Apparatus and method for cooling a gas turbine vane

An apparatus and method are provided for preventing the plugging of cooling air distribution holes in a hollow gas turbine vane by particles entrained in the cooling air. The portion of the cooling air is bled from the vane and discharged into the hot gas downstream of the vane, the shunted bleed air carrying the entrained particles.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to gas turbines. More specifically, the 
present invention relates to an apparatus and method for cooling a gas 
turbine vane which prevents the plugging, by airborne particles, of 
cooling air passages in the vane. 
2. Description of the Prior Art 
A gas turbine is comprised of a compressor section for compressing air, a 
combustion section for heating the compressed air by burning fuel therein, 
and a turbine section for expanding the heated and compressed gas 
discharged from the combustion section. 
The hot gas flow path of the turbine section of a gas turbine is comprised 
of an annular chamber contained within a cylinder and surrounding a 
centrally disposed rotating shaft. Inside of the annular chamber are 
alternating rows of stationary vanes and rotating blades arrayed 
circumferentially around the annular chamber. Hot gas discharged from the 
combustion section of the gas turbine flows over these vanes and blades. 
Since, to achieve maximum power output, it is desirable to operate the gas 
turbine so that this gas temperature is as high as feasible, the vanes and 
blades must be cooled. Cooling is obtained by causing relatively cool air 
to flow within and over the vanes and blades. To facilitate such cooling 
of the vanes, a hollow cavity is provided inside of each vane. The cavity 
is enclosed by the walls which form the airfoil portion of the vane. 
Cooling air enters the hollow cavity from an opening on the outboard end 
of the vane. The cooling air flows through the hollow cavity and then 
leaves the vane by flowing through holes in the walls of the vane 
enclosing the cavity. After discharging from these holes, the cooling air 
enters and mixes with the hot gas flowing over the vanes. 
To adequately cool the vane it is necessary to guide the cooling air 
flowing through the cavity to ensure that it is properly distributed over 
the entire surface of the walls forming the cavity. This distribution is 
accomplished by installing a thin-walled vessel, referred to as an insert, 
into the cavity. After entering the vane, the cooling air flows into the 
insert and is distributed around the cavity by a plurality of small 
distribution holes dispersed throughout the insert. 
Since to be effective the cooling air must be pressurized, it is bled from 
the compressed air discharged from the compressor. If the gas turbine is 
operating in a dirty or dusty environment, small particles entrained in 
the compressed air become deposited and accumulate in the small 
distribution holes in the insert, thereby plugging the holes. As a result, 
the ability of the insert to properly distribute the cooling air is 
impaired. 
It is therefore desirable to provide an apparatus which will prevent 
plugging of the cooling air distribution holes in the vane insert. 
SUMMARY OF THE INVENTION 
Accordingly, it is the general object of the present invention to provide a 
method and apparatus for cooling a gas turbine vane. 
More specifically, it is an object of the present invention to ensure 
proper distribution of cooling air within a gas turbine vane by preventing 
the plugging of holes in an insert used to distribute cooling air 
throughout the vane. 
Briefly these and other objects of the present invention are accomplished 
in a gas turbine having a plurality of stationary turbine vanes. Each vane 
is cooled by cooling air and has a cavity formed within it to facilitate 
cooling. An insert is disposed in the cavity to distribute the cooling air 
throughout the cavity by causing it to flow through a plurality of small 
holes dispersed throughout the insert. Plugging of these small holes by 
particles entrained in a cooling air is prevented by bleeding a portion of 
the air out of the cavity, the bleed air carrying with it the particles 
which entered the cavity along with the cooling air. Bleeding is 
accomplished through a tube which connects a large hole in the insert to a 
manifold formed on the inner shroud of the vane. From the manifold the 
bleed air is discharged into the hot gas flowing downstream of the vane 
through a hole in the inner shroud.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, wherein like numerals represent like elements, 
there is illustrated in FIG. 5 a schematic representation of a gas 
turbine. The gas turbine is comprised of a compressor section 47, a 
combustion section 48 and a turbine section 49. Atmospheric air 50 enters 
the compressor and exits as compressed air. The majority of the compressed 
air 8 is heated in the combustion section and forms the hot gas 30 which 
enters the turbine. A portion of this compressed air is bled for cooling 
purposes as explained below. There is shown in FIG. 1 a portion of the 
turbine section of a gas turbine in the vicinity of the row 1 stationary 
vanes 7. A plurality of vanes are contained within a turbine cylinder 1 
and are circumferentially arrayed around the turbine in a row. At the 
radially outboard end of each vane is an outer shroud 13, and at the 
radially inboard end an inner shroud 14. The portion of the vane between 
the shrouds comprises an airfoil 2. The inner and outer shrouds of each 
adjacent vane abut one another so that when combined over the entire row, 
the shrouds form a short axial section of the annular chamber through 
which the hot gas 30 flows. 
A shaft 5 forms a portion of the turbine rotor in the vicinity of the first 
row vanes 7 and is encased by a housing 4. Gas 30, which has been 
compressed in a compressor section and heated by burning fuel in a 
combustion section, neither in FIG. 1 of which are shown, is directed to 
the first row vanes by a duct, or transition 3. The first row vanes form 
the inlet to the turbine. 
Immediately downstream of the first row vanes are the first row rotating 
blades 32. The blades are affixed to a disc 6 which also forms a portion 
of the turbine rotor. 
The vanes 7 are cooled by compressed air 8 bled from the compressor 
discharge air through a bleed pipe, not shown. This cooling air 8 
penetrates the turbine cylinder 1 and retainer block attached thereto, 
through a plurality of holes 15, and enters the vanes. The majority 9 of 
the cooling air is discharged through holes in the trailing edges of the 
vanes and mixes with the hot gas downstream of the vanes. However, 
according to the present invention, a portion 10 of the cooling air is 
bled from the vanes and discharged into the hot gas flowing downstream of 
the vanes in the vicinity of the inner shroud. 
Since the static pressure of the hot gas downstream of the vanes is lower 
than that upstream of the vanes, there is a tendency for the hot gas to 
bypass the vanes by flowing along a path inboard of the inner shrouds, 
i.e., by flowing through the gap between the housing 4 and the inner 
shrouds 14. This is prevented by a seal 11 disposed in the housing 4. The 
seal is spring loaded and bears against the downstream portion 26 of the 
inner surface of the inner shroud, thereby blocking the flow of hot gas 
through the gap between the housing and the inner shrouds. 
Referring now to FIG. 2, the internal portion of a vane 7 can be seen. A 
hollow cavity 24 is formed inside of the airfoil portion 2 of the vane. A 
thin-walled vessel 22 referred to as an insert, is disposed within the 
cavity. The outboard end of the insert is affixed to the outer shroud 13 
and the inboard end is: supported by pins 19 which protrude from a closure 
plate 18. The closure plate forms a portion of the inner shroud and seals 
the inboard end of the cavity. A closure cap 16 seals the cavity at the 
outer shroud 13. Cooling air 8 enters the vane through a hole 17 in the 
closure cap 16. Referring also to FIG. 3, a plurality of small 
distribution holes, are dispersed throughout the insert 22 so that the 
majority of the cooling air is distributed into numerous small jets of air 
42 which impinge on the inner surfaces 40 of the walls forming the airfoil 
portion 2 of the vane. The diameter of these small distribution holes is 
typically in the range of 0.030 to 0.040 inch. After flowing over the 
inner surfaces 40 of the walls, this portion 9 of the cooling air exits 
the vane through a plurality of holes 27 in the walls forming the 
downstream edge of the airfoil, thereby cooling the downstream edge. It 
should be noted that since the cooling air is bled from the compressor 
discharge, its static pressure is higher than that of the hot gas flowing 
downstream of the vanes. A portion of the pressure drop between the 
cooling air and the hot gas is consumed in flowing through the small 
distribution holes in the insert and a larger portion is consumed in 
flowing through the holes 27 in the airfoil. 
As previously discussed, if the gas turbine is operating in a dusty or 
dirty environment, particles entrained in the cooling air are sometimes 
deposited in the small distribution holes in the insert 22 and accumulate 
until the holes become plugged. As result of this plugging, the cooling 
air is not properly distributed around the inner surfaces 40 of the 
airfoil walls, causing local overtemperature of the airfoil walls (hot 
spots). These hot spots result in deterioration of the material forming 
the airfoil walls and shorten the useful life of the vane. 
Referring again to FIG. 2, it can be seen that in accordance with the 
present: invention, air 21 is bled from the cavity 24 through a hole 44 at 
the inboard end of the insert 22. The bleed air 21 carries the particles 
entrained in the cooling air out of the cavity, preventing them from 
plugging the distribution holes. A hole 46, radially aligned with hole 44, 
is provided in the closure plate 18. The bleed air is directed through 
hole 46 by a tube 20. One end of the tube is affixed to the insert at hole 
44 and the other end penetrates into hole 46 in the closure plate. After 
passing through the closure plate 18, the bleed air enters a manifold 25 
from which it exits the vane through passageway 23 in the inner shroud. In 
effect, passageway 23 transports the bleed air past the seal 11, shown in 
FIG. 1, so that it discharges into the lower pressure zone downstream of 
the vane where it mixes with the hot gas, as previously explained. 
FIGS. 2 and 4 show a containment cover 12 which forms the manifold 25 and 
encloses a portion of the inner surface of the inner shroud 14 upstream of 
the portion 26 of the inner shroud upon which the seal 11 bears. 
In accordance with the invention, the diameter of bleed hole 44, and the 
inside diameter of tube 20, is in the range of four to six times larger 
than the diameter of the small distribution holes in the insert and they 
permit about 10% to 15% of the air supplied to the insert to be bled from 
the vane. The pressure drop between the air inside the insert and the hot 
gas flowing downstream of the vane to which the air is bled is larger than 
the pressure drop across the small distribution holes as a result of the 
aforementioned large pressure drop across the holes 27 in the downstream 
edge of the airfoil. As a result of the large bleed air pressure drop, due 
to the large size of bleed hole 44 and the significant quantity of cooling 
air bled, the particles entrained in the cooling air are preferentially 
bled from the insert and do not accumulate around the small distribution 
holes. 
In addition, it should be noted that the flow area of the manifold 25 and 
the passageway 23 are larger than that of bleed hole 44, thus insuring 
that the bleed hole controls the quantity of cooling air bled from the 
insert. Also the diameter of hole 17 in the closure cap 16 is increased so 
that additional cooling air enters the vane, thereby compensating for the 
air bled from the insert.