BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a typical gas turbine engine.
FIG. 2 schematically shows a method according to this invention.
FIG. 3 shows a first table of information that allows the prediction of a crack of certain length in an airfoil.
FIG. 4 shows an alternative table of information for predicting a crack when based upon a second system condition.
FIG. 5 shows yet another alternative table for predicting a crack of certain length.
FIG. 6 shows a remaining life table based upon a crack length, and various stress levels which may be applied to the blade.
FIG. 7 shows a monitored stress condition that would be indicative of a failure in a gas turbine engine.
FIG. 8 is a flowchart of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a gas turbine engine 10, such as a gas turbine used for power generation or propulsion, circumferentially disposed about an engine centerline, or axial centerline axis 12. The engine 10 includes a fan 14, a compressor 16, a combustion section 18 and a turbine 11. As is well known in the art, air compressed in the compressor 16 is mixed with fuel which is burned in the combustion section 18 and expanded in turbine 11. The air compressed in the compressor and the fuel mixture expanded in the turbine 11 can both be referred to as a hot gas stream flow. The turbine 11 includes rotors 13 and 15 that, in response to the expansion, rotate, driving the compressor 16 and fan 14. The turbine 11 comprises alternating rows of rotary blades 20 and static airfoils or vanes 19. FIG. 1 is a somewhat schematic representation, for illustrative purposes only, and is not a limitation of the instant invention, that may be employed on gas turbines used for power generation and aircraft propulsion. The compressor 16 and fan 14 also include rotors and removable blades.
FIG. 2 shows a method according to this invention in which remaining life for an airfoil such as turbine blade 30 is monitored. The invention extends to other blades, such as compressor, turbine or fan blades. A sensor 40 senses movement of blade 30. Conditions such as the time at which the leading edge of the airfoil passes a predetermined point, compared to an expected time, can be monitored. If the leading edge actually passes a predetermined point at a time different from the expected time an indication can be made that there is some problem with the particular airfoil.
The present invention has developed transfer functions which associate a relative frequency change, or other changes, with growing length of a crack in the airfoil. Different modes of monitoring the airfoil can be taken at different locations at the airfoil and can be utilized to predict the location and length of the crack. The transfer function such as shown in FIG. 2 can be determined experimentally and/or analytically, and are generally available to a worker of ordinary skill in this art. Over time, the damage to the airfoil will accumulate. Thus, a remaining life can be predicted given a particular crack length, and based upon the particular stresses on the airfoil in question.
FIG. 3 shows one embodiment of a table of information that associates a lean in the leading edge of the airfoil with a plurality of curves with different speeds of operation of the associated rotor. Now, a particular identified lean can be associated with the relative rotational speed, and in this manner a crack of certain length can be predicted. This information can be developed mathematically, and a worker of ordinary skill in the art would be able to develop the appropriate table. The Y axis is a measurement of blade deflection, or the “lean” of the leading edge measured in 1/1000 of an inch.
FIG. 4 shows another method of detecting a crack of certain length. Here, the tip of the leaning edge deflection is monitored. Again, the particular speed of operation is associated with a plurality of curves, and by finding the appropriate curve, and the appropriate amount of deflection, a prediction of a crack of certain length can be made. Again, the Y axis is measured as the leading edge deflection measured in 1/1000 of an inch.
Other deformations that can be measured include first bending mode, stiffwise bending mode, first torsion mode, chordwise bending mode, second leading edge bending mode, second bending mode, second torsion mode, chordwise second bending mode, and third trailing edge bending
FIG. 5 shows yet another embodiment, where model frequency shift is calculated and associated with a plurality of distinct measurements. Again, this can be utilized to predict a crack of certain length, as shown in the formula found in FIG. 5.
Once a crack of certain length has been detected, another family of curves can be used to associate various stress levels on the airfoil with a remaining life. Examples of such curves are shown in FIG. 6. Each curve represents the effect of different stress levels. In this figure, the remaining life is defined in “mini-sweeps” or times when the engine is accelerated and de-accelerated across a resonance frequency for the airfoil. Once the number of “mini-sweeps” remaining can be identified, a prediction can be made for the remaining useful life before failure of a particular airfoil. Essentially, the particular airfoil closest to failure would be a limitation on the amount of useful life for the entire engine and would suggest maintenance before the useful life has expired. Another measurement of useful life remaining would be cycles or missions. A computer associated with the sensors stores information with regard to each of the airfoils which are experiencing apparent cracks. The amount of damage which has been accumulated to that airfoil is stored in the computer, such that the computer has a running total of the amount of useful life remaining. As can be appreciated from this figure, at different stress levels, the useful life remaining changes. Thus, the computer must store not only the crack length and how often the particular engine has been operated, but also the operating conditions.
Further, with this invention and due to the various effects of different stress levels, it is apparent that by planning a particular flight schedule for an aircraft holding a particular jet engine, the number of flights remaining can be optimized. For example, in military applications there are high stress and low stress flights. An air to ground attack mission might be a relatively high stress flight in that it could involve frequent accelerations and decelerations. On the other hand, air cover under which an aircraft tends to remain high in the air at a relatively constant speed should be relatively low stress. A field commander might assign a particular aircraft to one of these flight schedules based upon an indicated remaining life indicated by this invention. This can lengthen the time between necessary maintenance.
The information provided in this invention also can provide an indication of an apparent immediate failure. As an example, FIG. 7 illustrates a series of mini-sweeps as each blade passes by the sensor. At points 1-2-3, a dramatic drop occurs. This may be indicative of a blade that is bent so badly that it has contacted the sensor, etc. At any rate, such an indication might require immediate maintenance.
FIG. 8 is a basic flowchart of the present invention. The blades rotation is monitored. A sensor and associated computer checks for flutter, etc. and determines that a particular blade has developed a crack. Once a crack has been detected, a crack length is determined. Once the crack length has been determined, a remaining life for the particular airfoil can be calculated. The computer then begins to store the actual conditions of operation for that airfoil such that a useful remaining life can be calculated in a continuous manner. The amount of remaining life can be utilized to schedule flights and maintenance, as mentioned above.
While the above embodiments of this invention are all disclosed utilizing a predicted crack length, other types of damage to a blade may also be utilized in connection with this invention.
While a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.
Patent Application
20070271023
