Abstract:
Disclosed is a combustor casing with an inner casing which defines a combustion chamber, an outer casing spaced apart from the inner casing for defining a passage between the inner and the outer casing, first and second effusion holes arranged in the inner casing, and dividing ribs connecting the inner and outer casings and forming at least first and second volumes for receiving part of a flow injected into the passage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of European Patent Application No. 08008606.9 EP filed May 7, 2008, which is incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a combustor casing with improved acoustic damping and cooling. 
     BACKGROUND OF THE INVENTION 
     Protecting our environment is an important responsibility. This is why authorities give limits for pollutant emission like NOx (oxides of nitrogen), CO (carbon monoxide) and UHC (unburned hydrocarbons) for gas turbines. 
     In lean burn combustors an increased flow of air into the combustor leads to fuel to air ratios below the level where high levels of NOx is formed. The drawback of increased air flow is that it can cause instabilities in the combustion process resulting in highly fluctuating pressure amplitudes at frequencies below 1000 Hz for a typical combustion system which can cause hardware damages to the combustion chamber. 
     Combustion chambers are usually cooled by a flow of air along the chamber and through perforations also known as effusion holes arranged in the casing of the chamber. Air penetrating through the effusion holes into the combustion chamber forms a cooling film over the inner surface of the combustion chamber, the film reducing convective heat transfer between the combustion flame and the inner casing of the combustion chamber. 
     It has been proposed to use the air for both film cooling and damping of instabilities in the combustion process. However, the flow of cooling air has usually different characteristics like volume and velocity to a flow providing damping. 
     EP 1666795 describes an acoustic damper component arranged on the wall of a combustor with multiple damping chambers. The acoustic damper component has a first metering passage, a first damping chamber, a first damping passage, a second damping chamber and a second damping passage. Air flows through the damper to be ejected into the combustion chamber from the second damping passage at a selected velocity and volumetric flow, the flow being sufficient to damp instabilities from the combustion process. 
     GB2104965 shows a multiple impingement cooled structure which is coupled to effusion holes in the wall of an element to be cooled such as a turbine shroud. The structure includes a plurality of baffles which define a plurality of cavities. 
     EP0896193 shows a combined impingement and convective cooling configuration of the combustion chamber where substantially all air flow into the combustion chamber passes through the cooling passage before entering the combustion chamber, i.e. that all of the air utilized is used for both cooling and for mixing with the fuel, assuring good cooling of components and producing a lean mixture which acts to keep the levels of pollutants such as nitrous oxides low. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a combustor casing with improved damping and cooling characteristics. 
     This object is achieved by the claims. The dependent claims describe advantageous developments and modifications of the invention. 
     An inventive combustor casing comprises an inner casing which defines a combustion chamber, an outer casing spaced apart from the inner casing for defining a passage between the inner and the outer casing, effusion holes arranged in the inner casing, and dividing ribs connecting the inner and outer casings and forming at least first and second volumes for receiving part of a flow injected into the passage. 
     The invention exploits the phenomenon of air resonance in a cavity. Air forced into a cavity will make the pressure inside the cavity increase. When the external force that forces the air into the cavity disappears, the air with higher-pressure inside the cavity will flow out. Since this surge of air flowing out of the cavity will tend to over-compensate due to the inertia of the air, the cavity will be left at a pressure slightly lower than outside. Air will then be drawn back in again. Each time this process repeats the magnitude of the pressure changes decreases, meaning that the air trapped in the chamber acts like a spring, wherein the spring constant is defined by the dimension of the chamber. 
     Preferably the at least first and second volumes defined by the dividing ribs differ in size allowing for multiple frequencies attenuation. 
     It is advantageous when the first volume has first effusion holes with a first effusion hole diameter and the second volume has second effusion holes with a second effusion hole diameter and the first and second effusion hole diameters are different since this allows to optimize the damping performance. 
     Furthermore, it is advantageous when a first spacing between the first effusion holes differs from a second spacing between the second effusion holes since this further improves the damping performance. 
     In one advantageous embodiment the effusion holes and the at least first and second volumes are arranged in areas of previously determined antinodes of dynamic acoustic waves to be damped during operation of the combustion chamber. This allows the maximum of the acoustic energy to enter and dissipate in the attenuation volume. 
     In a preferred arrangement and in order to give maximum cooling, turbulators are arranged in the cooling passage providing turbulence of the air flowing down the cooling passage. The turbulators are preferably arranged on the inner casing and extend around the combustor, i.e. in a direction traverse to a flow direction. The turbulators are applied on the cooling surface to energise the thermal boundary layer for enhancing convective heat transfer coefficient. 
     In one advantageous embodiment the ribs extend along the passage parallel to a centre line of the combustor casing. This is the most common solution and the easiest to manufacture. 
     In another advantageous embodiment the ribs extend along the passage following at least partially a helical curve, thus creating near ring shaped resonators. 
     The invention is not restricted to can-type combustors. It is also applicable to annular combustors or sequential/reheat burners which require cooling due to the high burner inlet temperatures generated by the combustion in the upstream first stage combustion chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be further described with reference to the accompanying drawings in which: 
         FIG. 1  represents a general combustor cooling scheme, 
         FIG. 2  represents a side view on a combustion dynamic attenuation and cooling scheme, 
         FIG. 3  represents the same scheme as shown in  FIG. 2  seen from a different angle, 
         FIG. 4  shows the hole pattern of the effusion holes, 
         FIG. 5  shows an axial mode dynamic pressure wave on a combustor casing and the best locations for the attenuation and cooling arrangement, and 
         FIG. 6  shows a circumference mode on a combustor. 
     
    
    
     In the drawings like references identify like or equivalent parts. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A gas turbine engine comprises a compressor section, a combustor section and a turbine section which are arranged adjacent to each other. In operation of the gas turbine engine air is compressed by the compressor section and output to the burner section with one or more combustors.  FIG. 1  shows a general combustor scheme. The combustor  1  comprises a burner  2  with a swirler portion  3  and a burner head portion  4  attached to the swirler portion  3 , a transition piece being referred to as a combustion pre-chamber  5  and a main combustion chamber  6  arranged in flow series. The main combustion chamber  6  has a larger diameter than the diameter of the pre-chamber  5 . The main combustion chamber  6  is connected to the pre-chamber  5  at the upstream end  7 . The burner  2  and the combustion chamber assembly show rotational symmetry about a longitudinal symmetry axis. 
     Moreover, the main combustion chamber  6  and the pre-chamber  5  comprise an inner casing  8  and an outer casing  9 . 
     There is an internal space  10  between the inner casing  8  and the outer casing  9  which is used as cooling passage  11  for cooling the inner casing  8 . Air enters the cooling passage  11  through the cooling air entrance  12  and convectively cools the combustor wall, particularly the inner casing  8  by arrangements of turbulators and effusion holes arranged in the inner casing  8  to allow the cooling air to penetrate into the main combustion chamber  6  and to form a cooling film that provides an insulating layer and protects the inner casing  8  by limiting the convective heat transfer. To obtain a uniform film over the length of the combustor facing surface a number of axially spaced parallel rows of effusion holes is provided. 
     Part of the air exits the cooling passage  11  and enters the burner hood  13  (see arrows  14 ). A fuel duct  15  is provided for leading a gaseous or liquid fuel to the burner  2  which is to be mixed with in-streaming air in the swirler  3 . The fuel-air-mixture  16  is then led towards the primary combustion zone  17  where it is burnt to form hot, pressurised exhaust gas flowing in a direction indicated by arrow  18  to a turbine of the gas turbine engine (not shown). 
       FIG. 2  shows the cooling passage  11  looking into the flow direction with the outer casing  9  of the combustor  1  on the left and the inner casing  8  of the combustor  1  on the right side of  FIG. 2 . Effusion holes  19  are arranged in the inner casing  8 . The flow of air  20  through the effusion holes  19  provides film cooling of the inner side  21  of the inner casing  8  and damping. When a sound wave passes an effusion hole  19  a vortex ring is generated and some of the energy of the sound wave is dissipated into vortical energy that is subsequently transformed into heat energy. 
     Dividing ribs  22  extend along the cooling passage  11  connecting the inner  8  and outer casings  9  and dividing the volume within the cooling passage  11  into the required dynamic attenuation volumes shown as at least first and second volumes  23 ,  24 . Since different dynamic frequencies need different damping volumes, multiple frequencies can be attenuated by dividing the cooling passage space into different patches for the intended attenuation frequencies. Cooling air passes through theses volumes of the cooling passage  11  and partly enters the combustion chamber  6  through the effusion holes  19 . The at least first and second volumes  23 ,  24  and the effusion holes  19  arranged in the at least first and second volumes  23 ,  24  act as Helmholtz resonators.  FIG. 2  also shows turbulators  25  arranged in the cooling passage  11  on the inner casing  8 . 
     With reference to  FIG. 3  a topview of the inner casing  8  with dividing ribs  22  and effusion holes  19  is shown. Again, the dividing ribs  22  are not equally spaced to form at least first and second volumes  23 ,  24  in the cooling passage  11 . The turbulators  25  arranged in the cooling passage  11  on the inner casing  8  are extending in a direction traverse to a flow direction of cooling air  26 . 
       FIG. 4  is a view onto the inner casing  8  of the combustor  1  and shows the at least first and second volumes  23 ,  24  defined by the ribs  22  and different effusion hole patterns with first and second effusion holes  27 ,  28  in the respective volumes. The patterns can differ in different ways. The effusion hole diameters can be different and the effusion hole spacing can be different. Both parameters can be adapted to specific frequencies to optimize damping performance and can of course differ between different volumes. The flow direction of the main air flow is shown by the arrows  26 . 
       FIG. 5  shows a sectional view of a combustor  1 . An example of an axial mode dynamic pressure wave  29  on the combustor casing is indicated with antinodes  30  and nodal points  31 . Similarly  FIG. 6  shows an example of a circumference mode  32  of a combustor  1 . The best locations to place the attenuation effusion hole patterns are the anti-nodes  30  of the corresponding dynamic acoustic wave  29 ,  32 . 
     Even though the figures focus on can-type combustors the invention is not restricted thereupon. It is also applicable to annular combustors or sequential/reheat burners.