Abstract:
A passive pressurizing air system for a gas turbine engine comprises a flow path for directing an air flow having a low temperature and low pressure, extending through a cavity to a pressurized area of the engine. The cavity contains pressurized air having a high temperature and high pressure. Means are provided for adding the pressurized air from the cavity into the flow path to provide a mixed air flow having an intermediate temperature and intermediate pressure.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates generally to gas turbine engines and more particularly, to an improved twin air source gas turbine pressurizing air system.  
       BACKGROUND OF THE ART  
       [0002]     Pressurizing air systems within gas turbine engines provide bleed air under pressure for many purposes including supplying auxiliary power, cooling air, etc. A pressurizing air system may extract bleed air from a compressor of the engine at more than one stage thereof to obtain air flows having different temperatures and pressures, in order to meet requirements for different purposes within the engine. However, for gas turbine engine operations the bleed airflow changes in both temperature and pressure at the individual stage ports of the compressor. For example, the temperature and pressure of the bleed air at the individual stage port of the compressor increase when the engine is operated at a full power level in contrast to an idling condition. In another example, as the demand of a bleed airflow extracted from a particular stage port of the compressor increases, the air pressure and temperature delivered from this particular stage port of the compressor will decrease. All these factors will result in fluctuations and variations causing transient thermal stresses on the engine components and transient rubbing (pinch point) in the non-contact air and air/oil seals.  
         [0003]     Accordingly, there is a need to provide an improved pressurizing air system for gas turbine engines to provide bleed airflows with relatively stable temperatures and pressures under most engine operating conditions.  
       SUMMARY OF THE INVENTION  
       [0004]     It is therefore an object of this invention to provide a twin-air source pressurizing air system for gas turbine engines in order to provide relatively stable bleed airflows.  
         [0005]     In one aspect, the present invention provides a passive pressurizing air system for a gas turbine engine which comprises a low pressure source of air and a high pressure source of air. An ejector is located in a cavity in fluid communication with the high pressure source of air. The ejector has a motive flow inlet thereof in fluid communication with the cavity, a secondary flow inlet thereof connected to the low pressure source of air and an outlet thereof connected to a pressurized area of the engine for delivery of a mixed air flow from the high and low pressure sources of air thereto.  
         [0006]     In another aspect, the present invention provides a passive pressurizing air system for a gas turbine engine which comprises a flow path for directing an air flow having a first temperature and a first pressure from a pressure stage of a compressor of the engine to a pressurized area of the engine. The flow path extends through a cavity containing pressurized air having a second temperature and a second pressure greater than the respective first temperature and first pressure. Means are provided for adding the pressurized air from the cavity into the flow path to provide a mixed air flow having a temperature and a pressure intermediate to the first and second temperatures and the first and second pressures. The mixed air flow flows along the flow path downstream of the cavity, to the pressurized area of the engine.  
         [0007]     In a further aspect, the present invention provides a method for reducing temperature variation of a pressurized air supply to a pressurized area of a gas turbine engine, which comprises directing a first air flow having a low temperature thereof from a low pressure source of air associated with the engine, to the pressurized area of the engine; and adding a second air flow having a high temperature thereof from a high pressure source of air associated with the engine, into the first air flow to provide a mixed pressurized air supply having an intermediate temperature thereof, to the pressurized area of the engine in a manner in which a ratio of energy distributed by the added second air flow in the mixed pressurized air supply varies to compensate for variations in the first air flow, thereby reducing variations in the intermediate temperature of the mixed pressurized air supply when the low temperature of the first air flow varies.  
         [0008]     Further details of these and other aspects of the present invention will be apparent from the detailed description and drawings included below. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0009]     Reference is now made to the accompanying drawings depicting aspects of the present invention, in which:  
         [0010]      FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine as an example illustrating an application of the present invention;  
         [0011]      FIG. 2  is a schematic illustration showing a twin-air source pressurizing air system, as one embodiment of the present invention illustrated in the engine of  FIG. 1 ;  
         [0012]      FIG. 3  is a schematic illustration of an ejector used in the embodiment of  FIG. 2 ;  
         [0013]      FIG. 4  is a chart illustrating air temperatures delivered by high pressure, low pressure ports and an ejector in the engine operation range according to the embodiment of  FIG. 2 ; and  
         [0014]      FIG. 5  is a schematic illustration showing another embodiment of the present invention illustrated in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     Referring to  FIG. 1 , a gas turbine engine incorporating an embodiment of the present invention is presented as an example of the application of the present invention and includes a housing or nacelle  10 , a core casing  13 , a low pressure spool assembly seen generally at  12  which includes a fan assembly  14 , a low pressure compressor assembly  16  and a low pressure turbine assembly  18 , and a high pressure spool assembly seen generally at  20  which includes a high pressure compressor assembly  22  and a high pressure turbine assembly  24 . The core casing  13  surrounds the low and high pressure spool assemblies  12  and  20  in order to define a main fluid path (not indicated) therethrough, including a chamber  26  containing and surrounding a combustor  28 . An air flow mixing apparatus  30  according to one embodiment of the present invention is located in the chamber  26  to be used for a twin-air source air pressurizing system of the gas turbine engine.  
         [0016]     Reference is now made to  FIGS. 1 and 2 . The low and high spool assemblies  12  and  20  of  FIG. 1  are simplified in  FIG. 2  for convenience of description. The twin-air source pressurizing air system is schematically illustrated and indicated generally by numeral  32  which includes an air flow path  34  connected to a low pressure source of air for example 2.5 P air from a stage of the low pressure compressor assembly  16 .  
         [0017]     The air flow path  34  extends to one or more pressurized areas  36  of the engine, for example a space defined between labyrinth seal  38  and the rotor shaft  40  located downstream of the turbine assembly  24 . The air flow mixing apparatus  30  is incorporated into and thus forms part of the air flow path  34 .  
         [0018]     The low pressure compressor assembly  16  as the low pressure source of air, provides an air flow having relatively low pressure and low temperature with respect to the pressurized air provided by the high pressure compressor assembly  22  as a high pressure source of air for the engine. The air flow extracted from the stage of the low pressure compressor assembly  16  which is represented by stage port  42 , is directed by the air flow path  34  to the pressurized area  36  for cooling or providing purging flow to the labyrinth seal  38  and other components downstream of the turbine assembly  24  which are located in a very high temperature environment.  
         [0019]     Nevertheless, the air flow extracted at the stage port  42  of the low pressure compressor assembly  16  varies during various power setting conditions of the engine, the flight regime and customer bleed air demand. Variations in temperature and pressure of the air flow delivered to the pressurized area  36  accompany variations in the air flow. These variations cause transient thermal stresses on the engine components and transient rubbing (pinch point) in the non-contact air and air/oil seals.  
         [0020]     Referring to  FIGS. 2 and 3  and according to an embodiment of the present invention, the air flow path  34  preferably includes a segment of a pipeline  44  extending through a cavity  46 , for example, an annular chamber defined by the core casing  13 , containing and surrounding the combustor  28  as illustrated in  FIG. 1 . The cavity  46  is in fluid communication with a stage of the high pressure compressor assembly  22  via a high pressure stage port  48 . High pressure air such as P3 air is therefore introduced into the cavity  46  for participating in combustion in the combustor  28  to generate combustion gases to drive the high pressure and low pressure turbine assemblies  24 ,  18 , as illustrated in  FIG. 1  (only high pressure turbine  24  is shown in  FIG. 2 ). This high pressure air filled in the cavity  46  has a temperature and a pressure greater than the temperature and pressure of the low pressure air delivered at the low pressure stage port  42 . Although the temperature of the high pressure air delivered at the high pressure stage port  48  also varies depending on the rotational speed of the high pressure compressor assembly  22 , the engine is designed to deliver the high pressure air at the high pressure stage port  48  with a relatively stable rate into the cavity  46 .  
         [0021]     The air flow mixing apparatus  30  preferably includes an ejector  50  profiled as a venturi tube and mounted on the segment of the pipeline  44  within the cavity  46 . The ejector  50  is a conventional device used to boost a low pressure stream to higher pressure streams, thereby effectively using available energy without waste. The ejector  50  includes a secondary flow inlet  52  and an outlet  54 . The secondary flow inlet and outlet  52 ,  54  are connected to the segment of the pipeline  44  in series, the ejector  50  thereby forming part of the pipeline  44 , and thus part of the air flow path  34 , in order to allow the air flow extracted from the low pressure stage port  42  to flow therethrough to be supplied to the pressurized area  36  of the engine.  
         [0022]     The ejector  50  further includes a motive flow inlet  56  which preferably includes a calibrated nozzle in fluid communication with the cavity  46  in order to allow the high pressure air filled within the cavity  46  to enter the ejector  50 . In such a configuration, high pressure air from a stage of the high pressure compressor assembly  22  can be extracted at the high pressure stage port  48  and added to the low pressure air flow through the air flow path  34  without any additional pipelines.  
         [0023]     Due to the engine high pressure compressor ratio, the expansion ratio of the high pressure air flow in the calibrated nozzle (motive flow inlet  56 ) ensures a steady hot motive air flow into the ejector  50  under any engine operating regime, and this steady hot motive air flow is not perturbed by pressure changes of the low pressure air flow in the air flow path  34 . On the other hand, as previously discussed, the pressure of the low pressure air flow delivered at the low pressure stage port  42  varies within the engine operation regime. Small reductions in pressure of the low pressure air flow delivered at the low pressure stage port  42 , result in large reductions in the low temperature and low pressure air flow delivered into the pressurized area  36  of the engine. Hence, at low engine power, the air flow delivered to the pressurized area  36  originates mainly from the high pressure source (high pressure stage port  48 ) while at high power of engine operation, the air delivered to the pressurized area is a mixture of high pressure and low pressure air. Therefore, the ratio of energy distributed by the high temperature and high pressure air into the mixed air flow varies when engine operating conditions vary. Neverthless, the mixture of the high and low pressure air always has a temperature intermediate to the high and low temperatures of the respective high pressure and low pressure air and a pressure intermediate to the high and low pressures thereof.  
         [0024]     The motive flow inlet  56  has a nozzle dimensioned such that the ejector  50  delivers the mixture of the high and low pressure air that provides the required temperature of the pressurized area  36  when the engine is operating at a high power. The low temperature and low pressure air flow will decrease at low power and thus the high temperature and high pressure air contribution will increase. Therefore, a ratio of energy distributed by the added high pressure air flow into the mixture of the high and low pressure air, varies to compensate for variation of the low pressure air flow delivered from the low pressure stage port  42 , thereby reducing variations in the intermediate temperature of the mixed pressurized air to be supplied to the pressurized area  36  when the temperature of the low pressure air flow extracted from the low pressure stage port  42  varies.  
         [0025]     Besides functioning as an air flow mixing apparatus, the ejector  50  also attenuates perturbations of the low pressure air flow occurring at a constant engine speed. Such perturbations can be caused by customer bleed air flow rate increases or the Handling Bleed Off Valve (HBOV) opening. Any perturbation that reduces the air pressure and temperature delivered by the low pressure stage port  42 , results in a reduced low pressure air flow rate into the ejector  50 . As previously discussed, the energy provided by the high pressure air through the motive inlet  56  at an increased proportion relative to the total energy of the mixed air flow, results in both temperature and pressure gain in the ejector  50 . The required degree of attenuation is preferably obtained by the effective mixing length of the ejector.  
         [0026]      FIG. 4  illustrates in chart form, the temperature changes at the high pressure stage port  48  (indicated by HP), low pressure stage port  42  (indicated by IP) and the output of the ejector  50  within the entire engine operating regime, from ground idling (indicated by GI) to taking off conditions (indicated by TO), in a temperature (indicated by T) and engines speed (indicated by N) coordinate system.  FIG. 4  clearly illustrates that variations in the temperature at the output of the ejector  50  are much smaller than temperature variations at the respective high pressure stage port  48  and the low pressure stage port  42  when engine operating conditions change.  
         [0027]     In accordance with another embodiment of the present invention illustrated in  FIGS. 2 and 5 , the ejector  50  in the previous embodiment is eliminated, and instead a calibrated hole  58  is defined in the segment of the pipeline  44  extending through the cavity  46 . The calibrated hole  58  functions as the motive flow inlet  56  of the ejector  50  of  FIG. 3  to introduce the high pressure air filled in the cavity  46  at a substantially stable rate, into the segment of the pipeline  44 . Thus, a part of the segment of the pipeline  44  downstream of the calibrated hole  58  functions as an air flow mixing apparatus, similar to the ejector  50  of  FIG. 2  in order to produce a mixed air flow having the relatively stable intermediate temperature and pressure required in the pressurized area  36  of the engine.  
         [0028]     Adjustment of the location of the calibrated hole  58  along the segment of the pipeline  44  within the cavity  46  will affect the intermediate temperatures of the mixed air flow delivered through the air flow path  34  into the pressurized area  36  of the engine when the low pressure air flow through the segment of the pipeline  44  is unchanged.  
         [0029]     Heat exchange occurs between said segment of the pipeline  44  and the cavity  46  because the temperature of the cavity  46  (the temperature of the high pressure air) is higher than the temperature of said segment of the pipeline  44 . However, said segment of the pipeline  44  has different temperatures at the upstream and downstream portions with respect to the location of the calibrated hole  58 . The temperatures of the upstream portion are mainly affected by the low temperature of the low pressure air extracted from the low pressure stage port  42  and the temperature of the downstream portion is mainly affected by the intermediate temperature of the mixed air flowing therethrough. Therefore, the heat exchange rates of the respective upstream and downstream portions of the segment of the pipeline  44  are different.  
         [0030]     The location change of the calibrated hole  58  varies the affected heat exchange contact areas at the different heat exchange rate portions, thereby affecting the resultant intermediate temperature of the mixed air flow eventually delivered into the pressurized area  36  of the engine. For example, the calibrated hole  58  moved to a downstream position will increase the heat exchange at the high exchange rate at the upstream portion of the segment of the pipeline  44  and will reduce the heat exchange at the relatively low heat exchange rate at a downstream portion of the segment of the pipeline  44 , resulting in more heat gain of the segment of the pipeline  44  within the cavity  46  and thus higher intermediate temperature of the mixed air flow delivered to the pressurized area  36  of the engine.  
         [0031]     In contrast to the conventional twin-source air systems using variable geometry ejectors, the present invention advantageously uses a fixed geometry flow mixing apparatus as a temperature control device for the twin-source air system. Therefore, there are no moving parts, control systems or valves needed for effective functioning, and thus no servicing is required. The present invention by advantageously positioning the flow mixing apparatus within a high pressure cavity eliminates the need for additional piping and thus reduces the high pressure flow temperature variations. The resultant relatively stable temperature of the pressurized area alleviates transient thermal stresses in the engine components and transient rubbing (pinch point) in the non-contact air and air/oil seals.  
         [0032]     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departure from the scope of the invention disclosed. For example, the cavity can be any cavities defined within the engine which are in fluid communication with a high pressure source of air of the engine other than the exemplary chamber surrounding a combustor of the engine. The ejector position may be changed along the segment of pipeline within the cavity, similar to the adjustment of the calibrated hole defined in the pipeline, in order to adjust the heat exchange between the pipeline and the surrounding hot cavity. The segment of pipeline extending through the hot air cavity may be entirely or partially insulated, and a check valve may be installed in the motive flow inlet upstream of the injection point. Individual ejectors may be installed and calibrated for each pressurized area of the engine, not limited to the space defined by labyrinth seals. The flow mixing apparatus of the present invention may be combined with heat exchangers to further improve the effectiveness of the arrangement. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.