EP0023025B1

EP0023025B1 - A turbine blade

Info

Publication number: EP0023025B1
Application number: EP19800104153
Authority: EP
Inventors: Takeshi Sato; Akira Uenishi; Norio Yasugahira; Katsukuni Hisano
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1979-07-18
Filing date: 1980-07-16
Publication date: 1989-03-15
Also published as: DE3072147D1; EP0023025A1; JPS6259203B2; JPS5614802A

Description

The present invention relates to a turbine blade of axial flow fluid machines according to the first part of the claim.
From the GB-A-550 393 it is known such a turbine blade having a thick rounded inlet edge to accelerate the income fluid flow to the highest speed immediately at the inlet portion of the flow channel upstream of the changing portion of the flow direction. The blades and the flow channels are so formed that the fluid flow maintains this highest speed downstream of said direction changing portions. But this known constructions has the drawbacks of large loss of flow energy and of increased costs of material and manufacture.
Turbine blade profiles are in general designed to obtain a desired inlet angle, a desired outlet angle and a desired blade width or chord length, but hydrodynamical conditions in the flow passage between the adjacent blades are not enough taken into consideration. The boundary layers are formed over the blade surfaces due to the viscosity of the fluid and flow past the outlet of the flow passage, resulting in the lack of velocity of the fluid at the downstream of the outlet. The degree of the lack of the velocity of the fluid at the downstream of the outlet determines the performance of the blade profile. The most important factor which must be taken into consideration in design of turbine blade profiles is the thickness of the boundary layer at the outlet of the flow passage between the adjacent blades. In general, the thinner the boundary layer at the outlet, the higher the performance becomes. It has been clarified that the development of the thickness of the boundary layer is closely correlated with the variations in velocity of the fluid passing through the flow passage. However so far the variations in velocity have not been taken into consideration in the design of a flow passage between the blades. As a result, no attempt has been made to suppress the formation of the boundary layer so that the separation of the boundary layer results, causing very serious adverse effects on the performance. Thus it has been difficult to obtain the turbine blade profiles which ensure the high performance.
It is the object of the invention to provide a turbine blade which can stabilize the boundary layers thereon to avoid a greater lack of the flow velocity of the fluid at the downstream of the blade outlet.
The solution of this object is defined by the characterizing features of the claim.
The turbine blade as defined in the claim has the advantage that the fluid flow at the outlet end of the flow channel has a more uniform velocity over the flow section of the channel than in the prior art turbine blades. The advantage results from the fact that the acceleration of the income fluid flow is substantially completed upstream of the flow direction changing point and that the relative long channel portion downstream of the flow direction changing point has a substantially uniform cross section and a wide curvature. Especially this wide curvature of the channel portion downstream of the changing point causes a reducing of the velocity difference between the fluid flow on the back or suction wall surface and the fluid flow on the front or pressure wall surface in a flow channel. Said uniformed velocity distribution has advantageous effects to the form and the development of the boundary layers and to the reducing of turbulences behind the trailing edges of the blade outlets.
The above and other objects, features and effects of the present invention will become more apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings, in which:-

Fig. 1 is a diagram of a turbine blade profile in accordance with the present invention;
Fig. 2 shows the pressure distributions on the surfaces of the turbine blade in accordance with the present invention;
Fig. 3 is a view used for the explanation of the behaviors of the boundary layer on the back surface of the turbine blade in accordance with the present invention; and
Fig. 4 shows the relationship between the deflection angle and the blade profile loss coefficient of the turbine blade in accordance with the present invention.

Referring first to Fig. 1, the features of a blade profile in accordance with the present invention will be described. A line H is first drawn which is in parallel with the axis of blade array (that is, the direction in which the blades 10 are mounted in a circular array) and which passes the point of intersection J between a first line F inclined to the axis of the blade array, at an inlet angle ₀₁ and a third line inclined to a fourth line in parallel with the above-mentioned axis at an outlet angle a₂. The position of this line H corresponds to the point P at which the fluid flow is deflected in direction within the passage between the back surface 10b of the turbine blade 10 and the front surface 10a of the adjacent blade 10. The inlet width of this passage i.e. the pitch of the blade array is denoted by t and the outlet width by S. The passage width Sp is the diameter of a circle around the point P at which the center line A of the flow passage intersects the line H. The distance 1. between the straight line H which passes the flow direction changing point P and the outlet of the blade is greater than one half of the chord length C of the blade 10. The portion of the blade profile above the straight line H is referred to as "the upstream portion" while the portion below the straight line H, "the downstream portion".
The width of the flow passage is drastically reduced at the upstream portion from the inlet to the flow direction changing point P (from A to P in Fig. 1) while the decrease in width is gradual in the downstream portion (from P to B in Fig. 1).
The radius of curvature R_N of the upstream portion of the back surface 10b (from the inlet to the straight line H in Fig. 1) is made smaller than 0.15 of the chord length C'. The radius of curvature R_NO of the downstream portion of the back surface 10b (from the straight line H to the outlet in Fig. 1) is expressed by R_NO/C>5.0. The radius of curvature R_NP of the downstream portion of the front surface 10a is expressed by R_NP/C>1.3. These conditions are summarized in Table 1 below.
The diameter Sp of a circle around said flow direction changing point P contacting the front surface (10a) of one blade (10) and the back surface (10b) of the next blade (10) is less than about 0.4 times as small as the blade pitch (1). Said diameter S_P of said circle is 0.9<S/Sp<1.0, whereby S is the smallest width at the outlet end of the flow channel. The curvature of the back surface upstream of the straight line H is made greater while the curvatures of the downstream portions of the front and back surfaces are made smaller or made substantially zero, so that an optimum acceleration of flow can be ensured and the acceleration of the fluid flow can be substantially completed before the fluid reaches the flow direction changing point P.
The increase in thickness of the upstream portion of the blade 10 results from the fact that the radius of curvature R_N of the upstream portion of the back surface 10b is reduced. As a result, the acceleration of the fluid can be substantially completed before the fluid reaches the flow direction changing point P without changing the inlet angle α₁. In addition, the acceleration stabilizes the boundary layers and decreases their thickness. The fluid flow is deflected along the concave front surface 10a and the convex back surface 10b so that satisfactory boundary layers are formed even after passing the flow direction changing point P. As a consequence, a uniform velocity distribution can be attained in the flow at the downstream of the outlet.
In summary, according to the present invention, the thickness d_m of the blade is given by the following dimensionless expression or parameter:
where d_m is the distance from the point M, at which the straight line Q is tangent to the back surface 10b, to the point at which a stragiht line constructed at the point M at right angle to the straight line Q intersects the front surface 10a of the blade.
The features of the present invention will be more clearly understood from Fig. 2 which shows the flow in the passage between the blades is expressed in terms of the pressure acting on the blade surfaces. The pressure acting on the back surface of the blade has a high pressure drop ΔPs in the upstream portion of the flow passage from the inlet to the point P at which the flow is deflected. Since the pressure drop ΔPs approaches AP which is a pressure drop in the overall portion of the flow passage, the stabilized boundary layers can be formed. At the throat (indicated by S in Fig. 1), a very gentle increase in pressure is observed. A sudden pressure rise (or the decrease in velocity) facilitates the formation of the boundary layers. That is, the pressure rise determines the conditions of the boundary layers formed and consequently the performance of the blade.
Shown in Fig. 3 are the velocity distribution V, displacement thickness δ and momentum thickness θ on the back surface 10b of the blade. The thicknesses 6 and θ are the measures in determining the thickness of the boundary layer and are calculated (according to "TN D-5681", published by NASA, May 1970) based upon the pressure distribution shown in Fig. 2. As described above, according to the present invention, the acceleration is almost completed before the fluid reaches the flow direction changing point P so that both the displacement thickness 6 and the momentum thickness θ can be decreased at the outlet of the blade (I_x/L=1.0), whereby a high performance blade profile can be obtained.
From the data shown in Fig. 3, the blade profile loss coefficeint e is obtained by the following equation.
where

e is the blade profile loss coefficient,
6 is the boundary layer displacement thickness,
δ*=δ/t,
8 is the boundary layer momentum thickness,
θ*=θ/t,
t is the blade pitch.

In Fig. 4 is shown the relationship between the blade profile loss coefficient e and the inlet and outlet angles a₁ and a₂. The blade profile loss coefficient e is plotted along the ordinate while the deflection angle [180^0-(a_l=a₂)], along the abscissa. It is seen that when the deflection angle is close to 100°, the blade profile loss coefficient can be made as little as about 0.02. Thus the present invention provides a blade profile with a mininum loss and a higher degree of performance.
In summary, according to the present invention, the acceleration is almost completed before the flow direction changing point so that the boundary layers can be highly stabilized and consequently the velocity enhancing and high performance blade profile can be provided.

Claims

Turbine blade for an axial flow fluid machine having an inlet portion of increased thickness so that the fluid will be accelerated to a high speed in the inlet portion of the flow channel and will maintain this high speed after changing its flow direction, characterized by the combination of the following features:
a) a flow direction changing point P defined by the intersection of the neutral line APB of the flow channel wiht a straight line H in parallel to the connecting line of the outlet ends of the blades (10) and intersecting the crossing point J of the extensions of the inlet angle a₁, and the outlet angle a₂ of the blade (10), said flow direction changing point P having a distance l_ax to said connecting line of more than 0.5 of the axial chord length C of the blade (10),

b) the radius of curvature R_N of the portion of the blade back surface (10b) upstream of the flow direction changing point P is less than 0.15 times of the chord length C of the blade (10),

c) the radius of curvature R_NO of the portion of the blade back surface (10b) downstream of the flow direction changing point P is greater than 5 times of the chord length C of the blade (10),

d) the radius of curvature R_NP of the portion of the blade front surface (10a) downstream of the flow direction point P is greater than 1.3 times of the chord length C of the blade (10),

e) the diameter Sp of a circle around said flow direction changing point P contacting the front surface (10a) of one blade (10) and the back surface (10b) of the next blade (10) is less than about 0.4 times as small as the blade pitch (t),

f) said diameter Sp of said circle is 0.9<S/Sp<1.0, whereby S is the smallest width at the outlet end of the flow channel.