Compressor Bleed Port

1. Technical Field

The invention relates to axial flow air compressors and in particular to a bleed port arrangement therein.

2. Background of the Invention

Axial flow air compressors are used in gas turbine engines for the purpose of establishing an appropriately high pressure to achieve substantial thrust by the discharge of the hot gas. Multiple stage compressors are used for this purpose. During certain periods of operation, particularly during start-up, these compressors experience stall or surge potential. This is experienced generally where the low pressure stages lose their lift effectiveness because they have to pump against a pressure too extreme for the flow condition. It is known to unload these stages by bleeding some air out downstream of the low pressure stages during certain periods of operation.

With these large airflow requirements during start-up or with high bypass requirements substantial bleed flow area is required. Much attention has been given to the design of these bleed flow ports to facilitate the passage of air with minimum energy loss. Little or no attention has been given to the effect of these openings, whether flowing or closed, on the compressor through flow.

The large openings inherently produce a loss of the supporting casing surface leading to a loss of effectiveness of the next stage of blades, from this lack of support of the blade upwash. Also, flow separation can occur at the casing surface where a portion of the flow passing into the opening then reenters the mainstream.

Such flow separation degrades stability, flow capacity and the efficiency of the succeeding blade stage. The opening when locating within the effective area of the upwash of the blades also causes such degradation in performance and energy losses due to recirculation in the openings.

The bleed port openings are located as is conventional in the compressor case between a row of stator vanes and a row of rotating blades. The port openings are each rectangular in shape with the length in the circumferential direction and the width in the axial direction. The upstream facing edge of each opening is shaped with a generous radius to avoid flow separation of air reentering from the opening into the main airstream.

A hypothetical streamline stagnation flowpath is established as a straight line extending through the opening at an angle of 15 degrees from the casing wall at the upstream edge of the opening. This imaginary flowpath intersects the upstream facing edge of the opening at a locus, whereby a reentrance surface is defined between the locus and the inside surface of the casing on this upstream facing edge.

From the flow standpoint it would be preferable to establish this surface as an elliptical shape with the minor radius greater than 25 percent of the width of the opening near the locus and a major radius of up to 50 percent of the width of the opening as this curved surface reaches the inside surface of the casing. From the manufacturing standpoint a uniform curvature is preferred and the ideal radius of such surface would be 33 percent of the width of the opening.

An upwash condition from the rotating blades exists which is a function of the blade tip spacing. The bleed port flow opening should be at a sufficient distance upstream of these blades so that the remaining wall provides support for the airfoil lift producing rotating pressure profile without flow moving in and out of the bleed ports causing energy losses and possibly flow instability. This is preferably a distance equal to 60 percent of the spacing between the blade tips and at least 40 percent of the spacing between the blade tips.

FIG. 1 is a sectional elevation showing a bleed port arrangement;

FIG. 2 is the detail section through the casing at the bleed port;

FIG. 3 is a view of the port from inside the casing;

FIG. 4 is an axial view showing the compressor blades;

FIG. 5 is a diagram illustrating blade upwash;

FIG. 6 is a detail of an alternate port with an elliptical upstream facing surface;

FIG. 7 is a sectional elevation showing a double opening bleed port arrangement;

FIG. 8 is a view of the double opening port arrangement from inside the casing; and

FIG. 9 is a section through the double port areas.

FIG. 1 illustrates an axial flow air compressor having a compressor casing 12. Secured to the casing is a circumferential row of fixed vanes 14 for guiding the airflow. Rotor 16 carries a circumferentially arranged row of blades 18 which receive air directly from the vanes 14 and which rotate to impart energy to the main airflow 20 passing therethrough. Downstream there is another row of vanes 22 and additional blade and vane sections not shown.

Bleed port 24 permits the extraction of bleed air 26 under the control of a flow valve not shown. Such bleed air may be used for cooling air at certain operating conditions, but the major bleed air requirement is for unloading the upstream stages (not shown) of the compressor during start-up. The overall effective opening of these bleed ports 24 is dictated by that maximum flow requirement.

Referring to FIGS. 2 and 3 the bleed port opening 24 has a downstream facing (leeward) edge 28, the shape of which is selected to facilitate the airflow through the bleed port, and which in itself is not a part of this invention.

During operation with no bleed flow a portion of the airflow 20 passes into the opening 24 and must reenter the airstream downstream of the opening. This also occurs to some extent when flow is passing through the bleed port. The upstream facing surface 30 of the opening is to be shaped and sized such that this reentrant airflow remains attached to the surface and does not separate, so as to prevent instability and inefficiency in the operation of blades 18. While an extremely large radius for surface 30 would be appropriate for this surface, it is inconsistent with the need to maintain the appropriate opening of the port for bleed flow purposes. The limits on the various dimensions set forth below are appropriate for providing a practical structure and avoiding flow separation.

An imaginary straight flowpath 32 is defined from the upstream side of the opening 24 at the casing inside surface 34. This represents the potential streamline stagnation line of the airflow at 15 degrees from the casing inside surface. While the actual line is believed to be somewhat less than 15 degrees, some additional tolerance is placed in this line to allow for some circumferential flow component established by the vanes in the flow passing over the openings. This imaginary flowpath 20 establishes a locus 35 on the upstream facing surface 30 of the opening. It is between this locus and the tangent point 36 where the curved surface 30 meets the inside surface 38 of the casing to which attention is given to avoid flow separation. The portion of the upstream facing surface between the locus 35 and the tangent point 36 is referred to as the reentrant surface 40.

From a manufacturing viewpoint this is most conveniently fabricated with the uniform radius. As illustrated in FIG. 2 with the width of opening W, the radius r is preferably 33 percent of the width of the port. The minimum radius is about 30 percent of the width of the port. It is the radius of this reentrant surface which is significant and the surface 40 outboard of locus 35 may be fabricated as convenient to optimize the overall flow through the port.

Each of the ports 24 is rectangular in shape having the length in the direction circumferentially around the casing and the width in the axial flow direction. These ports should be as rectangular as possible, with the corners being rounded only for the purpose of facilitating manufacturing and for minimizing stress concentrations. Should these ports approach the circular shape, flow entering the port will achieve a circumferential component as it nears the upstream facing edge. The component from each of the two sides of the opening would pass toward the center. This would tend to promote a flow peak at that one location thus exacerbating flow perturbations through the opening.

FIG. 4 is a view looking in the axial direction showing the plurality of blades 18 on rotor 16. Each blade has a leading edge 40 which is the most upstream portion and is near the bleed port 24. At the tips 42 of the blades the spacing between the leading edges is a distance .tau..

Upwash from these rotating blades, which is a function of the distance .tau., is illustrated in FIG. 5. As the blades move in the direction 44, local pressure peaks 46 occur as upwash axially upstream of the blades. As indicated by lines 47 and 48 these rotating pressure variations quickly die off in the upstream direction. The pressure drops off quickly with distance in accordance with the formula

When these pressure variations occur near the tip of the blade, the adjacent casing wall that normally exists is capable of sustaining this rotating pressure profile. Should, however, there be a recess such as a bleed port opening where these variations occur, recirculation in these openings will occur reducing the gas path gap-wise static pressure gradient. Flow will move radially into the holes in regions of high gas path static pressure and out of the holes in regions of low gas path static pressure. This transverse flow will consume momentum from the through flow reducing the efficiency of the compressor. In addition, the upwash will be reduced at the leading edge of the airfoil since the static pressure gradient supporting the flow curvature is reduced. Loss of the upwash will reduce the lift produced at the blade tip, reducing the overall performance of the compressor. Accordingly, referring to FIG. 2, the distance X between the opening 24 and the leading edge 40 of the blades should exceed 60 percent of the spacing .tau. between the blade tips. As a minimum it should exceed 40 percent of that spacing.

Referring now to FIG. 6, an alternate embodiment as illustrated where the only difference is the shape of the upstream facing surface 30. This upstream facing surface is now elliptical in shape with a minimum radius r.sub.x of 25 percent of the width W of the opening while it has a maximum radius r.sub.max of at least 33 percent of the width of the opening and possibly as much as the thickness of the casing.

FIGS. 7 and 8 illustrate an alternate embodiment wherein the flow port 24 is composed of two rows of narrower ports, with an upstream port 50 and a downstream port 52. The ports of each row are preferably staggered with respect to those of the other row. Each of these ports has a similar downstream facing edge 28 and an upstream facing surface 30. Each of these are guided by the same parameters described before with respect to the single opening. However, because of the decreased width of the openings, the extent of flow deviation and the required radii are less than with the single port. The upstream facing surfaces therefore may be appropriately radiused to avoid flow separation. The same guidelines apply as to the spacing between the leading edge 40 of blade 18 and the tangent point 36 of the downstream openings.

FIG. 9 is a section through the port area of FIG. 8 showing upstream 50 and a downstream port 52. Streamline 54 illustrates the airflow pattern. Radii r are each equal to 0.33 of the width b of the corresponding port.