Outer air seal

Abstract

An outer air seal circumscribing the tips of the rotor blades of a rotary machine is disclosed. Techniques for limiting the radial clearance between the tips of the blades and the outer air seal at steady state conditions are developed. Structure independent of the machine case for isolating the thermal response of the outer air seal from the thermal response of the machine case is discussed and illustrated.

Claims

Having thus described typical embodiments of my invention, that which I claim as new and desire to secure by Letters Patent of the United States is: 1. For a gas turbine engine having an engine case, a segmented, outer air seal of the type adapted to circumscribe the tips of the rotor blades, wherein the improvement comprises: a continuous ring positioned externally of the engine case for radially positioning the outer air seal, wherein said radial position is determined by a substantially fixed position of the continuous ring. 2. An outer air seal assembly of the type utilized to circumscribe the tips of the rotor blades of an axial flow rotary machine, wherein said seal assembly comprises: a machine case having a plurality of circumferentially spaced bushings disposed therein; a continuous ring positioned externally of the machine case and spaced radially apart therefrom; a plurality of arcuate segments which circumscribe the tips of the rotor blades, and wherein each segment has a circumferential extending track; two or more retention blocks slideably disposed within each circumferential track; and a plurality of pins extending radially inward from the continuous ring through said bushings to engage a corresponding block within the outer air seal. 3. The invention according to claim 2 wherein a sleeve disposed in the machine case is adapted to guide each of the radially extending pins. 4. The invention according to claim 3 wherein each sleeve further includes sealing means adapted to prevent the escape of working medium gases from the interior of the machine. 5. Means for supporting a segmented outer air seal of the type circumscribing the tips of rotor blades of a rotary machine, wherein said support means comprises; an essentially cylindrical case enclosing the machine rotor and the outer air seal; a continuous ring positioned externally of the engine case; and a plurality of rods attached to said ring and extending radially inward from said ring to engage the outer air seal. 6. The invention according to claim 5 wherein each of said seal segments is supported by at least two of said radially extending rods. 7. The invention according to claim 6 which further includes a cylindrical bushing supporting each rod at the engine case. 8. The invention according to claim 7 wherein said bushing has a cylindrical sealing surface and wherein each of said rods has a seal associated therewith to seal against the sealing surface of the corresponding bushing. 9. The invention according to claim 7 wherein each of said sealing segments has an arcuate track and wherein a slideable block is disposed in said track, each slideable block being engaged by one of said radially extending rods.
BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to axial flow rotary machines, and more particularly to outer air seals circumscribing the blade tips of a rotor stage. 2. Description of the Prior Art Gas turbine engines are illustrative of rotary machines in which the inventive concepts may be employed. In a gas turbine engine working medium gases are compressed by a first series of rotor mounted blades in the compression section and are flowed axially downstream to a combustion section. Fuel is combined with the compressed gases and burned in the combustion section to add thermal energy to the flowing medium. Downstream of the combustion section the gases are flowed across a second series of rotor mounted blades in the turbine section. Energy extracted from the medium by the second series of rotor blades is utilized to drive the blades of the compression section. The blades of both the compression and turbine sections are arranged in stages, or rows. The tips of the blades in each row are circumscribed by an outer air seal. Aerodynamic performance within the engine is largely dependent upon the radial clearance between the outer air seal and the circumscribed blade tips. Even the slightest clearance degrades performance as the working medium gases escape over the tips of the blades. The problem is particularly severe in high temperature machines having radical thermal fluctuations over the operating range of the machine. The blades of the rotor respond immediately to changes in the temperature of the working medium gases. The conventional outer air seal responds much more slowly to these changes. Substantial initial clearances are provided between the tips of the rotor blades and the outer air seals to protect the respective structures from destructive interference during transient thermal conditions. As the machine reaches thermal equilibrium the outer air seals tend to grow away from the rotor blade tips to a clearance which is on the same order of magnitude as the initial clearance discussed above. Past efforts for reducing clearances have primarily included elaborate case cooling systems for reducing the steady state temperature of the structure supporting the outer air seal. Reducing the temperature of the supporting structure limits the outward radial growth of the seal beyond the diameter required for transient clearance. An effective reduction in tip clearance results, although not without a substantial diversion of the working medium gases for case cooling. U.S. Pat. No. 3,583,824 to Smuland et al entitled "Temperature Controlled Shroud and Shroud Support" is representative of outer air seal structures employing cooling techniques. Other approaches to reduced seal clearance include that shown in U.S. Pat. No. 2,488,875 to Morley entitled "Gas Turbine Engine". In Morley the vanes of the stator stage engage an inner case structure at a spigot and socket arrangement. The outer air seals extend axially from the stator vanes to circumscribe the tips of the rotor blades. Although solutions proposed in the past for reducing blade tip clearance have been partially successful, scientists and engineers in the gas turbine art are continuing to search for structures offering improved engine performance. SUMMARY OF THE INVENTION A primary object of the present invention is to improve the aerodynamic efficiency of an axial flow rotary machine, such as a gas turbine engine. A reduction in the leakage of working medium gases over the tips of the rotor blades is sought, and in one particular embodiment a goal is to limit the radial outward growth of an outer air seal circumscribing the tips of the rotor blades. According to the present invention an outer air seal circumscribing the tips of the rotor blades of a rotary machine is supported radially by a continuous ring positioned externally of the machine case. A primary feature of the present invention is the ring disposed externally of the machine case. Other features include the rods which extend radially inward from the ring and the outer air seal segments which are engaged by the rods. In one embodiment the rods engage corresponding blocks which are slideable within the seal segments. The present invention has particular utility in stator constructions where isolation of the outer air seals from the machine case is desired. The diameter of the continuous ring establishes the radial position of the seal segments. The machine case expands and contracts without altering the radial position of the seal segments. The tendency of the machine case to pull the outer seal segments away from the circumscribed blade tips is avoided. Limiting the blade tip clearance at steady state conditions improves the aerodynamic efficiency of the machine as full interaction between the medium gases and the airfoil surfaces of the blades is achieved. The foregoing, and other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of the preferred embodiment thereof as shown in the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross section view taken through a portion of a gas turbine engine; and FIG. 2 is a sectional view taken along the line 2-2 as shown in FIG. 1. DESCRIPTION OF THE PREFERRED EMBODIMENT A gas turbine engine embodiment of the invention is shown in the drawing. The FIG. 1 simplified cross section view reveals a portion of the rotor assembly 10 including a plurality of rotor blades as represented by the single blade 12. Each blade 12 has a tip 14 at its radially outward extremity. A stator assembly 16, which principally comprising an engine case 18 and an outer air seal 20, houses the rotor assembly. The outer air seal has a cylindrical surface 22 which opposes the tips 14 of the rotor blades to prevent the leakage of working medium gases over the blade tips. The radial clearance between the tips of the rotor blades and the opposing cylindrical surface of the outer air seal is A. As is illustrated in FIG. 2, the outer air seal is comprised of a plurality of arcuate segments 24 disposed in end to end relationship to circumscribe the tips 14 of the rotor blades. Each segment 24 has an outwardly opening circumferential track 26. The segments of the outer air seal are supported by radially extending rods or pins 28 which extend inwardly from an outer continuous ring 30. The pins penetrate the case 18 to engage a slideable block 32 disposed within a circumferentially extending track 26 of each arcuate segment. Two rods or pins 28 supporting each segment 24 are shown. More than two of such rods or pins may be utilized. A cylindrical bushing 34 at each case penetration supports the corresponding rod. A seal element, such as the piston ring seal 36, prevents the leakage of working medium gases from the interior of the engine case. The radial position of the seal segments 24 is dependent upon the diameter of the continuous ring 30, and is largely independent of the working medium temperature. Consequently, as the tips of the blades grow radially outward toward the seal, the seal diameter remains fixed. As thermal equilibrium is reached the seal diameter again remains fixed. The initial clearance at A need only be so large as will enable operation through transient conditions without destructive interference. Seal clearance at equilibrium conditions, such as cruise, is not excessive as the seal is prevented from growing away from the blade tips by the ring 30. The case 18 moves freely between the seal 20 and the ring 30 in response to changes in the temperature of the medium flow path. The seal 36 prevents the leakage of working medium gases between each rod 28 and the corresponding bushing. Apparatus constructed in accordance with the present invention enables the attainment of relatively small clearances at the cruise condition without the need for case cooling. Although the invention has been shown and described with respect to preferred embodiments thereof it should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Description

Topics

Download Full PDF Version (Non-Commercial Use)

Patent Citations (5)

    Publication numberPublication dateAssigneeTitle
    DE-954835-CDecember 20, 1956Canadian Patents DevGehaeusering fuer Kreiselradmaschinen, insbesondere fuer Gasturbinen
    GB-689270-AMarch 25, 1953Rateau Soc, Anxionnaz ReneImprovements in axial flow turbines or compressors
    US-2494178-AJanuary 10, 1950Rateau SocGas turbine
    US-2638743-AMay 19, 1953Ruston & Hornsby LtdConstruction of turbine-inlet and stator elements of gas turbines
    US-3425665-AFebruary 04, 1969Curtiss Wright CorpGas turbine rotor blade shroud

NO-Patent Citations (0)

    Title

Cited By (31)

    Publication numberPublication dateAssigneeTitle
    EP-0192556-A1August 27, 1986Societe Nationale D'etude Et De Construction De Moteurs D'aviation, "S.N.E.C.M.A."Carter de turbomachine associé à un dispositif pour ajuster le jeu entre aubes mobiles et carter
    EP-0192557-A1August 27, 1986Societe Nationale D'etude Et De Construction De Moteurs D'aviation, "S.N.E.C.M.A."Turbine cylinder with a device to adjust the tip clearance between the turbine blades and the cylinder
    EP-0273790-A1July 06, 1988Societe Nationale D'etude Et De Construction De Moteurs D'aviation, "S.N.E.C.M.A."Compressor housing adapted for active regulated thermal dilatation, and method for its production
    EP-0651139-A1May 03, 1995Societe Nationale D'etude Et De Construction De Moteurs D'aviation, "S.N.E.C.M.A."Turbomaschine mit Einrichtungnen um das Schaufelspitzenspiel zwischen Rotor und Stator zu Regeln
    EP-1288444-A1March 05, 2003Snecma MoteursBefestigung von Statorelementen in einem Turbomaschinengehäuse
    FR-2467292-A1April 17, 1981SnecmaDispositif de reglage du jeu entre les aubes mobiles et l'anneau de turbine
    FR-2482662-A1November 20, 1981Mtu Muenchen GmbhCarter exterieur pour compresseur axial ou turbine axiale d'une machine fluidique
    FR-2522067-A1August 26, 1983Gen ElectricCarter de compresseur
    FR-2577281-A1August 14, 1986SnecmaCarter de turbomachine associe a un dispositif pour ajuster le jeu entre aubes mobiles et carter
    FR-2577282-A1August 14, 1986SnecmaCarter de turbomachine associe a un dispositif pour ajuster le jeu entre rotor et stator
    FR-2607198-A1May 27, 1988SnecmaCarter de compresseur adapte pour le pilotage actif de ses dilatations et son procede de fabrication
    FR-2711730-A1May 05, 1995SnecmaTurbomachine équipée de moyens de pilotage des jeux entre rotor et stator.
    FR-2829176-A1March 07, 2003Snecma MoteursCarter de stator de turbomachine
    GB-2238354-AMay 29, 1991Gen ElectricBlade tip clearance control apparatus
    JP-H03168303-AJuly 22, 1991General Electric Co Blade tip clearance control apparatus
    US-2004184912-A1September 23, 2004Francois Crozet, Daniel Didier, Christian Ducrocq, Sebastien Imbourg, Laurent Palmisano, Jean-Marc Rongvaux, Andre VerbruggeGas turbine stator housing
    US-2005067788-A1March 31, 2005Siemens Westinghouse Power CorporationOuter air seal assembly
    US-2006292001-A1December 28, 2006Siemens Westinghouse Power CorporationRing seal attachment system
    US-2014186152-A1July 03, 2014United Technologies CorporationBlade outer air seal system for controlled tip clearance
    US-4330234-AMay 18, 1982Rolls-Royce LimitedRotor tip clearance control apparatus for a gas turbine engine
    US-4343592-AAugust 10, 1982Rolls-Royce LimitedStatic shroud for a rotor
    US-4696619-ASeptember 29, 1987Societe Nationale D'etude Et De Construction De Moteurs D'aviation "S.N.E.C.M.A."Housing for a turbojet engine compressor
    US-4762462-AAugust 09, 1988Societe Nationale D'etude Et De Construction De Moteurs D'aviation (Snecma)Housing for an axial compressor
    US-4787817-ANovember 29, 1988Societe Nationale D'etude Et De Construction De Moteurs D-Aviation (Snecma)Device for monitoring clearance between rotor blades and a housing
    US-5616003-AApril 01, 1997Societe Nationale D'etude Et De Construction De Moteurs D'aviation "Snecma"Turbine engine equipped with means for controlling the play between the rotor and stator
    US-6382905-B1May 07, 2002General Electric CompanyFan casing liner support
    US-7070387-B2July 04, 2006Snecma MoteursGas turbine stator housing
    US-7494317-B2February 24, 2009Siemens Energy, Inc.Ring seal attachment system
    US-9447696-B2September 20, 2016United Technologies CorporationBlade outer air seal system for controlled tip clearance
    WO-03018962-A1March 06, 2003Snecma MoteursCarter de stator de turbomachine
    WO-2014200575-A3February 26, 2015United Technologies CorporationSystème de commande de dégagement à réaction rapide de moteur à turbine à gaz avec interface de segment de joint d'étanchéité à l'air