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Heat transfer enhancement using nanofluids in the compression exchanger in a solar Stirling engine
In this paper a improvement in the heat exchange of a solar Stirling engine
during the compression phase was studied. The viability of using nanoparticles as
ceramic oxide (Al2O3 in water) at different concentrations instead of conventional
refrigerants (water or air) was evaluated. Since these systems could behave as non-
Newtonian fluids the dynamic viscosity was measured, as well as other thermophysical
properties. The results showed that the convective heat transfer coefficient could raise
one order of magnitude respect to the conventional heat transfer fluids at moderately
volume fractions (over 0.15).Universidad de Málaga. Campus de Excelencia Internacional AndalucĂa Tec
Characterization and thermophysical properties of rutile and alumina nanofluids
The characterization in relation to shape and particle size of alumina and rutile
suspensions was performed. The intrinsic viscosity exhibited the presence of nonspherical
aggregates in both suspensions although TEM images showed non
monodisperse spherical shape of alumina particles. DLS indicated the existence of
particle aggregates for both systems. In all cases, the increase in thermal conductivity
with respect to the base fluid is verified. The thermal conductivities obtained
experimentally were compared with three mathematical models, which yielded lower
values than those measured. From rheological measurements and by means of the
Peclet number defined in colloidal suspensions, values of thermal conductivity were also
proposed.Universidad de Málaga. Campus de Excelencia Internacional AndalucĂa Tec
Rheological study of the aggregation state of alumina nanofluids
Se adjunta una comunicación, aunque se han publicado tres comunicaciones más.The presence of alumina solid particles in aqueous phase induces a change in the viscosity of the suspension from Newtonian to non- Newtonian flow. Besides, the presence of solid particles is adequate for the use as a heat exchanger fluid. The effect of nanoparticle size on thermal properties of nanofluids is still today a question, which is far from being answered. In this respect, the results reported in the literature are contradictory [1], probably due to the formation of aggregates when particles are dispersed in the liquid phase. Regarding to the nanoparticle shape influence in thermal conductivity, cylinders and spheres have been considered as the more effective in heat transfer.
Spherical nanoparticles of alumina, which is one of the most investigated nanofluid, dispersed in water were used in this study. TEM images showed a mean average diameter of 50 nm. However, DLS measurements showed monodispersed particles of 260 nm. Very recently [2], the relationship between shear rheology and aggregation state of suspensions has been reviewed. Mechanical and physical properties of the resultant materials depend on shape, size and size distribution, which are considered determining parameters in the formation of particle aggregates. The steady shear flow (figure 1) has shown that these clusters, when they are at rest, are formed by highly branched aggregates that erode when shear rate increases, until a suspension of individual particles is achieved. These results are in good agreement with the intrinsic viscosity obtained by Money and Krieger-Dougherty models. In both cases, these values are far from the 2.5 corresponding to spherical particles. The temperature effects were also taken into account.Universidad de Málaga. Campus de Excelencia Internacional AndalucĂa Tech
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