Hydrodynamics and boiling phenomena of water droplets impinging on hot solid

The collision of single water droplets with a hot Inconel 625 alloy surface was investigated by a two-directional flash photography technique using two digital still cameras and three flash units. The experiments were conducted under the following conditions: the pre-impact diameters of the droplets ranged from 0.53 to 0.60 mm, the impact velocities ranged from 1.7 m/s to 4.1 m/s, and the solid surface temperatures ranged from 170 °C to 500 °C. When a droplet impacted onto the solid at a temperature of 170 °C, weak boiling was observed at the liquid/solid interface. At temperatures of 200 or 300 °C, numerous vapor bubbles were formed. Numerous secondary droplets then jetted upward from the deforming droplet due to the blowout of the vapor bubbles into the atmosphere. No secondary droplets were observed for a surface temperature of 500 °C at the low-impact Weber numbers (∼30) associated with the impact inertia of the droplets. Experiments using 2.5-mm-diameter droplets were also conducted. The dimensionless collision behaviors of large and small droplets were compared under the same Weber number conditions. At temperatures of less than or equal to 300 °C, the blowout of vapor bubbles occurred at early stages for a large droplet. At a surface temperature of 500 °C, the two dimensionless deformation behaviors of the droplets were very similar to each other.