Observation of heat transfer due to variable thermophysical properties of sub-, near- and super- critical fluids in porous media by magnetic resonance imaging

Abstract Local velocity and temperature profiles are measured non-invasively using nuclear magnetic resonance (NMR) imaging (MRI) in porous media with a sub-, near-, and supercritical fluid. Heat transfer is driven by natural convection via heating from below. The C2F6 fluid fills the pore spaces in a packed bed of encapsulated phase change material (PCM). As the PCMs melt, 1H signal intensity increases with temperature. Physical properties of C2F6, e.g. thermal diffusivity, vary substantially at the critical point, affecting the rate and magnitude of heat transfer. The changes in bead signal, e.g. temperature, are imaged and measured over time at 27 bar (sub-critical), 32, 34 and 36 bar (near-critical), and 66 bar (supercritical). Despite velocity profile similarities, the temperature in the packed bed varies significantly with thermal diffusivity, the magnitude of which decreases and increases with temperature around the critical point. NMR spatially and temporally captures the resulting dynamic heat transfer processes.