DI

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Investigation of Heat-Transfer Characterization of EDA-MWCNT/DI-Water Nanofluid in a Two-Phase Closed Thermosyphon Mehdi Shanbedi, Saeed Zeinali Heris, Majid Baniadam,* Ahmad Amiri, and Morteza Maghrebi Department of Chemical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran ABSTRACT: In this study, the effect of addition of multiwalled carbon nanotubes (MWCNTs) functionalized with ethylenediamine (EDA) to deionized water (DI water) on the heat-transfer performance of a two-phase closed thermosyphon (TPCT) was studied experimentally. The functionalized MWCNTs were characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), NCHS elemental analysis, and transmission electron microscopy (TEM) and were compared with raw MWCNT to confirm the functionalization. Heat-transfer characterization of the nanofluid consisting of EDA-MWCNT/ DI-water was studied in different input powers (30
150 W) and various weight concentrations (0.2%
1.5%). The experimental results indicate that, when the input powers are 1%, the thermal efficiency of TPCT is maximized. This study shows that increasing the input power decreases the thermal resistance of TPCT. In addition, distribution of the wall temperature of TPCT is known to be the key parameter in calculating thermal resistance. The results also suggest that the vacuum-pressure drop of TPCT decreases when the concentration of nanofluid increases.

1. INTRODUCTION Thermosyphons have many applications in thermal engineering systems such as reboilers, air conditioning systems, heat exchangers, etc. The most important advantages of thermosyphon are simpler structure, higher efficiency, lower thermal resistance, and low manufacturing cost.1 Thermosyphons are considered to be a subset of heat pipes. Heat pipes have wick and closed end-cap tube while thermosyphons have no wick. Heat pipes can operate horizontally, because of the wick and the capillary force. However, in a wickless heat pipe or a thermosyphon, an evaporator must be located vertically below the condenser to ensure that the condensate returns to the evaporator under the effect of gravity.2 The ability of nanofluids to improve the heat transfer has been proved by various researchers in recent decades. This term was first introduced by Choi, who used the concept of nanofluid for nanoparticles suspension in a base fluid and obtained a high thermal conductivity of nanofluid, compared to the ordinary fluid.3 Zeinali Heris et al.4
6 compared convective heat-transfer enhancement of CuO/water and Al2O3/water nanofluid in laminar flow under constant wall temperature with different volume concentrations. They reported that the Al2O3/water nanofluids show more enhancement in the convective heattransfer coefficient, compared with CuO/water (increase of 10%
30%), for a Peclet number (Pe) ranging from 2500 to 6800 at an Al2O3 concentration of 2 vol %. In addition, Zeinali Heris et al.,7 reported similar results for laminar flow convective heat transfer using Cu/water nanofluid. Noie et al.8 reported the heat-transfer enhancement using Al2O3/water nanofluid at different input powers in a two-phase closed thermosyphon. They found that the thermal efficiency increased from 2% to 14.7% when they applied input powers in the range of 48.4 to 97.1 W, respectively. They also reported an increase of 2.7% for the base fluid when input power varied from 146.3 to 195.2 W. r 2011 American Chemical Society

Numerous researchers have studied the effect of filling ratio on performance of thermosyphons. These researchers have shown that the minimum heat resistance and maximum heat removal occurs in an optimal filling ratio. Paramatthanuwat et al.,1 working with Ag/DI-water nanofluid, investigated the effect of filling ratio and aspect ratio. Their experimental results confirmed that the optimal liquid filling ratio occurred at ∼50%. Cao et al.,9 Hopkins et al.,10 and Liu et al.11,12 showed that the filling ratio in the range of 40%
60% could significantly enhance the heattransfer rate. Carbon nanotubes (CNTs) are unique materials that have applications in different field of thermal sciences. However, the instability of aqueous and organic suspensions of CNT has limited their applications. Chemical functionalization can enhance the dispersibility of CNT in nanofluids.13 Liu et al.14,15 and Xie et al.16 compared the effects of CNTCuO and CNT-Al2O3 nanoparticles on the enhancement of thermal conductivity of nanofluids. Their results demonstrated that the type of base fluid has very little effect on the enhancement of the thermal conductivity of a nanofluid. However, the thermal conductivity enhancement of CNT suspensions was much higher than those of oxide nanoparticles suspensions with the same volume fraction of nanoparticles. Choi and Eastman17 confirmed the thermal conductivity enhancement of MWCNT/water nanofluid, according to the base fluid. This finding clearly suggests that conventional heat conduction models for solid-in-liquid suspensions are inadequate. Liu et al.12 investigated the effect of water-based CNT suspension in different operations pressures on thermal performance such as heat-transfer coefficient and total heat resistance of a miniature thermosyphon. Received: September 14, 2011 Accepted: December 20, 2011 Revised: December 5, 2011 Published: December 20, 2011 1423

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Figure 1. Schematic of the experimental setup.

Xue et al.18 studied the effect of purified CNT by nitric acid and sulfuric acid in aqueous solution on the thermal performance of the thermosyphon. Thermal resistance increased in a thermosyphon that contained CNT
water nanofluid, compared with water as base fluid. Therefore, the addition of CNT decreased the heat-transfer performance of the thermosyphon. In this study, first multiwalled carbon nanotubes (MWCNTs) were covalently functionalized with ethylenediamine (EDA) to produce EDA-MWCNTs. After that, transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, NCHS, and thermogravimetric analysis (TGA), are used to ensure the covalent functionalization of the MWCNTs. Then, the effect of the addition of EDA-MWCNT in deionized water on the thermal performance of a TPCT was investigated. Thermal efficiency, thermal resistance, vacuum pressure variations, and distribution of the wall temperature of TPCT are among the main operating parameters that are studied.

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Figure 2. Demonstration of dispersion of functionalized MWCNT dispersion in DI-water: (A) functionalized MWCNTs with EDA and (B) raw MWCNTs.

elimination of the noncondensable gases from TPCT. The maximum flow rate and vacuum pressure of pump could reach 142 L/min and
27 in.Hg, respectively. Four IC thermocouples (three thermocouples in the evaporator and one thermocouple into the condensing section) were mounted on TPCT to measure the wall temperature distribution. In addition, a thermometer pen measured the inlet and outlet water temperatures at the condenser section. The maximum precisions of the thermocouples, voltmeter, and ammeter were 0.1 °C, 1 V, and 0.001 A, respectively. The flow of cooling water through the condenser was 250 mL/min with an uncertainty of 2% in the measurement of results. The uncertainty was calculated according to the Holman19 correlation,

2. EXPERIMENTAL SECTION 2.1. Materials. MWCNTs with a diameter of 10
20 nm, a length of 5
15 μm, and purity of >95% were purchased from Shenzhen Nanotech Port Co. The EDA, N,N-dimethylformamide (DMF) (analytical grade), and NaNO2 used in this work were purchased from Merck, Inc. 2.2. Instruments. Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), NCHS, and transmission electron microscopy (TEM) are several analytical techniques that are used to determine the morphology and chemical groups on the MWCNTs. FTIR spectra (KBr pellets) were recorded on a Shimadzu Model 4300 FTIR spectrometer. TGA was performed using a Shimadzu Model TGA-50 thermogravimetric analyzer in air with a heating rate of 10 °C/min. TEM images were obtained using a LEO Model 912AB TEM microscope with an accelerating voltage of 120 kV. NCHS analysis determined the amount of nitrogen, carbon and hydrogen content of the functionalized and raw MWCNT samples, using a Flash EA1112 series NCHS Analyzer. Figure 1 shows the scheme of TPCT used in these experiments. TPCT with a length of 450 mm and external diameter of 20 mm was made of copper tubes and it has three sections. The evaporator, condenser, and adiabatic sections were 160, 200, and 90 mm in length, respectively. Peripheral equipment included in TPCT were DC power supply (Mastech (3 KV), China), water tank, and mechanical vacuum pump (ROBINAIR, USA) for

max Eη ¼ ( ½ðEQout Þ2 þ ð
EQin Þ2 1=2

ð1Þ

max EQout ¼ ( ½ðEin Þ2 þ ðEcp Þ2 þ ðEðTo
Ti Þ Þ2 Þ1=2

ð2Þ

max EQin ¼ ( ½ðEv Þ2 þ ðEI Þ2 1=2

ð3Þ

The maximum uncertainty was calculated at 5.77%. 2.3. Preparation of EDA-MWCNTs. Raw-MWCNTs were functionalized with EDA, according to the method reported by Chidawanyika et al.20 Marginal changes that are made in the procedure and reaction experiments are as follows. Two grams of raw MWCNTs were dispersed in NaNO2 (2.657 g), EDA (2.5 g), and 1.74 mL of H2SO4. The mixture is heated at 60 °C for 10 h. The mixture was cooled to room temperature, then filtered through a polytetrafluoroethylene membrane of 0.22 μm pore size. The mixture was washed several times with DMF and dried in an oven at 60 °C overnight. The dispersion of EDA-MWCNTs in DI-water is compared with the dispersion of raw MWCNT in DI-water in Figure 2. 2.4. Characterization. 2.4.1. FTIR. Figures 3A and 3B presents the FTIR spectra of the raw MWCNT and EDA-MWCNT. The FTIR spectrum of raw MWCNT does not display special peaks, but the FTIR spectrum of EDA-MWCNT displays several peaks, which are intensified after functionalization by EDA. The peaks at the range of 3200
3400 cm
1 are assigned to the primary amine stretching vibration. The peak at 1557 cm
1 corresponds with the
NH2 bending vibration. Increasing of peaks intensity at 2853 cm
1 and 1312 cm
1 are attributed to C
H and C
N stretching vibration. 1424

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Figure 3. FTIR spectra of (A) raw MWCNTs and (B) EDA-MWCNTs. Figure 5. NCHS analysis of the raw MWNTs and EDA-MWCNTs.

In eq 4, V and I represent the applied voltage and current, respectively. The rate of output heat transfer to the condenser can be calculated using eq 5 as follows:8,21 Q out ¼ mC _ P ðTout
Tin Þ

ð5Þ

Equation 5 expresses that the Qout is a function of the cooling water mass flow rate (m), _ outlet temperature (Tout), and inlet temperature, Tin: Q out ¼ f ðm, _ Tout , Tin Þ Figure 4. TGA analysis of the (A) raw MWNTs and (B) EDA-MWCNTs.

2.4.2. TGA. TGA curve of the raw MWCNT and EDAMWCNT are presented in Figure 4. The curve of the EDAMWCNT shows earlier mass loss, with regard to raw MWCNT; because EDA has a lower decomposition temperature. A significant mass loss can be seen in the temperature range of 50
150 °C for the EDA-MWCNT. The mass loss of the TGA is more evident for MWCNT functionalization. 2.4.3. NCHS. Nitrogen and hydrogen is determined before and after the functionalization of MWCNT with NCHS. The amount of nitrogen and hydrogen in the raw MWCNT and EDA-MWCNT is observed in Figure 5. Increasing the amount of nitrogen and hydrogen confirms functionalization of CNTs with EDA. 2.4.4. TEM. Figure 6 represents the TEM images of EDAMWCNT. The TEM images show that the MWCNT diameter ranges from 10 nm to 20 nm after functionalization, while the cylindrical structure of MWCNT remained perfect. These results confirm that functionalization with EDA have no defect on structure or morphology of treated MWCNTs. Note that thermal properties of CNT do not change if the basic structure of these materials is preserved, and the images advocate this statements.

3. DATA PROCESSING The thermal properties such as heat input and output, thermal efficiency, and thermal resistance were calculated using correlation presented in the literature. These formulas are outlined below. The rate of input heat transfer to the evaporator was obtained as8,21 Q in ¼ VI

ð4Þ

ð6Þ

The thermal efficiency of thermosyphon can be obtained as a ratio of output power to input power:8,22 η¼

Q out Q in

ð7Þ

In this study, the thermal resistance (Rth) is calculated as follows:11,12,22
25 Rth ¼

Te
Tc Q in

ð8Þ

In eq 8, Te and Tc are the temperatures of the evaporator and condenser sections, respectively.

4. RESULTS AND DISCUSSION In this study, experiments have been performed at six input powers:, 30, 45, 60, 90, 120, and 150 W with filling ratio of 60% and weight (volume) concentrations of EDA-MWCNT/DIwater nanofluids of 0.2% (0.095%), 0.5% (0.238%), 1% (0.475%), and 1.5% (0.71%). Figure 7 shows the thermal efficiency of TPCT as a function of input power. The increase in input power and concentration of nanofluid caused improvement in the thermal efficiency of TPCT. This trend continues to 90 W and 1 wt % of EDAMWCNT/DI-water nanofluid, but the increment of input power above 90 W and a concentration of nanofluid of >1 wt% reduced the thermal efficiency of TPCT. This may be attributed to the “geyser effect”, which is called impulsive boiling at low pressures. The maximum thermal efficiency of the thermosyphon occurred under the conditions of 90 W and 1 wt %, showing existence of an optimal concentration of nanoparticles in the TPCT. Also, decomposition of functional groups (EDA) on the surface of the MWCNT could play an important role in the reduction of thermal efficiency above 90 W. Meanwhile, an excessive increase 1425

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Figure 6. TEM images of the EDA-MWCNTs sample.

Figure 8. Thermal resistance of TPCT versus input powers for different weight concentrations of nanofluid. Figure 7. Thermal efficiency of TPCT versus input power for EDAMWCNT/DI-water nanofluid with different weight concentration.

in concentration leads to an increase of viscosity, which consequently reduces the thermal efficiency. Also, the formation of a vapor bubble at the solid/liquid interface affects the thermal resistance of thermosyphon. A larger bubble nucleation creates a higher thermal resistance, which prevents heat dissipation from the surface of the solid
liquid.26 Thermal resistance versus input power of thermosyphon is shown in Figure 8. Increasing the input power caused a decrease in thermal resistance. Also, Figure 8 shows that higher input power reduces the thermal resistance. Therefore, minimum thermal resistance occurred for nanofluid with 1 wt % of EDAMWCNT as the optimum concentration. On the other hand, increasing the concentration of functionalized MWCNT nanofluid decreases the average temperature of the evaporator wall, which has a direct relationship with the thermal resistance. A steady-state distribution of the average temperature of evaporator section wall in different input powers is shown in Figure 9. Meanwhile, the average temperature of the condenser wall versus different input powers and weight concentrations of nanofluids is presented in Figure 10. Based on eq 8, the average temperature of the evaporator and condenser play the basic role with regard to calculating the

Figure 9. Average temperature of evaporator versus input powers for different weight concentrations of nanofluid.

thermal resistance. Considering Figures 9 and 10, the evaporator and condenser wall temperatures increase as the input power increases. Comparison between the average temperature of the evaporator and the condenser in Figures 9 and 10 show that increasing the nanofluids concentration leads to significant temperature variations in evaporator section, while trivial temperature variation is obtained in the condenser section. According to the fixed Qin, the main parameter influencing the 1426

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Industrial & Engineering Chemistry Research

Figure 10. Temperature of condenser section versus different input powers in weight concentrations of nanofluid.

Figure 11. Decrease in vacuum pressure versus different stages of experiment in different weight concentrations of nanofluid.

thermal resistance is the average temperature of evaporator section. However, the average temperature of the evaporator section is dependent significantly on the concentration of nanofluid. This temperature is reduced with increasing concentration. However, to attain accurate thermal resistance, the condenser temperatures were considered in our results. In heating and cooling systems, the decrease in vacuum pressure is a very important parameter. In these experiments, changes in vacuum pressure versus input powers at different weight concentration of functionalized MWCNT are shown in Figure 11. For all samples, the vacuum pressure decreases at higher concentration of nanoparticles, but with different slopes. Obviously, the nanofluid with 1 wt % of nanoparticles has the minimum decrease in vacuum pressures. Therefore, this concentration is considered as the optimal concentration, according to the decrease in vacuum pressure. Decreasing the thermal resistance while increasing the concentration of functionalized MWCNT/water nanofluid might be attributed to the major thermal resistance of thermosyphon, which is, in turn, related to the formation of vapor bubbles at the fluid/solid interface. A larger bubble nucleation size creates a higher thermal resistance that prevents the transfer of heat from the solid surface to the liquid. The suspended MWCNT tend to bombard the vapor bubbles during the bubble formation. Therefore, it is expected that the nucleation size of vapor bubble would be much smaller for fluid with suspended nanoparticles than that without them.26
28 Besides the previously mentioned probable reason for the TPCT heat-transfer enhancement using nanofluid as working

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media, it may be also be because of the release of the bubbles that are accumulated on the inner wall of the evaporator, increasing the liquid mixing in evaporator dump, reduction of temperature fluctuation in the evaporation section, reduction of the temperature difference between the fluid and wall, and consequently a critical heat flux occurs at a shorter time. Also, during nucleate boiling, some nanotubes deposit on the heater surface to form a porous layer. This layer improves the wettability of the surface considerably.28 Random movement and dispersion effect of nanoparticles inside evaporator section can affect the temperature range along the inside wall of TPCT. The effect of nanoparticles on two-phase flow heat-transfer enhancement may be illustrated in two ways: the suspended nanoparticles, which increased the thermal conductivity of base fluid and the interactions among the nanotubes itself, on one hand, and between nanotubes and the inner surface of the heat pipe on the other hand. In addition, the diffusion and collision intensification of nanotubes in nanofluid near the duct wall, which is due to the increase in concentration of nanotubes leads to rapid heat transfer from the heat pipe wall to the nanofluid.

5. CONCLUSION In this study, MWCNT was functionalized with EDA by a simple ultrasonication procedure. Functionalized MWCNT was characterized by FTIR, TGA, NCHS and TEM analysis. These characterization techniques confirmed functionalized of MWCNT with EDA. The effect of addition of EDA-MWCNT in DI-water on the performance of a TPCT was then studied. Heat-transfer performance of the TPCT experimentally investigated in different weight concentrations ranging from 0.2% to 1.5% and different input powers ranging from 30 W to 150 W and compared with the base fluid. The main results are summarized as follows: (1) EDA-MWCNT/DI-water nanofluid with concentration of 1 wt % and input power of 90 W shows the maximum thermal efficiency of 93%. (2) In all input powers, EDA-MWCNT/DI-water nanofluid with concentration of 1 wt % has the minimum thermal resistance. (3) The use of EDA-MWCNT/DI-water nanofluids in TPCT caused the vacuum pressure to decrease. Nanofluid with 1 wt % EDA-MWCNT has the minimum decrease in vacuum pressure, compared with other nanofluids and DI-water. (4) The steady-state temperature distribution of the wall of the evaporator section corresponds with the thermal resistance of TPCT. In general, the addition of CNT functionalized with ethylenediamine improves the thermal performance of TPCT. The optimal concentration of nanofluids observed at 1 wt % EDA-MWCNT. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: (+98) (511) 8805031. Fax: (+98) (511) 8816840. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors would like to thank Iran Nanotechnology Initiative Council for financial support and the Chemical Engineering 1427

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Industrial & Engineering Chemistry Research Research Lab of Ferdowsi University of Mashhad (FUM), for performing the characterization analysis.

’ NOMENCLATURE A = external surface of evaporator section (m2) Cp = specific heat of water (J/(kg K)) I = current (A) L = length of evaporator section (m) m_ = water mass flow rate (kg/s) Q in = rate of input heat transfer by evaporation (W) Q out = rate of output heat transfer by condensation (W) Rth = thermal resistance (°C/W) Tin = inlet temperature at the condenser section (°C) Tout = outlet temperature at the condenser section (°C) Te = temperature of evaporator section (°C) Tc = temperature of condenser section (°C) Greek Letters

η = efficiency of TPCT ν = volume concentration of MWCNT in suspension (%) Abbreviations

TPCT = two-phase closed thermosyphon MWCNT = multiwalled carbon nanotubes EDA = ethylene diamine Subscripts

in = input out = output th = thermal e = evaporator section c = condenser section

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