The authors present a numerical and experimental study on heat and mass transfers by forced convection in a channel with a sinusoidal protuberance and by natural convection in a reservoir full of water. The numerical study has been carried out for Reynolds numbers in a range of 35 to 350, several densities of heat flux and protuberance amplitude range of 0.005 to 0.02 m. Results show that the vapour diffusion in the air modifies the stream function profiles which become convex over the free surface of the water. In addition, the evaporation reduces the perturbation caused by the protuberance and increases the heat transfer rate in the channel. The visualisation of the flow, using smoke, an argon laser and a videocamera, shows the complexity of the interaction between the flow of vapour caused by the evaporation, the flow in the channel and the heat losses across the lateral walls. Theoretical and experimental results are in good qualitative agreement.