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FIELD OF APPLICATION
The present invention generally relates to an improved lance with a central
subsonic nozzle for LD Steelmaking. In particular the invention relates to a
seven hole tance design with a cental separately-controllable subsonic nozzle.
BACKGROUND OF THE INVENTION
Steel is produced through many processes such as basic oxygen furnace (BOF)
process, electric arc furnace (EAF) process, Kaldo process, etc. Of these, the
bast oxygen furnace (BOF) or LD steelmaking process is widely used in the
world presently due to the effectiveness of the process and the quality of the
steal produced. LD steelmaking process is a purification process of Iiquk pig iron
that contains, along with very high percentage of iron, carbon, phosphorus,
magnesium, manganese, aluminium etc as principal impurities. These impurities
are removed by oxidation reactions using gaseous oxygen as the oxidier. The
oxygen gas is introduced into the LD vessel by means of multiple supersonic jets
through a water-cooled lance with a copper head. Further, argon gas is
introduced through tuyres at the bottom of the vessel to stir the liquid metal
thoroughly. This process of blowing oxygen gas from the top through the lance
and injecting argon through the bottom is called combined blowing process.
The refining process within the LD vessel can be summarized in the following
way. Liquid pig iron is charged into the vessel along with metal scraps. These
metal scraps can easily be melted because most of the reactions taking place in

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the LD vessel are exothermic reactions and the whole LD steelmaking process is
an auto-gene ration process, i.e., it requires no external supply of heat. Lime
(CaO), as a flux, is also added according to the required basicity ratio defined as
the gravimetric ratio of lime to silica (CaO/SiO2) and the blowing of oxygen gas
onto the liquid metal is started. The impurities are oxidized and thn oxides,
other than the oxides of carbon, form the liquid slag that floats on top of the
liquid metal. Carbon is oxidised as carbon mono-oxide (CO) gas that pass
through the liquid slag. Due to the, the slag layer swells in volume and forms
what in general, called as 'slag foam', The slag foam comprises of Iiquid slagr
gases evolving from the liquid metal and the liquid metal droplets thrown into
the vessel due to the impact of the oxygen jets on the liquid metal surface. The
foam thus formed, occupies a large volume of the vessel completely covering the
lance head and partly the lance itself. The foam creates a large interfacial area
between the liquid metal and the slag and thereby promotes inter facial eactions
such as dephoshorization.
Since the LD steelmaking process is highly dynamic and the conditions inside the
vessel continuously change during the oxygen blowing period, the control of the
oxygen lance is imperative. So, the oxygen lance is opera bed at different lance
heights to control the intensity of impingements of the supersonic jets. The
lance height is defined as the distance of the lance tip at any instance to the flat
liquid metal surface before the start of the blow. At the start of the blow, the
prine interest of the steelmaker is to form the liquid slag quickly and dissolve the
charged lime completely. It is felt that hard blow or lesser lance height will be
disadvantageous because the oxidation of the carbon is not preferred in this
stage. So, the lance is operated at a higher height, say for example, the initial
lance height is 2.2 m.

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During the initial period, the slag starts forming with the required chemical and
physical properties. Now, it is necessary to create a foamy slag by producing
more CO gas by oxidizing carbon since only the foamy slag can increase the
interfacial area between the slag and metal and thereby promoting the important
reaction of dephoshorization. So, the lance height re reduced to give a hard
blow. The reduced height can be around 1.5m. At this stage, the creation of
metal droplets is also of great importance as far as the dephoshorizatlor reaction
is concerned. Mostly, the lance is operated in this shorter height for most of the
blow to promote the oxidation of carbon.
During the last stages of the blow, the carbon percentage in the steel is very low
and the generation of CO gas is reduced to a great extent. The slag is no longer
foamy because of the absence of the generation of CO gas and it is understood
that a thick liquid slag layer is formed on top of the metal surface. The hard
blow end the creation of liquid metal droplets in this stage are not preferred due
to simillar reasons mentioned in the earlier stages of the blow. So, the lance
height is increased again to the initial lance height to give a softer blow.
From the above discussions, it is clear that the physical requirements of the
lance changes completely during the blow into the LD vessel. At some stages of
the blow, droplet generation is of prime importance and at some othe' stages,
the liquid metal droplet generation can be disadvantageous and detrimental to
the operation of LD vessel. It is clear that the lance plays a much greater role
than the simple supplier of oxygen gas into the vessel. Proper design of the
lame and control during the blow can greatly improve the efficiency of the
steelmaking process and enhance the quality of steel produced thereby.

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The lance is made up of copper and has a detachable head where the nozzles
are fixed. The oxygen is blown into the vessel with supersonic velocites in the
range of mach number 2.0-2.4 through the nozzles. The number of supersonic
nozzles in the lance is decided based on the size of the vessel, mass of the
charge and the other operating conditions. A typical lance can have 6 supersonic
nozzles with an angle of inclination from the vertical axis of 17.5o to minimize jet
coalescence. The nozzles are designed to produce the supersonic jets with the
exit mach number of 2.2. All the nozzles have a single supply of oxygen at the
pressure of 13.5 bar. The employed lance is water-cooled to protect it from the
hol temperatures within the LD vessels.
A need was felt to improve the dephosphorization within the LD vessel. As
already stated, the lance design and control during the blow wil have a
substantial effect on the steel making process and on improving the quality of
steel produced.
SUMMARY OF THE INVENTION
One object of the present invention is to improve the liquid metal droplet
generation to increase the slag-metal interfaciaI area for improving the
dephosphorisation within the LD vessel Since dephoshorization is essentially an
interfecial reaction between the slag and metal, increasing the metal droplets
would enhance the dephoshorization efficiency. In the present invention
therefore, an effort has been made to improve the droplet generation in LD
vessel. The metal droplets formation is essentially the function of the lance.
Thus, in order to improve the generation oF metal droplets, the function of the
oxygen jets have to be considered carefully under steelmaking conditions or very
close to such conditions.

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It has been found that the provision of a central hole in the oxygen lance creates
a lot of metal droplets and causes spitting. Spitting is disadvantageous because
it might cause the blockage of the vessel mouth and further reduces the life of
the lance and the vessel lining. So, though the central note can produce a lot of
droplets, it has disadvantages also.
Apart from improving the droplet generation, the central hole has a further
advantage that was hitherto unknown in steel industry. The effect of high
density slag foam on the supersonic jet characteristics in the LD vessel was
considered. It has been round that the slag foam absorbs all the momentum
supplied by the oxygen jets and the jets loose the momentum completely to the
slag. Thus the existing knowledge on the supersonic oxygen jet characteristics
within LD vessel can be considered to be wrong. Although the droplet
generation studies done using hydrodynamic models of 10 vessels will not reveal
the true mechanisms of droplet production within the LD vessel, they provide the
basis for improved understanding of the droplet formation. Since the peripheral
jets are exposed to the slag foam, they are expected to loose all the momentum
to the slag layer through the jet-slag Foam interface. Because the gas jets do
not have sufficient momentum when they reach the molten metal surface, they
cannot produce metal droplets as needed,
However, as the present reasoning suggests, the central jet will be covered by
very little or no stag foam as compared to the peripheral jets. The reasons for
this is that the peripheral jets will cover the central jet and make a protective
cover to the central jet from the high density slag foam. Further, there is a
positive pressure due to the presence of the central jet and this will also push
away the little entrainments of stag foam into the space amongst the peripheral
jets, This means that the central jet will not loose its momentum to the slag
layer and will each the metal surface with concentrated momentum, i.e., with
very high velocity that will tear the metal surface to produce the much needed
metal droplets for improving the dephoshorization,

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Thus, it is clear that having a central jet will be advantageous in augmenting the
metal droplet production that can improve the rate of dephoshorization
As explained earlier, when the foamy slag is absent during the initial and final
stages of the blow, the central jet would cause enormous spitting, i.e , ejection
of liquid metal through the vessel mouth. So, it is not advisable to have a very
strong blow through the central hole during all the phases of the LD steelmaking
process. Spitting or strong metal droplets generation during the initial and final
phases of the blow will damage the lance since there is no protectior from the
slag foam. It is expected that the presence of the slag foam slows down the
metal droplets and protects the lance and vessel refractory from the Impact of
the metal droplets. It is clear from the above arguments that having a strong
blow through the central hole is disadvantageous during these two stages of the
blow.
If the nozzle is operated at a lesser pressure ratio than the design pressure ratio
or in other words if the nozzle is underblown to reduce the flow rate to avoid
spitting during the Initial stage, shocks or strong discontinuities in pressure,
velocity, temperature and density of the gas can occur within the diverging
section of the nozzle itself. Such shocks formed at the diverging section can
severely effect the performance of the supersonic nozzle and will reduce the life
of the nozzle considerably. Moreover, under steelmaking conditions, such a
shock formed inside with diverging section of the nozzle, can suck the high
temperature slag foam and metal droplets into the nozzle and severe erosion and
failure of the lance can occur. It is clear mat it is not possible to have a great
degree of control of flow rates, as needed in the different phases of the LD
steelmaking process, through the supersonic nozzle.

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Because of the above mentioned considerations, it is necessary to have a
subsonic nozzle, i.e., nozzle with only a converging section, through which it is
easy to control the flow rate and a wide range of flow rates can also be achieved
by changing the supply pressure. Furthermore, the problem of shock formation
is not there with a nozzle providing subsonic velocities. The explanations also
make clear that it is not possible to control the flow rate through the central hole
alone if all the nozzle have the same gas supply line. As explained earlier, the
droplet generation needs to be augmented only during the middle duration of
the blow and a lot of droplet generation during the initial and final stag as of the
blow is not preferred. For such a lance operation, a control of the flow rate
through the central note Is required and as said above, it is not possible with the
same oxygen gas supply for all the nozzles. So, in the present intention a
separate and controllable gas supply has been provided for the central hole. All
the other six peripheral supersonic nozzles can share the high-pressure gas
supply.
Thus the present invention provides an improved lance for LD steelmaking,
comprising a plurality of peripheral supersonic jets; and a central subsonic jet;
wherein said plurality of peripheral subsonic jets are provided with a single inlet
gas supply line, and said central subsonic jet is provided with a separate gas
supply line; flow rate through said separate gas supply line for said central
subsonic jet being controllable, so that the volume flow rate through said
separate gas supply line is easily variable without sacrificing nozzle life.

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BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described referring to the figures of the drawing where
Figure 1 shows the schematic arrangement of a 6-hole lance design .
Figure 2 shows a typical geometric arrangement of a supersonic nozzle.
Figure 3 shows a schematic sketch of an LD Model vessel.
Figure 4 shows a schematic sketch of the 7 hole lance with separate air
supply Iine used in the hydrodynamic model experiments.
Figures 5(a)
and (b) are photographs showing extent of droplet generation in the
case of an existing 6 nozzle lance and a 7 hole lance of the
present invention.
Figure 6 shows a schematic representation of the droplet
generation mechanism.
Figure 7 shows the droplet generation rates with 7-hole lance with
different flow ratios,
Figure 8 shows the computational model and the mesh used
for numerical simulation.

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Figure 9 gives a closer look of the nozzles.
Figure 10 shows the velocity contours for 7- hole lance
with angle of inclination of 17.5o with the
presence of vessel walls end metal surface.
Figure 11 shows the temperature contours with shock at the
nozzles with 17.5° angle inclination angle of 17.5°.
Figure 12 shows velocity contours showing the impact
position on the metal surface.
Figure 13 shows velocity contours at different axial locations (a) X =0.5m
(b) X = 1.0 m (c) X = 1.5 m and (d) X = 2.0 m.
Figure 14 shows schematic diagram of the domain with boundary
conditions used for the high density ambient simulation.
Figure 15 shows mixture density contours near the nozzle exit
at an instant of time.
Figure 16 shows momentum flux rates profiles at different axial
locations (a) Nozzle exit; (b) 0.5 m; (c) 1.5 m and (d) 2.5 rr .
Figure 17 shows schematic representation of a 7-hole lance design.

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Since it has been found that the central hole will augment the droplet
genaration, the droplet generation mechanisms were studied through
hycrodynamic model experiments on a 1:6 scale down model with the central
hole as shown in the schematic sketch of figure 3, A 1:6 reduced scale model of
the LD vessel made with plexiglas is used. The reduced scale models of the
exiting and the proposed lance designs were made in order to study the
advantages of the central hole in augmenting droplet generation.
The top part of the vessel is made of stainless steel where the cylindrical and the
vessel bottom portions are made of plexiglas to have required transpaency for
visualization of the experiments. The lance is made up of copper with the facility
of putting different design of lance tips for investigation.
The scaled down lance was designed having six peripheral nozzles with a central
nozzle as shown in Figure 4. There are two separate air lines, line 1 is
connected to all the six outer peripheral nozzles whereas line 2 is corrected to
the central nozzle. The flow rate through the central hole 1 was controlled
separately by means of a set of pressure regulator and air flow rotameter
connected in series whereas the flow rate through the six peripheral nozzles was
controlled through another set of pressure regulator and air flow rotameter. The
inclination of the peripheral nozzles to the central axis were investigated at 17.5°
(as existing in practice) and 22° as well, by using two different lance tips 3.
The droplet generation mechanisms were investigated when all the 7-holes were
in operation and comparisons were made with only the six peripheral nozzles in
operation. In Figures 5 (a) and 5(b), the intensity of droplet generation s shown
for the cases of the blow through only the pheripheral nozzles and the blowing
through all the 7-holes respective. It can be seen visually that the extent of
droplet generation is much higher with the central hole in operation in
conjunction with the peripheral nozzles than that of only with the peripheral
nozzles.

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It was observed during the experiments that there is a critical flow rate after
which onset of the droplet generation starts. The mechanism for the accelerated
rate of droplet generation, due to the presence of the central jet, is Explained
schematically is Figure 6. The center jet impacts the liquid metel vertically and
creates the central strong depression of the liquid surface. The depression thus
formed is wavy in nature and provides 'lips out of the central water paddle' as
shown schematically in Figure 6. The water lips thus formed around the paddle
are then torn apart by the side jets and yield an improved droplet production.
These side jets also were thought to prevent the slag foam to enter the centre
space amongst the peripheral jets in the actual vessel and therefore ensure that
the central jet with its high momentum reaches the metal bath surface and
permits the droplet production similar to the one schematically sketched in
Figure 6.
The quantification of the droplet generation was studied in order to understand
the optimum flow rate to be given through the central nozzle to maximize the
droplet generation rate. The droplet generation rate is measured by putting a
collecting pan having dimension 400 x 100 x 50 mm3 and the measurements
were carried out for the existing 6-nozzle lance and the new 7-hole lance with a
central nozzle. The dimension of the pan was decided to measure the effective
droplet generation surrounding a single nozzle of the 6 peripheral nozzles. The
rate of droplet generation is expressed in terms of the rate of mass of droplets
collected (g/sec) on the pan.
The rate of droplet generation is studied for various flow rates though the
central nozzle for selecting the optimum Row rate through the central nozzle to
maximize the droplet generation, A flow rate ratio, X is defined as the ratio flow
rates through the central hole to that of one of the peripheral nozzles.

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The rate of droplet generation plotted against the flow rate ratio is shown in
Figure 7, The flow rate through the central nozzle was varied from a flow rate
ratio of as low as 25 % to as high as 125 %.
The optimum flow rate through the central nozzle is obtained by maintaining the
balance between an improved droplet generation and control of splashing and
spitting due to bath spilling out of the mouth of the vessel. It was quite
apoarent that as the flow rate through the center hole was increased
progressively, the rate of droplet generation was enhanced. Figure 7 shows that
for a flow rate ratio, X of 1 (100 %) given through the central nozzle, the droplet
generation almost doubles and reaches a maximum value. Beyond this flow
rate, there was vigorous splashing and spitting out of the mouth of the LD vessel
water model which is detrimental to the operation of LD vessel. Thus from the
hydrodynamic model experiments, the optimum flow rate ratio, X though the
central hole is decided that maximizes the droplet generation rate but without
splitting and splashing cut of the vessel.
Numerical simulations were performed using the commercial computational fluid
dynamics software, FLUENT to study the characteristics of the jets com ng out of
the 7-hole lance as explained earlier. The angle of inclination of the peripheral
jets was chosen to be 17.5° as the initial value and it is the same as that in
existing 6-hole lance designs. A centre subsonic nozzle was added to carry out
the jet flow predictions for the reasons discussed earlier.

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To reduce the computational time of the numerical simulations for the new lance
design, only half of the total flow domain was simulated by splitting lite whole
domain using the vertical mid-plane of the vessel. So, two complete supersonic
jets and two half supersonic jets were numerically simulated. The central
subsonic jet was also simulated as a half jet. The dimensions of the supersonic
nozzles were kept as the old dimensions, i.e., as the dimensions of the nozzles of
the existing lance, whereas the exit diameter of the central subsonic nozzle was
XXXX to be 54 mm.
To accommodate the bigger central nozzle, the lance pipe diameter had to be
increased by 100 mm compared to the existing lance dimensions. The volume
flow rate through the central subsonic nozzle was kept almost the same as that
of one of the peripheral supersonic jets. This means that mass flow rate through
one of the supersonic nozzles in the periphery when compared to the central
subsonic nozzle is different. This is due to the fact that because of the
supersonic Flow in the outer nozzles, the nozzle exit temperature fail down to
150 K. Due to this, the density of the gas at the exit of the supersonic nozzles
becomes much higher, given that the pressure is almost uniform everywhere in
the vessel. For the subsonic central jet, such low temperatures at the rozzle exit
do not reach.
Since it is intended to vary the flow through the central subsonic nozzle during
the blow, the ratio of flow rate through the subsonic nozzle to that through one
supersonic nozzle is kept as a variable. To keep the numerical efforts small, it
was decided to study the flow induced by the jets for only two volume flow rate
ratios. These were chosen to be 1.0 and 0.5, The results of the simulations are
given below for volume flow rate ratio of 1.0.

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In Figures 8 and 9, the computational model and the mesh user for the
numerical simulation of the 7-hole lance design suggested above are shown.
More than 1.3 million grid nodes were used in the simulations of the jet flows.
The simulations were performed with standard k-e model. 12 processors of a
one tera-flops Linux cluster were used for simulation and it took around 72-80 to
complete one flow simulation. It is well-known that k-s turbulence model
predicts the flow features of the multiple jets with some deviations from the real
flow but the deviations are not large. However, it is easy to get reasonable
solutions quickly with k-s model with short computational time. For this reason,
this model was used.
In Figure 10, the velocity contours in the symmetry plane, for the case of 7-hole
lance are shown in the presence of vessel walls and metal surface, for the
peripheral nozzles' angle of inclination of 17.5°. in the numerical simuation, the
metal surface was assumed to be a stress free horizontal layer. It is seen from
Figure 10 that the jets follow their geometrical path closely and the interaction
between them is small. It can be seen from Figure 10 that the jets interact only
in the middle elevations. There is only little interaction of the jets closer to the
metal surface. This is due to the central stagnation zone at the metal surface.
The higher stagnation pressure in this region pushes the jets away and reduces
the coalescence.
In Figure 11, the shocks formed at the nozzle tips of the 7-hole lance design are
shown by the temperature contours. It can be seen that there are smaller
stocks at the subsonic nozzle outlet also. This is because of differences in
temperature between the ambient and the nozzle outlet and the smaller
differences in pressure. This can be reduced by increasing the angle of the
convergent section of the nozzle. For the present simulations, the angle is kept
at 10°.

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In Figure 12, the velocity contours are plotted at the symmetry plane to show
the impact positions of the jets on the metal surface. The geometrical
projections of the jets are also shown on the liquid metal surface by dark circles.
It can be seen that the jets almost follow the geometrical path and the
coalescence is minimal due to the presence of the central jet and the bottom
stagnation region. In Figure 12, the velocity contours are shown only for the
velocity magnitudes less than 150m/s. It can be observed that the supersonic
jets and the central subsonic jet reach the liquid metal bath with almost the
same velocity magnitudes although the exit velocities at the respective nozzles
were different. Since the subsonic nozzle exit diameter is higher (54 mm) than
the supersonic nozzle exit diameter (37.3 mm), the velocities, closer to the metal
bath are matched.
In Figure 13, the velocity contours are plotted at different axial distances from
the nozzle tip for the 7-hole lance. It tan be seen from Figure 13 that up to the
axial distance of 1 m, the interactions between the jets are minimal. At 1.5 m
distance, there is considerable interaction amongst the jets. But the bottom
stagnation region pushes the jets away and the coalescence is reduced at 2 m.
The streaks shown in Figure 13 (d) are due to the presence of the central jet.
The gas in the central jet has to pass through the surrounding supersonic jets
since it cannot pass through the metal surface (in the simulation). This kind of
flow feature may not happen in the actual vessel because in the simulation, the
metal surface is assumed to be a stress free flat wall. In LD vessel, the impact
of the centra! jet will create a depression, which will change the flow
characteristics completely.

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In order to explain the effect of slag foam on the jet characteristics the single jet
results are discussed here. The likely range of ambient density (foam/emulsion)
values possible in the LD vessel has been calculated by assuming uniform
decarburization rate throughout the blow. It turns out that the average slag
volume fraction in the foam inside the vessel will be in the range of 12-15%
This would result in an average ambient density range of 360-450 Kg/m.
The numerical domain and the boundary conditions used are shown in Figure 14.
The vessel diameter required for a single axisymmetric nozzle has been
calculated by using 1/6 of the original vessel cross-sectional area (because out of
6 nozzles, only one is being simulated). Furthermore, the liquid metal surface has
been assumed to be a shear free flat wall. The lance height (distance between
nozile tip to the liquid metal surface) is taken to be 3.5 m, in order to study the
behaviour of the jet over a long axial distance. The actual fance height in the
vessel varies from 1.5-2.2 m,
The simulations have been carried out using a 2D axisymmetric unsteady RANS
with volume of fluid (VOF) multiphase model to track the interface between the
phases. No differentiation has been made between oxygen and carbon mono-
oxide gas. Hence only one gas phase has been considered. The Realizable k-s
turbulence model is used to close the system of equations. PISO algorithm has
been used for pressure - velocity coupling. Second order upwind discrotization
scheme has been used for all the flow variables except temperature for which
power law scheme is used. The average slag volume fraction (15 %), ccmputed
form the steady decarburisation rate, is patched in the vessel domain as an initial
guess. During the computation, the slag is free to move throughout the domain
depending on the local flow conditions unlike the earlier simulation. The surface
tension forces have also not been included in this simulation when entering into
a still ambient, the gas jet with high velocity invokes flow in the ambient also.

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Due to the momentum transferred tg the ambient, the ambient fluid adjacent to
the jet boundary starts moving in the predominant flow direction of the jet fluid.
So, the ambient fluid in the neighboring locations moves towards the je : because
of this flow induced by the jet. Slag along with the ambient gas rushes towards
the jet boundary owing to the flow induced by the jet. Here, slag accumulates
and the volume fraction/focal density increases. The momentum transferred
from the jet imposes movement to the slag and slowly, the stag covers the high-
speed jet core. The slag foam density contours near the nozzle tip are plotted in
Figure 15 in order to show the slag accumulation at the nozzle tip and its
movement along the jet.
The resultant momentum flux rate (pV2) at different axial locations is shown in
Figure 16 at a particular instant of time. It is worth noting that the maximum
momentum flux rate does not occur at the axis of the jet but away frorf it in the
radial direction as shown in Figure 16. The high speed core of the jet
continuously pumps momentum to the jet shear layer, both convectively and
diffusively. The velocity at the axis is still the maximum at any axial location.

radial velocity, v will be towards the shear layer within the jet so the net
connective transport of momentum in the radial direction (puv) is also towards
the shear layer. Since the density of the sheaf layer fluid (slag + gas) is very
high compared to the jet gas, the shear layer can store higher momentum fluxes
without increasing the velocity enormously just like storage of thermal energy in
a reservoir with higher specific heat/thermal capacitance without appreciable
temperature differences. Moreover, the gravity is aiding the slag layer to gain
momentum, i.e., the slag layer moves in the direction of the gravitational
acceleration.

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The momentum transferred from the high speed jet core to the high density
shew layer will be in addition to the momentum imparted by the gravitational
accelaratiion. From the momentum flux rates plots shown in Figure. 16, the
momentum flux rate at the high density shear layer is minimum 2 orders of
magnitude higher that the high speed jet core. It is clear from the above
discussions that the high density slag-gas foam present in the LD vessel poses
some interesting flow features of the supersonic gas jets. The understanding of
the depressions created during the blow might be changed completely.
It is important to note that the multiple supersonic jets inside the LD stell vessel
will also be subjected to such characteristics as shown above due to the
presence of high density slag foam. It is clear from the above discussions that
the peripheral supersonic jets will loose all their momentum to the slag layer
adjacent to them. The slag layers will move towards the liquid metal pool with
very high momentum and create complicated depression profiles, But due to the
presence of the central jet in the new 7-hole design, the pressure in side the
space amongst the supersonic jets will prevent the entrainment into this region.
So, the central jet will not see or see minimally the slag foam and urlike the
supersonic jets, it will not loose its momentum completely to the slag foam. So
the centra/ jet will reach the liquid metal surface with very high velocites as
compared to the supersonic jets and is expected to produce more droplets. This
kind of droplet production is not possible with the 6-hole design since all the 6
supersonic jets will completely loose their momentum to the slag larer that
moves relatively slowly. It is clear from the above discussions that the 7 hole
design is more efficient than the 6-hole conventional design.

-20-
The 7-hole design of the present invention is schematically shown in Figure 17.
It shows 6 peripheral supersonic jets with a central jet. The central jet is to be
controlled separately with a separate gas supply line whereas the peripheral
supersonic jets will have a single intet gas supply line. The central jet tan be put
into operation during different stages of the blow and the flow rate can also be
varied since it is a subsonic nozzle.
The flow rate through the central subsonic nozzle is kept as a variable. In the
numerical and experimental simulations, the ratio of the volume flow rate
though the central subsonic nozzle and that of the one of the supersonic nozzles
is kept as a variable. The maximum value of this ratio is kept as 1 in numerical
simulation. The dimensions of the central nozzle are calculated by keeping this
in mind. The outlet diameter of the subsonic nozzle is 54 mm and that of the
supersonic nozzle is 37.3 mm (existing value).
The angle of inclination of the peripheral jets is kept at 17.5° (the existing value).
In order to see the performance of a 7-hole lance with modified angle for the
peripheral jets, a study was carried out for a jet arrangement with the 22° angle
for the inclination of the side jets.
Through the numerical and experimental simulations and also by considering
different dynamics inside the LD steelmaking vessel, the following 7-hole lance
design was arrived at in a preferred embodiment. This design is much superior
to the existing designs and can perform better in steelmaking condition ;.
● 6 peripheral supersonic nozzles with a single gas supply line.
● A central bigger subsonic nozzle with a separate gas supply.
● The central nozzle can be put into operation during different stages of the
blow as required and the flow rate can also be varied easily without
paying the penalty of nozzle life.
● The angle of inclination of the peripheral jets is kept at 17.5°. This angle
can be increased in the further modifications.

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The advantageous features of the present invention provides better on / off
control of the central nozzle during the blow because of the separate gas supply
line. This will give a strong control of the spitting of metal droplets off the
vessal.
The bigger subsonic nozzle at the centre of the lance head is useful to control
the flow rate of oxygen through the central nozzle. This would mean more
flexibility and control over the process.
The system provides increased metal droplet generation. Since the central jet is
protected from the slag foam by the peripheral jets, this would reach the metal
bath with high velocities and promote increased droplet generation.
The improved effectiveness in dephosphorization is also improved. Improved
metal droplet generation will promote the interfacial reactions particularly
dephosphorization.