The melting process of a phase change material (PCM) enclosed in a horizontal, isothermal circular tube has been investigated analytically and by experiment for an interesting range of parameters. The physical process was analyzed by numerical methods, whereby the underlying mathematical model involves heat conduction as well as natural convection as the basic heat transport mechanisms. Difficulties associated with the complex and timewise changing melt region whose shape is also part of the solution, have been overcome by applying a numerical mapping technique. Computations and experiments were performed for Rayleigh numbers in the range 105 ≤ Ra ≤ 106. For lower Rayleigh numbers the numerical calculations predict a streamlined design of the PCM at later times, similar to the experiment. At higher Rayleigh numbers, three-dimensional Bernard convection was observed in the bottom region of the melt layer, which was unsteady in their timewise behaviour. The appearance of several roll-cells have also been predicted by the calculations, although the mathematical model was restricted to two-dimensional flow. The experiments were performed with n-octadecane (Pr ≃ 50) as PCM. The test cell basically consists of a short tube filled with the PCM. The tube is closed with plexiglass disks on both ends, thus allowing the melting front to be recorded photographically with time. As a result, the interface positions as well as the overall and local heat transfer coefficients are presented as function of time. The agreement between experimental and numerical data is reasonably good.

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