Numerical investigation on dynamic characteristics of the droplet impacting on a random rough surface

Water management is a significant problem affecting the reliability and stability of proton exchange membrane fuel cells (PEMFC), in which droplet wall collision is the critical factor affecting the liquid water transmission in the gas channel. The surface roughness directly affects the spreading deformation of the droplet after impacting. This study's influence of surface roughness on the dynamic process of a single droplet of micrometer level impacting random rough surfaces was numerically investigated. Based on the Weierstrass-Manderlbrot (W-M) fractal theory, three-dimensional random rough surfaces with five roughness levels were established, and the dynamic process of droplet impact on the random rough surface was captured. The contour features of the droplet were analyzed, and the results showed that rough peaks hinder the spreading and retracted process of droplets, resulting in a thicker liquid film deposited on the rough surface, which is not conducive to liquid water discharge. The higher roughness level makes it difficult for droplets to gather into liquid film after hitting the wall and further hinders the discharge of liquid water from the flow channel. Therefore, when micrometer-sized droplets collide with a wall velocity of about 10 m/s, the roughness Ra should be controlled below 0.4 μm, which is conducive to the formation of thin liquid film after droplet impact and the effective discharge of water, which is crucial to maintaining the performance and life of PEMFC.