Abstract
In this paper the mathematical and physical correlation between fundamental thermophysical properties of materials, with their structure, for nanofluid thermal performance in spray cooling applications is presented. The present work aims at clarifying the nanofluid characteristics, especially the geometry of their nanoparticles, leading to heat transfer enhancement at low particle concentration. The base fluid considered is distilled water with the surfactant cetyltrimethylammonium bromide (CTAB). Alumina and silver are used as nanoparticles. A systematic analysis addresses the effect of nanoparticles concentration and shape in spray hydrodynamics and heat transfer. Spray dynamics is mainly characterized using phase Doppler interferometry. Then, an extensive processing procedure is performed to thermal and spacetime symmetry images obtained with a high-speed thermographic camera to analyze the spray impact on a heated, smooth stainless-steel foil. There is some effect on the nanoparticles’ shape, which is nevertheless minor when compared to the effect of the nanoparticles concentration and to the change in the fluid properties caused by the addition of the surfactant. Hence, increasing the nanoparticles concentration results in lower surface temperatures and high removed heat fluxes. In terms of the effect of the resulting thermophysical properties, increasing the nanofluids concentration resulted in the increase in the thermal conductivity and dynamic viscosity of the nanofluids, which in turn led to a decrease in the heat transfer coefficients. On the other hand, nanofluids specific heat capacity is increased which correlates positively with the spray cooling capacity. The analysis of the parameters that determine the structure, evolution, physics and both spatial and temporal symmetry of the spray is interesting and fundamental to shed light to the fact that only knowledge based in experimental data can guarantee a correct setting of the model numbers.
Highlights
The dissipation of high thermal loads is still an obstacle to overcome in several applications such as in electronics cooling, UAV’s, among others [1].In this context, spray cooling is pointed as a high potential solution
The analysis performed here considered a constant temperature across the foil, which is valid for a Biot number Bi = hd/ks
The droplets may stick and spread, contributing to the liquid film that will spread on the surface, contributing to cooling the surface, they may bounce from the surface or disintegrate into secondary drops
Summary
The dissipation of high thermal loads is still an obstacle to overcome in several applications such as in electronics cooling, UAV’s (for military applications), among others [1]. Studied the effect of the use of nanoparticles in the thermophysical properties (viscosity) and convective thermal behavior in nanofluids utilized for heating buildings in cold zones Their results demonstrated an increase of the heat flux exchange for higher percentages of nanoparticles (considering the range between the 0% and the 6%). The main innovative aspects are the use of the PDI technique, coupled with an Infrared Analysis and the kind of percentages of nanoparticles considered to examine the effects in heat exchange and wetting surface processes. In this context, the present work combines the use of the PDI with time resolved thermographic analysis to obtain a full relation between the spray hydrodynamics and the heat transfer mechanisms at spray impact. The authors have attempted to improve the knowledge of these 2 nanofluids (due to their importance for several fields in chemistry and physics) and compare our conclusions with the scarce information reported in the literature
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