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ABSTRACT
In this experimental study, the effect of natural convection during the melting and solidification process on nano-enhanced phase change material in a designed cylindrical thermal energy storage system has been investigated. The temperature variations at both axial and radial locations are measured for stearic acid PCM with variation in mass fractions (0.1%, 0.3%, and 0.5%) of Al2O3 nano-additives. Impacts of Rayleigh number, Fourier number, and Nusselt number, as well as heat transfer coefficient, heat transfer rate, and total energy stored, have been considered. Results revealed that the charging time of stearic acid with 0.3% mass fraction of nanoparticles is 16.6%, 12%, and 15% lower compared to stearic acid, 0.1%, and 0.5% mass fraction-based nano-enhanced phase change materials. Also, the natural convection occurs in the designed thermal energy storage system based on the Rayleigh number, Fourier number, and Nusselt number. Stearic acid based 0.3% of mass fraction nano-additive has an increment of 11.08% in energy storage rate compared to stearic acid PCM-based TES system within a time interval of 10min. It also revealed that an increment in axial height from 2.33 to 4.66cm, the total energy stored would be decreased by 8.6%, 12.2%, 12.5%, and 7.9%, respectively, with 0-5% variation of nano-additive in stearic acid PCM. Thus, nano-enhanced PCM has been considered as better thermal storage compared to pure PCM.
Nomenclature
| g | = | Gravitational acceleration (m/s2) |
| cp | = | Specific heat capacity (J/kg.K) |
| l | = | Height(m) |
| t | = | Time(s) |
| r | = | Radius of copper tube |
| f | = | Liquid fraction |
| Ts | = | Temperature of solid region of PCM(K) |
| Tl | = | Temperature of liquid region of PCM(K) |
| Tm | = | Phase change temperature(K) |
| Tw | = | Inner wall temperature(K) |
| TR | = | Temperature of the PCM at the axis |
| k | = | Thermal conductivity(W/m-K) |
| D | = | Equivalent diameter(m) |
| A | = | Area(m2) |
| Fo | = | Fourier number |
| Ra | = | Rayleigh number |
| Nu | = | Nusselt number |
| = | Instantaneous heat transferred |
Greek symbols
| α | = | Thermal diffusivity(m2/s) |
| ρ | = | Density(kg/m3) |
| β | = | Expansion coefficient(1/K) |
| μ | = | Dynamic viscosity(Pa.s) |
| Ø | = | Mass fraction of nanoparticles |
Abbreviation
| PCM | = | Phase change material |
| NEPCM | = | Nano enhanced phase change material |
| TES | = | Thermal energy storage |
| np | = | Nano-PCM |