Melting of nano-PCM inside a cylindrical thermal energy storage system: Numerical study with experimental verification

In the current work, the melting process, heat transfer, and energy storage characteristics of a bio-based nano-PCM in a vertical Cylindrical Thermal Energy Storage (C-TES) system are numerically investigated and verified with experimental work. Mathematical models based on non-linear differential equations are developed to study the mass, momentum, and energy transport processes inside the C-TES system. The effects of nanoparticles volume fraction (i.e. ϕ = 0%, 3%, and 5%) and Rayleigh number (i.e. Ranl = 106, 107, and 108) on the melting process are investigated. To compare the numerical results, an experimental setup is developed and transient images are captured to identify the location and shape of solid-liquid interface. To prepare nano-PCM, the copper oxide (CuO) nanoparticles are dispersed into the bio-based coconut oil PCM. The C-TES system is insulated from the bottom, isothermally heated from its lateral walls and the top. Numerically obtained solid-liquid interface locations and melt fractions for base PCM and nano-PCM are compared with experimental analysis and a very good agreement is obtained. The numerical results are further compared with existing numerical and experimental results available in the literature. The work then explains the effects of Rayleigh number and volume fraction of nanoparticles on melt fraction, Nusselt number, and stored energy. The results indicate that adding nanoparticles do not change the patterns of melt fraction, Nusselt number, and energy storage capacity with time compared to the base PCM case. The effects of specific heat capacity of solid nano-PCM, liquid nano-PCM, and latent heat capacity of nano-PCM on energy stored are discussed. The results show that the difference in energy stored with Rayleigh number is less during the beginning of the melting; as melting reaches in the convection dominated regime, a larger difference is observed due to increased melting at larger Rayleigh number.