Investigation of single-droplet/wall collision heat transfer characteristics using infrared thermometry

• High-speed infrared thermometry allows to obtain time and space-resolved surface temperature distribution.
• Local surface heat flux is obtained using the measured surface temperatures.
• The mechanistic parameters relevant in analyzing collision heat transfer are measured.
• The effectiveness for a single-droplet/wall collision is determined.

This study experimentally investigated the heat transfer characteristics during the collision of a single droplet with a plate heated to temperatures ranging from 176 to 375 °C. The heated plate was made of infrared (IR)-transparent sapphire with a very thin IR-opaque platinum film on top, which allowed the top-surface temperature to be measured from below using an IR camera. The dynamics of the droplets and the corresponding top-surface temperature distribution of the heated plate during collision were acquired using synchronized high-speed video and IR cameras. The three-dimensional transient conduction equation for the heated plate was solved numerically using the measured surface temperatures as boundary conditions, and the time- and space-resolved surface heat flux was obtained. Various physical characteristics associated with the heat transfer during the collision of a single droplet with a heated plate were analyzed, including the local heat flux distribution, effective heat transfer area, instantaneous heat transfer rate, total heat transfer, and vapor film thickness. In addition, the crucial surface temperature at which the total heat transfer from a single-droplet collision is significantly degraded, known as the dynamic Leidenfrost point, was detected.