Supersonic Centrifugal Compressor

Abstract

The fixed portion of the rotor casing defines with the disc extension or a rotary disc carrying radially extending blades an annular deceleration space leading into a diffuser whose fixed vanes define passages inclined to the radial direction.

Parent Case Text

This is a Continuation, of application Ser. No. 104,876, filed Jan. 8, 1971, now abandoned.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a supersonic centrifugal compressor providing a high compression ratio and increased output.

2. Description of the Prior Art

It has long been known that a fairly high compression ratio may be obtained in a centrifugal compressor by giving to the fluid under compression a high rotational speed and then transforming the thus obtained kinetic energy into pressure.

It has already been proposed to transform this kinetic energy into pressure, very simply, by using a rotating diffuser whose profile is obtained by extending the radii of the rotating blade support disc beyond the blades. The expansion of the fluid between two walls both moving at speeds close to that of the fluid then takes place with a fairly high efficiency.

Such devices, however, cannot be used when a high compression ratio, about 5 for example, is required with a single compressor stage. For this, the peripheral speed of rotation of the rotor blades must be as high as 400 or 500 meters per second, requiring even higher speeds at the periphery of the rotating diffuser. Centrifugal stresses then become high enough to deform the blade support discs. To these mechanical stresses are added others particularly brought about by thermal expansion of the compressor elements.

It has subsequently been proposed to replace the rotary diffuser with a fixed diffuser with two walls prolonging the rotating blade support discs and whose radii are limited in this case to those of the blades. The supersonic movement of the fluid over the fixed walls, particularly the downstream wall on which the pressure is generally higher because of the curved path followed by the fluid, leads to losses in efficiency significantly reducing the available increase in pressure ratio.

It has also been proposed to mount a diffuser arranged to receive the fluid at supersonic speeds and having a convergent or convergentdivergent profile downstream of the rotor.

These various proposals have not, however, provided a significant increase in the available compression ratio in a single compressor stage, and certain of them bring about output instabilities or mechanical difficulties.

SUMMARY OF THE INVENTION

The invention is intended to provide an imrpoved supersonic centrifugal compressor.

In accordance with the invention, a supersonic centrifugal compressor has a rotor with substantially radially extending blades mounted on the face of a profiled support disc, the external diameter of the disc exceeding that of the blades to form a disc extension. A fixed portion of the rotor casing has an internal surface so arranged that an annular deceleration space is formed between that surface and the disc extension, this deceleration space leading into a diffuser with fixed vanes defining passages inclined to the radial direction.

Using the invention, it is possible to reduce the disadvantages of the previously proposed compressors and to provide a high compression ratio with a high and stable output.

The invention exploits the discovery that in providing a higher deceleration than that obtained with an equivalent rotary diffuser in a space situated beyond the periphery of the rotor, it is possible not only to eliminate the mechanical difficulties of providing a rotor with two mobile blade support discs, but also to combine the compressor with a supersonic diffuser at whose entry the fluid is moving at a speed only slightly supersonic such that a system of weak shock waves is obtained. Thus, in the diffuser, the efficiency of the transformation of kinetic energy into pressure is increased, thereby increasing the overall efficiency.

According to the invention, this deceleration is obtained between the fixed wall of the disc casing and the extension of the blade support disc.

The rotor casing is suitably adapted to define a fluid inlet, a rotor shroud conforming to the blade profile, said fixed portion of the rotor casing, and opposite walls of the diffuser supporting fixed vanes thereof.

The diffuser preferably provides divergent passages generating substantially plane re-compression waves.

The leading edges of the fixed vanes of the diffuser are suitably thin to be better adapted to supersonic operation.

The faces between which is defined the annular deceleration space, namely that of the fixed portion of the rotor casing and that of the blade disc extension, are preferably substantially parallel and provide a significant deceleration of the supersonic flow between their inside and outside diameters.

It should be noted that this deceleration, as opposed to the case with previously proposed techniques, is obtained with friction losses reduced to a minimum and with lowered mechanical stresses. Because of the form of the passage within which the fluid is accelerated by the rotor, the fluid in the rotary diffuser exerts a higher pressure on the moving blade support disc which is moving at a relatively slow speed than on the upstream wall which is fixed and with respect to which the fluid is moving at high speed. It appears that this property of the compressor significantly increases the output and is an important contribution to the very high pressure ratios obtained.

Moreover, it is known that the upstream wall when rotating at high speeds undergoes important deformations because of its shape.

In the compressor provided by the invention, this wall is fixed and only experiences thermal stresses which are easily mastered by one skilled in the art.

The diffuser thus receives a flow which is sufficiently decelerated that its speed is only slightly supersonic and it then becomes possible to set up a system of weak shockwaves and to obgain in the fixed diffuser and efficiency approaching unity.

The combination of the compressor rotor and diffuser is such that the increase in output obtained compensates the friction losses in the space between the moving blades of the rotor and the fixed blades of the diffuser. The stability of the flow is increased and the rotor comprises only a single disc of increased radius so that better mechanical strength is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying diagrammatic drawing, in which:

FIG. 1 is an axial section through the single stage supersonic centrifugal compressor of the present invention, showing only that part of the compressor above its rotor axis.

FIG. 2 is a section through the compressor of FIG. 1 taken perpendicularly to its axis of rotation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the present example, the compressor serves for compressing air and has a compression ratio of the order of 9.

Referring to the Figures, the compressor has a single rotor 1 with radially extending blades 2 mounted on the face of a profiled support disc 3. The external diameter of the disc 3 exceeds that of the blades 2 to form a disc extension 3'.

The rotor shroud 5 is shaped to define a fluid inlet 11 and a rotor shroud 5" conforming to the profile of the blades 2. The internal surface 5.sub.1 of this rotor shroud 5" lies very close to the forward faces of the blades 2, as is clearly seen in FIG. 1. This profile is that of a cone with curved generatrix.

The rotor casing continues to define a fixed wall portion facing the disc extension 3', an annular deceleration space 4 being defined between this fixed wall and the forward face of the extension 3' which, in use of the compressor, moves at a speed approaching that of the air under compression. This deceleration space 4 leads into a diffuser 6 with fixed blades 9 supported between parallel annular walls formed by sections of the rotor casing.

As is seen clearly in FIG. 2, th diffuser 6 has six diverging passages 7 defined by the fixed vanes 9 and arranged at an angle to the radial direction. At the diffuser entry, the air under compression is travelling at supersonic speed.

The distance between the rim of the profiled support disc 3 and the leading edges 9' of the fixed blades 9 of the diffuser 6 is made very small, and is chosen to take into account the radial expansion of the disc when turning at supersonic speed.

The rotor is keyed to a shaft shown schematically by its axis 10.

The internal face 5.sub.1 of the rotor shroud 5 is made as small as practicable so as to avoid loss of speed by the air. Where this face extends beyond the blades 2, to define the disc extension 3' the annular deceleration space 4, it runs substantially parallel to the front face of the extension 3'.

Where a particularly high output is required, any suitable form of flow distributor may be mounted upstream of the rotor so as to reduce the relative speed of the air at the fluid entry 11. When the rotor is turning at 33,000 r.p.m., the rotary speed of the blade periphery corresponds to a Mach number in the region of 1.3. The airspeed at the outlet from the blades 2 is then of the order of Mach 1.2.

In the deceleration space 4 the airspeed decreases to a value less than Mach 1.1 by the time it reaches the leading edge 9' of the fixed vanes 9 of the diffuser 6. The entry losses are negligible.

The air enters the diffuser 6 at a speed which is only slightly supersonic and accelerates gradually to provide a recompression wave, when it decelerates inside the diffuser because of its now subsonic speed.

The overall efficiency of a compressor stage such as just described may reach some 80 per cent.

It will be appreciated that a number of compressor stages such as just described may be combined to provide a multi-stage compressor.

In the compressor just described, because of the incurving form of the internal surface 5.sub.1 of the rotor shroud, the air exerts on the extension 3' a greater pressure than on the fixed wall of the rotor casing. It appears that this property of the compressor provides a significant increase in the output and has an important effect on the provision of a high compression ratio.

In the previously proposed rotary diffusers, the upstream diffuser wall suffers significant deformations because of its form, in the compressor just described this wall is fixed and so is subject only to thermal deformations which are more readily compensated.

In the compressor just described, the diffuser receives a fluid flow which is sufficiently decelerated that its speed is only slightly supersonic so that it is possible to obtain a system of weak shockwaves and consequently a diffuser efficiency approaching unity. Using the invention, the increased efficiency obtained largely compensates the friction losses encountered in the space between the moving blades of the rotor and the fixed blades of the diffuser. The stability of flow is improved Moreover, since the rotor features only a single profiled support disc, it presents a better mechanical resistance.

Claims

We claim:

1. A supersonic centrifugal compressor comprising:

a rotor casing,

a rotor mounted for rotation about its axis within said casing,

said casing and said rotor having spaced opposed, similarly curved surfaces defining therebetween a gas flow path which changes from axial to radial,

a plurality of circumferentially spaced blades carried by said rotor and extending outwardly of said rotor surface from the vicinity of the compressor axis throughout that portion of the flow path which changes from axial to radial,

said rotor surface being formed at its radially outboard end by a single radial disc whose periphery extends radially beyond the outboard edges of said blades to form a disc extension,

said rotor casing surface extending in a radial direction substantially parallel to said disc extension to form a gas deceleration chamber with said disc extension,

a diffuser chamber having fixed vanes defining passages leading from said deceleration chamber and inclined to the radial direction, and

said fixed vanes of said diffuser chamber extending outwardly from a point on said casing generally adjacent the periphery of said disc extension.

2. The compressor as claimed in claim 1, wherein said casing includes axially spaced fixed walls to define said diffuser chamber and to support said fixed diffuser vanes therebetween.

3. The compressor as claimed in claim 1, wherein the diffuser passages are divergent.

4. The compressor as claimed in claim 2, wherein the diffuser passages are divergent.

5. The compressor as claimed in claim 1, wherein the leading edges of the diffuser vanes have sharp edges for supersonic operation with low losses.

6. The compressor as claimed in claim 2, wherein the leading edges of the diffuser vanes have sharp edges for supersonic operation with low losses.

7. The compressor as claimed in claim 3, wherein the leading edges of the diffuser vanes have sharp edges for supersonic operation with los losses.

8. The compressor as claimed in claim 4, wherein the leading edges of the diffuser vanes have sharp edges for supersonic operation with low losses.