Abstract:
A method of stabilizing combustion in a gas turbine engine includes the steps of observing a combustor flame from a location upstream from said flame to detect spectral characteristics indicative of a condition affecting combustion stability, and tuning the engine to stabilize combustion based upon an indicated condition affecting combustion stability.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The subject invention is directed to optical sensors for gas turbine engines, and more particularly, to an apparatus, system and method for observing the spectral and thermal characteristics of a flame in the combustion chamber of a gas turbine engine to detect, in real-time, conditions indicative of combustion instabilities and the like.  
         [0003]     2. Background of the Related Art  
         [0004]     Combustion instability is a significant problem in the design of low-emission, high performing combustion chambers for gas turbines, boilers, heaters and furnaces. Combustion instability is generally understood as high amplitude pressure oscillations that occur as a result of the turbulent nature of a combustion process and the large volumetric energy release within the combustion chamber. Combustion instability diminishes engine system performance, and the vibrations resulting from pressure oscillations can damage hardware components, including the combustion chamber.  
         [0005]     There are many factors that contribute to combustion instability within the combustion chamber of a gas turbine. These include, for example, the fuel content, fuel and/or air injection speed or inlet pressure, fuel/air concentration/ratio, temperature changes within the combustion chamber, the stability of the flame, large scale coherent flow structures affecting mixing (i.e., vortex shedding), the coupling of acoustic pressure waves with combustion heat release at combustor resonance frequencies, and/or extinction/re-ignition phenomenon occurring at low flame temperature and high combustion pressure.  
         [0006]     In the past, passive control methods were employed to correct combustion instability, including, for example, modifying the fuel injection distribution pattern, or changing the shape or capacity of the combustion chamber. Passive controls are often costly and limit combustor performance. More recently, active controls have been used to correct combustion instability by modifying the pressure within the system and/or regulating the flow of fuel or air into the combustor in response to detected unstable conditions. An example of active control is disclosed in U.S. Pat. No. 5,784,300 to Neumeier et al.  
         [0007]     It has been determined through experimentation that direct observation of a combustor flame can provide information that may be used to actively control combustion instability. For example, combustion driven pressure oscillations can be detected by observing flame movement and variations in flame intensity. In addition, spectral radiation indicative of combustion by-products and emissions that effect flame temperature or other flame qualities may be observed. These observations may be analyzed and used by an active combustion control system to regulate the flow of fuel to the combustion chamber of a gas turbine or adjust the fuel/air ratio for combustion and thereby stabilize the combustion process.  
         [0008]     Optical sensors for observing combustion processes are known in the prior art, but they are limited in many respects. For example, U.S. Pat. No. 3,689,773 to Wheeler describes a flame monitoring system for a furnace wherein the flame is viewed from the side of the burner. Since the primary combustion zone within the burner is not stationary, the flame front can move out of the field of vision of the flame sensor. This can cause the system to obtain inaccurate measurements. U.S. Pat. No. 4,709,155 to Yamaguchi et al. describes an optical flame detector for use in a boiler that includes optical fibers protected from thermal damage by a forced air-cooling system. Such a system would have limited application in a gas turbine combustor where operating temperatures are far in excess of those present in a boiler.  
         [0009]     Clearly, there is a need in the art for an optical flame sensor that may be used in active combustion control which overcomes the deficiencies of prior art optical flame sensors. Moreover, there is a need in the art for an optical flame sensor that may be employed in the combustion chamber of a gas turbine engine, which has a wide field of view so that the combustor flame will remain within the line of sight of the sensor at all times during the combustion process, and which does not require cooling means to operate within the combustion chamber.  
       SUMMARY OF THE INVENTION  
       [0010]     The subject invention is directed to an apparatus for observing conditions within the combustion chamber of a gas turbine engine. More particularly, the subject invention is directed to a new and useful fuel injector for a gas turbine engine that includes, among other things, optical sensing means for observing characteristics of the combustor flame. Specifically, the optical sensing means is configured to detect spectral and thermal conditions that destabilize the combustion process.  
         [0011]     The fuel injector of the subject invention includes an elongated feed arm having means for mounting the injector within the combustion chamber, such as a flange for securing the injector to the interior wall or liner of the combustor. The fuel injector further includes a fuel nozzle or nozzle body, which depends from the feed arm for injecting or otherwise issuing atomized fuel into the combustion chamber for combustion. In accordance with a preferred embodiment of the subject invention, optical sensing means are provided within the fuel nozzle for observing combustion conditions within the combustion chamber, downstream from the fuel nozzle.  
         [0012]     In accordance with a preferred embodiment of the subject invention, the fuel nozzle includes an outer air swirler having a leading edge. A plurality of circumferentially spaced apart viewing ports are formed in the leading edge of the outer air swirler. For example, the leading edge of the outer swirler can include three or more viewing ports spaced equidistant from one another. Preferably, the optical sensing means of the subject invention includes a plurality of optical fiber bundles, and there is an optical fiber bundle accommodated within each of the viewing ports formed in the outer swirler. In addition, the optical sensing means may further include one or more optical fibers mounted or otherwise supported within a holder along the central axis of the fuel nozzle.  
         [0013]     It is envisioned that each embedded optical fiber bundle has a field of view of between about 14° to 30° (dependent on the fiber Numerical Aperture) relative to axis of fiber. Each optical fiber bundle preferably includes a plurality of optical fibers, and these optical fibers are oriented so as to extend substantially parallel to the central axis of the fuel nozzle. The optical fibers are preferably treated to withstand the operating temperatures within the combustor. For example, the optical fibers may be provided with a metal coating, such as gold or another precious metal, well suited for thermal protection.  
         [0014]     Each optical fiber bundle is preferably disposed within a temperature resistant guide tube. For example, the fiber bundles may be disposed within stainless steel guide tubes. Preferably, the distal end of each stainless steel guide tube is cemented within a corresponding viewing port in the outer air swirler of the fuel nozzle in a manner that accommodates thermal expansion and contraction. This will serve to maintain the structural integrity of the system through a multitude of engine operating cycles.  
         [0015]     The subject invention is further directed to a system for stabilizing combustion in a gas turbine engine by observing conditions within the combustion chamber. The system includes, among other things, optical sensing means, preferably embedded within each fuel injector, for observing conditions within the combustion chamber, means for detecting combustion instabilities based upon conditions observed by the optical sensing means, and means for modulating the rate of fuel flow to the combustion chamber to reduce the detected combustion instabilities or otherwise stabilize combustion.  
         [0016]     In accordance with a preferred embodiment of the subject invention, the means for detecting combustion instabilities is adapted and configured to analyze and process information relating to combustor flame characteristics, including thermal and spectral conditions. For example, the means for detecting combustion instabilities may include means for detecting changes in flame intensity at levels indicative of flame instability.  
         [0017]     Alternatively, the means for detecting combustion instabilities may include means for detecting spectral radiation indicative of stoichiometric instability. In one embodiment of the subject invention, the detecting means is configured to detect and analyze spectral radiation indicative of chemical emissions effecting flame temperature, NOx, CO, fuel/air ratio and equivalence ratio. It is also envisioned that the detecting means can be employed in turbine engines fueled by natural gas. In such instances, the detecting means is configured to detect variability in the natural gas composition that will affect fuel heat values. For example, the detecting means could be configured to detect spectral variations relating to ethane, propane, nitrogen and carbon dioxide. The system could also be configured to detect the presence of natural gas contaminants, such as sodium, which can have a corrosive effect on hot engine parts.  
         [0018]     The subject invention is also directed to a method of promoting stable combustion in a gas turbine engine. The method includes the steps of observing a combustor flame from a location upstream from the flame to detect spectral characteristics indicative of one or more conditions affecting combustion stability, and subsequently tuning the engine to stabilize combustion based upon an indicated condition effecting combustion stability. In one embodiment of the subject invention, the step of observing a combustor flame includes the step of detecting changes in spectral intensity indicative of flame instability. In another embodiment of the subject invention, the step of observing a combustor flame includes the step of detecting spectral radiation peaks or emissions indicative of stoichiometric instability. Those skilled in the art will readily appreciate that the step of tuning the engine to stabilize combustion includes adjusting the fuel flow and/or air flow to the combustion chamber, or otherwise the fuel to air ratio for combustion.  
         [0019]     These and other aspects of the apparatus, system and method of the subject invention will become more readily apparent to those having ordinary skill in the art from the following detailed description of the invention taken in conjunction with the drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     So that those having ordinary skill in the art to which the present invention pertains will more readily understand how to employ the novel apparatus, system and method of the present invention, embodiments thereof will be described in detail hereinbelow with reference to the drawings, wherein:  
         [0021]      FIG. 1  is a schematic representation of an active combustion control system for a gas turbine engine configured in accordance with a preferred embodiment of the subject invention;  
         [0022]      FIG. 2  is a graphical illustration of a spectral scan generated by a computer based spectrometer using optical data obtained by observing conditions within the combustion chamber of a gas turbine engine utilizing the optical sensing array of the subject invention.  
         [0023]      FIG. 3  is a side elevational view, in cross-section, of the combustion chamber of a conventional gas turbine engine, which includes fuel injectors containing the optical sensing array of the subject invention;  
         [0024]      FIG. 4  is a perspective view of a fuel injector constructed in accordance with a preferred embodiment of the subject invention, located within the combustion chamber of a gas turbine;  
         [0025]      FIG. 5  is an enlarged perspective view of the fuel nozzle, which forms part of the fuel injector of  FIG. 4 , with sections of the outer air swirler removed to reveal the optical fiber bundles for viewing conditions within the combustion chamber;  
         [0026]      FIG. 6  is a side elevational view of the lower portion of the fuel injector of  FIG. 4  depicting the field of view of the fiber bundles embedded within the fuel nozzle;  
         [0027]      FIG. 7  is a graphical illustration of a spectral ratio scan generated by a computer based spectrometer using optical data obtained by a pair of optical fibers located centrally within the fuel injector nozzle of the subject invention; and  
         [0028]      FIGS. 8   a  through  8   c  illustrate three different methods of terminating an optical fiber bundle in accordance with the subject invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     Referring now to the drawings wherein like reference numerals identify similar features or aspects of the subject invention, there is illustrated in  FIG. 1  an active combustion control system configured in accordance with a preferred embodiment of the subject invention, and designated generally by reference numeral  10 . The active combustion control system  10  is designed to reduce thermo-acoustic combustion instabilities within the combustion chamber of a gas turbine engine. The system is intended to lower engine emissions, improve engine dynamics and maximize operating efficiency. The active combustion control system of the subject invention is particularly well suited for use in combustion systems that are inherently unstable such as, for example, industrial gas turbine engines wherein lean premixed combustion is used to reduce NOx, and high power thrust augmented military aircraft engines (afterburners) which utilize locally rich combustion.  
         [0030]     Referring to  FIG. 1 , active combustion control system  10  includes a plurality of fuel injectors  20 , described in detail below, which deliver atomized fuel to the combustion chamber of a gas turbine engine at a controlled rate of flow. There are two primary types of atomizing fuel injectors, and either type of injector may be employed with the system of the subject invention. These devices include swirl pressure atomizers, which derive energy for atomization from fuel pressure, and air blast atomizers, which derive energy for atomization from high velocity compressor air. Examples of atomizing fuel injectors are disclosed in U.S. Pat. No. 6,688,534 to Bretz, the disclosure of which is incorporated herein by reference in its entirety.  
         [0031]     The fuel injectors  20  each include a plurality of optical sensors  30  in the form of an array for observing conditions within the combustion chamber of a gas turbine engine, such as, for example, the thermal and spectral stability of the combustor flame. These optical sensors  30  are embedded in or otherwise integral with the fuel injectors  20 , and are well adapted for prolonged service within the highly corrosive, high temperature environment of a gas turbine combustion chamber, as will be described in greater detail below.  
         [0032]     The active combustion control system  10  of the subject invention further includes an electronic signal processor  40 . Signal processor  40  is adapted and configured to analyze and process real-time data received from or otherwise transmitted by the optical sensor array  30  associated with each of the fuel injectors  20 . This real-time optical data is primarily used to detect combustion instability within the combustion chamber of the gas turbine engine, including conditions relating to spectral and/or thermal stability.  
         [0033]     Preferably, the signal processor  40  includes a signal-generating device  45 , which generates output signals based upon the data received from the optical sensors  30 . The signal generator  45  is preferably filtered so as to acquire energy in a narrow spectral band relating to an emission from a specific chemical species or radical. Alternatively, it is envisioned that the signal generator could be filtered in such a manner so as to acquire energy is a plurality of specific spectral bands, each of which relates to an emission from specific chemical species or radical.  
         [0034]     In one embodiment of the invention, the signal processor  40  may include one or more filtered photo multiplier tubes (PMT) for electronically amplifying a light signal. For example, the signal processor can include eight different filtered PMT&#39;s. Alternatively, signal processor  40  may include one or more filtered silicone photodiodes each with an integral operational amplifier.  
         [0035]     A photodiode is a solid-state device that detects light and creates a current flow between doped forms of silicon in response thereto. In comparison, the sensitivity of a PMT may be adjusted and it provides a faster response than a photodiode. Furthermore, PMT&#39;s are designed to detect relatively weak signals as compared to photodiodes. However, photodiodes are cheaper, more robust and more readily available in a wider variety of wavelengths than PMT&#39;s. Nevertheless, these types of signal generators are considered to be interchangeable and both provide reliable output signals for detecting conditions effecting combustion stability. The type of signal generator employed will depend upon the signal strength, wavelengths to be detected and cost.  
         [0036]     The output signal generated by signal processor  40  undergoes an analysis to detect conditions effecting spectral and/or thermal instability, which are root causes of combustion instabilities. The analysis is performed by a computer-based spectrometer, which is operatively associated with the signal processor  40 . A suitable spectrometer preferably has a range of between 180 and 800 nm, and is programmed or otherwise configured to derive flame quality parameters relating to combustion stability, such as flame stability and flame temperature, as well as the presence of fuel contaminants, fuel components and chemical emissions or radicals.  
         [0037]     In one embodiment of the subject invention, the signal processor  40  and associated spectrometer are programmed or otherwise configured to detect and analyze changes in flame intensity. These changes are indicative of the combustion driven pressure oscillations, which cause combustion instability in a gas turbine engine. It is envisioned that the optical sensor array  30  and/or signal processor  40  would be calibrated under normal engine operating conditions, so that variations in optical intensity or optical signal amplitude with respect to the variations in the intensity of the instability would be evaluated based upon calibrated values. A spectral scan generated with the optical sensor array  30  of the subject invention and depicting variations in spectral intensity with changes in fuel to air ratios is illustrated in  FIG. 2 . Upon detecting variations in spectral intensity indicative of an unstable operating condition, the fuel flow rate could be adjusted or modulated to stabilize combustion.  
         [0038]     In another embodiment of the subject invention, the signal processor  40  and associated spectrometer are configured to detect spectral radiation indicative of chemical emissions effecting combustion stability. More particularly, signal processor  40  is programmed to detect and analyze the ratio of OH chemiluminescence peaks (occurring at about 310 nm) and CH chemiluminescence peaks (occurring at about 430 nm) observed by optical sensor array  30 , as illustrated in  FIG. 2 . This information is then correlated to the flame temperature, NOx, CO, fuel/air ratio or equivalence ratio. This emissions feedback is then used to tune the engine to a stable operating condition, such as by adjusting the fuel flow, air flow and/or fuel to air ratio for combustion.  
         [0039]     In yet another embodiment of the subject invention, the signal processor  40  and associated spectrometer are programmed or otherwise configured to detect and analyze spectral radiation indicative of fuel contaminants or impurities effecting fuel heat values. This is of particular importance in turbine engines powered by natural gas, since natural gas is typically an unpurified fuel. It is well known that variability in the composition of natural gas can affect fuel heat values. For example, variations in the amount of ethane, propane, nitrogen and carbon dioxide can cause heat value to vary. It is also known that the presence of sodium in natural gas can have a corrosive affect on hot engine parts such as the combustor itself and turbine blades. Changes in fuel heat values can cause a gas turbine engine powered by natural gas to become de-tuned and can jeopardize the stability of the combustion process. Upon detecting spectral emissions or radicals indicative of the presence of certain fuel contaminants or impurities, the engine can be tuned to a stable operating condition.  
         [0040]     With continuing reference to  FIG. 1 , the signal processor  40  is operatively associated or otherwise in communication with a fuel injector controller  50 . The fuel injector controller  50  is adapted and configured to receive a conditioned signal from processor  40 . Based on this signal, the fuel injector controller  50  commands or otherwise controls the flow of fuel to each of the fuel injectors  20  to reduce or otherwise correct a detected condition effecting combustion stability. For example, the fuel controller  50  could command a pulsed or modulated fuel flow to the fuel injectors to stabilize combustion. Alternatively, fuel controller  50  could be configured to adjust the fuel to air ratio/concentration for combustion.  
         [0041]     Referring now to  FIGS. 3 and 4 , the fuel injectors  20  of the subject invention are mounted or otherwise supported within the combustion chamber  60  of gas turbine engine  70  in a conventional manner. More particularly, each fuel injector  20  includes an elongated feed arm  22  having a support flange  24  for mounting the injector within the combustion chamber  60 . The support flange  24  is particularly adapted to secure the injector to the interior wall or liner of the combustion chamber using conventional fasteners. The fuel injector  20  further includes an inlet port  25  for receiving fuel from a fuel pump at a desired flow rate. A fixed or variable displacement vane pump may be employed. A fuel nozzle  26  depends from the distal end of feed arm  22  and is designed to inject or otherwise issue atomized fuel into the combustion chamber  60 . As noted above, a fuel injector  20  can take the form of a pressure atomizer or an air blast atomizer. In either configuration, the fuel nozzle  26  includes an outer air swirler  28  configured to impart an angular component of velocity to the air flowing through the nozzle body.  
         [0042]     Referring now to  FIGS. 5 and 6 , in accordance with a preferred embodiment of the subject invention, the optical sensors  30  described briefly above are provided, located or otherwise embedded within the outer air swirler  28  of fuel nozzle  26  for observing combustion conditions within the combustion chamber  60  of gas turbine  70 , downstream from the fuel nozzle. To accommodate the optical sensors  30  in a non-intrusive manner, a plurality of circumferentially spaced apart viewing ports are formed in the leading edge  32  of the outer air swirler  28 , creating an optical sensor array. For example, as best seen in  FIG. 4 , the leading edge  32  of the outer swirler  28  can include three viewing ports  34 , which are preferably spaced substantially equidistant from one another (e.g., about 120° apart), and an optical sensor  30  is accommodated within each viewing port  34 .  
         [0043]     Those skilled in the art will readily appreciate that the number of viewing ports formed in the outer air swirler  28  of the fuel nozzle  26  can vary from one nozzle type to another, and/or from one engine type to another. For example, four viewing ports spaced 90° apart from one another can be provided in a particular nozzle body constructed in accordance with the subject invention. It should also be understood by those skilled in the art that the optical sensors disclosed herein can be embedded in other parts of the nozzle body, other than the outer air swirler, without departing from the spirit or scope of the subject invention. That is, depending upon the type and structure of the nozzle body, the location of the embedded sensors can vary, so long as they have an adequate field of view downstream from the fuel nozzle, and remain non-obtrusive in that they do not negatively affect the overall performance of the fuel nozzle.  
         [0044]     It has been found through experimentation that disposing the optical sensors  30  at the leading edge  32  of the fuel nozzle  26  is advantageous, since the combustor flame will remain within the line of sight or field of view sensors, thus providing a fairly accurate representative map of the flame zone. It has also been found that positioning the optical sensors  30  at a plurality of spaced apart locations around the leading edge  32  of the fuel nozzle  26 , thus forming an optical sensor array, will allow specific modes of combustion instability to be observed and determined. By knowing the specific form of the instability, more effective control can be achieved.  
         [0045]     The optical sensors  30  are defined, in pertinent part, by optical fiber bundles  36 . An optical fiber bundle  36  consists of a plurality of individual optical fibers  38 . This provides redundancy in case of failure, more efficient energy transfer to the signal processor  40 , and allows for a broader field of view relative to the combustor flame. In addition, an optical fiber bundle is easily bent to accommodate contours of the fuel nozzle.  
         [0046]     It is envisioned that each optical fiber bundle  36  has a field of view defined by angle θ of between about 14° and 30° relative to axis of fiber, as best seen in  FIG. 6 . It is also envisioned that the optical fiber bundles  36  can be split or otherwise configured such that individual fibers within each bundle will observe different spectral wavelengths within their field of view.  
         [0047]     In one embodiment of the subject invention, each optical fiber bundle  36  includes three individual optical fibers  38 . The optical fibers  38  are aimed or otherwise oriented so as to extend generally parallel to the central axis of the fuel nozzle  26 , as best seen in  FIGS. 4 and 5 . It has been determined that this orientation provides the broadest field of view with respect to the combustor flame. Those skilled in the art will readily appreciate however, that the specific orientation of the fiber bundles can vary depending, for example, upon the geometry of the fuel nozzle.  
         [0048]     The optical fibers  38  forming fiber bundles  36  can consist of 100μ silica (UV enhanced) fibers or the like. The fibers  38  are preferably coated or otherwise treated to withstand the operating temperatures within the combustion chamber  60 . These temperatures can exceed 500° C. For example, the optical fibers  38  may be provided with a coating, such as gold or a similar precious metal suited for thermal protection. Other coatings resistant to high temperatures may also be used.  
         [0049]     Each optical fiber bundle  36  is disposed within a temperature resistant guide tube  42  for additional thermal protection. For example, the fiber bundles  36  may be disposed within stainless steel guide tubes or a similar protective structure. The distal end of each guide tube  42  is swaged to secure the fibers therein, and cemented within a corresponding viewing port  34  in a manner that accommodates thermal expansion and contraction. For example, ceramic cement may be used to secure the distal end of each guide tube  42  within a viewing port  34 . This will ensure the integrity of the fiber bundles throughout a multiplicity of engine operating cycles. The guide tubes  42  are preferably embedded in or otherwise mounted to the feed arm  22  of fuel injector  20 . For example, the guide tubes may be positioned within channels formed in the feed arm  22  of fuel nozzle  20 . The proximal end of each fiber bundle  36  terminates at a location external to the combustion chamber  60  in a conventional optical connector (not shown), which is suitable for reception by signal processor  40 .  
         [0050]     Referring to  FIG. 5 , in addition to the plurality of optical fiber bundles  36  positioned within the viewing ports  34  formed in the outer air swirler  28  of the fuel nozzle  26 , an optical sensor  80  may be mounted along the central axis of fuel nozzle  26  to increase the overall field of view and functionality of the system. Optical sensor  80  can consist of one or more coated optical fibers. For example, optical sensor  80  can consist of one or more gold-coated 400μ silica (UV enhanced) fibers or the like.  
         [0051]     In one embodiment of the subject invention, optical sensor  80  is axially mounted within the fuel nozzle  26  by a supporting fixture associated with the inner air swirler (not shown). In this position, sensor  80  is utilized to detect variations in spectral ratios indicative of stoichiometric stability. More particularly, in such an embodiment, optical sensor  80  includes two optical fibers  82   a ,  82   b  that are axially positioned in side-by-side relationship to detect variations in spectral ratios of CH output to OH output generated over a period of time by the signal generator of signal processor  40 . For example, as illustrated in the spectral scan of  FIG. 7 , the relationship of the CH output signal to the OH output signal over time, generated by corresponding PMT&#39;s of signal generator  40 , can be correlated to changes in fuel to air ratio. Based on this information, the engine will be tuned to stabilize combustion. In another embodiment of the subject invention, the axially mounted optical sensor can include two optical fiber bundles, each containing four gold-coated 200μ silica fibers rated to 700° C.  
         [0052]     In accordance with another embodiment of the subject invention, the optical sensor array  30  of fuel injector  20  is adapted and configured to detect radiation in the infrared spectrum. In this case, the sensor array is embedded in the fuel nozzle  26  in the same manner described and illustrated above, but one or more of the optical fibers  38  are IR enhanced instead of UV enhanced. In one embodiment, the IR enhanced optical fibers  38  are adapted and configured to detect spectral radiation in the 1.7μ to 2.1μ range. This correlates to exhaust gas temperature. In contrast, exhaust gas temperature does not correlate to the chemiluminescence outputs in the UV range described previously.  
         [0053]     It has been determined that there is an absorption/emission band for water vapor generated by the combustion process within the 1.7μ to 2.1μ range. It is envisioned that the output from the infrared sensor array may be used to obtain flame temperature, in a manner similar to an optical pyrometer. It is also envisioned that the output from the infrared sensor array may be used to obtain turbine inlet vane temperature when in the field of view.  
         [0054]     Referring now to  FIGS. 8   a  through  8   c , there are illustrated three different methods of terminating the optical fibers  38 , which form the fiber bundles  36  within the stainless steel guide tubes  42  located in view ports  34 . In one embodiment shown in  FIG. 8   a , the optical fibers  38  terminate at a location spaced from the distal end of the guide tube  42 . A shaped lens  90  supported within a frame  92  is joined to the distal end of the guide tube  42 . The lens  90  is formed from a material such a sapphire and is used to focus or otherwise modify the field of view of the optical fibers  38 . It is envisioned that the exposed surface of the lens  90  would have a protective coating. For example, the lens can have a vapor deposited layer comprising a mixture of platinum and aluminum oxide which acts as a catalyst to promote oxidation of soot to a gaseous form and thereby reduce contamination of the lens, as disclosed in U.S. Pat. No. 4,521,088 to Masom, the disclosure of which is incorporated herein by reference in its entirety.  
         [0055]     In another embodiment shown in  FIG. 8   b , the optical fibers  38  terminate at a location spaced from the distal end of the guide tube  42 , and a window  94  supported within a frame  96  is joined to the distal end of the guide tube  42 . The window  94  may be formed from sapphire or a similar transparent material and would function to seal the end of the guide tube from contamination and combustion by-products, which could degrade the optical fibers. It is envisioned that the window  94  could also be frosted or otherwise treated and used as a diffuser to widen the viewing area of the optical fiber bundle.  
         [0056]     In the embodiment shown in  FIG. 8   c , the distal ends  38   a  of each optical fiber  38  in fiber bundle  36  is splayed or otherwise spread outwardly so that each individual fiber end  38   a  is oriented or directed in a different direction relative to the axis of the bundle. This enables the optical fibers to gather light from a broader or greater area. Protective coatings could be used to coat the ends of the fibers.  
         [0057]     In sum, the fiber optic instrumented fuel injector of the subject invention may be used for a multiplicity of purposes including, for example, engine control through detection of combustion instability; fuel to air ratio stability by analyzing spectral relationships; obtaining turbine inlet temperature correlated to flame temperature; detection of fuel impurities based on spectral anomalies; aircraft augmenter flame detection; obtaining turbine inlet vane temperature by way of a pyrometer; and advanced warning or prediction of lean blowout conditions.  
         [0058]     Although the apparatus, system and method of the subject invention have been described with respect to preferred embodiments, those skilled in the art will readily appreciate that changes and modifications may be made thereto without departing from the spirit and scope of the subject invention as defined by the appended claims.