MELTING AROUND A MIGRATING HEAT SOURCE (CONTACT, MOVING, MELT DOWN)

The problem of melting around a migrating heat source which arises in applications such as nuclear technology, geophysics and materials processing was studied. Experiments were performed with horizontal cylindrical heat sources (with either constant surface temperature or constant surface heat flux) which were designed to descend while melting the phase-change material surrounding it. The experiments with n-octadecane provided phenomenological understanding as well as quantitative data on the flow field and melting around the descending source. Important transport mechanisms were identified and the domain and extent of their influence were investigated. Approximate analytical and numerical solution methods were developed based on realistic models inferred from the experiments Conduction was found to be the dominant heat transfer mechanism around the lower stagnation point of the source where the source and the solid are separated by a thin melt layer. Thus, it was concluded that the source velocity is defined in this region. Melt flow in this thin film was essentially induced by the descent of the source. Whereas, in the melt pool above the source, natural convection also played a small role. Two boundary layers formed in the melt pool, one over the source and the other extended along the interface. These boundary layers were separated by a core of nearly stagnant and isothermal melt. The measured and predicted heat source velocities for the constant temperature source were found to depend on the Stefan number as well as the relative density of the source. For constant surface heat flux source, however, both the measured and predicted velocities were almost independent of the relative density of the source. For constant surface heat flux, the measured heat source of velocity was found to exceed the predicted upper limit due to the circumferential conduction of heat inside the source towards the lower stagnation point. Whereas, for constant surface temperature source, the heat losses at the end faces of the source resulted in nonuniform melting along the axis of the source, and thus, caused the measured velocity to be smaller than predicted