CROSS-REFERENCE TO RELATED APPLICATIONS

This specification is based upon and claims the benefit of priority from UK Patent Application Number 2019222.5 filed on Dec. 7, 2020, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates to combustion equipment, and in particular to lean burn combustors for gas turbine engines for aircraft, industrial, and marine applications.

2. Description of the Related Art

A gas turbine engine for aircraft applications typically comprises, in axial flow arrangement, a fan, one or more compressors, a combustion system and one or more turbines. The combustion system typically comprises 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. The mixture of air and atomised liquid fuel is then combusted in the combustion chamber and the resultant hot combustion products then expand through, and thereby drive, the one or more turbines.

There is a continual need to reduce the environmental impact of gas turbine engines in terms of carbon emissions and nitrous oxides (NOx), which begin forming at high temperatures and increase exponentially with increasing temperature.

In order to address the NOx emission issue, the “lean burn” combustion technology has been proposed. In lean burn combustion the air-to-fuel ratio (AFR) is higher than a stoichiometric ratio, which allows to keep the combustion temperature within limits known to reduce NOx production.

On the other hand, keeping the combustion temperature relatively low could lead to incomplete or weak combustion, which in turn may lead to producing other pollutants, such as carbon monoxide (CO) and unburned hydrocarbons (UHC), and/or flame instability and rumble, which in turn may cause fatigue failure of components in the engine and/or passenger discomfort, depending on the frequency of the rumbling.

Gas turbine engines for industrial and marine applications face similar challenges as gas turbine engines for aircraft applications.

There is therefore a need to provide a lean burn combustion system for aircraft, industrial, and marine engines which allows to reduce engine emissions of NOx, as well as CO and UHC, and improve engine operability.

SUMMARY

According to a first aspect, there is provided a lean burn combustor comprising: a plurality of lean burn fuel injectors, each comprising a fuel feed arm and a lean burn fuel injector head with a lean burn fuel injector head tip, wherein the lean burn fuel injector head tip has a lean burn fuel injector head tip diameter (d), the lean burn fuel injector head comprising a pilot fuel injector and a main fuel injector, the main fuel injector being arranged coaxially and radially outwards of the pilot fuel injector; and a combustor chamber extending along an axial direction and comprising a radially inner annular wall, a radially outer annular wall, and a meter panel provided upstream of the radially inner and radially outer annular walls with a plurality of apertures adapted for accommodating the lean burn fuel injector head tips. The radially inner annular wall, the radially outer annular wall, and the meter panel define the size and shape of the combustor chamber, wherein the combustor chamber has a combustor chamber length (L) and comprises a primary combustion zone with a primary combustion zone length (Z) and a primary combustion zone depth (D), and a secondary combustion zone with a secondary combustion zone length (L-Z) arranged downstream of the primary combustion zone. According to the first aspect, a ratio L/d of the combustor chamber length to the lean burn fuel injector head tip diameter is less than 5.

In the present disclosure, upstream and downstream are with respect to the fuel and air flow through the combustor, and front and rear is with respect to the lean burn combustor, i.e. the lean burn fuel injectors being in the front and the combustor chamber being in the rear.

The present inventors have found a unique non-dimensional parameter combination for the combustor chamber that allows combustor aerodynamics developing to optimise combustion efficiency and minimise NOX and Smoke. The lean burn combustor according to the disclosure allows for a so-called S-shaped recirculation zone to form in the primary combustion zone of the combustor chamber, which allows the pilot fuel nozzle to support the main fuel nozzle combustion. In particular, the present inventors have found that a combustor chamber according to the disclosure allows the burning mixture of pilot fuel and air coming from the pilot fuel injector to form an S-shaped flow recirculation. In detail, the burning mixture of pilot fuel and air coming from the pilot fuel injector may arrive at a stagnation point in the primary combustion zone, where the pilot fuel and air mixture local velocity is zero, go backwards towards the lean burn fuel injectors, and be diverted (due to low static pressure in the main flow stream) towards the radially inner and radially outer annular walls of the combustor chamber to join the burning mixture of main fuel and air coming from the main fuel injector and support the combustion thereof. In other words, the burning mixture of pilot fuel and air coming from the pilot fuel injector may flow along an S-shaped trajectory.

The skilled person would appreciate that when designing a combustor chamber for a lean burn combustor, aerodynamics study have to be carried out for any combustor chamber size in order to optimise the fuel and air mixture aerodynamics and combustion. The present inventors have surprisingly found that a lean burn combustor according to the disclosure can be scaled up and down without affecting the combustion efficiency. In other words, as the ratio L/d is non-dimensional, for a wide size range of the combustor chamber of the lean burn combustor according to the disclosure, the S-shaped recirculation zone can be effectively and efficiently formed within the primary combustion zone.

For example, a lean burn combustor according to the disclosure may be sized for engines adapted to be mounted on small, medium, and large aircrafts.

In embodiments, the ratio L/d of the combustor chamber length L to the lean burn fuel injector head tip diameter d may be less than 4.5, for example less than 4, or less than 3.5, or less than 3, or less than 2.8, or less than 2.6, or less than 2.5, or less than 2.45, or less than 2.4. The ratio L/d of the combustor chamber length L to the lean burn fuel injector head tip diameter d may be greater than 1.7, or greater than 1.8, or greater than 1.85, or greater than 1.9, or greater than 2.0.

The lean burn fuel injector head may generally extend along a longitudinal direction, the longitudinal direction forming a cant angle αcantwith the axial direction, the cant angle αcantbeing comprised between 0° and 10°.

The combustor chamber may extend axially between the meter panel (upstream) and an annular outlet (downstream), through which the combusted gas exits the combustor chamber. The annular outlet may be defined by, and between, the radially inner annular wall and the radially outer annular wall of the combustor chamber. In the present disclosure the combustor chamber length (L) may be defined as an axial distance between the meter panel and the annular outlet.

The radially outer annular wall may extend substantially axially between the meter panel and the annular outlet. In embodiments, the radially outer annular wall may form an outer angle αouterwith the axial direction, the outer angle αouterbeing comprised between 0° and 15°, for example between 0° and 12°, or between 0° and 10°, or between 3° and 15°, or between 5° and 15°.

The radially outer annular wall may comprise a first part and a second part. The first part of the radially outer annular wall may be arranged upstream of the second part of the radially outer annular wall. The first part and the second part of the radially outer annular wall may be mutually aligned.

The radially inner annular wall may comprise a first part and a second part. The first part of the radially inner annular wall may be arranged upstream of the second part of the radially inner annular wall. The first part of the radially inner annular wall may be connected to the meter panel. The second part of the radially inner annular wall and the second part of the radially outer annular wall may define the annular outlet of the combustion chamber. The first part of the radially inner annular wall may be arranged at an angle to the second part of the radially inner annular wall. The first part of the radially inner annular wall may be parallel to the radially outer annular wall. The first part of the radially inner annular wall may be parallel to the axial direction.

The first part of the radially inner annular wall, the first part of the radially outer annular wall, and the meter panel define the primary combustion zone.

In the present disclosure the primary combustion zone length (Z) may be defined as axial length of the primary combustion zone. The first part of the radially inner annular wall may define the primary combustion zone length (Z). The first part of the radially outer annular wall may define the primary combustion zone length (Z). The first part of the radially inner annular wall and the first part of the radially outer annular wall may have the same length along the axial direction.

In the present disclosure the primary combustion zone depth (D) may be defined as radial distance between the first part of the radially inner annular wall and the first part of the radially outer annular wall. The term “radial” as used herein may refer to the direction perpendicular to the first part of the radially inner annular wall and the first part of the radially outer annular wall.

The second part of the radially inner annular wall may be convergent towards the second part of the radially outer annular wall in a downstream direction. In embodiments, the second part of the radially inner annular wall may form an inner angle αinnerwith the first part of the radially inner annular wall, the inner angle αinnerbeing comprised between 15° and 50°, for example between 15° and 45°, or between 15° and 40°, or between 20° and 50°, or between 25° and 50°, or between 25° and 45°, or between 25° and 40°.

The second part of the radially inner annular wall and the second part of the radially outer annular wall may define the secondary combustion zone. The secondary combustion zone may extend between the primary combustion zone and the annular outlet of the combustion chamber. The secondary combustion zone is arranged downstream of the primary combustion zone. The secondary combustion zone extends for the secondary combustion zone length (L-Z). The second part of the radially outer annular wall may extend for a length equal to the secondary combustion zone length (L-Z). The second part of the radially inner annular wall may extend for a length equal to (L-Z)/cos(αinner).

Respective inner surfaces of the radially inner annular wall, radially outer annular wall, and meter panel may define the size and shape of the combustion chamber where combustion occurs. In some literature, the radially inner annular wall, radially outer annular wall, and meter panel are referred to as combustion liners. In embodiments, the radially inner annular wall, radially outer annular wall, and meter panel may each comprise respective tiles. The tiles may define the respective inner surfaces of the radially inner annular wall, radially outer annular wall, and meter panel, and therefore define the size and shape of the combustor chamber where combustion occurs. The tiles, or in other words the inner surfaces of the radially inner annular wall, radially outer annular wall, and meter panel may face the combustion process within the combustion chamber and may be in contact with the fuel and air mixture, and/or the combustion gases.

The inventors of the present disclosure have also found that other non-dimensional parameters may be advantageous when designing a combustor chamber for a lean burn combustor with improved combustion efficiency.

In embodiments, a ratio L/D of the combustor chamber length L to the primary combustion zone depth D may be less than 2.0, for example less than 1.9, or less than 1.8, or less than 1.75, or less than 1.70, or less than 1.65, or less than 1.60. The ratio L/D of the combustor chamber length L to the primary combustion zone depth D may be greater than 1.0, for example greater than 1.05, or greater than 1.10, or greater than 1.15, or greater than 1.20, or greater than 1.25.

In embodiments, a ratio D/d of the primary zone depth D to the lean burn fuel injector head tip diameter d may be less than 2.4, for example less than 2.3, or less than 2.2, or less than 2.1, or less than 2.0. The ratio D/d of the primary combustion zone depth D to the lean burn fuel injector head tip diameter d may be greater than 1.2, for example greater than 1.3, or greater than 1.4, or greater than 1.5.

In embodiments, a ratio Z/d of the primary combustion zone length Z to the lean burn fuel injector head tip diameter d may be less than 1.40, for example less than 1.35, or less than 1.30, or less than 1.25, or less than 1.20. The ratio Z/d of the primary combustion zone length Z to the lean burn fuel injector head tip diameter d may be greater than 0.70, for example greater than 0.75, or greater than 0.80, or greater than 0.85, or greater than 0.90.

The skilled person would appreciate that, as the ratios L/D of the combustor chamber length L to the primary combustion zone depth D, D/d of the primary zone depth D to the lean burn fuel injector head tip diameter d, and Z/d of the primary combustion zone length Z to the lean burn fuel injector head tip diameter d, are all non-dimensional, they all may apply to lean burn combustors, and relative combustor chambers, of a wide size range and may contribute to form the S-shaped recirculation zone within the primary combustion zone.

According to a second aspect, there is provided a lean burn combustor comprising: a plurality of lean burn fuel injectors, each comprising a fuel feed arm and a lean burn fuel injector head with a lean burn fuel injector head tip, wherein the lean burn fuel injector head tip has a lean burn fuel injector head tip diameter (d), the lean burn fuel injector head comprising a pilot fuel injector and a main fuel injector, the main fuel injector being arranged coaxially and radially outwards of the pilot fuel injector; and a combustor chamber extending along an axial direction and comprising a radially inner annular wall, a radially outer annular wall, and a meter panel provided upstream of the radially inner and radially outer annular walls with a plurality of apertures adapted for accommodating the lean burn fuel injector head tips. The radially inner annular wall, the radially outer annular wall, and the meter panel define the size and shape of the combustor chamber, wherein the combustor chamber has a combustor chamber length (L) and comprises a primary combustion zone with a primary combustion zone length (Z) and a primary combustion zone depth (D), and a secondary combustion zone with a secondary combustion zone length (L-Z) arranged downstream of the primary combustion zone. According to the second aspect, a ratio L/D of the combustor chamber length to the primary combustion zone depth is less than 2.0.

In embodiments, the ratio L/D of the combustor chamber length L to the primary combustion zone depth D may be less than 1.9, for example less than 1.8, or less than 1.75, or less than 1.70, or less than 1.65, or less than 1.60. The ratio L/D of the combustor chamber length L to the primary combustion zone depth D may be greater than 1.0, for example greater than 1.05, or greater than 1.10, or greater than 1.15, or greater than 1.20, or greater than 1.25.

In embodiments, a ratio L/d of the combustor chamber length L to the lean burn fuel injector head tip diameter d may be less than 5, for example less than 4.5, or less than 4, or less than 3.5, or less than 3, or less than 2.8, or less than 2.6, or less than 2.5, or less than 2.45, or less than 2.4. The ratio L/d of the combustor chamber length L to the lean burn fuel injector head tip diameter d may be greater than 1.8, for example greater than 1.85, or greater than 1.9, or greater than 2.0.

In embodiments, a ratio D/d of the primary zone depth D to the lean burn fuel injector head tip diameter d may be less than 2.4, for example less than 2.3, or less than 2.2, or less than 2.1, or less than 2.0. The ratio D/d of the primary combustion zone depth D to the lean burn fuel injector head tip diameter d may be greater than 1.2, for example greater than 1.3, or greater than 1.4, or greater than 1.5.

In embodiments, a ratio Z/d of the primary combustion zone length Z to the lean burn fuel injector head tip diameter d may be less than 1.40, for example less than 1.35, or less than 1.30, or less than 1.25, or less than 1.20. The ratio Z/d of the primary combustion zone length Z to the lean burn fuel injector head tip diameter d may be greater than 0.70, for example greater than 0.75, or greater than 0.80, or greater than 0.85, or greater than 0.90.

According to a third aspect, there is provided a lean burn combustor comprising: a plurality of lean burn fuel injectors, each comprising a fuel feed arm and a lean burn fuel injector head with a lean burn fuel injector head tip, wherein the lean burn fuel injector head tip has a lean burn fuel injector head tip diameter (d), the lean burn fuel injector head comprising a pilot fuel injector and a main fuel injector, the main fuel injector being arranged coaxially and radially outwards of the pilot fuel injector; and a combustor chamber extending along an axial direction and comprising a radially inner annular wall, a radially outer annular wall, and a meter panel provided upstream of the radially inner and radially outer annular walls with a plurality of apertures adapted for accommodating the lean burn fuel injector head tips. The radially inner annular wall, the radially outer annular wall, and the meter panel define the size and shape of the combustor chamber, wherein the combustor chamber has a combustor chamber length (L) and comprises a primary combustion zone with a primary combustion zone length (Z) and a primary combustion zone depth (D), and a secondary combustion zone with a secondary combustion zone length (L-Z) arranged downstream of the primary combustion zone. According to the third aspect, a ratio D/d of the primary combustion zone depth to the lean burn fuel injector head tip diameter is less than 2.4.

In embodiments, the ratio D/d of the primary combustion zone depth to the lean burn fuel injector head tip diameter may be less than 2.3, for example less than 2.2, or less than 2.1, or less than 2.0.

The ratio D/d of the primary combustion zone depth D to the lean burn fuel injector head tip diameter d may be greater than 1.2. In embodiments, the ratio D/d of the primary combustion zone depth D to the lean burn fuel injector head tip diameter d may be greater than 1.3, for example greater than 1.4, or greater than 1.5.

In embodiments, a ratio L/d of the combustor chamber length L to the lean burn fuel injector head tip diameter d may be less than 5, for example less than 4.5, or less than 4, or less than 3.5, or less than 3, or less than 2.8, or less than 2.6, or less than 2.5, or less than 2.45, or less than 2.4. The ratio L/d of the combustor chamber length L to the lean burn fuel injector head tip diameter d may be greater than 1.5, for example greater than 1.7, or greater than 1.8, or greater than 1.85, or greater than 1.9, or greater than 2.0.

In embodiments, a ratio L/D of the combustor chamber length L to the primary combustion zone depth D may be less than 2.0, for example less than 1.9, or less than 1.8, or less than 1.75, or less than 1.70, or less than 1.65, or less than 1.60. The ratio L/D of the combustor chamber length L to the primary combustion zone depth D may be greater than 1.0, for example greater than 1.05, or greater than 1.10, or greater than 1.15, or greater than 1.20, or greater than 1.25.

In embodiments, a ratio Z/d of the primary combustion zone length Z to the lean burn fuel injector head tip diameter d may be less than 1.40, for example less than 1.35, or less than 1.30, or less than 1.25, or less than 1.20. The ratio Z/d of the primary combustion zone length Z to the lean burn fuel injector head tip diameter d may be greater than 0.70, for example greater than 0.75, or greater than 0.80, or greater than 0.85, or greater than 0.90.

In embodiments, the lean burn combustor of the first, second, and third aspect described above may comprise a pre-diffuser, arranged upstream of the lean burn fuel injector head and adapted for providing the combustor chamber with compressed air. In some literature, the pre-diffuser is simply referred to as diffuser. The pre-diffuser may be generally annular and may include radially inner and radially outer walls defining an outlet for the compressed air. In the present disclosure a damp gap (g) may be defined as axial distance between a mid-point between the radially inner and radially outer walls of the pre-diffuser at said outlet and a mid-point between the radially inner and radially outer annular walls of the combustor chamber at the meter panel. A ratio g/d of the damp gap g to the lean burn fuel injector head tip diameter d may be less than 1.30, for example less than 1.25, or less than 1.2, or less than 1.15. The ratio g/d of the damp gap g to the lean burn fuel injector head tip diameter d may be greater than 0.65, for example greater than 0.7, or greater than 0.75, or greater than 0.8, or greater than 0.85.

The skilled person would appreciate that also the ratio g/d of the damp gap g to the lean burn fuel injector head tip diameter d is non-dimensional and may apply to lean burn combustors, and relative combustor chambers, of a wide size range and may contribute to form the S-shaped recirculation zone within the primary combustion zone.

According to a forth aspect, there is provided a lean burn combustor comprising: a plurality of lean burn fuel injectors, each comprising a fuel feed arm and a lean burn fuel injector head with a lean burn fuel injector head tip, wherein the lean burn fuel injector head tip has a lean burn fuel injector head tip diameter (d), the lean burn fuel injector head comprising a pilot fuel injector and a main fuel injector, the main fuel injector being arranged coaxially and radially outwards of the pilot fuel injector; and a combustor chamber extending along an axial direction and comprising a radially inner annular wall, a radially outer annular wall, and a meter panel provided upstream of the radially inner and radially outer annular walls with a plurality of apertures adapted for accommodating the lean burn fuel injector head tips. The radially inner annular wall, the radially outer annular wall, and the meter panel define the size and shape of the combustor chamber. The lean burn combustor of the forth aspect further includes a pre-diffuser, arranged upstream of the lean burn fuel injector head and adapted for providing the combustor chamber with compressed air.

The pre-diffuser is generally annular and includes radially inner and radially outer walls defining an outlet for the compressed air. A damp gap (g) is defined as axial distance between a mid-point between the radially inner and radially outer walls of the pre-diffuser at said outlet and a mid-point between the radially inner and radially outer annular walls of the combustor chamber at the meter panel, wherein a ratio g/d of the damp gap to the lean burn fuel injector head tip diameter is less than 1.30.

In embodiments, the ratio g/d of the damp gap g to the lean burn fuel injector head tip diameter d may be less than 1.25, for example less than 1.2, or less than 1.15. The ratio g/d of the damp gap g to the lean burn fuel injector head tip diameter d may be greater than 0.65, for example greater than 0.7, or greater than 0.75, or greater than 0.8, or greater than 0.85.

The combustor chamber of the lean burn combustor of the forth aspect has a combustor chamber length (L) and may define a primary combustion zone with a primary combustion zone length (Z) and a primary combustion zone depth (D), and a secondary combustion zone with a secondary combustion zone length (L-Z) arranged downstream of the primary combustion zone.

In embodiments, a ratio D/d of the primary zone depth D to the lean burn fuel injector head tip diameter d may be less than 2.4, for example less than 2.3, or less than 2.2, or less than 2.1, or less than 2.0. The ratio D/d of the primary combustion zone depth D to the lean burn fuel injector head tip diameter d may be greater than 1.2, for example greater than 1.3, or greater than 1.4, or greater than 1.5.

In embodiments, a ratio L/D of the combustor chamber length L to the primary combustion zone depth D may be less than 2.0, for example less than 1.9, or less than 1.8, or less than 1.75, or less than 1.70, or less than 1.65, or less than 1.60. The ratio L/D of the combustor chamber length L to the primary combustion zone depth D may be greater than 1.0, for example greater than 1.05, or greater than 1.10, or greater than 1.15, or greater than 1.20, or greater than 1.25.

In embodiments, a ratio L/d of the combustor chamber length L to the lean burn fuel injector head tip diameter d may be less than 5, for example less than 4.5, or less than 4, or less than 3.5, or less than 3, or less than 2.8, or less than 2.6, or less than 2.5, or less than 2.45, or less than 2.4. The ratio L/d of the combustor chamber length L to the lean burn fuel injector head tip diameter d may be greater than 1.5, for example greater than 1.7, or greater than 1.8, or greater than 1.85, or greater than 1.9, or greater than 2.0.

In embodiments, a ratio Z/d of the primary combustion zone length Z to the lean burn fuel injector head tip diameter d may be less than 1.40, for example less than 1.35, or less than 1.30, or less than 1.25, or less than 1.20. The ratio Z/d of the primary combustion zone length Z to the lean burn fuel injector head tip diameter d may be greater than 0.70, for example greater than 0.75, or greater than 0.80, or greater than 0.85, or greater than 0.90.

According to a fifth aspect, there is provided a gas turbine engine comprising a lean burn combustor according to any one of the aspects described above.

The gas turbine engine of the fifth aspect may be a gas turbine engine for an aircraft, or for industrial and marine applications.

In embodiments, the gas turbine engine may further comprise: an engine core comprising a compressor, a combustor, a turbine, and a core shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades, wherein the combustor is the lean burn combustor according to any one of the first, second, third, and forth aspect.

In embodiments, the compressor and turbine may rotate about an engine principal rotational axis, and the axial direction of the combustor chamber may be parallel to the engine principal rotational axis.

As previously noted, the lean burn combustor according to the disclosure may be sized for engines adapted to be mounted on small, medium, and large aircrafts. Accordingly, the fan of the gas turbine engine according to the fifth aspect may have a fan diameter greater than (or on the order of) any of: 220 cm, 230 cm, 240 cm, 250 cm (around 100 inches), 260 cm, 270 cm (around 105 inches), 280 cm (around 110 inches), 290 cm (around 115 inches), 300 cm (around 120 inches), 310 cm, 320 cm (around 125 inches), 330 cm (around 130 inches), 340 cm (around 135 inches), 350 cm, 360 cm (around 140 inches), 370 cm (around 145 inches), 380 (around 150 inches) cm, 390 cm (around 155 inches), 400 cm, 410 cm (around 160 inches) or 420 cm (around 165 inches). The fan diameter may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from 220 cm to 420 cm, or 240 cm to 380 cm, or 240 cm to 280 cm, or 330 cm to 380 cm.

Arrangements of the present disclosure may be particularly, although not exclusively, beneficial for fans that are driven via a gearbox. Accordingly, 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 input to the gearbox may be directly from the core shaft, or indirectly from the core shaft, for example via a spur shaft and/or gear. The core shaft may rigidly connect the turbine and the compressor, such that the turbine and compressor rotate at the same speed (with the fan rotating at a lower speed).

The gas turbine engine as described and/or claimed herein may have any suitable general architecture. For example, the gas turbine engine may have any desired number of shafts that connect turbines and compressors, for example one, two or three shafts. Purely by way of example, the turbine connected to the core shaft may be a first turbine, the compressor connected to the core shaft may be a first compressor, and the core shaft may be a first core shaft. The engine core may further comprise a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, second compressor, and second core shaft may be arranged to rotate at a higher rotational speed than the first core shaft.

In such an arrangement, the second compressor may be positioned axially downstream of the first compressor. The second compressor may be arranged to receive (for example directly receive, for example via a generally annular duct) flow from the first compressor.

The gearbox may be arranged to be driven by the core shaft that is configured to rotate (for example in use) at the lowest rotational speed (for example the first core shaft in the example above). For example, the gearbox may be arranged to be driven only by the core shaft that is configured to rotate (for example in use) at the lowest rotational speed (for example only be the first core shaft, and not the second core shaft, in the example above). Alternatively, the gearbox may be arranged to be driven by any one or more shafts, for example the first and/or second shafts in the example above.

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 2.5, for example in the range of from 3 to 4.2, or 3.2 to 3.8, for example on the order of or at least 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1 or 4.2. 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 3.1 or 3.2 to 3.8.

According to an aspect, there is provided an aircraft comprising a gas turbine engine as described and/or claimed herein. The aircraft according to this aspect is the aircraft for which the gas turbine engine has been designed to be attached.

The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and/or combined with any other feature or parameter described herein.

