Patent Application
20140150435
This application claims priority to European application 12195066.1 filed Nov. 30, 2012, the contents of which are hereby incorporated in its entirety.
The present invention relates to the field of gas turbines, in particular to lean premixed, low emission combustion systems having one or more devices to suppress thermo-acoustically induced pressure oscillations in the high frequency range, which have to be properly cooled to ensure a well-defined damping performance and a sufficient lifetime.
A drawback of lean premixed, low emission combustion systems in gas turbines is that they exhibit an increased risk in generating thermo-acoustically induced combustion oscillations. Such oscillations, which have been a well-known problem since the early days of gas turbine development, are due to the strong coupling between fluctuations of heat release rate and pressure and can cause mechanical and thermal damages and limit the operating regime.
A possibility to suppress such oscillations consists in attaching damping devices, such as quarter wave tubes, Helmholtz dampers or acoustic screens.
A reheat combustion system for a gas turbine with sequential combustion including an acoustic screen is described in the document US 2005/229581 A1. The acoustic screen, which is provided inside the mixing zone and/or the combustion chamber, consists of two perforated walls. The volume between both can be seen as multiple integrated Helmholtz volumes. The backward perforated plate allows an impingement cooling of the plate facing the hot combustion chamber.
To prevent hot gases to enter from the combustion chamber into the damping volume, an impingement cooling mass flow is required, which decreases the damping efficiency. If the impingement mass flow is too small, the hot gases recirculate passing through the adjacent holes of the acoustic screen. This phenomenon is known as hot gas ingestion. In case of hot gas ingestion the temperature rises in the damping volume. This leads to an increase of the speed of sound and finally to a shift of the frequency, for which the damping system has been designed. The frequency shift can lead to a strong decrease in damping efficiency. In addition, as the hot gas recirculates in the damping volume, the cooling efficiency is decreased, which can lead to thermal damage of the damping device. Moreover, using a high cooling mass flow, increases the amount of air, which does not take part in the combustion. This results in a higher firing temperature and thus leads to an increase of the NOx emissions.
A solution to the mentioned issues is described, for example, in the document EP 2295864. Here, a multitude of layers are braced together to form single compact Helmholtz dampers, which are cooled using an internal near-wall cooling technique close to the hot combustion chamber. Therefore, the cooling mass flow can be drastically reduced without facing the problem of hot gas ingestion, leading to less emissions and a higher damping efficiency. As single Helmholtz dampers are used, different frequencies can be addressed separately. Whether single or a cluster of Helmholtz dampers is used, the design is based on an appropriate implementation of a near wall cooling.
Another solution of a high-frequency damping system for a combustor in a gas turbine with a cooled wall part is disclosed in EP 2402658. A plurality of cooling paths extending in axial direction are formed in the combustor wall. The cooling paths are connected to a source of cooling medium, such as steam or cooling air, at the one end and to a cooling medium discharge channel at the other end. The cooling medium flowing through the cooling paths cools the peripheral portions of the through holes to avoid or minimize thermal stress, caused by the hot combustion gases when passing the through holes in case of hot gas ingestion.
The document EP 2362147 describes various solutions on how the near-wall cooling can be realized. The near-wall cooling passages are either straight passages or show coil shaped structures parallel to the laminated plates. A drawback of this solution is that due to the shape of the near wall cooling channels, the component is to be made from several layers, which in the end have to be brazed together. Brazing itself is a well-known technique in the turbo machinery business, but inherits disadvantages while compared to other joining methods.
Another way to realize different shapes of near wall cooling channels in wall structures would be to use a so-called “lost wax casting process”. With this technique, which is widely used to manufacture cooling passages in turbine blades, a ceramic core is used during the casting process to realize the later cooling channels. Compared to casting processes that can avoid the usage of ceramic cores, the production costs are multiple times higher.
It is an object of the present invention is to provide a near wall cooling system for a damping device of a gas turbine combustor with significantly reduced cooling air mass flow requirements, which eliminates the drawbacks of expensive casting techniques.
This object is obtained by a damping device for a gas turbine combustor according to claim 1.
The damping device for a gas turbine combustor, which is especially a damping device for a liner segment with a near wall cooling system, comprises a wall with a first inner wall, particularly the liner, and a second outer wall, arranged in a distance to each other, wherein said inner wall is subjected to high temperatures on a side with a hot gas flow, a plurality of cooling channels extending essentially parallel between the first inner wall and the second outer wall, and at least one damping volume bordered by said cooling channels, a first passage for supplying a cooling medium from a cooling channel into the damping volume and a second passage for connecting the damping volume to the combustion chamber, wherein an end plate is fixed to the inner wall, separating the damping volume from the combustion chamber, said end plate is provided with the neck passage and is additionally provided with at least one feed plenum for a cooling medium, at least one exit plenum for a cooling medium and at least one cooling passage enabling a flow of cooling medium from the at least one feed plenum to another feed plenum or to the at least one exit plenum.
The cooling passages between said plena act as near wall cooling channels.
According to a preferred embodiment the lateral edges of the end plate are provided with recesses. When connected to the inner wall, these recesses form the feed and exit plena.
The new invention enables an optimized cooling and lifetime performance of high frequency damping systems with reduced cooling air mass flow requirements. The described manufacturing process uses machining and welding techniques and, therefore, eliminates the said drawbacks of brazing and/or expensive casting techniques using ceramic cores. The novel near wall cooling design enables an efficient damping and reduces the manufacturing risks.
Usually, high-frequency dampers in can combustion systems are mounted around the circumference of the can liner with a limited extent in axial direction. If a near-wall cooling damper scheme is mounted in this way, difficulties in the manufacturing process arise because the bending process might influence the cooling channel geometries and thus cause a non-uniform cooling distribution. By arranging the near-wall damper volumes in axial direction these difficulties are overcome.
The present invention is now to be explained more closely by means of different embodiments and with reference to the drawings.
The interface between the burner section 2 and the combustion chamber 3 is characterized by a regularly sudden cross-sectional area change comprising a perpendicular front plate 2a, extending from the exit of the burner section 2 to the peripheral wall of the combustion chamber 3. At least a portion 4 of the combustor walls, including the burner section 2 and/or the combustion chamber 3 and/or the front plate 2a, are equipped with cooling means. For example, the combustor walls as a whole or any portions of the burner section 2 and/or the combustion chamber 3 and/or the front plate 2a comprise an inner liner 5 and, in a distance thereof, an outer cover plate 6, inner liner 5 and outer cover plate 6 defining an interposed cooling chamber. A cooling medium, such as air or steam, circulates through cooling channels 7 in this cooling chamber (as indicated by arrows F), thereby cooling the burner section 2, the combustion chamber 3 and the front plate 2a.
The advantage of this design is that the outer shape of the acoustic damper can be incorporated in the casting process of the liner 5. To define the needed acoustic volume 9 and to close the damping device, a machined end plate 10 is welded onto the liner 5 covering the molded-in recess. The end plate 10 is equipped with at least one through-hole 13, the neck passage for the interaction between the combustion chamber 3 and the damping volume 9.
The outer portions of the recessed liner 5 are charged with the cooling medium F, flowing through the cooling channels 7 and therefore are properly cooled. But as the damping device mainly consists of a damping volume 9, which has little or no purge air supply from the cooling circuit 7, the wall temperatures between the damping volume 9 and the combustion chamber 3 would outrun the material limits. As a consequence, an additional cooling means has to be incorporated in the end plate 10. One alternative for cooling this component is a near-wall cooling means.
To realize a near-wall cooling solution for a damping device according to the invention, a first passage 11 is established in liner 5. This passage 11 is connected to a cooling channel 7 at one end. And at the other end this passage 11 is connected to a first feed plenum 12 so that the cooling medium F can flow through passage 11 and supply cooling medium F from the cooling channel 7 into this plenum 12. This first plenum 12 is disposed between the liner 5 and the end plate 10. According to a preferred embodiment the plenum 12 is located in the end plate 10. In the region of its lateral edges a recess is milled into the end plate 10. When connected to the liner 5, these recesses form plenum 12. And this plenum 12 is the starting point of the near wall cooling system of the inventive damping device.
As can be seen from
As can be observed from
The small cooling mass flow (due to the high pressure drop over the near-wall cooling device) is used efficiently to pick up the heat load from the combustion chamber 3. As the design of the near-wall cooling device covers the end plate 10 completely, the wall temperature distribution is homogeneous. A homogenous temperature distribution reduces the thermal stresses and increases the lifetime.
It is an advantage of this structure that all feed plena and passages of the near-wall cooling device can be made by drilling, laser cut, water jet, milling and so on. Up to date, the realization of such a cooling technique requires expensive casting processes (including ceramic cores) or brazing techniques, which are difficult to handle. The advantage of the current invention is that it uses only machining and welding techniques.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12195066.1 | Nov 2012 | EP | regional |
