The present invention relates to an air bleed system for a compressor of a gas-turbine engine.
More specifically, the present invention relates to a centripetal airbleed system positioned between oppositely opposed first and second disks of a gas-turbine engine compressor. The air bleed system includes an annular support affixed to the first and second disks, and a plurality of airbleed tubes mounted in a substantially radial manner within orifices defined along the periphery of the support. Each tube is arranged with a vibration reduction device that reduces vibrations during operation of the engine.
An air bleed system is proposed in U.S. Pat. No. 5,472,313 wherein a damping tube is inserted within the radially inner part of each airbleed tube. At a radially outer portion, the damping tube is arranged with longitudinal slots defining axial strips which, during gas turbine engine operation, are centripetally urged against the inside wall of the bleed tube. Friction between the inner damping tube strips and the outer airbleed tube dissipates deformation energy and therefore lowers the dynamic stresses in the outer tube.
Through-holes are present at the end of the longitudinal slots and constitute stress concentration zones. This feature requires additional and intricate machining. Further, the service life of such inner damping tubes is less than 100,000 cycles.
The mode 1F frequency of the centripetal airbleed-tube of the GE turbojet engine 90-115B is 950 Hz. The aformentioned engine also includes 12 bleed tubes, and the 8F mode frequency operating at 7,125 rpm is (7,125xc3x978/60=) 950 Hz. This is the same frequency as for the 1F mode.
One object of the present invention is to lower the dynamic stress in the airbleed tube by dissipating the energy of deformation.
Another object of the present invention is to propose an airbleed system based on centrifugal outflow wherein the device damping the airbleed tube vibrations allows for substantially increasing the frequency of the first bending mode, the so-called 1F mode, in order to eliminate crossing of the 1F mode and drive source.
According to the present invention, there is provided a device for reducing the vibrations of an airbleed tube that includes a damping tube maintained within an annular support orifice and which encloses a radially outer part of an airbleed tube. It follows that the radially inner part of the damping tube encloses a portion of the radially outer part of the airbleed tube. An annular gap is defined between the median zone of the damping tube and the airbleed tube.
The damping of the centripetal airbleed system of the present invention relies on two principles: flexibility exhibited at the contact between the outer damping tube so that the airbleed tube can act as a spring; and the rigidity at the contact dissipates energy as if it were a spring. Moreover, the outer damping tube may be stressed by bending such that the bending strength of the damping tube enables energy dissipation.
The proposed system of the invention allows lowering the dynamic stress in the airbleed tube by dissipating energy of deformation and by an increase in the frequency of the first bending mode, the so-called 1F mode, on account of the increase in equivalent rigidity of the airbleed system.
In addition, the proposed outer damping tube is free of slots and therefor holes at the ends of the slots. As a result, the manufacture of the present invention is easier than the machining of a strip-fitted inner damping tube of the prior art.
In a first embodiment of the present invention, the radially inner end of the damping tube is constricted sufficiently to contact a peripheral wall of the airbleed tube along a plurality of axial zones defined between the damping and airbleed tubes.
In a second embodiment of the present invention, the airbleed tube includes a boss extending from a radially outer surface thereof that engages with the radially inner end of the damping tube. The boss includes a plurality of flat surfaces defined along thereof.
Other advantages and features of the invention are elucidated in the following illustrative description and in relation to the attached drawings.