GB2131100A - Diffuser - Google Patents

Abstract

A compact diffuser for high- power gas turbines is constituted by an initial substantially axial diffusion stage 5 defined by two conical coaxial walls 6, 7 and followed by a double curved diffuser with three walls 8, 9, 10, of which the intermediate wall 9 is cantilever-supported by a set of aerodynamically profiled ribs 11 disposed with their generating lines substantially parallel to the axis of the turbine and situated nearly at the end of the double curved diffuser. The diffuser is connected to the turbine exhaust casing by an envelope internal to the exhaust casing and having its contours smoothly joined to the diffuser. <IMAGE>

Description

SPECIFICATION Diffuser This invention relates to a diffuser, in particular to a diffuser suitable for high-power gas turbines (exceeding 10,000 kW) which enables very high diffusion efficiences to be obtained for small overall axial and radial dimensions, which do not exceed those allowable in current designs which are dictated by transportation considerations. In particular, the invention relates to a diffuser in respect of which, because of the high diffusion efficiency obtainable and the consequent lower exhaust gas velocity, a considerable reduction in vibration and noise is attained, thus making the construction of the exhaust silencer easier, with consequent reduced costs and bulk.

The diffusers most frequently used in power gas turbines are known to be derived from those designed and arranged for aeronautical turbines in which a small overall radial dimension is essential, and in which the diffusion duct must be traversed by aerodynamically profiled double-wall ribs which are cooled in the interspace with cold gas, and which support the shaft bearing which would otherwise not be reachable. These diffusers consists of two coaxial conical walls with an angle of about 7" between the cones.In this respect, a diffuser of this type has its maximum efficiency under conditions of the best compromise between the friction loss at the two walls (which is dependent on length for equal surface finishes and is thus smaller the shorter the diffuser) and the diffusion turbulence losses (which are smaller the more gradual the diffusion and thus the longer the diffuser). It has been found experimentally that the optimum compromise in length (dependent on degree of finish, velocity, etc.,) corresponds as a first approximation to an angle between the cones of about 7' for two-wall diffusers of prevalently axial extension.

On the other hand,the gas is still at high velocity when leaving the diffuser, and its energy is therefore lost, but as the exhaust is axial,and taking into account the aeronautical compromise between weight, overall size and efficiency, this loss is acceptable.

Land-based turbines, which derive from aeronautical experience, use similar diffusers, the only difference being that at the end of the diffuser the gas is made to curve in the radial direction due to the fact that the exhaust is radial in land-based turbines. In order to curve the gas with smaller losses and smaller radii, arrays of deflectors are often arranged in the bend, these deflectors having a cross-section in the form of parallel circular arcs. Gas diffusion is considered finished at the end of the conical portion, and the deflectors serve only to reduce the pressure drop through the bend, and not for diffusion purposes.

This type of diffuser does not exploit the larger range of alternatives offered by land installation rather than aeronautical installation, in that: (a) It retains the bearing support ribs at the diffuser inlet where the gas is of considerable velocity, so leading to a certain loss which becomes much greater if the turbine has to operate under other than design conditions, because in such a case the loss caused by the reduction in cross-section during passage through the ribs is supplemented by the loss caused by the impact of the gas against these latter, this impact occurring at an angle of incidence which is more removed from the optimum angle the more the operation deviates from the design point (in the case of land-based turbines, it is not unusual to have to operate at 50% of the initial design speed).

In an aeronautical turbine, the ribs are essential for overall size and weight reasons. In a land based turbine, the bearing could instead be supported from the outside if certain mechanical problems related to the shaft line are solved.

(b) It does not reduce the exhaust gas velocity to a minimum without worsening the efficiency and noise level.

One type of diffuser which is beginning to be adopted in land-based turbines is characterised by the elimination of these ribs in an attempt to improve diffusion in the final bend.

The ribs are eliminated by supporting the bearing from the outside, given that the exhaust is no longer axial, and the bend is made in the form of a truly curved diffuser which is much more complicated than a straight diffuser but which by careful design and experimental setting-up can attain a further worthwhile recovery.

In order to further improve this type of diffuser, it is necessary either to increase the axial conical portion so as to arrive at the bend with a greater diffusion ratio, which however causes an intolerable increase in the axial turbine length, or to dispose an intermediate wall in this portion so as to double the diffusion angle. This idea has been followed in particular by those manufacturers who retain the bearing support ribs so as to also support the intermediate wall by these latter. However, this design gives only significant results for obvious reasons.In this respect, by retaining the ribs, all the aforesaid losses under working conditions other than the design condition still occur, and in addition because of the aforesaid balance between friction losses and diffusion losses, the introduction of the double wall into the zone in which the gas is still at high speed leads to an increase in losses due to friction and entry impact,which strongly reduce the theoretical advantages of the increased diffusion.

A second idea would be to increase the curved diffuser portion, but this would lead to an increased overall radial dimension which in the case of large turbines is even less tolera ble, since it gives rise to transport problems, etc.

The object of the present invention is to obviate these size problems, and to obtain considerably improved diffusion and thus increased turbine efficiency, with decreased ex haust noise.

According to the present invention, there is provided a diffuser for a gas turbine, compris ing (a) an initial diffusion stage which is free from ribs and/or intermediate walls, which is of substantially axial path, and which is defined by two prevalently conical coaxial walls; and (b) a subsequent diffusion stage in the form of a double curved diffuser which is defined by three walls of which the intermediate curved wall is cantilever-supported by a set of ribs which have their generating lines substantially parallel to the axis of the turbine and which are situated near the end of the double curved diffuser.

Thus, the initial diffusion stage comprises two prevalently conical walls, and extends in a direction which forms a certain angle with the axis so as to better present itself to the bend.

This initial diffusion stage, which is free from ribs and intermediate walls and operates under optimum conditions, forms the most important part of the diffusion process, and is followed by the double curved diffuser comprising three walls, which allows optimum final diffusion in the bend, within the available overall dimensions. The intermediate wall, which enables the double diffuser to be formed, is cantilever-supported by a set of aerodynamically profiled ribs disposed at the final part of the diffuser where the gas has almost completely diffused to a velocity which is so low as not to create appreciable losses.

This arrangement, which enables the initial part of the intermediate wall to be cantileversupported, is made possible by the rigidity which the intermediate wall possesses by virtue of its curvature. The initial part of the intermediate wall may be machined so as to provide it with an aerodynamic profile suitable for dividing the stream which arrives from the initial diffusion stage into two streams without any impacts or sudden cross-section reductions being undergone, and in addition that part which has the largest extent of cantilever may be thinned down to a profile which is almost the optimum in eliminating the vibration modes at the various frequencies encountered under the different running conditions.

At high velocities, losses due to impacts and friction are very large (they increase in a square relationship) and this has determined the elimination of the ribs and the choice of a diffuser comprising only two walls in the initial stage.

In the curved portion, the sufficiently deuce! erated gas has a greater need for guidance (since diffusion through a bend is much more complicated). For this reason, the use of the ribless intermediate wall in the initial portion allows operation as two parallel curved diffus- ers with nearly double angles of diffusion, thus enabling the gas to enter the exhaust chamber at a velocity almost one half that obtainable with the final conventional curved diffuser.

The final ribs which support the intermedi ate wall are also designed and angularly dis posed in such a manner as to also perform an aerodynamically advantageous function. X pious, by creating a controlled final cross-section reduction (when the gas has almost completed its expansion), the uniformity of its circumfer ential outlet velocity is decisively improved.

In this respect, because the gas leaves the exhaust duct -radially, there is large disunifor mity in the paths of the exit stream threads, and in the absence of the ribs this can lead to an outlet velocity from the curved diffuser portion into the exhaust chamber which is several times greater in the part close to the exhaust mouth than in the diametrically oppo site part, and considering that pressure drop and noise vary with the square of the velocity, it can be understood that this sucking action of the exhaust mouth can have a highly negative influence.

This explains the apparently contradictory fact than an increase in efficiency can be obtained by introducing the ribs (i.e. ob stacles). This is because by causing a cross section reduction at the outlet of the curved portions, the ribs cause the gas to distribute uniformly along the outlet circumference by masking the sucking action of the radial ex haust mouth, this action being non-symmeti cal about the axis. The gas leaves almost perfectly distributed circumferentially, and blg providing a suitably shaped duct inside the exhaust chamber which conveys it in an or dered manner towards the outlet, it reaches the final silencer at a very low velocity, with practically no pressure pulsation, and thus with minimum aerodynamic noise.

This final outlet arrangement is important, because in many diffusers a large fraction of the pressure recovery which has been diffi culty attained in the diffuser is destroyed in the exhaust chamber in the form of a pressure drop. The effect is therefore an increase in turbine efficiency and a considerable reduc tion in the noise level of the exhaust gas which is known to represent one of the drawbacks most difficult to eliminate in band- based turbines (which require large and coo silencers which, given that their operating temperatures exceeds 450"C, have a short life).

Experimental tests have fully confirmed 'i.;1 phenomenon, and in fact the final noise re- duction is an indirect and immediate nlsas;o of the improved efficiency as the various para meters (number of ribs, initial profile, difference in curvatures and ratios) vary. Again experimentally, it has been shown that within the overall allowable dimensions, the concept of multiplying the walls in the curved diffusion portion cannot be further extended because if a second intermediate wall is provided, the consequent friction losses balance-out the improvements. If more are added, the efficiency begins to worsen.

The invention will now be described with reference to the accompanying drawings, which illustrate a preferred practical embodiment by way of non-limiting example only, in that technical and constructional modifications can be made thereto. In the drawings: Figure 1 is a partial longitudinal section through a power gas turbine incorporating a diffuser according to the invention; Figure 2 is a partial longitudinal section through the diffuser shown in Fig. 1, to a larger scale; and Figure 3 is a partial longitudinal section through a detail fo the diffuser of Fig. 2, to a much larger scale.

Referring to the Figs, there is shown a gas generator 1 for a gas turbine, which generator 1 feeds a power turbine 2 whose exhaust gas is conveyed into an exhaust casing 3 via a diffuser 4.

The diffuser 4 consists of an initial diffusion stage 5 of substantially axial extension defined by two prevalently conical, coaxial walls 6 and 7. The stage 5, which is inclined at an angle (see Fig. 2) to the horizontal in order to better present the gas stream to the bend, performs the most important part of the diffusion, and attains it in an optimum manner as it has no ribs or intermediate walls.

The stage 5 is followed by a subsequent diffusion stage in the form of a double curved diffuser comprising three walls 8,9 and 10, which completes the diffusion of the gas, now separated into two independent streams, and at the same time causes it to curve radially.

The intermediate curved wall 9 which enables the double diffuser to be formed is cantilever-supported by a set of aerodynamically profiled ribs 11 which have their generating lines parallel to the horizontal axis of the machine, and are arranged almost at the end of the double curved diffuser. The ribs 11 are dimensioned in such a manner as to create in the double diffuser a controlled cross-section reduction at the outlet from the curved portions, in order to obtain uniform circumferential distribution of the outlet velocity of the gas is it enters the exhaust causing 3, and thus substantially attenuate the non-symmetrical sucking action on the gas by mouth 1 2 of the exhaust casing 3. In order not to disturb the uniformity of the circumferential outflow of the gas from the diffuser and thus favour ordered gas flow towards the exhaust mouth 12, the diffuser 4 is connected to the exhaust casing 3 by an envelope 1 3 which has its contours smoothly joined to the diffuser and is contained in the exhaust casing itself.

Finally, the intermediate curved wall 9 commences with a portion 9' which is aerodynamically machined and profiled (see specifically Fig. 3) in such a manner as to divide the gas stream, which arrives from the initial stage of the diffuser at an already reduced velocity, into two streams without them under-going impact or sudden variations in cross-section.

Claims (5)

1. A diffuser for a gas turbine, comprising (a) an initial diffusion stage which is free from ribs and/or intermediate walls, which is of substantially axial path, and which is defined by two prevalently conical coaxial walls; and (b) a subsequent diffusion stage in the form of a double curved diffuser which is defined by three walls of which the intermediate curved wall is cantilever-supported by a set of ribs which have their generating lines substantially parallel to the axis of the turbine and which are situated near the end of the double curved diffuser.

2. A diffuser as claimed in claim 1, wherein the intermediate curved wall of the double curved diffuser commences with a portion which is aerodynamically machined and profiled in a manner such as in use to divide the gas stream in the diffuser without the stream undergoing impact or sudden variation in cross-section.

3. A diffuser as claimed in claim 1 or 2, wherein the ribs are aerodynamically profiled and dimensioned in a manner such as to create a controlled reduction in cross-section at the outlet from the curved portions of the diffuser.

4. A diffuser as claimed in any of claims 1 to 3, when connected to a turbine exhaust casing by means of an envelope which has its contours smoothly joined to the diffuser and which is contained in the exhaust casing itself.

5. A diffuser for a gas turbine, substantially as described and illustrated herein.