Patent Application
20070042307
This application is based on and claims priority under 35 U.S.C. §119 to Swiss Application No. 00210/04, filed Feb. 12, 2004 and is a continuation application under 35 U.S.C. §120 of International Application No. PCT/EP2005/050579, filed Feb. 9, 2005 designating the U.S., the entire contents of both of which are hereby incorporated by reference.
A premixer is disclosed 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 γ and air inlet slits directed tangentially to the length of the cone.
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, the contents of which are hereby incorporated by reference in their entireties, 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 of the premix flame 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 backflow zone, in the forward region of which, in the direction of flow, the premix flame forms.
The aerodynamic stability of the backflow zone forming depends 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 can occur to an increased extent within the combustion system, 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 A1 or DE 44 12 315 A1, the contents of which are hereby incorporated by reference in their entireties. 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 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 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 U.S. Pat. No. 5,588,826, also incorporated herein by reference) 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.
A premix burner is disclosed wherein, in spite of the increase in size of the burner dimensions, the optimized burner properties in the case of previously known premix burners can be retained virtually unchanged. In exemplary embodiments, the aim is to increase the size of the burner of previously known premix burner systems can be increased in order to reduce the number of burners in multiple burner arrangements, as described at the beginning, and also to reduce the associated system costs. Similarly, with the larger premix burner systems, previously known single diffusion burners, as are used for example for the firing of silo combustion chambers, can be replaced by a single premix burner.
According to an exemplary embodiment, a premix burner 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 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 can have 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 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 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 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.
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:
Shown in
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 conically in the direction of flow at a cone angle γ. 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 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
A graphic cross-sectional representation of the embodiment shown in
Depicted in
The connection 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.
It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 00210/04 | Feb 2004 | CH | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP05/50579 | Jan 2004 | US |
| Child | 11503163 | Aug 2006 | US |
