Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 0221~673 1997-09-12
Canadian Patent Application Kurt Skoog
- KLS-016-CA PO/MC/hl
BURNER HEAD FOR LIOUID FUELS
The invention relates to a burner for introducing liquid
fuels to be mixed with air into a combustion chamber,
preferably for the combustion of fuel oils of various
grades.
The US patent 4,726,760 discloses a burner head of the
present type, in which a tubular jet body terminates in a
nozzle piece for supplying fuel oil to be mixed with air
and introduced into a combustion chamber. Another
conventional apparatus is shown in Fig. 1 where the jet
body 36 is disposed along the axial or flow direction 26,
the jet body ending with a nozzle piece 50 provided with
fuel outlet ports. The jet body 36 is surrounded by two gas
and/or air passages 14, 18. The inner passage 14 opens into
the combustion chamber 16 via the annular inlet port 24.
The primary combustion air exiting the inlet port 24
diverges conically from the nozzle 50 when entering the
combustion chamber 16.
The outer annular passage 18 is defined by the outer jacket
39 and an inner jacket 38 disposed between the jet tube 36
and the outer jacket 39. The second annular outlet port 28
is defined by conical side walls so as to form a converging
conical flow of combustion air, which intersects the
expanding flow from the first outlet port 24. Swirl baffles
36, 37 are provided in the inner and outer passages 14, 18,
which give a vortex motion to the respective flows.
In the conventional device of Fig. 1, liquid fuel exits
from the nozzle 50 and atomization is caused by the
turbulence caused from the intersection of primary and
secondary air flows from the coaxially arranged inner and
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outer passages 14, 18. The fuel-air mixing takes place
relatively distant from the nozzle 50 as the gases are
already in the combustion chamber. At greater distances
from the outlet ports 24, 28, the mixing air has lost some
of its kinetic energy. The degree of atomization of the
liquid fuel is therefore reduced. In addition, the
provision of two air inlet passages 14, 18 requires the
additional fabrication of the inner jacket 38, associated
baffles. In operation, the air supply pressures in the air
supply passages 14, 18 as well as the fuel supply pressure
in the passage 54 of the jet body have to be carefully
adjusted with respect to one another to obtain optimal
combustion characteristics.
It is therefore an object of the present invention to
provide a burner apparatus for liquid fuels by which
atomization of the fuel can be improved and by which the
manufacturing costs and adjustment time in operating the
burner can be reduced.
Another object of the present invention is to provide
improved swirler means by which the internal circulation
and stabilization of the flame can be improved.
SUMMARY OF THE INVENTION
In accordance with the present invention, a burner for
liquid fuels is provided, preferably for heating oils or
oils of various grades. The burner comprises a nozzle
arranged on the downstream end of a jet tube, the nozzle
being adapted to supply a fuel air mixture to the
combustion chamber. An air tube is disposed about the jet
tube to form a single annular passage therebetween. The
annular passage terminates in an annular outlet port
defined by a constricting conical wall of the air tube. The
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primary combustion air is thereby introduced into the
combustion chamber in a converging flow. Swirler means are
disposed in the same annular passage and comprise a
plurality of blades arranged concentrically about the jet
tube. The swirler blades are arranged prior to the annular
outlet port to impart a vortex motion to the converging
combustion air flow.
The outer wall of the jet tube preferably comprises an
expanding conical wall portion located upstream of the
constricting conical wall defining the outlet port. The
expanding conical wall reduces the flow cross section in
the annular combustion air passage thereby creating a
Venturi effect. The exit flow velocity of the combustion
air is thereby increased to promote increased atomization.
The nozzle 50 of the jet tube 36 is preferably constructed
to supply a primary fuel-air mixture, whereas the
combustion air from the annular passage forms and
stabilizes the flame.
In addition to the reduction of the flow cross-section
caused by the walls of the annular passage, the blades of
the swirler means are preferably arranged to provide a
further Venturi effect. The outlet area between two
2s adjacent blades at their downstream end is 40 to 95~,
preferably 60 to 80~ of the area between the adjacent
blades at their upstream end. The swirler blades have a
pitch relative to the flow direction or to the axial
direction of the jet body which is preferably in the range
of 40~ to 70~. The length of the blades is selected such
that the amount of turn of each blade about the axial
direction, from the upstream end to the downstream end, is
in the range of 50~ to 70~. Furthermore, the downstream
ends of the individual blades are disposed at a position
prior to the outlet opening of the nozzle of the jet body.
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An under pressure produced by the swirler means at the tip
of the swirler blades enhances stabilization of the flame.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail with
reference to embodiments of the burner as illustrated in
the drawings.
Fig. 1 shows a cross-sectional view along the longitudinal
axis of a conventional burner.
Fig. 2 shows a cross-sectional view along the longitudinal
axis of a burner according to an embodiment of the
present invention.
Fig. 3 shows a side view of an embodiment of the swirler
means shown in Fig. 2.
Fig. 4 shows a cross-sectional view along the line I-I of
Fig. 3.
Fig. 5 shows a plan view looking down the longitudinal axis
of the swirler means of Fig. 3.
Fig. 6 shows a schematic illustration of the design of the
swirler blades.
Fig. 7 shows a schematic illustration of air flow between
the swirler blades.
A first embodiment of the present invention is illustrated
in Fig. 2. The jet body 36 comprises an outer tubular wall
41, the downstream end of which joins to an expanding
conical wall portion 26. A nozzle 50 is secured to the
downstream end of the expanding wall 26. A second tube 53
is coaxially mounted within the jet tube forming a passage
54 for supply of fuel oil to the nozzle 50. The outer
passage 55 within the jet tube 36 supplies gas and/or air
to side inlets 44 of the nozzle 50. Fuel and air are then
mixed within a mixing space 40, after which the mixture
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leaves the nozzle 50 through the nozzle opening 10. Various
arrangements of the fuel oil inlets and the side air inlets
44 are possible. Important is the premixture of fuel and
air performed by the nozzle. The creation of the fuel-air
mixture is such that it leaves the opening 10 with a vortex
motion as it moves towards the combustion chamber 16.
An air tube 40 is disposed about the outer wall 41 of the
jet tube 36, whereby a single annular passage 10 is formed
therebetween. At its downstream end, the air tube 40 is
formed with a constricting conical wall 24, which defines
one side wall of the annular outlet port 22. The conical
surface 24 causes the primary combustion air supplied by
the passage 10 to converge and intersect with the fuel air
mixture emanating from the nozzle opening 10.
The expanding conical wall 26 of the outer wall 41 iS
disposed upstream of the constricting conical wall 24 in
axial direction. In the embodiment of Fig. 2, the axial
extension of the expanding wall 26 partially overlaps the
axial extension of the constricting wall 24. In any case,
the expanding conical wall 26 should begin before the onset
of the constricting wall 24. The end wall 28 joining the
expanding wall 26 defines the other side wall of the
annular outlet port 22. As can be seen from the spacing
between the constricting wall 24 and the end wall 28 of the
outlet 22, the flow cross-section of the annular passage 10
is reduced compared to its value further upstream. The
Venturi effect produced increases the exit velocity of the
primary air to be provided for combustion and for flame
formation and other flame characteristics.
Swirler means are provided in the single annular passage 10
which comprise blades 20 fixed to a sleeve 30. The blades
20 have an axial extension which includes the expanding
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wall 26 as well as the constricting conical wall 24. The
blades 20 are concentrically arranged about the jet tube 36
and have a pitch to impart a vortex motion of the
converging flow exiting from the outlet 22. Thus the
combustion air is imparted with increased speed due to the
Venturi effect as well as a rotational component due to the
swirler means as it exits the air tube 40. With the nozzle
50 being adapted to supply a fuel-air mixture, the
converged vortex flow from the outlet port 22 is sufficient
for further atomization of the fuel-air mixture and is
particularly suited for flame formation.
A side view of the swirler means is shown schematically in
Fig. 3. Only the blades on the front side of the device are
shown. Fig. 4 shows a cross-section of the swirler means at
the line I-I. As can be seen, the blades 20 are welded to
the outer surface of the sleeve 30 at a predetermined
angular disposition. In this embodiment, eight blades are
arranged concentrically about the sleeve 30 at an equal
angular spacing of 45~. The number of blades will normally
depend upon the diameter of the jet tube 36. Preferably six
to twenty blades will be provided at equal spacing.
Fig. 5 shows a plan view looking down the axis of the jet
tube. As can be seen, the upstream end a of the blades has
a radial extension which corresponds approximately to the
radial dimension of the angular passage 10 shown in Fig. 2.
The radial extension of each blade then decreases toward
the downstream end d to match the reduced radial dimension
of the annular outlet port 22. Generally, the fit of the
individual blades to the dimensions of the annular passage
10 is close but not tight with the internal dimensions of
the annular passage 10.
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Fig. 6 shows an illustration of the geometry of the blades
individually and with respect to one another. The
illustration represents a composite of side views of
several blades taken to look down the edge of each blade as
one moves around the swirler means. As can be seen in
Fig. 6, each blade 20 is formed to have a pitch ~, which is
preferably in the range of 40~ to 70~. The solid lines of
Fig. 6 represent a 50~ pitch while the dashed lines
represent a 70~ pitch. The selection of the pitch will
influence the additional Venturi effect caused by the
blades. As seen in Fig. 4, the area A1 between two adjacent
blades at the upstream end a of the blades will be
approximately rectangular. Depending on the pitch of the
blades, the area A2 illustrated in Fig. 6 between the
adjacent blades at their downstream end will be smaller
than A1. The higher the pitch, the closer the two adjacent
blades are to one another as can be seen by comparing the
spacing A2 between two blades having a pitch of 50~ with
the spacing A'2 between adjacent blades having a pitch of
70~ (dashed lines of Fig. 6). The pitch of the blades is
selected such that the ratio between the outlet area A2
between two adjacent blades is 40~ to 95~ of the area A
between the adjacent blades at their upstream end.
Preferably, the ratio of the outlet area to inlet area is
60~ to 80~.
The length of the pitched or spiral portion b shown in
Fig. 6 determines the amount of turn c of each blade about
the axis of the swirler means. An amount of turn is best
illustrated in Fig. 5 where c shows the turn to be the
angular reach of a given blade 20 from its upstream end a
to the downstream end d. In the embodiment, with a spacing
of the blades being 45~, the turn of the blades is about
65~. This is also illustrated with the numeral c in Fig. 3.
As can be seen from the figures, one blade extends over and
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above at least a portion of its adjacent blade. According
to the invention, it has been found that the turn of each
blade should be in the range of 50~ to 70~. Expressed in
terms of the spacing between adjacent blades, the turn
should be between 1.5 and 2 times the spacing between the
blades.
As indicated above, the pitch ~ of the individual blades
should be in the range of 40~ to 70~. A higher pitch
creates a higher swirl number or vorticity which results in
a strong internal recirculation of the hot gases of the
flame. This makes the flame shorter and wider and provides
good fuel atomization. On the other hand, when the pitch is
higher than 70~, combustion noise increases caused by the
strong turbulent character of the flame.
When a longer and narrower flame is desired for a
particular combustion furnace, the pitch is made to be
lower. When the pitch is lower than 40~, the recirculation
of combustion gases in the flame is reduced leading to less
efficient combustion. According to the invention, it has
been found that when the swirler blades are dimensioned as
defined above, the flame has an optimal internal
recirculation which allows a high combustion efficiency.
Fig. 7 illustrates schematically the flow of combustion air
between adjacent swirler blades 20 and 20'. The air streams
entering at the inlet end a are urged by centrifugal force
toward the internal surface of the first blade 20. At the
same time, the air streams move away from the external
surface of the second blade 20'. This creates an over
pressure indicated with a + in Fig. 7. An under pressure is
created at the exit of the second blade 20'. This under
pressure holds the flame at the exit opening 10 of the
nozzle 50 as shown in Fig. 2. For this reason also, the
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downstream ends d of the swirler blades should be disposed
at a position upstream of the outlet opening 10 of the
nozzle 50 as shown in Fig. 2. The creation of the under
pressure with the swirler construction according to the
present invention thus provides flame stabilization in
addition to the improved internal gas recirculation of the
flame mentioned above.
