Melting of nano-PCM in an enclosed space: Scale analysis and heatline tracking

Nanoparticle enhanced phase change material (or nano-PCM) is an attractive option to enhance the performance of thermal energy storage systems (e.g., solar-thermal energy storage system). In this study, authors have investigated the melting performance of nano-PCMs inside a square enclosure which is a representative geometry of a thermal energy storage system. Three PCMs with different melting temperatures were experimented: (i) paraffin wax (T-m = 54 degrees C > T-infinity), (ii) coconut oil (T-m = 24 degrees C approximate to T-infinity), and (iii) Rubitherm (R) RT-18 (T-m =18 degrees C < T-infinity), where T-infinity is the average room or the surrounding temperature. The nano-PCMs inside the enclosure are: CuO nanoparticles dispersed in Rubitherm (R) RT-18 PCM (nano-PCM-1) and CuO nanoparticles dispersed in coconut oil PCM (nano-PCM-2). The two horizontal and right vertical walls of the enclosure are thermally insulated and the left vertical wall is kept at a constant temperature. The governing continuity equation, the momentum equation in the liquid region, the energy equations in both solid and liquid regions, and the heatfunction equation are solved numerically using the finite element method. Results are presented in terms of interface locations with time, temperature fields, velocity vectors, and heatlines for different values of volume fractions of nanoparticles (i.e., phi = 0%, 2%, and 4%) in the nano-PCM, Rayleigh number based on base PCM properties (i.e., Ra = Ra-phi=0% = 10(4), 10(5), 5 x 10(5), and 10(6)), and Stefan number (Ste). The influence of the volume fraction on the Nusselt number (Nu) for different values of Rayleigh number is presented. Results show the improvement in the heat transfer rate and fast advancement of the interface when adding nanoparticles. A scale analysis technique is applied for the first time to characterize different melting regimes (i.e., conduction, transition, convection, and shrinking solid regimes) during the melting processes of nano-PCM. Present results are also compared with results available in the literature and a good agreement is observed. Finally, experimental visualizations are carried out to compare melting processes of PCM and nano-PCM and to validate the scale analysis results. (C) 2017 Elsevier Ltd. All rights reserved.