Patent ID: 12241629

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG.1

Two aircraft101and102are illustrated inFIG.1in formation at substantially the same flight level and in substantially the same atmospheric conditions.

The aircraft101and102are of substantially the same configuration, save for their engines. Aircraft101comprises two engines103which, due to their configuration and operating point, are forming contrails104. Aircraft102comprises two engines105which are configured in accordance with the present invention, and are thus forming contrails106having a lower optical depth than contrails104. As will be described herein, the engines105include functionality so as to allow the optical depth of any contrails they produce to be modified.

As used herein, optical depth is a measure of how much electromagnetic radiation, optionally in certain wavelength ranges, is prevented from travelling through a region. In the case of a contrail or ice cloud, optical depth is influenced primarily by the ice particle number density, effective ice particle radius, and the physical thickness of the cloud. Since most contrails are optically thin their radiative forcing is approximately proportional to their optical depth.

Thus, in the example ofFIG.1, a determination has been made to the effect that, in terms of climate impact, it would be preferable for any contrails produced to have a lower optical depth. In turn, therefore measures are taken within the engines105to reduce the optical depth of the contrails106, so as to reduce the radiative forcing they cause.

As will also be described in further detail herein, the functionality of engines105is such that they may respond to the converse determination, i.e. that in terms of climate impact it would be preferable for any contrails produced to have a higher optical depth.

FIG.2

A general arrangement of one of the engines105for aircraft102is shown inFIG.2.

In the present embodiment, the engine105is a turbofan, and thus comprises a ducted fan201located in a nacelle202. The fan201receives intake air A and generates two airflows: a bypass flow B which passes axially through a bypass duct203and a core flow C which enters a core gas turbine.

The core gas turbine comprises, in axial flow series, a low-pressure compressor204, a high-pressure compressor205, a combustor206, a high-pressure turbine207, and a low-pressure turbine208.

In operation, the core flow C is compressed by the low-pressure compressor204and is then directed into the high-pressure compressor205where further compression takes place. The compressed air exhausted from the high-pressure compressor205is directed into the combustor206where it is mixed with fuel and the mixture is combusted.

In the present embodiment, the combustor206is an RQL combustor based on the rich-burn, quick-quench, lean-burn concept to reduce emissions, in particular oxides of nitrogen. In this configuration, fuel is injected into a rich primary zone, with the flame being rapidly quenched by cooling air admitted via quench ports, after which combustion continues under lean conditions. Such combustor systems will be familiar to those skilled in the art, and will be described further with reference toFIGS.11A,11B and11C.

Furthermore, in the present embodiment the combustor206is also a variable geometry combustor, As will be familiar to those skilled in the art, the term variable geometry in the context of gas turbine combustors refers to features which enable changes in airflow distribution to be made in operation. In the present embodiment, the variable geometry property of the combustor206is provided by a variable geometry airflow arrangement configured to vary the airflow through one or more of the fuel injectors or the quench ports, In this way, the fuel-air ratios in one or more of the rich primary zone or the lean secondary zone may be varied by an appropriate control scheme. This process will be described further with reference toFIGS.4A and4B. Examples of the variable geometry airflow arrangement will be described further with reference toFIG.13Aonward.

Following combustion, the resultant hot combustion products are discharged from the combustor206and expand through, and thereby drive, the high-pressure turbine207and in turn the low-pressure turbine208before being exhausted via a core nozzle209to provide a small proportion of the overall thrust.

The fan201is driven by the low-pressure turbine208via a reduction gearbox210. In the present embodiment, the reduction gearbox210takes the form of an epicyclic gearbox. In this specific embodiment, the reduction gearbox210is a planetary-type epicyclic gearbox and thus comprises a sun gear meshed with a plurality of planet gears located in a rotating carrier. In this example, five planet gears are provided. The planet gears are also meshed with a static ring gear. The rotating carrier is connected with the fan201. It will be appreciated that a star-type epicyclic gearbox could be used instead, with the planet gear carrier being static and the ring gear allowed to rotate to drive the fan201. In other embodiments, the gearbox210could be a layshaft-type gearbox or any other type of reduction gear. In further alternatives, the gearbox may be omitted and the engine105configured as a direct-drive engine, either in a two-spool or three-spool arrangement.

As described, in the present embodiment the engine105comprises a combustor206. Fuel is provided to fuel injectors by means of a fuel system controller, which in the present embodiment is provided by a fuel metering unit (FMU)211under control of an electronic engine controller (EEC)212. Fuel is delivered to the fuel metering unit211by a fuel pump213. In this embodiment, the fuel pump213is mechanically driven by an accessory gearbox214, itself driven via a high-pressure spool radial driveshaft of known configuration (not shown). In alternative configurations, for example in a more electric engine (MEE) configuration, the fuel pump213may be electrically-driven.

FIG.3

A block diagram illustrating the control scheme for the fuel injectors and the variable geometry combustor is shown inFIG.3.

In the present example, high-pressure fuel is delivered by the fuel metering unit211into a fuel manifold301for distribution to fuel injectors302.

The quantity of fuel to be injected is controlled by the electronic engine controller212, which provides control signals to the fuel metering unit211indicative of the total fuel that must be injected in the form of a fuel flow rate (WF). As is conventional, this is done on the basis of a control loop which derives a target fuel flow rate on the basis of a power lever angle (PLA) setting and a computed air mass flow rate into the combustor206.

In addition to controlling the fuel metering unit211to a target fuel flow rate WF, in the present embodiment the electronic engine controller212is configured to control a variable geometry airflow arrangement303comprised in the combustor206. As described previously, in the present embodiment the variable geometry airflow arrangement303is configured to vary the airflow through either or both of the fuel injectors302and quench ports in the combustor206. To this end, in the present embodiment the variable geometry airflow arrangement303comprises an actuation system configured to respond to a variable geometry setting command from the electronic engine controller212.

FIGS.4A &4B

As the formation of soot in aero engine combustors is primarily governed by the fuel-air ratio in the primary combustion zone and the degree of soot consumption in the secondary zone, the variable geometry airflow arrangement303therefore allows a degree of control over the quantity of soot produced at a given fuel flow rate.

FIGS.4A and4Bare charts showing the relationship between fuel flow rate and non-volatile particulate matter (nvPM) number. The various non-volatile particulate matter parameters are defined by the International Civil Aviation Organization. The dominant constituent of non-volatile particulate matter is soot, and so, in the present embodiments, non-volatile particulate matter number is used as the index of soot emissions, since the quantity of soot particles emitted per unit distance travelled by an aircraft has a substantial impact on contrail optical depth.

FIG.4Ashows nvPM number for lower fuel-air ratios thanFIG.4B. As can be seen in the charts, the quantity of non-volatile particulate matter emitted at the richer equivalence ratios ofFIG.4Bare far greater than that emitted during the leaner conditions ofFIG.4Afor the same fuel flow rate.

The inventors have determined that this characteristic may be utilised to vary the non-volatile particulate matter number. Thus, in the present embodiment, the electronic engine controller212is configured to determine the appropriate control for the variable geometry airflow arrangement303using a combination of the airflow into the combustor206, the fuel flow to the fuel injectors302, and a target index of soot emissions. In this way, the quantity of soot produced by combustion may be varied whilst respecting the power lever angle setting. In an embodiment, the target index of soot emissions is determined in dependence upon atmospheric conditions. In an embodiment, the said atmospheric conditions are those causative of contrails.

FIGS.5A &5B

FIGS.5A and5Bare charts adapted from B. Kärcher and F. Yu, “Role of aircraft soot emissions in contrail formation”,Geophysical Research Letters, vol. 36, no. 1, 2009, which is incorporated herein by reference. The charts show the dependence of contrail optical depth on nvPM number at a fixed ambient relative humidity over water. The contributing factors to the contrails' optical depth (solid) are ice particles formed from deposition on exhaust soot (dashed), and ice particles formed from emitted and/or existing ambient liquid particles (dot-dashed).FIG.5Ashows the relationship for ambient temperatures above some transition temperature (TAMB>TTRANS), andFIG.5Bshows the relationship for ambient temperatures at or below the transition temperature (TAMB≤TTRANS)

As shown inFIG.5A, with ambient temperatures greater than the transition temperature, and at higher nvPM numbers, exhausted non-volatile particulate matter is largely determinative of the contrail's optical depth. At lower nvPM numbers, emitted and/or existing ambient liquid particles which freeze in the exhaust plume begin to dominate the contribution to overall optical depth. As can be seen, above the transition temperature, the relationship between nvPM number and optical depth is one which is monotonically increasing.

Referring toFIG.5B, for ambient temperatures at or below the transition temperature, a much larger number of emitted and/or existing ambient liquid particles freeze and thus lead to an increase in contrail optical depth at low nvPM numbers. It will be seen that the relationship between nvPM number and optical depth is no longer monotonic. Hence there exists a stationary point at a transition value501.

Thus whilst an aero engine may be producing few soot emissions, it may still be forming a contrail having greater optical depth than an engine producing substantially greater soot emissions. Depending upon other factors, this could have a more detrimental climate impact.

In the research undertaken by Kärcher and Yu referenced above, modelling suggested that there is a transition from the monotonically-increasing relationship ofFIG.5Ato the non-monotonicity ofFIG.5B, As defined herein, the transition temperature TTRANSis, when considering a reduction in temperature, the temperature at which the relationship ceases to be monotonically increasing, with a stationary point appearing at the transition value501.

The transition temperature for any particular ambient relative humidity over water may be determined in accordance with the modelling approach set out in B. Kärcher, U. Burkhardt, A. Bier, L. Bock and I. Ford, “The microphysical pathway to contrail formation”,Journal of Geophysical Research: Atmospheres, vol. 120, no. 15, pp. 7893-7927, 2015, which is incorporated herein by reference.

As described with reference toFIGS.4A and4B, control of the variable geometry combustion system by way of a variable geometry airflow arrangement may be used to effect changes in nvPM number. This characteristic, in conjunction with the relationships between nvPM number and optical depth described with reference toFIGS.5A and5B, allows the electronic engine controller212to effect changes in optical depth.

FIG.6

A block diagram is shown inFIG.6that illustrates sensor inputs, memory registers and processing modules in the electronic engine controller212to control the variable geometry airflow arrangement303in accordance with a target index of soot emissions. As described previously, in the present embodiment this is performed in dependence upon atmospheric conditions.

In terms of processing functionality, in the present embodiment the electronic engine controller212comprises an atmospheric condition analysis module601, an optical depth response module602, and an airflow control module603. The atmospheric condition analysis module601will be described in further detail with reference toFIG.7. The optical depth response module602will be described in further detail with reference toFIGS.8and9. The airflow control module603will be described in further detail with reference toFIG.10.

The modules601,602, and603operate together to form an appropriate contrail optical depth response to an atmospheric condition in the form of variable geometry setting for implementation by the variable geometry airflow arrangement303.

It will be appreciated that whilst in the present embodiment the modules601,602and603are described as software running on the electronic engine controller212, they may be implemented as software running on separate control units or even implemented in dedicated hardware. It will also be appreciated that some or all of the processing steps described herein for the modules601,602and603could be carried out at a location which is physically remote from the aircraft, making use of suitable datalinks to transmit data from the aircraft to the processing location and vice versa.

In the present embodiment, each module601,602, and603is configured such that it has access to a plurality of registers storing various sensor outputs and/or downloaded data.

An optical sensor611is configured to produce image data621of the exhaust plume region of the engine105. This facilitates analysis of the optical depth of a contrail being generated by the engine105during flight, and thus closed loop control of optical depth. Additionally or alternatively, other types of sensors such as lidar or radar may be used to generate data suitable for analysis of the contrail optical depth.

Various ambient condition sensors may be provided to facilitate assessment of atmospheric conditions, In the present embodiment, a temperature sensor612(for example, an outside air temperature probe or similar), a pressure sensor613(for example, an aneroid barometer forming part of a pitot-static arrangement or similar), and a humidity sensor614(for example, a hygrometer or similar) write to respective registers for ambient temperature622, ambient pressure623(from which altitude may be derived), and ambient humidity624,

A positioning system615(for example Global Positioning System, Galileo, etc.) provides geolocation data625. A data downlink616(for example satellite communication) facilitates acquisition of satellite imagery626A and weather forecasts626B to allow identification of regions conducive to contrail formation and/or contrail persistence.

The output of a power lever angle sensor617in the cockpit of the aircraft102is converted into total fuel flow (WF) demand627by, for example, a surrogate engine model in the electronic engine controller212. In addition to this, the electronic engine controller212is configured to determine a corresponding total air mass flow quantity entering the combustor206(W30) to permit calculation of fuel-air ratio, etc. The design of such control loops will be familiar to those skilled in the art.

It will be appreciated that in alternative embodiments only a subset of the sensors and registers may be selected for implementation. For example, optical sensors may be deployed and be the sole means of detection of contrail formation. Conversely, only temperature, pressure and humidity instruments may be selected for use in detection of conditions which indicate that contrails will form, and so on.

FIG.7

The atmospheric condition analysis module601is shown in detail inFIG.7.

Input data obtained from some or all of the registers621-627is obtained by a contrail formation model701, The contrail formation model701is configured to determine whether or not contrail formation is likely under current ambient conditions and engine operating point, irrespective of subsequent persistence.

In an embodiment, the contrail formation model701uses the real- or near real-time atmospheric condition data measured by the sensors. For example, the measurement of ambient humidity624by the humidity sensor614may be used, or the image data621produced by the optical sensor611.

In a specific embodiment, the Schmidt-Appleman criterion is applied and coupled with an assumption of a linear or approximately linear mixing trajectory in the space defined by temperature and water-vapour partial pressure. To perform this processing, the contrail formation model701utilises the measurements of ambient temperature622, ambient pressure623, and ambient humidity624.

In an alternative embodiment, the contrail formation model701utilises the satellite imagery626A and/or the weather forecasts6268to determine whether atmospheric conditions are such that contrails will form. In a specific embodiment, the satellite imagery626A is used in conjunction with the geolocation data625and an altitude reading derived from the ambient pressure623to confirm whether or not other aircraft in the vicinity have caused contrails or not, In a specific embodiment, the weather forecasts626B are coupled to the Schmidt-Appleman criterion approach described above.

In the present embodiment, if the contrail formation model701determines that no contrail will form given current atmospheric conditions, then no action is taken and the atmospheric condition analysis module601proceeds to an idle process702where it waits until new input data are available.

If the contrail formation model701determines that a contrail will form, then control proceeds to a contrail type classifier703which is configured to determine, given ambient conditions, whether the contrail will persist or not. This may be achieved by assessing the ambient relative humidity with respect to ice: if the ambient air is supersaturated with respect to ice, then the contrail will persist. The output of the contrail type classifier703is the determined contrail type704, and is provided to the optical depth response module602.

FIG.8

The optical depth response module602receives the determined contrail type704from the atmospheric condition analysis module601, and proceeds to ask a question at a decision block801as to whether the contrail will be persistent, or not.

If this question is answered in the affirmative, then several models are invoked to assess the optimal response in terms of adjustments to the optical depth of the contrail.

A plume-wake interaction model802is provided which assesses the effect of the wake of the aircraft102on the exhaust plume of the engine105. This model will be described in further detail with reference toFIG.9. In the present embodiment, the model802outputs a set of predictions of the time-varying properties of a plurality of contrails, each caused by exhaust plumes having different nvPM numbers. The output of model802is provided to a long-wave forcing model803, which is configured to determine the time-integrated radiative forcing per unit length of each contrail due to long-wave (i.e. warming) effects over their expected lifetimes. In addition to model802, a solar radiation model804and a surface albedo model805are executed and their outputs combined. The solar radiation model804is configured to determine the strength and orientation of incoming sunlight over the expected lifetime of the contrail in the post-vortex regime. The surface albedo model805is configured to determine the albedo of surfaces (including other clouds) which would receive incoming sunlight in the absence of a contrail formed by the aircraft. In the present embodiment, models804and805utilise the satellite imagery626A and weather forecast data6268to perform this assessment.

The combined output of models804and805are supplied, along with the output of model802, to a short-wave forcing model806which is configured to determine the time-integrated radiative forcing due to short-wave (i.e. cooling) effects over the expected lifetime of the predicted set of contrails generated by the model802.

The outputs of the long-wave forcing model803and the short-wave forcing model806are supplied to a climate impact model807which determines the optimal optical depth to achieve the best balance between the magnitudes of the modelled short-wave cooling and long-wave warming effects.

In this way, in the present embodiment the model807determines that the optical depth of the condensation trail should be increased or decreased on the basis of a time-integrated effect of a persistent contrail over its lifespan given a current atmospheric condition and a predicted future atmospheric condition. The output of model807is an optical depth demand808, which is supplied to the airflow control module603. The optical depth demand808is a target optical depth for the contrail.

In practice, therefore, the target index of soot emissions is derived by identifying a condition to the effect that an optical depth of a condensation trail produced by the engine should be either reduced or increased. In response to identifying that the optical depth should be reduced, the target index of soot emissions is updated so as to reduce ice particle formation. In response to identifying that the optical depth should be increased, the target index of soot emissions is updated so as to increase ice particle formation.

In the present embodiment, the climate impact model807is configured to consider short-wave cooling to be a desirable effect, and long-wave warming to be an undesirable effect, and thus tends to produce an optical depth demand808which reduces the net warming impact of the contrail.

In the present example, if the question asked at decision block801is answered in the negative, then the idle process702is invoked until new input data are available. In alternative embodiments, measures may still be taken to alter the optical depth, possibly adopting a similar approach to that for persistent contrails.

FIG.9

The plume-wake interaction model802is shown in more detail inFIG.9.

The model802obtains input data pertaining to, in particular, the operating point901of the engine105(for example intake temperature and pressure, power lever angle setting, overall fuel flow rate, etc.) and the ambient conditions902at the current location of the aircraft102(for example ambient temperature622, ambient pressure623, ambient humidity624, etc.).

These input data are supplied to an ice crystal distribution model903, which is configured to determine, for each of a plurality of possible nvPM numbers, the initial particle size distribution of ice particles formed in the exhaust plume of the engine105. In the present embodiment, this is performed on the basis of factors including ambient temperature622, ambient pressure623, ambient humidity624, the mass of water vapour emitted by the engine per unit distance of travel, and the efficiency of the engine105. In this example the ice crystal distribution model903is configured to determine the mass of water vapour using the data pertaining to engine operating point901, i.e. power lever setting, fuel flow rate, fuel properties, calculated airspeed, etc.

The ice crystal distributions are supplied to an ice crystal entrainment model904, which is configured to determine the extent to which particles in the engine exhaust become captured by wingtip vortices of the aircraft102.

This is modelled because a significant proportion of ice crystals which would otherwise form a persistent contrail may be destroyed by heating in the vortices, thereby reducing the optical depth of the contrail.

The ice crystal entrainment model904is configured to determine the particle size distribution of ice particles initially captured within the wingtip vortex core, given an initial particle size distribution of a newly formed contrail, in dependence upon the location of the corresponding engine relative to the wingtip. It is further configured to determine the number or ratio of ice particles which remain after the adiabatic heating experienced within the wingtip vortex core during the lifetime of the wingtip vortex. The remaining ice particles also include those ice particles which were detrained from the vortex prior to its breakup.

In order to model this effect, the ice crystal entrainment model904calls upon one or more other models. In this example, a wingtip vortex lifetime model905is configured to determine the lifetime of a wingtip vortex in dependence upon such factors as the strength of ambient turbulence, the rate of change of ambient temperature with altitude, and/or the instantaneous aircraft weight (e.g. taking account of the amount of fuel burned so far during the flight). A vortex descent velocity model906is configured to determine the downward velocity of a wingtip vortex, in dependence upon factors including the instantaneous aircraft weight, and aircraft configuration. A vortex temperature rise model907is configured to determine the temperature change likely to be experienced within the vortex core as a result of the determined change in altitude during its descent and/or the speed of its descent.

It is contemplated that further models could be provided, for example a model from which can be determined the proportion, of those ice particles not captured/retained by the wingtip vortex core, which experience sufficient adiabatic heating in the region of downwash between the aircraft's wingtip vortices so as to be eliminated.

The output of ice crystal entrainment model904is then supplied to a wind shear model908which predicts the degree of horizontal spreading of the contrail over its expected lifetime. This prediction is performed using the vertical extent of the post-vortex contrail, and current and future weather conditions obtained from the weather forecasts626B, This is performed to account for the contrail's short- and long-wave effects.

The output from the plume-wake interaction model802is thus a set of contrail lifecycle data909for a plurality of nvPM numbers.

FIG.10

The airflow control module603is shown in greater detail inFIG.10.

Initially, the optical depth demand808is supplied to a soot formation model1001. Model1001is configured to determine a target nvPM number under the current ambient atmospheric conditions, in particular the ambient temperature, which will meet the optical depth demand808. In the present embodiment, this is achieved by use of the contrail lifecycle data909.

In an alternative embodiment, the model1001may implement a microphysical simulation to establish the target nvPM number.

This target nvPM number is then used as a set point for evaluating appropriate values for the variable geometry airflow arrangement303. In the present embodiment, a map1002is used, allowing an efficient lookup of the appropriate settings for the variable geometry airflow arrangement303to produce the requisite airflow settings for either the fuel injector302or the quench ports to achieve a target nvPM number, given an overall airflow rate into the combustor206and overall fuel flow rate WF. In an alternative embodiment, a surrogate model may be implemented to model the combustion process based on real time parameters.

As shown inFIGS.5A and5B, the relationship between optical depth and nvPM effectively splits into two regimes: one (FIG.5A) above a transition temperature where there is a generally monotonic relationship between nvPM and optical depth, and another (FIG.58) at or below the transition temperature where the minimum optical depth is found at the transition value501, with both a decrease and an increase in nvPM number resulting in optical depth increasing.

Hence the airflow control module603controls the variable geometry airflow arrangement303by taking these different regimes into account.

In functional terms, the airflow control module603compares a newly-received optical depth demand808with the current value thereof, Thus the airflow control module603determines whether the optical depth of a contrail should be reduced, or increased.

If the optical depth demand808is such that the optical depth of a contrail should be reduced, the airflow control module603evaluates a variable geometry setting1003that varies nvPM number (and thus soot production) to minimise ice particle formation,

The use of the soot formation model1001and map1002effectively compares a measurement of ambient temperature to the transition temperature. Then, if the ambient temperature is greater than the transition temperature, a variable geometry setting1003is determined that, if possible, decreases soot production to achieve the target optical depth. It will be appreciated that if the lowest possible level of soot production has already been reached, then the variable geometry setting1003will remain the same.

If the ambient temperature is less than or equal to the transition temperature, a variable geometry setting1003is determined that either decreases or increases soot production. This is performed in dependence upon whether the current variable geometry setting1003, and hence nvPM number, is above or below the transition value501. The minimum optical depth that may be achieved is that obtained when the variable geometry setting1003corresponds to the transition value501.

If the optical depth demand808is such that the optical depth of a contrail should be increased, the airflow control module603evaluates a variable geometry setting1003that varies nvPM number (and thus soot production) to increase ice particle formation.

The effect of the airflow control module's configuration is then to obtain a measurement of ambient temperature.

If the ambient temperature is greater than the transition temperature, a variable geometry setting1003is determined that increases soot production to achieve the target optical depth.

If the ambient temperature is less than or equal to the transition temperature, and the current variable geometry setting1003corresponds to an nvPM number that is greater than the transition value501, then a variable geometry setting1003is determined that increases the index of soot emissions.

If the ambient temperature is less than or equal to the transition temperature, and the current variable geometry setting1003corresponds to an nvPM number that is less than or equal to the transition value501, then a variable geometry setting1003is determined that decreases the index of soot emissions

In some embodiments, the decision as to whether to increase or decrease nvPM number to increase optical depth may be influenced by which operational change is associated with, for example, the lowest fuel consumption. In alternative embodiments, other parameters may influence this decision, such as emissions of unburnt hydrocarbons, oxides of nitrogen, or even impact on life-limited parts, etc.

FIGS.11A,11B &11C

Example configurations of the combustor206are shown inFIGS.11A,11B and11C.

As described previously, in the present embodiment, the combustor206is configured as an RQL combustor as illustrated inFIG.11A. In this configuration, fuel F is injected by the fuel injectors302into a rich-burn zone1101, in which the fuel-air ratio is fuel rich. In order to limit emissions, quench air Q is admitted to a quick-quench zone1102to quickly reduce the flame temperature. In the present embodiment, the combustor206comprises two rows of axially-separated quench ports1103. Amongst other things, this reduces formation of oxides of nitrogen. It also causes continued combustion to occur under lean conditions in a lean-burn zone1104. Due to the lean conditions, a large proportion of the soot produced in the rich-burn zone1101is consumed in the lean-burn zone1104. It will be understood that the net quantity of soot produced by the combustor206may be influenced by the fuel-air ratio in the rich-burn zone1101, which governs the total quantity of soot produced, and the amount of quench air Q admitted to the quick-quench zone1102, which governs the total quantity of soot burnt off in the lean burn zone1104.

Thus, in one embodiment, the fuel injectors302are variable geometry fuel injectors and comprise the variable geometry airflow arrangement303. In another embodiment, the quench ports1103are variable geometry quench ports and comprise the variable geometry airflow arrangement303.

An alternative combustor206′ is shown inFIG.11B, in which only one row of quench ports1103is provided. Another alternative combustor206″ is shown inFIG.11C, in which one row of quench ports1103is provided on both the inner and outer walls of the combustor206″. It will be appreciated that the principles disclosed herein may be applied to a variety of RQL combustor configurations.

FIG.12

A section through the combustor206is shown inFIG.12. The combustor206is mounted within a cavity1201formed by an inner air casing1202and outer air casing1203.

In operation, the high-pressure compressor205delivers pressurised core airflow C to the cavity1201via a diffuser1204. At this point, a quantity of the air enters the combustor206as combustion air D through the fuel injector302and/or mixing ports at the entrance to the combustor206into the rich-burn zone1101. The remaining air flows around the combustor206as cooling air E, a quantity of which is admitted into combustor206as quench air Q into the quick-quench zone1102via the quench ports1103. The remaining cooling air E may be directed to cool the liner of the combustor206and/or as dilution air prior to entering the high-pressure turbine207, etc.

In a conventional fixed geometry combustion system, the sizing of ports remains fixed and hence the relative proportion of combustion air and cooling air E remains fixed throughout the operational envelope. At a given airflow C and a given overall fuel flow rate WF, therefore, the fuel-air ratio for the fuel injectors302is essentially fixed.

As described previously, in the present embodiment the combustor206is a variable geometry combustor and comprises a variable geometry airflow arrangement303to vary the airflow through the fuel injectors302and/or the quench ports1103. This is because, if airflow is varied through the fuel injectors302, there is an opposing change in airflow through the quench ports1103. In this way, the fuel-air ratio for the fuel injectors302and the amount of quench air Q may be varied by the electronic engine controller212to achieve a target index of soot emissions.

In operation, a decrease in target nvPM number will be met by a decrease in airflow through the quench ports1103relative to airflow through the fuel injectors302. In this way, the fuel-air ratio in the rich-burn zone1101will be decreased, leading to a locally leaner mixture and hence a decrease in soot formation. The amount of quench air Q will be decreased, thereby increasing temperature in the lean-burn zone1104, and in turn increasing soot burn-off. The combination of decreased soot formation and increased soot burn-off leads to a net decrease in nvPM number in the products of combustion.

Conversely, an increase in target nvPM number will be met by an increase in airflow through the quench ports1103relative to airflow through the fuel injectors302. In this way, the fuel-air ratio in the rich-burn zone1101will be increased, leading to a locally richer mixture and hence an increase in soot formation. The amount of quench air Q will be increased, thereby decreasing temperature in the lean-burn zone1104, and in turn decreasing soot burn-off. The combination of increased soot formation and decreased soot burn-off leads to a net increase in nvPM number in the products of combustion.

As shown inFIG.12, in the present embodiment, the quench ports1103are variable geometry quench ports. The configuration of the variable geometry quench ports will be described further with reference toFIGS.13A,13B and130. In an alternative embodiment, the fuel injectors302are variable geometry fuel injectors. The configuration of such variable geometry fuel injectors will be described further with reference toFIGS.15A and15B.

FIGS.13A,13B &13C

The variable geometry quench ports1103are shown in plan view inFIGS.13A,13B and13C. In the present embodiment, the area of the quench ports1103are varied by an annular strip1301having apertures1302corresponding to the quench ports1103. Circumferential movement of the strip1301and hence alignment of the apertures1302and quench ports1103either opens them admitting a full quantity of quench air Q (FIG.13A), partially closes them admitting less quench air Q (FIG.13B), and possibly even fully closes them admitting no quench air Q (FIG.13C). It will be appreciated that in turn this also varies the airflow through the fuel injector302as described previously.

In this embodiment, the movement of the strip1301is effected by a rotary actuator1303. In the present embodiment the rotary actuator1303is rotated by a control rod (not shown), in turn rotated by hydraulic actuation (not shown). Such arrangements will be familiar to those skilled in the art, and others could be used, for example electrical or pneumatic.

It will be appreciated that the configuration and control of the variable geometry quench ports1103may be applied to combustor206′ or combustor206″.

FIG.14A

As described previously, in the present example combustor206comprises two rows of axially-separated quench ports1103. As shown inFIG.14A, it is possible for two sets of strips1301and actuators1303to be provided so as to allow modulation of quench air Q through each row independently.

In a specific embodiment, the electronic engine controller212is configured to control the strips1301such that airflow is varied through one row of quench ports1103in an opposite sense to variation of airflow through the other row of quench ports1103. This opposing variation may be inversely proportional or non-linear depending upon the implementation.

In a specific embodiment, the electronic engine controller212is configured to control the strips1301to maintain a constant total airflow through the quench ports1103for a given airflow C admitted to the combustor206.

It will be appreciated that these approaches to control of the variable geometry quench ports1103may also be applied to combustor206″, or any other arrangement with multiple port rows.

FIG.14B

An alternative embodiment for the variable geometry quench ports1103is shown inFIG.14B. In this embodiment, the area is varied by way of a variable area iris1401. The same control strategies may be applied, and indeed in such an arrangement, the geometry of each quench port could be varied independently.

FIGS.15A &15B

As described previously, as an alternative (or in addition to) the quench ports1103being variable geometry quench ports1103, the fuel injectors302may be configured as variable geometry fuel injectors and hence comprise the variable geometry airflow arrangement303, A section of one of the fuel injectors302is shown inFIGS.15A and158.

The fuel injector302comprises an inner duct1501for admitting a portion of combustion air DINNERand delivering it to the combustor206. Close to the outlet of the inner duct1501, fuel F is delivered into the airflow via a fuel circuit1502. Further airflow DOUTERis delivered into the combustor206via an outer duct1503concentric with the inner duct1501.

In the illustrated embodiment, the fuel injector302comprises a translatable ramp1504to provide the variable geometry function, which ramp is configured to move axially to vary the inlet area of the outer duct1503—note the difference in inlet area betweenFIG.14AandFIG.14B. In the present embodiment the translatable ramp1504is translatable by way of a lever arm driven by a shaft, in turn driven by a unison ring arrangement (none shown). Such arrangements are described in U.S. Pat. No. 5,664,412, which is assigned to the present applicant, and incorporated herein by reference. Other actuation systems may be utilised, for example other mechanical arrangements, or pneumatic or hydraulic pistons, for example. Other mechanisms may be employed, for example a variable area inlet to inner duct1501.

In operation, the ramp1504moves axially in accordance with the current variable geometry setting to vary the amount of airflow DOUTER, and hence the total airflow through the fuel injector302, As a consequence the amount of quench air Q is therefore also varied as described previously.

It will be appreciated that the system configuration disclosed herein may also lend itself for use in controlling nvPM parameters during different phases of flight, for example in the landing and take-off cycles. In such a context, the target index of soot emissions may be derived on the basis of visibility limits, local air quality limits, etc.

It will also be appreciated that other functionality may be enabled by the system configuration disclosed herein, for example improvements in extinction margin during transients,

Various examples have been described, each of which feature various combinations of features, It will be appreciated by those skilled in the art that, except where clearly mutually exclusive, any of the features may be employed separately or in combination with any other features and the invention extends to and includes all combinations and sub-combinations of one or more features described herein.