Remelting and interface dynamics during solidification of a eutectic solution in a top-cooled rectangular cavity: A numerical study M. Dhar, N. Barman ⇑ , S. Mandal, H. Chattopadhyay Department of Mechanical Engineering, Jadavpur University, Kolkata 700032, India article info Article history: Received 21 December 2013 Received in revised form 23 April 2014 Accepted 30 May 2014 Available online 30 June 2014 Keywords: Solidification Eutectic solution Remelting Thermal gradient abstract The present work is a numerical investigation on the occurrence of remelting phenomenon during solidi- fication of a eutectic aqueous ammonium chloride solution in a top-cooled rectangular cavity. The analysis is performed following a fixed-grid single-domain approach. Accordingly, the solidification process is rep- resented by a set of mass, momentum and energy conservation equations, where the enthalpy update scheme is used to trace the solid–liquid interface during solidification. The set of governing differential equations is solved using semi-implicit finite volume method, based on SIMPLER algorithm, applying line-by-line TDMA solver. The present simulation predicts the distribution of temperature and streamline along with fraction of solid in the computational domain, and evolution of the solidified layer during solid- ification. The simulation shows remelting of solidified layer at the solid–liquid interface during later stages of solidification due to irregular superheat of the bulk solution convected towards the solid–liquid interface. Hence, the occurrence of the remelting phenomenon during solidification is studied by evaluating the inter- face growth history as a function of the process parameters, namely, cavity height and thermal gradient imposed across the cavity. It is found that there is a small increase in the solidification growth rate with increase in cavity height, where the possibility of remelting is high. With the increase in imposed thermal gradient, the solidification growth rate increases significantly and the occurrence of remelting disappears. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The solidification process plays a vital role in casting, which determines the mechanical properties of cast products. In the last few decades, an endeavor has been made by many researchers [1–4] to improve the properties, where flow of the melt is enhanced during solidification, similar to the solidification of alloys in semisolid processing. In such a solidification process, non-den- dritic microstructures are found in the cast with enhanced mechanical and metallurgical properties [1]. The remelting phe- nomenon in the mush during solidification is an important issue towards formation of the non-dendritic structures in the cast, which is inadequately reported in the literature. Accordingly, in the present work, a detailed study on the remelting phenomenon in the mush during solidification is undertaken. This remelting occurs due to two reasons, namely, thermal and solutal superheats. Only the effect of thermal superheat on remelting is considered in this work implying solidification of a eutectic composition. In addi- tion, a fundamental study of solidification process demands a transparent binary solution freezing at low temperature. Hence, the solidification of a eutectic aqueous ammonium chloride solu- tion (19.7 wt.% NH 4 Cl + H 2 O) in a rectangular cavity is considered for understanding the remelting phenomenon of the solid–liquid interface. The top-cooled cavity is considered to induce prominent thermal gradient across the melt, which results in Rayleigh–Benard convection in the melt. Accordingly, the literature related to the work is reviewed. Though a large number of studies associated with solidification of binary aqueous solutions in a top-cooled cavity are available in lit- erature, a few studies reported on solidification process of a eutectic solution only. In this context, Turner et al. [5] reported a work regarding solidification of a eutectic solution cooled from the top considering heat balance among conduction heat transfer in the solid, latent heat released at the solid–liquid interface and convec- tive heat transfer in the melt. Several experiments on the solidifica- tion of eutectic systems in a top-cooled cavity are performed by Davis et al. [6]. Kumar et al. [7] conducted visualization studies on Rayleigh–Benard convective cells interacting with solid–liquid interface and evolution of interface growth during solidification of a eutectic aqueous ammonium chloride solution in a top-cooled cavity. These studies overlook the localized remelting phenomenon that occurs during solidification, which is the primary focus of the present work. However, the remelting phenomenon is preliminarily http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.05.064 0017-9310/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +91 9748012233. E-mail address: nilkanta@mech.jdvu.ac.in (N. Barman). International Journal of Heat and Mass Transfer 77 (2014) 730–737 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt