Performance enhancement of a two-phase closed thermosiphon with a thin porous copper coating

International Communications in Heat and Mass Transfer 82 (2017) 9–19

Contents lists available at ScienceDirect

International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt

Performance enhancement of a two-phase closed thermosiphon with a thin porous copper coating A. Brusly Solomon a,b, V. Arul Daniel a, K. Ramachandran c, B.C. Pillai a, R. Renjith Singh d, M. Sharifpur b,⁎, J.P. Meyer b a

Centre for Research in Material Science and Thermal Management, Department of Mechanical Engineering, Karunya University, Coimbatore, India Department of Mechanical and Aeronautical Engineering, University of Pretoria, Pretoria, South Africa Department of Physics, Bharathiar University, Coimbatore, India d Department of Aerospace Engineering, Karunya University, Coimbatore, India b c

a r t i c l e Available online xxxx Keywords: Porous deposition Thermosiphon Porous surface Metallic coating Dendrite structure Thermal performance

i n f o

a b s t r a c t In this study, the heat transfer augmentation of a two-phase closed thermosiphon (TPCT) with a thin, porous copper coating is studied and compared with an uncoated TPCT. The inner surface of the TPCT is coated using an electrochemical deposition process. The coated and uncoated TPCTs are filled with deionised water and tested with a heat input of 50 to 250 W. The heat transfer coefficient in the evaporator and condenser is assessed and compared with the thermal resistance of coated and uncoated TPCTs. The effects of the inclination angle, power input and thin copper coating on the performance of the TPCTs are explored. The heat transfer coefficient of the evaporator is found to be enhanced up to 44% at a heat flux of 10 W/m2 for an inclination angle of 45°. TPCTs with an oxide coating are also compared to those with a metallic coating and the metal-coated TPCT was found to perform better than the oxide-coated TPCT. The effect of non-dimensional numbers, such as Bond (Bo), Webber (We), Kutateladze (Ku) and condensation (Co) numbers, with the variation of heat flux, is also investigated. © 2017 Published by Elsevier Ltd.

1. Introduction A two-phase closed thermosiphon (TPCT) is a passive heat transfer device that operates on the basis of phase conversion heat transfer by means of boiling and condensation. As the heat is realised in the evaporator section, the working fluid absorbs the latent heat and becomes vapour. The vapour then moves towards the condenser section and becomes liquid by releasing its latent heat during the condensation process. The condensed liquid slides through the TPCT wall and reaches the evaporator section through gravity. These TPCTs are simple in construction and transfer a considerable amount of heat over a long distance with a minimum temperature drop. Hence, TPCTs are used in many industrial applications, such as electronic cooling, heat recovery systems, solar collectors and energy storage systems [1–5]. As miniaturisation technology grows faster than ever, the need for efficient cooling devices is also growing. To meet this requirement, heat transfer devices have to be enhanced in order to operate efficiently at small sizes. The performance of TPCTs mainly depends on the type of working fluid, inclination angle and geometry of the enclosure material. The boiling characteristics of the working

⁎ Corresponding author. E-mail address: [email protected] (M. Sharifpur).

http://dx.doi.org/10.1016/j.icheatmasstransfer.2017.02.001 0735-1933/© 2017 Published by Elsevier Ltd.

fluids and the nature of the evaporator surface also play a major role in TPCT performance. Hence, several investigations have been performed to access the heat transfer enhancement of TPCTs with various working fluids [6–18]. This includes traditional fluids such as water, ammonia and acetone [6]. Over the past few decades, nanofluids have been receiving much attention due to the enhancement in thermophysical properties. Hence, thermosiphons have been tested with working nanofluids like Al2O3-water and CuO-water [7], as well as carbon nanotube (CNT) nanofluids [8,9], and it was found that the performance of the TPCTs is enhanced by the nanofluid. Liu, Yang and Guo [8] and Liu, Yang, Wang and Guo [9] investigated the open thermosiphon to identify the optimum mass of nanoparticle concentration to attain the peak heat transfer. In this study, CuO-water and CNT-water nanofluids are used as heat transfer fluids. It was found that the required mass concentration for nanoparticles to attain the highest heat transfer capability was 1.0 and 2 wt% for CuO-water and CNT-water nanofluids [8,9] respectively. Sarafraz et al. [10] studied the thermal performance of a thermosyphon with biologically eco-friendly silver nanofluids. The nanofluid was prepared by green synthesis method with silver nitrate solution and fresh tea leaf extract. Their results showed that the use of nanoparticles in the thermosyphon reduces the evaporator temperature and also it enhances the thermal performance of thermosyphon. Yang and Liu [11] conducted a number of studies to understand the effects of surface

10

A. Brusly Solomon et al. / International Communications in Heat and Mass Transfer 82 (2017) 9–19

Nomenclature A ΔA Bo D g h h hfg Δh k Ku l Q_ ΔQ q Δq R r T We μ ρ σ

area (m2) change in area (m2) Bond number diameter (m) acceleration due to gravity (m/s2) heat transfer coefficient (W/m2-K) heat transfer coefficient (W/m2-K) heat of vaporization (J/kg-K) change in heat transfer coefficient (W/m2-K) thermal conductivity (W/m-K) Kutateladze number length (m) heat input (W) change in heat transfer (W) heat flux (W/m2) change in heat flux (W/m2) resistance (C/W) radius (m) temperature (°C) Webber number viscosity (Ns/m2) density (kg/m3) surface tension (N/m)

Subscripts c condenser e evaporator hp heat pipe t total l liquid v vapour

functionalised nanofluids and conventional nanofluids on the heat transfer characteristics of the TPCT. A thin, porous coating layer was found at the inner wall of the evaporating section of a TPCT with conventional nanofluid. However, no porous layer was found in the TPCT with surface-functionalised nanofluids. Noie et al. [12] observed an improvement in the thermal efficiency of a TPCT with a mixture of alumina nanoparticles and water. Parametthanuwat, Rittidech and Pattiya [13] and Paramatthanuwat, Boothaisong, Rittidech and Booddachan [14] analysed the thermal performance of a TPCT with silver nanofluids. Huminic et al. [15] analysed the heat transfer performance of a TPCT with Fe2O3-water nanofluids. All the abovementioned studies [8–15] show that the thermophysical properties of the working fluid play a major part in the heat transfer enhancement of TPCT. Recent literature [16–19] reveals that the surface structure formed through the operation of a TPCT is the key to enhancing the performance of a TPCT when nanoparticles are suspended in the base fluid. However, in recent years, better pool-boiling characteristics were obtained with a uniform thin porous coating [20–22]. Several researchers investigated the effect of a nanoporous coating on the heat transfer characteristics of the pool-boiling system. These coatings were prepared by the deposition of thin metallic oxides, such as CuO, Al2O3 and TiO2, and pure metals, such as copper, gold and silver. Vemuri and Kim [20] prepared a nanoporous Al2O3 coating with an anodising technique and investigated the pool-boiling characteristics of a nanoporous surface with a saturated dielectric fluid (FC-72) as the working fluid. Chen et al. [21] prepared a hydrophilic surface with a TiO2 coating on a flat surface and studied the pool-boiling heat transfer. El-Genk and Ali [22] found that nucleate boiling heat transfer enhancement was achieved with a copper microporous surface that is prepared on a

copper substrate. In this study, an electrochemical process is used to develop a coating. Kunugi et al. [23] presented a heat transfer enhancement through a nanoporous surface that was prepared by the chemical etching method. All these studies with improved surface properties on the device wall showed a significant improvement in heat transfer. Current experimental studies [24,25] reveal that when a thin oxide coating develops on the inner side of the TPCT enclosure, the heat transfer is enhanced significantly. Generally, the oxide coatings have poorer thermal conductivity when compared to metallic coatings (see Table 1). Hence, metallic coatings have attracted more interest in the areas of electronic cooling and compact heat exchangers. Therefore, Yong et al. [26] conducted a pool-boiling experiment and found that the heat transfer was enhanced using a novel metallic nanoporous surface. In recent times, the use of a metallic porous coating has been extended to many applications, such as heat exchangers, heat sinks, heat pipes and thermosiphons. Hanlon and Ma [27] showed that the thin-film evaporation, which occurs as a result of the sintered porous media, plays an important role in enhancing the evaporation heat transfer coefficient. Lee et al. [28] reported that the evaporation heat transfer enhancement of the heat exchanger with a sintered porous copper coating is twice as good as the plain uncoated heat exchanger. They recorded that this enhancement is mainly due to the thin spreading characteristics of the working fluid in the porous coating. Sun et al. [29] explained the mechanisms involved in the heat transfer enhancement in a porous layer. It was concluded that the microbubbles in the porous layer work like a pump in which they suck the working fluid into the porous layer for the bubble contraction and expel the fluid for expansion. From the abovementioned studies [27–29], it is clear that the heat transfer coefficient in the evaporation process is enhanced with the use of thin, porous coatings. Not only is the evaporation heat transfer enhanced, but the condensation heat transfer is also enhanced with the use of a thin, porous coating. Wang et al. [30] noticed a heat transfer enhancement in the film condensation process in a vertical fluted tube with a thin porous coating. Lee et al. [31] developed a micro-nanoporous coating with polyphenylene sulphide (PPS) and polytetrafluoroethylene (PTFE), which enhances the drop-wise condensation. Furthermore, Chien and Chang [32] investigated the effect of particle size and the porous surface's coating thickness on the evaporator thermal resistance of the flat thermosyphon. The porous coating consists of a sintered copper particle with two different thicknesses and particle sizes. The testing was done with two different saturation temperatures of 60 and 70 °C, and found that the best boiling surface has a thickness of 1 mm with 247 μm particles. Vasilieve et al. [33,34] also studied the performance of the grooved heat pipe with thin, porous deposited evaporators, and found that the heat transfer coefficient is enhanced to a factor of 1.5 to 2. Rahimi et al. [35] studied the thermal characteristics of a thermosiphon with a resurfaced evaporator and condenser. The evaporator surface was coated with SiO2 particles 154 μm in size to make the surface more hydrophilic. The condenser section was coated with (C6H5)2SiOn to make the surface more hydrophobic. By employing the above surface modifications, the thermal resistance of the resurfaced thermosiphon was 2.5 times lower than that of a traditional thermosiphon.

Table 1 Thermal conductivities of metals and metallic oxides. S·No

Material

Thermal conductivity (W/m-K)

1 2 3 4 5 6 7 8

Copper Titanium Aluminium Zinc Copper oxide Titanium oxide Aluminium oxide Zinc oxide

400 22 205 116 110 8 30 21

A. Brusly Solomon et al. / International Communications in Heat and Mass Transfer 82 (2017) 9–19

11

Fig. 1. Surface morphology of uncoated surface at (a) 600×, (b) 3000× and coated surface at (a) 600×, (b) 3000× magnification.

From the above literature, it is understood that the heat transfer enhancement in a pool-boiling experiment with a thin metallic coating is better than that of one with an oxide coating [22,26]. Furthermore, it is understood that most of the porous coatings were prepared using two-step methods via the preparation of particles and sintering or deposition on the target surface, which is a costly and time-consuming process. A few studies have also been conducted on the two-phase systems, such as a thermosiphon with a metallic coating. In these studies, experiments were conducted within limited temperature ranges [32]. Therefore, in the proposed study, a metallic coating is applied to the inner surface of the TPCT and the effect of a metallic coating on the TPCT's performance is investigated under various input powers and inclination angles. To manufacture the thin, metallic porous surface on the inner side of the TPCT, a simple electrodeposition process is adopted [36].

with a concentration of 50 g/lit for about 5 min, and then rinsing it with deionised water. After the pre-treatment process, electrodeposition is carried out for 5 h by maintaining a voltage of 0.5 V between the two electrodes. After coating, the prepared surface is rinsed and cleaned with deionised water and used for further testing. The thickness, pore size and surface morphology of the coated tube is analysed with a scanning electron microscope (SEM). Fig. 1 shows the surface morphology of the uncoated and coated surfaces. It is observed that a rough surface with a cauliflower-like structure is formed in the copper tube during coating. The pore size of the porous coating is in the range of 2 to 10 μm, and the average thickness of the coating is 15 μm. The contact angle of the surface is not measured due to its high fluid-spreading characteristics.

2. Experimental study

The experimental setup for testing the TPCT is presented in Fig. 2. The test facility consists of a resistance heater, wattmeter, variable transformer and data acquisition system (Agilent 34972A). A TPCT with an outer diameter of 19 mm and length of 350 mm is fabricated. The length of the evaporator is 100 mm, the adiabatic section is 100 mm and the condenser section is 150 mm. The temperature distributions of the TPCTs are measured by T-type thermocouples. Twelve thermocouples are fixed on the TPCT's outer wall and two thermocouples are used to measure the inlet and outlet temperatures of the coolant in the condenser. The thermocouples are connected to the data logger and computer to record and monitor the data. The condenser section consists of a cooling jacket, which is made of acrylic pipe. Water from the tank is passed through the chilling unit to maintain a constant temperature. Experimental conditions, such as flow rate, inclination angle, coolant temperature and heating conditions, are presented in Table 2. At the condenser, cooling water at a constant flow rate of 160 ml/min is supplied with a constant temperature of 15 °C. The TPCT is insulated with 4 cm-thick fibreglass to prevent heat loss from

2.1. Preparation of the thin metallic coating An electrodeposition process is performed to create a thin, metallic coating on the inner wall of a copper tube. A copper tube with a length of 350 mm, an inner diameter of 16 mm, and a wall thickness of 1.5 mm is used. The necessary cleaning processes are carried out before the coating process takes place. The copper tube is used as a cathode and a copper rod with a diameter of 5 mm is used as an anode. In order to form a uniform coating, the copper rod (anode) is kept inside the copper tube (cathode) and they are separated by Teflon sleeves. The cathode and anode are 5 mm apart so as to discharge the generated gas. They are immersed in an electrolytic solution. In order to avoid the coating on the outer side of the tube, a Teflon tape is wound over the copper tube. The electrolyte solution consists of a mixture of 0.8 mol of CuSO4 and 0.6 mol of H2SO4. Before performing the electrodeposition, pretreatment is done by rinsing it with a sodium dichromate solution

2.2. Experimental setup and details

12

A. Brusly Solomon et al. / International Communications in Heat and Mass Transfer 82 (2017) 9–19

(b)

(a)

Fig. 2. Schematic view of (a) experimental set up and (b) thermocouple positions.

the TPCT. The thermocouple's accuracy, including the uncertainty in the data logger, is ±0.2 °C. The uncertainties in the temperature and flow measurements are ± 0.5 and ± 3% respectively. The uncertainties in the heat flux and the heat transfer coefficients are calculated as: Δq ¼ q

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2   ΔQ ΔðΔAÞ 2 þ Q ΔA

Δh ¼ h

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2   Δq ΔðΔT Þ 2 þ q ΔT

ð1Þ

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi  2   Δ ΔT hp ΔQ þ Δτ Q

3. Data reduction The heat transfer coefficient of the evaporator and the condenser are calculated by Eqs. (4) and (5). he ¼

ð2Þ

The uncertainty in the total resistance of the TPCT is estimated using Eq. (3) as:

ΔR ¼ R

The estimated uncertainties in the heat flux, heat transfer coefficient and total resistance are found to be less than 6.5%.

ð3Þ

qe Te

ð4Þ

Table 2 Testing conditions. S·No

Fill ratio (%)

Heat input (W)

Inclination angle (°)

Flow rate (ml/min)

1 2 3 4

30 30 30 30

50–250 50–250 50–250 50–250

0 45 60 90

160 160 160 160

A. Brusly Solomon et al. / International Communications in Heat and Mass Transfer 82 (2017) 9–19

13

Fig. 3. Effect of inclination angle on the thermal resistance of (a) uncoated TPCT and (b) coated TPCT.

hc ¼

qc ; Tc

ð5Þ

where he and hc are the heat transfer coefficients of the evaporator and condenser respectively, and qe and qc are the heat flux of the evaporator and condenser, which are calculated as follows: qe ¼

Q_ 2πrle

ð6Þ

qc ¼

Q_ 2πrlc

ð7Þ

Q_ is the heat transferred by the heat pipe, which is calculated based on the energy balance at the condenser. The total resistance of the coated and uncoated TPCT is calculated by the following equation:

T e −T c ; Rt ¼ Q_

ð8Þ

where R t is the total resistance of the TPCT, and T e and T c are the average evaporator and average condenser temperatures respectively. 3.1. Estimation of non-dimensional numbers In order to understand and explain the effect of copper coating on the heat transfer enhancement process of a TPCT, non-dimensional parameters such as Webber (We), Bond (Bo), Kutateladze (Ku) and condensation (Co) numbers are used. For the calculation of non-dimensional numbers, the thermophysical properties of the working fluid are taken from Faghri [42] at average adiabatic temperature of the TPCT, which is considered as a vapour temperature. The Bo number is the ratio of buoyancy forces to the surface tension and is used to realise the importance of surface tension forces compared to the body forces. The Bo number is calculated using Eq. (9) as

Bo ¼

 h 1 ρ −ρv i2 D g l σ

ð9Þ

14

A. Brusly Solomon et al. / International Communications in Heat and Mass Transfer 82 (2017) 9–19

Fig. 4. Wall temperature profiles of (a) uncoated and (b) coated TPCT at the inclination angle of 45°.

A counter-current interaction is one of the important mechanisms for the successful operation of TPCTs as it indicates the available free space for the interaction of liquid film and vapour. The We number is !

Q2

We ¼

ð10Þ

2

ρv D3 hfg σ

The Ku number signifies the pool-boiling occurrence in the liquid pool of the TPCT evaporator and is calculated as

q 

v ρv hfg ρl −ρ ρ2v

7 7 i14 5

Co ¼



1 ! h μ2 3 k gρ2

ð12Þ

4. Results and discussion 4.1. The effects of the inclination angle on the performance of a TPCT

3

2 6 Ku ¼ 6 4h

The amount of liquid that returns from the condenser to the evaporator of the TPCT can be represented using the Co number and can be calculated as

ð11Þ

Fig. 3a and b show the thermal resistance variation of thin coppercoated TPCTs and uncoated TPCTs at different inclination angles. The inclination angle significantly affects the resistance of both coated and

A. Brusly Solomon et al. / International Communications in Heat and Mass Transfer 82 (2017) 9–19

15

Fig. 5. Wall temperature profile of TPCT at 60° inclination angle at (a) 50 W (b) 100 W and (c) 200 W.

uncoated TPCTs at a lower power level, and has a negligible effect at a higher power level. The mechanism that governs the heat transfer in the evaporator is the main reason for this performance variation. The heat is transferred by convection in the evaporator at low power levels due to the presence of a liquid pool. However, at higher power levels, the height of the liquid pool decreases as the boiling rate increases, which results in an increase in the active heat transfer area of the evaporation section. Furthermore, a thin liquid film will be formed at the inner wall of the TPCT due to improvements in the boiling and condensation processes. As a result, a thin-film evaporation heat transfer mechanism dominates at higher power levels when compared to lower power levels. Thus, a lower resistance is obtained at higher heat inputs. A similar observation is found in the previous experimental [25] and numerical [41] studies. Furthermore, it is seen from Fig. 3 that the thermal resistances of TPCTs at horizontal and vertical positions are higher than

the TPCT at an inclination angle of 45°. The thermal resistance of the TPCT at the inclination angle of 90° is also higher than that of the TPCT at a horizontal position. Conversely, a lower resistance is obtained at an inclination angle of 45° compared to the TPCT's other inclination angle. This is mainly due to the liquid inventory in the evaporator, which is caused by gravity. At low heat inputs, the increase in inclination angle leads to an increase in the gravity effect, which leads to an improvement in the liquid returning to the evaporator. Consequently, the thermal resistance decreases. The gravity effect is in a more vertical position, which leads to a higher liquid inventory in the evaporator, which creates a flooding condition. This ultimately results in a higher temperature in the evaporator section and higher resistance [41]. The wall temperature that was recorded along the elevation of the coated and uncoated TPCT at an inclination angle of 45° is presented

16

A. Brusly Solomon et al. / International Communications in Heat and Mass Transfer 82 (2017) 9–19

Fig. 6. Heat transfer coefficients at the evaporator of uncoated and coated TPCTs at an inclination angle of (a) 0° (b) 45° (c) 60° and (d) 90°.

in Fig. 4a and b, and it is observed that the temperature decreases from the evaporation section to the condenser section at all heat inputs. The temperature at the evaporator section of both the coated and uncoated TPCTs is also higher than the temperature at the other two sections. Furthermore, it is seen that the wall temperature increases as the heat input increases for both coated and uncoated TPCTs. At a lower heat input, the evaporator wall temperature is uniform for both coated and uncoated TPCTs. As the heat input increases, the evaporator temperature increases towards the adiabatic end for both TPCTs. This is mainly because of the change in the heat transfer mechanism in the evaporator section, as discussed earlier. 4.2. The effect of copper coating on the wall temperature of a TPCT To explain the effect of porous copper coating on heat transfer, the wall temperature profile of coated and uncoated TPCTs at an inclination angle of 60° is compared in Fig. 5. The average evaporator temperature of an uncoated TPCT at an inclination angle of 60° was found to be 79.3 °C, while that for a coated TPCT was found to be 56.6 °C at a heat input of 50 W. Hence, a reduction of almost 29% in the evaporator temperature is observed. This mainly occurs due to enhancement in the boiling process. It is also seen that there is a difference between the decrease in the evaporator wall temperatures of the coated and uncoated TPCTs as the heat input increases. A similar trend is found in the pool-boiling experiment conducted with a thin metallic coating [26]. The reduction in the coated TPCT's wall temperature may be due to the mutual effects of the variation in surface tension, the enhancement in the number of active nucleations, the bubble departure frequency and a resilient advection caused by the agitation of vapour bubbles [36–39]. Furthermore, the micro-pillar structure in the coating (see Fig. 1) acts as a fin structure when the

liquid moves through the coating, which leads to an enhancement in the single-phase convection [40]. Although the temperature at the evaporator of the coated TPCT showed significant variation, there is no variation in the adiabatic and condenser temperatures. 4.3. The effect of coating on the heat transfer coefficient The effect of the thin, porous coating on the heat transfer coefficient at various inclination angles of the TPCT is presented in Fig. 6. At all the inclinations, the heat transfer coefficient increases with an increase in the heat flux. The heat transfer coefficient of the coated TPCT is higher than that of the uncoated TPCT at all inclination angles. It is interesting to note that the inclination angle has a significant effect on the heat transfer coefficient with varying heat fluxes. In the horizontal position (zero inclination), the difference between the heat transfer coefficients of the coated and uncoated TPCTs increases with an increase in heat flux, and the temperature decreases by the same difference after 30 kW/m2. The maximum difference in heat transfer coefficient between the coated and uncoated TPCTs is 10% at this inclination angle for a heat flux of 30 kW/m2. At an inclination angle of 45°, the difference in heat transfer coefficient between the coated and uncoated TPCTs is higher at lower heat fluxes, and decreases by the same difference as the heat flux increases. The maximum difference in heat transfer coefficient between the coated and uncoated TPCT at an inclination angle of 45o is 44% at a heat flux of 10 kW/m2. However, at an inclination angle of 60o, the difference in the heat transfer coefficient between the coated and uncoated TPCTs is uniformly enhanced by 6% at all heat fluxes. In the vertical position (90° inclination), the difference in the heat transfer coefficient increases with heat flux and a maximum heat transfer enhancement of 11% is obtained at a heat flux of 50 kW/m2.

A. Brusly Solomon et al. / International Communications in Heat and Mass Transfer 82 (2017) 9–19

17

4.5. The effect of coating on the non-dimensional numbers The heat flux's effect on the Bo is presented in Fig. 8a. It is evident that the Bo increases as the heat flux increases for both uncoated and coated TPCTs. This reveals that the boiling process is enhanced with an increase in heat flux. Due to the increase in temperature, not only the buoyancy increases, but also the convection heat transfer. Therefore, if the Bo increases, the boiling process also increases. It is also noted that the variation between the Bo of the uncoated and coated TPCTs is insignificant. Fig. 8b shows the variation in the We with respect to the heat flux of both the uncoated and coated TPCTs. The We increases with the heat flux for both the uncoated and coated TPCTs. This indicates that the counter-current interaction improves with the heat flux and enhances the heat transportation. It has also been concluded that the We of the coated TPCT is higher than that of the uncoated TPCT, which indicates that the countercurrent interactions of the coated TPCTs are better than those of the uncoated TPCTs. This offers more free surfaces and leads to a higher heat transfer. The variations of the Ku with respect to the heat flux are presented in Fig. 8c. The Ku follows the same trend as the We. As the heat flux increases, the Ku also increases for both the uncoated and coated TPCTs, which indicates that the pool-boiling increases with the heat flux. Furthermore, the Ku of the coated TPCT is better than that of the uncoated TPCT, which indicates that the coated TPCT has better pool-boiling characteristics than the uncoated TPCT. The variations of the Co with heat flux are presented in Fig. 8c. It is seen that the Co of an uncoated TPCT increases up to a heat flux of 16 kW/m 2 and then decreases. This suggests that the condensate return of the uncoated TPCT increases up to a heat flux of 16 kW/m2 , after which it decreases. However, the Co of a coated TPCT increases linearly, even after 16 kW/m 2 , which indicates that the condensate return is improved over the uncoated TPCT. From the non-dimensional number analysis, it is clear that the counter-current interaction, pool-boiling enhancement and condensate return is enhanced with coating, which clearly indicates that the TPCT's performance is enhanced by using a thin porous copper coating.

Fig. 7. Total thermal resistance of (a) uncoated and (b) coated TPCT at various inclination angles.

4.4. The effect of coating on the total resistance of a TPCT The total thermal resistance of coated and uncoated TPCTs at different heat inputs is displayed in Fig. 7a and b. The thermal resistance decreases exponentially with an increase in power input for both the coated and uncoated conditions. However, the resistance of the coated TPCT is lower than that of the uncoated TPCT, although the inclination angles vary. The variation in the total resistance is significant at low power levels and less significant at high power levels. The reduction in the resistance and enhancement of the heat transfer coefficient clearly indicates that the metallic copper coating enhances the heat transfer performance of the TPCT. Furthermore, the anodised (Al2O3-coated) TPCT [24] is compared to the metal-coated TPCT (see Fig. 7b). The resistance of the copper-coated TPCT is lower than that of the anodised TPCT. This is mainly due to the variation in the thermal conductivity of the materials found in both TPCTs. As stated earlier, the thermal conductivity of the metal coating and copper wall of the metal-coated TPCT is higher than the Al2O3 coating and aluminium wall of the anodised TPCT. Thus, the metal-coated TPCT performs better than the anodised TPCT.

5. Conclusion The present study shows the influence of a thin porous copper coating on the inner side of the TPCT enclosure with deionised water as the working fluid. To form a thin copper coating on the interior of the TPCT enclosure, an electrodepositing process is performed. Due to this coating, the average evaporator temperature of an uncoated TPCT at an inclination angle of 60° is 79.3 °C, while it is 56.6 °C for a coated TPCT at a heat input of 50 W. This resulted in a reduction of almost 29% in the evaporator temperature. Furthermore, the maximum difference in heat transfer coefficients between the coated and uncoated TPCTs at an inclination angle of 45° is 44% at a heat flux of 10 kW/m 2. This thin, porous copper coating creates uniform dendritic pillars at the interior of the TPCT enclosure, and makes the interior wall surface more hydrophilic. This generates many small pits and peaks, which act as nucleation sites and augment the boiling heat transfer. Furthermore, the micro-pillar structure in the coating enhances the single-phase convection when the liquid moves through the pillars. Because of the thin copper coating that forms, the wall temperature of the evaporator is significantly reduced and the heat transfer coefficient is enhanced. Finally, the non-dimensional numbers, such as the We, Ku and Co numbers, clearly indicate that the performance of the TPCT increases with the thin copper coating. These thin metal-coated TPCTs are suitable for cooling high-density power electronic devices, since the coating dissipates huge amounts of heat.

18

A. Brusly Solomon et al. / International Communications in Heat and Mass Transfer 82 (2017) 9–19

Fig. 8. Effect of heat flux on the non-dimensional numbers (a) Bond (b) Webber (c) Kutateladze; and (d) condensation number.

References [1] M. Ivanova, Y. Avenas, C. Schaeffer, J.B. Dezord, J. Schulz-Harder, Heat pipe integrated in direct bonded copper (DBC) technology for cooling of power electronics packaging, IEEE Trans. Power Electron. 21 (6) (2006) 1541–1547. [2] H.N. Chaudhry, B.R. Hughes, S.A. Ghani, A review of heat pipe systems for heat recovery and renewable energy applications, Int. J. Renew. Sust. Energ. Rev. 16 (2012) 2249–2259. [3] L. Lu, Z.H. Liu, H.S. Xiao, Thermal performance of an open thermosyphon using nanofluids for high-temperature evacuated tubular solar collectors part 1: indoor experiment, Int. J. Sol. Energy 85 (2011) 379–387. [4] Y.C. Weng, H.P. Cho, C.C. Chang, S.L. Chen heat pipe with PCM for electronic cooling, Int. J. Appl. Energy 88 (2011) 1825–1833. [5] T.E. Tsai, H.H. Wu, C.C. Chang, S.L. Chen, Two-phase closed thermosyphon vapor-chamber system for electronic cooling, Int. Commun. Heat Mass Transfer 37 (2010) 484–489. [6] J.B. Junior, V.V. Vlassov, P.A. Candido, G. Genaro, Experimental performance comparison of axially grooved heat pipes charged with acetone and ammonia, 16th International Heat Pipe Conference, Lyon, France 2012, pp. 20–24. [7] S. Khandekar, Y. Joshi, B. Mehta, Thermal performance of closed two-phase thermosyphon using nanofluids, Int. J. Therm. Sci. 47 (2008) 659–667. [8] Z.H. Liu, X.F. Yang, G.L. Guo, Effect of nanoparticles in nanofluid on thermal performance in a miniature thermosyphon, Int. J. Appl. Phys. 102 (2007) 013526. [9] Z.H. Liu, X.F. Yang, G.S. Wang, G.L. Guo, Influence of carbon nanotube suspension on the thermal performance of a miniature thermosyphon, Int. J. Heat Mass Transf. 53 (9) (2010) 1914–1920. [10] M.M. Sarafraz, F. Hormozi, S.M. Peyghambarzadeh, Thermal performance and efficiency of a thermosyphon heat pipe working with a biologically ecofriendly nanofluids, Int. Commun. Heat Mass Transfer 57 (2014) 297–303. [11] X.F. Yang, Z.H. Liu, Application of functionalized nanofluid in thermosyphon, Nanoscale Res. Lett. 6 (2011) (Paper No. 494). [12] S.H. Noie, S.Z. Heris, M. Kahani, S.M. Nowee, Heat transfer enhancement using Al2O3-water nanofluid in a two-phase closed thermosyphon, Int. J. Heat Fluid Flow 30 (2009) 700–709.

[13] T. Parametthanuwat, S. Rittidech, A. Pattiya, A correlation to predict heat transfer rates of a two-phase closed thermosyphon (TPCT) using silver nanofluid at normal operating conditions, Int. J. Heat Mass Transf. 53 (21) (2010) 4960–4965. [14] T. Paramatthanuwat, S. Boothaisong, S. Rittidech, K. Booddachan, Heat transfer characteristics of a two-phase closed thermosyphon using deionised water mixed with silver nanoparticles, Int. J. Heat Mass Transf. 46 (2010) 281–285. [15] G. Huminic, A. Huminic, I. Morjan, F. Dumitrache, Experimental study of the thermal performance of thermosyphon heat pipe using iron oxide nanoparticles, Int. J. Heat Mass Transf. 54 (1–3) (2011) 656–661. [16] K.H. Do, S.P. Jang, Effect of nanofluids on the thermal performance of a flat micro heat pipe with a rectangular grooved wick, Int. J. Heat Mass Transf. 53 (9–10) (2010) 2183–2192. [17] J. Qu, H.Y. Wu, P. Cheng, Thermal performance of an oscillating heat pipe with Al2O3-water nanofluids, Int. Commun. Heat Mass Transfer 37 (2010) 111–115. [18] K.N. Shukla, A.B. Solomon, B.C. Pillai, M. Ibrahim, Thermal performance of cylindrical heat pipes using nanofluids, Int. J. Thermophys. Heat Transf. 24 (4) (2010) 796–802. [19] A.B. Solomon, K. Ramachandran, B.C. Pillai, Thermal performance of heat pipe with nanoparticles coated wick, Int. J. Appl. Therm. Eng. 36 (2012) 106–112. [20] S. Vemuri, K.J. Kim, Pool boiling of saturated FC-72 on nanoporous surfaces, Int. Commun. Heat Mass Transfer 32 (1–2) (2005) 27–31. [21] Y. Chen, D.C. Mo, H.B. Zhao, N. Ding, S.S. Lu, Pool boiling on the superhydrophilic surface with TiO2 nanotube arrays, Sci. China, Ser. E: Technol. Sci. 52 (2009) 1596–1600. [22] M.S. El-Genk, A.F. Ali, Enhanced nucleate boiling on copper micro-porous surfaces, Int. J. Multiphase Flow 36 (2010) 780–792. [23] T. Kunugi, K. Muko, M. Shibahara, Ultrahigh heat transfer enhancement using nanoporous layer, Int. J. Superlattice. Microst. 35 (3–6) (2004) 531–542. [24] A.B. Solomon, A. Mathew, K. Ramachandran, B.C. Pillai, V.K. Karthikeyan, Thermal performance of anodized two phase closed thermosyphon (TPCT), Int. J. Exp. Thermal Fluid Sci. 48 (2013) 49–57. [25] A.B. Solomon, R. Roshan, W. Vincent, V.K. Karthikeyan, L.G. Asirvatham, Heat transfer performance of an anodized two-phase closed thermosyphon with refrigerant as working fluid, Int. J. Heat Mass Transf. 82 (2015) 521–529.

A. Brusly Solomon et al. / International Communications in Heat and Mass Transfer 82 (2017) 9–19 [26] Y. Tang, B. Tang, Q. Li, J. Qing, L. Lu, K. Chen, Pool boiling enhancement by novel metallic porous surface, Int. J. Exp. Thermal Fluid Sci. 44 (2013) 194–198. [27] M.A. Hanlon, H.B. Ma, Evaporation heat transfer in sintered porous media, ASME J. Heat Transf. 125 (2003) 644–652. [28] S. Lee, B. Koroglu, C. Park, Experimental investigation of capillary-assisted solution wetting and heat transfer using a micro-scale, porous-layer coating on horizontaltube, falling-film heat exchanger, Int. J. Refrig. 35 (2012) 1176–1187. [29] H. Sun, Z. Kawara, Y. Ueki, T. Naritomi, T. Kunugi, Consideration of heat transfer enhancement mechanism of nano- and micro-scale porous layer via flow visualization, Heat Transfer Eng. 32 (2011) 968–973. [30] Z.Z. Wang, D. Wei, F. Hong, Experimental study of condensation heat transfer promotion on a fluted tube with thin porous coatings, Heat Transfer Eng. 21 (2000) 46–52. [31] S. Lee, K. Cheng, V. Palmre, M.M.H. Bhuiya, K. Kim, B.J. Zhang, H. Yoon, Heat transfer measurement during drop wise condensation using micro/nano-scale porous surface, Int. J. Heat Mass Transf. 65 (2013) 619–626. [32] L.H. Chien, C.C. Chang, Experimental study of evaporation resistance on porous surfaces in flat heat pipes, IEEE, International Society Conference on Thermal Phenomena, 2002 (ISBN: 0–7803–7152-6). [33] L.L. Vasil'ev, L.P. Grakovich, M.I. Rabetskii, D. Tulin, Investigation of heat transfer by evaporation in capillary grooves with a porous coating, J. Eng. Phys. Thermophys. 85 (2) (2012) 407–414.

19

[34] L. Vasil'ev, L. Grakovich, M. Rabetsky, V. Romanenkov, L. Vasil'ev, V. Ayel, Y. Bertin, C. Romestant, J. Hugon, Grooved heat pipes with nanoporous deposit in an evaporator, Heat Pipe Sci. Technol. 1 (3) (2010) 219–236. [35] M. Rahimi, K. Asgary, S. Jesri, Thermal characteristics of a resurfaced condenser and evaporator closed two-phase thermosyphon, Int. Commun. Heat Mass Transfer 37 (2010) 703–710. [36] C. Li, G.P. Peterson, Parametric study of pool boiling on horizontal highly conductive micro-porous coated surfaces, ASME J. Heat Transf. 129 (11) (2007) 1465–1475. [37] C.Y. Lee, M.M.H. Bhuiya, K.J. Kim, Pool-boiling heat transfer with nanoporous surface, Int. J. Heat Mass Transf. 53 (19–20) (2010) 4274–4279. [38] T.J. Hendricks, S. Krishnana, C. Choib, C.H. Chang, B. Paul, Enhancement of pool boiling heat transfer using nanostructured surfaces on aluminum and copper, Int. J. Heat Mass Transf. 53 (15–16) (2010) 3357–3365. [39] B. Stutz, C.H.S. Morceli, M.F.D. Silva, S. Cioulachtjian, J. Bonjour, Influence of nanoparticle surface coating on pool boiling, Int. J. Exp. Thermal Fluid Sci. 35 (7) (2011) 1239–1249. [40] R. Furberg, Enhanced Boiling Heat Transfer on a Dendritic and Micro-Porous Copper Structure(Doctoral Thesis) Royal Institute of Technology, Stockholm, Sweden, 2011. [41] H. Shabgard, B. Xiao, A. Faghri, R. Gupta, W. Weissman, Thermal characteristics of a closed thermosyphon under various filling conditions, Int. J. Heat Mass Transf. 70 (2014) 91–102. [42] A. Faghri, Heat Pipe Science and Technology, Taylor Francis, New York, NY, 1995.