Experimental and numerical investigation of melting of NePCM inside an annular container under a constant heat flux including the effect of eccentricity Nabeel S. Dhaidan a,1 , J.M. Khodadadi a,⇑ , Tahseen A. Al-Hattab b , Saad M. Al-Mashat c a Department of Mechanical Engineering, Auburn University, 1418 Wiggins Hall, Auburn, AL 36849-5341, USA b Department of Electrochemical Engineering, University of Babylon, Iraq c Department of Mechanical Engineering, University of Baghdad, Iraq article info Article history: Received 20 July 2013 Received in revised form 31 July 2013 Accepted 3 August 2013 Keywords: Annular cavity Eccentricity Melting Nanoparticles Natural convection Phase change materials Thermal conductivity enhancement Thermal energy storage abstract Melting of nano-enhanced phase change materials (NePCM) inside an annular cavity formed between two circular cylinders is investigated experimentally and numerically. The inner cylindrical tube is sub- jected to a constant heat flux, while the outer shell is thermally insulated. The phase change material (PCM) used is n-octadecane that is dispersed with CuO nanoparticles as thermal conductivity enhancer (TCE). The experiments involved recording of temperatures at different radial and angular locations inside the test cell, thus allowing for tracking the progress of the melting front under various rates of heat flux and multiple nanoparticle concentrations. The finite element approach is applied to solve the simul- taneous governing equations computationally. The computational model is validated and the results showed a good agreement with previous related work. Furthermore, the agreement between the exper- imental and simulated results is reasonable. The effects of the nanoparticle concentration and the amount of applied heat flux (Rayleigh number) on the melting process are examined. The characteristics of the melting process are described by temperature of NePCM, progress of the shape of the solid–liquid inter- face, melting rate, melt fraction and charging time. The experimental and numerical results reveal that there is an improvement in melting characteristics with the emulsion of more nanoparticles in PCM (intensifying the effective thermal conductivity) and raising the wall heat flux and the corresponding Rayleigh number (augmenting the role of natural convection). This enhancement in the melting process can be indicated by increasing the melting rate which leads to acceleration of the melting time. In early stages, melting is driven by conduction heat transfer identified by concentric isotherm patterns, high melting rate and lack of fluctuations in temperature data. As time progresses, natural convection will develop and increase the melting rate in the top half of the annulus with high rate of temperature fluc- tuations and thermal instabilities. On the other hand, melting at the bottom region is characterized by diffusion and thermal stability in liquid melt where no temperature oscillations are recorded. The melt fraction, melting rate and rate of enhancement in charging time are improved with the increasing of the Rayleigh number. Additionally, the rate of acceleration in melting is comparably high for low nano- particle concentration and it will degrade as the amount of nano-additives increases as the augmentation in viscosity, agglomeration and precipitation may outweigh the enhancement in thermal conductivity. The simulated findings exhibit that the subcooling has a negative effect on the melt fraction, solid–liquid interface progress and time required to complete melting. This effect is insignificant at higher supplied heat flux or higher values of Rayleigh number. Finally, the impact of eccentricity by lowering the center of the inside heated tube is also evaluated. The predicted results show that the eccentric mode has higher melt fraction in comparison with the concentric arrangement. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Uncertainties associated with utilizing conventional fossil-fuel energy sources such as stable supply, pricing, global warming and greenhouse gas emissions have long motivated researchers and technologists around the world to search for new renewable 0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.08.002 ⇑ Corresponding author. Tel.: +1 (334) 844 3333; fax: +1 (334) 844 3307. E-mail address: khodajm@auburn.edu (J.M. Khodadadi). 1 Karbala University, Iraq, Ph.D.-candidate at Baghdad University, Iraq and Visiting Research Scholar, Auburn University, USA. International Journal of Heat and Mass Transfer 67 (2013) 455–468 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt