14. Other aspects of Transport Phenomena

This study investigated the flow properties in a Leidenfrost droplet on a heated plate with the water-repellent treatment. Measurements of contact angle and surface roughness with and without water-repellent treatment showed that the contact angle changed from θ = 76° to 120° and surface roughness increased with water-repellent treatment. The evaporation time with temperature change showed a curve with a maximum value at 260℃ without the water-repellent treatment, indicating that 260℃ was the Leidenfrost temperature. On the other hand, the curve with the water-repellent treatment had no maximum value, and the Leidenfrost temperature was determined to be 140 - 160℃ based on the graph at the point where the curve began to form a linear graph. This was considered to be due to the fact that the microscopic irregularities caused by the water-repellent treatment promoted the formation of bubble nuclei and the transition to a film boiling state. Photographs of the bottom of the droplet showed that all particles at the bottom were mobile at 140℃, meaning that the droplet and the heating surface were not in contact with each other. Measurement of the flow velocity using a particle image velocimetry (PIV) method at the droplet surface showed that the velocity tended to increase with increasing temperature. This was attributed to the fact that the speed of vapor film formation increased with increasing temperature. Furthermore, the flow velocity with the water-repellent treatment was faster than that without the water-repellent treatment. This was also considered to be due to the fact that the formation speed of the vapor film increased due to the microscopic unevenness of the surface caused by the water-repellent treatment. This study revealed that the water-repellent treatment of the heating surface contributed to lower Leidenfrost temperatures and higher flow velocities in a droplet.

Prof.

Naoto Ohmura

Corresponding author