Ind. Eng. Chem. Res. 2007, 46, 4341-4346
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Bubbles in Nanofluids† Liang-Shih Fan,* Orin Hemminger, Zhao Yu, and Fei Wang Department of Chemical and Biomolecular Engineering, The Ohio State UniVersity, Columbus, Ohio 43210
The term “nanofluid” is used to describe a liquid that contains dispersed nanoparticles, which can have unique effects on the liquid, because of the enormous surface area of the nanoparticles and their interfacial forceinduced microstructures. The enhanced heat-transfer properties of such nanofluids have been extensively reported in the literature; however, little is known regarding the effects of nanofluids on bubble behavior. Such effects are examined experimentally in this study using hydrophilic nanoparticles with bubble flows in bubble columns and microchannels. The data reveal a significant increase in the gas holdup in bubble columns and a decrease in bubble sizes, signifying an intriguing bubble-nanofluid interactive behavior. Introduction The traditional operation of slurry bubble columns or gasliquid-solid three-phase fluidized beds is conducted with particles in the approximate size range of micrometers to millimeters. Extensive studies of the gas holdup in these systems have been reported in the literature.1-7 These studies involve mostly the air-water systems. The general findings of the particle size effects on the gas holdup can be described as follows: when the slurry particle size is small (e.g., ∼10-250 µm), the gas holdup in the slurry bubble column is less than that in the column without particles.4,6 On the other hand, when the fluidized particle size is large (e.g., ∼6 mm), the gas holdup in the three-phase fluidized bed is greater than that in the column without particles. The interpretation of the holdup behavior for the former is that the slurry particles increase the apparent viscosity of the liquid media and, consequently, the bubble size increases, leading to the decrease in the gas holdup. The interpretation for the latter is that the particle size is large enough to break up the bubbles, yielding an increased gas holdup. The operating conditions for the studies described above encompass the solid loadings of ∼0.4-50 vol %, and gas velocities of 1-12 cm/s. However, very limited experimental data exist5 for the slurry bubble column with particles 10 µm in size, as noted previously. However, because of the use of dispersant for the nanoparticle dispersion in their experiments, the specific effect on the gas holdup due to nanoparticles per se cannot be ascertained from this study. Therefore, the effect of nanoparticles on the gas holdup in the bubble-nanofluid system remains unknown. The subject of nanofluids has been extensively reported in the literature, primarily in association with its effect on heat transfer. The use of nanoparticles in liquid or nanofluid has been observed to increase the thermal-conductivity or heat-transfer properties considerably. For example, it was reported that using 0.3 vol % of copper nanoparticles with a mean diameter of 10 nm in liquids can increase the effective thermal conductivity of ethylene glycol by as much as 40%.9 The mechanisms underlining such enhancement have still been speculative. Among them are the effects of nanoparticle Brownian motion, the effects of the liquid layering at the nanoparticle surface, ballistic heat transport in nanoparticles, and the effects of nanoparticle clustering, forming microstructures.10 The work in this area is rapidly progressing. In this short communication, the experimental results for the air bubble holdup in nanofluids composed of hydrophilic nanoparticles in water are reported. The bubble shape and frequency in the nanofluid and in water are also studied through visualization. The hydrophilic nanoparticles can be dispersed in water without the aid of dispersants and, thus, the effect of the gas holdup due to nanoparticles alone in the liquid can be revealed. Experimental studies are also extended to examine the bubble dynamics in a microchannel that contains nanofluids. Experimental Section The nanofluid was prepared using hydrophilic Degussa Aerosil 90 nanoparticles that were suspended in water. The nanoparticles were 20 nm in diameter and were composed of silicon dioxide (SiO2). The Brunauer-Emmett-Teller (BET) area of the particle was 90 m2/g, and the bulk density was 30 kg/m3. Figure 1 shows the extent of dispersibility at different particle concentrations. It can be observed that, at concentrations of 0.48 and 1.4 wt %, these hydrophilic particles are generally well-dispersed in water, although a small amount of nanoparticle sediment can be observed at the bottom of the vials, signifying the clustering effects of the nanoparticles in suspension. To show the dispersibility of the hydrophobic particles, Degussa Aerosil R 974, which had a diameter of 12 nm and were composed of fumed silica, after being treated with dimethyl dichlorosilane (DDS), were also used. When hydrophobic particles are used,
10.1021/ie061532c CCC: $37.00 © 2007 American Chemical Society Published on Web 05/09/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007
Figure 1. Photograph showing water and nanofluids with different concentrations of hydrophilic and hydrophobic nanoparticles.
Figure 3. Experimental setup for the measurement of single-bubble injection. Figure 2. Plot of the viscosity of a nanofluid containing 1.40 wt % nanoparticles.
as shown in the figure, the nanoparticles remain only on the top of the liquid and little suspension of the nanoparticles is observed. Therefore, only the hydrophilic nanoparticles were used in our bubble column and microfluidic experiments. The nanofluid properties were measured to determine the interfacial tension or surface tension and viscosity. A SensaDyne surface tensiometer was used to obtain the surface tension between the nanofluid and air and between water and air, under ambient conditions. It was observed that the surface tensions of water and the nanofluid with 0.48, 0.96, and 2.0 wt % nanoparticles were 72.2, 72.2, 72.3, and 72.5 dyn/cm, respectively, which indicated negligible surface tension variations with the concentration of the nanoparticles under the concentration range considered. Das et al.11 also reported that the surface tension of the Al2O3-water nanofluid (without adding surfactants) was almost identical to that of water at particle volume fractions of 10 s-1. Prasher et al.12 compared the viscosity data for nanofluids reported by several research groups and found that the percentage increase in
viscosity is a factor of ∼10 times greater than the particle volume fraction. It is noted that the properties of the nanofluid vary with the type of the nanoparticle, as well as the base fluid. The surface tension and viscosity reported in this study are generally consistent with the data reported in the literature. The bubble column experiments were performed using both 2-in. (5.08-cm) and 4-in. (10.16-cm) columns. The nanoparticle weight percentages were in the range of 0.48-1.40 wt %, and the superficial gas velocities vary over a range of 4-50 cm/s. The gas holdup in the column is measured based on the pressure drop method, as well as the liquid expansion method. The liquid is operated in batch condition. Bubble injection experiments are performed to understand bubble formation and rise dynamics in water and in the nanofluid. The experiment setup is shown in Figure 3. The column is a plastic rectangular column (62.5 cm in height, 56 cm in length, and 30 cm in width). A PrecisionGlide needle, with an inner diameter of 0.495 mm, is placed at the bottom of the column. The tip of the needle is 20 mm above the bottom. A Masterflex Cole-Parmer drive pump is used to maintain and adjust the constant gas flow rate. A Photron high-speed CCD camera is used to record the bubble formation and motion, with a rate of 500 frames per second and a resolution of 512 × 240. The experiments are also conducted using microchannels. A schematic diagram and a photograph of the microchannel system are given in Figures 4a and b, respectively. In these experiments, flows were generated using a dual syringe pump with liquid in one side and air in the other side. The liquid is split into two
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Figure 5. Gas holdup in the 2-in. (5.08-cm) bubble column measured using the pressure-drop method: (a) water, nanoparticles, and FCC particles; (b) enlarged view of experimental data at low gas velocities. Figure 4. (a) Schematic diagram of the microfluidic experiment setup. (b) Photo of the microfluidic channel.
separate lines and enters the side channels with cross sections of 500 µm × 500 µm, which then converge into a central channel with a cross section of 500 µm × 500 µm. The gas is introduced into the middle channel with a cross section of 125 µm × 125 µm. The flow visualization of the gas bubbles in water and in the nanofluid is conducted using a Photron highspeed CMOS camera at a speed of 1000 frames/s, as observed through a 4× microscope objective. This experiment allows direct visualization and comparisons of bubble sizes for the different fluids. Results and Discussion For the bubble column experiments, the gas holdups that vary with gas velocity and nanoparticle concentrations in the 2-in. (5.08 cm) column are given for two different gas hold-up measurement techniques in Figures 5 and 6. Figure 5 uses the pressure-drop method, with nanoparticle concentrations that vary over a range of 0.48-1.40 wt %. Also shown in the figure are the gas holdups for 60-µm fluidized catalytically cracked (FCC) particles with the concentrations varying over a range of 1.052.03 wt %. The error in the gas holdup measurement using the pressure-drop method is within 1%. The results indicate that, for FCC particles, the gas holdups are consistently less than those for the nanofluid. At superficial gas velocities of >7 cm/s,
Figure 6. Gas holdup in a 2-in. (5.08-cm) bubble column measured using the liquid-expansion method.
the gas holdup in the nanofluid is consistently higher than that in water. At superficial gas velocities of
