Patent Publication Number: US-9429042-B2

Title: Acoustic damping device for chambers with grazing flow

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to EP application 14157239.6 filed Feb. 28, 2014, the contents of which are hereby incorporated in its entirety. 
     TECHNICAL FIELD 
     The invention relates to the acoustic damping of combustion dynamics. Combustion dynamics in the meaning of this application comprises pulsations, acoustic oscillations, pressure and velocity fluctuations and what is called in the everyday language “noise”. 
     BACKGROUND OF INVENTION 
     Combustion dynamics occur in combustors of gas turbines, for example, as a consequence of changes in the fuel supply. Excessive pressure fluctuations may result in damage of machine components. For reasons of simplification subsequently the term “chamber” is used and comprises all locations where combustion dynamics occur. In these chambers a gas (for example a mixture of fuel and air or a hot combustion gas) flows with a high velocity. 
     To reduce these combustion dynamics it is well known in the art, to install acoustic damping devices like Helmholtz resonators, half-wave tubes, quarter-wave tubes or other types of damping devices with or without flow through of gas. 
     These acoustic damping devices may have one or more resonance frequencies. If under operation of the gas turbine the combustion dynamics stimulate the resonance frequencies of the acoustic damping devices, the combustion dynamics are reduced or damped. 
       FIG. 1  illustrates the reflection coefficient (Y-Axis) and its dependency from the frequency. 
     The line  1  shows the theoretical reflection coefficient when using an acoustic damping device with a resonance frequency of approximately 300 Hertz. As can be seen, at a frequency of 300 Hertz the reflection coefficient has a relative minimum of approximately 0.5. At frequencies of approximately 225 Hertz and 375 Hertz, the reflection coefficient has a local maximum of about 0.75. 
     To give an example: a combustion chamber of a gas turbine is equipped with an acoustic damping absorber having a resonance frequency of 300 Hertz. Assuming that under operation in this combustion chamber fluctuations ensue comprising frequencies of 300 Hertz it can be expected that due to the local minimum of the reflection coefficient at 300 Hertz the fluctuations with a frequency of 300 Hertz are effectively damped and reduced. 
     In technical experiments the applicant made measurements and compared the theoretical reflection coefficient (line  1 ) with measurements taken at a frequency range between 50 Hertz and 400 Hertz. 
     The measured values are illustrated in  FIG. 1  by dots  3 . 
     By comparing the measured values with the theoretical reflection coefficient (line  1 ) it can be seen that in the range between 250 Hertz and 350 Hertz the measured values  3  do not show a local minimum as should be expected. In other words: The acoustic damping device does not work sufficiently. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention, to provide an acoustic damper that is capable of damping effectively in a gas turbine under operation and therefore effectively reduces combustion dynamics ensued from operation of the gas turbine at certain frequencies. 
     This objective has been achieved by using an acoustic damper comprising a neck and a damping volume, wherein the neck comprises a mouth being in fluid connection with a chamber that comprises adjacent to the mouth of the neck at least one opening for sealing gas. 
     The sealing gas, air or any other suitable gas that flows through the at least one opening into the chamber has the effect of a “fence” or a shield that protects the mouth of the damper from grazing flow. In conjunction with the claimed invention grazing flow is the flow of a gas more or less parallel to a wall that comprises the mouth of the damper. This grazing flow has a main or preferred direction more or less perpendicular to the neck of the damper and therefore may disturb the bias flow of gas through the neck and the mouth into the damping volume. 
     By means of the claimed opening or a number of openings located adjacent to the mouth of a damper the grazing flow is deflected and therefore does not disturb the bias flow through the neck and the mouth of the damper and as a result the performance of the damper is improved. 
     In a preferred embodiment of the claimed invention the at least one opening for sealing gas is located upstream of the mouth so as to deflect the grazing gas flow away from the mouth of the damper. If this opening is located upstream of the mouth it most efficiently protects the mouth from the grazing gas flow. 
     To even more efficiently deflect the grazing flow away from the mouth of the damper it may be advantageous to provide two or more openings upstream of the mouth. In embodiments where the preferred direction of the grazing flow may change it is preferred if three, four or even more openings are located around the mouth of the damper so as to deflect the grazing flow independent from its actual direction of flow and to protect the mouth of the damper from the grazing flow. 
     To optimize the effect of the claimed openings adjacent to the mouth the openings for sealing gas may have a circular, elliptic or square cross section. Of course, the selection of a specific cross section of the openings may be based on the efficiency, i.e. an optimal deflection of the grazing gas flow and little sealing gas consumption. Reducing the flow of sealing gas raises the overall efficiency of a gas turbine, since supplying a sealing gas with a higher pressure than the pressure inside the chamber requires energy. 
     In principle, any suitable source of a high pressure gas that is available may be used for the aerodynamic shielding of grazing flows according to the invention. In case the damper is a flow through damper the sealing gas that flows through the opening to the chamber may be the similar to that gas that flows through the damper into the chamber. 
     The claimed invention may be based on any type of acoustic damper, for example a resonator with one or more damping volumes, a half-wave tube a quarter-wave tube, a multi-volume damper, a liner or any kind of acoustic flow-through damper. 
     The claimed invention also may be applied to dampers with no flow through of the acoustic damper type. 
     The claimed invention may preferably be applied if the mouth of the damper opens into a combustor chamber, a mixing chamber a plenum and/or an air channel of a gas turbine. 
     Further advantages and details of the claimed invention are subsequently described in conjunction with the drawings and their description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures show: 
         FIG. 1  The reflection coefficient of an exemplary acoustic damper with a resonance frequency at 300 Hertz, 
         FIG. 2  a combustor chamber with an acoustic damper as known from the prior art and 
         FIGS. 3 to 7  several embodiments of the claimed invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a schematic cross section of a chamber  5 , for example a combustion chamber CC of a gas turbine that is limited by at least one wall  7  comprising an inner surface  9 . As can be seen from  FIG. 2 , the chamber  5  is equipped with an acoustic damper  11  comprising a neck  13  and a damping volume  15 . The neck  13  connects the damping volume  15  to the combustion chamber  5 . The opening of the neck  13  towards the combustion chamber  5  is referred to as “mouth”  17  of the neck  13 . 
     The damping device  11  in this exemplary embodiment may be a Helmholtz resonator, but the claimed invention is not limited to this type of acoustic damping device. The claimed invention may be used in conjunction with any type of acoustic damping device like a half-wave tube, a quarter-wave tube and the like. The claimed invention may be used in conjunction with flow through acoustic damping devices and acoustic damping devices without flow through. 
     As can be seen from  FIG. 2 , the mouth  17  of the neck  13  and the inner surface  9  of the wall  7  have the same level. 
     In the chamber  5  more or less parallel to the inner surface  9  a gas flows. This gas has a preferred direction of flow (illustrated by the arrow  19 ) and is also referred to as grazing flow  19 . The preferred direction of this grazing flow  19  is essentially perpendicular to a bias flow  21  between the damping volume  15  and the combustion chamber  5  and disturbs the bias flow  21  through the neck  13 . This negative effect of the grazing flow  19  on the bias flow  21  reduces the performance of the damper  11  as has been explained in conjunction with  FIG. 1  above. 
       FIG. 3  illustrates a first embodiment of the claimed invention. The reference numerals used are the same as in  FIG. 2  and therefore only the differences are described in detail. 
     In  FIG. 3  the bias flow has a preferred direction of flow from left to right and therefore upstream of the mouth  17  in  FIG. 3  means on the left side of the mouth  17 . 
     In this embodiment the damper  11  is a flow through damper which means that the damping volume  15  is connected via the neck  13  with the combustion chamber  5 . At the opposite end of the damping volume  15  the damping volume  15  is connected via a small bore  23  to a further chamber R 1 . 
     As can be seen from  FIG. 3 , adjacent to the mouth  17  and upstream of the mouth  17  there is a further bore  25  with an opening  27 . The bore  25  connects chambers  5  and R 1 . 
     Since the pressure p R1  in the chamber R 1  is higher than the pressure p 5  in the chamber  5  sealing air flows through the bore  25  and the opening  27  from chamber R 1  into chamber  5 . Since the bore  23  has a rather small diameter its flow resistance is great and consequently the bore  23  restricts the bias flow  21  through the damper  11 . 
     The resulting pressure difference Δp (=p R1 −p 5 ) causes not only the bias flow  21  through damper  11 , but a flow  29  of sealing gas through bore  25  and opening  27 . 
     The flow resistance of the tube  23  is greater than the flow resistance of the neck  13 . This means that the pressure reduction Δp 23  at the bore  23  is greater than the pressure reduction Δp 13  at the neck  13  of the damper. In other words: Δp 23 &gt;Δp 13 . 
     This means that the tube  23  due to its small diameter and/or its length acts as a flow restrictor reducing the bias flow  21  through the neck  13 . 
     The chamber R 1  may be any high pressure environment, for example the hood or the liner pressure or a reservoir for cooling air. In most appliances of the claimed invention the chamber  5  is the combustion chamber of a gas turbine, but the claimed invention is not restricted to that. 
     The sealing gas flowing through bore  25  and entering the chamber  5  via opening  27  in a direction more or less perpendicular to the grazing flow  19  it deflects the grazing flow away from the mouth  17  of the damper  11 . 
     As described in conjunction with  FIG. 2  in the chamber  5 , there may be a grazing flow  19  whose velocity is far greater than the velocity of the bias flow  21 . 
     The flow resistance of the bore  25  is smaller than the flow resistance of the bore  23 . This can be achieved by providing a larger diameter to bore  25  than to bore  23 . 
     Consequently, a gas flow  29 , illustrated by an arrow through the bore  25 , is far greater than the bias flow  21  although the damper  11  and the bore  25  are supplied from the same chamber R 1  with air or gas and open into the same chamber  5 . 
     As can be seen by comparison of the arrows  29  and  19 , the velocity of the sealing gas flow through the bore  25  is even higher than the velocity of the grazing flow  19 . 
     The great velocity of the air or gas flow  29  through the bore  25  deflects the grazing flow  19  away from the inner surface  9  and away from the mouth  17  of the damper  11 , as is illustrated by the arrow  19 . 2  in  FIG. 3 . This effect is illustrated by the arrow  19 . 2  (deflected grazing flow). 
     Doing so, the grazing flow  19  does not reach the mouth  17  of the damper  11  and therefore the bias flow  21  is not disturbed by the grazing flow  19  anymore. Consequently, the efficiency and effectiveness of the damper  11  is high and independent from the grazing flow  19 . 
     Going back to  FIG. 1 , the behavior of the damper  11  according to the claimed invention is similar to the line  1  in  FIG. 1 . Of course, this is only an example and the same invention may be applied to dampers  11  with damping frequencies different from 300 Hertz. 
     In  FIG. 4  the same arrangement is shown in another perspective. In the right part of  FIG. 4  it can be seen that the air or gas  29  that exits the opening  27  enters the chamber  5  with a high velocity and protects the mouth  17  of the damper  11  from the grazing flow  19  by deflecting the grazing flow  19  away from the inner surface  9  and the mouth  17 . The gas or air entering the chamber  5  to the bore  25  is a wind shield  31  that protects the mouth  17  and the bias flow  21  of the damper from the grazing flow  19 . 
     In other words: The mouth  17  is on the leeward side of the “windshield  31 ” that generated by the flow  29  of air or gas through the bore  25 . Since the mouth  17  should be on the leeward side of the windshield  31  in most cases it is preferred that the at least one opening  27  is located upstream of the mouth  17 . 
     On the left side of  FIG. 4  a top view from the chamber  5  onto the inner surface  9  with the mouth  17  and the opening  27  is illustrated. It can be seen that the grazing flow  19  is also deflected in a lateral direction which further improves the effectiveness of the windshield  31 . 
       FIG. 5  illustrates a second embodiment of the invention with two bore  25  and  32  adjacent to the mouth  17  of the damper  11 . In this case, one opening  27  is upstream of the mouth  17  and a further opening  35  is downstream of the mouth  17 . As can be seen from  FIG. 6 , the windshield  37  derived from the air or gas stream through the opening  35  supports and reinforces the windshield  31  starting from the first opening  27 . 
     Therefore, the bias flow  21  through the mouth  17  is even better protected from the grazing flow. 
     In  FIG. 7  several designs and arrangements of the bores that serve to supply sealing gas or air  29  for building up a windshield  31  are illustrated. 
     The embodiment  7   a ) has already been described in conjunction with  FIG. 4 . 
     In the embodiment illustrated in  FIG. 7 b   ) the opening  27  has an elliptic cross-section which broadens the windshield  31  and therefore results in a better protection of the bias flow  21 . 
     In the embodiment illustrated in  FIG. 7 c   ) there are two openings  27  with an elliptic cross-section arranged upstream of the mouth  17 . 
     According to the embodiment, illustrated in  FIG. 7 d   ), there are five openings  27  with circular cross-sections located upstream of the mouth  17 . 
     In  FIG. 7 e   ) there is one opening  7  with a rectangular cross-section and in  FIG. 70  an embodiment is illustrated with four openings  27  with rectangular cross-section. 
       FIG. 7 g   ) illustrates an embodiment with one opening  12  and  27  with a bent cross section. 
     The embodiment illustrated in  FIG. 7 h   ) is known from  FIGS. 5 and 6 . The embodiments illustrated in  FIGS. 7 e   ) and  7   j ) illustrate further embodiments with three and four opening  27 .