Power plants – Reaction motor – Method of operation
Reexamination Certificate
1998-12-30
2001-03-06
Freay, Charles G. (Department: 3746)
Power plants
Reaction motor
Method of operation
C060S223000, C060S039240, C415S119000
Reexamination Certificate
active
06195982
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an apparatus and method for preventing aeromechanical instabilities from occurring in turbofan engines, and more particularly relates to an apparatus and method for damping flutter dynamics in turbofan engines to prevent blade failure.
BACKGROUND INFORMATION
Turbofan engines are typically associated with running power plants or powering airplanes. With respect to airplanes, aeromechanical instabilities such as flutter may catastrophically lead to blade failure. Flutter is characterized by a resonance or elastic deformation of the turbofan blades generated by the coupling of the aerodynamics and the structural dynamics of the blades. The blades have natural and associated harmonic frequencies of resonance which are based on the blade structure or configuration. An axial turbomachinery blade is associated with structural mode shapes which are the natural patterns and frequencies in which the blade deflects and resonates when excited. A blade has more than one mode shape and each mode shape resonates at a particular frequency. When an instability such as flutter occurs, it is usually associated with one particular structural mode excited by the coupling with the unsteady aerodynamics. It is therefore vitally important to detect these instabilities in aeropropulsion compression systems and to dampen the instability dynamics to prevent such imminent blade failure.
Such aeromechanical instabilities impose significant constraints on the design and development of modern aero-engines. As shown in
FIG. 8
, of particular concern is flutter which has many occurances on a fan's pressure ratio (ordinate) versus mass flow (abscissa) operating map
400
. As shown in
FIG. 8
, curves
402
and
404
respectively correspond to the operating line and flutter lines of the operating map. The constraint on blade design is to keep the flutter boundaries outside of the operating envelope of the engine.
FIGS. 1
a
and
1
b
illustrate (in exaggerated form) blade resonance or energy waves generated in a turbofan
200
having eight blades
202
,
204
,
206
,
208
,
210
,
212
,
214
and
216
. The blades
200
-
216
are shown in solid form corresponding to a non-deflected state, and the blades
204
-
208
and
212
-
216
are also shown in phantom form corresponding to a deflected state during a resonance or elastic deformation of the blades which may arise due to flutter during blade rotation.
FIG. 1
b
maps the degree of deformation of each blade during an instant of time where the amount of blade deformation in the direction of blade rotation is a positive value and the amount of blade deformation in the direction opposite to blade rotation is a negative value.
At an instant of time during rotation of the turbofan
200
in the clockwise direction, the blade
202
is shown in
FIG. 1
a
to have no deformation which corresponds to a deformation value of zero units for the blade
202
as mapped in
FIG. 1
b
. The blade
204
is shown in
FIG. 1
a
to have a slight deformation in the direction of rotation which corresponds to a positive deformation of one unit for the blade
204
as mapped in
FIG. 1
b
. The blade
206
is shown in
FIG. 1
a
to have an even greater deformation relative to the blade
204
in the direction of rotation which corresponds to a positive deformation of two units for the blade
206
as mapped in
FIG. 1
b
. The blade
208
is shown in
FIG. 1
a
to have the same deformation as the blade
204
which corresponds to a positive deformation of one unit for the blade
208
as mapped in
FIG. 1
b.
The blade
210
is shown in
FIG. 1
a
to have no deformation which corresponds to a deformation value of zero units for the blade
210
as mapped in
FIG. 1
b
. The blade
212
is shown in
FIG. 1
a
to have a slight deformation in a direction opposite to blade rotation which corresponds to a negative deformation of one unit for the blade
212
as mapped in
FIG. 1
b
. The blade
214
is shown in
FIG. 1
a
to have an even greater deformation relative to the blade
212
in the direction opposite to blade rotation which corresponds to a negative deformation of two units for the blade
214
as mapped in
FIG. 1
b
. The blade
216
is shown in
FIG. 1
a
to have the same deformation as the blade
212
which corresponds to a negative deformation of one unit for the blade
216
as mapped in
FIG. 1
b
. The resonance pattern shown in
FIGS. 1
a
and
1
b
correspond at an instant of time to one sinusoidally shaped cycle of deformation of the blades as seen along a 360° path circumaxially about the turbofan
200
. However, other excitation patterns characterized by zero or multiple cycles contribute to flutter in aerocompression systems.
Flutter in axial turbomachinery typically occurs in specific nodal diameters dependent on the particular geometry of the turbomachinery. A nodal diameter is the wave number of the sinusoid that the blade deflection pattern represents.
FIGS. 2
a
-
2
c
illustrate various nodal diameters of the blade deflection pattern of turbofan blades. The length and direction of the arrows in each figure define respectively the degree and direction (positive or negative direction) of the turbofan blades as viewed at an instant of time about the rotational axis of the turbofan from a start point (0°) to the end point (360°). As is evident, the start and end points are the same physical position.
FIG. 2
a
illustrates a 0th nodal diameter pattern
227
of arrows
229
diagrammatically representing the direction and degree of blade deflection in which each turbofan blade exhibits no blade deflection or the same amount of blade deflection with respect to one another when viewed at an instant of time at any point around the axis of rotation of the turbofan.
FIG. 2
b
shows a 1st nodal diameter deflection pattern
231
of arrows
233
representing blade deflection at an instant of time in which the turbofan blades as viewed circumaxially about the turbofan exhibit a single cycle generally sinusoidal wave pattern. Such nodal diameter deflection patterns illustrate the general resonance deflection pattern of a turbofan in a manner which is independent of the actual number of blades comprising the turbofan. As can be seen, the 1st nodal diameter deflection pattern of
FIG. 2
b
corresponds to the deflection pattern embodied by the eight turbofan blades in
FIG. 1
a
.
FIG. 2
c
illustrates a 3rd nodal diameter deflection pattern
235
of arrows
237
representing blade deflection at an instant of time in which the turbofan blades as viewed circumaxially about the turbofan axis of rotation exhibit a three cycle generally sinusoidal wave pattern. The structural mode shape is the natural pattern in which an axial turbomachinery blade deflects and resonates when excited. A blade has more than one mode shape and each mode shape resonates at a particular frequency. When flutter occurs, it usually is associated with one particular structural mode. Flutter is difficult to predict analytically and expensive to investigate experimentally. Consequently, flutter is often encountered only in the final phases of engine development leading to expensive delays and often forcing a degradation in overall system performance.
In response to the foregoing, it is an object of the present invention to overcome the drawbacks and disadvantages of prior art apparatus and methods for preventing aeromechanical instabilities in aero-engines.
SUMMARY OF THE INVENTION
In one aspect, a system for damping the aeromechanical instability of flutter in a turbofan engine having a plurality of blades spaced substantially equidistant from each other about a rotational axis includes a sensor to be mounted on a turbofan engine outwardly from turbofan blades at an inlet of a rotor of the engine for generating a sensor signal indicative of resonance of the turbofan blades at frequencies associated with flutter. A controller is coupled to the sensor for generating from the sensor signal a command signal comprising a real time amplitude component and a spatial ph
Eveker Kevin M.
Feulner Matthew R.
Gysling Daniel L.
Freay Charles G.
McCormick Paulding & Huber LLP
United Technologies Corporation
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