Turbine blade shroud assembly

Abstract: A turbine blade shroud assembly (36) for a gas turbine engine includes a metal substrate ring (38) on the engine, a continuous ceramic barrier ring (70) inside the substrate ring (38) and exposed to hot gas in a hot gas flow path (26) of the engine, and a wire-mesh compliant ring (58) between the barrier and substrate rings (70,38). The temperature of the barrier ring (70) increases faster than the temperature of the substrate ring (38) as the temperature in the hot gas flow path (26) increases. THE COEFFICIENT OF THERMAL EXPANSION OF THE SUBSTRATE RING (38) IS LESS THAN THE COEFFICIENT OF THERMAL EXPANSION OF THE BARRIER RING (70) SO THAT THE BARRIER RING (70) EXPANDS RELATIVE TO THE SUBSTRATE RING (38) WITH INCREASING TEMPERATURE IN THE HOT GAS FLOW PATH (26) AND DEVELOPMENT OF TENSILE HOOP STRESS IN THE CERAMIC BARRIER RING (70) IS MINIMIZED.

Description: This invention relates to turbine blade shroud assemblies in gas turbine engines. In typical gas turbine engines, bypass of hot gas around turbine blades is minimised by blade shroud assemblies having metal substrate rings around the turbine blades and ceramic barrier rings bonded to the substrate rings to shield the latter from the hot gas. To avoid or minimise hoop stress in the ceramic ring due to thermal growth of the substrate ring relative to the barrier ring, segmented ceramic barrier rings are common. To the same end, a blade shroud assembly has been proposed in which a metal substrate ring is shrink-fitted around a continuous ceramic barrier ring. Still to the same end, another blade shroud assembly has been proposed in which a compliant cushioning ring is disposed between a continuous ceramic barrier ring and a metal substrate ring. GB-A-2,168,110 discloses a coolable stator assembly for a rotary machine which includes a plurality of arcuate seal segments mounted on a metal substrate and having a ceramic facing material attached to the metal substrate. US-A-4,273,824 discloses a method of applying a ceramic facing material to an underlying metallic substrate, in which a porous metal pad is placed between, and bonded to, the ceramic facing material and the metallic substrate. The ceramic faced structure may be used in gas turbine applications. According to an aspect of the present invention, there is provided a turbine blade shroud assembly as specified in claim 1. This invention is a new and improved gas turbine engine turbine blade shroud assembly of the type including a metal substrate ring, and a compliant ring between the substrate and barrier rings. In the blade shroud assembly according to this invention, the material of the substrate ring is selected to exhibit a coefficient of thermal expansion lower than that of the ceramic barrier ring throughout the operating temperature range of the engine so that the ceramic barrier ring expands relative to the substrate ring with increasing temperature. The invention and how it may be performed are hereinafter particular described with reference to the accompanying drawings, in which: FIGURE 1 is a partially broken-away side view of a gas turbine engine having a turbine blade shroud assembly according to this invention; FIGURE 2 is an enlarged view of a portion of Figure 1 showing the turbine blade shroud assembly according to this invention; FIGURE 3 is a fragmentary, broken-away perspective view of the turbine blade shroud assembly according to this invention; and FIGURE 4 is a graph depicting a gas turbine engine operating cycle during which the blade shroud assembly according to this invention may experience substantially maximum thermal growth excursions. Referring to Figures 1-3, a gas turbine engine 10 includes a case 12 having an inlet end 14, an exhaust end 16, and a longitudinal centreline 18. The case 12 has a compressor section 20, a combustor section 22, and a turbine section 24. Hot gas motive fluid generated in a combustor, not shown, in the combustor section 22 flows aft in an annular hot gas flow path 26 of the engine and expands through one or more stages of turbine blades on one or more turbine wheels supported on the case 12 for rotation about the centreline 18, only a representative stage 28 having a plurality of turbine blades 30 being shown in Figures 1-3. Each blade 30 is airfoil-shaped and has a flat tip 32 at the radially-outermost extremity of the blade. An annular stator assembly 34 is rigidly connected to the turbine section 24 of the engine case upstream of the turbine blades 30. In the plane of the turbine blade stage 28, the turbine blade tips 32 are closely surrounded by a stationary, annular blade shroud assembly 36 according to this invention. The blade shroud assembly 36 includes a metal substrate ring 38 having a cylindrical outer leg 40, a cylindrical inner leg 42, and an integral connecting web 44. An integral radial flange 46 extends out from the outer leg 40 about midway between the ends thereof. The flange 46 is retained in a slot 48 defined between a pair of structural annular flanges 50A, 50B of the engine case whereby the longitudinal position of the blade shroud assembly 36 on the case is established. The flange 46 has radial freedom in the slot 48 so that thermal growth of the substrate ring 38 is not impeded. The blade shroud assembly 36 is supported radially on the engine case through a plurality of conventional cross-keys arrayed around the substrate ring 38 which centre the substrate ring without impeding its thermal growth, only a representative cross-key 52 being illustrated in Figure 1-3. The representative cross-key 52 includes a radial lug 54 projecting inwards from the structural flange 50A of the engine case and a radial socket 56 on the outer leg 40 of the substrate ring 38 which slidably receives the lug 54. The blade shroud assembly 36 further includes a cylindrical, metal-mesh compliant ring 58 inside the substrate ring. The compliant ring 58 has an outside wall 60 brazed to an inside cylindrical wall 62 of the inner leg 42 of the substrate ring 38. An annular lip 64 of the inner leg 42 overlaps the upstream end of the compliant ring 58. The downstream end of the compliant ring 58 is open to the hot gas flow path 26 radially inwards of an annular rear face 66 of the substrate ring 38. A plurality of cooling air holes are formed in the inner leg 42 near the lip 64, only a representative cooling air hole 68 being shown in Figures 2 and 3. Seals, not shown, may be provided between the inner leg 42 of the substrate ring 38 and adjoining structure, such as the vane assembly 34, to minimize escape of hot gas from the flow path 26. A ceramic barrier ring 70 of the blade shroud assembly 36 is disposed inside the compliant ring 58. The barrier ring has a cylindrical full-density layer 72 adjacent the compliant ring 58 and an integral reduced-density layer 74 adjacent the blade tips 32. The barrier ring 70 has an integral lip 76 inside the lip 64 on the substrate ring 38 and covering the inner front edge of the compliant ring 58. The ceramic barrier ring 70 is a continuous, uninterrupted 360 degree ring which may be fabricated by spray application of liquid ceramic material onto an inner wall 78 of the compliant ring 58 to a radial depth of about 1.98 mm (0.078 inches). Migration of the ceramic material into the interstices in the compliant ring 58 mechanically connects the barrier ring 70 to the compliant ring 58. In the plane of the turbine blade stage 28, the reduced density layer 74 of the barrier ring defines the outer boundary of the hot gas flow path 26 and is, therefore, directly exposed to the gas in the flow path 26. The temperature of the gas in the flow path 26 typically varies from ambient temperature at engine start-up, to a maximum greater than 1371°C (2500°F) in a high-performance operating mode of the gas turbine engine 10. Cooling air from the compressor of the engine is ducted at elevated pressure to an annular plenum 80, Figures 1-2, the downstream end of which is closed by the substrate ring 38 of the blade shroud assembly 36. The cooling air circulates over both surfaces of the outer leg 40 and over an outer surface 82 of the inner leg 42. The pressure of the cooling air exceeds the pressure in the hot gas flow path 26 behind and downstream of the turbine blade stage 28 so that a continuous flow of cooling air is induced through the cooling air holes 68 in the inner leg 42, through the interstices of the compliant ring 58, and into the hot gas flow path 26 through the downstream end of the compliant ring 58. The circulation of cooling air maintains the substrate ring 38 at a lower temperature than the compliant ring 58 and the compliant ring 58 at a lower temperature than the barrier ring 70. Selection of the material for the substrate and barrier rings 38, 70 is an important feature of this invention. Specifically, the substrate and barrier ring materials are selected, respectively, to afford optimum structural integrity and thermal shielding and, in addition, to afford a thermal growth relationship characterized by expansion of the barrier ring relative to the substrate ring with increasing temperature in the operating temperature range of the engine. In a preferred embodiment, the required thermal growth relationship is achieved through material selection which yields a substrate ring having a lower coefficient of thermal expansion than the barrier ring. A preferred embodiment of the blade shroud assembly 36 is characterized by the following material selection: (a) the substrate ring 38 is a forging of niobium (also known as columbium) alloy FS 85 available commercially from Teledyne - Wah Chang Albany U.S.A; alloy FS 85 includes about 28% tantalum, 10.5% tungsten, and 0.9% zirconium; (b) the full-density and reduced-density layers 72,74 of the barrier ring 70 are formed from zirconium oxide (ZrO₂) and (c) the compliant ring (58) is a mesh of Hoskins 875 alloy metal wires each having a diameter of about 0.142 mm (0.0056 inches); such a ring is commercially available from Technetics, U.S.A. under the tradename Brunsbond Pad. Hoskins 875 alloy is an iron alloy having the following composition: 22.5% chromium, 5.5% aluminium, 0.5% silicon, 0.1% carbon and 71.4% iron. Figure 4 is a graph (turbine rotor speed vs. time) illustrating an operating cycle of the gas turbine engine 10 during which the blade shroud assembly 36 may experience substantially maximum thermal growth excursions. The operating cycle depicted in Figure 4 includes a normal acceleration from start-up to idle (points a-c) and stabilization at idle (points c-d), a first snap acceleration to and stabilization at super cruise and subsequent snap deceleration to idle (points d-e), and a second snap acceleration to and stabilization at super cruise (points e-g) and subsequent snap deceleration to idle (points g-i). Table I below is a tabulation of data reflecting the thermal growth at the inside diameters of the barrier ring 70 and the substrate ring 38 in a plane 84, see Figure 2, extending perpendicular to the centreline 18 during the engine operating cycle depicted in Figure 4. The data in Table I is for the preferred embodiment wherein the substrate ring 38 and barrier ring 58 are made of the materials described above, the inside diameter of the barrier ring 70 is 537.95 mm (21.179 inches) and the radial thickness of the barrier ring 70 is 1.98 mm (0.078 inches). Referring to Table I, column 1 identifies the point in the operating cycle depicted in Figure 4 for which the line data is applicable. Column 2 identifies the one of the substrate and barrier rings to which the line data pertains. Column 3 identifies the substrate and barrier ring temperatures at the corresponding engine operating points. Column 4 shows the respective coefficients of thermal expansion of the substrate ring 38 and of the barrier ring 70 at the corresponding temperatures. Column 5 shows the calculated thermal growths of the substrate ring 38 and the barrier ring 70 at the corresponding temperatures and coefficients of thermal expansion. Table I demonstrates that the temperature of the substrate ring 38 is always considerably lower than the temperature of the barrier ring 70 except immediately after engine start-up. The data in Table I, columns 4-5, further demonstrates that, throughout the operating cycle depicted in Figure 4, the coefficient of thermal expansion of the substrate ring 38 is always less than the coefficient of thermal expansion of the barrier ring 70 and that the barrier ring 70 expands relative to the substrate ring 30 with increasing temperature in the operating range of the engine. Expansion of the barrier ring 70 relative to the substrate ring 38 with increasing temperature minimizes the likelihood of tensile hoop stresses developing in the barrier ring 70 during thermal excursions of the blade shroud assembly 36.

Claims: A turbine blade shroud assembly (36) in a gas turbine engine (10) having an annular stage (28) of rotatable turbine blades (30) in a hot gas flow path (26) of said engine (10) wherein, during operation of said engine (10), the gas temperature varies in a range from ambient temperature to a maximum temperature in a high-performance operating mode of said engine (10), said turbine blade shroud assembly (36) comprising: a continuous ceramic barrier ring (70) around said turbine blades (30); a continuous metal substrate ring (38) on a case (12) of said gas turbine engine (10) around said barrier ring (70); and a compliant ring (58) between said barrier ring (70) and said substrate ring (38); said continuous ceramic barrier ring (70), during operation of said engine (10), having a plurality of operating temperatures increasing from ambient temperature with increasing temperature in said hot gas flow path temperature range; said continuous metal substrate ring (38) having a plurality of operating temperatures increasing from ambient temperature with increasing temperature in said hot gas flow path temperature range at a rate less than the rate of increase of temperature of said barrier ring for corresponding increases in temperature in said hot gas flow path temperature range; said continuous metal substrate ring (38) having a coefficient of thermal expansion selected with respect to the coefficient of thermal expansion of said barrier ring (70) such that said barrier ring (70) expands relative to said substrate ring (38) with increasing temperature in said hot gas flow path temperature range from ambient temperature to said maximum hot gas temperature; and said compliant ring (58) between said barrier ring (70) and said substrate ring (38) having an inside surface (78) attached to said barrier ring (70) and an outside surface (60) connected to said substrate ring (38) whereby said barrier ring (70) is connected to said substrate ring (38). A turbine blade shroud assembly (36) according to claim 1, in which said compliant ring (58) is a metal wire mesh ring having said outside surface (60) brazed to said metal substrate ring (38) and said inside surface (78) mechanically attached to said barrier ring (70) through migration of said barrier ring ceramic into interstices of said wire-mesh ring (58). A turbine blade shroud assembly (36) according to claim 2, in which cooling means (68,80) are provided for maintaining said operating temperature of said substrate ring (38) below said operating temperature of said barrier ring (70) when the temperature in said hot gas flow path (26) stabilises within said hot gas flow path temperature range. A turbine blade shroud assembly (36) according to claim 3, in which said cooling means includes means on said engine (10) defining a cooling air plenum (80) exposed to said substrate ring (38) and having pressurised cooling air therein, means on said substrate ring (38) defining a plurality of cooling air holes (68) for conducting cooling air from said cooling air plenum (80) to the interstices of said wire-mesh compliant ring (58), and means for conducting cooling air from the interstices of said wire-mesh compliant ring (58) to said hot gas flow path (26).

Inventors: BERGER JEFFREY LYNN, BERGER, JEFFREY LYNN
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1991
Main classification: F01D11
IPC codes: F01D11, F01D25