In this paper, we investigate the effects of laminar double-diffusive natural convection on directional solidification of binary fluids when cooled and solidified from the top. The study is performed using aqueous ammonium chloride solution as the model fluid. The experiments are performed with an initial concentration of ammonium chloride is less than the eutectic composition, leading to an aiding double-diffusive convection. In this case, solidification leads to the formation of a diffused matrix of dendritic crystals (mushy region) separating the pure solid and liquid regions. Below the two-phase mushy region, Rayleigh-Benard type of cellular convective motions are observed. The cellular motions, which are caused by thermal buoyancy, die once the thickness of the liquid layer falls below a critical value. The features of convective transport are visualised using a sheet of laser light scattered through neutrally buoyant glass particles seeded in the solution. Numerical simulations are also performed, and the agreement with experimental results is found to be good.

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