Experimental studies of the transient mixed convention in a vertical pipe. Applications to the heating radiators

The aim of this work is to observe the flow reversal that can appear at the time of interaction of hot and cold boundary layers in transient mixed convection in a vertical pipe.The instationary character of the flow is due to the application at the entry of the tube of a positive or negative temperature step. The transient mixed convection has a significant contribution in many industrial applications such as the air conditioning systems, the heat exchangers, nuclear plants, etc. The behavior of such apparatuses is well controlled in steady state, which is not yet the case in unsteady state. We designed and built an experimental installation which enables us to visualize the transient behavior of the flow in a vertical tube by laser tomography. The principle consists in creating a laser sheet parallel with the generatrix of the cylinder and observing the flow in this plan. The Plexiglas tube used to carry out these experiments has a 54 mm internal diameter, a height of 1200 mm and a thickness of 3 mm. The experiments were made for a Grashof number equal to 6.3*106 and for a Reynolds number of 600. When we apply a positive temperature step to the entry of the tube we note that zones of recirculation appear in the vicinity of the wall. This phenomenon is due to the instability of boundary layer.