Experimental investigation of heat transfer performance of molten HITEC salt flow with alumina nanoparticles

International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx

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

Experimental investigation of heat transfer performance of molten HITEC salt flow with alumina nanoparticles 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 4 September 2016 Received in revised form 4 November 2016 Accepted 4 November 2016 Available online xxxx Keywords: Molten salt flow Heat transfer Nanofluid Laminar flow

a b s t r a c t This study explores the effect of nanoparticle concentration on the laminar convective heat transfer performance of a molten nano-HITEC fluid in a mini circular tube. An innovative piston molten salt apparatus and a preparation process of molten HITEC nanofluid were developed to prevent the precipitation of nanoparticles during the measuring process. The results in this study demonstrate that the measurement of the mean Nusselt number of the pure HITEC fluid is in good agreement (within ±10%) with that published in the literature available. A concentration of nanoparticles of 0.25 wt.% in the nano-HITEC fluid, which maintains uniform dispersion for approximately 30 min, results in the maximum enhancement of the mean Nusselt number, i.e., 11.6%. A concentration of 0.063 wt.%, which is the concentration resulting in the maximum enhancement of the specific heat in our previous study (Ho and Pan, 2014), results in a 9.2% increase in the mean Nusselt number of HITEC nanofluid, and the precipitation phenomenon was not observed within an hour. In addition, a new correlation considering particle concentration for the laminar convective heat transfer performance of the nano-HITEC fluid is developed in this study, by which more than 93.9% of the experimental data can be predicted within ±10% of deviation. In conclusion, the HITEC fluid with concentration of alumina nanoparticles of up to 0.25 wt.% have all shown an increase in the heat transfer performance, and based on the previous investigation (Ho and Pan, 2014), also revealed positive effect on the specific heat capacity. Therefore, it may be an excellent working fluid for applications of thermal storage system in a concentrating solar thermal power system. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction A concentrating solar power (CSP) system is one of the significant energy technologies that can reduce carbon dioxide emission in the world. For example, the Desertec solar thermal power project, associated with a group of twelve companies in Africa and Europe, is aimed to provide as much as 13–15% of the electricity demand in Europe in 2050 using the huge renewable solar energy from the desert in North Africa [1]. On the other hand, the SunShot Initiative, started in 2011, is a collaborative national initiative of the United States, targeted to reduce total levelized cost of CSP generated electricity to less than USD $0.06/kWh by 2020 [2]. For a solar thermal power plant, energy storage and hence the working fluid is of critical importance for the performance. High temperature molten salt usually has high heat capacity, and it is quite suit-

⇑ Corresponding author at: Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC. E-mail address: [email protected] (C. Pan).

able in acting as the working fluid for CSP systems, though its high freezing point may result in solidification during a cloudy day or at night-time [3–5]. Kearney et al. [6] has pointed out that molten salt can increase the electrical efficiency and reduce the levelized electricity cost. There have been extensive studies on the fluid flow and heat transfer characteristics of molten salt flow. For example, Xiao et al. [7] studied heat transfer and pressure drop characteristics of molten salt flowing in a double tube helical heat exchanger using HITEC (53% KNO3 – 40% NaNO2 – 7% NaNO3, mol.%) as the hot side fluid, and deionized water that heated under subcritical and supercritical conditions as the cold side fluid. Their results demonstrated that the frictional pressure drop of molten HITEC flow in laminar and turbulent flow regimes agreed within 15% with that reported in related literatures, and the heat transfer characteristics of molten salt flow also showed good agreement, except for a part of flow rates with very low flow velocity. Wu et al. [8–10] conducted studies on the heat transfer coefficient for the turbulent and transition flows of molten lithium nitrate (LiNO3) salt and HITEC

http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.11.015 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved.

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Nomenclature C Cp D h(x)
h k L _ m Nu Pr T q00 Re U xL x

concentration, wt.% specific heat capacity, J/kg K tube diameter, m local convection heat transfer coefficient (W/m2K) mean convection heat transfer coefficient (W/m2K) thermal conductivity coefficient (W/mK) length, m mass flow rate, kg/s Nusselt number Prandtl number temperature, °C heat flux, W/m2 Reynolds number velocity (m/s) L xL ¼ DRePr , non-dimensional axial distance in entire length of test tube x x ¼ DRePr , non-dimensional axial distance

Greek symbols d thickness (m) g efficiency (%)

salt in a circular tube. Their data agreed well with the predictions from various classical correlations. Lu et al. [11,12] investigated the convective heat transfer in the horizontal and vertical annular ducts using HITEC salt as the working fluid. They found that the Nusselt numbers of HITEC flow in horizontal and vertical tubes are higher than that calculated from the Gnielinski correlation. Empirical correlations of molten salt flow in annual tube were thus developed based on their own experimental data. Shen et al. [13] reported the turbulent convective heat transfer of molten HITEC salt with non-uniform heating in a unilateral copper coating stainless steel tube. Based on a comparison of their data with the prediction of Sieder–Tate correlation, the Nusselt number of molten salt flow on the smooth side of the tube was mostly higher, while the Nusselt number on the copper coating side was mostly lower. A correlation of heat transfer performance of HITEC flow with nonuniform heating was thus established. Kunugi et al. [14] revealed that the molten salt exhibits poor heat transfer performance resulting in non-uniform heating and laminarization phenomena. In addition, Wu et al. [15] developed a low melting point (86 °C) and a high working temperature upper limit (550 °C) molten salt, and conducted a series of experiments on parabolic trough solar collector and heat transfer system. Their experimental data also indicated good agreement with the classical predictions of the Sieder–Tate and Gnielinski equations. He et al. [16] investigated heat transfer characteristics of molten HITEC flow in the shell and tube type heat exchanger with Reynolds number in the range of 400–2300 and operating temperatures of molten salt between 250 and 400 °C. Their results demonstrate that the heat transfer performance of molten salt flow was enhanced owing to the tube bundle structure of heat exchanger, and its Nusselt number increased by 3.5–5 times. Molten salts usually exhibit relatively poor heat transfer performance, and effectively improving its thermal properties is of significant interest. Several studies [17–19] investigated the structural surface design of heat receivers for the enhancement of heat transfer performance of molten salt flow. The results show that appropriately engraved transversally corrugation or spiral groove on the surface of heat receivers could increase heat transfer efficiency and reduce the losses due to natural convection and radiation. Empirical correlations based on the data of these experiments have

l q /

dynamic viscosity (Ns/m2) density, kg/m3 volume fraction

Subscripts ave average b bulk bf base fluid f fluid in inlet m mean max maximum nf nanofluid out outlet s solid p nanoparticles TC thermocouple w wall x axial location

also been proposed. These studies [20–22] utilized an alternative heat transfer salt, i.e., HITEC, to experimentally simulate lithium– beryllium fluoride salt (FLiBe) in a molten salt flow loop, with a packed-bed channel to enhance the heat transfer performance. They found that adding stainless steel spheres effectively enhanced heat transfer performance, but also increased the pressure drop in the circular tube. In fact, Maxwell [23] in 1892 had already proposed using high thermal conductivity particles (with sizes in the order of millimeter or micrometer) as additives to enhance thermal conductivity of fluids. Hamilton and Crosser [24] conducted an experimental study to verify Maxwell’s theoretical prediction, and found that the geometry of particles significantly influences the thermal conductivity of fluids. Choi and Eastman [25] first added nanometer size particles into a fluid, and their data revealed significant increase in the thermal conductivity of the fluid, and the fluid is called ‘‘nanofluids”. Many investigations have revealed similar enhancements in the heat transfer characteristics of nanofluids, for example, Refs. [26–29]. The methods to improve the effective thermal conductivity of nanofluids include the Brownian motion, aggregation, shape effect of nanoparticles, and liquid layering at the solid–liquid interfaces. The enhancement of thermal conductivity depends strongly on the nanoparticles size, shape, concentration, fluid temperature, dispersion, and the surface area of the solid–liquid interface [30–34]. The heat transfer application of nanofluids on the solar thermal system has also been studied extensively. For example, Sokhansefat et al. [35] used an Al2O3/synthetic oil nanofluid as the working fluid to numerically investigate the increase in heat transfer coefficient in a parabolic trough collector tube with non-uniform heating in the temperature range of 27–227 °C. They found that the heat transfer coefficient of the working fluid increases with increasing the particle concentration, but decreases with increasing the operational temperature. The numerical studies reported by Mwesigye et al. [36–37] demonstrated that synthetic oil doped with suitable nanoparticles improved its heat transfer characteristics. The simulated results demonstrated that the dispersion of alumina or copper nanoparticles in the synthetic oil increases the thermal efficiency of its receiver (approximately 8–13%) at high temperatures between 77 °C and 327 °C. Bellos et al. [38] found

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that the use of nanoparticles in thermal oil improved the heat transfer coefficient and thermal efficiency of solar parabolic trough collectors, and use of a converging–diverging absorber tube also improves the thermal efficiency of collectors. Kasaeian et al. [39] used a mineral thermal oil with dispersed multi-wall carbon nanotubes (MWCNT) as the working fluid to study the heat transfer and optical performances of a solar parabolic trough collector. They found that the heat transfer performance of MWCNT/mineral oil based nanofluids, ranging between 0.2 and 0.3 wt.%, increased nearly 4–7%, and a black chrome on the copper tube with vacuumed glass absorber presents the best optical performance. Specific heat capacity is an important thermal property in thermal storage and solar thermal power system. Generally, the specific heat capacity of low temperature (i.e., approximately room temperature to 100 °C) nanofluids decreases with lower specific heat nanoparticles (i.e., metal oxides, polymers, noble metal, and carbon nanotubes) being dispersed into the base fluids (i.e., water, glycol, alcohol, and ionic liquid) [40,41]. Interestingly, numerous studies [42–47] observed the anomalous increase in the specific heat capacity of molten salt nanofluids under high temperature conditions. The base salt is usually carbonate salt, chloride salt or nitrate salt, which is dispersed with nanoparticles such as Al2O3, SiO2, or SiO2-Al2O3 mixture. The particle size, geometry, and concentration of the nanoparticles are found to be significant variables influencing the enhancement of specific heat. Shin et al. [42–45] proposed three mechanisms of this enhancement, including the size effect of nanoparticles, higher solid–liquid layer area that increases the interfacial thermal resistance and layering of liquid molecules around the surfaces of nanoparticles forms a semisolid layer. Ho and Pan [47] experimentally studied the optimal concentration of alumina nanoparticles in doped molten HITEC that maximizes its specific heat capacity. An optimal concentration of 0.063 wt.% was identified that yields the highest enhancement in the specific heat capacity, i.e., 20%. The slight agglomeration of particles resulting in approximately half of the total interfacial area may be responsible for the maximum enhancement in the specific heat capacity. Conversely, some studies have reported that the molten salt doping with nanoparticles reduced its specific heat capacity. For example, Lu and Huang [48] reported 1.3% to 13% decrease in the specific heat capacity of nitrate salt (60% NaNO3-40% KNO3, mol.%) with doped alumina nanoparticles of 13 and 90 nm in size. Chieruzzi et al. [46] used nitrate salt as the base salt, and the alumina, titania or silica as the dopant particles at concentrations of 0.5, 1, and 1.5%. They observed that the dispersion of alumina and silica nanoparticles at 1 wt.% increased its specific heat by approximately 0.8–6% and reduction for the other two concentrations. Moreover, the addition of titania nanoparticles at all concentrations decreases the specific heat of molten salt nanofluids. From the previously mentioned studies, it is established that the heat transfer characteristics for molten salt flow in a pipe can be predicted well using the well-established correlations, and the heat transfer performance of nanofluids can be enhanced using suitable concentration of nanoparticles doped in the fluid. However, not many studies focus on the effect of nanoparticles on the heat transfer characteristics of molten salt flow. Therefore, the objective of the present study is to investigate the heat transfer characteristics of molten salt flow with and without nanoparticles in a minichannel. In recent years, heat exchangers with minichannels or microchannels with liquid flow as coolant are shown to be effective heat exchanging devices, and have good heat transfer characteristics. A small-scale channel in a heat exchanger system has advantages that include higher heat transfer area density and possibly larger heat transfer coefficient [49]. The molten HITEC flow in a small-scale channel is anticipated to provide thin boundary layer

3

in the channel, and therefore, higher heat transfer coefficient and potentially higher heat transfer area density. Therefore, it is of significant interest to investigate the heat transfer characteristics of molten HITEC flow in a minichannel. HITEC, a nitrate salt, is selected as the base fluid in this study, and Al2O3 as the material for the suspended nanoparticles. As indicted earlier, the previous study in our laboratory [47] revealed an optimal concentration of 0.063 wt.% yielding the maximum enhancement in specific heat capacity, i.e., 20%. Therefore, it is of significant interest to explore the convective heat transfer characteristics of this nanofluid. The nano-HITEC fluid in this study has no surfactant, allowing possible agglomeration and precipitation of nanoparticles. To solve these problems, an innovative rig for measuring the heat transfer characteristics of high-temperature molten salt flow was established. The heat transfer characteristic of pure HITEC flow was measured and compared with that reported in the literature to confirm the accuracy of the present measurements. The same method was then used to measure the convective heat transfer of HITEC flow doped with various concentrations of Al2O3 nanoparticles. 2. Experimental details 2.1. Synthesis and thermophysical properties of HITEC melt and nanoHITEC HITEC is the base fluid in this study. As indicated earlier, it is a nitrate salt, a ternary eutectic salt that is a mixture of sodium nitrate (NaNO3), potassium nitrate (KNO3), and sodium nitrite (NaNO2) in proportions of 7, 53, and 40 mol.%, respectively. Al2O3 nanoparticles were added as 20 wt.% Al2O3/water nanofluid, which was purchased from Sigma-Aldrich Co. The particles of mean size in the range of 40 nm are measured using a scanning electron microscope (SEM), as reported in our previous study [47]. The Al2O3/water nanofluid was stirred using a stirrer and sonicated in an ultrasonic cleaner (Tohama/LEO-3002S) for 1 h, respectively, and doped at a particular concentration into the HITEC eutectic in the molten salt tank. The HITEC that doped with a specific concentration was then stirred again using nitrogen bubbles of 500 ml/min for 2 h in a molten salt tank at 330 °C. The Al2O3 nanoparticles were well mixed with HITEC eutectic during this gas stirred process, and the moisture present in the nano-HITEC fluid subsequently dehydrated during this process. 2.2. Precipitation time of nano-HITEC melt The nano-HITEC fluid in this study is free of surfactant, allowing possible nanoparticles agglomeration and precipitation in the stand status. Therefore, before the experiments of heat transfer measurement, an experiment to observe the precipitation phenomenon of nano-HITEC fluid was conducted to determine the measuring time of heat transfer for the nano-HITEC flow in this experiment. In this test, the HITEC of 100 g was first doped with a particular concentration of nanoparticles using the Al2O3/water nanofluid in the beaker, and it was then melted using the heater in a furnace at 330 °C. Subsequently, nitrogen was fed into the fluid at a rate of 150 ml/min for 100 min and then it was put stationary. In the stationary status, the furnace shown in Fig. 1(a) was lift up by a mechanical device for visualization and the image of the prepared sample was taken every 10 min. The change in color would suggest precipitation. The schematic diagram of precipitation test is illustrated in Fig. 1(a). Fig. 1(b) shows the precipitation test of different nanoparticle concentrations for the HITEC nanofluid in the present study. The figure shows that for the concentration of nanoparticles less than

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Fig. 1. (a) Schematic diagram of the precipitation test of nano-HITEC fluid, (b)Observation of particle precipitation for nano-HITEC nanofluid with various particle concentration of 0.016–0.5 wt.%.

0.063 wt.%, there is no visible change in color and seems to be free of precipitation within 1 h, but when the concentration of nanoparticles increased to 0.125 wt.%, the precipitation takes place at approximately 1 h and the nanofluid appears to be stratified with the bottom layer being milky white. Such a layered and precipitation starts at approximately 30 min after preparation of the nano-HITEC melt with particles concentration of 0.25 wt.%, and only approximately 10 min if the concentration is further increased to 0.5 wt.%. Consequently, the installation process of piston is approximately 15 min, in order to be free of precipitation for particle concentrations of Al2O3 less than 0.5 wt.%. In this study, the performances of heat transfer of pure HITEC flow or HITEC with various concentrations (including 0.016 wt.%, 0.063 wt.%, 0.125 wt.% and 0.25 wt.%) of Al2O3 nanoparticles were observed and compared. 2.3. Measurement of heat transfer performance The agglomeration of nanoparticles in the nano-HITEC fluid, and the precipitation phenomena may cause inaccuracies in the measurement of convective heat transfer of molten salt flow. Therefore, the measurement of heat transfer coefficient of nano-HITEC fluid must be completed before the layered precipitation of nanoparticles takes place. To overcome this difficulty, an innovative reciprocating piston molten salt pump is designed in this study. The picture of system and schematic diagram of the experimental rig are shown in Figs. 2 and 3, respectively. The rig consists of a pair of molten salt tanks, a linear displacement piston system, and a bubble stirring system (including pressure regulator, needle valve, mass flow controller (Tokyo Keiso/NM1500), check valves, and gas tube). Both molten salt tanks are made of stainless steel 316. The tank with a volume of 2.4 L each can be filled with HITEC salt of 3480 g. Each salt tank was heated and maintained at a high temperature of 330 °C using a cylinder furnace. The linear displacement piston system was assembled with a server motor

Fig. 2. Experimental system picture with a piston molten salt pump.

(Mitsubishi/KQ43), a gear reducer, and a roller screw. This mechanism drives the piston with a stable displacement, and the minimum velocity of the piston is 15 lm/min. The piston ring, which is a critical component, is fabricated with a fireproof colloid (Chih Yi/RTV-SL3000), which has good flexibility and high stability under high temperature conditions. The piston ring can be maintained completely gastight with the tank wall. In addition, the bubble stirring system provides N2 bubbles to stir the nano-HITEC fluid in each molten salt tank. The mass flow rate of N2 was controlled using the needle valve with a mass flow controller (Tokyo Keiso/

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ࣟࣾउउࢽ ओࣾउओं

5

࣫आऑएऌऄंऋ ࣠अंऀईࢽओࣾउओं

࣪ऌउऑंऋࢽ ऐࣾउऑ ऑࣾऋई

࣠खउआऋँंए अंࣾऑंए ࣭आऐऑऌऋ

ࣰऑआएएआऋऄࢽ ࣿऒࣿࣿउंऐ

ࣱࣦࣥ࣢࣠ࢽःउऒआँ

ࣱंऐऑࢽऐंऀऑआऌऋ

ࣲ࣊ऑखऍं ࣥंࣾऑंए

Fig. 3. Schematic diagram of the convection heat transfer apparatus and the test section.

NM1500). The check valves prevent the flow of the molten salt from the molten salt tank into the gas tube. The outlet of the gas tube is installed 5 mm above the bottom of the molten salt tank. The installation of this piston molten salt pumping mechanism involves two major procedures. Before installing the piston, the pre-mixing process was carried out in the left-hand side tank. The nitrogen mixing bubbles of 500 ml/min was supplied to the left-hand tank for 1 h using the bubble stirring system to stir the nano-HITEC fluid present there. Subsequently, the ball valve at the top of the right-hand side tank was closed to contain the nano-HITEC fluid from the left-hand side tank through the action of the piston on top of the liquid level of the nano-HITEC fluid in the left-hand side tank. The O-ring of the piston, which was heated for 15 min, was expanded thermally, and provided sealing between the piston ring and the tank wall. Therefore, the installed piston can move freely in the left-side salt tank using a server motor displacement system, and the nitrogen mixing bubbles can be continuously injected to maintain favorable suspension of nanoparticles in the right-side salt tank. As shown in Fig. 3, the liquid HITEC salt is pushed to flow through the test tube from the pushing tank in the left-hand side to the mixing tank in the right-hand side. After completing a measurement each time, the piston that reaches the bottom of the tank is pulled to the initial location and the measurement is repeated. Such an innovative piston molten salt pump may provide more stable and smaller flow rates than traditional

commercial pumps, such as tubing, axial or centrifugal ones. The volume flow rate of pure Hitec or nano-HITEC flow in the present study ranged from 8 to 51 ml/min. The mass flow rate of the piston molten salt pump was calibrated accurately by utilizing an electronic balance (XS3250C/PRECISA) and room temperature deionized water was used as calibration fluid. The calibrated results demonstrate that the deviation of the mass flow rate ranges from 0.4% to 1.4%. The enlarged cross sectional diagram of the test tube is shown in Fig. 3. The length and inner diameter of the stainless steel test tube is 120 mm and 2.1 mm, respectively. The diameters of the front and after exit connecting pipeline of the test tube are 15 mm each. The connecting pipes and the test tube were heated using a U-type electric heater. Measurements of inlet and outlet temperature of the molten salt and wall temperatures are taken using K-type thermocouples with a diameter of 1.6 mm at 1.5 mm in front and after exit of the test tube, as shown in Fig. 3. The wall temperature of the test tube was measured at three axial locations (namely Tw1, Tw2, and Tw3, respectively) with equal distance of 30 mm, as shown in Fig. 3, to measure the axial variation in the wall temperature. The three thermocouples were embedded approximately 1.2 mm above the inner surface of the test tube, and were sealed with graphite glue (AREMCO/Graphi-Bond 669). The temperatures measured in the three locations are referred to as TTC in the following discussion. These temperature data were mon-

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itored and recorded using a data acquisition system (YOKOGAWA/ MX-100).

(knf), of the mixture of nano-HITEC is evaluated based on the Maxwell equation [23]:

2.4. Data reduction

knf ðkp þ 2kbf Þ  2/ðkbf  kp Þ ¼ kbf ðkp þ 2kbf Þ þ /ðkbf  kp Þ

Based on the energy balance for a constant heat flux tube, the heat flux (q00 ) applied to the fluid can be obtained from the measurement of inlet and outlet temperature of the fluid:

_ mCpðT out  T in Þ pDL

ð1Þ

_ Cp, D, and L are the mass flow rate, specific heat capacity, where, m, inner diameter, and length of the test tube, respectively. The local heat transfer coefficient hðxÞ and local Nusselt number (Nux) are calculated using:

q00 hðxÞ ¼ T w ðxÞ  T m ðxÞ Nux ¼

hðxÞD kf

ð2Þ

ð3Þ

q00 d ks

where, d, is the distance between the thermocouple location and the inner surface of the wall, and ks, is the thermal conductivity of the stainless steel [51]. The mean fluid temperature is evaluated based on the energy balance under the condition of constant heat flux; it can be shown as:

q00 pDx T m ðxÞ ¼ T in þ _ mCp

q00 ðT w;av e  T b Þ

ð4Þ

ð5Þ

Here, Tw,ave, denotes the average temperature of the inner wall surface, i.e., the average of the three inner wall temperatures, and the mean bulk temperature of the fluid, Tb, is the average of the inlet and outlet temperatures. Therefore, the mean Nusselt number may be obtained from the following equation:

Num ¼


hD kf

The dynamic viscosity of pure HITEC fluid, lbf, is evaluated using the equation mentioned in the study by Sohal et al. [50].

The measurement uncertainty in the volume flow rate of the fluid in the minichannel after calibration was estimated to be ±1.44%. The uncertainties in the specific heat capacity of the nano-HITEC fluid at different particle concentrations were ±1.7% to ±3.6%, based on the data reported by Ho and Pan [47]. The uncertainty in the temperature measurements was ±0.75% for K-type thermocouples. The uncertainties in the inner diameter and the length of the test tube were ±0.01 mm. The uncertainty in the wall heat flux is estimated to be ±2.5 to ±4.1%. The overall uncertainty in the Nusselt number, mainly attributed to uncertainties in the wall heat flux, and wall and fluid temperatures, are estimated to be ±4.4 to ±7.0%. 3. Results and discussions

Fig. 4 compares the mean Nusselt number of pure HITEC flow in the present study with the predictions of Shah and London equation [52]. This equation accounts for the thermal developing laminar convection heat transfer in a circular tube with a constant wall heat flux:

(

Num ¼

1:953ðxL Þ1=3 4:364 þ

for xL 6 0:03

0:0722=xL

ð10Þ

for xL > 0:03

where the, xL , is a non-dimensional length of the test section defined below:

xL ¼

L PrReD

ð11Þ

7

ð6Þ

The density (qnf) of the nano-HITEC fluid in this study is calculated using the volume fraction, (/), weighted bulk density of solid particles, and HITEC melt; it is shown as follows:

qnf ¼ /p qp þ ð1  /p Þqbf

ð9Þ

3.1. Heat transfer characteristics of HITEC flow

where, the specific heat capacity of the nano-HITEC fluid at different particle concentrations is based on the data reported by Ho and Pan [47].
can be obtained based The average heat transfer coefficient, h, on the average inner wall temperature and the mean bulk temperature of the fluid.


¼ h

lnf 1 ¼ lbf ð1  /Þ2:5

2.5. Uncertainty analysis

where, kf, is the thermal conductivity of the molten salt fluid, T w ðxÞ and T m ðxÞ , are the wall and fluid temperatures along the axial location x. The wall temperature on the inner surface can be extrapolated from the wall temperature, T TC ðxÞ; it can be done so based on the following equation:

T w ðxÞ ¼ T TC ðxÞ 

where the thermal conductivity of HITEC (kbf) was calculated using the equation mentioned in the study by Sohal et al. [50], and the thermal conductivity of Al2O3 particles was evaluated using the correlation for bulk material [51]. In addition, the dynamic viscosity of the HITEC fluid with nanoparticles (lnf ) was evaluated using the following equation [53]:

ð7Þ

where, the subscripts nf, bf, and p, refer to the nano-HITEC fluid, pure HITEC fluid, and alumina nanoparticle, respectively. The density of pure HITEC was calculated using the equation in Sohal et al. [50] and the density of Al2O3 particles was evaluated using the correlation for bulk material [51]. The thermal conductivity,

6.5

Num,exp

Q 00 ¼

ð8Þ

6

+10%

5.5 -10%

5 4.5 4 4

4.5

5

5.5

6

6.5

7

Num,SL Fig. 4. Comparison of the mean Nusselt number of pure HITEC eutectic measured with the predictions of Shah and London correlation [52].

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M.X. Ho, C. Pan / International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx

24

Cp l , k

Prandtl number defined as Pr ¼ in which the specific heat capacity is based on the data reported by Ho and Pan [47]. In the present study, the average temperature of the molten salt fluid ranges from 259 °C to 299 °C; the average Reynolds number and Prandtl number vary from 68 to 268 and 4.8 to 8.7, respectively. The figure shows that the data agree with the predictions of the correlation within ±10%. Such a good agreement in the mean Nusselt number of molten HITEC flow indicates that the present measurement procedure can be employed for the measurement of heat transfer performance of HITEC doped with Al2O3 nanoparticles.

20

ηCpnf (%)

η kf

nf

250 oC 285 oC 320 oC

250 oC 285 oC 320 oC

16

2 1.5

12

1

8

0.5

4 0

2.5

0.016 0.063 0.125

0.25

0

Concentration (wt.%) Fig. 6. Effect of nanoparticles concentration on the enhancement of volumetric heat capacity and thermal conductivity for nano-HITEC fluid.

3.2. Heat transfer characteristics of HITEC nanofluid Fig. 5 represents the mean Nusselt number as a function of nondimensional length, (xL ), and different nanoparticles concentration in the present study, in which the specific heat capacity of the nano-HITEC fluid is also based on the data reported by Ho and Pan [47]. As shown in Fig. 6, the solid square symbols illustrate the data for pure HITEC flow, and the hollow symbols represent the HITEC doped with various concentrations of nanoparticles. Fig. 6 indicates that the mean Nusselt number of pure HITEC eutectic decreases slowly from 6.2 to 4.2 as xL increases from 0.03 and 0.18 owing to the increase in the volume flow rate from 9.6 to 48.9 ml/min. The effect of the concentration of Al2O3 nanoparticles on the mean Nusselt number is quite uniform; however, a better enhancement is presented for a particle concentration of 0.063 wt.%. All the HITEC melts doped with a particular particle concentration up to 0.25 wt.% demonstrate positive enhancement on the mean Nusselt number, which also decreases slowly with, xL . Although, as shown earlier, the data of the mean Nusselt number for pure HITEC flow agrees well with the predictions of Shah and London [52]. The data of the mean Nusselt number for pure HITEC flow can also be fitted using non-linear regression as:

Num ¼ 4:36 þ

ηCp

ηkf (%)

where, Re, is the Reynolds number defined as Re ¼ qUD l , and Pr is the

0:092=xL

ð12Þ

1 þ 0:053ðxL Þ2=3

In the figure, the solid line stands for the fitted result for pure HITEC melt, and other dotted lines for the four different concentrations of nano-HITEC fluid. Similarly, the mean Nusselt number of nanoHITEC fluid for each concentration of nanoparticles can also be fitted using Eq. (12) multiplied by an enhancement factor C0. The fitted results show that a concentration of 0.25 wt.% demonstrates the largest enhancement of mean Nusselt number, followed by 0.063 wt.%. The effect of nanoparticles on the mean Nusselt number is relatively weak at concentrations of 0.016 and 0.125 wt.%. Table 1 lists the enhancement on the mean Nusselt number in percentage for various particle concentrations of the nano-HITEC fluid. The

Table 1 Enhancement and fitted correlation of nanoHITEC fluid with difference nanoparticles concentrations. Enhancement (%)

Pure eutectic 0.016 0.063 0.125 0.25

– 6.9 9.2 7.6 11.6

percentage enhancement of the mean Nussselt number is enhanced by 6.9–11.6% as the concentration of nanoparticles is increased from 0.016 to 0.25 wt.%. A concentration of 0.25 wt.% yields the maximum enhancement of 11.6% in this study. The enhancement achieved for the concentration of 0.063 wt.%, which demonstrates the highest enhancement in the specific heat as reported in Ho and Pan [47] is 9.2%. This value is higher than that for a lower concentration of 0.016 wt.% and that for a higher concentration of 0.125 wt.%, and it represents the maximum. A concentration of 0.063 wt.% of Al2O3 nanoparticles in the HITEC fluid not only enhances the specific heat capacity of the fluid, but also the mean Nusselt number. Fig. 6 shows a detailed examination of the effect of nanoparticle concentration on the specific heat capacity and thermal conductivity as the concentration of Al2O3 nanoparticles increases from 0.016 wt.% to 0.25 wt.% at three temperatures, i.e., 250 °C, 285 °C and 350 °C. The gCpnf and gkf in Fig. 6 stand for the percentage enhancement of specific heat capacity and thermal conductivity with different nanoparticle concentrations, respectively; it is defined below as follows:

gCpnf ¼ gknf ¼

10

Concentration (wt.%)

jCpnf  Cpbf j  100% Cpbf

jknf  kbf j  100% kbf

ð13Þ

ð14Þ

Fig. 6 shows that the, gCpnf , is significantly higher than, gknf , and pre-

Num

8

6

4 0

0.04

0.08

0.12

0.16

0.2

x*L Fig. 5. Effect of nanoparticle concentration on the mean Nusselt number (Num) versus the non-dimensional tube length (xL ).

sents a peak at the concentration of 0.063 wt.% with the peak value of 13–20%. On the other hand, the percentage enhancements of thermal conductivity for the nano-HITEC fluid are much smaller than that for specific heat capacity and increases monotonically from 0.02 to 0.34%. Clearly, the thermal conductivity of the nanoHITEC is only slightly increased, while the specific heat capacity is significantly enhanced. Therefore, the Prandtl number of the nano-HITEC fluid increases with increasing the addition of nanoparticles concentration, especially for the concentration of 0.063 wt.%. This enhancement in the Prandtl number yields a thinner thermal boundary layer for nano-HITEC fluid, and results in higher heat

Please cite this article in press as: M.X. Ho, C. Pan, Experimental investigation of heat transfer performance of molten HITEC salt flow with alumina nanoparticles, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.11.015

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M.X. Ho, C. Pan / International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx

transfer performance for nano-HITEC fluid especially for the particle concentration of 0.063 wt.%.

Num,predicted

8

3.3. Local Nusselt number of HITEC nanofluid Figs 7(a)–(c) represent the effects of particle concentration on the local Nusselt number against the non-dimensional axial distance (x ¼ x=LRePr) from the inlet and outlet of the test section for nano-HITEC fluid in the test section. The local Nusselt numbers (Nux1, Nux2, and Nux3) of nano-HITEC fluids decrease with increasing, x⁄, for various particle concentrations. As compared with pure HITEC melt, the Nux1 of nano-HITEC fluids demonstrate relatively high enhancement, up to 4.2–8.1% for x1 ranging from 0.008 to 0.03, after which the enhancement of Nux1 slightly decreases with x1 . The Nux2 and Nux3 also present the significant enhancement profile for x2 ranging from 0.01 to 0.05, and x3 ranging from 0.02 to 0.08, respectively, but less significant for x2 and x3 larger than 0.05 and 0.08, respectively.

11 Exp. result Pure 0.016 wt% 0.063 wt% 0.125 wt% 0.25 wt%

10 9 x1

8

5 0.02

0.03

0.04

0.05

x*1

(a) 8 Exp. result Pure 0.016 wt% 0.063 wt% 0.125 wt% 0.25 wt%

Nux2

7 6 5 4 3 0

0.02

0.04

0.06

0.08

0.1

x*2

(b)

8 Exp. result Pure 0.016 wt% 0.063 wt% 0.125 wt% 0.25 wt%

Nux3

7 6

-10%

6 5 4 4

MAPE=4.2 % 5

6

7

8

Num,measured Fig. 8. Comparison of the predicted mean Nusselt number of HITEC and nanoHITEC fluid using Eq. (15) with the present experimental data.

3.4. New correlation for the mean Nusselt number of nano-Hitec flow Based on the present experimental data with pure HITEC flow and nano-HITEC fluid, a correlation of heat transfer characteristics for the HITEC and the nano-HITEC fluid is proposed. This correlation, which is valid for experimental conditions under laminar flow with 55 < Re < 290 and 4.8 < Pr < 9.4, is obtained using a non-linear regression analysis; it is represented as follows:

   2 C C  9:558 1 þ 3:803 C max C max !1:49  3 ! Cpnf C Re0:289 Pr0:144 þ6:019 C max Cpbf

6

0.01

+10%

Num ¼

7

4 0

7

Pure 0.016 wt.% 0.063 wt.% 0.125 wt.% 0.25 wt.%

ð15Þ

A comparison of the present experimental data and predicted results of the mean Nusselt number given by Eq. (15) is illustrated in Fig. 8. The overall mean absolute percentage error (MAPE) of the developed correlation is 4.2%, and more than 93.9% of the experimental data were predicted within ±10%. The results in the present study demonstrate that the doping of alumina nanoparticles in molten HITEC can effectively enhance forced convective heat transfer performance in a circular tube under the condition of laminar flow. Although the concentration of nanoparticles of 0.25 wt.% presents the highest enhancement, it can maintain dispersed suspension of nanoparticles for approximately 30 min only (See Fig. 1). On the other hand, a concentration of 0.063 wt.% provides good suspension (up to 1 h in the present study) of nanoparticles, and the enhancement of Nusselt number can still be increased to around 9.2%. Consequently, this study suggests a better concentration of alumina nanoparticles of 0.063 wt. %, which improves the heat transfer performance of molten HITEC fluid, and maintained a uniform suspension of nanoparticles for a longer time. Moreover, this concentration coincides with the optimal concentration of alumina nanoparticles in molten HITEC salt maximizing the enhancement of its specific heat capacity to approximately 20%, as reported earlier by Ho and Pan [47]. Therefore, this investigation suggests that the nanoparticles concentration of 0.063 wt.% is the best for the enhancement of thermal– hydraulic properties of HITEC eutectic with Al2O3 nanoparticles.

5 4. Conclusions

4 3 0

0.04

0.08

0.12

0.16

x*3

(c)

Fig. 7. Effect of concentration on the local Nusselt number versus the nondimensional axial distance from the inlet of the test section (x ).

The effect of nanoparticle concentrations of the molten nanoHITEC flow on the heat transfer performance in a circular tube is investigated in this study. An innovative apparatus and preparation process for molten HITEC salt with Al2O3 nanoparticles salt have been developed to prevent the precipitation of nanoparticles during the measuring process. The experiment results and analysis reveal the following:

Please cite this article in press as: M.X. Ho, C. Pan, Experimental investigation of heat transfer performance of molten HITEC salt flow with alumina nanoparticles, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.11.015

M.X. Ho, C. Pan / International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx

(1) The precipitation phenomenon was not observed in the present study using the innovative apparatus for the concentration of Al2O3 nanoparticles in the nano-HITEC fluid of up to 0.063 wt.% within an hour or up to 0.25 wt.% within 30 min (i.e., 0.016 wt.%, 0.063 wt.%, 0.125 wt.% and 0.25 wt. %) which is long enough for conducting conduct steady state measurements of heat transfer coefficient. (2) On comparing the experimental data of the mean Nusselt numbers of the pure HITEC flow with the predictions of the correlation by Shah and London [52], it was found that the deviation was less than ±10%. (3) A concentration of Al2O3 nanoparticles of 0.25 wt.% in the nano-HITEC fluid demonstrated a maximum enhancement of the mean Nusselt number of up to 11.6%, but only maintained favorable uniform suspension for approximately 30 min. On the other hand, the concentration of 0.063 wt.% exhibited an enhancement in the Nusselt number of 9.2%, and precipitation phenomena was not observed within one hour. (4) The effect of nanoparticle on the local Nusselt numbers also presents positive effect for the nano-HITEC fluid with concentrations less than 0.25 wt.%. (5) Based on the data of this study, a new correlation is developed for the mean Nusselt number, which can predict 93.9% of the experimental data for the laminar forced convection of the nano-HITEC fluid with an accuracy of ±10%. In conclusion, this study demonstrates significant positive effect on the heat transfer performance of the HITEC melt doped with Al2O3 nanoparticles with concentrations less than 0.25 wt.%. The previous investigation by Ho and Pan [47] also revealed that the addition of alumina nanoparticles in molten HITEC melt may enhance its specific heat capacity. The HITEC salt doped with alumina nanoparticles with a concentration of 0.063 wt.% is suggested as a working fluid for applications in the thermal storage systems of a concentrating solar thermal power system, as it significantly enhances the specific heat capacity as well as heat transfer performance. Acknowledgments This work was undertaken as part of a boost program of the National Tsing Hua University (99N2929E1). This study is also supported by the National Science Council of Taiwan under contracts nos.: 101-2623-E-007-002-ET and 102-2623-E-007-004-ET. References [1] P. Viebahn, Y. Lechon, F. Trieb, The potential role of concentrated solar power (CSP) in Africa and Europe-A dynamic assessment of technology development, cost development and life cycle inventories until 2050, Energy Policy 39 (2011) 4420–4430. [2] M. Liu, N.H. Steven Tay, S. Bell, M. Belusko, R. Jacob, G. Will, W. Saman, F. Bruno, Review on concentrating solar power plants and new developments in high temperature thermal energy storage technologies, Renew. Sustain. Energy Rev. 53 (2016) 1411–1432. [3] D. Mills, Advances in solar thermal electricity technology, Sol. Energy 76 (2004) 19–31. [4] O. Behar, A. Khellaf, K. Mohammedi, A review of studies on central receiver solar thermal power plants, Renew. Sustain. Energy Rev. 23 (2013) 12–39. [5] K. Vignarooban, X. Xu, A. Arvay, K. Hsu, A.M. Kannan, Heat transfer fluids for concentrating solar power systems – a review, Appl. Energy 146 (2015) 383– 396. [6] D. Kearney, U. Herrmann, P. Nava, B. Kelly, R. Mahoney, J. Pacheco, R. Cable, N. Potrovitza, D. Blake, H. Price, Assessment of a molten salt heat transfer fluid in a parabolic trough solar field, J. Sol. Energy Eng. Trans. 125 (2003) 170–176. [7] P. Xiao, L. Guo, X. Zhang, Investigations on heat transfer characteristic of molten salt flow in helical annular duct, Appl. Therm. Eng. 88 (2015) 22–32.

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Please cite this article in press as: M.X. Ho, C. Pan, Experimental investigation of heat transfer performance of molten HITEC salt flow with alumina nanoparticles, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.11.015