Filament-Wound Tip-Shrouded Axial Compressor or Fan Rotor System

Abstract

A filament-wound tip-shrouded axial compressor or fan rotor system is described that employs a brush seal to minimize the leakage of flow that has passed through the rotor from re-entering the rotor inlet region. A circumferential array of small orifices through the outer casing just behind the brush seal is further provided to allow a small flow of air between the rotating shroud and the casing in order to limit the temperature rise due to aerodynamic heating of the fluid immediately adjacent to the filament winding.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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REFERENCE TO SEQUENCE LISTING, A TABLE, ETC.

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BACKGROUND OF THE INVENTION

This invention pertains primarily to a new sealing approach applied to a filament-wound tip-shrouded axial fan or compressor rotor used in an aircraft turbine engine. Filament-wound tip-shrouded rotors have been constructed and tested previously. However, they have never been incorporated in a production engine due to poor performance because of excessive leakage of flowpath air from back to front across the rotor tip resulting from inadequate sealing. Seals previously used to inhibit this leakage were of the labyrinth or knife-edge type. Those seals require a finite running clearance, which permits limited leakage, for three reasons. First, all rotors increase in diameter as a function of rotational speed due to stress as they rise in speed from zero to their design speed range, while the surrounding casing diameter changes less, and primarily only as a function of casing temperature. Second, rotor vibrations caused by imbalance and radial excursions resulting from aircraft maneuvers both cause transient local changes in clearance during operation. Third, any physical rotor contact with the casing causes wear, increasing clearance further, and a severe rub can lead to catastrophic failure of the rotor and/or casing leading to engine failure.

Brush seals have been used for several years to prevent leakage of fluids past rotating shafts. They offer an effectively zero-clearance option by incorporating a ring of tightly-packed flexible bristles that maintain light contact with the adjacent rotating surface. The flexibility of the bristle fibers allows for rotor growth, rotor vibration, and random radial perturbations while maintaining nearly zero clearance. Only recently have brush seals been developed to a degree that allows rubbing speeds as high as 500 meters/second (1640 feet/second). This is now adequate to provide an improved sealing solution for filament-wound, shrouded compressor or fan rotors as described in this invention.

A secondary issue with this type of design has been control of the temperature to which the filament winding is exposed. The carbon filaments typically used for such a filament winding can withstand relatively high temperatures. However, these filaments must be embedded in a polymer matrix to bind them together. The highest allowable continuous operating temperature for any currently available polymer matrix is about 400 degrees Celsius. A narrow radial gap exists between a rotor tip shroud and the surrounding compressor or fan casing. Air trapped in this gap between the stationary casing and the rotating tip shroud is subjected to viscous forces. The rotor, because of viscous forces, accelerates air in contact with the rotating tip shroud. Hence it adds energy to that fluid in the form of momentum. The angular momentum of that fluid causes it to be centrifuged outward where it contacts the stationary outer casing where viscous forces reduce that momentum and convert it into an increase in static temperature. This is variously known as aerodynamic heating or windage heating. In theory, because of recirculation, and neglecting heat transfer through the structure and leakage to the surroundings, the temperature of the small amount of air trapped in the cavity between the rotor shroud and the adjacent casing could quickly reach infinity. Although as a practical matter this temperature could not go that high, it could easily exceed 400 degrees Celsius in some engines at some operating points unless a small and continuous supply of fresh air is allowed to purge the gap. Use of a better quality seal such as a brush seal would aggravate this problem. Air leaking across the rotor tip and re-entering the inlet limits the temperature increase in the tip cavity but has been shown to greatly detract from compressor or fan performance. The solution offered by this invention is to incorporate small bleed passages in the outer casing just downstream of the brush seal to allow a small amount of air that has passed through the compressor or fan rotor to flow back through the cavity, exit the outer casing and rejoin the free stream external to the engine.

BRIEF SUMMARY OF THE INVENTION

This invention is aimed at solving two problems experienced by filament-wound tip-shrouded axial compressor or fan rotors. The first goal is to reduce the amount of air that has passed through the rotor that recirculates back into the rotor inlet. The invention accomplishes this by using a brush seal placed near the leading edge of the rotor between the rotor and the outer casing. The second goal is to limit the temperature rise caused by aerodynamic heating of the air in the cavity between the rotor shroud and the outer casing. The invention accomplishes this by incorporating a ring of bleed orifices in the outer casing close to, but downstream of, the brush seal that permit a controlled amount of air that has passed through the rotor to flow through the cavity and exit the engine flowpath.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross-section of a filament-wound tip-shrouded axial compressor or fan rotor that shows the relationship of the various parts important to this invention.

FIG. 2 is an enlarged partial view of FIG. 1 that shows the flowpath of the air used to limit the temperature of the filament winding.

FIG. 3 is an isometric drawing providing an external view of the front portion of the outer casing adjacent to the rotor and shows the approximately uniform circumferential distribution of the bleed orifices also shown in FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made to FIG. 1 of the drawing showing all elements of this invention. Air, flowing from left to right, enters the axial compressor or fan rotor consisting of a multiplicity of airfoils 10 distributed around a rotating hub 12. All airfoils are attached to a circumferential shroud 14 at their outer extremity. Shroud 14 is wound with high-strength fibers 16, usually composed of carbon or graphite, which are held together and cemented to shroud 14 with a polymer matrix.

The pressure developed by the compressor or fan is contained at the outer diameter by casing 18 that may consist of multiple parts. Since the purpose of a fan or compressor rotor is to increase the pressure of the gas flowing through it, in this case air, it is important to minimize the amount of air that has passed through the airfoils 10 from recirculating back into the inlet. Brush seal 20 serves this purpose, being clamped in casing 18 and with its brush fibers lightly contacting the outer surface of circumferential shroud projection 22.

Filament winding 16 and casing 18 are separated by a small and necessary gap. A circumferentially-distributed array of orifices 24 is provided in casing 18 to allow a small portion of the air that has passed through the rotor blades to ventilate the gap and exit the casing in order to limit the temperature that the air in the gap reaches as a result of aerodynamic heating. FIG. 2 presents an enlarged view of that region. Arrows 26 in this figure illustrate the flow path of air through orifices 24. FIG. 3 presents an isometric external view of casing 18 and illustrates the circumferential distribution of orifices 24. Although these orifices are depicted as drilled holes, their optimum shape and number will vary with the size and configuration of the engine to which this invention is applied.

Claims

1. A turbomachine for compressing air comprising:

a stationary casing disposed about a longitudinal axis,

a rotor assembly mounted for rotation about the longitudinal axis, the rotor assembly having a plurality of blades, tips of the blades being coupled to an annular shroud, the annular shroud containing an axially-narrow external cylindrical metallic surface at its leading edge, the remainder of the shroud being wound with carbon filaments embedded in a polymer matrix and disposed with respect to the stationary casing so as to define a gap extending continuously between an outer surface of the annular shroud and an inner surface of the stationary casing,

and a seal structure attached to the stationary casing, comprised of a plurality of metallic fibers or brushes in light contact with said axially-narrow external cylindrical metallic surface at the leading edge of the rotor shroud, said fibers inclined at an approximately 45 degree angle leaning in the direction of rotor rotation

2. The turbomachine of claim 1 wherein a circumferentially-distributed array of orifices passing through the stationary casing is located aft of but close to said seal structure.

3. The turbomachine of claims 1 and 2 wherein a manifold is provided, collecting air emerging from said circumferentially-distributed array of orifices, said manifold ducting said air to any other location within the engine.

4. The turbomachine of claims 1, 2 and 3 wherein the manifold of claim 3 ducts said air to a control valve modulating the quantity of flow of said air as a function of engine operating point.