Abstract:
A distributed sensing system for detecting a flashback condition in a combustor for a gas turbine engine The distributed sensing system includes one or more strategically positioned fiber optic cables provided upstream of the combustion area in the combustor. The distributed sensing system employs Rayleigh backscattering and swept-wavelength interferometry to measure temperature and reliably identify the location of the flashback condition The fiber optic cable is specially fabricated to have a high temperature resistance suitable for those temperatures existing during flashback conditions. The fiber optic cable can be wrapped on an inside of a combustion basket or on an outside of the combustion basket, and in a serpentine manner or otherwise.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates generally to a system and method for detecting flashback events in a combustor of a gas turbine engine and, more particularly, to a fiber optic distributed sensing system employing Rayleigh backscattering and swept-wavelength interferometry for measuring temperature and detecting flashback events at many locations within a combustor of a gas turbine engine. 
     Discussion of the Related Art 
     The world&#39;s energy needs continue to rise which provides a demand for reliable, affordable, efficient and environmentally-compatible power generation. A gas turbine engine is one known machine that provides efficient power, and often has application for an electric generator in a power plant, or engines in an aircraft or a ship. A typically gas turbine engine includes a compressor section, a combustion section and a turbine section. The compressor section provides a compressed airflow to the combustion section where the air is mixed with a fuel, such as natural gas. The combustion section includes a plurality of circumferentially disposed combustors that receive the fuel to be mixed with the air and ignited to generate a working gas. The working gas expands through the turbine section and is directed across rows of blades therein by associated vanes As the working gas passes through the turbine section, it causes the blades to rotate, which in turn causes a shaft to rotate, thereby producing mechanical work 
     Each combustor includes a fuel injector, orifices for receiving compressed air and an igniter for igniting the fuel/air mixture to create a flame in a combustion basket The pressure and volume of both the injected fuel and the air are carefully controlled for a particular combustor so that the flame is propelled forward into a transition duct to the turbine section. As the operating conditions of the turbine engine vary and change, a failure mode could occur where the pressure and flow volume of the fuel and/or air causes a flashback condition where the flame travels backwards in a direction away from the turbine section. If the engine operating parameters are not immediately changed to remove the flashback condition, the flame flashback could cause damage to components upstream of the combustion area in the combustion basket because many of those components are not designed for such high temperatures. 
     It is known in the art to provide various types of sensors, such as high temperature thermocouples or optical detectors, such as fiber Bragg grating (FBG) sensors, strategically positioned behind the combustion area in the combustion basket of a combustor to detect flame flashback by detecting higher than normal temperatures. If flame flashback is detected by one of the detectors, then the system engine controller will take some immediate action, possibly system shutdown, to remove the flashback condition. However, the number of thermocouples and/or optical sensors that can be provided in the combustor is limited because of limits of the ability to configure and position multiple thermocouple sensors in the combustion basket or the spatial resolution of the optical detectors provided in an optical sensor. Because the resolution is limited, the ability to quickly detect a flashback condition and specifically identify the location of the flashback condition is correspondingly limited. For example, the flame may flash back to a location in the combustion basket where a sensor does not exist, thus limiting the ability to detect that flashback condition. 
     SUMMARY OF THE INVENTION 
     The present disclosure describes a distributed sensing system for detecting a flashback condition in a combustor for a gas turbine engine, where the system is based on Rayleigh backscattering that can be detected at a very high spatial resolution The distributed sensing system employs swept-wavelength interferometry to measure temperature using the Rayleigh backscattering and reliably identify the location of the flashback condition. A fiber optic cable supporting the Rayleigh backscattering is specially fabricated to have a high temperature resistance suitable for those temperatures existing during flashback conditions. The fiber optic cable can be wrapped on an inside of a combustion basket or on an outside of the combustion basket, and in a serpentine manner or otherwise 
     Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cut-away, isometric view of a gas turbine engine, 
         FIG. 2  is a cut-away, cross-sectional type view of a portion of a combustor in the combustion section of the gas turbine engine; 
         FIG. 3  is an illustration of a distributed sensing system including a fiber optic cable; and 
         FIG. 4  is a block diagram of a flashback engine control system. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a distributed sensing system employing a fiber optic cable and Rayleigh backscattering for detecting temperature and a flashback condition in a combustor for a gas turbine engine is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is a cut-away, isometric view of a gas turbine engine  10  including a compressor section  12 , a combustion section  14  and a turbine section  16  all enclosed within an outer housing or casing  30 , where operation of the engine  10  causes a central shaft or rotor  18  to rotate, thus creating mechanical work. The engine  10  is illustrated and described by way of a non-limiting example to provide context to the invention discussed below. Those skilled in the art will appreciate that other gas turbine engine designs can also be used in connection with the invention Rotation of the rotor  18  draws air into the compressor section  12  where it is directed by vanes  22  and compressed by rotating blades  20  to be delivered to the combustion section  14 , where the compressed air is mixed with a fuel, such as natural gas, and where the fuel/air mixture is ignited to create a hot working gas. More specifically, the combustion section  14  includes a number of circumferentially disposed combustors  26  each receiving the fuel that is injected into the combustor  26  by an injector (not shown), mixed with the compressed air and ignited by an igniter  24  to be combusted to create the working gas, which is directed by a transition component  28  into the turbine section  16 . The working gas is then directed by circumferentially disposed stationary vanes (not shown in  FIG. 1 ) in the turbine section  16  to flow across circumferentially disposed rotatable turbine blades  34 , which causes the turbine blades  34  to rotate, thus rotating the rotor  18 . Once the working gas passes through the turbine section  16  it is output from the engine  10  as an exhaust gas through an output nozzle  36 . 
     Each group of the circumferentially disposed stationary vanes defines a row of the vanes and each group of the circumferentially disposed blades  34  defines a row  38  of the blades  34 . In this non-limiting embodiment, the turbine section  16  includes four rows  38  of the rotating blades  34  and four rows of the stationary vanes in an alternating sequence. In other gas turbine engine designs, the turbine section  16  may include more or less rows of the turbine blades  34  It is noted that the most forward row of the turbine blades  34 , referred to as the row 1 blades, and the vanes, referred to as the row 1 vanes, receive the highest temperature of the working gas, where the temperature of the working gas decreases as it flows through the turbine section  16 . 
       FIG. 2  is a cut-away, cross-sectional type view of a portion of one of the combustors  26  coupled to one of the transition components  28 . The combustor  26  includes a fuel injection system  40  mounted to a cover plate  42  enclosing a combustion shell  44 . The fuel injection system  40  includes fuel nozzles  46  and a pilot nozzle  48  An end of the fuel injection system  40  proximate the pilot nozzle  48  is coupled to a funnel-shaped combustion basket  50  including orifices  52  that allow pressurized air from the compressor section  12  to enter the combustion basket  50 . A combustion monitoring and control system  56  controls the fuel injection system  40  to cause the desired amount of fuel to be injected into the combustion basket  50  through the fuel nozzles  46  for a particular operating condition of the engine, where the fuel is mixed with the air and is ignited by the pilot flame to provide a high intensity flame. The flame generates the hot working gas that flows through the transition component  28  towards the first row of vanes in the turbine section  16 , represented here by vane  58 . 
     The present invention proposes a distributed sensing system that employs swept-wavelength interferometry for detecting Rayleigh backscattering in a fiber optical cable to detect elevated temperatures in a region in the combustor  26  upstream from the location where the fuel/air is ignited in the combustion basket  50  to generate the hot working gas, which could be an indication of a flashback condition. The distributed sensing system includes one or more fiber optic cables of a certain length strategically coupled to the combustion basket  50 , the pilot nozzle  48 , or some other suitable location in the combustor  26 . In this non-limiting example, a sensing fiber optic cable  60  is mounted to an inside surface of the combustion basket  50  upstream of the orifices  52  and thus upstream of the location where the main combustion event occurs. Additionally, or alternately, a distributed sensing fiber optic cable  62  is provided within the pilot nozzle  48 . The cables  60  and  62  provide Rayleigh backscatter reflectometry that will be measured using swept wavelength interferometry. In one non-limiting embodiment, the fiber  60  is about 1 meter long which can provide sub-millimeter spatial resolution and a high accuracy with a fast response time. 
     The fiber  60  can be mounted to the combustion basket  50 , or other suitable combustor component, in any desired strategic manner that allows it to effectively detect temperature depending on the particular combustor design. For example, the fiber  60  can be wound around an internal surface of the combustion basket  50  or wound around an external surface of the combustion basket  50 . Further, the cable  60  can be mounted to the inside or outside wall of the combustion basket  50  in a serpentine manner to provide even a higher degree of resolution for a particular application. By providing a single fiber in this manner, and internal to the combustion basket  50 , only a single hole needs to be drilled into the wall of the combustion basket  50  to allow the cable  60  to placed therein, where as with the tradition thermocouple sensors, a separate hole needed to be drilled for each separate thermocouple sensor The fiber optical cable  60  can be mounted to the wall of the combustion basket  50  in any suitable manner, such as by a high temperature adhesive or thermo-bonding 
       FIG. 3  is a representation of a distributed sensing system  70  including a distributed sensing fiber optic cable  72  of the type that can be used for the fiber optic cables  60  and  62  discussed above The fiber optic cable  72  includes an optical fiber core  74  surrounded by an outer cladding layer  76 . The index of refraction of the cladding layer  76  is greater than the index of refraction of the fiber core  74  so that a light beam at a low enough angle of incidence propagating down the fiber core  74  is reflected off of the transition between the fiber core  74  and the cladding layer  76  and is trapped therein. In one embodiment, the fiber core  74  is about 10 μm in diameter, which provides a multi-mode fiber for propagating multiple optical modes. Because the fiber optic cable  72  will be used in a high temperature environment, the fiber optic cable  72  is made of a high temperature material, such as quartz, so as not to be damaged in the high temperature environment. Further heat resistance can be provided by coating the cladding layer  76  with a high temperature coating  78 , such as gold, so as to withstand temperatures up to about 800° C. 
     The general idea of employing swept wavelength interferometry for detecting Rayleigh backscattering along the length of a fiber optic cable to detect temperature change is known to those skilled in the art. An analyzer  82  includes a swept wavelength interferometer having an optical reference path of a known length and an optical sensing path, which is the fiber optic cable  72  The analyzer  82  sends an optical signal of a predetermined wavelength down the core  74 . Rayleigh backscattering of the optical signal as it propagates along the cable  72  is caused by random profile fluctuations along the length of the cable  72 . The temperature of the cable  72  creates a particular reflection spectrum of the backscattering along the length of the fiber cable  72 , where changes in the temperature of the cable  72  cause a shift in that spectrum. The profile of the backscattering spectrum can be analyzed in segments along the length of the fiber cable  72  by Fourier transforming the spectrum to give the spatial resolution In one non-limiting embodiment, the backscattering analysis can provide a spatial resolution of about 0.5 mm and the analyzer response time of about 0.1 seconds 
       FIG. 4  is a block diagram of a distributed sensing control system  90  for responding to a flame flashback condition as discussed above. The system  90  includes a box  92  representing the gas turbine engine, which provides a signal to an analyzer  94  representing the optical signal from the distributed sensing fiber cable. Based on the numerous reflections from the sensing locations in the fiber optic cable, the analyzer  94  is able to determine if flame flashback is occurring, and if so, the location of the flashback, the intensity of the flashback and the rate of propagation of the flashback The analyzer  94  provides a signal indicative of all of these parameters to an engine control system  96  that will change the operating parameters of the engine  92 , including shutting the engine  92  down, if necessary, to limit the flashback condition if it exists. The engine control system  96  likely will provide a signal to the combustion monitoring and control system  56  for the particular combustor  26  that is experiencing the flashback condition. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the scope of the invention as defined in the following claims