Abstract:
A system for sensing at least one physical characteristic associated with an engine including a turbine having a plurality of blades turning inside a casing, the system including: a pressure sensor coupled substantially adjacent to the casing and including at least one output; a port in the turbine casing for communicating a pressure indicative of a clearance between the blades and casing to the pressure sensor; a cooling cavity substantially surrounding the pressure sensor; and, an inlet for receiving fluid from the engine and feeding the fluid to the cooling cavity to cool the pressure sensor; wherein, the pressure sensor output is indicative of the clearance between the blades and casing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 12/387,078, filed 28 Apr. 2009, which was a continuation application of U.S. patent application Ser. No. 11/063,205, filed 22 Feb. 2005, now U.S. Pat. No. 7,540,704, which issued on 2 Jun. 2009, which claimed priority to U.S. Provisional Patent Application No. 60/582,289, filed 23 Jun. 2004, the entire disclosures are hereby incorporated by reference in their entirety herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to gas turbine engines and their operation, and more particularly to gas turbine engine tip clearance measurement systems. 
     BACKGROUND OF THE INVENTION 
     It is well known that tip clearance leakage is one of the primary loss mechanisms in axial flow compressors and turbines of a gas turbine engine. Tip clearance loss translates into lost efficiency, higher fuel costs and thus higher operating costs. More particularly, over the operating life of an engine such as an aircraft engine, tip clearance increases over time, due at least in part to mechanical rubs between rotating blades and stationary casing and erosion. This clearance deterioration is a leading driver for engine performance deterioration, which often manifests in increased fuel burn and exhaust gas temperatures (EGT). The FAA mandates that an engine be removed for maintenance/overhaul once the EGT reaches an upper limit. 
     It is desirable therefore to maintain tip clearance as low as possible in an effort to minimize related losses throughout the engine-operating envelope. One way of achieving this is to use Active tip Clearance Control (ACC) systems, such that clearance levels are adjusted for engine operating conditions, and throughout the operating cycle. For any ACC concept to work effectively, real-time tip clearance data is required as part of the control algorithm. However, current tip clearance sensors are believed to be deficient in certain regards. 
     Accordingly, an alternative tip clearance measurement technique and system for accomplishing tip clearance measurement is highly desirable. 
     SUMMARY OF THE INVENTION 
     A system for sensing at least one physical characteristic associated with an engine including a turbine having a plurality of blades turning inside a casing, the system including: a pressure sensor coupled substantially adjacent to the casing and including at least one output; a port in the turbine casing for communicating a pressure indicative of a clearance between the blades and casing to the pressure sensor; a cooling cavity substantially surrounding the pressure sensor; and, an inlet for receiving a fluid such as compressed air from the engine and feeding the compressed air to the cooling cavity to cool the pressure sensor; wherein, the pressure sensor output is indicative of the clearance between the blades and casing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Understanding of the present invention will be facilitated by consideration of the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts, and: 
         FIG. 1  illustrates gas turbine tip clearance sensor system according to an aspect of the present invention; and, 
         FIG. 2  illustrates gas turbine tip clearance sensor system according to an aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding, while eliminating, for the purpose of clarity, many other elements found in typical gas turbine engines and methods of making and using the same, and pressure sensing systems and methods of making and using the same. Those of ordinary skill in the art may recognize that other elements and/or steps may be desirable in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. 
       FIG. 1  illustrates a schematic cross section of an exemplary turbine system  100  including a pressure transducer  110  mounted substantially adjacent to, including for example, within the interior of, turbine case  120 . Transducer  110  measures pressure on the turbine case  120 , and provides a signal indicative of the sensed pressure via leads (not shown) electrically coupled thereto. 
     Turbine system  100  may include an engine assembly that takes the form of a conventional gas turbine engine. In operation, blades  140  of engine  100  rotate past port  150  which communicates the pressure at the turbine casing  120  to transducer  110 . As a result of blade rotation, the pressure sensed by transducer  110  varies. As a blade passes and obscures port  150 , the inlet of port  150  is essentially closed and the pressure communicated to transducer  110  is essentially the ambient static pressure. The inlet to port  150  becomes un-obscured after the blade passes. At this point, the communicated and sensed pressure rises to a maximum pressure indicative of blade  140  loading. This cyclic process repeats as each of the turbine blades  140  passes port  150 . 
     As is understood, tip clearance size affects the blade loading. This is due to leakage flows from one side of the blade to the other across the clearance gap. Hence, the unsteady pressure field exerted upon port  150  is a function of tip clearance size. The functional dependence between the two tip clearance and the pressure signature as measured by the transducer may be established through computer modeling and/or calibration testing, for example. Thus, one may derive real-time tip clearance data from sensing the unsteady pressure signature resulting from turbine blades passing by a case mounted pressure transducer. 
     As will be understood by those possessing an ordinary skill in the pertinent arts, pressure transducer  110  may have a frequency response capability roughly 5-10 times that of the blade passing frequency in order to resolve the flow structure at the blade tip region. For example, the blade passing frequency for a high-pressure turbine in a typical modern gas turbine engine may be around ten kilohertz (10 KHz). Accordingly, transducer  110  may have a frequency response on the order of about 50 KHz-100 KHz. Such high frequency operation may require transducer  110  to be mounted close to turbine casing  120 —as a physically extending port  150  may serve to essentially low-pass filter the pressure signature resulting from turbine blades  140  passing port  150 . 
     The output of pressure transducer  110  may optionally be provided to a signal processing and conditioning electronics module  130  remotely located within the system  100 . Sensor  110  and/or signal processor  130  may provide one or more signals indicative of an operating condition of the engine assembly  100 , such as turbine tip clearance. 
     Signal processing and conditioning electronics module  130  may include a processor and memory, by way of example only. “Processor”, as used herein, refers generally to a computing device including a Central Processing Unit (CPU), such as a microprocessor. A CPU generally includes an arithmetic logic unit (ALU), which performs arithmetic and logical operations, and a control unit, which extracts instructions (e.g., code) from memory and decodes and executes them, calling on the ALU when necessary “Memory”, as used herein, refers to one or more devices capable of storing data, such as in the form of chips, tapes or disks. Memory may take the form of one or more random-access memory (RAM), read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), or electrically erasable programmable read-only memory (EEPROM) chips, by way of further non-limiting example only. The memory utilized by the processor may be internal or external to an integrated unit including the processor. For example, in the case of a microprocessor, the memory may be internal or external to the microprocessor itself. Of course, module  130  may take other forms as well, such as an electronic interface or Application Specific Integrated Circuit (ASIC). 
     As is well understood by those possessing an ordinary skill in the pertinent arts, in general an axial flow turbine engine includes a compressor, combustion area and turbine. In compressor applications, the casing temperature is at or below 1300 degrees Fahrenheit (1300° F.). In the turbine section, the metal temperature can reach as high as 2500° F. According to an aspect of the present invention, transducer cooling may used. According to another aspect of the present invention, pressure transducers for turbine clearance measurement may be air cooled, optionally using the same cooling air that may be used to cool the turbine casing. 
     Referring still to  FIG. 1 , there is shown a cooling chamber  160  substantially surrounding transducer  110  and having a cooling air inlet  170  and outlet  180 . Cooling air for inlet  170  may be drawn from a compressor of turbine system  100 , and thus have a temperature around 1300° F., for example. The cooling air may circulate through chamber  160 , cooling transducer  110  and/or the immediate environment it is subjected to, and then exit outlet  180 . A high temperature pressure transducer, such as model WCT-250 or WCT-312 cooled by air of water pressure sensor, commercially available from Kulite Semiconductor Products, Inc. the assignee hereof, may be used in combination with such a cooling scheme to provide a system that can reliably operate in a high temperature, high pressure turbine environment. 
     Referring now to  FIG. 2 , there is shown a turbine system  200  according to an aspect of the present invention. Like references have been used in  FIGS. 1 and 2  to designate like elements of the invention. Hence, a detailed discussion of those common elements will not be repeated. In system  200 , cooling air is again fed into a chamber  160  substantially surrounding pressure transducer  110  via inlet  170 . The cooling air is again discharged using an outlet  180 . However, outlet  180  of system  200  discharges spent cooling air into the main gas path, i.e., into the turbine. Outlet  180  may discharge into port  150 , such that pressure sensing port  150  of system  200  will have a net air outflow, forming a discharge jet into the turbine. This allows an interaction between the discharge jet and the passing turbine blades  140 . This interaction may enhance the sensing of the unsteady pressure as a function of tip clearance size. Parameters, such as discharge jet velocity and flow rate of the cooling chamber, inlet and outlet may be chosen to maximize the sensitivity of the sensed unsteady pressure signal as a function of tip clearance. 
     By way of further, non-limiting example only, the cooling airflow in  FIG. 2  is modulated by the relative motion of turbine blades or airfoils. The cooling air is first modulated by the interaction between the cooling air and unsteady pressure field around each turbine blade. The unsteady pressure fluctuations will modulate cooling airflow rate, therefore affecting air pressure measured by the pressure sensor. The cooling air is also modulated by the interaction with the turbine blades themselves. When the turbine blades periodically pass over the cooling air discharge jet, a blockage effect occurs when the turbine blade is aligned with the discharge jet, whereas no or little blockage is present without such an alignment. This on and off blockage effect modulates the cooling airflow rate, again impacting unsteady pressure measurements. The amount of blockage, and the resultant pressure fluctuations, will depend on blade geometry and tip clearance size. As blade geometry is known, tip clearance may be deduced. 
     According to an aspect of the present invention, by sizing the cooling air discharge and cooling chamber geometries, one may “acoustically tune” the effect on transducer  110  so as to maximize pressure fluctuations due to tip clearance changes, thus increasing tip clearance measurement accuracy. 
     According to an aspect of the present invention, transducer  110  may also be utilized to measure turbine rotational speed. Transducer  110  senses the turbine blade passing frequency, by sensing the unsteady pressure field generated each time a turbine blade  140  passes port  150 . Using this frequency, together with the known configuration of the turbine itself, such as the number of blades installed on the turbine wheel, one may readily deduce turbine shaft speed. Such a shaft speed sensor may prove more reliable, and physically lighter than conventional magnetic speed transducers. Further, as a same transducer may be used to provide multiple functionality according to an aspect of the present invention, additional cost savings to the engine system as a whole may be realized. 
     According to an aspect of the present invention, tip clearance may be adjusted using a conventional methodology responsively to the output of the pressure transducer. 
     Those of ordinary skill in the art may recognize that many modifications and variations of the present invention may be implemented without departing from the spirit or scope of the invention.