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 Andalucía 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 Andalucía 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 Andalucía Tech

Thermophysical and flow properties of coconut oil with silica fumed nanoparticles

This work is concerned with the rheological and thermal study of silica fumed dispersed in coconut oil. The dispersed phase behave as a non newtonian fluid during the phase change. The addition of nanoparticles complicates considerably the flow behaviour of the system. Therefore, a very thoughtful study of the flow and thermal properties with temperature when silica particles are added must be carried out.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

A comparative rheological study of amaranth, wheat and chickpea doughs

Legume flours due to their nutritional benefits (e.g., high levels of proteins, fiber, complex carbohydrates, micronutrients and vitamins) are ideal ingredients for improving the nutritional value of bread and bakery products. Although proteins has a major role in the quality breads, however, few studies related to the evaluation of doughs containing chickpea flour(or other legume flour) alone or blended with other ingredients have been published. This work performs a comparative study of the flow properties of three different flours: wheat, amaranth and chickpea, at two temperatures. The flow curves showed shear-thinning behaviour although amaranth flour presented the higher values of viscosity and wheat flour de lower values. The influence of the protein content in the three doughs was evaluated by means of step- up-stepdown tests and frequency sweeps (Fig. 1). Amaranth flour presented the higher elastic modulus and the wheat flour the lower. This can explain the compactation observed in gluten-free bread. The results of the flow studies are prone to the combination of cereal and legume flours in order to improve the taste and quality of gluten-free breads.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech