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Experimental study of the effect of a reduced graphene oxide coating on critical heat flux enhancement
International Journal of Heat and Mass Transfer 60 (2013) 763–771
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Experimental study of the effect of a reduced graphene oxide coating on critical heat flux enhancement Ho Seon Ahn a, Ji Min Kim b, Moo Hwan Kim c,⇑ a
Division of Mechanical System Engineering, Incheon National University, Incheon 406-772, Republic of Korea Department of Mechanical Engineering, POSTECH, Pohang 790-784, Republic of Korea c Division of Advanced Nuclear Engineering, POSTECH, Pohang 790-784, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 27 August 2012 Received in revised form 21 January 2013 Accepted 22 January 2013 Available online 17 February 2013 Keywords: Critical heat flux Graphene Reduced graphene oxide Pool boiling
a b s t r a c t Critical heat flux (CHF) nucleate pool boiling was investigated in a reduced graphene oxide (RGO) colloid consisting of 0.0005 wt.% flake-based RGO in distilled water using Ni–Cr thin-wire heaters 0.1 and 0.2 mm in diameter and 105 mm in length. One- and two-sided coatings were tested to study the RGO flake aggregation as a function of coating time using the Joule heating method. In the one-sided coating tests, RGO flake aggregation was biased on the anode so that the CHF occurred on the cathode side with slight enhancement. The RGO coating on the cathode increased with coating time; however, the coating on the anode did not appear to be affected by coating time. The biased deposition of the RGO coating was attributed to the negatively charged carboxyl group (–COOH) in the RGO flakes. In the two-sided coating tests, the RGO flake aggregation showed dramatic CHF enhancement with increasing coating time. The enhancement mechanisms were examined based on a surface analysis considering the deposition mechanism, thermal activity, water absorption, and the modulated Rayleigh–Taylor wavelength of the RGO coating layer on the wire heaters. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Phase-change heat transfer has high efficiency due to the large latent heat of the working fluids. Representative applications of boiling heat transfer (liquid–vapor phase change) include those found in nuclear power-plant energy transfer [1], electronic chip cooling [2], and various chemical processes [3]. The critical heat flux (CHF) is the thermal limit of a phenomenon in which the heating-induced phase change suddenly decreases the heat transfer efficiency, causing overheating of the heated surface. When the CHF occurs, the temperature of the surface suddenly increases, melting the heated material. Therefore, the CHF value is an important standard with regard to the safe performance of two-phase thermal systems (e.g., preventing nuclear meltdown). A number of models for CHF prediction in pool boiling have been proposed. Among them, Zuber’s CHF model [4] has been most widely accepted. In Zuber’s model, the vapor generated on a flat plate accumulates to form a continuous column of escape flow, with a diameter of half the Rayleigh–Taylor critical wavelength, kRT , at wavelength intervals. When the vapor–liquid interface of the escape flow passage becomes unstable due to Helmholtz instability, the CHF occurs. This phenomenon can be represented as follows:
⇑ Corresponding author. E-mail address: [email protected] (M.H. Kim). 0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.01.052
1=4 q00zuber ¼ 0:131hlg q0:5 ; g ½rgðql qg Þ
ð1Þ
where q00 zuber, hlg, r, g, ql, and qg are the Zuber CHF, latent heat, surface tension, gravity, liquid density, and vapor density, respectively. From Eq. (1), q00 zuber 1050 kW m–2 for boiling saturated water at 1 atm, which is close to the experimental value. In addition to efforts aimed at predicting the CHF value, considerable research has been directed at increasing the limit of boiling heat transfer (e.g., in nanofluids) to improve safety. Recent studies have shown that when boiled in a nanofluid, the nanoparticles on a heated surface cause CHF enhancement by changing the wettability characteristics of the surface [5]. Kim et al. [6] performed poolboiling experiments using pure water on nanoparticle-deposited surfaces that were produced while pool boiling water–TiO2 and water–Al2O3 nanofluids. The CHF of pure water boiling on the nanoparticle-deposited surface was greater than the nanofluid CHF in all cases. This clearly indicated that nanoparticles deposited on the heating surface were the main cause of CHF enhancement. Kim et al. [6] also conducted pool-boiling experiments in nanofluids and described the CHF enhancement in terms of surface wettability and the absorption of the nanoparticle deposition layer. They suggested a simple model consisting of an absorption layer of nanoparticles to describe how the nanoparticles changed the surface wettability. Using results from a thin-wire heater, Kim and Kim [7] postulated that the wettability and capillarity of the heating surface could influence CHF enhancement. They explained
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that the CHF increases the liquid supply on the heating surface due to the capillarity of the absorption layer of nanoparticles. Recently, Ahn et al. [8] reported that liquid spreading due to micro/nanostructures by the capillary force of water between the structures enhanced the CHF because of the increased liquid supply on the heating surface. Graphene has gained much attention recently due to its high thermal conductivity (900–5000 W K1 m1) [9], high transparency [10], ultra-high electric conductivity [11], and novel mechanical properties [12]. However, little information about the thermal characteristics or applications of graphene is available. Since graphene exists as ultra-thin and microscale flakes, considerable effort has been made to synthesize large-area films that retain the superb properties of graphene. Among them, soluble graphene in water solvent has been highlighted through the oxidation process of graphene. Through a chemical process with hydrazine, graphene oxide can be changed to a reduced graphene oxide, which can have properties similar to graphene. Jang et al. [13] estimated that the thermal conductivity of reduced graphene oxide (RGO) film was as 110– 1100 W K1 m1 at room temperature. They found that the measured thermal conductivity of RGO film on a silicon substrate increased as the RGO film became thicker (from 110 to 1100 W K1 m1 when its thickness increased from 1 to 100 nm at room temperature). Even though the thermal conductivity of graphene film is much lower than that of graphene flakes, it is still higher than that of other substrates; thus, graphene is still attractive for thermal applications. Recently, Park et al. [14] reported a dramatic CHF enhancement using graphene oxide (GO), graphene flakes (RGO), and alumina nanofluids. This was attributed to the reduced Rayleigh–Taylor (RT) wavelength due to the deposition of RGO and GO flakes on the heater surface. The RT wavelength was determined by measuring the distance between sessile water droplets on the tested heater after performing CHF experiments in GO/RGO nanofluids. However, some lingering doubts about the accuracy of this means of measuring the RT wavelength remain [4]. because the distance between the sessile water droplets on the wire are determined by the Plateau–Rayleigh instability wavelength [15], induced by the Laplace pressure due to surface tension as shown in Fig. 1 (cf. the RT instability is induced by the density difference between two fluids, and its wavelength on a thin cylinder/wire under boiling is defined by the distance between the generated vapor columns). Thus, some questions about their analysis still remain. In this study, we investigated CHF enhancement in a RGO colloid consisting of RGO flakes suspended in water (or RGO nanofluid) using a 0.2- and 0.1-mm-diameter Ni–Cr wire heaters under pool-boiling conditions. After the CHF tests, we examined the characteristics of the RGO coating layer on the wire heaters,
such as the wettability and surface modifications, to better understand the CHF enhancement. In contrast to previous measurements [14], we found direct representative evidence of a reduced RT wavelength during nucleate boiling due to the surface characteristics caused by the opaque RGO colloid. Here, the reduced RT wavelength could be defined by the distance between the generated vapor columns. We then examined the electrophoresis of the RGO colloid and its effect on the CHF enhancement and RGO coating mechanism during boiling experiments conducted using direct-current (DC) power. 2. Experiments 2.1. Preparation of reduced graphene oxide flakes suspended in water (RGO colloid) The procedure for synthesis of RGO flakes was as follows. A mixture of 55 mg of GO [16] in 55 mL of water was ultrasonicated in a high-intensity ultrasonic processor (Autotune Series, 750 W) for 30 min. The solution was then centrifuged (MF500 centrifuge, Hanil) at 3000 rpm for 30 min. Next, 50 mL of the GO solution was diluted with 50 mL of distilled water containing 50 lL of hydrazine solution (35 wt.%, Aldrich) and 250 lL of ammonia solution (30%). After vigorous mixing for a few minutes, the solution was maintained at 95 °C for 3 h [17]. The solubility of RGO is approximately 0.3 mg/mL. The amount of RGO in solution was measured by filtering a known amount of the solution and weighing the RGO and filter paper. A photograph of the RGO flakes suspended in deionized water is shown in Fig. 2(a). A transmission electron microscopy (TEM) image of typical RGO flakes synthesized by the above method is shown in Fig. 2(b). The inset image of Fig. 2(b). indicates the selected-area electron diffraction (SAED) performed over the area indicated by the box in the TEM image. The region was oriented along the [0 0 1] zone axis. The 24 spots in the first ring, corresponding to reflections from the (1 1 0 0) plane, revealed hexagonal symmetry in the [0 0 0 1] diffraction pattern [18]. These 24 bright spots represent four highly crystalline sheets of graphene that were overlapped. The thickness and size of the suspended RGO flakes in water, as characterized using atomic force microscopy (AFM) measurements, were 0.675 nm and 0.5– 1 lm, respectively. The thickness of the RGO was in agreement with RGO samples prepared using other processes [17,19]; however, this thickness exceeded that of a single graphene layer (0.334 nm). The apparent increase in the height of the graphene on mica was caused by residual epoxy and carbonyl groups that had not been fully reduced. The RGO colloid based on water was 0.0005 wt.%.
Fig. 1. Schematic diagrams of RT and PR instability wavelengths.
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Fig. 2. Characterization of RGO colloid: (a) 0.0005 wt.% RGO colloid, (b) TEM image of an RGO flake, and (c) AFM image of an RGO flake.
2.2. Pool-boiling critical heat flux experimental facility A schematic diagram of the experimental facility is shown in Fig. 3. The main test pool consisted of a 250 140 250-mm rectangular Pyrex glass vessel and a 50-mm-thick Teflon cover. The simple geometry and glass material of the test chamber ensured that clean conditions could be maintained for each experiment. The working fluid was pre-heated using a Corning hot plate, and the pool temperature was measured with a Pt 100-ohm-resistance temperature detector (RTD) sensor. A reflux condenser cooled with tap water prevented the loss of vapor from the test chamber. Accordingly, the volume concentration of the working fluid did not change during the experiments. The opening on the top of the condenser maintained the system pressure at atmospheric. The heating material was a horizontally suspended Ni–Cr wire with a diameter of 0.2 mm. The surface conditions of all test wires were uniform because they were commercially mass-produced. The ends of the wire were clamped to 20-mm-diameter cylindrical copper electrodes. The electrodes were connected to a DC power supply (HP-Agilent 6575A). A standard resistance of 1 X (±0.001 X) was connected to the entire circuit to measure the current. The vessel was filled with R-113 (dielectric liquid), which was maintained at 10 °C using a constant-temperature bath circulator (Jeiotech, HTRC-30) for the constant value of standard resistance.
All pool boiling experiments were conducted while the bulk temperature of the working fluids (distilled water, RGO colloid) was maintained at the saturated temperature under atmospheric pressure. Increasing electric power was supplied to the wire at 100 kW m–2 increments. After a heat flux of 800 kW m–2, which is close to the CHF in distilled water, the power was increased at 50 kW m–2 increments. When the CHF occurred, the resistance of the wire increased suddenly, resulting in an instantaneous breakage of the wire. The CHF was calculated using data obtained immediately before the steep increase of wire resistance as follows:
q00CHF ¼
V max Imax ; pDL
ð2Þ
where q00CHF , Vmax, Imax, D, and L are the CHF value, voltage and current at the CHF point, and wire diameter and length, respectively. The experimental uncertainty was determined using the method proposed by Holman [20] as follows:
U q00CHF q00CHF
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 U V max U Imax UD UL ¼ þ þ þ : V max Imax D L
ð3Þ
The main sources of uncertainty were the applied voltage and length of the wire. There was also the contact resistance between the wire and the copper electrodes, as they were connected only by mechanical clamping. The uncertainties of the applied voltage
Fig. 3. Schematic diagrams of the pool-boiling experimental facility.
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and the length of wire were estimated to be less than ±4.0% and ±1.7%, respectively. From the above analysis, the maximum uncertainty at the pool-boiling CHF was estimated at ±4.4%.
3. Results and discussion In this study, we investigated CHF pool boiling in distilled water with RGO flakes (RGO colloid with 0.0005 wt.% using a Ni–Cr thinwire heater that was 0.2 mm in diameter and 105 mm in length for most of the tests. Through careful review of previous literature, we excluded the effect of the improved thermal conductivity of the RGO colloid on CHF enhancement because this was insignificant for the nanofluid concentration considered in this study [21]. Therefore, we investigated the CHF enhancement in RGO colloids due to the RGO flake coating and the effect of the coating during nucleate boiling. Accordingly, the following experiments were conducted: pool-boiling CHF experiments using distilled water to obtain a reference CHF value, pool boiling in 0.0005 wt.% RGO colloid to determine the resulting CHF enhancement, and pool boiling in distilled water after pre-coating by nucleate boiling until the 90% of RGO colloid CHF value had been reached to confirm the effect of the RGO coating. The CHF value for the RGO colloid (1140 kW m2) was slightly enhanced, by 20%, compared with the CHF value of deionized water (950 kW m2, comparable to Zuber’s prediction [4]). Here, an interesting phenomenon was observed: the CHF in the RGO colloid always occurred on the cathode side of the Ni–Cr wire, as shown in Fig. 4(a). whereas the CHF under normal conditions (e.g., distilled water) occurred randomly over the wire’s length. We used scanning electron microscopy (SEM) to investigate this phenomenon on the cathode and anode sides of the wire heater. The anode side of Ni–Cr wire had more RGO flakes than the cathode side did, as indicated in Fig. 4(b) and (c), which show the RGO deposition and structure on the cathode and anode, respectively.
3.1. Electrophoresis of RGO flakes during nucleate boiling Before examining why the CHF increased during RGO colloid boiling, we investigated the biased RGO coating on the electrodes under the supplied DC power. The carboxyl group (–COOH) on the RGO surface was negatively charged. Because the pool-boiling experimental facility used electrically biased power, the Ni–Cr wire had an electric potential difference between the anode and cathode sides. This electric potential induced a Coulomb force due to the electric field (E = rV/L, where E and V are the electric field and potential difference, respectively). The Coulomb force attracted the RGO flakes to the anode side in a direction perpendicular to the wire orientation. The Coulomb force (electrophoresis) of the electric field can be written as
F c ¼ C RGO E;
ð4Þ
where Fc and CRGO are the Coulomb force on the RGO flake due to the electric field and the electric charge of the RGO flake, respectively. Thus, the localized concentration of the RGO colloid close to the anode was slightly higher than that on the cathode, as shown in Fig. 5. 3.2. CHF enhancement during RGO colloid boiling In the previous section, we postulated electrophoresis of RGO flakes during nucleate boiling. The CHF of the RGO colloid was slightly enhanced by 20% compared with that of deionized water. The fact that the CHF always occurred on the cathode side of the Ni–Cr wire heater indicates that the RGO coating surfaces of the cathode and anode sides during the CHF test were different, as shown in Fig. 4(b) and (c). This could explain the biased deposition, in addition to the location of the CHF phenomenon. On the cathode side of the Ni–Cr wire was a thinner layer that always induced the CHF occurrence, whereas the thicker layer on the anode side did not. Therefore, we examined the thin and thick coating layers using Raman spectroscopy and found that the RGO had nearly similar peaks for the cathode and anode sides of the heater, as shown in Fig. 6(a). The RGO structure on the anode side (thick layer) was capable of enduring higher heat flux than was the cathode side. We decided to first investigate the 20% CHF increase on the cathode side due to the thin RGO-layer deposit. As shown in Fig. 6(b), TEM and SEM of the thin RGO layer revealed a
Fig. 4. Ni–Cr wire heater breakage after a CHF test in RGO colloid: (a) image of the wire heater, (b) SEM image of the cathode side, and (c) SEM image of the anode side.
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Fig. 5. Electrophoresis of the RGO coating during pool boiling.
Fig. 6. Surface investigation of RGO flakes after a CHF test: (a) Raman spectroscopy measurement, (b) cross-section TEM view of RGO the multilayers on the cathode side of the Ni–Cr wire, and (c) contact angle of the bare surface and the thin RGO layer.
well-aligned 1-lm-thick RGO multilayer (thin RGO layer) on the cathode that could be responsible for the thermal characteristics and CHF enhancement. We focused on the role of this multilayer because of the high thermal conductivity of an in-plane graphene flake. In out-of-plane graphene, the thermal conductivity is less than 5 W/m K. According to Jang et al. [13], the thermal conductivity of well-aligned multilayer graphene on silicon oxide increases with the number of graphene layers up to a maximum of 1100 W m1 K1 at room temperature. The thin RGO layer did not influence surface wettability, as shown in Fig. 6(c). We then considered the thermal activity of the thermal conductivity of the thin RGO layer. The influence of thermal conductivity on CHF enhancement [22,23] is given as follows:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi S ¼ d qh ch kh ; q00CHF 00 qCHF;asy
/
S ; S þ 0:8
ð5Þ
ð6Þ
where S, d, qh, ch, kh, and q00CHF;asy are the thermal activity, heater characteristic dimension, density, specific heat, thermal conductivity of the heater material, and asymptotic CHF value for reference, respectively. The asymptotic CHF value from Zuber’s CHF model was used in this study. High thermal activity may dissipate hot/ dry spot formations on the heater by heat conduction removal, delaying the CHF [23]. The thermal activity (S) of the Ni–Cr wire heater was 2.2632; adding the thin 1-lm-thick RGO multilayer (from the TEM images, Fig. 6(b)) yielded a thermal activity of 2.4042. Using Eq. (5), the estimated CHF enhancement ratio for the thin RGO layer was 20%, comparable to the experimental values measured on the cathode side.
3.3. Effect of the RGO flake coating on the CHF enhancement As a result of the findings in the previous section, a closer examination of the RGO structure on the anode side of the wire heater was required. The boiling experiment was stopped at 1000 kW m–2, before the CHF occurred in the RGO colloid, and the polarity of the electrodes was reversed. The boiling experiment was then repeated, resulting in a two-sided coating on the heater. The resulting CHF value was 1588 kW m–2, representing an enhancement of 52%. SEM images of the anode and cathode Fig. 7. clearly showed the RGO-deposition structure responsible for enhancing the CHF. We then varied the coating time for the one-sided wire coating. The heat flux remained constant at 1200 kW m–2 for coating times of 10, 30, and 60 min. Fig. 8 shows clear evidence that the RGO flake coating on the wire was strongly influenced by the electric polarity and coating time. Because the electric field inside the conductive wire was too weak for RGO flake attachment, the thin RGO layer and structure of the RGO flakes were attributed to nucleate boiling [24]. The electric polarity of the RGO flakes changed the localized RGO concentration of the colloid suspension, possibly inducing a structured formation. Furthermore, all RGO-coated wires with different coating times had CHF values in the range of 1150–1310 kW m–2, as shown in Fig. 9, because the thin RGO layer on the cathode determined the CHF. Next, we conducted two-sided coating experiments in RGO using coating times of 10, 30, and 60 min. The coating method was the same as for the previous procedure. Unlike the one-sided coating results, the CHF of the two-sided RGO coating increased with coating time. As the coating time increased from 10 to 30 and 60 min, the CHF increased from 1690 to 2151 and 3355 kW m–2, respectively. The increase in the CHF for the two-sided RGO coating can be
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Fig. 7. SEM image of a two-sided coating.
Fig. 8. One-sided coating versus time.
explained in terms of RGO thickness: as the coating time increased, the RGO structures became thicker, and the CHF increased even more. The question then arises as to why the CHF increased on the RGO structured surface, despite the improved thermal effusivity of the thin RGO layer. This interesting phenomenon was observed during contact-angle measurements with a 2 lL water droplet, as
shown in Fig. 10. The thin RGO layer on the cathode had nearly the same contact angle as the bare Ni–Cr wire (below 90°). The RGO structure on the anode and the two-sided-coated heater exhibited hydrophobic properties. RGO flakes, which consist of a few layers of graphene with a carboxyl group, are more soluble in water compared with graphene, which is hydrophobic [25]. Observations by Liang et al. [26] through filtration with an external
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Fig. 9. CHF versus coating time.
vacuum indicated that RGO flakes in a water solution settle on the bottom surface, forming a well-aligned horizontal structure on the bottom. They reported that the horizontal alignment of the RGO layers indicate the hydrophilic nature of the RGO flakes (i.e., the mixed alignment indicates a hydrophobic response from RGO). We conducted our own study to resolve how a water droplet could stick on hydrophobic material while working against gravity. After waiting 2 min, the water droplet was absorbed into the structures, as shown in Fig. 10. This can be explained in terms of the effect of the carboxyl group on the wetting phenomenon: the microchannels in the RGO structure reduced the contact angle for water absorption. The dual wettability and water absorption of the RGO structures could therefore be responsible for the CHF enhancement [8,27]. But although the water absorption phenomenon could play a role in the saturated porous layer during nucleate boiling, the permeability due to capillary wicking did not seem sufficient to support the observed 320% CHF enhancement. 3.4. Additional observations and further discussion of the CHF enhancement Immediately after the two-sided coating CHF tests, the test specimens (Ni–Cr wires with RGO structure) were carefully pulled out of the main test pool. We observed interesting marks in the RGO aggregations, which were aligned at nearly constant spacing, as shown in Fig. 11. As the coating time increased, the spacing became shorter. For the two-sided coatings obtained after 10, 30, and 60 min, the averaged distance between RGO aggregations were 3.73, 2.16, and 1.28 mm, respectively. We supposed that these RGO aggregations were evidence of vapor columns along the Ni– Cr wire heater. Through fouling effects [28] and the nanoparticle deposition mechanism [29], nanoparticles or additives in the
Fig. 11. Evidence of a vapor column during RGO colloid boiling.
solution were deposited near the triple line during the vigorous nucleate boiling. According to Zuber [4], the distance between the vapor columns during transition boiling can be assumed as the critical wavelength (kcr ) of the RT instability. He extrapolated the relationship between the minimum heat flux and the critical wavelength to the CHF situation by taking the intermediate values as the vapor-column spacing between kcr and the most dangerous wavelength 31=2 kcr : fastest growing disturbance). kcr can be calculated analytically from the balance between the buoyancy and the surface tension as follows:
kcr ¼ 2p
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r ; gðql qv Þ
ð7Þ
kcr of saturated water under atmospheric pressure is 15.74 mm on a plane surface. According to Polezhaev and Kovalev [30], the CHF enhancement on a porous layer by the modulate RT wavelength is
q00CHF porous ¼ q00CHF bare
sffiffiffiffiffiffiffiffiffiffiffiffiffi kbare ; kporous
ð8Þ
where kbare and kporous are the RT wavelengths on the bare and the porous structures, respectively. Using kbare for kcr of water, we predicted the CHF for the two-sided coatings obtained after 10, 30, and 60 min based on the measured kporous . These CHF values were
Fig. 10. Contact angle and water absorption.
2
Critical heat flux (kW/m )
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4000
experiments with different diameter wire heaters (0.2 and 0.1 mm) to introduce a change in the resistance. The applied heat flux of the wire heater is given by:
3200
q00 ¼
2400
1600
CHF experiments Prediction values of CHF using the modulated RT wavelength 0
20
40
60
Two-sided coating time (minutes) Fig. 12. CHF values versus predictions using the RT wavelength.
1951, 2564, and 3331 kW m–2, respectively, and are in good agreement with the experimental CHF values, as shown in Fig. 12. Therefore, the modulated RT wavelength due to nucleate boiling of RGO colloid could be responsible for the CHF enhancement of the twosided coatings. 3.5. Effect of electrophoresis on the RGO coating and CHF enhancement A question still remains regarding the deposition mechanism of the negatively charged RGO flakes and nucleate boiling. The Coulomb force attracts the RGO flakes to the anode side in a direction perpendicular to the wire orientation. Thus, the localized concentration of the RGO colloid close to the anode may be slightly higher than that on the cathode, as shown in Fig. 5. To distinguish the effect of localized distribution concentrations on the deposition of RGO flakes from nucleate boiling, we performed nucleate boiling
krV
2
2
2L
;
ð9Þ
where k and r are the electric conductivity and radius of the wire heater, respectively. The Coulomb force (electrophoresis) of the electric field is given as Eq. (4). Fig. 13 shows the change in the electric field produced by changing the diameter of the wire heater while the heat flux remained constant (1000 kW m2, which is under the CHF value). The 0.1-mm-diameter wire heater should have more electric potential to maintain the same heat flux compared with the larger (0.2-mm diameter) wire heater; thus, the Coulomb force of the 0.1-mm wire should be larger than that of the 0.2mm wire. The effect of nucleate boiling on the deposition of RGO flakes should be same for both wires because the applied heat fluxes were the same. However, SEM images revealed that the RGO flake deposition on the anode of the 0.1-mm wire heater was greater than that of the 0.2-mm wire heater, as shown in Fig. 13, due to the higher electric potential and greater Coulomb force on the RGO flakes. 4. Conclusions In this study, the CHF enhancement mechanism of pool boiling in RGO colloid was investigated experimentally. The RGO colloid was prepared from GO using a chemical reduction process with hydrazine and diluted to 0.0005 wt.%. The CHF experiments were carried out by increasing heat flux on Ni–Cr heaters with 0.2and 0.1-mm wire diameters. The results showed that the RGO coating layer was biased by the electric polarity on the wire heater surface. There was a slight CHF enhancement of 20% in the RGO colloid compared with deionized water. Additional coating experiments were performed to prepare uniform RGO coating layers and analyze their effects on the CHF enhancement. The analyses
Fig. 13. Same heat flux and different electrophoresis experiment results.
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were focused on the deposition mechanism, thermal activity, water absorption, and modulated RT wavelength of the RGO coating layers on the wires. The following conclusions were drawn. 1. A biased deposition characteristic was found on the wire heater after the CHF experiment with RGO colloid. The RGO flakes were negatively charged after the synthesis process, so that they preferred to coat the anode side of the heater. This demonstrated that the RGO coating was strongly affected by DC power-induced electrophoresis. 2. The CHF was enhanced by 20% on the cathode side of the wire, which had a thin well-aligned RGO layer coating. We postulated that the thermal activity of the thin RGO layer on the cathode explained the CHF enhancement based on the reported thermal conductivity of well-aligned RGO layers from the literature. 3. Uniform RGO coating layers were prepared using a two-sided coating method. The coating time was varied, yielding a maximum CHF enhancement of 320%. 4. During wettability tests of the RGO coating layer, the thick anode layer was observed to have hydrophobic characteristics. After the 2 min of water dispensing, water absorption was observed, even though the thick layer was hydrophobic. Graphene is hydrophobic by nature, but its wetting characteristics were altered by activating a carboxyl group on the RGO flakes so that the layer could absorb water. This water absorption phenomenon is one possible mechanism of the CHF enhancement. 5. After the CHF experiments using the two-sided coating method were completed, interesting marks were observed on the RGOcoated wires. These marks were made by RGO aggregations formed at constant spacing. We postulated that these marks could represent the distance between vapor columns when the CHF phenomena occurred and that the distance was determined by the critical wavelength of the Rayleigh–Taylor instability. The average distance between the marks decreased as the CHF was enhanced, and a modulated wavelength analysis showed good agreement with these measured results.
Acknowledgement This research was supported by WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-30005). This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (2012R1A2A1A01003376 and 2012R1A1A2002900). References [1] M.G. Kang, Experimental investigation of tube length effect on nucleate pool boiling heat transfer, Ann. Nucl. Energy 25 (4–5) (1998) 295–304. [2] I. Mudawar, Assessment of high-heat-flux thermal management schemes, IEEE Trans. Compon. Packag. Technol. 24 (2) (2001) 122–141. [3] J.R. Thome, Enhanced Boiling Heat Transfer, Hemisphere, New York, 1990.
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[4] N. Zuber, Hydrodynamic aspects of boiling heat transfer, Ph.D. thesis, University of California, Los Angeles, AECU-4439, 1959. [5] S.J. Kim, I.C. Bang, J. Buongiorno, L.W. Hu, Effects of nanoparticle deposition on surface wettability influencing boiling heat transfer in nanofluids, Appl. Phys. Lett. 89 (15) (2006) 153107. [6] H.D. Kim, J.B. Kim, M.H. Kim, Effect of nanoparticles on CHF enhancement in pool boiling of nano-fluids, Int. J. Heat Mass Transfer 49 (25–26) (2006) 5070– 5074. [7] H.D. Kim, M.H. Kim, Effect of nanoparticle deposition on capillary wicking that influences the critical heat flux in nanofluids, Appl. Phys. Lett. 91 (1) (2007) 014104. [8] H.S. Ahn, H.J. Jo, S.H. Kang, M.H. Kim, Effect of liquid spreading due to nano/ microstructures on the critical heat flux during pool boiling, Appl. Phys. Lett. 98 (7) (2011) 071908. [9] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (3) (2008) 902–907. [10] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A.A. Firsov, Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (2005) 197–200. [11] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [12] C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388. [13] W. Jang, Z. Chen, W. Bao, C.N. Lau, C. Dames, Thickness-dependent thermal conductivity of encased graphene and ultrathin graphite, Nano Lett. 10 (10) (2010) 3909–3913. [14] S.D. Park, S.W. Lee, S. Kang, I.C. Bang, J.H. Kim, H.S. Shin, D.W. Lee, D.W. Lee, Effects of nanofluids containing graphene/graphene-oxide nanosheets on critical heat flux, Appl. Phys. Lett. 97 (2010) 023103. [15] R.J. Roe, Wetting of fine wires and fibers by a liquid film, J. Colloid Interface Sci. 50 (1) (1975) 70–79. [16] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (6) (1958) 1339. [17] D. Li, M.B. Müller, S. Gilje, R.B. Kaner, Processable aqueous dispersions of graphene nanosheets, Nat. Nanotechnol. 3 (2008) 101–105. [18] M.J. McAllister, J.L. Li, D.H. Adamson, H.C. Schniepp, A.A. Abdala, J. Liu, M. Herrera-Alonso, D.L. Milius, R. Car, R.K. Prud’homme, I.A. Aksay, Single sheet functionalized graphene by oxidation and thermal expansion of graphite, Chem. Mater. 19 (18) (2007) 4396–4404. [19] I.K. Moon, J. Lee, S.R. Ruoff, H. Lee, Reduced graphene oxide by chemical graphitization, Nat. Commun. 1 (2010) 73. [20] J.P. Holman, Experimental Methods for Engineers, seventh ed., McGrew-Hill, New York, 2001 (Chapter 3). [21] S.M.S. Murshed, K.C. Leong, C. Yang, Enhanced thermal conductivity of TiO2– water based nanofluids, Int. J. Therm. Sci. 44 (4) (2005) 367–373. [22] M. Arik, A. Bar-Cohen, Effusivity-based correlation of surface property effects in pool boiling CHF of dielectric liquids, Int. J. Heat Mass Transfer 46 (20) (2003) 3755–3764. [23] I.C. Bang, J. Buongirno, L.W. Hu, H. Wang, Measurement of key pool boiling parameters in nanofluids for nuclear applications, J. Power Energy Syst. 2 (1) (2008) 340–351. [24] H.D. Kim, H.S. Ahn, M.H. Kim, On the mechanism of pool boiling critical heat flux enhancement in nanofluids, J. Heat Transfer 132 (6) (2010) 061501. [25] J. Rafiee, M.A. Rafee, Z.Z. Yu, N. Korakar, Superhydrophobic to superhydrophilic wetting control in graphene films, Adv. Mater. 22 (19) (2010) 2151–2154. [26] Q. Liang, X. Yao, W. Wang, Y. Liu, C.P. Wong, A three-dimensional vertically aligned functionalized multilayer graphene architecture: an approach for graphene-based thermal interfacial materials, ACS Nano 5 (3) (2011) 2392– 2401. [27] R. Chen, M.C. Lu, V. Srinivasan, Z. Wang, H.H. Cho, A. Majumdar, Nanowires for enhanced boiling heat transfer, Nano Lett. 9 (2) (2009) 548–553. [28] H. Yu, R. Sheikholeslami, W.O.S. Doherty, Effect of thermohydraulic conditions on fouling of calcium oxalate and silica, AlChE J. 51 (2) (2005) 641–648. [29] S.M. Kwark, R. Kumar, G. Moreno, J. Yoo, S.M. You, Pool boiling characteristics of low concentration nanofluids, Int. J. Heat Mass Transfer 53 (5–6) (2010) 972–981. [30] Y.V. Polezhaev, S.A. Kovalev, Modeling heat transfer with boiling on porous structures, Therm. Eng. 37 (12) (1990) 617–620.
Contents lists available at SciVerse ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Experimental study of the effect of a reduced graphene oxide coating on critical heat flux enhancement Ho Seon Ahn a, Ji Min Kim b, Moo Hwan Kim c,⇑ a
Division of Mechanical System Engineering, Incheon National University, Incheon 406-772, Republic of Korea Department of Mechanical Engineering, POSTECH, Pohang 790-784, Republic of Korea c Division of Advanced Nuclear Engineering, POSTECH, Pohang 790-784, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 27 August 2012 Received in revised form 21 January 2013 Accepted 22 January 2013 Available online 17 February 2013 Keywords: Critical heat flux Graphene Reduced graphene oxide Pool boiling
a b s t r a c t Critical heat flux (CHF) nucleate pool boiling was investigated in a reduced graphene oxide (RGO) colloid consisting of 0.0005 wt.% flake-based RGO in distilled water using Ni–Cr thin-wire heaters 0.1 and 0.2 mm in diameter and 105 mm in length. One- and two-sided coatings were tested to study the RGO flake aggregation as a function of coating time using the Joule heating method. In the one-sided coating tests, RGO flake aggregation was biased on the anode so that the CHF occurred on the cathode side with slight enhancement. The RGO coating on the cathode increased with coating time; however, the coating on the anode did not appear to be affected by coating time. The biased deposition of the RGO coating was attributed to the negatively charged carboxyl group (–COOH) in the RGO flakes. In the two-sided coating tests, the RGO flake aggregation showed dramatic CHF enhancement with increasing coating time. The enhancement mechanisms were examined based on a surface analysis considering the deposition mechanism, thermal activity, water absorption, and the modulated Rayleigh–Taylor wavelength of the RGO coating layer on the wire heaters. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Phase-change heat transfer has high efficiency due to the large latent heat of the working fluids. Representative applications of boiling heat transfer (liquid–vapor phase change) include those found in nuclear power-plant energy transfer [1], electronic chip cooling [2], and various chemical processes [3]. The critical heat flux (CHF) is the thermal limit of a phenomenon in which the heating-induced phase change suddenly decreases the heat transfer efficiency, causing overheating of the heated surface. When the CHF occurs, the temperature of the surface suddenly increases, melting the heated material. Therefore, the CHF value is an important standard with regard to the safe performance of two-phase thermal systems (e.g., preventing nuclear meltdown). A number of models for CHF prediction in pool boiling have been proposed. Among them, Zuber’s CHF model [4] has been most widely accepted. In Zuber’s model, the vapor generated on a flat plate accumulates to form a continuous column of escape flow, with a diameter of half the Rayleigh–Taylor critical wavelength, kRT , at wavelength intervals. When the vapor–liquid interface of the escape flow passage becomes unstable due to Helmholtz instability, the CHF occurs. This phenomenon can be represented as follows:
⇑ Corresponding author. E-mail address: [email protected] (M.H. Kim). 0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.01.052
1=4 q00zuber ¼ 0:131hlg q0:5 ; g ½rgðql qg Þ
ð1Þ
where q00 zuber, hlg, r, g, ql, and qg are the Zuber CHF, latent heat, surface tension, gravity, liquid density, and vapor density, respectively. From Eq. (1), q00 zuber 1050 kW m–2 for boiling saturated water at 1 atm, which is close to the experimental value. In addition to efforts aimed at predicting the CHF value, considerable research has been directed at increasing the limit of boiling heat transfer (e.g., in nanofluids) to improve safety. Recent studies have shown that when boiled in a nanofluid, the nanoparticles on a heated surface cause CHF enhancement by changing the wettability characteristics of the surface [5]. Kim et al. [6] performed poolboiling experiments using pure water on nanoparticle-deposited surfaces that were produced while pool boiling water–TiO2 and water–Al2O3 nanofluids. The CHF of pure water boiling on the nanoparticle-deposited surface was greater than the nanofluid CHF in all cases. This clearly indicated that nanoparticles deposited on the heating surface were the main cause of CHF enhancement. Kim et al. [6] also conducted pool-boiling experiments in nanofluids and described the CHF enhancement in terms of surface wettability and the absorption of the nanoparticle deposition layer. They suggested a simple model consisting of an absorption layer of nanoparticles to describe how the nanoparticles changed the surface wettability. Using results from a thin-wire heater, Kim and Kim [7] postulated that the wettability and capillarity of the heating surface could influence CHF enhancement. They explained
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that the CHF increases the liquid supply on the heating surface due to the capillarity of the absorption layer of nanoparticles. Recently, Ahn et al. [8] reported that liquid spreading due to micro/nanostructures by the capillary force of water between the structures enhanced the CHF because of the increased liquid supply on the heating surface. Graphene has gained much attention recently due to its high thermal conductivity (900–5000 W K1 m1) [9], high transparency [10], ultra-high electric conductivity [11], and novel mechanical properties [12]. However, little information about the thermal characteristics or applications of graphene is available. Since graphene exists as ultra-thin and microscale flakes, considerable effort has been made to synthesize large-area films that retain the superb properties of graphene. Among them, soluble graphene in water solvent has been highlighted through the oxidation process of graphene. Through a chemical process with hydrazine, graphene oxide can be changed to a reduced graphene oxide, which can have properties similar to graphene. Jang et al. [13] estimated that the thermal conductivity of reduced graphene oxide (RGO) film was as 110– 1100 W K1 m1 at room temperature. They found that the measured thermal conductivity of RGO film on a silicon substrate increased as the RGO film became thicker (from 110 to 1100 W K1 m1 when its thickness increased from 1 to 100 nm at room temperature). Even though the thermal conductivity of graphene film is much lower than that of graphene flakes, it is still higher than that of other substrates; thus, graphene is still attractive for thermal applications. Recently, Park et al. [14] reported a dramatic CHF enhancement using graphene oxide (GO), graphene flakes (RGO), and alumina nanofluids. This was attributed to the reduced Rayleigh–Taylor (RT) wavelength due to the deposition of RGO and GO flakes on the heater surface. The RT wavelength was determined by measuring the distance between sessile water droplets on the tested heater after performing CHF experiments in GO/RGO nanofluids. However, some lingering doubts about the accuracy of this means of measuring the RT wavelength remain [4]. because the distance between the sessile water droplets on the wire are determined by the Plateau–Rayleigh instability wavelength [15], induced by the Laplace pressure due to surface tension as shown in Fig. 1 (cf. the RT instability is induced by the density difference between two fluids, and its wavelength on a thin cylinder/wire under boiling is defined by the distance between the generated vapor columns). Thus, some questions about their analysis still remain. In this study, we investigated CHF enhancement in a RGO colloid consisting of RGO flakes suspended in water (or RGO nanofluid) using a 0.2- and 0.1-mm-diameter Ni–Cr wire heaters under pool-boiling conditions. After the CHF tests, we examined the characteristics of the RGO coating layer on the wire heaters,
such as the wettability and surface modifications, to better understand the CHF enhancement. In contrast to previous measurements [14], we found direct representative evidence of a reduced RT wavelength during nucleate boiling due to the surface characteristics caused by the opaque RGO colloid. Here, the reduced RT wavelength could be defined by the distance between the generated vapor columns. We then examined the electrophoresis of the RGO colloid and its effect on the CHF enhancement and RGO coating mechanism during boiling experiments conducted using direct-current (DC) power. 2. Experiments 2.1. Preparation of reduced graphene oxide flakes suspended in water (RGO colloid) The procedure for synthesis of RGO flakes was as follows. A mixture of 55 mg of GO [16] in 55 mL of water was ultrasonicated in a high-intensity ultrasonic processor (Autotune Series, 750 W) for 30 min. The solution was then centrifuged (MF500 centrifuge, Hanil) at 3000 rpm for 30 min. Next, 50 mL of the GO solution was diluted with 50 mL of distilled water containing 50 lL of hydrazine solution (35 wt.%, Aldrich) and 250 lL of ammonia solution (30%). After vigorous mixing for a few minutes, the solution was maintained at 95 °C for 3 h [17]. The solubility of RGO is approximately 0.3 mg/mL. The amount of RGO in solution was measured by filtering a known amount of the solution and weighing the RGO and filter paper. A photograph of the RGO flakes suspended in deionized water is shown in Fig. 2(a). A transmission electron microscopy (TEM) image of typical RGO flakes synthesized by the above method is shown in Fig. 2(b). The inset image of Fig. 2(b). indicates the selected-area electron diffraction (SAED) performed over the area indicated by the box in the TEM image. The region was oriented along the [0 0 1] zone axis. The 24 spots in the first ring, corresponding to reflections from the (1 1 0 0) plane, revealed hexagonal symmetry in the [0 0 0 1] diffraction pattern [18]. These 24 bright spots represent four highly crystalline sheets of graphene that were overlapped. The thickness and size of the suspended RGO flakes in water, as characterized using atomic force microscopy (AFM) measurements, were 0.675 nm and 0.5– 1 lm, respectively. The thickness of the RGO was in agreement with RGO samples prepared using other processes [17,19]; however, this thickness exceeded that of a single graphene layer (0.334 nm). The apparent increase in the height of the graphene on mica was caused by residual epoxy and carbonyl groups that had not been fully reduced. The RGO colloid based on water was 0.0005 wt.%.
Fig. 1. Schematic diagrams of RT and PR instability wavelengths.
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Fig. 2. Characterization of RGO colloid: (a) 0.0005 wt.% RGO colloid, (b) TEM image of an RGO flake, and (c) AFM image of an RGO flake.
2.2. Pool-boiling critical heat flux experimental facility A schematic diagram of the experimental facility is shown in Fig. 3. The main test pool consisted of a 250 140 250-mm rectangular Pyrex glass vessel and a 50-mm-thick Teflon cover. The simple geometry and glass material of the test chamber ensured that clean conditions could be maintained for each experiment. The working fluid was pre-heated using a Corning hot plate, and the pool temperature was measured with a Pt 100-ohm-resistance temperature detector (RTD) sensor. A reflux condenser cooled with tap water prevented the loss of vapor from the test chamber. Accordingly, the volume concentration of the working fluid did not change during the experiments. The opening on the top of the condenser maintained the system pressure at atmospheric. The heating material was a horizontally suspended Ni–Cr wire with a diameter of 0.2 mm. The surface conditions of all test wires were uniform because they were commercially mass-produced. The ends of the wire were clamped to 20-mm-diameter cylindrical copper electrodes. The electrodes were connected to a DC power supply (HP-Agilent 6575A). A standard resistance of 1 X (±0.001 X) was connected to the entire circuit to measure the current. The vessel was filled with R-113 (dielectric liquid), which was maintained at 10 °C using a constant-temperature bath circulator (Jeiotech, HTRC-30) for the constant value of standard resistance.
All pool boiling experiments were conducted while the bulk temperature of the working fluids (distilled water, RGO colloid) was maintained at the saturated temperature under atmospheric pressure. Increasing electric power was supplied to the wire at 100 kW m–2 increments. After a heat flux of 800 kW m–2, which is close to the CHF in distilled water, the power was increased at 50 kW m–2 increments. When the CHF occurred, the resistance of the wire increased suddenly, resulting in an instantaneous breakage of the wire. The CHF was calculated using data obtained immediately before the steep increase of wire resistance as follows:
q00CHF ¼
V max Imax ; pDL
ð2Þ
where q00CHF , Vmax, Imax, D, and L are the CHF value, voltage and current at the CHF point, and wire diameter and length, respectively. The experimental uncertainty was determined using the method proposed by Holman [20] as follows:
U q00CHF q00CHF
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 U V max U Imax UD UL ¼ þ þ þ : V max Imax D L
ð3Þ
The main sources of uncertainty were the applied voltage and length of the wire. There was also the contact resistance between the wire and the copper electrodes, as they were connected only by mechanical clamping. The uncertainties of the applied voltage
Fig. 3. Schematic diagrams of the pool-boiling experimental facility.
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and the length of wire were estimated to be less than ±4.0% and ±1.7%, respectively. From the above analysis, the maximum uncertainty at the pool-boiling CHF was estimated at ±4.4%.
3. Results and discussion In this study, we investigated CHF pool boiling in distilled water with RGO flakes (RGO colloid with 0.0005 wt.% using a Ni–Cr thinwire heater that was 0.2 mm in diameter and 105 mm in length for most of the tests. Through careful review of previous literature, we excluded the effect of the improved thermal conductivity of the RGO colloid on CHF enhancement because this was insignificant for the nanofluid concentration considered in this study [21]. Therefore, we investigated the CHF enhancement in RGO colloids due to the RGO flake coating and the effect of the coating during nucleate boiling. Accordingly, the following experiments were conducted: pool-boiling CHF experiments using distilled water to obtain a reference CHF value, pool boiling in 0.0005 wt.% RGO colloid to determine the resulting CHF enhancement, and pool boiling in distilled water after pre-coating by nucleate boiling until the 90% of RGO colloid CHF value had been reached to confirm the effect of the RGO coating. The CHF value for the RGO colloid (1140 kW m2) was slightly enhanced, by 20%, compared with the CHF value of deionized water (950 kW m2, comparable to Zuber’s prediction [4]). Here, an interesting phenomenon was observed: the CHF in the RGO colloid always occurred on the cathode side of the Ni–Cr wire, as shown in Fig. 4(a). whereas the CHF under normal conditions (e.g., distilled water) occurred randomly over the wire’s length. We used scanning electron microscopy (SEM) to investigate this phenomenon on the cathode and anode sides of the wire heater. The anode side of Ni–Cr wire had more RGO flakes than the cathode side did, as indicated in Fig. 4(b) and (c), which show the RGO deposition and structure on the cathode and anode, respectively.
3.1. Electrophoresis of RGO flakes during nucleate boiling Before examining why the CHF increased during RGO colloid boiling, we investigated the biased RGO coating on the electrodes under the supplied DC power. The carboxyl group (–COOH) on the RGO surface was negatively charged. Because the pool-boiling experimental facility used electrically biased power, the Ni–Cr wire had an electric potential difference between the anode and cathode sides. This electric potential induced a Coulomb force due to the electric field (E = rV/L, where E and V are the electric field and potential difference, respectively). The Coulomb force attracted the RGO flakes to the anode side in a direction perpendicular to the wire orientation. The Coulomb force (electrophoresis) of the electric field can be written as
F c ¼ C RGO E;
ð4Þ
where Fc and CRGO are the Coulomb force on the RGO flake due to the electric field and the electric charge of the RGO flake, respectively. Thus, the localized concentration of the RGO colloid close to the anode was slightly higher than that on the cathode, as shown in Fig. 5. 3.2. CHF enhancement during RGO colloid boiling In the previous section, we postulated electrophoresis of RGO flakes during nucleate boiling. The CHF of the RGO colloid was slightly enhanced by 20% compared with that of deionized water. The fact that the CHF always occurred on the cathode side of the Ni–Cr wire heater indicates that the RGO coating surfaces of the cathode and anode sides during the CHF test were different, as shown in Fig. 4(b) and (c). This could explain the biased deposition, in addition to the location of the CHF phenomenon. On the cathode side of the Ni–Cr wire was a thinner layer that always induced the CHF occurrence, whereas the thicker layer on the anode side did not. Therefore, we examined the thin and thick coating layers using Raman spectroscopy and found that the RGO had nearly similar peaks for the cathode and anode sides of the heater, as shown in Fig. 6(a). The RGO structure on the anode side (thick layer) was capable of enduring higher heat flux than was the cathode side. We decided to first investigate the 20% CHF increase on the cathode side due to the thin RGO-layer deposit. As shown in Fig. 6(b), TEM and SEM of the thin RGO layer revealed a
Fig. 4. Ni–Cr wire heater breakage after a CHF test in RGO colloid: (a) image of the wire heater, (b) SEM image of the cathode side, and (c) SEM image of the anode side.
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Fig. 5. Electrophoresis of the RGO coating during pool boiling.
Fig. 6. Surface investigation of RGO flakes after a CHF test: (a) Raman spectroscopy measurement, (b) cross-section TEM view of RGO the multilayers on the cathode side of the Ni–Cr wire, and (c) contact angle of the bare surface and the thin RGO layer.
well-aligned 1-lm-thick RGO multilayer (thin RGO layer) on the cathode that could be responsible for the thermal characteristics and CHF enhancement. We focused on the role of this multilayer because of the high thermal conductivity of an in-plane graphene flake. In out-of-plane graphene, the thermal conductivity is less than 5 W/m K. According to Jang et al. [13], the thermal conductivity of well-aligned multilayer graphene on silicon oxide increases with the number of graphene layers up to a maximum of 1100 W m1 K1 at room temperature. The thin RGO layer did not influence surface wettability, as shown in Fig. 6(c). We then considered the thermal activity of the thermal conductivity of the thin RGO layer. The influence of thermal conductivity on CHF enhancement [22,23] is given as follows:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi S ¼ d qh ch kh ; q00CHF 00 qCHF;asy
/
S ; S þ 0:8
ð5Þ
ð6Þ
where S, d, qh, ch, kh, and q00CHF;asy are the thermal activity, heater characteristic dimension, density, specific heat, thermal conductivity of the heater material, and asymptotic CHF value for reference, respectively. The asymptotic CHF value from Zuber’s CHF model was used in this study. High thermal activity may dissipate hot/ dry spot formations on the heater by heat conduction removal, delaying the CHF [23]. The thermal activity (S) of the Ni–Cr wire heater was 2.2632; adding the thin 1-lm-thick RGO multilayer (from the TEM images, Fig. 6(b)) yielded a thermal activity of 2.4042. Using Eq. (5), the estimated CHF enhancement ratio for the thin RGO layer was 20%, comparable to the experimental values measured on the cathode side.
3.3. Effect of the RGO flake coating on the CHF enhancement As a result of the findings in the previous section, a closer examination of the RGO structure on the anode side of the wire heater was required. The boiling experiment was stopped at 1000 kW m–2, before the CHF occurred in the RGO colloid, and the polarity of the electrodes was reversed. The boiling experiment was then repeated, resulting in a two-sided coating on the heater. The resulting CHF value was 1588 kW m–2, representing an enhancement of 52%. SEM images of the anode and cathode Fig. 7. clearly showed the RGO-deposition structure responsible for enhancing the CHF. We then varied the coating time for the one-sided wire coating. The heat flux remained constant at 1200 kW m–2 for coating times of 10, 30, and 60 min. Fig. 8 shows clear evidence that the RGO flake coating on the wire was strongly influenced by the electric polarity and coating time. Because the electric field inside the conductive wire was too weak for RGO flake attachment, the thin RGO layer and structure of the RGO flakes were attributed to nucleate boiling [24]. The electric polarity of the RGO flakes changed the localized RGO concentration of the colloid suspension, possibly inducing a structured formation. Furthermore, all RGO-coated wires with different coating times had CHF values in the range of 1150–1310 kW m–2, as shown in Fig. 9, because the thin RGO layer on the cathode determined the CHF. Next, we conducted two-sided coating experiments in RGO using coating times of 10, 30, and 60 min. The coating method was the same as for the previous procedure. Unlike the one-sided coating results, the CHF of the two-sided RGO coating increased with coating time. As the coating time increased from 10 to 30 and 60 min, the CHF increased from 1690 to 2151 and 3355 kW m–2, respectively. The increase in the CHF for the two-sided RGO coating can be
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Fig. 7. SEM image of a two-sided coating.
Fig. 8. One-sided coating versus time.
explained in terms of RGO thickness: as the coating time increased, the RGO structures became thicker, and the CHF increased even more. The question then arises as to why the CHF increased on the RGO structured surface, despite the improved thermal effusivity of the thin RGO layer. This interesting phenomenon was observed during contact-angle measurements with a 2 lL water droplet, as
shown in Fig. 10. The thin RGO layer on the cathode had nearly the same contact angle as the bare Ni–Cr wire (below 90°). The RGO structure on the anode and the two-sided-coated heater exhibited hydrophobic properties. RGO flakes, which consist of a few layers of graphene with a carboxyl group, are more soluble in water compared with graphene, which is hydrophobic [25]. Observations by Liang et al. [26] through filtration with an external
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Fig. 9. CHF versus coating time.
vacuum indicated that RGO flakes in a water solution settle on the bottom surface, forming a well-aligned horizontal structure on the bottom. They reported that the horizontal alignment of the RGO layers indicate the hydrophilic nature of the RGO flakes (i.e., the mixed alignment indicates a hydrophobic response from RGO). We conducted our own study to resolve how a water droplet could stick on hydrophobic material while working against gravity. After waiting 2 min, the water droplet was absorbed into the structures, as shown in Fig. 10. This can be explained in terms of the effect of the carboxyl group on the wetting phenomenon: the microchannels in the RGO structure reduced the contact angle for water absorption. The dual wettability and water absorption of the RGO structures could therefore be responsible for the CHF enhancement [8,27]. But although the water absorption phenomenon could play a role in the saturated porous layer during nucleate boiling, the permeability due to capillary wicking did not seem sufficient to support the observed 320% CHF enhancement. 3.4. Additional observations and further discussion of the CHF enhancement Immediately after the two-sided coating CHF tests, the test specimens (Ni–Cr wires with RGO structure) were carefully pulled out of the main test pool. We observed interesting marks in the RGO aggregations, which were aligned at nearly constant spacing, as shown in Fig. 11. As the coating time increased, the spacing became shorter. For the two-sided coatings obtained after 10, 30, and 60 min, the averaged distance between RGO aggregations were 3.73, 2.16, and 1.28 mm, respectively. We supposed that these RGO aggregations were evidence of vapor columns along the Ni– Cr wire heater. Through fouling effects [28] and the nanoparticle deposition mechanism [29], nanoparticles or additives in the
Fig. 11. Evidence of a vapor column during RGO colloid boiling.
solution were deposited near the triple line during the vigorous nucleate boiling. According to Zuber [4], the distance between the vapor columns during transition boiling can be assumed as the critical wavelength (kcr ) of the RT instability. He extrapolated the relationship between the minimum heat flux and the critical wavelength to the CHF situation by taking the intermediate values as the vapor-column spacing between kcr and the most dangerous wavelength 31=2 kcr : fastest growing disturbance). kcr can be calculated analytically from the balance between the buoyancy and the surface tension as follows:
kcr ¼ 2p
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r ; gðql qv Þ
ð7Þ
kcr of saturated water under atmospheric pressure is 15.74 mm on a plane surface. According to Polezhaev and Kovalev [30], the CHF enhancement on a porous layer by the modulate RT wavelength is
q00CHF porous ¼ q00CHF bare
sffiffiffiffiffiffiffiffiffiffiffiffiffi kbare ; kporous
ð8Þ
where kbare and kporous are the RT wavelengths on the bare and the porous structures, respectively. Using kbare for kcr of water, we predicted the CHF for the two-sided coatings obtained after 10, 30, and 60 min based on the measured kporous . These CHF values were
Fig. 10. Contact angle and water absorption.
2
Critical heat flux (kW/m )
770
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4000
experiments with different diameter wire heaters (0.2 and 0.1 mm) to introduce a change in the resistance. The applied heat flux of the wire heater is given by:
3200
q00 ¼
2400
1600
CHF experiments Prediction values of CHF using the modulated RT wavelength 0
20
40
60
Two-sided coating time (minutes) Fig. 12. CHF values versus predictions using the RT wavelength.
1951, 2564, and 3331 kW m–2, respectively, and are in good agreement with the experimental CHF values, as shown in Fig. 12. Therefore, the modulated RT wavelength due to nucleate boiling of RGO colloid could be responsible for the CHF enhancement of the twosided coatings. 3.5. Effect of electrophoresis on the RGO coating and CHF enhancement A question still remains regarding the deposition mechanism of the negatively charged RGO flakes and nucleate boiling. The Coulomb force attracts the RGO flakes to the anode side in a direction perpendicular to the wire orientation. Thus, the localized concentration of the RGO colloid close to the anode may be slightly higher than that on the cathode, as shown in Fig. 5. To distinguish the effect of localized distribution concentrations on the deposition of RGO flakes from nucleate boiling, we performed nucleate boiling
krV
2
2
2L
;
ð9Þ
where k and r are the electric conductivity and radius of the wire heater, respectively. The Coulomb force (electrophoresis) of the electric field is given as Eq. (4). Fig. 13 shows the change in the electric field produced by changing the diameter of the wire heater while the heat flux remained constant (1000 kW m2, which is under the CHF value). The 0.1-mm-diameter wire heater should have more electric potential to maintain the same heat flux compared with the larger (0.2-mm diameter) wire heater; thus, the Coulomb force of the 0.1-mm wire should be larger than that of the 0.2mm wire. The effect of nucleate boiling on the deposition of RGO flakes should be same for both wires because the applied heat fluxes were the same. However, SEM images revealed that the RGO flake deposition on the anode of the 0.1-mm wire heater was greater than that of the 0.2-mm wire heater, as shown in Fig. 13, due to the higher electric potential and greater Coulomb force on the RGO flakes. 4. Conclusions In this study, the CHF enhancement mechanism of pool boiling in RGO colloid was investigated experimentally. The RGO colloid was prepared from GO using a chemical reduction process with hydrazine and diluted to 0.0005 wt.%. The CHF experiments were carried out by increasing heat flux on Ni–Cr heaters with 0.2and 0.1-mm wire diameters. The results showed that the RGO coating layer was biased by the electric polarity on the wire heater surface. There was a slight CHF enhancement of 20% in the RGO colloid compared with deionized water. Additional coating experiments were performed to prepare uniform RGO coating layers and analyze their effects on the CHF enhancement. The analyses
Fig. 13. Same heat flux and different electrophoresis experiment results.
H.S. Ahn et al. / International Journal of Heat and Mass Transfer 60 (2013) 763–771
were focused on the deposition mechanism, thermal activity, water absorption, and modulated RT wavelength of the RGO coating layers on the wires. The following conclusions were drawn. 1. A biased deposition characteristic was found on the wire heater after the CHF experiment with RGO colloid. The RGO flakes were negatively charged after the synthesis process, so that they preferred to coat the anode side of the heater. This demonstrated that the RGO coating was strongly affected by DC power-induced electrophoresis. 2. The CHF was enhanced by 20% on the cathode side of the wire, which had a thin well-aligned RGO layer coating. We postulated that the thermal activity of the thin RGO layer on the cathode explained the CHF enhancement based on the reported thermal conductivity of well-aligned RGO layers from the literature. 3. Uniform RGO coating layers were prepared using a two-sided coating method. The coating time was varied, yielding a maximum CHF enhancement of 320%. 4. During wettability tests of the RGO coating layer, the thick anode layer was observed to have hydrophobic characteristics. After the 2 min of water dispensing, water absorption was observed, even though the thick layer was hydrophobic. Graphene is hydrophobic by nature, but its wetting characteristics were altered by activating a carboxyl group on the RGO flakes so that the layer could absorb water. This water absorption phenomenon is one possible mechanism of the CHF enhancement. 5. After the CHF experiments using the two-sided coating method were completed, interesting marks were observed on the RGOcoated wires. These marks were made by RGO aggregations formed at constant spacing. We postulated that these marks could represent the distance between vapor columns when the CHF phenomena occurred and that the distance was determined by the critical wavelength of the Rayleigh–Taylor instability. The average distance between the marks decreased as the CHF was enhanced, and a modulated wavelength analysis showed good agreement with these measured results.
Acknowledgement This research was supported by WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-30005). This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (2012R1A2A1A01003376 and 2012R1A1A2002900). References [1] M.G. Kang, Experimental investigation of tube length effect on nucleate pool boiling heat transfer, Ann. Nucl. Energy 25 (4–5) (1998) 295–304. [2] I. Mudawar, Assessment of high-heat-flux thermal management schemes, IEEE Trans. Compon. Packag. Technol. 24 (2) (2001) 122–141. [3] J.R. Thome, Enhanced Boiling Heat Transfer, Hemisphere, New York, 1990.
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