Internship : Melting Dynamics of a non-Ideal Phase Change Material

Description 1. Context Thermal energy storage using Phase Change Materials (PCMs) relies on the absorption and release of latent heat during melting and solidification (Fig. 1a). Because large amounts of energy can be stored within a limited temperature range, PCMs are used in a wide variety of applications, including solar thermal systems, building thermal regulation, heating networks, and cooling technologies[1] . However, many organic PCMs do not undergo a well-defined phase transition at a given temperature, but rather over an extended temperature interval. Their phase-change behavior can depend on the applied thermal and dynamical conditions and may involve thermal hysteresis between melting and solidification points, the formation of mushy layers (i.e., partially melted regions), strong changes in rheological properties, and a significant influence of interfaces on nucleation and crystallization processes [2] . These non-ideal effects directly impact heat, mass, and momentum transfer, as well as the overall performance of PCM-based energy storage systems. Moreover, these properties play a particularly important role when the phase change occurs in a convective-dominated regime. In such configurations, buoyancy-driven flows that develop in the liquid phase can interact directly with the phase transition process, further affecting melting and solidification dynamics. An accurate understanding of the coupled effects of PCMs properties near the phase transition and thermal convection in the liquid PCM is therefore essential to properly predict the actual behavior of PCM-based systems under realistic operating conditions. 2. Proposed Work Characterization of thermomechanical properties The first part of the study will focus on the behavior of the PCM close to the phase transition temperature range using a coupled Rheometer-Raman experimental setup, which enables simultaneous access to bulk rheological properties and local structural information. Rheological measurements will be performed to investigate how bulk properties depend on imposed heating/cooling rates and on applied stresses during melting and solidification. In parallel, Raman spectroscopy will provide local, small-scale information on structural changes occurring during the phase change. The combined analysis will allow bulk rheological behavior to be directly correlated with microscopic structural evolution, providing useful insight into the solidification process. Study of thermo-convective flows In a second step, the melting of the same PCM will be studied in a convective-dominated regime when embedded in a macroscopic porous matrix. A Rayleigh-Bénard type configuration involving a porous matrix will be used to promote buoyancy-driven convection in the liquid phase. Experiments will be carried out using Magnetic Resonance Imaging (MRI), which enables noninvasive measurements inside opaque systems to identify solid and liquid phases, measure local flow velocity fields, and obtain temperature fields (Fig. 1b). These measurements will allow the characterization of convective flow structures and their interaction with mushy regions, as well as the quantification of how natural convection influences melting dynamics and heat transfer compared with purely conductive regimes 3. Expected skills This project is primarily experimental, but the work can be extended to include numerical modeling. The candidate is expected to have skills in materials physics and/or fluid mechanics and thermal science. The project is intended to lead to a possible PhD continuation.