Enhancement of thermal conductivity of ethylene glycol based silver nanofluids

Abstract

Nanofluid is a kind of new engineering material consisting of solid particles with size typically of 1–100 nm suspended in base fluids. Nanofluids offer excellent scope of enhancing thermal conductivity of common heat transfer fluids. In the present study, nanofluids are synthesized using silver nitrate (precursor), ethylene glycol (reducing agent), and poly(acrylamide-co-acrylicacid) (dispersion stabilizer). The different concentrations of silver nanofluid (1000–10,000 ppm) were synthesized. The silver particles present in colloidal phase have been characterized by EDX, XRD, UV–visible spectroscopy, Zeta potential and transmission electron microscopy (TEM). The stability as well as thermal conductivity of these nanofluids was determined with a transient hot-wire apparatus, as a lapse of time after preparation. Typically, 10,000 ppm silver nanofluid exhibited rapid increase in the particle size with the passage of time. Thermal conductivity of silver nanofluids increased to 10, 16, and 18% as the amount of silver particles in nanofluid were 1000, 5000, and 10,000 ppm, respectively. After 30 days of preparation, the thermal conductivity of 1000 and 5000 ppm silver nanofluids decreased slightly from 10% and 15% to 9% and 14%, respectively. In addition, the thermal conductivity of 10,000 ppm nanofluid was decreased from 18% to 14% after 30 days. It is very interesting to report that the silver particles were aggregated in early stage of preparation (up to 15 days), which leads to the increase in the size of silver particles. However, no significant change was observed after 15 days which indicates the stability of silver nanofluids.

Introduction

Researches in heat transfer have been carried out over the previous several decades, leading to the development of currently used heat transfer enhancement techniques. Conventional heat transfer fluids have inherently poor thermal conductivities. One of the most commonly used techniques to improve the thermal conductivity of fluids is to suspend small solid particles with high thermal conductivity in fluids. The fluid suspensions of solid particles provide useful advantages in industrial fluid system, including heat transfer fluids, magnetic fluids and lubricant fluids [1], [2], [3], [4], [5], [6], [7], [8], [9]. In the past, mili or micro-sized particles in fluids had not only interfered in the flow of fluids, but also increased the risk of clogging, sedimentation and erosion of pipes and channels.

However, recent developments on nano-sized particles in fluids suggest solutions for these problems [10], [11]. The principal idea is to exploit the very high thermal conductivities of solid particles which can be even hundred or thousand folds greater than those of conventional heat transfer fluids. Nanofluids engineered on the atomic or molecular scale to produce either new or enhanced physical properties were not exhibited by conventional micro-sized bulk solids. Therefore, nanofluids not only solve the problems such as sedimentation of large particles, clogging flow channels, cohesion, corrosion, erosion of pipelines and causing pressure drop which happens conventionally in heterogeneous solid/liquid mixture with millimeter or micrometer particles, but also increase the thermal performance of base fluids remarkably [7], [8], [9], [11], [12], [13]. Several other unique and remarkable properties of nanoparticles such as electronic, electric, optical, chemical, and mechanical properties can attributed to the advancement of modern techniques [14], [15]. Typically, metal nanoparticles are used industrially and are applied to many fields. Contrary to the milli- and micro-sized particle slurries explored in the past, nanoparticles are relatively close in size to the molecules of the base fluid, and thus can realize very stable suspension with little gravitational setting over long periods of time.

Nanofluid is a kind of new engineering material consisting of solid nanoparticles with sizes typically of 1–100 nm suspended in base fluids. The term “nanofluid” was first proposed by Choi in 1995 of the Argonne National Laboratory, U.S.A. [16] as a route for surpassing the performance of common heat transfer liquids. Nanofluids prepared by dispersing nanoparticles into conventional heat transfer fluids are proposed as the next generation heat transfer fluids as they offer exciting new possibilities to enhance heat transfer performance compare to pure liquids. Therefore, nanofluids have attracted great interest due to their potential benefits for numerous applications such as microelectronics, energy supply, transportation and HVAC. As a result, nanofluid technology becomes a new challenge for the heat transfer fluid because of their higher thermal conductivity [17] and stability than those of the conventional heat transfer fluid or the suspensions of the micro-sized particles. Keblinski et al. [11] listed four possible explanations for the cause of an anomalous increase of thermal conductivity such as Brownian motion of the nanoparticles, molecular-level layering of the liquid at the liquid/particle interface, the nature of heat transport in the nanoparticles, and the effects of nanoparticles clustering.

Preparation of nanofluids is the key step in the use of nanoparticles to improve the thermal conductivity of fluids. Researchers have used different types of solid nanoparticles like: (1) metallic particles (Cu, Al, Fe, Au, and Ag); (2) nonmetallic particles (Al₂O₃, CuO, Fe₃O₄, TiO₂, and SiC); (3) carbon nanotube; and (4) nanodroplet with high thermal conductivity as additives of nanofluid. The base fluids commonly used are water, acetone, decene, engine oil and ethylene glycol. There are mainly two techniques used to produce nanofluids; (i) the single-step direct evaporation method represents the direct formation of the nanoparticles inside the base fluids, and (ii) the two-step method represents the formation of nanoparticles and the subsequent dispersion of the nanoparticles in the base fluids. In either case, the preparation of a uniformly dispersed nanofluid is essential for obtaining stable reproduction of physical properties or superior characteristics of the nanofluids. The single-step process was developed by Akoh et al. [18] and is called the VEROS (Vacuum Evaporation onto a Running Oil Substrate) technique. A modified VEROS process was proposed by Wagener et al. [19] a little later. The processes of drying, storage, transportation, and the dispersion of nanoparticles are avoided, so the agglomeration of nanoparticles is minimized and the stability of the fluids is increased. However, only low vapour pressure fluids are compatible with this process. This disadvantage limits the application of this method. Whereas, the two-step process is extensively used in the synthesis of nanofluids considering the available commercial nanopowders supplied by several companies. Eastman et al. [20], Lee et al. [9], and Wang et al. [15] used this method to produce Al₂O₃ nanofluids. Also, Murshed et al. [21] prepared TiO₂ suspension in water using the two-step method. In this process, nanoparticles were first produced and then dispersed with the base fluids. Generally, ultrasonic treatment, control of pH or addition of surface active agents are some techniques which are used to attain stability of the suspension of the nanofluids against sedimentation by suppressing formation of particle cluster. But, the addition of surface active agents can affect the heat transfer performance of the nanofluids, especially at higher temperature. However, these efforts to improve the thermal conductivity have been interfered with the stability of particle size. When the heat transfer particles were dispersed in solvents, nanoparticles tend to aggregate into large clusters [22], [23], [24].

Literature survey reveals that nanofluids dispersing carbon nanotubes [25], [26], copper[27], aluminium [28], tin [29], iron [30]and their oxide, titania [21], and gold [31] nanoparticles have been widely investigated with water, ethylene glycol, toluene, engine oil, poly oil, decene and FC-72 as base fluid and their findings are published in the peer reviewed journals. However, very few reports are available on silver [32], [33] based nanofluids, which has excellent chemical and physical stability, even though it is not widely investigated. Therefore, in the present study, the chemical reduction method, which is simple single-step process involving simultaneous preparation and direct dispersion of nanoparticles in the base fluid, was applied to prepare silver nanofluid [34]. Among different polymers, as a dispersion stabilizer, poly(acrylamide-co-acrylicacid) (PAA-co-AA) was a final selection due to its good affinity with silver particles. Ethylene glycol that has already been reported was used as the reduction agent for silver nitrate as well as solvent [35]. The size distribution of particles and the stability of silver nanofluids were investigated. The silver particles were characterized by energy dispersive X-ray (EDX) and X-ray diffraction (XRD) techniques. UV–vis spectroscopy and Zeta potential were adopted to observe the trend of particle size distribution with the passage of time. Transmission electron microscopy (TEM) was used to observe the morphology of nanoparticles. Thermal conductivities of silver nanofluids were measured by a transient hot-wire method each after 1, 3, 7, 15, and 30 days of preparation.