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Orientation effects of nanoparticle-modified surfaces with interlaced wettability on condensation heat transfer
Applied Thermal Engineering 98 (2016) 1054–1060
Contents lists available at ScienceDirect
Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Orientation effects of nanoparticle-modified surfaces with interlaced wettability on condensation heat transfer You-An Lee a, Long-Sheng Kuo a, Tsung-Wen Su a, Chin-Chi Hsu b, Ping-Hei Chen a,* a b
Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan Department of Mechanical Engineering, National United University, Miaoli 36063, Taiwan
H I G H L I G H T S
• • •
The surface with interlaced-wettability can enhance the condensation heat transfer. Wider superhydrophobic strips increase the heat transfer for horizontal surfaces. Narrower superhydrophobic strips increase the heat transfer for vertical surfaces.
A R T I C L E
I N F O
Article history: Received 24 September 2015 Accepted 3 January 2016 Available online 11 January 2016 Keywords: Condensation heat transfer Interlaced wettability Nanoparticle-modified surface Surface orientation
A B S T R A C T
This study investigated the effects of surfaces with superhydrophobicity-based interlaced wettability on condensation heat transfer. Experiments were conducted on various types of surface with different modified strip widths under horizontal and vertical surface orientations. The experimental results revealed that the condensation heat-transfer on surfaces with interlaced wettability could be highly influenced by the surface pattern, surface orientation, and wall subcooling. Opposite trends of heat transfer were observed under different surface orientation. The experimental data of horizontal surfaces showed that the heat transfer can be enhanced when the spacing between the unmodified strips is getting wider, while the narrower spacing would increase the heat transfer more efficiently for vertical surfaces. Such the facts imply that the interlaced surface holds the potential of heat transfer enhancement, especially in the situation without the sweeping of condensates under the gravity force. © 2016 Elsevier Ltd. All rights reserved.
1. Background In the past decade, researchers have put a lot effort into the fabrication of superhydrophobic surface to achieve dropwise condensation [1–3] since dropwise condensation could yield a heat transfer coefficient one order higher in magnitude compared with filmwise condensation [4–7]. However, researchers found out that they cannot maintain the dropwise condensation and the tiny size of the condensates on hierarchically structured superhydrophobic surface when the heat flux and the supersaturation of the process were increased to a certain level, leading to the flooding of the surface and the degradation of the heat transfer performance [7–9]. To solve the problem, researchers conducted the experiments on surfaces with micro-scale divisions with different wettability under ESEM, trying to control the distribution of the nucleation site on the surfaces [10,11]. Based on nucleation theory, the energy barrier for nucleation is lower on a hydrophilic surface [12,13], making it possible to restrict the nucleation only on the hydro-
* Corresponding author. Tel.: +886 233662689; fax: +886 223631755. E-mail address: [email protected] (P.-H. Chen). http://dx.doi.org/10.1016/j.applthermaleng.2016.01.003 1359-4311/© 2016 Elsevier Ltd. All rights reserved.
philic regions of the surface. Their experimental results show that both the size and the distribution of the condensate can be controlled well the type of heterogeneous surfaces they fabricated. The condensation phenomena on the back of the Namib desert beetles [14] inspired the research of heterogeneous wettability. Surfaces with heterogeneous wettability take the advantages of both hydrophilicity and hydrophobicity. Thus, connecting and striking a balance for each phase of condensation is the key to enhancing condensation further [15]. In addition to nucleation control, different implementations for such surfaces are used. Chatterjee et al. used surface with matrix-patterned wettability, finding out that the heat transfer coefficient would increase with the reduction of the diameter of the hydrophilic circles [16,17]. Peng et al.’s team utilized hybrid surface consisted of hydrophobic and hydrophilic region to conduct the research numerically [18] and experimentally [19]. The results indicate that the heat transfer coefficient increases with the decreasing width of the hydrophilic strips while an optimum of the width of the hydrophobic region exists for vertical condensing surface. They attributed the heat transfer enhancement to the hybrid mode of filmwise and dropwise condensation. Our previous research [20] focused on the improvement of the superhydrophobic surface, attempting to combine the
Y.-A. Lee et al./Applied Thermal Engineering 98 (2016) 1054–1060
advantages of both superhydrophobic and hybrid surfaces. We concluded that the superhydrophobic/hydrophobic interlaced surface is also capable of raising heat transfer coefficient for horizontal surfaces, especially for those with wider superhydrophobic regions. In addition to the surface morphology, surface orientation is another key factor affecting condensation heat transfer performance for various types of surface. Generally speaking, the removal of droplets relies on the gravity force. Koch et al. pointed out that the inclination of the condensing surface would decrease the gravity effect, and thus, the heat transfer coefficient [21]; nonetheless, researches attempted to utilize the advantage of the recently developed patterned surfaces to enhance or even replace the function of gravity force. Bonner found out that the ratio of heat transfer enhancement on gradient surface is higher when the gravity force is reduced by inclining condensing surface [22]. Boreyko et al. applied the hierarchically structured surface inducing jumping condensates to heat transfer devices, confirming that the heat transfer enhancement by surface induced jumping condensates could function well against gravity force [23,24]. Macner et al. concluded that the surface with gradient wettability could achieve stable dropwise condensation on surface horizontally facing upward [25]. We studied the condensation heat transfer enhancement on the surface horizontally facing downward previously [20], which is uncommon among the recent researches of surface orientation. We attempt to further examine the effect of surface orientation for enhanced condensation on surfaces with superhydrophobicitybased interlaced wettability in this study. We fabricated interlaced surface with different modified strip widths under different surface orientation, aiming at finding out the principle of designing interlaced surface for condensation heat transfer enhancement.
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(a)
g (b)
2. Methods Figure 1 illustrates the experimental setup. A copper cylinder 35 mm in diameter and 30 mm in height was used as a heat transfer medium. One end of the cylinder functioned as the condensing surface. Four thermocouples were inserted into the copper cylinder at two different axial locations to measure the temperature and obtain average temperature gradient, followed by a calculation of surface temperature by employing extrapolation as well as heat flux by using Fourier’s law. A pressure transducer was inserted to determine the concentration of the non-condensable gas, which was approximately 1.2% calculated using W = (P − Patm)/Patm in the configuration of the system. The other details for the experimental setup can be found in our previous research [20]. The instrument error for the thermocouples, U T , was ±0.4 K , and for the location of temperature measurement, U x , was ±0.5 mm. The uncertainties of the heat flux, wall subcooling, and heat transfer coefficient within 10K wall subcooling are determined by the following method according to Taylor [26]: 2 2 Uq ⎛U ⎞ ⎛U ⎞ = ⎜ T ⎟ + ⎜ x ⎟ = 5 − 10% ⎝ ⎠ ⎝ ⎠ ΔT Δx q
2
2 U ΔTsub ⎛ U ⎞ ⎛ Uq ⎞ = ⎜ T ⎟ + ⎜ ⎟ = 2 − 10% ⎝ ΔT ⎠ ⎝ q ⎠ ΔTsub
Fig. 1. Experimental setup for (a) horizontal surface, and (b) vertical surface.
solution is presented in Hsu and Chen’s paper [27]. We heated the copper surfaces for 1.5 h at 120 °C after spinning. The thickness of the coated layer is less than 7 μm. The second step was the preparation of hydrophobic materials by fluoro-containing mixtures. Trichloro (1H, 1H, 2H, 2Hperfluorooctyl) silane was mixed with methyl alcohol (1/100 in v/v). After spin coating the surface with the solution, the surfaces were heated at 90 °C for 40 min. The interlaced wettability on the surface was produced by masking the specific strips before coating. The masks were removed afterward to obtain plain copper strips, which were fixed at a width of 0.5 mm in this study. Contact angle measurements were conducted using the Sindatek Model 100SB. The contact angles (CAs) of the surfaces in our experiments are listed in Table 1, and the illustrations of the interlaced surfaces are shown in Fig. 2. 3. Results and discussion
2
2
Uh ⎛ Uq ⎞ ⎛ U ⎞ = ⎜ ⎟ + ⎜ ΔTsub ⎟ = 5 − 15% ⎝ q ⎠ ⎝ ΔTsub ⎠ h
3.1. Experiments on surfaces with homogeneous wettability
The sol-gel method was used to spin-coat 40 nm silica nanoparticles on a polished copper surface to fabricate superhydrophobic and interlaced surfaces. Table 1 lists the surface types and the corresponding parameters. The detailed procedure of preparing the
Figure 3 presents the heat transfer coefficients varying with wall subcooling for both horizontal and vertical surfaces with homogeneous wettability, indicating that vertically oriented surfaces showed a superior heat transfer coefficient compared with their corre-
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Table 1 List of the surface types. Surface type
Modified strip width
Plain strip width
Modified area ratio
Modified region CA
Plain region CA
Plain Super hydrophobic 1.5/0.5 2.5/0.5 4.5/0.5 5.5/0.5
N/A Whole surface 1.5 mm 2.5 mm 4.5 mm 5.5 mm
Whole surface N/A 0.5 mm 0.5 mm 0.5 mm 0.5 mm
0 1 0.750 0.835 0.896 0.920
154.9° ± 3.1°
Heated: 104.7° ± 3.8°
sponding cases on horizontal surfaces. The sweeping down of the condensates under gravity force makes it easier for the smaller droplets to shed, leading to a more efficient droplet removal process on vertical surfaces. In addition, although the superhydrophobic surface made by sol-gel coated nano-particle could not exhibit the jumping droplets phenomena mentioned earlier, we found out that the superhydrophobic surface could enhance the heat transfer performance of horizontally oriented surface to surpass the unmodified vertical surface, which can be indicated by the black circle and white rectangle in Fig. 3. The experiments on the homogeneous plain and modified surfaces determined the data of h0 and hm respectively for the following analysis. 3.2. Droplets morphology Once the condensation process reached the steady state, there are two mechanisms for the condensates to leave the interlaced surface: one is the direct removal of tiny droplets from the modified superhydrophobic regions, and the other can be attributed to the lateral movements of small droplets near the interfaces from modified strips to plain strips. Such movements of small droplets across the interfaces are driven by the difference of surface tension between neighboring regions with different wettability. For horizontally oriented surfaces, the droplets near the interfaces coalesce with larger droplets on the plain strips, suspending and continu-
Fig. 2. Dimensions of the interlaced surfaces.
ally growing until overcoming the surface tension force. On the other hand, for the vertical condensing surface, condensates aggregating to the interfaces would coalesce with the droplets sweeping down the nearby plain strips, preventing the surfaces from being blocked by the large suspended droplets. The difference of droplets morphology can be seen in Fig. 4. 3.3. Effect of surface pattern and surface orientation Fig. 5 presents the heat transfer coefficient as functions of the ratio of the modified areas on a surface when wall subcooling was 5K. The ratio increased as the width of the modified strips increased and attained a value of 1 on the superhydrophobic surface. The overall heat transfer coefficient of an interlaced surface can be expressed as:
A ⎞ A ⎛ h = h0 ⎜ 1 − m ⎟ + hm m + Δhinterlace ⎝ Atotal ⎠ Atotal where A m is the sum of the area of modified regions, A total is the m is the modified area ratio, which area of the condensing surface, AAtotal can be seen in Table 1. h is the overall heat transfer coefficient, h0 is the heat transfer coefficient of the plain copper surface, hm is the heat transfer coefficient of the homogeneous superhydrophobic
Fig. 3. Variation of heat transfer coefficient with wall subcooling for plain and superhydrophobic surfaces.
Y.-A. Lee et al./Applied Thermal Engineering 98 (2016) 1054–1060
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Fig. 4. CCD images of the condensation processes on different types of surface. The suspended droplets can be seen on the horizontally oriented interlaced surfaces, and the surface wider spacing possessed less suspended droplets.
surface, and Δhinterlace is the source of heat transfer enhancement, a variable additionally contributing to the heat transfer depending on the surface pattern and the wall subcooling. h0 and hm could be decided by the experiments on the homogeneous surfaces. The two lines in Fig. 5 present the relation only considering the modified area ratio, functioning as standard value for examining the effect of interlaced wettability on condensing surfaces. Δhinterlace then can be determined by the difference between the measured heat transfer coefficient, h and the corresponding value on the line. As in Fig. 5, we can observe opposite trends of heat-transfer enhancement under different surface orientation. The experimental data of the horizontal surfaces showed that the heat transfer could be enhanced more when the width of the modified strips getting wider. The results indicate that the spacing between the unmodified strips should be wide enough to avoid the reduction of heat transfer due to the surface blockage. Moreover, the suspended
Fig. 5. Heat transfer enhancement ratio as a function of the modified area ratio for the various types of modified surface when wall subcooling = 5 K. The five increasing modified area ratios correspond to the 1.5/0.5, 2.5/0.5, 4.5/0.5, 5.5/0.5 interlaced surfaces, and the fully modified surface respectively.
droplets on the unmodified strips would induce the lateral movements of the droplets, accelerating the droplet removal process on both the superhydrophobic and the unmodified regions. Such effect of the suspended droplets could enhance heat transfer most effectively when the spacing between the 0.5 mm unmodified strips are around 5.5 mm or slightly wider. Therefore, the efficient droplet removal mechanism makes the interlaced surface reach higher heat transfer coefficient than the superhydrophobic one, which can also be seen in Fig. 5. In contrast, the negative value of Δhinterlace for vertical surfaces with wider spacing between the unmodified strips might result from the formation of the thicker liquid film on the unmodified strips. As in Fig. 4, the homogeneous unmodified surface demonstrates dropwise condensation; however, the lateral movements on the interlaced surfaces would lead to the transition of condensation mode to filmwise condensation. From Fig. 5 we found out that this problem could be addressed by decreasing the width of the modified strips. The narrower the modified strips on vertical surfaces means the more amount of the unmodified strips and the interfaces, leading to the increase of sweeping down of droplets on the unmodified strips and the accompanied coalescence with the droplets on the neighboring interfaces. The sweeping effect on the vertical condensing surfaces would compensate for the reduction of performance due to transition of condensation mode on the unmodified strips, dominating the heat transfer enhancement under the gravity force. Figure 5 also indicates that the heat transfer enhancement by the interlaced wettability seems more effective on horizontally oriented surfaces, which could be inferred from the condensation on homogeneous surfaces. The vertically oriented unmodified surface exhibited dropwise condensation in Fig. 4, which showed better heattransfer performance than the filmwise condensation on the horizontal unmodified surface, making the h0 larger and the ratio h h0 smaller. Moreover, we compare the heat transfer coefficient of both surface orientations for different types of modified surface in Fig. 6. The 1.5/0.5 vertically oriented surface showed a superior heat transfer coefficient compared to its corresponding case on horizontal surface; however, as the width of modified strips increases, the heat transfer coefficient of the horizontally oriented condensing surface gradually achieve and even slightly surpass that of the vertical surface. Figure 6 reveals that the interlaced pattern could demonstrate its advantage much more when the surface is horizontally oriented, that is to say, without the sweeping down of condensates
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Fig. 6. Orientation effect on heat transfer coefficient for different types of interlaced surface. The heat transfer coefficient of the horizontal interlaced surfaces change from smaller to larger than the corresponding vertical cases as the width of the modified strips increases.
under gravity force. The phenomena might result from the lateral movements of droplets on the interlaced surfaces, which could be found on both horizontal and vertical surfaces, as illustrated in Fig. 7. The lateral movements not only increased the growth rate of the droplets on the unmodified strips, but also decreased the size of the droplets on the modified regions. The suspended droplets created larger interfaces for the lateral movements. The large suspended droplets on the horizontal surfaces could “absorb” more droplets from the neighboring modified regions, while the sweeping effect on the vertical surface merely removed the small droplets adjacent to the interfaces. For the vertical surfaces, the departure
Fig. 7. Illustration of the lateral movements on the (a) horizontal and (b) vertical surfaces with interlaced wettability.
diameter of the droplet is smaller than the smallest studied spacing; thus, it was too fast for the vertical surfaces with sweeping effect to utilize the lateral movements.
3.4. Effect of wall subcooling As mentioned earlier, Δhinterlace is the parameter evaluating the effect of interlaced wettability on heat transfer enhancement, and it can be influenced not only by the modified area ratio but the wall subcooling. In addition, the heat transfer performance of the surface with interlaced wettability should be evaluated within sufficient range of wall subcooling to test its applicability to reduce the flooding on superhydrophobic surface. According to the simulation of Peng et al. [18], the hybrid surface is more effective within 10K wall subcooling, therefore we choose the value. Figure 8 shows the results of the effect of wall subcooling on additional enhancement ratio Δhinterlace. For the horizontally oriented surfaces with narrower modified strips (1.5/0.5 and 2.5/0.5), the suspended droplets would block the condensing surface and reduce the heat transfer performance more seriously with the increase of wall subcooling. As the wall subcooling raises, the enhancement on 4.5/0.5 surface increases and finally surpasses that on 5.5/0.5 surface, which having larger modified area ratio. The results indicate that the optimum of the modified area ratio for the 0.5 mm unmodified strips would decrease with the rising cooling intensity because of the degradation of heat transfer coefficient on superhydrophobic regions mentioned in previous research [8,9]. Moreover, as shown in Fig. 8, the 4.5/0.5 surface exhibited different trend from the other surfaces. That is to say, for relatively larger wall-subcooling (>6K) with other similar operating conditions, the interlaced surface with area ratio and configuration close to 4.5/0.5 surface is suggested to achieve optimal efficiency.
Y.-A. Lee et al./Applied Thermal Engineering 98 (2016) 1054–1060
(a)
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Fig. 8. Additional heat transfer enhancement ratio of various types of modified surface as a function of wall subcooling for the (a) horizontal and (b) vertical surfaces.
On the other hand, the frequent sweeping effect on the 1.5/0.5 and 2.5/0.5 vertical surfaces leads to the enhancement within small wall subcooling (3K). However, larger wall subcooling would lead to both stronger sweeping effect and bigger condensates, which possess opposite effect on heat transfer. We can deduce that the sweeping effect was strong enough only on the 1.5/0.5 surface, which enabled the surface to sustain further increase of wall subcooling to 9K, and kept the value of Δhinterlace positive. 4. Conclusions This study experimentally investigated the effects of superhydrophobic-based interlaced wettability on condensation heat transfer. The variation of surface orientation, surface pattern and wall subcooling could all alter the heat transfer performance of the interlaced surface. The existence of the unmodified strips would be the most effective on the interlaced-patterned horizontal surfaces with wider modified strips, which possess the enhancement ratio of 2.8 and even increased the condensation heat transfer coefficient to be 1.2 times higher than that of the superhydrophobic surface; however, the increase of wall subcooling would reduce the optimum width of the modified regions. On the other hand, instead of exhibiting similar trend, the sweeping effect on the vertical surfaces let the surfaces with narrower modified strips acquire higher enhancement ratio, and the effect of the lateral movements is less significant when the condensing surface is vertically oriented. Those results show that the nanoparticlemodified surface with interlaced wettability pattern can be applied for better performance for condensation heat transfer, especially in the situation without the help of the droplet-sweeping effect by the gravity force. Acknowledgements This work was supported by the Ministry of Science and Technology, Taiwan under project number MOST 102-2221E-002-133-MY3, MOST 102-2221-E-002-088-MY3, and MOST 102-2218-E-002-022. References [1] C. Dorrer, J. Ruhe, Advancing and receding motion of droplets on ultrahydrophobic post surfaces, Langmuir 22 (2006) 7652–7657.
[2] C.H. Chen, Q.J. Cai, C.L. Tsai, C.L. Chen, G.Y. Xiong, Y. Yu, et al., Dropwise condensation on superhydrophobic surfaces with two-tier roughness, Appl. Phys. Lett. 90 (2007). [3] C. Dietz, K. Rykaczewski, A.G. Fedorov, Y. Joshi, Visualization of droplet departure on a superhydrophobic surface and implications to heat transfer enhancement during dropwise condensation, Appl. Phys. Lett. 97 (2010). [4] J.W. Rose, Dropwise condensation theory and experiment: a review, Proc. Inst. Mech. Eng. A-J. Power. 216 (2002) 115–128. [5] E. Schmidt, W. Schurig, W. Sellschopp, Condensation of water vapour in film- and drop form, Z. Ver. Dtsch. Ing. 74 (1930) 544. [6] J.B. Boreyko, C.H. Chen, Self-propelled dropwise condensate on superhydrophobic surfaces, Phys. Rev. Lett. 103 (2009). [7] N. Miljkovic, R. Enright, Y. Nam, K. Lopez, N. Dou, J. Sack, et al., Jumping-dropletenhanced condensation on scalable superhydrophobic nanostructured surfaces, Nano Lett. 13 (2013) 179–187. [8] J.T. Cheng, A. Vandadi, C.L. Chen, Condensation heat transfer on two-tier superhydrophobic surfaces, Appl. Phys. Lett. 101 (2012). [9] M. Sbragaglia, A.M. Peters, C. Pirat, B.M. Borkent, R.G.H. Lammertink, M. Wessling, et al., Spontaneous breakdown of superhydrophobicity, Phys. Rev. Lett. 99 (2007). [10] K.K. Varanasi, M. Hsu, N. Bhate, W.S. Yang, T. Deng, Spatial control in the heterogeneous nucleation of water, Appl. Phys. Lett. 95 (2009). [11] C.W. Lo, C.C. Wang, M.C. Lu, Spatial control of heterogeneous nucleation on the superhydrophobic nanowire array, Adv. Funct. Mater. 24 (2014) 1211– 1217. [12] V.P. Carey, Molecular dynamics simulations and liquid-vapor phase-change phenomena, Microsc. Thermophys. Eng. 6 (2002) 1–2. [13] R. Xiao, N. Miljkovic, R. Enright, E.N. Wang, Immersion condensation on oil-infused heterogeneous surfaces for enhanced heat transfer, Scientific Reports 3 (2013). [14] A.R. Parker, C.R. Lawrence, Water capture by a desert beetle, Nature 414 (2001) 33–34. [15] S.S. Beaini, V.P. Carey, Strategies for developing surfaces to enhance dropwise condensation: exploring contact angles, droplet sizes, and patterning surfaces, J. Enhanc. Heat Transf. 20 (2013) 33–42. [16] C.W. Yao, J.L. Alvarado, C.P. Marsh, B.G. Jones, M.K. Collins, Wetting behavior on hybrid surfaces with hydrophobic and hydrophilic properties, Appl. Surf. Sci. 290 (2014) 59–65. [17] A. Chatterjee, M.M. Derby, Y. Peles, M.K. Jensen, Enhancement of condensation heat transfer with patterned surfaces, Int. J. Heat Mass Transf. 71 (2014) 675–681. [18] B.L. Peng, X.H. Ma, Z. Lan, W. Xu, R.F. Wen, Analysis of condensation heat transfer enhancement with dropwise-filmwise hybrid surface: droplet sizes effect, Int. J. Heat Mass Transf. 77 (2014) 785–794. [19] B.L. Peng, X.H. Ma, Z. Lan, W. Xu, R.F. Wen, Experimental investigation on steam condensation heat transfer enhancement with vertically patterned hydrophobic-hydrophilic hybrid surfaces, Int. J. Heat Mass Transf. 83 (2015) 27–38. [20] Y.A. Lee, L.S. Kuo, T.W. Su, C.C. Hsu, P.H. Chen, Condensation heat transfer enhancement on surfaces with interlaced wettability, J. Adv. Therm. Sci. Res. 2 (2015) 27–32. [21] G. Koch, D.C. Zhang, A. Leipertz, Condensation of steam on the surface of hard coated copper discs, Heat Mass Transfer 32 (1997) 149–156. [22] R.W. Bonner, Dropwise condensation on surfaces with graded hydrophobicity, ASME Paper No. HT2009-88516, 3 (2009) 491–495.
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[23] J.B. Boreyko, Y.J. Zhao, C.H. Chen, Planar jumping-drop thermal diodes, Appl. Phys. Lett. 99 (2011). [24] J.B. Boreyko, C.H. Chen, Vapor chambers with jumping-drop liquid return from superhydrophobic condensers, Int. J. Heat Mass Transf. 61 (2013) 409–418. [25] A.M. Macner, S. Daniel, P.H. Steen, Condensation on surface energy gradient shifts drop size distribution toward small drops, Langmuir 30 (2014) 1788–1798.
[26] J.R. Taylor, An introduction to error analysis: the study of uncertainties in physical measurements, second ed., University Science Books, Sausalito, California, 1997. [27] C.C. Hsu, P.H. Chen, Surface wettability effects on critical heat flux of boiling heat transfer using nanoparticle coatings, Int. J. Heat Mass Transf. 55 (2012) 3713–3719.
Contents lists available at ScienceDirect
Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Orientation effects of nanoparticle-modified surfaces with interlaced wettability on condensation heat transfer You-An Lee a, Long-Sheng Kuo a, Tsung-Wen Su a, Chin-Chi Hsu b, Ping-Hei Chen a,* a b
Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan Department of Mechanical Engineering, National United University, Miaoli 36063, Taiwan
H I G H L I G H T S
• • •
The surface with interlaced-wettability can enhance the condensation heat transfer. Wider superhydrophobic strips increase the heat transfer for horizontal surfaces. Narrower superhydrophobic strips increase the heat transfer for vertical surfaces.
A R T I C L E
I N F O
Article history: Received 24 September 2015 Accepted 3 January 2016 Available online 11 January 2016 Keywords: Condensation heat transfer Interlaced wettability Nanoparticle-modified surface Surface orientation
A B S T R A C T
This study investigated the effects of surfaces with superhydrophobicity-based interlaced wettability on condensation heat transfer. Experiments were conducted on various types of surface with different modified strip widths under horizontal and vertical surface orientations. The experimental results revealed that the condensation heat-transfer on surfaces with interlaced wettability could be highly influenced by the surface pattern, surface orientation, and wall subcooling. Opposite trends of heat transfer were observed under different surface orientation. The experimental data of horizontal surfaces showed that the heat transfer can be enhanced when the spacing between the unmodified strips is getting wider, while the narrower spacing would increase the heat transfer more efficiently for vertical surfaces. Such the facts imply that the interlaced surface holds the potential of heat transfer enhancement, especially in the situation without the sweeping of condensates under the gravity force. © 2016 Elsevier Ltd. All rights reserved.
1. Background In the past decade, researchers have put a lot effort into the fabrication of superhydrophobic surface to achieve dropwise condensation [1–3] since dropwise condensation could yield a heat transfer coefficient one order higher in magnitude compared with filmwise condensation [4–7]. However, researchers found out that they cannot maintain the dropwise condensation and the tiny size of the condensates on hierarchically structured superhydrophobic surface when the heat flux and the supersaturation of the process were increased to a certain level, leading to the flooding of the surface and the degradation of the heat transfer performance [7–9]. To solve the problem, researchers conducted the experiments on surfaces with micro-scale divisions with different wettability under ESEM, trying to control the distribution of the nucleation site on the surfaces [10,11]. Based on nucleation theory, the energy barrier for nucleation is lower on a hydrophilic surface [12,13], making it possible to restrict the nucleation only on the hydro-
* Corresponding author. Tel.: +886 233662689; fax: +886 223631755. E-mail address: [email protected] (P.-H. Chen). http://dx.doi.org/10.1016/j.applthermaleng.2016.01.003 1359-4311/© 2016 Elsevier Ltd. All rights reserved.
philic regions of the surface. Their experimental results show that both the size and the distribution of the condensate can be controlled well the type of heterogeneous surfaces they fabricated. The condensation phenomena on the back of the Namib desert beetles [14] inspired the research of heterogeneous wettability. Surfaces with heterogeneous wettability take the advantages of both hydrophilicity and hydrophobicity. Thus, connecting and striking a balance for each phase of condensation is the key to enhancing condensation further [15]. In addition to nucleation control, different implementations for such surfaces are used. Chatterjee et al. used surface with matrix-patterned wettability, finding out that the heat transfer coefficient would increase with the reduction of the diameter of the hydrophilic circles [16,17]. Peng et al.’s team utilized hybrid surface consisted of hydrophobic and hydrophilic region to conduct the research numerically [18] and experimentally [19]. The results indicate that the heat transfer coefficient increases with the decreasing width of the hydrophilic strips while an optimum of the width of the hydrophobic region exists for vertical condensing surface. They attributed the heat transfer enhancement to the hybrid mode of filmwise and dropwise condensation. Our previous research [20] focused on the improvement of the superhydrophobic surface, attempting to combine the
Y.-A. Lee et al./Applied Thermal Engineering 98 (2016) 1054–1060
advantages of both superhydrophobic and hybrid surfaces. We concluded that the superhydrophobic/hydrophobic interlaced surface is also capable of raising heat transfer coefficient for horizontal surfaces, especially for those with wider superhydrophobic regions. In addition to the surface morphology, surface orientation is another key factor affecting condensation heat transfer performance for various types of surface. Generally speaking, the removal of droplets relies on the gravity force. Koch et al. pointed out that the inclination of the condensing surface would decrease the gravity effect, and thus, the heat transfer coefficient [21]; nonetheless, researches attempted to utilize the advantage of the recently developed patterned surfaces to enhance or even replace the function of gravity force. Bonner found out that the ratio of heat transfer enhancement on gradient surface is higher when the gravity force is reduced by inclining condensing surface [22]. Boreyko et al. applied the hierarchically structured surface inducing jumping condensates to heat transfer devices, confirming that the heat transfer enhancement by surface induced jumping condensates could function well against gravity force [23,24]. Macner et al. concluded that the surface with gradient wettability could achieve stable dropwise condensation on surface horizontally facing upward [25]. We studied the condensation heat transfer enhancement on the surface horizontally facing downward previously [20], which is uncommon among the recent researches of surface orientation. We attempt to further examine the effect of surface orientation for enhanced condensation on surfaces with superhydrophobicitybased interlaced wettability in this study. We fabricated interlaced surface with different modified strip widths under different surface orientation, aiming at finding out the principle of designing interlaced surface for condensation heat transfer enhancement.
1055
(a)
g (b)
2. Methods Figure 1 illustrates the experimental setup. A copper cylinder 35 mm in diameter and 30 mm in height was used as a heat transfer medium. One end of the cylinder functioned as the condensing surface. Four thermocouples were inserted into the copper cylinder at two different axial locations to measure the temperature and obtain average temperature gradient, followed by a calculation of surface temperature by employing extrapolation as well as heat flux by using Fourier’s law. A pressure transducer was inserted to determine the concentration of the non-condensable gas, which was approximately 1.2% calculated using W = (P − Patm)/Patm in the configuration of the system. The other details for the experimental setup can be found in our previous research [20]. The instrument error for the thermocouples, U T , was ±0.4 K , and for the location of temperature measurement, U x , was ±0.5 mm. The uncertainties of the heat flux, wall subcooling, and heat transfer coefficient within 10K wall subcooling are determined by the following method according to Taylor [26]: 2 2 Uq ⎛U ⎞ ⎛U ⎞ = ⎜ T ⎟ + ⎜ x ⎟ = 5 − 10% ⎝ ⎠ ⎝ ⎠ ΔT Δx q
2
2 U ΔTsub ⎛ U ⎞ ⎛ Uq ⎞ = ⎜ T ⎟ + ⎜ ⎟ = 2 − 10% ⎝ ΔT ⎠ ⎝ q ⎠ ΔTsub
Fig. 1. Experimental setup for (a) horizontal surface, and (b) vertical surface.
solution is presented in Hsu and Chen’s paper [27]. We heated the copper surfaces for 1.5 h at 120 °C after spinning. The thickness of the coated layer is less than 7 μm. The second step was the preparation of hydrophobic materials by fluoro-containing mixtures. Trichloro (1H, 1H, 2H, 2Hperfluorooctyl) silane was mixed with methyl alcohol (1/100 in v/v). After spin coating the surface with the solution, the surfaces were heated at 90 °C for 40 min. The interlaced wettability on the surface was produced by masking the specific strips before coating. The masks were removed afterward to obtain plain copper strips, which were fixed at a width of 0.5 mm in this study. Contact angle measurements were conducted using the Sindatek Model 100SB. The contact angles (CAs) of the surfaces in our experiments are listed in Table 1, and the illustrations of the interlaced surfaces are shown in Fig. 2. 3. Results and discussion
2
2
Uh ⎛ Uq ⎞ ⎛ U ⎞ = ⎜ ⎟ + ⎜ ΔTsub ⎟ = 5 − 15% ⎝ q ⎠ ⎝ ΔTsub ⎠ h
3.1. Experiments on surfaces with homogeneous wettability
The sol-gel method was used to spin-coat 40 nm silica nanoparticles on a polished copper surface to fabricate superhydrophobic and interlaced surfaces. Table 1 lists the surface types and the corresponding parameters. The detailed procedure of preparing the
Figure 3 presents the heat transfer coefficients varying with wall subcooling for both horizontal and vertical surfaces with homogeneous wettability, indicating that vertically oriented surfaces showed a superior heat transfer coefficient compared with their corre-
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Table 1 List of the surface types. Surface type
Modified strip width
Plain strip width
Modified area ratio
Modified region CA
Plain region CA
Plain Super hydrophobic 1.5/0.5 2.5/0.5 4.5/0.5 5.5/0.5
N/A Whole surface 1.5 mm 2.5 mm 4.5 mm 5.5 mm
Whole surface N/A 0.5 mm 0.5 mm 0.5 mm 0.5 mm
0 1 0.750 0.835 0.896 0.920
154.9° ± 3.1°
Heated: 104.7° ± 3.8°
sponding cases on horizontal surfaces. The sweeping down of the condensates under gravity force makes it easier for the smaller droplets to shed, leading to a more efficient droplet removal process on vertical surfaces. In addition, although the superhydrophobic surface made by sol-gel coated nano-particle could not exhibit the jumping droplets phenomena mentioned earlier, we found out that the superhydrophobic surface could enhance the heat transfer performance of horizontally oriented surface to surpass the unmodified vertical surface, which can be indicated by the black circle and white rectangle in Fig. 3. The experiments on the homogeneous plain and modified surfaces determined the data of h0 and hm respectively for the following analysis. 3.2. Droplets morphology Once the condensation process reached the steady state, there are two mechanisms for the condensates to leave the interlaced surface: one is the direct removal of tiny droplets from the modified superhydrophobic regions, and the other can be attributed to the lateral movements of small droplets near the interfaces from modified strips to plain strips. Such movements of small droplets across the interfaces are driven by the difference of surface tension between neighboring regions with different wettability. For horizontally oriented surfaces, the droplets near the interfaces coalesce with larger droplets on the plain strips, suspending and continu-
Fig. 2. Dimensions of the interlaced surfaces.
ally growing until overcoming the surface tension force. On the other hand, for the vertical condensing surface, condensates aggregating to the interfaces would coalesce with the droplets sweeping down the nearby plain strips, preventing the surfaces from being blocked by the large suspended droplets. The difference of droplets morphology can be seen in Fig. 4. 3.3. Effect of surface pattern and surface orientation Fig. 5 presents the heat transfer coefficient as functions of the ratio of the modified areas on a surface when wall subcooling was 5K. The ratio increased as the width of the modified strips increased and attained a value of 1 on the superhydrophobic surface. The overall heat transfer coefficient of an interlaced surface can be expressed as:
A ⎞ A ⎛ h = h0 ⎜ 1 − m ⎟ + hm m + Δhinterlace ⎝ Atotal ⎠ Atotal where A m is the sum of the area of modified regions, A total is the m is the modified area ratio, which area of the condensing surface, AAtotal can be seen in Table 1. h is the overall heat transfer coefficient, h0 is the heat transfer coefficient of the plain copper surface, hm is the heat transfer coefficient of the homogeneous superhydrophobic
Fig. 3. Variation of heat transfer coefficient with wall subcooling for plain and superhydrophobic surfaces.
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Fig. 4. CCD images of the condensation processes on different types of surface. The suspended droplets can be seen on the horizontally oriented interlaced surfaces, and the surface wider spacing possessed less suspended droplets.
surface, and Δhinterlace is the source of heat transfer enhancement, a variable additionally contributing to the heat transfer depending on the surface pattern and the wall subcooling. h0 and hm could be decided by the experiments on the homogeneous surfaces. The two lines in Fig. 5 present the relation only considering the modified area ratio, functioning as standard value for examining the effect of interlaced wettability on condensing surfaces. Δhinterlace then can be determined by the difference between the measured heat transfer coefficient, h and the corresponding value on the line. As in Fig. 5, we can observe opposite trends of heat-transfer enhancement under different surface orientation. The experimental data of the horizontal surfaces showed that the heat transfer could be enhanced more when the width of the modified strips getting wider. The results indicate that the spacing between the unmodified strips should be wide enough to avoid the reduction of heat transfer due to the surface blockage. Moreover, the suspended
Fig. 5. Heat transfer enhancement ratio as a function of the modified area ratio for the various types of modified surface when wall subcooling = 5 K. The five increasing modified area ratios correspond to the 1.5/0.5, 2.5/0.5, 4.5/0.5, 5.5/0.5 interlaced surfaces, and the fully modified surface respectively.
droplets on the unmodified strips would induce the lateral movements of the droplets, accelerating the droplet removal process on both the superhydrophobic and the unmodified regions. Such effect of the suspended droplets could enhance heat transfer most effectively when the spacing between the 0.5 mm unmodified strips are around 5.5 mm or slightly wider. Therefore, the efficient droplet removal mechanism makes the interlaced surface reach higher heat transfer coefficient than the superhydrophobic one, which can also be seen in Fig. 5. In contrast, the negative value of Δhinterlace for vertical surfaces with wider spacing between the unmodified strips might result from the formation of the thicker liquid film on the unmodified strips. As in Fig. 4, the homogeneous unmodified surface demonstrates dropwise condensation; however, the lateral movements on the interlaced surfaces would lead to the transition of condensation mode to filmwise condensation. From Fig. 5 we found out that this problem could be addressed by decreasing the width of the modified strips. The narrower the modified strips on vertical surfaces means the more amount of the unmodified strips and the interfaces, leading to the increase of sweeping down of droplets on the unmodified strips and the accompanied coalescence with the droplets on the neighboring interfaces. The sweeping effect on the vertical condensing surfaces would compensate for the reduction of performance due to transition of condensation mode on the unmodified strips, dominating the heat transfer enhancement under the gravity force. Figure 5 also indicates that the heat transfer enhancement by the interlaced wettability seems more effective on horizontally oriented surfaces, which could be inferred from the condensation on homogeneous surfaces. The vertically oriented unmodified surface exhibited dropwise condensation in Fig. 4, which showed better heattransfer performance than the filmwise condensation on the horizontal unmodified surface, making the h0 larger and the ratio h h0 smaller. Moreover, we compare the heat transfer coefficient of both surface orientations for different types of modified surface in Fig. 6. The 1.5/0.5 vertically oriented surface showed a superior heat transfer coefficient compared to its corresponding case on horizontal surface; however, as the width of modified strips increases, the heat transfer coefficient of the horizontally oriented condensing surface gradually achieve and even slightly surpass that of the vertical surface. Figure 6 reveals that the interlaced pattern could demonstrate its advantage much more when the surface is horizontally oriented, that is to say, without the sweeping down of condensates
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Fig. 6. Orientation effect on heat transfer coefficient for different types of interlaced surface. The heat transfer coefficient of the horizontal interlaced surfaces change from smaller to larger than the corresponding vertical cases as the width of the modified strips increases.
under gravity force. The phenomena might result from the lateral movements of droplets on the interlaced surfaces, which could be found on both horizontal and vertical surfaces, as illustrated in Fig. 7. The lateral movements not only increased the growth rate of the droplets on the unmodified strips, but also decreased the size of the droplets on the modified regions. The suspended droplets created larger interfaces for the lateral movements. The large suspended droplets on the horizontal surfaces could “absorb” more droplets from the neighboring modified regions, while the sweeping effect on the vertical surface merely removed the small droplets adjacent to the interfaces. For the vertical surfaces, the departure
Fig. 7. Illustration of the lateral movements on the (a) horizontal and (b) vertical surfaces with interlaced wettability.
diameter of the droplet is smaller than the smallest studied spacing; thus, it was too fast for the vertical surfaces with sweeping effect to utilize the lateral movements.
3.4. Effect of wall subcooling As mentioned earlier, Δhinterlace is the parameter evaluating the effect of interlaced wettability on heat transfer enhancement, and it can be influenced not only by the modified area ratio but the wall subcooling. In addition, the heat transfer performance of the surface with interlaced wettability should be evaluated within sufficient range of wall subcooling to test its applicability to reduce the flooding on superhydrophobic surface. According to the simulation of Peng et al. [18], the hybrid surface is more effective within 10K wall subcooling, therefore we choose the value. Figure 8 shows the results of the effect of wall subcooling on additional enhancement ratio Δhinterlace. For the horizontally oriented surfaces with narrower modified strips (1.5/0.5 and 2.5/0.5), the suspended droplets would block the condensing surface and reduce the heat transfer performance more seriously with the increase of wall subcooling. As the wall subcooling raises, the enhancement on 4.5/0.5 surface increases and finally surpasses that on 5.5/0.5 surface, which having larger modified area ratio. The results indicate that the optimum of the modified area ratio for the 0.5 mm unmodified strips would decrease with the rising cooling intensity because of the degradation of heat transfer coefficient on superhydrophobic regions mentioned in previous research [8,9]. Moreover, as shown in Fig. 8, the 4.5/0.5 surface exhibited different trend from the other surfaces. That is to say, for relatively larger wall-subcooling (>6K) with other similar operating conditions, the interlaced surface with area ratio and configuration close to 4.5/0.5 surface is suggested to achieve optimal efficiency.
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(a)
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(b)
Fig. 8. Additional heat transfer enhancement ratio of various types of modified surface as a function of wall subcooling for the (a) horizontal and (b) vertical surfaces.
On the other hand, the frequent sweeping effect on the 1.5/0.5 and 2.5/0.5 vertical surfaces leads to the enhancement within small wall subcooling (3K). However, larger wall subcooling would lead to both stronger sweeping effect and bigger condensates, which possess opposite effect on heat transfer. We can deduce that the sweeping effect was strong enough only on the 1.5/0.5 surface, which enabled the surface to sustain further increase of wall subcooling to 9K, and kept the value of Δhinterlace positive. 4. Conclusions This study experimentally investigated the effects of superhydrophobic-based interlaced wettability on condensation heat transfer. The variation of surface orientation, surface pattern and wall subcooling could all alter the heat transfer performance of the interlaced surface. The existence of the unmodified strips would be the most effective on the interlaced-patterned horizontal surfaces with wider modified strips, which possess the enhancement ratio of 2.8 and even increased the condensation heat transfer coefficient to be 1.2 times higher than that of the superhydrophobic surface; however, the increase of wall subcooling would reduce the optimum width of the modified regions. On the other hand, instead of exhibiting similar trend, the sweeping effect on the vertical surfaces let the surfaces with narrower modified strips acquire higher enhancement ratio, and the effect of the lateral movements is less significant when the condensing surface is vertically oriented. Those results show that the nanoparticlemodified surface with interlaced wettability pattern can be applied for better performance for condensation heat transfer, especially in the situation without the help of the droplet-sweeping effect by the gravity force. Acknowledgements This work was supported by the Ministry of Science and Technology, Taiwan under project number MOST 102-2221E-002-133-MY3, MOST 102-2221-E-002-088-MY3, and MOST 102-2218-E-002-022. References [1] C. Dorrer, J. Ruhe, Advancing and receding motion of droplets on ultrahydrophobic post surfaces, Langmuir 22 (2006) 7652–7657.
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