A “hourglass” system for transient thermal management based on dynamic close-contact melting of a phase-change material

High latent heat of phase change materials (PCMs) can be utilized for thermal management of electronic devices operating in transient conditions, either over a limited period of time or in an intermittent fashion. However, because of the low thermal conductivity of PCMs and associated thermal resistance, the device can exceed its allowed operation temperature. A common approach to deal with this problem is by using extended surfaces and their analogs, like porous structures or conductive inclusions. An alternative approach to mitigating thermal resistance in PCM-based systems, explored in the present study, is based on a novel concept termed “dynamic PCM”, in which a load applied to the solid PCM causes the latter to move towards the heat source and melt in such a manner that the melted PCM is squeezed away and the melted region retains its rather small, and practically constant, thickness. Being an extension of close-contact melting (CCM), caused only by the PCM's own gravity, in “dynamic PCM” the resulting heat transfer is both enhanced and may be controlled via the regulated applied load. A novel device, termed an “hourglass” system because of its shape and its way of operation, is introduced and explored experimentally. Its dimensions and features, specifically the transparent envelope, are chosen to obtain a clear picture of the process via visualization. The force exerted on the PCM is created by completely passive means, namely, a weight which moves freely through the liquid PCM but pushes the solid PCM. The system is sealed and can be used for cyclic operation, without the need to be opened or refilled. It contains up to 220 g of eicosane, whereas the stainless steel weight is 250 or 500 g. The explored heat inputs are up to 60 W, corresponding to the heat fluxes of up to 3.8 W/cm². The results clearly demonstrate the advantages of the dynamic PCM: at the heat input of 30 W, the excess temperatures of the heated body over the PCM melting point are about 11 and 8 centigrade for the weight of 250 g and 500 g, respectively, while in pure CCM without a weight this value is about 24 °C. Robust and repeatable performance of the system is demonstrated for various heat inputs, PCM amounts and added weights. A complete periodicity of the operation is achieved, with turning the system around for recharge being the one and only mechanical action required because of the cyclic character of its operation. To provide further insights into the physical phenomena involved, an in-house numerical model is formulated, allowing for the inclusion of the added weight in the force balance. Then, a physically-meaningful dimensional analysis is performed, based on the appropriately defined Stefan, Fourier and Archimedes numbers and involving the mass ratio of the added weight and the PCM itself. This analysis successfully generalizes the predicted and experimentally achieved melt fractions and Nusselt numbers. It indicates that for the systems of this type, it is possible to define system parameters needed to cope with expected heat loads in a prospective application. The ways of practical implementation of the explored type of systems are outlined and discussed.