Measurements of the temperature field of mushy and liquid regions during solidification of aqueous ammonium chloride

Abstract

Experiments are conducted to study the solidification from below of aqueous ammonium chloride. Thermochromic liquid crystal paints are used to visualize the temperature field simultaneously in both the liquid and the mushy layers. In a quasi-two-dimensional cell (thickness 10 mm), mushy-layer and boundary-layer convection are revealed as bumps in isotherms within and above the mushy layer, respectively. The onset, growth and decay of these convective modes are measured by monitoring the progression of the bumps during an experiment. The small-wavelength boundary-layer mode is short-lived (approximately 20–30 min), whereas the larger-wavelength mushy-layer mode survives for several hours, dominating the flow even long after the growth has stopped. Experiments in a Hele-Shaw cell (thickness 2.0 mm) enable simultaneous visualization of both the temperature field and the solid fraction. A coarsening mechanism is observed in which the flow spontaneously changes, reducing the strength of plume convection in one of the channels, and leading to growth of dendrites into the channel. An oscillatory convective mode is also observed, perhaps an indication of one of the oscillatory modes recently predicted by Chen, Lu & Yang (1994) and by Anderson & Worster (1995).