Patent Description:
A gas turbine engine typically comprises, in axial flow arrangement, a fan, one or more compressors, a combustion system and one or more turbines. The combustion system may comprise a plurality of fuel injectors having fuel spray nozzles which combine fuel and air flows and generate sprays of atomised liquid fuel into a combustion chamber. Correct production of the atomised sprays has a significant impact on combustion efficiency.

Conventional injectors for lean-burn combustion systems typically have a pilot fuel circuit and a mains fuel circuit (see for example <CIT> and <CIT>). The pilot fuel circuit produces a central fuel spray from the injector, while the mains fuel circuit produces a coaxial, radially outward fuel spray. In addition to the two fuel flows (within the pilot fuel circuit and the mains fuel circuit), the injectors each have one or more swirling air flows. As well as atomising the fuel, the air flows serve to maintain separation of the pilot and mains fuel flows until the point of ignition, and to define the flow fields and resulting flame shape in the combustion chamber.

The fuel flow in each of the pilot fuel circuit and mains fuel circuit is typically varied throughout the combustion cycle of the combustion system. At certain times during the combustion cycle (i.e. during engine ignition and at low power operation), the mains fuel flow is staged out (i.e. shut off) whilst the pilot fuel flow is maintained.

Another example of injectors is illustrated in <CIT> which relates to an injector including a main nozzle body defining a central axis and having a main fuel circuit. A pilot nozzle body is mounted inboard of the main nozzle body. The pilot nozzle body includes a pilot air circuit on the central axis with fuel circuitry radially outboard of the pilot air circuit for delivering fuel to a fuel outlet in a downstream portion of the pilot nozzle body. The fuel circuitry includes a primary pilot fuel circuit configured and adapted to deliver fuel to the fuel outlet and a secondary pilot fuel circuit configured and adapted to deliver fuel to the same fuel outlet.

Selected features of a conventional fuel spray nozzle <NUM> are herein described with reference to a schematic partially cut-away view of such a nozzle shown in <FIG>. The fuel spray nozzle has a mains fuel circuit and an annular prefilming surface <NUM> downstream of it. The mains fuel circuit has in flow series a gallery <NUM> circumferentially wrapped around the nozzle, plural circumferentially spaced passages <NUM> arranged in a row around the nozzle, and an annular spin chamber <NUM>. The gallery can include multiple branches, each branch <NUM> supplying fuel to a number of the passages <NUM>. Although only two branches are shown in <FIG>, a typical fuel spray nozzle may have a gallery including, e.g. four branches, each branch supplying fuel flow to three passages.

Each of the passages <NUM> can have an upstream portion <NUM> and a downstream conditioning portion <NUM>. The upstream portions <NUM> of the passages <NUM> are arranged to evenly distribute the fuel flow between the passages <NUM> for the entire range of flow conditions of the mains fuel flow. The conditioning portions <NUM> then impart a circumferential component to their respective portions of the mains fuel flow.

During operation, the mains fuel flow enters the fuel circuit at an inlet port <NUM>, and then flows into the gallery <NUM>. The upstream portions <NUM> of the passages <NUM> receive respective portions of the mains fuel flow from the gallery via inlets <NUM>. The portions of the fuel flow are then delivered into the conditioning portions <NUM> of the respective passages, and from there, into the spin chamber via respective metering orifices <NUM> of the passages. In <FIG>, the white circles at the orifices <NUM> signify their respective sizes which, as shown, are the same for all the passages. The fuel flow from all the passages is recombined in the spin chamber <NUM>. The mains 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 <NUM> of the nozzle for atomisation at a trailing edge of the surface into a spray of fine droplets.

When the fuel flow to the mains circuit is staged out for pilot-only operation, the temperature of the mains fuel circuit can quickly rise. Consequently, any stagnant fuel retained within the fuel circuit under these circumstances may attain a temperature at which it breaks down into coking products, which in turn may form lacquer on the surface of the injector rendering it susceptible to blockage. Such blockages can cause a non-uniform heat traverse to the turbine across the combustor. This can encourage high cycle fatigue and turbine failure. Additionally, the blockages can lead to undesirably high back pressures in the fuel system.

Aerodynamic nozzle modifications for purging fuel by means of differential static pressures at the prefilmer exit of the nozzle are known in the art (e.g. <CIT>). Such modified nozzles are configured to introduce a static pressure differential across the mains fuel circuit when the flow of liquid fuel to the circuit is shut off, which creates a propulsive force acting on the remaining fuel in the circuit. This promotes purging the circuit of fuel and thus decreases the risk of coking of fuel residues therein.

However, in both conventional and modified spray nozzles, when the fuel flow to the mains fuel circuit is shut off, air flows preferentially follow paths of least resistance within the mains fuel circuit. This causes the circumferentially spaced passages which feed into the annular spin chamber to drain unequally. Consequently, this promotes retention of fuel in some of the passages bypassed by the purging flows of air. Thus, the mains fuel circuit may not be consistently and completely purged of fuel, leading to coking and its associated negative consequences.

It is therefore desirable to provide an improved fuel spray nozzle configured to more consistently and completely purge all such passages of a fuel circuit of fuel when the flow of liquid fuel is staged out.

In a first aspect, according to claim <NUM>, the present invention provides a lean burn 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:.

Advantageously, by configuring selected of the passages to develop different differential static pressures to the remaining passages, it is possible to generate 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 via the gallery. Consequently, the gallery and all passages can be completely purged of fuel, which reduces the risk of fuel coking therein. This can improve the reliability and longevity of the fuel spray nozzle, and of the engine (e.g. its turbines) more generally.

Effectively, by configuring selected of the passages to develop different differential static pressures to the remaining passages, a syphonic purge of the passages 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 can be achieved.

This improved effectiveness of the purging process can eliminate a need for a separate heat exchanger between a mains fuel circuit and a pilot fuel circuit at the fuel spray nozzle tip. This is advantageous as such heat exchangers can be complex to design, difficult to manufacture and add weight to lean-burn fuel spray nozzles.

The selected passages of the fuel spray nozzle extend further axially into the annular spin chamber than the remaining passages to develop the different differential static pressure. This enhances the static pressure differential across the selected passages during periods of low or no fuel supply to exert a propulsive force on the fuel that drains it from the passages and gallery into the spin chamber. However, this configuration also enables the metering orifices of the selected passages to occupy locations within the spin chamber which are more exposed to compressor discharge air, whereas the metering orifices of the remaining passages occupy locations which remain fuel-wetted at the outset of purge. In this way, the surface tension of the fuel at the metering orifices of the selected passages can be reduced relative to that at the metering orifices of the remaining passages. This effectively reduces the threshold differential pressure across the selected passages needed to overcome surface tension and friction. Coupled with the enhanced pressure differential across the selected passages, when the flow of liquid fuel to the inlet port is shut off, this also helps air to preferentially enter through the selected passages and exit through the remaining passages to purge all the passages. As a result, purging can occur at lower nozzle pressure drops, or more rapidly for a given pressure drop.

Preferably, an internal geometry of the selected passages may be 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.

In this way also, it is possible to generate 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 via the gallery.

For example, in a fuel spray nozzle of the first aspect, a flow cross-sectional area of the metering orifices of the selected passages may be 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. This configuration can also enhance the static pressure across the selected passages during periods of low or no fuel supply, which in turn can exert a propulsive force on the fuel to drain it from the passages and gallery and into the spin chamber.

Additionally or alternatively, the internal geometry of the selected passages may be 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. For example, edges of the inlets to the selected passages may be more chamfered than edges of the inlets to the remaining passages and/or edges of outlets from the selected passages to the spin chamber may be more chamfered than edges of outlets from the remaining passages to the spin chamber to vary the stagnant liquid fuel meniscus contact angle. In this way, meniscus adhesion to the surface of the selected passages can be reduced at such locations, decreasing the resistance for the meniscus to move through the selected passages.

The fuel spray nozzle of the first aspect may be further configured such that: 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. This configuration ensures that there is at least one selected passage per branch and therefore when the flow of liquid fuel to the inlet port is shut off, the air flow necessarily passes through each branch to purge the fuel therein. Additionally, as the subgroups of passages are mutually exclusive, and the stagnant fuel remaining in each branch is substantially isolated from the stagnant fuel remaining in the other branches, each subgroup and its respective branch can be purged of fuel independently of the others.

Preferably, each subgroup may contain just one of the selected passages and just one or just two of the remaining passages. A ratio of one selected passage to one or two of the remaining passages helps to ensure more complete purging.

Advantageously, 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 and the metering orifice of the two passages when the flow of liquid fuel to the inlet port is shut off (e.g. caused by uneven fluid flow conditions) can produce a lower resistance air path and drive syphonic purging from one passage to the other via the respective branch 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.

The fuel spray nozzle of the first aspect is a lean burn nozzle including a mains fuel circuit, and the nozzle may further include a pilot fuel circuit, the mains fuel circuit being stageable to effect pilot-only and pilot-and-mains staging control.

In a second aspect, according to claim <NUM>, the present invention provides a gas turbine engine including in flow series:.

For example, the gas turbine engine of the second aspect may further include:.

As noted elsewhere herein, the gas turbine engine may comprise 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.

The gearbox may be a reduction gearbox (in that the output to the fan is a lower rotational rate than the input from the core shaft). Any type of gearbox may be used. For example, the gearbox may be a "planetary" or "star" gearbox, as described in more detail elsewhere herein. The gearbox may have any desired reduction ratio (defined as the rotational speed of the input shaft divided by the rotational speed of the output shaft), for example greater than <NUM>, for example in the range of from <NUM> to <NUM>, or <NUM> to <NUM>, for example on the order of or at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The gear ratio may be, for example, between any two of the values in the previous sentence. Purely by way of example, the gearbox may be a "star" gearbox having a ratio in the range of from <NUM> or <NUM> to <NUM>. In some arrangements, the gear ratio may be outside these ranges.

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

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> has a split flow nozzle <NUM>, <NUM> meaning that the flow through the bypass duct <NUM> has its own nozzle <NUM> that is separate to and radially outside the core engine nozzle <NUM>. 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 <NUM> and the flow through the core <NUM> 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 <NUM> may not comprise a gearbox <NUM>.

The combustion equipment <NUM> of the engine <NUM> 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.

<FIG> are schematic partially cut-away views of selected features of respective variants of a fuel spray nozzle of one of the injectors. The embodiments according to the invention of <FIG> and <FIG> and the variants of <FIG>, <FIG> and <FIG> each have a mains fuel circuit and an annular prefilming surface <NUM> downstream of it. The fuel circuit has in flow series: a gallery <NUM> circumferentially wrapped around the nozzle, plural circumferentially spaced passages 60a, 60b (<FIG>, <FIG> and <FIG>) or plural circumferentially spaced passages <NUM> (<FIG>) arranged in a row around the nozzle, and an annular spin chamber <NUM>.

The gallery <NUM> includes multiple branches, each branch <NUM> supplying fuel to a number of the passages 60a, 60b; <NUM>. 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 <NUM>. When the flow of liquid fuel to the inlet port <NUM> 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. Although only two branches are shown in each of <FIG>, the gallery <NUM> typically includes more branches <NUM>. For example, the spray nozzles of <FIG>, in which each branch supplies three passages 60a, 60b, may have four such branches, and the spray nozzles of <FIG>, in which each branch supplies two passages 60a, 60b; <NUM>, 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.

Next, each of the passages 60a, 60b; <NUM> has an upstream portion 53a, 53b; <NUM> and a downstream conditioning portion 54a, 54b; <NUM>. The upstream portions of the passages extend axially and end at respective metering orifices 58a, 58b; <NUM>, 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.

During operation, the fuel flow enters the fuel circuit at an inlet port <NUM>, and then flows into the gallery <NUM>. The upstream portions 53a, 53b; <NUM> of the passages 60a, 60b; <NUM> receive respective portions of the fuel flow from the gallery via inlets 57a, 57b; <NUM>. The portions of the fuel flow are then delivered into the conditioning portions 54a, 54b; <NUM> of the respective passages, and from there, into the spin chamber. In <FIG>, the white circles signify the respective sizes of the respective metering orifices 58a, 58b; <NUM> of the passages. The fuel flow from all the passages is recombined in the spin chamber <NUM>. 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 <NUM> of the nozzle for atomisation at a trailing edge of the surface into a spray of fine droplets.

When the flow of liquid fuel to the inlet port <NUM> is shut off, a respective differential static pressure develops across stagnant liquid fuel remaining between the inlet 57a, 57b; <NUM> and the metering orifice 58a, 58b; <NUM> of each passage 60a, 60b; <NUM>. Additionally, in the embodiment of <FIG>, 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.

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 <NUM>. 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 <NUM>, and of the engine <NUM> (e.g. its turbines <NUM>, <NUM>) more generally.

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 <NUM> is achieved.

In the embodiment according to the invention of <FIG>, each branch <NUM> of the gallery <NUM> 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 <NUM> 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 <NUM> into the spin chamber <NUM>.

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 <NUM> 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.

In the variant of <FIG>, each branch <NUM> of the gallery <NUM> 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>, 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 <NUM>. 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>.

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 <NUM> to purge the passages 60a, 60b completely of any stagnant fuel. 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 <NUM> 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 <NUM> 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.

Although not illustrated, a fuel spray nozzle can combine the approach of the embodiment of <FIG>, 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>, in which the selected passages have an internal geometry which reduces their threshold differential static pressure.

<FIG> shows a further variant where each branch <NUM> of the gallery <NUM> supplies fuel to just two passages <NUM>. Unlike the embodiment of <FIG> and the variant of <FIG>, the passages <NUM> in the variant of <FIG> 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 <NUM> and the metering orifice <NUM> of the two passages when the flow of liquid fuel to the inlet port <NUM> is shut off can produce a lower resistance air path and drive syphonic purging from one passage to the other via the respective branch <NUM> 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. However, a consequence of this approach is that more branches <NUM> of the gallery <NUM> are required to maintain the same number of passages <NUM> around the nozzle.

The embodiment according to the invention of <FIG> and the variant of <FIG> the approach of the embodiment of <FIG> and the variant of <FIG> with the approach of the variant of <FIG>. More particularly, in the embodiment according to the invention of <FIG> and the variant of <FIG>, each branch <NUM> of the gallery <NUM> supplies fuel to just two passages, one of which is a selected passage 60a and the other of which is a remaining passage 60b.

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

In <FIG>, 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>, to reduce the threshold differential static pressure of the selected passage.

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 scope of the claims.

Claim 1:
A lean burn fuel spray nozzle (<NUM>) for generating a spray of atomised liquid fuel in a combustor of a gas turbine engine, wherein the fuel spray nozzle includes:
a mains fuel circuit, and a pilot fuel circuit, the mains fuel circuit being stageable to effect pilot-only and pilot-and-mains staging control, the mains fuel circuit having an inlet port (<NUM>) 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 (<NUM>) 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 mains fuel circuit has in flow series:
a gallery (<NUM>) which wraps circumferentially around the nozzle and receives the fuel flow from the inlet port;
plural circumferentially spaced passages (60a, 60b) arranged in a row around the nozzle, each passage having an inlet (57a, 57b) for receiving a respective portion of the fuel flow from the gallery, a metering orifice (58a, 58b) 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 (<NUM>) 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 (60a, 60b) 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 (57a, 57b) and the metering orifice (58a, 58b) of each passage,
characterized in that
the passages are further configured such that one or more selected passages (60a) extend further axially into the annular spin chamber (<NUM>) than the remaining passages (60b) to 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 (60b), thereby purging the gallery and the passages of fuel.