1. Field of the Invention
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.
2. Background of the Related Art
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.
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.
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.
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.
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.
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.