Interaction between Double Diffusive Convection and Solidification in Ammonium-Chloride Solutions

This paper presents an experimental investigation of thermally and chemically driven convection with simultaneous crystallisation in a concentrated aqueous ammonium-chloride solution. Measurements were performed in a transparent Hele-Shaw cell (200·100·10mm3) between two massive copper blocks equipped with internal water channels controlling the thermal boundary conditions at the upper and lower horizontal boundaries. The temperatures were regulated by thermostats in a range between -20°C and +40°C giving Rayleigh numbers up to around 0.2·10 9 . A double-wall construction with climatisation was implemented in order to avoid thermal losses through the side walls. Temperatures were monitored by thermocouples calibrated to an accuracy better than 0.05K.
Various flow regimes were studied by choosing different temperature boundary conditions.
The focus was on configurations with negative vertical temperature gradients where thermal convection occurs once a critical temperature difference is exceeded. At sufficient supercooling free crystals nucleate in the upper part of the cell, grow and descend due to their higher density compared to the ambient fluid.
The flow field in the liquid was measured by PIV using fluorescent tracer-particles and laser illumination. PTV with LED-background lighting was used to determine the size-evolution and the trajectories of the free-moving crystals. The application of alternating lighting methods and advanced digital image filtering allows for simultaneous operation of PIV and PTV. This approach enables a quantitative study of the interplay of different convection regimes and the solidification process. For example the relation between drag-coefficient, crystal size and crystal growth was investigated.
As a reference case a stable stratification resulting from parallel cooling of the top and bottom walls was investigated. The following solidification phenomena were observed during the experiments: columnar growth at the walls, nucleation and growth of equiaxed crystals in the bulk, chimney-formation in the mushy layer as well as the remelting of columnar and equiaxed dendrites.