stability and thermal conductivity characteristics of

Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering

2012

Stability and Thermal Conductivity Characteristicsof Nanofluids (H2O/CH3OH + NaCl + Al2O3Nanoparticles) for CO2 Absorption ApplicationChangwei [email protected]

Yong Tae Kang

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International Refrigeration and Air Conditioning Conference at Purdue, July 16-19, 2012

Stability and Thermal Conductivity Characteristics of Nanofluids (H2O/CH3OH + NaCl + Al2O3 Nanoparticles) for CO2 Absorption Application

Changwei Pang1, Yong Tae Kang1*

1*Kyung Hee University, Department of Mechanical Engineering, Gyeong-Gi 446-701, Korea Tel: +82-31-201-2990, Fax: +82-31-202-3260, E-mail: [email protected]

1Kyung Hee University, Department of Mechanical Engineering, Gyeong-Gi 446-701, Korea

E-mail: [email protected]

ABSTRACT In this study, the nanofluids are produced by dispersing Al2O3 nanoparticles in the H2O/CH3OH binary mixtures using an ultrasonic equipment. Solid Sodium Chlorides (NaCl) with a fixed mass fraction are dissolved in the binary mixture for CO2 absorption. The main objective of this paper is to measure the thermal conductivity of the Al2O3 nanofluids using the transient hot-wire method. The experimental uncertainties in repeatability are obtained as 1.95% for DI water and 1.34% for pure methanol, respectively. The results show that the dispersed nanoparticles can always enhance the thermal conductivity of the basefluid and the highest enhancement is observed to be 6.3% in the concentration of 0.1vol% of Al2O3 nanoparticles, 40vol% of CH3OH and 10wt% of NaCl at 293.15 K. In addition, the zeta potential, visualization and Tyndall effect are investigated to discuss the stability of the present nanofluids.

1. INTRODUCTION

Recently, the integrated gasification combined cycle (IGCC) is attracting extensive attention as the next generation energy plant by using synthetic natural gases (SNG). The Rectisol system (physical absorption type) is prevalently applied to remove the sour gases in the SNG production system. Methanol (CH3OH) are commonly used as an absorbent in Rectisol system due to its low cost and the selective absorption of acid gases such as CO2, H2S and COS (Ranke and Mohr, 1985; Korens et al., 2002). The temperature of methanol absorbent should be maintained at least about -40 oC to increase the absorption rate according to the Henry’s solubility law. That means, a huge amount of energy is required in the Rectisol process. Nanofluids (nanoparticles dispersed in the basefluid) are explored as the promising next generation heat transfer fluid due to the high enhancement in thermal conductivity as described in the review literatures (Yu et al., 2008; Özerinç et al., 2010). To improve the energy efficiency in the sour gases removal system, the methanol-based nanofluids have been developed to enhance the CO2 absorption rate in recent researches (Kim et al., 2008; Lee et al., 2011). Kim et al. (2008) reported that the capacity coefficient of CO2 absorption in water-based silica (SiO2) nanofluid was 4 times higher than water without nanoparticles because the small bubble sizes in the nanofluid had large mass transfer areas and high solubility. In another previous report (Lee et al., 2011), it was proposed that the pH variation was closely related with the absorption enhancement by the dissociation of the particle surface, and then bonding the hydroxyl group to nanoparticles in the nanofluids. In this study, the methanol/aqueous Sodium Chloride (NaCl) solution based Al2O3 nanofluids are prepared for the application in the CO2 removal system. The original viewpoint for the addition of solid NaCl to prepare the methanol based nanofluids is attributed to the high availablity of sea water and its low cost in this application. The

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suspension stability and thermal characteristics of nanofluids should be known to optimize the CO2 absorption systems (Jodecke et al., 2004; Lee et al., 2011). Therefore, in this study, the suspension stabilities of nanofluids are estimated by analyzing the visualization photos and Tyndall effect and the measurements of zeta potential, the thermal characterizations are investigated by measuring the thermal conductivity of nanofluids using the transient hot-wire method.

2. EXPERIMENT

2.1 Preparation for Nanofluids The two-step method is applied to prepare nanofluids in this study. Al2O3 nanoparticles (Alfa Aesar, Johnson Matthey Korea, Korea) are used for the production of nanofluids with the particle concentrations of 0.1vol%. Figure 1 shows the FE-SEM image of Al2O3 nano-powder, which has the primary particle size ranging from 40 - 50 nm. The concentrations of 10, 20, 30 and 40vol% for methanol and 3.5, 10 wt% for NaCl are chosen for the preparation. Arabic Gum (AG) (Dae Jung Chemicals & Metals Co. Ltd., Korea) is used as a steric stabilizer and has the weight concentration of 0.05 wt%.

Figure 1: FE-SEM image of Al2O3 nano powders (40 - 50 nm) The detailed procedures for nanofluids preparation is as follows: (1) Mixing the deionized (DI) water and pure methanol with a specific vol%, which is followed by adding the Al2O3 nanoparticles, AG and solid NaCl to produce a primary suspension; (2) The suspension is mixed using the ultrasonic vibration for 1 hr, and then, finally the nanofluids are prepared. Table 1 shows the description of the experimental conditions in detail.

Table 1: Experimental conditions Basefluid H2O/CH3OH + NaCl

Concentration of Methanol 10/20/30/40 vol%

Concentration of NaCl 0/3.5/10 wt%

Nanoparticle Al2O3 (0.1 vol%)

Powder diameter 40 - 50 nm

Surfactant Arabic Gum (AG)

Stirring condition 700 rpm

Total vol. of sample 600 mL

Ultra sonication

Time 1 h

Frequency 20 kHz

Power 750 W

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2.2 Transient Hot-Wire System This work presents the application of transient hot-wire method to measure the thermal conductivity of nanofluids. Figure 2 describes the schematic diagram of the transient hot-wire system, includes the Wheatstone bridge (a) and the test section (b). In principle, using a Wheatstone bridge circuitry, while the hot-wire is electrically heated, the change in the temperature is measured as a function of time and the results are transfered to the data acquisition system. Finally, the thermal conductivity is determined by the heating power and the slope of temperature change in logarithm time. The Pt-wire (isonel insulated platinum wire, A-M System, Inc., WA, US) of 185 mm in length and 25 µm in diameter are utilized as the hot-wire. The bridge input and output voltages are measured using the computerized data acquisition system (LabVIEW 8.5) and the input voltage is configured at 5 V. The fluid tank (test section) has a diameter of 50 mm and the Pt-wire is vertically located in it. The transient hot-wire method for the thermal conductivity measurement of nanofluids is introduced in detail in the references (Kostic et al., 2009; Jung et al., 2011).

(a) Wheatstone bridge

(b) Test section

Figure 2: Schematic diagram of the transient hot-wire system In this work, DI water and pure methanol are used as the test samples prior to the main experiments to validate the transient hot-wire system. The measurements in thermal conductivity were carried out at the temperature of 293.15 K. Figure 3 shows the verification results for the present experimental apparatus. With the respective reference thermal conductivities of 0.6018 W/m-1.K-1 for DI water, 0.2040 W/m-1.K-1 for pure methanol, the measurements uncertainties in repeatability were carried out by the calculation of bias error and precision error, which was finally obtained as 1.95% for DI water and 1.34% for pure methanol, respectively.

Figure 3: Experimental verification results

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3. RESULTS AND DISCUSSION

3.1 Visualization and Tyndall Effect The Tyndall effect, which comes from light scattering by particles in a colloid or a fine suspension, is used to confirm the existence of particles in nanofluids. The level of particles dispersion in the test section can be judged by taking photos of the Tyndall effect. Figure 4 shows the photos of visualization and Tyndall effect for the prepared nanofluids. These pictures were taken in two cases: just right after preparation and one week later. In the case of just right after preparation, it is clearly shown that the light could not pass through the nanofluids due to the high particle concentration. Since one week later after the preparation, it is obviously found that the nanofluids display a weak stability when the NaCl salts are added into the suspension. Especially at the 3.5 wt% NaCl, a large amount of nanoparticles settle down at the bottom of the bottles. This result suggests that the NaCl reduces the nanofluids stability and the steric stabilizer, Arabic Gum, does not work well in the present Al2O3 nanofluids.

Figure 4: Photos of visualization and Tyndall effect for the Al2O3 (0.1vol%) nanofluids

3.2 Zeta Potential The absolute value of zeta potential is one of the evaluation parameters for colloidal stability (Hunter, 2011). The dielectric constant, which is an essential parameter to measure the zeta potential for suspensions, has to be known prior to the measurement. However, the measurement in this work becomes difficult since there is no available reference on the dielectric constants of the basefluid. Therefore, the prevalent method is suggested to estimate the dielectric constant for the H2O/CH3OH + NaCl solution in two steps. Firstly, the dielectric constant for the mixtures of H2O/CH3OH could be obtained based on the previous report (Harvey and Prausnitz, 1987) as shown in Table 2.

Table 2: Dielectric constant for H2O/CH3OH mixture

Volume fraction of methanol (vol%)

10 20 30 40

Dielectric Constant

75.80 71.67 68.78 65.21

Fawcett and Tikanen (1996) proposed the dielectric constant ( bf ) of the electrolyte solution depending on the

electrolyte concentration ( c ), the correlation is given as follows,

/bf bm c bc 3 2 (1)

where bm is the dielectric constant of H2O/CH3OH mixture, is the dielectric decrement for the electrolyte

solution and b is a parameter describing the curvature. Also the values of the constants and b are obtained by

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considering the electrolyte activity as 19 and 5, respectively, which has been well described in the literature (Fawcett and Tikanen, 1996). Therefore, in the second step, the dielectric constant of the basefluid could be determined based on the dielectric constant estimation of H2O/CH3OH mixture and the calculation with eq. (1). Finally, the zeta potential would be measured by using a dynamic light scattering device (ELS-Z, Otsuka, JPN). Figure 5 shows the absolute zeta potential of the present Al2O3 (0.1vol%) nanofluids. The corresponding measurement at the concentration of 3.5 wt% NaCl and 40 vol% CH3OH is not obtained due to the bad nanofluid stability.. It shows that the cases without NaCl have relatively higher values of the absolute zeta potential (hereafter just zeta potential). It confirms that the NaCl added in the suspension would decrease the nanofluids stability as described in the literature Jung et al. (2011). In the case of NaCl at 3.5 wt%, the values of the zeta potentials for nanofluids get close to the point of zero charge (PZC), at which the electrical charge density on a surface is zero and most of the particles have the potential to settle down. This represents that the nanofluids with the addition of NaCl have a weak stability and which agrees well with the description of the Visualization and Tyndall effect as shown in Fig. 4.

Figure 5: Absolute zeta potential of the Al2O3 (0.1vol%) nanofluids 3.3 Thermal Conductivity Enhancement Figure 6 shows thermal conductivities of the basefluid (the mixtures of aqueous NaCl solutions and pure methanol) and the Al2O3 nanofluids. It is found that the thermal conductivities of both the basefluid and nanofluids gradually decrease with respect to the concentration of salt NaCl (0/3.5/10 wt%) or pure methanol (10/20/30/40 vol%). It confirms the reduction in thermal conductivity with the addition of salt in aqueous solution as shown in the previous literature (Yusufova et al., 1975), which also proposes the possible explanation for the decrement results from the formed sodium ions which promote stiffening of the solvent structure and hinder translational thermal motion of both the ions themselves and of the solvent molecules and thereby reduce the thermal conductivity of the solution. With the effect of dispersed Al2O3 nanoparticles in the suspension, it is clearly shown in Fig. 6, the thermal conductivity of nanofluids are always enhanced over the basefluid. The enhancement in thermal conductivity is defined in eq. (2) and the corresponding results are shown in Fig. 7.

100nf bfk

bf

k kE

k

(2) Where Ek is thermal conductivity enhancement, knf is thermal conductivity of the nanofluid and kbf is thermal conductivity of the basefluid. It indicates that the thermal conductivity enhancement goes up with an increase of either the weight percent of NaCl or the volume fraction of pure methanol in the mixtures. The highest enhancement of thermal conductivity for the mixtures (H2O/CH3OH + NaCl) based Al2O3 nanofluids is obtained to be 6.34% at a concentration of 10 wt% of NaCl and 40 vol% of CH3OH.

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Figure 6: Thermal conductivities of the basefluid and the Al2O3 (0.1vol%) nanofluids

Figure 7: Thermal conductivity enhancements for the Al2O3 (0.1vol%) nanofluids In the recent years, there are various theories proposed to discuss the thermal conductivity enhancement of nanofluids such as the aggregation of nanoparticles, Brownian motion, nanoscale convection and nanolayer, which are described well in the literatures (Murshed et al., 2008; Lee et al., 2010; Özerinç et al., 2010). However, the mechanism for thermal conductivity enhancement remains unclear and is intensely debated. In this study, it is clearly found that adding CH3OH and NaCl results in a decrease in thermal conductivities of the H2O/CH3OH and H2O/CH3OH/NaCl solutions (shown in Fig. 6). It represents that the thermal conductivity ratio of nanoparticle to basefluid (knp/kbf) increases with increasing the concentrations of CH3OH and NaCl. Therefore, it could be suggested that thermal conductivity enhancements of the nanofluids are heavily affected by the ratio of knp/kbf.

4. CONCLUSIONS

In this study, the Al2O3 nanofluids are prepared and the estimation for stability and thermal conductivity for the nanofluids are carried out. The following conclusions are obtained;

The dispersion stability of H2O/CH3OH/NaCl based Al2O3 (0.1vol%) nanofluids deteriorates one week later after the preparation.

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It is found that the thermal conductivity enhancement of nanofluids increases with increasing either the concentration of NaCl or CH3OH, which is mainly caused by the increment of the thermal conductivity ratio, knp/kbf.

The highest enhancement in thermal conductivity of the present nanofluids is obtained as 6.3% for the concentration of 10 wt% NaCl, 40 vol% CH3OH and 0.1 vol% Al2O3 nanoparticles.

NOMENCLATURE

b parameter describing the dielectric curvature - Subscripts c electrolyte concentration (%) bf Base fluid k thermal conductivity (W.m-1.K-1) bm Base mixture

dielectric decrement for the electrolyte solution - np nanoparticle Ek thermal conductivity enhancement - nf nanofluids

bf dielectric constant of the base fluid -

bm dielectric constant of the H2O/CH3OH mixture -

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ACKNOWLEDGEMENT

This work was supported by the National Research Foundation (NRF) grant (No. 20100029120).

stability and thermal conductivity characteristics of

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