Claims
- 1. A multistage axial flow turbine for compressible fluids in which the fluid leaving the antepenultimate stage is separated into coaxial radially inner and radially outer flows which are subject to substantially the same decrease in enthalpy between the outlet from the antepenultimate stage and the outlet of the turbine, the inner flow passing through the penultimate and final stages with respective decreases in enthalpy of h.sub.1 and h.sub.2 and the outer flow passing through only the final stage, the rotor blades of the last stage including a wall serving as a barrier between the inner flow and the outer flow, the rotor blades of the last stage having a discontinuity in variation in profile from root to tip with the profile of the blade radially outward of the wall between the two flows being more highly curved than that radially inward of said wall, and the degree of reaction of said rotor blades at this barrier when operating under optimum conditions having a value x.sub.2 on the outer flow side which is substantially equal to x.sub.1 .multidot.h.sub.2 /(h.sub.2 +h.sub.1) where x.sub.1 is the value of the degree of reaction on the inner flow side such that the pressure on the radially outward side of the wall is maintained substantially equal to the pressure on the radially inward side of the wall across said last stage rotor blades.
- 2. A turbine for compressible fluids according to claim 1, wherein x.sub.2 is between 0.3 and 0.7 times x.sub.1.
- 3. A turbine for compressible fluids according to claim 1 wherein the rotor blades of the final stage are made of titanium.
- 4. A turbine for compressible fluids according to claim 1, wherein the wall separating the coaxial inner and outer flows in the rotor of the final stage being at a mean distance r.sub.p from the turbine axis which is substantially equal to ##EQU3## where r.sub.b is the distance of the blade roots from the turbine axis and x.sub.0 is the degree of reaction at the base of the inner flow in the final stage.
- 5. A turbine for compressible fluids according to claim 4 wherein the wall between the coaxial inner and outer flows in the rotor of the final stage is at a mean distance r.sub.p from the turbine axis and r.sub.p is between 1.25 and 1.55 times r.sub.b where r.sub.b is the distance of the blade roots from the turbine axis.
- 6. A turbine for compressible fluids according to claim 1, wherein the decrease of enthalpy of the inner flow as it passes through the penultimate stage is substantially equal to the decrease of enthalpy as it flows through the final stage.
- 7. A turbine for compressible fluids according to claim 6, wherein x.sub.2 is substantially equal to x.sub.1 /2.
- 8. A turbine for compressible fluids according to claim 6, wherein the wall between the coaxial inner and outer flows in the rotor of the final stage is at a mean distance r.sub.p from the turbine axis which is substantially equal to r.sub.b .multidot..sqroot.2.multidot.(1-x.sub.0)/(1-x.sub.2), where r.sub.b is the distance of the blade roots from the turbine axis and x.sub.0 is the degree of reaction at the base of the inner flow in the final stage.
- 9. A turbine for compressible fluids according to claim 6, wherein the wall between the coaxial inner and outer flows in the rotor of the final stage is at a mean distance r.sub.p from the turbine axis which is substantially equal to r.sub.b .multidot..sqroot.2, where r.sub.b is the distance of the blade roots from the turbine axis.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 76 24765 |
Aug 1976 |
FR |
|
FIELD OF THE INVENTION
The present invention concerns a turbine for compressible fluids in which the flow of fluid at the outlet of the antepenultimate stage is separated into coaxial inner and outer flows which are subject to substantially the same decrease in enthalpy between the outlet from the antepenultimate stage and the outlet of the turbine, the inner flow passing through the penultimate and final stages of the turbine with respective decreases in ethalpy of h.sub.1 and h.sub.2, and the outer flow passing through only said final stage.
In known Baumann-type turbines, the outer flow passes through the penultimate stage of the turbine.
Baumann turbines have the advantage of a large outlet cross-section, but they also have a number of disadvantages, especially in so far as they require two rotors with long blades which are difficult to construct and very expensive, and in that the rotor blades of the final stage are subject to severe erosion when the fluid is a condensable one.
Preferred embodiments of the invention provide turbines for compressible fluids which do not have these disadvantages.
The present invention provides a turbine for compressible fluids in which the fluid leaving the antepenultimate stage is separated into coaxial inner and outer flows which are subject to substantially the same decrease in enthalpy between the outlet from the antepenultimate stage and the outlet of the turbine, the inner flow passing through the penultimate and final stages with respective decreases in enthalpy of h.sub.1 and h.sub.2 and the outer flow passing through only said final stage, the turbine being characterized in that the rotor blades of the last stage include a wall serving as a barrier between the inner flow and the outer flow and in that at this barrier the degree of reaction of the said rotor blades when operating under aerodynamic adaptation conditions has a value x.sub.2 on the outer flow side which is substantially equal to x.sub.1 .multidot.h.sub.2 /(h.sub.1 +h.sub.2) where x.sub.1 is the value of the degree of reaction on the inner flow side.
In the turbine in accordance with the invention, the long blades of the final stage rotor are not so wide as the blades of the final stage of a conventional turbine producing the same rate of fluid flow for the same length of blade.
As is well known, the rotor blades of the final stage generally have a pronounced curvature at the root, straightening out towards the tip.
To minimize the traction and flexing loads on the blades, it is necessary to have large blade cross-sections, especially at the root, and this can lead to prohibitive axial dimensions.
In the turbine in accordance with the invention, the variation in the profile of the final stage rotor blades from the root to the tip incorporates a discontinuity, and the profile of the blade outside the interface between the two flows is more highly curved than that inside this interface.
This geometrical feature of the blades is a direct result of the fact that in the final stage the flow degree of reaction decreases suddenly across the interface separating the inner and outer flows.
The local increase in curvature, and the consequent increase in rigidity, decreases over the length of the rotor blade section in the outer flow, so that while the bending stresses are kept equal to those in conventional rotor blades, the width of the rotor blade in accordance with the invention can be significantly reduced. The degree of reduction in cross-section is determined at the point which the aerodynamic loads produce the highest bending stresses, the cross-section at other points being reduced in proportion, especially at the root, without changing the stress due to centrifugal tractive loads on the blade.
Furthermore, the decrease in enthalpy in that part of the final stage through which the outer flow passes can always be made equal or nearly equal to the enthalpy decrease corresponding to the optimum aerodynamic efficiency, as the enthalpy decrease in the outer flow is sufficiently high. Also, in the case of condensable fluids, the blades are then protected against erosion by droplets of condensed vapour.
The impact energy of droplets of the fluid in the inner flow is considerably less, and the decrease in enthalpy in this section of the final stage can be close to the value producing the maximum efficiency, even though considerably less than the decrease in ethalpy of the outer flow.
Finally it is known that if the flow rate of a compressible fluid through the last stage of a turbine drops below its optinum value there is a large disturbance and the fluid tends to flow only through the outer portions of the blades and ceases to fill their inner portions. This phenomenon, which may go so far as to cause a turning detachment on the upper profiles of the blades, causes a rapid drop in efficiency and also produces pulsed forces on the blades which are very dangerous if the turning detachment occurs. These drawbacks become worse as the ratio between the blade length/the base diameter is increased.
The invention tends to reduce these injurious effects since the inner flow remains filled at low flow rates because the h.sub.1 +h.sub.2 decrease is maintained over the last two stages and because the outer flow behaves as if it were acting on a rotor blade whose length/base diameter ratio is more favourable than that of the last stage taken as a whole.
If in accordance with the invention the degree of reaction x.sub.2 =x.sub.i .multidot.h.sub.2 /(h.sub.2 +h.sub.1), then the pressure in the final stage rotor is the same on each side of the interface between the two flows, which means that no fluid leaks across the interface. Generally speaking the value of x.sub.2 is between 0.3 and 0.7 times x.sub.1. If the decrease in enthalpy in the penultimate and final stages is the same, in the part of the turbine through which the inner flow passes, the pressure in the final stage rotor is the same on each side of the interface if x.sub.2 =x.sub.1 /2.
In accordance with another preferred feature of the invention, the interface between the coaxial inner and outer flows in the final stage rotor is at a mean distance r.sub.p from the turbine axis which is between 1.35 and 1.55 times r.sub.b, which is the distance of the blade roots from the turbine axis.
The value of r.sub.p is preferably substantially equal to ##EQU1## where x.sub.0 is the degree of reaction of the inner flow at the root of the final stage rotor blades. This is because with this relationship between the values of r.sub.p and r.sub.b, it is possible to achieve enthalpy decreases of h.sub.1 +h.sub.2 in the outer flow and h.sub.2 in the final stage inner flow which means that the efficiencies of the two flows are substantially equal and correspond to the maximum possible aerodynamic efficiency.
If h.sub.1 =h.sub.2, the value of r.sub.p must be substantially equal to r.sub.b .multidot..sqroot.2.multidot.(1-x.sub.0)/(1-x.sub.2), which in practice is not very different from a value of r.sub.b .multidot..sqroot.2.
The advantages of the turbine in accordance with the invention are of increasing importance as the blade length is increased, the result of which is that the blades are subject to high centrifugal inertia loads requiring the use of materials such as titanium which have a high value of the ratio elastic limit/density.
The present invention will be better understood from the following description and drawings.
US Referenced Citations (6)
Foreign Referenced Citations (3)
| Number |
Date |
Country |
| 574106 |
Mar 1924 |
FR |
| 731766 |
May 1932 |
FR |
| 719236 |
Dec 1954 |
GB |
Patent Grant
4165949
