Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 4.3.2 - Experimental investigation of delaying splashing in the jet wiping process by means of a side jet
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 Material Information
Title: 4.3.2 - Experimental investigation of delaying splashing in the jet wiping process by means of a side jet Industrial Applications
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Myrillas, K.
Gosset, A.
Rambaud, P.
Anderhuber, M.
Mataigne, J.M.
Buchlin, J.M.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: jet wiping
liquid film
splashing
side jet
 Notes
Abstract: Jet wiping is used in coating techniques as a method of controlling the thickness of the applied coating film. A turbulent slot jet is used to wipe the coating film dragged by a moving substrate, often after dipping in a liquid bath. This process is limited by a rather violent film instability called splashing. Splashing is characterized by the ejection of droplets from the film, resulting in its detachment form the substrate. This instability occurs at high jet velocities and high substrate velocities and limits the production line speed. In the present study a technique to delay the occurrence of splashing is investigated experimentally. An additional side jet is used close to the main wiping jet in order to stabilize the runback flow and avoid the splashing. The mean film thickness after wiping is measured using a light absorption method and the results are compared for the single jet wiping and two jet configuration. It is shown that using the side jet, stronger wiping can be applied resulting to lower values of the final film thickness not achievable with a single jet.
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
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Bibliographic ID: UF00102023
Volume ID: VID00105
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: 432-Myrillas-ICMF2010.pdf

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


Experimental investigation of delaying splashing in the jet wiping process by
means of a side jet


K. Myrillas* A. Gosset* P. Rambaud* M. Anderhuber! J.-M. Mataignet

and J.-M. Buchlin*
von Karman Institute for Fluid Dynamics, B-1640 Rhode-Saint-Genbse, Belgium
SArcelorMittal Maizibres Research SA, 57283 Maizibres-ls-Metz Cedex, France
myrillas@vki.ac.be, rambaud@vki.ac.be, buchlin@vki.ac.be
Keywords: jet wiping, liquid film, splashing, side jet




Abstract

Jet wiping is used in coating techniques as a method of controlling the thickness of the applied coating film. A
turbulent slot jet is used to wipe the coating film dragged by a moving substrate, often after dipping in a liquid
bath. This process is limited by a rather violent film instability called splashing. Splashing is characterized by the
ejection of droplets from the film, resulting in its detachment form the substrate. This instability occurs at high jet
velocities and high substrate velocities and limits the production line speed. In the present study a technique to delay
the occurrence of splashing is investigated experimentally. An additional side jet is used close to the main wiping
jet in order to stabilize the runback flow and avoid the splashing. The mean film thickness after wiping is measured
using a light absorption method and the results are compared for the single jet wiping and two jet configuration. It is
shown that using the side jet, stronger wiping can be applied resulting to lower values of the final film thickness not
achievable with a single jet.


Nomenclature

Roman symbols
A splashing correlation coefficient (-)
B splashing correlation coefficient (-)
d nozzle slot opening (m)
E total wiping energy per unit time (W)
g gravitational constant (ms 2)
h local film thickness (m)
n splashing correlation coefficient (-)
P pressure (Pa)
Q total volumetric flow rate (n3s 1)
R cylinder radius (m)
Re Reynolds number, p1Uhf /p(-)
U substrate velocity (ms-1)
V jet velocity (ms-1)
We Weber number, pg V 1,,/ (-)
x spatial coordinate normal to the substrate (m)
y spatial coordinate along the moving substrate (m)
Z standoff distance (m)
Greek symbols
a nozzle tilt angle (deg)
P dynamic viscosity (Pas)


density (kgm 3)
surface tension (Nm 1)
shear stress from impinging jet (Pa)


Subscripts
g g
j je
1 li
max n
ref r
wj
0 w


as
t
quid
maximum
reference
all jet
without wiping


Superscripts
* critical for splashing



Introduction

In coating techniques, jet wiping is a hydrodynamic
method of controlling the final coating film thickness.
It is used in various industrial processes such as paper
and photographic film manufacturing, wire coating and
steel strip finishing. In the dip coating method a continu-
















ous web is dipped in a bath and getting covered with the
coating liquid (initial thickness ho). The gas jet wip-
ing method is a kind of air knife used to reduce and
control the thickness of the coating film, using a turbu-
lent slot jet to wipe the excess liquid. This method is
preferred when physical contact with the coating liquid
should be avoided, such as in the hot dip galvanizing
process. The steel strips emerge from a bath of liquid
zinc and the gas jet impinges on the dragged liquid film
reducing its thickness while forming a runback flow to
the bath, as illustrated in Figure 1. The film thickness
after wiping hf, depends on the substrate velocity U,
the nozzle pressure P,, the nozzle to substrate standoff
distance Z, the nozzle slot width d, as well as the liq-
uid properties. The process has been studied in the past
[1-7] and some useful analytical models have been pro-
posed for the film thickness distribution at the wiping re-
gion. The pressure gradient and shear stress distributions
from the gas jet have been identified as the governing pa-
rameters. Moreover, numerical studies mainly using the
Volume-of-Fluid model for multiphase flow simulations
have been carried out to predict the final film thickness
and wiping behavior [8,9].


The jet wiping process is limited by a rather vio-
lent instability, characterized by the ejection of droplets
from the runback flow and resulting in the complete de-
tachment of the runback flow from the substrate. This
phenomenon is called splashing and occurs usually at
high jet velocities and high substrate velocities, posing
a limit for the production line speed. The occurrence of
splashing can have serious consequences since the liquid
droplets can block the slot of the nozzle and the produc-
tion line has to be stopped. An example of splashing in
an industrial galvanization line is shown in Figure 2.a.
The splashing phenomenon degrades the final coating
quality as the process becomes unstable and the main
wiping parameters (pressure gradient and shear stress
at the impingement) are negatively affected. Previous
studies of the phenomenon have provided some useful
tools for the prediction of the wiping conditions lead-
ing to splashing [5,10-13] and a first attempt to delay
the occurrence of splashing has been made by tilting the
wiping nozzle [11,12].


In the present study a new technique to delay the oc-
currence of splashing is investigated experimentally. A
side jet is used close to the main jet, modifying the gas
flow and having a stabilizing effect on the runback flow.
In this way the occurrence of splashing is delayed and
much stronger wiping can be used to achieve smaller
film thicknesses, not achievable with a single wiping jet.


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


Splashing phenomenon

The splashing mechanism has been approached by
Yoneda[5,13] through Hinze's model [14], which has
been developed for the breakup of liquid film sheared by
turbulent gas flow. A phenomenological approach has
been proposed by Buchlin (1997) [7], postulating that
splashing occurs when the shear effect produced by the
downward gas wall jet overcomes the stabilizing effect
of the surface tension (modeled as cT/ i.,, ii fig' b). Ex-
pressing the shear stress term Twj in terms of the typical
dynamic pressure of the wall jet and evaluating the ratio
of dominating forces controlling the splashing mecha-
nism leads to the effective jet Weber number.

We pgV2 1.,, /l (1)

The critical We number above which splashing occurs is
correlated with the critical film Reynolds number, based
on the strip velocity U and the final thickness h .


Re* = piUhf /i


The wall jet velocity can be modeled taking into account
the tilting angle a of the nozzle with respect to the hori-
zontal plane.

Vj + sin (3)
Zld

An empirical model is proposed [11,12] for prediction of
splashing, correlating the critical We* and Re* at which
splashing occurs, as shown in Equation 4.

We* eA a+BRe-n (4)

The value of coefficients A and B as well as the expo-
nent n depends on the nozzle design, with 0.018 < A <
0.066, 5.5 < B < 7.9 and 1.44 < n < 1.91. As it is
shown in Figure3, the empirical correlation represents a
limit above which splashing occurs. It has been shown
that tilting the jet nozzle downwards can displace this
splashing limit to higher We* for constant Re*.

Experimental facility and method

The experimental facility used for the study of the jet
wiping process is presented in Figure 4. It consists of
a transparent cylinder (R = 0.225m) made of Plexi-
glas that is partly submerged in a bath of coating liquid.
The liquid used in the experiments is dipropylene glycol
(density: p 1023kg/m3, viscosity: p 0.105Pa s,
surface tension: a 0.032N/m), colored with a dye
(methylene blue). The cylinder rotates getting covered
with a liquid film. A slot jet with a nozzle opening
d = 1mm at variable standoff distance (Z) is used to
wipe the film and reduce its thickness.














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


hf




ii (o d N o z z l e

'z .......



-- - -- -V3 I Bfrcto


pBifurcation
point

X


Shear


Figure 1: Schematic of jet wiping process presenting the interaction between the slot jet and the liquid film. The film
thickness is reduced and the excess liquid returns to the bath forming a runback flow. The main parameters
of wiping are the pressure gradient and shear stress from the jet.




















Figure 2: The splashing phenomenon, a) Splashing in industrial galvanization line, b) Schematic shown the formation
of droplets from the runback flow when the shearing effect from the gas flow overcomes the stabilizing effect
of the surface tension.
U

Nozzle



2h,

o/2ho *
droplets



Figure 2: The splashing phenomenon. a) Splashing in industrial galvanization line, b) Schematic shown the formation
of droplets from the runback flow when the shearing effect from the gas flow overcomes the stabilizing effect
of the surface tension.


We* Line condition
1 ----------- ------ -- No splashing ...........
0 Splashing
0.8 -.... ----------
0.6 ---- ...... ------ .......
0 .6 . ....... ........................................................
Correlation: a=00
0.4 .......................... ... ......... .........

0.2 ---------- .....
Re*
0 40 80 120 16


Figure 3: Splashing correlations, a) Splashing curves presenting the correlation between We* and Re* for two differ-
ent nozzle tilt angles. b) Validation of the empirical correlation for the conditions of the industrial production
line.
















The final film thickness hf is measured using a light
absorption technique [15]. A diffused light source is
placed in the transparent cylinder and a high speed cam-
era (Phantom v5, Vision Research, USA) is placed 1/8
of the cylinder rotation downstream wiping recording
images of the cylinder surface covered with the liquid
film. The light coming from the light source inside the
cylinder is partly absorbed by the dyed liquid film. The
technique is based on the association of the brightness of
the greyscale image with the thickness of the film. The
brightness of each pixel of the image is quantified, giv-
ing a value of the film thickness. The air jet is obtained
by supplying pressurized air to a nozzle, controlling the
pressure in the stagnation chamber of the nozzle. This
pressure is measured by a Validyne transducer.


High-speed camera



Diffused
light sourcecm

Side jet





Figure 4: Schematic of the experimental facility using
the light absorption technique. The light from
the diffused source inside the cylinder is par-
tially absorbed by the liquid film. The film
thickness is measured from images of the
film, correlating the level of gray with the lo-
cal film thickness.


In the experimental investigation splashing is detected
visually and the limit conditions for splashing are con-
sidered at the point where a complete detachment of the
runback flow occurs. Splashing begins with the ejection
of droplets from the runback flow, followed by a more
violent instability that starts at the strip edge and prop-
agates along the width of the strip, until the complete
detachment of the film. In these conditions the wip-
ing mechanism is destroyed and often the jet nozzle is
blocked by the ejected liquid, at which point the process
is stopped.
The new technique to delay the occurrence of splash-
ing consists of using a side jet close to the main wiping
jet. For simplicity in this study a side jet is placed paral-
lel to the main jet, upstream the wiping. Both the main
jet and the side jet have an opening of 1mm and are sep-
arated by a distance of 1mm. The nozzle pressure is con-


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


trolled independently for eachjet. A reference case is se-
lected for the study with a standoff distance of Z=10mm,
and nozzle pressure P, = 600Pa for the main jet. The
side jet nozzle pressure is increased in steps of 150 Pa.
A reference series of measurements with only the main
jet is taken increasing the main jet pressure in steps of
150 Pa.


Results and discussion

When final film thickness results are to be compared be-
tween cases with and without the side jet, a difficulty
rises as to with what reference the comparison can be
made. Since the effect of the side jet on the gas flow
and on the film thickness is not well described, a com-
parison between a case with only the mainjet and a case
with the addition of the side jet becomes problematic.
The nozzle pressure or the jet velocity is not suitable to
describe the changes of multiple jets. In this case the to-
tal energy of wiping (E) is introduced. This represents
the energy per unite time (in Watts) that is supplied to
the nozzle in form of compressed air. It is computed
as: E Q P,, where Q is the total volume flow rate
of air provided to the nozzle and P, is the nozzle pres-
sure. Therefore higher nozzle pressures lead to higher
wiping energy. Using the total wiping energy, different
jet configurations with different efficiency at wiping can
be compared.
The final film thickness results are presented in Figure
5, normalized by the film thickness of a reference case
using only the main jet with a nozzle pressure P,
600Pa, standoff distance Z = 10mm, jet slot opening
d = 1mm, strip velocity U 0.34m/s which gives a
mean film thickness h,,f = 380pm and wiping energy
Eef 9.5W.
The series without the side jet serves as a basis for the
comparison. Splashing occurs at relatively low wiping
energy, especially in short standoff distances, while the
lowest film thickness that can be reached is significantly
limited. In this specific case it cannot be lower than 80%
of the starting point value.
Activating the side jet leads to higher wiping energies
and eventually lower film thicknesses without the limita-
tion of splashing occurring so early in the process. In the
specific configuration a film thickness of less than 70%
of the starting point was reached without splashing. The
limit conditions of splashing by increasing the pressure
of the side jet could not be investigated due to limita-
tions of the provided flow rate. As it is observed from
the curve, the efficiency of wiping is lower in the case of
a strong side jet, since higher wiping energy is needed
to reach the same film thickness as the single jet case.
This is expected as the wiping mechanism is strongly
based on the pressure gradient and shear stress imposed















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


Mean film thickness with energy of wiping


1.1 -

1.0 I

0.9 -

" 0.8

S0.7
-
S0.6-

0.5

0.4

0.3
1


.0 1.5 2.0 2.5 3.0 3.5 4.0
EIEref [-]

S-- side jet -0- no side jet


4.5 5.0 5.5 6.0


Figure 5: Results of normalized mean film thickness versus the energy of wiping for the cases with and without a side
jet. Splashing limit conditions are depicted using a weak side jet with a strong main jet (splashing limit).


Mean film thickness with energy of wiping


1.1

1.0 I

0.9 -

S0.8

2 0.7 -

c 0.6

0.5

0.4

0.3
1


.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
EIEref [-]
S-- no side jet sidejet upstream -A-side jet downstream


Figure 6: Results of normalized mean final film thickness with energy of wiping for the configurations with upstream
and downstream placement of the side jet. The case of wiping with a single jet is used as a reference for the
comparison, with the splashing limit indicated.


............... sp la shing ............... ................ ................ .................






Ssplashing limit
--


S splashing

-- --------- --------- -- ---------- --------------- ---------------- -- --------- -- ---------- -------- ------- --------------- ------------
















by the jet. Normal impingement of the jet provides the
strongest wiping, while tilting the nozzle or activating a
side jet has a negative effect on the wiping efficiency.
A different wiping case is also presented in Figure 5,
showing the limit of splashing for a rather week side jet.
In the depicted conditions a side jet with a nozzle pres-
sure of 300 Pa is used with a mainjet with a nozzle pres-
sure of 1800 Pa, giving a total wiping energy 5.52 times
the wiping energy of the reference case. This configura-
tion seems to suppress the instability of the runback flow
that leads to splashing, allowing to reach much smaller
film thickness (in this case about 37% of the reference
case) without this limitation.
The effect of the secondary jet placement in the sym-
metric parallel configuration can be easily assessed by
changing the roles of the main and the side jet, thus
placing the side jet downstream the main jet. As it is
shown in Figure 6, the effect of the placement of the
secondary jet in this configuration has small effect on the
wiping behavior. It appears that in this case the jet is not
tilted downwards like when tilting the nozzle, but it is
the gas flow field over the runback flow that is affected.
Unsteady wiping conditions that appear close to splash-
ing are stabilized with the help of the side jet, avoid-
ing the splashing. This permits to reach much smaller
film thicknesses, not achievable when using a single jet.
It is noted that when the splashing limit conditions are
surpassed by using the side jet, deactivating the side jet
leads to splashing. This indicates that the continuous op-
eration of the side jet is needed for avoiding splashing in
the examined conditions.


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


Acknowledgements

The authors would like to thank ArcelorMittal for sup-
porting the study.


References

[1] J. A. Thornton,, and M. F. Graff, An Analytical
Description of the Jet Finishing Process for Hot-
Dip Metallic Coating on Strip, Metall. Trans. B,
7B (1976) 607-618.

[2] E. O. Tuck, Continuous Coating with Gravity and
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[3] C. H. Ellen, and C. V Tu, An analysis of Jet Strip-
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[4] E. O. Tuck, and J.-M. vanden Broeck, Influence of
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[5] H. Yoneda, Analysis of Air-Knife Coating, Master
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[6] H. Yoneda and L. E. Scriven, Air-Knife Coating:
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lanta, GA, April 17-21 (1994).


[7] J.-M. Buchlin, Modeling of Gas-Jet Wiping, Thin
Liquid Films and Coating Processes, VKI Lecture
Conclusions Series LS1997-06, von Karman Institute for Fluid
Dynamics, Brussels, Belgium, (1997).


In the present study a first experimental investigation is
presented about a technique to delay the occurrence of
splashing in the jet wiping process. By use of a side
jet close to the main wiping jet much stronger wiping
can be applied, resulting in smaller film thickness be-
fore splashing occurs. This technique to delay splashing
in thejet wiping process can be of high industrial interest
as it can be used to reach conditions that are not possi-
ble with a single jet. In the investigated conditions it is
shown that much lower film thicknesses can be reached
with the use of the side jet, not possible when using a
single wiping jet. The study can be extended to different
configurations and wiping parameters, as well as a fur-
ther investigation of the mechanism that results in this
effect of delaying the splashing can help to clarify the
physical phenomena that take place in this interaction.


[8] D. Lacanette, A. Gosset, S. Vincent, J.-M. Buch-
lin, E. Arquis and P Gardin, Macroscopic Analy-
sis of Gas-Jet Wiping: Numerical Simulation and
Experimental Approach, Phys. Fluids 18, 042103
(2006).

[9] K. Myrillas, A. Gosset, P Rambaud and J.-M.
Buchlin, CFD Simulation of Gas-Jet Wiping Pro-
cess, Eur. Phys. J. Special Topics 166 (2009) 93-
97.

[10] M. Dubois, M.L. Riethmuller, J.-M. Buchlin, M.
Arnalsteen, The gas-jet wiping limit: The splash-
ing phenomenon, Galvatech Conference, Chicago,
Iron & Steel Institute, 17-21 sept. (1995), pp 667-
673.












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


[11] A. Gosset, V Perrot, J. Anthoine, J.-M. Buchlin,
Effect of nozzle tilting on splashing in jet wiping,
Proceedings 5th European Coating Symposium on
Advances in Liquid Film Coating Technology, Fri-
bourg, Switzerland, September 17-19, (2003).

[12] A. Gosset and J.-M. Buchlin, Jet wiping in hot-dip
galvanizing, J. Fluids Eng. 126 (2007) 469-475.

[13] H. Yoneda and L. E. Scriven, Analysis of Spray
Generation in Air-Knife Coating, Proceedings 7th
Symposium on Coating Process Science and Tech-
nology at the AICHe Spring National Meeting, At-
lanta, GA, April 17-21 (1994).

[14] J. O. Hinze, Fundamentals of the Hydrodynamics
Mechanism of Splitting in Dispersion Processes,
AIChE J., 1 (3) (1995) 289-295.

[15] A. Gosset, Measurement techniques for unstable
film flows, VKI-LS 2006-07, Lecture Series on
Thermo-hydraulic instabilities, von Karman Insti-
tute, Belgium (2006).




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