An experimental study of the airside flow structure during natural convection

We report on an experimental study conducted to investigate the airside flow structure above an evaporative water surface during natural convection. Two-dimensional airside velocity fields were measured using particle image velocimetry for three different surface heat flux conditions. Detailed analysis of the turbulent velocity fields shows a complex flow structure due the local interactions of fluid motions in vertical, horizontal, and normal directions. The trends of turbulent intensity profiles on airside and waterside are found to be similar. However, the airside turbulent intensities are approximately 20 times stronger than that on the waterside. The spectral analysis of the turbulent velocities showed the existence of two distinct power law regimes. In low wavenumber range, the buoyancy subrange is observed with a slope of $−3$ whereas, in high wavenumber range, the inertial subrange with the classical slope of $−5/3$ is observed. The results also indicate that the airside turbulent velocity fields control the local evaporation rate, which in turn influences the water surface temperature field and the waterside velocity field.