Compressor inlet control ring

Abstract

A centrifugal compressor includes a stationary outer shroud and a centrifugal rotor with an inner hub wall located radially inwardly of the stationary shroud to define an axial inlet and a radial outlet. A plurality of centrifugal blades located on the hub wall define a plurality of circumferentially spaced flow passages for compression of working fluid drawn from the axial inlet for discharge through the radial outlet. Means are included at the inlet end of the stationary shroud to direct additional flow into the flow passages between each of the centrifugal blades on the rotor to relieve partial vacuum conditions existing between the stationary shroud wall and the rotor tips at the inlet end thereof to improve the rotor exit pressure and velocity gradient thereby to increase efficiency of a following diffuser located downstream of the radial outlet of the compressor.

Description

This invention relates to centrifugal compressors and more particularly to means in association with such compressors for controlling the pressure build-up therein.

In the design of centrifugal or mix flow compressors there are two principle objectives. The first is to achieve a certain flow rate at a predetermined pressure ratio to maximize efficiency at various operating speeds. The second objective is to achieve a broad flow range from the inlet to the outlet of the compressor rotor at any speed to provide satisfactory operation during transient compressor conditions.

In accordance with these objectives, it is assumed that there is an ideal uniform flow from the inlet through the outlet of the compressor. Other parameters such as inlet and outlet areas, diameter, pump and shroud contour, blade number distribution and blade thickness are designed to obtain a proper balance between preformance, the design material of the compressor components and manufacturing requirements.

However, under real environmental conditions there are viscous losses, compressibility build-up within the flow passages of the compressor rotor and boundary layer effects that cause a compressed working fluid to be distorted from the inlet to the outlet of the compressor thereby resulting in non-uniform pressure and velocity gradients at different points in a working flow passage through the rotor of the compressor.

Accordingly, an object of the present invention is to improve the uniformity of compressed fluid at an exit passage of a centrifugal rotor to obtain overall higher efficiency and broader flow range through the compressor.

Still another object of the present invention is to provide means for maintaining a uniform flow distribution at the inlet of a centrifugal compressor rotor so as to improve the uniformity of the compressed fluid at the radial outlet of a centrifugal compressor to maintain higher efficiency and broader flow range therethrough.

Yet another object of the present invention is to provide an improved centrifugal type compressor including a rotor and blade passages wherein a partial vacuum is produced between a stationary shroud wall and tips of blades defining the blade passages wherein means are provided to selectively direct an additional flow of ambient air into the inlet of the compressor to produce an increase in flow range between the inner shroud wall and the blade tips thereby to improve the rotor exit pressure and velocity gradients.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown.

In the Drawings:

FIG. 1 is a vertical sectional view of a centrifugal compressor shown in association with a downstream diffuser;

FIG. 2 is a fragmentary sectional view taken along the line 2--2 of FIG. 1 looking in the direction of the arrows;

FIG. 3 is a fragmentary vertical sectional view of a centrifugal compressor including another embodiment of the invention;

FIG. 4 is a graph showing the pressure profile across a flow passage in the compressor of FIG. 1 at select points thereon;

FIG. 5 is a graph of traverse points taken to indicate air angle across the width of a flow passage at select points in the flow passages of the compressor in FIG. 1;

FIG. 6 is a graph showing the pressure ratio between the shroud wall static pressure and the inlet pressure to the compressor of FIG. 1; and

FIG. 7 is a graph showing air flow characteristics and pressure ratios at various rotor speeds of a typical centrifugal compressor without the present invention.

Referring now to the drawings, in FIG. 1 a centrifugal compressor 10 is illustrated including a rotor 12 and a stationary shroud 14 for directing compressed fluid to a vane or vaneless type diffusers 16. Working fluid is introduced from an inlet 18 and compressed and pushed out by a combination of centrifugal and lift forces toward a radial annular exit passage 20 leading to the diffuser 16.

The rotor 12 more particularly includes a hub 22 having a central opening 24 through which a drive shaft is directed and connected to the rotor 12 for driving it at selected speeds of rotation. The hub 22 includes a radially outwardly directed rear wall 26 and an axial inlet surface 28 thereon joined through an annular curved surface 30 to a radially outwardly directed inner surface 32.

The rotor 12 includes a plurality of circumferentially located blades 34 thereon each including a root 36 thereof secured to the surfaces 28, 30, 32. Each of the blades 34 further includes an outer radial tip 38 which is curved from an axial direction to a radial direction as best seen in FIG. 1. As seen in FIG. 2, each of the blades 34 has a curvature or camber from a leading edge 40 directed generally perpendicularly with respect to the hub surface 28 at the inlet 18. Each blade 34 further includes a trailing edge 42 that is directed generally perpendicularly to the surface 32 adjacent the radial outlet 20. The stationary shroud 14 includes an annular, inwardly converging inlet lip 43 thereon that is joined to an axial wall surface 44 of the shroud 14 that overlies the rotor surface 28 in spaced parallelism therewith and in closely spaced relationship with the rotor blade tips 38. The inner wall surface of stationary shroud 14 is then directed radially outwardly at 46 in spaced relationship with the radially outwardly directed hub wall 26 and closely adjacent blade tips 38.

The inner wall surfaces 44, 46 of the stationary shroud 14 and the hub wall surfaces 28, 30, 32 along with the individual rotor blades 34 define a plurality of enclosed flow passages 48 each being opened at opposite ends thereof to the inlt 18 and the outlet passage 20.

Compressors of this type, under real environmental conditions, have viscous fluid losses and compressibility effects as well as boundary layer effects therein which can cause a compressed fluid to be distorted from the inlet 18 to the outlet 20 as the working fluid passes through passages 48.

As shown in FIG. 1, a plurality of pressure taps a through p directed through the stationary shroud 14 are utilized to demonstrate the degree of this non-uniform flow condition. As shown in FIG. 4, the pressure tap located at 20 and pressure traverses from the inner surface 46 of the shroud 14 to the hub surfaces 30, 32 at different flow rates D-G results in a plurality of pressure gradient curves represented by the family of curves 50. They show a reduced pressure in the vicinity of the interface between the shroud surface 46 and the blade tips 38 and an increased pressure toward the midpoint of the passages 48 for each rate D-G. The pressure is reduced toward the hub wall with this pressure still exceeding the pressure at the stationary shroud wall. As shown in FIG. 5, angle probes located at 20 produce indicated air angle from the shroud surface 46 toward the hub wall as shown by the family of curves 52. The angle increases from the region between the inner surface 46 and the blade tip 38 in the direction of the hub wall.

Furthermore, as is depicted in FIG. 6 by the family of curves 54, the pressure ratio between the shroud wall static pressure and the inlet pressure at 18 at different points along the camber line distance of the blades 34 increases from a minimum at the inlet edge of the passages 48 to a maximum at the outlet thereof.

Reference line 56 in FIG. 6 indicates ambient pressure. The data points were measured at a rotor speed of 44,000 rpm at the inlet end of the blades at a temperature of 784.degree.F. At this speed of operation, representative of compressor speeds found in turbine engines, it can be seen that for a range of flow rates represented by the family of curves 54 that there will be a negative pressure at the static pressure taps a through p through a substantial operating speed range. This discontinuity in the stream tube through the passages 48 will produce an equivalent total pressure and indicated air angle which is much lower at the outlet of the shroud wall 46 compared with the rest of the outlet opening 20 at the trailing edge 42 of each of the blades 34. The measurements also indicate that a smaller portion of the flow is passed through the stream tube at points close by the shroud surfaces 44, 46.

As best shown in FIG. 2, a plurality of slanted ports 58 shaped as parallelograms are provided in the stationary shroud 14 immediately downstream of the lip 43 thereof. Each of the ports 58 are located where the static pressure ratio is always less than unity as shown in the graph of FIG. 6. A control ring overlies the slanted ports 58 and includes a plurality of slanted ports 62 therein shaped like ports 58. Rotation of the ring 60 on the outer surface of the shroud 14 will cause ports 62 to overlap ports 58 to a greater or lesser degree to produce a variable control of communication between ambient air and the inner wall surface 44 of the shroud 14 at locations where the partial vacuum conditions exists as shown in FIG. 6. As a result, a controlled amount of additional flow will be sucked into the rotor stream at the shroud surface 46 due to pressure differential between atmosphere and the partial vacuum condition. This extra introduction of flow will increase the flow range of a given rotor and will also improve the rotor exit pressure and velocity gradient.

Because of this improved pressure profile and velocity gradient at the outlet passageway 20, the following diffuser 16 will have increased efficiency by virtue of the improvement of the entrance flow uniformity thereto.

The improved flow characteristics are set forth in the graph of FIG. 7 which shows equivalent air flow in pounds per second versus the pressure ratio across the compressor. There is a family of curves 64 shown thereon which represent various speeds of rotation of the rotor 12. Each of the curves 64 run from a surge limit line 68 and represent a fairly flat pressure ratio through most of the extent thereof through a wide range of air flow rates. By use of the present invention, each of the curves 64 will have a higher pressure ratio and flow range than shown in FIG. 7. The control ring 60 will also produce a substantially uniform pressure condition at the inlet of the vaneless diffuser throughout the full transverse extent thereof. FIG. 3 shows another embodiment with a slot 66 formed in a two piece shroud 70, 72 circumferentially, continuously therearound to define a gap in communication with atmosphere. A continuous ring 74 around shroud 72 has grooves 76 therein at circumferential points therearound which receive dowel pins 78 fixed to the shroud piece 72 to permit axial movement of ring 74 with respect to slot 66 to open and close the gap. This regulates additional inlet flow into the rotor stream at surfaces 80, 82, corresponding to surfaces 44, 46, respectively in the embodiment of FIG. 1.

While the embodiments of the present invention, as herein disclosed, constitute a preferred form, it is to be understood that other forms might be adopted.

Claims

What is claimed is:

1. A centrifugal compressor comprising a stationary shroud with an inner wall having an axial inlet and a radial outlet, a continuously open inwardly converging inlet lip on said shroud fixedly connected with respect to said axial inlet to form a flow transition therebetween, a rotor including a hub adapted to be connected to a drive shaft, said hub including a hub wall having an axial inlet and a radial outlet a plurality of rotor blades supported on said hub wall including a radially outer tip and an axial leading edge and a radial trailing edge thereon, each of said rotor blades defining a flow passage therebetween bound by the inner wall of said stationary shroud and the hub wall and having a flow discontinuity negative pressure region at said tip from the axial leading edge through a part of the chamber length of said tip, a gap in the inner wall of said stationary shroud located circumferentially therearound at the flow transition between said inlet lip and said axial inlet, said gap being in communication with atmospheric pressure exteriorly of said stationary shroud and overlying the rotor blade tips immediately downstream of the leading edge of each of said rotor blades, an annular control ring movable with respect to said gap for regulating the communication between atmosphere and the inner wall of said stationary shroud to produce a controlled bleed of atmosphere to said negative pressure region to produce a uniform pressure and velocity gradient from the inlet to the outlet of each of the said flow passages with the total pressure and indicated air angle of compressed flow through the passages being uniform from the radial outlet of the shroud inner wall to the hub wall.

2. A centrifugal compressor comprising a stationary shroud having an inner wall with an axial portion and a radial portion, a continuously open inwardly converging inlet lip on said shroud fixedly connected with respect to said axial inlet to form a flow transition therebetween, a rotor having a hub wall thereon with an axial portion located radially inwardly of said inner wall axial portion to define an annular axial inlet and a radial portion thereon in spaced relationship to the radial portion of said inner wall of said shroud to define an annular radial outlet, a plurality of centrifugal blades supported on said hub at circumferential points therearound, each of said centrifugal blades having a root secured to the rotor hub wall and including a tip portion thereon located in close spaced relationship to the inner wall of said shroud from the axial inlet thereof to the radial outlet thereof, each of said blades including a leading edge at the axial inlet and a trailing edge thereon at the radial outlet, each of said blades defining a flow passage therebetween into which working fluid is drawn from the axial inlet for centrifugal and lift discharge through the radial outlet, said blades producing a partial vacuum through a predetermined portion of the axial portion of the inner shroud wall which distorts the rotor exit pressure and velocity gradient across the radial outlet of said compressor, means forming a gap in said inner shroud wall at the flow transition between the inlet lip and the axial portion at a flow passage location wherein the static pressure ratio between the blade tip portions and the inner wall of said shroud is always less than unity during compressor operation, a control ring supported on the exterior of said stationary shroud radially outwardly of said gap movable to vary the flow area between atmospheric pressure and the flow passage to produce additional air flow into the regions of static pressure ratio less than unity to produce a uniform rotor exit pressure and velocity gradient at the radial outlet therefrom thereby to produce an improved efficiency of entrance flow to a diffuser located downstream of the radial outlet from said centrifugal impeller.