Patent Application
20160115971
The application relates generally to gas turbine engines and, more particularly, to compressor diffusers therefor.
Diffuser pipes are provided in gas turbine engines for directing flow of compressed air from a centrifugal compressor to an annular chamber containing the combustor, while diffusing the high speed air. The diffuser pipes are typically circumferentially arranged at a periphery of an impeller, and are designed to transform kinetic energy of the flow into pressure energy. Diffuser pipes may provide a uniform exit flow with minimal distortion, as it is preferable for flame stability, low combustor loss, reduced hot spots etc. While longer diffuser pipes may accomplish better diffusion, spatial constraints in the gas turbine engine may restrict their length. Large flow diffusion in diffuser pipes over insufficient pipe length may result in thick and weak boundary layers built up on the pipe wall. To compensate for a shorter length, many diffuser pipes have a tight bend. Turbulence and other non-streamline behavior of the flow at the bend may lead to pressure losses and decrease efficiency of the diffuser pipe.
In one aspect, there is provided a compressor diffuser for a gas turbine engine, the diffuser having a plurality of diffuser pipes each comprising: a diverging tubular body defining a flow passage extending fully therethrough, the tubular body including a first portion extending in a first direction, a second portion extending in a second direction different from the first direction, and a curved portion interconnecting the first portion and the second portion; and at least one splitter vane extending into the flow passage and disposed at least partially within the curved portion of the tubular body.
In another aspect, there is provided a gas turbine engine comprising a centrifugal compressor including an impeller case and a plurality of diffuser pipes downstream of the impeller and receiving compressed air therefrom, each of the diffuser pipes having a diverging tubular body defining a flow passage extending therethrough, the tubular body of the diffuser pipes extending from the periphery of the impeller case and including a radial portion and an axial portion connected by a curved portion, the curved portion having at least one splitter vane disposed at least partially within the flow passage.
In a further aspect, there is provided a method of manufacturing a diffuser pipe for a centrifugal compressor of a gas turbine engine, the method comprising: forming a tubular body out of a sheet metal, the tubular body having a first portion extending in a first direction, a second portion extending in a second direction different from the first direction, and a curved portion between the first portion and the second portion; inserting a splitter vane at least partially into the curved portion of the tubular body and aligning sides of the splitter vane in a desired position between opposed walls of the curved portion; and fixing the sides of the splitter vane to the opposed walls within the curved portion.
Reference is now made to the accompanying figures in which:
Turning now to
A second portion 26 is disposed generally axially and is connected to the first portion 24 by an out-of-plane bend or curved portion 28. The second portion 26 includes an open end forming an outlet O (shown in
High swirl of the flow F exiting the impeller 17, and therefore entering the first portion 24 of each of the diffuser pipes 20, may be removed by shaping the diffuser pipe 20 with the curved portion 28, such that the flow F is redirected axially before existing to the combustor 16. For a given impeller exit Mach number and swirl of the flow F, the effectiveness of a diffuser pipe may be dependent upon its length. For a fishtail pipe type diffuser, such as the one described herein, the greater the length the easier it is for the pipe to diffuse flow efficiently without, or with minimal, flow separation at the curved portion 28. Length can be obtained by growing pipe radially or axially or both. Longer diffuser pipes are however disadvantaged in that they can potentially increase both weight and size of the engine. In addition, a required gap between the outlet and fuel nozzle locations is another constraint that put a physical limit on radial/axial extension of the diffuser pipes 20. As a result, the diffuser pipe 20 may be designed to have a tight 90 degrees bend to compensate for a reduced length.
In the depicted embodiment, the cross-sectional area of the diffuser pipe 20 increases gradually and continuously along its length, from the inlet I to the outlet O. The first portion 24 has a generally circular cross-section C1 (shown in
Referring now to
The curvature of the curved portion 28 may tend to detach the flow F from the walls 28a, 28b, which can result in pressure losses and non-uniform flow at the outlet O. Mixing loss may contribute to overall diffuser performance. Flow separation in the diffuser pipe 20 starting at the curved portion 28 may not only be potentially detrimental to the compressor section 17 performance and operability, but also to its structural integrity as flow separation can be destructive in nature and can lead to premature pipe breakage, fatigue, cracking, noise, flame instability etc.
The diffuser pipe 20 of the present disclosure may relieve the pressure gradient at the curved portion 28 by the presence of the splitter vane 30. While the splitter vane 30 may provide additional aerodynamic friction loss, the reduction in overall mixing loss may more than offset this increase.
As seen in
The splitter vane 30 extends across the diffuser pipe 20, wall-to-wall. In the example shown in
Referring now to
The splitter vanes 130, 130′ are disposed in a curved portion 128 of the diffuser pipe 120, with the splitter vane 130 being upstream relative to the splitter vane 130′. The curved portion 128 of the diffuser pipe 120 may be longer than the curved portion 28 of the diffuser pipe 20, in order to accommodate the multitude of splitter vanes 130, 130′. The splitter vanes 130, 130′ have a same orientation and disposition as the splitter vane 30. As best seen in
Because of the diffusion process, the diffuser pipes 20, 120 experience adverse pressure gradients in the direction of flow F, with endwall boundary layer being built up as the result. The buildup may lead to increased blockage, diminished pressure recovery and eventually lead to flow separation. The flow separation usually starts at the diffuser bend 28, 128 where the curvature is at its maximum. The splitter vane(s) 30, 130, 130′ may reduce pressure gradient across the curved portion 28, 128 and help the flow F to negotiate the tight turn more efficiently. The airfoil splitter vanes 30, 130, 130′ described herein may also facilitate swirl removal. Computational fluid models can be used to optimize the splitter vane 30, 130, 130′ length and/or location, while the inner and outer walls 28a, 28b, 128a, 128b can be shaped in accordance with the splitter vane 30, 130, 130′ to best conform to a stator pitch.
The diffuser pipes 20, 120 with splitter vane(s) 30, 130, 130′ at the curved portions 28, 128 thereof may at least reduce flow separation from initiating. Since mixing losses may be a prominent contributor to diffuser pipe loss and is initiated mostly at the curved portion 28, 128, employing splitter vane(s) 30, 130, 130′ at that location may be more effective than anywhere else in the diffuser pipe 20, 120.
One way to manufacture any of the above sheet metal diffuser pipes with internal vanes is to laser drill slots on the sheet metal forming the diffuser pipes, at a location where the splitter vane is to be disposed in the curved portion. The splitter vane(s) may then be inserted inside the diffuser pipe, for example from the outlet end O thereof, and brazed at both ends onto the inner wall(s) of the diffuser pipe where the slots are formed. Alternatively, no slots may be need to be formed and the splitter vanes may be simply brazed in place within the portion of each diffuser pipe.
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. 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.
