Experimental investigation of the natural and forced convection on solidification of Sn-3wt. %Pb alloy using a benchmark experiment

International audienceWe deal with the development of a solidification benchmark experiment in order to investigate the structure formation as well as solute macro-mesosegregation, by means of a well-controlled solidification experiment. The experiment consists in solidifying a rectangular ingot of Sn-3wt.%Pb alloy, by using two lateral heat exchangers which allow extraction of the heat flux from one or two vertical sides of the sample. The domain is a quasi two dimensional parallepipedic ingot (100x60x10) mm. The temperature difference ΔT between the two lateral sides is 40 K and the cooling rate CR= 0.03 K/s. The instrumentation consists in recording the instantaneous temperature maps by means of an array of 50 thermocouples in order to provide the time evolution of the isotherms. After each experiment the patterns of the segregations have been obtained by X-ray radiograph and confirmed by eutectic fraction measurements. The local solute distribution determined by means of induction coupled plasma analysis is provided. The originality of the present study is to examine the effect of the forced convection driven by a travelling magnetic field (TMF) induced by a linear inductor located on the bottom part of the sample. A periodically reversed stirring with a modulation frequency equal to 0.5 Hz stirring have been investigated. This study allows us to evaluate the evolution due to the forced convection induced by a TMF field, as well as its influence on the initial conditions, the solidification macrostructure and the segregation behavior. Measurements of the velocity field by ultrasonic Doppler velocimetry (UDV) method in a Ga-In-Sn pool were performed and transposed to the tin-lead alloy case before solidification. Post-mortem patterns of the macro-mesosegregations have been obtained by X-ray radiography. The results show the transport effects of the flow on both the macrosegregations and the channel formation. The reversal of the TMF produces a decrease of the level of mesosegregations, namely channel formation

Buoyant-thermocapillary instabilities in extended liquid layers subjected to a horizontal temperature gradient

We report experiments on buoyant-thermocapillary instabilities in differentially heated liquid layers. The results are obtained for a fluid of Prandtl number 10 in a rectangular geometry with different aspect ratios. Depending on the height of liquid and on the aspect ratios, the two-dimensional basic flow destabilizes into oblique traveling waves or longitudinal stationary rolls, respectively, for small and large fluid heights. Temperature measurements and space–time recordings reveal the waves to correspond to the hydrothermal waves predicted by the linear stability analysis of Smith and Davis @J. Fluid Mech. 132, 119 ~1983!#. Moreover, the transition between traveling and stationary modes agrees with the work by Mercier and Normand @Phys. Fluids 8, 1433 ~1996!# even if the exact characteristics of longitudinal rolls differ from theoretical predictions. A discussion about the relevant nondimensional parameters is included. In the stability domain of the waves, two types of sources have been evidenced. For larger heights, the source is a line and generally evolves towards one end of the container leaving a single wave whereas for smaller heights, the source looks like a point and emits a circular wave which becomes almost planar farther from the source in both directions

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Numerical study of convection in the horizontal Bridgman configuration under the action of a constant magnetic field .2. Three-dimensional flow

International audienceThe effects of a constant magnetic field on electrically conducting liquid-metal flows in a parallelepiped cavity are investigated using a spectral numerical method involving direct numerical solution of the Navier-Stokes and Ohm equations for three-dimensional hows. Three horizontal Bridgman configurations are studied: buoyancy-driven convection in a confined cavity and in a cavity where the top boundary is a stress-free surface and thirdly, thermocapillary-driven how in a cavity where the upper boundary is subjected to effects of surface tension. The results of varying the Hartmann number (Ha) are described for a cavity with A(x) = L/H = 4 and A(y) W/H = 1, where L is the length, W is the width and H is the height of the cavity. In general, an increase in the strength of the applied magnetic field leads to several fundamental changes in the properties of thermal convection. The convective circulation progressively loses its intensity and when Ha reaches a certain critical value, which is found to depend on the direction (longitudinal or vertical) of the applied magnetic field, decrease of the flow intensity takes on an asymptotic form with important changes in the structure of the flow circulation. The flow structure may be separated into three regions: the core flow, Hartmann layers which develop in the immediate vicinity of the rigid horizontal boundaries or at the endwalls, and parallel layers appearing in the vicinity of the sidewalls. The behaviour of the maxima of velocity and of the overall how circulation is found to depend on both the boundary conditions used and the direction of the applied magnetic field. Furthermore, the interaction of the electric current density with the applied magnetic field which leads to the structural reorganization described above can also create more subtle how modifications, such as flow inversions which are observed mainly in the central region of the cavity