Investigations on thermal storage systems containing micron-sized conducting particles dispersed in a phase change material

Abstract

The purpose of the present study is to investi-gate the performance enhancement of a shell and tube, latent heat thermal storage (LHTS) unit due to the addition of microsize copper particles in the phase change material (PCM). The thermo-physical properties of composites were experimentally obtained using Temperature-History tech-nique. Numerical model was developed to explore the heat transfer characteristics of composites during both charging and discharging modes. The numerical model also features a transport equation for particle flux. The commercial computational fluid dynamics (CFD) code, FLUENT, was employed to solve the enthalpy-based two dimensional transient equations. In addition, exergy models were developed to demonstrate the effect of particle dispersion on exergy performance of LHTS systems. The thermal behavior and exergy performance in terms of exergy effi-ciency and total exergy stored of particle-dispersed PCM units are compared with those of pure PCM unit and nanoparticles composites. The heat transfer performance of PCM is significantly high throughout the discharging when microparticles are added. However, the enhancement could be observed only during the earlier stages of charging as particles are found to be hampering natural convection in the liquid PCM. When it comes to exergy performance, particles help in increasing the exergy efficiency of pure PCM system only when the natural convection is weak during charging. Also, the exergy recovered from com-posites is higher as compared to that of pure PCM and it is found that particles addition results in reduced storage capacity in terms of exergy. Nomenclature a Particle diameter (m) A Area (m 2), Coefficient in Eq. (19) b Tube wall thickness (m) Bi Biot number C Mushy zone constant c p Specific heat (J/kg K) e Particle volume fraction Ex Total exergy stored/recovered (J) _ Ex Exergy rate (W) f l Liquid fraction g Gravitational acceleration (m/s 2) h Enthalpy (J/kg) or heat transfer coefficient (W/m 2 K) k Thermal conductivity (W/m K) K c, K l Constants in Eq. (13) L Length (m) m mass (kg) _ m Mass flow rate of HTF (kg/s) P Pressure (Pa) r Radial co-ordinate (m) r i , R Radius of test tube (m) r int Radius of solid/liquid interface (m) R i Tube radius (m) R o Shell radius (m) S Source term Ste Stefan number t Time (s) T Temperature (o C or K) T atm Atmospheric temperature (o C or K) T m Melting temperature (o C or K) u Velocity component in x-direction (m/s) V Velocity vector (m/s) x Axial co-ordinate (m) Greek letters a Thermal diffusivity (m 2 /s) b Coefficient of volume expansion (1/K) e Computational constant in Eq. (20) q Density (kg/m 3) k Latent heat of fusion (J/kg) l Dynamic viscosity (kg/m s) _ c Strain rate (1/s) w Exergy efficiency U Diffusive coefficient