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Experimental study of pool boiling characteristic of an aluminized copper surface
International Journal of Heat and Mass Transfer 85 (2015) 239–246
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Experimental study of pool boiling characteristic of an aluminized copper surface D. Saeidi a,⇑, A.A. Alemrajabi a, N. Saeidi b a b
Department of Mechanical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 11 August 2014 Received in revised form 21 January 2015 Accepted 22 January 2015
Keywords: Boiling Critical heat flux (CHF) Heat transfer coefficient Aluminize
a b s t r a c t Aluminizing is an appealing method of surface modification. Although this method is widely used in industry for surface modification, its effect on boiling heat transfer has not been reported yet. In the present study, effect of aluminizing of a copper surface on its pool boiling characteristic in the presence of De-Ionized water as working fluid is investigated experimentally. To perform the process of aluminizing the copper specimen was sunk in a powder that contained Al2O3, Al, and NH4Cl and was heated thereafter. The experimental results revealed that critical heat flux increases about 37% on aluminized surface compared with untreated copper surface while changes in boiling heat transfer coefficient is inconsiderable. Therefore some undesirable characteristic of copper like oxidation, and relatively low critical heat flux can be diminished by applying this coating method. In addition, AFM images were used to study some surface characteristics such as roughness, surface height and area ratio. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Wettability, high critical heat flux (CHF), and high heat transfer coefficient are among the most desirable characteristics of a surface in the field of boiling heat transfer. So far tremendous amount of research has been done to investigate the boiling characteristic of different materials. Many researchers have tried to improve the boiling characteristics of surfaces by nanostructure coating. For instance, Saeidi and Alemrajabi [1] used nano structured surface which were prepared by anodizing method and surveyed contact angle and boiling traits like CHF and heat transfer coefficient. They employed atomic force microscope (AFM) images to study surface characteristics and observed enhancement in heat transfer coefficient of up to 159%. Wetting under different conditions was studied by Jo et al. [2]. Their observation in nucleate pool boiling indicated that hydrophobic surfaces have better boiling heat transfers attribute in very low heat flux regimes than do hydrophilic surfaces. Fagerholm et al. [3] studied boiling heat transfer in the presence of R144 as the working fluid. They worked on porous tubes and surfaces and reported enhanced boiling heat transfers of up to 10 times relative to that of smooth surfaces. White et al. [4] used ⇑ Corresponding author. E-mail addresses: [email protected] (D. (A.A. Alemrajabi), [email protected] (N. Saeidi).
Saeidi),
http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.01.110 0017-9310/Ó 2015 Elsevier Ltd. All rights reserved.
[email protected]
coated stainless steel surface to investigate heat transfer coefficient under atmospheric pressure using deionized water. Their result showed up of to 200% improvement in boiling heat transfer coefficient. They employed a ZnO–propylene glycol-based nanofluid to produce coated surfaces. Pastuszko [5] studied boiling heat transfer in narrow tunnel structures experimentally. He implemented pure water, ethanol, and R-123 at atmospheric pressure. Finally he proposed a theoretical model on the basis of his investigation and previous studies. Piasecka and Maciejewska [6] carried out experimental study in order to investigate boiling heat transfer characteristics in mini channels. In another work, Lu et al. compared boiling characteristics of plain Si surfaces and nano-coated Si wire [7]. Measurement of CHF and heat transfer coefficient values on nano-coated wire exhibited the highest values reported in the literature so far. In addition, Cieslinski [8], and Frieser and Reeber [9] worked on the sandblasted surfaces. While Cieslinski reported no significant CHF enhancement compared to smooth tubes, Frieser and Reeber reported enhanced boiling heat transfer compared to that of polished surfaces due to the increase in the number of nucleation sites in the modified surfaces. Phan et al. [10] studied the influence of surface wettability on pool boiling heat transfer. In order to change the contact angle they used nano-coating techniques and modified the surface to achieve the best heat transfer coefficient in the samples that had contact angle close to either 0° or 90°. Stutz et al. [11] studied heat transfer on the nanostructured platinum wire and investigated the effect of
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Nomenclature A D g h I k K Q T V z z Ra AFM BSE DI
heated surface area (=pD2/4) (m2) diameter of heated section (m) gravity acceleration (m/s2) heat transfer coefficient (W/m2 K) current (A) thermal conductivity (W/m k) empirical constant, Eq. (2) (–) power input (W) temperature (°C) voltage (V) height average height roughness atomic force microscope Back Scattering Electron De-Ionized
coating duration on boiling phenomenon. Their outcomes revealed significant enhancement in CHF and attributed this behavior to nanoparticles deposition on the heated surface. In another study, Tang et al. [12] worked on the novel metallic nanoporous surface which was fabricated by using hot-dip galvanizing/dealloying method. The results showed dramatic improvement in heat transfer coefficient in nanostructured surface compared to those of unstructured surface. Lee et al. [13] studied aluminum coated surfaces that were prepared by anodizing method. Their results indicated that although heat transfer coefficient improved at low heat fluxes, it showed no meaningful enhancement for heat fluxes greater than 60 kW/m2. Forest et al. [14] covered boiling surfaces with a thin layer of silica nanoparticles by using the layer-by-layer method. Their results showed significant enhancements in pool boiling critical heat flux and heat transfer coefficient. In some cases they observed CHF enhancements of up to 100% and over 100% increase in boiling heat transfer coefficient. Ji et al. [15] also
SEM
Scanning Electron Microscope
Greek letters q density (kg/m3) r surface tension (N/m) h contact angle (°) u angle of heater orientation (°) Subscripts CHF critical heat flux f fluid fg fluid–gas g gas w wall
investigated on uniform and non-uniform porous coating surfaces. They observed poor heat transfer in plain surface, moderate heat transfer improvement in uniform porous coating surface, and significant heat transfer enhancement in 2-D/3-D porous coating surfaces. Yang and Liu [16] experimented micro porous coated surface with different layer thickness in order to distinguish heat transfer characteristics in those surfaces. They used confined and unconfined space to implement their studies and found optimum coating layer thickness for micro porous surface. Copper has some beneficial properties like high thermal conductivity and fairly high strength which make this substance favorable especially in thermal industry. It also has some unfavorable characteristics like surface oxidation [17]. On the other hand, several boiling experiments have shown that critical heat flux is noticeably delayed on the aluminum surfaces compared with copper surfaces. Having this in mind, it seems that aluminum coated copper surfaces can be used to prevent surface oxidation and to
Fig. 1. Schematic diagram of the experimental apparatus and copper block containing 6 holes for cartridge heaters.
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postpone critical heat flux; while high thermal conductivity of copper is maintained. In the present study, boiling characteristics of aluminized copper surface is investigated experimentally. 2. Experimental apparatus and procedures 2.1. Experimental apparatus Fig. 1 shows the experimental set-up. The copper block comprises 6 cartridge heaters, 200 W each, to which is attached the test section. To minimize heat contact resistance between the copper block and heaters or test specimen, high thermal conductive paste (4.5 W/m K) was used. The cartridge heaters are, 6.35 mm in diameter and 37.5 mm in length. The copper block’s geometry was such designed to provide approximately one-dimensional heat transfer near the test section. In order to measure the surfaces’ temperature a T-type thermocouple was used. This thermocouple was fixed into a hole, 3 mm beneath the surface by using a high thermal conductive paste. The Fourier’s law of heat conduction in conical shapes has been employed to extrapolate the surface temperature. The test chamber and the cooper block were both thermally insulated to prevent heat loss to the surrounding environment. The test section was positioned at the bottom of a cylindrical transparent container so as to observe the boiling phenomena. Deionized (DI) water was employed as the working fluid in all the pool boiling experiments. The test section used in this study was made of aluminum alloy 2011 and copper as a frustum with a height of 10 mm while the larger and smaller radii of cone are 24 mm and 20 mm, respectively (Fig. 1). 2.2. Preparation of the test section Initially two copper samples and one aluminum sample were prepared and their surfaces which boiling test will be run on them were polished by 1200 grit SiC paper and washed with DI water and acetone. To make aluminized coating surface on the copper sample, a powder consisting of 88 wt.% AL2O3, 10 wt.% Al as inter material and 2 wt.% NH4Cl as an activator were used. Aluminizing was performed in an austenitic stainless steel container which was sealed by using a paste of kaoline and liquid sodium silicate. Thereafter, heating process was carried out for 3 h at 850 °C. Then the specimen was cooled gradually in the oven. Finally, in order to eliminate the surface polish effects on the experiment, aluminized surface was polished in the same manner as aluminum and copper surfaces.
Fig. 2. (a) Top view of aluminized surface (b) cross section of aluminized surface.
Table 2 Comparison of measured values for contact angle, CHF, and predicted CHF values. Surface
Aluminum (Al) Copper (Cu) Aluminized
Advancing contact angle (°) 55 58 66
CHF (Exp.) (kW/m2)
1610 896 1227
Kandlikar correlation, Eq. (2) (kW/m2) Predicted CHF
Error (%)
1137 1090 977
29.4 21.7 20.3
2.3. Test procedure Considering that surface characteristics may have principal roles in boiling heat transfer features, contact angle on all specimens was measured. Measuring contact angle was placed after cleaning entire specimens by acetone. All tests and measurements were carried out at room conditions, i.e. 23 °C and atmospheric pressure of 82,526 Pa (616 mmHg). At this pressure the saturation temperature of water is 94.1 °C [18]. Prior to each test, the surface of the test section was cleaned with acetone and washed by DI water. During the tests, heat flux was gradually increased until critical heat flux was occurred,
Table 1 Uncertainty values of wall superheat and heat flux for the copper surface. Wall superheat (°C) Uncertainty of DT% Heat flux (kW/m2) Uncertainty of heat flux%
6.6 ±0.3 90 ±3.9
10.4 ±0.5 214 ±4.1
14.0 ±0.9 389 ±4.9
18.0 ±1.3 616 ±5.6
Fig. 3. Heat flux vs. wall superheat. 21.5 ±1.8 896 ±6.6
where a jump in temperature was monitored. By measuring the applied voltage and current, i.e. power input to the heaters, heat transfer coefficient can be calculated using Eq. (1):
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2.4. Uncertainty analysis Considering that in addition to the errors in individual measurements, there are random errors that need to be considered, uncertainty analysis is an unavoidable part of any experimental investigation. To perform this analysis, systematic errors in measuring devices for temperature, voltage, length, and current were evaluated; then uncertainty analysis [20] was performed for wall superheat and heat flux and the results are presented in Table 1. Uncertainty levels for the wall superheat and heat flux were less than 2% and 7%, respectively. These processes including measuring the contact angles, boiling tests, and measuring heat transfer were repeated three times and the averaged data for each sample are reported. The standard deviations of different contact angle measurements were in the range of 3° to 6°.
Fig. 4. Heat transfer coefficient vs. heat flux.
h¼
Q =A T w T sat
ð1Þ
where Q is the power input to the heaters. Heat losses are negligible due to good insulation. Heat losses in a similar setup which was examined by one of the authors accounted for only 6% of the consumed power near the critical heat flux [19]. Considering that better quality insulation materials have been employed in the current investigation, lower heat losses are expected. However, the estimated heat loss has been taken out from the measured power in the calculations of heat transfer rate and CHF.
3. Results and discussion 3.1. Quality of coated surface In order to identify the quality of the aluminized surface, one can study the Back Scattering Electron (BSE) mode on a Scanning Electron Microscope (SEM). The top view of aluminized surface is shown in Fig. 2a. In the BSE image, the dark spots are indicative of heavier element which is copper in the current case. The low numbers of these spots show that aluminum has almost entirely
Fig. 5. 3D AFM image for (a) and (b) aluminum (c) and (d) aluminized (e) and (f) copper.
D. Saeidi et al. / International Journal of Heat and Mass Transfer 85 (2015) 239–246
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Test case
Average height (nm)
Roughness (nm)
Area ratio real area/projected area
section of aluminized specimen after polishing. By using this figure one can find out that the thickness of aluminized layer is less than 50 lm and has nearly uniform thickness along the surface. Both of these features are indicative of good quality of coating process.
Aluminum (Al) Copper (Cu) Aluminized
41 62 406
12 8 41
1.6 1.3 1.1
3.2. Contact angle
Table 3 Summary of surface properties from AFM image.
covered the copper surface and aluminized coating has been implemented correctly. On the other hand, Fig 2b shows the cross
Contact angle and surface wettability for three specimens, i.e. aluminum, copper, and aluminized copper, were measured. Results of measurements are listed in Table 2. Contact angle readings are
Fig. 6. Height of proses along transverse of surfaces for (a) aluminum (b) aluminized (c) copper.
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fairly close for untreated aluminum and copper specimens whereas the reading for the aluminized copper specimen is relatively higher which is accompanied by lower wettability. Despite close readings of contact angles for untreated aluminum and copper specimens, the CHF value for aluminum surface is about 80% higher than that of copper surface. This indicates that contact angle is not the only controlling parameter on CHF. The CHF for the aluminized surface is 37% higher than that of copper surface.
The role of contact angle can be further visualized by employing a well-known empirical correlation. This correlation is usually used to evaluate the CHF and was derived by Kandlikar [21] as Eq. (2):
q00CHF ¼ q1=2 g hfg
1=2 h i1=4 1 þ cos h 2 p þ ð1 þ coshÞcosu g rðqf qg Þ 16 p 4 ð2Þ
where u represents the heater slope angle relative to the horizontal.
Fig. 7. Cumulative height distribution of surfaces for (a) aluminum (b) aluminized (c) copper.
D. Saeidi et al. / International Journal of Heat and Mass Transfer 85 (2015) 239–246
Table 2also summarizes the predictions of CHF based on this correlation. It can be seen that Kandlikar’s correlation predicts CHF values in the range of ±30% of the experimental values. The maximum difference between the experimental results and the Kandlikar’s prediction is about 30% which corresponds to the aluminum sample. Considering that contact angle may not be the only parameter which governs CHF, the difference between the correlation predictions and the measured quantities is not surprising. Previous measurements of the advancing and receding contact angle for specimens showed that there is no considerable difference between them and in the worst case they differ by about 5° [1]. Considering that the Kandlikar’s model is based upon the receding contact angle, the 5° difference between the advancing and receding contact angles will lead to at most 6% differences in the Kandlikar’s correlation prediction which is negligible. 3.3. Boiling curves Fig. 3 shows heat flux vs. wall superheat for the three surfaces. Results show that at a specific heat flux, aluminized surface has higher wall superheat compared to aluminum and copper surfaces. Fig. 4 depicts changes of heat transfer coefficient vs. heat flux. It can be observed that heat transfer coefficient for both the copper and aluminized surfaces follow the same trend and are so close to each other. It is interesting to note that CHF on aluminized surface has been delayed until 1227 kW/m2 which is about %37 higher than that of the copper. This makes the aluminized surface more favorable compared with copper surface.
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sured for the aluminized surface. This difference is depicted in Fig. 6. Considering the fact that smaller contact angle is expected to be accompanied by higher CHF, the value of CHF for copper surface in comparison with aluminized surface is unexpected. Taking other characteristics into account, one possible explanation to this ambiguity is as following. As Fig. 6 shows, the copper surface is fairly flat and has few ups and downs. The vapor generated in the nucleation sites on a fairly flat surface with few narrow peaks and valleys can join to each other to overcast the surface easily, hence lower CHF. A combination of high area ratio, low roughness and high average height imply that the proses are too small to work as nucleation sites and vapor is just trapped in them which finally leads to a low CHF value. Aluminizing the copper surface enhances the peaks and valleys. These ups and downs which work as nucleation sites postpone CHF to the higher values of heat flux. Considering that apart from contact angle and roughness, other factors such as chemical constituency and porosity are also likely to affect CHF, more investigation is needed in further studies. In Fig. 7 cumulative distribution of height for aluminum, aluminized and copper specimens have been shown. Based on this figure it is readily observed that for Aluminum and copper specimens height distribution is relatively homogenous and it is scattered between 20 and 100 nm, whereas, in aluminized sample height ranges between 200 and 500 nm which is remarkably higher than raw aluminum and copper surfaces. This distribution of valleys and peaks on aluminized surfaces is an indication of enhanced nucleation sites which can interfere with CHF happening. 4. Conclusion
3.4. Surface roughness In order to survey the surfaces’ characteristics precisely, atomic force microscope (AFM) (Device Model: Bruker Nanos 1.1) was used. Some features such as real surface area and surface roughness can be measured by using AFM images. Hence three different surfaces, i.e. aluminum surface, copper surface and aluminized surface, have been compared with each other by using the 3-D pictures of AFM. Three-D images are shown in Fig. 5 for squares of 20 and 1 lm in lengths. Results for these three surfaces have been extracted for squares of 20 micron length and have been summarized in Table 3. The average height is defined as [22]:
zðN; MÞ ¼
M X N 1 X zðx; yÞ NM y¼1 x¼1
ð3Þ
while the roughness is defined as:
Ra ðN; MÞ ¼
M X N 1 X ðzðx; yÞ zðN; MÞÞ NM y¼1 x¼1
ð4Þ
The aluminum and copper specimens have relatively close roughness and contact angles, whereas the aluminized surface has higher roughness and higher contact angle. It is worth to mention although Wenzel’s equation provides a direct relation between roughness and contact angle and indicates increasing roughness leads to a reduction in the apparent contact angle and it is in contrast with our achievements, there are clues that cast doubts on this equation. Gao and McCarthy [23] have mentioned that all of their data indicate that contact angle behavior (advancing, receding, and hysteresis) is determined by interactions of the liquid and the solid at the three-phase contact line alone and that the interfacial area within the contact perimeter is irrelevant which questions the Wenzel’s equation. In addition, although the average height for copper and aluminum surfaces are relatively low and close to each other, a higher average height has been mea-
In the present investigation, aluminization as a method of surface treatment was applied to a copper surface and the boiling heat transfer characteristic of the treated surface was acquired and compared with those of untreated copper and aluminum surfaces. Aluminization combines the advantageous high thermal conductivity of copper and relatively high critical heat flux of aluminum surfaces but eliminates undesirable oxidation of copper. It also expands the wall superheat at which critical heat flux occurs while leaves heat transfer coefficient almost intact. AFM images of the surfaces of 3 specimens were studied. These images revealed that aluminizing treatment increases microscopic valleys and peaks on the surface which might be one of the reasons for higher CHF. Measurements of contact angle and critical heat flux were performed on copper, aluminum, and aluminized copper surfaces. The CHF increased 37% for the aluminized copper surface with respect to untreated copper surface. Further investigation is needed to find out the most affecting factors on CHF. Conflict of interest None declared. References [1] D. Saeidi, A.A. Alemrajabi, Experimental investigation of pool boiling heat transfer and critical heat flux of nanostructured surfaces, Int. J. Heat Mass Transfer 60 (2013) 440–449. [2] H. Jo, H.S. Ahn, S. Kang, M.H. Kimb, A study of nucleate boiling heat transfer on hydrophilic, hydrophobic and heterogeneous wetting surfaces, Int. J. Heat Mass Transfer 54 (2011) 5643–5652. [3] N.E. Fagerholm, A.R. Ghazanfari, K. Kivioja, E. Jarvinen, Boiling heat transfer performance of plain and porous tubes in falling film flow of refrigerant R114, Wärme - und Stoffübertragung 21 (6) (1987) 343–353. [4] S.B. White, A.J. Shih, K.P. Pipe, Boiling surface enhancement by electrophoretic deposition of particles from a nanofluid, Int. J. Heat Mass Transfer 54 (2011) 4370–4375. [5] R. Pastuszko, Pool boiling for extended surfaces with narrow tunnels– visualization and a simplified model, Exp. Therm. Fluid Sci. 38 (2012) 149–164.
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[6] M. Piasecka, B. Maciejewska, The study of boiling heat transfer in vertically and horizontally oriented rectangular minichannels and the solution to the inverse heat transfer problem with the use of the Beck method and Trefftz functions, Exp. Therm. Fluid Sci. 38 (2012) 19–32. [7] M.C. Lu, R. Chen, V. Srinivasan, V.P. Carey, A. Majumdar, Critical heat flux of pool boiling on Si nanowire array-coated surfaces, Int. J. Heat Mass Transfer 54 (2011) 5359–5367. [8] J.T. Cieslinski, Nucleate boiling on porous metallic coatings, Exp. Therm. Fluid Sci. 25 (2002) 557–564. [9] R.G. Frieser, M.D. Reeber, Surface treatments of silicon to enhance thermal nucleation, J. Appl. Electrochem. 10 (1980) 449–457. [10] H.T. Phan, N. Caney, P. Marty, S. Colasson, J. Gavillet, Surface wettability control by nano coating: the effects on pool boiling heat transfer and nucleation mechanism, Int. J. Heat Mass Transfer 52 (2009) 5459–5471. [11] B. Stutz, C.H.S. Morceli, M.D.F.D. Silva, S. Cioulachtjian, J. Bonjour, Influence of nanoparticle surface coating on pool boiling, Exp. Therm. Fluid Sci. 35 (2011) 1239–1249. [12] Y. Tang, B. Tang, Q. Li, J. Qing, L. Lu, K. Chen, Pool-boiling enhancement by novel metallic nanoporous surface, Exp. Therm. Fluid Sci. 44 (2013) 194– 198. [13] C.Y. Lee, M.M.H. Bhuiya, K.J. Kim, Pool boiling heat transfer with nano-porous surface, Int. J. Heat Mass Transfer 53 (2010) 4274–4279.
[14] E. Forrest, E. Williamson, J. Buongiorno, L.W. Hu, M. Rubner, R. Cohen, Augmentation of nucleate boiling heat transfer and critical heat flux using nanoparticle thin-film coatings, Int. J. Heat Mass Transfer 53 (2010) 58–67. [15] X. Ji, J. Xu, Z. Zhao, W. Yang, Pool boiling heat transfer on uniform and nonuniform porous coating surfaces, Exp. Therm. Fluid Sci. 48 (2013) 198–212. [16] C.Y. Yang, C.F. Liu, Effect of coating layer thickness for boiling heat transfer on micro porous coated surface in confined and unconfined spaces, Exp. Therm. Fluid Sci. 47 (2013) 40–47. [17] K.T. Chiang, K.J. Kallenborn, J.L. Yuen, Aluminization of copper for oxidation protection, Surf. Coat. Technol. 52 (1992) 135–139. [18] R.E. Sonntag, C. Borgnakke, G.J. Van Wylen, Fundamentals of Thermodynamics, sixth ed., Wiley, 2002. [19] A. Abbassi, A.A. AlemRajabi, R.H.S. Winterton, Effect of confined geometry on pool boiling at high temperature, Exp. Therm. Fluid Sci. 2 (1989) 127–133. [20] S.J. Kline, F.A. McClintock, Describing uncertainties in single-sample experiments, Mech. Eng. 75 (1953) 3–8. [21] S.G. Kandlikar, A theoretical model to predict pool boiling CHF incorporating effect of contact angle and orientation, J. Heat Transfer 123 (2001) 1071–1079. [22] M. Raposo, Q. Ferreira, P.A. Ribeiro, in: A. Méndez-Vilas, J. Díaz (Eds.), A Guide for Atomic Force Microscopy Analysis of Soft Condensed Matter, Modern Research and Educational Topics in Microscopy, 2007. [23] L. Gao, T.J. McCarthy, How Wenzel and Cassie were wrong, Langmuir 23 (2007) 3762–3765.
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Experimental study of pool boiling characteristic of an aluminized copper surface D. Saeidi a,⇑, A.A. Alemrajabi a, N. Saeidi b a b
Department of Mechanical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 11 August 2014 Received in revised form 21 January 2015 Accepted 22 January 2015
Keywords: Boiling Critical heat flux (CHF) Heat transfer coefficient Aluminize
a b s t r a c t Aluminizing is an appealing method of surface modification. Although this method is widely used in industry for surface modification, its effect on boiling heat transfer has not been reported yet. In the present study, effect of aluminizing of a copper surface on its pool boiling characteristic in the presence of De-Ionized water as working fluid is investigated experimentally. To perform the process of aluminizing the copper specimen was sunk in a powder that contained Al2O3, Al, and NH4Cl and was heated thereafter. The experimental results revealed that critical heat flux increases about 37% on aluminized surface compared with untreated copper surface while changes in boiling heat transfer coefficient is inconsiderable. Therefore some undesirable characteristic of copper like oxidation, and relatively low critical heat flux can be diminished by applying this coating method. In addition, AFM images were used to study some surface characteristics such as roughness, surface height and area ratio. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Wettability, high critical heat flux (CHF), and high heat transfer coefficient are among the most desirable characteristics of a surface in the field of boiling heat transfer. So far tremendous amount of research has been done to investigate the boiling characteristic of different materials. Many researchers have tried to improve the boiling characteristics of surfaces by nanostructure coating. For instance, Saeidi and Alemrajabi [1] used nano structured surface which were prepared by anodizing method and surveyed contact angle and boiling traits like CHF and heat transfer coefficient. They employed atomic force microscope (AFM) images to study surface characteristics and observed enhancement in heat transfer coefficient of up to 159%. Wetting under different conditions was studied by Jo et al. [2]. Their observation in nucleate pool boiling indicated that hydrophobic surfaces have better boiling heat transfers attribute in very low heat flux regimes than do hydrophilic surfaces. Fagerholm et al. [3] studied boiling heat transfer in the presence of R144 as the working fluid. They worked on porous tubes and surfaces and reported enhanced boiling heat transfers of up to 10 times relative to that of smooth surfaces. White et al. [4] used ⇑ Corresponding author. E-mail addresses: [email protected] (D. (A.A. Alemrajabi), [email protected] (N. Saeidi).
Saeidi),
http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.01.110 0017-9310/Ó 2015 Elsevier Ltd. All rights reserved.
[email protected]
coated stainless steel surface to investigate heat transfer coefficient under atmospheric pressure using deionized water. Their result showed up of to 200% improvement in boiling heat transfer coefficient. They employed a ZnO–propylene glycol-based nanofluid to produce coated surfaces. Pastuszko [5] studied boiling heat transfer in narrow tunnel structures experimentally. He implemented pure water, ethanol, and R-123 at atmospheric pressure. Finally he proposed a theoretical model on the basis of his investigation and previous studies. Piasecka and Maciejewska [6] carried out experimental study in order to investigate boiling heat transfer characteristics in mini channels. In another work, Lu et al. compared boiling characteristics of plain Si surfaces and nano-coated Si wire [7]. Measurement of CHF and heat transfer coefficient values on nano-coated wire exhibited the highest values reported in the literature so far. In addition, Cieslinski [8], and Frieser and Reeber [9] worked on the sandblasted surfaces. While Cieslinski reported no significant CHF enhancement compared to smooth tubes, Frieser and Reeber reported enhanced boiling heat transfer compared to that of polished surfaces due to the increase in the number of nucleation sites in the modified surfaces. Phan et al. [10] studied the influence of surface wettability on pool boiling heat transfer. In order to change the contact angle they used nano-coating techniques and modified the surface to achieve the best heat transfer coefficient in the samples that had contact angle close to either 0° or 90°. Stutz et al. [11] studied heat transfer on the nanostructured platinum wire and investigated the effect of
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Nomenclature A D g h I k K Q T V z z Ra AFM BSE DI
heated surface area (=pD2/4) (m2) diameter of heated section (m) gravity acceleration (m/s2) heat transfer coefficient (W/m2 K) current (A) thermal conductivity (W/m k) empirical constant, Eq. (2) (–) power input (W) temperature (°C) voltage (V) height average height roughness atomic force microscope Back Scattering Electron De-Ionized
coating duration on boiling phenomenon. Their outcomes revealed significant enhancement in CHF and attributed this behavior to nanoparticles deposition on the heated surface. In another study, Tang et al. [12] worked on the novel metallic nanoporous surface which was fabricated by using hot-dip galvanizing/dealloying method. The results showed dramatic improvement in heat transfer coefficient in nanostructured surface compared to those of unstructured surface. Lee et al. [13] studied aluminum coated surfaces that were prepared by anodizing method. Their results indicated that although heat transfer coefficient improved at low heat fluxes, it showed no meaningful enhancement for heat fluxes greater than 60 kW/m2. Forest et al. [14] covered boiling surfaces with a thin layer of silica nanoparticles by using the layer-by-layer method. Their results showed significant enhancements in pool boiling critical heat flux and heat transfer coefficient. In some cases they observed CHF enhancements of up to 100% and over 100% increase in boiling heat transfer coefficient. Ji et al. [15] also
SEM
Scanning Electron Microscope
Greek letters q density (kg/m3) r surface tension (N/m) h contact angle (°) u angle of heater orientation (°) Subscripts CHF critical heat flux f fluid fg fluid–gas g gas w wall
investigated on uniform and non-uniform porous coating surfaces. They observed poor heat transfer in plain surface, moderate heat transfer improvement in uniform porous coating surface, and significant heat transfer enhancement in 2-D/3-D porous coating surfaces. Yang and Liu [16] experimented micro porous coated surface with different layer thickness in order to distinguish heat transfer characteristics in those surfaces. They used confined and unconfined space to implement their studies and found optimum coating layer thickness for micro porous surface. Copper has some beneficial properties like high thermal conductivity and fairly high strength which make this substance favorable especially in thermal industry. It also has some unfavorable characteristics like surface oxidation [17]. On the other hand, several boiling experiments have shown that critical heat flux is noticeably delayed on the aluminum surfaces compared with copper surfaces. Having this in mind, it seems that aluminum coated copper surfaces can be used to prevent surface oxidation and to
Fig. 1. Schematic diagram of the experimental apparatus and copper block containing 6 holes for cartridge heaters.
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postpone critical heat flux; while high thermal conductivity of copper is maintained. In the present study, boiling characteristics of aluminized copper surface is investigated experimentally. 2. Experimental apparatus and procedures 2.1. Experimental apparatus Fig. 1 shows the experimental set-up. The copper block comprises 6 cartridge heaters, 200 W each, to which is attached the test section. To minimize heat contact resistance between the copper block and heaters or test specimen, high thermal conductive paste (4.5 W/m K) was used. The cartridge heaters are, 6.35 mm in diameter and 37.5 mm in length. The copper block’s geometry was such designed to provide approximately one-dimensional heat transfer near the test section. In order to measure the surfaces’ temperature a T-type thermocouple was used. This thermocouple was fixed into a hole, 3 mm beneath the surface by using a high thermal conductive paste. The Fourier’s law of heat conduction in conical shapes has been employed to extrapolate the surface temperature. The test chamber and the cooper block were both thermally insulated to prevent heat loss to the surrounding environment. The test section was positioned at the bottom of a cylindrical transparent container so as to observe the boiling phenomena. Deionized (DI) water was employed as the working fluid in all the pool boiling experiments. The test section used in this study was made of aluminum alloy 2011 and copper as a frustum with a height of 10 mm while the larger and smaller radii of cone are 24 mm and 20 mm, respectively (Fig. 1). 2.2. Preparation of the test section Initially two copper samples and one aluminum sample were prepared and their surfaces which boiling test will be run on them were polished by 1200 grit SiC paper and washed with DI water and acetone. To make aluminized coating surface on the copper sample, a powder consisting of 88 wt.% AL2O3, 10 wt.% Al as inter material and 2 wt.% NH4Cl as an activator were used. Aluminizing was performed in an austenitic stainless steel container which was sealed by using a paste of kaoline and liquid sodium silicate. Thereafter, heating process was carried out for 3 h at 850 °C. Then the specimen was cooled gradually in the oven. Finally, in order to eliminate the surface polish effects on the experiment, aluminized surface was polished in the same manner as aluminum and copper surfaces.
Fig. 2. (a) Top view of aluminized surface (b) cross section of aluminized surface.
Table 2 Comparison of measured values for contact angle, CHF, and predicted CHF values. Surface
Aluminum (Al) Copper (Cu) Aluminized
Advancing contact angle (°) 55 58 66
CHF (Exp.) (kW/m2)
1610 896 1227
Kandlikar correlation, Eq. (2) (kW/m2) Predicted CHF
Error (%)
1137 1090 977
29.4 21.7 20.3
2.3. Test procedure Considering that surface characteristics may have principal roles in boiling heat transfer features, contact angle on all specimens was measured. Measuring contact angle was placed after cleaning entire specimens by acetone. All tests and measurements were carried out at room conditions, i.e. 23 °C and atmospheric pressure of 82,526 Pa (616 mmHg). At this pressure the saturation temperature of water is 94.1 °C [18]. Prior to each test, the surface of the test section was cleaned with acetone and washed by DI water. During the tests, heat flux was gradually increased until critical heat flux was occurred,
Table 1 Uncertainty values of wall superheat and heat flux for the copper surface. Wall superheat (°C) Uncertainty of DT% Heat flux (kW/m2) Uncertainty of heat flux%
6.6 ±0.3 90 ±3.9
10.4 ±0.5 214 ±4.1
14.0 ±0.9 389 ±4.9
18.0 ±1.3 616 ±5.6
Fig. 3. Heat flux vs. wall superheat. 21.5 ±1.8 896 ±6.6
where a jump in temperature was monitored. By measuring the applied voltage and current, i.e. power input to the heaters, heat transfer coefficient can be calculated using Eq. (1):
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2.4. Uncertainty analysis Considering that in addition to the errors in individual measurements, there are random errors that need to be considered, uncertainty analysis is an unavoidable part of any experimental investigation. To perform this analysis, systematic errors in measuring devices for temperature, voltage, length, and current were evaluated; then uncertainty analysis [20] was performed for wall superheat and heat flux and the results are presented in Table 1. Uncertainty levels for the wall superheat and heat flux were less than 2% and 7%, respectively. These processes including measuring the contact angles, boiling tests, and measuring heat transfer were repeated three times and the averaged data for each sample are reported. The standard deviations of different contact angle measurements were in the range of 3° to 6°.
Fig. 4. Heat transfer coefficient vs. heat flux.
h¼
Q =A T w T sat
ð1Þ
where Q is the power input to the heaters. Heat losses are negligible due to good insulation. Heat losses in a similar setup which was examined by one of the authors accounted for only 6% of the consumed power near the critical heat flux [19]. Considering that better quality insulation materials have been employed in the current investigation, lower heat losses are expected. However, the estimated heat loss has been taken out from the measured power in the calculations of heat transfer rate and CHF.
3. Results and discussion 3.1. Quality of coated surface In order to identify the quality of the aluminized surface, one can study the Back Scattering Electron (BSE) mode on a Scanning Electron Microscope (SEM). The top view of aluminized surface is shown in Fig. 2a. In the BSE image, the dark spots are indicative of heavier element which is copper in the current case. The low numbers of these spots show that aluminum has almost entirely
Fig. 5. 3D AFM image for (a) and (b) aluminum (c) and (d) aluminized (e) and (f) copper.
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Test case
Average height (nm)
Roughness (nm)
Area ratio real area/projected area
section of aluminized specimen after polishing. By using this figure one can find out that the thickness of aluminized layer is less than 50 lm and has nearly uniform thickness along the surface. Both of these features are indicative of good quality of coating process.
Aluminum (Al) Copper (Cu) Aluminized
41 62 406
12 8 41
1.6 1.3 1.1
3.2. Contact angle
Table 3 Summary of surface properties from AFM image.
covered the copper surface and aluminized coating has been implemented correctly. On the other hand, Fig 2b shows the cross
Contact angle and surface wettability for three specimens, i.e. aluminum, copper, and aluminized copper, were measured. Results of measurements are listed in Table 2. Contact angle readings are
Fig. 6. Height of proses along transverse of surfaces for (a) aluminum (b) aluminized (c) copper.
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fairly close for untreated aluminum and copper specimens whereas the reading for the aluminized copper specimen is relatively higher which is accompanied by lower wettability. Despite close readings of contact angles for untreated aluminum and copper specimens, the CHF value for aluminum surface is about 80% higher than that of copper surface. This indicates that contact angle is not the only controlling parameter on CHF. The CHF for the aluminized surface is 37% higher than that of copper surface.
The role of contact angle can be further visualized by employing a well-known empirical correlation. This correlation is usually used to evaluate the CHF and was derived by Kandlikar [21] as Eq. (2):
q00CHF ¼ q1=2 g hfg
1=2 h i1=4 1 þ cos h 2 p þ ð1 þ coshÞcosu g rðqf qg Þ 16 p 4 ð2Þ
where u represents the heater slope angle relative to the horizontal.
Fig. 7. Cumulative height distribution of surfaces for (a) aluminum (b) aluminized (c) copper.
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Table 2also summarizes the predictions of CHF based on this correlation. It can be seen that Kandlikar’s correlation predicts CHF values in the range of ±30% of the experimental values. The maximum difference between the experimental results and the Kandlikar’s prediction is about 30% which corresponds to the aluminum sample. Considering that contact angle may not be the only parameter which governs CHF, the difference between the correlation predictions and the measured quantities is not surprising. Previous measurements of the advancing and receding contact angle for specimens showed that there is no considerable difference between them and in the worst case they differ by about 5° [1]. Considering that the Kandlikar’s model is based upon the receding contact angle, the 5° difference between the advancing and receding contact angles will lead to at most 6% differences in the Kandlikar’s correlation prediction which is negligible. 3.3. Boiling curves Fig. 3 shows heat flux vs. wall superheat for the three surfaces. Results show that at a specific heat flux, aluminized surface has higher wall superheat compared to aluminum and copper surfaces. Fig. 4 depicts changes of heat transfer coefficient vs. heat flux. It can be observed that heat transfer coefficient for both the copper and aluminized surfaces follow the same trend and are so close to each other. It is interesting to note that CHF on aluminized surface has been delayed until 1227 kW/m2 which is about %37 higher than that of the copper. This makes the aluminized surface more favorable compared with copper surface.
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sured for the aluminized surface. This difference is depicted in Fig. 6. Considering the fact that smaller contact angle is expected to be accompanied by higher CHF, the value of CHF for copper surface in comparison with aluminized surface is unexpected. Taking other characteristics into account, one possible explanation to this ambiguity is as following. As Fig. 6 shows, the copper surface is fairly flat and has few ups and downs. The vapor generated in the nucleation sites on a fairly flat surface with few narrow peaks and valleys can join to each other to overcast the surface easily, hence lower CHF. A combination of high area ratio, low roughness and high average height imply that the proses are too small to work as nucleation sites and vapor is just trapped in them which finally leads to a low CHF value. Aluminizing the copper surface enhances the peaks and valleys. These ups and downs which work as nucleation sites postpone CHF to the higher values of heat flux. Considering that apart from contact angle and roughness, other factors such as chemical constituency and porosity are also likely to affect CHF, more investigation is needed in further studies. In Fig. 7 cumulative distribution of height for aluminum, aluminized and copper specimens have been shown. Based on this figure it is readily observed that for Aluminum and copper specimens height distribution is relatively homogenous and it is scattered between 20 and 100 nm, whereas, in aluminized sample height ranges between 200 and 500 nm which is remarkably higher than raw aluminum and copper surfaces. This distribution of valleys and peaks on aluminized surfaces is an indication of enhanced nucleation sites which can interfere with CHF happening. 4. Conclusion
3.4. Surface roughness In order to survey the surfaces’ characteristics precisely, atomic force microscope (AFM) (Device Model: Bruker Nanos 1.1) was used. Some features such as real surface area and surface roughness can be measured by using AFM images. Hence three different surfaces, i.e. aluminum surface, copper surface and aluminized surface, have been compared with each other by using the 3-D pictures of AFM. Three-D images are shown in Fig. 5 for squares of 20 and 1 lm in lengths. Results for these three surfaces have been extracted for squares of 20 micron length and have been summarized in Table 3. The average height is defined as [22]:
zðN; MÞ ¼
M X N 1 X zðx; yÞ NM y¼1 x¼1
ð3Þ
while the roughness is defined as:
Ra ðN; MÞ ¼
M X N 1 X ðzðx; yÞ zðN; MÞÞ NM y¼1 x¼1
ð4Þ
The aluminum and copper specimens have relatively close roughness and contact angles, whereas the aluminized surface has higher roughness and higher contact angle. It is worth to mention although Wenzel’s equation provides a direct relation between roughness and contact angle and indicates increasing roughness leads to a reduction in the apparent contact angle and it is in contrast with our achievements, there are clues that cast doubts on this equation. Gao and McCarthy [23] have mentioned that all of their data indicate that contact angle behavior (advancing, receding, and hysteresis) is determined by interactions of the liquid and the solid at the three-phase contact line alone and that the interfacial area within the contact perimeter is irrelevant which questions the Wenzel’s equation. In addition, although the average height for copper and aluminum surfaces are relatively low and close to each other, a higher average height has been mea-
In the present investigation, aluminization as a method of surface treatment was applied to a copper surface and the boiling heat transfer characteristic of the treated surface was acquired and compared with those of untreated copper and aluminum surfaces. Aluminization combines the advantageous high thermal conductivity of copper and relatively high critical heat flux of aluminum surfaces but eliminates undesirable oxidation of copper. It also expands the wall superheat at which critical heat flux occurs while leaves heat transfer coefficient almost intact. AFM images of the surfaces of 3 specimens were studied. These images revealed that aluminizing treatment increases microscopic valleys and peaks on the surface which might be one of the reasons for higher CHF. Measurements of contact angle and critical heat flux were performed on copper, aluminum, and aluminized copper surfaces. The CHF increased 37% for the aluminized copper surface with respect to untreated copper surface. Further investigation is needed to find out the most affecting factors on CHF. Conflict of interest None declared. References [1] D. Saeidi, A.A. Alemrajabi, Experimental investigation of pool boiling heat transfer and critical heat flux of nanostructured surfaces, Int. J. Heat Mass Transfer 60 (2013) 440–449. [2] H. Jo, H.S. Ahn, S. Kang, M.H. Kimb, A study of nucleate boiling heat transfer on hydrophilic, hydrophobic and heterogeneous wetting surfaces, Int. J. Heat Mass Transfer 54 (2011) 5643–5652. [3] N.E. Fagerholm, A.R. Ghazanfari, K. Kivioja, E. Jarvinen, Boiling heat transfer performance of plain and porous tubes in falling film flow of refrigerant R114, Wärme - und Stoffübertragung 21 (6) (1987) 343–353. [4] S.B. White, A.J. Shih, K.P. Pipe, Boiling surface enhancement by electrophoretic deposition of particles from a nanofluid, Int. J. Heat Mass Transfer 54 (2011) 4370–4375. [5] R. Pastuszko, Pool boiling for extended surfaces with narrow tunnels– visualization and a simplified model, Exp. Therm. Fluid Sci. 38 (2012) 149–164.
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