BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a device and a method for measuring the dynamic pressure of a combustion chamber of, for example, a gas turbine machine.  
           [0002]    As part of the monitoring controls and diagnostic tools for an operating combustion system in a rotary machine such as a gas turbine, it is necessary to measure and acquire various data including combustion chamber dynamic pressure. This data is used to confirm proper operational health of the combustion system, and is also used to tune the gas turbine engine so that it is operating with an appropriate balance between combustion dynamics and emissions. Measuring dynamic pressure directly in a combustion chamber requires a sensor that functions in operating environments having temperatures in the range of 2000-3000° F. Currently, existing dynamic pressure probes are designed to withstand no more than about 1000° F. As a result, existing combustion dynamic pressure measurement methods do not utilize sensors located directly on the combustion chamber. Rather, current systems use metal tubing called wave guides to transmit the pressure signal from the combustion chamber to a remotely located dynamic pressure sensor. The long length of the metal tubing from the combustion chamber to the remotely located sensor results in significant attenuation of the pressure signal, so that it is not possible to measure the true dynamic pressure of the combustion system. In these systems, several factors affect the degree of signal attenuation, including, the internal diameter of the tubing; the length of the tubing; the temperature profile within the tubing; the static pressure within the tubing; and the frequency content of dynamic pressure signature. In some systems, a damping coil wound around an axis is used to prevent the formation of standing waves in the measurement system. This type of system results, however, in the formation of condensate in the wound damping coil. Condensation build up in the coils results in standing waves being formed in the tubing which attenuates the true source signal and prevents it from being measured accurately.  
           [0003]    Thus, in order for an acoustic damping system to work continuously, the formation of condensation in a coil system must be prevented. To address this issue, conventional systems must be periodically purged to remove the condensate from the damping coils.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0004]    Rather than periodically purging condensate in the coil system at least one mechanism is provided to prevent condensation formation in the acoustic damping system. According to an embodiment of the invention, it is ensured that the temperature inside the damping coil(s) is high enough to prevent condensation. This may be achieved by providing a dedicated heat source close to the damping system to maintain an elevated temperature or by locating the acoustic damping system in a location that is sufficiently hot so as to prevent condensation, whereby an additional heat source is not required.  
           [0005]    In an embodiment of the invention, the dynamic pressure signal is transmitted from the high temperature environment, such as the inside of a combustion chamber, via a wave guide to a damping coil that is wound around a horizontal axis of the sensor holder. In this example, the coil comprising the acoustic damping system is wound around the pressure sensor holder in a heat exchange configuration whereby the heat of the media disposed in the wave guide is conducted to the damping coil by convection.  
           [0006]    Thus, the invention may be embodied in a dynamic pressure probe holder for a combustor comprising: a holder body having a pressure sensing passage and housing a pressure sensor operatively coupled to said pressure sensing passage; and an elongated acoustic damping coil coupled to so as to be in flow communication with said pressure sensing passage, said damping coil being disposed in heat exchange relation to a heat source so as to substantially avoid condensation formation in said coil.  
           [0007]    The invention may also be embodied in a dynamic pressure probe holder for a combustor comprising: a holder body having a pressure sensing passage and defining a pressure sensor chamber operatively coupled to said pressure sensing passage; and an elongated acoustic damping coil having a bore operatively coupled to said pressure sensing passage, said damping coil being wound about said holder body so as to be disposed in heat exchange relation thereto so as to substantially avoid condensation formation in said coil.  
           [0008]    Additionally the invention may be embodied in a method of obtaining a dynamic pressure signal from a combustor while preventing the formation of condensation, comprising: providing a dynamic pressure probe device comprising a holder body having a pressure sensing passage and housing a pressure sensor operatively coupled to said pressure sensing passage; and an elongated damping coil coupled to said pressure sensing passage, said damping coil being disposed in heat exchange relation to a heat source; supplying a dynamic pressure signal from the combustor through said pressure sensing passage; detecting said dynamic pressure signal with said pressure sensor; and transmitting said pressure signal downstream from said pressure sensor to a signal damping mechanism comprising said coil; whereby heat from said heat source prevents the formation of condensation in said coil.  
           [0009]    In addition, or in the alternative, passive continuous purging with hot air is provided to prevent condensation in the damping coil. According to this embodiment, one end of the wound-damping coil is coupled to the wave guide via an attenuation line and the other end of the wound-damping coil is connected to a source of hot air via a purge coil.  
           [0010]    According to one embodiment, the source of hot air is compressor discharge and, provides a continuous gentle purge of the system.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic view, partly in cross-section taken through a combustor, illustrating a dynamic pressure sensor mounted to the outer combustor wall by means of a pressure sensor holder in accordance with an exemplary embodiment of the invention;  
         [0012]    [0012]FIG. 2 is a schematic cross-section of the sensor holder of FIG. 1;  
         [0013]    [0013]FIG. 3 is a schematic cross-section of a sensor holder according to a second embodiment of the invention; and  
         [0014]    [0014]FIG. 4 is a schematic cross-section of a sensor holder according to a third embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    With reference to FIG. 1, a probe holder  10  embodying the invention is schematically shown attached to the outer wall or casing  12  of a combustor  15  via a conventional compression fitting, such as a Swagelok® 17. As explained further below, the forward tip of the probe is seated in an aperture in the combustion liner  14 , which is concentric with, and spaced radially inward from, the outer wall or casing  12 . The dynamic pressure signal is transmitted through a passage in the holder to a sensor located within the holder but relatively remote from the forward tip, as described in further detail below. The pressure signal is damped in an acoustic damping system  19 , also described further below.  
         [0016]    With reference to FIG. 2, which illustrates in greater detail a first embodiment of the invention, the probe holder includes a generally cylindrical or other suitably shaped holder body  16  formed with a first through-bore or wave guide passage  18  extending from a rearward end  20  to a forward end  22  of the holder body. The forward end  22  includes a reduced thickness forward extension  24  and the rearward end  20  includes a reduced thickness rearward extension  26 . The forward end is adapted to project through an aperture in the combustor liner  14  so that the inlet to passage  18  is exposed to the combustor dynamic pressure. Passage  18  is counterbored in the rearward extension  26  to permit attachment (via a conventional compression fitting  29 ) of a metal tube or attenuation line  28  having an inside diameter equal to the inside diameter of bore  18 , so that the inside diameter of the bore  18  and tube  28  is uniform throughout.  
         [0017]    A sensor housing portion  30  of the holder body  16  is joined to (or is integral with) the holder body adjacent the rearward end  20 , and extends perpendicular thereto. The housing portion  30  is formed with a cylindrical interior that extends into the wall of the body  16 , such that only a relatively small thickness wall  32  separates the interior of the housing portion from the through bore or first passage  18 , with a pressure feed hole or aperture  34  centrally located in the wall  32 .  
         [0018]    The outer end of the housing portion  30  includes a radial flange  36  with a plurality of screw holes  38  therein. Within the housing portion  30 , a metal sleeve  40  is fitted such that the base of the sleeve  40  is seated on the bottom wall  32  of the housing portion  30 . An O-ring  42  seals the sleeve relative to wall  32 , and a second O-ring  44  at the opposite or outer end of the sleeve  40  seals the sleeve relative to a radial flange connector  46  of the sensor  48 .  
         [0019]    The inner or sensing portion  50  of the sensor  48  is received within the sleeve  40  with its innermost end, defined by diaphragm  52 , spaced from bottom wall  32  of the housing portion  30 , establishing a pressure chamber  54  between the diaphragm  52  and the wall  32 . The sensor  48  also includes a cable connector  70  that extends out of the flange connector  46 , and to which a cable (not shown) is attached, connecting the sensor with suitable monitoring and/or control apparatus. Reference is made in this regard to co-pending application Ser. No. 09/989,102 (Attorney Docket 839-1149), the entire disclosure of which is incorporated herein by this reference, and which discloses particulars of an exemplary sensor in greater detail.  
         [0020]    In an embodiment of the invention, the dynamic pressure signal is transmitted from a high temperature environment such as in the inside of a combustion chamber via a wave guide passage to a damping coil. Thus, after the pressure signal passes the aperture  34  (having been exposed to diaphragm  52 ), it continues into the attenuation line  28  as shown in FIGS. 1 and 2. In order to prevent the formation of a standing wave at the probe holder/attenuation line interface, the internal diameter of the attenuation line  28  and the probe holder pressure signal (wave guide) passage  18  are identical, as mentioned above. Attenuation line  28  is connected in turn to damping coil  86 .  
         [0021]    To prevent the formation of condensate, the damping coil is in heat exchange relation with a heat source which may be dedicated heat source or an existing heated environment. Recognizing that the sensor holder itself is at an elevated temperature due to its communication with the combustor, in the illustrated embodiment, the damping coil  86  is wound around the horizontal axis of the pressure sensor holder, as schematically depicted in FIG. 2, so as to be disposed in heat exchange relation with the sensor holder. In the illustrated embodiment, the damping coil  86  is made of metal tubing with substantially the same internal diameter as the metal tubing comprising the attenuation line  28 . Collectively the damping coil  86  on the probe holder and the attenuation line  28  may be referred to as a local mounted acoustic damping system.  
         [0022]    As will be understood, by ensuring that the temperature inside the damping coil(s) is high enough to prevent condensation by disposing the damping coil in a heat exchange relation to a heat source, condensation is prevented from forming so that the system may work continuously. If deemed necessary, or desirable, to further prevent condensation, the end of the damping coil remote from its communication to the attenuation line may be in flow communication with a source of hot air to provide a continuous gentle purge of the system.  
         [0023]    The alternative of providing passive continuous purge with hot air is schematically illustrated in FIG.  1 . According to this option, a second bore or passage  72  is provided to extend from within the outer wall  12  of the combustor, such that its inlet is exposed to compressor discharge air in the radial space  84  between the outer wall  12  and liner  14 , to an outlet bushing  76  generally aligned with the housing portion  30  and perpendicular to the passage  72 . A tube  78  is secured within the outlet bushing  76  via e.g. a compression fitting and includes a bore that communicates with bore  72 . This second axial bore or passage  72  is used to extract compressor discharge air from the radial space  84  and to supply the compressor discharge air to the top side of the wound damping coil  86  of the acoustic damping system. This hot compressor discharge air is used to provide a continuous passive purging of the horizontally wound damping coil  86  and thereby prevent formation of any condensate in the damping coil.  
         [0024]    When the dynamic pressure signal leaves the source location and travels down in the inside of the metal tubing, it is gradually attenuated due to friction between the signal and the sidewalls of the tubing. The further down the tubing the signal travels, the more attenuation results. When the signal gets to the end of the tubing (including the damping coil) it is reflected and starts to travel back towards the signal source. Accordingly, the system is advantageously sized such that the distance from the measurement point (at  34 ) to the end of the acoustic damping system  28 , 86  is sufficiently long to ensure that any reflected signal will be completely damped away before it can travel back to the measurement point. Also, in an exemplary embodiment, the distance from the measurement point to the dynamic pressure source is kept to an absolute minimum so that at the point of measurement, a minimum amount of damping has occurred.  
         [0025]    In the illustrated embodiment, to accommodate the damping coil, the holder may be pre-formed to provide a coil winding segment. In the alternative, as schematically shown in FIG. 2, a coil holder may be detachably secured to, e.g., the body  16  of a holder of the type illustrated and disclosed in the &#39;102 application. In this regard, winding the damping coil about the holder has the advantage that the heat exchange will reduce or minimize condensation. However, a length of tubing or a dedicated portion of the holder is required to accommodate the wound coil. In the embodiment illustrated in FIG. 2, a Tee holder of the type disclosed in copending application Ser. No. 09/989,102 has been modified to accommodate a coil holder. Thus, in this embodiment, a coil holder  60  defined as a tube is secured with a fitting  62  to the main body  16  of the probe holder and projects distally therefrom concentrically to the wave guide passage  18 . A wave guide extension  64  is provided to extend the wave guide passage  18  to the liner wall  14 . To support the coil holder tube  60  with respect to the wave guide extension, a support tube  66  is provided in spaced relation to the wave guide extension and secured, e.g., with a fitting  68  to the distal end of the coil holder tube  60 .  
         [0026]    As will be understood, FIG. 2 illustrates how a Tee holder of the type generally shown in the co-pending &#39;102 application may be modified to provide for heat exchange to the damping coil  86 . It is to be understood that rather than providing a retrofit configuration comprising a damping coil holder  60  and extensions  64  and  66 , the probe holder body may be formed in the first instance to have a length and damping coil support portion to receive the damping coil thereto.  
         [0027]    A further, alternate embodiment of the invention is illustrated in FIG. 3. In this embodiment, the probe holder  110  has a straight configuration rather than a T-shaped configuration. Thus, in the FIG. 3 embodiment, the pressure sensor housing portion  130  is disposed coaxially with the axis of the wave guide  118  and the attenuation line  128  is defined to extend generally perpendicular with respect to the axis of the wave guide  118 . A mounting tube  166 , which extends through the casing wall (not shown) is coupled to the main body  116  of the probe holder  110  with e.g. a Swagelok® structure  117 . A stop ring  165  may be mounted to the wave guide tubing  164  to insure that the probe tip does not go into the machine.  
         [0028]    In the embodiment illustrated in FIG. 3, the damping coil  186  is wound about the housing portion  130  of the pressure sensor  148 , which simplifies the configuration and allows the sensor  148  to be disposed at a minimal distance from the pressure source. It is to be appreciated, however, that the heat exchange effect of the pressure sensor itself may be less than provided by mounting the damping coil so as to be concentric to the wave guide  118 , as in the embodiment of FIG. 2. Thus, as a further alternative (not illustrated) the coil may be disposed coaxially and concentrically to the wave guide  118  with the pressure sensor  148  disposed downstream from and axially aligned with the wave guide  118 .  
         [0029]    Yet a further embodiment of the invention is illustrated in FIG. 4. As will be noted, this embodiment has a straight configuration similar to the FIG. 3 embodiment, so that the pressure sensor housing portion  230  is disposed coaxially with the axis of the wave guide  218  and the attenuation line  228  is defined to extend generally perpendicular to the axis of the wave guide  218 . As in the FIG. 3 embodiment, in the FIG. 4 embodiment a mounting tube  266  extends through the casing wall  212  and the wave guide tube  264  terminates at the combustion liner  214 . The mounting tube  266  is coupled to the main body  216  of the probe holder  210  with e.g. a Swagelok® structure  217 . A stop ring  265  may be mounted to the wave guide tubing  264  to insure that the probe tip does not go into the machine.  
         [0030]    In the embodiment illustrated in FIG. 4, the damping coil  286  is wound about the housing portion  230  of the pressure sensor  248 . More specifically, the damping coil is wound on a metal spool  287  disposed co-axially to housing portion  230  and having radially projecting end walls to delimit the coil. To accommodate the spool and to simplify the assembly process, the outer end of the housing portion  230  includes an inner, e.g., left hand thread for engaging a complementary thread provided on end piece  246  and an outer, e.g., right hand thread for engaging a complementary thread provided in locking nut  236 . A lock plate  290  is slidably disposed on end piece  246  to be locked between the flange of end piece  246  and lock nut  236 . As will be understood, the end piece  246  and the lock nut  236  cooperatively lock to position and hold the sensor  248  in housing portion  230 .  
         [0031]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
A component adapted to transmit a dynamic pressure signal from a high temperature environment to a location where it can be measured without causing significant attenuation of the signal. In particular, the component provides for a dynamic pressure signal transmission in the manner that will not result in the formation of condensation in the measurement system and thus eliminates the need to periodically purge condensate from the system.