Natural convection heat transfer of alumina-water nanofluid in vertical square enclosures: An experimental study
Introduction
Nanofluid, a colloid composed of nanoparticles or carbon nanotubes in a base fluid, has been proposed as a highly-effective heat transfer medium in view of its abnormally higher thermal conductivity [1]. Considerable works have been focused mainly on identifying and modeling physical mechanisms for the unusual enhancement in effective thermal conductivity that arises at low particle fractions [2], [3], [4], [5], [6], [7].
For feasibility and efficacy of using various nanofluids for natural convection heat transfer, relatively few research efforts have been undertaken. Khanafer et al. [8] numerically investigated natural convection heat transfer in a two-dimensional vertical enclosure utilizing nanofluids. They concluded that the heat transfer rate increases with the particle fraction at any given Grashof number. Similar finding for heat transfer enhancement with suspended nanoparticles was reported in a recent numerical study [9] concerning natural convection heat transfer characteristics of a two-dimensional rectangular enclosure. However, the foregoing numerical predictions appear contradictory to the existing experimental studies [10], [11], [12], [13], [14] that dispersion of nanoparticles in base fluid was observed to result in marked reduction, instead of enhancement, of natural convection heat transfer in enclosures. Putra et al. [10] conducted an experiment on natural convection of Al2O3- and CuO-water nanofluids inside a horizontal cylinder heated from one end and cooled from the other. They found a systematic and definite deterioration in natural convective heat transfer. The deterioration increases with particle fraction and appears more significant for CuO-water nanofluid. For a TiO2-water nanofluid within a rectangular enclosure heated from below, Wen and Ding [11] reported that for the Rayleigh number less than 106, the natural convection heat transfer rate increasingly decreases with the increase of particle fraction, particularly at low Rayleigh numbers. In an experimental study [12] for natural convection in a vertical square enclosure filled with Al2O3-water nanofluid of mass fraction ranging from 0.36 wt.% to 10.04 wt.%, substantial heat transfer reduction was observed except the cases of the particle mass fraction not higher than 1 wt.%, for which marked heat transfer enhancement was detected though within the experimental uncertainty. In a vertical tall enclosure of aspect ratio 6.14, Nnanna [13] experimentally investigated the heat transfer behavior of natural convection of Al2O3-water nanofluid. At low volume fractions in the range of 0.2 vol.% ≤ cv ≤ 2 vol.%, it was found that the presence of the nanoparticles does enhance, instead of impede, the natural convection heat transfer. However, for higher volume fractions above 2 vol.%, the natural convective heat transfer was found deteriorated due to reduction in the Rayleigh number associated mainly with a significant increase in effective dynamic viscosity of the nanofluid. Most recently, an experimental work [14] was reported for natural convection of Al2O3-water nanofluid in a vertical cylindrical enclosure heated from below. Compared with the base fluid (distilled water), the heat transfer rate across the enclosure was found increasingly deteriorated with the volume fraction of the nanoparticles in the nanofluid, which ranges from 0.5 vol.% to 6 vol.%. To further explain the possible causes, other than the relatively enhanced viscosity of the nanofluid, for the deteriorated heat transfer efficacy of using the nanofluid, a flow visualization experiment of submicron polystyrene-water suspension in rectangular enclosure was performed to infer the detrimental influences due to the Brownian motions and thermophoresis movements of the particles.
For the above-mentioned diversity among the numerical predictions and experimental results concerning natural convection heat transfer efficacy of using nanofluid in enclosures, the possible contributing factors may include variations of the size and shape of particle, clustering of particles, particle–fluid interactions, and uncertainties in thermophysical properties of nanofluid, in particular the effective thermal conductivity and dynamic viscosity. Incorporating various models for effective thermal conductivity and dynamic viscosity, Hwang et al. [15] theoretically investigated by means of correlation analysis for natural convection heat transfer coefficient of the Al2O3-water nanofluid in a rectangular enclosure heated from below. The effect of the volume fraction on the ratio of heat transfer coefficient of nanofluids to that of base fluid was shown to be dependent on the two models adopted for calculating the effective viscosity of nanofluid. More recently, in a numerical study concerning natural convection in a vertical square enclosure filled with Al2O3-water nanofluid [16], it was shown that significant difference in the effective dynamic viscosity enhancement of the nanofluid calculated from the two adopted formulas, other than that in the thermal conductivity enhancement, could act as a major factor, thereby leading to contradictory results concerning the heat transfer efficacy of using nanofluid in the enclosure.
The present study is a follow-up to the earlier experiment [12], aiming to provide supplementary data for thermal characterization as well as for the natural convection heat transfer characteristics of Al2O3-water nanofluid in the vertical enclosures.
Section snippets
Formulation and characterization of nanofluids
Nanoparticles of Al2O3 with a density of 3600 kg/m3 and an averaged particle size about 33 nm (Nanotech, Kanto Chemical Co. Inc., Japan) were used to formulate the nanofluid with ultra-pure Milli-Q water as the base fluid in the present study. The following steps were adopted to prepare the nanofluid: (1) Dispersing the required amount of nanoparticles into the bottle of base fluid while stirring with a magnetic stirrer for 2 h; (2) Adjusting the pH value of the nanofluid to pH = 3. The
Thermophysical properties of nanofluid
Firstly, the volume-mean diameter, dp, of the alumina particles dispersed in the water-based nanofluid formulated was found to increase significantly with the volume particle fraction of the nanofluid, from 129 nm at cv = 0.1 vol.% to 167 nm at cv = 4 vol.%.
In Fig. 2 dependence of the measured results of the dynamic viscosity of the nanofluid prepared on the volume particle fraction and temperature is illustrated. Also included in Fig. 2(a) are the experimental data from Wang et al. [17], the
Concluding remarks
In the present study, we have experimentally investigated the natural convection heat transfer of Al2O3-water nanofluid in vertical square enclosures of three different sizes together with measurements for the thermophysical properties of the nanofluids formulated. A correlation analysis based on the thermophysical properties for the nanofluids formulated shows that the heat transfer efficacy of using nanofluid for natural convection in enclosure is dependent on the net influences by the
Acknowledgement
The present study is supported by the National Science Council of ROC in Taiwan through the projects: NSC93-2212-E006-013, NSC94-2212-E006-101, and NSC95-2212-E006-232. The constructive comments of the reviewer are sincerely appreciated.
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