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Nanofluids and critical heat flux, experimental and analytical study
Applied Thermal Engineering 29 (2009) 1281–1288
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Nanofluids and critical heat flux, experimental and analytical study M.N. Golubovic, H.D. Madhawa Hettiarachchi, W.M. Worek *, W.J. Minkowycz Mechanical and Industrial Engineering Department, University of Illinois at Chicago, 842 West Taylor, Chicago, IL 60607, USA
a r t i c l e
i n f o
Article history: Received 13 November 2007 Accepted 10 May 2008 Available online 20 May 2008 Keywords: Nanofluids Boiling Critical heat flux
a b s t r a c t In recent years, nanofluids have been attracting significant attention in the heat transfer research community. These fluids are obtained by suspending nanoparticles having sizes between 1 and 100 nm in regular fluids. It was found by several researchers that the thermal conductivity of these fluids can be significantly increased when compared to the same fluids without nanoparticles. Also, it was found that pool boiling critical heat flux increases in nanofluids. In this paper, our objective is to evaluate the impact of different nanoparticle characteristics including particle concentration, size and type on critical heat flux experimentally at saturated conditions. As a result, this work will document our experimental findings about pool boiling critical heat flux in different nanofluids. In addition, we will identify reasons behind the increase in the critical heat flux and present possible approaches for analytical modeling of critical heat flux in nanofluids at saturated conditions. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, nanofluids have attracted considerable attention due to their enhanced thermal properties. These novel types of heat transfer fluids are usually obtained by suspending small concentrations of nanoparticles in regular fluids. Choi [1] termed these solid/liquid suspensions ‘‘nanofluids”. The primary idea behind suspending such small particles in regular fluids is to increase as much as possible the thermal conductivity of such suspensions. Lee et al. [2]) demonstrated a maximum increase in the thermal conductivity of approximately 20% when CuO nanoparticles were suspended in ethylene glycol. Suspending copper nanoparticles, average diameter size less than 10 nm, Eastman et al. [3] were able to increase thermal conductivity of ethylene glycol up to 40%. In the same year, Choi et al. [4] were able to report 160% increase of a synthetic poly oil thermal conductivity when metallic multi-wall nanotubes where suspended. An interesting result was reported by You et al. [5]. You and his group investigated critical heat flux in nanofluids obtained by dispersing alumina oxide nanoparticles in distilled and deionized water. It was found that critical heat flux increased 200% for nanofluids that contained less than 0.005 g/l of alumina oxide nanoparticles. Vassallo et al. [15] presented the data that showed a marked increase, approximately 60%, in critical heat flux (CHF) for silica–water solutions compared to water. Kim et al. [16] showed increase in CHF for TiO2–water suspensions compared
* Corresponding author. E-mail addresses: [email protected] (M.N. Golubovic), [email protected] (H.D. Madhawa Hettiarachchi), [email protected] (W.M. Worek), [email protected] (W.J. Minkowycz). 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.05.005
to water. Bang and Chang [18] published similar findings for Al2O3–water suspensions on a plain surface in a pool. For the horizontal test section they obtained 32% increase in peak heat flux with addition of nanoparticles in water. Kim et al. [17,19] showed that buildup of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly improved the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces compared with the pure-water-boiled surfaces. They did not measure the latent heat of nanofluids or showed the influence of nanoparticle presence in nanofluids on CHF. Also, they did not show the influence of surface roughness on peak heat flux. Kim et al. [17,19] concluded that the reason for significant CHF enhancement of nano-fluids was the surface modification due to nanoparticle coating. Sefiane [20] suggested that the analysis of boiling heat transfer of nanofluids must account for the important effect of nanoparticles on the contact line region through the structural disjoining pressure to provide accurate interpretation of critical heat flux enhancement during boiling of nanofluids. The work reported in this paper is motivated by this increase in critical heat flux. Our goal is to find mechanisms through which the presence of nanoparticles in regular fluids affects the critical heat flux phenomenon. The critical heat flux occurs at the point of transition from nucleate boiling, characterized by moderate surface temperatures that go along with high heat fluxes, to film boiling characterized by high surface temperatures which is a consequence of vapor enveloping the heat source. As result of its importance, a vast amount of research work is done in this area. Models that deal with critical heat flux in pool boiling can be classified into four main groups:
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Nomenclature Ah, Aj
area of the heater that feeds jet, cross sectional area of the jet, m2 APS average particle size, nm CHF critical heat flux, W/m2 hfg latent heat, kJ/kgk radius of the jet, m Rj critical velocity of vapor in the jet, m/s UH Vp, Vf, Vnanofluid particle volume, fluid volume and nanofluid volume, m3
1. The hydrodynamic instability model was proposed by Zuber [6]. Zuber assumed that near the critical heat flux, the heating surface is covered by rising vapor jets with counter flow of liquid jets flowing downward. He assumed that on an infinite flat heater the vapor jets would form on the nodes of the square two dimensional collapsing Taylor waves. When velocity of the vapor in the vapor jets reaches a critical value, Kelvin–Helmholtz instability occurs which will cause the jets to collapse. The vapor cannot escape the surface and the critical heat flux condition will set in. Zuber derived critical heat flux expression in the form 0:25 qmaxf ¼ 0:131q0:5 : g hfg ½rgðqf qg Þ
ð1Þ
Later, Lienhard and Dhir [7] modified Zuber’s theory to account for both size and geometrical effects. They provided hydrodynamic predictions of peak pool boiling heat fluxes from different finite bodies. These predictions matched experimental results. 2. The macrolayer consumption model was proposed by Haramura and Katto [8]. They postulated that important instabilities occur not at the walls of the vapor jets away from the heater surface, but rather at the walls of the tiny vapor stems imbedded in the liquid macrolayer on the heater surface. Due to Kevin–Helmholtz instability these stems are going to collapse. If the heat flux is sufficient to evaporate the macrolayer before liquid resupply of the evaporating layer, the critical heat flux will be reached. They proposed that peak pool boiling heat flux can be calculated as
qmax ¼ ð1 aM ÞdO ql hfg =s;
ð2Þ
where aM is the void fraction in the macrolayer, dO is the macrolayer thickness and s is the macrolayer hovering period. 3. Bubble crowding at heated surface was proposed by [9]. In this model, close packing of bubbles on a heater surface is responsible for the stopping of the liquid flow toward the surface leading to critical heat flux. They proposed the following equation for CHF
qmax =hfg qg ¼ Cðg=g s Þ0:25 ½ðql qg Þ=qg 0:6 ;
critical heat flux, W/m2 critical heat flux for an infinite flat plate, W/m2
qmax qmaxf
Greek symbols critical Kevin–Helmholtz wavelength, m kH qf, qg, qp liquid density, vapor density and particle density, kg/m3 r surface tension, N/m
versy about the mechanism of peak pool boiling heat flux. The primary focus of this paper was to find an explanation for the influence of nanoparticles in regular fluids on the critical heat flux. With this in mind and taking into account our decision to use cylindrical heaters in our experimental set up, the Lienhard and Dhir [7] and Ramilison et al. [11] models were used as the starting point of our analysis. All parameters relevant for evaluation of critical heat flux recognized by these models are evaluated in the case of nanofluids.
2. Experimental set up and procedure The experimental set up consists of two coaxial square chambers, as shown in Fig. 1. The inner chamber has dimensions 7 cm 7 cm 10 cm and the outer chamber has dimensions 13 cm 13 cm 10 cm. Nanofluids are boiled in the inner chamber. In the outer chamber tap water is kept at the same temperature as the temperature of the fluid in the inner chamber. In this way, the temperature in the inner chamber is maintained constant over time. To maintain the temperature constant, tap water is circulated in the outer chamber and two high density cartridge heaters are used to control the temperature to a predefined set point. Two heaters are used in the boiling chamber. The primary heater is a NiCr resistance wire. The diameter of the wire is 0.64 mm and it has an active length of 50 mm. The ends of primary heater are soldered to short copper cylindrical ends to minimize any additional electrical resistance. The primary heater is mounted on two copper cylindrical posts. These posts are connected to a DC electrical circuit. A Hantrex DC power supply, 7.5 V – 80 A, operating in
Pressure transducer T thermocouple
Tap water
P
ð3Þ
where C is equal to 0.012 m/s, g is the local gravitational acceleration and gs corresponds to the standard g value. 4. Hot spot heating was proposed by [10]. This model proposes that the temperature at the center of a dry patch on the heater surface is an important parameter that can trigger the peak pool boiling heat flux. When this temperature is high enough so that liquid can no longer be in contact with the surface, a gradual increase in surface temperature can be expected and the CHF condition will be reached. Unal did not propose any formulations for peak pool boiling heat flux but rather looked at critical heat flux from the phenomenological point of view. While going through critical heat flux literature, it was obvious to the authors that there still seems to be considerable contro-
Condenser
Heater
Heater Primary heater
+ -
+ +
-
Fig. 1. Schematic of experimental setup.
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constant current mode provided the power to the heater. A DC shunt of known resistance, 0.001 X, is connected to DC circuit for current measurements. Hewlett-Packard multimeters are used to measure voltage drops across active length of primary heater and over the DC shunt. The secondary heater is a high density cartridge heater mounted vertically in the inner chamber. It is used for heating the boiling fluid up to its boiling temperature and for degassing purposes. The pressure inside the inner chamber is maintained by mounting a coil condenser on the lid of the inner chamber. Temperature controlled water passes through the heat exchanger coil. This approach is sufficient to keep the pressure inside the inner chamber constant and equal to the atmospheric pressure at all times during the boiling experiments. The pressure inside the boiling chamber is measured using a pressure transducer. Type-T thermocouples are mounted in the chamber to evaluate temperature of boiling fluid. All temperatures were the same throughout during the boiling experiments which indicated that boiling fluid temperature was volumetrically uniform. Before the beginning of each experiment the primary heater was cleaned with soap and water and rinsed with acetone. The secondary heater was used to bring the boiling fluid to its saturation temperature. The fluid was maintained at this temperature for several minutes for degassing purposes. The secondary heater was turned off and the primary heater was turned on. The current through the primary heater was increased steadily until the critical heat flux condition was observed. The peak heat flux was calculated from the measured voltage, current and primary heater dimensions. For different runs, new heater wires and fresh nanofluids were used. 3. Analytical model and direction of analysis In this work the models by Lienhard and Dhir [7] and Ramilison et al. [11] were used as a starting point. In the model by Lienhard and Dhir [7] they evaluated critical heat flux by starting with the simple equation
qmax =qg hfg ¼ U H
Aj ; Ah
ð4Þ
where Aj is the combined area of the vapor jets leaving the heater surface of area Ah and Uh is the critical vapor velocity within the jets which will cause the jets to collapse. This velocity can be evaluated as
UH ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2p=qg kH :
ð5Þ
Substituting the Eq. (5) into Eq. (4) and knowing that 1=4 qmaxf ¼ 0:131q1=2 g hfg ½rgðqf qg Þ
ð6Þ
it can be obtained
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qmax 24 u 2p Aj u ¼ t qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi : qmaxf p kH g=gðq q Þ=r Ah f g
ð7Þ
For small cylinders it is proposed by Lienhard and Dhir [7] that critical Helmholtz wavelength can be obtained as
kH ¼ 2pRj ;
ð8Þ
where
Rj ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðAh =pÞðAj =Ah Þ:
ð9Þ
Substituting Eq. (9) into Eq. (8) and then into Eq. (7) it can be obtained
qmax 24 ¼ qmaxf p
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4
p
r
Aj Ah gðqf qg Þ Ah
3=4 :
ð10Þ
Analyzing Eqs. (6) and (10) it is obvious that there are two groups of parameters that influence the critical heat flux, these are the thermophysical properties of the fluid and the configuration of vapor jets above the surface. This model does not recognize the influence of surface characteristics on critical heat flux. This has been shown in the literature to be an important factor. Berenson [12] showed a surface roughness dependence of ±10%. Also, Liaw and Dhair [13] showed that the surface contact angle can have a great influence on peak pool boiling heat flux. These results motivated Ramilison et al. [11] to modify the critical heat flux equation for flat plates to account for surface characteristics, surface roughness and surface contact angle. Ramilison et al. [11] accounted for these influences through a correction factor that is a function of surface roughness and surface contact angle. The influence of surface characteristics does not come naturally from the physical model. Based on our understanding of these two models, our approach was first to evaluate thermo-physical properties of nanofluids and heater surface characteristics during the boiling of nanofluids. If there is some detected change of surface characteristics of the heater as result of nanofluid boiling, then it is hypothesized these should influence the jet configuration above the heater and the critical heat flux given by Eq. (10). This hypothesis is based on two observations. First, if there is an increase of the CHF as result of change of surface characteristics then that should produce greater amount of vapor. Vapor is defined through jet configuration as given by Eq. (4). Second, Han and Griffith [14] recognized the influence of surface contact angle on bubble departure diameter. It seems reasonable to think that a change in the surface contact angle, and with that a change in the bubble departure diameter, would change the diameter of the vapor jet away from the surface and with that, the jet configuration as whole. This paper is the first part of our analysis, in which our goal is to find the reasons behind
Fig. 2. Pictures of nanoparticles obtained with transmission electron microscope.
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the increase of critical heat flux in nanofluids. If the reasons behind the increase of peak heat flux include changes of surface characteristics, then in second part of our analysis, our goal will be to connect these changes of surface characteristics with the change of jet configuration above the heater. 4. Results and discussion Two types of nanoparticles are used in this study, alumina oxide in two sizes and bismuth oxide. These particles were purchased from Nanophase Technologies Corp. in powder form. The base fluid is distilled water. Nanofluids are obtained by a two-step method. First the nanoparticles’ weight is measured using a Mettler micro weight balance. Then particles are suspended in a specified volume of the base fluid. This suspension is then put in an ultrasonic bath with the purpose of deaglomerating the nanoparticles. After approximately one hour, the suspension is taken into the chamber, where it is heated up to the temperature just below the saturation temperature of water at atmospheric pressure. At this temperature, approximately 98 °C, the nanofluid is highly unstable and natural mixing within the fluid takes place. The suspension is kept for half an hour at this temperature, which is enough to obtain good dispersion of the nanoparticles over the entire nanofluid volume. In order to properly classify the nanofluids, the average particle size and the particle distribution for the two different alumina oxides was done using an electron transmission microscope (JEM3010). Based on a 900 particle sample, it was found that the average particle size of Al2O3 – Nano Dur is 46 nm and the average particle size of Al2O3 – Nano Arc is 22.6 nm. The shape of alumina particle is spherical, as is shown in Fig. 2a and b. The Bismuth Oxide average particle size is provided by the producer (Nanophase Technologies). It is 38 nm and it is obtained using the specific surface area method. This method can only provide information on average particle size not on particle size distribution. The shape of the BiO2 nanoparticles is not spherical, as is shown in Fig. 2c. That is why taking pictures of these particles with a transmission electron microscope cannot lead to average particle size and particle distribution as in the case of perfectly spherical Al2O3 nanopraticles. Particles are also checked for their purity by doing element analysis with help of a TEM. Results confirmed the purity of the particles. In order to confirm the distribution of the particles over the nanofluid volume, several samples were produced for the transmission electron microscope by dropping drops of nanofluid taken from different places in the suspension on carbon grids. These samples under the microscope proved that nanoparticles are uniformly present in every portion of the nanofluid volume.
4.1. Influence of different nanoparticles, particle size and concentration on critical heat flux in nanofluids Every point obtained in Fig. 3 represents average of several peak heat flux measurements. Fluctuations of critical heat flux around the average value, for every point in Fig. 3, were within the heat flux measurement uncertainty. This is estimated to be 5.6%. From Fig. 3 it can be seen that with the increase of particle concentration in nanofluids, the critical heat flux increases up to the point when further increase of particle concentration does not have further influence on the peak heat flux. The maximum obtained increase is 50% in the case when alumina oxides are used as nanoparticles and 33% when bismuth oxide particles are used. These experimental results are obtained at saturated conditions. It is interesting to note that the presence of nanoparticles in water does not change the saturation temperature of the nanofluid as compared to water. The boiling point for all nanofluids considered at atmospheric pressure is 100 °C. The average particle size of the nanoparticles also seems to have negligible influence on peak heat flux of nanofluids. This can be observed from Fig. 3 by looking at two curves obtained when Al2O3 nanoparticles of average particle size 46 nm and 22.6 nm are suspended in water. As is shown in Fig. 3, the choice of nanoparticles to be suspended in the base fluid does have an influence on maximum possible increase in critical heat flux. Just suspending alumina oxide nanoparticles rather than bismuth oxide particles in water leads to 17% increase in maximum achievable peak heat flux. 4.2. Physical and thermal properties of nanofluids The apparatus for measuring surface tension is a capillary tube. If a small open tube is immersed into fluid, the fluid level in the tube will rise or fall in relation to the liquid level outside the tube. The height of the liquid in the small tube depends on surface tension of the liquid, the radius of the tube, and the weight of the liquid. The apparatus consists of a 250 mm long borosilicate glass capillary tube, of 0.5 mm nominal bore, graduated from zero to 10 cm in 0.1 cm increments, a glass cylinder with tabulation. The value of surface tension for distilled water obtained by the apparatus was compared to the same value from the literature. It is found that the relative error is 1.7%. In Fig. 4, the experimentally obtained values of the nanofluid’s surface tension as a function of nanoparticle concentration are presented. It can be seen from Fig. 4 that the surface tension does not change for concentrations of nanoparticles considered in this work,
260
240
max
q
[W/cm
2 ]
220 200 180 Al 2 O 3 + Water -- APS 46 nm Al 2 O 3 + Water -- APS 22.6 nm BiO 2 + Water -- APS 39 nm
160 140 0.000
0.002
0.004
0.006
0.008
0.010
0.012
Particle Concentration [gr/l] Fig. 3. Influence of nanoparticle concentration, size and material on critical heat flux in nanofluids.
Fig. 4. Surface tension of nanofluids at Tnanofluid = 24 °C.
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Nanofluid density
Relative increase
(g)
(g/cc)
(cc)
(l)
(cc)
(g/cc)
(g/cc)
(%)
0 0.0005 0.0015 0.003
3.6 3.6 3.6 3.6
0 0.000138889 0.000416667 0.000833333
0.3 0.3 0.3 0.3
300 300 300 300
0.958 0.958 0.958 0.958
0.958000 0.958001 0.958004 0.958007
0.000000 0.000128 0.000383 0.000766
Nanofluid density
Relative increase
Water
BiO2 (g)
(g/cc)
(cc)
(l)
(cc)
(g/cc)
(g/cc)
(%)
0 0.0005 0.0015 0.003
8.9 8.9 8.9 8.9
0 5.61798E05 0.000168539 0.000337079
0.3 0.3 0.3 0.3
300 300 300 300
0.958 0.958 0.958 0.958
0.958000 0.958001 0.958004 0.958009
0.000000 0.000155 0.000466 0.000931
h fg = 2257 kJ/kgK at T sat = 100ο C for water from Keenan et al. (1969)
2280
Latent Heat [kJ/kgK]
Al2O3
2300
2260
2240
2220 AL 2O3 (APS 46 nm)+ Water BiO 2 + Water 2200 0.000
0.002
0.004
0.006
0.008
0.010
Particle Concentration [gr/l] Fig. 5. Latent heat of nanofluids at Tsat = 100°C.
and the surface tension is approximately equal to surface tension of pure water. In Eqs. (6) and (10), the difference in the density between the liquid and vapor phase of given fluid is an important parameter for the evaluation of peak heat flux. This difference in densities directly influences the buoyancy force that acts on bubbles at the heater surface. Usually the density of the vapor phase can be neglected when compared to the density of the liquid phase. With this assumption being made, the qf qg term can be evaluated through evaluation of nanofluid density in liquid phase. Table 1 shows the change of nanofluid density as a function of particle concentration. The density of suspension can be obtained from
qnanofluidliquid ¼ qp
Vp Vf þ qf : V nanofluid V nanofluid
ð11Þ
From Table 1 it can be observed that for the particle concentrations in this study, there is a negligible increase in nanofluid density as result of the nanoparticles. This leads us to conclude the qf qg term for nanofluids in this study and pure water is the same. The latent heat of nanofluids is measured in a classical way. The experimental setup consists of a boiling chamber, reflux condenser and pot for gathering condensed vapor. Vapor produced by boiling nanofluids is taken to the reflux condenser where it is condensed and the condensate is gathered in the pot. Measuring the power
Fig. 6. Scanning electron microscope views of heater surface and nanoparticle layer formed on the surface.
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supplied to the fluid over a period of time and the mass of condensate gathered, it is possible to calculate the latent heat. Results of these experiments are presented in Fig. 5. It can be seen from Fig. 5 that latent heats of pure water and nanofluids considered in this study are the same. 4.3. Influence of surface characteristics on critical heat flux in nanofluids A closer look at the heater surfaces at the end of the boiling experiments revealed that the surfaces were covered with a thin layer of material. Discovery of this thin layer indicated that there was a change of heater surface characteristics between the start and the end of the boiling experiments. A change of surface roughness and surface contact angle can have an influence on the peak heat flux according to Ramilison et al. [11]. Accordingly, this section investigates the heater surface characteristics. A scanning electron microscope (Hittachi 3000S) was used to obtain a closer look at the thin layer on the heater surface, as shown in Fig. 6. Comparing Fig. 6a and b, it can be observed that the heater surface just before critical heat flux has many more dark dots than the clean surface. Because these pictures are taken with a scanning electron microscope that obtains pictures by doing a topography of the surface, these black dots on the surface indicate that during the boiling experiment the surface becomes more rough, with more peaks and valleys. Fig. 6c and d show a close-up of the thin layers for Al2O3 and BiO2, respectively. These layers look different, and their look depends on the type of nanoparticles used in the nanofluids. In order to show that these are actually nanoparticles on the heater surface, an elemental analysis of the heater surfaces was performed. From Fig. 7a it can be seen that clean heater surface consists of three elements, Ni, Cr and C. If the nanofluid with Al2O3 nanoparticles is boiled with this heater, then Fig. 7b shows that element analysis indicates that beside Ni, Cr and C, Al and O elements will be present at the heater surface. This indicates that during the boiling of nanofluids, nanoparticles coat the heater surface. This conclusion is also supported by Fig. 7c that is obtained for the heater that boiled nanofluid with BiO2 nanoparticles and has Bi and O elements in the element analysis of the heater surface. To obtain a baseline for comparison, the static surface contact angles were measured at the beginning and just before the critical heat flux takes place on the boiling surface. Results are presented in Fig. 8. Surface contact angles are measured by dropping drops of nanofluid on the dry heater surface and then taking photos of drops. From these photos it is easy to measure the static surface contact angle. A clean heater wire is used in Fig. 8a, while Fig. 8b and c show the surface contact angle on a heating wire that has been used to boil the nanofluids of different concentrations (0.00257 g/l and 0.00646 g/l of Al2O3 in suspension) up to the CHF. On the clean heater in Fig. 8a, the droplet is pure water, and the contact angle is h is 90°. On the heaters coated with nanoparticles in Fig. 8b and c, for illustrative purposes the droplets consist of the same fluids that have deposited the thin film of nanoparticles on the heater surface during the boiling process. However, this droplet composition is not a variable, since droplets of pure water would form the same contact angle with the respective surfaces as these droplets of nanofluids. The surface contact angle decreases with an increase of the Al2O3 nanoparticle concentration in the solution, from h = 46.5° for the 0.00257g/l Al2O3 concentration (Fig. 8b) to h = 33° for the 0.00646 g/l concentration (Fig. 8c). Fig. 8 above illustrates the surface contact angle h on heater surfaces that have been coated with nanoparticles in the process of boiling Al2O3 nanofluids of 0.00257 g/l and 0.00646 g/l particle
Fig. 7. Element analysis on different heater surfaces. (a) Element analysis on the clean heater surface. (b) Element analysis of heater surface just before CHF, nanofluid made of Al2O3 nanoparticles (46 nm) and water. (c) Element analysis of heater surface just before CHF, nanofluid made of BiO2 nanoparticles (39 nm) and water.
concentrations. Fig. 9a below plots these two data points plus the contact angles for a lower concentrations of Al2O3 nanofluid. Fig. 9b shows the same correlation between surface contact angle and the concentration of nanofluid boiled by the heater for the BiO2 nanofluid. From Fig. 9 it can be observed that with an increase in the particle concentration in the nanofluids boiled by the heater, which affects the nature of the surface layer of nanoparticles deposited on the heater, the static surface contact angle of fluid droplets on the dry heater outside the test rig decreases. In the case of water and Al2O3 nanoparticles, the surface contact angle decreases from 90° to 35.5°. This is a significant decrease that decreases the vertical component of the surface tension force that keeps the bubble on the heater surface. As result, bubbles will spend less time on the boiling surface and the radius of bubbles at the point of departure will be smaller. This can translate into an increase in the peak heat flux. From Fig. 9b it can be seen that for water and BiO2 nanoparticles change of static surface contact angle is between 90° and 56.1°. This change is smaller than in the case of water and Al2O3 nanoparticles. That is why the maximum increase in critical heat
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Fig. 8. Water and Al2O3 nanoparticles drops of different particle concentration on heater surface boiled in the corresponding nanoparticle concentration nanofluid.
Fig. 9. Surface contact angles just before peak heat flux for different particle concentrations in nanofluids. (a) Surface contact angle just before CHF takes place, water and Al2O3 nanoparticles. (b) Surface contact angle just before CHF takes place, water and BiO2 nanoparticles.
flux is greater in the case of a suspension with Al2O3 nanoparticles than in the case of a suspension with BiO2 nanoparticles. Fig. 10 shows that changing the size of the nanoparticles (here Al2O3) in the solution, keeping everything else the same, does not change the static surface angle. Fig. 10 shows that nanoparticle size has little or no influence on surface contact angle. Fig. 3 above also shows that nanoparticle size has little influence on peak heat flux. Taken together, these two figures show that the effect on the critical heat flux by the surface roughness (which increases with nanoparticle size) is small, and that the surface characteristic that primary affects the critical heat flux is static surface contact angle. In order for this analysis to be complete, the influence of nanoparticles in solution (as distinct from the nanoparticles deposited
on the heater surface) on the peak heat flux has to be evaluated. This is done by boiling the nanofluid of a certain nanoparticle concentration up to the point just below the critical heat flux. The nanofluid is then taken out of the test rig and replaced with pure distilled water. By doing this, the surface characteristics of the heater at the point of critical heat flux may be assumed to be similar for the nanofluid and for pure water. If there is some change of peak heat flux, this should be the result of nanoparticle presence in the fluid, not a change of surface characteristics. The results of these experiments are shown in Table 2. Nanofluids used to treat the heater surface had water as the base fluid and Al2O3 (APS 46 nm) as nanoparticles. It can be seen from Table 2 that if the surface characteristics are similar, there is a small difference in the peak heat flux when pure water is used instead of a nanofluid that
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Al2O3 nanoparticles, surface contact angle can decrease from 90° to 35.5° depending on the nanoparticle concentration in the nanofluids. The change in static surface contact angle is between 90° and 56.1° when nanofluids with BiO2 nanoparticles are boiled. The influence of surface roughness and the presence of suspended nanoparticles in nanofluids on the peak heat flux is found to be minor. The surface tension, latent heat and density difference between the liquid and the vapor phase of nanofluids are found to be the same as for pure distilled water. As result, it can be concluded that the major reason behind the increase of critical heat flux in nanofluids is the decrease of static surface contact angle. Finally it is hypothesized that a change of surface contact angle should influence the jet size and jet spacing above the heater surface. It is our hope that this influence of surface contact angle on critical heat flux can be incorporated into hydrodynamic CHF model proposed by Lienhard and Dhir [7]. This hypothesis is currently being investigated. Fig. 10. Influence of particle size on static surface contact angle.
References Table 2 Influence of nanoparticle presence in nanofluid on peak heat flux Nanofluid
Water
(g/l)
CHF (W/cm2)
CHF (W/cm2)
0.0005714 0.001143 0.002570 0.0064615
180 205.3 219.4 230.45
187.6 205.4 219.4 221.1
has water as its base (the fact that the CHF for the nanofluid is sometimes slightly less than, sometimes equal to, and sometimes slightly greater than the CHF for water suggests that the variations are within the range of experimental error). These results lead us to conclude that nanoparticle presence in fluid has a minor influence on the critical heat flux. With all results presented it can be said that the main cause of the increase of peak heat flux in nanofluids is nanoparticle coating of heater surface during the boiling process. As result, static surface contact angle decreases. This change of contact surface angle is primary force behind the increase in the CHF. 5. Conclusion The critical heat flux is experimentally evaluated for two average particle sizes of Al2O3 and one size of BiO2 nanoparticles suspended in pure distilled water. It is found that the increase of nanoparticle concentration in the nanofluid increases the peak heat flux up to a certain point, after which further increase does not affect CHF. Experimental results showed that particle size has negligible influence on the peak heat flux. The critical heat flux can be increased 50% when Al2O3 nanoparticles are suspended in pure distilled water and 33% when BiO2 particles are suspended. The type of nanoparticles used in nanofluids affects the maximum critical heat flux that can be achieved. It is found that during the boiling of nanofluids, nanoparticles are continuously coating the heater surface. This results in the change of static surface contact angle. In the case of nanfluids with
[1] U.S. Choi, Developments and Applications of Non-Newtonian Flows, ASME FED-Vol. 231/MD, vol. 66, 1995, pp. 99–105. [2] S. Lee, S.U.S. Choi, S. Li, J.A. Eastman, ASME Journal of Heat Transfer 121 (1999) 280. [3] J.A. Eastman, S.U.S. Choi, S. Li, W. Yu, L.J. Thompson, Anomalously increased effective thermal conductivities ofethylene glycol based nanofluids containing copper nanoparticles, Applied Physics Letters 78 (2001) 718–720. [4] S.U.S. Choi, Z.G. Zhang, W. Yu, F.E. Lockwood, E.A. Grulke, Anomalous thermal conductivity enhancement in nanotube suspensions, Applied Physics Letters 79 (2001) 2252–2254. [5] S.M. You, J. Kim, K.H. Kim, Effect of nanoparticles on critical heat flux of water in pool boiling heat transfer, Applied Physics Letters 83 (2003) 3374–3376. [6] N. Zuber, Hydrodynamic Aspects of Boiling Heat Transfer, Ph.D Dissertation, Department of Mechanical Engineering, University of California at Los Angeles, California, USA, 1959. [7] J.H. Lienhard, V.K. Dhir, Hydrodynamic prediction of peak pool boiling heat fluxes from finite bodies, ASME Journal of Heat Transfer 95 (1973) 477–482. [8] Y. Haramura, Y. Katto, A new hydrodynamic model of CHF applicable widely to both pool and forced convection boiling on submerged bodies in saturated liquids, International Journal of Heat and Mass Transfer 26 (1983) 389–399. [9] W. Rosenhow, P. Griffith, Correlation of maximum heat flux data for boiling of saturated liquids, Chemical Engineering Progress Symposium Series 52 (1956) 47–49. [10] C. Unal, V. Daw, R.A. Nelson, Unifying the controlling mechanism for the critical heat flux and quenching: the ability of liquid to contact the hot surface, ASME Journal of Heat Transfer 114 (1993) 972–982. [11] J.M. Ramilison, P. Sadasivan, J.H. Lienhard, Surface factors influencing burnout on flat heaters, ASME Journal of Heat Transfer 114 (1992) 287–290. [12] P.J. Berenson, Experiments on pool boiling heat transfer, International Journal of Heat and Mass Transfer 5 (1962) 985–999. [13] V.K. Dhir, S.P. Liaw, Framework for a unified model for nucleate and transition pool boiling, ASME Journal of Heat Transfer 111 (1989) 3739–3746. [14] C.Y. Han, P. Griffith, The mechanism of heat transfer in nucleate pool boilingPart I: Bubble initiation, growth and departure, International Journal of Heat and Mass Transfer 8 (1965) 887–904. [15] P. Vassallo, R. Kumar, S.D. Amico, Pool boiling heat transfer experiments in silica–water nanofluids, International Journal of Heat and Mass Transfer 47 (2004) 407–411. [16] H. Kim, J. Kim, M. Kim, Experimental study on CHF characteristics of water– TiO2 nanofluids, Nuclear Engineering and Technology 38 (1) (2006). [17] S.J. Kim, I.C. Bang, J. Buongiorno, L.W. Hu, Surface wettability change during pool boiling of nanofluids and its effect on CHF, International Journal of Heat and Mass Transfer 50 (2007) 4105–4116. [18] I.C. Bang, S.H. Chang, Boiling heat transfer and phenomena of Al2O3 –water nanofluids from a plain surface in a pool, International Journal of Heat and Mass Transfer 48 (2005) 2407–2419. [19] H. Kim, J. Kim, M. Kim, Experimental studies on CHF characteristics of nanofluids at pool boiling, International Journal of Multiphase Flow 33 (2007) 691–706. [20] K. Sefiane, On the role of structural disjoining pressure and contact line pinning incritical heat flux enhancement during boiling of nanofluids, Applied Physics Letters 89 (4) (2006).
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Nanofluids and critical heat flux, experimental and analytical study M.N. Golubovic, H.D. Madhawa Hettiarachchi, W.M. Worek *, W.J. Minkowycz Mechanical and Industrial Engineering Department, University of Illinois at Chicago, 842 West Taylor, Chicago, IL 60607, USA
a r t i c l e
i n f o
Article history: Received 13 November 2007 Accepted 10 May 2008 Available online 20 May 2008 Keywords: Nanofluids Boiling Critical heat flux
a b s t r a c t In recent years, nanofluids have been attracting significant attention in the heat transfer research community. These fluids are obtained by suspending nanoparticles having sizes between 1 and 100 nm in regular fluids. It was found by several researchers that the thermal conductivity of these fluids can be significantly increased when compared to the same fluids without nanoparticles. Also, it was found that pool boiling critical heat flux increases in nanofluids. In this paper, our objective is to evaluate the impact of different nanoparticle characteristics including particle concentration, size and type on critical heat flux experimentally at saturated conditions. As a result, this work will document our experimental findings about pool boiling critical heat flux in different nanofluids. In addition, we will identify reasons behind the increase in the critical heat flux and present possible approaches for analytical modeling of critical heat flux in nanofluids at saturated conditions. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, nanofluids have attracted considerable attention due to their enhanced thermal properties. These novel types of heat transfer fluids are usually obtained by suspending small concentrations of nanoparticles in regular fluids. Choi [1] termed these solid/liquid suspensions ‘‘nanofluids”. The primary idea behind suspending such small particles in regular fluids is to increase as much as possible the thermal conductivity of such suspensions. Lee et al. [2]) demonstrated a maximum increase in the thermal conductivity of approximately 20% when CuO nanoparticles were suspended in ethylene glycol. Suspending copper nanoparticles, average diameter size less than 10 nm, Eastman et al. [3] were able to increase thermal conductivity of ethylene glycol up to 40%. In the same year, Choi et al. [4] were able to report 160% increase of a synthetic poly oil thermal conductivity when metallic multi-wall nanotubes where suspended. An interesting result was reported by You et al. [5]. You and his group investigated critical heat flux in nanofluids obtained by dispersing alumina oxide nanoparticles in distilled and deionized water. It was found that critical heat flux increased 200% for nanofluids that contained less than 0.005 g/l of alumina oxide nanoparticles. Vassallo et al. [15] presented the data that showed a marked increase, approximately 60%, in critical heat flux (CHF) for silica–water solutions compared to water. Kim et al. [16] showed increase in CHF for TiO2–water suspensions compared
* Corresponding author. E-mail addresses: [email protected] (M.N. Golubovic), [email protected] (H.D. Madhawa Hettiarachchi), [email protected] (W.M. Worek), [email protected] (W.J. Minkowycz). 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.05.005
to water. Bang and Chang [18] published similar findings for Al2O3–water suspensions on a plain surface in a pool. For the horizontal test section they obtained 32% increase in peak heat flux with addition of nanoparticles in water. Kim et al. [17,19] showed that buildup of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly improved the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces compared with the pure-water-boiled surfaces. They did not measure the latent heat of nanofluids or showed the influence of nanoparticle presence in nanofluids on CHF. Also, they did not show the influence of surface roughness on peak heat flux. Kim et al. [17,19] concluded that the reason for significant CHF enhancement of nano-fluids was the surface modification due to nanoparticle coating. Sefiane [20] suggested that the analysis of boiling heat transfer of nanofluids must account for the important effect of nanoparticles on the contact line region through the structural disjoining pressure to provide accurate interpretation of critical heat flux enhancement during boiling of nanofluids. The work reported in this paper is motivated by this increase in critical heat flux. Our goal is to find mechanisms through which the presence of nanoparticles in regular fluids affects the critical heat flux phenomenon. The critical heat flux occurs at the point of transition from nucleate boiling, characterized by moderate surface temperatures that go along with high heat fluxes, to film boiling characterized by high surface temperatures which is a consequence of vapor enveloping the heat source. As result of its importance, a vast amount of research work is done in this area. Models that deal with critical heat flux in pool boiling can be classified into four main groups:
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Nomenclature Ah, Aj
area of the heater that feeds jet, cross sectional area of the jet, m2 APS average particle size, nm CHF critical heat flux, W/m2 hfg latent heat, kJ/kgk radius of the jet, m Rj critical velocity of vapor in the jet, m/s UH Vp, Vf, Vnanofluid particle volume, fluid volume and nanofluid volume, m3
1. The hydrodynamic instability model was proposed by Zuber [6]. Zuber assumed that near the critical heat flux, the heating surface is covered by rising vapor jets with counter flow of liquid jets flowing downward. He assumed that on an infinite flat heater the vapor jets would form on the nodes of the square two dimensional collapsing Taylor waves. When velocity of the vapor in the vapor jets reaches a critical value, Kelvin–Helmholtz instability occurs which will cause the jets to collapse. The vapor cannot escape the surface and the critical heat flux condition will set in. Zuber derived critical heat flux expression in the form 0:25 qmaxf ¼ 0:131q0:5 : g hfg ½rgðqf qg Þ
ð1Þ
Later, Lienhard and Dhir [7] modified Zuber’s theory to account for both size and geometrical effects. They provided hydrodynamic predictions of peak pool boiling heat fluxes from different finite bodies. These predictions matched experimental results. 2. The macrolayer consumption model was proposed by Haramura and Katto [8]. They postulated that important instabilities occur not at the walls of the vapor jets away from the heater surface, but rather at the walls of the tiny vapor stems imbedded in the liquid macrolayer on the heater surface. Due to Kevin–Helmholtz instability these stems are going to collapse. If the heat flux is sufficient to evaporate the macrolayer before liquid resupply of the evaporating layer, the critical heat flux will be reached. They proposed that peak pool boiling heat flux can be calculated as
qmax ¼ ð1 aM ÞdO ql hfg =s;
ð2Þ
where aM is the void fraction in the macrolayer, dO is the macrolayer thickness and s is the macrolayer hovering period. 3. Bubble crowding at heated surface was proposed by [9]. In this model, close packing of bubbles on a heater surface is responsible for the stopping of the liquid flow toward the surface leading to critical heat flux. They proposed the following equation for CHF
qmax =hfg qg ¼ Cðg=g s Þ0:25 ½ðql qg Þ=qg 0:6 ;
critical heat flux, W/m2 critical heat flux for an infinite flat plate, W/m2
qmax qmaxf
Greek symbols critical Kevin–Helmholtz wavelength, m kH qf, qg, qp liquid density, vapor density and particle density, kg/m3 r surface tension, N/m
versy about the mechanism of peak pool boiling heat flux. The primary focus of this paper was to find an explanation for the influence of nanoparticles in regular fluids on the critical heat flux. With this in mind and taking into account our decision to use cylindrical heaters in our experimental set up, the Lienhard and Dhir [7] and Ramilison et al. [11] models were used as the starting point of our analysis. All parameters relevant for evaluation of critical heat flux recognized by these models are evaluated in the case of nanofluids.
2. Experimental set up and procedure The experimental set up consists of two coaxial square chambers, as shown in Fig. 1. The inner chamber has dimensions 7 cm 7 cm 10 cm and the outer chamber has dimensions 13 cm 13 cm 10 cm. Nanofluids are boiled in the inner chamber. In the outer chamber tap water is kept at the same temperature as the temperature of the fluid in the inner chamber. In this way, the temperature in the inner chamber is maintained constant over time. To maintain the temperature constant, tap water is circulated in the outer chamber and two high density cartridge heaters are used to control the temperature to a predefined set point. Two heaters are used in the boiling chamber. The primary heater is a NiCr resistance wire. The diameter of the wire is 0.64 mm and it has an active length of 50 mm. The ends of primary heater are soldered to short copper cylindrical ends to minimize any additional electrical resistance. The primary heater is mounted on two copper cylindrical posts. These posts are connected to a DC electrical circuit. A Hantrex DC power supply, 7.5 V – 80 A, operating in
Pressure transducer T thermocouple
Tap water
P
ð3Þ
where C is equal to 0.012 m/s, g is the local gravitational acceleration and gs corresponds to the standard g value. 4. Hot spot heating was proposed by [10]. This model proposes that the temperature at the center of a dry patch on the heater surface is an important parameter that can trigger the peak pool boiling heat flux. When this temperature is high enough so that liquid can no longer be in contact with the surface, a gradual increase in surface temperature can be expected and the CHF condition will be reached. Unal did not propose any formulations for peak pool boiling heat flux but rather looked at critical heat flux from the phenomenological point of view. While going through critical heat flux literature, it was obvious to the authors that there still seems to be considerable contro-
Condenser
Heater
Heater Primary heater
+ -
+ +
-
Fig. 1. Schematic of experimental setup.
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constant current mode provided the power to the heater. A DC shunt of known resistance, 0.001 X, is connected to DC circuit for current measurements. Hewlett-Packard multimeters are used to measure voltage drops across active length of primary heater and over the DC shunt. The secondary heater is a high density cartridge heater mounted vertically in the inner chamber. It is used for heating the boiling fluid up to its boiling temperature and for degassing purposes. The pressure inside the inner chamber is maintained by mounting a coil condenser on the lid of the inner chamber. Temperature controlled water passes through the heat exchanger coil. This approach is sufficient to keep the pressure inside the inner chamber constant and equal to the atmospheric pressure at all times during the boiling experiments. The pressure inside the boiling chamber is measured using a pressure transducer. Type-T thermocouples are mounted in the chamber to evaluate temperature of boiling fluid. All temperatures were the same throughout during the boiling experiments which indicated that boiling fluid temperature was volumetrically uniform. Before the beginning of each experiment the primary heater was cleaned with soap and water and rinsed with acetone. The secondary heater was used to bring the boiling fluid to its saturation temperature. The fluid was maintained at this temperature for several minutes for degassing purposes. The secondary heater was turned off and the primary heater was turned on. The current through the primary heater was increased steadily until the critical heat flux condition was observed. The peak heat flux was calculated from the measured voltage, current and primary heater dimensions. For different runs, new heater wires and fresh nanofluids were used. 3. Analytical model and direction of analysis In this work the models by Lienhard and Dhir [7] and Ramilison et al. [11] were used as a starting point. In the model by Lienhard and Dhir [7] they evaluated critical heat flux by starting with the simple equation
qmax =qg hfg ¼ U H
Aj ; Ah
ð4Þ
where Aj is the combined area of the vapor jets leaving the heater surface of area Ah and Uh is the critical vapor velocity within the jets which will cause the jets to collapse. This velocity can be evaluated as
UH ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2p=qg kH :
ð5Þ
Substituting the Eq. (5) into Eq. (4) and knowing that 1=4 qmaxf ¼ 0:131q1=2 g hfg ½rgðqf qg Þ
ð6Þ
it can be obtained
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qmax 24 u 2p Aj u ¼ t qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi : qmaxf p kH g=gðq q Þ=r Ah f g
ð7Þ
For small cylinders it is proposed by Lienhard and Dhir [7] that critical Helmholtz wavelength can be obtained as
kH ¼ 2pRj ;
ð8Þ
where
Rj ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðAh =pÞðAj =Ah Þ:
ð9Þ
Substituting Eq. (9) into Eq. (8) and then into Eq. (7) it can be obtained
qmax 24 ¼ qmaxf p
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4
p
r
Aj Ah gðqf qg Þ Ah
3=4 :
ð10Þ
Analyzing Eqs. (6) and (10) it is obvious that there are two groups of parameters that influence the critical heat flux, these are the thermophysical properties of the fluid and the configuration of vapor jets above the surface. This model does not recognize the influence of surface characteristics on critical heat flux. This has been shown in the literature to be an important factor. Berenson [12] showed a surface roughness dependence of ±10%. Also, Liaw and Dhair [13] showed that the surface contact angle can have a great influence on peak pool boiling heat flux. These results motivated Ramilison et al. [11] to modify the critical heat flux equation for flat plates to account for surface characteristics, surface roughness and surface contact angle. Ramilison et al. [11] accounted for these influences through a correction factor that is a function of surface roughness and surface contact angle. The influence of surface characteristics does not come naturally from the physical model. Based on our understanding of these two models, our approach was first to evaluate thermo-physical properties of nanofluids and heater surface characteristics during the boiling of nanofluids. If there is some detected change of surface characteristics of the heater as result of nanofluid boiling, then it is hypothesized these should influence the jet configuration above the heater and the critical heat flux given by Eq. (10). This hypothesis is based on two observations. First, if there is an increase of the CHF as result of change of surface characteristics then that should produce greater amount of vapor. Vapor is defined through jet configuration as given by Eq. (4). Second, Han and Griffith [14] recognized the influence of surface contact angle on bubble departure diameter. It seems reasonable to think that a change in the surface contact angle, and with that a change in the bubble departure diameter, would change the diameter of the vapor jet away from the surface and with that, the jet configuration as whole. This paper is the first part of our analysis, in which our goal is to find the reasons behind
Fig. 2. Pictures of nanoparticles obtained with transmission electron microscope.
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the increase of critical heat flux in nanofluids. If the reasons behind the increase of peak heat flux include changes of surface characteristics, then in second part of our analysis, our goal will be to connect these changes of surface characteristics with the change of jet configuration above the heater. 4. Results and discussion Two types of nanoparticles are used in this study, alumina oxide in two sizes and bismuth oxide. These particles were purchased from Nanophase Technologies Corp. in powder form. The base fluid is distilled water. Nanofluids are obtained by a two-step method. First the nanoparticles’ weight is measured using a Mettler micro weight balance. Then particles are suspended in a specified volume of the base fluid. This suspension is then put in an ultrasonic bath with the purpose of deaglomerating the nanoparticles. After approximately one hour, the suspension is taken into the chamber, where it is heated up to the temperature just below the saturation temperature of water at atmospheric pressure. At this temperature, approximately 98 °C, the nanofluid is highly unstable and natural mixing within the fluid takes place. The suspension is kept for half an hour at this temperature, which is enough to obtain good dispersion of the nanoparticles over the entire nanofluid volume. In order to properly classify the nanofluids, the average particle size and the particle distribution for the two different alumina oxides was done using an electron transmission microscope (JEM3010). Based on a 900 particle sample, it was found that the average particle size of Al2O3 – Nano Dur is 46 nm and the average particle size of Al2O3 – Nano Arc is 22.6 nm. The shape of alumina particle is spherical, as is shown in Fig. 2a and b. The Bismuth Oxide average particle size is provided by the producer (Nanophase Technologies). It is 38 nm and it is obtained using the specific surface area method. This method can only provide information on average particle size not on particle size distribution. The shape of the BiO2 nanoparticles is not spherical, as is shown in Fig. 2c. That is why taking pictures of these particles with a transmission electron microscope cannot lead to average particle size and particle distribution as in the case of perfectly spherical Al2O3 nanopraticles. Particles are also checked for their purity by doing element analysis with help of a TEM. Results confirmed the purity of the particles. In order to confirm the distribution of the particles over the nanofluid volume, several samples were produced for the transmission electron microscope by dropping drops of nanofluid taken from different places in the suspension on carbon grids. These samples under the microscope proved that nanoparticles are uniformly present in every portion of the nanofluid volume.
4.1. Influence of different nanoparticles, particle size and concentration on critical heat flux in nanofluids Every point obtained in Fig. 3 represents average of several peak heat flux measurements. Fluctuations of critical heat flux around the average value, for every point in Fig. 3, were within the heat flux measurement uncertainty. This is estimated to be 5.6%. From Fig. 3 it can be seen that with the increase of particle concentration in nanofluids, the critical heat flux increases up to the point when further increase of particle concentration does not have further influence on the peak heat flux. The maximum obtained increase is 50% in the case when alumina oxides are used as nanoparticles and 33% when bismuth oxide particles are used. These experimental results are obtained at saturated conditions. It is interesting to note that the presence of nanoparticles in water does not change the saturation temperature of the nanofluid as compared to water. The boiling point for all nanofluids considered at atmospheric pressure is 100 °C. The average particle size of the nanoparticles also seems to have negligible influence on peak heat flux of nanofluids. This can be observed from Fig. 3 by looking at two curves obtained when Al2O3 nanoparticles of average particle size 46 nm and 22.6 nm are suspended in water. As is shown in Fig. 3, the choice of nanoparticles to be suspended in the base fluid does have an influence on maximum possible increase in critical heat flux. Just suspending alumina oxide nanoparticles rather than bismuth oxide particles in water leads to 17% increase in maximum achievable peak heat flux. 4.2. Physical and thermal properties of nanofluids The apparatus for measuring surface tension is a capillary tube. If a small open tube is immersed into fluid, the fluid level in the tube will rise or fall in relation to the liquid level outside the tube. The height of the liquid in the small tube depends on surface tension of the liquid, the radius of the tube, and the weight of the liquid. The apparatus consists of a 250 mm long borosilicate glass capillary tube, of 0.5 mm nominal bore, graduated from zero to 10 cm in 0.1 cm increments, a glass cylinder with tabulation. The value of surface tension for distilled water obtained by the apparatus was compared to the same value from the literature. It is found that the relative error is 1.7%. In Fig. 4, the experimentally obtained values of the nanofluid’s surface tension as a function of nanoparticle concentration are presented. It can be seen from Fig. 4 that the surface tension does not change for concentrations of nanoparticles considered in this work,
260
240
max
q
[W/cm
2 ]
220 200 180 Al 2 O 3 + Water -- APS 46 nm Al 2 O 3 + Water -- APS 22.6 nm BiO 2 + Water -- APS 39 nm
160 140 0.000
0.002
0.004
0.006
0.008
0.010
0.012
Particle Concentration [gr/l] Fig. 3. Influence of nanoparticle concentration, size and material on critical heat flux in nanofluids.
Fig. 4. Surface tension of nanofluids at Tnanofluid = 24 °C.
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M.N. Golubovic et al. / Applied Thermal Engineering 29 (2009) 1281–1288 Table 1 Change of nanofluid density with the change of particle concentration Water
Nanofluid density
Relative increase
(g)
(g/cc)
(cc)
(l)
(cc)
(g/cc)
(g/cc)
(%)
0 0.0005 0.0015 0.003
3.6 3.6 3.6 3.6
0 0.000138889 0.000416667 0.000833333
0.3 0.3 0.3 0.3
300 300 300 300
0.958 0.958 0.958 0.958
0.958000 0.958001 0.958004 0.958007
0.000000 0.000128 0.000383 0.000766
Nanofluid density
Relative increase
Water
BiO2 (g)
(g/cc)
(cc)
(l)
(cc)
(g/cc)
(g/cc)
(%)
0 0.0005 0.0015 0.003
8.9 8.9 8.9 8.9
0 5.61798E05 0.000168539 0.000337079
0.3 0.3 0.3 0.3
300 300 300 300
0.958 0.958 0.958 0.958
0.958000 0.958001 0.958004 0.958009
0.000000 0.000155 0.000466 0.000931
h fg = 2257 kJ/kgK at T sat = 100ο C for water from Keenan et al. (1969)
2280
Latent Heat [kJ/kgK]
Al2O3
2300
2260
2240
2220 AL 2O3 (APS 46 nm)+ Water BiO 2 + Water 2200 0.000
0.002
0.004
0.006
0.008
0.010
Particle Concentration [gr/l] Fig. 5. Latent heat of nanofluids at Tsat = 100°C.
and the surface tension is approximately equal to surface tension of pure water. In Eqs. (6) and (10), the difference in the density between the liquid and vapor phase of given fluid is an important parameter for the evaluation of peak heat flux. This difference in densities directly influences the buoyancy force that acts on bubbles at the heater surface. Usually the density of the vapor phase can be neglected when compared to the density of the liquid phase. With this assumption being made, the qf qg term can be evaluated through evaluation of nanofluid density in liquid phase. Table 1 shows the change of nanofluid density as a function of particle concentration. The density of suspension can be obtained from
qnanofluidliquid ¼ qp
Vp Vf þ qf : V nanofluid V nanofluid
ð11Þ
From Table 1 it can be observed that for the particle concentrations in this study, there is a negligible increase in nanofluid density as result of the nanoparticles. This leads us to conclude the qf qg term for nanofluids in this study and pure water is the same. The latent heat of nanofluids is measured in a classical way. The experimental setup consists of a boiling chamber, reflux condenser and pot for gathering condensed vapor. Vapor produced by boiling nanofluids is taken to the reflux condenser where it is condensed and the condensate is gathered in the pot. Measuring the power
Fig. 6. Scanning electron microscope views of heater surface and nanoparticle layer formed on the surface.
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supplied to the fluid over a period of time and the mass of condensate gathered, it is possible to calculate the latent heat. Results of these experiments are presented in Fig. 5. It can be seen from Fig. 5 that latent heats of pure water and nanofluids considered in this study are the same. 4.3. Influence of surface characteristics on critical heat flux in nanofluids A closer look at the heater surfaces at the end of the boiling experiments revealed that the surfaces were covered with a thin layer of material. Discovery of this thin layer indicated that there was a change of heater surface characteristics between the start and the end of the boiling experiments. A change of surface roughness and surface contact angle can have an influence on the peak heat flux according to Ramilison et al. [11]. Accordingly, this section investigates the heater surface characteristics. A scanning electron microscope (Hittachi 3000S) was used to obtain a closer look at the thin layer on the heater surface, as shown in Fig. 6. Comparing Fig. 6a and b, it can be observed that the heater surface just before critical heat flux has many more dark dots than the clean surface. Because these pictures are taken with a scanning electron microscope that obtains pictures by doing a topography of the surface, these black dots on the surface indicate that during the boiling experiment the surface becomes more rough, with more peaks and valleys. Fig. 6c and d show a close-up of the thin layers for Al2O3 and BiO2, respectively. These layers look different, and their look depends on the type of nanoparticles used in the nanofluids. In order to show that these are actually nanoparticles on the heater surface, an elemental analysis of the heater surfaces was performed. From Fig. 7a it can be seen that clean heater surface consists of three elements, Ni, Cr and C. If the nanofluid with Al2O3 nanoparticles is boiled with this heater, then Fig. 7b shows that element analysis indicates that beside Ni, Cr and C, Al and O elements will be present at the heater surface. This indicates that during the boiling of nanofluids, nanoparticles coat the heater surface. This conclusion is also supported by Fig. 7c that is obtained for the heater that boiled nanofluid with BiO2 nanoparticles and has Bi and O elements in the element analysis of the heater surface. To obtain a baseline for comparison, the static surface contact angles were measured at the beginning and just before the critical heat flux takes place on the boiling surface. Results are presented in Fig. 8. Surface contact angles are measured by dropping drops of nanofluid on the dry heater surface and then taking photos of drops. From these photos it is easy to measure the static surface contact angle. A clean heater wire is used in Fig. 8a, while Fig. 8b and c show the surface contact angle on a heating wire that has been used to boil the nanofluids of different concentrations (0.00257 g/l and 0.00646 g/l of Al2O3 in suspension) up to the CHF. On the clean heater in Fig. 8a, the droplet is pure water, and the contact angle is h is 90°. On the heaters coated with nanoparticles in Fig. 8b and c, for illustrative purposes the droplets consist of the same fluids that have deposited the thin film of nanoparticles on the heater surface during the boiling process. However, this droplet composition is not a variable, since droplets of pure water would form the same contact angle with the respective surfaces as these droplets of nanofluids. The surface contact angle decreases with an increase of the Al2O3 nanoparticle concentration in the solution, from h = 46.5° for the 0.00257g/l Al2O3 concentration (Fig. 8b) to h = 33° for the 0.00646 g/l concentration (Fig. 8c). Fig. 8 above illustrates the surface contact angle h on heater surfaces that have been coated with nanoparticles in the process of boiling Al2O3 nanofluids of 0.00257 g/l and 0.00646 g/l particle
Fig. 7. Element analysis on different heater surfaces. (a) Element analysis on the clean heater surface. (b) Element analysis of heater surface just before CHF, nanofluid made of Al2O3 nanoparticles (46 nm) and water. (c) Element analysis of heater surface just before CHF, nanofluid made of BiO2 nanoparticles (39 nm) and water.
concentrations. Fig. 9a below plots these two data points plus the contact angles for a lower concentrations of Al2O3 nanofluid. Fig. 9b shows the same correlation between surface contact angle and the concentration of nanofluid boiled by the heater for the BiO2 nanofluid. From Fig. 9 it can be observed that with an increase in the particle concentration in the nanofluids boiled by the heater, which affects the nature of the surface layer of nanoparticles deposited on the heater, the static surface contact angle of fluid droplets on the dry heater outside the test rig decreases. In the case of water and Al2O3 nanoparticles, the surface contact angle decreases from 90° to 35.5°. This is a significant decrease that decreases the vertical component of the surface tension force that keeps the bubble on the heater surface. As result, bubbles will spend less time on the boiling surface and the radius of bubbles at the point of departure will be smaller. This can translate into an increase in the peak heat flux. From Fig. 9b it can be seen that for water and BiO2 nanoparticles change of static surface contact angle is between 90° and 56.1°. This change is smaller than in the case of water and Al2O3 nanoparticles. That is why the maximum increase in critical heat
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Fig. 8. Water and Al2O3 nanoparticles drops of different particle concentration on heater surface boiled in the corresponding nanoparticle concentration nanofluid.
Fig. 9. Surface contact angles just before peak heat flux for different particle concentrations in nanofluids. (a) Surface contact angle just before CHF takes place, water and Al2O3 nanoparticles. (b) Surface contact angle just before CHF takes place, water and BiO2 nanoparticles.
flux is greater in the case of a suspension with Al2O3 nanoparticles than in the case of a suspension with BiO2 nanoparticles. Fig. 10 shows that changing the size of the nanoparticles (here Al2O3) in the solution, keeping everything else the same, does not change the static surface angle. Fig. 10 shows that nanoparticle size has little or no influence on surface contact angle. Fig. 3 above also shows that nanoparticle size has little influence on peak heat flux. Taken together, these two figures show that the effect on the critical heat flux by the surface roughness (which increases with nanoparticle size) is small, and that the surface characteristic that primary affects the critical heat flux is static surface contact angle. In order for this analysis to be complete, the influence of nanoparticles in solution (as distinct from the nanoparticles deposited
on the heater surface) on the peak heat flux has to be evaluated. This is done by boiling the nanofluid of a certain nanoparticle concentration up to the point just below the critical heat flux. The nanofluid is then taken out of the test rig and replaced with pure distilled water. By doing this, the surface characteristics of the heater at the point of critical heat flux may be assumed to be similar for the nanofluid and for pure water. If there is some change of peak heat flux, this should be the result of nanoparticle presence in the fluid, not a change of surface characteristics. The results of these experiments are shown in Table 2. Nanofluids used to treat the heater surface had water as the base fluid and Al2O3 (APS 46 nm) as nanoparticles. It can be seen from Table 2 that if the surface characteristics are similar, there is a small difference in the peak heat flux when pure water is used instead of a nanofluid that
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Al2O3 nanoparticles, surface contact angle can decrease from 90° to 35.5° depending on the nanoparticle concentration in the nanofluids. The change in static surface contact angle is between 90° and 56.1° when nanofluids with BiO2 nanoparticles are boiled. The influence of surface roughness and the presence of suspended nanoparticles in nanofluids on the peak heat flux is found to be minor. The surface tension, latent heat and density difference between the liquid and the vapor phase of nanofluids are found to be the same as for pure distilled water. As result, it can be concluded that the major reason behind the increase of critical heat flux in nanofluids is the decrease of static surface contact angle. Finally it is hypothesized that a change of surface contact angle should influence the jet size and jet spacing above the heater surface. It is our hope that this influence of surface contact angle on critical heat flux can be incorporated into hydrodynamic CHF model proposed by Lienhard and Dhir [7]. This hypothesis is currently being investigated. Fig. 10. Influence of particle size on static surface contact angle.
References Table 2 Influence of nanoparticle presence in nanofluid on peak heat flux Nanofluid
Water
(g/l)
CHF (W/cm2)
CHF (W/cm2)
0.0005714 0.001143 0.002570 0.0064615
180 205.3 219.4 230.45
187.6 205.4 219.4 221.1
has water as its base (the fact that the CHF for the nanofluid is sometimes slightly less than, sometimes equal to, and sometimes slightly greater than the CHF for water suggests that the variations are within the range of experimental error). These results lead us to conclude that nanoparticle presence in fluid has a minor influence on the critical heat flux. With all results presented it can be said that the main cause of the increase of peak heat flux in nanofluids is nanoparticle coating of heater surface during the boiling process. As result, static surface contact angle decreases. This change of contact surface angle is primary force behind the increase in the CHF. 5. Conclusion The critical heat flux is experimentally evaluated for two average particle sizes of Al2O3 and one size of BiO2 nanoparticles suspended in pure distilled water. It is found that the increase of nanoparticle concentration in the nanofluid increases the peak heat flux up to a certain point, after which further increase does not affect CHF. Experimental results showed that particle size has negligible influence on the peak heat flux. The critical heat flux can be increased 50% when Al2O3 nanoparticles are suspended in pure distilled water and 33% when BiO2 particles are suspended. The type of nanoparticles used in nanofluids affects the maximum critical heat flux that can be achieved. It is found that during the boiling of nanofluids, nanoparticles are continuously coating the heater surface. This results in the change of static surface contact angle. In the case of nanfluids with
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