Experimental velocity and temperature measurements for natural convection in a highly porous medium

We present natural convection experiments at low-to-moderate porous media Rayleigh number Ra in a fluid-saturated structured plastic foam with porosity of 0.83 heated from below. This is motivated by the lack of detailed experiments on thermal convection in these systems, although they are of great interest for many engineering applications. Our experiments were carried out using Magnetic Resonance Imaging (MRI), which allowed us to obtain quantitative measurements of velocity vectors together with temperature fields in the bulk of the fluid-saturated sample. The results obtained are compared with those for classical porous media (e.g. packed beds of spheres) and with theoretical predictions. For our system, we found the onset of convection at Ra-c = 41.9, with a critical wavenumber a(c) approximate to pi. For Ra < Ra-c, velocities always remain below the accuracy of the measurement and temperature fields remain homogeneous and linear in the vertical direction. For Ra-c <= Ra < 308, the flow has a stable convective patterns in forms of axisymmetric, straight or deformed rolls. At about Ra = 308, the flow enters in a transitional regime in which the convective rolls begin to deform. At the highest Ra tested, the convective pattern takes the form of cross-rolls. The intensity of convection is estimated through the large-scale mean squared velocity U-2. For Ra > 70, we obtain U similar to Ra-1/2. The heat transfer is investigated by evaluating the Nusselt number from temperature fields. We found that the local Nu reflects the flow structure: higher values of local Nu correspond to cold down-welling regions, and smaller Nu to warm up-welling regions. Finally, we found that the averaged Nusselt number for the whole system increases linearly with Ra with a slope equal to 2.