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Smart surface in pool boiling: Thermally-induced wetting transition
International Journal of Heat and Mass Transfer 109 (2017) 231–241
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Smart surface in pool boiling: Thermally-induced wetting transition Jin Man Kim a,b,⇑, Dong In Yu b, Hyun Sun Park a, Kiyofumi Moriyama a, Moo Hwan Kim a a b
Division of Advanced Nuclear Engineering, POSTECH, Pohang 37673, Republic of Korea Korea Atomic Energy Research Institute, Daejeon 34057, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 November 2016 Received in revised form 4 February 2017 Accepted 4 February 2017
Keywords: Critical heat flux Heat transfer coefficient Wettability TiO2 Pool boiling
a b s t r a c t The boiling heat transfer coefficient (HTC) and critical heat flux (CHF) of a TiO2-coated surface (TCS) were investigated in pool boiling with increasing saturated temperature at the pressurized conditions ranged from 1.0 to 4.1 bar. TCS increased HTC in comparison with a reference surface coated with SiO2 (SCS) under the pressure ranged from 1.0 to 4.1 bars. CHF of TCS was higher than SCS at pressure of 4.1 bar while lower below 4.1 bar. Measurement of the contact angle of a water droplet on the tested surfaces after heat treatment showed a wettability increase of TCS, a contact angle reduction from 83.1° to 32.7° when the heat treatment temperature changed from 100 °C to 200 °C. No such change was observed for SCS. This contact angle change after heating suggests that the wetting transition of TCS is a key factor in the enhancement of both HTC and CHF in boiling. TCS is hydrophobic at a low wall temperature and becomes hydrophilic as the wall temperature increases. Hydrophobicity of TCS at low wall temperatures explains the improved HTC over SCS near the boiling inception point and low heat flux regime, and hydrophilicity at high wall temperatures explains the increase of CHF. The transition in the wettability of TiO2 appeared to be involved in CHF enhancement. The thermally-induced wetting transition of TiO2 provides a simple and innovative means for enhancing both HTC and CHF with no additional treatment; as such, we refer to TiO2 as a ‘smart’ surface. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Nucleate boiling is one of the most efficient heat-transfer methods. In evaluating boiling performance, heat transfer coefficient (HTC) and critical heat flux (CHF) are important measures. HTC is an index for the efficiency of the heat transfer and CHF is a limitation on the available nucleate boiling regime. Thus, many techniques have been assessed to enhance boiling performance for the efficiency and safety of heat transfer systems. Recently, the wettability of the heating surface has been a focus because it has an important effect on boiling performance. A hydrophobic surface activates bubble generation more vigorously than a hydrophilic surface; thus, it contributes to higher HTC. However, a hydrophilic surface induces liquid supply to the dry area of a heating surface, delaying CHF. Wettability can be controlled by modifying the morphology and/or chemical composition of a surface [1–3]. There are many reports on enhancing boiling performance by controlling wettability in pool boiling. Forrest et al. [4] controlled the wettability of nickel wires using a layer-by-layer (LbL) method and examined the boiling character⇑ Corresponding author at: Division of Advanced Nuclear Engineering, POSTECH, Pohang 37673, Republic of Korea. E-mail address: [email protected] (J.M. Kim). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2017.02.009 0017-9310/Ó 2017 Elsevier Ltd. All rights reserved.
istics of these wires. Hydrophobic wires showed much higher HTCs, but lower CHF value than hydrophilic wires. Superhydrophilic wires showed the highest CHF (enhanced by 101%) although they had lower HTCs than hydrophobic wires. Bourdon et al. [5] investigated the wettability effect using surfaces having nanometer roughness to isolate the effect; bronze plates were highly polished and grafted using an alkanethiol self-assembled monolayer (SAM) to control wettability. From pool boiling results, hydrophobic surfaces showed higher HTCs, but lower CHFs than hydrophilic surfaces [5]. Bourdon et al. [6] conducted similar experiments using glass substrates with chemical grafting using SAMs of octadecyltrichlorosilane (OTS) and identified higher HTCs and lower CHFs than hydrophilic surfaces; their results were consistent with previous work [5]. From the literatures, hydrophobic surfaces induce active bubble generation at low wall superheat, but cause premature CHFs due to excessive bubbles, despite low wall superheat. Thus, if the premature CHF of hydrophobic surfaces could be overcome, then it would be practical for higher heattransfer-performance surfaces with higher HTCs than hydrophilic surfaces. Hydrophilic surfaces promote liquid supply to the surface, contributing to a higher CHF than a hydrophobic surface. In You et al. [7], a nanofluid was examined because it enhanced CHF. Bang et al.
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Nomenclature A1 A2 Ah B cpl C simple CHF F g hlv HTC I kl m P Psat q00 q00CHF Rs SCS Tb Th T sat Tw
experimental constant experimental constant effective heating area, m2 experimental constant heat capacity, kJ/kgK simplified experimental constant critical heat flux, kW/m2 experimental constant gravity acceleration, m/s2 latent heat, kJ/kg heat transfer coefficient, kW/m2K current, A thermal conductivity, W/mK exponent pressure, bar saturated pressure, bar heat flux, kW/m2 critical heat flux, kW/m2 resistance of shunt resistor, X SiO2-coated surface bulk temperature of water, °C temperature of the heating element, °C saturated temperature, °C wall temperature, °C
[8] used an Al2O3 nanofluid as a working fluid and conducted pool boiling experiments; although nanofluid showed poorer HTC than pure water, the CHF increased regardless of surface orientation. These results were attributed to the deposition of nanoparticles on the heating surface [8]. Kim et al. [9] used pool boiling to examine the effects of deposited nanoparticles on a wire heater; they reported that CHF increased not only on the bare wire in nanofluid boiling but also on nanoparticle-deposited wire in pure water boiling. Kim et al. [10] demonstrated increased wettability due to nanoparticle deposition after boiling in a nanofluid. They reported that nanoparticle deposition enhanced wettability and caused a CHF increase. Stutz et al. [11] and Jo et al. [12] also used nanoparticle-deposited surfaces in pool boiling and reported that the enhanced wettability, due to nanoparticle deposition, increased CHF. Since noting the importance of ‘good’ wettability of nanoparticle-deposited surfaces in increasing CHF, other types of surface modification techniques have been suggested. Forrest et al. [4] also identified CHF increases on a hydrophilic surface by the LbL method in pool boiling. Kim et al. [13] fabricated nanoand microstructures using a microelectromechanical system (MEMS) technique and conducted pool boiling on these structured surfaces. Microstructures showed the highest HTC, and/or nano/ micro-combined structures showed the highest CHF (107% enhancement). The increase in CHF was facilitated by enhanced wettability due to the nanostructures [13]. Ahn et al. [14] made micro/nano-multiscale structures on zircaloy-4 surfaces using anodic oxidation; the contact angle on the surface was <10° due to increased wettability. From pool boiling experiments, they also reported increased CHF and attributed that to a spreading effect of the liquid. From experimental evidences [4–6,15,16], a hydrophobic surface can trigger nucleate boiling at low wall superheat, but it also brings premature CHF due to bubble coalescence and sticking to the surface. In contrast, a hydrophilic surface can promote liquid supply to the dry area to delay CHF. In this respect, heterogeneous wetting control techniques have been developed. Betz et al. [17,18]
TCS Vh Vr
TiO2-coated surface voltage of the heating element, V voltage of the shunt resistance, X
Greek symbols b receding contact angle, ° / orientation of heating surface, ° ll viscosity, lPas h static contact angle ql liquid density, kg/m3 qv vapor density, kg/m3 r surface tension, N/m Subscripts l liquid v vapor phase change from liquid to vapor lv w wall of heated surface b bulk sat saturated condition h heating element CHF CHF
developed heterogeneous wetting surfaces that had hydrophobic islands on a hydrophilic network or hydrophilic islands on a hydrophobic network. Their investigation of pool boiling performance confirmed that hydrophobic islands with hydrophilic networks showed the best boiling performance and enhancement of the HTC and CHF by 100% and 65%, respectively. Jo et al. [15,19] also prepared heterogeneous wetting surfaces and examined their boiling performance; they showed that the hydrophobic surface had a higher HTC with sustaining CHF. Using a surface with hydrophobic dots on a hydrophilic substrate, they enhanced HTC without degradation of CHF. Although wettability can affect boiling performance, there is a trade-off between HTC and CHF. In this respect, Bertossi et al. [20] used switchable polymers coating for the improvement of boiling heat transfer. The polymer coating showed wetting transition of hydrophilic to hydrophobic when a temperature is above 108 °C. Since hydrophobic surface promotes initiation of bubble nucleation while hydrophilic surface enhanced bubble detachment, the polymer coating increased boiling heat transfer in the nucleate boiling regime. In the present study, TiO2 thin film was used to enhance both HTC and CHF. Sun et al. [21] fabricated thin films of TiO2 on glass substrates to investigate wettability changes after heat treatment at various temperatures in air. The initial water contact angle on TiO2 was 54°. The contact angle decreased as the heat treatment temperature increased. When TiO2 was annealed at 200 °C, the contact angle was 20°. In addition, the contact angle decreased to <10° with increasing heat treatment temperature, up to about 250 °C. They reported that oxygen vacancies in the crystal structure of TiO2 primarily improved wettability. Such oxygen vacancies can be generated in the crystal structure of TiO2 when it is treated by ultraviolet (UV) irradiation, heat treatment, and Ar+ sputtering. They explained that oxygen vacancies were kinetically favorable for adsorption of hydroxyl, and water molecules were adsorbed dissociatively on the oxygen vacancies. The special characteristics of TiO2 have been reported in numerous studies [22–29].
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We manufactured a TiO2-coated surface (TCS) that had a wetting transition temperature; it showed hydrophobic characteristics before the transition temperature, while it became hydrophilic above the wetting transition temperature. Thus, under lowpressure conditions, the TCS had a higher HTC and lower CHF than hydrophilic surface because it did not reach the transition temperature. However, it had a higher HTC in the low-wall-superheat region due to its hydrophobicity and also enhanced the CHF because it reached wetting transition temperature and became a hydrophilic surface. The thermally-induced wetting change of TCS is unique characteristics for enhancing both HTC and CHF with no additional treatment; as such, we refer to TCS as a ‘smart’ surface. 2. Experimental 2.1. Surface fabrication A silicon wafer with the thickness of 500 lm was used for a heating substrate. A roughness effect was neglected in the present study because the silicon substrate has a nano-meter scale roughness. To use the Joule heating method, platinum patterned layer of 120 nm was deposited on the backside using MEMS technique after SiO2 (SCS) was deposited using thermal growth method for electrical insulation as depicted in Fig. 1. On the boiling side, SiO2 or TiO2 layer was deposited: SiO2 layer of 500 nm was deposited as a reference surface and TiO2 of a 200 nm layer was deposited using RF sputtering. 2.2. Pool for a regulation of saturated temperature Since the TCS had a wetting transition temperature around 200 °C, a pool boiling facility was constructed to achieve the wetting transition temperature near CHF conditions by increasing system pressure from 1.0 to 4.1 bar. The whole body was made of stainless steel pipe depicted (Fig. 2). The inside diameter, height and wall thickness of the pool were 200 mm, 500 mm and 8.2 mm, respectively. There were some penetrations for the measurement of the bulk water temperature and water level. In the pool, 12 L of deionized (DI) water was used as the working fluid. An immersion heater was used to make saturated temperature of the bulk water for each pressure condition. A test section assembly was positioned at the bottom of the pool. This assembly consisted of the heating surface and a polyether ether ketone (PEEK) jig, which is an insulating material. There were two lids for the pool. A degassing lid was connected to the reflux condenser for the degassing process (not shown in Fig. 2). For pressurization, another lid equipped with three relief
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valves (VRV1MH, HY-LOK) was used; the relief valves were adjusted to be opened at 2.0, 3.3 and 4.1 bar, respectively. The relief valves discharged excessive steam during boiling, so the pressure in the pool was not beyond the set value of the relief valve. Under saturated conditions, the pressure in the pool was estimated using a correlation [30]. The experiment was initiated once the pressure remained at the set pressure for more than 10 min. 2.3. Experimental facilities The experimental system is shown in Fig. 3. From the bottom of the pool, wires soldered to the electrodes of the heating element were connected to the main circuit and data acquisition system (34970a, Agilent Technologies). To calculate a current in the circuit during boiling, the circuit was connected to a shunt resistor in the thermostat. Because the thermostat maintained a constant temperature of 10 °C, the resistance of shunt resistor was also kept at a constant value. The current in the circuit was calculated from the voltage drop at the shunt resistor. Then, the heat flux was estimated based on the current value using Eq. (1), where Vh is the voltage drop at the heating element (V), Ah is the effective heating area (m2), I is the current (A), Vr is the voltage drop at the shunt resistance (V), Rs is the resistance of the shunt resistor (X). Experiments were conducted at 1.0, 2.0, 3.3, and 4.1 bar to identify boiling characteristics of TCS and its wetting transition. The experimental range and uncertainty, by Holman [31], are provided in Table 1, where Psat is a saturated pressure, Tsat is a saturated temperature of bulk water, Th is a temperature of heating element, and q00 is a heat flux.
q00 ¼
V hI V h V r ¼ Ah Ah Rs
ð1Þ
2.4. Experimental procedures To estimate the heating element temperature during boiling, the resistance of the heating element was calibrated in the convection oven (OF-02GW, JEIO TECH) before experiments. The resistance of the heating element increased linearly with the temperature. During boiling, the temperature of the heating element was calculated using the resistance-temperature correlation. For the degassing, DI water was boiled for 3 h using an immersion heater with the degassing lid connecting to a reflux condenser, so that the water level was maintained during the degassing step. After degassing was complete, the degassing lid was replaced with a pressurized lid with the assembled relief valves and fastened to the body of the pool. DI water was boiled continuously via the
Fig. 1. Schematic images of the surface fabrication.
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Fig. 2. Schematic images of the pressurized pool.
Fig. 3. Experimental system.
Table 1 Experimental range and uncertainty. Test #
Experimental parameters Psat
P1 P2 P3 P4
Tsat
q00max
Th,max
Value (bar)
Error (%)
Value (°C)
Error (%)
Value (°C)
Error (%)
Value (kW/m2)
Error (%)
1.0 2.0 3.3 4.1
3.58 3.17 2.88 2.76
99.6 120.2 136.8 144.9
1.0 0.8 0.7 0.7
153.1 167.4 192.3 203.4
0.2 0.2 0.2 0.2
918 1246 1629 1912
0.59 0.58 0.58 0.58
immersion heater, and the temperature and pressure were increased to specific values. Because a relief valve opened to release excess steam at the set pressure, steady state was achieved under saturated conditions. When the pressure and temperature were maintained a constant value for more than 10 min, the experiment was initiated and the heat flux was increased step-by-step slowly. At specific levels of the heat flux, heat flux was maintained and recorded for
2 min to estimate averaged data under steady-state conditions. When the wall temperature increased suddenly, by >18 °C in 1 s, the power supply was turned off, and this point was deemed CHF. 2.5. Data reduction Because the heating element was exposed to air during the experiment, the heat loss to air was estimated. The heating ele-
J.M. Kim et al. / International Journal of Heat and Mass Transfer 109 (2017) 231–241
ment was downward facing; heat was transferred from the heating element to air by conduction [32]. For all of the experimental conditions, the maximum heat loss to air was 0.43%. Thus, the heat loss to air was ignored. Additionally, we measured the temperature of the heating element to estimate surface temperature during the experiment. Using the thermal conductivity of 130 W/mK for silicon, the surface temperature was evaluated at each heat flux condition using the Fourier’s law. 2.6. Validation of the facility To validate the experimental facility, we compared the experimental CHF with Eq. (2) suggested by Zuber [33] and Eq. (3) suggested by Haramura and Katto [34], where qv is the density of vapor, ql is the density of liquid, hlv is the latent heat, r is the surface tension, and g is the gravity acceleration; dc was taken to be 0.056 mm that was used in the work [34]. For the calculation, experimental properties were used. The results (Fig. 4) showed that the correlations overestimated the experimental CHF of SCS. Zuber’s correlation [33] showed differences in 20% at 1.0 bar and 11% at 4.1 bar. The correlation of Haramura and Katto [34] also showed differences in 3% at 1.0 bar and 19% at 4.1 bar. Although the correlations overestimated the experimental data, they captured the trend of increasing CHF with the saturated pressure. This result means that the pool boiling facility showed a general trend of pool boiling.
q00CHF ¼
q00CHF
p
q1=2 hlv ½rgðql qv Þ1=4 24 v
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rq qv 0:4 q ¼ hlv 0:00536 v 1þ v dc ql ql
ð2Þ
ð3Þ
3. Results and discussion To examine an effect of the thermally-induced wetting conversion of TCS, pool boiling experiments were conducted with increasing saturated temperature. After TiO2 thin film was successfully deposited on the heating substrate, the contact angle changes of TCS and SCS were investigated using heat treatment test. Furthermore, HTC of TCS was increased for all experimental conditions, and CHF of TCS was enhanced at 4.1 bar. With analysis of wetting conversion and boiling characteristics for TCS, we postulated that the enhancement was attributed to the wetting transition.
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3.1. Contact angle & receding contact angle The thermally-induced conversion in wettability with temperature was key to enhancing boiling performance with TCS. Thus, to confirm the wettability change in TCS with temperature, heat treatments were conducted under various temperature conditions using a furnace. The temperature was controlled from 100 to 200 °C, which corresponded to the wall temperature for the boiling experiment. SCS and TCS were annealed under each prescribed temperature condition in atmosphere for more than 10 h to guarantee a sufficient wettability change and were not reused for the heat treatment at different temperatures. After the heat treatment, the surfaces were cooled to room temperature (25 °C), and the contact angles were measured. Fig. 5 shows static and receding contact angles on SCS and TCS after heat treatment. The SCS surfaces show a significant change in contact angles with heat treatment. However, static and receding contact angles on TCS decreased with increasing heat treatment temperature. After the surfaces were annealed at 100 °C, the static contact angles were 68.4° and 83.1° on SCS and TCS, respectively. When the surfaces were annealed at 200 °C, the static contact angle of TCS decreased to 32.7°, unlike SCS, which showed almost the same contact angles. The receding contact angles of TCS also changed from 53.0 to 14.3° at 100 °C and 200 °C, respectively. SCS didn’t show the significant change of the receding contact angle. 3.2. Changes in the crystal structure after heat treatment The decrease in the wettability of TiO2 with temperature is attributed to a change in the crystal structure of TiO2. Park et al. [28] investigated the crystal phase of TiO2 with heat treatment at various temperatures. When TiO2 was annealed at 500 °C, the intensity of peaks for the anatase, rutile, and brookite phases were enhanced in X-ray diffraction (XRD) analyses. This indicates that there was a change in the crystal structure. Liu et al. [35] also studied wettability of TiO2 with temperature variation. From XRD patterns, the TiO2 film had showed an amorphous structure even when it was annealed at 300 °C. The heat treatment at a temperature of 400 °C resulted in an anatase structure; the intensity of the peak was enhanced at a heat treatment temperature of 650 °C. They found that the contact angle decreased as the anatase structure appeared and concluded that anatase TiO2 showed the highest hydrophilicity in their study. We investigated crystal structure of TiO2 before and after heat treatment was conducted at 200 °C. In Fig. 6(a), SiO2 showed an amorphous structures, regardless of heat treatment at 200 °C. TiO2 showed an amorphous structure in the as-deposited condition; when annealed at 200 °C, three peaks appeared (Fig. 6(b)). These peaks, (101), (004), and (200), corresponds to the anatase phase structure (JCPDS No. 21-1272). Thus, these results confirm a change in the crystal structure of TiO2 with temperature, resulting in enhancement in the wettability of TiO2. 3.3. Heat transfer coefficient
Fig. 4. A comparison of experimental data with correlations of Zuber [33] and Haramura and Katto [34].
Fig. 7 shows HTC curves as a function of the wall superheat at pressures of 1.0, 2.0, 3.3, and 4.1 bars. From the wettability characteristics (Fig. 5), it was expected that TCS had a higher HTC than SCS because TCS was hydrophobic at the low-wall-temperature regime. Experimental data clearly showed higher HTCs on TCS than SCS, regardless of the pressure. Specially, the TCS showed an abrupt degradation of heat transfer for the TCS at approximately 45 K. In that point, HTC deteriorated because the boiling regime changed from nucleate boiling to film boiling due to CHF. At 3.3 bar, HTC of TCS was higher than of SCS due to the hydrophobic-
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Fig. 5. Measured (a) static contact angles and (b) receding contact angles after heat treatment.
Fig. 6. X-ray diffraction analysis of (a) SiO2 and (b) TiO2.
ity of TCS. Likewise, the HTC of TCS improved at 4.1 bar due to its hydrophobicity, and CHF (CHF is discussed in the Section 3.5) was enhanced in comparison with SCS in high-wall-superheat regime due to the transition in wettability to a hydrophilic surface. For the analysis of HTC with wettability, we used the correlation in Eq. (4) suggested by Wang et al. [36]. The correlation is a function of properties, wall superheat, contact angle, and empirical constants in Eqs. (5) and (6), where Tw is the wall temperature (°C), Tb is the bulk temperature, h is the contact angle, ll is the viscosity of bulk water, hlv is the latent heat, r is the surface tension, g is the gravity acceleration, ql is the density of bulk water, and qv is the density of vapor. Eq. (4) also includes empirical constants of C and m; thus, the correlation can be rewritten in the form of Eq. (7) with a constant of Csimple for simplicity. In the present analysis, Csimple and m were fitted to the experimental data of SCS with the measured contact angle (Fig. 5) at each pressure condition. The correlation reflected the wettability effect; thus, the heat flux was estimated with increasing wall superheat using specific contact angles.
q00 ¼ B½FðT w T b Þmþ1 Cð1 cos hÞll hlv " F
mþ1
¼
1=2
17=8 19=8 m23=8 m15=8 cpl hlv v 9=8 m11=8 m15=8 Þ T l v b
kl ql
ll ð q
q
1=2 B ¼ A2=3 1 A2
q
2 p1=2 g 9=8
r
r
1=2
gðql qv Þ
ð4Þ
# ð5Þ
ð6Þ
q00 ¼ C simple ½FðT w T b Þmþ1 ð1 cos hÞll hlv
r
1=2
gðql qv Þ
ð7Þ
In Fig. 8, the experimental data are plotted using black and blue points for SCS and TCS, respectively. Dashed lines were calculated using Eq. (7) with changing contact angles. When the contact angle of 67° was used for SCS, the experimental data for SCS did not interfere with the other dashed lines, which were calculated with different contact angles. In contrast, the boiling curve of TCS crossed other dashed lines. This suggests that the boiling curve of TCS can be explained with multiple values of contact angle. As in Fig. 5, the contact angle for TCS decreased as the wall temperature increased. 3.4. Roughness effect For the validation of a negligible effect of a roughness on boiling, atomic force microscopy (AFM) was used to characterize the roughness of the SCS and TCS surfaces (Fig. 9). Before experiments were conducted, the SCS and TCS had roughness of 1 nm. After the experiments were conducted up to a heat flux near CHF at 4.1 bar, the roughnesses were 1.5 and 4.3 nm, for SCS and TCS, respectively. If the roughness of the present surfaces acted as nucleation cavities during boiling, then the boiling characteristics would be attributed to not only the wettability but also a roughness effect. To estimate the theoretical cavity size that can act as a nucleation seed, the range of active nucleation cavities was calculated using the theory by Hsu et al. [37]; they suggested that the cavity size range depended on the wall temperature (°C), bulk tem-
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Fig. 7. Heat transfer coefficient curves at (a) 1.0, (b) 2.0, (c) 3.3, and (d) 4.1 bars.
Fig. 8. Boiling curves with the correlation of Wang et al. [36] at (a) 1.0, (b) 2.0, (c) 3.3, and (d) 4.1 bars, respectively.
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Fig. 9. Measured roughness of (a) before, (b) after experiment, and (c) comparison with a theoretical minimum cavity size [37].
perature (°C), latent heat (kJ/kg), density of vapor (kg/m3), surface tension (N/m), and the thermal boundary layer (m). The thermal boundary layer was assumed to be 0.2 mm at atmospheric pressure [16]. Using the experimental properties, the calculated range of nucleation cavities ranged from 1.03 to 100 lm at 1 bar. With the same boundary layer thickness, the range of nucleation cavities was from 0.25 to 100 lm at 4.1 bar. Thus, the minimum cavity size was 250 times larger than the roughness of the surfaces studied. As such, there was no contribution of the roughness to the nucleation cavity because the roughness in the present study was much smaller than the theoretical value. Kang [38] also showed the effects of nanometer-scale roughness on HTC during pool boiling. From their experimental results, roughnesses of 60.9 and 15.1 nm had no significant effect on boiling characteristics on a horizontal tube. Thus, it is reasonable that the roughness effect on HTC was also negligible in the present study by cavity theory [37] and an experimental report on nanometer roughness effects [38].
3.5. Critical heat flux Kandlikar suggested a theoretical prediction model of CHF in the pool boiling [39]. The model in Eq. (8) includes not only properties of working fluid but also a receding contact angle as a parameter. Fig. 10 shows a comparison of experimental CHF values with Kandlikar’s predictions. At each pressure condition, a correction factor in Fig. 10 was applied to the model for fitting of SCS experimental data. The dashed lines are CHFs based on calculations using the prediction model for various receding contact angles; the experimental CHFs are shown as solid points for pressure conditions of 1.0, 2.0, 3.3 and 4.1 bars. The blue solid line is an expectation line of CHF based on the receding contact angle by the heat treatment test in Fig. 5. At the pressures below 4.1 bar, there was no enhancement in CHF with TCS because it did not experience a sufficient change in wettability. However, CHF of TCS surpassed that of SCS due to the transition in the wettability of TCS at 4.1 bar. At high wall temperature regime under this pressure, the
J.M. Kim et al. / International Journal of Heat and Mass Transfer 109 (2017) 231–241
Fig. 10. Critical heat flux comparison with the correlation of Kandlikar [39] and C is a correction factor.
surface temperature was so high that TCS became hydrophilic and enhanced CHF.
q00CHF ¼ hlv q1=2 v ð
1 þ cos b 2 p Þ½ þ ð1 þ cos bÞ cos / 16 p 4
239
the crystal structure of TiO2. From previous reports [40–42], the decrease in the contact angle of TiO2 was affected not only by the intensity of UV irradiation but also by its duration. When the UV irradiation exposure time was not sufficiently long to change the crystal structure of TiO2, the contact angle did not decrease completely. In the present study, it was possible that 2 min of maintained heat flux for each steady-state condition was short to change the crystal structure of TCS, although the type of treatment differed from that in the literatures. Thus, additional tests were conducted to identify the CHF with a complete wettability change. For this test, the heat flux was increased using the same procedure described in Section 2.4. When the heat flux reached 80–90% of the CHF at each pressure, the heat flux was sustained for about 20 min to guarantee complete wetting change in TCS. After 20 min, the experiment was resumed until CHF occurred. As a result, there were additional enhancements in CHF with TCS in comparison with the CHF in the ‘normal’ test. These results are plotted as crossed hollow points in Fig. 10. There were also changes in CHF for SCS, but the CHFs for SCS deteriorated at 3.3 and 4.1 bar. From the additional tests, we suggest that CHF could be enhanced further given enough time, even though the CHF of the TCS did not perfectly match the predictions of Kandlikar’s model. 3.6. Boiling curves and contact angles
1=2
ð8Þ
From Fig. 10, it was confirmed that the CHF of the TCS was enhanced. Nevertheless, the enhancement of CHF was lower than the predictions (the blue solid line). In previous works [21,28,29,40–42], TCS became hydrophilic with a change in crystal structure when various types of energy, such as UV irradiation, heat treatment, and bombardment with Ar+, were applied. Additionally, the treatment time is also an important factor in changing
Overall boiling characteristics of TCS can be explained by the variation of contact angles from the heat treatment. Fig. 11 shows boiling curves of the surfaces with their contact angles from the heat treatment versus wall temperature. The curves under pressure of 1.0 bar was clearly understood with wettability. In the nucleate boiling regime, TCS was hydrophobic; thus HTC of TCS was higher than SCS. However, sticking of bubbles to TCS caused lower CHF than SCS due to its hydrophobicity. The curves under pressure of 2.0 bar showed similar boiling characteristics of
Fig. 11. Boiling curves and contact angles with wall temperature under pressures of (a) 1.0, (b) 2.0, (c) 3.3, and (d) 4.1 bars, respectively. Solid points are the experimental data from the normal operation, and hollow points are CHFs from the time effect operation.
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1.0 bar. In specific, HTC of TCS deteriorated due to the transition to vapor film regime. The boiling curves for 3.3 bar also showed similar characteristics; HTC of TCS was higher than SCS because nucleate boiling was initiated in hydrophobic regime of TCS. Although the wall temperature near CHF was in hydrophilic regime of TCS, the change of wettability was not sufficient to enhance CHF because of the short period of steady state time. Under the pressure of 4.1 bar, nucleate boiling was initiated in the hydrophobic regime of TCS, so HTC for TCS was improved in comparison to SCS. The wall temperature was high enough to achieve a sufficient wetting transition of TCS near CHF point, and CHF for TCS was enhanced in comparison to SCS due to hydrophilicity of TCS. Furthermore, maintaining the heat flux of 80–90% of CHF at the pressure of 3.3 and 4.1 bars caused additional enhancement of CHF for TCS by the complete wetting transition. Consequently, there was the enhancement of both HTC and CHF for TCS by its wetting transition.
[5] [6]
[7]
[8]
[9] [10]
[11]
[12]
4. Conclusions In the present study, the pool boiling HTC and CHF of TiO2coated surface (TCS) that has unique characteristics of the wetting transition with temperature were examined in a pool boiling. A TiO2 thin film of 200 nm thickness was deposited on a silicon substrate using RF sputtering, and SiO2 film of 500 nm thickness was coated on the substrate as a reference surface (SCS) using a thermal growth technique. Contact angle variation after a heat treatment was investigated for both surfaces with changing the treatment temperature in an oven. Pool boiling experiments with the fabricated test surfaces were conducted at 1.0, 2.0, 3.3, and 4.1 bar. The boiling HTCs of TCS samples were higher than those of SCS, regardless of pressure conditions. The CHF of TCS was higher than SCS at 4.1 bar, while no enhancement was observed at 1.0, 2.0, and 3.3 bar. Based on the observed temperature dependence of the contact angle, we hypothesize that TCS is hydrophobic at the initial stage of boiling and becomes hydrophilic with increasing wall temperature. Since the roughnesses of the surfaces were extremely low (below 5 nm); the roughness effect should be negligible in the present study. Therefore, wettability change depending on temperature is probably the key parameter affecting the boiling characteristics of TCS. When a heat flux of 80–90% of the CHF was maintained for 20 min, additional enhancement of CHF was observed on TCS, which can be attributed to a complete wetting transition. Thus, the boiling characteristics of the surfaces are well explained by the characteristics of the wettability change observed in the heat treatment test. We suggest TCS as a ‘smart’ surface that enhances both HTC and CHF due to the wetting transition, which is completely passive and does not need additional external control.
[13]
[14]
[15]
[16] [17] [18]
[19]
[20] [21]
[22]
[23]
[24]
Acknowledgement
[25]
This work was supported by National Research Foundation of Korea (NRF) grants funded by the Korean Government (MSIP) (2015M2A8A2074795). The fabrication of the test sections was supported by Ulsan National Institute of Science and Technology (UNIST), and National Nanofab Center (NNFC).
[26]
[27]
[28]
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Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Smart surface in pool boiling: Thermally-induced wetting transition Jin Man Kim a,b,⇑, Dong In Yu b, Hyun Sun Park a, Kiyofumi Moriyama a, Moo Hwan Kim a a b
Division of Advanced Nuclear Engineering, POSTECH, Pohang 37673, Republic of Korea Korea Atomic Energy Research Institute, Daejeon 34057, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 November 2016 Received in revised form 4 February 2017 Accepted 4 February 2017
Keywords: Critical heat flux Heat transfer coefficient Wettability TiO2 Pool boiling
a b s t r a c t The boiling heat transfer coefficient (HTC) and critical heat flux (CHF) of a TiO2-coated surface (TCS) were investigated in pool boiling with increasing saturated temperature at the pressurized conditions ranged from 1.0 to 4.1 bar. TCS increased HTC in comparison with a reference surface coated with SiO2 (SCS) under the pressure ranged from 1.0 to 4.1 bars. CHF of TCS was higher than SCS at pressure of 4.1 bar while lower below 4.1 bar. Measurement of the contact angle of a water droplet on the tested surfaces after heat treatment showed a wettability increase of TCS, a contact angle reduction from 83.1° to 32.7° when the heat treatment temperature changed from 100 °C to 200 °C. No such change was observed for SCS. This contact angle change after heating suggests that the wetting transition of TCS is a key factor in the enhancement of both HTC and CHF in boiling. TCS is hydrophobic at a low wall temperature and becomes hydrophilic as the wall temperature increases. Hydrophobicity of TCS at low wall temperatures explains the improved HTC over SCS near the boiling inception point and low heat flux regime, and hydrophilicity at high wall temperatures explains the increase of CHF. The transition in the wettability of TiO2 appeared to be involved in CHF enhancement. The thermally-induced wetting transition of TiO2 provides a simple and innovative means for enhancing both HTC and CHF with no additional treatment; as such, we refer to TiO2 as a ‘smart’ surface. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Nucleate boiling is one of the most efficient heat-transfer methods. In evaluating boiling performance, heat transfer coefficient (HTC) and critical heat flux (CHF) are important measures. HTC is an index for the efficiency of the heat transfer and CHF is a limitation on the available nucleate boiling regime. Thus, many techniques have been assessed to enhance boiling performance for the efficiency and safety of heat transfer systems. Recently, the wettability of the heating surface has been a focus because it has an important effect on boiling performance. A hydrophobic surface activates bubble generation more vigorously than a hydrophilic surface; thus, it contributes to higher HTC. However, a hydrophilic surface induces liquid supply to the dry area of a heating surface, delaying CHF. Wettability can be controlled by modifying the morphology and/or chemical composition of a surface [1–3]. There are many reports on enhancing boiling performance by controlling wettability in pool boiling. Forrest et al. [4] controlled the wettability of nickel wires using a layer-by-layer (LbL) method and examined the boiling character⇑ Corresponding author at: Division of Advanced Nuclear Engineering, POSTECH, Pohang 37673, Republic of Korea. E-mail address: [email protected] (J.M. Kim). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2017.02.009 0017-9310/Ó 2017 Elsevier Ltd. All rights reserved.
istics of these wires. Hydrophobic wires showed much higher HTCs, but lower CHF value than hydrophilic wires. Superhydrophilic wires showed the highest CHF (enhanced by 101%) although they had lower HTCs than hydrophobic wires. Bourdon et al. [5] investigated the wettability effect using surfaces having nanometer roughness to isolate the effect; bronze plates were highly polished and grafted using an alkanethiol self-assembled monolayer (SAM) to control wettability. From pool boiling results, hydrophobic surfaces showed higher HTCs, but lower CHFs than hydrophilic surfaces [5]. Bourdon et al. [6] conducted similar experiments using glass substrates with chemical grafting using SAMs of octadecyltrichlorosilane (OTS) and identified higher HTCs and lower CHFs than hydrophilic surfaces; their results were consistent with previous work [5]. From the literatures, hydrophobic surfaces induce active bubble generation at low wall superheat, but cause premature CHFs due to excessive bubbles, despite low wall superheat. Thus, if the premature CHF of hydrophobic surfaces could be overcome, then it would be practical for higher heattransfer-performance surfaces with higher HTCs than hydrophilic surfaces. Hydrophilic surfaces promote liquid supply to the surface, contributing to a higher CHF than a hydrophobic surface. In You et al. [7], a nanofluid was examined because it enhanced CHF. Bang et al.
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Nomenclature A1 A2 Ah B cpl C simple CHF F g hlv HTC I kl m P Psat q00 q00CHF Rs SCS Tb Th T sat Tw
experimental constant experimental constant effective heating area, m2 experimental constant heat capacity, kJ/kgK simplified experimental constant critical heat flux, kW/m2 experimental constant gravity acceleration, m/s2 latent heat, kJ/kg heat transfer coefficient, kW/m2K current, A thermal conductivity, W/mK exponent pressure, bar saturated pressure, bar heat flux, kW/m2 critical heat flux, kW/m2 resistance of shunt resistor, X SiO2-coated surface bulk temperature of water, °C temperature of the heating element, °C saturated temperature, °C wall temperature, °C
[8] used an Al2O3 nanofluid as a working fluid and conducted pool boiling experiments; although nanofluid showed poorer HTC than pure water, the CHF increased regardless of surface orientation. These results were attributed to the deposition of nanoparticles on the heating surface [8]. Kim et al. [9] used pool boiling to examine the effects of deposited nanoparticles on a wire heater; they reported that CHF increased not only on the bare wire in nanofluid boiling but also on nanoparticle-deposited wire in pure water boiling. Kim et al. [10] demonstrated increased wettability due to nanoparticle deposition after boiling in a nanofluid. They reported that nanoparticle deposition enhanced wettability and caused a CHF increase. Stutz et al. [11] and Jo et al. [12] also used nanoparticle-deposited surfaces in pool boiling and reported that the enhanced wettability, due to nanoparticle deposition, increased CHF. Since noting the importance of ‘good’ wettability of nanoparticle-deposited surfaces in increasing CHF, other types of surface modification techniques have been suggested. Forrest et al. [4] also identified CHF increases on a hydrophilic surface by the LbL method in pool boiling. Kim et al. [13] fabricated nanoand microstructures using a microelectromechanical system (MEMS) technique and conducted pool boiling on these structured surfaces. Microstructures showed the highest HTC, and/or nano/ micro-combined structures showed the highest CHF (107% enhancement). The increase in CHF was facilitated by enhanced wettability due to the nanostructures [13]. Ahn et al. [14] made micro/nano-multiscale structures on zircaloy-4 surfaces using anodic oxidation; the contact angle on the surface was <10° due to increased wettability. From pool boiling experiments, they also reported increased CHF and attributed that to a spreading effect of the liquid. From experimental evidences [4–6,15,16], a hydrophobic surface can trigger nucleate boiling at low wall superheat, but it also brings premature CHF due to bubble coalescence and sticking to the surface. In contrast, a hydrophilic surface can promote liquid supply to the dry area to delay CHF. In this respect, heterogeneous wetting control techniques have been developed. Betz et al. [17,18]
TCS Vh Vr
TiO2-coated surface voltage of the heating element, V voltage of the shunt resistance, X
Greek symbols b receding contact angle, ° / orientation of heating surface, ° ll viscosity, lPas h static contact angle ql liquid density, kg/m3 qv vapor density, kg/m3 r surface tension, N/m Subscripts l liquid v vapor phase change from liquid to vapor lv w wall of heated surface b bulk sat saturated condition h heating element CHF CHF
developed heterogeneous wetting surfaces that had hydrophobic islands on a hydrophilic network or hydrophilic islands on a hydrophobic network. Their investigation of pool boiling performance confirmed that hydrophobic islands with hydrophilic networks showed the best boiling performance and enhancement of the HTC and CHF by 100% and 65%, respectively. Jo et al. [15,19] also prepared heterogeneous wetting surfaces and examined their boiling performance; they showed that the hydrophobic surface had a higher HTC with sustaining CHF. Using a surface with hydrophobic dots on a hydrophilic substrate, they enhanced HTC without degradation of CHF. Although wettability can affect boiling performance, there is a trade-off between HTC and CHF. In this respect, Bertossi et al. [20] used switchable polymers coating for the improvement of boiling heat transfer. The polymer coating showed wetting transition of hydrophilic to hydrophobic when a temperature is above 108 °C. Since hydrophobic surface promotes initiation of bubble nucleation while hydrophilic surface enhanced bubble detachment, the polymer coating increased boiling heat transfer in the nucleate boiling regime. In the present study, TiO2 thin film was used to enhance both HTC and CHF. Sun et al. [21] fabricated thin films of TiO2 on glass substrates to investigate wettability changes after heat treatment at various temperatures in air. The initial water contact angle on TiO2 was 54°. The contact angle decreased as the heat treatment temperature increased. When TiO2 was annealed at 200 °C, the contact angle was 20°. In addition, the contact angle decreased to <10° with increasing heat treatment temperature, up to about 250 °C. They reported that oxygen vacancies in the crystal structure of TiO2 primarily improved wettability. Such oxygen vacancies can be generated in the crystal structure of TiO2 when it is treated by ultraviolet (UV) irradiation, heat treatment, and Ar+ sputtering. They explained that oxygen vacancies were kinetically favorable for adsorption of hydroxyl, and water molecules were adsorbed dissociatively on the oxygen vacancies. The special characteristics of TiO2 have been reported in numerous studies [22–29].
J.M. Kim et al. / International Journal of Heat and Mass Transfer 109 (2017) 231–241
We manufactured a TiO2-coated surface (TCS) that had a wetting transition temperature; it showed hydrophobic characteristics before the transition temperature, while it became hydrophilic above the wetting transition temperature. Thus, under lowpressure conditions, the TCS had a higher HTC and lower CHF than hydrophilic surface because it did not reach the transition temperature. However, it had a higher HTC in the low-wall-superheat region due to its hydrophobicity and also enhanced the CHF because it reached wetting transition temperature and became a hydrophilic surface. The thermally-induced wetting change of TCS is unique characteristics for enhancing both HTC and CHF with no additional treatment; as such, we refer to TCS as a ‘smart’ surface. 2. Experimental 2.1. Surface fabrication A silicon wafer with the thickness of 500 lm was used for a heating substrate. A roughness effect was neglected in the present study because the silicon substrate has a nano-meter scale roughness. To use the Joule heating method, platinum patterned layer of 120 nm was deposited on the backside using MEMS technique after SiO2 (SCS) was deposited using thermal growth method for electrical insulation as depicted in Fig. 1. On the boiling side, SiO2 or TiO2 layer was deposited: SiO2 layer of 500 nm was deposited as a reference surface and TiO2 of a 200 nm layer was deposited using RF sputtering. 2.2. Pool for a regulation of saturated temperature Since the TCS had a wetting transition temperature around 200 °C, a pool boiling facility was constructed to achieve the wetting transition temperature near CHF conditions by increasing system pressure from 1.0 to 4.1 bar. The whole body was made of stainless steel pipe depicted (Fig. 2). The inside diameter, height and wall thickness of the pool were 200 mm, 500 mm and 8.2 mm, respectively. There were some penetrations for the measurement of the bulk water temperature and water level. In the pool, 12 L of deionized (DI) water was used as the working fluid. An immersion heater was used to make saturated temperature of the bulk water for each pressure condition. A test section assembly was positioned at the bottom of the pool. This assembly consisted of the heating surface and a polyether ether ketone (PEEK) jig, which is an insulating material. There were two lids for the pool. A degassing lid was connected to the reflux condenser for the degassing process (not shown in Fig. 2). For pressurization, another lid equipped with three relief
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valves (VRV1MH, HY-LOK) was used; the relief valves were adjusted to be opened at 2.0, 3.3 and 4.1 bar, respectively. The relief valves discharged excessive steam during boiling, so the pressure in the pool was not beyond the set value of the relief valve. Under saturated conditions, the pressure in the pool was estimated using a correlation [30]. The experiment was initiated once the pressure remained at the set pressure for more than 10 min. 2.3. Experimental facilities The experimental system is shown in Fig. 3. From the bottom of the pool, wires soldered to the electrodes of the heating element were connected to the main circuit and data acquisition system (34970a, Agilent Technologies). To calculate a current in the circuit during boiling, the circuit was connected to a shunt resistor in the thermostat. Because the thermostat maintained a constant temperature of 10 °C, the resistance of shunt resistor was also kept at a constant value. The current in the circuit was calculated from the voltage drop at the shunt resistor. Then, the heat flux was estimated based on the current value using Eq. (1), where Vh is the voltage drop at the heating element (V), Ah is the effective heating area (m2), I is the current (A), Vr is the voltage drop at the shunt resistance (V), Rs is the resistance of the shunt resistor (X). Experiments were conducted at 1.0, 2.0, 3.3, and 4.1 bar to identify boiling characteristics of TCS and its wetting transition. The experimental range and uncertainty, by Holman [31], are provided in Table 1, where Psat is a saturated pressure, Tsat is a saturated temperature of bulk water, Th is a temperature of heating element, and q00 is a heat flux.
q00 ¼
V hI V h V r ¼ Ah Ah Rs
ð1Þ
2.4. Experimental procedures To estimate the heating element temperature during boiling, the resistance of the heating element was calibrated in the convection oven (OF-02GW, JEIO TECH) before experiments. The resistance of the heating element increased linearly with the temperature. During boiling, the temperature of the heating element was calculated using the resistance-temperature correlation. For the degassing, DI water was boiled for 3 h using an immersion heater with the degassing lid connecting to a reflux condenser, so that the water level was maintained during the degassing step. After degassing was complete, the degassing lid was replaced with a pressurized lid with the assembled relief valves and fastened to the body of the pool. DI water was boiled continuously via the
Fig. 1. Schematic images of the surface fabrication.
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Fig. 2. Schematic images of the pressurized pool.
Fig. 3. Experimental system.
Table 1 Experimental range and uncertainty. Test #
Experimental parameters Psat
P1 P2 P3 P4
Tsat
q00max
Th,max
Value (bar)
Error (%)
Value (°C)
Error (%)
Value (°C)
Error (%)
Value (kW/m2)
Error (%)
1.0 2.0 3.3 4.1
3.58 3.17 2.88 2.76
99.6 120.2 136.8 144.9
1.0 0.8 0.7 0.7
153.1 167.4 192.3 203.4
0.2 0.2 0.2 0.2
918 1246 1629 1912
0.59 0.58 0.58 0.58
immersion heater, and the temperature and pressure were increased to specific values. Because a relief valve opened to release excess steam at the set pressure, steady state was achieved under saturated conditions. When the pressure and temperature were maintained a constant value for more than 10 min, the experiment was initiated and the heat flux was increased step-by-step slowly. At specific levels of the heat flux, heat flux was maintained and recorded for
2 min to estimate averaged data under steady-state conditions. When the wall temperature increased suddenly, by >18 °C in 1 s, the power supply was turned off, and this point was deemed CHF. 2.5. Data reduction Because the heating element was exposed to air during the experiment, the heat loss to air was estimated. The heating ele-
J.M. Kim et al. / International Journal of Heat and Mass Transfer 109 (2017) 231–241
ment was downward facing; heat was transferred from the heating element to air by conduction [32]. For all of the experimental conditions, the maximum heat loss to air was 0.43%. Thus, the heat loss to air was ignored. Additionally, we measured the temperature of the heating element to estimate surface temperature during the experiment. Using the thermal conductivity of 130 W/mK for silicon, the surface temperature was evaluated at each heat flux condition using the Fourier’s law. 2.6. Validation of the facility To validate the experimental facility, we compared the experimental CHF with Eq. (2) suggested by Zuber [33] and Eq. (3) suggested by Haramura and Katto [34], where qv is the density of vapor, ql is the density of liquid, hlv is the latent heat, r is the surface tension, and g is the gravity acceleration; dc was taken to be 0.056 mm that was used in the work [34]. For the calculation, experimental properties were used. The results (Fig. 4) showed that the correlations overestimated the experimental CHF of SCS. Zuber’s correlation [33] showed differences in 20% at 1.0 bar and 11% at 4.1 bar. The correlation of Haramura and Katto [34] also showed differences in 3% at 1.0 bar and 19% at 4.1 bar. Although the correlations overestimated the experimental data, they captured the trend of increasing CHF with the saturated pressure. This result means that the pool boiling facility showed a general trend of pool boiling.
q00CHF ¼
q00CHF
p
q1=2 hlv ½rgðql qv Þ1=4 24 v
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rq qv 0:4 q ¼ hlv 0:00536 v 1þ v dc ql ql
ð2Þ
ð3Þ
3. Results and discussion To examine an effect of the thermally-induced wetting conversion of TCS, pool boiling experiments were conducted with increasing saturated temperature. After TiO2 thin film was successfully deposited on the heating substrate, the contact angle changes of TCS and SCS were investigated using heat treatment test. Furthermore, HTC of TCS was increased for all experimental conditions, and CHF of TCS was enhanced at 4.1 bar. With analysis of wetting conversion and boiling characteristics for TCS, we postulated that the enhancement was attributed to the wetting transition.
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3.1. Contact angle & receding contact angle The thermally-induced conversion in wettability with temperature was key to enhancing boiling performance with TCS. Thus, to confirm the wettability change in TCS with temperature, heat treatments were conducted under various temperature conditions using a furnace. The temperature was controlled from 100 to 200 °C, which corresponded to the wall temperature for the boiling experiment. SCS and TCS were annealed under each prescribed temperature condition in atmosphere for more than 10 h to guarantee a sufficient wettability change and were not reused for the heat treatment at different temperatures. After the heat treatment, the surfaces were cooled to room temperature (25 °C), and the contact angles were measured. Fig. 5 shows static and receding contact angles on SCS and TCS after heat treatment. The SCS surfaces show a significant change in contact angles with heat treatment. However, static and receding contact angles on TCS decreased with increasing heat treatment temperature. After the surfaces were annealed at 100 °C, the static contact angles were 68.4° and 83.1° on SCS and TCS, respectively. When the surfaces were annealed at 200 °C, the static contact angle of TCS decreased to 32.7°, unlike SCS, which showed almost the same contact angles. The receding contact angles of TCS also changed from 53.0 to 14.3° at 100 °C and 200 °C, respectively. SCS didn’t show the significant change of the receding contact angle. 3.2. Changes in the crystal structure after heat treatment The decrease in the wettability of TiO2 with temperature is attributed to a change in the crystal structure of TiO2. Park et al. [28] investigated the crystal phase of TiO2 with heat treatment at various temperatures. When TiO2 was annealed at 500 °C, the intensity of peaks for the anatase, rutile, and brookite phases were enhanced in X-ray diffraction (XRD) analyses. This indicates that there was a change in the crystal structure. Liu et al. [35] also studied wettability of TiO2 with temperature variation. From XRD patterns, the TiO2 film had showed an amorphous structure even when it was annealed at 300 °C. The heat treatment at a temperature of 400 °C resulted in an anatase structure; the intensity of the peak was enhanced at a heat treatment temperature of 650 °C. They found that the contact angle decreased as the anatase structure appeared and concluded that anatase TiO2 showed the highest hydrophilicity in their study. We investigated crystal structure of TiO2 before and after heat treatment was conducted at 200 °C. In Fig. 6(a), SiO2 showed an amorphous structures, regardless of heat treatment at 200 °C. TiO2 showed an amorphous structure in the as-deposited condition; when annealed at 200 °C, three peaks appeared (Fig. 6(b)). These peaks, (101), (004), and (200), corresponds to the anatase phase structure (JCPDS No. 21-1272). Thus, these results confirm a change in the crystal structure of TiO2 with temperature, resulting in enhancement in the wettability of TiO2. 3.3. Heat transfer coefficient
Fig. 4. A comparison of experimental data with correlations of Zuber [33] and Haramura and Katto [34].
Fig. 7 shows HTC curves as a function of the wall superheat at pressures of 1.0, 2.0, 3.3, and 4.1 bars. From the wettability characteristics (Fig. 5), it was expected that TCS had a higher HTC than SCS because TCS was hydrophobic at the low-wall-temperature regime. Experimental data clearly showed higher HTCs on TCS than SCS, regardless of the pressure. Specially, the TCS showed an abrupt degradation of heat transfer for the TCS at approximately 45 K. In that point, HTC deteriorated because the boiling regime changed from nucleate boiling to film boiling due to CHF. At 3.3 bar, HTC of TCS was higher than of SCS due to the hydrophobic-
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Fig. 5. Measured (a) static contact angles and (b) receding contact angles after heat treatment.
Fig. 6. X-ray diffraction analysis of (a) SiO2 and (b) TiO2.
ity of TCS. Likewise, the HTC of TCS improved at 4.1 bar due to its hydrophobicity, and CHF (CHF is discussed in the Section 3.5) was enhanced in comparison with SCS in high-wall-superheat regime due to the transition in wettability to a hydrophilic surface. For the analysis of HTC with wettability, we used the correlation in Eq. (4) suggested by Wang et al. [36]. The correlation is a function of properties, wall superheat, contact angle, and empirical constants in Eqs. (5) and (6), where Tw is the wall temperature (°C), Tb is the bulk temperature, h is the contact angle, ll is the viscosity of bulk water, hlv is the latent heat, r is the surface tension, g is the gravity acceleration, ql is the density of bulk water, and qv is the density of vapor. Eq. (4) also includes empirical constants of C and m; thus, the correlation can be rewritten in the form of Eq. (7) with a constant of Csimple for simplicity. In the present analysis, Csimple and m were fitted to the experimental data of SCS with the measured contact angle (Fig. 5) at each pressure condition. The correlation reflected the wettability effect; thus, the heat flux was estimated with increasing wall superheat using specific contact angles.
q00 ¼ B½FðT w T b Þmþ1 Cð1 cos hÞll hlv " F
mþ1
¼
1=2
17=8 19=8 m23=8 m15=8 cpl hlv v 9=8 m11=8 m15=8 Þ T l v b
kl ql
ll ð q
q
1=2 B ¼ A2=3 1 A2
q
2 p1=2 g 9=8
r
r
1=2
gðql qv Þ
ð4Þ
# ð5Þ
ð6Þ
q00 ¼ C simple ½FðT w T b Þmþ1 ð1 cos hÞll hlv
r
1=2
gðql qv Þ
ð7Þ
In Fig. 8, the experimental data are plotted using black and blue points for SCS and TCS, respectively. Dashed lines were calculated using Eq. (7) with changing contact angles. When the contact angle of 67° was used for SCS, the experimental data for SCS did not interfere with the other dashed lines, which were calculated with different contact angles. In contrast, the boiling curve of TCS crossed other dashed lines. This suggests that the boiling curve of TCS can be explained with multiple values of contact angle. As in Fig. 5, the contact angle for TCS decreased as the wall temperature increased. 3.4. Roughness effect For the validation of a negligible effect of a roughness on boiling, atomic force microscopy (AFM) was used to characterize the roughness of the SCS and TCS surfaces (Fig. 9). Before experiments were conducted, the SCS and TCS had roughness of 1 nm. After the experiments were conducted up to a heat flux near CHF at 4.1 bar, the roughnesses were 1.5 and 4.3 nm, for SCS and TCS, respectively. If the roughness of the present surfaces acted as nucleation cavities during boiling, then the boiling characteristics would be attributed to not only the wettability but also a roughness effect. To estimate the theoretical cavity size that can act as a nucleation seed, the range of active nucleation cavities was calculated using the theory by Hsu et al. [37]; they suggested that the cavity size range depended on the wall temperature (°C), bulk tem-
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Fig. 7. Heat transfer coefficient curves at (a) 1.0, (b) 2.0, (c) 3.3, and (d) 4.1 bars.
Fig. 8. Boiling curves with the correlation of Wang et al. [36] at (a) 1.0, (b) 2.0, (c) 3.3, and (d) 4.1 bars, respectively.
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Fig. 9. Measured roughness of (a) before, (b) after experiment, and (c) comparison with a theoretical minimum cavity size [37].
perature (°C), latent heat (kJ/kg), density of vapor (kg/m3), surface tension (N/m), and the thermal boundary layer (m). The thermal boundary layer was assumed to be 0.2 mm at atmospheric pressure [16]. Using the experimental properties, the calculated range of nucleation cavities ranged from 1.03 to 100 lm at 1 bar. With the same boundary layer thickness, the range of nucleation cavities was from 0.25 to 100 lm at 4.1 bar. Thus, the minimum cavity size was 250 times larger than the roughness of the surfaces studied. As such, there was no contribution of the roughness to the nucleation cavity because the roughness in the present study was much smaller than the theoretical value. Kang [38] also showed the effects of nanometer-scale roughness on HTC during pool boiling. From their experimental results, roughnesses of 60.9 and 15.1 nm had no significant effect on boiling characteristics on a horizontal tube. Thus, it is reasonable that the roughness effect on HTC was also negligible in the present study by cavity theory [37] and an experimental report on nanometer roughness effects [38].
3.5. Critical heat flux Kandlikar suggested a theoretical prediction model of CHF in the pool boiling [39]. The model in Eq. (8) includes not only properties of working fluid but also a receding contact angle as a parameter. Fig. 10 shows a comparison of experimental CHF values with Kandlikar’s predictions. At each pressure condition, a correction factor in Fig. 10 was applied to the model for fitting of SCS experimental data. The dashed lines are CHFs based on calculations using the prediction model for various receding contact angles; the experimental CHFs are shown as solid points for pressure conditions of 1.0, 2.0, 3.3 and 4.1 bars. The blue solid line is an expectation line of CHF based on the receding contact angle by the heat treatment test in Fig. 5. At the pressures below 4.1 bar, there was no enhancement in CHF with TCS because it did not experience a sufficient change in wettability. However, CHF of TCS surpassed that of SCS due to the transition in the wettability of TCS at 4.1 bar. At high wall temperature regime under this pressure, the
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Fig. 10. Critical heat flux comparison with the correlation of Kandlikar [39] and C is a correction factor.
surface temperature was so high that TCS became hydrophilic and enhanced CHF.
q00CHF ¼ hlv q1=2 v ð
1 þ cos b 2 p Þ½ þ ð1 þ cos bÞ cos / 16 p 4
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the crystal structure of TiO2. From previous reports [40–42], the decrease in the contact angle of TiO2 was affected not only by the intensity of UV irradiation but also by its duration. When the UV irradiation exposure time was not sufficiently long to change the crystal structure of TiO2, the contact angle did not decrease completely. In the present study, it was possible that 2 min of maintained heat flux for each steady-state condition was short to change the crystal structure of TCS, although the type of treatment differed from that in the literatures. Thus, additional tests were conducted to identify the CHF with a complete wettability change. For this test, the heat flux was increased using the same procedure described in Section 2.4. When the heat flux reached 80–90% of the CHF at each pressure, the heat flux was sustained for about 20 min to guarantee complete wetting change in TCS. After 20 min, the experiment was resumed until CHF occurred. As a result, there were additional enhancements in CHF with TCS in comparison with the CHF in the ‘normal’ test. These results are plotted as crossed hollow points in Fig. 10. There were also changes in CHF for SCS, but the CHFs for SCS deteriorated at 3.3 and 4.1 bar. From the additional tests, we suggest that CHF could be enhanced further given enough time, even though the CHF of the TCS did not perfectly match the predictions of Kandlikar’s model. 3.6. Boiling curves and contact angles
1=2
ð8Þ
From Fig. 10, it was confirmed that the CHF of the TCS was enhanced. Nevertheless, the enhancement of CHF was lower than the predictions (the blue solid line). In previous works [21,28,29,40–42], TCS became hydrophilic with a change in crystal structure when various types of energy, such as UV irradiation, heat treatment, and bombardment with Ar+, were applied. Additionally, the treatment time is also an important factor in changing
Overall boiling characteristics of TCS can be explained by the variation of contact angles from the heat treatment. Fig. 11 shows boiling curves of the surfaces with their contact angles from the heat treatment versus wall temperature. The curves under pressure of 1.0 bar was clearly understood with wettability. In the nucleate boiling regime, TCS was hydrophobic; thus HTC of TCS was higher than SCS. However, sticking of bubbles to TCS caused lower CHF than SCS due to its hydrophobicity. The curves under pressure of 2.0 bar showed similar boiling characteristics of
Fig. 11. Boiling curves and contact angles with wall temperature under pressures of (a) 1.0, (b) 2.0, (c) 3.3, and (d) 4.1 bars, respectively. Solid points are the experimental data from the normal operation, and hollow points are CHFs from the time effect operation.
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1.0 bar. In specific, HTC of TCS deteriorated due to the transition to vapor film regime. The boiling curves for 3.3 bar also showed similar characteristics; HTC of TCS was higher than SCS because nucleate boiling was initiated in hydrophobic regime of TCS. Although the wall temperature near CHF was in hydrophilic regime of TCS, the change of wettability was not sufficient to enhance CHF because of the short period of steady state time. Under the pressure of 4.1 bar, nucleate boiling was initiated in the hydrophobic regime of TCS, so HTC for TCS was improved in comparison to SCS. The wall temperature was high enough to achieve a sufficient wetting transition of TCS near CHF point, and CHF for TCS was enhanced in comparison to SCS due to hydrophilicity of TCS. Furthermore, maintaining the heat flux of 80–90% of CHF at the pressure of 3.3 and 4.1 bars caused additional enhancement of CHF for TCS by the complete wetting transition. Consequently, there was the enhancement of both HTC and CHF for TCS by its wetting transition.
[5] [6]
[7]
[8]
[9] [10]
[11]
[12]
4. Conclusions In the present study, the pool boiling HTC and CHF of TiO2coated surface (TCS) that has unique characteristics of the wetting transition with temperature were examined in a pool boiling. A TiO2 thin film of 200 nm thickness was deposited on a silicon substrate using RF sputtering, and SiO2 film of 500 nm thickness was coated on the substrate as a reference surface (SCS) using a thermal growth technique. Contact angle variation after a heat treatment was investigated for both surfaces with changing the treatment temperature in an oven. Pool boiling experiments with the fabricated test surfaces were conducted at 1.0, 2.0, 3.3, and 4.1 bar. The boiling HTCs of TCS samples were higher than those of SCS, regardless of pressure conditions. The CHF of TCS was higher than SCS at 4.1 bar, while no enhancement was observed at 1.0, 2.0, and 3.3 bar. Based on the observed temperature dependence of the contact angle, we hypothesize that TCS is hydrophobic at the initial stage of boiling and becomes hydrophilic with increasing wall temperature. Since the roughnesses of the surfaces were extremely low (below 5 nm); the roughness effect should be negligible in the present study. Therefore, wettability change depending on temperature is probably the key parameter affecting the boiling characteristics of TCS. When a heat flux of 80–90% of the CHF was maintained for 20 min, additional enhancement of CHF was observed on TCS, which can be attributed to a complete wetting transition. Thus, the boiling characteristics of the surfaces are well explained by the characteristics of the wettability change observed in the heat treatment test. We suggest TCS as a ‘smart’ surface that enhances both HTC and CHF due to the wetting transition, which is completely passive and does not need additional external control.
[13]
[14]
[15]
[16] [17] [18]
[19]
[20] [21]
[22]
[23]
[24]
Acknowledgement
[25]
This work was supported by National Research Foundation of Korea (NRF) grants funded by the Korean Government (MSIP) (2015M2A8A2074795). The fabrication of the test sections was supported by Ulsan National Institute of Science and Technology (UNIST), and National Nanofab Center (NNFC).
[26]
[27]
[28]
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