Note: Descriptions are shown in the official language in which they were submitted.
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LIQUID SPRAYER USING ATOMISING GAS MIXED WITH THE LIQUID IN A
SWIRL CHAMBER
Field of the Invention
The present invention relates to a two-phase sprayer for spraying a liquid
using an atomising gas, in particular to such a sprayer in which the atomising
gas is mixed with the liquid in a swirl chamber.
Background of the Invention
The atomisation of a liquid by an atomising gas is a process that has diverse
applications in many fields of technology. Examples of such applications
include the atomisation of liquid fuel in combustion processes for power
plants, internal combustion engines and gas turbines; in chemical and
pharmaceutical industries; in agriculture (spraying of water and pesticides,
for example); sprayers of paint or other liquid coating in various
industries; to name but a few.
The two-phase fluid produced by such atomising processes essentially
comprises a suspension of liquid particles or droplets in the atomising gas.
One of the basic characteristics required of liquid atomisers, also referred
to
herein as sprayers, is that the size of the liquid particles, obtained by the
dispersion process inherent in the atomisation; be within preset parameters.
For example, in the context of liquid hydrocarbon fuels, small liquid particle
size, say about 40-60 ~,m, for the dispersed liquid ensures substantially
complete combustion of the fuel, leading to high fuel efficiency as well as
low levels of emissions including pollutants.
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One type of two-phase sprayer known in the art consists of an inner tube for
the liquid, and an outer tube concentric with the inner tube, in which the
annular space between the tubes is used for the gas. An axial annular outlet
for the gas flow is concentric with an inner axial outlet for the liquid flow,
and these outlets may be coplanar or axially displaced from one another. As
gas and liquid are discharged from their respective outlets, the shear
pressure
induced by the gas flow at the gas-liquid interface induces an
unstable gas-liquid interaction leading on the liquid flow shears
the liquid and leads to dispersal of the fluid flow into small fluid
particles. In US 4,171,091 an improvement to this type of sprayer is disclosed
in which the outer tube extends substantially beyond the liquid outlet and
further comprises an annular inner wall at an angle of between 70° and
90° to
the sprayer axis downstream of the liquid outlet. As the liquid is discharged
axially from the liquid outlet, it is dispersed by the annular airflow, which
comprises a radial velocity component arising from the change in flow
direction induced by the annular inner wall. However, such devices have poor
atomisation characteristics, and very large gas flows are required to atomise
a
relatively small amount of liquid. Furthermore, such sprayers only provide an
axially-directed spray, and cannot provide 2-phase fluid spray at angles to
the
sprayer axis, necessary for applications such as mixing injectors in burners,
combustion chambers and the like.
In US 3,963,178, a number of vanes are arranged so as to produce
a spirally converging flow of air, which becomes a vortex
surrounding the axial liquid outlet end, shearing the liquid and
spreading the sheared liquid on the vanes, such that the liquid is further
sheared into smaller particles. While providing high liquid dispersion at
relatively low airflow rates, this arrangement does not enable a high gas exit
. . . _ _ __
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55121 W0199 PCT/ILOOI00182
- 3-
velocity to be achieved. Since the liquid is essentially discharged into the
part of the sprayer comprising the vanes, the air flowing therethrough
immediately merges with the liquid transferring to it part of its angular
momentum. Thus, the amount of angular momentum available in the
airflow for dispersing the liquid is reduced, thereby reducing the extent of
dispersion hitherto potentially available. Furthermore, the tangential
velocity component imparted to the airflow by the vanes does not enable
predetermined spray angles to be obtained, particularly for viscous
liquids. Rather, only a limited range of angles is possible, related to the
relative magnitudes of the tangential and axial velocity components of the
airflow and 2-phase flow, respectively, the value of achievable tangential
velocity in such a system being relatively low, as explained above.
USP 3,790,086 discloses an atomizing nozzle in which gas is conically
swirled by means of a swirler along the front face of a nozzle end while
liquid is injected into the gas flow, at a substantial angle with respect to
the gas flow direction, about the outlet from the swirler. The nozzle
includes means for preventing the recombination of the liquid to a liquid
stream along the nozzle surface adjacent to the liquid ejection ports.
It is therefore an aim of the present invention to provide a sprayer device
that overcomes the limitations of prior art two-phase sprayers.
It is another aim of the present invention to provide a sprayer that may be
adapted for widely ranging uses.
It is another aim of the present invention to provide such a sprayer that
may be adapted to be used with liquids of different viscosities.
AMENDED SHEET
14-v~=-200'( - - a ooooo; ~ s?
55121 W0199 PCTIIL00100182
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It is another aim of the present invention to provide such a sprayer that is
relatively simple mechanically and thus economic to produce as well as to
maintain.
It is another aim of the present invention to provide such a device that
incorporates means for providing a predetermined spray angle relative to the
sprayer axis
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It is another aim of the present invention to provide such a device in which
the required liquid pressure is not much greater than ambient pressure.
It is another aim of the present invention to provide such a device in which
cavitation and erosion in the ducts for the liquid flow are substantially
reduced or altogether eliminated.
It is another aim of the present invention to provide such a device having a
longer service life than corresponding prior art devices.
It is another aim of the present invention to provide such a device requiring
lower gas flow rates for liquid atomisation than corresponding prior art
devices.
It is another aim of the present invention to provide such a device for fuel
atomisation applications providing improved ecological characteristics of
combustion products.
The present invention achieves these and other aims by providing a
two-phase sprayer device comprising a liquid feed tube and a gas feed tube.
Each of these tubes is blanked off at its downstream end and provides the
corresponding fluid, liquid and air, respectively, to a swirl chamber, via
corresponding lateral fluid outlets between each respective tube end and the
swirl chamber. The lateral air outlets are located upstream of the liquid
outlets, and are configured to provide a swirl motion to the airflow entering
the swirl chamber. The swirl chamber is configured to maintain the swirl
motion of the gas and comprises a downstream annular throat region into
which liquid is laterally injected via the liquid outlets. The radial extent
of the
swirl chamber is generally greater than that of the annular throat region.
Thus, as the airflow progresses axially downstream in the swirl chamber to
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the annular throat region, the angular velocity of the airflow increases to
conserve angular momentum, and thus the static pressure of the airflow
decreases. At an optimum radius r (typically for the throat region), the
static
pressure of the air flowing in the swirl chamber becomes equal to that of the
ambient fluid (e.g. air, for applications in which the sprayer discharges to
the
atmosphere) into which the liquid is to be dispersed, while the tangential or
angular velocity reaches a maximum value. As a result, the liquid static
pressure required to enable liquid to be inject into the swirl chamber becomes
a minimum under these conditions, and thus the liquid outlets to the swirl
chamber are optimally positioned in the throat region. Furthermore, as the
airflow angular velocity reaches its maximum value, airflow turbulence is at
maximum thereby enabling high atomisation quality or fineness to be
achieved for the liquid. In a further improvement of the sprayer, the liquid
outlets to the throat region are adapted to provide a swirl motion to the
liquid
in the same direction as the swirling air, thereby reducing losses in airflow
angular velocity in the area of interaction with the liquid, thereby improving
the rate of atomisation. The provision of a suitable angled diffuser
downstream of the annular throat region enables predetermined spray angles
for the two-phase fluid to be attained.
The present invention also overcomes specific problems hitherto present in
prior-art centrifugal atomisers used for liquids of high viscosity, such as
for
example high viscosity fuels. In such prior art atomisers, high velocities for
the liquid flow were difficult to attain because of angular momentum losses
in the vortex chamber due to considerable friction forces between the liquid
and the chamber's walls. In the present invention, fine liquid dispersion is
achieved not by imparting high velocities to the liquid, but by making use of
the interaction between a high velocity air jet and the low velocity liquid
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flow. Therefore, viscosity has little influence on dispersion quality, and
thus
the present invention may be used for a range of liquids, particularly fuels,
having widely varying viscosities.
Summary of Invention
The present invention relates to a two-phase sprayer for spraying a liquid
using an atomising gas, comprising: -
an annular swirl chamber having a downstream end comprising a
substantially annular throat region, the throat region having an inner
diameter and an outer diameter;
a gas feed tube adapted for the supply of said atomising gas, said gas
feed tube having a downstream gas feed tube end comprising at least one
lateral gas port in fluid communication with said swirl chamber, said at
least one lateral gas port adapted to impart a tangential velocity
component to gas fed from said gas feed pipe to said swirl chamber at
least within said throat region;
a liquid feed tube adapted for the supply of said liquid to be sprayed
and having a downstream liquid feed tube end comprising at least one
lateral liquid port in fluid communication with said swirl chamber, said at
least one liquid port being in fluid communication with said annular
throat region.
The present invention further comprises a method for producing a
spray of liquid, which comprises the following steps:
feeding a stream of gas;
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imparting to said stream of gas a swirling motion;
concurrently advancing said swirling stream of gas in an axial
direction, said swirling stream of gas having a first annular
cross-sectional area perpendicular to said direction;
constraining said stream of gas to a second annular
cross-sectional area smaller than said first area, whereby to
accelerate said swirling stream of gas both axially and
tangentially; and
injecting a stream of said liquid tangentially into said
accelerated swirling gas stream. whereby to generate the spray of
liquid.
Preferably, the stream of liquid is injected into the swirling gas
stream immediately after the gas stream has been constrained to
said second cross-sectional area and therefore has been
accelerated.
Brief Description of the Drawings
Figure 1 shows, in side elevational cross-sectional view, a first embodiment
of the present invention;
Figure 2 shows, in cross-sectional view, the embodiment of Figure 1 along
A-A;
Figures 3(a) and 3(b) show, in cross-sectional view, two alternative
configurations for the liquid ports of embodiment of Figure 1 along B-B;
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Figure 4 shows, in cross-sectional view, the embodiment of Figure 1 along
C-C;
Figure 5 shows the embodiment of Figure 1 comprising an alternative
configuration of the liquid ports;
Figure 6 shows the embodiment of Figure 1 comprising an alternative
configuration of the liquid ports comprising a liquid swirl chamber;
Figure 7 shows, in cross-sectional view, the embodiment of Figure 6 along
E-E;
Figure 8 shows, in side elevation cross-sectional view, a second embodiment
of the present invention incorporating a diffuser;
Figures 9(a) and 9(b) shows, in side elevation cross-sectional view and in end
view, respectively, the embodiment of Figure 8 incorporating baffles;
Figures 10(a) and 10(b) shows, in side elevation cross-sectional view and in
end view, respectively, an alternative embodiment of the present invention
incorporating a diffuser and baffles; and
Figure 11 shows, in side elevation cross-sectional view, an alternative
embodiment of the present invention, comprising axial alignment adjustment
means.
Detailed Description of Preferred Embodiments
The present invention is defined by the claims, the contents of which are to
be read as included within the disclosure of the specification, and will now
be
described by way of example with reference to the accompanying Figures.
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_g_
The present invention relates to a sprayer device for spraying a two-phase
fluid, typically produced as a result of atomising processes, the two-phase
fluid essentially comprising a suspension of the desired liquid in particle or
droplet form in the atomising gas.
As will be described hereinbelow, the present invention essentially comprises
an annular swirl chamber having at least one upstream lateral gas port
providing fluid communication between the swirl chamber and the
downstream end of a gas feed tube, and at least one lateral liquid port
providing fluid communication between the downstream end of a liquid
supply tube and the swirl chamber. The gas port is adapted such that gas
entering the swirl chamber is swirled around the chamber, eventually meeting
liquid introduced into the swirl chamber radially, or preferably tangentially
in
the same direction as the swirling gas, shearing the liquid and atomising it.
The relative positional term "upstream" and "downstream" respectively refer
to axial directions along and away from the direction of flow of gas or liquid
within the sprayer. The terms "upstream" and "downstream" are respectively
designated (U) and (D) in the figures.
Referring to the figures, Figures 1 to 6 illustrate a first embodiment of the
present invention. The sprayer device, also referred to herein as the sprayer,
designated by the numeral ( 10), comprises a gas feed tube (20), a liquid feed
tube (30) and a swirl chamber (50).
The liquid feed tube (30) is adapted for the supply of the desired liquid to
be
sprayed from any suitable source. The liquid feed tube (30) has a downstream
liquid feed tube end (32) comprising at least one lateral liquid port (35) in
fluid communication with said swirl chamber (50). In the first embodiment,
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the said downstream liquid feed tube end (32) is substantially coaxial with
said swirl chamber (50), though in other embodiments the said downstream
liquid feed tube end (32) may be in any suitable configuration, requiring
simply that it comprises at least one lateral liquid port providing
communication with the swirl chamber. Thus, in other embodiments, the said
downstream liquid feed tube end (32) may comprise, for example, a tube end
having an axis parallel to and radially displaced from the axis of the swirl
chamber (50), having at least one lateral liquid port (35) in fluid
communication with said swirl chamber (50).
The term "lateral" in relation to said at least one lateral liquid port (35)
and to
the at least one lateral gas port (25), described hereinbelow, relates to the
direction of the axes thereof being non-aligned with respect to the axis (
100)
of the swirl chamber (50).
Thus, in the first embodiment, the downstream liquid feed tube end (32)
typically comprises a substantially cylindrical wall (34) having an outer
surface (36) and is blanked off at the downstream end thereof (38).
The swirl chamber (SO) has an upstream end (51), axially bounded by annular
wall (58), having a relatively large axial cross-sectional flow area, and a
downstream open end comprising a substantially annular throat region (52)
having a smaller axial cross-sectional flow area. The throat region (52)
terminates in a downstream annular opening, (54), and the throat region (52)
is defined by an inner diameter db and an outer diameter d~.
The swirl chamber (50) in the first embodiment is radially bounded by at least
a portion of the outer surface (36) of said downstream liquid feed tube end
(32) and by the inner surface (56) of an outer substantially cylindrical swirl
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chamber wall (55) that is substantially concentric with respect to said
downstream liquid feed tube end (32).
The swirl chamber wall (55) typically comprises a downstream portion (53)
corresponding to said annular throat region (52) and having a diameter
corresponding to said outer diameter for said annular throat region. The said
swirl chamber (50) further comprises an upstream portion having a diameter
greater than said outer diameter of said annular throat region (52).
The said gas feed tube (20) is adapted for the supply of the atomising gas
from any suitable source. The gas feed tube (20) has a downstream gas feed
tube end (22) and comprises at least one lateral gas port (25) in fluid
communication with said swirl chamber (50). In the first embodiment, the
said downstream gas feed tube end (22) is substantially coaxial with said
swirl chamber (50), though in other embodiments the said downstream gas
feed tube end (22) may be in any suitable configuration, requiring simply that
it comprises at least one lateral gas port providing communication with the
swirl chamber (50). Thus, in other embodiments, the said downstream gas
feed tube end (22) may comprise, for example, a tube end having an axis
parallel to and radially displaced from the axis of the swirl chamber (50),
having at least one lateral gas port (25) in fluid communication with said
swirl chamber (50).
In the first embodiment, said downstream gas feed tube end (22) is radially
bounded by at least a portion of the outer surface (57) of said swirl chamber
wall (55) and by at least a portion of the inner surface (26) of a
substantially
cylindrical outer gas feed tube wall (24) substantially concentric with
respect
to said swirl chamber wall.
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The said at least one lateral gas port (25) is characterized in being adapted
to
impart a tangential velocity component to gas fed from said gas feed pipe
(20) to said swirl chamber (50) at least within said throat region (52).
Referring to Figure 2, this is typically accomplished by arranging the said at
least one lateral gas port (25) as a tangential aperture into the swirl
chamber
(50), i.e., the axis of the lateral gas port (25) being substantially
perpendicular
both to the axis of the swirl chamber and to a radial line taken form this
axis.
Alternatively, the said at least one gas port (25) could take the form of
corresponding spaces between suitable radially spaced vanes mounted in a
circumferential slot on said swirl chamber wall (55) (not shown).
Furthermore, the said at least one gas port (25) is located upstream of said
annular throat region (52). Thus, gas entering the swirl chamber (50)
comprises a tangential velocity component due to the said geometrical
configuration of the inlet thereto, i.e., the lateral gas port (25), as well
as an
axial velocity component due to the general flow direction of the gas along
the axis ( 100). Both these velocity components together provide a swirling
gas flow in the swirl chamber (SO), which is maintained at least until exit
thereof, and therefore through the said annular throat region (52); due to the
annular geometry of the swirl chamber. To further enhance the gas swirl in
the swirl chamber (50), the said gas feed tube end (22) comprises a plurality
of said lateral gas ports (25) in fluid communication with said swirl chamber
(50). Optionally, and preferably, the said plurality of gas ports (25) is
arranged in at least two axially spaced groups. Further optionally, each of
these groups comprises an equal number of said gas ports (25), for example 4
ports, substantially uniformly distributed circumferentially. Each of these
gas
ports of one row may be substantially aligned axially, or alternatively
angularly displaced, with corresponding gas ports of an adjacent group of gas
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ports, as for example described hereinbelow with reference to the said liquid
ports (35), mutatis mutandis. Typically, the number of lateral gas ports (25),
in particular the number of axially spaced groups, and the number of gas
ports (25) per group, as well as the dimensions of each gas port (25) of the
sprayer ( 10) may be chosen advantageously taking into consideration factors
such as the flow rate characteristics of the device as well as the available
total
pressure head.
In the first embodiment, the said at least one liquid port (35) may be axially
aligned with said annular throat region (52) as illustrated in Figures 1 to 6.
Optionally, and referring in particular to Figures 3(a) and 4, said at least
one
lateral liquid port (35) is adapted to impart a radial velocity component to
liquid fed from said liquid feed pipe (30) to said swirl chamber (50) at least
within said throat region (52), and thus the liquid port (35) is in the form
of a
radial aperture into the swirl chamber (50).
Alternatively, and referring in particular to Figure 5, the at least one
liquid
port (32) may optionally be angled to the axis (100) of the swirl chamber (50)
so as to reduce flow losses. In other words, said at least one lateral liquid
port
(35) is adapted to impart a radial velocity component and an axial velocity
component to liquid fed from said liquid feed pipe (30) to said swirl chamber
(50) at least within said throat region (52).
Alternatively, and preferably, said at least one lateral liquid port (35) is
adapted to impart a tangential velocity component to liquid fed from said
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liquid feed pipe (30) to said swirl chamber (50) at least within said throat
region (52), as illustrated in Figure 3(b), said tangential velocity being in
the
same direction as the tangential velocity component of the swirling gas
within the swirl chamber (50). In the first embodiment, the latter option is
adopted, and a tangential velocity component is imparted to the liquid flow
on entering the swirl chamber by arranging the said at least one lateral
liquid
port (35) as a tangential aperture into the swirl chamber (50), i.e., the axis
of
the lateral liquid port (35) being substantially perpendicular both to the
axis
of the swirl chamber and to a radial line taken form this axis.
Optionally, and referring in particular to Figures 6 and 7, the said device (
10)
comprising tangential liquid ports (35) may further comprise a substantially
cylindrical sleeve (70) radially displaced from said liquid feed end (32),
i.e.,
intermediate said outer surface (36) of said downstream liquid feed tube end
(32) and the inner surface (56) of said swirl chamber wall (55). The annular
space (76) between the said outer surface (36) and the sleeve (70) acts as a
swirl chamber for the liquid exiting the tangential liquid ports (35) of the
liquid feed tube end (32). In this situation, angular momentum losses of the
gas flow are reduced, providing larger values of the tangential velocity of
the
two-phase medium at exit from the sprayer, thereby improving atomisation
quality on account of the stronger centrifugal forces acting to fragment
liquid
drops. The minimum clearance 8min between the sleeve (70) and the said outer
surface (36) is related to the diameter d of the at least one liquid port (35)
by
the expression:-
Smin = ~4
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The said sleeve (70) extends axially downstream from the said swirl chamber
wall (58) and may extend into the throat region (52), with the said liquid
ports (35) axially aligned with the throat region (52), as illustrated in
Figure
6, or alternatively situated at any suitable location upstream thereof.
Optionally, the inner and outer diameters of the throat region (52), db and
d~,
respectively, may be adjusted to compensate for the presence of the sleeve
(70) within the throat region (52). Alternatively, the sleeve (70) only
extends
as far as the entry to the throat region (52), and the said liquid ports (35)
are
axially disposed upstream with respect to the throat region (52), in which
case, liquid communication between the said liquid ports (35) and the throat
region (42) is still maintained via said annular space (76) provided by the
sleeve (70).
Preferably, the said liquid feed tube end (32) comprises a plurality of said
lateral liquid ports (35) in fluid communication with said swirl chamber (50).
In particular, the plurality of lateral liquid ports (35) are advantageously
arranged in at least two axially spaced groups. Further optionally, each of
these groups comprises an equal number of said liquid ports (35), for
example 4 ports, substantially uniformly distributed circumferentially. Each
port on one row may be substantially aligned axially with corresponding
liquid ports (35) of an adjacent group of liquid ports (35), though
preferably,
the liquid ports (35) on one row are angularly displaced with respect to the
liquid ports (35) of an adjacent row, as illustrated in Figure 4. In the
latter
case, the , precise angular displacement between adjacent rows may be
advantageously chosen such as to maximize homogenous introduction of
liquid into the swirl chamber (50). Thus, for example, if there are two rows
of
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four liquid ports (35), the liquid ports (35) in one row could be displaced by
45° with respect to the liquid ports (35) of the adjacent row. If there
were
three such rows, the liquid ports could be angularly displaced by 30°
rather
than 45°. Typically, the number of lateral liquid ports (35), in
particular the
number of axially spaced groups, and the number of liquid ports (35) per
group, as well as the dimensions of each liquid port (35) of the sprayer (10)
may be chosen advantageously taking into consideration factors such as the
flow rate characteristics of the device as well as the viscosity of the
liquid.
It is seen that in the embodiment described gas is fed through tube
(20) and is imparted a swirling motion by feeding it into the swirl
chamber (50) through ports (25) that are lateral (as this term is
defined hereinbefore), viz. have axes that do not intersect the axis
(100) of the swirl chamber. Axis (100) is also the general axis of the
device and the swirling gas stream generally, or in the average,
progresses in the direction of said axis, while not being directed
along it at any point. Lateral ports (25) are tangential, in the sense
that their axes are tangential to circles concentric with the inner
cylindrical surface of the swirl chamber. While this is the simplest
way of imparting to the stream of gas a swirling motion, different
means could be used for imparting said motion without departing
from the invention as claimed. Concurrently with its swirling
motion, therefore, the gas stream is advanced in the direction
defined by the axis (100) of the swirl chamber and of the device as
a whole. The cross-sectional area of the swirling gas stream
perpendicular to said axial direction is initially that of the swirling
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chamber (50), but as the stream progresses, it is constrained within
the throat region (52), and its cross-sectional area is decreased.
Due to the conservation of angular momentum, this causes the
intensification of the rotational gas motion and the reduction in gas
pressure due to acceleration in both the axial and the tangential
velocity components of the gas. The gas is thus brought to a high
rotational velocity and low pressure, which pressure is preferably
close to that prevailing downstream of the sprayer outlet. The
stream of liquid is then injected into the gas stream, preferably in
the throat region and tangentially to it. The liquid injection ports
(35) are lateral, and preferably tangential, in the sense that their
axes are tangential to circles concentric with the inner cylindrical
surface of the throat region. It is seen, in the embodiment
described, that ports (35) positioned shortly after the beginning of
the throat region, viz. the stream of liquid is injected tangentially
into the swirling gas stream immediately after said stream has
been constrained to the second cross-sectional area and has
therefore been accelerated. This is desirable because, on the one
hand, the changes in rotational velocity of the gas stream and in
pressure have already occurred before the injection of the liquid,
and on the other hand the liquid will participate in the swirling
motion of the gas stream and its atomizing effects before issuing
from the throat through the sprayer outlet. The injection requires
low feed pressure, because of the low pressure prevailing in the
throat region. The gas mass and flow rate required for atomization
of the liquid are smaller than in other atomizing devices because of
said low pressure and of the high gas velocity at the points of liquid
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injection. Preferred, but not limiting, dimensional relationship are
set forth hereinafter.
At an inner diameter db of the throat, given by the expression:
db=d~'~( 1-~P )
(the terms d~ and cp being defined below) the static pressure of the gas flow
in
the swirl duct (50) becomes substantially equal to that of the medium into
which the two-phase liquid is dispersed downstream of the sprayer outlet,
while the tangential velocity of the airflow in the swirl chamber (50) reaches
a maximum. Thus, under these conditions, the required liquid pressure
differential with respect to the swirl chamber static pressure for injecting
liquid into the swirl chamber (50) is a minimum, while the shear forces on the
liquid (due to the tangential velocity component of the swirling gas) is at a
maximum leading to high atomisation quality for relatively low gas flow rates
and low liquid pressure values. By providing tangential liquid ports (35), as
illustrated in Figure 3(b) for the first embodiment, the liquid is injected
into
the swirl chamber with a tangential velocity in the same direction as the
swirling air, thereby reducing air rotation losses in the axea of interaction
with
the liquid, thereby improving further the atomisation characteristics of the
sprayer ( 10).
Thus, in the first embodiment, the optimal relative magnitudes of said inner
diameter and said outer diameter of said annular throat region are
substantially related by the expression:
db _<< d~ 1- ~p
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wherein
db is said inner diameter
d~ is said outer diameter
cp is a parameter linked to the throat flow rate coefficient ~. by the
expression:
i~=~~~P3~(2-~P) ~
The throat flow rate coefficient ~ is a measure of the effective flow area Fx
relative to the actual flow area Fc.
For the swirled gas stream outflow, the flow rate coefficient ~, at the throat
(52) may be determined by the characteristic "A" of the swirl chamber (50).
The characteristic "A" is a complex of geometrical dimensions determining
the intensity of the gas stream swirling, and is a similarity criterion in
which
theoretically different dimensional representations of vortex chambers
characterized by the same value of "A" feature the same value of flow rate
coefficient ~.. "A" may be determined from the relationship: -
A = 7Lr~R~,in~G
wherein: -
r~ is the outer radius of the throat region (52).
Rmin is the minimum distance between the axis (200) of each gas
port (32) and the axis (100) of the swirl chamber (50) (see
Figure 2).
F~ is the total geometrical area of the said gas ports (25).
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The "A" characteristic is related to the parameter cp by the expression:-
A-f(1-~)~21~L ~ ~~ J
Thus, the flow rate coefficient ~. may be determined given the "A"
characteristic of the swirl chamber (50).
The following hydrodynamic considerations are offered to provide
background information and facilitate the understanding of phenomena that
may have some relationship to the process and device of the invention, but
they are only approximate and in any case not relevant and not binding in the
definition of the invention.
At precritical gas flow regimes, i.e., where the Mach number of the gas flow
at the throat region is less than unity, the gas mass flow rate "m" may be
determined from the following relationship:-
2 k+1
P;~r k Pot,r k _ Pain k
m = ,uFa. 2g
RT~n k -1 C Pin ) ( P~.
At a critical flow regime, i.e., with sonic flow conditions at the throat
region
(52), the gas mass flow rate may be determined from the relationship:-
2
Pr~ 2 k 2 k-''
m = ,uF~. g
RT~n k+lCk+1~
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wherein
P;" is the total gas pressure at inlet to the swirl chamber (50).
T;" is the total gas temperature at inlet to the swirl chamber (50).
Po"t is the total pressure of the medium into which the two-phase
flow is being discharged.
F~ is the geometric area of the throat (52).
k is the adiabatic index of the gas, which for air is typically about
1.4.
R is the Universal Gas Constant.
The total area FL of the liquid ports (35) may be determined from the
following relationship:-
FL=ml.,~~~L~(2g~'.OP)~
wherein:-
mL is the liquid flow rate.
~,LL 1S the flow rate coefficient of the liquid ports (35), typically
about 0.5 to about 0.6.
g is the acceleration due to gravity constant.
y is the specific weight of the liquid.
OP is the difference between the total pressure of the liquid and the
total pressure of the gas at entry to the swirl chamber (SO).
A second embodiment of the invention will now be described.
Typically, the two-phase flow flows axially from the annular exit (54) of the
swirl chamber (50). In some applications, it is important for the two-phase
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flow to flow at an angle to the axis of the sprayer ( 10). Thus, turning to
Figure 8, this figure illustrates a second embodiment of the present invention
incorporating a diffuser (90) downstream of and in fluid communication with
said annular throat region (52). The diffuser (90) is adapted to impart a
radial
velocity component to two phase fluid flowing from said downstream
opening of said swirl chamber. The said radial velocity component may be
chosen to be such as to provide a predetermined spray angle for said two
phase fluid exiting said diffuser (90). In this embodiment, said diffuser (90)
comprises an outer substantially frustoconical diverging wall (92) which is at
an angle a 1 to the axis ( 100) of the diffuser (and thus to the axis of the
sprayer ( 10)) at planes through this axis. The diffuser (90) also comprises a
substantially coaxial inner substantially frustoconical diverging plug (94)
which is at an angle a2 to the axis ( 100) of the diffuser (and thus to the
axis
of the sprayer ( 10)) at planes through this axis. The diverging plug (94) and
the diverging wall (92) define therebetween an axially diverging conical
channel (96) having an average cone angle or apex angle a, defined as the
average value of the angles {twice a 1 } and {twice a2 } . Said angles a 1 and
a2 may be equal, in which case the said conical channel (96) is parallel but
of
increasing area in the downstream direction. Said average apex angle a is
typically between about 70° and about 120°, i.e., about
35° and about 60° to
the axis ( 100).
Structurally, and with reference to Figure 8, the said diverging plug (94) may
be optionally integrally connected to the downstream end (3 8) of said liquid
feed tube , end (32), and the said diverging wall (92) may be optionally
integrally connected to the said outer gas feed tube wall (24) of said gas
feed
end (22).
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In high temperature applications such as in fuel atomisation for combustion
processes, the high velocity two-phase fluid flow through the diffuser (90)
provides cooling thereof, leading to longer service life of the sprayer ( 10).
Another requirement that must usually be met for sprayers used as fuel
atomisers is that of enabling a range of velocities to be provided for the
two-phase flow. The axial flow velocity within the diffuser is reduced as a
result of the diffusion therein. The degree of flareout or diffusion, DK, may
be determined from the relationship:-
DK= Fao /F~
wherein:-
F~ is the geometric area of the throat (52).
Fdo is the geometric area of the conical channel (96) at the
downstream end of the diffuser (90).
The value of DK may be advantageously chosen to be between 2 and 5 for
many applications, and depends on many factors including gas pressure,
properties of the liquid, and so on. Typically, reducing DK increases the
value of the outflow velocity, which may worsen the index of combustion
completeness in such applications. On the other hand, excessive diffusion
lowers the dispersion quality and increases the size of the dispersed liquid
particles.
Optionally, the said sprayer ( 10) comprises at least one baffle (80) for
separating two-phase fluid flow exiting said sprayer ( 10) into at least two
streams arid for enhancing mixing thereof with ambient fluid external to the
said sprayer ( 10). In particular, if the said sprayer ( 10) incorporates a
diffuser
(90), the said at least one baffle (80) is comprised at the downstream end of
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the diffuser, as illustrated in Figure 9. Preferably, a plurality (e.g. 4) of
circumferentially equi-spaced baffles (80) are comprised at the downstream
end of the sprayer ( 10) or said conical channel (96), as appropriate. The
baffles (80) separate the annular or conical flow exiting the sprayer (10) or
diffuser (90), respectively, into a number of adjacent streams, promoting
mixing with the ambient fluid. In combustion applications, where the sprayer
( 10) (with or without the said diffuser (90)) is used for atomising fuel,
thorough mixing of the atomized fuel with ambient oxidizer gas (typically
compresses air or oxygen) enables the fuel to be more fully combusted,
resulting in a more efficient combustion process and in lower emissions,
particularly pollutants.
Optionally, said at least one baffle (80) is in the form of a vane-like member
(82) radially extending the width of said conical channel at the downstream
end thereof, and having a substantially blunt trailing edge. The said vane-
like
member (82) may be optionally integrally connected to said conical plug (94).
Alternatively, the vane-like member (82) may be optionally integrally
connected to said diverging wall (92).
Alternatively, and with reference to Figure 10, said at least one baffle (80)
may be integrally connected at an inner radial end thereof to said inner plug
(94), said inner plug comprising a free upstream end (97) comprising a
diameter substantially equal to the said inner diameter of said throat region
db,
said upstream end (97) being optionally abuttable against the downstream
end of said swirl chamber (50), i.e. of said blank (38). The said at least one
baffle (80) may also be integrally connected at the outer radial end thereof
to
a suitable annular ring (98) having an inner diameter substantially equal to
the diameter do of the downstream end of said diverging wall (92). Said
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annular ring (98) is integrally connected to a downstream end portion of said
outer gas feed tube wall (24). This configuration for the diffuser and baffles
is particularly advantageous with respect to the manufacture of these
components.
Referring to Figure 11, the sprayer (10) comprises suitable adjustment means
for enabling said downstream liquid feed end (32) to be axially movable with
respect to said throat region (52). Such adjustment means may be necessary
for compensating for manufacturing errors which may otherwise bring into
misalignment the said liquid ports (35) with respect to the said annular
throat
region (52).Preferably, said suitable adjustment means comprises mutually
engaging complementary screwthread surfaces (72), (74) respectively
comprised on the said outer surface (36) of an upstream portion of said inner
liquid feed tube wall (34), and on a lower radial portion (59) of said swirl
chamber wall (58), which swirl chamber wall (58) may further comprise an
axial extension (59) in the upstream direction, as illustrated in Figure 1
l.The
said adjustment means may advantageously comprise a locking nut (60) to
lock the said liquid feed end (32) in any particular axial alignment with
respect to said swirl chamber (50). While the said adjustment means has been
illustrated and described with respect to the embodiment of Figure 11, it may
also be comprised in each of the other embodiments of the present invention
described herein, and with reference to Figures 1 to 10, mutatis mutandis.
While in the foregoing description describes in detail only a few specific
embodiments of the invention, it will be understood by those skilled in the
art
that the invention is not limited thereto and that other variations in form
and
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details may be possible without departing from the scope and spirit of the
invention herein disclosed.
