TECHNICAL FIELD
[0001] The present invention relates to a mixer and a method for operating the same.
[0002] In particular the mixer is part of a gas turbine, e.g. a gas turbine of a power plant.
BACKGROUND
[0003] Figure 1 schematically shows an example of a gas turbine; the gas turbine 1 has a
compressor 2, a first combustion chamber 3, a second combustion chamber 4 and a turbine
5. Possibly between the first combustion chamber 3 and the second combustion chamber
4 a high pressure turbine is provided. During operation air is compressed at the compressor
2 and is used to combust a fuel in the first combustion chamber 3; the hot gas (possibly
partly expanded in the high pressure turbine) is then sent into the second combustion
chamber 4 where further fuel is injected and combusted; the hot gas generated at the
second combustion chamber 4 is then expanded in the turbine 5.
[0004] Between the first combustion chamber 3 and the second combustion chamber 4 a mixer
7 can be provided in order to dilute with air (or other gas) the hot gas coming from
the first combustion chamber 3 and directed into the second combustion chamber 4.
This allows a correct fuel injection and mixing with the hot gas at the second combustion
chamber.
[0005] Figure 2 schematically shows the section of the gas turbine including the first and
the second combustion chambers 3, 4. Figure 2 shows a first burner 3a of the first
combustion chamber 3 where the compressed air coming from the compressor 2 is mixed
with the fuel and a combustor 3b where the mixture is combusted generating hot gas
(reference 20a indicates the flame). The hot gas is directed via a transition piece
3c into the mixer 7, where air is supplied into the hot gas to dilute it. The diluted
(and cooled) hot gas is thus supplied into the burner 4a of the second combustion
chamber 4 where further fuel is injected into the hot gas via a lance 8 and mixed
to it. This mixture combusts in the combustor 4b by auto combustion (reference 20b
indicates the flame), after a "delay time" from the injection into the second burner
4a.
[0006] The temperature at the inlet of the second burner 4a can fluctuate, typically because
of mass flow fluctuation of the air coming from the mixer 7 and directed into the
second burner 4a.
[0007] The delay time depends on, inter alia, the temperature within the second burner 4a,
such that temperature fluctuations in the second burner 4a cause increase/decrease
of the delay time and thus axial upstream/downstream oscillations of the flame in
the combustor 4b.
[0008] In order to counteract these axial oscillations of the flame, the temperature in
the second burner 4a has to be maintained constant and thus the temperature of the
flow emerging from the mixer 7 has to be maintained constant.
[0009] The flow temperature at the exit of mixer 7 can vary because within the mixer 7 pressure
fluctuations exist (e.g. due to the combustion in the combustor 3b and/or 4b); these
pressure fluctuations cause diluting air fluctuating mass flow injection into the
mixer.
[0010] In order to maintain the diluting air mass flow substantially constant, multiple
injectors can be provided at different axial locations of the mixer 7, in such a way
that fluctuating air mass flow supplied through upstream injectors compensate for
fluctuating air mass flow supplied trough downstream injectors. In other words, air
is injected in such a way that the dilution air mass flow injected from upstream injectors
reaches the downstream injectors in phase opposition with respect to the dilution
air injected through them (and vice versa); this way the upstream/downstream mass
flows compensate for one another and air mass flow fluctuations are counteracted.
[0011] Nevertheless the mass flow fluctuations amplitude of air injected at different axial
positions of the mixer can differ because of the acoustic mode within the mixer. The
acoustic mode is the maximum fluctuation amplitude of the acoustic pressure over the
axial axis of the mixer. The acoustic pressure derives by the relationship: Pi=Pm+Pa,
wherein Pi is the pressure inside of the mixer, Pm is the average pressure in the
mixer (i.e. the operating nominal pressure); Pa is the acoustic pressure, being the
pressure fluctuations around the average pressure.
[0012] In this respect, figure 3A shows an example of an acoustic mode in connection with
the axial position x along the mixer; references 17a and 17b indicate the injectors,
which respectively inject the mass flow Ma and mass flow Mb. Aa and Ab identify the
maximum amplitude of the acoustic pressure fluctuations at the injector axial locations
17a and 17b.
[0013] Figures 3B and 3C respectively show the mass flow Ma and Mb and their fluctuations;
the mass flows Ma and Mb propagate along the mixer towards the mixer exit; the fluctuation
course is defined with respect to the mean flow (which is in general different but
could also be the same) and is shown as a wave that moves from the inlet to the outlet
of the mixer.
[0014] Figure 3D shows the total mass flow Mtot and the fluctuations thereof, resulting
from the overlapping of the mass flows Ma and Mb; as shown, since the fluctuation
amplitude of the mass flow Mb is larger than the fluctuation amplitude of the mass
flow Ma, the overlapping of the mass flows Ma and Mb does not result in fluctuation
cancellation, but only in attenuated fluctuations.
SUMMARY
[0015] An aspect of the invention includes providing a mixer and a method by which the mass
flow fluctuation cancellation for the fluid injected into the mixer can be improved.
[0016] Advantageously, by adjusting the pressure drop of the mass flow injected through
different injectors, the fluctuations amplitude can be made comparable, such that
overlapping of the mass flows injected through the different injectors can result
in a large reduction or also cancellation of the mass flow fluctuations.
[0017] These and further aspects are attained by providing a mixer and a method in accordance
with the accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Further characteristics and advantages will be more apparent from the description
of a preferred but non-exclusive embodiment of the mixer and method, illustrated by
way of non-limiting example in the accompanying drawings, in which:
Figures 1 and 2 show a gas turbine and a part thereof;
Figures 3A through 3D show the acoustic mode in the mixer (figure 3A), the mass flow
injected through the different injectors and the fluctuations thereof (figures 3B
and 3C), the total mass flow injected into the mixer and the fluctuations thereof
(figure 3D);
Figures 4 and 5 show different embodiments of the mixer;
Figures 6A through 6D show the acoustic mode in the mixer (figure 6A), the mass flow
injected through the different injectors and the fluctuations thereof (figures 6B
and 6C), the total mass flow injected into the mixer and the fluctuations thereof
(figure 6D).
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0019] With reference to the figures, these show a mixer 7. The mixer 7 comprises a housing
15, a duct 16 within the housing 15, a first injector 17a and a second injector 17b
for injecting a fluid (such as compressed air from the compressor, possibly cooled)
into the duct 16; the fluid is injected by the first and second injector 17a, 17b
with a fluctuating mass flow.
[0020] The first injectors 17a and 17b can be provided around the periphery of the duct
16 and can open in one or more points into the duct, as explained in the following.
[0021] The first injector 17a and the second injector 17b are at a distance D such that
the fluid mass flow injected through the first injector 17a reaches the second injector
17b in phase opposition with the fluid mass flow injected through the second injector
17b. E.g. when the first injector 17a injects a large fluid mass flow, the large fluid
mass flow axially travels through the duct 16 and reaches the second injector 17b
when the second injector 17b is injecting fluid with a small fluid mass flow.
[0022] Advantageously, the first and the second injectors 17a, 17b are configured and arranged
for injecting a mass flow (e.g. instantaneous mass flow) having substantially the
same fluctuation amplitude. This allows a large reduction or also cancellation of
the mass flow fluctuation for the mass flow resulting from the sum of the mass flow
Ma from the first injector 17a and the mass flow Mb from the second injector 17b.
[0023] The first injector 17a can comprise a plenum 19a with at least an inlet 20a and at
least a nozzle 21a for injecting the fluid into the duct 16. For example, the plenum
19a can be annular in shape and can embrace and be connected to the duct 16, the inlet
20a can be provided on any surface of the plenum 19a and the inlet 21a can protrude
into the duct 16 or not.
[0024] Likewise, the second injector 17b can comprise a plenum 19b with at least an inlet
20b and at least a nozzle 21b for injecting the fluid into the duct 16. Also the plenum
19b can be annular in shape and can embrace and be connected to the duct 16, the inlet
20b can be provided on any surface of the plenum 19b and the inlet 21b can protrude
into the duct 16 or not.
[0025] The injector can also be defined by a plurality of nozzles without the need of a
plenum connected to it, as e.g. shown in figure 5.
[0026] In different embodiments, the first and/or the second injector can have any of the
described structures. In the following reference to an embodiment with a plenum at
both the first and the second injector 17a, 17b is made.
[0027] The inlet 20a of the first injector 17a and the inlet 20b of the second injector
have different features in order to cause a different pressure drop for the fluid
moving from the housing 15 into the plena 19a, 19b.
[0028] For example, these features of the inlets 20a, 20b include the inlet cross section
and/or the inlet surface rugosity; other means are possible.
[0029] In addition, the nozzle 21a of the first injector 17a and the nozzle 21b of the second
injector 17b can have different features in order to cause a different pressure drop
for the fluid moving from the plena 19a, 19b into the duct 16. In case the injector
17a or 17b has no plenum, the nozzle 21a or 21b causes a pressure drop in the fluid
moving from the housing 15 into the duct 16.
[0030] These features can include the nozzle cross section and/or the nozzle surface rugosity;
other means are possible.
[0031] Naturally all combinations are possible, e.g. the following embodiments are possible:
- mixer with first injector 17a having the plenum 19a and second injector 17b having
only nozzles (i.e. the second injector 17b does not have the plenum 19b) or vice versa;
the nozzles 21a and 21b can have same or different features;
- mixer with both the first injector 17a and the second injector 17b having the plena
19a, 19b; the inlets into the plena have different features; the nozzles 21a and 21b
can have same or different features;
- mixer with the first nozzle 21a and the second nozzle 21b having different features;
the first injector 17a and the second injector 17b can have the plena 19a, 19b or
not or either only the first injector 17a or the second injector 17b can be provided
with the plenum; the inlets 20a and 20b (if provided) can have same or different features.
[0032] The operation of the mixer is apparent from that described and illustrated and is
substantially the following.
[0033] Hot gas G coming from the first combustion chamber 3 enters the duct 16 and passes
through it, to be then discharged into the second combustion chamber 4.
[0034] The first injector 17a injects a fluid (compressed air e.g. from the compressor 2
possibly cooled) into the duct 16 to dilute and cool the hot gas; the fluid is injected
into the duct 16 with a fluctuating mass flow Ma. After injection the mass flow (while
mixing with the hot gas) travels through the duct 16 and reaches (completely or partly
mixed to the hot gas) the second injector 17b (figure 6B).
[0035] Likewise, the second injector 17b injects a fluid (compressed air) into the duct
16 to dilute and cool the hot gas; the fluid is injected into the duct 16 with a fluctuating
mass flow Mb (figure 6C).
[0036] The mass flow Ma reaches the second injector 17b in phase opposition with the mass
flow Mb.
[0037] The fluid (compressed air) passes from the inside of the housing 15 into the plenum
19a of the first injector and 19b of the second injector. While passing through the
inlets 20a and 20b the fluid undergoes a different pressure drop, such that the pressure
inside the plena 19a and 19b is different and the flow injected through the injectors
17a and 17b and in particular the flow fluctuation amplitude thereof is different.
[0038] In addition, also the nozzles 21a and 21b can cause pressure drop for the fluid passing
through them, to cause or contribute to cause injection of a different mass flow and
thus different flow fluctuation amplitudes through the first and second injectors
17a, 17b.
[0039] As shown in figure 6D, the flow fluctuation amplitude for the mass flow Ma and Mb
is made substantially equal; in addition, since the flow fluctuations are in phase
opposition, their overlapping causes fluctuation cancellation.
[0040] The present invention also refers to a method for operating a mixer 7.
[0041] The method comprises injecting through the first and the second injectors 17a, 17b
a mass flow (e.g. instantaneous mass flow) having substantially the same fluctuation
amplitude.
[0042] According to the method:
- different pressure within the plenum 19a of the first injector 17a and plenum 19b
of the second injector 17b can be provided. E.g., the fluid contained in the housing
15 can enter the plenum 19a of the first injector 17a and plenum 19b of the second
injector 17b by passing through the inlet 20a of the first injector 17a and inlet
20b of the second injector 17b, and while passing through the inlet 20a of the first
injector 17a and inlet 20b of the second injector 17b the fluid undergoes a different
pressure drop, and/or
- the fluid contained in the plenum 19a of the first injector 17a and plenum 19b of
the second injector 17b enters the duct 16 by passing through the nozzle 21a of the
first injector 17a and nozzle 21b of the second injector 21b, and while passing through
the nozzle 21a of the first injector 17a and nozzle 21b of the second injector 17b
the fluid undergoes a different pressure drop.
[0043] The different pressure drop within the plena 19a, 19b and/or through the nozzles
21a, 21b causes injection of fluid with different fluctuation amplitude with respect
to what would be imposed by the acoustic mode; therefore the fluctuations being in
phase opposition and with substantially the same amplitude are cancelled following
their overlapping.
[0044] Naturally the mixer can have more than two axially spaced injectors, which are at
distances such as to reduce or cancel different frequencies. E.g. a mixer could have
a first, a second and a third injectors, the first and the third injectors cooperating
to cancel a fluctuation at a frequency and the second and third injectors cooperating
to cancel fluctuations at another frequency. In a further example the mixer can have
four injectors, with a first and a second injectors that cancel a frequency and a
third and a fourth injectors that cancel another frequency. Naturally other examples
with any number of injectors are possible.
[0045] Naturally the features described may be independently provided from one another.
For example, the features of each of the attached claims can be applied independently
of the features of the other claims.
[0046] In practice the materials used and the dimensions can be chosen at will according
to requirements and to the state of the art.
REFERENCE NUMBERS
[0047]
1 gas turbine
2 compressor
3 first combustion chamber
3a first burner
3b combustor
3c transition piece
4 second combustion chamber
4a second burner
4b combustor
5 turbine
7 mixer
8 lance
9 turbine
15 housing
16 duct
17a, 17b injector
19a, 19b plenum
20a, 20b inlet
21a, 21b nozzle
Aa, Ab maximum amplitude for the mass flow fluctuations
D distance between the first and the second injectors
G hot gas
20a, 20b flame
1. A mixer (7) comprising a housing (15), a duct (16) within the housing (15), at least
a first injector (17a) and a second injector (17b) for injecting a fluid having a
fluctuating mass flow into the duct (16), wherein the first injector (17a) and the
second injector (17b) are at a distance (D) such that the fluid mass flow injected
through the first injector (17a) reaches the second injector (17b) in phase opposition
with the fluid mass flow injected through the second injector (17b), characterized in that the first and the second injectors (17a, 17b) are configured and arranged for injecting
a mass flow having substantially the same fluctuation amplitude.
2. The mixer (7) of claim 1, characterized in that the first injector (17a) comprises a plenum (19a) with at least an inlet (20a) and
at least a nozzle (21a) for injecting the fluid into the duct (16).
3. The mixer (7) of claim 1 or 2, characterized in that the second injector (17b) comprises a plenum (19b) with at least an inlet (20b) and
at least a nozzle (21b) for injecting the fluid into the duct (16).
4. The mixer (7) of claim 2 and 3, characterized in that the inlet (20a) of the first injector (17a) and the inlet (20b) of the second injector
(17b) have different features in order to cause a different pressure drop for the
fluid moving from the housing (15) into the plenum (19a) of the first injector (17a)
and plenum (19b) of the second injector (17b).
5. The mixer (7) of claim 4, characterized in that the features of the inlet (20a) of the first injector (17a) and/or inlet (20b) of
the second injector (17b) include the inlet cross section.
6. The mixer (7) of claim 4 or 5, characterized in that the features of the inlet (20a) of the first injector (17a) and/or inlet (20b) of
the second injector (17b) include the inlet surface rugosity.
7. The mixer (7) of claim 1 or 2 or 3, characterized in that a nozzle (21a) of the first injector (17a) and a nozzle (21b) of the second injector
(17b) have different features in order to cause different pressure drop for the fluid
moving from the plena (19a, 19b) of the first and/or second injector (17a, 17b) into
the duct (16).
8. The mixer (7) of claim 7, characterized in that the features of the nozzle (21a) of the first injector (17a) and/or nozzle (20b)
of the second injector (17b) include the nozzle cross section.
9. The mixer (7) of claim 7 or 8, characterized in that the features of the nozzle (21a) of the first injector (17a) and/or nozzle (21b)
of the second injector (17b) include the nozzle surface rugosity.
10. A method for operating a mixer (7) comprising a housing (15), a duct (16) within the
housing (15), at least a first injector (17a) and a second injector (17b) for injecting
a fluid having a fluctuating mass flow into the duct (16), wherein the first injector
(17a) and the second injector (17b) are at a distance (D) such that the fluid mass
flow injected through the first injector (10a) reaches the second injector (17b) in
phase opposition with the fluid mass flow injected through the second injector (17b),
characterized by injecting through the first and the second injectors (17a, 17b) a mass flow having
substantially the same fluctuation amplitude.
11. The method of claim 10, characterized in that the first and second injectors (17a, 17b) each comprises: a plenum (19a, 19b) with
at least an inlet (20a, 20b) and at least a nozzle (21a, 21b) for injecting the fluid
into the duct (16), and by providing different pressure within the plenum (19a) of
the first injector (17a, 17b) and plenum (19b) of the second injector (17b).
12. The method of claim 11, characterized in that fluid contained in the housing (15) enters the plenum (19a) of the first injector
(17a) and plenum (19b) of the second injector (17b) by passing through the inlet (20a)
of the first injector (17a) and inlet (20b) of the second injector (20b), and in that while passing through the inlet (20a) of the first injector (17a) and inlet (20b)
of the second injector (17b) the fluid undergoes a different pressure drop.
13. The method of claim 11 or 12, characterized in that fluid contained in the plenum (19a) of the first injector (17a) and plenum (19b)
of the second injector (17b) enters the duct (16) by passing through the nozzle (21a)
of the first injector (17a) and nozzle (21b) of the second injector (17b), and in that while passing through the nozzle (21a) of the first injector (17a) and nozzle (21b)
of the second injector (17b) the fluid undergoes a different pressure drop.