Three-dimensional numerical simulations are carried out for the study of freeudconvection in a layered porous enclosure heated from below and cooled from theudtop. The system is defined as a cubic porous enclosure comprising three layers,udof which the external ones share constant physical properties and the internaludlayer is allowed to vary in both permeability and thermal conductivity. Theudmodel is based on Darcy's law and the Boussinesq approximation. A parametricudstudy to evaluate the sensitivity of the Nusselt number to a decrease in theudpermeability of the internal layer shows that strong permeability contrasts areudrequired to observe an appreciable drop in the Nusselt number. If additionallyudthe thickness of the internal layer is increased, a further decrease in the Nusseltudnumber is observed as long as the convective modes remain the same, if theudconvective modes change the Nusselt number may increase. Decreasing theudthermal conductivity of the middle layer causes first an increment in the Nusseltudnumber and then a drop. On the other hand, the Nusselt number decreases inudan approximately linear trend when the thermal conductivity of the layer isudincreased.
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