Di Giulio, Elio (2023) Thermo-fluid dynamic properties of porous materials for energy conversion. [Tesi di dottorato]

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Abstract

Thermoacoustics combines thermodynamics, fluid dynamics and acoustics to describe the interaction that exists between heat and sound. Two types of devices can be realized which are based on the thermoacoustic conversion of energy: thermoacoustic refrigerator (or heat pump) that convert sound wave energy in refrigeration (or heating), thermoacoustic engine that convert heat in useful work. Nevertless, thermoacoustic technology could play a significant role in the development of renewable energies because of its advantages over conventional energetic technologies (environmentally friendly working fluid, low-grade energetic inputs can be used as driving sources, no moving part, low manufacturing and maintenance costs), actually there are still some challenges left that need to be resolved before thermoacoustic devices can be used competitively on a large scale. The core of thermoacoustic engines and heat pumps (or refrigerators), in which the energy conversion takes place, is represented by a particular porous material, named stack (or regenerator), suitably designed to allow the correct viscous and thermal interactions between the oscillating fluid and its solid surface in order to convert a mechanical energy (as a sound wave) into heat, and vice versa. To make these devices more efficient, it is crucial to understand the phenomena that occur in that porous core better and then identify the ideal geometry for each unique working condition. Therefore, the characterization of a porous material, used as stack, is made through the study of the thermo-fluid dynamic fields inside its solid skeleton fulfilled by a fluid. In particular, fluid mechanics balance equations for fluids need to be solved in the harmonic regime, because of the fluid moves under an oscillating flow (as sound wave is) stimulus. The goal of this thesis work is to expand the frontiers of knowledge in thermoacoustics by employing unconventional materials as stacks. Unconventional thermoacoustic stacks, such as Tetragonal Pin Array, Wire Mesh and 3D-Membrane Foams, have been investigated. New semi-phenomenological models, inspired by classically used models used to predict the sound absorption (or sound transmission loss) applications, are used to mathematically describe their behaviour. These predictive models of the viscous and thermal behaviours are based on their micro-geometrical features. Furthermore, the experimental validations of the predictive models have been carried out. Two new measuring approaches have been devised to address the shortcomings of the existing experimental methodologies, particularly in the low frequency range, for evaluating the dynamic behaviour of porous materials. Finally, an energetic criterion to select the core which maximize the heat-to-acoustic energy conversion has been presented.

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