As is well known, surge or stall is a phenomenon that is characteristic to all types of axial flow compressors and occurs at a given engine operating condition and that if gone unattended could be deleterious or harmful to not only the engine's performance but to the engine itself.
Hence, throughout the entire evolution from the original design through the development and the improvement stages of a gas turbine engine, those involved in this technology pay great heed to the surge characteristics of the rotating machinery to assure that the compromise between the safe operation of the engine and its performance is optimized.
Since the point at which stall may occur limits the blades operating pressure ratio for a given corrected air weight flow and since higher pressure ratios enhance its performance, the engine's operating line is dictated by a compromise between the stall line and performance. Hence, it is always desirable to be able to raise the stall line to a higher pressure ratio for a given engine operation. For example, raising the stall line can increase the stall margin between the engine's operating line, or raising the stall line permits raising of the operating line without changing the stall margin, which obviously would result in an increase in engine's performance. Other alternatives can use up the increased stall margin with reduced rotor speed, reduced blade count, reduced rotor chord length, or eliminated variable geometry to improve component efficiency or lower component weight and complexity.
Experience has shown that beoause there are so many factors affecting stall it is not surprising that the stall line may not match its design point. In these situations the engine's hardware is typically modified to satisfy the stall margin requirement so as to meet engine specifications. It is also not surprising that this cannot always be done without degrading engine performance. This is not to say that there aren't other advantages that are attendant an increase in stall margin.
Thus, it is ideal to be able to increase the stall margin and at the same time obtain corresponding increase in engine performance. 0f course, the next best would be to be able to increase stall margin without incurring an engine performance deficit.
As is well known, and as to be understood for purposes of understanding the invention, rotating stall is a phenomenon that occurs whenever sufficient blades or regions of the blades stall so as to occasion a complete blockage or reversal of flow of air through the compressor. Also flow separation on the airfoils can lead to compressor stall or rotation stall which, in turn, can lead to an overall system breakdown of the flow, i.e., surge.
Hence, whenever stall occurs and is allowed to propagate throughout the entire or nearly entire blading, surge can ensue. It is important to understand that the surge problem can be corrected by either providing means for handling an incipient surge or design the engine so that the engine never operates where a stall can manifest. For example, an incipient stall may be corrected simply by reducing engine power as compared to designing the engine so that its operating parameters assure that the engine always operates below a given stall line.
Also, it is well-known in the art that surge may manifest in many different forms and stall may occur in one or more blades and at different regions. The most limiting stall characteristics often occur at the tip of the blade which essentially is the type of stall being addressed by this invention. More particularly, this invention is directed to enhance the stall line so as to avoid the manifestation of an incipient stall. This will serve to prevent compressor stall although it will be understood that the treatment of the casing does not affect whether or not a rotating stall could degenerate into a surge condition.
Treatment of the casing, which sometimes is referred to as shroud or tip seal or outer air seal, to enhance the stall line is exemplified in the prior art, for example, by U.S. Pat. No. 4,239,452 granted to F. Roberts, Jr. on Dec. 16, 1980, and assigned to the assignee of this patent application. This patent discloses that axially extending skewed grooves and circumferentially extending grooves in the blade tip shroud enhance stall characteristics and is particularly efficacious in use with a fan.
U.S. Pat. No. 3,580,692 granted to A. Mikolajczak on May 25, 1971 also assigned to the assignee common to this patent application teaches a honeycomb structure casing treatment for enhancing the stall characteristics.
Other casing treatments that are known in the prior art are, for example, disclosed in the ASME paper reported in the Journal of Fluid Engineering Vol. 109 dated May 1987 entitled "Improvement of Unstable Characteristics of an Axial Flow Fan by Air-Separator Equipment" authored by Y. Mijake, T. Inola and T. Kato, and in a paper from The School of Mechanical Engineering, Cranfield Institute of Technology in Great Britain entitled "Application of Recess Vaned Casing Treatment to Axial Flow Compressor", dated Feb. 1988 and authored by A. R. Aziman, R. L. Elder and A. B. McKenzie. The work presented in these papers is based in part on earlier work of S. K. Ivanov disclosed in his U.S. Pat. No. 3,189,260 granted on Jun. 15, 1965.
The Ivanov patent and the Miyski et al paper, supra, both investigate properties of air separators for industrial fans that operate at relatively low speeds and low aerodynamic loadings while the Aziman et al paper, supra, investigates properties of air separators operating at similar low speeds but with aerodynamic loadings that are encountered in aerospace applications.
In the main, the teachings disclosed in the two papers and the Ivanov patent, supra, relate to mechanisms that collect rotating stall cells in post-stall operation in a significantly large recess formed in the casing, turn and reorient the flow and then reintroduce the collected air back into the main compressor flow upstream of the rotor.
Obviously, since rotating stall is a mass of cells of stalled and highly turbulent air that processes around the rotor at a rate that is nearly half the rotating speed of the rotor and extends upstream of the rotor a significant axial distance, one skilled in the art armed with these teachings is led to believe that in order to enhance the stall line it follows that the recess should be large enough to swallow the rotating stall. Hence, knowing that rotating stall extends a significant distance upstream of the rotor and since it is a collection of a large mass of stalled air cells, a significantly large recess would be necessary in order to swallow the rotating stall. These teachings, while particularly relevant to industrial types of fans and compressors are not relevant to aircraft application inasmuch as a large recess in the casing at the inlet of the engine or in front of a compressor is intolerable. In a sense, these papers teach away from the present invention, notwithstanding the fact that both the prior art and the present invention teach means for enhancing the stall line.
A particular problem inherent in axial compressor design is the limited space available for treatment of the case. It should be understood that the amount of air removed from the compressor is significantly different, percentage wise, than the amount of air for treatment of the fan, and the amount is deemed critical. It was found that a suitable amount of air used for treatment is substantially up to 12 percentage of the total air ingested by the compressor blades in a given compression stage. The amount of air recirculated depends on the ratio of the hub radius to the tip radius; and a larger percentage of the total air ingested is required for the higher hub-to-tip radius ratios found in high pressure compressors. Moreover, it was found that it was abundantly important to remove or even reverse the swirl in a single turning of the treated air. In this light, the passage used for treating the air is judiciously oriented so that the air flows angularly relative to the main flow stream through the blading.