1. Description of the Prior Art
In recent years, widespread legal and environmental requirements have imposed strict limits on nitrous oxide emissions in power generating equipment. Low temperature combustion chambers may be used to attain low nitrous oxide emissions in gas turbines. Current gas turbine combustion systems producing low nitrous oxide emissions typically use both a premix or primary zone and a secondary zone where reduced temperature combustion takes place as a direct result of the enhanced air fuel mixing. The combustion takes place only in the secondary zone at base load, then in either or both combustion zones on a strict start-up and shut-down schedule in order to avoid hardware damage.
To control flame presence in the proper zone or zones, one must sense flame independently in either zone. Typically, flame sensors continuously sense the presence of infrared, visible, ultraviolet or some combination of these three wavelengths of flame radiation (hereinafter sometimes collectively referenced as "light" radiation), and then announce that presence to a control system which then acts immediately when the flame improperly appears in either combustion zone. The sensor itself must be physically located some distance away from the intense heat generated by the combustion chambers, while retaining high sensitivity to the generated radiation.
In FIG. 1 a typical flame detection system 10 similar to that disclosed in U.S. Pat. No. 4,855,718 to Cholin et al is shown. In effect, such prior art approaches have coupled a sensor to a combustion chamber via a multi-mode light radiation waveguide having a relatively wide view angle or effective aperture.
System 10 includes a flame detector sensor 12 at one end of an elongate, hollow, cylindrical tube (the waveguide) 14 which is located external to the combustion chamber 16 to be viewed. Tube 14 and chamber 16 are sealed by connector 17 to both seal in combustion chamber pressure and to prevent unwanted environmental radiation from being detected. The inner surface 18 of tube 14 reflects radiation, thus increasing the effective view angle for the detector by permitting many multiple transmission modes. At the end of tube 14 opposite sensor 12 is an aperture 20 through which is admitted light radiation from aperture 22 located in wall 24 of combustion chamber 16. Light radiation from inside the combustion chamber 16 which falls within a radiation-detection zone defined by cone angle .theta. emanates from secondary aperture 22, enters primary aperture 20 and is transmitted throughout the length of the interior of tube 14 and ultimately detected by sensor 12. Since the inner surface 18 of tube 14 is reflective (the reflectance being a function of tube material, surface finish, wavelength, temperature, etc.), light radiation present within a relatively large cone angle field of view within the combustion chamber is admitted through apertures 20, 22. In effect, the reflectivity of the internal walls of the waveguide 14 greatly increases the number of transmission modes--thus increasing the effective view angle of the sensor in the chamber. This ensures detection of a flame virtually anywhere within the combustion chamber under view--but it also increases the risk of false flame detections and makes it essentially useless in modern day dual zone gas turbine combustors.
Typically, during start up of a gas turbine, two spark plugs are temporarily inserted into two combustion chambers and fired until the combustible air fuel mixture is ignited and cross fires to the adjacent combustion chambers. This occurrence is detected by the flame detection system and at that point the spark plugs are shut down and retracted. Reflected radiation from the spark plugs can, however, arc directly to the flame sensor either directly or through combustion chamber interconnecting crossfire tubes, which can cause the control system to make a premature false flame-present indication prior to actual ignition. In the past this problem has been avoided by prudent placement of the flame sensors and the spark plugs on opposite sides of the machine, or by having one or more combustion chambers located between the spark plugs and the flame sensors and by avoiding sight lines toward the crossfire tubes. These arrangements effectively attenuate undesirable reflected radiation by providing a long tortuous path between the two devices. With the advent of dual-zone, dry low NOx combustors, however, the continuous presence of an intense radiation source within all chambers precludes avoidance by these existing techniques and requires resolution.
Prior to the use of flame sensors such as that depicted in FIG. 1, bundled thermocouples known as thermo-piles were used to detect the presence of flame. This technique was replaced with the more reliable and faster flame sensor of FIG. 1. Although thermo-piles are not significantly affected by spurious, reflected radiation, and their insufficient time-constant can be overcome by using a combination flame detector-thermo-pile system, major technical limitations exist which preclude their application in dual-zone gas turbine combustors. The reliability of thermocouples in close proximity to flame does not meet present control standards, and the combination of two devices which will provide contradictory signals to the control system increases the complexity of the overall system.
Selective filtering of the primary wavelength of the desirable radiation is not feasible because the detrimental reflected radiation which produces the unwanted portion of the signal to be attenuated has the same wavelength as the radiation which produces the required signal.
One solution is to restrict the viewing area by eliminating radiation rays outside a narrow cone angle within the line-of-sight, thus controlling the amount of radiation to the sensor and eliminating a resultant false flame signal. Radiation detection systems which employ some sort of directional sensitivity are disclosed in:
U.S. Pat. No. 3,689,773--Wheeler (1972);
U.S. Pat. No. 4,037,113--Moore (1977);
U.S. Pat. No. 4,163,903--Robertson (1979);
U.S. Pat. No. 4,317,045--Coe et al (1982);
U.S. Pat. No. 4,328,488--Yanai et al (1982);
The optical field-narrowing systems disclosed in the prior art, however, are either too complex, provide inadequate radiation collimation or are otherwise unsuited for use in a dual-zone gas turbine combustor where line-of-sight collimation must be selected at an angle within the viewed combustion chamber zone which will cut directly across the flame path at all operating conditions when source radiation is present in that combustion chamber zone, while yet eliminating any extraneous direct or reflected radiation from any other of the combustion chamber zones.
2. Summary of the Invention
This invention attenuates detrimental, reflected flame radiation which, when received by flame sensors, otherwise results in false or biased information of flame presence and/or flame location within gas turbine combustors. Attenuation is accomplished without filtering or otherwise reducing the flame sensor sensitivity to flame radiation received through a clear aperture. The foremost application of the invention is for dual chamber or dual zone, dry low NOx, combustors. In this type of combustion system, knowledge of flame presence in either or both zones is critical for proper operation. Unattenuated, reflected radiation from either zone to the sensor of the other zone results in spurious flame indication and ineffectual system control.
One exemplary system provides a spark or flame detector contained at one end aperture of an elongate, hollow cylindrical tube having side walls with mechanically-reduced effective reflectance and thus, a more restricted view angle. The detector converts light energy reaching it in the infrared, visible and ultraviolet bands to an electric signal which can be conventionally used to control operation of the gas turbine system. The other end of the tube contains an aperture arrangement which allows impinging radiation to be transmitted along the length of the tube to the detector at the other end. The open end of the tube is secured to view into an opening in the lining of the combustion chamber. The combination of size of the aperture and the reduced effective sidewall reflectance provides restriction in the zone of radiation detection or view angle of the flame detector.
Impinging radiation from the larger and unwanted view angles strikes the walls of the tube and is attenuated by the physical configuration of the inner surface of the tube. For example, the inner surface of the tube may be lined with a plurality of annular ridges (e.g., internal threads) along the length of the tube. These ridges reflect backward and scatter unwanted reflected radiations entering the tube from wider view angles, while leaving direct line-of-sight radiation unaffected as it travels the length of the tube to the detector.
In another exemplary embodiment, at least one section of the interior of the tube may, in effect, be cut out by having its diameter enlarged. This also can be used to effect backward reflection and scattering of unwanted reflected light radiation impinging upon that section from larger view angles.
In still another exemplary embodiment, at least one section of the tube contains singular raised annular ridges separated by relatively smooth interior sections,--while other sections of the inner lining of the tube may contain smaller, multiple annular ridges of the first embodiment (e.g., threading), with the same effect as above.
It will thus be appreciated that the present invention enables a zone flame detector (in a dual zone gas turbine combustion) to selectively attenuate unwanted reflected radiation from larger view angles while admitting collimated radiation received through the clear aperture formed by the first and secondary apertures. This prevents false flame detection system response from other sources while admitting, without attenuation, the direct radiation from a significant source at which the flame detection system is aimed.