Abstract:
A combustion system of a flow engine has at least one combustion chamber, a shared manifold to feed a flow medium via at least two injectors to the at least one combustion chamber and at least one resonator with at least one resonator cavity, wherein the at least one resonator is arranged functionally in/at the manifold. To obtain good combustion performance with a homogenous fuel flow rate and high combustion stability, the resonator includes at least one perforated section with at least two orifices, wherein the at least two orifices provide access to the at least one resonator cavity of the at least one resonator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2014/056803 filed Apr. 4, 2014, and claims the benefit thereof. The International Application claims the benefit of European Application Nos. EP13164908 filed 23 Apr. 2013, and EP13165158 filed 24 Apr. 2013. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a combustion system of a flow engine comprising at least a resonator having a resonator cavity and a method for determining a dimension of this resonator cavity of the aforementioned resonator. 
       ART BACKGROUND 
       [0003]    Combustors of a flow engine, like a gas turbine, comprise a combustion chamber provided with a combustion air inlet and with a fuel passage connected to a fuel injector arranged to discharge fuel into the combustion chamber. Combustion air as well as fuel enters the combustion chamber under pressure. The resultant combustion is basically very fast and generates dynamic forces in the form of pressure fluctuations, which may manifest on a casing of the combustor as vibrations. 
         [0004]    Moreover, combustion dynamics also occur due to a coupling between pressure oscillations and unsteady heat release. One of the major causes of unsteady heat release is due to fuel flow rate perturbations. These perturbations emerge from an acoustic pressure wave, entering the fuel passage as well as a downstream (direction in reference to entering direction) arranged fuel gallery and/or manifold through the fuel injector. This depends on the impedance at the fuel injector, which in turn is influenced by running conditions of the combustor, namely operation e.g. under full or part load or characteristics of the flame, by properties of the fuel, e.g. Wobbe indices, temperature or gaseous or liquid state of the fuel, by characteristics of the injector, like size or shape of its orifices, or by characteristics of the manifold, e.g. size or shape. Thus, the impedance at the fuel injection point may vary to a great deal. Consequently, it is highly possible that the acoustic wave penetrates the fuel gallery/manifold and establishes itself into a standing or rotating acoustic wave to perturb the fuel flow rate and thus combustion dynamics. 
         [0005]    When designing a combustion system, it is difficult to predict the frequency, amplitude and wave form of the vibrations influenced by the fuel flow rate and the combustion dynamics. As a consequence, it is particularly difficult to design a combustion system which is not subjected to undesirable vibration for part of its range of operation. Apart from having a mechanical effect on the hardware of the combustion system, the combustion gas dynamics also influence combustion stability and can cause extinguishing of the combustion flame—so-called “flame-out”—with the result that the engine stops producing power. 
         [0006]    From WO 93/10401 it is for example known to provide a fuel supply line of a gas turbine system with a so-called Helmholtz resonator to suppress combustion-cause vibrations. Such classical Helmholtz type resonators provided damping at a very specific frequency. Problems arise with this resonator types in case of the necessity to attenuate a broad range of frequencies. 
         [0007]    GB 1 274 529 discloses reheat equipment in an exhaust of a gas turbine engine the equipment comprising a V-section flame stabilizing ring. A fuel ring manifold having injection orifices is supported within the stabilizing ring, fuel being injected from the orifices through aligned orifices. Spaced plates are located within the stabilizing ring, the downstream plate being formed with orifices so that the assembly of plates forms a vibration damping device for damping combustion instabilities in the wake of the V-section flame stabilizing ring within the exhaust. 
         [0008]    US2006/0000220A1 discloses a resonator assembly in the form of a T-shaped pipe and having an inlet end and an outlet end and is connected to fuel flow passage of a fuel nozzle. The resonator assembly is used to produce the effect of wave shifting. 
         [0009]    EP 2 273 096 A2 discloses an engine having an exhaust gas pipeline for discharging of combustion gas from a cylinder, and a suction pipeline for delivering of fresh air into the cylinder. The cylinder is equipped with a plastic charge air cooler, which is arranged in the suction pipeline. A resonator is designed as a sound damper and provided downstream of the cylinder. The resonator is integrated in the charge air cooler. The suction pipeline comprises a charge air hose that is arranged between the cylinder and the charge air cooler. 
       SUMMARY OF THE INVENTION 
       [0010]    It is a first objective of the present invention to provide a combustion system of a flow engine, which provides damping of a broad range of frequencies, a good combustion performance with a homogenous fuel flow rate and high combustion stability as well as less combustion noise and vibrations. 
         [0011]    It is a further objective of the present invention to provide a method for determining a dimension of a resonator cavity to effectively suppress fuel flow fluctuations and damp combustion dynamics of the combustion system. 
         [0012]    These objectives may be solved by a combustion system and a method according to the subject-matter of the independent claims. 
         [0013]    According to a first aspect of the present invention, a combustion system of a flow engine is presented, comprising at least one combustion chamber, a shared manifold to feed a flow medium via at least two injectors to the at least one combustion chamber and at least one resonator with at least one resonator cavity, wherein the at least one resonator is arranged functionally in/at the manifold. 
         [0014]    It is proposed, that the resonator comprises at least one perforated section with at least two orifices, wherein the at least two orifices provide access to the at least one resonator cavity of the at least one resonator. 
         [0015]    Due to the inventive matter, fluctuations of a flow medium, like fuel, may be suppressed effectively and successfully. This results advantageously in a suppression of pressure fluctuations in the fuel flow which leads to reduction in combustion dynamics. Compared to a classical Helmholtz type resonator the inventive resonator has a very broad response to frequency, which is a very useful feature to have since the frequency of oscillations varies according to operating conditions and fuel composition. Moreover, equivalence ratio fluctuations in a combustor mixing region may be beneficially reduced leading to a better control of emissions, e.g. NOx emissions. Furthermore, a damage of pieces of the combustion system, like a combustor can, caused by combustion dynamic pulsations may be prevented. In addition, the life of downstream components such as turbine blades or vanes may be improved compared to state of the art systems through a reduction in heat release fluctuations. Advantageously, the combustion system may be operated over a wide range of Wobbe indices, resulting in flexibility concerning the used fuel. Further, by placing the resonator at the source of the flow fluctuation of the flow medium, namely the manifold, combustion dynamics may be effectively reduced compared to a state of the art system. Hence, this invention is also concerned with the interaction between the dynamic forces caused by combustion and those caused by the flow of fuel, air and other gases. 
         [0016]    In this context a flow engine is intended to mean any engine or machine suitable for a person skilled in the art, e.g. a thermal heating plant, a gas turbine or an internal combustion engine. Further, an injector is intended to mean a nozzle for e.g. fuel, air and/or other gases and especially a fuel nozzle. Moreover, the term “shared manifold” should be understood as “shared by the at least two injectors” or in case of an embodiment with at least two combustion chambers alternatively and/or additionally as “shared by the at least two combustion chamber” and the term “manifold” as a passage for a flow medium and especially as fuel manifold and/or fuel gallery. A flow medium may be any medium feasible for a person skilled in the art, like a fuel (gaseous), air or other gases. 
         [0017]    Furthermore, a resonator is intended to mean a device for modulating and/or damping perturbations, especially of a flow rate of the flow medium e.g. in the manifold and/or that is used to provide damping to an established standing or rotating wave e.g. in the manifold. A resonator should be understood as a functional unity, thus it may comprise more than one piece or may be a selected arrangement of several pieces. The resonator is especially a so called cavity resonator. The statement the “resonator is arranged functionally in/at the manifold” should be understood as a functional interaction of the parts and may be independent of a spatial arrangement of the resonator and the manifold. 
         [0018]    Moreover, a perforated section is intended to mean a part of the resonator embodied with a perforation, wherein the perforation has at least two orifices. Alternatively, the perforation may have more than two orifices or a plurality of orifices. The wording to “provide access to the at least one resonator cavity of the at least one resonator” should be understood as an access for a wave propagation of a wave that may be established in or travel in the manifold and/or flow medium during operation of the combustion system. A direction of the wave propagation is in particular from the manifold, precisely from its lumen, to the resonator cavity of the resonator. In the following text the phrases “at least one resonator/at least one resonator cavity/at least one perforated section/at least one combustion chamber/the at least two injectors/at least two orifices” are referred to as “the resonator/the resonator cavity/the perforated section/the combustion chamber/the two injectors/the two orifices”. 
         [0019]    An orifice may have any size or shape, like circular, oval, triangular, rectangular square, etc., feasible for a person skilled in the art. The dimensions of the orifices (like a height, a length or a diameter of an orifice) as also for example a number of orifices (see below) and/or a volume of the resonator cavity define and hence also depend on a target value of a peak resonance which is: 
         [0000]    
       
         
           
             
               
                 f 
                 peak 
               
               = 
               
                 
                   C 
                   
                     2 
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                      
                     π 
                   
                 
                  
                 
                   
                     V 
                     Sl 
                   
                 
               
             
             , 
           
         
       
     
         [0020]    (C=speed of sound, V=resonator volume, S=area of resonator orifices, l=height or length of orifices). Thus, all the above stated parameters could take different and/or any suitable values. 
         [0021]    It is further provided, that said orifice is embodied with a circular shape. Thus, the perforation can be easily manufactured. Advantageously, both orifices or all orifices, in case of a plurality of orifices, have a circular shape. In general, it would be also possible to embody the two orifices or all orifices or groups of orifices of one perforated section with different orifice shapes. 
         [0022]    In particular, the resonator has a shape of a linear body, like a pipe or a box, which has an axial extension. A slight bent in a part or the shape of the linear body should not hinder the definition of body as linear and with an axial extension. Advantageously, the perforated section is arranged basically along or basically in parallel to the axial extension of the linear body of the resonator. In the scope of an arrangement of the perforated section as “basically parallel” to an axial extension of the linear body should also lie a divergence of the strictly parallel arrangement of about 30°. In particular, the perforated section is oriented parallel to the axial extension of the linear body. Furthermore, the perforated section is in radial direction of the resonator spatially arranged between a part, like the lumen or a jacket, of the manifold and a part, e.g. a wall, of the resonator cavity of the resonator. Hence, access to the resonator cavity through the orifices can be easily provided. 
         [0023]    The resonator and the manifold may be constructed out of separate pieces or they may share pieces. In another embodiment the perforated section is formed in a part of the manifold. In other words, the perforated section is a part of the manifold or it is formed integrally with the manifold or the perforated section continues the shape of the manifold. Thus, parts, space, costs as well as construction efforts may be saved. 
         [0024]    Advantageously, the perforated section has a shape of a hollow cylinder. Consequently, a robust a reliable part that is easy to manufacture can be provided. The cylinder is e.g. a wall of the manifold. According to a further embodiment the perforated section comprises a plurality of orifices, which are distributed along a whole circumference of a jacket of the hollow cylinder. Due to this, the resonator has a very broad response to frequency, especially compared to a resonator with a hollow space, like a classical Helmholtz type resonator, where the resonator provides damping only at a very specific frequency. The inventive resonator is especially useful, since the frequency of oscillations varies according to operating conditions of the combustion system and the fuel composition. The inventive resonator thus may be called a resonator of a perforated liner type. These perforated liner type resonators provide broadband absorption in the frequency domain therefore would be adequate to provide damping to combustion dynamics of varying frequencies as the Wobbe index of the fuel varies. 
         [0025]    As stated above the resonator has a shape of a linear body having an axial extension and the perforated section may comprise a plurality of orifices. Beneficially, plurality of orifices is distributed along the whole extension of the linear body. This results in a reliable and good damping of frequencies of the combustion system. The linear body may be a rectangular box or a cylinder or a tubular pipe. Both the resonator cavity and the perforated section may be embodied as a linear body with the same direction of their axial extension or the directions of the axial extensions may diverge from one another (see below). 
         [0026]    To restrict the volume of the resonator the resonator cavity of the resonator is encased by walls. The resonator comprises at least one wall (referred to as the wall in the following text), which is arranged basically along the axial extension of the linear body of the resonator (for the definition of basically along see the definition of basically in parallel above). To provide a system that is easily assembled the wall of the resonator cavity and a part of the manifold, which is arranged basically along the axial extension of the linear body (definitions see above), are arranged like in a so-called—known in the art—pipe in pipe system. In this context the term “pipe in pipe system” should not be applied to strictly tubular arrangements. Also a combination of one or more circular pipe(s) with one or more rectangular pipe(s) or box(es) as well as the combination of two or more rectangular shapes should be understood as a “pipe in pipe system”. 
         [0027]    It is further provided, that the orifices of the plurality of orifices are evenly distributed over the at least one perforated section. Hence, the damping effect may be established homogeneously. In general, it would also be feasible to distribute the plurality of orifices in any matter a person skilled in the art sees as practicable, like randomly, in clusters, in lines (arranged axially, diagonally or circumferentially) etc. 
         [0028]    Furthermore, it has been shown that the feeding of flow medium to the combustion chamber is constructively easy and most reliable, if the manifold has, seen in feeding direction of the combustion chamber, a circular shape or in other words has an overall shape of a closed ring. Thus, in the exemplary and further embodiment, where the perforated section is a part of the manifold, the perforated section has a shape of a part cycle. Consequently, in that case, an axial extension of the perforated section is also a circumferential extension/direction (referred to the circumference of the manifold). 
         [0029]    Moreover, by an arrangement of the wall of the resonator cavity and the part of the manifold, which is arranged basically along the axial extension of the linear body, as a strictly pipe in pipe system, the axial extensions are both oriented in direction of the circumference of the manifold. Whereas the axial extensions differ slightly by an arrangement of a circular manifold (e.g. with integrated perforated section) and a linear body of the resonator embodied as a rectangular box or as a cylindrical pipe. Here the axial extension of the manifold is in direction of the circumference of the manifold and the resonator box/cylindrical pipe has a classical axial extension. 
         [0030]    According to a further exemplary embodiment the manifold is embodied as a torus, advantageously, as a ring torus. Thus, the transporting of flow medium in the manifold can be accomplished homogeneously. 
         [0031]    Furthermore, the combustion system comprises a selected number of injectors and a selected number of resonators, wherein the number of resonators is equal or less than the number of injectors. Due to this, the damping of the perturbations is most effective. Moreover, the combustion system may comprise one or more combustion chambers and a selected number of resonators, wherein the number of resonators is equal or less than the number of combustion chambers. Further, measures may be taken for each injector and/or combustion chamber or its feeding region in the manifold, respectively, individually by selectively choose or adjust different resonators depending on their position in reference to the different injectors and/or combustion chambers. 
         [0032]    According to a further aspect of the present invention, the resonator has at least one restriction device (referred to as the restriction device in the following text) that is constructed to prevent a flow of flow medium through the two orifices from the manifold into the resonator cavity. Thus, a leakage of flow medium and therewith a possibility of further unwanted perturbations may advantageously be prevented. The restriction device may be any feature feasible for a person skilled in the art, like a membrane or a pressure. 
         [0033]    Beneficially, the restriction device is a pressure in the resonator cavity. 
         [0034]    The advantages of the restriction device may be constructively easy accomplished if a pressure in the resonator cavity is higher than a pressure in the manifold to prevent a flow of flow medium through the two orifices from the manifold into the resonator cavity. This is not required during resonance condition as pressure in the cavity will always be higher than pressure in the manifold during resonance. However, a pressure difference across the perforation may be required, say during shut down, to purge any flow that may be in the cavity out to the manifold. 
         [0035]    When the combustion system comprises at least a second resonator (referred to as second resonator in the following text) damping could be increased compared to the use of only one resonator. Moreover, two or multiple resonators in the same fuel line avoid placing a resonator in a pressure node in the case of an established standing wave in the manifold pipe. 
         [0036]    In an advantageously embodiment of the invention the first resonator and the second resonator differ in at least one characteristic. This results in the possibility to adjust each resonator specifically for its function and/or position. This characteristic may be any function, feature, property, or parameter of the resonator or parts thereof or their arrangement or function together feasible for a person skilled in the art. This may be for example a feature of the perforated section, like an axial (circumferential) length, a diameter, a shape, a size or a pattern of the orifices etc.; or of the linear body, like an axial length, a diameter etc.; or of the resonator cavity of the resonator, like a volume, a shape etc.; or of a special arrangement of a resonator in respect to a specific injector and/or combustion chamber. 
         [0037]    Hence, different resonators could have e.g. different orifice sizes, shapes and lengths depending on the frequencies that should be attenuated. For example if multiple frequencies should be damped and they should be attenuated at the same time, then resonators may be designed with different orifice sizes/shapes and resonator lengths such that each resonator deals with a particular frequency. Moreover, since in case of a circular manifold the feeding of fuel to the successively arranged injectors and/or combustion chambers is consecutively, the conditions differ for each injector and/or combustion chamber. This could be considered by designing each resonator differently and individually. 
         [0038]    The resonator (s) will be designed to provide optimum damping around the combustion dynamics frequency, using the Helmholtz equation, and specifically in dependency of beforehand determined conditions using the Helmholtz equation. 
         [0039]    During operation of the combustion system the perturbations establish themselves as a standing or rotating wave e.g. in the manifold. If it is a rotating wave then the position of the resonator (s) may not be of a concern. In turn, if it is a standing wave, the location of the resonator had to be purposefully chosen. Advantageously, the resonator is placed in a location of a pressure anti-node of the standing wave established in the manifold due to perturbations of a flow rate of a flow medium travelling in the manifold. However, multiple resonators in such locations may be required to damping the perturbations to an acceptable level. 
         [0040]    In a further advantageous embodiment the resonator cavity has a volume, which is adjustable. By way of this the frequency of damping of the resonator may be controlled. Hence, the combustion system is allowed to operate over a wide range of Wobbe indices i.e. providing a high flexibility in the choice of fuel. It would be either possible to vary the volume beforehand of the operation of the combustion system according to expected conditions or during the operation of the combustion system depending of in situ measurements. 
         [0041]    In particular, the resonator has an adjustable wall to adjust the volume and the resonator capacity of the resonator. This would be a very easy and effective construction. 
         [0042]    According to a further aspect of the present invention, a method for determining a dimension of a resonator cavity of a resonator of a combustion system of a flow engine is presented. 
         [0043]    It is provided, that the method comprises at least the following steps: determining at least one frequency that will be established in the combustion system under defined conditions and that needs modulation to provide optimum damping around a combustion dynamics frequency of the combustion system, evaluating the dimension of the resonator cavity of the resonator using the Helmholtz equation in dependency of the at least one determined frequency and adjusting the dimension according to the evaluation. 
         [0044]    The method can be extended where two or more cavities are provided either in one resonator or two or more resonators. 
         [0045]    The method comprises the steps of determining a range of frequencies that will be established in the combustion system under defined conditions and that needs modulation to provide optimum damping around a combustion dynamics frequency of the combustion system, evaluating the dimension of each resonator cavity of the resonator using the Helmholtz equation in dependency of the determined at least one frequency and adjusting the dimension of each resonator cavity according to the evaluation to provide different cavity characteristics and which cover the range of frequencies. 
         [0046]    Due to this inventive matter, the resonator will be designed to provide optimum damping around the combustion dynamics frequency. This results advantageously in a suppression of pressure fluctuations in the fuel flow. Thus, a combustion system may be operated even if frequency of oscillations varies according to operating conditions and fuel composition. Moreover, equivalence ratio fluctuations in a combustor mixing region may be beneficially reduced leading to a better control of emissions, e.g. NOx emissions. 
         [0047]    Furthermore, a damage of pieces of the combustion system, like a combustor can, caused by combustion dynamic pulsations may be prevented. In addition, the life of downstream components such as turbine blades or vanes may be improved compared to state of the art systems through a reduction in heat release fluctuations. Advantageously, the combustion system may be operated over a wide range of Wobbe indices, resulting in flexibility concerning the used fuel. Further, by placing the resonator at the source of the flow fluctuation of the flow medium, namely the manifold, combustion dynamics may be effectively reduced compared to a state of the art system. Hence, this invention is also concerned with the interaction between the dynamic forces caused by combustion and those caused by the flow of fuel, air and other gases. 
         [0048]    All steps may be executed beforehand of the operation or during the operation of the combustion system. For example, it would be either possible to adjust the dimension of the resonator cavity beforehand of the operation of the combustion system according to expected conditions or during the operation of the combustion system depending of in situ measurements, in other words, to have a dynamic system. 
         [0049]    The above-described characteristics, features and advantages of this invention and the manner in which they are achieved are clear and clearly understood in connection with the following description of exemplary embodiments which are explained in connection with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0050]    The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited. 
           [0051]      FIG. 1 : shows a schematic view of a flow engine with an inventive combustion system comprising a manifold with a resonator feeding a flow medium to combustion chambers via injectors, 
           [0052]      FIG. 2 : shows a schematic back view of a part of the combustion system from  FIG. 1  with six combustion chambers and a shared manifold, 
           [0053]      FIG. 3 : shows a schematic view of the manifold from  FIG. 2  with two resonators and an established wave, 
           [0054]      FIG. 4 : shows a schematic view of a resonator from  FIG. 3  with a perforated section as a part of the manifold from  FIG. 3  and 
           [0055]      FIG. 5 : shows a perspective view of the part of the manifold and the resonator with the perforated section from  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0056]    The illustrations in the drawings are schematically. It is noted that in different figures, similar or identical elements are provided with the same reference signs. 
         [0057]      FIG. 1  shows in a schematically view a flow engine  12 , e.g. a gas turbine. The flow engine  12  comprises a compressor  48 , a combustion system  10  and a turbine  50 , which are arranged one after another in a flow direction  52  of a first flow medium, e.g. air (see arrows pointing from left hand side to right hand side of  FIG. 1 ). In the compressor  48  the incoming first flow medium is compressed for application to one or more combustors of the combustion system  10 . The combustion system  10  may comprise one combustion chamber e.g. of an annular type or several combustion chambers  14 ,  16  e.g. of a can type (see also  FIG. 2 ). A second flow medium, like fuel (referred to as fuel in the following text), is introduced into the combustion chambers  14 ,  16  from a shared manifold  18  via a separate fuel line  54 ,  54 ′ and an injector  56  for each combustion chamber  14 ,  16  separately. After injection the fuel is mixed with a part of the compressed first flow medium leaving the compressor  48 . Hot gases created by combustion in the combustion chambers  14 ,  16  are directed to the turbine  50  having a set of turbine blades  60 , being guided in the process by a set of guide vanes  58 , and the turbine blades  60  and the shaft forming an axis are turned as a result. The turbine blades  60  in turn rotate the blades of the compressor  48 , so that the compressed flow medium is supplied by the flow engine  12  itself once this is in operation. 
         [0058]      FIG. 2  shows a schematic back view of the combustion system  10  in direction of the arrows II in  FIG. 1 . The combustion system  10  has in this exemplary embodiment six combustion chambers  14 ,  16  (only two combustion chambers are equipped with reference signs), which are arranged one after another evenly spaced in a circumferential direction  62  around the turbine shaft. The manifold  18  is arranged, in respect to the combustion chambers  14 ,  16 , with a greater radial distance from the shaft than the combustion chambers  14 ,  16 . Moreover, it has the shape of a closed ring and/or is a circular pipe and, as could be seen in  FIG. 1 , is embodied as a torus. The manifold  18  is a shared manifold  18 . Thus, all combustion chambers  14 ,  16  are fed via the same manifold  18  with fuel from the same source. 
         [0059]    During operation of the combustion system  10  e.g. self-excited combustion oscillations may manifest in the combustion chambers  14 ,  16 . These oscillations or vibrations travel through the fuel lines  54 ,  54 ′ and into the manifold  18 . Inside the manifold  18 , either a travelling (rotating) or standing acoustic wave  46  is formed. The wave  46  may fluctuate the fuel being fed to the fuel manifold  18 . 
         [0060]    Consequently, the flame in the combustion chamber  14 ,  16  is fed with an unsteady (fluctuating) fuel supply which causes even more combustion dynamics. This situation is schematically shown in  FIG. 3  that depicts a snap-shot of an established wave  46  in the manifold  18  due to perturbations of the flow rate of the fuel travelling in the manifold  18 . 
         [0061]    To reduce or in particular suppress the fuel flow fluctuations and thus to obtain a reduction in combustion dynamics the combustion system  10  comprises in this exemplary embodiment two resonators  20 ,  20 ′ (In the following text also specified as the first resonator  20  and the second resonator  20 ′ to distinguish when needed between the two resonators  20 ,  20 ′)/wherein each resonator  20 ,  20 ′ has a resonator cavity  22  (for details see  FIGS. 4 and 5 ). The resonators  20 ,  20 ′ are arranged functionally in the manifold  18 . Hence, by placing the resonators  20 ,  20 ′ in the manifold  18  they are placed directly at the source of fuel flow fluctuations. 
         [0062]    The combustion system  10  comprises a selected number of injectors  56  as well as combustion chambers  14 ,  16 , specifically in this exemplary embodiment in each case six, and a selected number of resonators  20 ,  20 ′, namely in this exemplary embodiment two, thus the number of resonators  20 ,  20 ′ is less than the number of injectors  56  and combustion chambers  14 ,  16 . If the established wave  46  is a standing wave  46  at least one of the resonators  20  is placed in a location of a pressure anti-node  44  of the standing wave  46 . By placing at least two resonators  20 ,  20 ′ non-periodically it could be easily avoided to have a resonator  20 ,  20 ′ accidently in a location of a pressure node (not shown). 
         [0063]      FIGS. 4 and 5  show exemplarily the first resonator  20  from  FIG. 3  in more detail and in a linear configuration for better presentability. Generally all the features described for resonator  20  could also be applied to resonator  20 ′. The resonator  20  has a shape of a linear, rectangular, box-like body  34  having an axial extension  36 . The linear body  34  is only shown partially in  FIG. 1  (without reference sign) and in phantom in  FIG. 1  and  FIG. 5  for better presentability of the perforated section  24  (see below). In general, it would also be possible to construct the linear body  34  with a slight bent to follow the shape or the circumference of the manifold  18  or as a cylindrical linear or bended pipe. 
         [0064]    Moreover, to provide a broad response to frequency the resonator  20  comprises a perforated section  24  with at least two or a plurality of orifices  26 . The orifices  26  provide access for a wave propagation of the wave  46  from a lumen  64  of the manifold  18  to the resonator cavity  22  of the resonator  20 . All orifices  26  are embodied with a circular shape. Further, the perforated section  24  has a shape of a hollow cylinder  28  (see  FIG. 5 ). Hence, it is a perforated liner. 
         [0065]    The orifices  26  of the plurality of orifices  26  are evenly distributed over the perforated section  24  and along a whole circumference  30  of a jacket  32  of the hollow cylinder  28  (see  FIG. 5 ). Furthermore, they are distributed along the whole axial extension  36  of the linear body  34 . The perforated section  24  is formed in a part of the manifold  18  or is a part of the manifold  18 . 
         [0066]    To prevent that fuel enters through the orifices  26  the resonator cavity  22  of the resonator  20 , the resonator  20  is embodied with a restriction device. The restriction device is a pressure P in the resonator cavity  22 , wherein the pressure P in the resonator cavity  22  is higher than a pressure p in the manifold  18 . A pressure difference of  0 . 5  bar would for example be sufficient. 
         [0067]    The resonator cavity  22  of the resonator  20  is encased by six walls, wherein two of these walls are walls  38  that are arranged along or in parallel to the axial extension  36  of the linear body  34  of the resonator  20 . These walls  38  and a part  40  of the manifold  18 , which is also arranged along or in parallel to the axial extension  36  of the linear body  34 , are arranged like in a so-called pipe in pipe system  42 . The part  40  of the manifold  18  is at least a region of the perforated section  24 . Thus, the axial extension  36 , the walls  38  and the part  40 —the region of perforated section  24 , are arranged in parallel to one another. 
         [0068]    As could be seen in  FIG. 3  the first resonator  20  and the second resonator  20 ′ differ in a characteristic, specifically, as an exemplary embodiment, in their axial length L. Thus, both resonators  20 ,  20 ′ are designed specifically for their location and function in reference to the locations and properties of the injectors  56  and the combustion chambers  14 ,  16 . In general, the resonators  20 ,  20 ′ may differ in more than one characteristic and/or in another characteristic (see listing above). 
         [0069]    The resonators  20 ,  20 ′ will be designed to provide optimum damping around the combustion dynamics frequency, using the Helmholtz equation, and specifically in dependency of beforehand determined conditions using the Helmholtz equation, which is: 
         [0000]    
       
         
           
             
               
                 f 
                 peak 
               
               = 
               
                 
                   C 
                   
                     2 
                      
                     
                         
                     
                      
                     π 
                   
                 
                  
                 
                   
                     V 
                     Sl 
                   
                 
               
             
             , 
           
         
       
     
         [0070]    C=speed of sound, V=resonator volume, S=area of resonator orifices, l=height or length of orifices. Thus, the determines conditions and the peak resonance frequency that will need damping define the dimensions of the orifices  26  (like a height, a length or a diameter), a number of orifices  26  and/or a volume V of the resonator cavity  22 . 
         [0071]    Hence, to determine a dimension (volume) of the resonator cavity  22  of exemplarily the resonator  20  at least one frequency and in particular a broad range of frequencies that will be established in the combustion system  10  under defined conditions and that need(s) modulation to provide optimum damping around the combustion dynamics frequency of the combustion system  10  is determined. Subsequently, the dimension of the resonator cavity  22  of the resonator  20  using the Helmholtz equation in dependency of the at least one determined frequency or the determined range of frequencies is evaluated and finally the dimension according to the evaluation is adjusted (not shown in detail). 
         [0072]    Alternatively and/or additionally the resonator cavity  22  has a volume V, which is adjustable. Therefore, a moveable wall  66  is provided. That may be any of the walls encasing the resonator volume  22 . As an exemplary embodiment the moveable wall  66  is one of the walls  38 , arranged in parallel to the axial extension  36  of the linear body  34 . This is shown in  FIG. 4  as a dashed line with a bidirectional arrow. 
         [0073]    It should be appreciated that there is no overall or net fuel flow through the present resonator arrangement ( 20 ,  20 ′), in other words, there is no inlet or outlet. The present resonator acts as an accumulator for a fluid such as the fuel. The resonator is an accumulator of a set volume and pressure balances as it acts as an expansion volume to damp the fuel flow perturbations and pressure variances or waves. 
         [0074]    Where two or more resonators ( 20 ,  20 ′) are provided on the manifold  18  each resonator may define different volumes from one another. Each volume may be determined to attenuate different frequencies. The different volumes of each resonator can be set by virtue of setting a ‘height’ or ‘length’ in a direction away from the orifices  26 ; in the case of  FIG. 4 , the length of the resonator is a (radial) distance from the orifices  26  to the wall  38 . The distance can be a radial distance with respect to the central axis of the manifold  18 . Thus an even broader range of frequencies can be attenuated. 
         [0075]    In another embodiment, the area of the orifices can be different between the at least two resonators, such that a first resonator has a first total area of orifices and a second resonator has a second total area of orifices. The first area of orifices is greater than the second area of orifices. A greater total area of orifices can be achieved by virtue of a greater number of orifices or a greater area of each or some of the orifices in a plurality of orifices associated to each volume. 
         [0076]    As can be seen in  FIGS. 4 and 5  the perforated section  24  is a wall of the manifold  18  and therefore defines the flow path of the fuel. The resonator  20 ,  20 ′ is shown as completely surrounding an axial extension or length  36  of the manifold  18 . With respect to the central axis of the manifold  18 , the resonator&#39;s  20 ,  20 ′ volume  22  is defined by radially outer walls and radially inwardly by the wall of the manifold  18 . Thus the fuel flow in the manifold has direct access to the volume  22  from the manifold  18  via the perforated section  24 . 
         [0077]    Although  FIGS. 4 and 5  show a resonator  20 ,  20 ′ having a single cavity or volume  22 , it is possible that the volume  22  is circumferentially segmented. Each circumferential sub-volume may comprise a different volume than the other (s) sub-volume (s). For example there may be two, three or more sub-volumes. In this example, the perforated section  24  may have a constant perforation size and density and thus the same area of resonator orifices. Alternatively, the perforated section associated with each sub-volume may have a different area of resonator orifices. This may be achieved by different density of orifices and/or a different size of orifices. 
         [0078]    It should be noted that the term “comprising” does not exclude other elements or steps and “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims. 
         [0079]    Although the invention is illustrated and described in detail by the preferred embodiments, the invention is not limited by the examples disclosed, and other variations can be derived therefrom by a person skilled in the art without departing from the scope of the invention.