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A Noble Gas Wiping System to Prevent the Edge Overcoating in Continuous Hot-dip Galvanizing

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A noble method is proposed to prevent the edge overcoating (EOC) that may develop near the edge of the steel strip in the gas wiping process of continuous hot-dip galvanizing. In our past study (ISIJ International, Vol. 27 (2003), No. 10, pp. 1495-1501), it was found that the EOC is caused by the alternating vortices which are generated by the collision of two opposing jets in the region outside the steel strip. In the present study, the flow field around the gas wiping system has been analyzed numerically and it was found that when the two opposing jets collide at an angle much less than 180 degrees, the alternating vortices disappear and the impinging pressure on the steel strip surface becomes nearly uniform. In order to deflect both jets downward by a certain angle, a cylinder with small diameter is installed tangentially to each exit of the lower lips of the two-dimensional opposing jets. The three dimensional flow field with the proposed device is analyzed numerically by using the commercial CFD software, STAR-CD. And the coating thickness is calculated by solving the boundary layer momentum equation with an integral analysis method. In order to compare the present noble method with the conventional edge baffle plate method to prevent the EOC, the flow field with edge baffle plates is also calculated. The calculation results show that the tangentially installed cylinder at the lower lip of the jet exit is significantly more effective than the edge baffle plate.
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1. Introduction
Since galvanized steel strip has very good properties for
corrosion-resistance, paintability, workability and weldabil-
ity, it has been widely used over a variety of industrial fields
such as buildings, bridges, automobiles, and electronic
equipments. In general, the galvanized steel strip is pro-
duced through a continuous hot-dip galvanizing process.
The thickness of the adhered zinc film is controlled by a gas
wiping process. Due to its good productivity and ease of
thickness control, the gas wiping process that was commer-
cialized in the mid-20th century has been used in most of
the continuous hot-dip galvanizing system.1) Usual working
fluid for the wiping process is nitrogen gas. But the gas
wiping process causes two technically serious problems;
namely the edge overcoating (EOC) and check mark.2)
Frequently, the zinc film near the steel strip edge is 1.4–1.8
times thicker than that in the steel strip center region. This
EOC is a chronic problem that causes troubles in coiling
and poor flatness of the steel strip after uncoiling. Ac-
cordingly, a number of researches have been carried out to
reduce the EOC.3–6)
Recently Kim et al.7) investigated the mechanism of the
EOC through a numerical analysis. They found that the
two-dimensional opposing jets that are not blocked by the
steel strip establish alternating vortices in the region outside
the steel strip (z0 in Fig. 1(a)), and that due to such alter-
nating vortices the impinging pressure of the wiping gas
near the steel strip edge becomes significantly lower than
that in the central region of the steel strip. They showed that
this drop in impinging pressure is the main cause of the
EOC. In order to prevent the collision of the two opposing
jets in the region outside the steel strip in z0, an edge baf-
fle plate parallel to the steel strip has often been used like
the one in the experiment of Park et al.2) As the edge baffle
plate and the steel strip are getting closer, the EOC was ob-
served to be reduced significantly. However, technically it is
very difficult to maintain the small gap between the edge
baffle plate and the steel strip. Due to the inevitable vibra-
tion of the up-lifting steel strip, the edge baffle plate and the
steel strip clash each other. This causes unwanted disper-
sion of molten zinc toward the exit of the air knife nozzle
that eventually blocks the nozzle exit.
Gilchrist et al.8) demonstrated that a jet, discharged tan-
gentially to a cylindrical surface, flows along the surface as
shown in Fig. 2. The stream of fluid emerging from a noz-
zle tends to attach the nearby curved surface due to the
Coanda effect. In the present study, this effect is used to
prevent the direct collision of the two opposing jets at
180°C. As shown in Fig. 1(a), the flow direction of the wip-
ing gas is deflected by a cylinder with small diameter in-
stalled tangentially to the exit of the lower lip of the two-di-
mensional jet, which is placed in the region z0 where the
two opposing jets are not blocked by the steel strip.
ISIJ International, Vol. 46 (2006), No. 4, pp. 573–578
573 ©2006 ISIJ
A Noble Gas Wiping System to Prevent the Edge Overcoating in
Continuous Hot-dip Galvanizing
Ki Jang AHN and Myung Kyoon CHUNG
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1, Kusong-dong,
Yusong-gu, Daejon 305-701, Korea. E-mail: gijang.ahn@kaist.ac.kr
(Received on December 12, 2005; accepted on January 23, 2006)
A noble method is proposed to prevent the edge overcoating (EOC) that may develop near the edge of
the steel strip in the gas wiping process of continuous hot-dip galvanizing. In our past study (ISIJ
International, Vol. 27 (2003), No. 10, pp. 1495–1501), it was found that the EOC is caused by the alternating
vortices which are generated by the collision of two opposing jets in the region outside the steel strip. In
the present study, the flow field around the gas wiping system has been analyzed numerically and it was
found that when the two opposing jets collide at an angle much less than 180°, the alternating vortices dis-
appear and the impinging pressure on the steel strip surface becomes nearly uniform. In order to deflect
both jets downward by a certain angle, a cylinder with small diameter is installed tangentially to each exit of
the lower lips of the two-dimensional opposing jets. The three dimensional flow field with the proposed de-
vice is analyzed numerically by using the commercial CFD software, STAR-CD. And the coating thickness is
calculated by solving the boundary layer momentum equation with an integral analysis method. In order to
compare the present noble method with the conventional edge baffle plate method to prevent the EOC, the
flow field with edge baffle plates is also calculated. The calculation results show that the tangentially in-
stalled cylinder at the lower lip of the jet exit is significantly more effective than the edge baffle plate.
KEY WORDS: continuous hot-dip galvanizing; edge overcoating; impinging jet; gas wiping.
In the present study, the 3-D unsteady compressible tur-
bulent flow field with the proposed configuration with a
pair of cylinders in the continuous hot-dip galvanizing sys-
tem is simulated by using a commercial CFD software, to
find out how the cylinder affects the flow behavior of the
wiping gas. The simulation results for the impinging pres-
sure and the shear stress on the steel strip surface are used
to calculate the thickness of the adhered zinc film by the
noble integral analysis method of the boundary layer mo-
mentum equation.7) In addition, the flow field generated by
the edge baffle plate is also analyzed to compare its perfor-
mance for reduction of the EOC with that of the cylinder
installed at the lower lip of the air knife nozzle.
2. Numerical Analysis
2.1. Numerical Analysis of 3-D Unsteady Compressible
Flow
In the continuous hot-dip galvanizing process, a heat-
treated steel strip is passed through a molten zinc bath and
the molten zinc is adhered on the steel strip surfaces.
Usually, the thickness of the adhered zinc film is nearly 10
times thicker than desired one, and the excessive zinc is re-
moved by nitrogen gas jets. This wiping gas jet system is
often called the air knife system. Usually, the Mach number
of the nitrogen gas flow is in a range of 0.3–0.6. Therefore,
in addition to the continuity and momentum equations, the
equation of state and the energy equation are needed. Large
Eddy Simulation (LES) is used to solve the turbulent equa-
tions, of which forms can be found in Kim et al.7) The nu-
merical solutions of these equations for 3-D unsteady com-
pressible turbulent flow are obtained by using a commercial
CFD software. Reliability and numerical accuracy of LES
for complex turbulent flows have been well established
through many comparative studies between LES and exper-
imental measurements. Recently, for example, even a sepa-
rated turbulent flow over a forward-backward facing step
has been successfully simulated with LES by Addad et al.9)
They also demonstrated in a different study that the same
LES could be applied to accurately simulate a buoyant
wall-jet.10) Since the present flow geometry is a combina-
tion of separated flow and curved wall-jet, it is expected
that LES is capable of producing reliable computational re-
sults for the flow geometry of our concern.
The calculation domain is shown in Fig. 1(b). The steel
strip is lifted up at the middle of the two air knives. The
width of the steel strip is nearly half of the two-dimensional
nitrogen jet width. At each of the two opposing jets, a cylin-
der of small diameter is installed tangentially to the lower
lip of the jet exit so that the jet flow is deflected downward
in the region outside the steel strip in z0. The boundary
conditions and the flow Mach number in the present study
are given the same values as described in Kim et al.7)
2.2. Numerical Computation of the Coating Thickness
Nitrogen gas jet injected normally onto the surface of the
moving steel strip is divided into two different wall jets as
depicted in Fig. 3. In the upper region (y0), since the
steel strip moves upward, the molten zinc moves in the
same direction at both the wall boundary and the free
boundary. On the other hand, in the lower region (y0), the
molten zinc moves upward near the steel strip surface while
it moves downward near the free boundary. This feature has
been verified by the experiment of Ellen et al.11) Due to the
velocity profile of the nitrogen gas flow near the interface
between the molten zinc and the nitrogen gas, the shear
stress in the nitrogen gas wall jet exerts a wiping force in
the upper region while that in the lower region contributes
to augmenting the film thickness. This difference makes the
film thickness of the upper region thinner than that of the
lower region as shown in Fig. 3. The governing equations
determining the thickness of the molten zinc film are the
ISIJ International, Vol. 46 (2006), No. 4
©2006 ISIJ 574
Fig. 1. Schematic diagram of air knife system and calculation
domain.
Fig. 2. Deflection of air knife jet due to the curved wall surface.
continuity and Navier-Stokes equations. The noble integral
analysis method proposed by Kim et al.7) is also used for
calculation of the film thickness in the present study.
3. Computation Results and Discussion
3.1. Deflection of the Air Knife Jet
In order to find the effect of the cylinder, that is tangen-
tially attached at the lower lip of a jet exit, on the deflection
of the jet flow, the two-dimensional flow field in the im-
pinging region between two opposing jets in the absence of
a steel strip is calculated for different cylinder diameters.
For comparison, the flow field without installing the cylin-
der was calculated first. When there is no cylinder installed
at the lower lip of the air knife nozzle, the jet alternates its
flow direction cyclically upward and downward as shown in
Fig. 4 where T represents the period of such a cyclic
change. However, when a cylinder of small diameter larger
than 3 mm is installed in place, Fig. 5 shows that the alter-
nating vortices do not appear. In this case, a steady flow
field is established, and the jet flow direction does not
change. It can be seen that the deflection angle of the jet de-
pends on the cylinder diameter. When d5mm, the jet flow
deflects more than 90°.
ISIJ International, Vol. 46 (2006), No. 4
575 ©2006 ISIJ
Fig. 5. Effect of cylinder diameter on the deflection of air knife jet (unit in m/s).
Fig. 4. Variation of flow directions of two opposing air knife jets without cylinders at the lower lips (unit in m/s).
Fig. 3. Schematics of gas wiping mechanism.
3.2. Three Dimensional Flow Field in the Impinging
Region
As depicted in Fig. 1(a), two opposing jets are discharged
onto each other and a steel strip moves upward at the mid-
dle of the two jet exits. Since the jet width is twice larger
than the steel strip, the jets impinge normally on the steel
strip in the central region (z0) and they collide with each
other in the outer free space in z0. Figure 6 demonstrates
the velocity contours of the jet flows at various locations in
span-wise direction. Figure 6(a) shows the velocity contour
at the symmetric center plane. Since the jets are discharged
normally onto the steel strip, the velocity contours are near-
ly symmetric with respect to the moving steel strip. At
z1.75 mm the effect of colliding jets in the free space
begins to appear in the velocity contour. Slightly more wip-
ing gas flows downward due to the deflection of the jets in
the free space caused by the presence of the cylinder whose
diameter is d5mm. At z1.75 mm, a large percentage of
the wiping gas flows downward, and at z175mm, almost
all of the wiping gas flows downward. There are no alter-
nating vortices in the flow field.
3.3. Effect of Cylinder Diameter on the Impinging
Pressure and the Coating Thickness
Figure 7(a) compares the mean impinging pressure on
the steel strip surface for different cylinder diameters.
When there is no cylinder, as the steel strip edge is ap-
proached the mean impinging pressure significantly de-
creases from 21 to 16.5 kPa within 100 mm from the steel
strip edge. But, if a cylinder of small diameter is present at
each exit of the air knife system, the impinging pressure re-
mains at almost the same level up to the point 25 mm from
the steel strip edge. When the cylinder diameter is larger
than 3 mm, it was found that such constant pressure level
does not depend on the cylinder diameter. Similar compari-
son can be observed with Fig. 7(b) where the span-wise dis-
tributions of the coating thickness are compared for differ-
ent cylinder diameters. From these comparisons, it may be
ISIJ International, Vol. 46 (2006), No. 4
©2006 ISIJ 576
Fig. 6. Velocity contours at various span-wise locations (unit in m/s).
Fig. 7. Mean pressure and coating thickness distributions for
various cylinder diameters.
concluded that the problem of the EOC can be effectively
removed by installing a cylinder tangentially to each of the
lower lips of the opposing jet exits. It is worth noting that
the coating thickness becomes gradually thinner approach-
ing to the strip edge. Investigating the static pressure field
on the strip surface at the impinging region, depicted in
Fig. 8, it was observed that the cyclic pressure distribution
moves alternatively left and right. The pressure distribution
shown in Fig. 7(a), is the average pressure field of such an
alternatively moving cyclic field. At the end of the strip,
such movement is restricted due to the presence of the
cylinder. This may cause the higher static pressure near the
strip edge.
3.4. Effect of Gap Distance between the Cylinder and
the Steel Strip
At the exit of the two-dimensional jet, the nitrogen gas
spreads up and down two-dimensionally in the central re-
gion (z0), whereas in the free space (z0) the nitrogen
gas stream deflects downward due to the presence of the
cylinder. These two gas streams disperse into each other
span-wisely at the interface of the two streams. Therefore it
is important to investigate the effect of the gap distance be-
tween the cylinder and the steel strip on the coating thick-
ness distribution. Figures 9(a) and 9(b) compare the mean
impinging pressure and coating thickness distributions for
gap distances of g0mm, 2.5 mm and 5 mm. The diameter
of the cylinder is fixed at 5 mm for all cases. As can be seen
in Fig. 9(b), the case with g5mm yields most desirable
thickness distribution among the three cases. When the gap
becomes larger than 5 mm, the EOC begins to appear. This
trend is noted even with the gap distance of g5mm in Fig.
9(b).
3.5. Cylinder and Edge Baffle Plate
Finally in order to compare the EOC performance of the
cylinder installation method with the conventional method
that employs the edge baffle plates, the same analysis has
been carried out for the case of the edge baffle plate.
Figures 10(a) and 10(b) compare the EOC performances of
these two methods. As can be seen from the figures, the
cylinder installation method is far superior to the edge baf-
fle plate. Moreover, it is certain that the installation of
cylinders is much more convenient in maintenance and op-
eration. Experimental data of coating thickness distribution
obtained by Takeish4) are shown in Fig. 10(b). Although the
device to improve EOC is different, the computed coating
thickness distribution with the edge baffle is similar to that
obtained experimentally with the edge mask. Also note the
favorable comparison between the experimental and com-
puted thickness distributions for the case without baffle or
cylinder in Fig. 10(b). This agreement of the computed re-
sult with the experimental measurement may justify the use
of LES to study the EOC in the present work.
4. Conclusions
The EOC is a chronic problem in the continuous hot-dip
galvanizing process that is caused by the alternating vor-
tices which are generated by the collision of the two oppos-
ing jets in the free outside region of the moving steel strip.
In this research, a noble method was devised to prevent the
collision of two impinging jets in the free outside space. In
other words, a small diameter cylinder was installed tangen-
tially to the exit of the lower lip of the two-dimensional gas
wiping jet to deflect the jet flow direction downward in the
region where two opposing jets are not blocked by the steel
strip so that the two jets collide at an angle less than 180°.
In order to investigate the possibility to remove the EOC by
such a flow configuration 3-D unsteady compressible turbu-
lent flow around the gas wiping system has been calculated
using a commercial CFD software, STAR-CD. The flow
structure of the computed three dimensional flow field was
investigated in detail and the calculated impinging pressure
and shear stress data on the steel strip surface were used to
find the galvanized film thickness.
ISIJ International, Vol. 46 (2006), No. 4
577 ©2006 ISIJ
Fig. 8. Instantaneous pressure distribution on the strip surface.
Fig. 9. Mean pressure and coating thickness distributions for
various distances between cylinder and steel strip.
The effect of cylinder installation on the flow deflection
was first investigated by carrying out two-dimensional flow
field analysis. It was found that when the cylinder diameter
is greater than 3 mm, the wiping gas jet indeed is deflected
enough to prevent the occurrence of the alternating vor-
tices. In the three-dimensional calculation, it was concluded
that when the cylinder diameter is greater than 3 mm, the
EOC problem can be greatly improved. Another parameter
in concern was the gap distance between the cylinder and
the steel strip. When the gap distance is about 5 mm, the
EOC performance of the proposed method is most suitable.
Finally, the EOC performance of the cylinder installation
method was compared with the conventional method of in-
stalling edge baffle plates near both edges of the steel strip.
Computed film thickness distributions reveal that the cylin-
der installation to the lower lip of the gas wiping jet exit
shows much better EOC performance than the edge baffle
plate method.
Acknowledgements
This work was supported by the Brain Korea 21 Project
in 2005.
Nomenclature
d:Cylinder diameter
g:Distance between cylinder and steel strip
P
d
:Molten zinc pressure at y
d
T:Period of a cyclic change
v:Absolute molten zinc velocity component
Vs:Steel strip moving velocity
x,y,z:Cartesian coordinate
Greek symbols
d
:Molten zinc thickness
t
q
:Molten zinc shear stress at y
d
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ISIJ International, Vol. 46 (2006), No. 4
©2006 ISIJ 578
Fig. 10. Comparisons of mean pressure and coating thickness
distributions between cylinder and edge baffle plate.
... The excess air jets from two opposing air knives collide with each other and create turbulence. This turbulence in the excess air region alters the wiping pressure near the edges of the steel strip, which results in an uneven coating [7][8][9]. As the zinc coating thickness depends on the wiping air jet parameters, investigating the wiping air flow parameters on the steel strip surface is inevitable to know the cause of coating defects. ...
... In the most air knife designs, air knives are positioned horizontally opposite each other [9][10][11][12]. In one of the studies of EOC with a similar air knife arrangement, Kim et al. [7] spotted the formation of alternate vortices in a region where excess air collides. ...
... They observed mean pressure distribution near strip edges for various nozzle-strip distances and stated that EOC is caused by a pressure drop after the formation of alternate vortices. In a similar study, Ahn and Chung [9] showed with velocity contours that alternate clockwise and anti-clockwise vortices are generated in excess air regions where air jets collide at an angle slightly less than 180°. They used an analytical model from Kim et al. [7] to predict the zinc film thickness. ...
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In a continuous hot-dip galvanizing process, the opposite air jets from the air knife nozzles collide with each other in the region near the steel strip edges, causing complex 3D von Kármán vortices. The vortices alter the pressure gradient of wiping air jet over the steel strip edge surfaces, causing uneven coating thickness. Here, the effects of the excess air vortices on zinc coating distribution at the edges are investigated with 3D eddy structures in the region of collision using detached eddy simulation (DES). The wiping pressure and shear stresses, obtained numerically, were used as boundary conditions in an analytical model to predict the stagnation line – a bottleneck region in the zinc wiping process. In comparison with experiments, the proposed analytical model successfully computes the quantity of zinc dragged upward by wiping air and the transient variations in coating values near steel strip edges.
... Additionally, they are used to provide humidity to foods in the food sector [1][2][3]. Air knives are commonly used in the continuous hot dip galvanizing procedure, a coating technique in the steel industry [4][5][6][7][8][9][10][11]. The paper industry is one with effective use of air knives in the thermal paper production process where a very thin layer of chemicals is spread on paper [12]. ...
... Researchers made a variety of recommendations to prevent errors and control the coating thickness for zinc-coated surfaces in continuous hot dip galvanizing processes [4,6,8,10]. So et al. [4], analyzed surface flaws (which called sag lines) on steel sheet surfaces and developed a new model to predict the variation in coating thickness. ...
... They performed 3-D CFD simulations. Another study by Ahn and Chung [6], recommended a configuration to prevent flaws on the edges of steel strips, edge overcoating (EOC) by simulating a 3-D unstable compressible turbulent flow area. Inaccurately determined air-knife parameters caused bouncing off the strip edge with negative effect on performance of the galvanizing process and product quality. ...
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Air knives with very different nominal values are used for various purposes in the industry. The uses of air knives include the painting and coating stages of automobile semi-finished products, as well as many stages related to food production. One of the important sectors among these is the continuous hot-dip galvanizing process, which is a coating technique in the steel industry. In addition, the chemical coating process on paper as a very thin layer is another research topic. The most critical process needed in these sectors is to ensure a homogeneous air-flow at the expected air speed of the air-knife. These determine whether the coating on the steel or the chemical layer on the paper is homogeneous at the desired thickness. In this study, it is aimed that the air-knife can reach the expected values (speed and homogeneous distribution) with the expected tolerances, the shortest time, cost, and the least production process error. First, a design has been made so that this knife can blow air at the desired speed and homogeneously. For this, the most appropriate modeling and design values were created and analyzed with the CFD. The analysis and evaluations of the design were confirmed as a result of the measurements made on the prototype. This study shows that the inclusion of this type of modeling and analysis in the rapid product development process has an important role in minimizing cost and time.
... However, increasing the pressure can cause some industrial difficulties such as higher tonal noise generation, 6-9 splashing 6,10,11 and coating nonuniformity. [12][13][14][15] Moreover, zinc coating quality is an impor-tant industrial issue, especially in the automobile industry, which requires exposed (i.e., parts exposed directly to the consumer view-e.g., closures such as hoods and door panels) sheet steels to have a defectfree, uniform coating and excellent corrosion resistance. ...
... It has been found that EOC is caused by alternating vortices shedding from the jets during gas jet wiping process. 14 It was found that the vortices caused a gradual decrease in the surface pressure approaching the strip edge. Such reductions in the average surface pressure near the edge region Fig. 1: Schematic of the conventional single-slot gas jet wiping process for coating control weakened the gas wiping force, which was shown to be a major contributor to edge over-coating. ...
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... Delphineet et al. [6] simulated the flow of air flow and considered the flow of liquid and gas at the same time. Aha and Chung [7] focused on the study of the flow field in the edge of the strip and discussed the reason of excessive zinc in the strip. Gosset and Buchlin [8] analyzed the effect of gas nozzle shape on gas flow field and discussed the splash behavior. ...
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Air knife is the key equipment to determine zinc coating thickness in the hot-dip galvanizing process. The process parameters related to air knife directly determine whether the desired coating thickness can be obtained. Therefore, setting the optimal process parameters is the key step to improve hot dip galvanizing process capability and zinc coating quality. Previous research on zinc layer thickness was based on the principle of physics and simulation to build models and few studies were carried about the interaction between parameters of air knife. In this research, three process parameters (stripe velocity, air pressure and air knife-stripe distance) were considered to optimize coating thickness by Response Surface Method (RSM), Taguchi method and Genetic Algorithm (GA). This paper firstly used response surface method (RSM) to establish polynomial model and discuss interaction effect between air pressure and the range of air knife. Then, this paper used Taguchi method to do the robustness analysis of air knife and validate the polynomial model from response surface method. Finally, this paper built suitable a fitness function and then uses genetic algorithm to find the optimal combination of parameters for a given thickness of zinc layer. This proposed integrated method is a good try to combine RSM, Taguchi method and GA.
... Gosset [11] analyzed the influence of nozzle shape of air knife on its flow field and simulated flow field in the cavity of air knife, but they failed to design the outlet. Ahn [12] studied the influence of internal guide plates on the flow field of air knife and conducted a simulation analysis on the two-dimensional flow field of air knife which was greatly different from the inner flow field distribution of actual three-dimensional air knife structure. Farkas [6] analyzed the flow field of air knife of glass washers, and he thought that Dumur air knife adopted a rectangular cross-section and produced a large airflow vortex, which hindered airflow and thus resulted in energy loss and the vibration of air knife. ...
Article
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This paper conducted a parametric modeling for air knife structure in a printing factory, used HYPERMESH software to divide the meshes of air model and combined with actual conditions to define various boundary conditions in the inner flow field of air knife. Meanwhile, this paper adopted fluid dynamics software Fluent to conduct numerical simulation for the internal airflow of air knife, obtained the distribution regulation of flow field, conducted a parametric modeling for air knife structure under many internal structural proposals through ANSYS design module based on the simulation computational result, conducted optimization design for the position of guide plates, the number of outlets and the size of return air tank in the detailed structure in the air knife in order to determine specific dimension parameters and optimal proposals. Based on the computational results of simulation, this paper found that the original air knife structure had a non-uniform flow field and low velocity at the inlet and outlets. With the increase of length of air knife, the velocity of the middle outlet reduced to zero and did not have obvious effects any more. Guide plates in the air knife had a great influence on the inner flow field of air knife. Through optimization design, the inner flow field of air knife became uniform when there was only one guide plate. When the guide plate was close to the front end of the air knife, the inner flow field of air knife was relatively uniform and velocity at the inlet and outlets was relatively high. This paper conducted a model design for air knives with different structural types and determined proposal 4 as the optimal design through repeated analysis. The design method in this paper could provide guidance for studying and designing air knife structures in the aspect of technological approach and theory.
... This software can be used to simulate and analyze fluid flow and heat transfer in areas with complex geometries. Fluent can also simulate sophisticated flow fields ranging from incompressible flow to moderately and highly compressible flow by using a variety of methods, such as multi-grid acceleration and convergence technologies, to achieve optimal convergence accuracy [7] . The grid-based 2D model established through a numerical simulation is a quad mesh with meshes refined at the air knife outlet near the wall with a total of approximately 200,000 meshes. ...
Chapter
Much of the productivity, and a great deal of the profitability, of a hot dip galvanizing line is governed by the performance of the coating thickness control step. The customer demands a coating weight at least equal to the amount specified, the producer seeks to minimize cost by providing this coating weight, but as little as possible in excess of it, because doing so consumes excess zinc, the largest consumable cost of a galvanizing operation. This chapter begins by describing the liquid metal properties influencing coating weight, namely density, viscosity and surface tension. A short history of coating weight controls is then given. The equations governing the gas wiping process are derived from the Navier-Stokes equation and solved to show the effects of process variables and liquid metal properties on the predicted coating weight. Sources of instability are then described, including strip shape, vibration and gas jet instabilities. Coating analysis and control techniques are described, followed by consideration of nitrogen wiping and other coating regulation techniques. Standard specifications for coating weight are then described.
Article
In hot-dip galvanizing process, air jet wiping control is so crucial to determine the coating thickness and uniformity of the zinc layer on the steel strip. A numerical simulation of gas-jet wiping in hot-dip galvanizing was conducted to minimize the occurrence of edge over coating (EOC). The causes of EOC were identified by contrasting and analyzing the airflow fields on the strip edge with and without a baffle. The factors influencing the airflow field on the strip edge during the change in the gap between the baffle and the strip edge were also analyzed. The effect of the distance between the air knife and the strip was evaluated. Technological parameters with on-site guidance role were obtained by combining them with the actual production to elucidate the role of the baffle in restraining the occurrence of EOC. The uniform distribution of pressure and coating thickness on the strip is achieved when the distance of the baffle from the strip edge is about 0.3 times of the jetting distance.
Article
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The edge over-coating (EOC) developed near the edge of a galvanized steel strip is numerically analyzed. The 3-D flow field with alternating vortices in the region away from the strip edge is investigated and the distributions of mean impinging pressure and shear stress on the strip surface are obtained by using a commercial 3-D flow analysis code, STAR-CD. It was found that the appearance of alternating vortices causes the surface pressure to decrease gradually approaching the strip edge. The coating thickness is calculated by one integral analysis method of the boundary layer momentum equation. A sample calculation of a benchmark experiment on EOC problem shows that the present analysis method yields a reasonably accurate prediction of EOC for a given plant operating condition. And the result demonstrates theoretically that EOC can be effectively remedied by adjusting the distance of the 2-D air-knife nozzle and by installing a baffle plate parallel to the end of the steel strip.
Article
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The feasibility of using a commercial CFD code for large eddy simulation (LES) is investigated. A first test on homogeneous turbulence decay allows a fine-tuning of the eddy viscosity with respect to the numerical features of the code. Then, a flow over forward–backward facing step at Reynolds number Reh=1.7×105 is computed. The results found show good agreement with the new LDA data of Leclercq et al. [Forward backward facing step pair: aerodynamic flow, wall pressure and acoustic characterization. AIAA-2001-2249]. The acoustic source term, recorded from the LES and to be fed into a following acoustic propagation simulation, is found to be largest in the separation from the forward step. The source terms structures are similar to the vortical structures generated at the front edge of the obstacle and advected downstream. Structures generated from the backward step rapidly break down into smaller scale structures due to the background turbulence.
Article
In order to clarify the mechanism of edge over coating (EOC) for continuous hot-dip galvanizing, a visualization test of the gas flow on strip and a cold model test to measure the profile of the coating thickness at the strip edge were carried out. Outward deflected gas flow was observed at the strip edge and EOC developed in the absence of gas wiping. With gas wiping, EOC developing below the wiping position is reduced by the impinging pressure of the gas wiping jet, and the film thickness becomes approximately uniform at the gas wiping position. However, upward of the gas wiping position, EOC increases again and the outward deflected gas flow on the strip edge sweeps the liquid film to the strip edge. EOC is considered to develop at the location where the dynamic pressure of the outward deflected gas flow balances with the surface tension. For the prevention of EOC, edge masking was devised and the effects which reduce EOC were measured in the cold model test and on a commercial line test. The edge mask which can be kept farther away from the strip edge is more effective for preventing EOC than the edge plates. The optimum dimension of the edge mask is 30 mm in width and 75-100 mm in depth, and installing it at 4-10 mm away from the strip edge is most effective. It was confirmed by the commercial line test that the edge mask can reduce EOC from 45% to less than 10%.
Article
This paper presents a new analysis of the jet stripping process, as used to control coating thicknesses in the paper, photographic and galvanizing industries, and demonstrates that the inclusion of a surface shear stress term, acting in conjunction with the pressure gradient on the coating, gives theoretical predictions of coating behavior quite different from those based on stripping which allow only for pressure gradient effect. Measurements of coating mass, taken from galvanizing line trials, have shown good agreement with the revised theory.
Article
The results of a Large-Eddy Simulations (LES) of a downward hot wall-jet injected against a cold upward channel flow are presented. Based on the experiment of He et al. (2002) [Int. J. Heat Fluid Flow 23 (2002) 487], this flow was suggested as an “application challenge” by the power generation industrial sector to the Qnet-CFD EU network. Indeed, numerical predictions vary significantly with the type of RANS model used, with only the most advanced models yielding reasonable agreement with the experiment as presented in a companion paper by Craft et al. [Int. J. Heat Fluid Flow 25 (2004), this issue]. The present LES was attempted to hopefully confirm and complete the experimental data, which in some areas can be sparse. As resources limited the LES to 1/2 million nodes, an optimal LES mesh was defined from RANS derived scales. Then to reduce uncertainties, two independent codes are used to perform the simulations: the commercial code Star-CD and an industrial one, Code_Saturne. The statistical quantities compared with the experimental data show that both codes are able to return fairly satisfactory results for the isothermal and moderately buoyant cases. For the third and strongly buoyant case comparison was only qualitative.
Article
A two-dimensional underexpanded jet blowing tangentially over a cylindrical surface was studied experimentally and reported in a previous paper. The curved jet has a shock cell structure similar to that of a plane jet, but with a separated region on the curved surface. This region grows with increasing upstream blowing pressure until reattachment fails to take place and the jet breaks away from the surface. This paper reports progress in calculating the jet structure so that predictions may be made of the jet development and of breakaway conditions. The inviscid core of the jet was calculated by the method of characteristics: the outer shear layer and surface boundary layer were ignored. When compared with experiments, this gave good predictions for the structure of the first shock cell for low blowing pressures. However, the neglect of the separated region on the surface caused increasing error in the predictions as blowing pressures increased. The program was modified to replace the wall condition by a specified pressure boundary, the pressures being obtained experimentally. The predictions then agreed well with experimentally observed flow patterns for the first one or two shock cells. After that the growth of the shear layer encroaches into the core so that the shock cell structure disappears. Although of wide application, this work is particularly related to the design of Coanda flares where the jet is axisymmetric. As well as extension of the method to axisymmetric geometry, further work is required to develop methods to predict the outer shear layer and the separated region. This would enable predictions of complete jet development and breakaway conditions to be made.
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