Cinética da transferência de calor em materiais com mudança de fase


Potential applications of phase change materials in thermal regulations and heat or cold storage have been addressed extensively in the literature. However, low thermal conductivity limits a wide variety of prospective applications requiring short response time or high power delivery in charge/discharge of latent heat. These limitations determined the contents of this thesis. One developed numerical codes to obtain solutions for the kinetics of heat transfer with endo/exothermic phase change, for representative geometries, i.e., for planar, spherical and cylindrical symmetries-. Simpler solutions were also derived for quasi steady state solutions, and one established criteria to validate these approximate solutions, based on properties of the phase change materials, and taking into account spatial and time scales. These solutions allowed one to identify the relevant factors, which determined those limitations, to quantify corresponding effects, and to establish quality criteria for different types of prospective applications. These criteria were establish by analogy with generic selection guidelines proposed by Ashby and co-authors, seeking optimum performance for representative applications with specific main requirements in terms of power density, response time, power under latent heat changing/discharging, and temperature range. These selection maps include some of the phase change materials developed in the actual work. The analysis of impact of kinetic limitations set the framework to develop composite materials for heat or cold storage, based on incorporation of a suitable highly thermal conducting phase on the phase change material. One developed models for the optimum spatial distribution of the highly conducting phase, to overcome percolation limits imposed by random distribution, and to seek enhanced thermal response for low fractions of the conducting phase, with minimum impact on energy density. The developed models correspond to core-shell composites, based on cellular microstructures of highly conducting phase impregnated with the phase change material. In addition to seeking minimization of the conducting phase and corresponding costs, the model composites were designed to facilitate versatile processing, with emphasis on methods based on emulsification of organic liquids in aqueous suspensions, or other processes of low complexity and based on low cost precursors (phase change material and thermal conductor). Design of these model composites also considered the possibility of preferential orientation of highly anisotropic thermal conductors (e.g. graphite), by self organization. Other tasks of this work were determined by those objectives of development of latent heat storage composites with enhanced thermal response, in close agreement agreement with predictions by the model core-shell composites; this comprised development of 3 different types of composites with cellular organization of the highly thermal conducting phase, as follows: i) cellular paraffin-graphite composites for heat storage, prepared by emulsification of graphite suspensions in melted paraffin; ii) cellular paraffin-alumina latent heat storage composites, prepared by impregnation of cellular alumina ceramics with paraffin; iii) cellular composites for cold storage, prepared by impregnation of a cellular graphite skeleton with collagen aqueous solution. Composites with cellular ceramic skeleton required development of a dedicated processing method, which was based on emulsification of alumina suspensions with melted paraffin; with the required additives (dispersant, tensioactive, and consolidation agent); this allowed preparation of a selfsupported green ceramic skeleton, which did not collapse during subsequent steps of elimination of the paraffin phase and final firing at high temperatures, to obtain cellular ceramics with suitable mechanical strength. The developed composites yielded very important gains in thermal response, reaching increase in thermal conductivity by more than one order of magnitude for volumes fractions of conducting phase in the order of 10 vol.% (4 W m-1 K-1); this can be ascribed to the core-shell organization with additional contribution of preferential orientation of highly anisotropic conductor (graphite). One also developed composites for heat storage, with random distribution of conducting phase, by gelling graphite aqueous suspensions with additions of collagen. Collagen exerted the double effect as a dispersant for graphite in aqueous medium, and as gelling additive, yielding microstructural stability and shape consolidation. Yet, gains in thermal conductivity were rather small, when compared with corresponding composites with cellular organization of the conducting phase.

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Nuno Miguel Dias Vitorino

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Jorge Ribeiro Frade; João Carlos de Castro Abrantes


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