Patent Application
20180135525
The application relates generally to gas turbine engines and, more particularly, to bleed air configuration for engines with a centrifugal compressor.
In an aircraft, bleed air is compressed air that is taken/bled from the compressor section of an engine. Bleed air is used to provide pressurized air to various parts of an aircraft, such as the environmental control system (hereinafter referred to as “ECS”).
In gas turbine engines where the last (most downstream) compression stage is a centrifugal compressor/impeller (hereinafter referred to as “Engines with a Downstream Centrifugal Impeller”), it is well known to take ECS bleed air from 2 locations along the engines' annular gas path: one at a (1st) location positioned upstream of the last compression stage of the compressor (i.e. upstream of the centrifugal impeller), and another at a (2nd) location positioned downstream of the centrifugal impeller and upstream of the combustor, at a location known as P3. Typically, when the engines are operating in a low pressure (Low Power) range environment (i.e. when operating at low thrust or at high altitude), Low Power ECS bleed air is taken at the 2nd location (P3), and when the engines are operating in a high pressure (High Power) range environment (i.e. when operating at higher thrust or at lower altitude), High Power ECS bleed air is taken at the 1st location. A switching valve, operating on detected pressure, normally ensures the alternation between the two (Low Power & High Power) ECS bleed sources.
There is an ongoing need for ever more efficient ECS bleed air configurations for gas turbine engines, more specifically for Engines with a Downstream Centrifugal Impeller.
In one aspect, there is provided a method of bleeding air from a gas turbine engine having a compressor section with a downstream centrifugal compressor, the method comprising, when the compressor is operating in a first pressure range, bleeding air from a first location positioned upstream of the centrifugal compressor's outlet; and, when the engine is operating in a second pressure range, the second pressure range being lower than the first pressure range, bleeding air from a second location positioned upstream of the centrifugal compressor's outlet, the second location being positioned downstream of the first location. The method may supply air to an ECS of an aircraft.
In another aspect, there is provided a compressor section of a gas turbine engine for providing compressed air to a combustor, the compressor section comprising: an annular gas path, positioned around the engine's centerline, for conveying air across the compressor section; a centrifugal impeller, positioned in the compressor section's downstream extremity, the centrifugal impeller comprising a plurality of blades protruding within the annular gas path, wherein air enters the centrifugal impeller in a generally axial direction and exits the centrifugal impeller in a generally radial direction; a centrifugal impeller shroud surrounding the blades and acting as a portion of the annular gas path's radially outer boundary; a first bleed opening arrangement, for bleeding air from the annular gas path when the engine is operating in a first pressure range; and a second bleed opening arrangement, positioned in the impeller shroud, for bleeding air from the annular gas path when the engine is operating in a second pressure range, the second pressure range being lower than the first pressure range. When in operation, the first and second bleed opening arrangements may provide bleed air to an ECS of an aircraft.
In a further aspect, there is provided a centrifugal compressor shroud for use in a compressor section of a gas turbine engine, the centrifugal compressor shroud comprising an ECS (ECS) bleed opening arrangement, for providing bleed air to an ECS, the ECS bleed opening arrangement comprising orifices aligned partially tangential to the anticipated compressed air flow direction.
Further details of these and other aspects of the subject matter of this application will be apparent from the detailed description and drawings included below.
Reference is now made to the accompanying figures in which:
Bleed air is compressed air, taken/bled from compressor section 14 of an engine when in operation, for purposes other than combustion in the combustor 16. Bleed air is used to provide pressurized air to various parts of an aircraft, such as the environmental control system (ECS), the anti-icing system and various systems of the engine itself, examples of the latter being air used to preserve the stability of the compressor system (known as compressor handling bleed air) or air used to maintain internal engine functions (known as engine secondary air). Each such system has air pressure and/or temperature requirements, which has an effect on where and how bleed air may or may not be taken to satisfy the requirements of each such system.
In engines with a Downstream Centrifugal Impeller, it is well known to take air that will be supplied to the ECS, known as ECS bleed air, at the centrifugal impeller's outlet (i.e. at the outlet of compressor section 14/inlet of the combustor 16, a station known as P3), as this is where ECS bleed air requirements (mass flow, static pressure, temperature . . . ) are met for all engine operation conditions. It is also well known to take ECS bleed air at P3 only when an Engine with a Downstream Centrifugal Impeller is operating in a low pressure range environment (i.e. when operating at low thrust and/or at high altitude, an engine state known as “Low Power”) and to take ECS bleed air more upstream of the P3 station, but still downstream of what is known as the P2 station (which refers to compressor section 14's inlet), when an Engine with a Downstream Centrifugal Impeller is operating in a high pressure range environment (i.e. when operating at higher thrust and/or at lower altitude, an engine state known as “High Power”).
Indeed, when designing bleed air configurations, one must always strive to use as low a supply pressure as possible (i.e. as much upstream compressor section 14 as possible), because the energy that is used by an engine to compress the bleed air is not available for propulsion purposes, with the consequent fuel consumption/efficiency loss. Therefore, a well-known ECS bleed air configuration is such that, as engine operating pressure increases and a predetermined crossover point between the low pressure range and the high pressure range is reached, the ECS bleed air stops being taken from the P3 station and starts being taken more upstream, at stations known as P2.# (with “#” typically being numbers between 0 and 9, 0 referring to a location in the compression section close to its inlet and 9 referring to a location in the compression section close to its outlet); the reverse happens as engine operating pressure decreases. In other words, when an Engine with a Downstream Centrifugal Impeller is at Low Power, (Low Power) ECS bleed air is taken at P3 and, when an Engine with a Downstream Centrifugal Impeller is at High Power, (High Power) ECS bleed air is taken at P2.#.
Drawing ECS bleed air at P3 is however problematic as it sometimes contributes to Cabin Fume events (also known as “smoke in the cabin” events or “cabin air contamination” events). Cabin Fume events refer to instances when an aircraft air cabin is contaminated by chemicals. In aircraft powered by Engines with a Downstream Centrifugal Impeller, this sometimes occur because of the compressor shaft bearing cavity's (shown as item 50 in
Pursuant to an embodiment of the invention, an Engine with a Downstream Centrifugal Impeller will be described, more specifically, as shown in
Air thereafter flows through compressor section 14's last compression stage, in the current embodiment centrifugal impeller 30. As is represented schematically by box 60 and is well known in the art, centrifugal impeller 30 is linked to the remaining rotating portion of compressor section 14, such as axial compressor rotor 25 in the current embodiment. As also shown in
When air exits centrifugal impeller 30, and more generally compression section 14, it enters a diffuser 40, which purpose is to slow down the air flow, more specifically to convert the air flow's pressure from a predominantly dynamic pressure form to a predominantly static pressure form. Air exits diffuser 40 into P3 volute 44. Volute 44 is fluidly linked to combustor 16's entrance, resulting in compressed air from compressor section 14 being fed to combustor 16. P3 Volute 44 is where Low Power ECS bleed air is taken in previously known Engines with a Downstream Centrifugal Impeller. As will be seen in more details below, no ECS bleed air is taken at the P3 station or anywhere downstream centrifugal impeller 30's exit. Therefore, the Cabin Fume event issue outlined above is addressed.
In more recent Engines with a Downstream Centrifugal Impeller, the Overall Compression pressure Ratio (OPR), which is the factor by which compressor section 14 can increase the air pressure (i.e. air pressure at P3/ambient air pressure), is large enough so that the total air pressure at certain points between centrifugal impeller 30's entrance and exit is sufficient to meet current Low Power ECS bleed air requirements. Also, because of more recent aircraft's move towards more electric architectures, ECS bleed air requirements are lower, such that, again, the total air pressure at certain points between centrifugal impeller 30's entrance and exit is sufficient.
As shown in
In Engines with a Downstream Centrifugal Impeller, the location immediately upstream of centrifugal impeller 30 is known as P2.7 and the location over centrifugal impeller 30 (i.e. between the inlet and outlet of this last compression stage), is known as P2.8. In the current embodiment, 1st location is positioned at P2.7, where 1st bleed opening arrangement 21 can be found. High Power ECS bleed air H is taken from the 1st location, more specifically via 1st bleed opening arrangement 21, and directed into P2.7 Volute 24. No further details is given regarding P2.7 Volute 24, or any other volute referred to elsewhere in this description as such details will be apparent to someone skilled in the art.
High Power ECS bleed air H is then taken from P2.7 Volute 24 to the ECS in the way that will be apparent to those skilled in the art (not shown). In the current embodiment, High Power ECS bleed air H is sufficient to meet the compressor handling bleed air and engine bleed air requirements; therefore, air bled at this 1st location is utilised for all three purposes (High Power ECS, compressor handling and engine secondary air system). One skilled in the art will however understand that the invention is not limited to requiring that ECS bleed air taken at the 1st location having to meet all 3 such purposes; other purposes besides, or no purposes save for, High Power ECS system requirements are possible pursuant to the invention. As air pressure exiting axial compressors are generally in the form of static pressure and hence not in need to be significantly converted any further, 1st bleed opening arrangement 21 are orifice(s), through radial outer wall 23, aligned perpendicularly to the air flow direction (similarly to a static pressure tap hole).
In the current embodiment, 2nd location is positioned on centrifugal impeller shroud 38, in the current embodiment at station P2.8, where 2nd bleed opening arrangement 31 can be found. Low Power ECS bleed air L is taken from the 2nd location, more specifically via 2nd bleed opening arrangement 31, and directed into P2.8x Volute 34.
Low Power ECS bleed air L is then taken from P2.8x Volute 34 to the ECS in the way that will be apparent to those skilled in the art (not shown). In the current embodiment, Low Power ECS bleed air L is sufficient to meet the aircraft anti-icing system bleed air requirements; therefore, air bled at this 2nd location is utilised for both purposes (Low Power ECS and anti-icing system). One skilled in the art will however understand that the invention is not limited to requiring that ECS bleed air taken at the 2nd location having to meet both purposes; other purposes besides, or no purposes save for, Low Power ECS system requirements are possible pursuant to the invention.
The air passing through centrifugal impeller 30, more specifically the air flowing over centrifugal impeller shroud 38, fluctuates in terms of its characteristics, such as total pressure, ratio of dynamic vs static pressure, turbulence etc. . . . . The choice of the 2nd location, more specifically the location of 2nd bleed opening arrangement 31, as well as the characteristics of such 2nd bleed opening arrangement 31, will therefore depend on such characteristics. The 2nd location must be positioned, along centrifugal impeller shroud 38, where sufficient total energy (air pressure) is available to meet current Low Power ECS bleed air requirements and 2nd bleed opening arrangement 31 must have enough dynamic energy (pressure) recovery characteristics to ensure the necessary dynamic to static energy (pressure) conversion. Indeed, the dynamic energy (i.e. pressure) of air flowing through a centrifugal impeller represents a large proportion of the total energy (i.e. pressure) of such flow. As only the static energy (i.e. pressure) is useful for bleed air purposes, one must ensure, when taking bleed air from a passing air flow, that the static energy (pressure) is high enough to meet the bleed requirements, and/or to sufficiently convert dynamic energy (pressure) into static energy (pressure) to meet such requirements. Pursuant to the invention, this means that, when adding the static pressure at the 2nd location plus the dynamic recovery characteristics of the 2nd bleed opening arrangement 31, the Low Power ECS bleed air requirements are met. As one travels upstream along centrifugal impeller shroud 38 and the static pressure increases, this means that the dynamic recovery needs of the 2nd bleed opening arrangement 31 decrease
A shown in
wherein
with R1: radial distance of shroud 38, at the entrance of centrifugal impeller 30
all radial distances being from engine centerline A.
A 100% RNS position would mean that 2nd bleed opening arrangement 31 would be positioned at centrifugal impeller 30's exit. As outlined above, no ECS bleed air is to be taken at the P3 station or anywhere downstream centrifugal impeller 30's exit (to address Cabin Fume event issues). In order to add a certain margin of safety in that regard (i.e. to avoid taking P3 air), it is preferred to avoid taking ECS bleed air in the immediate upstream vicinity of centrifugal impeller 30's exit. Furthermore, taking ECS bleed air in the immediate upstream vicinity of centrifugal impeller 30's exit could raise some tip impeller to shroud clearance issues. More specifically, having bleed opening arrangements and/or dynamic pressure recovery mechanisms near the shroud's exit could affect such shroud tip's stiffness, with a negative impact on the impeller-shroud clearance at that location. Shroud stiffness design is contingent upon anticipated impeller and shroud displacement during operation. Consequently, it has been found to be preferred to have 2nd bleed opening arrangement 31 positioned upstream of the 75% RNS position.
A 0% RNS position would mean that 2nd bleed opening arrangement 31 would be positioned at centrifugal impeller 30's entrance. That position, or in its immediate downstream vicinity, would not be preferred because of the insufficient pressure rise in the air. Furthermore, at least until a point downstream of splitter blade leading edge plane 36, air flow would present stability issues. Consequently, it has been found to be preferred to have 2nd bleed opening arrangement 31 positioned downstream of the splitter blade leading edge plane 36 or, alternatively, downstream of the 10% RNS position. In the embodiment shown in
Turning now to the characteristics of 2nd bleed opening arrangement 31, it is understood the more upstream it is positioned, the greater the dynamic pressure recovery will be needed. This is because, as one travels upstream on centrifugal impeller shroud 38, the lower the available static pressure of the air is. In the embodiment shown in
In the embodiment shown in
Low Power ECS bleed air L is taken at a 2nd location, more specifically from 2nd bleed opening arrangement 131, which is positioned more downstream of 2nd bleed opening arrangement 31 described in
A 3rd bleed air location, positioned on centrifugal impeller shroud 38 and upstream of 2nd location, is utilised for compressor handling bleed air purposes. This is where 3rd bleed opening arrangement 231 can be found. One skilled in the art will recognise that 3rd bleed opening arrangement 231 is positioned upstream of splitter blade leading edge plane 36, with the consequent air flow turbulence and lower total pressure issues that arise therefrom; the characteristics of 3rd bleed opening arrangement 231 will be consequently adjusted to meet such compressor handling bleed air purposes.
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 departing from the scope of the invention disclosed. For example, in the embodiment outlined above, a number of axial compressors are found upstream of centrifugal impeller 30. One, or no, axial compressor may be found (for example in Engines with a Downstream Centrifugal Impeller where centrifugal impeller 30 provides all of the necessary air compression). Alternatively, other types of compressors, such as one or more centrifugal impellers, may be found upstream of centrifugal impeller 30. Furthermore, other bleed opening arrangements, over and above the 2 ECS bleed air opening arrangements, may be found pursuant to the invention. 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.
