreased too. But for the gradient surface with continuous contact angle, the gradient driving force does not change along with the displacement of the working liquid. Therefore, the micro heat pipe with gradient wetting surface can improve the recycling capacity of the working liquid. Especially in the condition of the micro heat pipe with long axial length, the gradient wettability force is particularly excellent. 3.2. Thermal performance test method The main parameters of micro heat pipe heat conduction performance are the maximum amount of thermal conductivity Qmax, temperature difference 4T, thermal resistance R, heat flux q and heat conductivity co-efficiency k. The transfer of heat in the pipe from the evaporation to condensation section Qmax is estimated as

Q max ¼ Q Q s

ð11Þ

where Q is the total load in the quantity of heat of evaporation, Qs is the air heat dissipation losses. The temperature difference 4T is the difference between the evaporator temperature T1 and the condenser temperature T2

DT ¼ T 1 T 2

ð12Þ

The thermal resistance R, heat flux q and heat conductivity coefficient k are given respectively as:

R ¼ DT=Q

ð13Þ

q ¼ Q=Av

ð14Þ

k ¼ q L=DT

ð15Þ

where Av is the cross-sectional area of steam chamber, L is the micro heat pipe length.

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Each part of the micro heat pipe is measured by the temperature information acquisition system. Method of maximum power measurement is used to measure the thermal conductivity of micro heat pipe. The way is that the 4T is measured by the increase of each power of 1 W, until the micro heat pipe burns out. In this time the heating power is the maximum thermal conductivity of the micro heat pipe Qmax. The thermal resistance R is expressed as:

R ¼ DT=Q max

ð16Þ

The thermal performance of different capillary structure, spacing of capillary structure, wettability surface and the wettability gradient surface are compared with each other. 4. Results and discussion 4.1. Laser ablated copper surface microstructure morphology Table 2 shows the SEM images and the contact angle of microgroove structure processed by direct laser ablation at the power of 20 W, scanning speed of 200 mm/s, pulse width of 100 ns, pulse repetition rate of 40 kHz and scanning number of 10 times. The morphology of the grooves is about 140 lm deep and 40 lm wide. The contact angle of the substrate was detected for 5 times. All the surfaces are placed in air for about two weeks When the groove interval increases from 50 lm to 200 lm, the surface contact angle decreases from 145° to 125°. The variation of the surface wettability is explained by the surface roughness factor according to the Wenzel model [18]. From the SEM images of different groove interval, when the groove interval is equal to 50 lm, the copper surface is covered by the fluffy ball particles completely and the surface roughness factor is higher than other groups obviously, which means its surface contact angle reaches the maximum value of 145.25°. With the increasing of groove interval, few fluffy globular particles are covered on the surface and debris-free capillary grooves can be observed clearly, which leads to the decrease of the roughness and the contact angle. When the groove interval is equal to 200 lm, the morphology of the grooved surface is almost totally debris-free and the lowest contact angle of 125.45° is achieved. Fig. 4 shows that the cross section of the micro groove is close to U-shape after laser fabrication. In the direct laser ablation, no matter what designed parameters are used for processing, the copper surface shows hydrophilic characteristic initially. Although the physical microstructure is an important factor for wettability, it still does not exist independently from the surface chemical properties. In addition to structural factors, laser on copper substrate surface also causes certain chemical contents changes like cupric oxide formation [19–21].

Fig. 6. The illustrations for gradient surface regulation on micro-grooved substrate.

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Fig. 7. Gradient wettability surface process by laser.

Fig. 8. The XRD of the copper composition after air placement and laser processing.

The remelting layer near the groove after laser processing is transformed into hydrophobic cuprous oxide when placed in air [22], making the surface becomes hydrophobic. In Table 2, the surface is placed in air for about two weeks and the content of the cupric oxide was deoxidized to cuprous oxide, which makes the surface show better hydrophobicity. Besides, different interval of the micro grooves will obtain different hydrophobic surface.

Fig. 10. Driving force at different position of heat pipe under different wettability.

4.2. Capillary gradient structure regulation After laser processing, the copper substrate presents superhydrophilic in the initial state, and the hydrophilic properties are irrelevant to the structural shape. But when the copper substrate is placed in atmosphere or vacuum environment, the surface con-

Fig. 9. SEM photos of the grooved morphology (a) after laser processing and (b) air exposure.

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Fig. 12. Heat transfer property of heat pipe with different groove interval. Fig. 11. Driving force at different position of heat pipe under different gradient surface.

tact angle increases gradually with time and reaches the constant value finally, which is shown in Fig. 5. The contact angle increment on the copper surface in vacuum is faster than that in air. The contact angle reaches to almost the same value of 140° after being placed in air or vacuum for two weeks. Thus the different hydrophobic surfaces can be regulated through controlling the exposure time of the copper substrate. In the process of gradient surface preparation, the wettability of the parts of the capillary structure can be accurately obtained through controlling the exposure time of each part of the surface. The gradient wettability surface of the capillary structure is constructed by using the rule. In order to guarantee a stable hydrophilic section, a length of substrate is immersed in a concentration of 15% hydrogen peroxide solution with immersion depth of about 10 mm. Different gradient wettability surface can be obtained by adjusting the soak time. Finally, successful preparation of the surface contact angle varying from 0° to 130° is obtained as shown in Fig. 7. And the detailed illustrations is showed in Fig. 6. Figs. 8 and 9 show the XRD and SEM in composition and morphology after laser processing and two weeks air exposure respectively. In the XRD diagram, the main component of the sample after laser processing is copper oxide and copper single substance. Because the copper in the surface layer is easily deoxidized when exposed in air for two weeks, the main component is cuprous oxide besides the peak of copper and copper oxide, which implies that the outer layer material of the copper substrate is converted into Cu2O and the copper surface presents in hydrophobic state. The inside wall of the groove is smooth after laser processing. Some micro/nano particles are attached to the inner wall of the groove after being exposed in air for two weeks which increases the contact angle.

Fig. 13. The wettability surface with the contact angle of 130°.

Fig. 14. Thermal resistance of heat pipe with different wettability.

4.3. Gradient driving force analysis By substituting these parameters (surface tension r = 0.0212 N/m, h = 4 lm, q = 970 kg/m3, L = 100 mm) into Eq. (9), the axial

Table 3 Heat transfer properties of heat pipe with different capillary structure. Interval a (lm)

Starting power P0 (W)

Maximum thermal power Pm (W)

Minimum temperature difference DT (°C)

Minimum thermal resistance R (°C/W)

50 80 100 150 200 0

15 25 20 20 25 25

45 30 30 40 30 30

0.56 0.11 0.77 0.79 0.92 4.2

0.025 0.003 0.024 0.039 0.03 0.25

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Table 4 Heat transfer properties of heat pipe with different wettability. Contact angle (°)





5 45 ± 5 90 ± 5 130 ± 5

15 30 35 35

45 45 40 35

0.56 0.11 0.87 4.33

0.012 0.002 0.022 0.124

Fig. 15. The gradient surface with the contact angle of 0°–90°.

capillary driving force of micro heat pipe at each position is obtained, shown in Fig. 10. When the micro groove surface of FMHP is hydrophilic (CA < 90°), the capillary force is greater than zero. The groove has a relatively strong driving force for working liquid flow. Simultaneously, the contact area of liquid working fluid and capillary structure is large on the low contact angle surface. Therefore, the heat transfer area is also large which is favorable for heat exchange of the heat pipe. When the micro groove contact angle is equal to 90°, the driving force of the micro groove is reduced to zero. When the surface of the capillary structure is hydrophobic (CA > 90°), the surface tension prevents the working liquid from flowing smoothly in the micro groove. The capillary structure is playing a negative role in the recycling of working liquid. Because the working liquid is hard to flow in the micro groove, the working liquid exposed to the steam cavity will increase the sheer force between the liquid and gas block. From Eq. (10), the working liquid is mainly driven by grooved capillary force Pc and gradient surface force Pa if the gravity force is neglected. By substituting actual parameters into Eq. (10), the driving force profile with different gradient surface is calculated, shown in Fig. 11. When the CA of gradient surface is in the range of 45°–0°, it exists a great driving force at the axial direction, and the driving force value is 5 times higher than the CA range of 90°–5° and 90°–45°. When the CA of gradient surface is in the range of 130°–5° or 130°–45°, it encounters a resistance at one distance along the heat pipe, and the maximum driving force is still extremely small. The driving force of all the gradient surface in Fig. 11 is positive and increases to a large value, much greater than that of the hydrophobic surface (CA > 90°) with negative value in Fig. 10, which verifies the gradient surfaces can remarkably enhance the capillary flow in theory.

150 lm and 200 lm, respectively. The heat transfer experiment was repeated for 3 times. When the groove interval a = 50 lm, the starting power is only 15 W and the maximum thermal power is 45 W, which means the power working range of this structure is larger than the others. When the groove interval a > 50 lm, the temperature difference DT between the evaporation section and condensation section increases with the increase of groove interval. When the groove interval comes up to 200 lm, the heat pipe dries up immediately after 2 min, as a result of worsening the heat performance of the heat pipe. Taking results of Table 3 and Fig. 12 into account, the minimum starting power, the maximum thermal power and minimum thermal resistance are obtained when the groove interval is of 50 lm. The reason is that the interval of 50 lm can obtain more microgrooves per unit area and a larger specific surface area to enhance the heat transfer. The smaller the groove interval is, the greater the specific surface area of capillary structure surface is. Meanwhile, a number of micro/nano spherical particles also produced during the laser-interaction with materials covering around the groove will further increase the specific surface area of the capillary surface. It is known that the capillary force increases with the increase of specific surface area of the capillary surface, so the FMHP with denser micro groove has better thermal performance. Copper surface content is transferred into cupric oxide when placed in air after laser fabrication. More contents of cupric oxide mean the more hydrophilic surface, the higher capillary force, and the better heat

4.4. Thermal performance affected by different grooved interval Table 3 shows the thermal properties test of the capillary structural FMHP with the groove interval of 0, 50 lm, 80 lm, 100 lm,

Fig. 16. Heat transfer property of heat pipe with different gradient surface.

Table 5 Heat transfer properties of heat pipe with different gradient surface. Contact angle (°)





5° 45 ± 5°–0° 90 ± 5°–0° 130 ± 5°–0° 90 ± 5°–45 ± 5° 130 ± 5°–45 ± 5°

15 20 25 30 25 20

45 50 45 45 40 40

0.56 0.098 0.21 0.44 0.79 2.12

0.012 0.0002 0.005 0.009 0.020 0.025


transfer performances. Therefore, in order to improve the drive force of the working liquid, the interval of micro groove for laser processing is preferably selected as 50 lm.

Fig. 17. The thermal properties of the micro heat pipe with different gradient surface under the same CA of condensing section.

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4.5. Thermal performance affected by different wettability surfaces The optimal capillary groove structure with groove depth of 140 lm, groove width of 40 lm and the groove interval of 50 lm is chosen to investigate. According to the FMHP surface wettability regulation mentioned above, when the substrate surface is placed 0, 2 days, 5 days and 10 days respectively, the surface wettability obtained is less than 5°, 45 ± 5°, 90 ± 5° and 130 ± 5°. All the samples has almost the same contact angle respectively, as shown in Fig. 13. The FMHP heat transfer performances are shown in Fig. 14 and Table 4. From Table 4, when the micro groove surface is hydrophilic (CA < 90°), the heat transfer properties is better than the hydrophobic surface (CA > 90°). When the contact angle is greater than 90°, the thermal resistance and temperature difference increase sharply due to the surface tension impeding the working liquid into the micro grooves. When the contact angle is less than 90°, the heat transfer of hydrophilic surface is obviously better than that of the hydrophobic surface. When the contact angle is equal to 45°, a minimum thermal resistance can be obtained. Because the lower contact angle surface makes the working liquid completely contacted with the micro grooved surface, it can increase the heat exchange area remarkably. When the surface wettability changes from hydrophilic to hydrophobic, the thermal properties like starting power increasing,

Fig. 18. The thermal properties of the micro heat pipe with different gradient surface under the same CA of evaporator section.

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the maximum thermal power decreasing, and the minimum temperature difference increasing are occurred. When the micro grooved surface contact angle is 90°, the FMHP starts work under high power input, but is easily invalid if continuous increasing the input power. The tendency of the thermal resistance of the hydrophilic surface (CA = 5°, CA = 45°) is considerably few than that of the hydrophobic surface (CA = 90°, CA = 130°). The reason of these experimental results can be clarified in driving force theory of wettability surface in Eq. (9). The capillary groove with a hydrophilic surface (CA < 90°) has a stronger liquid driving force and a larger contact area. When the surface contact angle CA 90°, the capillary structure basically plays a negative role in the backflow of the working liquid. Because the working liquid cannot enter the capillary groove, the working liquid exposed to the steam chamber will increase the shearing force of liquid and the liquid vapor to prevent the backflow. In order to improve the heat performance of FMHP without using gradient wetting surface, the reduction of the contact angle on the surface capillary structure can improve not only the working medium backflow of capillary force, but also the heat transfer area and the heat flow, meanwhile, it can also reduce the sheer force between the vapor and the working liquid simultaneously. Thus the reduction of the capillary structure surface contact angle can improve the FMHP heat transfer performance. 4.6. The thermal performance affected by gradient wetting surface The groove interval of 50 lm of copper substrate is used to prepare the different gradient wettability surface by the method of controlling the exposure time in air. All the samples has the gradient distribution, as shown in Fig. 15. The experimental FMHP heat transfer properties are shown in Table 5 and Fig. 16 respectively. Each group of Table 5 has a good thermal power (40 W). When the evaporative contact angle is equal to zero, it can obtain a lager thermal power and lower thermal resistance. In order to describe the thermal resistance of different gradient surface, we divide the several groups of above-mentioned micro heat pipe with gradient wettability surface into a sub-group based on the same contact angle of the evaporation section or the condensation section. As shown in Fig. 17, by comparing the thermal performance of several groups of gradient surface with the same condensing section wettability, the thermal performance of gradient wettability on the surface of the micro heat pipe is better than that without gradient in various aspects of the thermal performance, and the maximum thermal power can be increased to 50 W. Among the same condensing section contact angle with the different gradient surface, the gradient surface with the greater range of contact angle can achieve a larger maximum thermal power and a minimum thermal resistance R. The reason is mainly explained from the gradient surface driving force theory, the liquid is mainly driven by the capillary force and the greater gradient surface can provide the additional driving force. From the gradient surface driving force theory, we can get the comparison diagram of driving force of the working liquid in micro heat pipe with different gradient wettability capillary structure. And as for the evaporator section of FMHP with the same contact angle, when the moving distance of working fluid increases, the greater the additional driving force provided by the gradient surface is obtained. Therefore, under the same condensation section, the greater gradient variation range has a higher driving force. As shown in Fig. 18(a), when the evaporator section of the contact angle is less than or equal to 5°, the increase of contact angle gradient range cannot improve the thermal performance. When the evaporator section varies from 0° to 45°, the micro heat pipe heat transfer performance is improved. The input power is increased from 45 W to 50 W, and the minimum thermal resis-

tance and resistance is reduced to 0.098 from 0.56. But when the contact angle of the condensing section increases to 45°, the maximum thermal power drops and the starting power increases, which makes the range of working power reduced and the micro heat pipe easily dried up. As shown in Fig. 18(b), the same phenomenon also appears in the evaporator section of the micro heat pipe when the contact angle is 45°, and the heat performance of the heat pipe is poor with the increasing contact angle of the condensing section. Anyhow, in micro heat pipe condensation section, there exists an optimal contact angle value under the same evaporator section contact angle. The over high condensation period of contact angle value will reduce the heat transfer performance of micro heat pipe. According to the Eq. (10), when the contact angle of the evaporator section is fixed as a constant value (>90°) in the gradient wettability surface, the increase of CA of condensing section will make the capillary structure become a continuously variable gradient wettability surface. On the one hand, the gradient surface provides a driving force that can promote the recycling of the working liquid. On the other hand, with the incensement of the gradient range at the same time, the micro groove surface of the capillary force component will be reduced. Because the cos hx of the x axial location with any values is less than evaporative section of cos he (cos hx < cos he), the gradient surface will reduce the micro grooved capillary force. As the contact angle of condensing section increases, it will also improve axial shear force between the steam and the working liquid and hinder the working liquid backflow. Therefore, whether the gradient driving force is greater than the capillary force is decided by gradient surface, which is beneficial to the working liquid backflow. When Pa > DPc, the gradient wettability of the driving force is larger than the surface without gradient surface. If Pa < DPc, the gradient wettability on the micro grooves will reduce the driving force and the heat transfer performance. By contrasting Fig. 10 with Fig. 11, when the evaporation section approaches to the optimum contact angle (CA 5°), the driving force of the condensing section (CA = 45°) is equal to the same hydrophilic surface (CA 5°), and has a better capillary force in gradient surface of evaporation section, which explains that the thermal conduction of the evaporation section with the gradient surface (CA = 45°) is better than the same hydrophilic surface (CA 5°). As shown in Fig. 11, when the condensation section contact angle is greater than or equal to 5°, the driving force of the working liquid is obviously lower than that of the surface contact angle of less than 5°, and the driving force decreases with the increase of the condensing section contact angle. Under the experimental conditions and the geometry size of micro heat pipe in this paper, when the contact angle of evaporating segment is less than 5° and condensation section is 45°, the micro heat pipe will have the optimal heat transfer properties with the minimum thermal resistance of 0.002 °C/W and the maximum thermal power of 50 W. Compared with the heat pipe without wettability gradient surface, the heat pipe with wettability gradient surface decreased the minimum thermal resistance by 10 times.

5. Conclusion The strengthening the FMHP heat transfer performance is investigated in this paper. The pulse fiber laser is used to fabricate different interval micro-grooves copper substrates, and the different gradient surfaces are regulated. The gradient driving force model is established to analyze the change of driving force on different gradient wettability surface. The thermal performances of the FMHP with different capillary structure as well as different gradi-


ent wettability surface are carried out. Main conclusions are made as followers: (1) The gradient wettability groove wick prepared by method of theory and experiment can improve the micro heat pipe working fluid driving force and enhance the heat transfer performance. (2) The different groove interval of capillary structure is fabricated by the wavelength of 1064 nm fiber laser. Smaller groove interval of 50 lm with low launch power can achieve a larger capillary force. The wettability surface with the contact angle of 45° can obtain a minimal thermal resistance. (3) The surface wettability gradient can offer additional driving force for liquid working medium backflow. But oversized the gradient range (condenser CA > 90°) will seriously reduce the capillary force of micro-heat pipe grooves and affect the heat transfer performance, therefore, the gradient range should be controlled in a reasonable scope. When the evaporator section is less than or equal to 0, and the condensation section contact angle is equal to 45°, the FMHP has the minimum thermal resistance of 0.002 °C/W and maximum thermal power of 50 W. Its minimum thermal resistance is decreased by 10 times than that without gradient wettability surface. Conflict of interest None. Acknowledgements Financial assistance for this work is granted by the National Natural Science Foundation of China (No. 51575114), Guangzhou Municipal Science and Technology Project (No. 201607010156), Ordinary University Characteristics Innovation Project of Guangdong Province (Natural Science, No.2014KTSCX059) and Guangdong Natural Science Foundation (No. S2013010014070). References [1] G.M. Grover, T.P. Cotter, G.F. Erickson, Structures of very high thermal conductance, J. Appl. Phys. 35 (10) (1964), 3072-3072.

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