Two-dimension experimental study of the reverse flow in a free convection channel with active walls differentially heated

An experimental apparatus dedicated to the study of the reverse flow in a vertical channel with active walls differentially heated is presented. This study explores the influence of two parameters over this air flow, namely the Rayleigh number (6 × 109 up to 4.5 × 1010) and the aspect ratio (10 up to 12). The axial velocity u and fluctuations urms are obtained from laser doppler velocimetry at different positions along the channel axis.As results, conditions of existence of the reverse flow are precised and partially qualified. Three classes of downflows have been confirmed according to an analysis of the velocity histogram: absence, intermittence and permanence. The maximum velocity has been found to vary linearly with downstream position in the channel. An original analysis locates the center of the vortex-like structure and provides the width and the maximum tangential velocity, which are evaluated against the global parameters of the experiment.