Patent Description:
Gas turbine engines (used in propulsion and power applications and broadly inclusive of turbojets, turboprops, turbofans, turboshafts, industrial gas turbines, and the like) use PSR in several situations.

A PSR seals between an inner member and an outer member. The inner member and outer member may be static structure such as case components. Or, the inner member and the outer member may be rotating structure such as components of a spool or rotor. The inner member and the outer member may be subject to small excursions relative to each other. For example, torque loads may cause small rotational movements; thrust loads or differential thermal expansion may cause small axial movements. Vibration may also cause small rotational or axial movements. Such small or transient rotational movements, however, are distinguished from continuous relative rotational movement such as in face seal or shaft seal between two relatively rotating components (e.g., two different spools or a spool and a static structure).

The PSR is accommodated in an outer diameter groove in the inner member. Under pressure loading, the PSR seals against a sidewall of the groove and an inner diameter surface of the outer member. In one example of such a situation involving a rotor, the inner member is a shaft and the outer member is a rotor stack of the associated spool. In a more particular example, the outer member is a seal runner protruding axially from a protuberant bore of a disk of the rotor stack. Tension in the shaft holds the rotor stack in precompression. Small rotational, axial, and/or radial displacements of the shaft and seal runner may be caused by factors including transients and changes in operational conditions such as torque and thrust loads (which will vary between one steady state condition and another steady state condition). In such an example, the PSR is accommodated in an outer diameter (OD) groove in the shaft. In an operational condition where there is a pressure difference across the PSR, optimally, one axial end face of the PSR will bear against and seal against the adjacent sidewall face of the groove and the outer diameter (OD) surface of the PSR will bear and seal against the inner diameter (ID) surface of the seal runner. Specifically, if a first axial end face of the PSR is subject to higher pressure than the opposite second axial end face, the pressure difference will shift the PSR so that the second axial end face seats and seals against the associated/adjacent sidewall face of the groove. However, as discussed below, the optimal situation is not always the case.

PSRs are often small in cross section so as to be relatively compliant compared to the contacting structure (e.g., members forming the groove and runner). The ring is split for assembly purposes. The ring circumferential ends may form an overlapping joint (e.g., a shiplap joint). The small cross section and split provide the ring with little hoop strength and twist resistance. For example, with a nickel alloy shaft and nickel alloy runner, ring material may be nickel or cobalt alloy and may have generally similar material hardness (at least of a substrate if coated). Example coatings are an aluminum bronze (CuAl) layer directly atop the substrate outer diameter surface and a molybdenum sulfide (MoS<NUM>) layer thereatop and extending onto the substrate axial end surfaces.

The manufacturing process as well as operational loads and wear can cause the ring's sealing surfaces to have flatness (of axial end faces) and/or circularity (of OD surface) errors that allow for local leakage through the seal. Non-axisymmetric leakage from the end gap (joint) or due to such PSR form errors leads to local thermal hotspots on the contacting structure (or coldspots such as if there as a low temperature pressurized fluid). These thermal hotspots may, in turn, lead to non-axisymmetric distortion of the contacting structure resulting in mass unbalance, localized stress concentrations, and reduced sealing performance. Other factors such as coning of the runner under load may also contribute to either a general axisymmetric unseating or a non-axisymmetric unseating, exacerbating the situation.

To combat flatness errors of the ring, some PSR designs have incorporated a secondary (spacer) ring that under centrifugal load provides a wedging action to seat a primary (seal) ring against the groove sidewall. See, <CIT>, and entitled "Rotating turbomachine seal". That patent discloses slots in the face of the secondary ring to vent high pressure gas into the groove. The wedging may also correct some departures from circularity. This type of configuration requires additional parts, introduces additional wear surfaces, can be difficult to design with tolerances and varying PSR loads, and is only applicable to rotating applications.

<CIT> discloses a prior art seal as set forth in the preamble of claim <NUM>.

From one aspect of the disclosure, there is provided a split ring seal as recited in claim <NUM>.

There is also provided a machine as recited in claim <NUM> that includes the split ring seal.

There is also provided a method for using the machine, as recited in claim <NUM>.

There is also provided an apparatus as recited in claim <NUM>.

To reduce the severity of the thermal asymmetry associated with a non-axisymmetric leakage (e.g., a locally mis-seated seal ring (PSR)), features providing a relatively axisymmetric base leakage may be added around the circumference of the PSR. The term "base" is used to limit confusion with "baseline" (used to identify a prior art PSR or other PSR lacking the features). The symmetric leakage is sufficient to at least partially offset any non-axisymmetric leakage and thus reduces the severity of the thermal asymmetry.

The example PSR features are radial channels machined on both axial end faces of the ring. These channels allow leakage through the channels on the high pressure, side around the PSR, and through the channels on the low pressure side. Based on the system requirements the channels' size, shape, and location can be adjusted to maintain an adequate seal.

For example, if a given condition introduces a non-axisymmetric leakage, superimposing a more axisymmetric leakage due to the channels, reduces the relative thermal asymmetry even if there is a further increase in heating at the hot spot. A similar consideration may occur if the high pressure area is relatively cool and leakage causes local cooling (cold spot).

<FIG> shows a piston seal ring (PSR) formed as a split ring seal <NUM> having a first circumferential end <NUM>, a second circumferential end <NUM>, an inner diameter (ID) surface <NUM>, an outer diameter (OD) surface <NUM>, a first axial end face <NUM>, and a second axial end face <NUM>. The PSR has a nominal central longitudinal axis (centerline) <NUM> shared with the members it seals when in a nominally centered condition. <FIG> shows a radius RI at the ID surface <NUM> and an outer radius RO at the OD surface <NUM>. <FIG> shows the ID surface <NUM> as having a straight (circular cylindrical) central portion <NUM> and more frustoconical transitions <NUM> and <NUM> to the axial end faces <NUM> and <NUM>, respectively. In the example, there are more radiused transitions at the extremes of the transitions <NUM> and <NUM> than centrally within those transitions. Viewed in section, junctions between the axial end faces and ID and OD surfaces are shown as radiused corners. However, other junctions are possible. The PSR also has a pair of orthogonal longitudinal centerplanes <NUM> and <NUM> arbitrarily defined so that <NUM> extends centrally through the joint <NUM>.

In the example, first circumferential end <NUM> and second circumferential end <NUM> form a joint or junction <NUM> (<FIG>). The example joint <NUM> is a shiplap joint with a projecting portion <NUM> of the first circumferential end received in a rebate <NUM> of the second circumferential end and a projecting portion <NUM> of the second circumferential end received in a rebate <NUM> in the first circumferential end. The example projecting portions have mating faces/surfaces <NUM>, <NUM> which, in the example, closely face or contact along a transverse radial centerplane <NUM> (<FIG>) of the PSR.

As discussed further below, the PSR may consist of a single alloy piece or may comprise a single alloy piece substrate with one or more coating layers along portions of its exterior surface (and thus forming associated portions of the exterior surface of the PSR). Coating thicknesses discussed below are small enough that overall dimensions and substrate dimensions may be within ranges given.

To provide venting (discussed further below), the PSR has a circumferentially distributed plurality of vents. In the <FIG> example, vents are channels, more particularly, open channels (or grooves or slots) <NUM>, <NUM> (e.g., vs. closed channels being through-holes) along the first axial end face and similar open channels along the second axial end face both extending from the ID surface to the OD surface. Channel axial depth or height is labeled H<NUM> (<FIG>) and channel width is labeled W (<FIG>).

The illustrated channels extend essentially the full radial span of the sealing portions of axial end faces of the seal (exclusive of further relieved areas) that would potentially contact mating faces of the groove accommodating the seal. In the illustrated example the channels extend fully to the OD extreme of the cylindrical OD surface inboard of rounded transitions with the axial ends. At the ID, the channels do not extend all the way radially to the ID surface portion <NUM> due to the relief provided by the transitions <NUM> and <NUM>. This is distinguished from radial channel segments in pressure balanced carbon seals (sealing between components subject to continuous relative rotation) that feed circumferential channels where the circumferential channel has a sidewall opposite the radial channel segment.

The PSR may consist of or comprise a metallic substrate <NUM> (<FIG>) (e.g., with coating on one or more surface regions). The example PSR has a coated substrate with a coating <NUM> (<FIG>) on its OD surface (forming the PSR OD surface) and a coating <NUM> (<FIG>) on its axial end surfaces (forming the PSR axial end surfaces). The example OD coating <NUM> is a multi-layer coating with a base layer <NUM> and an anti-gallant layer <NUM> atop the base layer. The example axial end coatings <NUM> consist of just the anti-gallant layer <NUM>.

The base layer <NUM> may have a generally uniform thickness along the cylindrical region of the OD surface then tapering/feathering out over the rounded transitions to the axial end faces. This may be achieved such as via shadow masking or spray gun movement. Alternatively, a more abrupt termination of the base layer may occur such as via contact masking or fixturing. Similarly, the anti-gallant layer <NUM> may taper/feather out along a transition to the ID surface of the substrate such as along the frustoconical portions of transitions <NUM> and <NUM> or rounded transition portions between the frustoconical transition portions and the axial end surfaces.

The example base layer <NUM> is a CuAl coating applied to the substrate <NUM> OD surface by plasma spray (e.g., wire spray) preserved from a prior art baseline. An example CuAl aluminum bronze coating is CuAl10 (e.g., <NUM>-<NUM> weight percent Al). Example CuAl coating thickness is <NUM> micrometer to <NUM> micrometer, more broadly <NUM> micrometers to <NUM> micrometers. Cu is the majority by weight component and typically essentially the balance with small amounts of alloyants (if any (e.g., up to <NUM> weight percent individually and up to <NUM> weight percent combined alloyants and impurities, more narrowly up to <NUM> weight percent combined)). One known alloyant is Fe at about <NUM> weight percent.

The example anti-gallant is a dry film coating similarly preserved from a prior art baseline. Example dry film coating is a molybdenum sulfide (MoS<NUM>). Example MoS<NUM> coating includes a silicate binder and is applied (over the outer diameter surface of the base layer <NUM> and over the substrate along the substrate axial end surfaces including transitions) as a water-based mixture by spraying (e.g., plasma spray) and baking. Example anti-gallant layer <NUM> thickness is <NUM> micrometers to <NUM> micrometers, more broadly <NUM> micrometers to <NUM> micrometers.

In the example PSR, the anti-gallant also covers substrate surface at the channels <NUM>, <NUM> (and thus is applied only after channel formation). Nevertheless, alternative PSRs that involve post-coating channel formation may have exposed channel surfaces (exposed substrate material) recessed below coated axial end surface regions.

In one example of distributing the channels, the channels are evenly circumferentially spaced relative to the center of the joint with the joint representing an omitted channel from the even spacing. For example, if the joint is at twelve o'clock (in a rotating frame of reference for a rotor seal), there could be channels at every other hour on the clock for a total of eleven channels per face at an even spacing of <NUM>°. A similar example would be nine channels at a spacing (on-center) of <NUM>°. However, opening and closing of the PSR for mounting imposes the highest stress near the six o'clock position. Accordingly, the illustrated example omits a channel from the six o'clock position. With the omission of a channel from the six o'clock position, <FIG> shows an example where channels on opposite axial end faces <NUM>, <NUM> are paired in exact registry with each other. The example PSR is symmetric between its two axial end faces allowing interchangeable installation in either orientation. The example pairing involves: pairs 70A, 70B, 70C, 70D, 70E, and 70F of channels <NUM> at the <NUM>°, <NUM>°, <NUM>°, <NUM>°, <NUM>°, and <NUM>° positions; and pairs 72A and 72B (discussed further below) of channels <NUM> at <NUM>° and <NUM>° positions (where twelve o'clock is <NUM>°). This leaves eight channels per side in two groups of four with even on-center spacing of <NUM>° within each group. There is <NUM>° on-center spacing between the groups at both diametric opposite locations centered at twelve o'clock and six o'clock. Thus, each pair of channels leaves a reduced thickness web <NUM> of seal material (<FIG>) having a thickness labeled T<NUM>. Example T<NUM> is <NUM>% to <NUM>% of the overall PSR thickness TS, more particularly, <NUM>% to <NUM>% or <NUM>% to <NUM>%. An example number of channels per side is at least six (e.g., six to twelve or six to ten).

However, leakage through the joint <NUM> still presents an asymmetry of flow if there is no diametrically opposite channel. Accordingly, to compensate for that asymmetry, on each axial end face, the two channels <NUM> closest to diametrically opposite the joint are of greater cross-section than the other channels to more evenly distribute the net venting effect. In the illustrated example, these two channels <NUM> have the same channel depth or axial height H<NUM> as the channels <NUM> (<FIG>) but a greater width or circumferential span W. The example channels <NUM> and <NUM> are of arcuate cross-section. For example, if machined with a circular profile bit, the channels <NUM> have a greater radius of curvature than the channels <NUM>.

In the examples of channels whose cross-section are circular segments, the cross-sectional areas may be calculated via known formulas. In one example, the cross-sectional area (normal to a central radius from the centerline <NUM>) of each channel <NUM> is about <NUM>% larger than that of each channel <NUM>. This may reflect an approximation that leakage through the joint <NUM> will be similar to leakage through a pair of channels <NUM> so that the combined cross-sectional area of the two pairs 72A, 72B of channels <NUM> equals the net cross-sectional venting provided by the joint <NUM> and the pairs 70A and 70F of channels <NUM>. More broadly, example channel <NUM> cross-sectional area is <NUM>% to <NUM>% or <NUM>% to <NUM>% of the cross-sectional area of channel <NUM>. Due to relative thinness of the coating, these numbers may apply to an uncoated substrate, a substrate coated both within the channels and beyond, and a substrate coated only beyond the channels. And they may be measured pre-coating or post-coating.

<FIG> and <FIG> show the PSR <NUM> seated in an outer diameter groove <NUM> in an inner member (e.g., shaft or shaft section) <NUM> and sealing against an ID surface <NUM> of an outer member <NUM> (e.g., a seal runner). The groove <NUM> has a first sidewall or end wall <NUM>, a second sidewall or end wall <NUM>, and a base <NUM> joining the two. Example junctions between the sidewalls and the base are shown as chamfers or bevels <NUM>. However, right angle junctions or other transitions are possible. The groove <NUM> extends radially inward from an outer diameter (OD) surface section or portion <NUM> of the inner member.

<FIG> shows the PSR in a gas turbine engine rotor <NUM> including the shaft section <NUM>. Such a situation is discussed as background above. The example rotor is the high pressure compressor (HPC) portion of a high pressure spool of a two-spool engine. The rotor includes a stack of blade disks <NUM>. Each blade disk includes a protuberant inner diameter (ID) bore <NUM> having an ID surface <NUM>. A radial web <NUM> extends outward from the bore to a rim structure <NUM>. A circumferential array of blades <NUM> (shown with airfoil tips cut away) may be mounted to the rim (e.g., via fir tree or dovetail mounting). Or, blade airfoils may be unitarily formed with the rim and the rest of the disk (e.g., an integrally bladed rotor (IBR)).

The example PSR seals between the rotor shaft section <NUM> and one of the disk bores <NUM>. The example seal runner <NUM> is unitarily formed with the particular disk bore and protrudes axially from the disk bore near the ID surface thereof to a free distal end/rim of the seal runner. This is one non-limiting example of one baseline situation in which the channeled PSR may replace a baseline unchanneled PSR.

In the example, there is axial play (i.e., the axial length WG between the groove <NUM> and <NUM> sidewalls is greater than the axial length or thickness TS (<FIG>) of the PSR between the axial end faces <NUM> and <NUM>). This axial play allows the presence of an axial gap between the PSR axial end face(s) and the adjacent groove sidewall(s). It potentially allows cocking of the PSR cross-section to contact both sidewalls and, thus, have one radially diverging gap and one radially converging gap if the aligned clearance is small enough (groove width WG is only slightly greater than the PSR thickness TS).

In the example, there also is radial play with an ID radial gap <NUM> between the ID surface <NUM> of the PSR and the base of the groove. In the example, this is a static gap and a dynamic gap discussed below.

The example PSR has a relaxed condition wherein the circumferential ends <NUM> and <NUM> are not completely nested/bottomed against each other (there is a slight circumferential gap <NUM> (<FIG>). When assembled over the shaft and seated in the groove, there may be a small ID gap <NUM>. However, the PSR OD surface <NUM> may be close to flush and even potentially sub-flush to the adjacent shaft OD surface <NUM>. Thus, when the seal runner is assembled over the shaft and PSR, there is an at least local OD radial gap between the PSR OD surface <NUM> and the seal runner ID surface <NUM>. However, when the shaft rotates, centrifugal action will radially expand the PSR, closing the OD gap and expanding the ID gap <NUM> (or creating the ID gap) and expanding the circumferential gap <NUM>. This centrifugal action biases the PSR OD surface <NUM> into sealing engagement with the ID surface <NUM> of the seal runner.

Nevertheless, other variations include the PSR having a relaxed diameter of the OD surface <NUM> greater than the diameter of the seal runner ID surface <NUM> so as to have a static radial sealing bias not merely a dynamic radial sealing bias.

In use, various static and dynamic factors may cause uneven seating.

<FIG> and <FIG> show the sealing between a first region or volume <NUM> and a second region or volume <NUM>. In an example dynamic operating condition, the first region is a high pressure region and the second region is a low pressure region so that the pressure difference biases the PSR so that the axial end face to the low pressure side of the PSR contacts the groove sidewall <NUM> to the low pressure side and there is an axial gap <NUM> between the high pressure side axial end face of the PSR and the high pressure side sidewall of the groove. However, due to the asymmetries noted above, this dislodgement from low pressure side engagement may be at a limited circumferential extent. Other factors may overcome the pressure bias and cause unseating at the low pressure side (creating an axial gap <NUM> (<FIG>)) and even potential contact at the high pressure side. <FIG>, by way of example, shows local contact at the high pressure side.

For purposes of illustration, <FIG> show channels <NUM> or <NUM> at particular circumferential locations of seating or mis-seating. However, this is merely for purposes of illustration. Thus, a PSR might have a condition fully seated against the low pressure sidewall <NUM> like <FIG> but at a location away from the channels. Similarly, a <FIG> fully mis-seated condition may be away from the channels <NUM> and <NUM> and the more intermediate condition of <FIG> may be away from the channels <NUM> and <NUM>.

Additionally, although not shown, there may be cocked conditions where the PSR axial end faces are not parallel to the adjacent groove sidewalls. Similarly, whereas the OD surface <NUM> of the PSR may be in full axial contact with the runner ID surface <NUM>, other non-contacting or more locally contacting situations may be present.

From <FIG>, it is seen that in a baseline PSR without the channels <NUM> and <NUM> there would still be leakage flow <NUM> in the intermediate condition of <FIG> circumferential location. Thus, particularly with gaps <NUM>, <NUM> to both sides (<FIG>), in a baseline PSR there may be a leakage flow imposing a circumferential thermal asymmetry. Even if seated against the high pressure sidewall <NUM>, increased local friction due to increased contact forces against the runner may locally increase temperature causing thermal asymmetry. In the <FIG> example, the PSR is unseated at both sides and there is a leakage flow with the prior art baseline. The presence of the channels <NUM> and <NUM> increases this leakage flow slightly relative to an otherwise similar baseline PSR lacking the channels. However, at another location (<FIG>) where the PSR is seated, the essentially zero leakage flow of the baseline is replaced with a leakage flow through the channels. If the leakage flow is a high pressure and high temperature gas, even though the leakage flow at the <FIG> location is less than that at the <FIG> location, the net difference in heating between the different circumferential locations may be reduced relative to the baseline.

Similar considerations attend cooling depending upon the relative temperatures involved (e.g., when the high pressure fluid is relatively cool). Cooling may also be relevant where cocking of the PSR increases frictional heating. Thus, the venting flow may cool. Particularly, the venting flow may disproportionately cool the frictionally heated area.

In an example of dimensions, for a range of gas turbine engine compressor rotor sealing uses a generally linear relationship between seal radius or diameter on the one hand and effective venting area on the other hand has been found beneficial. At takeoff conditions example high pressure side pressure is about <NUM> MPa and low pressure side about <NUM> kPa for a difference of about <NUM> MPa. More broadly, example difference is <NUM> kPa to <NUM> MPa or <NUM> MPa to <NUM> MPa. This may typically involve seal OD radii RO between <NUM> and <NUM>, more particularly <NUM> and <NUM>. Example total channel area per side is k*RO where k is a constant of value of about <NUM><NUM>/cm. A broader k range is <NUM><NUM>/cm to <NUM><NUM>/cm or <NUM><NUM>/cm to <NUM><NUM>/cm. So the total slot area (two sides) is thus double the per side area.

Example channel depth or height H<NUM> is <NUM>, more broadly <NUM> to <NUM> or <NUM> to <NUM>, but this will depend on PSR thickness. Example channel width W is <NUM> for the smaller channels and <NUM> for the larger channels. More broadly, a generic width may be an example <NUM> to <NUM> or <NUM> to <NUM>. The wider channels may be an example at least <NUM>% wider or an example <NUM>% wider to <NUM>% wider (<NUM> to <NUM> times the width). Due to relative thinness of the coating, these numbers may apply to an uncoated substrate, a substrate coated both within the channels and beyond, and a substrate coated only beyond the channels. And they may be measured pre-coating or post-coating.

The arcuate form of the channel surface limits flow losses and stress. A wide shallow arc limits stress (both by having greater web thickness T<NUM> and reducing stress concentration areas such as sharp internal corners). But a wide channel also removes more end/face contact area than a narrower deeper channel of the same cross-sectional (flow) area.

In terms of the number of channels per side, more channels allows tailoring for countering asymmetries. But more smaller channels may increase manufacturing costs relative to a smaller number of larger channels. Small channels, particularly shallow channels, may suffer from manufacturing tolerance which may be exacerbated by wear of the seal (e.g., a given wear will impose a greater area reduction with a shallow channel which is exacerbated if the channel is undersized due to machining tolerance). Use of a larger number of smaller cross-section channels may require the channels get closer to the six o'clock position, thereby bringing potential weak spots closer to the area most stressed by seal installation.

Component materials and manufacture techniques and assembly techniques may be otherwise conventional. Additionally, in one example, the channels are machined into an otherwise fully formed baseline PSR by machining (e.g., via end mill). Other machining examples include abrasive quills, abrasive wheels, and electrodischarge machining (EDM). In other examples, a baseline manufacture process is preserved with the channel machining added before the coating process.

In one example of a baseline process modified to add the channels, initial blanks may be cut from raw stock of nickel-based superalloy or cobalt-based superalloy. Example stock is barstock or tube form of alloy such as AMS <NUM> cobalt alloy (particularly a cobalt-based, chromium-tungsten alloy) or Inconel <NUM> ™ nickel-based superalloy.

The blanks are then rough cut (e.g., via lathe) to form a rough inner diameter (ID) hole and a rough outer diameter (OD) surface.

A further lathe may precision cut OD and ID surfaces, axial end faces, ID chamfers/bevels, and radii.

A side grinding process may further finish PSR substrate thickness/width, corner fillets, and surface finish.

An engine lathe may finish outer diameter (OD) corner radii.

The joint <NUM> gap and circumferential end features may be cut via a CNC mill including rounding of edges and crowning as in the prior art baseline.

The ring may be formed back down to a reduced OD associated with nesting the shiplapped end features (e.g., via progressive rolling).

Heat treating may relieve residual stress.

Then, departing from the baseline, the channels may be cut. As noted above, example cutting is via CNC mill and may involve one or more radial passes per channel. As noted above, there may be separate profiled tools for the different sizes of channel. Alternatively, one tool may cut both sizes of channel via additional passes for the larger channels <NUM>.

Then, as with the example baseline there may be deburring (e.g., hand deburring) and inspection.

There may be partial masking of the PSR substrate prior to application of the base layer <NUM> (e.g., via fixturing, hard masking or in situ formed masking). Then, optionally after demasking, there may be optional precision machining (e.g., grinding/polishing) of the base layer for dimensions and/or finish.

Again, after masking, then anti-gallant layer <NUM> may be applied.

Alternatively to sealing a disk bore to a shaft, such venting means may be applied to static structures such as cases.

Alternatively, applications beyond gas turbine engines include pumps, turbochargers, and other turbomachines.

<FIG> shows an example gas turbine engine <NUM> as a two-spool turbofan engine. Although shown as a high bypass turbofan, a low bypass turbofan may have similar features. The engine <NUM> has an engine case <NUM> surrounding a centerline or central longitudinal axis <NUM>. An example engine has a fan section <NUM> including a fan <NUM> within a fan case <NUM>. The example engine includes an inlet <NUM> at an upstream end of the fan case receiving an inlet flow along an inlet flowpath <NUM>. The fan <NUM> has one or more stages <NUM> of fan blades (typically one in a high bypass turbofan and more in a low bypass turbofan). Downstream of the fan blades, the flowpath <NUM> splits into an inboard portion <NUM> being a core flowpath and passing through a core of the engine and an outboard portion <NUM> being a bypass flowpath exiting an outlet <NUM> of the fan case.

The core flowpath <NUM> proceeds downstream to an engine outlet <NUM> through one or more compressor sections, a combustor, and one or more turbine sections. The example engine has two axial compressor sections and two axial turbine sections, although other configurations are equally applicable. From upstream to downstream there is a low pressure compressor section (LPC) <NUM>, a high pressure compressor section (HPC) <NUM>, a combustor section <NUM>, a high pressure turbine section (HPT) <NUM>, and a low pressure turbine section (LPT) <NUM>. Each of the LPC, HPC, HPT, and LPT comprises one or more stages of blades which may be interspersed with one or more stages of stator vanes. In many low bypass turbofan configurations, the core and bypass flows rejoin to exit a nozzle (e.g., a variable nozzle).

In the example engine, the blade stages of the LPC and LPT are part of a low pressure spool mounted for rotation about the axis <NUM>. The example low pressure spool includes a shaft (low pressure shaft) <NUM> which couples the blade stages of the LPT to those of the LPC and allows the LPT to drive rotation of the LPC. In the example engine, the shaft <NUM> also drives the fan. In the example implementation, the fan is driven via a transmission (not shown, e.g., a fan gear drive system such as an epicyclic transmission) to allow the fan to rotate at a lower speed than the low pressure shaft.

The example engine further includes a high pressure shaft <NUM> (of which the shaft section <NUM> forms a section) mounted for rotation about the axis <NUM> and coupling the blade stages of the HPT to those of the HPC to allow the HPT to drive rotation of the HPC. In the combustor <NUM>, fuel is introduced to compressed air from the HPC and combusted to produce a high pressure gas which, in turn, is expanded in the turbine sections to extract energy and drive rotation of the respective turbine sections and their associated compressor sections (to provide the compressed air to the combustor) and fan.

Claim 1:
A split ring seal (<NUM>) comprising:
a first circumferential end (<NUM>) and a second circumferential end (<NUM>);
an inner diameter surface (<NUM>) and an outer diameter surface (<NUM>);
a first axial end face (<NUM>) and a second axial end face (<NUM>);
a circumferentially distributed first plurality of open channels (<NUM>; <NUM>) along the first axial end face (<NUM>); and
a circumferentially distributed second plurality of open channels (<NUM>; <NUM>) along the second axial end face (<NUM>),
wherein the first circumferential end (<NUM>) and the second circumferential end (<NUM>) form a joint (<NUM>),
characterised in that:
of the first plurality of open channels (<NUM>; <NUM>), two channels (<NUM>) closest to
diametrically opposite the joint (<NUM>) are the largest in cross-sectional area; and of the second plurality of open channels (<NUM>; <NUM>), two channels (<NUM>) closest to
diametrically opposite the joint (<NUM>) are the largest in cross-sectional area.