Experimental Characterization of Liquid Film Behavior during Droplets-Polyethylene Particles Collision Behavior

Xiang Ren, Jingyuan Sun, Jingdai Wang, Yongrong Yang

State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China

The collision of droplets with solid particles is a common phenomenon in nature and industrial applications. For example, in the ethylene polymerization fluidized bed process under the condensed-mode operation, a large amount of condensate is sprayed into the fluidized bed, and the atomized droplets collide with high-temperature polyethylene particles, forming liquid films to wrap the particles or traversing the particles from the outside. The droplet-particle interaction significantly affects the heat and mass transfer characteristics, and even provides a basis for the formation of the liquid bridge and particle agglomeration. Therefore, it is essential to understand the complex hydrodynamic and thermal behaviors and the underlying mechanisms of droplet-polyethylene particle collisions.

In our study, the collision behaviors of water and n-hexane droplets with polyethylene particles at different temperatures have been studied. By controlling the falling velocity of the droplets to obtain different Weber numbers, high-speed camera and image analysis methods were used to investigate the liquid film thickness of the north pole point of the particle, the central angle of the liquid film extending farthest point, the dynamic contact angle, and the arc length. Characterize liquid film behavior.

At low Weber numbers, the water droplets collide with the polyethylene particles and stay dynamically on the particle surfaces. Under the atmospheric temperature, the underdamped oscillation of kinetic energy is gradually attenuated. With the increase of particle temperature, the underdamped oscillation amplitude is first attenuated and then enhanced. The horizontal axis of the underdamped oscillation is linearly correlated with temperature. In the case of high Weber number, the droplets of water collide to form liquid film-coated particles, and the liquid film drops slightly after rebounding to the particles. The film thickness of the regurgitation is correlated with time.

At high Weber numbers, the n-hexane droplets exhibit the collision behaviors as the water droplets at low Weber numbers, and the liquid film-coated particles are formed after the collision. Unlike water, no regurgitation phenomenon appears for the n-hexane liquid films at the at the atmospheric temperature.

Key words: droplets-particles collision, liquid film, high-speed camera , fluidized bed reactor