Experimental measurement of frozen and partially melted water droplet impact dynamics

High-speed video of single frozen water droplets impacting a surface was acquired. The frozen particles had a diameter ranging from 0.4 mm to 0.9 mm and impacted at velocities varying from 140 m/sec to 309 m/sec. The technique used to freeze the droplets and launch the particles against a surface is described in this paper. High-speed video was used to quantify the ice accretion area to the surface for varying impact angles (30°, 45°, 60°), and impacting velocities. An oxygen /acetylene cross-flow flame used to partially melt the traveling frozen particles is also discussed. A linear relationship between impact angle and ice accretion is identified for fully frozen particles. The slope of the relationship is affected by impact speed. Higher impact angles closer to perpendicularity between the surface and the particle trajectory, e.g. 60°, exhibited small differences in ice accretion with varying velocities. Increasing velocity from 161 m/sec to 259 m/sec nearly doubled the ice accretion area at a shallower impact angle of 30°. The increase accretion area highlights the importance of impact angle and velocity on the accretion process of partially melted ice crystals. It was experimentally observed that partial melting was not a pre-requisite for accretion at the tested velocities when impact angles of 45° and 30° were used. Partially melted droplets using just 0.0023 Joules of energy also doubled the ice accretion area. The partially melted state of the particles and a method to quantify the percentage increase in the ice accretion area is also described in the paper.