Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: P3.29 - Analysis of time-dependent spray structures of spray processes in enclosures
ALL VOLUMES CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00102023/00523
 Material Information
Title: P3.29 - Analysis of time-dependent spray structures of spray processes in enclosures Fluid Structure Interactions
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Aljoscha, L.
Udo, F.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: droplet clustering
PIV
image processing
particle-laden gas
spray inhomogeneity
 Notes
Abstract: The effect of spray chamber design and the spray development is scarcely analyzed in the literature. In this paper the influence of different spray chamber designs with square cross sections on unsteady flow structures in the spray of twin-fluid atomizers is discussed. The product properties of a spray process highly depend on the local mass and heat transfer within the spray. These mechnisms are affected by particle tragectory as well as local droplet concentration. Both phenomena are reviewed experimentally by using Particle Image methods in planar light sheets. The effect of coherent structures on the local particle concentration is the central point in this investigation.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
 Record Information
Bibliographic ID: UF00102023
Volume ID: VID00523
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: P329-Lampa-ICMF2010.pdf

Full Text


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010



Analysis of time-dependent spray structures of spray processes in enclosures


Lampa Aljoscha, Fritsching Udo

University of Bremen, Production Technology, Particles and Process Engineering
Badgasteiner Str. 3, Bremen, 28359, Germany
fritsching@iwt. uni-bremen. de


Keywords: droplet clustering, PIV, image processing, particle-laden gas, spray inhomogeneity



Abstract

The effect of spray chamber design and the spray development is scarcely analyzed in the literature. In this paper the
influence of different spray chamber designs with square cross sections on unsteady flow structures in the spray of twin-fluid
atomizers is discussed. The product properties of a spray process highly depend on the local mass and heat transfer within the
spray. These mechnisms are affected by particle tragectory as well as local droplet concentration. Both phenomena are
reviewed experimentally by using Particle Image methods in planar light sheets. The effect of coherent structures on the local
particle concentration is the central point in this investigation.


Introduction

Unsteady phenomena in sprays greatly affect the outcome
of the spray process. In this paper the large-scale
movement of the jet and the ambient air as well as local
clustering of droplets in a spray is analysed. In enclosed
spray processes flapping accompanied by precession of the
spray can occur. The strength and size of entrainment and
recirculation zones influences the retention time of
particles and are analysed. Additionally the meso-scale
clustering of droplets within the spray cone is reviewed
quantitatively. The interactions of particles with large-scale
eddy structures within the gas are therefore discussed. A
Particle Image method was applied to planar light sheets in
the spray to examine the correlation between particle
concentration and the flow characteristic. It can be
described as a combination of
2D-Particle-Image-Velocimetry measurements and image
analysis techniques to investigate the concentration
patterns of the spray droplets (Figure 1).

The following parameters are changed systematically to
estimate their influence on the time-dependent structures of
the spray:

- conical spray chamber design with square cross sections
- twin-fluid atomizer type
- loading of spray


my& particle velocities


laser light
sheet pictures


I4vorticity, shearflux,
velocity fluctuations


!Tlc~ ':


dsrbtointensity~ T size and position of
particle cluster

Figure 1: Particle Imaging: velocity- and concentration
fields

Instabilities in the gas flow, caused by high shear rates in
the boundary region of the spray for example, have a great
influence on the formation and transport of particle cluster.
There are several hypothesis why clusters in particulate
systems occur (Heinlein 2007, Aliseda 2001). Here only
the big coherent flow structures are of concern. The
method to quantify the Inhomogeneities within the spray is
derived from Gamncarek (Czainski 1994). In this work a
practical approach with midscale laboratory equipment has
been performed to induce more application-like conditions.












































































a /mm a/o h /mm

200 10 900
200 15 900
400 10 900


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


'"


starting cross section (mm)
end cross section (mm)
spray chamber height (mm)
mass flux air (g min )
mass flux water (g min- )
sum of gray values in interrogation
window (-)
sum of gray values of picture (-)
pressure atomizer gas (bar)
pressure atomizing fluid (bar)

interrogation window size (mm)
characteristic scale of analysed clusters
(mm)
Inhomogeneity Index (-)
side length of picture (mm)
Particle-Image- Velocimetry
Volume flux atomizer air (1 min- )
Volume flux water (1 min )




opening angle (")
loading of spray (riha /rilwate)
Measurement Scale (-)


a
b
h


riwatre,
n,

n
pair
pwater

B
Bcluster

H
L
PIV
i,,,
water


Greek letters
a
r
x


Figure 2: experimental setup and coordinate system

In order to estimate the effect of the disintegration process
on the formation of particle inhomogeneities an external
and an internal atomizer is analyzed. Both nozzles produce
very fine water droplets with particle diameters around 10
Clm. The mass flux of water is kept constant, while the
atomizer pressure is varied to investigate the influence of
the gas phase on the trajectory und the structure of the
particle collectives.


Pair PwJnatac Vjj are V wteC 9 arjlj tIl wteC If
I bar / bar/ Ilmin-1 Imin-1 Igminl I gmin-l [ ]

0.5 0.7 48 0.1 7.13 100 0.0713
1 0.7 62 0.1 18.41 100 0.1841

2 0.7 80 0.1 47.53 100 0.4753
^ ^^ ^' ^^ ^ ^ ^


Subsripts
RMS_cluster Root Mean Square value of cluster
property
RMSlmean Root Mean Square value of mean flow
property


81.99


0.8199


4 0.7 104 0.1 123.58 100 1.2358

Figure 3: parameters for external mixing atomizer

The investigated Spray chamber designs vary in the size of
starting cross sections and opening angles (Figure 3).


The outer spray chamber has a cross section of about 1 m2
(Figure 2). In the center of this chamber the conical,
tranparent spray chambers are built in. The spray is
removed from the chamber by a vacuum outlet. The
bottom side of the transparent spray chambers is open to
minimize the influence of the vacuum on the spray flow.


Figure 4: spray chamber designs


Nomenclature


Experimental Facility























0 Intensity oflnhomogeneity

random

Figure 5: Garncarek-algorithm (Kuno et al. 2007)

The Inhomogeneity Index is defined as the ratio of the
distribution of objects in a specific state and the
distribution of objects in a random state. The
Inhomogeneity Index increases when there are structures
in the spray that show clustering (Figure 1). The
Inhomogeneity index is dependent on a certain
Measurement Scale K. The Inhomogeneity Index H is
expressed as follows


H(x, n) = + nij0


where n is the the sum of all grey or binary values within
the processed picture and K is the scale on which the
Inhomogeniety Index is evaluated (Figure 5).
For each interrogation window with the side length
B= L/G the sum of gray values n, is counted. H is a scalar
value for the inhomogeneity in the whole picture. For the
variation of B the Inhomogeneity Index H yields a
maximum where the characteristic cluster size BCluster is
to be found.







I lse


Iiue6 hrkeitcsaewti pa





I Bctuste


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


A dual-cavity Nd:YAG laser (60 mJ/pulse at 532 nm) was
used to illuminate the droplets within the center plane of
the spray (Figure 1). The CCD-camera recording the light
scattered by the particles has a 12 bit image sensor with
2048 x 2048 pixels. The laser sheet has a thickness of
approximately 1 mm at the centre of the images. A small
laser light thickness is needed to focus the laser light
properly, yielding enough intensity in large light sheets. If
the laser sheet thickness is extended multiple scattering
within the spray cone can be observed. This leads to the
blurring of particle collectives. Image analysis of particle
cluster requires an optimized laser light intensity. There
must be a minimum of intensity to illuminate the particles
for the PIV method and there must be enough light to
detect particle concentration differences. On the other hand
a maximum of intensity leads to multiple scattering effects
resulting in blurring, which prevents PIV and cluster
recognition from being applicable.

Requirement for a narrow laser sheet is that the time
difference between two images is kept short. Otherwise
out-of-plane losses of particles take place which falsify the
PIV measurements. In this setup a time difference of 100
Cls was chosen. The temporal resolution of the camera is
7.4 Hz which is too low to track the dynamics of the spray.
Therefore only qualitative statements on the oscillation
behaviour for example can be made.



Unsteady particle clusters are determined quantitatively by
evaluating the spatial inhomogeneity within planar light
sheets. First of all it is necessary to distinguish the particle
clusters from the background noise. Common image
processing techniques are used for filtering. For an 8 bit
grayscale image the following methods were applied:

- gamma correction
- bandpass filtering
- local statistical filtering (Wiener filter)
- conversion to binary picture

The result is a binary picture with segregated particle
cluster (Figure 1). These particle structures are processed
with the Garncarek-Algorithm. In this study, the
inhomogeneity of the spray is estimated by using the
Inhomogeniety Index defined by Czainski (Czainski et al.
1994).


L
.
:'


S= 4: 2




a,
*

* *






7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Results and Discussion

Flow patterns of the investigated spray process are
complex. If the spray cone is close to the boundary
complex interactions between gas and particles take place.
The velocity field of the particles is far from being
stationary (Figure 6). The spray cone flow shows
characteristic flapping combined with a precession
movement. Flapping can be estimated by PIV
measurements, while precession can only be recorded
qualitatively by high-speed videography.


Figure 8: transport of particles clusters into recirculation
area, Variant V2, Z = 100 mm

In most cases the droplets at the spray cone edge are
re-entrained (Figure 8). But once the oscillations are too
strong the particles remain in the recirculation area and are
transported upward to the nozzle. This effect has great
influence on the distribution of the retention times of the
particles. Especially the retention time of larger particles
could be increased because they have a bigger centrifugal
moment which keeps them in the recirculation area. The
re-entrainment at the top could lead to undesired product
properties.
To get a stationary spray flow the starting cross-section
should be as large as possible. This geometry is already
being used by default in most spraying processes. Variant
VI with a smaller opening angle, but with the same
starting cross section area as variant V2, shows less
destabilization of the spray flow. This is probably due a to
a lower pressure rise in the propagation direction. Also
the recirculation zones are smaller and the particles in the
recirculation zone are entrained before they reach the
nozzle zone.


A comparison between the cluster structures and the
instantaneous velocity field of the particles (Figure 9)
shows that particles in a cluster have a higher velocity than
the particles around the clusters. This has also been found
by Aliseda (Aliseda 2001).


0 ms-l 22 ms-


Figure 7: instantaneous particle velocity field pair = 4 bar,
variant V2, Z =100 mm


The strength of these effects is mainly dependent on the air
pressure and the spray chamber design. Before discussing
the influence of these parameters on the inhomogeneity of
the spray and the characteristic scale of the cluster the
unsteady behaviour of the jet is reviewed.

Most stationary results can be found for Variant V3. This
means that the oscillation amplitude is low and no large
scale destabilization of the jet takes place. For Variant V2
one gets a high oscillation amplitude and destabilization of
the jet, which in some cases leads to the transport of
particle clusters into the recirculation area (Figure 8).





i i f


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010



The result is that cluster are not significantly faster than the
average flow at the cluster's position. This might be due to
the fact, that the spray flow itself is strongly unsteady in
space and time. Thus the validity of average values is
limited. This is also expressed by the large error bars in
Figure 10. The same results can be found for the analysis
of the shear rate and the vorticity.

The influence of the spray chamber design and the nozzle
parameters are analysed. Therefore the measured cluster
size for the different variants in spray chamber design are
compared. It shows that the inhomogeneity of the spray
and the cluster scale (Figure 10) increases slightly with
increasing gas flow flux.


C~sj I
,
i:::::tr
i I
,
rii
Irirui r
"*''1
rrrii Ilrr
) I Ir I I I Ilr I I
1 ITII1 (I
~ll~)l(llr~(
Ir r I I r r I I r~
111)111111)1
I I I 1 I ( ( 1 ( I Ill
Irlli~rlll(
,( I I ~ ( ) I I I 1 1
1II1II)1))Ii
)I 1(1
~~Llll(llr
111111111I
)(1)(1(1 11(
I 1 I I ( 1 i ) 1 I 1 I
rr r~r, r r rf i I I r 1 ,


,,,
,,, '
,,, I
B
I ~
j
I IIIIL i i
i 1:11
I
rI ~t i ijj
iilili
I ,
,
i
1 r
I r
i;l
iI i
ia
Iir


2 bar


4bar


Figure 9: instantaneous cluster structures and velocity of
particles p,, = 4 bar, variant V2


The Particle Image method has been used to find out
whether the particle cluster properties differ from the mean
state of the spray at the cluster's position,

The properties that are examined are RMS-velocity, shear
rate flux and vorticity. One has to keep in mind, that only
the motion of the particulate phase can be tracked,


Atomizer pressure
Figure 11: cluster size and atomizer pressure, variant Vl


Particle clusters tend to differ a lot in size and shape. This
is obvious looking at Figure 8 and considering the standard
deviation in Figure 10. Stretching of clusters (Figure 8)
does not significantly increases the cluster size detected by
the Garncarek method. This method is based on quadratic
interrogation windows and is therefore only accurate for
quadratic or circular cluster shapes. Despite of this the
Inhomogeneity Index and the cluster scale are simple
quantitative meaSures suitable to detect particle
inhomogeneities relevant for the optimization of spray
processes.


The other factor to influence particle clusters is the spray
chamber design. An increase of the cross section area leads
to a slight increase of the particle cluster size (Figurell1).


1.2D


1 bar


2 bar


Figure 10: Comparison between cluster velocity and mean
velocity at 4 cluster positions, variant VI





7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

Acknowledgements

This work has been performed within the project SPP 1423
C2 "Process Spray" funded by the Deutsche
Forschungsgemeinschaft (DFG). The support from the DFG
is gratefully acknowledged.

References


Heinlein J. and Fritsching U. Droplet clustering in sprays,
Experiments in Fluids, Vol. 40, pp. 464-472 (2006).

Scholler M. and Fritsching U. O ,iil us; J~et Flow in
Enclosures with Non-circular Cross Section Int. Joural of
Flow Control, Vol. 1, No. 2., pp. 167-173 (1 II I9~

Kuno K. and Tokuoka N. Transition of Spatial
hnhomogeneity of Droplets in Spray, Proceedings of 21th
ILASS, Europe Meeting 2007

Czainski, A.: Ouantitive Characterization of inhomogeneity
in Thin 1letallic Films Using Garncarek 's 1lethod J. Phys.
D: Appl. Phys. 27, pp 616-622, 1994

Aliseda, A; Cartellier, A.; Hainaux, F.; Lasheras, J.C.:
Effect of preferential concentration on the settling velocity
ofheavy particles in homogeneous isotropic turbulence J.
of Fluid Mechanics Vol. 468 ('1 II1), 77-105


V2 V3


Spray chamber design


Figure 12: cluster size and spray chamber design, pgas = 4
bar

There is no strong trend to be found when the influences
on the cluster size are examined. Same thing can be
assessed for the atomizer typ. The internal and external
mixing atomizers show same particle cluster structures.
The only difference for these two atomizers is that the
internal mixing atomizer requires less air flow to achieve
same quality of atomization. This leads to less
destabilisation of the jet in spray chambers with small
cross section areas.


Conclusions

The cluster formation, the entrainment flow and the
recirculation movement in sprays have great influence on
the retention time, heat- and mass transfer and the
agglomeration of particles. The product properties are
thereby dependent not only the nozzle parameters but also
on the spray chamber design.

By means of Particle Image methods several results for the
given experimental setup have been found:

-the spray flow structures are highly time
dependent (flapping, precession, .. )
-the cluster size is depend on the spray chamber
design and the nozzle parameters
-cluster velocity is faster than the velocity of
surrounding particles
-the state of the cluster does not differ significantly
in one direction from the mean spray quantities
-no significant influence of atomizer type on
cluster formation

The influence of straight spray chambers will be included
in future analysis.




University of Florida Home Page
© 2004 - 2010 University of Florida George A. Smathers Libraries.
All rights reserved.

Acceptable Use, Copyright, and Disclaimer Statement
Last updated October 10, 2010 - Version 2.9.7 - mvs