Title: Combustion chamber
Abstract: A three-stage lean burn combustion chamber (28) comprises a primary combustion zone (36), a secondary combustion zone (40) and a tertiary combustion zone (44). Each of the combustion zones (36, 40, 44) is supplied with premixed fuel and air by respective fuel and air mixing ducts (54, 70, 92). The fuel and air mixing ducts (54, 70, 92) have a plurality of air injection slots (62, 64, 76, 98) spaced apart transversely to the direction of flow through the fuel and air mixing ducts (54, 70, 92). The air injection slots (62, 64, 76, 98) extend in the direction of flow through the fuel and air mixing ducts (54, 70, 92) to the reduce the magnitude of the fluctuations in the fuel to air ratio of the fuel and air mixture supplied into the at least one combustion zone (36, 40, 44). This reduces the generation of harmful vibrations in the combustion chamber (28).
Patent Number: 6,959,550 Issued on 11/01/2005 to Freeman, et al.
| Inventors:
|
Freeman; Christopher (Nottingham, GB);
Day; Ivor J (Cambridge, GB);
Scarinci; Thomas (Quebec, CA)
|
| Assignee:
|
Rolls-Royce plc (London, GB)
|
| Appl. No.:
|
747401 |
| Filed:
|
December 30, 2003 |
Foreign Application Priority Data
| Current U.S. Class: |
60/725; 60/737; 60/746 |
| Intern'l Class: |
F02C 007/00; F02C 007/22 |
| Field of Search: |
60/725,737,746,747,748
|
References Cited [Referenced By]
U.S. Patent Documents
| 2560074 | Jul., 1951 | Bloomer.
| |
| 4263780 | Apr., 1981 | Stettler.
| |
| 4412414 | Nov., 1983 | Novick.
| |
| 5235814 | Aug., 1993 | Leonard.
| |
| 5319935 | Jun., 1994 | Toon et al.
| |
| 5475979 | Dec., 1995 | Oag et al.
| |
| 5797267 | Aug., 1998 | Richards.
| |
| 6016658 | Jan., 2000 | Willis et al.
| |
| 6164055 | Dec., 2000 | Lovett et al.
| |
| 6532742 | Mar., 2003 | Scarinci et al.
| |
| 6732527 | May., 2004 | Freeman et al.
| |
| Foreign Patent Documents |
| 0 863 369 | Sep., 1998 | EP.
| |
| 0924463 | Dec., 1998 | EP.
| |
| 1 108 957 | Jun., 2001 | EP.
| |
| 1489339 | Oct., 1977 | GB.
| |
| WO 92/0722/1 | Apr., 1992 | WO.
| |
Primary Examiner: Kim; Ted
Attorney, Agent or Firm: Taltavull; W. Warren, Manelli Denison & Selter PLLC
Parent Case Text
This is a Continuation of National Application No. 10/135,690 filed May 1, 2002,
now U.S. Pat. No. 6,732,527.
Claims
1. A combustion chamber comprising at least one combustion zone defined by at
least one peripheral wall, at least one fuel and air mixing duct for supplying
a fuel and air mixture to the at least one combustion zone, the at least one fuel
and air mixing duct having an upstream end and a downstream end, fuel injection
means for supplying fuel into the at least one fuel and air mixing duct, air injection
means for supplying air into the at least one fuel and air mixing duct, the pressure
of the air supplied to the at least one fuel and air mixing duct fluctuating, the
air injection means comprising a plurality of air injectors spaced apart transversely
to the direction of flow through the at least one fuel and air mixing duct, each
air injector comprising a slot extending in the direction of flow through the at
least one fuel and air mixing duct to reduce the magnitude of the fluctuations
in the fuel to air ratio of the fuel and air mixture supplied into the at least
one combustion zone; the volume of the fuel and air mixing duct being arranged
such that the average travel time from the fuel injection means to the downstream
end of the fuel and air mixing duct is greater than the time period of the fluctuation
wherein the combustion chamber is a tubular combustion chamber, the tubular combustion
chamber having a length and the slots in the fuel and air mixing duct having a
length in the direction of flow through the at least one fuel and air mixing duct,
the length of the slots in the direction of flow through the at least one fuel
and air mixing duct being greater than four times the length of the tubular combustion
chamber multiplied by the velocity of the fuel and air in the at least one fuel
and air mixing duct divided by the average speed of sound inside the tubular combustion chamber.
2. A combustion chamber as claimed in claim 1 wherein the at least one fuel and
air mixing duct comprises at least one wall, the air injectors comprise a plurality
of slots extending through the wall.
3. A combustion chamber as claimed in claim 1 wherein the combustion chamber
comprises a primary combustion zone and a secondary combustion zone downstream
of the primary combustion zone.
4. A combustion chamber as claimed in claim 3 wherein the combustion chamber
comprises a primary combustion zone, a secondary combustion zone downstream of
the primary combustion zone and a tertiary combustion zone downstream of the secondary
combustion zone.
5. A combustion chamber as claimed in claim 4 wherein the at least one fuel and
air mixing duct supplies fuel and air into the tertiary combustion zone.
6. A combustion chamber as claimed in claim 3 wherein the at least one fuel and
air mixing duct supplies fuel and air into the primary combustion zone.
7. A combustion chamber as claimed in claim 3 wherein the at least one fuel and
air mixing duct supplies fuel and air into the secondary combustion zone.
8. A combustion chamber as claimed in claim 1 wherein the at least one fuel and
air mixing duct comprises a single annular fuel and air mixing duct, the air injection
means being circumferentially spaced apart and the air injection means extending axially.
9. A combustion chamber as claimed in claim 8 wherein the annular fuel and air
mixing duct comprises an inner annular wall and an outer annular wall, the air
injector means being provided in at least one of the inner and outer annular walls.
10. A combustion chamber as claimed in claim 9 wherein the air injector means
are arranged in the inner and outer annular walls.
11. A combustion chamber as claimed in claim 10 wherein the air injection means
in the inner annular wall are staggered circumferentially with respect to the air
injection means in the outer annular wall.
12. A combustion chamber as claimed in claim 1 wherein the fuel and air mixing
duct comprises a radial fuel and air mixing duct, the air injection means being
circumferentially spaced apart and the air injection means extending radially.
13. A combustion chamber as claimed in claim 12 wherein the radial fuel and air
mixing duct comprises a first radial wall and a second radial wall, the air injector
means being provided in at least one of the first and second radial walls.
14. A combustion chamber as claimed in claim 13 wherein the air injector means
are provided in the first and second radial walls.
15. A combustion chamber as claimed in claim 13 wherein the air injection means
in the first radial wall are staggered circumferentially with respect to the air
injection means in the second radial wall.
16. A combustion chamber as claimed in claim 1 wherein the fuel and air mixing
duct comprises a tubular fuel and air mixing duct, the air injector means being
circumferentially spaced apart and the air injection means extending axially.
17. A combustion chamber as claimed in claim 1 wherein the fuel injector means
is arranged at the upstream end of the fuel and air mixing duct and the air injector
means are arranged downstream of the fuel injector means.
18. A combustion chamber as claimed in claim 1 wherein the fuel injector means
is arranged between the upstream end and the downstream end of the at least one
fuel and air mixing duct, at least a portion of the air injector means are arranged
upstream of the fuel injector means and at least a portion of the air injector
means are arranged downstream of the fuel injector means.
19. A combustion chamber as claimed in claim 18 wherein each air injector means
at the downstream end of the fuel and air mixing duct is arranged to supply more
air into the fuel and air mixing duct than each air injector means at the upstream
end of the fuel and air mixing duct.
20. A combustion chamber as claimed in claim 18 wherein each air injector means
at a first position in the direction of flow through the fuel and air mixing duct
is arranged to supply more air into the fuel and air mixing duct than said air
injector means upstream of the first position in the fuel and air mixing duct.
21. A combustion chamber as claimed in claim 1 wherein each air injector means
at the first position in the fuel and air mixing duct is arranged to supply less
air into the fuel and air mixing duct than said air injector means downstream of
the first position in the fuel and air mixing duct.
22. A combustion chamber comprising at least one combustion zone defined by at
least one peripheral wall, at least one fuel and air mixing duct for supplying
a fuel and air mixture to the at least one combustion zone, the at least one fuel
and air mixing duct having an upstream end and a downstream end, fuel injection
means for supplying fuel into the at least one fuel and air mixing duct, air injection
means for supplying air into the at least one fuel and air mixing duct, the pressure
of the air supplied to the at least one fuel and air mixing duct fluctuating, the
air injection means comprising a plurality of air injectors spaced apart transversely
to the direction of flow through the at least one fuel and air mixing duct, each
air injector comprising a slot extending in the direction of flow through the at
least one fuel and air mixing duct to reduce the magnitude of the fluctuations
in the fuel to air ratio of the fuel and air mixture supplied into the at least
one combustion zone; the volume of the fuel and air mixing duct being arranged
such that the length of the fuel and air mixing duct multiplied by the frequency
of fluctuations divided by the velocity of the fuel and air leaving the downstream
end of the fuel and air mixing duct is at least one wherein the combustion chamber
is a tubular combustion chamber, the tubular combustion chamber having a length
and the slots in the fuel and air mixing duct having a length in the direction
of flow through the at least one fuel and air mixing duct, the length of the slots
in the direction of flow through the at least one fuel and air mixing duct being
greater than four times the length of the tubular combustion chamber multiplied
by the velocity of the fuel and air in the at least one fuel and air mixing duct
divided by the average speed of sound inside the tubular combustion chamber.
23. A combustion chamber comprising at least one combustion zone defined by at
least one peripheral wall, at least one fuel and air mixing duct for supplying
a fuel and air mixture to the at least one combustion zone, the at least one fuel
and air mixing duct having an upstream end and a downstream end, fuel injection
means for supplying fuel into the at least one fuel and air mixing duct, air injection
means for supplying air into the at least one fuel and air mixing duct, the pressure
of the air supplied to the at least one fuel and air mixing duct fluctuating, the
air injection means comprising a plurality of air injectors spaced apart transversely
to the direction of flow through the at least one fuel and air mixing duct, each
air injector comprising a slot extending in the direction of flow through the at
least one fuel and air mixing duct to reduce the magnitude of the fluctuations
in the fuel to air ratio of the fuel and air mixture supplied into the at least
one combustion zone; the volume of the fuel and air mixing duct being arranged
such that the length of the fuel and air mixing duct multiplied by the frequency
of fluctuations divided by the velocity of the fuel and air leaving the downstream
end of the fuel and air mixing duct is at least two wherein the combustion chamber
is a tubular combustion chamber, the tubular combustion chamber having a length
and the slots in the fuel and air mixing duct having a length in the direction
of flow through the at least one fuel and air mixing duct, the length of the slots
in the direction of flow through the at least one fuel and air mixing duct being
greater than four times the length of the tubular combustion chamber multiplied
by the velocity of the fuel and air in the at least one fuel and air mixing duct
divided by the average speed of sound inside the tubular combustion chamber.
24. A combustion chamber comprising at least one combustion zone defined by at
least one peripheral wall, at least one fuel and air mixing duct for supplying
a fuel and air mixture to the at least one combustion zone, the at least one fuel
and air mixing duct having an upstream end and a downstream end, fuel injection
means for supplying fuel into the at least one fuel and air mixing duct, air injection
means for supplying air into the at least one fuel and air mixing duct, the pressure
of the air supplied to the at least one fuel and air mixing duct fluctuating, the
air injection means comprising a plurality of air injectors spaced apart transversely
to the direction of flow through the at least one fuel and air mixing duct, each
air injector comprising a slot extending in the direction of flow through the at
least one fuel and air mixing duct to reduce the magnitude of the fluctuations
in the fuel to air ratio of the fuel and air mixture supplied into the at least
one combustion zone; the length of an air injector in the direction of flow through
the at least one fuel and air mixing duct multiplied by the frequency of the fluctuations
divided by the velocity of the fuel and air inside the at least one mixing duct
is at least one wherein the combustion chamber is a tubular combustion chamber,
the tubular combustion chamber having a length and the slots in the fuel and air
mixing duct having a length in the direction of flow through the at least one fuel
and air mixing duct, the length of the slots in the direction of flow through the
at least one fuel and air mixing duct being greater than four times the length
of the tubular combustion chamber multiplied by the velocity of the fuel and air
in the at least one fuel and air mixing duct divided by the average speed of sound
inside the tubular combustion chamber.
25. A combustion chamber as claimed in claim 24 wherein the length of an air
injector in the direction of flow through the at least one fuel and air mixing
duct multiplied by the frequency of the fluctuations divided by the average velocity
of the fuel and air inside the at least one mixing duct is at least two wherein
the length of the slots in the direction of flow through the at least one fuel
and air mixing duct is greater than eight times the length of the tubular combustion
chamber multiplied by the velocity of the fuel and air in the at least one fuel
and air mixing duct divided by the average speed of sound inside the tubular combustion chamber.
26. A combustion chamber comprising at least one combustion zone defined by at
least one peripheral wall, at least one fuel and air mixing duct for supplying
a fuel and air mixture to the at least one combustion zone, the at least one fuel
and air mixing duct having an upstream end and a downstream end, fuel injection
means for supplying fuel into the at least one fuel and air mixing duct, air injection
means for supplying air into the at least one fuel and air mixing duct, the pressure
of the air supplied to the at least one fuel and air mixing duct fluctuating, the
air injection means comprising a plurality of air injectors spaced apart transversely
to the direction of flow through the at least one fuel and air mixing duct, each
air injector comprising a slot extending in the direction of flow through the at
least one fuel and air mixing duct to reduce the magnitude of the fluctuations
in the fuel to air ratio of the fuel and air mixture supplied into the at least
one combustion zone; the volume of the fuel and air mixing duct being arranged
such that the average travel time from the fuel injection means to the downstream
end of the fuel and air mixing duct is greater than the time period of the fluctuation
wherein the combustion chamber is an annular combustion chamber, the annular combustion
chamber having a diameter and the slots in the fuel and air mixing duct having
a length in the direction of flow through the at least one fuel and air mixing
duct, the length of the slots in the direction of flow through the at least one
fuel and air mixing duct being greater than π times the diameter of the annular
combustion chamber multiplied by the velocity of the fuel and air in the at least
one fuel and air mixing duct divided by the average speed of sound inside the annular
combustion chamber.
27. A combustion chamber comprising at least one combustion zone defined by at
least one peripheral wall, at least one fuel and air mixing duct for supplying
a fuel and air mixture to the at least one combustion zone, the at least one fuel
and air mixing duct having an upstream end and a downstream end, fuel injection
means for supplying fuel into the at least one fuel and air mixing duct, air injection
means for supplying air into the at least one fuel and air mixing duct, the pressure
of the air supplied to the at least one fuel and air mixing duct fluctuating, the
air injection means comprising a plurality of air injectors spaced apart transversely
to the direction of flow through the at least one fuel and air mixing duct, each
air injector comprising a slot extending in the direction of flow through the at
least one fuel and air mixing duct to reduce the magnitude of the fluctuations
in the fuel to air ratio of the fuel and air mixture supplied into the at least
one combustion zone; the volume of the fuel and air mixing duct being arranged
such that the length of the fuel and air mixing duct multiplied by the frequency
of fluctuations divided by the velocity of the fuel and air leaving the downstream
end of the fuel and air mixing duct is at least one wherein the combustion chamber
is an annular combustion chamber, the annular combustion chamber having a diameter
and the slots in the fuel and air mixing duct having a length in the direction
of flow through the at least one fuel and air mixing duct, the length of the slots
in the direction of flow through the at least one fuel and air mixing duct being
greater than 7t times the diameter of the annular combustion chamber multiplied
by the velocity of the fuel and air in the at least one fuel and air mixing duct
divided by the average speed of sound inside the annular combustion chamber.
28. A combustion chamber comprising at least one combustion zone defined by at
least one peripheral wall, at least one fuel and air mixing duct for supplying
a fuel and air mixture to the at least one combustion zone, the at least one fuel
and air mixing duct having an upstream end and a downstream end, fuel injection
means for supplying fuel into the at least one fuel and air mixing duct, air injection
means for supplying air into the at least one fuel and air mixing duct, the pressure
of the air supplied to the at least one fuel and air mixing duct fluctuating, the
air injection means comprising a plurality of air injectors spaced apart transversely
to the direction of flow through the at least one fuel and air mixing duct, each
air injector comprising a slot extending in the direction of flow through the at
least one fuel and air mixing duct to reduce the magnitude of the fluctuations
in the fuel to air ratio of the fuel and air mixture supplied into the at least
one combustion zone; the volume of the fuel and air mixing duct being arranged
such that the length of the fuel and air mixing duct multiplied by the frequency
of fluctuations divided by the velocity of the fuel and air leaving the downstream
end of the fuel and air mixing duct is at least two wherein the combustion chamber
is an annular combustion chamber, the annular combustion chamber having a diameter
and the slots in the fuel and air mixing duct having a length in the direction
of flow through the at least one fuel and air mixing duct, the length of the slots
in the direction of flow through the at least one fuel and air mixing duct being
greater than π times the diameter of the annular combustion chamber multiplied
by the velocity of the fuel and air in the at least one fuel and air mixing duct
divided by the average speed of sound inside the annular combustion chamber.
29. A combustion chamber comprising at least one combustion zone defined by at
least one peripheral wall, at least one fuel and air mixing duct for supplying
a fuel and air mixture to the at least one combustion zone, the at least one fuel
and air mixing duct having an upstream end and a downstream end, fuel injection
means for supplying fuel into the at least one fuel and air mixing duct, air injection
means for supplying air into the at least one fuel and air mixing duct, the pressure
of the air supplied to the at least one fuel and air mixing duct fluctuating, the
air injection means comprising a plurality of air injectors spaced apart transversely
to the direction of flow through the at least one fuel and air mixing duct, each
air injector comprising a slot extending in the direction of flow through the at
least one fuel and air mixing duct to reduce the magnitude of the fluctuations
in the fuel to air ratio of the fuel and air mixture supplied into the at least
one combustion zone; the length of an air injector in the direction of flow through
the at least one fuel and air mixing duct multiplied by the frequency of the fluctuations
divided by the velocity of the fuel and air inside the at least one mixing duct
is at least one wherein the combustion chamber is an annular combustion chamber,
the annular combustion chamber having a diameter and the slots in the fuel and
air mixing duct having a length in the direction of flow through the at least one
fuel and air mixing duct, the length of the slots in the direction of flow through
the at least one fuel and air mixing duct is greater than it times the diameter
of the annular combustion chamber multiplied by the velocity of the fuel and air
in the at least one fuel and air mixing duct divided by the average speed of sound
inside the annular combustion chamber.
30. A combustion chamber as claimed in claim 29 wherein the length of an air
injector in the direction of flow through the at least one fuel and air mixing
duct multiplied by the frequency of the fluctuations divided by the average velocity
of the fuel and air inside the at least one mixing duct is at least two wherein
the length of the slots in the direction of flow through the at least one fuel
and air mixing duct is greater than 2 π times the diameter of the annular
combustion chamber multiplied by the velocity of the fuel and air in the at least
one fuel and air mixing duct divided by the average speed of sound inside the annular
combustion chamber.
31. A combustion chamber as claimed in any of claims
1,
22,
23,
24,
25 and
26-
28 wherein the at least one fuel and
air mixing duct comprises a swirler.
32. A combustion chamber as claimed in claim 31 wherein the swirler is a radial
flow swirler.
Description
The present invention relates generally to a combustion chamber, particularly
to a gas turbine engine combustion chamber.
In order to meet the emission level requirements, for industrial low emission
gas turbine engines, staged combustion is required in order to minimise the quantity
of the oxide of nitrogen (NOx) produced. Currently the emission level requirement
is for less than 25 volumetric parts per million of NOx for an industrial gas turbine
exhaust. The fundamental way to reduce emissions of nitrogen oxides is to reduce
the combustion reaction temperature, and this requires premixing of the fuel and
a large proportion, preferably all, of the combustion air before combustion occurs.
The oxides of nitrogen (NOx) are commonly reduced by a method, which uses two stages
of fuel injection. Our UK patent no. GB1489339 discloses two stages of fuel injection.
Our International patent application no. WO92/07221 discloses two and three stages
of fuel injection. In staged combustion, all the stages of combustion seek to provide
lean combustion and hence the low combustion temperatures required to minimise
NOx. The term lean combustion means combustion of fuel in air where the fuel to
air ratio is low, i.e. less than the stoichiometric ratio. In order to achieve
the required low emissions of NOx and CO it is essential to mix the fuel and air uniformly.
The industrial gas turbine engine disclosed in our International patent application
no. WO92/07221 uses a plurality of tubular combustion chambers, whose axes are
arranged in generally radial directions. The inlets of the tubular combustion chambers
are at their radially outer ends, and transition ducts connect the outlets of the
tubular combustion chambers with a row of nozzle guide vanes to discharge the hot
gases axially into the turbine sections of the gas turbine engine. Each of the
tubular combustion chambers has two coaxial radial flow swirlers, which supply
a mixture of fuel and air into a primary combustion zone. An annular secondary
fuel and air mixing duct surrounds the primary combustion zone and supplies a mixture
of fuel and air into a secondary combustion zone.
One problem associated with gas turbine engines is caused by pressure fluctuations
in the air, or gas, flow through the gas turbine engine. Pressure fluctuations
in the air, or gas, flow through the gas turbine engine may lead to severe damage,
or failure, of components if the frequency of the pressure fluctuations coincides
with the natural frequency of a vibration mode of one or more of the components.
These pressure fluctuations may be amplified by the combustion process and under
adverse conditions a resonant frequency may achieve sufficient amplitude to cause
severe damage to the combustion chamber and the gas turbine engine. Alternatively
the amplitude of the pressure fluctuations may be sufficiently large such as to
induce damage to the combustion chamber and the gas turbine engine in their own right.
It has been found that gas turbine engines, which have lean combustion, are particularly
susceptible to this problem. Furthermore it has been found that as gas turbine
engines which have lean combustion reduce emissions to lower levels by achieving
more uniform mixing of the fuel and the air, the amplitude of the resonant frequency
becomes greater. It is believed that the amplification of the pressure fluctuations
in the combustion chamber occurs because the heat released by the burning of the
fuel occurs at a position in the combustion chamber, which corresponds, to an antinode,
or pressure peak, in the pressure fluctuations.
Our European patent application No. 00311040.0 filed 11 Dec. 2000, which claims
priority from UK patent application 9929601.4 filed 16 Dec. 1999 discloses a combustion
chamber arranged to reduce this problem. The combustion chamber has at least one
fuel and air mixing duct for supplying a fuel and air mixture to a combustion zone
in the combustion chamber. Fuel injection means is arranged to supply fuel into
the at least one fuel and air mixing duct. Air injection means is arranged to supply
air into the at least one fuel and air mixing duct. The air injection means comprises
a plurality of air injectors spaced apart in the direction of flow through the
at least one fuel and air mixing duct to reduce the magnitude of the fluctuations
in the fuel to air ratio of the fuel and air mixture supplied into the at least
one combustion zone.
However, although the fuel to air ratio fluctuations have been reduced there
is a risk of auto ignition of the fuel in the fuel and air mixing duct in the wakes
from the air injectors due to the possibility of excessively long residence times
in the fuel and air mixing duct. The risk of excessively long residence time is
a function of the gas turbine engine pressure ratio. The higher the pressure ratio,
the higher the risk of autoignition.
Accordingly the present invention seeks to provide a combustion chamber
which reduces or minimises the above-mentioned problem.
Accordingly the present invention provides a combustion chamber comprising
at least one combustion zone defined by at least one peripheral wall, at least
one fuel and air mixing duct for supplying a fuel and air mixture to the at least
one combustion zone, the at least one fuel and air mixing duct having an upstream
end and a downstream end, fuel injection means for supplying fuel into the at least
one fuel and air mixing duct, air injection means for supplying air into the at
least one fuel and air mixing duct, the pressure of the air supplied to the at
least one fuel and air mixing duct fluctuating, the air injection means comprising
a plurality of air injectors spaced apart transversely to the direction of flow
through the at least one fuel and air mixing duct, each air injector comprising
a slot extending in the direction of flow through the at least one fuel and air
mixing duct to reduce the magnitude of the fluctuations in the fuel to air ratio
of the fuel and air mixture supplied into the at least one combustion zone.
Preferably the at least one fuel and air mixing duct comprises at least
one wall, the air injectors comprise a plurality of slots extending through the wall.
Preferably the combustion chamber comprises a primary combustion zone
and a secondary combustion zone downstream of the primary combustion zone.
Preferably the combustion chamber comprises a primary combustion zone,
a secondary combustion zone downstream of the primary combustion zone and a tertiary
combustion zone downstream of the secondary combustion zone.
The at least one fuel and air mixing duct may supply fuel and air into the primary
combustion zone. The at least one fuel and air mixing duct may supply fuel and
air into the secondary combustion zone. The at least one fuel and air mixing duct
may supply fuel and air into the tertiary combustion zone.
The at least one fuel and air mixing duct may comprise a single annular fuel
and air mixing duct, the air injection means being circumferentially spaced apart
and the air injection means extending axially. The annular fuel and air mixing
duct may comprise an inner annular wall and an outer annular wall, the fuel injector
means being provided in at least one of the inner and outer annular walls. The
air injector means may be arranged in the inner and outer annular walls. The air
injection means in the inner annular wall may be staggered circumferentially with
respect to the air injection means in the outer annular wall.
Preferably the fuel and air mixing duct comprises a radial fuel and air
mixing duct, the air injection means being circumferentially spaced apart and the
air injection means extending radially. Preferably the radial fuel and air mixing
duct comprises a first radial wall and a second radial wall, the air injector means
being provided in at least one of the first and second radial walls. Preferably
the air injector means are provided in the first and second radial walls. The air
injection means in the first radial annular wall may be staggered circumferentially
with respect to the air injection means in the second radial wall.
Alternatively the fuel and air mixing duct comprises a tubular fuel
and air mixing duct, the air injector means being circumferentially spaced apart.
Preferably the fuel injector means is arranged at the upstream end of
the fuel and air mixing duct and the air injector means are arranged downstream
of the fuel injector means.
Alternatively the fuel injector means is arranged between the upstream
end and the downstream end of the at least one fuel and air mixing duct, a portion
of the air injector means are arranged upstream of the fuel injector means and
a portion of the air injector means are arranged downstream of the fuel injector means.
Preferably each air injector means at the downstream end of the fuel and
air mixing duct is arranged to supply more air into the fuel and air mixing duct
than said air injector means at the upstream end of the fuel and air mixing duct.
Preferably each air injector means at a first position in the direction
of flow through the fuel and air mixing duct is arranged to supply more air into
the fuel and air mixing duct than said air injector means upstream of the first
position in the fuel and air mixing duct.
Preferably each air injector means at the first position in the fuel and
air mixing duct is arranged to supply less air into the fuel and air mixing duct
than said air injector means downstream of the first position in the fuel and air
mixing duct.
Preferably the volume of the fuel and air mixing duct being arranged such
that the average travel time from the fuel injection means to the downstream end
of the fuel and air mixing duct is greater than the time period of the fluctuation.
Preferably the volume of the fuel and air mixing duct being arranged such
that the length of the fuel and air mixing duct multiplied by the frequency of
the fluctuations divided by the velocity of the fuel and air leaving the downstream
end of the fuel and air mixing duct is at least one.
Preferably the volume of the fuel and air mixing duct being arranged such
that the length of the fuel and air mixing duct multiplied by the frequency of
the fluctuations divided by the velocity of the fuel and air leaving the downstream
end of the fuel and air mixing duct is at least two.
Preferably the plurality of air injectors extend in the direction of flow
through the at least one fuel and air mixing duct over a length equal to half the
wavelength of the fluctuations of the air supplied to the at least one fuel and
air mixing duct.
Preferably the length of an air injector in the direction of flow through
the at least one fuel and air mixing duct multiplied by the frequency of the fluctuations
divided by the velocity of the fuel and air inside the at least one mixing duct
is at least one.
Preferably the length of an air injector in the direction of flow through
the at least one fuel and air mixing duct multiplied by the frequency of the fluctuations
divided by the average velocity of the fuel and air inside the at least one mixing
duct is at least two.
Preferably the at least one fuel and air mixing duct comprises a swirler.
Preferably the swirler is a radial flow swirler.
The present invention also provides a fuel and air mixing duct for a combustion
chamber, the fuel and air mixing duct comprising fuel injection means for supplying
fuel into the fuel and air mixing duct, air injection means for supplying air into
the fuel and air mixing duct, the air injection means comprising a plurality of
air injectors spaced apart transversely to the direction of flow through the fuel
and air mixing duct, the air injectors comprise a plurality of slots extending
in the direction of flow through the fuel and air mixing duct.
The present invention will be more fully described by way of example with reference
to the accompanying drawings, in which:
FIG. 1 is a view of a gas turbine engine having a combustion chamber according
to the present invention.
FIG. 2 is an enlarged longitudinal cross-sectional view through the combustion
chamber shown in FIG. 1.
FIG. 3 is an enlarged cross-sectional view of part of the primary fuel and air
mixing duct shown in FIG. 2.
FIG. 4 is an enlarged cross-sectional view of part of the secondary fuel and
air mixing duct shown in FIG. 2.
FIG. 5 is a cross-sectional view of an alternative fuel and air mixing duct.
FIG. 6 is a cross-sectional view in the direction of arrows W—W in FIG. 5.
FIG. 7 is a cross-sectional view in the direction of arrows X—X in FIG. 5.
FIG. 8 is a cross-sectional view of an alternative fuel and air mixing duct.
FIG. 9 is a cross-sectional view in the direction of arrows Y—Y in FIG. 8.
FIG. 10 is a cross-sectional view in the direction of arrows Z—Z in FIG. 8.
FIG. 11 is a graph comparing the fuel to air ratio fluctuation with radial distance
in a radial flow fuel and air mixing duct according to the present invention and
a radial flow fuel and air mixing duct according to the prior art.
FIG. 12 is a graph of the fuel to air ratio of a fuel and air mixing duct according
to the present invention divided by the fuel to air ratio of a fuel and air mixing
duct according to the prior art against the frequency of fluctuation multiplied
by the length of the fuel and air mixing duct divided by the velocity of the fuel
and air mixture leaving the fuel and air mixing duct.
FIG. 13 is a cross-sectional view of an alternative fuel and air mixing duct.
FIG. 14 is cross-sectional view in the direction of arrows T—T in FIG. 13.
FIG. 15 is a cross-sectional view of a further fuel and air mixing duct.
FIG. 16 is a graph of the fuel to air ratio of fuel and air mixing ducts according
to the present invention against the frequency of the fluctuation multiplied by
the length of the fuel and air mixing duct divided by the velocity of the fuel
and air mixture leaving the fuel and air mixing duct.
An industrial gas turbine engine
10, shown in FIG. 1, comprises in axial
flow series an inlet
12, a compressor section
14, a combustion chamber
assembly
16, a turbine section
18, a power turbine section
20
and an exhaust
22. The turbine section
18 is arranged to drive the
compressor section
14 via one or more shafts (not shown). The power turbine
section
20 is arranged to drive an electrical generator
26 via a
shaft
24. The operation of the gas turbine engine
10 is quite conventional,
and will not be discussed further. Alternatively, the turbine section
18
may drive part of the compressor section
14 via a shaft (not shown) and
the power turbine section
20 may be arranged to drive part of the compressor
section
14 via a shaft (not shown) and is arranged to drive an electrical
generator
26 via a shaft
24. However, the power turbine section
20
may be arranged to provide drive for other purposes.
The combustion chamber assembly
16 is shown more clearly in FIGS. 2,
3
and
4. The combustion chamber assembly
16 comprises a plurality of,
for example eight or nine, equally circumferentially spaced tubular combustion
chambers
28. The axes of the tubular combustion chambers
28 are arranged
to extend in generally radial directions. The inlets of the tubular combustion
chambers
28 are at their radially outermost ends and their outlets are at
their radially innermost ends.
Each of the tubular combustion chambers
28 comprises an upstream wall
30 secured to the upstream end of an annular wall
32. A first, upstream,
portion
34 of the annular wall
32 defines a primary combustion zone
36, a second, intermediate, portion
38 of the annular wall
32
defines a secondary combustion zone
40 and a third, downstream, portion
42 of the annular wall
32 defines a tertiary combustion zone
44.
The second portion
38 of the annular wall
32 has a greater diameter
than the first portion
34 of the annular wall
32 and similarly the
third portion
42 of the annular wall
32 has a greater diameter than
the second portion
38 of the annular wall
32.
A plurality of equally circumferentially spaced transition ducts
46 are
provided, and each of the transition ducts
46 has a circular cross-section
at its upstream end
48. The upstream end
48 of each of the transition
ducts
46 is located coaxially with the downstream end of a corresponding
one of the tubular combustion chambers
28, and each of the transition ducts
46 connects and seals with an angular section of the nozzle guide vanes.
The upstream wall
30 of each of the tubular combustion chambers
28
has an aperture
50 to allow the supply of air and fuel into the primary
combustion zone
36. A radial flow swirler
52 is arranged coaxially
with the aperture
50 in the upstream wall
30.
A plurality of fuel injectors
56 are positioned in a primary fuel and
air
mixing duct
54 formed upstream of the radial flow swirler
52. The
walls
58 and
60 of the primary fuel and air mixing duct
54
are provided with a plurality of circumferentially spaced slots
62 and
64
respectively which form a primary air intake to supply air into the primary fuel
and air mixing duct
54. Each circumferentially spaced slot
62 and
64 extends radially, longitudinally, in the direction of flow, of the primary
fuel and air mixing duct
54 over a distance D. The slots
62 and
64
extend purely radially.
A central pilot igniter
66 is positioned coaxially with the aperture
50.
The pilot igniter
66 defines a downstream portion of the primary fuel and
air mixing duct
54 for the flow of the fuel and air mixture from the radial
flow swirler
52 into the primary combustion zone
36. The pilot igniter
66 turns the fuel and air mixture flowing from the radial flow swirler
52
from a radial direction to an axial direction. The primary fuel and air is mixed
together in the primary fuel and air mixing duct
54.
The primary fuel and air mixing t
54 reduces cross-sectional area from
the intake
62,
64 at its ups end to the aperture
50 at its
downstream end. The shape of the primary fuel and air mixing duct
54 produces
a constantly accelerating flow through the duct
54.
The fuel injectors
56 are supplied with fuel from a primary fuel manifold
68.
An annular secondary fuel and air mixing duct
70 is provided for each
of
the tubular combustion chambers
28. Each secondary fuel and air mixing duct
70 is arranged circumferentially around the primary combustion zone
36
of the corresponding tubular combustion chamber
28. Each of the secondary
fuel and air mixing ducts
70 is defined between a second annular wall
72
and a third annular wall
74. The second annular wall
72 defines the
inner extremity of the secondary fuel and air mixing duct
70 and the third
annular wall
74 defines the outer extremity of the secondary fuel and air
mixing duct
70. The second annular wall
72 of the secondary fuel
and air mixing duct
70 has a plurality of circumferentially spaced slots
76 which form a secondary air intake to the secondary fuel and air mixing
duct
70. Each circumferentially spaced slot
76 extends axially, longitudinally,
in the direction of flow, of the secondary fuel and air mixing duct
70.
The slots
76 extend purely axially.
At the downstream end of the secondary fuel and air mixing duct
70, the
second and third annular walls
72 and
74 respectively are secured
to a frustoconical wall portion
78 interconnecting the wall portions
34
and
38. The frustoconical wall portion
78 is provided with a plurality
of apertures
80. The apertures
80 are arranged to direct the fuel
and air mixture into the secondary combustion zone
40 in a downstream direction
towards the axis of the tubular combustion chamber
28. The apertures
80
may be circular or slots and are of equal flow area.
The secondary fuel and air mixing duct
70 reduces in cross-sectional area
from the intake
76 at its upstream end to the apertures
80 at its
downstream end. The shape of the secondary fuel and air mixing duct
70 produces
a constantly accelerating flow through the duct
70.
A plurality of secondary fuel systems
82 are provided, to supply fuel
to
the secondary fuel and air mixing ducts
70 of each of the tubular combustion
chambers
28. The secondary fuel system
82 for each tubular combustion
chamber
28 comprises an annular secondary fuel manifold
84 arranged
coaxially with the tubular combustion chamber
28 at the upstream end of
the secondary fuel and air mixing duct
70 of the tubular combustion chamber
28. Each secondary fuel manifold
84 has a plurality, for example
thirty two, of equi-circumferentially-spaced secondary fuel apertures
86.
Each of the secondary fuel apertures
86 directs the fuel axially of the
tubular combustion chamber
28 onto an annular splash plate
88. The
fuel flows from the splash plate
88 through an annular passage
90
in a downstream direction into the secondary fuel and air mixing duct
70
as an annular sheet of fuel.
An annular tertiary fuel and air mixing duct
92 is provided for each of
the tubular combustion chambers
28. Each tertiary fuel and air mixing duct
92 is arranged circumferentially around the secondary combustion zone
40
of the corresponding tubular combustion chamber
28. Each of the tertiary
fuel and air mixing ducts
92 is defined between a fourth annular wall
94
and a fifth annular wall
96. The fourth annular wall
94 defines the
inner extremity of the tertiary fuel and air mixing duct
92 and the fifth
annular wall
96 defines the outer extremity of the tertiary fuel and air
mixing duct
92. The tertiary fuel and air mixing duct
92 has a plurality
of circumferentially spaced slots
98 which form a tertiary air intake to
the tertiary fuel and air mixing duct
92. Each circumferentially spaced
slot
98 extends axially, longitudinally, in the direction of flow, of the
tertiary fuel and air mixing duct
92. The slots
98 extend purely axially.
At the downstream end of the tertiary fuel and air mixing duct
92, the
fourth and fifth annular walls
94 and
96 respectively are secured
to a frustoconical wall portion
100 interconnecting the wall portions
38
and
42. The frustoconical wall portion
100 is provided with a plurality
of apertures
102. The apertures
102 are arranged to direct the fuel
and air mixture into the tertiary combustion zone
44 in a downstream direction
towards the axis of the tubular combustion chamber
28. The apertures
102
may be circular or slots and are of equal flow area.
The tertiary fuel and air mixing duct
92 reduces in cross-sectional area
from the intake
98 at its upstream end to the apertures
102 at its
downstream end. The shape of the tertiary fuel and air mixing duct
92 produces
a constantly accelerating flow through the duct
92.
A plurality of tertiary fuel systems
104 are provided, to supply fuel
to
the tertiary fuel and air mixing ducts
92 of each of the tubular combustion
chambers
28. The tertiary fuel system
104 for each tubular combustion
chamber
28 comprises an annular tertiary fuel manifold
106 positioned
at the upstream end of the tertiary fuel and air mixing duct
92. Each tertiary
fuel manifold
106 has a plurality, for example thirty two, of equi-circumferentially
spaced tertiary fuel apertures
108. Each of the tertiary fuel apertures
108 directs the fuel axially of the tubular combustion chamber
28
onto an annular splash plate
110. The fuel flows from the splash plate
110
through the annular passage
112 in a downstream direction into the tertiary
fuel and air mixing duct
92 as an annular sheet of fuel.
As discussed previously the fuel and air supplied to the combustion zones is
premixed
and each of the combustion zones
36,
40 and
44 is arranged
to provide lean combustion to minimise NOx. The products of combustion from the
primary combustion zone
36 flow into the secondary combustion zone
40
and the products of combustion from the secondary combustion zone
40 flow
into the tertiary combustion zone
44.
Some of the air, indicated by arrow A, for primary combustion flows to a chamber
114 and this flow through the slots
62 in wall
58 into the
primary fuel and air mixing duct
54. The remainder of the air, indicated
by arrow B, for primary combustion flows to a chamber
116 and this flow
through the slots
60 in wall
56 into the primary fuel and air mixing
duct
54. The air, indicated by arrow C, for secondary combustion flows to
the chamber
116 and this flow through the slots
76 in wall
72
into the secondary fuel and air mixing duct
70. The air, indicated by arrow
E, for tertiary combustion flows to the chamber
118 and this flow through
the slots
98 in wall
94 into the tertiary fuel and air mixing duct
92.
The combustion process amplifies the pressure fluctuations for the reasons discussed
previously and may cause components of the gas turbine engine to become damaged
if they have a natural frequency of a vibration mode coinciding with the frequency
of the pressure fluctuations. Alternatively the amplitude of the pressure fluctuations
may be sufficiently great to cause damage to the components of the gas turbine engine.
The pressure fluctuations, or pressure waves, in the combustion chamber produce
fluctuations in the fuel to air ratio at the exit of the fuel and air mixing ducts.
The pressure fluctuations in the airflow and the constant supply of fuel into the
fuel and air mixing ducts of the tubular combustion chambers results in the fluctuating
fuel to air ratio at the exit of the fuel and air mixing ducts.
Consider the equation:
Where U is the velocity of the air, M is the mass, P is the pressure, Δu
is the change in velocity, Δp is the change in pressure, FAR is the fuel
to air ratio and Δ(FAR) is the change in the fuel to air ratio.
Thus in a typical fuel and air mixing duct, if Δp/P is about 1%, then
Δu/U is about 30% and hence the Δ(FAR)/FAR is about 30% into the combustion chamber.
The present invention seeks to provide a fuel and air mixing duct which supplies
a mixture of fuel and air into the combustion chamber at a more constant fuel to
air ratio. The present invention provides at least one point of fuel injection
into the fuel and air mixing duct and a plurality of points of air injection into
the fuel and air mixing duct. The air injection points are spaced apart longitudinally,
along the slots, in the direction of flow of the fuel and air mixing duct. The
pressure of the air at the longitudinally spaced air injection points at any instant
in time is different. Thus as the fuel and air mixture flows along the fuel and
air mixing duct the fuel and air mixture becomes weaker due to the additional air.
More importantly the maximum difference between the actual fuel to air ratio and
the average fuel to air ratio becomes relatively low, see line F in FIG.
11.
However for a single fuel injection point and a single air injection point the
maximum difference between the actual fuel to air ratio and the average fuel to
air ratio remains relatively high, see line G in FIG.
11.
A single point of fuel injection means that there is one or more fuel injectors
arranged at the same distance from the combustion zone, or alternatively one or
more fuel injectors are arranged at a fixed time delay from the combustion zone.
Thus the fuel injectors are arranged at a position such that the time of travel
from the point of fuel injection to the combustion zone is the same for all of
the fuel injectors.
Calculations show, see FIG. 12, that the variation in the fuel to air
ratio for a fuel and air mixing duct with a single fuel injection point and multiple
air injection points are a few percent of the variation in the fuel to air ratio
for a fuel and air mixing duct with a single fuel injection point and a single
air injection point if the volume of the fuel and air mixing duct is such that
the following equation is satisfied
Where L is the length of the fuel and air mixing duct, F is the frequency,
U is the exit velocity of the fuel and air mixture and X is a number greater than
2. The greater the number X, the lower the variation in the fuel to air ratio.
For example with X=2, the variation is about 7% for X=3, the variation is about
4%, for X=4, the variation is about 3%. Preferably X is a number greater than 3,
more preferably X is a number greater than 4 and more preferably X is a number
greater than 5.
For a tubular combustion chamber, the frequency of the lowest acoustic mode of
the combustion chamber is
Where F is the frequency of the pressure fluctuations, c is the average speed
of sound inside the combustion chamber and L is the overall length of the tubular
combustion chamber.
For an annular combustion chamber, the frequency of the lowest acoustic mode
of the combustion chamber is
Where F is the frequency of the pressure fluctuations, c is the average speed
of sound inside the combustion chamber and D is the diameter of the annular combustion chamber.
For the present invention to work effectively the air injectors, slots, need
to extend over a length X such that
Where X is the length of the slots and U is the average velocity of the air
inside the mixing duct. Preferably FX/U>2.
This results in the following design rules, for a tubular combustion chamber
X>4LU/c or more preferably X>8LU/c and for an annular combustion chamber
X>πDU/c or more preferably X>2πDU/c.
The above equations indicate that as the operating temperature of the combustion
chamber increases, the speed of sound increases and therefore the amount of damping
by the invention increases. This is an advantage of the present invention.
The progressive introduction of air along the length of the fuel and air mixing
duct through the slots results in a number of physical mechanisms which contribute
to the reduction, preferably elimination, of the pressure fluctuations, pressure
waves or instabilities, in the combustion chamber. The physical mechanisms are
the creation of a low velocity region, integration of the fuel to air ratio fluctuations,
damping of pressure waves and destruction of phase relationships. The advantage
of the slots over apertures is that there is a narrow residence time distribution,
hence a reduced risk of autoignition of the fuel, while maintaining excellent fuel
to air ratio characteristics.
The airflow in the vicinity of the fuel injector experiences fluctuations in
its bulk velocity due to the pressure fluctuations in the fuel and air mixing duct.
This creates a local fluctuation in fuel concentration, a local fuel to air ratio,
which then flows downstream at the bulk velocity of the air in the fuel and air
mixing duct. Due to the mixing of the fuel and air in the fuel and air mixing duct
these fuel to air ratio fluctuations normally diffuse out, although the process
is quite slow. However, if the local convective velocity is low and the local turbulent
intensity is high, as in the present invention, any fuel to air ratio fluctuations
are substantially dissipated by the time the fuel to air ratio fluctuations reach
the combustion chamber.
Any fluctuation in the local fuel to air ratio in the vicinity of the fuel injector
flows downstream and the progressive introduction of air along the length of the
fuel and air mixing duct integrates out any fluctuations in the local fuel to air
ratio due to the fuel injector. This is because the pressure of the air supplied
along the length of the slots of air injectors fluctuates with time. If the average
time of travel of a fluid particle from the vicinity of the fuel injector to the
downstream end of the fuel and air mixing duct is longer than the time period of
the pressure fluctuations, then the fluid particle originating from the vicinity
of the fuel injector is subjected to a number of cycles of becoming leaner and
richer that average out the initial fuel concentration fluctuation. This determines
the spatial extent of the air injectors, i.e. the length D of the fuel and air
mixing duct containing air injectors. This also determines the width, or cross-sectional
area, of the fuel and air mixing duct as this affec
