Patent Publication Number: US-6705428-B2

Title: Exhaust gas system with helmholtz resonator

Description:
FIELD OF THE INVENTION 
     The invention relates to an exhaust gas system for industrial gas turbines with an exhaust gas conduit and a chimney connected to it, as described in the preamble to claim  1 . Residential zones and installations which are operated by gas turbines, such as combined heat and power stations, are becoming increasingly close together. In order to keep the noise annoyance to the population at a low level, noise emission restrictions have become more and more severe in recent years. In many places, restrictions on low-frequency noise have been introduced in addition to the existing restrictions on high and medium frequencies. The noise emission from a gas turbine installation principally takes place via its exhaust gas system. The occurrence of the low-frequency noise, which is difficult to deal with, has many causes and may be attributed inter alia to pulsations in the combustion space. 
     BACKGROUND OF THE INVENTION 
     So that restrictions on low-frequency noise emissions can be met, absorption noise suppressors have been installed in the exhaust gas system of gas turbine installations, as is mentioned for example in DE-A1-44 19 604 and DE-A1-40 09 072. This is intended to reduce the low-frequency noise at the location at which its radiation into the surroundings takes place. Whereas, however, noise in the high and medium frequency ranges can be relatively successfully absorbed with absorption noise suppressors, low-frequency noise is difficult to deal with because conventional noise suppressors only exhibit a slight noise suppression effect at low frequencies. In order to permit reduction in low-frequency noise, it is therefore necessary to install large absorption noise suppressors with suppression mats of up to 800 mm thickness in the exhaust gas system of the installation. This increases the space requirements of the exhaust gas installation, reduces its power in some circumstances because of the pressure drop in the system and is, in addition, very complicated with respect to assembly and maintenance. In consequence, the exhaust gas system becomes very expensive. 
     SUMMARY OF THE INVENTION 
     The object of the invention is therefore to create an exhaust gas system of the type mentioned at the beginning in which low-frequency noise emissions are efficiently reduced without the power of the installation being essentially impaired and which, in addition, is simple and economical with respect to assembly and maintenance. This object is achieved by means of an exhaust gas system with the features of claim 1. In an exhaust gas system for industrial gas turbines, an exhaust gas conduit and a chimney connected to it together form a continuous flow duct. A Helmholtz resonator is acoustically coupled on the flow duct in the exhaust gas system. The Helmholtz resonator is precisely tuned to the low frequency which has to be suppressed. For this purpose, it demands less space than an absorption noise suppressor. The assembly of a Helmholtz resonator is very simple and, at large flow velocity, its useful life is much higher than that of absorption noise suppressors. In addition, the employment of Helmholtz resonators does not cause any decrease in the power of the installation. For these reasons, the exhaust gas system can be more easily assembled and maintained and the overall installation can be operated more economically. 
     If the inlet opening of the Helmholtz resonator is located in the region of the pressure maximum of an acoustic mode in the exhaust gas system, its efficiency is at a maximum. 
     It is very advantageous to locate the Helmholtz resonator in the transition region between the exhaust gas duct and the chimney because, as a rule, there are hardly any space problems in this area. It is particularly favorable to provide the Helmholtz resonator on the chimney rear wall, which bounds the exhaust gas duct in the flow direction, because this permits particularly simple assembly. 
     In a preferred embodiment, the dimensions of the exhaust gas duct and the chimney are selected in such a way that a pressure maximum of the acoustic mode occurs in the transition region between the exhaust gas duct and the chimney. In this way, the Heimholtz resonator can be very simply assembled, as described above, and is in addition extremely efficient. 
     Thermal insulation of the Helmholtz resonator from the outside ensures an approximately constant temperature of the Helmholtz resonator and, therefore, frequency stability of its absorption properties. 
     If the Helmholtz resonator has a throat which can be adjusted in its length and/or its cross section, the Helmholtz resonator can be better adjusted to the frequencies to be absorbed. 
     In a further preferred embodiment, the Helmholtz resonator has an adjustable volume. This again provides a simple possibility for matching to the frequencies to be absorbed. The adjustable volume can be very simply realized if the height of the side walls is arranged to be adjustable by means of a displaceable base. 
     The Helmholtz resonator can be matched particularly simply to the frequency to be absorbed if its temperature is adjustable. The temperature adjustment capability can, for example, be simply realized by attaching heating elements to the outer walls of the Helmholtz resonator. Another low-cost possibility consists in designing the Helmholtz resonator so that medium can flow around it in such a way that, for the purpose of temperature regulation, either hot exhaust gas is branched from the exhaust gas system and guided around the outer walls of the Helmholtz resonator or cold air flows around the latter. 
     In a further preferred embodiment, the Helmholtz resonator is screened in an acoustically transparent manner from the flow in the flow duct. This permits improved noise absorption by the Helmholtz resonator. Such screening can be very simply and expediently realized by means of an absorption noise suppressor located between the inlet opening of the Helmholtz resonator and the flow. 
     It is particularly advantageous to use an absorption noise suppressor which has the following approximate construction: A first perforated cover is part of a wall bounding the flow duct. A flow-resistant fabric and a layer of absorption material, which is located on the side of the perforated cover facing away from the flow duct, adjoins this first perforated cover. A second perforated cover follows this layer of absorption material on the side facing away from the flow duct. The absorption noise suppressor is laterally enclosed by side walls. Such an absorption noise suppressor can accept loads satisfactorily when bounding a flow duct with high flow velocities. 
     If a hollow space is arranged between the absorption noise suppressor and the inlet opening of the Helmholtz resonator, this has a positive effect on the vibration behavior of the Helmholtz resonator and therefore on its absorption capability. 
     It is very advantageous to provide a plurality of Helmholtz resonators in the exhaust gas system. These can then be located at different locations in the exhaust gas system, for example where respective maxima of the sonic modes occur. They can also be tuned to different low frequencies and, in this way, contribute to an even more effective reduction in the low-frequency noise. For this purpose, they can be located at different locations in the exhaust gas system or also close together. In order to ensure good noise absorption, however, the Helmholtz resonators should be separated from one another in a gas-tight manner. 
     Other preferred embodiments are the subject matter of further sub-claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter is explained in more detail below using preferred embodiment examples, which are represented in the attached drawings. In these, and purely diagrammatically: 
     FIG. 1 shows an exhaust gas system according to the invention with Helmholtz resonator; 
     FIG. 2 shows, in a diagrammatic section along the longitudinal axis of the flow duct, a part of an exhaust gas system according to the invention with Helmholtz resonators arranged beside one another; 
     FIG. 3 shows a view along the section line III—III in FIG. 2 of the Helmholtz resonators from FIG. 2 arranged beside one another; and 
     FIG. 4 shows a diagrammatic section through a Helmholtz resonator with throat adjustable in length and adjustable volume. 
     The designations used in the drawings and their significance are listed in summarized fashion in the list of designations. Fundamentally, the same parts are provided with the same designations in the figures. The embodiments described represent an example of the subject matter of the invention and have no limiting effect. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a sketch of an exhaust gas system  10  for a gas turbine installation (not shown) with an exhaust gas duct  12  and a chimney  14 . Exhaust gas duct  12  and chimney  14  together form a flow duct  16 . The flow direction of the exhaust gas  18  in the flow duct  16  is designated by arrows S. In a transition region  20  between exhaust gas duct  12  and chimney  14 , the exhaust gas duct  12  is bounded in its flow direction S by a rear wall  22  of the chimney  14 . In the transition region  20 , a Helmholtz resonator  24  is located on the rear wall  22  of the chimney  14 . The Helmholtz resonator  24  is screened from the flow in the flow duct  16  by a perforated cover  26 , which forms a part of the rear wall  22  of the chimney  14 , and by an acoustically transparent fabric  28  arranged behind the perforated cover  26  viewed from the flow duct  16 . 
     The exhaust gas duct  12  and the chimney  14  are dimensioned in such a way that a pressure maximum of a sonic mode is located in the transition region  20  or in the inlet region  30  of the Helmholtz resonator  24 . The Helmholtz resonator  24  is thermally insulated from the outside so that it takes up an approximately constant temperature during operation. In the exhaust gas system  10 , absorption noise suppressors  32  are located in a known manner in the exhaust gas system  10 , in addition to the Helmholtz resonator  24 , in order to absorb noise in the high and medium frequency ranges. 
     As is indicated by dashed lines in FIG. 1, it is also possible to locate the Helmholtz resonator  24  at other positions in the exhaust gas system  10  or even to locate a plurality of Helmholtz resonators  24 ,  24 ′,  24 ″, . . . at various positions in the exhaust gas system  10 . In order to achieve a good noise absorption efficiency, the Helmholtz resonator or Helmholtz resonators  24 ,  24 ′,  24 ″, . . . should be located in the exhaust gas system  10  where a pressure maximum of a sonic mode is located. 
     FIGS. 2 and 3 show, in various views, a part of an exhaust gas system  10  in which three Helmholtz resonators  24 ,  24 ′,  24 ″ are located beside one another in the transition region  20  between exhaust gas duct  12  and chimney  14  on the rear wall  22  of the chimney  14 . The dimensions of the exhaust gas duct  12  and the chimney  14  are in turn designed in such a way that the pressure maximum of a sonic mode is located in the transition region  20  or in the inlet region  30  of the Helmholtz resonators  24 ,  24 ′,  24 ″. The three Helmholtz resonators  24 ,  24 ′,  24 ″ are configured in a cylindrical hollow body  34 . The hollow cylinder  34  is screened from the flow duct  16  by an upstream absorption noise suppressor  36 . An intermediate wall  38 , which together with the absorption noise suppressor  36  encloses an intermediate space  44 , is arranged in the hollow cylinder  34  at a distance from this absorption noise suppressor  36 . On the side opposite to the intermediate wall  38 , the hollow cylinder  34  is closed in a gas-tight manner relative to the outside by a base  40 . The whole of the hollow cylinder  34  and also the base  40  are thermally insulated from the outside so that, during operation, the hollow cylinder  34  approximately adopts the temperature which is present in the flow duct  16 . 
     The absorption noise suppressor  36  has, essentially, the usual construction. The absorption noise suppressor  36  is bounded, relative to the flow duct  16 , by a perforated cover  26 , which forms a part of the rear wall  22  of the chimney  14 . Behind the perforated cover  26  is a flow-resistant and wear-resistant fabric  28 , for example a metal fabric, but one which is acoustically transparent. Following in layer construction on the fabric  28 , there is a layer of absorption material  46 , which can be constructed in one or a plurality of layers to match the frequency range to be absorbed. The material and the thickness of the absorption material  46  are respectively determined by the requirement. Finally, a further perforated cover  48  is located towards the intermediate space  44 . The shell of the hollow cylinder  34  also forms the side walls for the absorption noise suppressor  36 . 
     The hollow space of the hollow cylinder  34  remaining between the intermediate wall  38  and the base  40  is subdivided into three sectors by means of walls  42 , which sectors form the volumes  25 ,  25 ′,  25 ″ of the three Helmholtz resonators  24 ,  24 ′,  24 ″. The walls  42  close off the Helmholtz resonators  24 ,  24 ′,  24 ″ in a gas-tight manner relative to one another. Each Helmholtz resonator  24 ,  24 ′,  24 ″ is acoustically connected, by means of a tubular throat  47  which is led through the intermediate wall  38 , to the intermediate space  44  located between the upstream absorption noise suppressor  36  and the intermediate wall  38 . Low-frequency noise which is not absorbed by the absorption noise suppressor  36  is fed into the intermediate space  44  and on into the three Helmholtz resonators  24 ,  24 ′,  24 ″. The number and shape of the Helmholtz resonators  24 ,  24 ′,  24 ″ shown here can be altered as required. A Helmholtz resonator  24  with two, three, four or also more resonators  24 ,  24 ′,  24 ″, . . . can therefore be located beside one another. The shape can also be arbitrarily varied. A plurality of cylinders can be located beside one another instead of the cylinder sectors or also, however, arbitrary polygonal shapes. In addition, one or a plurality of Helmholtz resonators  24 ,  24 ′,  24 ″, . . . can also be located beside one another at other positions in the exhaust gas system  10 . 
     In a particular embodiment, the three Helmholtz resonators  24 ,  24 ′,  24 ″ are adjusted by means of throats  47 , which can be adapted in length and/or in cross section, and by means of an adjustable volume  25 ,  25 ′,  25 ″ to slightly different low frequencies, which preferably also differ from the frequency which is suppressed in the intermediate space  44 . The low-frequency noise can, in this way, be reduced highly efficiently. The principle of an adaptable Helmholtz resonator  24   a  is shown in section in FIG.  4 . As may be seen from FIG. 4, the throat  47   a  has two tubes  50 ,  52  which are pushed one into the other. Arbitrary other cross-sectional shapes can also, however, be selected. The outer tube  50  with the larger diameter is firmly anchored in the intermediate wall  30 . It can, for example, be welded to the intermediate wall  30 . On its inner surface, the outer tube  50  has, in each of its two end regions, protrusions  54  which extend radially inward and are located on circular disks. A seal  56 , which surrounds in a gas-tight manner the inner tube  52  with the somewhat smaller diameter, is located between the protrusions  54 . The inner tube  52  is concentrically supported in the outer tube  50  and can be displaced against the resistance of the seal  56 . The inner tube  52  has ends  53 , which are bent radially outward and which, when brought into contact with the protrusions  54 , prevent the inner tube  52  from being extracted too far from the outer tube  50 . The throat  47   a  of the Helmholtz resonator  24   a  can be displaced in its length by displacing the inner tube  52  in the outer tube  50 . The throat diameter can, for example, be made adjustable by configuring the throat with a polygonal cross section and by configuring the side walls of the polygon so that they can be moved relative to one another by means of linkages. 
     The volume  25   a  of the Helmholtz resonator  24   a  can be adjusted by means of the side walls  58 , which can be adjusted in height. The height of the side walls  58  can be altered with the aid of a displaceable base  60 . The displaceable base  60  has a pot-shaped configuration and comprises a base plate  62  and base walls  64  protruding approximately at right angles from the base plate  62 , which base walls  64  laterally surround the side walls  58  of the Helmholtz resonator  24   a . At their ends  66  opposite to the base plate  62 , the base walls  64  are bent radially inward. A collar  68  extending radially inward is provided on the base walls  64  at a distance from the bent-up ends  66 . A base seal  70 , which surrounds the side walls  58  of the Helmholtz resonator  24   a  in a gas-tight manner, is located between the collar  68  and the bent-up ends  66  of the base walls  64 . At their end facing towards the base  60 , the side walls  58  have radially outwardly bent edges  72 , which can be brought into contact with the collar  68  and in this way prevent the base being withdrawn from the side walls  58  of the Helmholtz resonator  24   a . The volume  25   a  of the Helmholtz resonator  24   a  can therefore be adjusted during the displacement of the base  60  from contact between the base plate  62  and the rims  72  of the side walls  58  to contact between the rims  72  of the side walls  58  with the collar  68  of the base wall  64 . The Helmholtz resonator  24   a  can therefore be adjusted precisely to the frequency to be suppressed by means of the adjustable throat  47   a  and the adjustable volume  25   a.    
     For greater clarity, the distance between the two tubes  54 ,  56  and between the base walls  64  and the side walls  58  of the Helmholtz resonator  24   a  are shown exaggeratedly large in FIG.  4 . 
     LIST OF DESIGNATIONS 
       10  Exhaust gas system 
       12  Exhaust gas duct 
       14  Chimney 
       16  Flow duct 
       18  Exhaust gas 
       20  Transition region 
       22  Rear wall 
       24 ,  24 ′,  24 ″ Helmholtz resonator 
       25 ,  25 ′,  25 ″ Volume 
       26  Perforated cover 
       28  Fabric 
       30  Inlet region 
       32  Absorption noise suppressor 
       34  Hollow cylinder 
       36  Upstream absorption noise suppressor 
       38  Intermediate wall 
       40  Base 
       42  Walls 
       44  Intermediate space 
       46  Absorption material 
       48  Further perforated cover 
       50  Outer tube 
       52  Inner tube 
       54  Protrusion 
       56  Seal 
       58  Side walls 
       60  Displaceable base 
       62  Base plate 
       64  Base wall 
       66  Bent-up ends 
       68  Collar 
       70  Base seal 
       72  Bent-up rims