Natural Convection in an Open Vertical Channel with Heated Walls at high Rayleigh Number
Résumé
Photovoltaic double-skin fac¸ades can be modelled by considering natural convection in an open-ended vertical channel heated under isoflux conditions. Such a system is particularly challenging for numerical simulations, where turbulence may be observed for high Rayleigh numbers (Ra* > 10^5, Ra* being the modified Rayleigh number based on channel width, aspect ratio, and wall heat flux). We present an experimental investigation of the flow along a symmetrically-heated vertical channel with a modified Rayleigh of Ra* = 6.7 × 107, a configuration for which few studies have been carried out to date. These results thus represent important reference data for future numerical studies. The experimental apparatus is described and characterized in order to quantify the measurement uncertainties. The flow behaviour is studied through direct measurements of velocities and temperatures in the channel. In the bottom part of the channel, heat is located inside the thermal boundary whereas a shear layer develops outside this layer. A zone is defined in which the shear is responsible for the increase in velocity fluctuations. At the two third of the channel (from the bottom), fluctuations are strong enough to create an efficient turbulent heat transport from the boundary layer to the channel centre. It results in an increase in the temperature at the centre of the channel and a decrease in the wall temperature. A transition from laminar to turbulent heat transfer is found to be driven by the shear layer.