ETD

• Conduction
• Cooling Systems
• CubeSat
• Design
• Dimensionless Map
• Experimental Analysis
• Heat Transfer
• IR Thermography
• Jany-Bejan
• Lattice Structure
• Melting Front
• Metal Foam
• Modified Rayleigh Number
• Natural Convection
• PCM
• Permeability
• Porosity
• Porous Media
• Reduced Gravity
• Regime Transition
• Space Applications
• Spacecraft
• Stefan-Fourier Number
• Themal Control
• Thermal Conductivity
• Thermal Management

The present Master's thesis focuses on the experimental investigation of the melting process of phase change materials integrated with porous structures, with particular attention to the analysis of the melting front and the transition between conduction and natural convection thermal regimes. The study specifically investigates the use of metallic foams and lattice structures, characterized by various geometric configurations and base materials, aiming to understand the influence of key parameters such as porosity, permeability, and thermal conductivity of the porous medium.
The entire experimental campaign was carried out at the BEARS Laboratory of the Department of Energy, Systems, Territory and Construction Engineering (DESTEC) at the University of Pisa. The test apparatus included a long-wave infrared (LWIR) thermal camera and thermocouples to record the evolution of the melting front under controlled conditions. The data collected were processed through custom-developed MATLAB codes, enabling the synchronization, processing, and analysis of the measurements to extract both dimensional and dimensionless parameters. Among these, particular attention was given to the Stefan (Ste), Fourier (Fo), and modified Rayleigh (RaD) numbers, according to the theoretical framework proposed by Jany and Bejan.
Several PCMs were considered, including eicosane used in the current experimental campaign, and previously acquired data were reanalyzed from an earlier study involving a commercial paraffin and two PEGs with different molecular weights. These were tested in combination with aluminum 6101 foams and lattice structures in AlSi10Mg and resin, with different cell sizes. Experimental observations clearly highlighted the critical role of porosity and material on the melting dynamics: the foam with lower porosity and the aluminum lattices exhibited faster front propagation, whereas the resin lattices showed strongly convective behavior despite their low thermal conductivity, confirming the dominant influence of permeability.
The scope of the analysis was further extended by introducing an actively cooled wall opposite the heated side, simulating more realistic application conditions. The results revealed a significant slowdown in the melting process for high-conductivity materials, with a reduced effect observed in resin-based structures.
The main contribution of this thesis lies in the construction of dimensionless RaD-SteFo maps, which synthesize the experimental findings and clearly identify the conduction, convection, and transition regimes. These maps highlighted a hyperbolic behavior of the regime transitions, suggesting that the activation threshold for convection can be effectively represented by the product RaD ⋅ SteFo.
Since the modified Rayleigh number explicitly includes gravitational acceleration, these maps also offer a predictive tool for the design of PCM-porous systems operating under varying gravitational conditions, making them particularly relevant for advanced space applications.
The study also revealed limitations related to the geometric scale of the structures and opened the way to future developments, including testing under altered gravity environments and the exploration of new lattice topologies.