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New Measurements of the Apparent Thermal Conductivity of Nanofluids and Investigation of Their Heat Transfer Capabilities Georgia J. Tertsinidou, Chrysi M. Tsolakidou, Maria Pantzali, and Marc J. Assael* Laboratory of Thermophysical Properties & Environmental Processes, Chemical Engineering Department, Aristotle University, Thessaloniki 54636, Greece
Laura Colla, Laura Fedele, and Sergio Bobbo Consiglio Nazionale delle Ricerche, Istituto per le Tecnologie della Costruzione, Corso Stati Uniti 4, Padova 35127, Italy
William A. Wakeham Chemical Engineering Department, Imperial College London, London SW7 2BY, U.K. ABSTRACT: The aim of this paper is to investigate in depth whether adding nanoparticles or nanotubes to a fluid enhances its heat transfer capabilities. For this reason, the thermal conductivities and viscosities of a selection of nanofluids were thoroughly examined. The systems studied were (a) ethylene glycol with added CuO, TiO2, or Al2O3 nanoparticles and (b) water with TiO2 or Al2O3 nanoparticles or multiwall carbon nanotubes (MWCNTs). All of the measurements were conducted at 298.15 K. In a very recent paper, it was shown that instruments employing the transient hot-wire technique can produce excellent measurements when a finite element method (FEM) is employed to describe the instrument for the geometry of the hot wire. Furthermore, it was shown that an approximate analytic solution can be employed with equal success over the time range from 0.1 to 1 s, provided that four specific criteria are satisfied. Subsequently a transient hot-wire instrument was designed, constructed, and employed for the measurement of the thermal conductivities of nanofluids with an uncertainty of about 2%. A second, validated technique, namely, a hot-disk instrument, was also employed to conduct measurements on some of the systems to provide mutual support for the results of the thermal conductivity measurements. To investigate the effect of any enhancement of the thermal conductivity of the fluids on their application in practical heat transfer, the viscosities of typical concentrations of several of the nanofluids were also measured. A parallel-plate rotational rheometer, able to measure the viscosities of Newtonian and non-Newtonian liquids with an uncertainty of better than 5%, was employed for these measurements because most of the nanofluids considered showed behavior comparable to a Bingham plastic. All of these measurements have allowed an investigation of the change in the heat transfer capability of the base fluid when nanoparticles or MWCNTs are added to it for a typical heat exchanger. It is shown that in general the combined changes in physical properties that accompany suspension of nanoparticles in fluids mean that the heat transfer benefits are all rather modest, even when they are achieved.
1. INTRODUCTION The aim of this paper is to investigate in depth whether adding nanoparticles or nanotubes to a fluid enhances its heat transfer capabilities. For this reason, the thermal conductivities and viscosities of a selection of nanofluids were thoroughly examined. The systems studied were (a) ethylene glycol with added CuO, TiO2, or Al2O3 nanoparticles and (b) water with added TiO2 or Al2O3 nanoparticles or multiwall carbon nanotubes (MWCNTs) at 298.15 K. In a very recent paper,1 the conditions that are necessary to secure accurate measurements of the apparent thermal conductivities of two-phase systems comprising nanoscale particles of one material suspended in a fluid phase of a different material © XXXX American Chemical Society
were carefully examined. It was shown that instruments operating according to the transient hot-wire technique can produce excellent measurements with an uncertainty of better than 0.5% when a finite element method (FEM) is employed to describe the operation of the instrument for the geometry of the hot wire. Furthermore, it was shown that an approximate analytic solution can be employed with equal success over the time range from 0.1 to 1 s provided that (a) two wires are employed, so that end effects are canceled; (b) each wire is very thin (less than 30 μm in Received: August 30, 2016 Accepted: October 25, 2016
A
DOI: 10.1021/acs.jced.6b00767 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
case of MWCNTs, a nonionic dispersant (no further information was provided by the supplier) was employed. 2.1.1. Ready-Made Nanofluids. All of the ready-made nanofluids were purchased from US Research Nanomaterials, Inc. The original nanoparticles’ concentration was 20 mass % in all cases except case 6 in Table 1, while the MWCNT concentration was 3 mass %. In order to produce fluids with different particle volume fractions, dilution with deionized water or ethylene glycol followed by a stirring action was employed. 2.1.2. Nanofluids Prepared in Our Laboratory. The CuO and Al2O3 nanoparticles were obtained from US Research Nanomaterials Inc., while the TiO2 nanoparticles were purchased from Nanostructured & Amorphous Materials, Inc. Desired solutions were made using the two-step method. First of all, the masses of the solid and liquid phases were determined by means of a precision balance. It may be mentioned here that the true density of particles (neglecting the mass of air trapped inside), can be up to 20 times the apparent bulk density, as shown in Table 2.
diameter), so that the line source model and the corresponding corrections are valid; (c) low values of the temperature rise (less than 4 K) are employed in order to minimize the effect of convection on the heat transfer by conduction in the time of measurement of 1 s; and (d) insulated wires are employed for measurements in electrically conducting or polar liquids to avoid current leakage or other electrical distortions. According to these criteria, a transient hot-wire instrument was designed, constructed, and commissioned.1 Because the systems included in this study are not a single phase, the “apparent” thermal conductivity is not a thermophysical property of the system dependent only on the thermodynamic state parameters for a single phase, and the quantity measured may depend upon the method of measurement. For that reason, we also employed a second validated technique, namely, a hot-disk instrument, able to measure thermal conductivity with an uncertainty of 5%. The principal interest in nanofluids has been driven by an argument that their enhanced thermal conductivity, relative to that of the base fluid, makes them superior agents for heat transfer in a variety of circumstances. In order to test this argument rigorously, we also measured the viscosities of typical concentrations of the above nanofluids. A parallel-plate rotational rheometer, able to measure the viscosities of Newtonian and non-Newtonian liquids with an uncertainty of better than 5%, was employed for these measurements because the nanofluids considered display the character of a Bingham plastic. Finally, we examined the performance of the studied nanofluids in a typical heat exchanger relative to the performance of the base fluid on its own.
Table 2. True and Bulk Densities of Nanopowders As Given by the Corresponding Suppliers density/(kg·m−3) nanopowder
true
bulk
CuO (40 or >80 nm) TiO2 (5 nm) Al2O3 (5 nm)
6400 3900 3890
790 200−300 180
Because it is the volume fraction of the particles in the suspension that is the relevant parameter for comparison with the most relevant theories of transport in nanofluids, e.g., the Hamilton−Crosser model,2 we need the true density of the nanoparticles to derive this quantity from the known masses of the materials. No dispersant was added. Following weighing, the liquid and solid phases were mixed, and homogenization was implemented with an ultrasonic vibrator (HF-generator GM2200, Bandelin) which was employed to sonicate the solution continuously for about 1 h in order to break down any existing agglomerations. The time period of 1 h was found to be the optimum sonication time because additional sonication time had no further effect on the stability of the sample. During the sonication period, it was crucial to avoid dissipative heating of the sample, and thus, an external water bath was employed to keep the temperature at ambient conditions.
2. PREPARATION AND CHARACTERIZATION OF NANOFLUIDS 2.1. Nanofluid Preparation. In the present study, two different groups of nanofluids were examined, which are detailed in Table 1. The first group included ready-made nanofluids provided by a commercial source, while the second group consisted of nanofluids prepared in our laboratory using the twostep method. In the case of ready-made nanofluids, poly(vinylpyrrolidone) (PVP) was employed as a dispersant in the samples provided by the supplier. We note here that although PVP produces very good and stable solutions, as is necessary for industrial use, it also increases the viscosity of the material compared with those of systems without such a dispersant. In the
Table 1. Nanofluids (in Water or Ethylene Glycol) and Nanopowders Purchaseda 1 2 3 4 5 6 7 8 9 10 11 12
nanopowder + nanofluid
diameter/nm
dispersant or stabilizerb
companyc
CuO CuO rutile TiO2 (20%) + H2O rutile TiO2 (15%) + H2O rutile TiO2 (20%) + C2H6O2 TiO2 γ-Al2O3 (20%) + H2O γ-Al2O3 (20%) + H2O γ-Al2O3 (20%) + C2H6O2 hydrophilic γ-Al2O3 MWCNTs (3%) + H2O MWCNTs (3%) + H2O
