Experimental investigation on mixed convection in a horizontal parallel-plate channel heated from below

In this paper, mixed convection in a horizontal channel, with the lower wall heated at uniform heat flux is studied experimentally. The upper wall is unheated and allows a heat transfer with the external ambient. The experiments are performed with air as working fluid. The investigated Reynolds numbers (Re) are between 5.0 and 1000 and the Rayleigh (Ra) number ranges from 4.60 x 105 to 1.85 x 106. The effect of the secondary motion along the heated wall of the channel is detected by measuring the lower wall temperatures along the longitudinal axis for Re >= 5.0 and by flow visualization for Re <= 100. Results show that the separation from the lower heated wall strongly depends on the buoyancy force and forced velocity. In fact, at lower Reynolds numbers the higher the wall heat flux, the smaller the axial coordinate where the flow separation takes place. Different flow structures due to secondary motions are observed. Correlations for average Nusselt number and for separation point as a function of Richardson and Grashof numbers are proposed in the ranges 0.65 <= Ri <= 1.04 x 10^5 and 4.60 x 10^5 <= Ra <= 1.85 x 10^6 (5.0 <= Re <= 1000).