Power plants – Combustion products used as motive fluid – Combustion products generator
Reexamination Certificate
2000-10-10
2002-12-10
Freay, Charles G. (Department: 3746)
Power plants
Combustion products used as motive fluid
Combustion products generator
C060S737000, C060S738000
Reexamination Certificate
active
06490864
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the field of combustion technology. It concerns a combustion apparatus, in particular for driving gas turbines, in which combustion apparatus a gaseous fuel in a burner is sprayed through a plurality of separate fuel-injection devices into a gas flow containing combustion air, and the resulting mixture flows into a combustion chamber for combustion where the mixture is burned.
U.S. Pat. No. 4,932,861, for example, discloses such a combustion apparatus, which is based in particular on a double-cone burner.
Thernoacoustic combustion instabilities can seriously impair safe and reliable operation of modern gas turbines with premixing. One of the mechanisms responsible for these instabilities is based on a feedback loop which includes the pressure and velocity fluctuations during the fuel-injection, the (convective) fuel in homogeneity transported by the flow, and the heat-release rate.
A fundamental stability criterion for the occurrence of thermnoacoustic combustion instabilities is the Rayleigh criterion, which can be formulated as follows:
As soon as a flame is enclosed in an acoustic resonator, thermoacoustic self-excited vibrations may occur if
∫
0
τ
Q
′
p
′
ⅆ
t
>
0
(
1
)
where Q′ is the instantaneous deviation of the integral heat-release rate from its average (steady) value, p′ designates the pressure fluctuations, and T designates the period of the vibrations (1/T=f is the frequency of the vibrations). In the formula (1), it is assumed that the spatial extent of the heat-release zone is sufficiently small in order to work with integral values of Q′ and p′. Extension to the general situation with a distributed heat-release rate Q′ (x) and a small acoustic wavelength is obtained directly and leads to a so-called Rayleigh index. The Rayleigh criterion (1) states that, an instability can only occur if fluctuations in the heat release and in the pressure are at least in phase up to a certain degree.
In a combustion apparatus with premixing, the instantaneous heat-release rate depends, inter alia, on the instantaneous fuel concentration in the premixed fuel/air mixture which enters the combustion zone. The fuel concentration in turn may be influenced by (acoustic) pressure and velocity fluctuations in the vicinity of the fuel-injection device, provided that the air feed and the fuel-injection device are not acoustically stiff. This last-mentioned condition is normally fulfilled, i.e. the pressure drop of the air flow along the fuel-injection region of the burner is normally quite small, and even the pressure drop along the fuel-injection device is generally not large enough in order to uncouple the fuel-feed line from the acoustics in the combustion apparatus. The relationship between the acoustics at the fuel-injection device and the heat release in the flow can be formulated with the simplest expressions as follows:
Q
′
(
t
)
Q
=
u
′
(
x
1
,
t
-
τ
)
u
(
x
1
)
-
1
2
p
′
(
x
1
,
t
-
r
)
Δ
p
(
2
)
where x, designates the location of the fuel-injection and u(x) and u′ (x) designate the flow velocity and, respectively, its instantaneous time change, whereas &tgr; designates the time delay, which expresses the fact that fuel in homogeneity which occurs at the fuel-injection device is not immediately felt at the flame but only after it has been transported by the average flow from the injection location to the flame front. In a self-igniting combustion apparatus, &tgr; is determined by the kinematics of the chemical reactions, which determine the location of the flame. In a conventional combustion apparatus with premixing, however, the flame is anchored with a flame holder, which may assume different configurations (bluff body, V-gutter, recirculation zone or the like). In this case, the time delay depends on the average flow velocity and the distance between injection location and flame holder. In each case, the time delay can be described approximately by
τ
=
∫
0
l
ⅆ
x
U
(
x
)
(
3
)
where l designates the distance between the injection location and the flame front, whereas U(x) is the average flow velocity in the premix zone of the burner, with which average flow velocity the fuel in homogeneity in the flow is transported from the injection device to the flame.
In summary, it may be stated that the equation (2) expresses the fact that an instantaneous increase in the velocity of the air flowing past the fuel-injection device (first term on the right-hand side of the equation) leads to a dilution of the fuel/air mixture and to a corresponding reduction in the heat release, whereas a pressure increase at the fuel-injection device (second term on the right-hand side of the equation) reduces the instantaneous fuel mass flow and thus likewise reduces the heat-release rate. Even if the fuel-injection device is acoustically “stiff” (i.e. &Dgr;→∞) - fuel in homogeneity can be produced at the injection device.
As far as the thermoacoustic stability is concerned, a time delay, as occurs in equation (2), generally permits a resonant feedback and an amplification of infinitesimal disturbances. Of course, the exact conditions and frequencies during which self-excited vibrations occur also depend on the average flow conditions, to be precise in particular on the flow velocities and temperatures, and on the acoustics of the combustion apparatus, such as, for example, the boundary conditions, natural frequencies, damping mechanisms, etc. Nonetheless, the relationship between the acoustic properties and the fluctuations in the heat release, as described in equation (2), constitute a serious threat to the thermoacoustic stability of the combustion apparatus. A way of suppressing this mechanism from the very start is therefore desirable.
In principle, it is conceivable within the limits of the above-mentioned considerations to suppress thermoacoustic instabilities by a distribution of different time delays on the time axis. In this case, the injected fuel is split up into two or more individual flows or “lots” which all have different time delays and correspondingly different phases with respect to one another. Ideally, such splitting-up into various fuel flows should result in fluctuations in the heat release Q
i
(i=1, 2, . . . ) in such a way that
∑
i
∫
0
T
Q
i
(
t
)
ⅆ
t
=
0
(
4
)
would apply. This would ensure that the Rayleigh criterion (1) cannot be fulfilled. In practice, such an exact extinction is neither possible nor necessary; it is sufficient to reduce the intensity of the resonant feedback to such an extent that the dissipative effects within the system are greater than the amplification mechanisms.
It has been proposed (DE-A1-198 09 364), for a burner or a plurality of burners working in parallel in a combustion chamber, to inject fuel in an axially graduated manner at different axial distances from the location of the heat release in order to uncouple the fuel from the combustion and reduce the dynamic pressure amplitude of the combustion flame. However, such a solution has the disadvantage that the desired graduated fuel-injection requires complicated equipment to achieve the axial graduation. This is because, if fuel is injected in an axially graduated manner inside a burner, a plurality of separate injection openings arranged one behind the other are necessary. On the other hand, if a plurality of parallel burners having different axial injection locations are used, the burners must be produced individually on account of their different configurations, which makes manufacture and stock-keeping considerably more expensive.
SUMMARY OF THE INVENTION
In one aspect of the invention, a combustion apparatus is provided that achieves a distribution of delay times in the injection of fuel without having to change the location of fuel injection.
The various fuel-injection devices are provided
Joos Franz
Ni Alexander
Polifke Wolfgang
Alstom (Switzerland) LTD
Burns Doane Swecker & Mathis L.L.P.
Freay Charles G.
Rodriguez William
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