US2970807A

US2970807A - Rotor blades for gas turbines

Info

Publication number: US2970807A
Application number: US662161A
Authority: US
Inventors: Endres Johann
Original assignee: Messerschmitt AG
Current assignee: Messerschmitt AG
Priority date: 1957-05-28
Filing date: 1957-05-28
Publication date: 1961-02-07
Anticipated expiration: 1978-02-07

Description

Feb. 7, 1961 J. ENDRES ROTOR BLADES FOR GAS TURBINES Filed May 28, 1957 IN V EN TOR. .30 #9 AW Eva/9.5.5 BY My WW ate ROTOR BLADES FOR GAS TURBINES Filed May 28, 1957, Ser. No. 662,161

'1 Qlaim. (Cl. 253-3915) This invention relates to improvements in rotor blades for gas turbines.

The efiiciency of a gas turbine engine depends upon the pressure and temperature generated during the combustion process, the temperature being subject to an upper limit which is determined mainly by the nature of the material employed. A further increase in temperature will then call for the provision of special cooling means for cooling the exposed structural parts, such as more particularly the nozzles and rotor blades of the turbine.

It has been proposed to construct the rotor blades in the form of hollow blades and to subject the hollow interior to the action of a coolant for the purpose of extracting heat from the structural parts that are principally exposed to high temperatures. However, these measures have only been partly successful because the available heat-transferring surfaces inside the blades are too small in relation to the quantities of heat that must be withdrawn. Furthermore, the expenditure in additional structural means is considerable.

German patent specification No. 821,881 and French patent specification No. 924,012 both disclose methods of cooling gas turbine blades in which liquid fuel is discharged through porous or perforated walls of hollow blades on to the surfaces of the blades where they form an evaporative cooling film which is intended to protect the blades from the high temperatures.

This method of cooling is open to very grave objections which preclude its application with any prospect of operational reliability. Centrifugal forces push the liquid fuel to the outside through the openings in the blade walls. The weight of fuel thus conveyed to the outside surface depends upon the pressures set up by the centrifugal field in the hollow blade interior, and this in turn depends upon the radial distance of the openings from the axis of revolution and upon the square of the angular speed.

The blade dimensions of modern gas turbines are such that pressure differentials of 100 ats. and more will arise between the openings that are nearer and those that are more remote from the axis, a circumstance which prevents an evenly distributed film of evaporating fuel to form on the blade surfaces. Furthermore, considerable quantities of fuel are ejected at high speed in the shape of liquid jets, especially from the tips of the blades, whereas only small quantities of fuel issue from the openings in the central portions and near the root of the blades. This non-uniformity in the delivery of the fuel from the openings leads to the development of high combustion temperatures at the tips and consequent local overheating of the blade material, which sets up critical thermal stresses that cannot be withstood by the blade and causes its destruction.

An earlier proposal likewise relates to the problem of creating a uniformly distributed cooling effect on the blade surface and to this end provides tapering ducts in the blade interior. However, the more particular purpose of this earlier proposal is to eliminate the effect of differ- 2,970,807. Patented Feb. 7, 1961 ent degrees of heat transfer at various points of the blade upon the uniformity of the cooling effect, whereas the aim of the present invention is to eliminate the difiiculties that arise as a result of the distribution of the centrifugal forces.

In other words the present invention relates to a sur face-cooled gas turbine rotor which is provided with hollow blades comprising a porous material and/or blades comprising a casing perforated by outlet openings, the interior of the blades having a radial fuel deliveryduct, and the principal feature of the invention consists in that the factors which govern the specific rate of fuel discharge per external surface element of the blade, such as the conformation of the fuel duct, the density of the porous material and/or the wall thickness of the porous material and/or the diameter of the openings in the blade casing, are determined and possibly relatively balanced and concerted in such a way that the specific fuel dis-' charge over the entire radius of the blade, i.e., over the entire blade surface will everywhere be substantially the same when the turbine runs at its rated operational speed.

According to one proposal of the invention the blade is constructed in the known manner of a hollow blade with an outer perforated sheet metal wall, and in a manner likewise already known the interior of the blade consists of a porous material with a radially disposed fuel delivery duct, the perforations in the sheet steel casing of the hollow blade being provided with nozzle-shaped bores and the fuel duct inside the internal porous material being a convergent duct which tapers towards the tip of the blade.

Alternatively the invention proposes to construct the rotor blade throughout of a porous material with an internal convergent fuel delivery duct which tapers towards the blade tip.

Another proposal-according to the invention is to construct the rotor blade throughout of a porous material of non-uniform density more particularly of such a nature that the density of the material increases and its porosity diminishes radially outwards.

A special technical possibility of compensating differentials in fuel pressure due to the centrifugal field in the blade consists in producing the blade as a centrifugal casting in which the density of the material increases and its porosity decreases radially outwards so that the velocity of diffusion of the fuel along the length of the blade can likewise be compensated and a uniform rate of fuel discharge on to the outer blade surface secured.

As a result of the proposed construction of the blade the fuel will be forced into the hollow blade and through the ducts or through the porous wall to the outside on to the blade surface where it will form a boundary film consisting of vaporised fuel which will have a cooling effect on the blade material. The desired uniformity of fuel discharge on to the blade surface is thus achieved by the selection of different nozzle section and by differences in the thickness of the wall as well as by differences in the porosity of the porous material.

Coincidentally with the cooling of the blade by the fuel the subsequent combustion of the fuel in the gas stream will have an intermediate heating effect and maintain an approximately isothermal combustion process with a resultant considerable increase in power.

Illustrative embodiments of the invention are shown in the accompanying drawing and it will be understood that these embodiments disclosed herein are not intended in any way to limit the invention and merely serve to illustrate the technical features of the invention so that the latter may be the more readily understood.

Fig. 1 is a cross-section of a hollow rotor blade with a porous core; and

Fig. 2 is a section of a rotor blade consisting throughout of a porous material.

As shown in Fig. 1, the rotor blade 1 is constructed in the manner of a hollow body 1a of revolution made of sheet steel with an internal porous filling material 2 which contains a duct 3 of conical convergent shape in axial section which is filled with fuel by the effect of the centrifugal forces and mounted on the turbine shaft 7. Fuel is delivered through duct'lt) and the fuel entry The fuel diffuses to the outside through the porous filler material 2 and nozzles 5 in the sheet steel casing 6 to form a cooling boundary film 6a. Owing to the convergence of the conical duct the distances 8, S, 8" the particles of fuel have to travel from the interior to the outside, differ at different radial distances 9, 9', 9" from the axis of revolution. More particularly, they are proportional to the pressure of the fuel in the centrifugal field. By thus varying these distances of travel the effect of the radially varying centrifugal force upon the discharge speed and specific volume of fuel discharge is compensated and the cooling boundary film 6a will be uniformily maintained.

As shown in Fig. 2, the rotor blade 11 mounted on turbine shaft 15 has an axis of rotation and consists throughout of a porous material with a convergent duct 12 for the introduction of fuel from a duct 16 through a fuel entry 13 into the porous blade material. The effect is the same as that described with reference to the embodiment of Fig. 1, with the exception that the fuel will diffuse through the porous blade wall into a cooling boundary film 14 in very fine distribution so that a completely uniform distribution of the fuel on the blade surface will be ensured. The porous material may be of differing densities which increase and differing porosities which diminish radially towards the outer surface of blade 11.

Unlike known cooling systems the heat is not extracted to the outside but transferred to the inner gas stream so that no heat losses occur and the intermediate heating effect will produce an approximately isothermal combustion process with a resultant increase in power.

A special advantage offered by the present invention is the simple construction of the rotors.

The proposed arrangement provides basic technical conditions for achieving a combustion process in gas turbines operating at very high temperatures which will permit substantial increases inefficiency and power.

Various changes and modifications may be made Without departing from the spirit and scope of the present invention and it is intended that such obvious changes and modifications be embraced by the annexed claim.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent, is:

A surface-cooled rotor blade for a gas turbine, said blade having an axis of rotation and consisting throughout of a porous material, said blade having a radially disposed conical-shaped fuel delivery duct in the interior thereof so that the specific fuel discharge over the entire blade surface will everywhere besubstanti'ally-the same when said blade is rotated as the turbine runs at its rated operational speed, said fuel delivery duct being a convergent duct which tapers towards the tip of said blade for radially directing fuel discharge equally over the periphery of said blade with the dimensions of said duct being substantially inversely proportional to the distance thereof from said axis of rotation, said porous material being of differing densities which increase and differing porosities which diminish radally toward the outer surface of said blade.

References (Iited in the file of this patent UNITED STATES PATENTS 2,807,437 Roush Sept. 24, 1957 FOREIGN PATENTS 619,634 Great Britain Mar. 11, 1949 760,734 Great Britain Nov. 7, 1956 774,499 Great Britain May 8, 1957 924,012 France Mar. 3, 1947