4,847 research outputs found
Diffuse interface modeling of a radial vapor bubble collapse
Full text linkA diffuse interface model is exploited to study in details the dynamics of a cavitation vapor bubble, by including phase change, transition to supercritical conditions, shock wave propagation and thermal conduction. The numerical experiments show that the actual dynamic is a sequence of collapses and rebounds demonstrating the importance of nonequilibrium phase changes. In particular the transition to supercritical conditions avoids the full condensation and leads to shockwave emission after the collapse and to successive bubble rebound
Fluctuating hydrodynamics as a tool to investigate nucleation of cavitation bubbles
Full text linkVapor bubbles can be formed in liquids by increasing the temperature over the boiling threshold (evaporation) or by reducing the pressure below its vapor pressure threshold (cavitation). The liquid can be held in these tensile conditions (metastable states) for a long time without any bubble formation. The bubble nucleation is indeed an activated process, in fact a given amount of energy is needed to bring the liquid from that local stable condition into a more stable one, where a vapor bubble is formed. Crucial question in this field is how to correctly estimate the bubble nucleation rate, i.e. the amount of vapor bubbles formed in a given time and in a given volume of liquid, in different thermodynamic conditions. Several theoretical models have been proposed, ranging from classical nucleation theory, to density functional theory. These theories can give good estimate of the energy barriers but lack of a precise estimate of the nucleation rate, especially in complex systems. Molecular dynamics simulations can give more precise results, but the computational cost of this technique makes it unfeasible to be applied on systems larger than few tenth of nanometers. In this work the approach of fluctuating hydrodynamics has been embedded into a continuum diffuse interface modeling of the two-phase fluid. The resulting model provides a complete description of both the thermodynamic and fluid dynamic fields enabling the description of vapor-liquid phase change through stochastic fluctuations. The continuum model has been exploited to investigate the bubble nucleation rate in different metastable conditions. Such an approach has a huge impact since it reduces the computational cost and allows to investigate longer time scales and larger spatial scales with respect to more conventional techniques
Unraveling low nucleation temperatures in pool boiling through fluctuating hydrodynamics simulations
Full text linkWhen dealing with numerical simulations of boiling phenomena, the spontaneous appearance of vapor bubbles is one of the most critical feature to be addressed. Capturing bubble formation during the dynamics, instead of patching vapor regions as initial conditions, is crucial for the correct evaluation of nucleation rates and nucleation site density, two of the most important parameters characterizing boiling. In this work the Diffuse Interface modeling for vapor–liquid systems is coupled with Fluctuating Hydrodynamics Theory to properly address this aspect and to analyze the detailed nucleation mechanism during boiling inception on a hot surface. The simulations revealed a new enhancing mechanism of bubble formation that is able to explain the low onset temperature measured in boiling experiments on ultra-smooth, wettable surfaces: the interaction and coalescence between sub-critical vapor embryos plays a fundamental role in lowering the onset temperature, increasing the lifetime of the embryos and their probability to trigger the phase change.</p
Thermally activated vapor bubble nucleation: The Landau-Lifshitz--Van der Waals approach
Full text linkVapor bubbles are formed in liquids by two mechanisms: evaporation (temperature above the boiling threshold) and cavitation (pressure below the vapor pressure). The liquid resists in these metastable (overheating and tensile, respectively) states for a long time since bubble nucleation is an activated process that needs to surmount the free energy barrier separating the liquid and the vapor states. The bubble nucleation rate is difficult to assess and, typically, only for extremely small systems treated at an atomistic level of detail. In this work a powerful approach, based on a continuum diffuse interface modeling of the two-phase fluid embedded with thermal fluctuations (fluctuating hydrodynamics), is exploited to study the nucleation process in homogeneous conditions, evaluating the bubble nucleation rates and following the long-term dynamics of the metastable system, up to the bubble coalescence and expansion stages. In comparison with more classical approaches, this methodology allows us on the one hand to deal with much larger systems observed for a much longer time than possible with even the most advanced atomistic models. On the other, it extends continuum formulations to thermally activated processes, impossible to deal with in a purely determinist setting
Dynamics of a vapor nanobubble collapsing near a solid boundary
Full text linkIn the present paper a diffuse interface approach is used to address the collapse of a sub-micron vapor bubble near solid boundaries. This formulation enables an unprecedented description of interfacial flows that naturally takes into account topology modification and phase changes (both vapor/liquid and vapor/supercritical fluid transformations). Results from numerical simulations are exploited to discuss the complex sequence of events associated with the bubble collapse near a wall, encompassing shock-wave emissions in the liquid and reflections from the wall, their successive interaction with the expanding bubble, the ensuing asymmetry of the bubble and the eventual jetting phase
Shock-induced collapse of a vapor nanobubble near solid boundaries
Full text linkThe
collapse
of
a
nano-bubble
near
a
solid
wall
is
addressed
here
exploiting
a
phase
field
model
recently
used
to
describe
the
process
in
free
space.
Bubble
collapse
is
triggered
by
a
normal
shock
wave
in
the
liquid.
The
dynamics
is
explored
for
different
bubble
wall
normal
distances
and
triggering
shock
inten-
sities.
Overall
the
dynamics
is
characterized
by
a
sequence
of
collapses
and
rebounds
of
the
pure
vapor
bubble
accompanied
by
the
emission
of
shock
waves
in
the
liquid.
The
shocks
are
reflected
by
the
wall
to
impinge
back
on
the
re-expanding
bubble.
The
presence
of
the
wall
and
the
impinging
shock
wave
break
the
symmetry
of
the
system,
leading,
for
su
ffi
ciently
strong
intensity
of
the
incoming
shock
wave,
to
the
poration
of
the
bubble
and
the
formation
of
an
annular
structure
and
a
liquid
jet.
Intense
peaks
of
pres-
sure
and
temperatures
are
found
also
at
the
wall,
confirming
that
the
strong
localized
loading
combined
with
the
jet
impinging
the
wall
is
a
potential
source
of
substrate
damage
induced
by
the
cavitation
Shock wave formation in the collapse of a vapor nano-bubble
Full text linkIn this paper a diffuse-interface model featuring phase change, transition to supercritical conditions, thermal conduction, compressibility effects and shock wave propagation is exploited to deal with the dynamics of a cavitation bubble. At variance with previous descriptions, the model is uniformly valid for all phases (liquid, vapor and supercritical) and phase transitions involved, allowing to describe the non-equilibrium processes ongoing during the collapse. As consequence of this unitary description, rather unexpectedly for pure vapor bubbles, the numerical experiments show that the collapse is accompanied by the emission of a strong shock wave in the liquid and by the oscillation of the bubble that periodically disappears and reappears, due to transition to super/sub critical conditions. The mechanism of shock wave formation is strongly related to the transition of the vapor to supercritical state, with a progressive steepening of the compression wave to form the shock which is eventually reflected as an outward propagating wave in the liquid
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