Optimal concentration of alumina nanoparticles in molten Hitec salt to maximize its specific heat capacity

International Journal of Heat and Mass Transfer 70 (2014) 174–184

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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt

Optimal concentration of alumina nanoparticles in molten Hitec salt to maximize its specific heat capacity Ming Xi Ho a, Chin Pan a,b,c,⇑ a

Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC c Low Carbon Energy Research Center, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC b

a r t i c l e

i n f o

Article history: Received 20 June 2013 Received in revised form 31 October 2013 Accepted 31 October 2013

Keywords: Thermal storage Molten salt Specific heat capacity Nanofluid

a b s t r a c t The investigation experimentally studies the optimal concentration of alumina nanoparticles in doped molten Hitec that maximizes its specific heat capacity. A simplified model of the interfacial area is developed to explain the optimal concentration. The specific heat capacities of pure Hitec and nano-Hitec fluid are measured using a differential scanning calorimeter (DSC), and the microstructures following solidification are observed using a scanning electron microscope (SEM). A novel sampling apparatus and process for preparing molten Hitec nanofluids were developed to prevent the precipitation of nanoparticles. An optimal concentration of 0.063 wt.% is identified as yielding the greatest enhancement of specific heat capacity of 19.9%. At a concentration of 2 wt.%, the detrimental effect of the dopant nanoparticles on the specific heat capacity is evident at all temperatures. The negative effect is more significant than that predicted by the thermal equilibrium model. The SEM images following the solidification of samples and the developed model reveal the uniform dispersion of nanoparticles with negligible agglomeration at concentrations of under 0.016 wt.%. The agglomeration becomes significant and the particle clusters seem to be inter-connected at high concentrations. Moreover, the optimal concentration is approximately the concentration at which the contributions of isolated particles and clusters of sizes from 0.2 to 0.6 lm in the interfacial area to the specific heat capacity are equal. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Like photovoltaic systems, concentrating solar power systems (CSP) effectively supply renewable energy to help reduce global carbon dioxide emissions. For example, the DESERTEC solar thermal power project, associated with a group of 12 companies in Africa and Europe, aims to meet as much as 15% of European electricity demand by 2050 using amount of renewable solar energy from the desert in North Africa [1]. A solar thermal power plant requires effective energy storage and so the working fluid critically affects its performance. High-temperature molten salt typically has a high heat capacity and is effective as a working fluid for CSP systems [2,3]. Kearney et al. [4] noted that molten salt can increase electrical efficiency and reduce the levelized cost of electricity. However, the high freezing point of molten salt may result in its solidification on a cloudy day or at night. Some studies of the fluid flow and heat transfer of molten salts have been published. Kungi et al. [5] revealed that the molten salt ⇑ Corresponding author at: Low Carbon Energy Research Center, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC. Tel.: +886 35715131x34320; fax: +886 35720724. E-mail address: [email protected] (C. Pan). 0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.10.078

exhibits poor heat transfer performance resulting in which cause non-uniform heating and laminarization phenomena. Wu et al. [6] experimentally investigated heat transfer characteristics of molten salt (LiNO3) in a circular tube, and compared them with well-known empirical results. They found that the correlations of Sieder-Tate, Petukhov and Hausen agree closely with their experimental data for Prandtl numbers from 0.7 to 59.9. Wu et al. [7] determined the heat transfer coefficient for the turbulent and transition flows of molten Hitec salt (53% KNO3 – 40%NaNO2 – 7%NaNO3, mol.%) in a circular tube. The correlation of Hausen and Gnielinski agreed closely well with their data on transition flow and the correlation of Sieder-Tate agreed excellently with their data on turbulent flow. Molten salt normally exhibits with low thermal conductivity, and effectively improving its thermal properties is of great interest. Several studies [8–10] utilized alternative heat transfer salt, i.e., Hitec, experimentally to simulate lithium-beryllium fluoride salt (FLiBe) in a molten salt flow loop, and developed a packed-bed channel to enhance the heat transfer performance of Hitec flow. Their experimental results demonstrated that adding stainless steel spheres effectively enhanced heat transfer performance but also clear obviously increased the pressure drop in a circular tube. Yang et al. [11] experimentally demonstrated that the Nusselt

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Nomenclature At C Cp dc dp Nc Np Np,count Nt mp,t mp,i msalt

total interfacial area, m2 concentration, wt.% specific heat capacity, kJ/kg K diameter of cluster, m diameter of nanoparticle, m number of clusters number of isolated particles number of isolated particles determined from SEM image total number of nanoparticles total weight of particles, kg weight of single nanoparticle, kg weight of Hitec eutectic, kg

numbers of Hitec that flows in a spiral surface on the tube of such a tube are on average three times those of Hitec in smooth tubes. In 1902, Maxwell [12] proposed enhancing the thermal conductivity of fluids by dispersing solid particles with diameter of the order of millimeters or micrometer with high thermal conductivity. The experimental study of Hamilton and Crosser [13] confirmed this Maxwell’s theoretical prediction, and determined that the geometry of particles considerably influenced their thermal conductivity. Choi and Eastman [14] were first to reveal the abnormal enhancement of the thermal conductivity of liquid by doping with nanometer scale particles and they called such a fluid that contained nanoparticles ‘‘nanofluids’’. Numerous investigations have reported similar thermal enhancements of nanofluids [15–18]. The mechanisms of enhancement of the effective thermal conductivity of nanofluids include the Brownian motion, aggregation, the shape effect of nanoparticles and layer of the solid–liquid interfaces [19–22]. The enhancement of thermal conductivity depends strongly on the size, shape, concentration and dispersion of nanoparticle, the fluid temperature and the surface area of the solid–liquid interface [23–27]. The literatures include some investigations of the specific heat capacity of nanofluids. Wang et al. [28] used mathematical models of statistical thermodynamics to examine the specific heat capacity of nanoparticles. They found that it increased as the size of the particles was reduced to the nanoscale. For example, the specific heat capacity of nanoscale CuO particles exceeds that of coarse particles above 225 K. The experimental results by Wang et al. [29] demonstrated that the specific heat of Al2O3 particles in nanoscale was increased around 15% in the temperature range 200 to 370 K, and especially at higher temperatures within that range. Snow et al. [30] shows that the specific heat capacity of 13 nm hematite (aFe2O3) was higher than that of bulk particles with temperature is increased. Nieto de Castro et al. [31] reported that the specific heat capacity of ionic fluid that was doped with multiwall carbon nanotubes was anomalously enhanced at temperatures between 60 and 90 °C. Nelson and Banerjee [32] observed that the dispersion of graphite nanoparticles (0.6 and 0.3 wt.%) in polyalphaolefin increased its specific heat capacity by approximately 50%. Conversely, several studies have reported doping with nanoparticles reduced its specific heat capacity. For example, Zhou and Ni [33] and O’Hanley et al. [34] found that the specific heat capacity of nanofluid falls as the concentration of nanoparticles increases between 20 and 55 °C. This finding is consistent with predictions based on the mixing theory based on the assumption of thermal equilibrium between particles and fluid. This thermal equilibrium model can be expressed as [35]:

Greek symbols q density, kg/m3 u volume of isolated particles as fraction of total solid volume Subscripts f base fluid max maximum nf nanofluid p nanoparticle

Cpnf ¼

/ðqn Cpn Þ þ ð1  /Þðqf Cpf Þ /qn þ ð1  /Þqf

ð1Þ

where Cp is the specific heat capacity, / is the volume fraction of nanoparticles, and the subscripts nf, f, n, refer to the nanofluid, base fluid and nanoparticle, respectively. The studies of above mentioned literature reveal that particle size and operating temperature may be the significant factors affecting the specific heat capacity of nanofluid. Bridges et al. [36] studied the specific heat capacity of the ionic liquid with that doped nanoparticles of Al2O3. Their results reveal that the specific heat capacity of ionic liquid with doped nanoparticles of Al2O3 is enhanced about 30%. Shin and Banerjee [37–41] and Tiznobaik and Shin [42] investigated the specific heat capacity of carbonate and alkali chloride salts with dispersed nanoparticles of SiO2. They found that the specific heat of molten salt nanofluid was enhanced by 15–124% for concentrations from 1 to 1.5 wt.%, respectively. Shin and Banerjee proposed three mechanisms of this anomalous enhancement, including the size effect of nanoparticles and the high thermal resistance between solid–fluid interfaces and the semi-solid layer around the nanoparticles. Special needle-like structures and a percolation network in the salt eutectic have been observed in the scanning electron microscopic (SEM) and transmission electron microscopic (TEM) images of solidified molten salts with nanoparticles [37,42], in which the needle-like structures may be formed by the effect of thermophoresis on the semi-solid layers. Jung et al. [43,44] developed a simple analytical model for the specific heat capacity of water-based nanofluid and alkaline metal carbonate salt eutectic mixture with dispersed nanoparticles with different geometries. The model proposed that the density, size and geometry of the nanoparticle and the compressed phase at the solid–liquid interface are significant factors for the enhancement of specific heat capacity of nanofluid. Dudda and Shin [45] showed that the dispersion of SiO2 nanoparticle of size 5, 10, 30, 60 nm in binary nitrate eutectic increased its specific heat capacity by 8–24% in liquid state. The present study investigates the optimal concentration of dopant nanoparticles that maximizes the specific heat capacity of molten salt (Hitec). Hitec (53%KNO3 + 40%NaNO2 + 7%NaNO3, mol.%), a nitrate salt, is the base fluid that is used in this study, and the suspended nanoparticles are Al2O3. The nano-Hitec fluid in this study has no surfactant, allowing possible agglomeration and precipitation of nanoparticles. To solve these problems, an innovative sampling rig for preparing a high-temperature molten was developed. Uniform and reproducible molten salt nanofluids were made in a high-temperature sampling process. The specific

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heat capacity of molten Hitec with or without nanoparticles was measured using a differential scanning calorimeter (DSC) at the Industrial Technology Research Institute (ITRI) of Taiwan. The specific heat of pure Hitec was firstly measured and compared with that reported in the literature to confirm the accuracy of the measurement method that was utilized in this investigation. The same method, DSC, was then used to measure the specific heat capacity of molten Hitec that contained various concentrations of Al2O3 nanoparticles. The nanostructures of solidified samples of pure Hitec or Hitec with various concentrations of nanoparticles of Al2O3 are observed using a SEM. This study develops a simplified model of the interfacial area of the nano-Hitec fluid to explain the optimal concentration of nanoparticles. 2. Experimental 2.1. Preparation and sampling Hitec is a ternary eutectic salt that is a mixture of sodium nitrate (NaNO3), potassium nitrate (KNO3) and sodium nitrite (NaNO2) in proportions of 7, 53, 40 mol%, respectively. In this study, reagent-grade salt is used without purification. The Hitec with nanoparticles is prepared as follows; 17.9 g of NaNO3, 135.7 g of KNO3 and 102.4 g of NaNO2 were measured out using a high-precision balance (Precisa/XS 625 M) and mixed. Al2O3 nanoparticles were added as a 20 wt.% Al2O3/water nanofluid, which was purchased from Sigma–Aldrich Co. LCC. The particle sizes are less than 50 nm. The Al2O3/water nanofluid was sonicated in an ultrasonic cleaner (Tohama/LEO-3002S) for 100 min, and doped at a particular concentration into the Hitec eutectic. The Hitec that contained nanoparticles of Al2O3 was then melted using a heater at 350 °C for one hour, before being stirred for three hours. Subsequently, the novel sampling rig, described in detail below, was used in the preparation of well-mixed and reproducible samples. In this study, nothing prevented the possible agglomeration of nanoparticles in the Hitec salt that was doped with Al2O3 nanoparticles. Accordingly, a favorable suspension of nanoparticles was not guaranteed and the precipitation of nanoparticles could result in

Pneumatic Cylinder

Linking hose

Ball valve

Crucible Heater Molten salt

Sampling tubes

the loss of non-repeatability of the sampling. Therefore, an new method of preparation and sampling rig for high-temperature molten salt with nanoparticles was developed. Fig. 1 schematically depicts this experimental apparatus for the preparation of Hitec and its nanofluid. The special design consisted of a crucible material that was made of stainless steel 316 (SS316) and filled with nano-Hitec fluid, and a stirrer with a surface of SS316 to mix the Hitec with the nanoparticles. To reduce the required sampling time of the nano-Hitec fluid, three sampling tubes were dipped to the bottom of crucible after the nanofluid had been mixed for around 180 min of stirring. Subsequently, the ball valve was closed, the Hitec melt that was collected in the sampling tubes was moved away from the high-temperature crucible using an automatic elevating system. Finally, the nanofluid samples in the three tubes were cooled through forced convection by a cooling fan. This approach significantly reduced the cooling time and eliminated the significant precipitation of nanoparticles. The prepared samples were consistent and the repeatability of the measurements was ensured. All of the samples were heated to 120 °C for three hours to vaporize the moisture in the sample before each measurement of specific heat capacity was made. The microstructures of pure Hitec and Hitec that was doped with Al2O3 nanoparticles were investigated after solidification using a SEM (JEOL/JSM-6330F) at the National Tsing Hua University (NTHU). 2.2. Measurement of specific heat capacity In this investigation, specific heat capacity was measured using a DSC (Perkin Elmer/DSC 7) at the Industrial Technology Research Institute (ITRI) of Taiwan. The measurements were made according to the ASTM (American Society for Testing and Materials) standard E1269. Perkin Elmer U-type aluminum pans (No. 02190041) were used as sample pans. The mass of sapphire was 27.850 mg and the mass of the sample was measured using an analytical balance to five decimal places (Mettler Toledo/AG245) at the ITRI. The specific heat of three pure Hitec samples was measured and the temperature was then ramped up to 350 °C at a fixed heating rate of 20 °C/ min. The masses of the pure Hitec samples were 11.100, 13.840, and 12.720 mg, respectively. The samples with added Al2O3 nanoparticles at a concentration of under 2 wt.%, and their specific heat capacities were measured. Four samples for each concentration of nanoparticles are measured. Their temperature was ramped up from room temperature to 350 °C at a heating rate of 40 °C/min. The heating rate was higher than that applied to the pure Hitec to minimize the precipitation of nanoparticles during measurement. The sample mass of the nano-Hitec fluid ranged from 10.150 to 23.630 mg. Mean values and standard error of the mean from three or four measurements are presented. New samples and aluminum pans were always used to ensure the independence of all measurements and to obtain base, standard and sample line. From these lines, each specific heat capacity was calculated using the computer software that was incorporated in the DSC. Furthermore, at the ITRI, before each measurement, the DSC was calibrated with the melting point of indium (156.66 °C). The results indicate that the mean melting point of indium is 156.63 °C, which is well within the range of acceptable temperatures 156.6 ± 2 °C.

Stirrer 3. 3.Results and discussion 350°C

Stirring Hotplate

Fig. 1. Schematic diagram of preparation and sampling rig for high-temperature molten salt.

3.1. Specific heat capacity of Hitec with Al2O3 nanoparticles Fig. 2 compares the specific heat data of pure Hitec salt in this study with those in the literature [7,46–48]. The study that was performed at Oak Ridge National Laboratory [46] and that by Tufeu

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Fig. 2. Specific heat capacity measurement of pure Hitec salt and comparison with that reported in literatures.

et al. [47] both demonstrated that the specific heat of Hitec melt is nearly constant around 1.56 (kJ/kg K), as indicated by the dash line. However, Wu et al. [7] in 2012 reported a lower specific heat capacity of pure Hitec of about 1.424 (kJ/kg K). The four rhombuses in the figure that reveal that the specific heat decreases slowly as the temperature increases represent experimental data obtained by Janz et al. in 1983 [48]. The solid line with the error bars represents the data that are obtained herein investigation, which reveal that the specific heat of pure Hitec is 1.37–1.48 (kJ/kg K) and the standard error of the mean is 0.01–0.05 (kJ/kg K). At T = 200 °C, the specific heat of liquid Hitec obtained in this investigation is approximately 1.38 (kJ/kg K) and decreases slowly as temperature is increased to 270 °C, after which it increases with temperature to about 1.5 (kJ/kg K) at 350 °C. Fig. 2 shows the results herein provide a lower bound, and are in good agreement with the most recent data of Wu et al. [7], which they published in 2012. This good consistency between the measured of the melting/solidification temperatures, chemical decomposition temperatures, thermal conductivities and thermal diffusivities [49] verify that the Hitec salt that was prepared in the present study was pure eutectic and not polluted during the preparation process. The good comparison in the measurement of specific heat capacity also indicates that the

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present measurement procedure may be employed to measure the specific heat of Hitec that contains Al2O3 nanoparticles. Fig. 3 demonstrates that the addition of less than 2 wt.% Al2O3 nanoparticles significantly increases the specific heat of Hitec melt at low temperatures. Additionally, the specific heat of Hitec melt that contains Al2O3 nanoparticles generally declines slowly as the temperature increase. An examination of Fig. 3 reveals that the effect of the concentration of Al2O3 particle concentration on the specific heat is quite non-uniform. Furthermore, the temperature influences on the effect of the concentration. At temperatures between 200 °C and approximately 330 °C, increasing the concentration of nanoparticles tends to increase the specific heat, as long as the concentration does not exceed than or equal to 1 wt.%. At temperature about 335 °C, adding of Al2O3 nanoparticles at concentrations of greater than or equal to 0.5 wt.% has a negative effect on specific heat. At all temperatures, a concentration of 0.063 wt.% provides the largest enhancement of specific heat. This concentration may be regarded as the optimal concentration for the Hitec melt that contains Al2O3 nanoparticles. Fig. 4 illustrates the repeatability of the measurement of specific heat capacity of Hitec salt for the pure eutectic and the addition of 0.063 wt.% Al2O3 nanoparticles, respectively. The measurements for the three samples of pure Hitec indicate that the specific heat capacity varies from 1.31 to 1.43 (kJ/kg K) at 200 °C and is relatively independent of temperature for temperature below 270 °C, after which the specific heat increases with temperature and the data for the three samples tend to converge together to about 1.5 (kJ/kg K) at 350 °C. On the other hand, the measurements for the four samples of nano-Hitec with addition of 0.063 wt.% Al2O3 nanoparticles reveal that the specific heat capacity varies from 1.58 to 1.73 (kJ/kg K) for temperature below 250 °C, after which the specific heat decreases slightly as temperature is increased to 350 °C. Two unexpected fall-and-rise herein occurs at 330 °C and 340 °C for sample 4 and sample 1, respectively. Fig. 4 indicates that the repeatability is within 10%, which may be considered as the uncertainty of measurement. The standard error of the mean for the measurement of specific heat of nano-Hitec with various concentrations of nanoparticles at three specific temperatures is given in Table 1, as will be discussed in the following paragraph. Table 1 presents a detailed examination of the effect of the concentration of Al2O3 nanoparticles on the specific heat of Hitec melt at three temperatures: 200 °C, 275 °C and 350 °C. The table presents experimental data, including standard error of the mean from

2 Pure 0.016 wt.% 0.0625 wt.% 0.125 wt.% 0.25 wt.% 0.5 wt.% 1 wt.% 2 wt.%

Specific Heat (kJ/kgK)

1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 200

225

250

275

300

325

350

Temperature (oC) Fig. 3. Variation of specific heat capacity with temperature for the pure and the different nanoparticle concentration of Hitec nanofluid.

Fig. 4. Repeatability of specific heat capacity measurements with temperature for the pure Hitec and that with nanoparticles of Al2O3 at optimal concentration of 0.063 wt.%.

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Table 1 The specific heat capacity and the enhancement of the pure and Hitec nanofluid in temperature of 200 °C, 275 °C, and 350 °C, respectively. Standard error of the mean is shown in the parenthesis under the specific heat capacity. Specific heat capacity (kJ/kg K) (standard error of the meana)

Enhancement (%)

Concentration (wt.%) Pure eutectic 0.016 0.063 0.125 0.25 0.5 1 2

a

200 °C 275 °C 350 °C 200 °C 275 °C 350 °C 1.37 (0.03) 1.36 (0.02) 1.48 (0.01) – – – 1.50 (0.02) 1.49 (0.02) 1.51 (0.02) 9.5 9.6 1.9 1.64 (0.03) 1.63 (0.03) 1.57 (0.02) 19.9 19.9 6.1 1.57 (0.03) 1.53 (0.02) 1.51 (0.01) 14.7 12.7 6.1 1.50 (0.01) 1.49 (0.01) 1.49 (0.01) 9.8 9.6 0.3 1.48 (0.02) 1.46 (0.02) 1.46 (0.02) 7.8 7.8 1.7 1.45 (0.01) 1.45 (0.01) 1.44 (0.01) 6.1 6.5 2.8 1.31 (0.05) 1.32 (0.04) 1.40 (0.02) 4.1 2.7 5.7 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Pn 2 , where Standard error of the mean is defined as: C is the mean value of specific heat capacity, n is the number of measurements of the sample. ðC p;i C p Þ p i¼1 nðn1Þ

measurements of three samples of pure eutectic or four samples of Hitec with nanoparticles and percentage enhancements. At 200 °C and 275 °C, the enhancement achieved using the lowest concentration of 0.016 wt.% is 9.5 to 9.6%. The enhancement further increases to 19.9% as the concentration is increased to 0.063 wt.%. Further increasing the concentration slowly reduces the enhancement. A concentration of 1 wt.% yields an enhancement of only 6.1 to 6.5%. In this investigation, this negative effect on the specific heat was observed all temperatures when the concentration was increased to 2 wt.%. The effect of concentration is much weaker at 350 °C than at low temperatures. The peak enhancement at concentrations of 0.063 wt.% and 0.125 wt.% was only 6.1% at 350 °C. As indicated earlier, increasing the concentration above 0.5 wt.% has a negative effect on specific heat capacity, apparently because at high temperatures increasing the temperature positively affects the specific heat capacity of pure Hitec melt but somewhat reduces that of the Hitec melt with Al2O3 nanoparticles. This negative effect of adding nanoparticles with a smaller specific heat is consistent with the theoretical prediction based on the traditional theory. Hence, a comparison of the data obtained herein with the theoretical prediction is of interest. In the calculation made using traditional mixing theory, i.e., Eq. (1), the specific heat capacity, according to the thermal equilibrium model, decreases from 0.10 to 0.41% at 275 °C as the concentration of the Al2O3 nanoparticles increases from 0.5 to 2 wt.%. The density of Hitec was calculated using the equation that was used by Sohal et al. [50] and the specific heat of pure Hitec was as measured in the present investigation. The density and heat capacity of Al2O3 particles were evaluated using the correlation for bulk material that was used by Millers [51]. Since the specific heat of nanoparticles may exceed that of the corresponding bulk material, if the specific heat of nanoparticles is assumed to be 15% higher than the corresponding bulk material, then, as suggested in the literature [29], the specific heat of the nano-Hitec, according to the thermal equilibrium formula decreases from 0.04% to 0.17% at 275 °C as the concentration is varied from 0.5 to 2 wt.%. Therefore, the thermal equilibrium model seems to underestimate the detrimental effect of adding nanoparticles at high temperatures. To understand the cause of the enhancement of specific heat of Hitec melt by adding a low concentration of Al2O3, the micro-structures of solidified Hitec melts with and without Al2O3 nanoparticles are investigated using scanning electron micrography, as will be discussed in the following subsection. 3.2. Scanning electron micrography (SEM) Fig. 5 displays an SEM image of pure Hitec eutectic that contains no dopant nanoparticles. The figure reveals that the surface of the salt is rough because the grains had sizes of about 1 to 3 lm. Figs. 6–14 show the images of the microstructures of the nano-Hi-

Fig. 5. SEM image of pure Hitec solid.

Fig. 6. SEM image of Hitec with 0.016 wt.% of nanoparticles of Al2O3 after solidification (10,000x magnification). Yellow arrows indicate isolated particles. Red circles indicate clusters with diameter in microns under or above the circle. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

tec fluids with various concentrations of Al2O3 nanoparticles. A comparison of these images reveals that the grains in Hitec that contains 0.016 wt.% or 0.25 wt.% are much smaller than those in pure Hitec or a mixture with other concentrations. Figs. 6 and 7

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Fig. 7. SEM image of Hitec with 0.016 wt.% of Al2O3 after solidficatioin (30,000x magnification). Yellow arrows indicate isolated particles. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

Fig. 9. SEM image of Hitec with 0.063 wt.% of nanoparticles of Al2O3 after solidification (30,000x magnification). Yellow arrows indicate isolated particles. Red circles indicate clusters. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

Fig. 8. SEM image of Hitec with 0.063 wt.% of nanoparticles of Al2O3 after solidification (10,000x magnification). Yellow arrows indicate isolated particles. Red circles indicate clusters with diameter in microns under or above the circle. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

Fig. 10. SEM image of Hitec with 0.125 wt.% of nanoparticles of Al2O3 after solidification (10,000x magnification). Yellow arrows indicate isolated particles. Red circles indicate clusters with diameter in microns under or above the circle. This SEM was taken using the original sample after the review of the original manuscript in October, 2013. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

present SEM images of Hitec with 0.016 wt.% Al2O3 nanoparticles after solidification under 10,000x and 30,000x magnifications, respectively. The image at 10,000x magnification presents a few isolated nanoparticles, indicated by yellow arrows, randomly distributed in the salt. Small clusters with minor agglomerations of nanoparticles are also observed, and are roughly indicated by red circles, the number under or above which represents the diameter of the cluster in microns. At a magnification of 30,000x, Fig. 7 clearly indicates that isolated nanoparticles with mean diameter of 30 to 50 nm are dispersed in the salt approximately uniformly. Section 3.4 will discuss the optimal concentration of enhancement of specific heat capacity in detail based on a simplified model of the interface area which consists the surface area of clusters and isolated particles. Figs. 8 and 9 present SEM images of Hitec with 0.063 wt.% nanoparticles, which is the concentration that maximizes the specific heat, under 10,000x and 30,000x magnifications, respectively.

Again, the yellow arrows and red circles in the SEM images indicate the location of isolated particles and clusters, respectively. These two figures show that, even at a relatively low concentration, nanoparticles aggregate as clusters with sizes of 0.2 to 0.6 lm in the grain boundaries of Hitec. The clusters seem not to form networks. This agglomeration appears to be greater and the formed clusters become larger and increasingly inter-connected as the concentration of Al2O3 nanoparticles in Hitec is increased, as indicated by Figs. 10–14, which correspond to nanoparticle concentrations of 0.125 wt.%, 0.25 wt.%, 0.5 wt.%, 1 wt.% and 2 wt.%, respectively. 3.3. Mechanism of enhancement of specific heat capacity The above SEM images for Hitec with various concentrations of nanoparticles seem to reveal no particular physical structure at

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Fig. 11. SEM image of Hitec with 0.25 wt.% of nanoparticles of Al2O3 after solidification (10,000x magnification). Yellow arrows indicate isolated particles. Red circles indicate clusters with diameter in microns under or above the circle. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

1

3

1

2

1

1

1

1

2

1

Fig. 13. SEM image of Hitec with 1 wt.% of nanoparticles of Al2O3 after solidification (10,000x magnification) Yellow arrows indicate isolated particles. Red circles indicate clusters with diameter in microns under or above the circle. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

1 1

1

2

1

1

1

1

d0.2 d1

1

d0.4

d1

1 d0.2

d0.4

d0.2

d1 d0.4

d1

d1 1

d1

1

1 1

d0.2

d0.4

d0.2 d1

d0.6

d0.4

d1

d1

d0.8 1

Fig. 12. SEM image of Hitec with 0.5 wt.% of nanoparticles of Al2O3 after solidification (10,000x magnification). Yellow arrows indicate isolated particles. Red circles indicate clusters with diameter in microns under or above the circle. This SEM was taken using the original sample after the review of the original manuscript in October, 2013. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

concentration of 0.063 wt.% that would explain why that concentration yields the highest specific heat capacity. The works of Shin and Banerjee [37–39] on the synthesis of silica-nanofluids in alkali chloride salt eutectics proposed three potential mechanisms for the enhancement of the specific heat of (chloride) salt upon the addition of nanoparticles. They are as follows; (1) the specific heat capacity of silica nanoparticles exceeds that of bulk silica because the former have a high specific surface energy; (2) a high interfacial thermal resistance exists between the nanoparticles and the surrounding liquid molecules, and (3) layering of the liquid molecules around the surfaces of nanoparticles forms a semi-solid layer. Indeed, their SEM images obtained after melting/solidification [38] seem to show a percolation network substructure of intercon-

Fig. 14. SEM image of Hitec with 2 wt.% of nanoparticles of Al2O3 after solidification (10,000x magnification).

nected the SiO2 nanoparticles. Additionally, the literature [37,42] on carbonate salt with SiO2 nanoparticles reveals the needle-like structures of nanoparticles in the molten salt eutectic. They proposed that this needle-like structure may enhance the specific heat capacity. Neither the interconnecting network of nanoparticles nor the needle-like structure is observed in the solidified nano-Hitec fluid in the present investigation. Therefore, the apparently significant factors that significantly influence the specific heat capacity of the nano-Hitec fluid in this study may be the pattern of the nanoparticles and the temperature. The groups of particles with sizes of tens of microns and the moderate degree interconnection in the high-concentration solution, presented in Figs. 13 and 14. The findings of this present study suggest that the concentration that yields favorable uniform dispersion and the optimal pattern of particles or clusters may maximize the specific heat. To confirm

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this explanation, a simplified model of the interfacial area is developed, as will be elucidated in the next subsection. Figs. 6 and 7 display the superior dispersion of nanoparticles in the solidified Hitec mixture at a concentration of 0.016 wt.% and they demonstrate that the addition of such small concentration of nanoparticles increases the specific heat capacity of Hitec by a significant 9.5–9.6%. However, this concentration is too low to provide enough nanoparticles and sufficient particle–fluid interfaces to improve the specific heat capacity. As the concentration of nanoparticles is further increased to 0.063 wt.%, although some degree of particle clustering occurs, forming clusters with sizes of around 0.2 to 0.6 lm. The formed clusters are small enough, and the number of clusters and the interfacial area density between the solid clusters and the basic fluid are large enough, to increase the specific heat capacity. As the concentration is increased, the agglomeration becomes more intense and the formed clusters become larger. The specific heat capacity starts to decline. At concentration as high as 1 to 2 wt.%, the clustering of particles after solidification is highly significant and clusters with size of several microns appear; the interfacial area density between the cluster and the basic fluid decreases as does the specific heat of the mixture, especially at high temperatures. 3.4. Simplified model of interfacial area For a specified concentration of nanoparticles, the total area of the solid–fluid interface (At) in a nano-Hitec can be calculated from the areas of isolated particles and clusters in SEM images, as follows: 2

2

At ¼ N p pdp þ Nc pdc

3

up ¼

N p ¼ N t up

ð3Þ

In the above equation, Nt is determined as the total weight of nanoparticles (mp,t) divided by the weight of a single nanoparticle (mp,i) and is given by:

  mp;t 6msalt 0:01C ¼ mp;i qp pd3p 1  0:01C

"

ð4Þ

dc (lm) up (%)

0.2 0.4 0.6 0.8 1 1.5 2 – –

0.016

0.063

0.125

0.25

0.5

1

269 4412 7 0 0 0 0 0 0 0.2 83.5

283 4761 11 2 1 0 0 0 0 0.26 41.4

87 811 8 4 2 0 0 0 0 0.31 3.0

164 2100 18 5 4 2 2 0 0 0.37 3.0

29 156 5 5 1 1 9 0 0 0.64 0.10

73 624 5 6 0 0 10 4 2 0.87 0.10

dc

!# ð6Þ

3

dp

Therefore, the number of clusters at a particular concentration can be determined using the following equation:

Nc ¼

Nt  Np 0:68ðdc =dp Þ

ð7Þ

3

From the data in Table 2, the diameter of the clusters and the volume fraction of the isolated particles can be fitted as power functions of concentration, plotted in Figs. 15 and 16, respectively:

dc ¼ a1 C b1

ð8Þ

up ¼ a2 C b2

ð9Þ

1

data in the present study Fitted line 0.8

dc(10-6m)

0.04

Concentration (wt.%)

ð5Þ

3

þ N c dc

3

Nt ¼ N p þ Nc 0:68

Table 2 Quantification of isolated nanoparticles and clusters formed of the nano-Hitec fluid from the SEM images.

Np,count (Np,count)2/3 Nc

3 ðNp;count Þ3=2 dp

where Np,count is the number of isolated particles that are identified in an SEM image. To account for the fact that isolated particles may be embedded in the salt grains, the number of isolated particles that is determined from the 2-D SEM image is raised to the power of 3/2 in Eq. (5). The number of clusters identified in the SEM image is not changed because as the much larger than the isolated nanoparticles so the number of clusters that are observed in the 2-D image should be close to that in the real 3-D sample. The data in Table 2. reveal that Np,count decreases from 269 to 29 and the mean diameter of the clusters increases from 0.2 to 0.87 lm as the concentration of nanoparticles increases from 0.016 wt.% to 1 wt.%. The conservation of the number of nanoparticles requires that for any concentration, the sum of the number of isolated particles and the number of particles in the clusters equals the total number of particles. Additionally, although particles in the cluster may be stacked orderlessly, it is assumed to be arranged to form the body-centered cubic (BCC) structure such that the packing factor is 0.68. Further discussion on this assumption will be addressed later after the illustration of model prediction in Fig. 17 and associated discussion. Therefore,

where C denotes the concentration of nanoparticles in nano-Hitec fluid (wt.%); qp is the density of Al2O3 particles, and msalt is the mass

Size (lm)

ðNp;count Þ3=2 dp

ð2Þ

where Np and Nc are the total number of isolated particles and clusters in the nano-Hitec fluid, respectively; dc is the mean diameter of the clusters and dp = 40 nm is the mean diameter of the isolated particles. The total number of isolated particles, Np, can be determined as the total number of nanoparticles in nano-Hitec (Nt) multiplied by the volume of the isolated particles as a fraction of the total solid volume of all isolated particles and clusters up :

Nt ¼

of the pure Hitec salt. The volume of isolated particles as a function of the total solid volume, including that of the isolated particles and clusters, in the mixture, may be estimated from the number of isolated particles and the number of clusters, counted in the SEM image, for a known concentration:

dc=7.49x10 C0.359 (m) -7

0.6

0.4

0.2

0 0

0.2

0.4

0.6

0.8

Concentration (wt.%) Fig. 15. Mean cluster diameter as a function to the particle concentration.

1

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data in the present study Fitted line

Volume fraction (ϕ p)

0.8

ϕ p=9.95x10-4C-1.84 0.6

0.4

0.2

0 0

0.2

0.4

0.6

0.8

1

Concentration (wt.%) Fig. 16. Volume fraction of isolated particles in all solid volume as a function of the particle concentration.

Fig. 17. The area of solid–fluid interfaces, isolated particles and clusters of nanoHitec fluid with concentration less than 0.25 wt.%.

where a1 ¼ 7:49  107 , b1 ¼ 0:359, a2 ¼ 9:95  104 and b2 ¼ 1:84. In Eqs. (8), (9), dc is in meters and C is in wt.% Eq. (7) suggests that if Nt = Np, corresponding to up ¼ 1, no cluster is formed in the mixture. The concentration Cmax is considered to be the upper limit on particle concentration, above which particle agglomeration occurs and clusters are formed. From Eq. (9), 2 C max ¼ a1=b 2

ð10Þ

Substituting the fitting constants a2 and b2 that were obtained in this investigation yields Cmax = 0.023 wt.%. Consequently, the total solid–fluid interfacial area in the nano-Hitec fluid can be expressed as, 2

At ¼ Nt pdp for C  C max

ð11Þ

" # 0:06msalt a2 C 1þb2 ð1  a2 C b2 Þ 1b1 ; for C At ¼ þ C 0:68a1 qp ð1  0:01CÞ dp > C max

the concentration increases over of Cmax, clusters are formed and the interfacial area decreases. As the nanoparticle concentration increases above 0.023 wt.%, the number and the mean diameter of the clusters increase and the number of isolated particles declines. A particle concentration of 0.048 wt.% minimizes the interfacial area. Assuming the particles stacking in FCC or HCP structure will result in a packing factor 0.74. However, as it can be seen from discussion above, it does not affect the explanation on the concentration with the maximum enhancement of the specific heat capacity for nano-Hitec in the present study. The only distinct difference due to different structure is that the solid–fluid interfacial area in the nano-Hitec slightly smaller for clusters with the FCC or HCP structure than that with BCC structure. The region that is obliquely shaded in Fig. 17 is the region in which all particles are isolated without the formation of clusters. These isolated nanoparticles may have been responsible for the enhancement of the specific heat capacity of the nano-Hitec fluid by its increasing the total interfacial area. The agglomeration above Cmax may initially reduce the interfacial area. The interfacial area may rise again as the concentration increases due to increase number and diameter of clusters formed. Fig. 17 demonstrates that for the optimal concentration of 0.063 wt.%, which maximizes the specific heat capacity in this study, is approximately the concentration at which the solid–fluid interfacial area contributes to the increase in heat capacity as much as do the isolated particles and clusters. As can be seen from Fig. 17, the total interfacial area at the optimal concentration is slightly less than the maximum value that corresponds at Cmax. As observed above from the SEM images, the diameters of clusters that are formed at a concentration of 0.063 wt.% are 0.2 to 0.6 lm suggesting that the equal contributions of isolated nanoparticles and clusters to the total interfacial area may yield the highest specific heat capacity. The other possible explanation is that the concentration that maximizes the enhancement of the specific heat capacity is close to 0.023 wt.%, which is the concentration at which nanoparticles agglomerate. The specific heat capacity increases with the concentration of isolated nanoparticles up to 0.023 wt.%. Increasing the particle concentration above 0.023 wt.% causes particle agglomeration that may reduce the increase in specific heat capacity. According to this analysis, the experimental optimal particle concentration of 0.063 wt.% may not be the actual optimal concentration. However, the enhancement of specific heat capacity for the concentration of 0.016 wt.%, which is very close to 0.023 wt.%, is only 9.5%, about a half of that for the optimal concentration revealed in this study. This further suggests that some agglomeration of nanoparticles forming submicrometer clusters may be the best for the enhancement of specific heat capacity.

ð12Þ

Fig. 17 shows the solid–fluid interfacial area, including isolated particles and clusters in the nano-Hitec fluid that contains particles at a concentration from 0 to 0.25 wt.%. The figure demonstrates that interfacial area is maximal at a concentration of 0.023 wt.%. As

4. Conclusions The optimal concentration of nanoparticles of Al2O3 in molten Hitec salt that maximizes its specific heat capacity is investigated. To prevent the precipitation of nanoparticles in the sampling process, a novel sampling apparatus and preparation process for molten Hitec salt with alumina nanoparticles were developed. The following conclusions are drawn from the results herein. (1) A concentration of Al2O3 nanoparticles of 0.063 wt.% in nano-Hitec fluid maximizes the enhancement of its specific heat capacity to 19.9%. Nano-Hitec fluids with lower or higher concentrations of nanoparticles of Al2O3 all have exhibited less enhanced specific heat capacities. In this investigation, a nanoparticle concentration of 2 wt.% has a

M.X. Ho, C. Pan / International Journal of Heat and Mass Transfer 70 (2014) 174–184

detrimental effect on specific heat capacity at all temperatures. The negative effect is larger than predicted from the thermal equilibrium model. (2) SEM images of particle concentration of 0.016 wt.% in nanoHitec fluid reveal the relatively uniform dispersion of nanoparticles with negligible agglomeration at concentrations, which is identified as the upper concentration limit at which nanoparticles remain isolated, based on both the simplified model and the data obtained herein in the present study. The mixture with the optimal concentration of 0.063 wt.% exhibits the most enhanced specific heat capacity, owing to the agglomeration of nanoparticles into from clusters with sizes of about 0.2 to 0.6 lm at the grain boundaries. (3) The agglomeration of nanoparticles in Hitec becomes stronger and the enhancement of specific heat capacity may even become negative as the concentration of the nanoparticles increases above 2 wt.%. (4) The simplified model of the interfacial area that is developed in this study demonstrates that the isolated nanoparticles may have enhanced the specific heat capacity of nano-Hitec by increasing its total interfacial area. Slight agglomeration of particles to form clusters with sizes from 0.2 to 0.6 lm, which provide about half of the total interfacial area, may be responsible for the maximal improvement of specific heat capacity at the optimal concentration of 0.063 wt.%.

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