Abstract:
A recirculation passageway for a turbine engine provides stall protection in a booster by directing high pressure airflow from a flow path of the booster to the passageway. The high pressure airflow loses energy and decreases in pressure while traveling through the passageway until re-entry into the booster flow path. The airflow recirculates in the passageway until the airflow is discharged through a high pressure compressor.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to turbine engines and, more particularly, to apparatus and methods for preventing stall in a compressor. 
     A turbine engine typically includes a fan in front of a core engine having, in serial flow relationship, a low pressure compressor, or a booster, and a high pressure compressor. The low pressure compressor and the high pressure compressor each include an inlet section and a discharge section. 
     During engine power reductions, the inlet section of the high pressure compressor may generate an airflow blockage resulting from a flow differential between airflow through the high pressure compressor inlet section and the airflow through the booster discharge section. The airflow blockage generates a back pressure in the booster which causes the booster operating line to migrate closer to a stall limit. Migration of the booster operating line closer to the stall limit restricts the operating range of the turbine engine because less air continues to flow through the booster. 
     If the booster stalls, loud banging noises and flames or smoke may be generated at the booster inlet and/or discharge section. A booster stall condition results in excessive wear, degradation of performance, and a reduction in engine reliability and durability. In order to compensate for booster stall, the booster is typically over constructed, leading to more parts that in turn make the booster, and the resulting engine, heavier. 
     Booster stall is mitigated in existing engines by the use of complex variable bleed doors, or valves, which open during unsteady airflow conditions and allow a portion of the booster airflow to bypass the high pressure compressor. However, the bleed doors may fail or malfunction due to the complexity of the doors and valves. 
     Accordingly, it would be desirable to provide efficient booster stall protection without the added complexity of variable bleed doors. Additionally, it would be desirable to provide improved reliability of booster stall protection. 
     BRIEF SUMMARY OF THE INVENTION 
     A booster which includes a stator casing, a rotor shroud, and stator and rotor hub treatments extends the booster stall limit capability, and eliminates the need for variable bleed, or bypass, doors. More particularly, and in an exemplary embodiment, the booster includes a passageway which extends from a higher pressure portion of the booster to a lower pressure portion of the booster. The passageway includes angular slots which extend along an airflow path from the higher pressure portion of the booster to the lower pressure portion of the booster. 
     In operation, an airflow enters the passageway at a higher pressure portion of the booster. The airflow travels through the passageway from the higher pressure portion of the booster to the lower pressure portion of the booster, and expends energy and decreases in pressure while traveling through the passageway. The airflow then exits the passageway at the lower pressure portion of the booster and returns to the airflow path. 
     Recirculation of the airflow from the higher pressure portion of the booster to the lower pressure portion of the booster extends a booster stall free operating region and reduces the likelihood that the booster will reach a stall limit during engine power reductions. As back pressure diminishes, the recirculation lessens and the booster returns to a more normal operation. By eliminating the bypass doors or valves, the passageway increases engine and booster stall protection reliability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a turbine engine including a low pressure compressor; 
     FIG. 2 is an enlarged axial sectional view of the low pressure compressor shown in FIG. 1 including a recirculating passageway; 
     FIG. 3 is an enlarged perspective view of a portion of the recirculating passageway shown in FIG. 2; 
     FIG. 4 is an enlarged axial sectional view of the low pressure compressor shown in FIG. 1 including a plurality of circumferential grooves; and 
     FIG. 5 is an enlarged axial sectional view of the low pressure compressor shown in FIG. 1 including an alternative recirculating passageway. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a cross sectional view of a turbine engine  10  symmetrical about a central axis  20 . Engine  10  includes, in serial flow communication, a front fan  30 , a multistage low pressure compressor, or booster  40 , a multistage high pressure compressor  116  which supplies high pressure air to a combustor  120 , a high pressure turbine  130 , and a low pressure turbine  140 . 
     During operation of engine  10 , air flows downstream through fan  30  and into multistage booster  40 . The booster compresses the air and the air continues to flow downstream through high pressure compressor  116  where the air becomes highly pressurized. A portion of the highly pressurized compressed air is directed to combustor  120 , mixed with fuel, and ignited to generate hot combustion gases which flow further downstream and are utilized by high pressure turbine  130  and low pressure turbine  140  to drive high pressure compressor  116 , front fan  30 , and booster  40 , respectively. 
     FIG. 2 illustrates a portion of the engine shown in FIG.  1 . As shown in FIG. 2, booster  40  includes a plurality of stator vanes  42  and a plurality of rotor blades  44  surrounded by a stator casing  46  and a plurality of rotor shrouds  48 . A first passageway, or flow path,  50  extends through booster  40  and is formed, and defined, by stator vanes  42 , rotor blades  44 , stator casing  46 , and rotor shrouds  48 . 
     A second passageway, or flow path,  52  in booster  40  extends through a portion of rotor shroud  48  adjacent a forward rotor blade  54 . Second passageway  52  is in flow communication with flow path  50 . Booster  40  includes a first wall  56 , stator casing  46 , a leading edge  60 , and a trailing edge  62  which form second passageway  52 . First wall  56  and stator casing  46  extend substantially  360  degrees around central axis  20  of turbine engine  10  (shown in FIG.  1 ). First wall  56  is connected to leading edge  60  and trailing edge  62 , which are also connected to stator casing  46 . 
     Forward rotor blade  54  also includes a leading edge  64  and a trailing edge  66 . A plurality of openings  68  extend through stator casing  46  and are in flow communication with second passageway  52 . Openings  68  in stator casing  46  extend from leading edge  60  to a portion  69  of rotor blade  54  between leading edge  64  and trailing edge  66 . First passageway  50  of booster  40  further includes an inlet, or a lower pressure portion,  70  and a discharge, or a higher pressure portion,  72 . 
     In operation, airflow moves downstream through booster  40  along flow path  50  and increases in pressure and temperature. When fuel and high pressure airflow are decreased to combustor  120  (shown in FIG.  1 ), fan  30  (shown in FIG.  1 ), booster  40 , and high pressure compressor  116  (shown in FIG. 1) decelerate. Due to a lower inertia and a higher pressure ratio, high pressure compressor  116  decelerates faster than fan  30  and booster  40 . The faster deceleration of high pressure compressor  116  generates an airflow blockage that results in an increased back pressure at discharge  72 , forcing an operating line of booster  40  to migrate towards a stall limit line. 
     The increased back pressure causes a portion of the high pressure airflow to recirculate and exit passageway  50  at a higher pressure portion of booster  40  through openings  68  and enter passageway  52 . The recirculating airflow re-enters flow path  50  at a lower pressure portion of booster  40 , i.e., extends the booster stall limit line. Recirculating a portion of the high pressure airflow beyond the raised operating line of booster  40  allows airflow to freely move from the higher pressure portion of booster  40  to the lower pressure portion of booster  40 . The amount of recirculation varies depending on the amount of booster back pressure. For example, an increased booster back pressure results in an increased recirculating airflow and a decreased booster back pressure results in a decreased recirculating airflow. 
     FIG. 3 illustrates a perspective view of openings  68  shown in FIG.  2 . As shown in FIG. 3, openings  68  in stator casing  46  include a plurality of angled slots  74  which extend from leading edge  60  to portion  69 . 
     In operation, high pressure airflow enters angled slots  74  between rotor blade leading edge  64  and portion  69 . The high pressure airflow travels through passageway  52  (shown in FIG. 2) until the airflow exits passageway  52  through angled slots  74  at leading edge  60 . The airflow then travels downstream in flow path  50  and increases in pressure. 
     FIG. 4 illustrates a portion of booster  40  including a plurality of circumferential grooves  76 . Circumferential grooves  76  extend from leading edge  60  to trailing edge  62  in rotor shroud  48 . Booster  40  includes first wall  56  and circumferential grooves  76  extend from opening  68  to first wall  56 . 
     In operation, a portion of a wake fluid enters a downstream circumferential groove  76  between rotor blade leading edge  64  and trailing edge  66  at openings  68  when the high pressure airflow reverses flow direction and flows upstream in booster  40 . The wake fluid then progresses upstream in booster  40  and enters an adjacent groove  76 . The upstream progression of the wake fluid continues until either the high pressure airflow again flows downstream or the wake fluid extends upstream beyond grooves  76  and booster stall occurs. Grooves  76  extend the stall line of booster  40  and increase the operating range of booster  40 . 
     FIG. 5 illustrates a booster  77  including a plurality of hub stator vanes  78  and a plurality of hub rotor blades  80  surrounded by a hub stator casing  82  and a plurality of hub rotor shrouds  84 . 
     A first passageway, or flow path,  86  extends through booster  77  and is formed, or defined, by hub stator vanes  78 , hub rotor blades  80 , hub stator casing  82 , and hub rotor shrouds  84 . Booster  77  further includes a second passageway  88  and an aft hub rotor blade  90  connected to a rotor shaft  91 . Second passageway  88  extends through a portion of rotor shaft  91 . Rotor shaft  91  includes a first wall  92  and a second wall  94  which extend  360  degrees. Second passageway  88  is in flow communication with flow path  86  and is bounded by first wall  92  and second wall  94 . 
     Rotor shaft  91  further includes a leading edge  96  and a trailing edge  98 . First wall  92  is connected to leading edge  96  and trailing edge  98  which are connected to second wall  94 . First wall  92 , second wall  94 , leading edge  96 , and trailing edge  98  form second passageway  88 . Aft hub rotor blade  90 , located in the hub of booster  77 , includes a leading edge  100  and a trailing edge  102 . Second wall  94  comprises a plurality of openings  104  in flow communication with second passageway  88  and an opening  106  in hub stator vane  78  adjacent aft hub rotor blade  90 . 
     In one embodiment, openings  104  and  106  in second wall  94  and in hub stator vane  78  adjacent aft hub rotor blade  90  comprise a plurality of circular apertures (not shown). Booster  77  also includes an inlet  112  located at an area of lower pressure, and a discharge  114  located at an area of higher pressure. 
     The embodiment of Booster  77  shown in FIG. 5 maintains stability in boosters that have their aerodynamic stability limitations in the hub region. When booster  77  has raised operating line conditions, increased recirculation through second passageway  88  keeps the hub region pressure at trailing edge  102  of hub rotor blades  80  from attaining a stability limit level. This increased recirculation maintains booster  77  in a stable, i.e., a stall free, operation at the raised operating line condition. 
     The recirculation passageway is formed in the existing structure of the turbine engine and adds minimal cost and complexity to the booster. The inclusion of the recirculating passageway in the booster protects against booster stall and improves the reliability of operation when compared to variable bleed valves or doors which may stick or function improperly. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.