| Resumo: | The effect of using non-Newtonian fluids as a heat transfer medium in microchannel heat exchangers (MCHE) is numerically investigated. The study of Non-Newtonian fluids for heat transfer purposes is a research area with growing interest as a result of the development of nanofluids with enhanced heat transfer characteristics, as these usually show a complex rheologic behaviour. In the present work, water-based carbon multi-walled carbon nanotubes (MWCNT) fluids, showing a shear-thinning rheological behaviour were considered. The heat transfer performance of MWCNTs nanofluids in microchannels is assessed for a wide range of operating conditions and comprising the parameters known to directly influence the thermophysical and transport properties of this sort of heat transfer fluid, namely base fluid, nanoparticle geometry and concentration. The overall problem is computationally solved using CFD tools, considering a single-phase, 2D, laminar, steady state flow numerical model for a specific micro heat exchanger geometry. The thermophysical properties of the considered MWCNTs nanofluids, experimentally obtained, were made available and conveniently modelled. As the physical properties of the considered fluids are directly related to the fluids morphology, the study provides a means to establish the relative influence of MWCNTs nanofluids properties on the overall heat transfer and fluid flow effectiveness of such systems, providing a mean to indirectly support the tailoring of the heat transfer fluid to specific heat exchanger applications, e.g.: micro-electromechanical systems, solar energy, aerospace applications. This thesis addresses several goals of the 2030 Agenda for Sustainable Development, adopted by all United Nations Member States in 2015, namely those directly and indirectly related with energy use and availability (7, 9, 11 and 13, see: https://sustainabledevelopment.un.org/sdgs), since heat transfer effectiveness dictates the overall energy efficiency of most thermal systems, broadening the scope towards more rational use of energy sources and hence mitigate the environmental impact of the system’s life cycle.
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