Patent Description:
In the field of gas turbine engines, it is well known that combustion instabilities may arise in certain operating conditions. Such conditions may depend on the response of the complex structure and dynamics of fluids in the gas turbine engines and may widely vary according to the kind and the size of gas turbines.

Critical acoustic vibrating modes are known, because normally they become apparent during the steps of design and test. It is therefore possible to implement protective measures that avoid or reduce effects of critical acoustic vibrating modes. Known measures, that include acoustic dampers and controlling fuel supply to change operating conditions, are not completely satisfactory, however.

Acoustic dampers, such as Helmholtz dampers, occupy relatively large space and require mechanical and fluidic coupling to the flow path of the gas turbine engine. Moreover, damping action of the acoustic dampers may depend on the specific location where the dampers are connected and optimal positioning may not be achieved because of geometrical or mechanical constraints.

Controlling fuel supply to all or part of the burners often results in an effective protective action against critical acoustic vibrating modes, but the change of combustion conditions may lead to an inadmissible increase of pollutant emissions, especially carbon monoxide.

Therefore, there is a general interest in improving protection of gas turbines engines from critical operation conditions, in which dangerous acoustic vibration modes (pulsations) may arise.

<CIT> discloses a gas turbine engine comprising a combustor having a natural vibration frequency and comprising a plurality of first burners and a plurality of second burners. The first burners are configured to produce first flames with a first time delay and the second burners are configured to produce second flames with a second time delay. Other examples of known gas turbines are disclosed in <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

It is an aim of the present invention to provide a a method for operating a gas turbine engine, which allows to overcome or to attenuate at least in part the limitations described.

According to the present invention, there is provided a method for operating a gas turbine engine according to claim <NUM>.

A combustor of the method according to the invention has first burners that generate first flames with a first time delay τ<NUM> and second burners that generate second flames with a second time delay τ<NUM>. The difference between the first time delay τ<NUM> and the second time delay τ<NUM> is equal to the reciprocal of the natural vibration frequency, i.e.: τ<NUM> - τ<NUM> = <NUM>/f<NUM>, where τ<NUM> is the first time delay, τ<NUM> is the second time delay and f<NUM> is the natural vibration frequency f<NUM> of the combustor (i.e. the resonance frequency of the combustor).

When the first and second time delays meet the above condition, the acoustic vibration modes (i.e. pulsations) that usually are generated in a combustion chamber of a gas turbine during operation are attenuated and at least partly cancelled. In particular, by use of the above equation, attenuation and cancellation are made to occur at the natural vibration frequency f<NUM>, i.e. at the critical frequency where acoustic vibration modes may amplify and cause damage of the gas turbine engine.

The attenuation is achieved without use of additional components, such as acoustic dampers, which are bulky and need fluid coupling to the hot gas path from outside. Moreover, the overall fuel supply is not altered by throttling to either the first burners or to the second burners.

The present invention will now be described with reference to the accompanying drawings, which show some non-limitative embodiments thereof, in which:.

<FIG> shows a simplified view of a gas turbine engine, designated as whole with numeral <NUM>. The gas turbine engine <NUM> comprises a compressor <NUM>, a first combustor <NUM>, optionally a high-pressure turbine <NUM>, a second combustor <NUM> (also referred to as sequential combustor) and a low-pressure turbine <NUM>. The example of <FIG> is not limitative, as the invention may be advantageously exploited also in gas turbine engines having different structure, such as with a single combustor or with two combustors and no high-pressure turbine between the first combustor and the second combustor. A diluter, to introduce diluting air in the hot gas passing through the combustors, may also be provided between the first and the second combustors, in addition to or as an alternative to the high pressure turbine. The two combustors may also be directly coupled, i.e. without any components in-between.

The gas turbine engine further comprises a fuel supply system <NUM> and a controller <NUM>.

The fuel supply system <NUM> delivers fuel flowrates for operation of the first combustor <NUM> and second combustor <NUM> and comprises a first supply system <NUM>, coupled to the first combustor <NUM>, and a second supply system <NUM>, coupled to the second combustor <NUM>. Both the first supply system <NUM> and the second supply system <NUM> are controlled by the controller <NUM>.

The controller <NUM> receives state signals from system sensors <NUM> and operates the gas turbine through actuators to provide a controlled power output. The actuators include orientable inlet guide vanes <NUM> of the compressor <NUM> and valves of the first supply system <NUM> and second supply system <NUM>.

A flow of compressed air supplied by the compressor <NUM> is added with fuel and the air/fuel mixture thus obtained is burnt in the first combustor <NUM>. The exhaust gas of the first combustor <NUM> is partly expanded in the high-pressure turbine <NUM>; then additional fuel is mixed and burnt in the second combustor <NUM>. The exhaust gas is finally expanded in the low-pressure turbine <NUM> and discharged either to the outside or e.g. to a heat recovery steam generator. The amount of fuel delivered by the first supply system <NUM> and second supply system <NUM> is controlled by the controller <NUM>.

The invention will be hereinafter described in detail with reference to the first combustor <NUM>. It is however understood that the invention is also applicable to the second combustor <NUM> or a single combustor gas turbine engine without any substantial change.

The first combustor <NUM> is schematically shown in <FIG> and comprises an annular combustion chamber <NUM>, extending about a longitudinal combustor axis A of the gas turbine engine <NUM>, a plurality of first burners <NUM> and a plurality of second burners <NUM>, circumferentially distributed around the combustor axis A at a common radial distance therefrom.

The first burners <NUM> and the second burners <NUM> may define a first asymmetric group of burners and a second asymmetric group of burners, respectively. In other words, although the first burners <NUM> and the second burners <NUM> can be symmetrically distributed as a whole, the sole first burners <NUM> and the sole second burners <NUM> may be not. Such a configuration helps promoting cancellation of the vibrating modes that propagate in the combustion chamber and counteracting their amplification.

The first combustor <NUM> has a natural vibration frequency f<NUM>. The natural vibration frequency is the resonance frequency of the first combustor, such that acoustic vibration modes (i.e. pulsations) having that frequency do not attenuate when propagating through the first combustor, but are amplified. Therefore, pulsations having the natural vibration frequency need to be dampened to avoid structural damages and loss of efficiency.

The first burners <NUM> and the second burners <NUM> may be all operated with a same fuel flowrate by the controller <NUM>.

The first burners <NUM> are configured to produce first flames with a first time delay τ<NUM> and the second burners <NUM> are configured to produce second flames with a second time delay τ<NUM>, where the second time delay τ<NUM> is different from the first time delay τ<NUM>.

The time delay is a characteristic time required for the fuel to be conveyed from a fuel injection point to the flame front.

The first burners <NUM> and the second burners <NUM> are structured so that a difference between the first time delay τ<NUM> and the second time delay τ<NUM> is equal to the reciprocal of the natural vibration frequency f<NUM>: <MAT>.

The first burners <NUM> and the second burners <NUM> may comprise respective first stages <NUM>, <NUM> and respective second stages <NUM>, <NUM>. The first stages <NUM>, <NUM> may be pilot stages (e.g. arranged for generating a diffusion flame) that extend along a burner axis B and the second stages <NUM>, <NUM> may be main premix stages that extend around the respective first stages <NUM>, <NUM>.

The time delay of the first burner preferably refers to the time delay of the second (main) stage <NUM> and likewise the time delay of the second burner preferably refers to the time delay of the second (main) stage <NUM>. Anyway, it is also possible that the time delay of the first burner <NUM> refers to an average of the time delay of the first and second stages <NUM>, <NUM> and likewise the time delay of the second burner <NUM> refers to an average of the time delay of the first and second stages <NUM>, <NUM>; such a solution may be preferred in case a substantial amount of fuel, e.g. <NUM>% or more, is fed via the first (pilot) stages <NUM>, <NUM>.

In one embodiment, the first burners <NUM> have first air passages <NUM> and the second burners <NUM> have second air passages <NUM>. The second air passages <NUM> are different from the first air passages <NUM>. Differences in air passages determine different air supply, that in turn results in different time delays.

For example, the first burners <NUM> may have first air passages <NUM> with respective air inlets and first inlet grids <NUM> at the air inlets. The second burners <NUM> may likewise have second air passages <NUM> with respective air inlets and second inlet grids <NUM> at the air inlets. The first inlet grids <NUM> and the second inlet grids <NUM> are different from one another and e.g. they are configured to differently affect inlet airflows and cause different first time delay τ<NUM> and second time delay τ<NUM>. Use of different inlet grids is a simple and cheap, yet effective solution to differentiate air supply and obtain different time delays.

As an alternative or additional measure, the first burners <NUM> may have swirlers <NUM>, <NUM>; the second burners <NUM> may have swirlers <NUM>, <NUM>, which are different from the swirlers <NUM>, <NUM>.

In another embodiment (not shown), air splitters may be arranged to differently divide airflows in the first air passages <NUM> of the first burners <NUM> and in the second air passages <NUM> of the second burners <NUM>.

With reference to <FIG>, where parts substantially identical to those already shown are identified by the same numerals, in another embodiment the first burners <NUM> have a first fuel split ratio between the respective first stage <NUM> and second stage <NUM> and the second burners <NUM> have a second fuel split ratio between the respective first stage <NUM> and second stage <NUM>, whereby the second fuel split ratio is different from the first fuel split ratio.

For example, the first supply system <NUM> may comprise independent fuel valves <NUM>, <NUM> for the first stage <NUM> and for the second stage <NUM> of the first burners <NUM>, and further independent fuel valves <NUM>, <NUM> for the first stage <NUM> and for the second stage <NUM> of the second burners <NUM>. The fuel valves <NUM>-<NUM> are controlled by the controller <NUM> to supply fuel flowrates F<NUM>, F<NUM> to the first stage <NUM> and to second stage <NUM> respectively of the first burners <NUM> and fuel flowrates F<NUM>', F<NUM>' to the first stage <NUM> and to second stage <NUM> respectively of the second burners <NUM>.

The fuel flowrates F<NUM>, F<NUM> and the fuel flowrates F<NUM>', F<NUM>' are selected such that a first fuel split ratio F<NUM>/F<NUM> of the first burners <NUM> is different from a second fuel split ratio F<NUM>'/F<NUM>' of the second burners <NUM>: <MAT>.

In one embodiment, however, each of the first burners <NUM> and second burners <NUM> receives the same total fuel flowrate FT: <MAT>.

The fuel split ratio between the first and second burners may be used to control flame characteristic (shape, location) and thus the time delay, without any structural modification of the first and second burners, as damping of the target frequencies may be obtained through gas turbine engine control.

In one embodiment, shown in <FIG>, the first burners <NUM> and the second burners <NUM> have respective different outlets. As for air inlets, also burner outlets may be exploited to differentiate the behavior of the first burners <NUM> and second burners <NUM>. Differences may reside e.g. in shape, length and width of the outlets.

With reference to <FIG>, the first burners <NUM> are provided with respective first outlets <NUM>, which project in an axial direction and are defined by conical or generally convergent or cylindrical sections having a first length L<NUM> and a first width W<NUM>.

The second burners <NUM> are provided with respective second outlets <NUM>, which project in an axial direction and are defined by conical or generally convergent or cylindrical sections having a second length L<NUM>, different from the first length L<NUM>, and/or a second width W<NUM>, different from the first width W<NUM>.

In other embodiments not shown, only the first burners <NUM> or the second burners <NUM> are provided with projecting outlets.

The first burners <NUM> may also be configured to cause respective first flame anchorage locations and the second burners <NUM> may be configured to cause respective second flame anchorage locations, the second flame anchorage locations being axially different from the first flame anchorage locations.

The effect may be achieved in a simple and cost effective manner e.g. by using lance injectors of different length at the first burners <NUM> and second burners <NUM>. For example, the first burners <NUM> include respective first lance injectors <NUM> having a first length L<NUM>' and the second burners <NUM> include respective first lance injectors <NUM> having a second length L<NUM>', where the second length L<NUM>' is different from the first length L<NUM>' (<FIG>).

According to another embodiment shown in <FIG>, another way to cause different flame axial anchoring locations and delay times in the first burners <NUM> and second burners <NUM> relies on burners with stabilizing actuators differently operated.

Specifically, the burners have a first flame stabilizer <NUM>, configured to trigger a first flame configuration and make the burners to operate as the first burners <NUM>, and a second flame stabilizer <NUM>, configured to trigger a second flame configuration and make the burners to operate as the second burners <NUM>. The first flame stabilizers <NUM> and the second flame stabilizers <NUM> may be e.g. spark plugs or plasma generators. The first flame stabilizers <NUM> and the second flame stabilizers <NUM> are controlled by the controller <NUM>.

The present invention refers to a method for operating a gas turbine engine.

According to the method, first burners <NUM> of a gas turbine engine combustor are operated to produce first flames with a first time delay τ<NUM> and second burners <NUM> of the gas turbine combustor are operated to produce second flames with a second time delay τ<NUM>.

The difference between the first time delay τ<NUM> and the second time delay τ<NUM> is equal to a reciprocal of the natural vibration frequency f<NUM>: <MAT> where τ<NUM> is the first time delay, τ<NUM> is the second time delay and f<NUM> is the natural vibration frequency.

In a first example, the first flames have a first flame shape and the second flames have a second flame shape, different from the first flame shape.

In another example, the first flames are set at a first distance D<NUM> from the respective first burner assemblies <NUM> and the second flames are set at a second distance D<NUM> from the respective second burner assemblies <NUM>, the second distance D<NUM> being different from the first distance D<NUM>.

In a further example, the first burners <NUM> have a first fuel split ratio F<NUM>/F<NUM> between a first stage <NUM> and second stage <NUM> thereof and the second burners <NUM> have a second fuel split ratio F<NUM>'/F<NUM>' between a first stage <NUM> and second stage <NUM> thereof, the second fuel split ratio F<NUM>'/F<NUM>' being different from the first fuel split ratio F<NUM>/F<NUM>.

The solutions exampled above may also be combined together.

A gas turbine engine may also be retrofitted to achieve suppression of natural vibration frequency as described above. The gas turbine engine comprises a combustor having a natural vibration frequency f<NUM>. The combustor <NUM>, <NUM> comprises a plurality of first burners <NUM>. The first burners are configured to produce flames with a first time delay τ<NUM>.

The retrofitting method comprises replacing one or more components of at least one of the first burners <NUM> with a modified component to obtain a second burner <NUM>. The second burners <NUM> are configured to generate flames with a second time delay τ<NUM>. The second time delay τ<NUM> is different from the first time delay τ<NUM>.

The difference between the first (native) time delay τ<NUM> and the second (modified) time delay τ<NUM> is equal to the reciprocal of the natural vibration frequency f<NUM>, as explained above.

The replacement component may be at least one of inlet grids (<NUM>, <NUM>); swirlers (<NUM>, <NUM>, <NUM>, <NUM>); air splitters; outlets (<NUM>, <NUM>); lance injectors (<NUM>, <NUM>); stabilizing actuators (<NUM>, <NUM>); etc..

The controller <NUM> contains a computer program configured to control operation of the gas turbine engine <NUM>. As herein understood, component replacement to achieve suppression of natural vibration frequency may also include replacing the controller <NUM> or replacing the computer program loaded in the controller <NUM> with a modified computer program or replacing or adding code portions to the computer program.

For example, the native computer program that controls the fuel split ratio F<NUM>/F<NUM> of the first stage <NUM> and second stage <NUM> of one or more of the first burners <NUM> may be replaced with a modified computer program that controls the fuel split ratio F<NUM>'/F<NUM>'.

Finally, it is evident that the described method may be subject to modifications and variations, without departing from the scope of the present invention, as defined in the appended claims.

Claim 1:
A method for operating a gas turbine engine comprising a combustor (<NUM>; <NUM>) having a natural vibration frequency (f<NUM>), wherein:
the combustor (<NUM>; <NUM>) comprises a plurality of first burners (<NUM>) and a plurality of second burners (<NUM>);
the first burners (<NUM>) are operated to produce first flames with a first time delay (τ<NUM>) and the second burners (<NUM>) are operated to produce second flames with a second time delay (τ<NUM>);
characterized in that a difference between the first time delay (τ<NUM>) and the second time delay (τ<NUM>) is equal to a reciprocal of the natural vibration frequency (f<NUM>): <MAT>
where τ<NUM> is the first time delay, τ<NUM> is the second time delay and f<NUM> is the natural vibration frequency of the combustor.