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Measurements of the contact angle between R134a and both aluminum and copper surfaces
Experimental Thermal and Fluid Science 31 (2007) 979–984 www.elsevier.com/locate/etfs
Measurements of the contact angle between R134a and both aluminum and copper surfaces Bhavin Vadgama, Daniel K. Harris
*
Department of Mechanical Engineering, College of Engineering, Auburn University, 201 Ross Hall, Auburn University, AL 36849-5341, USA Received 3 August 2006; accepted 8 October 2006
Abstract Measurements of quasi-static advancing contact angles of refrigerant R134a on copper and aluminum surfaces are reported over a temperature range from 0 C to 80 C. The metal surfaces tested were aluminum (alloy 3003) and copper (alloy 101) plates. Measurements were done using a direct optical observation technique where the liquid meniscus at the surface of a vertical plate was captured using a high magnification camera system. The contact angle of solid–liquid interface was deduced by enhancing and manipulating the digital image using solid modeling software by drawing a tangent line to the meniscus at the intersection location of the solid, liquid and vapor. Values of the contact angle were found to vary between 8.3 and 5.6 for aluminum and between 5.1 and 6.5 for copper when the temperature rose from 0 C to 80 C. Maximum standard deviation amongst the measured values of contact angles was 1.3. 2006 Elsevier Inc. All rights reserved. Keywords: R134a; Contact angle; Heat pipes; Refrigerants; Contact angle measurements; Wettability; R134a properties
1. Introduction and background Contact angle is an extremely important parameter that defines the wetting phenomenon important in several industrial processes as well as in our daily life. It can be used to know how well a fluid will wet a particular surface, penetration of liquids in porous structures, fluid–fluid displacement as well as to characterize solid surfaces [1]. In most of the heat transfer processes involving a twophase system, vaporization and condensation occurs at the liquid–vapor interface. The energy required for the phase change is transferred through the liquid and/or vapor in contact with the solid boundary of this two-phase system. The way these phases are in contact with the boundary or in other words, the degree to which the liquid or the vapor wets this solid boundary will, as one would expect, affect the heat transfer [2]. The influence of contact *
Corresponding author. Tel.: +1 334 844 3337; fax: +1 334 844 3307. E-mail addresses: [email protected], [email protected] (D.K. Harris). 0894-1777/$ - see front matter 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2006.10.010
angle on nucleate boiling, nucleation site density, bubble departure diameter and the critical heat flux have been well studied [3,4]. Hong et al. [5] studied the effects of surface oxidation which enhances the wettability on pool boiling of R11, and found that oxidation increased the heat transfer coefficient. The influence of contact angle of the working fluid on the performance of heat pipes is evident considering the high heat flux applications for which they are designed. Recently, R134a has been studied as a candidate fluid for use in heat pipes [6,7]. A heat pipe gives large heat transport rates with relatively small temperature differences by use of evaporation and condensation within a closed system containing a saturated liquid–vapor mixture. The heat pipe absorbs heat in the evaporator section and vaporizes the working fluid. The vapor pressure then drives the vapor to the condenser section where it condenses; thereby transporting the thermal energy through latent heat interactions. The liquid is returned to the evaporator, against a natural pressure gradient, through a porous wick structure. A liquid meniscus formed within the wick creates
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B. Vadgama, D.K. Harris / Experimental Thermal and Fluid Science 31 (2007) 979–984
Nomenclature F g M P RTD V Dq
force exerted on a plate gravitational constant mass of a plate perimeter of a plate resistance temperature detector volume of liquid displaced is the density difference between the two liquid phase and surrounding air.
the capillary pressure which pumps the liquid back to the evaporator section. This osmotic pressure depends on the contact angle between the liquid and the surface of the wick material. In general, the lower the contact angle, the higher the osmotic pressure. In fact, the exact behavior is that the osmotic pressure varies directly with the cosine of the contact angle. If the capillary pumping head is not larger than the total pressure difference within the heat pipe between the evaporator and condenser sections, the evaporator section will dry out and the heat pipe will fail to operate. One of the basic factors that govern the capillarity or the return of the working fluid from condenser to the evaporator is the contact angle of the working fluid. This makes it is necessary to have the values of contact angle available for a better prediction of the heat pipe performance and optimization of the heat pipe. The contact angle values for more common working fluids including refrigerants that were developed in 1930s such as R11, R12, and R22 are well reported in the open archival literature [5,8,9]. However, the use of R134a (introduced as a commercial refrigerant in 1990) as the working fluid for heat pipes has given rise to an interest in R134a contact angles, which are not easily found in the literature. Hence, the measurements of contact angle of R134a were undertaken in this work. 2. Measurement techniques Contact angle h is defined [2] as the angle between the liquid–vapor interface and the solid surface, measured through the liquid at the point on the surface where all the three phases meet. The familiar Young’s equation which defines contact angle is given by Eq. (1) where, rsv, rsl, and rlv are interfacial surface tensions at the solid–vapor, solid– liquid and liquid–vapor interfaces, respectively, rsv rsl cos h ¼ ð1Þ rlv The contact angle is not a fluid property but rather a function of the fluid’s free surface energy as compared to the solid surface and vapor surface energies [10]. A smaller contact angle means that more of the fluid spreads over an area for a given fluid volume; hence the fluid has a high wettability on that surface. Fluids with h = 0 are called
h rsv rsl rlv
contact angle interfacial surface tensions at the solid–vapor interface interfacial surface tension at the solid–liquid interface interfacial surface tension at the liquid–vapor interface
highly wetting and with h = 180 are called highly non-wetting. In case of heat pipes, maximum capillary pressure is reached when h = 0 for a working fluid [11]. The familiar Young’s equation relates the contact angle, interfacial surface tension, and capillary pressure, to the pore size. Young’s equation assumes the solid surface to be ideal (smooth and homogeneous), which is usually not the case. Values of observed (or apparent) contact angles are highly sensitive to the surface finish [1,12,13]. The angle given by Young’s equation is also known as the intrinsic contact angle [13]. For surfaces that are real or surfaces that are rough and/or heterogeneous, the contact angle depends on the point where it is measured. Such a contact angle observation is known as the actual contact angle [13]. The angle that is measured in this work is known as the apparent contact angle which is defined as the angle between the tangent to the liquid–vapor interface, as seen by a comparatively low magnification optical device, and the solid surface [13]. Several methods have been proposed and used to measure contact angles depending on the geometry and the materials in question [14–23], each of them with advantages and disadvantages. One of the common methods used is the tilting plate method [14]. In this method a plate is partially immersed in the liquid and the meniscus is formed at both the faces of the plate. The plate is then tilted until the liquid surface on either of the sides is perfectly horizontal up to the solid–liquid interface. The angle of the plate with the flat liquid surface at this point is the contact angle. The accuracy of this method depends on the means utilized to detect the flatness of the meniscus. This method gives good reproducibility however the distinction between advancing and receding contact angles is difficult. Another well known method is the Wilhelmy Plate method which is a force based method. A plate which is suspended from a balance is brought in contact with a liquid and the downward force exerted by the liquid on the plate is measured [14]. This force is related to the contact angle by F ¼ P rsl cos h DqgV þ mg
ð2Þ
where P is the perimeter of the plate, rsl is the liquid surface tension, V is the volume of liquid displaced, m is the mass of the plate, Dq is the density difference between the fluid’s
B. Vadgama, D.K. Harris / Experimental Thermal and Fluid Science 31 (2007) 979–984
liquid phase and the surrounding ambient. The accuracy of this method is high since the measurement of contact angle is reduced to the measurement of a force which can be done with comparatively higher accuracies. However, the reproducibility of the measurements is sensitive to the consistency in plate geometry and composition. The simplest and most widely used method is the sessile drop method. A drop of the fluid is placed on the test surface and the angles are determined with a telescope equipped with a goniometer eyepiece or by taking a picture and then measuring the contact angle later. A major advantage of this method is that it requires very small quantities of liquid and small test surface. A disadvantage with this method is that it is not highly accurate with an overall error of ±2 [14] and is subject to the observer. Several modifications have been proposed to improve the accuracy and reproducibility of this method [17,24,25]. Another added difficulty with this method is when the fluid does not exist in liquid phase at lab pressures and temperatures, as is the case with R134a. 3. Experiment methodology All the methods discussed above are most easily implemented for fluids that are liquid at room conditions. However, due to the thermodynamic properties of R134a, these methods cannot be used without suitable modifications to the setup. Any method that involves movement of the test surface will be complicated and/or expensive to incorporate in a closed system with R134a. Use of such equipment was out of the scope of this work and hence, a more appropriate method was sought. In this work, the measurements were done by direct visual inspection through a view port in a pressure vessel. This method is based on a visual observation of the capillary rise on a vertical plate [14] and a polynomial fitting approach [17], used in combination. A digital image of the liquid meniscus formed at the test metal surface was captured by a close focus microscopic lens and camera. The shape of the meniscus from the digital image was traced by a polynomial curve and contact angles were estimated directly by drawing a tangent to this polynomial at the metal surface using solid modeling software. Measurements were done over the temperature range of 0–80 C with increments of 10 C. Laboratory grade R134a manufactured by Aspen and containing no oil was used in this experimental program. The test metal surfaces were aluminum (alloy 3003) and copper (alloy 101) plates of dimensions 80 mm (L) · 10 mm (W) · 1 mm (t). These test surfaces were deoxidized and degreased in an ultrasonic bath for 15 min at 44 C (recommended by the manufacturer) using formulated cleaning solution by Branson. Surface finishes of these surfaces were not measured and were assumed to be identical to that of a typical heat pipe container. The pressure variation of R134a over the temperature range is significantly high and hence the test had to be carried out in a pressure vessel. A cylindrical stainless
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steel pressure vessel of dimensions 300 (OD) · 100 (ID) · 600 (L) with a maximum pressure rating of 10 000 psi was used for this purpose. The test metal samples were cleaned in an ultrasonic bath prior to testing so as to remove any oxidation or grease from the surface. Fig. 1 shows the experimental setup used. A clean test surface was suspended inside the pressure vessel. The pressure vessel had flat glass view ports on opposite sides of the cylindrical face. The test surface was placed such that only its thickness was visible through the view ports (Fig. 1 – exploded view). The pressure vessel was then sealed with a cover which contained an RTD for temperature measurement, a pressure gauge, a charging port and a bleed port. It was then placed in a chiller bath set at 20 C while connected to the vacuum pump of the charging system and evacuated to a vacuum of 102 torr to remove any moisture as well as to facilitate the charging process. The boiling point of R134a at atmospheric pressure is 26 C. The temperature of the pressure vessel had to be lowered so that during the charging process the vessel was filled by liquid R134a rather than its vapor. R134a was then charged in the pressure vessel using a MasterCool Automotive Charging system through the charging port, which was a schrader valve commonly used in refrigeration systems. Once the pressure vessel was completely filled, it was removed from the chiller and mounted on a stand and leveled. Excess R134a was bled through the bleed port until the liquid level Pressure Vessel CCD Camera
A
35x Lens
Light Source
Stand View Port
A
Charging Port Left Meniscus
Test Surface
Right Meniscus Vapor R134a
Liquid R134a 1 mm Pressure Sensor
RTD
Exploded view of the view port
Section A-A
Fig. 1. Schematic of experimental setup.
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was visible through the view ports. The bleed port was located and the highest point in the system. A cooling system for the setup was unavailable once the pressure vessel was removed from the chiller however, the thermal mass of the pressure vessel allowed for a quasi-static rise in the temperature since the thermal mass of the vessel was three orders of magnitude greater than that of the fluid/plate combination. Once the liquid inside the pressure vessel reached 0 C, a digital image of the menisci formed on both the sides of the test surface was captured using a close focus microscopic lens having a 35· magnification which was mounted on a SONY DFW-V500 CCD camera. This continues as the system slowly rose to room temperature with data taken at several temperature levels. Once the system rose above room temperature a flexible heating tape was used to heat the system and an image was taken at every required temperature level. The same procedure was repeated for both test surfaces. Since the rise in the temperature and the pressure were quasi-static, the meniscus was also advancing in a quasi-static manner and hence the measured angle would be considered as a quasi-static advancing contact angle. 4. Data reduction Once all the photographs were captured, they were analyzed using photo editing software and CAD software (SolidEdge 14). The visible distortion of the liquid–vapor interface of the menisci and the edges of the test surface were sharpened (Fig. 2) using the photo editor. The vapor inside the pressure vessel and the meniscus that was formed at the view port surface tended to blur the view of the required meniscus at the test surface and so the line of sight of the camera was slightly moved up to bypass this distortion. It also reduced the uncertainty of tracing the actual meniscus. Using SolidEdge, the edges were traced along the meniscus using a splined curve (polynomial of order 3), and a tangent was drawn for each meniscus giving the contact angle reading (Fig. 3). Readings were taken on
Fig. 2. Digital image of the meniscus on the test sample.
Fig. 3. Contact angle measured by drawing tangent using solid edge.
both the left and the right side of the test surface and were averaged to eliminate any error due to a tilt of the test surface with respect to the R134a liquid level. For each temperature value the procedure of tracing the menisci and drawing the tangent was repeated five times so that the uncertainty in choosing the correct pixel while tracing the curve was reduced. These five readings for both left and right side were averaged out to final left and right values. Again, these left and right values for a given temperature were averaged to get final contact angle value. The maximum standard deviation of the obtained values was 1.3. Actual error in the measured values could not be measured since the interfacial surface tension data for R134a is not available. 5. Results Values of the measured contact angle for aluminum and copper for a temperature range 0–80 C are given in Tables 1 and 2, respectively. Contact angle values are plotted versus temperature for both aluminum and copper in Figs. 4 and 5, respectively. It can be seen that the average values of measured contact angles were in the range of 5.6–8.3 for aluminum and 5.1–6.5 for copper, meaning the refrigerant is highly wetting on both the test surfaces. Table 3 shows the values of reported contact angles of various refrigerants. These values were fairly comparable to those reported for R11 and R113 [12], as would be expected given the similarity of the chemistries. However, these values significantly differ from those reported in a similar work by Reale [8]. For copper, the values were slightly lower than that for aluminum, albeit within the margin of error reported in this work. This could be due to the difference in the surface finishes between the test-specimens used, or in the subjective observed contact angle. Furthermore, the variation in the values over the temperature range tested was not significant. This could be reasoned that the values were already very small for any noticeable change with temperature or the contact angle is not a strong function of temperature over the range tested.
B. Vadgama, D.K. Harris / Experimental Thermal and Fluid Science 31 (2007) 979–984
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Table 1 Measured contact angles values for R134a on aluminum 3003 Temperature (C)
10 20 30 40 50 60 70 80
Analysis-1
Analysis-2
Analysis-3
Analysis-4
Analysis-5
L
R
L
R
L
R
L
R
L
R
6.7 8.4 6.4 7.0 7.7 5.3 5.6 5.7
9.7 7.9 8.9 7.4 6.4 6.9 4.7 6.3
8.6 6.9 6.8 6.9 8.3 5.6 5.7 6.6
8.3 9.0 7.5 7.7 7.6 7.3 6.0 5.2
7.5 7.0 9.3 7.2 6.5 5.0 5.3 5.1
9.0 8.9 8.2 7.4 7.8 5.7 6.9 7.0
8.3 8.0 8.6 6.0 7.6 5.4 6.4 5.6
9.1 8.0 8.4 9.0 6.9 7.2 5.1 6.3
6.6 8.1 9.3 7.8 7.7 6.5 5.3 5.5
9.3 8.7 8.0 8.6 8.4 7.6 5.3 6.4
Lavg
Ravg
Average
7.5 7.7 8.1 7.0 7.6 5.6 5.7 5.7
9.1 8.5 8.2 8.0 7.4 6.9 5.6 6.2
8.3 8.1 8.1 7.5 7.5 6.3 5.6 6.0
Lavg
Ravg
Average
5.7 4.8 5.3 5.3 5.6 4.9 4.2 4.3
5.7 5.8 7.8 7.1 6.0 5.3 5.3 8.0
5.7 5.3 6.5 6.2 5.8 5.1 4.7 6.2
Table 2 Measured contact angles values for R134a on copper 101 Temperature (C)
0 10 20 30 40 50 60 80
Analysis-1
Analysis-2
Analysis-3
L
R
L
R
L
R
L
R
L
R
5.5 4.5 5.3 5.4 5.4 4.3 4.8 3.3
6.6 5.3 7.4 5.0 5.6 4.9 5.3 7.2
6.6 4.6 5.9 5.9 5.3 5.8 3.9 4.7
5.0 6.2 8.4 7.5 5.4 5.7 4.7 8.7
6.2 5.3 4.5 4.9 6.5 5.6 4.4 4.7
5.8 5.9 7.2 6.7 7.1 4.3 5.4 8.5
5.6 5.1 5.0 4.3 5.3 4.3 4.0 3.2
5.9 6.0 7.8 7.9 5.6 6.0 5.0 7.0
4.8 4.7 5.8 6.2 5.6 4.7 4.0 5.6
5.2 5.8 8.0 8.2 6.3 5.7 5.9 7.8
Contact Angle, θ (degrees)
Right Side
Analysis-5
Table 3 Reported contact angle values of other refrigerants
15 Left Side
Analysis-4
Combined Average
Realea [8]
10
Copper Aluminum a b
5
Hong et al. Alb [12]
R11
R12
R22
R11
R113
29–38 –
27–35 –
20–27 –
3–5 2–7
3–6 4–6
Reported over a temperature range of 0–40 C. Reported over different surface finishes of test surfaces.
6. Conclusions 0 0
20
40 Temperature (oC)
60
80
Fig. 4. Measured contact angle values for aluminum 3003.
Contact Angle, θ (degrees)
15 Left Side
Right Side
Combined Average
10
Actual error in the measured values of contact angle could not be estimated due to unavailability of the interfacial surface tension data. Furthermore, the repeatability of measured values is highly dependent on the user. Reported values of contact angles for R134a still provides for a good aid in the prediction of performance of a heat pipe. Data shows that the wettability of R134a is comparable to other refrigerants and can be a promising working fluid for heat pipes in its operating temperature range. Receding contact angles were not reported in this work since the movement of the test surface in the experimental setup used was not possible.
5
Acknowledgements 0 0
20
40 Temperature (oC)
60
Fig. 5. Measured contact angle values for copper 101.
80
This work was performed partially under a contract with Dana Fluids Systems Products. Sincere appreciation is expressed to Christopher Kitchens and Dr. Christopher Roberts of the Department of Chemical Engineering at
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Auburn University for providing the experimental pressure vessel, their valuable knowledge and time.
[14]
References [15] [1] R.J. Good, Contact angles and the surface free energy of solids, in: R.J.S. Good, R.R. Stromberg (Eds.), Surface and Colloid Science, vol. 11, Plenum Press, New York, 1979. [2] V.P. Carey, Liquid–Vapor Phase-Change, Taylor & Francis, Phenomena, 1992. [3] W. Tong, A. Bar-Cohen, T.W. Simon, S.M. You, Contact angle effects on boiling incipience of highly-wetting liquids, International Journal of Heat and Mass Transfer 33 (1990) 91–103. [4] C.H. Wang, V.K. Dhir, Effect of surface wettability on active nucleation site density during pool boiling of water on a vertical surface, in: Proceedings of the 28th National Heat Transfer Conference, July 28–31 1991, Minneapolis, MN, USA, 1991. [5] K.T. Hong, H.E. Imadojemu, R.L. Webb, Pool boiling of R-11 refrigerant and water on oxidized enhanced tubes, in: Proceedings of the 6th AIAA/ASME Thermophysics and Heat Transfer Conference. Part 3 (of 10), Jun 20–23 1994, Colorado Springs, CO, USA, 1994. [6] T. Kobayashi, T. Ogushi, S. Haga, E. Ozaki, M. Fujii, Heat transfer performance of a flexible looped heat pipe using R134a as a working fluid: proposal for a method to predict the maximum heat transfer rate of FLHP, Heat Transfer – Asian Research 32 (2003) 306–318. [7] K.S. Ong, M. Haider-E-Alahi, Performance of a R-134a-filled thermosyphon, Applied Thermal Engineering 23 (2003) 2373–2381. [8] F. Reale, Measurements of liquid on solid contact angles of refrigerants, in: Symposium on Thermophysics Properties, 6th Proceeding, August 6–8 1973, 1973, pp. 376–386. [9] F. Reale, M. Cannaviello, Wetting and surface properties of refrigerants to be used in heat pipes, in: International Heat Pipe Conference, 2nd March 31st–April 2 1976, 1976, pp. 773–792. [10] D.P. Woodruff, The Solid–Liquid Interface, Cambridge University Press, 1973. [11] A. Faghri, Heat Pipe Science and Technology, Taylor & Francis, Washington, DC, 1995. [12] K.T. Hong, H. Imadojemu, R.L. Webb, Effects of oxidation and surface roughness on contact angle, Experimental Thermal and Fluid Science 8 (1994) 279–285. [13] A. Marmur, Equilibrium contact angles: theory and measurement, Colloids and Surfaces A: physicochemical and engineering aspects, in:
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Measurements of the contact angle between R134a and both aluminum and copper surfaces Bhavin Vadgama, Daniel K. Harris
*
Department of Mechanical Engineering, College of Engineering, Auburn University, 201 Ross Hall, Auburn University, AL 36849-5341, USA Received 3 August 2006; accepted 8 October 2006
Abstract Measurements of quasi-static advancing contact angles of refrigerant R134a on copper and aluminum surfaces are reported over a temperature range from 0 C to 80 C. The metal surfaces tested were aluminum (alloy 3003) and copper (alloy 101) plates. Measurements were done using a direct optical observation technique where the liquid meniscus at the surface of a vertical plate was captured using a high magnification camera system. The contact angle of solid–liquid interface was deduced by enhancing and manipulating the digital image using solid modeling software by drawing a tangent line to the meniscus at the intersection location of the solid, liquid and vapor. Values of the contact angle were found to vary between 8.3 and 5.6 for aluminum and between 5.1 and 6.5 for copper when the temperature rose from 0 C to 80 C. Maximum standard deviation amongst the measured values of contact angles was 1.3. 2006 Elsevier Inc. All rights reserved. Keywords: R134a; Contact angle; Heat pipes; Refrigerants; Contact angle measurements; Wettability; R134a properties
1. Introduction and background Contact angle is an extremely important parameter that defines the wetting phenomenon important in several industrial processes as well as in our daily life. It can be used to know how well a fluid will wet a particular surface, penetration of liquids in porous structures, fluid–fluid displacement as well as to characterize solid surfaces [1]. In most of the heat transfer processes involving a twophase system, vaporization and condensation occurs at the liquid–vapor interface. The energy required for the phase change is transferred through the liquid and/or vapor in contact with the solid boundary of this two-phase system. The way these phases are in contact with the boundary or in other words, the degree to which the liquid or the vapor wets this solid boundary will, as one would expect, affect the heat transfer [2]. The influence of contact *
Corresponding author. Tel.: +1 334 844 3337; fax: +1 334 844 3307. E-mail addresses: [email protected], [email protected] (D.K. Harris). 0894-1777/$ - see front matter 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2006.10.010
angle on nucleate boiling, nucleation site density, bubble departure diameter and the critical heat flux have been well studied [3,4]. Hong et al. [5] studied the effects of surface oxidation which enhances the wettability on pool boiling of R11, and found that oxidation increased the heat transfer coefficient. The influence of contact angle of the working fluid on the performance of heat pipes is evident considering the high heat flux applications for which they are designed. Recently, R134a has been studied as a candidate fluid for use in heat pipes [6,7]. A heat pipe gives large heat transport rates with relatively small temperature differences by use of evaporation and condensation within a closed system containing a saturated liquid–vapor mixture. The heat pipe absorbs heat in the evaporator section and vaporizes the working fluid. The vapor pressure then drives the vapor to the condenser section where it condenses; thereby transporting the thermal energy through latent heat interactions. The liquid is returned to the evaporator, against a natural pressure gradient, through a porous wick structure. A liquid meniscus formed within the wick creates
980
B. Vadgama, D.K. Harris / Experimental Thermal and Fluid Science 31 (2007) 979–984
Nomenclature F g M P RTD V Dq
force exerted on a plate gravitational constant mass of a plate perimeter of a plate resistance temperature detector volume of liquid displaced is the density difference between the two liquid phase and surrounding air.
the capillary pressure which pumps the liquid back to the evaporator section. This osmotic pressure depends on the contact angle between the liquid and the surface of the wick material. In general, the lower the contact angle, the higher the osmotic pressure. In fact, the exact behavior is that the osmotic pressure varies directly with the cosine of the contact angle. If the capillary pumping head is not larger than the total pressure difference within the heat pipe between the evaporator and condenser sections, the evaporator section will dry out and the heat pipe will fail to operate. One of the basic factors that govern the capillarity or the return of the working fluid from condenser to the evaporator is the contact angle of the working fluid. This makes it is necessary to have the values of contact angle available for a better prediction of the heat pipe performance and optimization of the heat pipe. The contact angle values for more common working fluids including refrigerants that were developed in 1930s such as R11, R12, and R22 are well reported in the open archival literature [5,8,9]. However, the use of R134a (introduced as a commercial refrigerant in 1990) as the working fluid for heat pipes has given rise to an interest in R134a contact angles, which are not easily found in the literature. Hence, the measurements of contact angle of R134a were undertaken in this work. 2. Measurement techniques Contact angle h is defined [2] as the angle between the liquid–vapor interface and the solid surface, measured through the liquid at the point on the surface where all the three phases meet. The familiar Young’s equation which defines contact angle is given by Eq. (1) where, rsv, rsl, and rlv are interfacial surface tensions at the solid–vapor, solid– liquid and liquid–vapor interfaces, respectively, rsv rsl cos h ¼ ð1Þ rlv The contact angle is not a fluid property but rather a function of the fluid’s free surface energy as compared to the solid surface and vapor surface energies [10]. A smaller contact angle means that more of the fluid spreads over an area for a given fluid volume; hence the fluid has a high wettability on that surface. Fluids with h = 0 are called
h rsv rsl rlv
contact angle interfacial surface tensions at the solid–vapor interface interfacial surface tension at the solid–liquid interface interfacial surface tension at the liquid–vapor interface
highly wetting and with h = 180 are called highly non-wetting. In case of heat pipes, maximum capillary pressure is reached when h = 0 for a working fluid [11]. The familiar Young’s equation relates the contact angle, interfacial surface tension, and capillary pressure, to the pore size. Young’s equation assumes the solid surface to be ideal (smooth and homogeneous), which is usually not the case. Values of observed (or apparent) contact angles are highly sensitive to the surface finish [1,12,13]. The angle given by Young’s equation is also known as the intrinsic contact angle [13]. For surfaces that are real or surfaces that are rough and/or heterogeneous, the contact angle depends on the point where it is measured. Such a contact angle observation is known as the actual contact angle [13]. The angle that is measured in this work is known as the apparent contact angle which is defined as the angle between the tangent to the liquid–vapor interface, as seen by a comparatively low magnification optical device, and the solid surface [13]. Several methods have been proposed and used to measure contact angles depending on the geometry and the materials in question [14–23], each of them with advantages and disadvantages. One of the common methods used is the tilting plate method [14]. In this method a plate is partially immersed in the liquid and the meniscus is formed at both the faces of the plate. The plate is then tilted until the liquid surface on either of the sides is perfectly horizontal up to the solid–liquid interface. The angle of the plate with the flat liquid surface at this point is the contact angle. The accuracy of this method depends on the means utilized to detect the flatness of the meniscus. This method gives good reproducibility however the distinction between advancing and receding contact angles is difficult. Another well known method is the Wilhelmy Plate method which is a force based method. A plate which is suspended from a balance is brought in contact with a liquid and the downward force exerted by the liquid on the plate is measured [14]. This force is related to the contact angle by F ¼ P rsl cos h DqgV þ mg
ð2Þ
where P is the perimeter of the plate, rsl is the liquid surface tension, V is the volume of liquid displaced, m is the mass of the plate, Dq is the density difference between the fluid’s
B. Vadgama, D.K. Harris / Experimental Thermal and Fluid Science 31 (2007) 979–984
liquid phase and the surrounding ambient. The accuracy of this method is high since the measurement of contact angle is reduced to the measurement of a force which can be done with comparatively higher accuracies. However, the reproducibility of the measurements is sensitive to the consistency in plate geometry and composition. The simplest and most widely used method is the sessile drop method. A drop of the fluid is placed on the test surface and the angles are determined with a telescope equipped with a goniometer eyepiece or by taking a picture and then measuring the contact angle later. A major advantage of this method is that it requires very small quantities of liquid and small test surface. A disadvantage with this method is that it is not highly accurate with an overall error of ±2 [14] and is subject to the observer. Several modifications have been proposed to improve the accuracy and reproducibility of this method [17,24,25]. Another added difficulty with this method is when the fluid does not exist in liquid phase at lab pressures and temperatures, as is the case with R134a. 3. Experiment methodology All the methods discussed above are most easily implemented for fluids that are liquid at room conditions. However, due to the thermodynamic properties of R134a, these methods cannot be used without suitable modifications to the setup. Any method that involves movement of the test surface will be complicated and/or expensive to incorporate in a closed system with R134a. Use of such equipment was out of the scope of this work and hence, a more appropriate method was sought. In this work, the measurements were done by direct visual inspection through a view port in a pressure vessel. This method is based on a visual observation of the capillary rise on a vertical plate [14] and a polynomial fitting approach [17], used in combination. A digital image of the liquid meniscus formed at the test metal surface was captured by a close focus microscopic lens and camera. The shape of the meniscus from the digital image was traced by a polynomial curve and contact angles were estimated directly by drawing a tangent to this polynomial at the metal surface using solid modeling software. Measurements were done over the temperature range of 0–80 C with increments of 10 C. Laboratory grade R134a manufactured by Aspen and containing no oil was used in this experimental program. The test metal surfaces were aluminum (alloy 3003) and copper (alloy 101) plates of dimensions 80 mm (L) · 10 mm (W) · 1 mm (t). These test surfaces were deoxidized and degreased in an ultrasonic bath for 15 min at 44 C (recommended by the manufacturer) using formulated cleaning solution by Branson. Surface finishes of these surfaces were not measured and were assumed to be identical to that of a typical heat pipe container. The pressure variation of R134a over the temperature range is significantly high and hence the test had to be carried out in a pressure vessel. A cylindrical stainless
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steel pressure vessel of dimensions 300 (OD) · 100 (ID) · 600 (L) with a maximum pressure rating of 10 000 psi was used for this purpose. The test metal samples were cleaned in an ultrasonic bath prior to testing so as to remove any oxidation or grease from the surface. Fig. 1 shows the experimental setup used. A clean test surface was suspended inside the pressure vessel. The pressure vessel had flat glass view ports on opposite sides of the cylindrical face. The test surface was placed such that only its thickness was visible through the view ports (Fig. 1 – exploded view). The pressure vessel was then sealed with a cover which contained an RTD for temperature measurement, a pressure gauge, a charging port and a bleed port. It was then placed in a chiller bath set at 20 C while connected to the vacuum pump of the charging system and evacuated to a vacuum of 102 torr to remove any moisture as well as to facilitate the charging process. The boiling point of R134a at atmospheric pressure is 26 C. The temperature of the pressure vessel had to be lowered so that during the charging process the vessel was filled by liquid R134a rather than its vapor. R134a was then charged in the pressure vessel using a MasterCool Automotive Charging system through the charging port, which was a schrader valve commonly used in refrigeration systems. Once the pressure vessel was completely filled, it was removed from the chiller and mounted on a stand and leveled. Excess R134a was bled through the bleed port until the liquid level Pressure Vessel CCD Camera
A
35x Lens
Light Source
Stand View Port
A
Charging Port Left Meniscus
Test Surface
Right Meniscus Vapor R134a
Liquid R134a 1 mm Pressure Sensor
RTD
Exploded view of the view port
Section A-A
Fig. 1. Schematic of experimental setup.
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was visible through the view ports. The bleed port was located and the highest point in the system. A cooling system for the setup was unavailable once the pressure vessel was removed from the chiller however, the thermal mass of the pressure vessel allowed for a quasi-static rise in the temperature since the thermal mass of the vessel was three orders of magnitude greater than that of the fluid/plate combination. Once the liquid inside the pressure vessel reached 0 C, a digital image of the menisci formed on both the sides of the test surface was captured using a close focus microscopic lens having a 35· magnification which was mounted on a SONY DFW-V500 CCD camera. This continues as the system slowly rose to room temperature with data taken at several temperature levels. Once the system rose above room temperature a flexible heating tape was used to heat the system and an image was taken at every required temperature level. The same procedure was repeated for both test surfaces. Since the rise in the temperature and the pressure were quasi-static, the meniscus was also advancing in a quasi-static manner and hence the measured angle would be considered as a quasi-static advancing contact angle. 4. Data reduction Once all the photographs were captured, they were analyzed using photo editing software and CAD software (SolidEdge 14). The visible distortion of the liquid–vapor interface of the menisci and the edges of the test surface were sharpened (Fig. 2) using the photo editor. The vapor inside the pressure vessel and the meniscus that was formed at the view port surface tended to blur the view of the required meniscus at the test surface and so the line of sight of the camera was slightly moved up to bypass this distortion. It also reduced the uncertainty of tracing the actual meniscus. Using SolidEdge, the edges were traced along the meniscus using a splined curve (polynomial of order 3), and a tangent was drawn for each meniscus giving the contact angle reading (Fig. 3). Readings were taken on
Fig. 2. Digital image of the meniscus on the test sample.
Fig. 3. Contact angle measured by drawing tangent using solid edge.
both the left and the right side of the test surface and were averaged to eliminate any error due to a tilt of the test surface with respect to the R134a liquid level. For each temperature value the procedure of tracing the menisci and drawing the tangent was repeated five times so that the uncertainty in choosing the correct pixel while tracing the curve was reduced. These five readings for both left and right side were averaged out to final left and right values. Again, these left and right values for a given temperature were averaged to get final contact angle value. The maximum standard deviation of the obtained values was 1.3. Actual error in the measured values could not be measured since the interfacial surface tension data for R134a is not available. 5. Results Values of the measured contact angle for aluminum and copper for a temperature range 0–80 C are given in Tables 1 and 2, respectively. Contact angle values are plotted versus temperature for both aluminum and copper in Figs. 4 and 5, respectively. It can be seen that the average values of measured contact angles were in the range of 5.6–8.3 for aluminum and 5.1–6.5 for copper, meaning the refrigerant is highly wetting on both the test surfaces. Table 3 shows the values of reported contact angles of various refrigerants. These values were fairly comparable to those reported for R11 and R113 [12], as would be expected given the similarity of the chemistries. However, these values significantly differ from those reported in a similar work by Reale [8]. For copper, the values were slightly lower than that for aluminum, albeit within the margin of error reported in this work. This could be due to the difference in the surface finishes between the test-specimens used, or in the subjective observed contact angle. Furthermore, the variation in the values over the temperature range tested was not significant. This could be reasoned that the values were already very small for any noticeable change with temperature or the contact angle is not a strong function of temperature over the range tested.
B. Vadgama, D.K. Harris / Experimental Thermal and Fluid Science 31 (2007) 979–984
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Table 1 Measured contact angles values for R134a on aluminum 3003 Temperature (C)
10 20 30 40 50 60 70 80
Analysis-1
Analysis-2
Analysis-3
Analysis-4
Analysis-5
L
R
L
R
L
R
L
R
L
R
6.7 8.4 6.4 7.0 7.7 5.3 5.6 5.7
9.7 7.9 8.9 7.4 6.4 6.9 4.7 6.3
8.6 6.9 6.8 6.9 8.3 5.6 5.7 6.6
8.3 9.0 7.5 7.7 7.6 7.3 6.0 5.2
7.5 7.0 9.3 7.2 6.5 5.0 5.3 5.1
9.0 8.9 8.2 7.4 7.8 5.7 6.9 7.0
8.3 8.0 8.6 6.0 7.6 5.4 6.4 5.6
9.1 8.0 8.4 9.0 6.9 7.2 5.1 6.3
6.6 8.1 9.3 7.8 7.7 6.5 5.3 5.5
9.3 8.7 8.0 8.6 8.4 7.6 5.3 6.4
Lavg
Ravg
Average
7.5 7.7 8.1 7.0 7.6 5.6 5.7 5.7
9.1 8.5 8.2 8.0 7.4 6.9 5.6 6.2
8.3 8.1 8.1 7.5 7.5 6.3 5.6 6.0
Lavg
Ravg
Average
5.7 4.8 5.3 5.3 5.6 4.9 4.2 4.3
5.7 5.8 7.8 7.1 6.0 5.3 5.3 8.0
5.7 5.3 6.5 6.2 5.8 5.1 4.7 6.2
Table 2 Measured contact angles values for R134a on copper 101 Temperature (C)
0 10 20 30 40 50 60 80
Analysis-1
Analysis-2
Analysis-3
L
R
L
R
L
R
L
R
L
R
5.5 4.5 5.3 5.4 5.4 4.3 4.8 3.3
6.6 5.3 7.4 5.0 5.6 4.9 5.3 7.2
6.6 4.6 5.9 5.9 5.3 5.8 3.9 4.7
5.0 6.2 8.4 7.5 5.4 5.7 4.7 8.7
6.2 5.3 4.5 4.9 6.5 5.6 4.4 4.7
5.8 5.9 7.2 6.7 7.1 4.3 5.4 8.5
5.6 5.1 5.0 4.3 5.3 4.3 4.0 3.2
5.9 6.0 7.8 7.9 5.6 6.0 5.0 7.0
4.8 4.7 5.8 6.2 5.6 4.7 4.0 5.6
5.2 5.8 8.0 8.2 6.3 5.7 5.9 7.8
Contact Angle, θ (degrees)
Right Side
Analysis-5
Table 3 Reported contact angle values of other refrigerants
15 Left Side
Analysis-4
Combined Average
Realea [8]
10
Copper Aluminum a b
5
Hong et al. Alb [12]
R11
R12
R22
R11
R113
29–38 –
27–35 –
20–27 –
3–5 2–7
3–6 4–6
Reported over a temperature range of 0–40 C. Reported over different surface finishes of test surfaces.
6. Conclusions 0 0
20
40 Temperature (oC)
60
80
Fig. 4. Measured contact angle values for aluminum 3003.
Contact Angle, θ (degrees)
15 Left Side
Right Side
Combined Average
10
Actual error in the measured values of contact angle could not be estimated due to unavailability of the interfacial surface tension data. Furthermore, the repeatability of measured values is highly dependent on the user. Reported values of contact angles for R134a still provides for a good aid in the prediction of performance of a heat pipe. Data shows that the wettability of R134a is comparable to other refrigerants and can be a promising working fluid for heat pipes in its operating temperature range. Receding contact angles were not reported in this work since the movement of the test surface in the experimental setup used was not possible.
5
Acknowledgements 0 0
20
40 Temperature (oC)
60
Fig. 5. Measured contact angle values for copper 101.
80
This work was performed partially under a contract with Dana Fluids Systems Products. Sincere appreciation is expressed to Christopher Kitchens and Dr. Christopher Roberts of the Department of Chemical Engineering at
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Auburn University for providing the experimental pressure vessel, their valuable knowledge and time.
[14]
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