WO2019143366A1

WO2019143366A1 - Exhaust diffuser for a gas turbine engine

Info

Publication number: WO2019143366A1
Application number: PCT/US2018/014646
Authority: WO
Inventors: Kai Kadau; Jose Antonio PASCUAL-GUTIERREZ; Bogdan R. Raica
Original assignee: Siemens AG; Siemens Corp
Current assignee: Siemens AG; Siemens Corp
Priority date: 2018-01-22
Filing date: 2018-01-22
Publication date: 2019-07-25
Anticipated expiration: 2020-07-22

Abstract

A turbine exhaust diffuser (12) for a gas turbine engine having a turbine section, the exhaust diffuser (12) includes a flowpath (14) downstream of the turbine section. The flowpath (14) is defined at least in part by a turbine casing (16) having an inner casing (18) forming an ID flowpath boundary (20) and an outer casing (22) forming an OD flowpath boundary (24). A least one strut (26) is positioned within the flowpath (14) and spans between the inner casing (18) and the outer casing (22). The exhaust diffuser (12) includes an undulating portion (32) located along at least one of the inner casing (18) and the outer casing (22) along the exhaust diffuser (12). The undulating portion (32) includes at least a first point (34) upstream of the at least one strut (26) and a second point (36) approximate to a halfway span of the at least one strut (26). The undulating portion (32) includes radially and axially extending crests (42) and troughs (44).

Description

EXHAUST DIFFUSER FOR A GAS TURBINE ENGINE BACKGROUND 1. Field

[0001] The present invention relates to turbine engines, and more specifically to flow paths in exhaust diffusers in gas turbine engines.

2. Description of the Related Art

[0002] In an industrial gas turbine engine, hot compressed gas is produced. A combustion system receives air from a compressor and raises it to a high energy level by mixing in fuel and burning the mixture, after which products of the combustor are expanded through a turbine. The hot gas flow is passed through the turbine and expands to produce mechanical work used to drive an electric generator for power production. The turbine generally includes multiple stages of stator vanes and rotor blades to convert the energy from the hot gas flow into mechanical energy that drives the rotor shaft of the engine. Both the turbine section and the compressor section have stationary or non-rotating components, such as vanes, for example, that cooperate with rotatable components, such as blades, for example, for compressing and expanding the hot working gas.

[0003] An exhaust diffuser is located at an extreme aft end of the gas turbine, bolts to, and is supported by the exhaust frame. An important element affecting the efficiency of a gas turbine is the aerodynamic performance of the exhaust diffuser (ED). The role of the exhaust diffuser is to slow down the high speed flow exiting the last stage of the turbine as isentropically as possible in order to bring up its pressure back to ambient values.

[0004] Traditionally, an exhaust diffuser is the volume left between an inner cylinder and an outer cone concentrically surrounding the former. The functionality of the ED is to gradually increase the flow path area so as to slow down the flow (See FIG. 1).

SUMMARY [0005] In one aspect of the present invention, a turbine exhaust diffuser for a gas turbine engine having a turbine section, the exhaust diffuser comprises: a flowpath located downstream of the turbine section; wherein the flowpath is defined at least in part by a turbine casing having an inner casing forming an inner-diameter (ID) flowpath boundary and an outer casing forming an outer-diameter (OD) flowpath boundary; at least one strut positioned within the flowpath and spanning between the inner casing and the outer casing, wherein the at least one strut comprises a leading edge towards the upstream direction and a trailing edge towards the downstream direction; and an undulating portion located along at least one of the inner casing and the outer casing along the exhaust diffuser, wherein the undulating portion includes at least a first point positioned upstream of the at least one strut, a second point positioned approximately near a halfway span of the at least one strut, the undulating portion including radially and axially extending crests and troughs.

[0006] These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention is shown in more detail by help of figures. The figures show preferred configurations and do not limit the scope of the invention. [0008] FIG. 1 is a partial cross section of a turbine engine having features according to the prior art.

[0009] FIG. 2 is a detailed axial-radial cross section view of a turbine exhaust diffuser system of an exemplary embodiment of the present invention with straight segments. [0010] FIG. 3 is a detailed axial-radial cross section view of a turbine exhaust diffuser system of an exemplary embodiment of the present invention with curved segments.

[0011] FIG. 4 is a detailed axial-radial cross section view of a turbine exhaust diffuser system of an exemplary embodiment of the present invention.

[0012] FIG. 5 is a perspective view of a turbine exhaust diffuser system of an exemplary embodiment of the present invention.

[0013] FIG. 6 is a chart comparing an exemplary embodiment of the present invention and the prior art.

[0014] FIG. 7 is a chart comparing an exemplary embodiment of the present invention and the prior art.

DETAILED DESCRIPTION [0015] In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention.

[0016] Broadly, an embodiment of the present invention provides a turbine exhaust diffuser for a gas turbine engine having a turbine section, the exhaust diffuser includes a flowpath downstream of the turbine section. The flowpath is defined at least in part by a turbine casing having an inner casing forming an ID flowpath boundary and an outer casing forming an OD flowpath boundary. A least one strut is positioned within the flowpath and spans between the inner casing and the outer casing. The exhaust diffuser includes an undulating portion located along at least one of the inner casing and the outer casing along the exhaust diffuser. The undulating portion includes at least a first point upstream of the at least one strut and a second point approximate to a halfway span of the at least one strut. The undulating portion includes radially and axially extending crests and troughs.

[0017] A gas turbine engine may comprise a compressor section, a combustor and a turbine section. The compressor section compresses ambient air. The combustor combines the compressed air with a fuel and ignites the mixture creating combustion products comprising hot gases that form a working fluid. The working fluid travels to the turbine section. Within the turbine section are circumferential rows of vanes and blades, the blades being coupled to a rotor. The turbine section comprises a fixed turbine casing, which houses the vanes, blades and rotor. A blade of a gas turbine receives high temperature gases from a combustion system to produce mechanical work of a shaft rotation. Along the aft end of the turbine section is an exhaust diffuser (ED). An ED includes a turbine exit casing (TEC) followed by a turbine exit manifold (TEM). Within the TEC an aft engine bearing sits and airfoil-shaped vanes, called struts, run from an inner casing, or hub, to an outer casing, or shroud. These struts that are hollow provide pathways for oil feed pipes, drain pipe and the like to reach the bearing housing from outside the exhaust diffuser.

[0018] Losses can occur in the ED caused by large turbulence regions that evolve from early onset of flow instabilities upstream. Further, computational fluid dynamics (CFD) simulations show that turbulence in the ED is initiated in the area of the struts. With high flow velocities having high Reynolds numbers, an uneven radial velocity and pressure profile, and a high tangential component of the velocity, which can be up to roughly thirty percent of the total, causes flow to meet the strut with high incidence angles.

[0019] Relatively small disturbances caused by flow recirculation in the strut areas of the exhaust diffuser can have a large influence on the flow dynamics downstream of the strut areas within the TEC and further downstream within the turbine exhaust manifold (TEM). Large scale turbulence evolves from these small disturbances through propagation and strengthens as the flow moves downstream. The creation of all these disturbances of turbulence affect the overall performance in terms of pressure recovery, and therefore, engine efficiency.

[0020] A reduction in turbulence and an increase in overall efficiency is desired. Embodiments described below provide elements that allow for a turbulence reduction so as to maintain a high aerodynamic efficiency in the exhaust diffuser. [0021] Referring now to FIGS. 1 through 5, a portion of a gas turbine engine 10 is illustrated. As illustrated, the gas turbine engine 10 includes one or more turbine assemblies 46. A longitudinal axis 50 of the turbine engine 10 is shown. A flowpath 14 of turbine exhaust gases generally runs downstream from the one or more turbine assemblies 46 towards a far aft end of the turbine engine 10 through an exhaust diffuser 12.

[0022] Referring now again to FIGS. 1 through 5, the exhaust diffuser includes a turbine exit casing (TEC) 16 followed downstream by a turbine exit manifold (TEM) 48. The exhaust diffuser 12 is roughly a volume that is left between an inner cylinder and an outer cone concentrically surrounding the former. The exhaust diffuser 12 may include one or more flowpaths 14 downstream of the one or more turbine assemblies 46. Within the TEC 16, the flowpath 14 is defined by an outer casing 22 forming an OD flowpath boundary 24 and an inner casing 18 forming an ID flowpath boundary 20. The exhaust diffuser 12 includes at least one strut 26. Within the TEC 16 the at least one strut 26 runs in length from the inner casing 18 to the outer casing 22. The at least one strut 26 can be hollow in order to provide pathways for oil feed pipes and drain pipes to reach a bearing housing from outside the exhaust diffuser 12. A similar configuration of the at least one strut 26 may be found in the TEM 48 as well. Each strut 26 includes a leading edge 28 facing an upstream direction of the exhaust diffuser 12 towards the turbine section and a trailing edge 30 facing a downstream direction of the exhaust diffuser 12.

[0023] In this description, the relative terms“upstream” and“forward” refer to a direction along the longitudinal axis 50 toward the turbine section or beginning of the exhaust diffuser 12, while the relative terms“downstream” and“aft” refer to a direction along the longitudinal axis 50 toward an end of the exhaust diffuser 12 and turbine engine 10. The upstream and downstream directions are in reference to the flowpath and how it relates to the location along the turbine engine.

[0024] Illustrated in FIGS. 2 through 5 are embodiments of the exhaust diffuser 12 that include an undulating portion 32. The undulating portion 32 of the exhaust diffuser 12 is in reference to a portion of the ED 12 where at least one of the inner casing 18 and the outer casing 22 in the area of the exhaust diffuser 12 surrounding and covering the area including the at least one strut 26. Within the undulating portion 32 of the exhaust diffuser 12, at least one of the inner casing 18 and the outer casing 22 includes radially and axially extending crests 42 and troughs 44. The combination of crests 42 and troughs 44 can be designated as waves. Each crest 42 can initiate a decline section. While each trough 44 can start an incline section.

[0025] Predefined degrees of freedom provide an optimization. As an example, points of the outer casing 22 can be allowed to move axially and radially within given bounds to work towards a target of pressure recovery. Manufacturing considerations may include whether only truncated cone-like structures are used for the outer casing 22, how many of these structures, and the like, can be considered.

[0026] The following is a description of the undulating portion 32 around one specific strut 26, however, this undulating portion 32 can be repeated for additional struts 26 within the exhaust diffuser 12 that includes within the TEC 16 and the TEM 48. FIG. 3 also shows an example of where the ID flowpath boundary and OD flowpath boundary would be positioned in a conventional exhaust diffuser. The undulating portion 32 is broken up into different sections that either increase or decrease the cross-sectional area increase. Each section begins with a change in the either expansion or narrowing of the rate of the flowpath cross sectional area increase such as one of the crests 42 or troughs 44 indicating a high point and low point respectively.

[0027] Undulations in the ID flowpath boundary 20 and/or OD flowpath boundary 24 vary the function describing the rate of cross-sectional area increase with respect to axial location. Undulations may or may not narrow the cross-sectional area itself. The cross-sectional area is, in principle, always increasing so as to slow down the flow. A conventional TEC maintains the rate of cross-sectional area increase determined by a unique function of axial location. Using the undulating ID flowpath boundary 20 and/or OD flowpath boundary 24, however, the function describing the rate of cross-sectional area increase is changed. The struts 26 pose a reduction of cross-sectional area or area blockage. If the ID flowpath boundary 20 or OD flowpath boundary 24 did not show undulations, the flow will experience a decrease in its deceleration as it encounters the struts 26. To keep the deceleration unaffected by the stmts 26, or even increase its deceleration, so that the tendency to separate at a suction side of the at least one stmt 26 is removed, the ID flowpath boundary 20 and/or OD flowpath boundary 24 open up to counteract the otherwise decrease in cross-sectional area. Inversely, as the flow approaches the trailing edge 30 of the stmts 26, to reduce flow separation tendencies, the flow may be pinched, i.e. its rate of deceleration is reduced (not necessarily accelerated). This is achieved by reducing the rate of cross- sectional area increase.

[0028] The undulating portion 32 has at least a first point 34 and a second point 36 that initiates a first section 54 and second section 56 respectively. The first point 34 can be positioned upstream of the stmt 26. The first point 34 initiates an increasing rate of the cross-sectional area moving downstream from the one or more turbine assemblies 46 to allow for the decleration to be unaffected by the at least one stmt 26 or to increase the deceleration. Approximately half way along a span of the stmt 26 is the second point 36. At the second point 36 the rate of the cross-sectional area starts to narrow, pinching the flow reducing the rate of deceleration.

[0029] In certain embodiments, an additional third point 38 may be added to the undulating portion 32. The third point 38 may be positioned towards the trailing edge 30 of the stmt 26. Here at the third point 38, a trough 44, the rate of the cross- sectional area may start to increase again for a third section 62. Additionally in certain embodiments, a fourth point 40 may be added downstream of the third point 38 and downstream of the stmt 26 as the beginning of a fourth section 64. At this optional fourth point 40, the rate of the cross-sectional area increase starts again to narrow before the cross-sectional area once again expands at a particular rate for the remainder of the exhaust diffuser 12. Additional points may be chosen if computational capacity allows and is compatible with the desired manufacturing processes.

[0030] Illustrated in FIG. 3 is the radial-axial cross section with a wave like modification in that plane. This modification can also extend tangentially in an axisymmetric representation shown in FIG. 5. The wave-like modifications shown in FIG. 3 and FIG. 5 can be on multiple length scales l_ί and with variable amplitude Ai. For example a first and second wave-length and amplitude can be applied with li > l₂ and Ai > A_2. Amplitudes of applied wavelengths can decrease further downstream when flow velocity is reduced (as indicated in FIG. 3 and FIG. 5). Furthermore, there can be an additional tangential modulation on multiple length-scales that extends circumferentially. This particular modulation influences the rotational direction of the gas flow.

[0031] How these points are connected along the at least one of the inner casing 18 and the outer casing 22 can be adjusted. For example, between the first point 34 and the second point 36 may be a straight line (FIG. 2), a curved line (FIG. 3), or some combination of the two. As mentioned above, the resulting shape can include a wave- like modulation on multiple length scales and can also have degrees of freedom circumferentially such as shown in FIG. 5. undulating portion is axisymmetric with the radially and axially extending crests and troughs on multiple wavelength and amplitudes. Strategies need to be made compatible with desired manufacturing and assembly processes. The general narrowing or expanding of the cross-sectional area between two points creates the undulation portion 32 of the exhaust diffuser 12.

[0032] As mentioned above, high flow velocities exit the turbine assemblies 46 into the exhaust diffuser 12. The first point 34 of the undulating portion 32 of the exhaust diffuser 12 can provide a reduction of the flow acceleration. The expansion of the rate of the cross-sectional area increase after the first point 34 slows down the flow 14 at it move towards the at least one strut 26. The flow acceleration is caused by area blockage in the flow path due to the presence of the at least one strut 26. This flow acceleration is on top of the high velocities coming out of the turbine assemblies 46. The geometry of the at least one strut 26 brings on flow separation at the leading edge 28 and trailing edge 30 of the strut 26. With a possible flow acceleration, there is a tendency for the flow 14 to separate prior to the strut 26. In order to continue the reduction or removal of the flow separation, the second point 36 initiates a pinching of the flow 14 that compresses the flow eliminating wall separation from the leading edge 28 of the strut 26.

[0033] Flow tendencies are to separate at the trailing edge 30 of the strut 26 as well due to the abrupt strut geometry in the flow path. The third point 38 may provide a minimization of the separation of the flow 14 along the trailing edge 30 of the strut 26 by slowing down the flow 14 with an expansion of the rate of the cross-sectional area near the trialing edge 30 of the strut 26. With the fourth point 40, a pinching of the flow 14 is produced again reducing or eliminating the onset of flow instabilities prior to the eventual expansion downstream of the strut 26. The waviness, or undulating features of the undulating portion 32 of the exhaust diffuser 12 compensates for the potential for flow separation leading from the abrupt strut geometry in the flow path.

[0034] FIG. 6 and 7 illustrate the increase in pressure recovery and decrease in pressure loss percentages when comparing a conventional exhaust diffuser with an embodiment that includes an undulating portion 32 as described above. The undulating portion 32 provides a reduction in the affect of the at least one strut 26 within the exhaust diffuser 12.

[0035] While specific embodiments have been described in detail, those with ordinary skill in the art will appreciate that various modifications and alternative to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims, and any and all equivalents thereof.

Claims

CLAIMS What is claimed is:

1. A turbine exhaust diffuser (12) for a gas turbine engine having a turbine section, the exhaust diffuser comprising:

at least one flowpath (14) located downstream of the turbine section; wherein the at least one flowpath (14) is defined at least in part by a turbine exhaust casing (16) having an inner casing (18) forming an ID flowpath boundary (20) and an outer casing (22) forming an OD flowpath boundary (24);

a least one strut (26) positioned within the flowpath (14) and spanning between the inner casing (18) and the outer casing (22), wherein the at least one strut (26) comprises a leading edge (28) towards an upstream direction and a trailing edge (30) towards a downstream direction of the flowpath; and an undulating portion (32) located along at least one of the inner casing (18) and the outer casing (22) along the exhaust diffuser (12), wherein the undulating portion (32) includes at least a first point (34) positioned upstream of the at least one strut (26) connected to a second point (36) positioned approximately halfway along a span of the at least one strut (26), the undulating portion (32) including radially and axially extending crests (42) and troughs (44).

2. The exhaust diffuser (12) according to claim 1, wherein the undulating portion (32) further comprises a third point (38) positioned approximate to the trailing edge (30) of the at least one strut (26).

3. The exhaust diffuser (12) according to claim 2, wherein the undulating portion (32) further comprises a fourth point (40) positioned downstream of the at least one strut (26).

4. The exhaust diffuser (12) according to any of claims 1 through 3, wherein the first point (34) of the undulating portion (32) is a crest that initiates an incline section expanding the rate of a flowpath cross sectional area increase.

5. The exhaust diffuser (12) according to any of claims 1 through 4, wherein the second point (36) of the undulating portion (32) is a trough that initiates a decline section narrowing the rate of a flowpath cross sectional area increase.

6. The exhaust diffuser (12) according to any of claims 2 through 5, wherein the third point (38) of the undulating portion (32) initiates an incline section expanding the rate of a flowpath cross sectional area increase.

7. The exhaust diffuser (12) according to any of claims 2 through 6, wherein the fourth point (40) of the undulating portion (32) initiates a decline section narrowing the rate of a flowpath cross sectional area increase.

8. The exhaust diffuser (12) according to any of claims 1 through 7, wherein the undulating portion (32) has varying amplitude.

9. The exhaust diffuser (12) according to any of claims 1 through 8, wherein each successive point has a reduced amplitude compared to the prior point.

10. The exhaust diffuser (12) according to any of claims 1 through 9, wherein straight lines connect points (34, 36, 38, 40) of the undulating portion (32).

11. The exhaust diffuser (12) according to any of claims 1 through 10, wherein curved lines connect points (34, 36, 38, 40) of the undulating portion (32).

12. The exhaust diffuser (12) according to any of claims 1 through 11, wherein the undulating portion (32) is axisymmetric with the radially and axially extending crests (42) and troughs (44) on multiple wavelength and amplitudes.

13. The exhaust diffuser (12) according to any of claims 1 through 12, wherein the undulating portion (32) further comprises a tangential modulation on multiple length-scales that extends circumferentially.