Thermo-physical properties of water based SiC nanofluids for heat transfer applications

International Communications in Heat and Mass Transfer 84 (2017) 94–101

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Thermo-physical properties of water based SiC nanofluids for heat transfer applications

MARK

Gabriela Huminica,⁎, Angel Huminica,⁎, Claudiu Fleacab, Florian Dumitracheb, Ion Morjanb a b

Transilvania University of Brasov, Mechanical Engineering Department, 29, Bulevardul Eroilor, 500036, Brasov, Romania National Institute for Laser, Plasma and Radiation Physics, 409, Atomistilor Street, PO Box MG-36, 077125, Magurele, Bucharest, Romania

A R T I C L E I N F O

A B S T R A C T

Keywords: SiC nanoparticles nanofluids thermo-physical properties heat transfer characteristics

The aim of current paper consists in the fabrication, characterization and preparation of water based on SiC nanofluids and the experimental investigation of their thermo-physical properties. Thermal conductivity, viscosity and surface tension of SiC/water nanofluids were measured for two two weight concentrations of nanoparticles 0.5 and 1.0 wt% respectively, within the range 20 °C to 50 °C. Concerning the thermal-properties of studied nanofluids, the experimental results show that the thermal conductivity increases with the increasing both of the weight concentration of the nanoparticles and temperature. Also, the dynamic viscosity of the SiC/ water nanofluids increases with increasing nanoparticles concentration and decreases with the increasing temperature. Furthermore, the surface tension of studied nanofluids increases with the increase of the weight concentrations of the nanoparticles, but the results show that at a concentration of the nanoparticles of 0.5 wt%, the surface tension was lower than the surface tension of the water, while at 1.0 wt% nanoparticles, the surface tension of the nanofluids was close to the surface tension of the water. Measurements were compared with experimental data available in literature and theoretical models. Finally, SiC/water nanofluids were used as working fluid inside of the two-phase closed thermosyphon in order to study of the heat transfer from point of view both of operating temperature and the nanoparticles concentration.

1. Introduction The nanofluids can be defined as systems containing very small particles with sizes (under 100 nm) (nanoparticles) suspended in conventional liquids as water, oils or glycols. Generally, the introduction of nanoparticles in the base fluid enhances the thermal properties of the system, such as the thermal conductivity and the heat transfer coefficient. Silicon nanoparticles have attracted attention due their intriguing physical properties, active surface state and biocompatibility [1] being used in various fields as thermoelectrics, photovoltaics, nanoelectronics, and nanomedicine [2]. Combining Si structure with a conducting carbonaceous layer can get many benefits, such as excellent flexibility, high conductivity, lightweight, electrochemical and thermal stability [3,4]. Thermal conductivity and dynamic viscosity of the silicon carbide were experimental investigated in Refs [4–11]., but researches concerning the surface tension of the SiC/water nanofluids not are available in the literature. Furthermore, only few papers [12–14] investigated the heat transfer in two-phase thermosyphons using silicon carbide. On the other side, silicon nanoparticles were seldom reported

⁎

Corresponding authors. E-mail address: [email protected] (G. Huminic).

http://dx.doi.org/10.1016/j.icheatmasstransfer.2017.04.006

0735-1933/ © 2017 Elsevier Ltd. All rights reserved.

for nanofluid applications. We can cite the study of silicon nanoparticles prepared by pulsed-laser ablation in deionized water, where the resulted low concentration (0.01 and 0.001% vol.) nanofluids presenting slightly lower surface tension and a very weak viscosity increasing compared to the corresponding values from pure water [15]. Also, the silicon-containing nanofluids reveal a slightly higher CHF (critical heat flux) than that for the water [15]. Moreover, polydisperse (40–250 nm) silicon nanoparticles (made by thermal plasma) were successfully used as aqueous nanofluis for enhanced heating and vaporization of water under sunlight due to the broadband absorption spectrum from UV to NIR of the silicon nanoparticles [16]. The observed vaporization rates were enhanced for a wide range of concentrations (from 0.001 to 0.1% vol.) with a maximum efficiency at 0.01% vol. [16] possibly due to the solar radiation shielding at the highest tested concentration. The current study is divided in two parts: first part is dedicated to fabrication, characterization and preparation of water based on SiC nanofluids and second part for the investigation the effects of the temperature and weight concentration on the thermo-physical (thermal conductivity, dynamic viscosity and surface tension) and heat transfer characteristics of SiC/water nanofluids used in two-phase closed

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tion, based on their relative maxima of as synthesized SiC nanoparticles. The X ray diffractograms (XRD) of both sample (see Fig. 1) showed a nanometric nature of analyzed materials revealed by the presence of broad peaks. The identified diffraction maxima correspond to Si and most probable 3C–SiC phase. The main peaks were well fitted by pseudo-Voight function and this fact was a sign for a narrow size distribution of crystallite dimensions. The presence of nanocrystalline Silicon was attested only in SiC-1 sample (Fig. 1, the black line) by the peaks placed at 2θ = 28.6, 47.4, 56.2, 69.1, 76.4 and 88.1°. Based on full width half maximum (FWHM) of the (111) Si peak centered at 2θ = 28.6° and using the Debye –Scherrer equation the mean crystallite size was calculated to be 23.4 nm. In the same SiC-1 sample using again the FWHM, yet for the (111) 3C–SiC centered at 2θ = 35.7°, the mean crystallite size of this phase was 5.3 nm. For the SiC-2 sample the X ray pattern (Fig. 1, red line) showed only SiC peaks, their mean crystallite size being measured in the same manner and the resulted value was 10.7 nm. The absence of silicon phase in this sample corresponds with the near unitary silicon to carbon atomic ratio extracted from EDX measurement. EDX analyses evaluate the following mediated elemental proportions: 42.02 at.% C, 4.44 at.% O and 54.54 at. % Si for SiC1sample, and 52.55 at.% C, 1.24 at.% O and 46.21 at.% Si for SiC-2 sample. The presence of both elemental silicon and silicon carbide (as (β)-3C SiCpolytype) crystalline phases, in conjunction with the observed higher atomic silicon content vs. the carbon content in the SiC-1 nanopowder can be related with the introduction of an excess of silane reactant. Also, even if the synthesis of those nanopowders was performed in anoxic atmosphere and without oxygen-containing precursors, the EDX analyses show the presence of a small percent of atomic oxygen in both samples due to their post-synthesis exposure at ambient air. The higher oxygen percent in SiC-1 sample can be correlated with the presence of elemental silicon which is much more reactive towards oxygen than silicon carbide. Due to reaction of these nanoparticles with molecular oxygen amorphous silica and/or silicon oxycarbide superficial thin layer can be formed [23]. TEM image of SiC nanoparticles is shown in Fig. 2. As depicted, some nanoparticles are arranged in ramified/chained aggregates or small clusters. Their sizes are in more or less correlated with the mean crystallite sized extracted from X-ray measurements, generally under 25 nm for both samples. The aggregation of these nanoparticles can be explained by the welding of hot fresh nanoparticles due to their collision in the crowded laser pyrolysis flame.

thermosyphon. 2. Experimental procedure 2.1. Synthesis of SiC nanoparticles The nanopowders were synthesized using the laser pyrolysis technique. The laser pyrolysis method requires the presence of a sensitizer (laser energy transfer agent) besides the nanoparticles precursors. For the synthesis of the nanopowders, a mixture of silane and acetylene was used. The silane plays the double role as sensitizer and as silicon donor. Also, acetylene (C2H2) plays the role of carbon donor. Due to the strong absorption of silane (SiH4) molecules at the 10P(20) line (wavelength = 10.59 μm)of CO2 laser [17], they become excited on higher vibrational levels and transfer this excess energy by collisions with other molecules from the gas mixture(such as C2H2 molecules that not have vibrational modes near CO2 laser frequency [17]) which is rapidly heated and pyrolysed with the formation of silicon-based nanoparticles. The high productivity gas-phase laser pyrolysis method was widely reported for the nano-SiC synthesis from this precursor in the presence of acetylene or other hydrocarbons [18–22]. A Coherent CO2 pulsed laser with 400 W nominal power, working at 10.59 μm was used for the synthesis of the nanopowders (80 KHz frequency and a duty factor around 60%). Some parametric studies were performed in order to find proper conditions for nanoparticles production with elemental composition as SiC at stoichiometric composition and then as SiCx with x less than 1.In order to obtain a near stoichiometric SiC composition, the Si to C atomic ratio in precursors must to be 1/1 which is translated in a SiH4/C2H2flow ratio of 2/1. In the first experiment, for the SiC-1 nanopowder synthesis, a mixture consisting in 24.1 sccm C2H2 and 110 sccm SiH4 was injected through a central thinner nozzle with the diameter of 2.0 mm, whereas 4015 sccm Argon was simultaneously introduced through an external annular nozzle of 10 mm diameter. The velocity values for both inner gas mixture (C2H2 + SiH4) and outer Argon were kept equal in order to preserve the laminar flow of the reaction mixture in the laser irradiation zone. In the secondexperiment, for the SiC-2 nanopowder synthesis, a 50 sccm C2H2 + 100 sccm SiH4 mixture was injected through the central nozzle and 5000 sccm Argon through the annular one. The SiC nanopowders synthesis occurred at a pressure of 450 mbar (kept constant with the aid of a throttle valve) and under higher laser power of 400 W.

2.3. Preparation of SiC nanofluids 2.2. Characterization of SiC nanoparticles In this study, homogeneous and stable water-based SiC nanofluids with different weight fractions were prepared. For the aqueous nanofluid preparation, the low viscosity dry carboximethylcellulose white

The SiC nanoparticles have been characterized using XRD and TEM techniques. Fig. 1 shows the XRD patterns along with phase's identifica(111)

SiC-1

1

SiC-2

Intensity (a.u.)

1 3C-SiC 2 (111) 2 Si

(220)

2

(220) (113) (200)

20

30

40

1

(113)

1

1

(400)

50

2

60

Fig. 1. Superposed X ray pattern for the SiC-1 and SiC-2 powders.

95

70

2

1

(331) 1 (222)

2

80

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Fig. 2. TEM image of SiC nanopaticles: a) SiC-1; b) SiC-2.

powder (CMCNa) was mixed with SiC nanopowder and introduced in very small portions in the vessel containing 250 ml distilled water under the action of a strong ultrasonic homogenizer (Hielscher UIP 1000hd endowed with a 22 mm diameter sonotrode having a 1000 W nominal power and working at 20 kHz frequency) with a supplementary mechanical mixing using a vibrating thin rod terminated with a orthogonally positioned toroidal spiral. After introducing the powders, the rod was removed and the ultrasonical treatment was continued for another hour, without external cooling. The powders concentration in distilled water was 0.5 wt% and 1.0 wt% respectively. Following this procedure, no settlement of nanoparticles was observed after 1 year. For this study, thermo-physical properties and heat transfer characteristics of the TPCT were measured for the SiC-2 simple.

2.4. Experimental setup and procedure Thermal conductivity of the water-based SiC nanofluids was measured using a KD 2 Pro Thermal Properties Analyzer. The measurement principle of KD2 is based on the transient hot-wire technique. The instrument has a specified accuracy of ± 5%. Before measurements, the calibration of the sensor needle was carried out by measuring the thermal conductivity of the distilled water at the room temperature of 293 K. Thus, the measured value of the distilled water was 0.600 W/ mK, which is in good agreement with the value in literature of 0.598 W/mK at a temperature of 293 K. The viscosity and the surface tension of the water-based SiC nanofluids were measured using a Brookfield programmable viscometer and a Sigma force tensiometer respectively. The Du Noüy ring method was employed for the measurement of the surface tension. Instruments were computer-controlled and calibrated with the distilled water. The computed maximum uncertainty was lower than 2%. The installation used for the thermal performances measurement of copper two-phase closed thermosyphon is illustrated in Fig. 3. The geometrical dimensions of the TPCT are: the outer diameter and the thickness, 10 mm and 1 mm; evaporator and adiabatic sections, 121 mm and 54 mm; the length 305 mm. The measurement of the temperatures on evaporator, adiabatic and condenser sections were performed with five thermo-resistances, type–Pt-100 (uncertainty: ± 0.1 °C). The thermo-resistances were placed on TPCT as follows: two on the evaporator, one on the adiabatic section and two on the condenser. The temperatures on evaporator and condenser sections were keeping at a constant value using two thermostatic bath, GD 120S26 and Haake C10 – P5/U types. The two-phase closed thermosyphons were tested at an inclination angle of 90° (vertical) and a filling ratio of 25%.

Fig. 3. The experimental set-up for measuring the thermal performance of the TPCT.

3. Results and discussion 3.1. Influence of the CMCNa concentration on thermo-physical properties In order to establish the optimal amount of surfactant, the low viscosity CMCNa solutions in distilled water were prepared in conditions similar, with concentrations of 3.0, 6.0 and 10 g /l, because it assume that to the viscosity of the nanofluid contributes to both nanoparticles and surfactant (even if a part was bound nanoparticle). Also, for comparison a suspension of 3 g/l of medium viscosity CMCNa was prepared. The thermo-physical properties of the distilled water with surfactant (CMCNa) were shown in Figs. 4-6. As shown in Fig. 4, thermal conductivity of the water + CMCNa (low viscosity) decreases with the increase of surfactant concentration. Also, it can be seen that the thermal conductivity value of the medium viscosity CMCNa was less than the low viscosity CMCNa for the same concentration. The effect of the surfactant concentration on the dynamic viscosity of distilled water with surfactant is shown in Fig. 5. The results indicate that the viscosity of the distilled water + CMCNa (low viscosity) solution increases with the increase of the concentration of the surfactant, and as expected the viscosity of the water + CMCNa (medium viscosity) solution was higher than the viscosity of water + CMCNa (low viscosity) solution. Also, it is can see from Fig. 6, that the surface tension of the low viscosity CMCNa was higher than the medium viscosity CMCNa for the 96

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Fig. 4. The effect of the surfactant concentration on thermal conductivity. Fig. 7. The thermal conductivity versus temperature at various weight concentrations.

thermal conductivity ratio of the sodium dodecylbenzenesulfonate (SDBS) decreases slowly with increase of the SDBS concentration from 0.03 wt% to 0.2 wt%, and then decreases very quickly with increase of the SDBS concentration. Also, the thermal conductivity ratios of the SDS and CTAB surfactants had the same trend as SDBS surfactant, namely the thermal conductivity ratio decreases with the increase of the surfactant concentration. Concerning the effect of the surfactant concentration on viscosity, the same authors showed that the viscosities of SDS and SBDS surfactants increase up to 0.05 wt%, decrease slowly from 0.05 wt% to 0.3 wt% and then increases with the increase of the surfactant concentration. 3.2. Influence of the temperature on thermal conductivity of the nanofluids In the current study, the thermal conductivity was measured for two weight concentrations (0.5 and 1.0%) of SiC nanoparticles within the range of temperature from 20 °C to 50 °C. The effect of temperature on the thermal conductivity of SiC/water nanofluids is shown in Fig. 7. As shown in Fig. 7, for all studied cases, the thermal conductivity of nanofluids linearly increases with the increase of the temperature. The maximum relative thermal conductivity (100(knf − kw)/kw) was 17.62% for a concentration of 1.0 wt% and a temperature of 50 °C. At this temperature, the relative thermal conductivity for a concentration of the nanoparticles of 0.5% was 14.53%. The results from this study are similar with those found in literature [4,6,9]. The effect of the weight concentrations on thermal conductivity ratio defined as the ratio of thermal conductivity of nanofluid (knf) to the thermal conductivity of base liquid (kbf) for various temperatures is shown in Fig. 8. From the experimental data, higher thermal conductivity ratio can be observed with increasing of the concentration of nanoparticles in distilled water and the temperature of nanofluid. The maximum thermal conductivity ratio was 1.176 at 1.0 wt% and 50 °C, respectively. The experimental data for the thermal conductivity of the SiC-water nanofluids with weight concentrations of 0.5 wt% and 1.0 wt% respectively in within range 20–50 °C were compared with both experimental data from previous research and theoretical models, as shown in Tables 1 and 2. In current study, two theoretical models for the estimation of effective thermal conductivity of nanofluids at the standard temperature (298 K) were considered: Murshed and Bruggeman models. The model proposed by Murshed et al. [26] for the estimation the effective thermal conductivity of nanofluids is defind as:

Fig. 5. The effect of the surfactant concentration on viscosity.

Fig. 6. The effect of the surfactant concentration on surface tension.

same concentration, and increases with the increase of surfactant concentration. Following the measurements, the concentration of the surfactant for each type of nanofluids was established to 3 g/l. The results similar were founded by Tavman and Turgut [24]. They studied the effect of the sodium dodecylbenzenesulfonate (SDBS) used as surfactant on the thermal conductivity and founded that the thermal conductivity of the SDBS – water mixture decreases with the increasing SDBS ratios. Also, Mingzheng et al. [25] measured the thermal conductivity and viscosity of the various surfactants with different concentrations in pure water and in nanofluids. They founded that the

keff , Murshed =

⎡ kbf ⎢1 + 0.27ϕ4 3 ⎣ 1+

97

ϕ4 3

(

ks kbf

(

ks kbf

−1

⎤⎡ − 1 ⎥ ⎢1 + ⎦⎣

)(

)

0.52ϕ 1 − ϕ1 3

+

0.52ϕ 1 − ϕ1 3

0.27ϕ1 3

(

⎤ −1 ⎥ ⎦

) + 0.27) ks kbf

(1)

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Table 2 Experimental thermal conductivity in comparison with the theoretical. Nanoparticles concentration (wt%)

Current Murshed Model study [26] Thermal conductivity [W/m K]

Bruggeman Model [27]

0.5 1.0

0.681 0.702

0.608 0.612

0.685 0.828

3.3. Influence of the temperature on dynamic viscosity of the nanofluids Fig. 9 shows the effect of the temperature on dynamic viscosity of SiC/water nanofluids. It is can be seen in Fig. 9 that the viscosity of the SiC/water nanofluid decreases with the increasing temperature. The dynamic viscosity at a concentration of 0.5 wt% decreases with increasing temperature from 31.58% to 24.72% compared to distilled water in the temperature range of 20 °C–50 °C. Also, at a concentration of 1.0 wt%, the dynamic viscosity in the same temperature range decreases from 40.89% to 35.92%. It is can be seen that at a temperature of 50 °C decrease of the viscosity was more pronounced than at 20 °C. The main reason in increases of the dynamic viscosity could be adding of the surfactant in the base fluid for to prevent agglomeration/sedimentation nanoparticles. Similar results were found in Refs [5–9].These results were shown in Table 3 compared with the results from the current study. The comparison between thermal conductivity enhancement expressed by thermal conductivity ratio and viscosity increase expressed by dynamic viscosity ratio defined as the ratio of viscosity of nanofluid (μnf) to the viscosity of base liquid (μbf) for two temperatures, 20 °C and 50 °C respectively, is shown in Fig. 10. The values of the thermal conductivity ratio of the SiC/water nanofluids increase with increasing temperature. When the thermal conductivities of SiC/water nanofluids were compared on a common scale, one can see that the addition of nanoparticles in the base fluid, the thermal conductivity of the working fluid is improving at the same temperature. Also, as shown in Fig. 10, the dynamic viscosity ratio of the SiC/water nanofluids increases with increasing nanoparticles concentration and decreases with the increasing temperature.

Fig. 8. Thermal conductivity ratio of SiC/water nanofluids.

where kbf is the thermal conductivity of the base fluid, ks represents the thermal conductivity of the solid particles and ϕ is the volume concentration of the nanoparticles. The Bruggeman model [27] is given by Eq. (2):

keff , Bruggeman =

kbf 1 [ (3ϕ − 1) ks + (2 − 3ϕ) kbf ] + Δ, 4 4

(2)

where Δ is defined as

⎡ ⎛ k ⎞2 ⎛ k ⎞⎤ Δ = ⎢ (3ϕ − 1)2 ⎜ s ⎟ + (2 − 3ϕ)2 + 2 (2 + 9ϕ − 9ϕ2 ) ⎜ s ⎟ ⎥ . ⎢⎣ ⎝ kbf ⎠ ⎝ kbf ⎠ ⎥⎦

(3)

where kbf is the thermal conductivity of the base fluid, ks represents the thermal conductivity of the solid particles and ϕ is the volume concentration of the nanoparticles. As can be seen from Table 1 the thermal conductivity of nanofluids, for all experimental data, was higher than the thermal conductivity of the base fluid. The enhancement of the thermal conductivity of the nanofluids depends on many factors: the preparation method, the base fluid, the nanoparticles concentration, the particle size, presence or absence of the surfactants. Also, as shown in Table 3, the experimental data were in good agreement with the results of Murshed model, the deviations between experimental data and theoretical model were 0.58% and 15.2%, respectively. For comparison of the experimental results with those provided by theoretical models, the weight concentrations were converted to the corresponding volume concentration.

3.4. Influence of the temperature on surface tension of the nanofluids The best of authors' knowledge the studies concerning the measurement of the surface tension of those nanofluids not are available in the literature. Only few studies were performed on the surface tension of the nanofluids [30–34]. The surface tension of SiC/water nanofluids was measured within the range of the temperature of 20 °C to 50 °C, for the same two weight concentrations of nanoparticles 0.5 and 1.0%, respectively. The influence of the temperatures on surface tension is shown in Fig. 11. The results indicate that the surface tension of SiC/ water nanofluids increase with increasing weight concentration and

Table 1 Experimental thermal conductivity in comparison with the literatures. Reference

Concentration

Nanofluid

Temperature

Maximum enhancement in thermal conductivity (%)

Current study Li et al. [4] Li et al. [5] Singh et al. [6] Timofeeva et al. [8] Lee et al., [9] Manna et al. [10] Nikkam et al. [11]. Xie [28]

0.5–1.0 wt% 0.2–1.0 vol% 0.1 vol% 1.8–7.0 vol% 4.0 vol% 0.001–3.0 vol%. 0.1 to 0.8 vol%. 9.0 wt% 0.8–4.2 vol%

SiC/water SiC/EG SiC/oil SiC/water α-SiC /EG-H2O β −SiC/water SiC/water α-SiC /EG-H2O SiC 26 (26 nm)/EG SiC 26 (26 nm)/water SiC 600 (600 nm)/EG SiC 600 (600 nm)/water

20–50°C 20–50°C 25–55 °C 15–55 °C Not reported 22–23.5 °C Room temperatures 20°C 4 °C

17.62% 16.21% 34% (30 °C) 28% ~ 17% 7.2% 26% 20% 15.8%

4 °C

22.9%

1.0–4.0 vol%

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Table 3 Experimental dynamic viscosity in comparison with the literatures. Reference

Concentration

Nanofluid

Temperature

Maximum increase in dynamic viscosity (%)

Current study Singh et al. [6] Timofeeva et al. [7] Timofeeva et al. [8] Lee et al. [9]

0.5–1.0 wt% 1.8–7.4 vol% 4.0 vol% (16 nm) 4.0 vol% (16 nm) 3.0 vol%.

SiC/water SiC/water α-SiC / water α-SiC /ethylene glycol + water β −SiC/water

20–50°C 15–55 °C 22.5 °C Not reported 72–28 °C

17.62% 158.46% 60.0% 52.0% 102%

Fig. 9. The dynamic viscosity versus temperature at various weight concentrations. Fig. 11. The surface tension versus temperature at various weight concentrations.

Fig. 12. Surface tension ratio of SiC/water nanofluids.

nanoparticle surface imparting an electrostatic repulsive force between the particles in the nanofluid [31,32].

Fig. 10. Comparison of the thermal conductivity enhancement and viscosity increase for SiC/water nanofluids.

3.5. Heat transfer

decrease with increasing temperature. As shown in Fig. 11, at a concentration of the nanoparticles of 0.5%, the surface tension of nanofluids was lower than the surface tension of the water. For a concentration of the nanoparticles of 1.0%, the surface tension of the nanofluids within the range of 20 °C to 50 °C was close to the surface tension of the water. Also, at temperature of 50 °C and 0.5% concentration of the nanoparticles, the surface tension ratio defined as ratio between the surface tension of the nanofluids and the surface tension of the water was 0.923. At this temperature, surface tension ratio for a concentration of the nanoparticles of 1.0% was 1.005 (Fig. 12). The results from this study were similar with the results other researchers [29–32]. A concentration of 0.1 vol% TiO2 nanoparticles in water can decrease the surface tension of the nanofluid due to Brownian motion of nanoparticles in the base fluid and tadsorption of nanoparticles at the interfaces [30]. Another reason for reducing the surface tension of nanofluids is due to the adsorption of ionic surfactant on the

Finally, the heat transfer characteristics of the two-phase closed thermosyphon (TPCT) using water based SiC nanofluids were investigated. Thermal performances of the TPCT were characterized by the heat transfer rate, the evaporator heat transfer coefficient and the thermal resistance. Fig. 13 shows the heat transfer rate of the TPCT using water and SiC/water nanofluids with two weight concentrations of nanoparticles (0.5–1.0%). Heat transfer rate was defined as:

Q̇ = ṁ ⋅cp⋅(TC 2 − TC1 ) [W ]

(4)

where Q̇ is heat transfer rate, in W, ṁ is mass flow rate of the cooling water, in kg/m3, cp is specific heat, in J/kgK and Tc1, Tc2 are inlet and outlet temperatures from condenser section, in K. As shown in Fig. 13, the heat transfer rates of the TPCT using nanofluids were much higher than those of the water. Also, it can be seen that the heat transfer rate of the TPCT increases significantly with 99

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Fig. 13. Heat transfer rate of the TPCT for water and SiC/water nanofluids.

Fig. 15. Thermal resistance of the TPCT for water and SiC/water nanofluids.

increase the operating temperature and with the increase of the nanoparticles concentration. The heat transfer rate increases up to 24.4% for the TPCT with SiC/water nanofluid at a weight concentration of 1.0% compared with that of the TPCT using distilled water, in conditions of TPCT operation at maximum temperature difference. At the same the temperature difference and a concentration of nanoparticles of 0.5 wt%, the enhancement of the heat transfer rate in TPCT was 18.5%. At lower temperatures differences (~ 39 K), the increases of the heat transfer rate at concentrations of 0.5 and 1.0 wt% were 45.5% and 145.5% respectively. Based on the analysis of the experimental data it can be seen that the better thermal performances of the TPCT using SiC/water nanofluids were obtained at the lower temperatures differences. Fig. 14 shows the evaporation heat transfer coefficients of the TCPT at various operating temperatures of the TCPT. The evaporator heat transfer coefficient was calculated by:

It can be seen that the evaporation heat transfer coefficients of the TCPT using SiC/water nanofluids for two concentrations studied, 0.5 and 1.0 wt% respectively, were much higher than those of the distilled water. The evaporation heat transfer coefficient increases with the increase of both weight concentration and operating temperature, in all cases studied. Initially, it increases rapidly at the lower temperature differences and then, the increase was gradually. This increase of the evaporator heat transfer coefficients is correlated with improvement of the thermal properties of the working fluid inside the TCPT. The thermal resistance of the TPCT at various operating temperatures of the TPCT with water and SiC/water nanofluids is shown in Fig. 15. The thermal resistance was computed by the Equation [35]:

hE =

⎡ W ⎤ ⎢ ⎥ AE (TEm − TS ) ⎣ m2K ⎦

R=

TEave − TS ⎡ K ⎤ ⎢⎣ ⎥⎦ W Q̇

(6)

As shown in Fig. 15, the thermal resistance of the TPCT decreases with the increase operating temperature of the TPCT, and the thermal resistance for TPCT using water at various temperatures differences was much higher than the thermal resistance for TPCT using SiC/water nanofluids. Also, it can be seen that the increases of the nanoparticles concentration decrease the thermal resistance of TPCT using SiC/water nanofluids thus providing a better performance. Thus, the thermal resistance decreases up to 32.8% for the TPCT with SiC/water nanofluid at a weight concentration of 1.0% compared with that of the TPCT using distilled water, in conditions of TPCT operation at higher temperature differences. At the same temperature difference and a concentration of nanoparticles of 0.5 wt%, the reduction of the thermal resistance in TPCT was 16.6%. The experimental data from current study were compared with experimental data, for the thermosyphons and heat pipes which used SiC-water nanofluids as working fluid, from previous research, as shown in Table 4. As can be seen from Table 4, the results of this study were very close to the obtained results by Ghanbarpour et al. [13]. In both studies, for a concentration of nanoparticles of 1.0 wt%, the maximum decrease in thermal resistance was approximately of 30%.

Q̇

(5)

where AE is evaporator surface area, in m2, TEm is mean temperature on evaporator section, in K, TS is vapor temperature, in K. The vapor temperature,TS, was measured on adiabatic section.

4. Conclusions In this study, fabrication, characterization and preparation of water based on SiC nanofluids were performed. The thermo-physical properties, thermal conductivity, viscosity and surface tension, were investigated within the range of the temperature of 20 °C to 50 °C, for two weight concentrations, 0.5 and 1.0 wt% respectively. Finally, the heat

Fig. 14. Evaporator heat transfer coefficient of the TPCT for water and SiC/water nanofluids.

100

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Table 4 Experimental data in comparison with the literatures. Reference

Thermosyphons or heat pipes

Inner diameter/length [mm]

Nanofluid

Concentration

Maximum decrease in thermal resistance (%)

Current study Abbasi et al. [12] Ghanbarpour et al. [13] Kim et al. [14]

Copper thermosyphon Copper thermosyphon Copper heat pipes Stainless steel heat pipes

8/305 28/1500 6.36/250 19/1000

SiC/water β− SiC /water α-SiC / water SiC/water

0.5–1.0 wt% 0.5, 1.0, 1.5, 2.0 vol% 0.35, 0.7, 1.0 wt% 0.01%, 0.1% vol%

32.8% ~ 25% 30% Not reported improvement of the thermal performances

transfer characteristics of the two-phase closed thermosyphon (TPCT) using water based SiC nanofluids were studied. The main conclusions were:

[9]

[10]

1. With the increase of surfactant concentration in distilled water, the thermal conductivity decreases, the viscosity and surface tension of the mixture increases. 2. The thermal conductivity increases with the increase both of nanoparticle concentration and temperature. The increase of the thermal conductivity was 17.62% for a concentration of 1.0 wt% and a temperature of 50 °C. 3. The dynamic viscosity of the SiC/water nanofluids increases with increasing nanoparticles concentration and decreases with the increasing temperature. The dynamic viscosity at a concentration of 1.0 wt% decreases with increasing temperature from 40.89% to 35.92% compared to distilled water in the temperature range of 20 °C–50 °C. 4. The surface tension of SiC/water nanofluids increase with increasing weight concentration and decrease with increasing temperature. At a concentration of the nanoparticles of 0.5%, the surface tension of nanofluids was lower than the surface tension of the water and at 1.0 wt% nanoparticles, the surface tension of the nanofluids was close to the surface tension of the water within the range of 20 °C to 50 °C. 5. The heat transfer rates of the TPCT using SiC/water nanofluids were much higher than those of the water. The heat transfer rate increases up to 24.4% for the TPCT with SiC/water nanofluid at a weight concentration of 1.0%, in conditions of TPCT operation at maximum temperature difference. 6. The evaporation heat transfer coefficient of the TPCT increases with the increase of both weight concentration and operating temperature, in all cases studied. 7. The thermal resistance of the TPCT decreases with the increase of both weight concentration and operating temperature of the TPCT. The thermal resistance decreases up to 32.8% for the TPCT with SiC/water nanofluid at a weight concentration of 1.0% compared with that of the TPCT using water.

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