Secondary atomization of a droplet in diverse interaction settings

includes bibliographical references and index

PhD; 2023; Mechanical Engineering

Secondary atomization refers to the process by which liquid droplets, which are already in
a dispersed state, are further atomized down into smaller droplets. This process occurs after
primary atomization, which is the initial breakup of a bulk liquid into droplets. Understanding
and controlling secondary atomization is crucial for optimizing various industrial and natural
applications. In this study, we investigate the secondary atomization of a single droplet in three
different interaction settings: shock waves, vortices, and porous surfaces (such as facemasks).
In shock-droplet interaction (őrst setting), the multiscale phenomenon is classiőed into
two stages: wave dynamics (stage I) and droplet breakup dynamics (stage II). Stage I involves
the formation of different wave structures after an incident shock impacts the droplet surface.
Stage II involves induced airŕow interaction with the droplet, leading to its deformation and
breakup. Primarily, two modes of droplet breakup, i.e., shear-induced entrainment (SIE) and
RayleighśTaylor piercing (RTP) (based on the modes of surface instabilities) are observed
for the studied range of Weber numbers (We ∼ 30 − 15000). A criterion for the convenient
transition between two breakup modes is also discussed. For measurement of drop sizes during
a shock-drop interaction process, the two-sensor depth from defocus (DFD) technique is further
developed to achieve higher spatial and temporal resolution, to improve estimates of spatial size
distribution and number concentration, and to provide additional guidelines for the calibration
and design of the optical system for a speciőc application. Furthermore, an investigation of the
secondary atomization of liquid metal droplets has also been conducted using Galinstan as a
test ŕuid. The study explores crucial questions, such as the applicability of atomization results
obtained from conventional ŕuids like DI water to liquid metal atomization. The study sheds
light on how surface oxidation of liquid metal plays a signiőcant role in regulating atomization dynamics and the shape of fragmented droplets.
In vortex-droplet interaction (second setting), we elucidate the mechanism of co-axial
interaction of a droplet with a vortex ring of different circulation strengths(Γ = 45−161cm2
s
−1
).
We focus on both the droplet and the vortex dynamics, which evolve spatially and temporally
during different stages of the interaction, as in a two-way coupled system. In the droplet
dynamics, different regimes of interaction are identiőed, including deformation (regime-I),
stretching and engulfment (regime-II), and droplet breakup (regime-III). In vortex dynamics, we
compare the interaction’s effect on different characteristics of the vortex rings. Vortex-droplet
interaction leads to a reduction in these parameters.
In droplet porous-surface (facemask) interaction (third setting), we show that high-momentum,
large-sized (>250 µm) surrogate cough droplets can penetrate single- or double-layer mask
material to a signiőcant extent. The penetrated droplets can atomize into numerous much
smaller (< 100µm) droplets, which could remain airborne for a signiőcant time. The possibility
of secondary atomization of high-momentum cough droplets by hydrodynamic focusing and
extrusion through the microscale pores in the őbrous network of the single/double-layer mask
material must be considered in determining mask efficacy. The results of droplet atomization
are compared in terms of droplet penetration, size distribution, and volume transmission. Theoretical models for the criteria of droplet penetration, breakup time, and droplet size prediction
agree well with the experimental data.
To conclude the discussion, we investigate an interaction test case at a low Weber number
value. In this scenario, we examine a periodic interaction between a vortex ring and a droplet,
where surface tension force dominates over inertial force (low Weber number), and secondary
atomization does not occur. This type of interaction can potentially modify the droplet’s
evaporation and crystallization characteristics. Our őndings reveal that the droplets’ evaporation
characteristics depend on the strength of the vortex, while the crystallization dynamics remain
independent of it.