Inter-turbine ducts with variable area ratios

A turbine section of a gas turbine engine is annular about a longitudinal axis. The turbine section includes a first turbine with a first inlet and a first outlet; a second turbine with a second inlet and a second outlet; and an inter-turbine duct extending from the first outlet to the second inlet and configured to direct an air flow from the first turbine to the second turbine. The inter-turbine duct has a first station with a first meridional area, a second station with a second meridional area, and a third station with a third meridional area. The first station is upstream of the second station and the second station is upstream of the third station, and the second meridional area is less than or equal to the first meridional area.

TECHNICAL FIELD

The present invention generally relates to gas turbine engines, and more particularly relates to inter-turbine ducts between the turbines of gas turbine engines.

BACKGROUND

A gas turbine engine may be used to power various types of vehicles and systems. A gas turbine engine may include, for example, five major sections: a fan section, a compressor section, a combustor section, a turbine section, and an exhaust nozzle section. The fan section induces air from the surrounding environment into the engine and accelerates a fraction of this air toward the compressor section. The remaining fraction of air induced into the fan section is accelerated through a bypass plenum and exhausted. The compressor section raises the pressure of the air it receives from the fan section and directs the compressed air into the combustor section where it is mixed with fuel and ignited. The high-energy combustion products then flow into and through the turbine section, thereby causing rotationally mounted turbine blades to rotate and generate energy. The air exiting the turbine section is exhausted from the engine through the exhaust section.

In some engines, the turbine section is implemented with one or more annular turbines, such as a high pressure turbine and a low pressure turbine. The high pressure turbine may be positioned upstream of the low pressure turbine and configured to drive a high pressure compressor, while the low pressure turbine is configured to drive a low pressure compressor and a fan. The high pressure and low pressure turbines have optimal operating speeds, and thus, optimal radial diameters that are different from one another. Because of this difference in radial size, an inter-turbine duct is arranged to fluidly couple the outlet of the high pressure turbine to inlet of the low pressure turbine and to transition between the changes in radius. It is advantageous from a weight and efficiency perspective to have a relatively short inter-turbine duct. However, decreasing the length of the inter-turbine duct increases the radial angle at which the air must flow between the turbines. Increasing the angle of the duct over a relatively short distance may result in boundary layer separation of the flow within the duct, which may adversely affect the performance of the low pressure turbine. Accordingly, the inter-turbine ducts are designed with a compromise between the overall size and issues with boundary layer separation. As a result, some conventional gas turbine engines may be designed with elongated inter-turbine ducts or inter-turbine ducts that do not achieve the optimal size ratio between the high pressure turbine and the low pressure turbine.

Accordingly, it is desirable to provide gas turbine engines with improved inter-turbine ducts. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.

BRIEF SUMMARY

In accordance with an exemplary embodiment, a turbine section of a gas turbine engine is annular about a longitudinal axis. The turbine section includes a first turbine with a first inlet and a first outlet; a second turbine with a second inlet and a second outlet; and an inter-turbine duct extending from the first outlet to the second inlet and configured to direct an air flow from the first turbine to the second turbine. The inter-turbine duct has a first station with a first meridional area, a second station with a second meridional area, and a third station with a third meridional area. The first station is upstream of the second station and the second station is upstream of the third station, and the second meridional area is less than or equal to the first meridional area.

In accordance with another exemplary embodiment, an inter-turbine duct is provided extending between a first turbine having a first radial diameter and a second turbine having a second radial diameter, the first radial diameter being less than the second radial diameter. The inter-turbine duct includes a hub and a shroud circumscribing the hub to form a flow path fluidly coupled to the first turbine and the second turbine. The hub and shroud converge or maintain a constant separation with respect to meridional area in a first portion, and the hub and shroud diverge relative to one another in a second portion with respect to meridional area.

DETAILED DESCRIPTION

Broadly, exemplary embodiments discussed herein provide gas turbine engines with improved inter-turbine ducts. In one exemplary embodiment, the inter-turbine duct is positioned between a high pressure turbine with a relatively small radial diameter and a low pressure turbine with a relatively large radial diameter. The inter-turbine duct may be defined by a shroud forming an outer boundary and a hub forming an inner boundary. As such, the meridional area of the flow path of the inter-turbine duct is defined between the hub and the shroud at any axial position, and the change in the meridional area along the length of the inter-turbine duct may be referred to as the meridional area ratio. In one exemplary embodiment, the meridional area ratio distribution is constant or decreases (e.g., the areas are constant or converge in a downstream direction) in a forward portion of the inter-turbine duct, and the meridional area ratio distribution increases (e.g., the areas diverge in a downstream direction) at an aft portion of the inter-turbine duct. These configurations may reduce separation of the air flow from the shroud wall, and thus, reduce pressure losses in the inter-turbine ducts.

FIG. 1a schematic cross-sectional view of a gas turbine engine100in accordance with an exemplary embodiment. As shown, the engine100may be an annular structure about a longitudinal or axial centerline axis102. In the description that follows, the term “axial” refers broadly to a direction parallel to the axis102about which the rotating components of the engine100rotate. This axis102runs from the front of the engine100to the back of the engine100. The term “radial” refers broadly to a direction that is perpendicular to the axis102and points towards or away from the axis of the engine100. A “circumferential” direction at a given point is a direction that is normal to the local radial direction and normal to the axial direction. As such, the term “axial-circumferential” plane generally refers to the plane formed by the axial and circumferential directions, and the term “axial-radial” plane generally refers to the plane formed by the axial and radial directions. An “upstream” direction refers to the direction from which the local flow is coming, while a “downstream” direction refers to the direction in which the local flow is traveling. In the most general sense, flow through the engine tends to be from front to back, so the “upstream direction” will generally refer to a forward direction, while a “downstream direction” will refer to a rearward direction.

The engine100generally includes, in serial flow communication, a fan section110, a low pressure compressor120, a high pressure compressor130, a combustor140, and a turbine section150, which may include a high pressure turbine160and a low pressure turbine170. During operation, ambient air enters the engine100at the fan section110, which directs the air into the compressors120and130. The compressors120and130provide compressed air to the combustor140in which the compressed air is mixed with fuel and ignited to generate hot combustion gases. The combustion gases pass through the high pressure turbine160and the low pressure turbine170. As described in greater detail below, an inter-turbine duct180couples the high pressure turbine160to the low pressure turbine170.

The high pressure turbine160and low pressure turbine170are used to provide thrust via the expulsion of the exhaust gases, to provide mechanical power by rotating a shaft connected to one of the turbines, or to provide a combination of thrust and mechanical power. As one example, the engine100is a multi-spool engine in which the high pressure turbine160drives the high pressure compressor130and the low pressure turbine170drives the low pressure compressor120and fan section110.

FIG. 2is a schematic, partial cross-sectional view of a turbine assembly with an inter-turbine duct, such as the inter-turbine duct180of the turbine section150of the engine100ofFIG. 1in accordance with an exemplary embodiment.

As shown, the turbine section150includes the high pressure turbine160, the low pressure turbine170, and the inter-turbine duct180fluidly coupling the high pressure turbine160to the low pressure turbine170. Particularly, the inter-turbine duct180includes an inter-turbine duct inlet202coupled to a high pressure turbine outlet162and an inter-turbine duct outlet204coupled to a low pressure turbine inlet172. The annular structure of the inter-turbine duct180is defined by a hub210and a shroud220to create a flow path230for air flow (e.g., air flow232and234) between the high pressure and low pressure turbines160and170.

As noted above, the inter-turbine duct180transitions from a first radial diameter250at the inlet202(e.g., corresponding to the radial diameter at the outlet162of the high pressure turbine160) to a larger, second radial diameter252(e.g., corresponding to the radial diameter at the inlet172of the low pressure turbine170). In one exemplary embodiment, as shown inFIG. 2, the radial diameters are measured from the mid-point of the inter-turbine duct180although such diameters may also be measured from the hub210and/or the shroud220. This transition is provided over an axial length254. The hub210and shroud220may have various shapes to transition between the radial diameters250,252along the axial length254. For example, the shroud220may extend at constant or changing angles (e.g., angles212,222), as described in greater detail below.

In general, it is advantageous to minimize the axial length254of the inter-turbine duct180for weight and efficiency. For example, a shorter axial length254may reduce the overall axial length of the engine100(FIG. 1) as well as reducing friction losses of the air flow232,234. However, as the axial length254is decreased, the orientation of the air flow232,234must be more aggressively transitioned over a shorter length.

During operation, the inter-turbine duct180functions to direct the air flow232,234along the radial transition between turbines160and170. It is generally advantageous for the air flow232,234to flow smoothly through the inter-turbine duct180. Particularly, it is advantageous if the air flow232adjacent to the shroud220maintains a path along the shroud220instead of undergoing a boundary layer separation. However, as the axial length254decreases and the angle between the turbines160,170increases, the air flow232tends to maintain an axial momentum through the inlet202and, if not addressed, attempts to separate from the shroud220, particularly in upstream regions of the shroud220. Such separations may result in vortices or other turbulence that result in undesirable pressure losses through the inter-turbine duct180as well as inefficiencies in the low pressure turbine170.

The inter-turbine duct180may be considered with respect to duct or flowpath stations positioned along the axial length254. InFIG. 2, examples of duct stations are labeled as duct stations290,292,294,296. Any number of stations may be defined and considered, and a station line for each station290,292,294,296, as shown, extends between a station point at a predetermined fractional distance along the surface of the hub210(e.g., 10% of the hub210along the surface from the inlet) and a corresponding station point at the same predetermined fractional distance along the surface of the shroud220(e.g., 10% of the shroud220along the surface from the inlet). Since the hub210and the shroud220may have different shapes, the station lines are not necessarily radial.

The area at any duct station290,292,294,296may be defined with respect to meridional area. Reference is briefly made toFIG. 3, which is a schematic representation of a turbine section illustrating the meridional area of an inter-turbine duct at a station300, e.g., station290,292,294,296ofFIG. 2.

InFIG. 3, the station300is defined by a shroud302and a hub304, as above. A shroud angle or slope (ΦSHROUD)) of the shroud302relative to an axial reference and a hub angle or slope (ΦHUB) relative to the axial reference are labeled. The meridional flow corresponds to the air flow characteristics of the gases flowing through the station. Typically, a pitch or meridional angle (ΦM) of the meridional flow is evaluated. Since such a flow may vary between the shroud302and hub304, a simplified meridional angle (ΦM) may be considered. The meridional angle (ΦM) may be calculated by averaging the flow vector along a station line height (hs) or by taking the average of the shroud angle (ΦSHROUD)) and the hub angle (ΦHUB) at the axial endpoints of the station. In general, the station line height (hs) is the length of the line segment defining the given station300. In other words, any number of stations may be defined, and a station line height (hs) extends between a station point at a predetermined fractional distance along the surface of the hub304(e.g., 10% of the hub304along the surface from the inlet) and a corresponding station point at the same predetermined fractional distance along the surface of the shroud302(e.g., 10% of the shroud302along the surface from the inlet). Since the hub304and the shroud302may have different shapes, the station line is not necessarily radial. The slope of a normal vector to the station line and the engine axis (or the angle between a radial vector (hA) and the station line (hs)) is labeled as the station angle (Φs). The radial midpoint of the duct at the station from an engine centerline is labeled as radius (RM). From these relationships, the meridional flow height (hM) may be defined as follows in Equation (1):
hM=hS*cos(ΦM−ΦS)  Eq. (1).
where
hMis the station meridional height;
hSis the station height;
ΦMis the meridional angle; and
ΦSis the station angle.

Meridional area is defined as the projection of the station area normal to the notional flow direction as projected on the meridional view, as expressed in Equation (2):
AM=2π*RM*hMEq. (2).
where
AMis the meridional area;
RMis the radial radius of the duct; and
hMis the meridional height.

Inter-turbine duct characteristics may be expressed as a meridional area ratio of the meridional area at a station relative to the meridional area of the duct inlet, as expressed in Equation (3):
MAR=AM_STATION/AM_INLETEq. (3)
where
MAR is the meridional area ratio;
AM_STATIONis the meridional area at a given station; and
AM_INLETis the meridional area at the duct inlet.

Now that meridional area and meridional area ratio have been defined, reference is again directed toFIG. 2. In accordance with an exemplary embodiment, a distribution of meridional area along the length254of the duct180may be defined to provide advantageous flow characteristics. In conventional inter-turbine ducts, the area ratio (e.g., the meridional area at any given station location divided by the meridional area at the inlet) is continuously increasing through the entire length of the duct. However, in one exemplary embodiment, the area distribution may be manipulated in a predetermined manner to impact the characteristics of the air flow through the inter-turbine duct180, and in particular, to prevent or reduce separation of air flow from the shroud220.

As an example,FIG. 2depicts a meridional area ratio distribution that is “converging/diverging.” In other words, the meridional area ratio decreases (or converges) from the inlet to a predetermined distance from the inlet202. In this embodiment, the meridional area ratio then increases (or diverges) along the axial length254to the outlet204of the duct180. Using the depicted embodiment as an example, the meridional area at station290is greater than the meridional area at station292, which is greater than the meridional area at station294. As such, the meridional areas are decreasing or converging through the first three stations290,292, and294. However, the meridional area at station294is less than the meridional area at station296, thus resulting in an increasing or diverging meridional area distribution.

The position at which the meridional area ratio transitions from converging to diverging may be at any axial length. In the depicted embodiment, the axial position is about 20% of the axial length, although in other embodiments, the axial position may be about 15% to about 80%. The length of duct180from the inlet202to the predetermined distance may be referred to as the first portion and the length of the duct180from the predetermined distance may be referred to as the second portion, such that the first portion has a converging area ratio distribution and the second portion has a diverging area ratio distribution. In the depicted exemplary embodiment, the position of station294may correspond to the predetermined distance at which the duct180starts to diverge. In one exemplary embodiment, the meridional area ratio distribution in the first portion is continuously converging and the meridional area ratio distribution in the second portion is continuously diverging.

FIG. 4is a schematic, partial cross-sectional view of a turbine section450with an inter-turbine duct480that may be implemented, for example, in the gas turbine engine100ofFIG. 1in accordance with a second exemplary embodiment.

As in the embodiments ofFIGS. 1 and 2, the turbine section450includes a high pressure turbine460, a low pressure turbine470, and the inter-turbine duct480fluidly coupling the high pressure turbine460to the low pressure turbine470. Particularly, the inter-turbine duct480includes an inlet402coupled to the outlet462of the high pressure turbine460and an outlet404coupled to the inlet472of the low pressure turbine470. The annular structure of the inter-turbine duct480is defined by a hub410and a shroud420to create a flow path430for air flow (e.g., air flow432and434) between the high pressure and low pressure turbines460,470.

As noted above, the inter-turbine duct480transitions from a first radial diameter450at the inlet402to a larger, second radial diameter412over an axial length. The hub410and shroud420may have various shapes to transition between the radial diameters450,452along the axial length. For example, the shroud420may extend at constant or changing angles (e.g., angles422,424) to prevent boundary layer separation, as described in greater detail below.

In the view ofFIG. 4, the stations490,492,494,496are schematically depicted between the hub410and shroud420at various positions along the inter-turbine duct480. As an example,FIG. 4depicts a meridional area ratio distribution that is “constant/diverging.” In other words, the meridional area ratio is constant from the inlet402to a predetermined axial distance from the inlet402. In this embodiment, the meridional area ratio then increases (or diverges) along the axial length to the outlet of the duct480. Using the depicted embodiment as an example, the meridional area at station490is equal than the meridional area at station492and the meridional area at station494. As such, the meridional areas are constant through the first three stations490,492, and494. However, the meridional areas at stations490,492, and494are less than the meridional area at station496, thus resulting in an increasing or diverging meridional area distribution.

The position at which the meridional area ratio transitions from converging to diverging may be at any axial length. In the depicted embodiment, the axial position is about 20% of the axial length, although in other embodiments, the axial position may be about 15% to about 80%. In one exemplary embodiment, the meridional area ratio distribution in the first portion is continuously constant and the meridional area ratio distribution in the second portion is continuously diverging.

FIG. 5is a chart500depicting meridional area ratio as a function of axial distance of the duct section (e.g., duct sections180and480ofFIGS. 2 and 4) in accordance with an exemplary embodiment. As noted above, the meridional area ratio may be defined as the meridional area at a given axial station location divided by the meridional area at the inlet of the duct. As also above, the meridional area ratio may be designed to provide advantageous flow characteristics.

In another exemplary embodiment represented by line502, the meridional area ratio distribution is “converging/diverging,” such as the duct180shown inFIG. 2. In other words, the meridional area ratio decreases (or converges) from the inlet to a predetermined axial distance from the inlet. In this embodiment, the meridional area ratio then increases (or diverges) along the axial length to the outlet of the duct. The position at which the meridional area ratio transitions from converging to diverging may be at any axial length. In the depicted embodiment, the axial position is about 20% of the axial length, although in other embodiments, the axial position may be about 15% to about 80%.

In one exemplary embodiment represented by line512, the meridional area ratio distribution is “constant/diverging,” such as the duct480shown inFIG. 4. In other words, the meridional area ratio is constant from the inlet to a predetermined axial distance from the inlet. In this embodiment, the meridional area ratio then increases (or diverges) along the axial length to the outlet of the duct. The position at which the meridional area ratio transitions from constant to diverging may be at any axial length. In the depicted embodiment, the axial position is about 20% of the axial length, although in other embodiments, the axial position may be about 15% to about 80%.

As a comparison, the area ratio distribution of an exemplary conventional duct is illustrated by line522. Typically, the area ratio distribution is not a consideration for conventional duct designers, although the resulting ducts general have continuously diverging meridional area ratios along the entire length.

The particular location at which the area ratio increases, as well as the amount of increase, may be selected based on computational fluid dynamics (CFD) analysis of various flow rates through the inter-turbine duct and/or weight, installation, cost or efficiency considerations. In general, active devices, such as flow injectors, and additional structures, such as guide vanes, are not necessary to maintain a smooth flow through the inter-turbine duct, particularly along the shroud. In one exemplary embodiment, the flow negotiates both the sharp bend (radially outward) in flow direction in the early portion of the duct and an area increase as the flow progresses from the inlet to the exit. In the depicted embodiments, maintaining constant area or, in fact, a converging area in the region of the initial bend, and then later having the area increase, enables flow to negotiate the bend without separating along the shroud. By constricting or maintaining area through these area functions, mean flow velocities are higher and peak velocity in the flow near the shroud is reduced, thereby lowering the total diffusion on the shroud and reducing the risk of separation. In particular, the radial angle of the inter-turbine duct may be increased and/or the axial length may be decreased to reduce the overall length and weight of the engine and to reduce friction and pressure losses in the turbine section.

The relatively compact nature of the flow control scheme also enables retrofitting of existing engines and engine designs with a minimum of additional complexity. In general, the techniques described above can be applied either during the design of a new engine to take advantage of the shorter duct length and optimized area-ratio made possible by the boundary layer control, or to retrofit an existing engine or engine design in order to improve the efficiency of the engine while changing the design as little as possible. Additionally, existing engines may be shortened for weight reduction and rotor-dynamic improvement enables by shortening the bearing span. Although reference is made to the exemplary gas turbine engine depicted inFIG. 1, it is contemplated that the inter-turbine ducts discussed herein may be adapted for use with other types of turbine engines including, but not limited to steam turbines, turboshaft turbines, water turbines, and the like. Moreover, the turbine engine described above is a turbofan engine for an aircraft, although exemplary embodiments may include without limitation, power plants for ground vehicles such as locomotives or tanks, power-generation systems, or auxiliary power units on aircraft.