Flow Sensor Rake Assembly

Abstract

A flow sensor rake assembly for taking flow field measurements in flow machine, such as a compressor or turbine of gas turbine engine. Axial flow machine has rotor arranged to rotate about an axis to interact with fluid flow through the rotor in use, and static annular wall surrounding axis. Annular wall is mounted axially upstream or downstream of rotor to be washed by fluid flow in use. Flow sensor rake has elongate stem having first and second ends and plurality of probes depending therefrom at spaced locations along stem. Elongate stem is mounted to annular wall at first end and depending away from annular wall to the second end. Elongate stem is obliquely angled relative to annular wall when viewed in axial direction and second end of elongate stem is offset in circumferential direction relative to first end. Stem is slanted relative to radial direction with respect to axis.

Claims

1. A flow sensor rake assembly for taking flow field measurements in a flow machine, the flow machine comprising a rotor arranged to rotate about an axis so as to interact with a fluid flow through the rotor in use, and a static annular wall surrounding the axis, the annular wall being mounted axially upstream or downstream of the rotor so as to be washed by the fluid flow in use, wherein the flow sensor rake assembly comprises: an elongate stem having first and second ends and a plurality of probes depending therefrom at spaced locations along the stem; the elongate stem being mounted to the annular wall at a first end and depending away from the annular wall to the second end; and the elongate stem being obliquely angled relative to the annular wall when viewed in the axial direction and the second end of the elongate stem is offset in a circumferential direction relative to the first end.

2. A flow sensor rake assembly according to claim 1, the elongate stem being obliquely angled relative to the annular wall and/or its surface normal vector at the first end.

3. A flow sensor rake assembly according to claim 1, the elongate stem being obliquely angled relative to the annular wall and/or its surface normal vector along a majority or all of the length of the stem between the first and second ends.

4. A flow sensor rake assembly according to claim 1, the flow machine having an axis of rotation and the elongate stem being obliquely angled relative to a radial direction with respect to said axis of rotation.

5. A flow sensor rake assembly according to claim 4, the elongate stem being obliquely angled relative to the radial direction of the axial flow machine at the first end.

6. A flow sensor rake assembly according to claim 1, arranged for location in an annular flow field having circumferentially periodic flow field regions and the flow sensor rake assembly is obliquely angled so as to span at least a half or one of said periodic flow field regions.

7. A flow sensor rake assembly according to claim 6, wherein the second end of the elongate stem is offset in the circumferential direction relative to the first end of the elongate stem such that the length of the elongate stem traverses at least one or two of said periodic flow field regions.

8. A flow sensor rake assembly according to claim 1, wherein the static annular wall of the axial flow machine is a first annular wall of the assembly, the assembly further comprising a second static annular wall surrounding the axis, the first static annular wall and the second static annular wall defining an annular flow passage, wherein the elongate stem may be obliquely angled relative to the surface normal of the first static annular wall and/or the surface normal of the second static annular wall.

9. A flow sensor rake assembly according to claim 8, the elongate stem being obliquely angled relative to the second static annular wall at the second end and/or along a majority or all of the length of the stem between the first and second ends.

10. A flow sensor rake assembly according to claim 8, wherein the elongate stem depends away from the first static annular wall in a direction that intersects with the second static annular wall.

11. A flow sensor rake assembly according to claim 8, wherein the second end of the elongate stem being mounted to the second annular wall such that the elongate stem extends across the entire annular flow passage radial height from the first annular wall to the second annular wall.

12. A flow sensor rake assembly according to claim 8, comprising a first and second elongate stems the second end of the first elongate stem terminating within the annular flow passage part way between the first and second annular wall, and the second elongate stem being mounted to the second annular wall and depending away from the second annular wall to a location within the annular flow passage part way between the first and second annular wall.

13. A flow sensor rake assembly according to claim 1, the elongate stem having a straight line profile between the first end and the second end.

14. A flow sensor rake assembly according to claim 1, the plurality of probes comprising any or any combination of pitot tubes, pitot-static tubes, thermocouple probes, Kiel probes, yaw probes, and hot wire anemometers/probes.

15. A flow sensor rake assembly according to claim 1, the flow field measurements comprising any or any combination of total pressure, static pressure and temperature

16. An axial flow machine comprising a rotor arranged to rotate about an axis so as to interact with a fluid flow through the rotor in use, and a static annular wall surrounding the axis, the annular wall being mounted axially upstream or downstream of the rotor so as to be washed by the fluid flow in use, and at least one flow sensor rake assembly according to claim 1.

17. The axial flow machine of claim 16, comprising a plurality of flow sensor rake assemblies.

18. The axial flow machine of claim 16, further comprising one or more static structure mounted to the annular wall and the flow sensor rake assembly is located in a wake region downstream of the one or more static structure.

19. The axial flow machine of claim 18, wherein the one or more static structure comprises a circumferential array of static structures.

20. A gas turbine engine for an aircraft comprising: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor, a fan located upstream of the engine core, the fan comprising a plurality of fan blades; a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft; and a flow sensor rake assembly for taking flow field measurements for the flow through the engine core, wherein the flow sensor rake assembly comprises: an elongate stem having first and second ends and a plurality of probes depending therefrom at spaced locations along the stem; the elongate stem being mounted to a first annular wall of the engine core and depending away from the first annular wall to the second end; and the elongate stem being obliquely angled relative to the first annular wall when viewed in the axial direction and the second end of the elongate stem being offset in a circumferential direction relative to the first end.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0043] FIG. 1 is a sectional side view of a gas turbine engine;

[0044] FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

[0045] FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

[0046] FIG. 4 is a cross-section of a first flow sensor rake assembly geometry within an axial flow machine as known in the state of the art;

[0047] FIG. 5 is a cross-section of a second flow sensor rake assembly geometry within an axial flow machine as known in the state of the art;

[0048] FIG. 6 is a cross-section of a third flow sensor rake assembly geometry within an axial flow machine as known in the state of the art;

[0049] FIG. 7 is a perspective view of a flow sensor rake assembly;

[0050] FIG. 8 is a partial cross-sectional view of an axial flow passage in which a flow sensor rake assembly is implemented;

[0051] FIG. 9 is a partial cross-sectional view of an axial flow machine passage in which a further example of a flow sensor rake assembly is implemented;

[0052] FIG. 10 is a partial cross-sectional view of flow field measurements having circumferential periodicity and the flow sensor rake assembly arranged relative to said flow field;

[0053] FIG. 11 is a schematic partial cross-section view of a flow passage comprises a variety of different examples of flow sensor rakes;

[0054] FIG. 12 shows two total pressure plots using measurements taken by a conventional rake and a rake according to an example of the invention; and

[0055] FIG. 13 shows a plot of cumulative probability distribution against average total pressure for a plurality of rakes at different slant angles.

DETAILED DESCRIPTION

[0056] FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

[0057] In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

[0058] An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

[0059] Note that the terms low pressure turbine and low pressure compressor as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the low pressure turbine and low pressure compressor referred to herein may alternatively be known as the intermediate pressure turbine and intermediate pressure compressor. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

[0060] The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

[0061] The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

[0062] It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

[0063] Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

[0064] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0065] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 20, 22 meaning that the flow through the bypass duct 22 has its own nozzle that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

[0066] In other examples, the invention may be applied to different types of gas turbine engine, e.g. for different uses, such as thrust or power generation for marine, nuclear or other power generation, or other land applications, such as industrial pumping and the like.

[0067] The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

[0068] The flow sensor rake assembly described hereinbelow may be positioned anywhere within the path of core airflow A and/or bypass flow B. The flow sensor rake assembly described herein may be positioned in the vicinity of, e.g. downstream of, a flow machine within the engine, such as any of the low pressure compressor 14, the high pressure compressor 15, the high pressure turbine 17, and the low pressure turbine 19. Additionally or alternatively, the flow sensor rake assembly described herein may be positioned downstream of static/non-rotating structures such as any of, or any combination of, stator vanes and outlet guide vanes positioned in the path of core airflow A. Any such structures may be downstream of a rotor of the relevant flow machine.

[0069] In more general terms, the flow sensor rake assemblies described herein may be located anywhere in a continuous flow machine, typically in a complex flow field, where minimisation of sampling uncertainty is desired for a given number of measurement probes. Gas turbine engines are described as being an example of a flow machine to which the sensor rake assembly is relevant. The rake assembly may be used in axial or radial flow machines more generally, or else in a flow machine for which the direction of flow is between axial and radial directions.

[0070] FIG. 7 illustrates a flow sensor rake assembly 50. The flow sensor rake assembly 50 comprises an elongate stem 52 having a plurality of probes 54, 56 protruding therefrom. At a first end of the flow sensor rake assembly 50 there is a mounting flange 58 for mounting the flow sensor rake assembly 50 to an annular wall of an axial flow machine.

[0071] The mount 58 may comprise a connection interface 59 for electrical/electronic and/or flow connections with the relevant sensing and/or signal processing equipment. The interface typically comprises a plurality of connectors.

[0072] The probes 54, 56 protrude substantially perpendicularly from the elongate stem 52 such that the flow sensor rake assembly 50 has a rake-like structure. The probes 54, 56 each have a proximal end attached to the stem 52, i.e. at spaced locations along the stem, and a free/distal end. The probes are generally straight and oriented in parallel in this example such that each probe faces the same direction with respect to the stem 52, i.e. towards the oncoming flow in use. In other examples, the probes need not be straight and could be oriented in a direction that is not perpendicular to the stem. The probes need not be parallel depending on the expected flow conditions. For example, probes could be individually or collectively oriented to follow the direction of flow in the immediate vicinity of the probe, e.g. being at different angles and/or having a varying direction along the length of each probe. Probes could be curved, if desired.

[0073] The probes comprise pitot tubes 54, e.g. including pitot-static tubes, for measuring pressure (total pressure and/or static pressure). The probes may comprise conventional pressure sensing transducers/equipment coupled with the tubes 54. In this example, thermocouple probes 56 are also provided on the rake for measuring temperature. Each of the pressure probes 54 are positioned at spaced locations along the elongate stem 52 relative to one another, and each of the thermocouple probes 56 are positioned at spaced locations along the elongate stem 52 relative to one another. One pitot-static probe 54 and one thermocouple probe 56 is positioned, e.g. as a probe pair, at each spaced location.

[0074] Each of the pressure probes 54 and thermocouple probes 56 are configured to record a measurement signal and transmit the measurement signal via wiring and/or fibres running along the elongate stem 52. Alternatively, the probes could provide a fluid connection to a transducer that is spaced from the stem, e.g. with tubes running along the stem to another suitable location for sensing.

[0075] The mounting flange 58 comprises thermocouple plugs and/or pressure/flow connections at interface 59. The signals measured using the pressure probes 54 and thermocouple probes 56 may be transmitted to a processing/control unit such that the results can be recorded and analysed. The thermocouple plugs and pressure probe connections provide individual measurement signals for each of the individual probes 54, 56 such that suitable processing equipment can determine the corresponding values of relevant variables, such as total pressure, static pressure, dynamic pressure, flow rate, temperature, etc. in a conventional manner. The processing equipment will typically determine a corresponding averaged measurement signal for each of the probes 54 and an averaged signal for each of the thermocouple probes 56.

[0076] The mounting flange 58 may also comprise a plurality of apertures located around its circumference, or other fixing formations, as a means of fixing the flow sensor rake assembly 50 to the relevant annular wall of an axial flow machine.

[0077] FIG. 8 illustrates a flow sensor rake assembly according to the present disclosure positioned within an axial flow machine passage having an inner annulus wall 60, an outer annulus wall 62 and a central axis of rotation 64, which may correspond with the principal rotational axis 9 (shown in FIG. 1) for the engine as a whole. The flow passage is thus an annular flow space bounded by the inner 60 and outer 62 walls, e.g. bounded in a radial direction relative to the axis 64. The axial flow machine is viewed as a cross-section, as if looking along the central axis of rotation 64. The flow sensor rake assembly 50 is positioned within the axial flow machine such that a first end A of the elongate stem 52 is mounted to the inner annulus wall 60, and the elongate stem 52 depends away from the inner annulus wall 60.

[0078] FIG. 8 illustrates that the elongate stem 52 is slanted relative to a radial vector 66 and a plane 68 perpendicular to the radial vector 66. The slanting rake stem 52 forms an angle relative to the radial vector 66, where the radial vector 66 extends radially from the central axis of rotation 64. The elongate stem 52 is therefore obliquely angled relative to the inner annulus wall 60 and/or the radial direction of vector 66 such that the second end B of the elongate stem 52 is offset from the first end A of the elongate stem 52 in a circumferential direction.

[0079] The elongate stem 52 is slanted such that the angle has a value of greater than 0 degrees and greater than a first/minimum threshold. In some examples, the first threshold may be dependent on the circumferential periodicity of the flow sensor rake assembly or features in the flow field, which will be explained in greater detail in relation to FIG. 10. In simpler terms, must be greater than 0 degrees to ensure that the elongate stem 52 is slanted relative to radial vector 66, and must be greater than a first threshold such that the elongate stem 52 is slanted enough to traverse across at least one angular period of static structures and/or associated periodic flow features/regimes of the flow through the passage.

[0080] FIG. 9 illustrates a flow sensor rake assembly akin to that of FIG. 8 positioned in the same axial flow machine as FIG. 8 and viewed along the same axis.

[0081] FIG. 9 illustrates that the elongate stem 52 is slanted relative to a surface normal vector 74, forming an angle relative to the surface normal vector 74, where the surface normal vector 74 extends normally from the inner annulus wall 60, i.e. perpendicular to a tangent or tangential plane of the inner annulus wall 60 at the point of intersection with the vector 74. The elongate stem 52 is therefore obliquely angled relative to the inner annulus wall 60 such that the second end B of the elongate stem 52 is offset from the first end A of the elongate stem 52 in a circumferential direction.

[0082] The elongate stem 52 is slanted such that 1 takes a value of less than 90 degrees, a value of greater than 0 degrees, and more than a second threshold, the second threshold being dependent on the circumferential periodicity of the flow sensor rake assembly, which will be explained in greater detail in relation to FIG. 10 In simpler terms, must be less than 90 degrees to ensure that the elongate stem 52 is slanted relative to the tangential of the inner annulus 60, must be greater than 0 degrees to ensure that the elongate stem 52 is slanted relative to vector 74 and may be greater than a second threshold such that the elongate stem 52 is slanted enough to traverse across at least one angular period of static structures and/or associated periodic flow features/regimes of the flow through the passage.

[0083] In the examples described herein, the angle or is sufficiently large that the rake stem 52 spans at least one angular period. In the particular examples herein, the optimal arrangement may be a span in the region of 1.5 times the period. However it has been found that at least some benefit can be derived by spanning only a fraction of a period, such as a quarter, third, half or three-quarters of the period. The disclosure encompasses an angle of the rake stem that is greater than any engineering tolerance produced when assembling a perpendicular rake. A minimum angle of or may be as little 2, 3 or 5. The angle may be at least 10, 15 or 20 in other examples.

[0084] The angle or will be less than 90. The maximum angle of or may be limited by the length of the stem, e.g. such that the stem has the requisite strength. Diminishing benefit will also typically be achieved with the stem spanning larger numbers of periodic flow features. The maximum angle of or may be in the region of 80, 70, 60 or 50.

[0085] The concept of slanting the elongate stem relative to any of the abovementioned vectors and/or planes is limited to the elongate stem 52 (e.g. a longitudinal axis of the elongate stem) extending between an inner annulus 60 and an outer annulus 62, where the inner annulus 60 and outer annulus 62 have differing radii. This is achieved by the elongate stem 52 extending between the inner annulus 60 and the outer annulus 62, either by the elongate stem 52 extending across the entire flow passage so as to contact both the inner annulus 60 and the outer annulus 62, or where the elongate stem 52 does not extend the whole way between the inner 60 and outer 62 walls, but the linear extrapolation of the elongate stem 52 would intersect with the opposing annulus. Thus, the angles and must be such that physical extension of the elongate stem 52 or the linear extrapolation of the elongate stem 52 extends from the inner 60 to the outer 62 annulus wall or vice versa.

[0086] Whilst a single stem 52 is shown in FIGS. 8-10, spanning the full radial height of the flow passage, the stem may be split into opposing portions/halves, e.g. with one portion depending outwardly from the inner annular wall 60 and the other portion depending inwardly from the outer annular wall 62. The distal ends of such opposing stem portions may terminate adjacent each other, e.g. being spaced by only a small gap such that the two opposing portions approximate a single stem for the purpose of taking sensor measurements across the passage.

[0087] The difference reference vectors in FIGS. 8 and 9 is to show that in some configurations, the inner 60 and/or outer 62 annular walls may not be symmetrical about the axis 64. Whilst in some examples, the inner and/or outer walls may be circular in section and aligned symmetrically about the central axis 64, in other examples the inner and/or outer annular walls could be offset from the axis in alignment (i.e. misaligned) and/or could be non-circular in shape such that the normal vector to wall surface is not a radial line with respect to the axis 64.

[0088] In FIGS. 8 and 9, the stem 52 is shown as following a straight line vector, i.e. such that the entire stem is oriented at the relevant angle. However in other examples the stem 52 could be curved in form along its length, i.e. following a curved longitudinal axis. If a curved stem is used, it is intended that the angle of the stem relative to the radial 66 and/or normal 74 vectors will always lie within the thresholds disclosed herein, e.g. for all points along its length.

[0089] FIG. 10 illustrates the nature of the circumferential periodicity of an example flow field 76 which is downstream of a circumferential array of static structures within the axial flow machine. The circumferential array of static structures may comprise stator vanes or outlet guide vanes in a flow machine of a gas turbine engine of the type described herein, for example, although it will be appreciated that such static structures are used in other flow machine applications to redirect or accelerate/decelerate flow either upstream or downstream of a rotor.

[0090] A conventional circumferential array of such vanes comprises multiple angularly spaced vanes, each extending in a generally radial direction between inner and outer ends. The vanes typically span an annular flow passage of the type shown in FIGS. 8 and 9.

[0091] The flow field 76 immediately downstream of the vane array thus comprises flow variations reminiscent of the geometry of the vane array, i.e. including regions of high flow rate (and higher pressure) where flow passes freely through the gap between adjacent vanes, and lower flow rate (and lower pressure) behind the vanes themselves in the direction of fluid flow. Similar repeating variations in temperature also occur.

[0092] The flow field thus comprises periodic regions that repeat, e.g. regularly, in the circumferential direction dependent on the geometry of the vane array. The periodicity of the flow field variations/regions is thus described in FIG. 10 by the angular extent of each region and/or the angular spacing between corresponding flow features in adjacent regions. This angular value is denoted . For an array of n vanes, the annular flow passage can be split into angular regions .sub.0-.sub.N-1 (e.g. for a regular array: =360/n).

[0093] A general principle of the disclosure is that the angular spacing between one end of the rake stem 52 and its opposing end will be at least 0.5 or , and may be at least 1.5 or 2. It is to be noted that the angular spacing may not always be constant and so it may be necessary to define this orientation or slant of the stem 52 according to a mean average value for , namely .sub.N.

[0094] In FIG. 10, relative to a common datum angle 78, the angular orientation of the opposing ends of the rake stem 52 are given as .sub.A and .sub.B respectively. The angular region covered by the flow sensor rake assembly (.sub.B.sub.A) is defined by the first end A and the second end B of the elongate stem 52.

[0095] The circumferential periodicity of the flow sensor rake assembly can be defined as the ratio of the angular region covered by the flow sensor rake assembly (.sub.B.sub.A) to the average value, .sub.N.

[0096] The flow sensor rake assembly of the present disclosure has a circumferential periodicity of greater than 1. The flow sensor rake assembly of FIG. 10 illustrates a circumferential periodicity between 2 and 3, ie the elongate stem traverses across 2-3 angular periods of the flow field regions and/or the static structures located upstream of the flow sensor rake assembly.

[0097] FIG. 11 illustrates a variety of different rake stems that may be accommodated either individually or collectively in accordance with the present disclosure. It can be seen that any single assembly could accommodate rakes at different angles and/or of different lengths. Rakes may or may not extend across the full radial height of the flow passage. Furthermore, angled rakes according to the present disclosure could be used in combination with conventional rakes in a single assembly.

[0098] FIG. 12 illustrates the total pressure profile of five flow sensor rake assemblies according to the present disclosure on the right, compared with the total pressure profile of five standard flow sensor rake assemblies on the left. The flow sensor rake assemblies used to measure the profile on the right of FIG. 12 had a periodicity of between 2 and 3, ie the elongate stems of the flow sensor rake assemblies traversed across at least 2 angular periods of the static structures located upstream of the flow sensor rake assemblies.

[0099] It can be seen from FIG. 12 that for each flow sensor rake assembly according to the present disclosure, two troughs in the curves are apparent, suggesting that the flow sensor rake assemblies traversed across two wakes behind respective vanes. However, for each of the standard flow sensor rake assemblies, only one trough is apparent, suggesting that the standard flow sensor rake assemblies provide a less accurate measurement of total pressure across the cross-section of the axial flow machine.

[0100] Although not shown in the left hand plot of FIG. 12, it is also possible that a single rake could be almost entirely held in a wake region such that its readings would be considerably different from the other rakes. This could further skew the measurements taken collectively by all the rakes, e.g. when calculating average flow/pressure/temperature.

[0101] FIG. 13 shows a plot illustrating potential benefits associated with the degree of slant of the rake. For perpendicular rakes distributed at circumferential spacing around an annular flow passage, it can be seen that the average total pressure readings can differ between the rakes by a relatively large margin x, thereby leading to the greatest degree of uncertainty in the average pressure readings. Each plot in FIG. 13 concerns a rake stem leaning at a different angle to accommodate a different periodicity as described herein. As the angle of lean of the rake is increased to accommodate different periodicity, the margin of uncertainty between the rakes in the circumferential array is reduced. For example, it can be seen that the margin of uncertainty y for a rake periodicity of 1 is less than the margin x. In the example plot of FIG. 13, the discrepancy between average total pressure readings reaches a minimum around a periodicity of 4. However it will be appreciated that there may be other practical factors affecting the optimal rake length and angle, and also that, in other applications and flow regimes, a different optimal value may be found. Some of the features and embodiments described herein may provide one or more of the following advantages: [0102] A reduction in the number of flow sensor rake assemblies required to accurately determine average fluid flow properties with a predetermined level of certainty. [0103] Increased accuracy in the determination of average flow properties for a flow field using a common number of rakes. [0104] Reduced sampling uncertainty in the determination of average or overall fluid flow properties for a flow field. [0105] Simple and easy to manufacture flow sensor rake geometry. [0106] Robustness in flow sensor rake circumferential positioning.

[0107] Although the present disclosure has been described above in relation to the measurement and characterisation of complex flow fields within axial flow machines, and more specifically gas turbine engines, the present disclosure may be used for measurement and characterisation of any complex flow field, particularly complex flow fields in which there is a need to capture elaborate flow fields. The present disclosure may be especially applicable, for example, to marine, nuclear and land applications.

[0108] Although the flow sensor rake assembly of FIG. 7 has been described as having pitot-static probes and thermocouple probes, it will be understood that the flow sensor rake assembly may comprise other types of sensing probe (such as Kiel probes, yaw probes, hot wire probes), or only pitot-static probes, or only thermocouple probes, or any combination of sensing probes. The principle of the disclosure herein may be relevant to any such probes.

[0109] Although the flow sensor rake assembly of FIG. 7 has been described as having pressure probes and thermocouple probes positioned at specific spaced locations along the elongate stem, it will be understood that the probes may be positioned at any locations along the elongate stem provided the probes can provide a sufficient average of the measured flow field. The probes may be evenly/uniformly spaced or with varying spacing.

[0110] Although FIGS. 8 to 10 have been described in relation to just one flow sensor rake assembly being positioned within the axial flow machine, it will be understood that any number of flow sensor rake assemblies may be positioned around the circumference of the axial flow machine. For example, 2-20, 4-15, or 6-12 flow sensor rake assemblies may be positioned around the circumference of the axial flow machine.

[0111] Although the flow sensor rake assemblies of FIGS. 8-10 have been described in relation to the elongate stem having only its first end mounted at the inner annulus wall, it will be understood that the elongate stem may have only its first end mounted to the outer annulus wall, or the elongate stem may extend between, and mount to both of, the inner annulus wall and the outer annulus wall. In this respect it will also be understood that where the elongate stem has its first end mounted to the outer annulus wall, the elongate stem may be slanted relative to the surface normal vector of the outer annulus wall.

[0112] Although the elongate stems of FIGS. 8 to 10 have been described independently as being slanted relative to a radial vector and a plane perpendicular to the radial vector (FIG. 8), and slanted relative to a surface normal vector of an annulus wall (FIG. 9), it will be understood that the elongate stem of the rake sensor assembly of the present disclosure may be slanted relative to all of a radial vector, a plane perpendicular to the radial vector and a surface normal vector of one or both annulus walls.

[0113] Whilst certain benefits of the rake assembly have been described in relation to its positioning immediately downstream of a circumferential array of static structures, the rake assembly may also be beneficial when placed in a free stream, i.e. a generally non-rotating flow, in order to assess/capture flow field variations in a circumferential direction, e.g. upstream of a rotor. The rake assembly may also be beneficial when placed in a rotating/vortical flow downstream of a rotor, regardless of whether it is immediately preceded by a vane array or other static structures, such as struts or the like.

[0114] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.