NATURAL CONVECTIVE HEAT TRANSFER OF WATER-IN-FC72 NANOEMULSION FLUIDS
Zenghu Han Nano Heat Transfer & Energy Conversion Lab
Department of Mechanical Engineering University of Maryland College Park, MD 20742
Bao Yang Nano Heat Transfer & Energy Conversion Lab
Department of Mechanical Engineering University of Maryland College Park, MD 20742
ABSTRACT Recently, the authors have proposed a radically new design
Proceedings of MNHT2008 Micro/Nanoscale Heat Transfer International Conference
January 6-9, 2008, Tainan, Taiwan
MNHT2008-52351
The use of SOLID-particles has long been a common way of increasing fluid thermal conductivity. In this paper, nanoemulsion fluids–dispersions of LIQUID-nanodroplets–are proposed. As an example, water-in-FC72 nanoemulsion fluids are developed, and their thermophysical properties and impact on natural convective heat transfer are investigated experimentally. A significant increase in thermal conductivity–up to 52% for 12vol% of water nanodroplets (or 7.1wt%)–is observed in the fluids. The enhancement in conductivity and viscosity of the fluids is found to be nonlinear with water loading, indicating an important role of the hydrodynamic interaction and aggregation of nanodroplets. However, the relative viscosity is found to be about two times the relative conductivity if compared at the same water loading. The presence of water nanodroplets is found to systematically increase the natural convective heat transfer coefficient in these fluids, in contrast to the observation in several conventional nanofluids containing solid nanoparticles.
INTRODUCTION Over the past few decades, there has been much interest in
developing more efficient heat transfer fluids for use in power plants, motor engines, and in industrial and computer equipment.[1-15] The coolants, lubricants, oils, and other heat transfer fluids used in today’s thermal systems typically have inherently poor heat transfer properties. The idea of adding SOLID particles to improve thermal conductivity has been pursued since Maxwell’s theoretical work more than 100 years ago,[16] but early studies were confined to large (mm to micron-sized) particles.[17] About a decade ago, this strategy was put forward by Choi et al. with the use of nanometer-sized SOLID particles.[1, 2]
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for thermal fluids that completely eliminates solid particles, and instead, uses liquid nanostructures.[10, 18] Dispersions of liquid nanostructures (e.g., spherical droplets) in thermal fluids can be called “nanoemulsion fluids.” Nanoemulsion fluids are part of a broad class of multiphase colloidal dispersions, and can be formed by shear-induced rupturing or self-assembly. Such fluids possess long-term stability and can be mass produced. Initial experiments were done with dispersions of water nanodroplets in FC72. FC72 is one of a line of Fluorinert™ Electronic Liquids.[19] This paper focuses on an experimental study of thermophysical properties, i.e. thermal conductivity, viscosity and specific heat, of water-in-FC 72 nanoemulsion fluids. The natural convective heat transfer of such fluids is also investigated.
RESULTS AND DISCUSSION As reported in another of the authors’ papers, thermal
conductivity of the nanoemulsion fluids is measured at room temperature using a 3ω-wire technique.[10] This technique is a combination of the hot-wire method[20] and the 3ω method,[9, 10, 21-23] and so, it determines the fluid conductivity by detecting the frequency dependence of temperature oscillation in a metal wire. The relative thermal conductivity, NF Ok k , as a function of water loading is plotted in Fig. 1(a). Results estimated from the Maxwell Model are also shown for comparison.[16, 24] The conductivity calculation in the Maxwell Model involves parameters including interfacial thermal conductances and droplet radius. The water nanodroplets are found to be 9.8 nm in diameter by dynamic light scattering (DLS). In the experiment, a significant increase in thermal conductivity–up to 52% for 12vol% of water nanodroplets (or 7.1wt%)–is observed in the water-in-FC72
nanoemulsion fluids. The conductivity enhancement appears to be nonlinear with the loading of water nanodroplets, while the Maxwell Model clearly predicts a linear relationship between the conductivity enhancements and the water fraction.
Both thermal conductivity and viscosity of emulsions could be strongly and similarly associated with the microstructure and dynamics of the fluids. In order to comprehend thermal conductivity of the nanoemulsion fluids, their dynamic viscosity is investigated experimentally. A commercial viscometer (Brookfield DV-I Prime) is used for the viscosity measurement. The dynamic viscosity is found to be 0.65 cP in the pure FC72, which compares very well with the literature values.[19] Figure 1(b) shows the relative dynamic viscosity, NF Oμ μ , for the water-in-FC72 nanoemulsion fluids with varying water loading. A nonlinear relationship is observed between the viscosity increase and the loading of water nanodroplets, a trend similar to thermal conductivity plotted in Fig. 1(a). However, the relative viscosity is found to be about two times the relative conductivity if compared at the same water loading. It is worth noting that the fluid viscosity is measured at a range of shear rate or spindle rotational speed (50-100RPM), and the viscosity is found to be independent of shear rate. This indicates the nanoemulsion fluids in this experiment are Newtonian in nature.
The viscosity increase of dilute colloids has been successfully predicted using the Einstein Equation,
1 2.5rμ φ= + , where r NF Oμ μ μ= is the relative viscosity
and φ is the volume fraction of the dispersed phase.[25] This equation, however, underpredicts significantly the viscosity increase in the water-in-FC72 nanoemulsion fluids at relatively high water loadings, as can be seen in Fig. 1(b). This is because the Einstein Equation is derived based on dilute systems (assuming volume fraction <0.01). For more highly concentrated systems where the hydrodynamic interaction and aggregation of nanoparticles become important, the Einstein Equation must be augmented by higher order terms of the volume fraction as .[26] Although the coefficient for the first-order term, 2.5, can be strictly derived, it is not a simple task to theoretically determine those for the higher order terms because of the difficulty in accounting for the effects of increased concentrations. By fitting the experimental data in Fig. 1(b), B is found to be about 117, indicating a strong nonlinear behavior. This nonlinear increase in viscosity is common in colloidal systems, and has been interpreted by the hydrodynamic interaction and/or aggregation of nanoparticles.[26, 27] These effects could also give an explanation for the deviation of the conductivity increase from the Maxwell Model (Fig. 1(a)), as could be inferred from the corresponding trends observed in viscosity (Fig. 1(b)) and thermal conductivity (Fig. 1(a)).
21 2.5r Bμ φ φ= + + +
In addition to thermal conductivity and viscosity, specific heat is also an important property in determining the thermal performance of heat transfer fluids. In this experiment, the specific heat of the nanoemulsion fluids and the pure FC72 are
2
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measured using a Differential Scanning Calorimeter (DSC, Model TA-Q100). DSC measurements are taken at an ordinary cyclic ramp mode, and the scan rate is 10 oC/min. Figures 2(a) and (b) show the cyclic DSC heating and cooling curves in the scanning temperature ranges of 0 to 50oC and -40 to 50 oC, respectively, for the water-in-FC72 nanoemulsion fluids. The volumetric specific heat can be directly extracted from these curves with a given scanning rate. The volumetric specific heat of the pure FC72 is experimentally found to be 1.88 J/ml⋅K at room temperature, which is in good agreement with literature data.[19] In the case of no phase transition of water nanodroplets, as shown in Fig. 2(a), the volumetric specific heat of the nanoemulsion fluids at room temperature is found to be increased by 5.29%, 9.52%, 11.64%, and 17.90% for 3 vol%, 6 vol%, 9 vol%, and 12 vol% of water loading, respectively. Little deviation is observed from the prediction of the simple mixing rule, i.e., (1 )a bC C Cφ φ= + − . This enhancement is simply because the specific heat of water is higher than that of FC72.
To achieve a better understanding of transport mechanisms in the water-in-FC72 nanoemulsion fluids, natural convective heat transfer is investigated experimentally in these fluids. In this experiment, a 25 μm-diameter, 3 cm-long, 6 μm-thick-
0 1 2 3 4 5 6 7 8
ExperimentMaxwell Model
Weight Fraction (%)
k NF/
k o
G=65 MW/m2 K15 MW/m2 K
(a)
(b)
1.0
1.2
1.4
1.6
1.8
2.0
0 5 10 15
ExperimentEinstein Model
μ NF/μ o
Volume Fraction (%)1.0
2.0
3.0
4.0
Fig. 1 (a) Relative thermal conductivity and (b) relative dynamic viscosity of the water-in -FC72 nanoemulsion fluids as a function of water nanodroplet concentration. In (a), the relative thermal conductivity estimated from the Maxwell Model is also shown for comparison. Two interfacial thermal conductances–G=65 MW/m2 K and 15 MW/m2 K–are used in the calculation. In (b), the prediction from the Einstein model is plotted.
Teflon coated Platinum-Iridium wire is immersed in the fluids to serve as a test heater. This heater can be kept at constant temperature by a feedback circuit consisting of a Wheatstone bridge, as in hotwire anemometry.[28] This method is so-called “temperature-controlled heating.” The heat dissipated by the wire heater can be calculated directly from its electrical voltage and resistance. The measurement is taken under steady-state conditions.
The natural convective curves are plotted in Fig.3 for the pure FC72 and the nanoemulsion fluids containing 9 vol% and 12 vol% water nanodroplets. It is evident in this figure that the presence of water nanodroplets can systematically increase natural convective heat transfer in these nanoemulsion fluids. The heat transfer is enhanced further at 12 vol% than at 9 vol%. The increased heat transfer in the nanoemulsion fluids, could be
-0.5
-0.25
0
0.25
0.5
0 10 20 30 40 50
pure FC3 vol %6 vol %9 vol %12 vol %
Hea
t Flo
w (m
W/μ
L)
Temperature (0C) Fig. 2 Cyclic DSC heating and cooling curves for the water-in-FC72 nanoemulsion fluids and the pure FC72 in the range of 0 to 50oC.
0
0.5
1
1.5
2
0 5 10 15 20 25 30
pure FC9 vol %12 vol %
q" (W
/cm
2 )
Twire-Tfluid (oC)
Fig. 3 Natural convective heat transfer curves for the pure FC 72 and the water-in-FC72 nanoemulsion fluids. The solid lines are drawn for eye guidance. Please note that Tfluid=24oC, the boiling point of FC72 is 56oC.
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explained using the classical correlation: 0.188D D
hDNu mRak
= = ,
where Nu is the Nusselt number, Ra the Rayleigh number, h the heat transfer coefficient, D the wire diameter, and m an empirical constants.[29] The natural convective heat transfer is predicted to be increased by 7.9% and 15%, in the water-in-FC72 nanoemulsion fluids at 9 vol% and 12 vol%, respectively, when the properties presented in the aforementioned sections are used. It can be found in Fig.3 that the corresponding increases are 6.8% and 13%, respectively. The prediction and the experimental results are in reasonable agreement. Higher enhancement in natural convective heat transfer is expected in these fluids when the water nanodroplets undergo melting-freezing phase transition. Please note that, in contrast to the observations in the water-in-FC72 nanoemulsion fluids, deterioration in natural convective heat transfer has been experimentally observed in several conventional nanofluids containing solid nanoparticles.[30, 31]
CONCLUSION In summary, the concept of nanoemulsion fluids–
dispersions of liquid nanostructures in heat transfer fluids–has been demonstrated via study of the water-in-FC72 nanoemulsion fluids. Thermophysical properties, including thermal conductivity, viscosity, and specific heat, have been investigated experimentally in these fluids. The viscosity increase appears to be nonlinear with the volume fraction of water nanodroplets, analogous to the trend observed in thermal conductivity. However, the relative viscosity is found to be about two times the relative conductivity if compared at the same water loading. The nonlinear rise in conductivity and viscosity at high water loadings could be accounted for by the hydrodynamic interaction and/or aggregation of water nanodroplets. The enhanced specific heat of the nanoemulsion fluids follows the simple mixing rule. These thermophysical properties along with the classical correlations could be used to explain the enhancement in natural convective heat transfer measured in these fluids.
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