Note: Descriptions are shown in the official language in which they were submitted.
CA 02555481 2006-08-08
Premix burner arrangement for operating a combustion
chamber and method for operating a combustion chamber
Technical field
The invention relates to a premix burner for operating
a combustion chamber with a gaseous and/or liquid fuel,
having a swirl generator for inducing a swirl flow in
an incoming combustion air flow and means for injecting
fuel into the swirl flow, the swirl generator having at
least two cone shell. segments which fit together to
form a flow body and together enclose a conically
formed swirl space with a cone angle y and air inlet
slits directed tangentially to the length of the cone.
Prior art
Premix burners of the aforementioned generic type are
known from many prior publications, such as for example
from EP 0 210 462 A1 and EP 0 321 809 B1, to mention
just two. Premix burners of this type are based on the
general operating principle of generating within a
generally conically formed swirl generator, which
provides at least two cone shell segments fitted
together in such a way that they appropriately overlap
one another, a swirl flow which comprises a mixture of
fuel and air and is ignited within a combustion chamber
downstream of the premix burner in the direction of
flow, thereby forming a premix flame which is spatially
as stable as possible. In this case, the spatial
position oz the premix tlame is determined by the
aerodynamic behavior of the swirl flow, the swirl
coefficient of which increases with increasing
propagation along the burner axis, consequently becomes
unstable and ultimately, as the result of a
discontinuous cross-sectional transition between the
burner and the combustion chamber, breaks down into an
annular swirl flow with the formation of a backf7nw
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zone, in the forward region of which, in the direction
of flow, the premix flame forms.
Of special importance is the aerodynamic stability of
the backflow zone forming, which however depends in a
highly sensitive way on the design, shape and size of
the swirl generator. For example, if the forwardmost
front in the direction of flow of the backflow zone
forming is not spatially stabilized successfully,
thermoacoustic vibrations or pulsations occur to an
increased extent within the combustion system,
significantly impairing the overall combustion and the
release of heat.
In consideration of this fact, the premix burner
systems previously known and in use are restricted to
overall sizes in which the maximum burner diameter at
the burner outlet is only 180 mm. Premix burners of
this type additionally have a relatively pointed, i.e.
small, cone angle, of less than or equal to 18°, so
that the length of the burner tends to be great in
relation to the diameter of the burner facing
downstream, but is still well able to be handled for
assembly and maintenance purposes.
However, whenever combustion chambers of large
dimensions are to be fired, so far use has been made of
so-called multiple burner arrangements, which provide
for use of the above premix burners. Multiple burner
arrangements of this type are disclosed for example by
DE 42 23 828 Al or DE 44 12 315 A1. To operate
multiple burner arrangements of this type, which are
suitable for example for firing a silo combustion
chamber, a sophisticated arrangement of the large
number of premix burners respectively serving as main
burners or pilot burners is required to achieve the end
effect of allowing the combustion chamber to continue
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to be operated with lowest possible emission values in
the entire load range.
However, there is the desire to reduce the complexity
and with it also the number of the individual premix
burners that are required for firing combustion
chambers of large dimensions, without losses of quality
in the combustion process having to be accepted at the
same time. In addition, for reasons concerning the
increasingly stringent environmental standards with
regard to the reduction of emission values, the aim is
to replace the previously operated single diffusion
burners that are primarily used for firing silo
combustion chambers of large dimensions by more modern,
environmentally more acceptable burner systems. In
particular with regard to the avoidance of high
conversion and first-time acquisition costs, it is
desirable to provide premix burners of the largest
possible dimensions, in order for example to be able to
continue to maintain the operation of such silo
combustion chambers of large dimensions with only a
single premix burner.
Theoretical studies and trials have shown that simply
scaling for example a double cone burner known from EP
0 321 809 B1 does not achieve the objective, especially
since, as already mentioned above, the length of the
burner would increase disproportionately. Added to
this is the fact that the width of the air inlet slits
which extend tangentially in the burner axis and
through which incoming combustion air flows into the
swirl generator to generate the desired swirl flow
would likewise increase proportionately, so that good
mixing of the fuel and incoming combustion air with
adequate quality can no longer be ensured.
A further very important and at the same time critical
aspect of a desired increase in size or increase in
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output of the previously known premix burner systems
concerns the downstream termination of the cone shell
segments enclosing the swirl space of the swirl
generator, which in the example of the double cone
burner described in EP 0 321 809 B1 end in axially
directed blocking-off elements. These blocking-off
elements contribute to the formation of undesired
separation vortices which, as coherent vortex
structures, lead to combustion instabilities and,
associated therewith, to thermoacoustic vibrations or
pulsations.
There are also known, premix burner arrangements (for
example US 5,588,826) which, as a difference from the
premix burner described above, have a transitional
geometry interposed between the swirl generator and the
combustion chamber, for example in the form of a
hollow-cylindrically formed mixing tube. However,
transitional geometries of this type are extremely
sensitive aerodynamically, since flow separations in
this zone can lead to flashback or spontaneous
ignition. Similarly, because of their complex
production, transitional geometries of this type
contribute decisively to the production costs.
Summary of the invention
The invention is based on the object of developing a
premix burner according to the features of the
precharacterizing clause of claim 1 in such a way that,
in spite of the increase in size of the burner
dimensions, the optimized burner properties in the case
of previously known premix burners are to be retained
virtually unchanged. So the aim is to increase the
size of the burner of previously known premix burner
systems in order to reduce the number of burners in
multiple burner arrangements, as described at the
beginning, and also to reduce the associated system
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costs. Similarly, it is intended to be possible with
the larger premix burner systems to replace previously
known single diffusion burners, as are used for example
for the firing of silo combustion chambers, by a single
premix burner.
The solution achieving the object on which the
invention is based is specified in claim 1. Features
which advantageously develop the idea of the invention
are the subject of the subclaims and are disclosed in
the description, in particular with reference to the
exemplary embodiments.
According to the invention, a premix burner according
to the precharacterizing clause of claim 1 is developed
in such a way that, at least at the downstream end
region of the swirl generator, a shaped element
enclosing the cone shell segments with an inside wall
facing the cone shell segments is provided, and that
the cone shell segments end in the inside wall in a
flush manner while maintaining their shape.
A large number of theoretical studies and
experimentally conducted trials to increase the size of
the previously known form of premix burners, which
usually have a maximum burner diameter on the burner
outlet side of 180 mm, led to the realization that the
downstream end structure of the cone shell segments has
a significant influence on the stability of the premix
burner flame forming, in particular in the endeavor to
form the premix burner with as large a volume as
possible.
So it is found that in most cases of the premix burners
that are in use the end regions of the cone shell
segments go over in the direction of flow into a
hollow-cylindrical flow channel, which is adjoined
either directly by the combustion chamber or by an
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additional mixing zone in the form of a mixing tube.
To avoid the separation vortices following on directly
from the cone shell segments in the direction of flow,
it has been realized according to the invention that,
immediately after leaving the swirl generator, the
swirl flow is able to spread out largely without making
the swirl flow unstable if, while maintaining their
shape, the individual cone shell segments keep in close
contact with the inside wall of the flow channel
adjoining the swirl generator. The concept of
"maintaining their shape" means for the purposes of the
invention that the shape of the cone shell segments
formed in the manner of segments of a cone remains
unchanged in the region where they come into contact
with the inside wall of the shaped element enclosing
the cone shell segments, as though the cone shell
segments would penetrate unhindered through the shaped
element in a radially outward direction.
The inside wall of the shaped element also serves for
forming a flow channel adjoining the cone shell
segments downstream. Depending on the shape and size
of the shaped element, it also serves as a mixing tube
or as a joining element, in the sense of a flanged
piece, by means of which the swirl generator can be
connected to a combustion chamber following on in the
direction of flow, such as for example a silo
combustion chamber, or some other channel structure.
To be able to make the premix burner as compact as
possible, i.e. with a burner length that is as small as
possible, cone angles of at least 11°, but preferably
in the range of 20° and greater, are used, at which
angles the cone shell segments surround the swirl space
in a conically widening manner. For example, burners
with a burner diameter in the outlet region of greater
than 500 mm which have a burner length of well below
one meter can be made possible . To ensure good mixing
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of the mixture of fuel and air within the swirl
generator, it is also advantageous to increase the
number of cone shell segments and, associated with it,
the number of air inlet slits, in order in this way to
achieve the smallest possible slit width per air inlet
slit.
Brief description of the drawings
Without restricting the general idea of the invention,
the invention is described below by way of example on
the basis of exemplary embodiments with reference to
the drawings, in which:
Figures 1 + 2 show a perspective representation of a
premix burner with a cylindrically formed
shaped element,
Figures 3 + 4 show a perspective representation of a
premix burner with a frustoconically
formed shaped element,
Figure 5 shows a sectional representation through
a premix burner with a frustoconically
formed shaped element, and
Figures 6 + 7 show a perspective representation of a
premix burner with a shaped element
having an inlet region formed in a
funnel-shaped manner.
Ways of implementing the invention, commercial
usability
Shown in Figure 1 is a perspective representation of a
premix burner, with the viewing direction from the
downstream side into the swirl space of a swirl
generator 2 that is enclosed by a multiplicity of cone
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shell segments 1. Figure 2 shows the same premix
burner, but from a different viewing angle, that is
looking at the swirl generator 2 from the outside, said
generator being enclosed by eight cone shell segments 1
in the exemplary embodiment represented. The further
refinements of the exemplary embodiment represented in
Figures 1 and 2 are the same in each case, so that
there is no further distinction between Figure 1 and
Figure 2.
The premix burner represented has a central receiving
unit 3 , which takes the form of a receiving sleeve and
is intended for a central fuel supply unit, for example
in the form of a fuel nozzle for liquid fuels or fuel
lance for a pilot flame (not represented), to be pushed
in and held. The cone shell segments 1, connected by
their upstream ends to the receiving unit 3, mutually
enclose air inlet slits 4 and are placed with respect
to a burner axis A extending centrally through the
premix burner in such a way that they delimit a swirl
space widening sonically in the direction of flow at a
cone angle y. Each individual cone shell segment 1
has, moreover, depending on the type of fuel, at least
one fuel supply line 5, by means of which fuel can be
admixed into the incoming combustion air flow passing
through the air inlet slits 2.
While maintaining their shape, the individual cone
shell segments 1 open out with their downstream end on
an inside wall 6 of a cylindrically formed shaped
element 7 surrounding,the cone shell segments 1. The
individual cone shell segments 1 are connected to the
inside wall 6 of the shaped element 7 along a line of
intersection 8, which is obtained by a virtual
penetration of each individual cone shell segment 1
with the inside wall 6 of the cylindrical shaped
element 7. In this way, the swirl flow formed inside
the swirl generator 2 does not undergo any disturbance
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after flowing over the individual cone shell segments
1.
Provided upstream of the shaped element 7, for purposes
of an improved incoming air flow through the air inlet
slits 4 of the swirl generator 2, is a second shaped
element 9, which is likewise formed in a hollow-
cylindrical manner and has a greater inside diameter
than the first shaped element 7. The transition
between the inside diameters of the shaped elements 7
and 9 takes place by means of a discontinuous stage 10,
which is adjoined, moreover, by the fuel supply lines 5
of each individual cone shell segment 1.
The downstream contour of the shaped element 7 is
formed in a cylindrical manner and offers the
possibility of a constructionally simple connection,
for example to a combustion chamber (not represented)
arranged downstream in the direction of flow of the
premix burner represented.
Shown in a perspective representation in Figures 3 and
4 is a further exemplary embodiment of a premix burner
formed according to the invention, in which, as a
difference from the exemplary embodiment that is
represented in Figures 1 and 2, the region of the
shaped element 7 is formed as a frustoconical portion
tapering conically in the direction of flow. To avoid
repetition, the reference numerals that have already
been introduced are not described. The exemplary
embodiment represented in Figures 3 and 4 has in the
region of the shaped element 7 an inside wall 6, which
is hatched in the perspective representations and is
adjoined by the downstream ends of the cone shell
segments 1, while maintaining their shape. Adjoining
the inside wall 6 of the shaped element 7 downstream is
a shaped element 11 which is formed in a hollow-
cylindrical manner and serves as a coupling piece or
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flanged piece for a following combustion chamber. The
inside wall 6 placed with respect to the cone shell
segments 1 has the effect that the lines of
intersection 8 of the cone shell segments 1 with which
they adjoin the inside wall 6 are smaller or shorter in
the direction of flow than in the case of the
aforementioned exemplary embodiment, in which the cone
shell segments adjoin along a cylindrically formed
inside wall directed soaxially in relation to the
burner axis.
A graphic cross-sectional representation of the
embodiment shown in Figures 3 and 4 can be seen in
Figure 5. Clearly evident is the shape-maintaining
adjoinment of each individual cone shell segment 1 to
the inside wall 6 of the shaped element 7 tapering
sonically in the direction of flow in the form of a
cone, which element goes over in the direction of flow
into a straight-cylindrical region 11.
Depicted in Figures 6 and 7 is a further exemplary
embodiment of a premix burner, provided with a shaped
element 7 which has an upstream region, the so-called
inflow region 12, which has an inside wall 6 tapering
in the form of a funnel in the direction of flow. The
curvature of the inside wall 6 in this inflow region 12
corresponds approximately to the contour of a quarter
ellipse. The inflow region 12 is adjoined in the
direction of flow as part of the shaped element 7 by a
flow region 12' with a largely constant flow cross
section, to which finally an inlet flange of a
combustion chamber can be attached (not represented).
As also in the variants described above, while
retaining their cone shell segment shape, the cone
shell segments 1 of the swirl generator 2 end in the
adjoining inside wall 6 of the shaped element 7 in a
flush manner, as though the cone shell segments would
penetrate unhindered through the inside wall 6, but the
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cone shell segments 1 terminate respectively by means
of the lines of intersection 8 at the inside wall 6.
In the case of the inflow region 12 formed as a quarter
ellipse, the downstream end regions of the cone shell
segments 1 closely follow the elliptical curvature of
the inside wall 6 in this region 12. For the meaning
of the reference numerals that have already been
introduced and are additionally provided in Figures 6
and 7, reference is made to the aforementioned figures.
The connection according to the invention of each
individual cone shell segment by its downstream end
region to an inside wall of a shaped element
surrounding the cone shell segments, while maintaining
its shape, leads to minimal irritation of the swirl
flow passing through the cone shell segments. As a
difference from the previously known premix burners,
there are no blocking-off effects in the axial
direction through the burner axis associated with the
way according to the invention in which the cone shell
segments end in the corresponding inside wall while
maintaining their shape.
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List of reference numerals
1 cone shell segments
2 swirl generator
3 receiving unit
4 air inlet slits
fuel supply line
6 inside wall
7 shaped element
8 plane of intersection
9 shaped element
step
11 hollow-cylindrically formed shaped element
12 inflow region
