Pipe Diffuser With Auxiliary Bleed System

Abstract

The disclosure illustrates a "pipe" diffuser having basic flow passages sized to handle the output requirements of a centrifugal compressor without a bleed requirement. The diffuser has a series of auxiliary bleed passages intersecting the inlet portions of the pipe diffuser passages. The bleed passages are sized to handle a supplementary bleed flow requirement when needed.

Description

The present invention relates to compressors and more particularly to centrifugal compressors.

In recent years the so-called "pipe" diffuser has been proposed for use with centrifugal compressors designed to discharge air at supersonic velocities. This type of diffuser is described in U.S. Pat. No. 3,333,762 in the name of J. C. Vrana. Briefly, the pipe diffuser comprises a series of circular passages drilled in an annular member surrounding a centrifugal impeller. The passages are oriented so that they intersect adjacent the periphery of the impeller to produce a series of scalloped leading edges to the inlets of the passages. This permits greater stability in the handling of transonic and supersonic flow discharged from the impeller. The reason for this is that the scalloped leading edges conform better to the velocity distribution of the flow than a straight leading edge of a normal diffuser. The net result is a centrifugal compressor with an improved efficiency capability at higher pressure ratios.

When such a diffuser is incorporated in a centrifugal compressor used in an auxiliary power unit (APU), certain problems may arise. Generally speaking, the output requirements of APU's range between several modes of operation which demand widely varying compressor performance. At one extreme the compressor bleed is closed and the APU engine produces maximum power output, for example, to drive an alternator. At the other extreme there is a no-load condition and a substantial amount of air is bled from the compressor for cabin pressurization and other uses. Differences in airflow requirements at these two operating conditions make it difficult to size the flow passages of the compressor and the diffuser to operate efficiently under the maximum power condition and the maximum bleed condition.

If the throat area of the diffuser is sized to provide adequate surge margin for the compressor at the maximum power output condition (minimum airflow), the diffuser may choke when the maximum bleed flow is called for. This is because the flow through the diffuser is greater than the throat area can handle at the chocking condition. This choking of the diffuser causes high losses and poor efficiency for operation in bleed mode.

Therefore it is an object of the present invention to provide a centrifugal compressor that operates at high efficiency over a wide range of pressure ratios and flow rates.

These ends are achieved by a pipe diffuser having flow passages intersecting to form scalloped inlet edges receiving flow from a centrifugal impeller. A series of auxiliary bleed passages extend from the diffuser passages to receive a portion of the air stream flowing through the diffuser. A means is provided to selectively permit or block flow through the bleed passages so that the effective throat area of the diffuser is varied.

The above and other related objects and features of the present invention will be apparent from a reading of the description of the disclosure shown in the accompanying drawings and the novelty thereof pointed out in the appended claims.

In the drawings:

FIG. 1 is a sectional fragmentary view of a centrifugal compressor embodying the present invention;

FIG. 2 is a view of the diffuser of FIG. 1 taken on line 2--2 of FIG. 1;

FIG. 3 is an alternate embodiment of the compressor diffuser shown in FIGS. 1 and 2;

FIG. 4 is a compressor MAP for the centrifugal compressor of FIG. 1.

Referring to FIG. 1 there is shown a centrifugal compressor, generally indicated by reference character 10, which incorporates a rotatable impeller 12 having a series of radial blades 14. An annular diffuser housing 16 has an inner diameter 20 closely adjacent the periphery 18 of impeller 12. A series of generally tangentially directed diffuser passageways 22 are formed around the periphery of diffuser housing 16. These passages each comprise inlet portions 24, a throat section 25 and a conical outlet portion 26. The longitudinal axes D of the passages 22 are oriented so that they are all tangent to a reference circle, herein shown as the periphery 18 of impeller 12. The inlet portions 24 of passages 22 intersect to form a series of scalloped shaped entrance edges 30, as particularly shown in FIG. 2.

A series of bleed passages 32 are formed in the diffuser housing 16 adjacent the diffuser passages 22. The passages 32 each comprise an inlet portion 34, a throat 35 and a generally conical outlet portion 36. The inlet portions 34 intersect the inlet portions 24 of passages 22 to form elliptically shaped inlets 38 (see FIG. 2). The outlet portions of the passages 32 connect through branch conduits 40 to a bleed manifold 42 having a suitably actuated flow control valve 44 in an overboard supply conduit 46. As seen in FIG. 1, the axes D of passages 22 and axes B of passages 32 intersect at an angle .theta. which is large enough to permit a separate bleed flow collecting conduit arrangement around the periphery of diffuser housing 16. However, .theta. is small enough so that the static pressure recovery in the auxiliary bleed passages 32 are of the same order as those of the main passages 22.

During operation of the compressor 10 the impeller 12 rotates at a high rate of speed which causes air to be discharged generally tangentially from the tips of blades 14 at a high velocity. The air thus discharged enters the diffuser passages 22 where it is de-accelerated to a subsonic level in the inlet portions 24 for passage through throats 25 and subsonic diffusion in the outlet portions 26. The flow areas of throats 25 are sized to provide an effective flow in impeller 12 for a no-bleed condition that permits a maximum efficiency.

During this state, as shown on FIG. 4, the compressor operating line C is as shown in relation to the surge line of the compressor. This line is at a relatively high level of efficiency, as seen by the constant speed operating line C.sub.s. It should be noted that the compressor operates well upon the horizontal portion of line C.sub.s so that it is away from a choked condition.

In the absence of the bleed system comprising passages 32, a condition where air would be bled downstream of diffuser passages 22 causes the compressor to operate on the lower portion of line C.sub.s and into the choked region. This greatly lowers the overall efficiency of the compressor 10.

However, when the bleed flow is demanded for compressor 10, valve 44 is opened, thus permitting flow into bleed passages 32. The flow areas of passages 32 are sized to efficiently handle the bleed open requirements of the compressor 10. This increases the effective throat area of the diffuser and thereby moves the operating lines to line 0 and the constant speed line to 0.sub.s on FIG. 4.

It can be seen that the compressor still operates at an efficient level well away from a choked condition. As a result, the compressor 10 efficiently handles a wide range of flow rates without the need for complicated and expensive variable passage geometry.

When bleed is no longer called for, valve 44 is closed to block off passages 32 and the diffuser functions in a normal fashion.

The bleed passages 32 may be positioned so that their center lines B lie in the same plane as the center lines D of the diffuser passages (see FIG. 2) or they may be positioned at an obtuse angle relative to this plane. This modification is shown in FIG. 3 where passages 32' have their longitudinal axes B' positioned at angle with respect to the plane containing the longitudinal axes D' of passages 22'. This permits an axial shifting of the outlets of the two systems and reduces the mechanical interference between them.

While a preferred embodiment of the present invention has been described, it should be apparent to those skilled in the art that other modifications may be performed with equivalent results without departing from the spirit and scope of the present invention.

Claims

Having thus described the invention what is claimed as novel and desired to be secured by Letters Patent of the United States is:

1. A compressor assembly comprising:

a rotatable bladed centrifugal impeller;

an annular diffuser housing surrounding the periphery of said impeller, said diffuser housing having a plurality of generally tangentially directed primary diffuser passages intersecting adjacent the periphery of said impeller to form a series of generally scalloped inlet edges to said diffuser passages, said diffuser having a series of auxiliary bleed passages extending from said diffuser passages; the intersection between them being forward of the primary passage throat; and

means for selectively permitting or blocking flow through said bleed passages whereby the effective total throat area of said diffuser is varied.

2. A compressor assembly as in claim 1 wherein the longitudinal axes of the diffuser and bleed passages intersect at an angle sufficiently small so that the static pressure recovery potential in the secondary bleed passages is of the same order as in the primary passages.

3. A compressor assembly as in claim 1 having predetermined flow requirements for bleed closed and bleed open conditions and wherein:

said primary diffuser passageways have a flow area to provide maximum efficiency for a bleed closed condition;

said auxiliary bleed passages have a flow area sized to provide maximum efficiency for a bleed open condition.

4. A compressor assembly as in claim 1 wherein the longitudinal axes of said diffuser passages substantially lie in a given plane and wherein the longitudinal axes of the bleed passages lie in the same plane and in between adjacent diffuser passages.

5. A compressor assembly as in claim 1 wherein the longitudinal axes of said diffuser passages lie in a given plane and the longitudinal axes of said bleed passageways extend from said diffuser passageways at an oblique angle relative to said plane.