THE NATURE AND INFLUENCE OF CONVECTION ON THE DIRECTIONAL DENDRITIC SOLIDIFICATION OF A METAL ALLOY ANALOG, NH4CL, AND H2O

Using optical techniques, the onset and development of convection was characterized in the liquid ahead of a (transient) solidifying dendritic interface during Bridgman growth of NH4Cl-H2O. Mushy zone growth rates were measured and correlated to three regimes: quiescent (diffusion dominated growth), cellular flow, and pluming flow. The growth rates were found to increase an order of magnitude as the system transitioned through the three regimes. Critical Rayleigh numbers, which are shown to be mushy zone height dependant, are calculated for the onset of the latter two regimes.