Blading For Axial Flow Turbo-machine

Abstract

Turbine blading for the rotors of turbo-machines of the axial-flow type in which the blades are twisted to allow for a high ratio of external to internal diameter of the flow passageway, the amount of the twist being so selected that it will neither untwist nor increase its twist when subjected to the centrifugal forces which act on the blade when the rotor is operating.

Description

This invention relates to turbine blading for use in axial-flow type turbo-machines such as compressors, turbines and the like and more particularly to an improved blade construction of the twisted type.

Twist type blading is often used on the rotors of turbo-machines, of the axial-flow type, for example, in steam turbines and compressors, which have a high ratio of external to internal diameter of the flow passageway in order to allow for the differing angular velocities which occur along the face of the blade between its root and tip. Experience has shown, however, that such twisted blades will tend to untwist due to the longitudinal pull exerted thereon as a result of the centrifugal forces to which the blade is subjected as it rotates at rather high speeds, with the result that the amount of the twist will differ, depending upon whether the rotor, and hence the blading is rotating, or at stand-still. This will result not only in some uncertainty as to the aerodynamic effect of the blading, especially during the starting period, and also at lower speeds, but also in the generation of additional stresses which will be superimposed on the already high centrifugal as well as bending stresses.

The object of this invention is to provide a twisted blade strurcture in which the amount of twist will remain the same, i.e., both at standstill and during operation. This objective is attained by selecting the degree of twist in such manner that it will neither untwist, nor increase its twist as a result of the influence thereon of centrifugal forces which obtain during operation.

The improved blade structure offers the advantage that it can follow aerodynamic aspects and that no provision need be made for any potential changes in shape of the blade during operation.

The invention will become more apparent from the following description of a preferred embodiment thereof and in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic presentation of a twisted blade illustrating the nature of the centrifugal forces acting upon it under dynamic conditions;

FIG. 2 is a view similar to FIG. 1 but wherein the blade is straight, i.e., non-twisted;

FIG. 3 illustrates a blade of the twist type when at rest, i.e., under static conditions; and

FIG. 4 illustrates a twisted blade constructed in accordance with the principles of this invention.

With reference now to FIG. 1, there is shown the tip 1 which represents a thin section of a turbine blade having an airfoil profile. The leading and trailing edges of the blade are designated by 2 and 3, respectively. At the center of mass 4, the tip 1 is secured to pin 5 which is located vertically to the axis of rotation 6, and thus in a radial direction. Centrifugal forces acting upon the masses in the center of the mass, exert a pull in the axial direction of the pin and are designated by vector 4-7. The centrifugal forces which originate from the masses at point 2, are depicted by a radial vector 2-8 which intersects the axis of rotation 6 vertically. Since point 2 is staggered in the peripheral direction in relation to point 4, the forces 2-8 and 4-7 are not parallel. Vector 2-8 can be resolved into two components, one such component 2-9 being parallel to vector 4-7, and the other component 9-8 perpendicular to the latter and functioning in a tangential direction. In the same manner, the radial forces 3-10 can be resolved into two components, one such component 3-11 being parallel to vector 4-7 and the other component 11-10 perpendicular to the latter. Those components which are parallel to the vector 4-7 will form a resultant which exerts a purely tensional force upon the pin 5 if, as stipulated, point 4 represents the center of mass for the tip 1. Under the same conditions will the perpendicular vector components exert a purely torsional force onto the pin 5.

FIG. 2 shows a rotor 12 upon which is fastened a circumferential array of blading. However, in order to simplify the disclosure only one such blade 13 has been included. The blade is not twisted about its longitudinal axis and it has a constant cross-sectional profile throughout its entire length. The axis 14 of the center of mass is located radially to the axis of rotation 6. When this blade system is rotated rapidly, the blades will stretch longitudinally and, as explained on the basis of FIG. 1, will twist in the direction of the arrows 15.

FIG. 3 shows a blade which has a sharp twist, the blade being in the rest, i.e., when the rotor is at standstill. The forces generated on the turbine blade are not parallel to each other, and are not perpendicular, but rather are inclined to the axis of rotation 6. During operation of the blading, the blades 16 tend to straighten out under the influence of the centrifugal forces, and to assume a position perpendicular to the axis of rotation, thus tending to untwist the blade, in known manner, in the direction of the arrows 17.

In accordance with the novel concept of this invention, the twist imparted to the blade is such that the twisting and untwisting forces just neutralize one another. FIG. 4 illustrates an embodiment of a blade structured in accordance with the invention. The blade, made of metal, is connected to the rotor 12 along the path 19-20-21. It is designed in the form of a control wing where all of the force generating vectors 19-22, 20-23 and 21-24 are located in a radial direction, that is to say, they intersect the axis of rotation 6 at an angle of 90.degree.. It will be readily apparent that such a blade structure can not and will not change its shape by twisting or untwisting when subjected to centrifugal forces during rotation of the rotor.

The invention is not limited to the specific blade design illustrated in FIG. 4 but rather other configurations may be adopted by which the same result can be obtained. For example, in contrast to the blade configuration depicted in FIG. 4, the thickness or the length of the cord of the profile diminishes from within to without.

The principle which determines the extent of the blade twist can be expressed by the equation

d.beta./dr = T/P .sup.. A/I.sub.1 + I.sub.2 wherein

r is the axial pitch of any blade section on which the following values are based.

.beta. is the angle between the principal inertia axis and the circumferential direction.

d.beta./dr is the degree of blade twist

T is the resultant torsion moment of the blade part from the distance r to the end of the blade

P is the resultant tension force of this part of the blade

A is the cross-sectional area of the profile, and

I.sub.1, i.sub.2 are the maximum and minimum moments of inertia of the blade profile

The shape of the blade can deviate somewhat in actual practice from the theoretical shape as defined by the above equation if a slight correction is indicated for purposes of flow-engineering.

Claims

I claim:

1. The improvement in turbine blading for the rotor of a turbo-machine of the axial-flow type wherein for the purpose of allowing a high ratio of external to internal diameter of the flow passageway each of the blades is given a twist about its horizontal axis which is determined at least approximately in accordance with the equation

d.beta./dr = T/P .sup.. A/I.sub.1 + I.sub.2 wherein

r is the axial pitch of any blade section on which the following values are based.

.beta. is the angle between the principal inertia axis and the circumferential direction.

d.beta./dr is the degree of blade twist

T is the resultant torsion moment of the blade part from the distance r to the end of the blade

P is the resultant tension force of this part of the blade

A is the cross-sectional area of the profile, and

I.sub.1, i.sub.2 are the maximum and minimum moments of inertia of the blade profile

thereby maintaining constant the amount of the blade twist under the influence of centrifugal forces acting thereon when the motor is operating.

2. Turbine blading as defined in claim 1 wherein all centrifugal force generating vectors along the surface of the blade between its leading and trailing edges extend in a radial direction from the axis of rotation of the rotor.