Cooling of porous metal surfaces by droplet impact

An experimental study was conducted on the impact of droplets of pure water and n-heptane on impervious and porous (5 pm and 100 pm pore size) stainless steel surfaces. Surface porosity, roughness and thermal conductivity were measured. Droplet diameter (2.5 mm) and impact velocity (0.9 m/s) were kept constant while surface temperature was varied from room temperature to 300 degrees C. Droplet impact was observed using a high-speed video camera. Droplet evaporation times were measured by recording the change in weight of the surface on which the droplet was deposited and surface temperature variation during droplet impact measured using a fast response surface thermocouple. Droplets spread out on porous surfaces during impact to form a thin film that was then drawn into pores by capillary forces. The rate of droplet spread decreased with greater surface roughness. The Leidenfrost temperature of n-heptane droplets increased with surface porosity while water droplets did not go into film boiling on the porous surfaces. A one-dimensional heat conduction model was used to estimate heat transfer coefficients between the impacting droplets and substrates. Heat transfer coefficients increased with surface temperature until sufficient vapor was generated at the liquid-solid interface to inhibit contact. Heat transfer coefficients then decreased with further increases in temperature. (C) 2020 Elsevier Ltd. All rights reserved.