Energy Storage in an Impure Paraffin Wax Embedded in the Finned-Annulus of Two Horizontal Tubes: Part-Il

Abstract

This Part-Il of the manuscript reports on the unsteady two-dimensional numerical investigations of melting of a paraffin wax (phase change material, PCM) which melts over a temperature range of 8.70C. The PCM is placed inside a circular concentric horizontal-finned annulus for the storage of thermal energy. The inner tube is fitted with three longitudinalfins which are placed strategically near the bottom part of the annulus to accelerate the melting process. The non-dimensional conservation equations for mass, momentum, and energy are solved employing a staggered control-volume based finite difference method using the previously verified in-house CFL) code. Under identical geometrical configuration and operating conditions, the melting characteristics of the finned annulus are compared with the annulus without fins presented in Ref [l]. The numerical results show that the average Nusselt number over the inner tube surface, the total melt fraction, the total stored energy all increased at every time instant in the internally finned annulus compared to the annulus without fins. This is due to the fact that in the finned annulus, the fins at the lower part of the annulus promotes buoyancy-driven convection as opposed to the slow conduction melting that prevails at the bottom part of the plain annulus. Fins with two different heights have been considered. It is found that by extending the height of the fin to 50% of the annular gap about 33.05 0 0 more energy could be stored compared to the bare annulus at the melting time of82.37 min for the identical operating conditions. The effects offins with different heights on the temperature distribution are found to be different. The present study can provide some useful guidelines for achieving a better energy storage system and also can aid in the design/optimization ofthe shell-and-tube LHTES devices.