Surfaces and Interfaces 36 (2023) 102499
Available online 19 November 2022
2468-0230/© 2022 Elsevier B.V. All rights reserved.
Surface treatment on metal foam wick of a ferrofluid heat pipe
Fitri H.S. Ginting
a,*, Anggito P. Tetuko
b,*, Nining S. Asri
b, Lukman F. Nurdiyansah
b, Eko A. Setiadi
b, Syahrul Humaidi
a,*, Perdamean Sebayang
b,*aDepartment of Physics, Universitas Sumatera Utara, Jl. Bioteknologi No. 1, Padang Bulan, Medan 20155, Indonesia
bResearch Center for Advanced Materials, National Research and Innovation Agency (BRIN), Bld. 440, KST. B. J. Habibie, Tangerang Selatan, Banten 15314, Indonesia
A R T I C L E I N F O Keywords:
Surface modification Metal foam Wick structure Ferrofluid Cylindrical heat pipe
A B S T R A C T
In this current investigation, two metal foams (stainless steel) were treated and cleaned to be modified into superhydrophilic and superhydrophobic surfaces. The surface modifications of the metal foams were prepared using acetone cleaning and superhydrophobic coating methods. The two metal foam samples were then char- acterized to analyze the morphology, physical properties, wettability and capillary. The superhydrophilic metal foam was chosen as the wick structure in cylindrical heat pipe because of high wettability, and capillary behavior as characterized previously. In the cylindrical heat pipe, ferrofluid was used as the working liquid. The cylin- drical heat pipe was tested at two heat input variations of 3 and 5 W in a horizontal direction. The heat pipe’s performance test proposed that the optimum thermal resistance is 2.47 ◦C/W at a heat input of 5 W.
1. Introduction
Heat pipes have been known for many years as the cooling medium in several types of equipment, such as fuel cells, batteries, flywheel generators, and other electronic devices [1–6]. Heat pipes offer many advantages, such as their flexibility to be installed in different orienta- tions (gravity assisted and opposed). The heat pipe could be bent and flattened and thus could be used in a small area [7–10]. Nowadays, the developments of heat pipes continue to increase. The improvements are focus on different aspects, such as the container materials, the heat pipe types (e.g., cylindrical, pulsating, flatten and loop heat pipes), working liquids (e.g., nanofluids), and the wick structure types (e.g., sintered powder, mesh, and fiber) [11–16]. There are several developments in the aspect of heat pipes’ wick structure, particularly porous material, such as those conducted by several authors.
Many researchers often used the sintering method to produce the wick structure with an optimum contact angle and capillarity installed in a heat pipe. However, several types of research were developed to simplify the fabrication process of a wick structure. An example is the modification of the wick surface with a hydrophilic characteristic that has a high capillary effect and a low thermal resistance [17–19].
Putra et al. [20] studied the effect of ambient conditions on the wettability of wick structure materials. The wick samples were synthe- sized from sintered copper powders with several variations: particle
sizes (100, 200, and 300 µm), compaction pressures (79.5, 119.25, and 159 bar), and sintering temperatures (600, 700, and 800 ◦C). The author proposed that the optimum wick material was obtained on the sample that has been synthesized using the parameters: particle size of 100 µm, compaction pressure of 119.25 bar, and sintering temperature of 700 ◦C.
The authors found that the hydrophobicity of the sample was enhanced from the contact angle of 75◦(day 1) to 129◦(day 28). The sample can only withstand its wettability (hydrophilic surface) for seven days (contact angle of 82◦). These wettability changes were caused by the increase of the carbon’s mass as detected by the EDAX measurement.
Midiani et al. [21] analyzed the wick structure’s contact angle’s effect on its capillarity. The wick structure was fabricated from sintered zeolite and sintered hybrid-copper using two particle size variations of 100 and 200 µm. The lowest contact angle of 32.7◦was obtained at the sample with a zeolite composition of 100% and particle size of 200 µm. How- ever, the optimum capillary result was found in the sample with zeolite composition (25%), copper (75%), and particle size of 100 µm with a contact angle of 83.8◦. The sintering method to be used in a wick structure fabrication was also conducted by Zhang et al. [22]. The effect of stainless steel powder sizes was investigated to evaluate the capillary performance of the wick structure. The capillary measurement was conducted using an IR camera to detect the fluid (ethanol) movement on the wick structure. Based on their investigations, the wick structure’s surface can be categorized as a superhydrophilic surface (contact angle of ~0◦). The powder’s particle size and the wick structure’s pore size
* Corresponding authors.
E-mail addresses: [email protected] (F.H.S. Ginting), [email protected] (A.P. Tetuko), [email protected] (S. Humaidi), perdamean.
[email protected] (P. Sebayang).
Contents lists available at ScienceDirect
Surfaces and Interfaces
journal homepage: www.sciencedirect.com/journal/surfaces-and-interfaces
https://doi.org/10.1016/j.surfin.2022.102499
Received 26 October 2021; Received in revised form 7 November 2022; Accepted 17 November 2022
affected the capillary performance. Szyma´nski and Mikielewicz [23]
used nickel-copper and nickel-aluminum powders and a sintering pro- cess to synthesize the wick structure, where water and acetone were utilized as the working fluid in a capillary pumped loop (CPL) system.
The author suggested that the mass flow rate of the fluid could reach 0.37 g/s using nickel-aluminum powder. On the other hand, a lower rate of 0.13 g/s was obtained on the wick structure fabricated using nickel-copper powder.
One of the current interests in wick structure manufacturing is using a porous material. Metal foam is a porous material utilized as the wick structure in heat pipes. Metal foam has high thermal conductivity and porous area that contributes to the convection heat transfer and high capillary pumping [24,25]. The researchers applied several modifica- tions to use the metal foam as the wick structure. The modification in- cludes a hybrid method to combine different materials, the number of pores and sizes, and the surface modifications [26,27]. Current research related to the surface coating processes was conducted by several re- searchers [28–30].
Jadhav et al. [31] developed six numerical models to analyze the heat transfer of partially filled aluminum in a horizontal tubular: a length of 1 m, a diameter of 0.10 m, and a wall thickness of 7 mm. The foams used in their models consist of 10 to 45 PPI with 0.90 to 0.95 porosity. The boundary condition consisted of inlet velocity, pressure outlet, adiabatic wall, heat flux on the metal foam, and axis symmetry.
The air velocity varied from 0.6639 to 3.0246 m/s, and Darcy Extended Forchheimer and turbulence k-ω were utilized to solve the models. The numerical models were validated using the experimental results inves- tigated by Garrity et al. [32]. Jadhav et al. proposed that the thermal performance of the 10 PPI aluminum foam has close results to the 10 PPI copper foam. The author also suggested that the 30 PPI aluminum foam shows the highest performance factor of 2.93 with a porosity of 0.92 (model 1) at a Reynolds number of 4500.
Arbak et al. [33] examined the effect of metal foam’s porosity (10 and 40 PPI) on heat transfer enhancement. In their research, the metal foam was placed in a cylindrical shell. The metal foam was brazed to the inner surface of the cylinder to minimize the thermal contact resistance.
The working fluid used as a heat transfer medium was flown into the aluminum foam. Then, the temperature was measured along the surface of the cylinder. Their experiments used two metal foam sizes: 10 and 40 PPI. The authors proposed that the optimum result was obtained on the aluminum foam with 40 PPI. Using a metal foam in 2020, Bao et al. [34]
investigated the heat transfer on a multi-channel heat pipe (MCHP). The metal foam was manufactured from copper with a porosity of 90% and 20 PPI. In their experiments, several parameters were used: heat flux of 18,750 W/m2 to 112,500 W/m2 and filling ratio of 10, 20, 30, 40, 50, and 60%, respectively. The author proposed that the optimum thermal resistance on the MCHP was obtained by applying the heat flux of 112, 500 W/m2 and a filling ratio of 20%. The heat flux increase enhances the fluid’s boiling process inside the MHCP’s evaporator section. However,
the filling ratio of the fluid inside the MHCP needs to be adjusted. A low fluid ratio could cause a drying condition at the evaporator section that decreases the performance of the MHCP. Hu et al. [26] analyzed the heat transfer on the pristine and hydrophobic coated copper foam. The number of pores varied from 5 to 40 PPI with a porosity of 90%. The hydrophobic layer was synthesized from 1% (weight) of 1-dodecane- thiol (C12H26S). The increase in the number of pores enhanced the heat transfer, and the author found that the optimum sample was ob- tained using 20 PPI of copper foam. Hydrophobic coated copper foam could improve its heat transfer performance (5–34%). Another researcher that tests the wettability characteristic of metal foam is Shirazy et al. [35]. The author tested the hydrophobicity of the metal foam at several conditions. The experiments were conducted with ambient and dried air and a nitrogen atmosphere in a desiccator. The metal foam used in their research was copper with 75 PPI and a porosity of 85%. The surface of the copper foam was modified into a hydrophilic surface using the hydrogen reduction method. This method increases the wettability of the copper foam surface from 136◦to ~0◦. The author suggested that the surface of the copper foam tested at an ambient condition changes to a superhydrophobic surface (contact angle of 136◦) after 48 h. On the other hand, the copper foam investigated inside the desiccator remains its superhydrophilic characteristic after 96 h of the test. The ambient and environmental conditions improve the adsorption of the organic compound into the copper foam.
In the current investigation, stainless steel foam was used as a wick structure in the heat pipe, serving as a capillary medium for the working liquid. In order to analyze the thermal behavior of the stainless steel, differential scanning calorimetry (DSC) reviews from previous studies were conducted. The purpose of the calorimetry measurement is to investigate the heat as there is a heat exchange that affects the tem- perature of the material. In the differential scanning calorimetry (DSC), a heat flow was applied to the material at different temperature ranges to predict the physical transitions and chemical reactions. In DSC, a peak occurred due to the steady state condition of the material disturbed by the added heat (endothermic) and the released heat (exothermic) [36].
In 2012, Petrovic et al. [37] conducted a differential scanning calo- rimetry (DSC) measurement of duplex stainless steel. The melting and solidification reactions were analyzed through the endothermic and exothermic peaks. Endothermic peaks at 485 and 818 ◦C suggested the dissolving of chromium-rich α′-phase and the transformation of tertiary austenite (γ3) into ferrite and the dissolving of precipitates. Further- more, at 1119.6 and 1407 ◦C, the solid-state transformation of second- ary austenite into ferrite γ2→δ and the disappearance of ferrite occurred, respectively. On the other hand, the exothermic reactions for the so- lidification process at the temperature of 1450 ◦C show the precipitation of primary δ-ferrite from the melt. Moreover, at the temperatures of 1374, 1136, and 1058 ◦C other exothermic peaks of δ→γ transformation are present [38,39]. Petrovic et al. [40] analyzed the austenitic stainless steel using differential scanning calorimetry (DSC). The author proposed that endothermic peaks relate to melting reactions. The reaction occurred at the temperature of 798 ◦C, suggesting the dissolving of precipitates in the austenite. Furthermore, at the temperature of 1426 ◦C, the primary endothermic reaction arises. On the other hand, exothermic peaks at 1448.7, 1440.1, and 1415.7 ◦C happened due to liquid-to-solid phase transformation. Mazur and Hebda [41] investi- gated the thermal behavior of austenitic stainless. The author proposed that the exothermic peaks occur at a high temperature (higher than 1100 ◦C). The peak arises due to the formation of oxides.
The novelty proposed in this current article is the combination of metal foam (stainless steel) as the wick structure and the ferrofluid used as the working liquid in a cylindrical heat pipe. The metal foams surface was treated with acetone cleaning and superhydrophobic coating. This paper focuses on the characterizations of metal foam and the heat pipes’ performance that utilized the metal foam as the wick structure. The analyses involve the metal foam’s material characterizations and fluid dynamics properties. The heat pipe installed with a metal foam wick Nomenclature
Pc capillary pressure (Pa)
γ surface tension of the liquid (N/m)
θ the contact angle between the solid and the liquid (◦) r the average pore radius (m)
R thermal resistance of the heat Pipe (◦C/W) Te temperature of the evaporator section (◦C) Tc temperature of the condenser section (◦C) Qin heat input applied to the heat pipe (W)
k effective thermal conductivity of the heat pipe (W/m◦C) L effective length of the heat pipe (m)
A area of the heat pipe (m2)
structure and utilized the ferrofluid as working liquid.
2. Methodology
2.1. Preparation of metal foams 2.1.1. Acetone cleaning-metal foam
The metal foam used in this research was manufactured by Recemat BV, Netherlands. The metal foam was cut into a sample with a size of 32.7 ×8.2 ×7.3 mm. The metal foam was then cleaned using sandpaper and immersed in acetone (CH3COCH3) liquid for 10 min. Then the sample was dried in an oven at the temperature of 110 ◦C for 60 min.
The sample was removed from the oven and dried at an ambient tem- perature for 15 min. The metal foam was immersed again in the acetone liquid for 30 min. The metal foam was then dried at an ambient tem- perature for 30 min before being characterized and analyzed.
2.1.2. Superhydrophobic coating-metal foam
In order to compare the metal foam characteristic that has been cleaned using acetone, another sample was coated using a spray coating method by utilizing the commercial superhydrophobic liquid (super nano hydrophobic liquid repellent). Sandpaper and acetone liquid im- mersion cleaned the metal foam’s surface. The metal foam was dried in an oven at the temperature of 110 ◦C for 60 min. Then, the metal foam was sprayed at a 300 mm distance between the spray and the sample.
One layer of the superhydrophobic surface was applied to the metal foam sample and then dried at an ambient temperature for 20 min.
2.2. Characterizations of metal foams
The metal foam samples were characterized and measured using several types of equipment at the Research Center for Advanced Mate- rials, National Research and Innovation Agency (BRIN). X-Ray Diffrac- tion (XRD) analysis was conducted using a Rigaku Smartlab X-ray diffractometer, Cu Kα radiation (λ = 1.5418 Å). Scanning Electron Microscope-Energy Dispersive X-Ray (SEM-EDX) characterization was measured using a JSM IT200 with an accelerating voltage of 10–15 kV.
Fourier transform infrared (FTIR) spectroscopy analysis was performed using a Thermoscientific Nicolet iS-10. The contact angle measurement was conducted using a 3D optic microscope, VHX 5000 series. The physical properties of the metal foam sample, such as density and porosity, were measured using the Archimedes method [42]. On the other hand, the capillary capability of the metal foam was measured using set-up equipment [43].
The capillary pressure on the metal foam as the wick structure can be calculated using Eq. (1) [10].
Pc=2γcosθ
r (1)
2.3. Experimental set-up of heat pipe
The cylindrical heat pipe was fabricated using a copper container with an outer diameter of 12.67 mm, a thickness of 2.08 mm, and a length of 150 mm. The metal foam cleaned with the acetone was inserted in the heat pipe to be used as the wick structure. Only the acetone cleaning-metal foam was used as the wick structure due to its superhydrophilic characteristic. Ferrofluid from our previous research that has been prepared using the two-step method [44,45] was used as the working liquid in the heat pipe with a 20% filling ratio. At the beginning of the installment, the heat pipe’s evaporator section was heated to remove the trapped air before being closed using a fitting and plug. A Ni-Cr wire was used to wrap the evaporator section of the heat pipe, and the heat was applied to the wire by utilizing the DC power supply (Joule heating method). Two heat input variations were used: 3 and 5 W to test the performances of the heat pipe. Six thermocouples
were attached to the evaporator and the heat pipe’s adiabatic and condenser sections. The thermocouples were connected to a data logger and computer to monitor the temperature distributions on the heat pipe.
Insulation material was used to minimize heat loss and to ensure the heat was transferred from the evaporator to the condenser sections. The heat pipe and the metal foam wick structure used in this research and the experimental set-up of the heat pipe test are presented in Figs. 1 and 2.
The heat pipe’s effective thermal resistance and thermal conductivity can be calculated using Eqs. (2) and 3 [10,11].
R=Te− Tc
Qin (2)
k= L
R.A (3)
2.4. Uncertainty analysis
Uncertainty analysis of various parameters in the experiments can be calculated using the equations below [12,13]
ΔQin Qin
=
̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
(ΔV
V )2
+ (ΔI
I )2
√
(4)
ΔR R =
̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
(ΔQin Qin
)2 +
(Δ(ΔT) ΔT
)2
√
(5)
Δk k =
̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
(ΔL
L )2
+
(ΔR
R )2
+
(ΔA
A )2
√
(6) The uncertainty of several parameters of voltage, current, heat input, temperature difference, length, area, thermal resistance and thermal conductivity are listed in the following: 1.7, 2, 2.6, 1.2, 1.3, 2.4, 3 and 4%, respectively.
3. Results and discussions 3.1. XRD analysis
XRD result of the metal foam sample is presented in Fig. 3, where the radiation of CuKα and λ =1.540598 Å was used in the investigation.
Based on the peaks analyses, it can be concluded that the major phase is austenitic stainless steel (SS 304). The results agree with the previous research conducted by Li et al. and Guo et al. [46,47]. The metal foam consists of nickel (Ni), chromium (Cr), and iron (Fe) as the main sub- stances of SS 304 with cubic crystals, as shown in Fig. 4, where the lattice parameters are a=b=c with angles of α=β=γ=90◦The nickel,
Fig. 1. Cylindrical heat pipe and metal foam used as the wick structure.
chromium, and iron lattice parameters are 3.5870 Å, 2.8839 Å, and 2.8860 Å, respectively. Table 1 shows the intensity and the full width at half maximum (FWHM) values for every major substance (Ni. Cr. Fe) obtained from the analysis. Based on the analysis as well, the index millers of nickel are presented as follows 43.60◦(111), 50.77◦(002), and 74.57 ◦(022). The index millers of chromium and iron are 44.53◦(011), 81.80◦(112), and 64.86◦(002).
3.2. Three dimensional (3D) optical microscope analysis
The pores’ number and average diameter size of the metal foam were
measured using the three-dimensional (3D) optical microscope and shown in Fig. 5a. The ligament surfaces and the pores affect the liquid movement, and capillary behavior as the surface has been treated as superhydrophilic and superhydrophobic. These properties, particularly with high wettability (superhydrophilic surface), could improve the evaporation and condensation process of the working liquid to dissipate the heat in the heat pipe [48–50]. Based on the analysis using Image J software, it is obtained that the size of the metal foam is 30 pores per inch (PPI). On the other hand, the average size of the pores is 443 µm, where the pore size distribution is presented in Fig. 5b. The metal foam’s pore sizes: diameter, and number used as the wick structure affects the overall performance of the heat pipe. The fluid’s capillary movement from the heat pipe’s condenser to the evaporator section is enhanced as the pore size decreases, thus improving the liquid circulation, perme- ability, and evaporation time.
3.3. Density and porosity analyses
Based on the current analysis, the density and porosity of the stain- less steel foam are 5.02 g/ cm3 and 37.5%. The material, ligament thickness, size and pores’ number (PPI) affect the density of the metal foam. On the other hand, the metal foam’s pore influences the capillary Fig. 2. Experimental set-up of the cylindrical heat pipe test.
Fig. 3.XRD analysis of the metal foam sample.
Fig. 4. Crystal structures of Cr, Ni, Fe.
Table 1
Intensity and FWHM of the difraction patterns.
Phase 2θ (◦) Intensity (cps) FWHM
Nickel 43.60 88 0.4092
50.77 30 0.6346
74.57 31 0.7594
Chromium, Iron 44.53 59 0.3400
64.86 8 1.0429
81.80 12 0.9956
movement of the liquid, as a smaller pore size enhances its capillary rate.
The comparisons of the stainless steel foam to other metal foam analyzed by previous researchers are presented in Table 2.
3.4. SEM-EDX analysis
The morphologies of the metal foams (acetone cleaning and super- hydrophobic coating) were analyzed using SEM and presented in Fig. 6 (a) and (b). Based on the images, additional material is attached to the surface of the metal foam due to the superhydrophobic coating compared to the acetone cleaning foam. This current finding agrees with the previous analysis conducted by Shi et al. where the superhydrophilic and superhydrophobic surfaces were obtained by oxidation and chem- ical modification methods. The superhydrophilic surface microstructure is smaller than the superhydrophobic surface [54]. EDX analysis pre- sented in Fig. 7a and Table 3 suggested that the main substances of the foam are Cr, Fe, Ni, and O. Thus, stainless steel can be confirmed as the material of the foam and has a good agreement with the XRD analysis.
Based on the characterization, the acetone cleaning also does not affect the substances on the metal foam. On the other hand, on the metal foam that has been coated using the superhydrophobic layer, several sub- stances, such as flour (F), silicon (Si), and carbon (C), were obtained (Fig. 7b and Table 4). Another material layer due to the super- hydrophobic spray covered the metal foam surface and could make the surface rough compared to the other foam’s sample (acetone cleaning).
Previous researchers also suggested similar results, where a low surface energy material (fluoropolymer) was used in their research [55–57].
3.5. FTIR analysis
FTIR characterizations on both of the metal foam samples (acetone cleaning and superhydrophobic coating) were measured at 400 – 4000 cm−1 and shown in Fig. 8(a) and (b). In the acetone cleaning-metal foam, the FTIR spectrum was detected and showed their stretching vi- bration at 3441.70 (O–H), 1629.91 (C=O), 1017.5 (C–O–C), 648.32, and 463.66 cm−1 (M-O). Absorption bands that arose in the wave number less than 1000 cm−1 can be classified as metal oxide (M-O). On
the other hand, similar stretching vibrations and identical trends occurred on the superhydrophobic coated metal foam, where the stretching vibrations were obtained at 3441.70, 1629.81, 1004.68, 653.64, and 463.61 cm−1 which correspond to O–H, C=O, C–O–C, and M-O, respectively [58–60].
3.6. Contact angle analysis
The contact angles of the metal foam at two different treatments of acetone cleaning and superhydrophobic coating are presented in Table 5 and Figs. 9 and 10. The images captured using the 3D optical microscope show that the acetone cleaning-metal foam has the superhydrophilic characteristic from day 1 to day 10. Wettability can be measured from the contact angle of water dropped on a solid surface [61–63]. The contact angle on the acetone cleaning-metal foam could not be measured as the liquid was completely wetting and spread across the surface. The solid surface wettability indicator is derived from surface roughness and energy. Hydrophilic properties are determined from lower surface roughness and higher surface energy than hydrophobic properties [64, 65]. The surface changes its wettability on day 15 with the contact angle value of 107◦. The changes occur due to the change in the chemical composition of the surface affected by environmental conditions, organic compound contamination, and oxidation [35]. On the other hand, the superhydrophobic coating-metal foam suggested high contact angle values within a range of 132–150◦from day 1 to 15, and this was due to the high surface roughness of the coating layer [61]. The hy- drophilic character of the wick structure with a low contact angle is needed to be utilized inside the heat pipe [48,49]. On the super- hydrophilic surface, the adhesion force of the surface is higher than the cohesive force of the liquid’s molecules [66,67]. This phenomenon could improve the capillary movement of the liquid from the heat pipe’s condenser to the evaporator section and enhance the heat pipe’s per- formance [68]. Similar behavior is also suggested by previous research conducted by Putra et al. [20] and Shirazy et al. [35]. Both of these studies showed a change in the wettability of metal foam. As time goes on, the carbon content (Volatile Organic Compounds) increases, which causes the wettability to change from hydrophilic to hydrophobic. Putra et al. [20] proposed that the contact angle on their copper powder wick increase from 75◦to 125◦after 14 days. Shirazy et al. [35] also proposed a similar argument, where their copper foam wick’s contact angle in- creases significantly from 0◦to 136◦after two days.
3.7. Capillary analysis
The capillary analyses of the two samples: acetone cleaning and superhydrophibic coating metal foams are presented in Fig. 11 and Fig. 5.(a) Morphology of the metal foam. (b) Pore size distribution of the metal foam.
Table 2
Porosity and pore size of the metal foam.
Material Porosity
(%) Pore per inch (PPI)
Stainless steel (curent research) 37.5 30
Stainless steel [51] 90 10, 30, and 70
Copper [52] 90 15
Alumunium [53] 88.5 10 and 40
Table 6. Water was used as the fluid to be used in the capillary mea- surement. The analysis was conducted by weighting the absorbed fluid mass where the value changes due to the capillary movement. The absorbed fluid mass increases rapidly for the acetone cleaning-metal foam, particularly up to 275 s, and starts becoming steady until the end of the measurement test at 350 s. However, the absorbed fluid mass shows a relatively steady value on the superhydrophobic coating-metal
foam. The capillary pressure was calculated using Eq. (1) and suggested that positive values were obtained for the acetone cleaning-metal foam, particularly for the complete wetting cases from day 1 to day 10. The difference in absorbed fluid mass and capillary pressure occurs due to Fig. 6. SEM of metal foam; (a) Acetone cleaning and (b) Superhydrophobic coating.
Fig. 7. EDX spectra of metal foam; (a) Acetone cleaning and (b) Superhydrophobic coating.
Table 3
EDX analysis of the metal foam (acetone cleaning).
Elements Weight (%) Atomic (%)
O 1.82 6.04
Cr 33.99 34.71
Fe 25.78 24.51
Ni 38.41 34.74
Table 4
EDX analysis of the metal foam (superhydrophobic coating).
Elements Weight (%) Atomic (%)
C O 41.27
11.67 62.54
13.27
F 9.90 9.49
Si 8.60 5.57
Cr 1.81 0.63
Fe 12.65 4.12
Ni 14.11 4.37
the wettability of the two samples. The surface of the acetone cleaning- metal foam has a superhydrophilic characteristic that could improve its capillary movement by having high adhesion force on the surface [53].
On the other hand, for the superhydrophobic coating-metal foam, the
capillary movement is slow and restricted due to the high cohesive force of the liquid [54]. Jafari et al. [69] conducted previous research on the stainless steel foam capillary, where the fluid absorbed mass due to the capillary movement reached a steady value at 430 mg/s. The value is higher than this current finding due wettability, pores homogeneity and size. Large pore size enhances the permeability of the liquid and reduces the liquid’s velocity, pressure drop, and capillary movement [22]. For the heat pipe’s application, the capillary movement of the liquid on the wick structure affects the performance of the heat pipe for removing the heat. The higher capillary capability of the wick structure improves the liquid distribution from the condenser to the evaporator section and enhances the boiling process of the liquid [48,70].
Fig. 8. FTIR spectra of metal foam; (a) Acetone cleaning and (b) Superhydrophobic coating.
Table 5
Contact angle analysis of the metal foams.
Treatment Contact angle (◦)
Day-
1 Day-
2 Day-
5 Day-
10 Day-
15
Acetone cleaning-metal foam 0 0 0 0 107
Superhydrophobic coating-metal
foam 132 146 149 150 150
Fig. 9.Contact angle on the metal foam’surface (acetone cleaning). a. Day 1. b. Day 2. c. Day 5. d. Day 10 and e. Day 15.
3.8. Performances of the cylindrical heat pipe
For the heat pipe test, only one type of metal foam was used as the wick structure due to the hydrophilic characteristic (acetone cleaning- metal foam). Two heat input variations of 3 and 5 W were chosen due
to the low heat input and the fact slower heat transfer rate could be gained. The temperature distribution on the heat pipe at two heat input variations of 3 and 5 W as a function of length is shown in Fig. 12.
Similar phenomena occur in both conditions of heat inputs, where the temperature decreases as the length of the heat pipe increases, where the heat is transferred from the heat pipe’s evaporator to the condenser section. At the heat input of 3 W, the temperature of the heat pipe’s evaporator section is within a range of ~55–60 ◦C, and the condenser section’ temperature is ~50 ◦C. On the other hand, by increasing the heat input to 5 W, the temperature of the heat pipe increases, and the value at the evaporator and condenser sections are ~70–75 ◦C and
~60 ◦C, respectively. The temperature distribution as a function of time at 3 and 5 W of heat inputs are presented in Fig. 13. Based on the graph, it can be concluded that the heat pipe temperature in all sections Fig. 10.Contact angle on the metal foam’surface (superhydrophobic coating). a. Day 1. b. Day 2. c. Day 5. d. Day 10 and e. Day 15.
Fig. 11.Absorbed fluid mass on the metal foams.
Table 6
Capillary pressure on the metal foams.
Treatment Capillary presure (Pa)
Day-1 Day-2 Day-5 Day-
10 Day-
15 Acetone cleaning-metal foam 0.65 0.65 0.65 0.65 −0.19 Superhydrophobic coating-
metal foam − 0.43 − 0.53 −0.55 −0.56 −0.56
Fig. 12.Temperatures distribution on the heat pipe as a function of length at heat inputs of 3 and 5 W.
increased rapidly at the beginning of the test and reached a steady state condition after the 2000s by applying the heat inputs of 3 and 5 W. The heat transfer occurs from the heater to the outer surface of the heat pipe through joule heating, and the heat is flown through conduction (outer to the pipe’s inner surface) and convection to the liquid inside the heat pipe. Therefore there is time for the liquid to change its phase into vapor at the evaporator section and reach the condenser section to dissipate the latent heat. These current temperature distribution trends agree with the research conducted by Goshayeshi et al. [71] and Setyawan et al.
[72].
Figs. 14 and 15 show the heat pipe’s thermal resistance and effective thermal conductivity at two heat inputs of 3 and 5 W. Higher heat input applied to the heat pipe’s evaporator section could reduce the overall heat pipe’s thermal resistance and improve the effective thermal con- ductivity. These current results agree well with previous research con- ducted by Shen et al. [51] and Zhou et al. [73]. The phenomenon occurs due to more available heat at the evaporator section that was needed by the heat pipe to change the liquid into a gas phase (vapor) to be used as a transport medium to remove the latent heat at the condenser section [74]. However, other factors could affect the heat pipe’s thermal resis- tance and effective thermal conductivity, such as the container and wick materials, the diameter, thickness, and length of the heat pipe, and the working liquid (type and filling ratio) [34,68,73]. Another critical factor is the maximum heat that could be applied in a heat pipe, a dried evaporator section occurs due to a high heat input that enhances the evaporation process in a short time, and unbalanced fluid circulation could arise [22].
4. Conclusions
The analyses and characterizations of two metal foam samples with different wettability have been conducted. The metal foams (stainless steel) surfaces were treated using acetone cleaning and super- hydrophobic coating. The acetone cleaning and the superhydrophobic coating-metal foams have superhydrophilic and superhydrophobic characteristics. The superhydrophilic metal foam sample was chosen as the wick structure to be tested in the cylindrical heat pipe. The metal foam’s superhydrophilic behavior could enhance the capillary effect that the liquid needs to circulate in the heat pipe. Higher capillary movement means the liquid could travel back from the condenser to the evaporator sections of the heat pipe in a short time. Thus, it could eliminate the dried condition in the heat pipe’s evaporator section and enhance the evaporation process to dissipate the latent heat. Another
parameter that affects the liquid movement in the heat pipe is the porosity, where a smaller pore generates higher velocity, permeability, and pressure drop. The heat pipe’s performance test with ferrofluid
Fig. 13.Temperatures distribution on the heat pipe as a function of times; (a) heat input of 3 W. (b) heat input of 5 W.
Fig. 14.Thermal resistance of the heat pipe.
Fig. 15.Effective thermal conductivity of the heat pipe.
suggested that higher heat input contributes to less thermal resistance and higher effective thermal conductivity in the heat pipe.
CRediT authorship contribution statement
Fitri H.S. Ginting: Conceptualization, Methodology, Funding acquisition, Writing – review & editing. Anggito P. Tetuko: Concep- tualization, Methodology, Data curation, Formal analysis, Writing – review & editing. Nining S. Asri: Funding acquisition, Data curation, Formal analysis, Writing – review & editing. Lukman F. Nurdiyansah:
Funding acquisition, Writing – review & editing, Writing – original draft.
Eko A. Setiadi: Funding acquisition, Writing – original draft, Writing – review & editing. Syahrul Humaidi: Funding acquisition, Writing – original draft, Writing – review & editing. Perdamean Sebayang:
Conceptualization, Methodology, Data curation, Formal analysis, Writing – review & editing, Writing – original draft.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
All persons who have made substantial contributions to the work reported in the manuscript (e.g., technical help, writing and editing assistance, general support), but who do not meet the criteria for authorship, are named in the Acknowledgements and have given us their written permission to be named. If we have not included an Acknowl- edgements, then that indicates that we have not received substantial contributions from non-authors.
The authors would like to thank the Research Center for Advanced Materials, National Research and Innovation Agency (BRIN) for the fa- cilities used in this research.
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