Melting and solidification of PCM enhanced by radial conductive fins and nanoparticles in cylindrical annulus A. Ali Rabienataj Darzi a,⇑ , Mahmoud Jourabian b , Mousa Farhadi c a Department of Mechanical Engineering, University of Mazandaran, P.O. Box 416, Babolsar, Iran b SEDITRANS Network, Doctoral School in Earth Science and Fluid Mechanics (ESFM), University of Trieste (UNITS), Trieste, Italy c Faculty of Mechanical Engineering, Babol University of Technology, Iran article info Article history: Received 20 January 2016 Received in revised form 12 March 2016 Accepted 3 April 2016 Available online 6 April 2016 Keywords: Elliptical cylinder Melting Solidification Nanoparticles Radial fins abstract The main disadvantage of phase change materials (PCM) is related to their low thermal conductivities. In this study, the melting and solidification of a PCM within three various horizontal annulus configurations including two circular cylinders, one elliptical cylinder in a circular cylinder and one finned cylinder in a circular cylinder are investigated numerically in terms of the aspect ratio and the orientation of the ellipse and the number of fins. Different volume fractions of the copper nanoparticles are added to the base PCM to examine the effect of nano-particles on the heat transfer rate. Results indicate natural con- vection plays important roles in the melting process where the melting rate at the bottom section of the annulus is lower than that at the top section. By using the vertical-oriented tube instead of the circular one the melting rate increases. Adding nanoparticles to the base PCM enhances the melting and solidifi- cation rate as well. However, it does not eliminate the stable heat transfer at the bottom section of the annulus. Inserting fins leads to the significant enhancement of the melting and solidification rate. It is more efficient during the solidification process due to the suppression of the natural convection effect during the melting process. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction The thermal energy storage (TES) is used for balancing between the supply and the demand of the thermal energy when they are mismatched. Solar collectors integrated with a TES system can be practiced for the utilization of the solar energy during 24 h. The excess heat in the day time can be stored in a TES system and then it is used for night hours. There are several techniques for a ther- mal energy storage system such as the electric thermal storage heaters, chemical storage, sensible heat storage, PCM and so forth. The utilization of a PCM is one of the best techniques of a TES which has been widely studied in last three decades. PCMs absorb and release high amount of the thermal energy at a fixed temper- ature as the latent heat of fusion. Their favorite properties includ- ing chemical stability, heat source at fixed temperature, heat small temperature drop during heat recovery, high storage density allow them to be used in many industrial applications such as cooling of electronic devices [1–4], solar energy [5–7], thermal management in building [8–10] and thermal management of photovoltaic cells [11,12]. The major undesirable property of PCMs is related to their low thermal conductivities. Different heat transfer enhancement techniques have been implemented by researchers to overcome this deficiency. Some of most applicable methods are the encapsu- lation of PCM [13], inserting metal foam [14], adding fins [15,16] and recently, dispersing nanoparticles with high thermal conduc- tivities [17–19]. Rabienataj Darzi et al. [20] conducted a numerical study on the melting of N-eicosane in concentric and eccentric cylinders. They found that the melting rate grows by decreasing the space beneath the hot cylinder where pure heat conduction is dominant. Results indicated that the melting rates for different arrays of cylinders are identical before 15 min when the PCM on the top section of the hot cylinder is melted completely. Rabienataj Darzi et al. [21] conducted experimental and numer- ical studies on the melting of stearic acid as a PCM inside a shell and tube heat storage unit for solar application. Results showed that the effect of the inlet temperature of the heat transfer fluid (HTF) is more influential than that of the mass flow rate of the HTF on the heat transfer augmentation. They reported that the solidification rate increased up to 43.6% by using three longitudinal fins. http://dx.doi.org/10.1016/j.enconman.2016.04.016 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: a.rabienataj@umz.ac.ir, ar.darzi@yahoo.com (A.A. Rabienataj Darzi), mahmood.jourabian@gmail.com (M. Jourabian), mfarhadi@nit.ac.ir (M. Farhadi). Energy Conversion and Management 118 (2016) 253–263 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman