Time periodic natural convection heat transfer in a nano-encapsulated phase-change suspension

Abstract

The natural convection of a Nano Encapsulated Phase Change Materials (NEPCMs) suspension in a cavity with a hot wall having a time-periodic temperature is investigated. The top and bottom walls of the enclosure are insulated, the right side wall is kept at a constant temperature of Tc while the left side wall is considered as hot wall and is subjected to a time-periodic temperature. The NEPCM consist of capsules with phase change material PCM in their core. The phase change core of the capsules is covered by a shell. During the fusion or the solidification of the core, the phase change will absorb or release heat in the surrounding, when the temperature is close to the PCM fusion temperature. The partial differential equations governing the flow and heat transfer in the enclosure are formulated in the dimensionless form, and affective key dimensionless numbers and parameters are introduced. The finite element method is used to solve the governing equations. The accuracy of the results is verified by comparison to the benchmark solutions available in the literature. The average Nusselt number in the enclosure, as an indicator of the heat transfer performance, is analyzed. It is shown that the average Nusselt number in the enclosure follows a periodic variation with the same frequency of the temperature of the hot wall and with an amplitude that varies correspondingly with the temperature amplitude. The heat transfer in the cavity is enhanced when a higher fraction ϕ of the NEPCM is used, and a fraction of 5% provides the highest heat transfer. Increasing the volume fraction of nanoparticles from 2.5% to 5% enhanced the average Nusselt number by 21% and the maximum value of Nusselt number by 18.5%. The fusion temperature of nanocapsules is an important parameter affecting the thermal performance of the enclosure, mainly, when the fusion temperature is notably different from cold or hot-wall temperatures.