Abstract
In this study, the thermal conductivity of aqueous nanofluids containing clusters of iron oxide (Fe3O4/γ-Fe2O3) nanoparticles has been investigated experimentally for the first time, with the aim of assessing the role of a controlled aggregation of nanoparticles in these final nanofluids. For that, clusters of iron oxide nanoparticles of different cluster size (46–240 nm diameter range) were synthesized by a solvothermal method and fully characterized by transmission electron microscopy, X-ray diffraction and Raman spectroscopy. The rheological behavior of the optimal nanofluids was also studied by rotational rheometry. The nanofluids were obtained by dispersing the clusters of iron oxide nanoparticles in water taking into account different solid volume fractions (from 0.50 to 1.5 wt%) and the experiments were conducted in the temperature range from 293.15 K to 313.15 K. The study reveals and quantifies enhancements in the thermal conductivity of nanofluid with increase of cluster size and temperature. Furthermore, a 0.50 wt% concentration of clusters of iron oxide nanoparticles within the whole range of proposed nanofluids offers great stability and improved thermal conductivity for heat transfer applications with an small dynamic viscosity increase. In addition, the larger the size of the clusters of iron oxide nanoparticles, the greater the increase in thermal conductivity for the designed Fe3O4/γ-Fe2O3 cluster-based nanofluids, with thermal conductivity values following a constant upward trend and reaching a maximum increase of 4.4% for the largest synthesized clusters (average size of 240 nm). These results open the door for the development of iron oxide-based nanofluids on which taking advantage of an optimized aggregation of nanoparticles by using size-customized clusters.
Highlights
The increasing demand of energy across the world has become a critical issue in our nowadays society
The clusters of iron oxide (Fe3O4/c-Fe2O3) nanoparticles studied were obtained through a solvothermal method, by which the mechanism of formation proceeds via a two-stage growth process, with nucleation of primary nanocrystals followed by uniform and controlled aggregation into larger secondary nanostructures [27]
The X-ray diffraction (XRD) analysis of these clusters shows five similar patterns (Fig. 2a), on which we observe well-defined peaks that correspond to a spinel structure, though with different noise attending to the different size of the crystalline domains forming part of the nanoparticles in the final clusters [27]
Summary
The increasing demand of energy across the world has become a critical issue in our nowadays society. A TC enhancement upon the dispersion of nanoparticles into a base fluid has been previously studied for heat transfer [8,12,13,14,15], and there are many different examples reporting improvements in the values of TC when using metallic (Fe, Cu) or metal oxide (Fe3O4) nanoparticles, at different concentrations, at different temperatures, using different average sizes and even taking into account the influence of an external magnetic field [2,11,16] In this regard, different authors reported non-dependence in the enhancement ratios of this property with the increasing temperature. The chosen clusters were dispersed in water to obtain aqueous nanofluids using different concentrations, of which the thermal conductivities and dynamic viscosities were obtained through the detailed careful analysis
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