231 research outputs found
Shear instability of an axisymmetric air-water coaxial jet
We study the destabilization of a round liquid jet by a fast annular gas
stream. We measure the frequency of the shear instability waves for several
geometries and air/water velocities. We then carry out a linear stability
analysis, and show that there are three competing mechanisms for the
destabilization: a convective instability, an absolute instability driven by
surface tension, and an absolute instability driven by confinement. We compare
the predictions of this analysis with experimental results, and propose scaling
laws for wave frequency in each regime. We finally introduce criteria to
predict the boundaries between these three regimes
Preferential concentration of inertial sub-kolmogorov particles. The roles of mass loading of particles, Stokes and Reynolds numbers
Turbulent flows laden with inertial particles present multiple open questions
and are a subject of great interest in current research. Due to their higher
density compared to the carrier fluid, inertial particles tend to form high
concentration regions, i.e. clusters, and low concentration regions, i.e.
voids, due to the interaction with the turbulence. In this work, we present an
experimental investigation of the clustering phenomenon of heavy sub-Kolmogorov
particles in homogeneous isotropic turbulent flows. Three control parameters
have been varied over significant ranges: ,
and volume fraction . The scaling of clustering characteristics, such as the distribution
of Vorono\"i areas and the dimensions of cluster and void regions, with the
three parameters are discussed. In particular, for the polydispersed size
distributions considered here, clustering is found to be enhanced strongly
(quasi-linearly) by and noticeably (with a square-root
dependency) with , while the cluster and void sizes, scaled with the
Kolmogorov lengthscale , are driven primarily by . Cluster
length scales up to , measured
at the highest , while void length
scaled also with is typically two times larger ().
The lack of sensitivity of the above characteristics to the Stokes number lends
support to the "sweep-stick" particle accumulation scenario. The non-negligible
influence of the volume fraction, however, is not considered by that model and
can be connected with collective effects
Buoyancy driven bubbly flows: scaling of velocities in bubble columns operated in the heterogeneous regime
The hydrodynamics of bubble columns in the heterogeneous regime is revisited.
Focusing on air-water systems at large aspect ratio, we show from dimensional
analysis that buoyancy equilibrates inertia, and that velocities scale as
, where is the bubble column diameter,
the void fraction and the gravitational acceleration.
From new experiments in a m diameter column with
particle Reynolds number bubbles and from a detailed analysis of published
data, we confirm the self-organization prevailing in the heterogeneous regime,
and that the liquid flow rate is only set by the column diameter . Besides,
direct liquid and gas velocity measurements demonstrate that the relative
velocity increases above the terminal velocity in the heterogeneous
regime, and that it tends to at very large gas superficial
velocities . The proposed velocity scaling is shown to hold for liquid
and gas mean velocities and for their standard deviations. Furthermore, it is
found to be valid over a wide range of conditions, corresponding to Froude
numbers from 0.02 to 0.5. Then, the relevance of this
scaling for coalescing media is discussed. Moreover, following the successful
prediction of the void fraction with a Zuber \& Findlay approach at the
beginning of the heterogeneous regime, we show how the void fraction is
correlated with . Further investigations are finally suggested to connect
the increase in relative velocity with meso-scale structures known to exist in
the heterogeneous regime
Do finite size neutrally buoyant particles cluster?
We investigate the preferential concentration of particles which are
neutrally buoyant but with a diameter significantly larger than the dissipation
scale of the carrier flow. Such particles are known not to behave as flow
tracers (Qureshi et al., Phys. Re. Lett. 2007) but whether they do cluster or
not remains an open question. For this purpose, we take advantage of a new
turbulence generating apparatus, the Lagrangian Exploration Module which
produces homogeneous and isotropic turbulence in a closed water flow. The flow
is seeded with neutrally buoyant particles with diameter 700\mum, corresponding
to 4.4 to 17 times the turbulent dissipation scale when the rotation frequency
of the impellers driving the flow goes from 2 Hz to 12 Hz, and spanning a range
of Stokes numbers from 1.6 to 24.2. The spatial structuration of these
inclusions is then investigated by a Voronoi tesselation analysis, as recently
proposed by Monchaux et al. (Phys. Fluids 2010), from images of particle
concentration field taken in a laser sheet at the center of the flow. No matter
the rotating frequency and subsequently the Reynolds and Stokes numbers, the
particles are found not to cluster. The Stokes number by itself is therefore
shown to be an insufficient indicator of the clustering trend in particles
laden flows
Stability of a swirled liquid film entrained by a fast gas stream
International audienceWe study the liquid flow inside a recessed gas-centered swirl coaxial injector, where a swirled liquid flowing against an outer wall is destabilized by a central fast gas stream. We present measurements of the liquid intact length inside the injector, as a function of swirl number and dynamic pressure ratio. We propose a simple model to account for the effect of these parameters.We next study the surface instability inside the injector: its frequency ismeasured for several swirl angles, and as a function of gas velocity. Results are first confronted to the predictions of an inviscid linear stability analysis including swirl, and second to the predictions of a viscous linear stability analysis where swirl is not included. The viscous analysis captures the experimental frequency
Flapping instability of a liquid jet
International audienceIn air assisted atomization, small droplets arise from the stripping of a liquid jet (or a film) by a fast gas stream (Lasheras & Hopfinger 2000, Eggers & Villermaux 2008). Yet, the incoming liquid jet is seemingly never fully atomized by the stripping process alone. Instead, the remaining jet experiences a flapping instability, similar to the instability observed on liquid sheet configurations: the resulting large scale structures break into large liquid lumps some distance downstream the injection. Little is known on the underlying mechanism of this instability and on the characteristics of the large drops it produces, though these large drops probably control flame extent in combustion devices. We suggest in the present study that this instability could be triggered by non-axisymmetric Kelvin-Helmholtz modes. Indeed, in coaxial injector configuration, non-axisymmetric modes of the KH instability can be observed. First, we study the dependence of KH modes upon two control parameters, namely the liquid and gas velocities, and discuss the symmetry of these modes. Secondly, we investigate a possible link between non-symmetric modes of KH instability and the large scale instability. Finally, amplitude of the large scale oscillation is measured as a function of gas and liquid velocity
Monofiber optical probe using Doppler signals detection for Drop Size and Velocity measurement in air assisted atomization
Reliable measurement of droplet/bubble size and velocity distributions in
dense flows is desired in a variety of research fields, both for laboratory and
industrial use. A new type of single-mode monofiber optical probe manufactured
by A2 Photonic Sensors is introduced in this paper: it combines traditional
phase detection with the collection of a Doppler signal returned by an incoming
gas-liquid interface to provide information on residence times, drop
concentration and velocity, which afford then drop chords and liquid flux
measurements. Compared with classical optical probes, that new sensor does not
require any calibration. The purpose of the present work is to test this
technique in assisted atomization in order to provide a mean for spray
characterization and ultimately to improve our understanding of atomization
mechanisms. The probe has been tested downstream of a coaxial air-assisted
atomizer operated at liquid velocity =. to. / , and gas velocity from = to
about /. We first analyzed raw signals in various flow conditions. It happens
that, when increasing the gas velocity and the number density of drops, the
signal experiences very strong fluctuations of the gas level, making the
identification of individual droplets more difficult. That leads us to develop
a new signal processing routine specifically adapted to such complex working
conditions. At = / and for three liquid flow rates, the spatial integration of
local liquid fluxes represents 92 to 99% of the injected liquid flow rate.
These good results demonstrate that the Doppler probe provides reliable
statistics on drops velocity and size
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