Flow instability caused by temperature boundary conditions in a model of Czochralski crystal growth

A weakly turbulent buoyancy-driven flow is studied experimentally and numerically in a liquid metal cylinder with small height-to-diameter ratio. The cell represents a model of the Czochralski crystal growth process and includes heating at the bottom and non-uniform cooling at the top. Transition to a non-axisymmetric roll-like flow structure (wind) is observed experimentally under conditions, which still produce a nearly axisymmetric flow in the computations. It is shown that the contradiction can be explained by the flow sensitivity to temperature boundary conditions at the cell bottom. It is made of a massive copper disk to approximate isothermal temperature conditions. Due to a finite heat conductivity, the bottom is unable to sufficiently equalize the temperature over large distances. This deviation from isothermal conditions is amplified in a separate experiment by a thin heat barrier at the cell's bottom. In this case, the transition to wind is observed at a nearly ten times lower heat flux both in numerics and experiment. The transition then produces a very long-period flow oscillation as the wind erratically changes flow direction.