Radial split diffuser

The invention relates to a radial split diffuser with an inner casing and an outer casing joined together along a cylindrical joint. Each diffuser passage is intersected by and extends transversely across the joint. The advantages of this design include: the elimination of a transition within the initial portion of the passages where air flow speeds are supersonic and minute surface discontinuities can significantly effect performance; and the simplification of manufacturing through use of metal castings to replace sheet metal fabrications in the manufacture of diffusers. The joint in the present invention can be located downstream from the diffuser inlet a sufficient distance in a lower velocity area. The joint is located to enable access for precise machining of the critical initial portion of the passages within the inner casing, and to minimize air flow disturbance in the initial portion.

TECHNICAL FIELD
 The invention is directed to an annular diffuser for a gas turbine engine
 that is split into an inner diffuser casing and an outer diffuser casing
 along a cylindrical joint, thereby simplifying the fabrication and
 machining of the highly accurate compressed air passages over conventional
 diffuser designs, reducing vibration, reducing air flow efficiency losses
 in the diffuser passages and enabling the structural integration of the
 diffuser with the gas generator casing structure.
 BACKGROUND OF THE ART
 The compressor section of a conventional gas turbine engine usually
 includes a diffuser located downstream of the centrifugal compressor
 turbines and impeller, and upstream of the combustor. The function of a
 diffuser is to reduce the velocity of the compressed air and
 simultaneously increase the static pressure thereby preparing the air for
 entry into the combustor at a low velocity and high pressure.
 High-pressure low velocity air presented to the combustor section is
 essential for proper fuel mixing and efficient combustion.
 Gas turbine engines that include a centrifugal impeller as the
 high-pressure stage of the compressor are suitable for application of the
 present invention. Centrifugal impellers are used generally in smaller gas
 turbine engines. A compressor section may include axial or mixed flow
 compressor stages with the centrifugal impeller as the high-pressure
 section or alternatively a low-pressure impeller and high-pressure
 impeller may be joined in series.
 The centrifugal compressor impeller draws air axially from a low diameter.
 Rotation of the impeller increases the velocity of the air flow as the
 input air is directed over impeller vanes to flow in a radially outward
 direction under centrifugal force. In order to redirect the radial flow of
 air exiting the impeller to an annular axial flow for annular presentation
 to the combustor, a diffuser assembly is provided to redirect the air from
 radial to axial flow and to reduce the velocity and increase static
 pressure.
 A conventional diffuser assembly generally comprises a machined ring which
 surrounds the periphery of the impeller for capturing the radial flow of
 air and redirecting it through generally tangential orifices into an array
 of separate diffuser tubes. The diffuser tubes are generally horn-shaped
 with an increasing internal cross-section and bend to direct air from the
 radial to axial direction. The diffuser tubes are formed of sheet metal
 with a longitudinal seam. The narrow end of the diffuser tubes are brazed
 or mechanically connected to the central ring and have an increasing
 cross-section rearwardly. As a result, the narrow stream of air at high
 pressure taken into the orifices in the ring are expanded in volume as the
 air travels axially through the diffuser tubes. The increase in air volume
 results in a reduced velocity and corresponding increase in static
 pressure, (i.e.: kinetic energy is converted to pressure energy, where
 total energy of a fluid flow remains constant being the sum of the
 pressure energy, potential energy and kinetic energy, Bernoulli theorem)
 Fabrication of the conventional diffuser with individual tubes is extremely
 complex since the tubes have a flared internal pathway that curves from a
 generally radial tangential direction to an axial rearward direction. Each
 tube must be manufactured to close tolerances individually and then
 assembled to the machined central ring. Complex tooling and labour
 intensive manufacturing procedures result in a relatively high cost for
 preparation of the diffusers.
 During engine operation, diffusers often cause problems resulting from the
 vibration of the individual diffuser tubes. Vibration can cause a
 reduction in service life due to metal fatigue, causes instability in the
 engine compressed air flow, and adds to the engine noise. To remedy
 vibration difficulties, the diffuser tubes may be joined together or may
 be balanced during routine maintenance. However, such operations are
 labour intensive, and involve costly downtime for the engine. From an
 aerodynamic standpoint, the joining of individual diffuser tubes to the
 machined central ring results in interior surface transitions that
 inevitably effect the efficiency of the engine detrimentally due to the
 high velocity of air flow. On the interior of the tube as it joins the
 orifice in the ring, there is a step or transition caused by manufacturing
 tolerances in the assembly and brazing procedures. Since the air in this
 section flows at supersonic velocity, even minute disturbances in air flow
 and increases in drag as the air flows over such transitions can result in
 very high losses in efficiency.
 In general, the design of diffusers is not optimal since their complex
 structure requires a compromise between the desired aerodynamic properties
 and the practical limits of manufacturing procedures. For example, the
 orifices in the impeller surrounding ring are limited in shape to
 cylindrical bores or conical bores due to the limits of economical
 drilling procedures. To provide elliptical holes for example, would
 involve prohibitively high costs in preparation and quality control. The
 shape of the diffuser pipes themselves is also limited by the practical
 considerations of forming their complex geometry. In general, the diffuser
 tubes are made in a conical shape and bent to their helical final shape
 prior to brazing. Whether or not this conical configuration is optimal for
 aerodynamic efficiency becomes secondary to the practical considerations
 of economical manufacturing.
 Diffuser designs incorporating multiple diffuser tubes have the advantage
 that oil lines can easily be passed between adjacent tubes through the
 diffuser. As a result, bearings can be located adjacent the combustor area
 to support the high pressure shaft where loading is most critical. The
 disadvantages inherent in a complex diffuser design are justified by the
 advantages inherent in centrifugal compressor efficiency and preferred
 bearing locations, particularly in small engines.
 Due to the radial extent of the combined centrifugal compressor and
 diffuser, together with any external bypass ducts, the diameter of the
 diffuser assembly contributes significantly to the overall diameter of the
 entire engine. Reduction in the diameter of the diffuser assembly can
 result in reduction of engine diameter which significantly effects the
 drag and fuel efficiency of an aircraft.
 It is an object of the invention to provide a diffuser assembly which
 significantly reduces the tooling and manufacturing costs associated with
 prior art diffuser assemblies, thereby reducing costs and manufacturing
 time.
 It is an object of the invention to significantly reduce the losses in
 efficiency that result from locating transitions in areas of the diffuser
 passages carrying high velocity air flow.
 It is an object of the invention to reduce the vibration difficulties and
 the number of parts resulting from use of multiple independent diffuser
 tubes.
 It is an object of the invention to rationalize the various components of a
 conventional diffuser design and adjacent engine structures into a more
 compact structurally integral robust unit, preferably with reduced overall
 diameter.
 Further objects of the invention will be apparent from review of the
 disclosure and description of the invention below.
 DISCLOSURE OF THE INVENTION
 The invention relates to a radial split diffuser with an inner casing and
 an outer casing joined together along a cylindrical joint. Each diffuser
 passage is intersected by and extends transversely across the joint.
 An advantage of this design include: the elimination of a transition within
 the initial portion of the passages where air flow speeds are supersonic
 and minute surface discontinuities can significantly effect performance.
 Further advantage is achieved through the simplification of manufacturing
 by use of robust low cost metal castings to replace labour intensive sheet
 metal fabrications in the manufacture of diffusers.
 The joint in the present invention can be located downstream from the
 diffuser inlet a sufficient distance within a lower velocity area. A
 conventional diffuser directs a radially outward flow of compressed air
 from an impeller of a centrifugal compressor to an axially rearward
 diffused annular flow.
 The split diffuser of the invention has an inner and outer casing
 manufactured as a casting and machined to be joined along a manufacturing
 joint by brazing for example into an annular diffuser assembly having a
 central impeller opening and an outer rim. A number of discrete diffuser
 passages are cast and machine finished in a circumferential spaced apart
 array through the diffuser assembly, each passage extending through the
 diffuser assembly from an inlet in the central opening, across the brazed
 joint and to an outlet in the rim.
 The joint is located to enable access for precise machining of the critical
 initial portion of the passages within the inner casing, and to minimise
 air flow disturbance in the initial portion. For example, in conventional
 diffusers, the initial portion of the passages is machined in a conical
 shape within a narrow inner ring. The sheet metal diffuser tubes are
 fitted within the ring with a transition at the joint between tubes and
 ring relatively close to the inlet where air flow speeds are extremely
 high. In contrast, the invention provides a relatively wide inner casing
 with a longer initial portion of the passages machined conically.
 The joint with the cast metal outer casing is positioned at a distance from
 the inlets where the passages have widened to a stage where air flow
 speeds are lower and the air flow losses resulting from the transition
 joint are much lower. In addition, the outer casing includes cast passages
 that are machined conically adjacent the joint to match the passages in
 the inner casing, and that arc to redirect flow from a radial to an
 annular flow. The arc portions of the passages are wider and carry air of
 much lower speed. Hence the requirements of dimensional accuracy and
 surface finish in the arc passage profile are much reduced permitting the
 casting of passages and extrude honing in manufacture.
 The casting of the outer casing reduces manufacturing costs in
 significantly reducing the number of parts, however, several other
 advantages also result such as the freedom to cast diffuser passages of
 differing profiles. The dynamic instability of separate conventional
 diffuser tubes is eliminated by the superior structural integrity of the
 robust inner and outer ring-like casings.
 In addition, the casings themselves can be used to form part of the engine
 gas generator casing structure, thereby eliminating weight and increasing
 structural strength by combining the diffuser function with the pressure
 vessel function of the gas generator casing in a single unitary structure.
 In some cases, the combining of the diffuser with the gas generator casing
 structure will reduce the overall diameter of the engine assembly.
 Further details of the invention and its advantages will be apparent from
 the detailed description and drawings included below.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 FIG. 1 illustrates a gas turbine engine structure that is conventional
 apart from the novel annular diffuser assembly in accordance with the
 present invention. The engine depicted includes an outward bypass air duct
 1, which directs external air rearwardly (as indicated by the arrows)
 through the action of a forward fan (not shown). An internal flow of air
 is passed through the compressor section of the engine. The high-pressure
 centrifugal impeller 2 directs compressed air radially outwardly as
 indicated by the arrows into the annular diffuser assembly 3.
 The diffuser assembly 3 redirects the compressed air from a radial
 direction to an annular rearward flow into the gas generator casing 4. The
 diffuser assembly 3 and gas generator casing 4 serve to reduce the
 velocity of the compressed air thereby increasing its static pressure and
 containing the high-pressure compressed air within the pressure vessel of
 the gas generator casing 4. The compressed air within the casing 4 flows
 through apertures into the combustor 5 where it is mixed with fuel sprayed
 from the fuel nozzle 6. The ignited fuel and compressed air mixture
 produces hot gas which is directed as indicated by the arrow rearwardly
 towards high-pressure turbines (not shown).
 In the embodiment shown, the rotating high-pressure shaft 7 is supported on
 bearings 8 located between the high pressure compressor section and high
 pressure turbine section. In order to provide oil lubrication to the
 bearings 8, the embodiment illustrated shows an oil line 9 which passes
 through the annular diffuser assembly 3 and through a vane 10 in the
 bypass air duct 1 where it is connected with other components of the oil
 circulation system such as an oil pump and filter.
 Referring to FIG. 2, a detailed view of one embodiment of the annular
 diffuser assembly 3 is illustrated. Unlike conventional diffusers, the
 external wall 11 of the diffuser assembly 3 is continuous with the gas
 generator casing 4 and is secured to other engine structural components
 12. Conventional diffusers are independent of the adjacent gas generator
 casing and are supported only at their central ring giving rise to
 vibration considerations in operation.
 As a result, the external wall 11 of the invention serves as a pressure
 vessel wall to contain compressed air in a continuous pressure vessel
 formed by the gas generator casing 4, the external wall 11 and other
 engine structures 12. Also the external wall 11 together with the engine
 structure components 12 serve to carry loads between the engine supports
 and shafts for example. The diffuser assembly 3 therefore serves as a
 pressure vessel, an engine support structure component and as a compressed
 air diffuser. Conventional diffuser assemblies are substantially
 independent of the engine structure and serve merely to diffuse compressed
 air. Conventional diffusers suffer from vibration due to their
 independence from adjacent engine structures.
 In contrast, the annular diffuser assembly 3, in accordance with the
 invention, is constructed of an inner casing 13 and an outer casing 14
 which are joined together along a manufacturing joint 15 which is very
 accurately machined and press fit, braced or secured with fasteners 16 to
 ensure structural and pressure vessel integrity.
 Referring to FIG. 4, the sectional views shown in FIGS. 2 and 3 are
 indicated with lines 2--2 and 3--3 in FIG. 4. The diffuser assembly 3 has
 a central impeller opening 17 adjacent to the impeller 2 and an outer rim
 18. The diffuser assembly 3 includes a plurality of discrete diffuser
 passages 19 disposed in a circumferentially spaced apart array through the
 diffuser assembly 3. Each passage 19 extends through the diffuser assembly
 3 from an inlet 20 in the central opening 17 to an outlet 21 in the rim
 18. Each diffuser passage 19 is intersected by and extends transversely
 across the joint 15. In the embodiment illustrated, the inner and outer
 casings 13 and 14 have cylindrical mating joint surfaces coaxial the
 impeller opening 17. It will be apparent that any joint configuration can
 be utilized, however for ease of machining and assembly mating surfaces
 with surfaces of revolution are most advantageous.
 In the embodiment illustrated, the passages 19 have a circular
 cross-section at the inlet 20 and a rectangular cross-section at their
 outlet 21. This geometric configuration is familiar to designers and is
 utilized in conventional diffuser designs. It will be apparent however
 that the separate casting and machining of the inner casing 13 and outer
 casing 14 frees the designer to utilize any desired geometry for the
 passage ways 19. In conventional diffuser assemblies, a central ring
 includes a conically machined opening into which individual sheet metal
 diffuser tubes are braced. Machining of a conical shape is readily
 accomplished with conventional machinery and methods, and serves the
 diffusing purpose by expanding the cross-section of the diffuser passages
 19 in a controlled predictable manner.
 In the embodiment illustrated, each passage 19 has a conical internal
 surface 22 which extends from the circular inlet 20. To simplify machining
 of the inner casing 13, the embodiment illustrated includes a conical
 surface 22 extending through the entire inner casing 13 to the joint 15.
 Also in the particular embodiment illustrated, for simple machining and
 accurate fit of passages across the joint 15, the conical surface 22
 extends across the joint 15 into the initial portion of the passage way 19
 in the outer casing 14.
 Each passage 19 has a circular to rectangular cross-section transition
 surface 23 from the outward boundary 24 of the conical surface 23 to the
 rectangular outlet 21. The conical surface 22 can have a highly accurate
 machined finish utilizing conventional machining methods. In contrast to
 conventional diffusers, the invention provides a relatively long conical
 machined surface 22 and locates the joint 15 at a position in the passage
 19 where airflow speeds are relatively low compared to the speeds
 immediately adjacent to the inlet 20. Due to the low speed in the circular
 to rectangular cross-section transition surface area 23, the outer casing
 14 can be manufactured from a metal casting and the transition surface 13
 can be finished with exude honing methods.
 With reference to FIGS. 3 and 4, the diffuser assembly 3 can include lands
 25 between adjacent passages 19 due to the geometry of the passages 19.
 Conventional diffusers constructed of multiple individual diffuser tubes
 have an opening between diffuser tubes through which oil lines are passed
 through the diffuser. In the present invention, the lands can include a
 perforation through which an oil line 9 can be passed preferably secured
 with mounting flanges 26.
 With reference to perspective views in FIGS. 5 and 6, the separate
 construction of the inner casing 13 and outer casing 14 is best
 illustrated. The outer casing 14 can be cast with conventional methods as
 a metal ring with the external wall 11 being continuous and individual
 passages 19 formed within the unitary casting integral with the external
 wall 11. An internal cylindrical joint mating surface 27 is accurately
 machined to ensure close diametrical fitting with the external cylindrical
 joint surface 28 of the inner casing 13. The inner casing 13 can be formed
 as a casting or a machined ring. The conical surfaces 22 of passage ways
 19 are machined in the inner casing 13 prior to fitting and securing the
 joint 15.
 As described above, the present invention has distinctive advantages over
 conventional diffuser assemblies. The casting of the outer casing and
 inner casing significantly reduces the number of parts that must be
 accurately manufactured and fitted. The relocation of the transition
 within passages to the joint 15 radially outward relative to conventional
 diffusers reduces the airflow losses across the transition. Conventional
 diffusers include a transition relatively close to the inlet and suffer
 from the potential for significant airflow loses. The diffuser assembly 3
 can be used as a structural component of the engine and a portion of a
 pressure vessel. The outer wall 11 of the present invention can serve to
 reinforce the structure of the engine, serve to act as a pressure vessel
 wall for the gas generator casing 4 and is significantly reinforced by the
 walls of the passages 19 which serve as reinforcing ribs for the outer
 wall 11 shell structure. The structural integrity of the outer casing 14
 in particular eliminates the dynamic instability experienced with
 conventional diffusers that use individual diffuser tubes. Due to the use
 of castings, the geometry of the passageways 19 can have greater
 independence from the fabrication methods than conventional diffuser
 tubes.
 As described above therefore, the present invention provides several
 advantages over conventional diffusers at reduced costs and reduced
 complexity. Although the above description and accompanying drawings
 relate to a specific preferred embodiment as presently contemplated by the
 inventors, it will be understood that the invention in its broad aspect
 includes mechanical and functional equivalents of the elements described
 and illustrated.