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Café thésard : Natural convection in Phase Change Materials
4 mars 2021 @ 14h00 - 15h00
Résumé : Phase change materials (PCM) are extensively studied nowadays in the con- text of energy storage. These materials can store or release a significant amount of energy with latent heat. In this context, optimizing heat transfer is a key objective. One solution consits in performing the phase change in convective regime within the liquid phase. Natural convection in PCM is widely studied by theoretical and numerical means 1,2,3,4. However, scientific locks still exist due to the complexity to deal with both buoyancy-driven convection and an unsteady background state due to the phase change. Very few experimental in- vestigations exist because PCM are opaque in solid phase. In the present work, we present new experimental results obtained by Magnetic Resonance Imaging (MRI) which allows to image through opaque materials and to measure velocity fields inside the liquid part.
We consider a PCM, the hexadecane with melting point of Tm = 17.9◦C, initially in solid phase at homogeneous temperature Tc < Tm. Then, the ma- terial is heated from below leading to its melt. The melting interface remains planar as long as the regime is conductive in the growing liquid layer. When the fluid depth h, and therefore the Rayleigh number, are large enough convection occurs in the liquid, coupled with a distorted interface (Fig. 1(a)). With the onset of convection, the fluid height can increase by a factor of 4 compared with the conductive regime (Fig. 1(b)). We find that heat transfer is also clearly enhanced with convection, even close to the threshold, according to the power law Nu ∝ Ra0.3.