Power plants – Combustion products used as motive fluid – Combustion products generator
Reexamination Certificate
2002-09-09
2003-06-24
Casaregola, Louis J. (Department: 3746)
Power plants
Combustion products used as motive fluid
Combustion products generator
C060S746000
Reexamination Certificate
active
06581385
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of combustion technology. It refers to a combustion device, particularly for driving gas turbines, comprising a plurality of burners of identical thermal power output, which work parallel to an axis into a common combustion chamber.
2. Description of the Related Art
Such a combustion device is known, for example, from the applicant's EP-B1-0571782.
Thermoacoustic combustion instabilities may severely impede 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 in the fuel injection, the (convective) fuel inhomogeneities transported by the flow and the heat release rate.
A fundamental stability criterion for the occurrence of thermoacoustic combustion instabilities is the Rayleigh criterion which may be formulated as follows:
As soon as a flame is enclosed in an acoustic resonator, thermoacoustic self-starting oscillations may occur when the following applies
∫
0
T
Q
′
p
′
ⅆ
t
>
0
(
1
)
Here, Q′ is the instantaneous deviation of the integral heat release rate from its mean (stationary) value, p′ designates the pressure fluctuations and T designates the period of the oscillations (1/T=f is the frequency of the oscillations). Formula (1) that the spatial extent of the heat release zone is sufficiently small to operate with integral values of Q′ and p′. An extension to the more general situation with a distributed heat release Q′ (x) and a small acoustic wavelength results directly and leads to a so-called Rayleigh index. The Rayleigh criterion (1) states that an instability can occur only when fluctuations in the heat release and the pressure are in phase with one another at least to a particular degree.
In a combustion device 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, presupposing that the air supply and the fuel injection device are not acoustically rigid. This lastmentioned condition is usually fulfilled, that is to say the pressure drop of the airflow along the fuel injection region of the burner is usually relatively slight, and even the pressure drop along the fuel injection device is generally not sufficient to uncouple the fuel feed line from the acoustics in the combustion device. The relation between the acoustics at the fuel injection device and the heat release in the flow may be formulated by means of the simplest possible explanations as follows:
Q
′
(
t
)
Q
=
-
u
′
(
x
1
,
t
-
τ
)
u
(
x
1
)
-
1
2
p
′
(
x
1
,
t
-
τ
)
Δ
p
(
2
)
Here, x
1
designates the location of fuel injection and u(x) and u′ (x) designate the flow velocity and its instantaneous change in time, while &tgr; is the time delay which expresses the fact that fuel inhomogeneities occurring at the fuel injection device are not detected immediately by the flame, but only after they have been transported from the injection location to the flame front by the mean flow. In a self-igniting combustion device, &tgr; is determined by the kinetics of the chemical reactions defining the location of the flame. By contrast, in a conventional combustion device with premixing, the flame is anchored by a flame holder which may assume different configurations (bluff body, V-gutter, recirculation zone or the like). The time delay depends in this case on the mean flow velocity and the distance between the injection location and the flame holder. At all events, the time delay may be described approximately by
τ
=
∫
0
l
dx
U
(
x
)
,
(
3
)
l designating the distance between the injection location and the flame front, while U(x) is the mean flow velocity in the premixing zone of the burner with which the fuel inhomogeneities are transported in the flow from the injection device to the flame.
It may be stated, in summary, that 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 side of the equation) leads to a dilution of the fuel/air mixture and a corresponding reduction in the heat release, while a pressure increase at the fuel injection device (second term on the right side of the equation) reduces the instantaneous fuel mass flow and therefore likewise lowers the heat release rate. It may be pointed out that, even when the fuel injection device is acoustically “rigid” (that is to say &Dgr;p→∞), fuel inhomogeneities may be generated at the injection device.
As regards thermoacoustic stability, a time delay, such as occurs in equation (2), generally makes it possible to have a resonant feedback and an intensification of infinitesimal disturbances. The exact conditions and frequencies at which self-starting oscillations occur also depend, of course, on the mean flow conditions, specifically, in particular, the flow velocities and temperatures, and on the acoustics of the combustion device, such as, for example, the boundary conditions, natural frequencies, damping mechanisms, etc. The relation between the acoustic properties and the fluctuations in the heat release, such as is described in equation (2), is nonetheless a threat to the thermoacoustic stability of the combustion device which is to be taken seriously. A way should therefore be found to suppress this mechanism from the outset.
In principle, within the framework of the considerations referred to above, it is conceivable to bring about a suppression of thermoacoustic instabilities by a distribution of different time delays on the time axis. In this case, the injected fuel is divided into two or more individual streams or “parcels” which all have time delays different from one another and correspondingly different phases. Ideally, such a division into various fuel streams would result in fluctuations in the heat release Q′
i
(i=1, 2, . . . ), such that
∑
i
∫
0
T
Q
i
(
t
)
ⅆ
t
=
0
(
4
)
would be applicable. This would ensure that the Rayleigh criterion (1) cannot be fulfilled. In practice, such an exact cancellation is neither possible nor necessary; it is sufficient to lower the intensity of the resonant feedback to an extent such that the dissipative effects within the system are greater than the reinforcing mechanisms.
It was already proposed in the past (DE-A1-198 09 364), within a burner or in a plurality of burners working in parallel into a combustion chamber, to inject fuel in an axially graduated manner at different axial distances from the location of heat release. Part quantities of the fuel are thus transported with convective delay times of differing length from the location of injection to the flame, thus resulting in unequal phase relationships and therefore an attenuation of the resonant feedback. Such a solution has the disadvantage, however, that fuel injection is comparatively complicated in terms of apparatus because of the axial graduation: to be precise, if axially graduated injection takes place within a burner, it is necessary to have a plurality of separate injection orifices located one behind the other. If, on the other hand, a plurality of parallel burners are used with different axial injection locations, the burners have to be manufactured individually because of their different configuration, thus making production and stockkeeping considerably more expensive. At all events, in the case of fuel injection which is far upstream, there is also the increased risk of a so-called flame flashback which may lead to thermal overloading and
Joos Franz
Ni Alexander
Polifke Wolfgang
Alstom
Burns Doane Swecker & Mathis L.L.P.
Casaregola Louis J.
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