Photographic study of melting about an embedded horizontal heating cylinder

INTRODUCTION THE OBJECTIVE of this note is to provide direct visual information to supplement and extend the results of [l] relevant to the role of natural convection in the melting of solids. In [l], experiments were described in which a horizontal heating cylinder was embedded in a solid at its fusion temperature. By energizing the heater with a constant power input, a steadily growing melt region surrounding the cylinder was created. The heating cylinder was instrumented to provide information about the surface heat-transfer coefficient, and a grid of 92 thermocouples was deployed throughout the phase change medium to detect the passage of the solid-liq~d interface. The experiments were performed with a eutectic mixture of sodium hydroxide and sodium nitrate [fusion temperature u 244°C (471”F)l. The experiments demonstrated that natural convection was the dominant mode of heat transfer. One of the evidences of this dominance was the shapes of the melt region. These shapes were inferred from temperature measurem~ts in the phase change medium. Owing to mechanicat constraints, it was not possible to make detailed temperature measurements below and at the sides of the cylinder, so that the shapes presented in [l] were confined to the region above the cylinder. In the present investigation, photographic evidence of actual meh layer shapes was obtained to corroborate those inferred in [l]. The photographs show the melt Iayer both below and above the cylinder, thereby extending the results of [l]. Another extension was made in connection with the initial state of the solid at the onset of melting. Whereas in [I] all of the experiments were concerned with a solid at its fusion temperature, the present experiments encompassed both that condition and an initially subcooled condition (i.e. initial temperature of the solid lower than the fusion temperature). The general approach used in the investigation was to embed a horizontal heating cylinder in a solid phasechange material and to create a growing melt region about the cylinder by a steady input of heat. At a preselected time, the heating was discontinued and the liquid was extracted from the melt region by means of suction and gravity. The unmetted solid was allowed to cool to room temperature and then was removed from the test apparatus and cut along a vertical plane, thereby revealing the cross section of the melt cavity. To obtain a clear photographic record of the shape of the cavity, it was filled with a dark sand. The experiments were performed using the same eutectic salt as in [l J.