Cooled rotor blade for a gas turbine

Abstract

The rotor blade body has a first flow path extending parallel to and immediately downstream of the blade front. This path passes through a trough-shaped indentation in the blade tip before terminating in an air exit in the zone of the trailing edge. One or more secondary flow paths also pass through the blade body and terminate in the zone of the trailing edge. These secondary flow paths reverse the flow either by 90.degree. or 180.degree. and are of larger intermediate flow cross-sectional area. Restrictors can be provided in the exits for the secondary paths to obtain high exit velocities.

Description

This invention relates to a cooled rotor blade for a gas turbine.

Generally, in order to achieve cooling of the rotor blades, both guide blades and rotor blades, in an operating turbine, two contradictory conditions must be basically satisfied. First, good cooling requires high coefficients of heat transmission which, in turn, involve high flow velocities and relatively high pressure losses. Second, the amount of cooling air required by each blade should be as small as possible because the cooling air branched off, for example, from a compressor represents a loss in a certain sense and results in a deterioration of the efficiency of the entire process. Moreover, in practice it frequently occurs that the pressure gradient available between the cooling air entry into the blade and the cooling air exit from the blade is relatively low. Thus, the required velocities cannot be obtained or, if obtained because a high consumption of cooling air can be tolerated, is done only with difficulty.

In order to overcome these basic problems, it has been known to provide constructions in which the cooling air is passed once through the blade through a plurality of ducts radially of the machine from the interior to the exterior. The required velocities can be easily obtained with this system but require relatively large quantities of air. Moreover, the cooling capacity of the air in this system is only very incompletely utilized.

It has also been known to provide rotor blades which have shroud bands with labyrinth projections in a gap between the casing in which the rotor blades are installed and the blade tips in order to reduce gap losses. If these blades are cooled in the manner described above, the cooling air is discharged into the inner cavity of the shroud band after flowing through the individual blades in the radial direction before being discharged rearwardly in the same direction as the working gas.

Accordingly, it is an object of the invention to achieve an optimum cooling of rotor blades with a relatively low consumption i.e. a small quantity of cooling air and a relatively low available pressure gradient.

It is another object of the invention to provide a rotor blade of cast construction which can be cooled effectively.

Briefly, the invention provides a rotor blade having a blade front or nose, a trailing edge and a blade tip at one radial end with a coolant flow path located immediately downstream of and parallel to the blade front or nose which terminates in a trough-shaped indentation in the blade tip. In addition, a coolant exit is provided at the end of the indentation on the suction side of the blade body in the zone of the trailing edge.

The trough-shaped indentation, whose air exit on the trailing edge has a relatively large exit cross-section, enables the low pressure prevailing at the trailing edge of the blade to be transposed practically directly to the end of the flow path so that the entire pressure gradient is available for this relatively short distance. By contrast to known constructions, this results in increased velocities and therefore better coefficients of heat transfer. A further advantage of the construction is due to the fact that the provision of the air exit on the suction side of the blade allows the maximum available pressure gradient to be utilized for cooling the blade front. The maximum duct velocity, at the minimum cooling duct cross-section, and therefore the maximum cooling effect may thus be produced with a given amount of cooling air. The entire pressure gradient is thereby available for the actual cooling section in the aforementioned rotor blades with shroud bands. In this known construction, however, all the disadvantages associated with a shroud band, such as an additional mass which is subject to centrifugal forces and as a result imposes a substantial load on the blade root, must be tolerated. Furthermore, in this known construction it is not possible to transfer the lowest pressure prevailing on the suction side of the blade onto the end of the flow path. A further disadvantage of the known construction is due to the increased manufacturing costs because blades with a shroud band cannot be produced as an integral casting i.e. as a one-piece casting.

While the above described flow path particularly cools the blade front, two possibilities are available for cooling the remaining parts of the blade. To this end, means are provided to define a second flow path which extends parallel to the first flow path and leads to an air exit in the zone of the trailing edge. The exit for this flow path extends over the height of the blade and is located downstream of a reversal in the flow path through which the flow can be reversed 180.degree.. In the other case, a second and third flow path are provided, parallel to the first flow path in the blade, in order to lead the air to exits after being deflected through 90.degree.. In this case, each of the exits for the secondary flow path cover a part of the blade height and are disposed in the zone of the trailing edge. Which of the two possibilities is more favorable must be determined in relation to the circumstances governing the individual case, for example, in accordance with the amount of heat to be dissipated, the available pressure gradient and the blade length. connection, have

Dividing the air over the two or three flow paths may be advantageously performed by means of restrictors in the air exits of the second and third flow paths. These restrictors can also be adapted for individual adjustment on the basis of tests. In this connection, it is advantageous if the second and thirid flow paths, have a flow with the least possible losses as far as the restrictors and if, practically, the entire nominal pressure gradient occurs at the restrictor positions of the aforementioned flow paths.

If any adequate pressure gradient is available for the second and third flow paths, it will be advantageous to provide the air exits thereof on the delivery side of the blade because flow discharge on this side is more advantageous and simpler in terms of flow. By contrast, the provision of the aforementioned air exits on the suction side will provide a better cooling action because, on the one hand, a higher pressure gradient is available which enables higher velocities and higher thermal transfer coefficients to be achieved while, on the other hand, film cooling on the suction side is more effective because of the higher thermal transfer coefficients on the suction side. This film cooling is known to be the result of the cooling air which flows along the surface of the blade.

It is, of course, also possible to provide the air exits in the trailing edge itself.

To achieve a further improvement of heat dissipation, it may be advantageous if the second and third flow paths have boundary walls at least over a part of their length which are provided, at least partially, with fins.

It is generally known that cast blades are preferable to forged and welded blades more particularly because of their higher high-temperature resistance, their materials and because of the greater simplicity of manufacture or because of the absence for any need of finish-machining. The blade of the invention is therefore advantageously constructed so that the blade represents a precision casting in its entirety i.e. the blade is a one-piece casting.

These and other objects and advantages of the invention will become more apparent from the following detailed description and appended claims taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates a longitudinal sectional view taken on line I--I of FIG. 2 of a rotor blade according to the invention;

FIGS. 2 and 3 illustrate views taken on line II--II and III--III of FIG. 1, respectively;

FIG. 4 illustrates a sectional view taken on line IV--IV of FIG. 3;

FIG. 5 illustrates a plan view of the rotor blade of FIG. 1 taken in the direction of the arrow A in FIG. 1;

FIG. 6 illustrates a plan view of the rotor blade of FIG. 7 in the direction of the arrow A of FIG. 7;

FIG. 7 illustrates a longitudinal sectional view taken on line VI--VI of FIG. 8 of a second embodiment of a rotor blade according to the invention;

FIG. 8 illustrates a sectional view taken on line VII--VII of FIG. 7; and

FIG. 9 illustrates a detail view of a modification of the construction shown in FIG. 8.

Referring to FIG. 1, the cooled rotor blade is disposed to move in a flow duct 1 indicated by an outer filler ring segment 4 of a casing (not shown) and which receives flow from the right (arrow B) as viewed. The rotor blade is secured in a rotor ring which is screened relative to the duct 1 by heat exchange segments 5 which provide protection against hot gases. The heat exchange segments 5 in the same way as the filler ring segment 4 comprise material of high-temperature resistance while the ring is constructed of less expensive ferritic material.

The rotor blade is made of one-piece construction and has three separate parallel flow paths for a coolant such as cooling air which extend from an aperture 8 disposed in the blade root and fed with cooling air by channels. The first relatively narrow flow duct 9 extends parallel to and directly downstream of the blade front or nose 10. The duct 9 also extends into a trough-shaped indentation 11 at the blade tip. The indentation 11 communicates with a relatively wide air exit 12 on the suction side of the blade in a socket 13 (FIG. 5) which surrounds the air exit 12 in the manner of a wall following the blade profile contour. The flow cross-section of the indentation 11 and of the air exit 12 are broad. This means that the pressure at the duct end 14 is low because the gas-side pressure at the suction side of the trailing edge 17 comes into effect practically at the duct end 14. The entire pressure gradient between the aperture 8 and the flow duct 1 on the suction side of the trailing edge 17 of the blade may thus be utilized for cooling the particularly hot front of the blade.

Second and third flow paths 15 and 16 are provided which are less direct and in this example, while being simultaneously deflected through 90.degree., extend from the aperture 8 to air exits 19, 20 in the zone of the traling edge 17. The second flow path 15, separated from the third flow path 16 by a bulkhead 18, cools the trailing edge 17 of the outer zone of the blade while the third path 16 supplies the inner part of the trailing edge 17, closer to the blade root, with cooling air.

In the blade illustrated in FIG. 1, the air exits 19, 20 are disposed on the delivery side of the blade so that the flow conditions are improved. It is, of course, also possible to dispose the air exits 19, 20 on the suction side of the blade if it is necessary to utilize the maximum available pressure gradient in order to achieve adequate cooling of the trailing edge (see FIG. 8). For the sake of completeness, it should be mentioned that the air exits 19, 20 may also be disposed in the trailing edge itself (FIG. 9). In order to improve the cooling action, it is also possible to provide the flow paths 15, 16 with cooling fins, at least over a part of the boundary walls defining the paths but this is not separately shown.

The flow paths 15, 16 are constructed so that flow therethrough as far as the air exits 19, 20 takes place at relatively low velocities and therefore substantially without pressure loss. The terminology "practically without pressure loss" means that the second, and on occasion, the third flow path in the middle part of the blade is constructed, relative to the quantity of cooling air flowing through, so that the pressure produced by centrifugal force at least approximately compensates the flow resistance as far as the radially outer deviation. The aforementioned available pressure gradient is therefore almost entirely consumed in the air exits 19, 20. To this end, the air exits 19, 20 are provided with restrictor and guide elements 21, 22 so that the air distribution in the air exits 19, 20 is at least approximately uniform over the blade height, accompanied by high flow velocities and therefore large coefficients of heat transfer. All these features result in uniform and good cooling of the trailing edge 17 of the blade with relatively small quantities of cooling air and with relatively low available pressure gradients.

Distribution of the air over the three different flow paths may be varied to some extent by varying the restrictor and guide members 21, 22. This distribution must be defined from case to case and depends on given conditions, for example flow, pressures, given temperature and their distribution.

Referring to FIG. 7, the rotor blade can also be constructed without the need of the third flow path. As shown, apart from the inflow from the left (arrow C) such a rotor blade differs from the blade described in FIG. 1 by the absence of the third flow path 16. Also, the second flow duct 15 incorporates a reversal through 180.degree.. To this end, the flow duct 15 initially extends radially outwardly in the middle part of the blade and then leads through a reversal chamber 23 through 180.degree. into a second portion 25 disposed over the entire height of the blade to return to the blade root. The two part flow paths 15, 25 are separated from each other by a bulkhead 24. In this embodiment, the second flow path is subject to low pressure losses, at least as far as the reversal. Thus, practically the entire gradient is available for uniformly distributed discharge on the trailing edge over the entire height of the blade, a feature which cannot readily be achieved against the action of centrifugal force. As shown in FIG. 6, the blade tip is of similar construction as above a indicated by like reference characters.

The embodiment of FIG. 7 which involves more difficulties for the uniform distribution of the second stream of cooling air is suitable more particularly for smaller machines with blades of lower height and for machines with a lower peripheral velocity.

Both embodiments (FIGS. 1 and 7) are advantageously produced as separate castings by the precision casting method so that in addition to the advantages already mentioned, it is possible to utilize the advantages of cast as against forged blades.

Claims

What is claimed is:

1. A cooled rotor blade for a gas turbine comprising

a one-piece rotor blade body having a blade front, a trailing edge and a blade tip at one radial end extending from said blade front to said trailing edge, said blade tip having a trough-shaped indentation extending longitudinally therein;

means defining a first flow path through said blade body extending immediately downstream of and parallel to said blade front and terminating in said trough-shaped indentation;

a first coolant exit in said blade body in the zone of said trailing edge, said exit being in communication with said trough-shaped indentation on the suction side of said body for the passage of coolant therethrough; and

means defining a second flow path and a third path through said blade body, each of said second and third flow paths having a portion parallel to said first flow path and including portions for deflecting respective flows of coolant 90.degree. therein; a second coolant exit in communication with said second flow path and a third coolant exit in communication with said third flow path, each of said second and third coolant exits being disposed over a part of the height of said blade body in the zone of said trailing edge on the pressure side of said body.

2. A cooled rotor blade as set forth in claim 1 wherein each of said second and third flow paths have boundary walls defining portions of said respective flow paths and wherein fins are disposed on said walls projecting into said respective second and third flow paths.

3. A cooled rotor blade as set forth in claim 1 which further comprises means in each of said second and third exits for throttling the flow of coolant therefrom.