Effects of surfaces and nozzles on hydrodynamic characteristics of water film during spray impingement

The hydrodynamic behaviors during the spray impingement onto a solid surface are greatly different from the single or multiple droplets impingement since the spray is polydisperse and the interaction between droplets is indispensable. The present paper investigated the hydrodynamic characteristics of the water film formed on the different solid surfaces including the radial flow and wave, which characterized the spray impingement to some extent. Experimental results showed that a bulge on the solid surfaces under the pressure-swirl nozzle was observed, and the bulge phenomenon became indistinct with the increase of spray heights. The water film was subdivided into the raised zone, annular zone and free flow zone according to the radial film distribution on the surfaces. The maximum Reynolds number of the radial flow was 1099 and lower than the critical Reynolds number (5 × 10⁵) of the flow along a flat plate, indicating that the radial motion of the water film along the solid surfaces belonged to the laminar flow. The effects of water flow rates and spray heights on the radial flow and wavy characteristics of the water film formed on the PMMA surface were relatively more significant than the polished copper surface. Besides, via comparison, it was found that the fluctuation of the water film produced by the air-atomized nozzle was more dramatic than the pressure-swirl nozzle due to the direct droplet impingement. The experimental findings provide some references for further research on hydrodynamic behaviors of spray impingement.