Fuel Spray Nozzle

Abstract

A fuel spray nozzle includes a fuel circuit having in series a gallery, circumferentially spaced passages arranged in a row around the nozzle, and an annular spin chamber. Each passage has an inlet for receiving a respective portion of the fuel from the gallery and a metering orifice for discharging its portion of the fuel. The passages are configured such that, when the flow of liquid fuel to the inlet port is shut off, a respective differential static pressure develops across stagnant liquid fuel remaining between the inlet and the metering orifice of each passage, and the passages are further configured such that one or more selected passages develop a different differential static pressure to the remaining passages causing a flow of purging air to enter the gallery from the combustor through the selected passages and exit through the remaining passages, thereby purging the gallery and the passages of fuel.

Claims

1. A fuel spray nozzle for generating a spray of atomised liquid fuel in a combustor of a gas turbine engine, wherein the fuel spray nozzle includes: a fuel circuit having an inlet port for receiving a flow of liquid fuel and having an annular exit port for discharging the received fuel as a swirling fuel flow; and an annular prefilming surface downstream of the annular exit port, and configured such that the swirling fuel flow discharged from the exit port spreads, as a film of fuel, across the prefilming surface, whereupon one or more swirling air flows generated by the nozzle shear the fuel film towards a trailing edge of the prefilming surface and atomise the fuel film into a spray of fine droplets; wherein the fuel circuit has in flow series: a gallery which wraps circumferentially around the nozzle and receives the fuel flow from the inlet port; plural circumferentially spaced passages arranged in a row around the nozzle, each passage having an inlet for receiving a respective portion of the fuel flow from the gallery, a metering orifice for discharging its portion of the fuel flow, and being configured to impart a circumferential component to its discharged portion of the fuel flow; and an annular spin chamber which receives the respective discharged portions of the fuel flow from the metering orifices of the passages to form the swirling fuel flow which is discharged at the exit port; and wherein: the passages are configured such that, when the flow of liquid fuel to the inlet port is shut off, a respective differential static pressure develops across stagnant liquid fuel remaining between the inlet and the metering orifice of each passage, and the passages are further configured such that one or more selected passages develop a different differential static pressure to the remaining passages, the different differential static pressure causing a flow of purging air to enter the gallery from the combustor through the selected passages and exit through the remaining passages, thereby purging the gallery and the passages of fuel.

2. The fuel spray nozzle according to claim 1, wherein the selected passages extend further axially into the annular spin chamber than the remaining passages to develop the different differential static pressure.

3. The fuel spray nozzle according to claim 1, wherein an internal geometry of the selected passages is different from a corresponding internal geometry of the remaining passages to reduce a threshold differential static pressure of the selected passages relative to a corresponding threshold differential static pressure of the remaining passages, whereby a given differential static pressure developed across stagnant liquid fuel remaining between the inlets and the metering orifices of the selected and remaining passages causes a flow of purging air to enter the gallery from the combustor through the selected passages and exit through the remaining passages, thereby purging the gallery and the passages of fuel.

4. The fuel spray nozzle according to claim 3, wherein a flow cross-sectional area of the metering orifices of the selected passages is larger than a flow cross-sectional area of the metering orifices of the remaining passages to reduce the threshold differential static pressure of the selected passages.

5. The fuel spray nozzle according to claim 3, wherein the internal geometry of the selected passages is different from the corresponding internal geometry of the remaining passages to vary a stagnant liquid fuel meniscus contact angle in the selected passages relative to a corresponding stagnant liquid fuel meniscus contact angle of the remaining passages to reduce the threshold differential static pressure of the selected passages.

6. The fuel spray nozzle according to claim 1, wherein: the passages are divided into plural mutually exclusive subgroups such that each subgroup contains plural of the passages and each subgroup receives its fuel from a respective branch of the gallery; the gallery is configured such that, when the flow of liquid fuel to the inlet port is shut off, the stagnant fuel remaining in each branch of the gallery is substantially isolated from the stagnant fuel remaining in the other branches of the gallery; and each subgroup contains one of the selected passages and one or more of the remaining passages.

7. The fuel spray nozzle according to claim 1, which is a lean burn nozzle in which the fuel circuit is a mains fuel circuit, and the nozzle further includes a pilot fuel circuit, the mains fuel circuit being stageable to effect pilot-only and pilot-and-mains staging control.

8. A fuel spray nozzle for generating a spray of atomised liquid fuel in a combustor of a gas turbine engine, wherein the fuel spray nozzle includes: a fuel circuit having an inlet port for receiving a flow of liquid fuel and having an annular exit port for discharging the received fuel as a swirling fuel flow; and an annular prefilming surface downstream of the annular exit port, and configured such that the swirling fuel flow discharged from the exit port spreads, as a film of fuel, across the prefilming surface, whereupon one or more swirling air flows generated by the nozzle shear the fuel film towards a trailing edge of the prefilming surface and atomise the fuel film into a spray of fine droplets; wherein the fuel circuit has in flow series: a gallery which wraps circumferentially around the nozzle and receives the fuel flow from the inlet port; plural circumferentially spaced passages arranged in a row around the nozzle, each passage having an inlet for receiving a respective portion of the fuel flow from the gallery, a metering orifice for discharging its portion of the fuel flow, and being configured to impart a circumferential component to its discharged portion of the fuel flow; and an annular spin chamber which receives the respective discharged portions of the fuel flow from the metering orifices of the passages to form the swirling fuel flow which is discharged at the exit port; and wherein: the passages are configured such that, when the flow of liquid fuel to the inlet port is shut off, a respective differential static pressure develops across stagnant liquid fuel remaining between the inlet and the metering orifice of each passage, and an internal geometry of one or more selected passages is different from a corresponding internal geometry of the remaining passages to reduce a threshold differential static pressure of the selected passages relative to a corresponding threshold differential static pressure of the remaining passages, whereby when a given differential static pressure develops across the stagnant liquid fuel remaining between the inlets and the metering orifices of the selected and remaining passages exceeds the threshold differential static pressure of the selected passages, a flow of purging air is enters the gallery from the combustor through the selected passages and exits through the remaining passages, thereby purging the gallery and the passages of fuel.

9. The fuel spray nozzle according to claim 8, wherein a flow cross-sectional area of the metering orifices of the selected passages is larger than a flow cross-sectional area of the metering orifices of the remaining passages to reduce the threshold differential static pressure of the selected passages.

10. The fuel spray nozzle according to claim 8, wherein the internal geometry of the selected passages is different from the corresponding internal geometry of the remaining passages to vary a stagnant liquid fuel meniscus contact angle in the selected passages relative to a corresponding stagnant liquid fuel meniscus contact angle of the remaining passages to reduce the threshold differential static pressure of the selected passages.

11. The fuel spray nozzle according to claim 10, wherein edges of the inlets to the selected passages are more chamfered than edges of the inlets to the remaining passages and/or edges of outlets from the selected passages to the spin chamber are more chamfered than edges of outlets from the remaining passages to the spin chamber to vary the stagnant liquid fuel meniscus contact angle.

12. The fuel spray nozzle according to claim 8, wherein: the passages are divided into plural mutually exclusive subgroups such that each subgroup contains plural of the passages and each subgroup receives its fuel from a respective branch of the gallery; the gallery is configured such that, when the flow of liquid fuel to the inlet port is shut off, the stagnant fuel remaining in each branch of the gallery is substantially isolated from the stagnant fuel remaining in the other branches of the gallery; and each subgroup contains one of the selected passages and one or more of the remaining passages.

13. The fuel spray nozzle according to claim 12, wherein each subgroup contains just one of the selected passages and just one or just two of the remaining passages.

14. The fuel spray nozzle according to claim 8, which is a lean burn nozzle in which the fuel circuit is a mains fuel circuit, and the nozzle further includes a pilot fuel circuit, the mains fuel circuit being stageable to effect pilot-only and pilot-and-mains staging control.

15. A fuel spray nozzle for generating a spray of atomised liquid fuel in a combustor of a gas turbine engine, wherein the fuel spray nozzle includes: a fuel circuit having an inlet port for receiving a flow of liquid fuel and having an annular exit port for discharging the received fuel as a swirling fuel flow; and an annular prefilming surface downstream of the annular exit port, and configured such that the swirling fuel flow discharged from the exit port spreads, as a film of fuel, across the prefilming surface, whereupon one or more swirling air flows generated by the nozzle shear the fuel film towards a trailing edge of the prefilming surface and atomise the fuel film into a spray of fine droplets; wherein the fuel circuit has in flow series: a gallery which wraps circumferentially around the nozzle and receives the fuel flow from the inlet port; plural circumferentially spaced passages arranged in a row around the nozzle, each passage having an inlet for receiving a respective portion of the fuel flow from the gallery, a metering orifice for discharging its portion of the fuel flow, and being configured to impart a circumferential component to its discharged portion of the fuel flow; and an annular spin chamber which receives the respective discharged portions of the fuel flow from the metering orifices of the passages to form the swirling fuel flow which is discharged at the exit port; and wherein: the passages are divided into plural mutually exclusive subgroups such that each subgroup contains just two of the passages and each subgroup receives its fuel from a respective branch of the gallery; and the gallery is configured such that, when the flow of liquid fuel to the inlet port is shut off, the stagnant fuel remaining in each branch of the gallery is substantially isolated from the stagnant fuel remaining in the other branches of the gallery.

16. The fuel spray nozzle according to claim 15, which is a lean burn nozzle in which the fuel circuit is a mains fuel circuit, and the nozzle further includes a pilot fuel circuit, the mains fuel circuit being stageable to effect pilot-only and pilot-and-mains staging control.

Description

DESCRIPTION OF THE DRAWINGS

[0041] Embodiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0042] FIG. 1 is a schematic partially cut-away view of selected features of a conventional fuel spray nozzle for a gas turbine engine;

[0043] FIG. 2 is a sectional side view of a gas turbine engine;

[0044] FIG. 3 is a close-up sectional side view of an upstream portion of a gas turbine engine;

[0045] FIG. 4 is a partially cut-away view of a gearbox for a gas turbine engine; and

[0046] FIGS. 5 to 9 are schematic partially cut-away views of selected features of respective variants of a fuel spray nozzle.

DETAILED DESCRIPTION

[0047] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0048] FIG. 2 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.

[0049] 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.

[0050] An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 3. The low pressure turbine 19 (see FIG. 2) 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.

[0051] 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.

[0052] The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 4. 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. 4. 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.

[0053] 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.

[0054] 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. 3 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. 3. 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. 3.

[0055] 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.

[0056] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0057] 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. 2 has a split flow nozzle 18, 20 meaning that the flow through the bypass duct 22 has its own nozzle 18 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.

[0058] 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. 2), and a circumferential direction (perpendicular to the page in the FIG. 2 view). The axial, radial and circumferential directions are mutually perpendicular.

[0059] The combustion equipment 16 of the engine 10 includes a plurality of fuel injectors having lean burn fuel spray nozzles which combine pilot and mains fuel flows, and swirling air flows to generate sprays of atomised liquid fuel into a combustion chamber. The mains fuel flow can be staged in and out to provide, as required, pilot-only operation and pilot-and-mains operation.

[0060] FIGS. 5 to 9 are schematic partially cut-away views of selected features of respective variants of a fuel spray nozzle of one of the injectors. The variants of FIGS. 5 to 9 each have a mains fuel circuit and an annular prefilming surface 56 downstream of it. The fuel circuit has in flow series: a gallery 52 circumferentially wrapped around the nozzle, plural circumferentially spaced passages 60a, 60b (FIGS. 5, 6, 8 and 9) or plural circumferentially spaced passages 60 (FIG. 7) arranged in a row around the nozzle, and an annular spin chamber 55.

[0061] The gallery 52 includes multiple branches, each branch 59 supplying fuel to a number of the passages 60a, 60b; 60. The passages are divided into plural mutually exclusive subgroups such that each subgroup contains plural of the passages and each subgroup receives its fuel from a respective branch of the gallery 52. When the flow of liquid fuel to the inlet port 51 is shut off, the stagnant fuel remaining in each branch of the gallery is substantially isolated from the stagnant fuel remaining in the other branches of the gallery. Thus, each subgroup and its respective branch are purged of fuel independently of the others.

[0062] Although only two branches are shown in each of FIGS. 5 to 9, the gallery 52 typically includes more branches 59. For example, the spray nozzles of FIGS. 5 and 6, in which each branch supplies three passages 60a, 60b, may have four such branches, and the spray nozzles of FIGS. 7 to 9, in which each branch supplies two passages 60a, 60b; 60, may have six such branches. However, the number of passages receiving fuel from the same branch of the gallery is not thus limited, and neither is the number of branches of the same gallery.

[0063] Next, each of the passages 60a, 60b; 60 has an upstream portion 53a, 53b; 53 and a downstream conditioning portion 54a, 54b; 54. The upstream portions of the passages extend axially and end at respective metering orifices 58a, 58b; 58, and are configured to evenly distribute the fuel flow between the passages for the entire range of flow conditions of the mains fuel flow. The conditioning portions then extend circumferentially from the ends of the upstream portions to impart a circumferential component to their respective portions of the mains fuel flow.

[0064] During operation, the fuel flow enters the fuel circuit at an inlet port 51, and then flows into the gallery 52. The upstream portions 53a, 53b; 53 of the passages 60a, 60b; 60 receive respective portions of the fuel flow from the gallery via inlets 57a, 57b; 57. The portions of the fuel flow are then delivered into the conditioning portions 54a, 54b; 54 of the respective passages, and from there, into the spin chamber. In FIGS. 5 to 9, the white circles signify the respective sizes of the respective metering orifices 58a, 58b; 58 of the passages. The fuel flow from all the passages is recombined in the spin chamber 55. The fuel flow is then discharged from an annular exit port at the downstream end of the spin chamber as a swirling flow onto the annular prefilming surface 56 of the nozzle for atomisation at a trailing edge of the surface into a spray of fine droplets.

[0065] When the flow of liquid fuel to the inlet port 51 is shut off, a respective differential static pressure develops across stagnant liquid fuel remaining between the inlet 57a, 57b; 57 and the metering orifice 58a, 58b; 58 of each passage 60a, 60b; 60. Additionally, in the variant of FIG. 5, one or more selected passages 60a are configured to develop a different differential static pressure to the remaining passages 60b. The different differential static pressure causes a flow of purging air to enter the gallery from the combustor through the selected passages 60a and exit through the remaining passages 60b, thereby purging the gallery and the passages of fuel.

[0066] More particularly, this configuration of the selected passages 60a generates paths of least resistance within the fuel circuit such that when the flow of liquid fuel to the circuit is shut off, the purging air flow necessarily passes through all the passages 60a, 60b via the gallery 52. Consequently, the gallery and all passages are completely purged of fuel, which reduces the risk of fuel coking therein. This can improve the reliability and longevity of the fuel spray nozzle 50, and of the engine 10 (e.g. its turbines 17, 19) more generally.

[0067] Effectively, by configuring selected of the passages 60a to develop different differential static pressures to the remaining passages 60b, a syphonic purge of the passages 60a, 60b is promoted in which a propulsive force on the fuel inside the passages is exerted and a faster and more complete purge of the passages and the gallery 52 is achieved.

[0068] In the variant of FIG. 5, each branch 59 of the gallery 52 supplies fuel to three passages, one of which is a selected passage 60a and the other two are remaining passages 60b. The selected passage 60a extends further axially into the spin chamber 55 than the two remaining passages 60b to develop the different differential static pressure as a result of a spin chamber medium internal flow field. This enhances the static pressure differential across the selected passage 60a during periods of low or no fuel supply to exert a propulsive force on any stagnant liquid fuel, the propulsive force draining the fuel from the passages and gallery 52 into the spin chamber 55.

[0069] Additionally, the metering orifice 58a of the selected passage 60a occupies a location within the spin chamber, which is more exposed to compressor discharge air, whereas the metering orifices 58b of the two remaining passages 60b occupy locations which are fuel-wetted at the outset of purge. In this way, the surface tension of the fuel at the metering orifice of the selected passage is reduced relative to that at the metering orifices of the remaining passages. This effectively reduces the threshold differential pressure across the selected passage needed to overcome surface tension and friction. Coupled with the enhanced pressure differential across the selected passage, when the flow of liquid fuel to the inlet port 51 is shut off, air preferentially enters through the selected passage and exits through the remaining passages to purge all the passages 60a, 60b. As a result, fuller purging occurs at lower nozzle pressure drops, or more rapidly for a given pressure drop.

[0070] In the variant of FIG. 6, each branch 59 of the gallery 52 supplies fuel to three passages, one of which is a selected passage 60a and the other two are remaining passages 60b. However, unlike in FIG. 5, in this variant a flow cross-sectional area of the metering orifice 58a of the selected passage 60a is greater than the corresponding flow cross-sectional area of the metering orifices 58b of the remaining passages 60b in the branch 59. In other words, the selected passage has a different internal geometry such that the internal diameter of the metering orifice 58a of the selected passage 60a is larger compared to the diameters of the metering orifices 58b of the remaining passages 60b in the branch. This is illustrated by the differently sized white circles representing the metering orifices 58a, 58b in FIG. 6.

[0071] With this configuration, air preferentially enters through the selected passage 60a and exits through the remaining passages 60b of the branch, because the larger internal diameter of the metering orifice 58a of the selected passage causes it to have a lower threshold differential static pressure for air to enter than the remaining passages 60b. Thus the air necessarily passes through all the passages 60a, 60b and across their respective branch 59 to purge the passages 60a, 60b completely of any stagnant fuel.

[0072] Another option for changing the internal geometry of the selected passages 60a from a corresponding internal geometry of the remaining passages 60b to lower the threshold differential static pressure for the selected passages is to change a geometry that affects a stagnant liquid fuel meniscus contact angle in the passages when the flow of liquid fuel to the inlet port 51 is shut off. This can be achieved, for example, by forming the edges of the inlets 57a to the selected passages 60a to be more chamfered than the edges of the inlets 57b to the remaining passages 60b and/or by forming the edges of outlets from the selected passages 60a to the spin chamber 55 to be more chamfered than the edges of the corresponding outlets from the remaining passages 60b. Such chamfered edges vary the contact angle that a fuel meniscus forms with the inlet/outlet of the selected passage 60a.

[0073] Although not illustrated, a fuel spray nozzle can combine the approach of the variant of FIG. 5, in which one or more selected passages are configured to develop a different differential static pressure to the remaining passages, and the approach of the variant of FIG. 6, in which the selected passages have an internal geometry which reduces their threshold differential static pressure.

[0074] FIG. 7 shows a further variant where each branch 59 of the gallery 52 supplies fuel to just two passages 60. Unlike the variants of FIGS. 5 and 6, the passages 60 in the variant of FIG. 7 are nominally identical in terms of their lengths and flow cross-sectional areas, i.e. metering orifice diameters. However, by arranging the passages in mutually exclusive subgroups, each of which contains just two of the passages, any small difference in differential static pressures across stagnant liquid fuel remaining between the inlet 57 and the metering orifice 58 of the two passages when the flow of liquid fuel to the inlet port 51 is shut off can produce a lower resistance air path and drive syphonic purging from one passage to the other via the respective branch 59 connecting the two passages. As there are no other passages fed by the branch, there is little danger of unpurged fuel being left behind in those passages.

[0075] However, a consequence of this approach is that more branches 59 of the gallery 52 are required to maintain the same number of passages 60 around the nozzle.

[0076] The variants of FIGS. 8 and 9 combine the approach of the variants of FIGS. 5 and 6 with the approach of the variant of FIG. 7. More particularly, in the variants of each of FIGS. 8 and 9, each branch 59 of the gallery 52 supplies fuel to just two passages, one of which is a selected passage 60a and the other of which is a remaining passage 60b.

[0077] Thus, in FIG. 8, the selected passage 60a extends further axially into the spin chamber 55 than the remaining passage 60b, as described in detail in relation to FIG. 5, to develop the enhanced static pressure differential across the selected passage needed to overcome surface tension and flow losses (e.g. friction and turbulence).

[0078] In FIG. 9, by contrast, a flow cross-sectional area of the metering orifice 58a of the selected passage 60a is different from a corresponding flow cross-sectional area of the metering orifice 58b of the remaining passage 60b, as described in detail in relation to FIG. 6, to reduce the threshold differential static pressure of the selected passage.

[0079] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0080] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0081] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0082] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0083] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0084] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +/−10%.