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ISSN Online: 2379-1748
ISBN Flash Drive: 978-1-56700-483-0
ISBN Online: 978-1-56700-482-3
4th Thermal and Fluids Engineering Conference
VERTICAL COALESCENCE CHARACTERISTICS OF LIQUID DROPLETS PLACED OVER A HYDROPHOBIC SURFACE
Abstract
Coalescence of liquid droplets is an important and interesting physical phenomenon encountered in several applications. The configuration of two liquid droplets with one initially above the other, while the lower one is placed over a horizontal hydrophobic surface is investigated numerically and from experiments. Three liquids considered are water, glycerin, and Cs-alloy. Coalescence process takes place under atmospheric conditions. The equilibrium contact angle for the three liquids relative to the substrate is taken to be 150°. The bond number based on the volume of the combined drop is ~0.6. Quantities of interest are the interfacial shapes generated in time, velocity and timescales, and wall properties such as footprint radius, shear stress and heat flux. The numerical study is carried out in an axisymmetric coordinate system. Heat transfer simulations are carried out with the drop above at an initially higher temperature than the surface while the lower drop and the surface are both cold. Experiments are performed in water using high-speed imaging. For water, experiments as well as numerical simulation clearly reveal that the lower drop bounces off the surface before the combined drop spreads and attains equilibrium. Two distinct timescales emerge during the coalescence process. The instantaneous surface-averaged wall shear stress and wall heat flux are zero initially, increase rapidly to a maximum and slowly decrease to zero with multiple changes in sign. The instantaneous peak in the wall shear stress is the greatest for glycerin while the peak wall heat flux is the greatest for Cs-alloy.
KEY WORDS:
Vertical coalescence, Droplets, Water, Glycerin, Cs-alloy, Hydrophobic surface, Wall heat flux, Wall shear stress