Natural convection in an air-filled cavity: Experimental results at large Rayleigh numbers

A large-scale experimental setup is built and instrumented. It consists in a 4 m-high cavity with a horizontal cross-section equal to 0.86 × 1.00 m². Two opposite vertical walls are heated and cooled down; other walls (lateral walls, ceiling and floor) are made of insulating medium covered with a thin and low-emissivity film designed to minimize radiative effects into the cavity. The temperatures of active walls are imposed, constant and equally distributed around the ambient temperature in order to reduce heat losses. The temperature difference between the hot and cold walls is chosen to respect the Boussinesq approximation. Under these assumptions, Rayleigh number values up to 1.2 × 10¹¹ (ΔT = 20 °C) can be obtained. The centre-symmetry is verified on the thermal stratification. Influence of the temperature difference and of wall emissivities on the stratification parameter (dimensionless vertical temperature gradient) is discussed. Velocity measurements allow the velocity field to be obtained and provide information on flows encountered in the cavity. Temperature measurements are also carried out in the whole cavity. In the paper, a complete experimental characterization is provided: airflow inside the cavity is analyzed and the Nusselt number along the hot and the cold wall is presented.