Rayleigh-Bénard convection (RB) in porous media has received a lot of attention recently because of its applications in the representation of convective geothermal systems and its interesting nonlinear properties, such as the development of quasi-columnar flow structures at high Rayleigh number when the ambient fluid has a constant dynamic viscosity. Although the theoretical advances, the experimental research has been limited by the image analysis technique, which can be substantially improved by using a variational formalism. In this work, we present the design and results of a two-dimensional experiment using the Background Oriented Schlieren (BOS) technique to obtain the temperature field when RBD convection is fully developed inside a Hele-Shaw cell, heated from below and above. Sophisticated imaging techniques were implemented to obtain visual results of the development of thermal convective plumes at laboratory scale, improving the detection of thermal structures. The observation of two regions where thermal fingers has different characteristic length scales shows the importance of viscosity in analogous experiments, which needs more studies using complementary techniques such as velocimetry methods to characterize the viscous transition zone observed in our experiments.

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