(488b) An Experimental Study of a Liquid Drop Impinging on a Liquid Pool

An experimental study of a liquid drop
impinging on a liquid pool

Manas Ranjan Behera ^a, Anirvan Dasgupta ^b, Sudipto Chakraborty ^a

^aDepartment of Chemical
Engineering, Indian Institute of Technology Kharagpur,
India.

^bDepartment of Mechanical
Engineering, Indian Institute of Technology Kharagpur,
India.

E-mail:
manaswebmail@gmail.com, anir@mech.iitkgp.ernet.in,
sc@che.iitkgp.ernet.in

The
coalescence of a drop into a liquid pool has been studied in the context of a
vortex ring¹, foam², emulsion², and
microfluidic device³. The research on the drop coalescence is
motivated due to their relevance to various industrial applications; viz.
transport of oxygen across air-water interface⁴, and spray cooling⁵,
as well as its fundamental importance in fluid mechanics. Generally, when a
drop impinges on the surface of the liquid pool having similar properties at a
low velocity, a vortex ring is formed. A vortex ring is a locale
where the fluid mostly turns around an imaginary axis line that forms a closed
loop. The formation of the ring is a commonly
occurring phenomenon which is seen in both natural occurring processes (e.g.
volcano⁶) as well as artificial processes (e.g. blast of atomic bomb⁷, generated at wings of the aeroplane).

The
current work attempts to study the formation of vortex ring for the miscible
liquid system by impinging a liquid drop on a liquid pool.  It is the
simplest and relatively inexpensive method to generate vortex ring. A schematic diagram of the experimental setup for the generation
of vortex ring has been shown in Figure 1. A
rectangle tank [Plexiglas, Volume: 25x25x45 cm³] was used for all
these experiments. Distilled water was filled up to 35 cm of total tank volume
to create the liquid pool. The microsyringe assembly
was attached to a stepper motor system for dispensing drops in a controlled
manner. A fine vertical adjustment system was used for impingement of liquid at
different heights. The impingement height was varied from 10-40 mm. The size of
the drop was maintained 2.96 mm. A high-speed digital camera (manufacturer:
Phantom, Vision Research Inc. (USA), model: V 7.3)
was used to capture the post impact drop dynamics. The video was recorded in
800x600 resolutions at 1500 frames per second with an exposure time of 150 µs.

The
formation and growth of the ring are described and presented in a sequence of
images at the different time interval in Figure 2. When a particle laden drop
impinges on a liquid pool, it creates a crater which is marked as the onset
point for vortex ring generation. The dynamics of crater influences the
characteristics of the ring. The impact of crater dynamics on vortex ring
formation is investigated by measuring the variation of depth and velocity of
the crater with time. The effect of drop impact height on the formation and
characterization of vortex ring is also studied. How the impingement height
affects the eccentricity of the drop, crater dynamics, translational velocity,
and diameter of the vortex ring is investigated. From the observation, it is
concluded that good vortex rings are formed over 10-30 mm height. The effect of
drop eccentricity on the characterization of the ring is also investigated. It
is concluded that the maximum penetrating vortex rings are achieved when the
drop shape is prolate at the time of impact whereas
minimum penetrations are obtained for the oblate shaped drop. From our
observation, it can be concluded that the eccentricity of the drop also affects
the crater dynamics. The shape and motion of the ring are measured and the
variations are depicted with respect to time. The effects of various forces on
penetration and formation of the ring were characterized by different
dimensionless numbers such as Reynolds number, Weber number, Ohnesorge number, and Froude number.

Keywords:
Interfacial Transport Phenomena, Droplet Dynamics, Crater Dynamics, Vortex
Ring, High-speed imaging.

Figure
1.
Experimental setup for generation of vortex ring.

Figure 2.  Formation and growth of vortex ring.

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