GB2115487A

GB2115487A - Double wall compressor casing

Info

Publication number: GB2115487A
Application number: GB08300210A
Authority: GB
Inventors: Joseph Clayton Burge; Julius Bathori
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1982-02-19
Filing date: 1983-01-06
Publication date: 1983-09-07
Also published as: DE3305170C2; GB2115487B; DE3305170A1; FR2522067A1; GB8300210D0; FR2522067B1; IT1168258B; IT8319599A0; JPS58152108A; JPH0261603B2; IT8319599A1

Abstract

In a gas turbine engine, the casing 25 is provided with a nonstructural inner wall removably attached to a thin, structural outer casing. The inner wall isolates the thin stator outer casing during transient turbine operations of throttle burst (heavy throttle) and throttle chop (reduced throttle). During throttle burst and chop, the nonstructural inner wall delays rapid heating and cooling of the relatively thin-walled outer casing, and reduces radial clearance d between the stator casing and blade tips due to uneven thermal expansion and contraction. To fine tune the actual clearances d between stator and blade tips and prevent the casing outer wall from overheating, thermal insulation material 27, 29, 31 is used between the nonstructural inner wall and outer casing. <IMAGE>

Description

SPECIFICATION Compressor casing Background of the Invention The present invention relates to a gas turbine engine, and in particular to such an engine having improved compressor performance during periods of transient engine operation.

A current problem existing in turbomachinery, such as, for example, gas turbine compressors, relates to transient thermal response during periods of engine operation known as throttle burst and throttle chop. During these periods of transient engine operation, large radial excursions occur in both stator and rotor components. To prevent interference between the compressor stator and rotor during these transient excursions, clearances are provided between the stator and rotor blades. These clearances in typical compressors are undesirably large during both transient and nontransient operation, thus adversely affecting compressor efficiency and stall margin.More particularly, the outer casing wall of a typical gas turbine compressor stator is relatively thin walled metal, and it responds rapidly to temperature changes during periods of transient engine performance such as throttle burst (advanced or heavy throttle) or throttle chop (reduced throttle).

Accordingly, it is an object of the present invention to improve gas turbine performance by reducing clearance during transient operation.

It is another object of the present invention to improve gas turbine engine performance by isolating casing from excessive heating and cooling effects during transient operation.

It is another object of the present invention to introduce a thermal delay into the outer casing in order to reduce a temperature gradient across its wall.

It is a further object of the invention to optimize clearances between stator casing and rotor to improve engine efficiency and stall margins of the compressor.

It is an additional object of subject invention to delay the thermal response of the outer wall in order to obtain a better stator-rotor match for optimum clearance.

It is another object of this invention to provide a turbomachine casing for surrounding a rotor wherein an inner wall is attached to and thermally insulated from the casing, for tuning the radial clearances between the rotor and the inner wall to provide a predetermined clearance during operation of the turbomachine.

It is another object of this invention to improve gas turbine performance by cutting the load paths of pressure and temperature from the inner wall to the load carrying outer wall.

Summary of the Invention In one form of the invention, a turbomachine casing for surrounding a rotor is provided. The casing includes an outer structural wall. An inner nonstructural wall is attached to and thermally insulated from the outer wall for tuning a radial clearance between the rotor and the inner wall to provide a predetermined clearance during operation of the turbomachine.

Brief Description of the Drawings.

FIGURE 1 is a sectional view of part of a compressor taken in an axial direction and embodying one form of the present invention.

FIGURE 2 is a sectional view of a compressor stage support rail in relationship to the outer casing.

FIGURE 3 is a plan view of a sector support rail.

FIGURE 3A is a sectional view taken on lines 3A-3A.

FIGURE 4 is an isometric view of a sector support rail retainer lug.

FIGURE 5 is a graph comparing transient clearances in a compressor stage with transient clearance achieved in the same stage by one form of the present invention.

FIGURE 6 is another embodiment of the present invention, taken as in FIGURE 1.

Detailed Description of the Invention Referring now to FIGURE 1 , there is shown a portion of a compressor section 10 of a gas turbine engine in sectional view. The compressor 10 is comprised of an axially extending, generally cylindrical rotor spool (not shown) disposed radially inward of and spaced from a relatively thin casing wall 25 to form an annular gas flow passage (not shown). The casing wall 25 comprises an upper and lower half (not shown) which are joined together by means of flanges and bolts (not shown). Depending radially outward from such a rotor spool are a plurality of staged rotor blades 12, 14, 1 6 which extend across the gas flow passage.The spool and the rotor blades 12,14, 1 6 are rotatably driven by drive shaft means (not shown) for the purpose of compressing gas flow within the gas passage.

Located directly opposite respective rotor blades 12, 14, 1 6 are support rail end retaining lugs 24, 26, 28, which are fixedly attached to casing 25 by means of respective threaded bolts 30, 32, 34. Tips of the rotor blades 12, 14, 1 6 are separated from lugs 24, 26, 28 by a distance d.

Spacers 31, 33, 35 are interposed between the casing 25 and the respective retainer lugs 24, 26, 28 in order to maintain a proper spatial relationship between the casing 25 and lugs 24, 26, 28. Retainer lugs 24, 26, 28 are shown in greater detail in the isometric view of FIGURE 4 and clearly show side slots 40, 41 formed respectively between ledges 42, 43 and sloping members 44, 45. A step 87 is provided on lug 24 whose purpose will be discussed in a later paragraph.

Returning to FIGURE 1, stator airfoils or vanes 1 8, 20, 22 include respective mounting tangs 50, 52, 54, 56, 58, 60, 62. Mounting tangs 50, 52, 54, 56, 58, 60, 62 are respectively provided for mating engagement with said slots 40, 41, 47, 49, 51, 53, 55 whereby stator airfoils 18, 20, 22 are attached to the stator casing 25. Immediately above the stator mounting platform or tangs 52, 54, 56, 58, 60, 62 and an inner surface of the casing 25 are respective spaces 64, 66, 68, wherein insulation 27, 29, 31 may be inserted.It is noted that vane 22, which is an outlet guide vane is of larger size than stators 1 8, 20. The outlet guide vane is located in the casing's aft end and is the last vane in the compressor section.

Slot 55 for mating with tang 62 is provided in a ring 95 sandwiched between the casing flange 25a and a frame flange 97. The ring 95 is maintained in place with flange member 25a, 97 by means of bolt and nut combination 98.

The compressor 10 consists of one or more stages wherein each stage is comprised of a rotating multi-bladed rotor and a nonrotating multi-vane stator, and an axial compressor is normally of a multi-stage construction. Within each stage, air flow is accelerated and decelerated with resulting pressure rise. To maintain the axial velocity of the air as pressure increases, the crosssectional flow area is gradually decreased with each compressor stage from the low to high pressure end. The net effect across the compressor is a substantial increase not only in air pressure, but also in temperature.

Referring now to FIGURE 2 which is a radial cross-sectional view of an exemplary support rail as utilized in this invention, support ring or rail 70 (see Figs. 3, 3A) is shown attached to casing 25 via a tapped-through hole for retention bolt 74 in retainer lug 73. Support rail 70, which is made of Inconel 718 (RTM), a well known nickel based alloy, has a high tolerance to heat and also a high coefficient of thermal expansion. Additional retention lugs 72, 76 are provided along rail 70 so that they interface with an inner radial surface 80 of the casing 25.Ends 82, 84 of the sectored rail 70 are fabricated with a respective step 83, 85 which is adapted to mate with the respective steps 87, 89 formed on the support rail end retainer lugs 24, 24a. It should be noted that circumference clearances 92, 94 are provided for ends 82, 84 with respect to support rail lugs 24, 24a to allow for circumferential expansion of the rail sector 70. In other words, during throttle burst when engine temperature increases the sectored Inconel (RTM) rail 70 will move circumferentially by increasing its length which will be absorbed into clearances 92. 94. Furthermore, the Inconel (RTM) rail will be restrained radially in view of the positioning of retention lugs 72, 76 against the casing wall 25.In effect, the thermal time constant of the casing 25 has been delayed after the application of heat in view of the delaying functions provided by the inner sectored rail 70.

Eleven lightening pockets 71 are provided along the length of the rail sector 70 in order to reduce its weight to a minimum. Additional space 91 is provided above the lightening pockets 71 to allow for insulation e.g., blanket type, to be placed between the outer casing wall 25 and the rail sector 70. This insulation is used to thermally protect the outer casing walls as well as to thermally insulate the support rails from the outer casing walls. It should be appreciated that only one rail sector 70 has been discussed, whereas in actual practice sufficient rails will be utilized to cover two sections covering 1 80 degrees of circumference each.

Preferably, the insulation 91 comprises a glasswool type insulator enclosed in a stainless steel sheet holder for handling and installation. For example, a glass-wool type insulator commercially available under the designation KAO--WOOL (RTM) from Babcock 8 Wilcox, Co. can be utilized.

If desired, the insulator material may be in powder form such as the one commercially avilable as MlN-K from Johns-Manville Company. Also, in place of the shown blanket type insulation, a flame sprayed thermal barrier coating such as nickel, chromium, aluminum/bentonite (NiCrAL Bentonite) from METCO, Inc., can be used. A ceramic such as Yttria-Zirconia may also be used to thermally insulate the outer casing wall.

In accordance with one form of the present invention, the steel outer casing wall 25 as shown in FIGURE 2 is a structural wall i.e., hoop load carrying, whereas the Inconel inner casing wall 70, which is attached to the outer casing wall, is a nonstructural wall. In view of the relative thinness of the outer steel casing 25, use of single wall casings have responded rapidly to changes in air temperature especially during periods of engine transience, for example, application of throttle burst or throttle chop. During throttle burst, the casing 25 thermally responds to an increase in air temperature by radial expansion faster than does the thermal response of the rotor. Consequently, the radial clearance "d" between the stator casing and the rotor blade tips increases substantially whereby the turbine engine becomes inefficient.

This phenomenon can be seen by referring to a dotted curve in FIGURE 5, which is a graph of a typical compressor stage and indicates average transient clearance between a rotor tip blade and the stator casing over a period of engine performance. A hump in the dotted curve illustrates increased rotor clearances as a result of throttle burst. A dip in the dotted curve just prior to the hump formation is due to growth of the rotor dimensions with respect to the stator casing because of stress, which is related to an elasticity characteristic of the metal.

During throttle chop, the casing wall 25 will conventionally try to thermally shrink faster than that of the rotor. Also, there is an initial rapid decrease of the rotor dimensions at this time because of the elasticity factor. These considerations will cause the clearance to increase after a steady state take-off condition has been reached, and will cause a fip in the dotted curve around a point where chop is initiated.

In can be appreciated from the dotted line (prior art) curve of FIGURE 5 that there is great clearance variation with respect to steady-state ground idle in the compressor during engine operation, which is not conducive to optimum engine performance. The solid curve represents compressor clearance variations in accordance with one form of the invention discussed herein. It can be readily appreciated that extreme clearance variations during transient operation have been substantially eliminated resulting in improved engine operation. In addition, the presence of the insulation material desirably reduces clearances during steady state operation, e.g., cruise and ground idle.

Referring now to FIGURE 6, another embodiment of the present invention is shown wherein a different arrangement is provided near the aft end of the compressor in the vicinity of stator vane 101, and rotor blades 102, 103. The variation near the aft end of the compressor comprises using an integral liner 11 3 having two rub liners 100, 104, as well as two support rails 105, 106. Located within support rails 105, 106 are two oppositely positioned slots 114, 11 5 which are adapted to mate with respective tangs 107, 108 for holding the stator blade 101 in position. The integral liner 11 3 incorporates two pockets 109, 110 for locating insulation 111, 112 therein.In the manner previously described, the integral liner 113 is a nonstructural member which is attached to an outer structural casing wall 25, i.e., hoop load carrying The integral liner 113 in conjunction with the insulation 111, 112 is designed to thermally insulate the outer casing wall 25 during transient operation to thereby minimize radial misalignment between the outer casing and the rotor.

The nonstructured inner wall arrangement of this invention increases the thermal time constant of the outer steel casing 25 thereby minimizing radial misalignment. The thermal time constant is that time that it takes the casing 25 to reach 66% of applied heat temperature after application thereof. In the prior art use of thin wall casings, the time constant was small, that is, the casing would heat up to 66% of the applied heat quite rapidly. This rapid heating would cause concomitant radial aberrations such as racial misalignment due to the above discussed thermal expansion or shrinkage of the casing.

In the present invention, during throttle bursts and chops, the circumferential end gaps in the sectored casing inner wall close and open freely.

This cuts the load paths of both pressure and temperature from the inner wall to the casing outer wall. Cutting these load paths improves the stress and deflection characteristics of the casing outer wall while allowing the tuning of the radial clearances between the rotor blade tips and the casing inner wall.

Although the present invention has been described in connection with a compressor, it is applicable to other forms of turbomachinery, such as, for example, high and low pressure turbines.

Also, it is to be appreciated that various forms of insulation may be employed to provide the desired engine operating characteristics. For example, thermal barrier coatings and other types of insulation may be employed.

It will be understood that the foregoing suggested apparatus as exemplified by the Figures, is intended to be illustrative of a preferred embodiment of the subject invention and that many options will readily occur to those skilled in the art without departure from the spirit or the scope of the principles of the subject invention.

Claims

1. A turbomachine casing for surrounding a rotor, comprising: (a) an outer structural wall; and (b) an inner nonstructural wall attached to and thermally insulated from said outer wall for tuning a radial clearance between said rotor and said inner wall to provide a predetermined clearance during operation of said turbomachine.

2. A turbomachine casing in accordance with Claim 1 wherein said inner wall comprises at least two vane platform support rails removably attached to said outer wall, and having at least one vane platform supported therebetween in spaced relation to said outer wall.

3. A turbomachine casing in accordance with Claim 1 wherein said inner wall comprises at least one vane platform support rail removably attached to said outer wall, and having at least one vane platform supported thereon in spaced relation to said outer wall.

4. A turbomachine casing in accordance with Claim 2 wherein said space between said outer wall and said vane platform contains a thermal insulating material having a predetermined thermal resistance value.

5. A turbomachine casing in accordance with Claim 4 wherein said thermal insulating material comprises blanket-type insulations.

6. A turbomachine casing in accordance with Claim 5 wherein said thermal blanket-type insulating material comprises glass wool.

7. A turbomachine casing in accordance with Claim 5 wherein said thermal blanket-type insulating material comprises a powder.

8. A turbommachine casing in accordance with Claim 4 wherein said thermal insulating material comprises an integral type of coating of NiCrAL Bentonite.

9. A turbomachine casing in accordance with Claim 4 wherein said thermal insulating material comprises Yttria-Zirconia ceramic.

10. A turbomachine casing for surrounding a rotor comprising: (a) an outer structural wall, said outer wall having a relatively low tolerance to high temperature, and a relatively low coefficient of thermal expansion; (b) an inner nonstructural wall attached to said outer wall, said inner wall having a relatively high coefficient of thermal expansion, and a relatively high tolerance to high temperature: and (c) said inner wall being thermally insulated from said outer wall for tuning a radial clearance between said rotor and said inner wall during operation of said turbomachine.

11. A turbomachine casing in accordance with Claim 10 wherein said inner wall comprises a plurality of sectors with adjacent ones of said sectors being separated by circumferential end gaps.

12. In a turbomachine casing for surrounding a rotor comprising: (a) means for radially tuning clearances between a stator casing and rotor blade tips of said turbomachine in order to maintain uniform clearances during transient operation thereof; (b) said means comprising a sectored, nonstructural inner wall connected to and insulated from said casing; (c) such that during transient operation of said turbomachine expansion of said inner wall initially occurs in a circumferential direction, after which said casing and inner wall expand radially in a uniform manner, and wherein said rotor in said turbomachine radially expands in concert with said double casing wall.

13. A method of tuning radial clearances between a substantially cylindrical stator casing and a rotor of a turbomachine, said method comprising the steps of: (a) providing a nonstructural inner wall supported by and radially spaced from said stator casing; (b) reducing the magnitude of the thermal response of said casing resulting from a change in temperature of said inner wall, (c) thereby delaying, for a predetermined period of time, said thermal response of said casing.

14. A method of tuning radial clearances in accordance with Claim 13 wherein step (a) comprises removably attaching two vane platform support rails to said stator casing, said platform support rails having at least one vane platform support therebetween in spaced relation to said stator casing.

1 5. A method of tuning radial clearances in accordance with Claim 13 wherein step (b) comprises the steps of providing a thermal insulating material having a predetermined thermal resistance value in said space intermediate said stator casing and said vane platform.

1 6. A method of tuning radial clearances in accordance with Claim 1 5 wherein step (b) comprises providing blanket-type insulations in said space intermediate said stator casing and said vane platform.

1 7. A method of tuning radial clearances in accordance with Claim 1 5 wherein step (b) comprises depositing a flame sprayed thermal barrier in said space intermediate said stator casing and said vane platform.

18. A turbomachine casing substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 5 or 6 of the drawings.

1 9. A method of tuning radial clearances as claimed in claim 13 and substantially as hereinbefore described.