Effects of surface tension and wood surface roughness on impact splash of a pure and multi-component water drop

Concerning the deeper understanding of the mechanisms on fire suppression with multi-component water mist/spray, the dynamical process of a water drop with or without additives impacting upon wood surfaces is preliminarily studied. The initial diameters of the pure water drop and the water drop with NaCl additive are about 2.4±0.1 mm, and the diameter of the water drop with AFFF (Aqueous Film-Forming Foam) additive is about 1.8±0.1 mm. The drop impact velocities are varied from 1.13 m/s to 2.80 m/s. A Photorn FASTCAM high-speed video camera coupled with a Nikon 200 mm micro-lens is used to record the dynamical process of the drop impacting. The results show that the critical impact Weber number of the water drop with additives is obviously larger than that without additives, and the critical impact Weber number increases with decrease of the wood surface roughness. In addition, the current empirical models both on predicting the critical Weber number and the maximum spread factor just partially agree with the experimental results. The current results are limited to the interaction of a single water drop impacting upon a horizontal wood surface.