Study of droplet impact on constant surface temperature

Abstract

The phenomenon of liquid droplets impinging on solid surface is encountered in a number of areas related to various types of combustion engines, cooling systems as well as many fire safety situations. By studying the dynamic process of droplet spreading upon impingement on a heated surface, will further allow the author to relate the physical droplet dynamics to its heat transfer characteristic. The author has designed and fabricated a constant copper surface temperature set-up with a chamber to simulate a one dimensional heating surface. A circular heating surface with an area of 490mm² made of copper and powered by a single 300W heater cartridge was designed and built. Solid State Relay is use to enable the heater cartridge to maintain constant temperature through a feedback from the thermocouple. The impact of a water droplet upon a heated copper surface was investigated experimentally using a high-speed digital camera. The droplet impact height was varied from 20mm to 60mm and the collision dynamics were investigated with the temperature of the copper surface varied from 26ºC to 170ºC. For each height, the evolution of the maximum spreading diameter and maximum recoil height was measured as a function of surface temperature. When temperature increases above 100ºC, the liquid film was observed to recoil higher and the maximum droplet spreading starts to become unstable. At certain point, the Leidenfrost effect was observed.