Fluid sprinkling – spraying – and diffusing – One fluid stream impinges upon another
Reexamination Certificate
1999-11-12
2002-03-26
Scherbel, David A. (Department: 3752)
Fluid sprinkling, spraying, and diffusing
One fluid stream impinges upon another
C239S544000, C239S545000, C239S406000, C239S420000
Reexamination Certificate
active
06360971
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to method and apparatus for atomizing and burning liquid fuel, and more particularly for forming a fuel/air mixture for a combustion chamber of a gas turbine installation.
BACKGROUND OF THE INVENTION
In addition to a large number of parameters which determine the efficiency of a gas turbine and which affect both the design layout of all the individual components of a gas turbine and its mode of operation, the atomization process, by means of which the liquid fuel is to be atomized to form a fuel/air mixture which is as homogeneous as possible, plays a very decisive role in the fuel firing. In order to make it possible to carry out the combustion of the liquid fuel as completely as possible, it is the task of the fuel nozzles to atomize the liquid fuel into the finest possible fuel droplets in order to achieve, in this way, the largest possible fuel surface.
The simplest and lowest-cost fuel atomizers for liquid fuel are represented by pressurized fuel atomizers by means of which the fuel is driven through a nozzle opening at high pressure. Such, so-called SIMPLEX, atomizer nozzles are employed in combustion chamber operating concepts with burner staging and are suitable for the complete power range of a gas turbine, i.e. from the ignition process to the point where basic load operation is achieved. However, the employment of burner staging is very greatly limited because of the severe requirements with respect to the ignition process and with respect to the average temperature difference factor (OTDF) in the region of the turbine inlet. Thus the following relationship applies for the temperature difference factor OTDF:
OTDF
T
MAX
T
_
H
T
_
H
-
T
C
with
T
MAX
Maximum temperature at the turbine inlet
{overscore (T)}
H
Average temperature at the turbine inlet
T
C
Air temperature at the combustion chamber inlet (before combustion)
As a consequence of this, single-stage atomizers are exclusively employed in so-called silo combustion chambers in which one burner stage is provided, whereas multistage atomizer units, such as air-supported and compressed-air-supported atomizers are frequently employed in annular combustion chambers.
The fundamental problem in the design and layout of liquid fuel atomizer units is the quite different fuel flow rates at which the atomizer units are supplied during the operation of a gas turbine installation, starting with the ignition event and extending to the achievement of basic load operation. Thus, fuel flow rates under typical ignition conditions are less by a factor of between 10 and 20 than those under base load conditions. Also associated with this is the fact that the pressure ratios within the gas turbine installation are subjected to large changes, changing lay up to more than a factor of 100. Thus, typical pressure values for the atomization of liquid fuel under base load conditions are approximately 60 bar, whereas the atomization pressure under ignition conditions drops to between 300 and 600 m/bar. Pressure conditions are therefore reached which make it impossible to employ atomization nozzles designed for operation under base load conditions.
SUMMARY OF THE INVENTION
The invention is based on the object of providing a method and an appliance for atomizing liquid fuel for a firing installation, preferably for a combustion chamber of a gas turbine installation, having a nozzle arrangement through which the pressurized liquid fuel passes and is atomized to form a fuel/air mixture, in such a way that despite the large pressure differences described above, a single atomization unit is sufficient for undertaking the atomization necessary for optimized combustion of liquid fuel. This arrangement is to dispense with multiple staging, known per se, of the atomizing units. In particular, the atomizer appliance necessary for this purpose is to be of simple construction and be associated with only low manufacturing costs. It shall be possible to match the atomization rate and the achievable fuel droplet diameters in an optimum manner for both the ignition process and base load operation.
The invention derives from the basic concept that the minimum droplet size which can be achieved during atomization of a fluid by means of a pressurized atomizer unit is determined by the equilibrium between the surfaces tension, which holds a droplet together in its spherical shape, and the aerodynamic forces acting on the droplet from the outside, which aerodynamic forces can destroy the shape of the droplet. Thus, in the case of large droplet diameters, the aerodynamic forces are dominant so that, after the atomization process, the large droplets are really torn apart and disintegrate into smaller droplets. This process of bursting asunder into smaller droplets takes place until the surface tension becomes sufficiently large relative to the aerodynamic forces for further disintegration into even smaller fuel droplets to be prevented. This disintegration process leads to a droplet diameter which can be described by the following relationship:
D
=
C
ρ
LIQUID
γ
ρ
GAS
u
R
2
(
1
)
with
&ggr; Kinematic surface tension
P
LIQUID
Density of the atomized liquid
P
GAS
Density of the surrounding gas
U
2
R
Relative velocity between droplets and surrounding gas
C Constant
It may be seen from the above relationship that the droplet diameter D varies as the reciprocal of the square of the relative velocity between the atomized droplets and the gas surrounding the droplets. If, on the other hand, the supply pressure necessary for the atomization process (with which, for example, the liquid fuel is supplied to the atomization nozzle) is limited, only small relative velocities u are achieved so that the reduction in droplet size is unsatisfactory in terms of the finest possible atomization. This applies particularly in gas turbines during their ignition phase, in which the supply pressure within the turbine is relatively low.
In order, nevertheless, to achieve satisfactory atomization of the liquid fuel under the pressure conditions which make the atomization process difficult, a method in accordance with the preamble to claim
1
is developed, in accordance with the invention, in such a way that—after passage of the fuel through the atomization unit configured as a nozzle arrangement—at least two, spatially separated fuel sprays are formed in which the fuel is mainly present in the form of individual fuel droplets. The fuel droplets each have a relative propagation direction such that the fuel droplets of one fuel spray collide with the fuel droplets of the other fuel spray in such a way that, during this collision of the fuel droplets, new fuel droplets are formed whose diameter is smaller than that of the colliding fuel droplets.
In contrast to the widespread concept of natural droplet disintegration due to the interaction between the surface tensions and the aerodynamic forces acting on the individual droplets, the method in accordance with the invention makes use of deliberate collision between fuel droplets after their formation as part of the atomization process.
By means of deliberate collision between fuel droplets which are formed when using a simple nozzle arrangement and an atomization pressure of approximately 500 mbar, i.e. a pressure which is usual for ignition in a gas turbine (these droplets having typical droplet diameters in the range between 2 and 5 mm), it is possible to obtain very small droplets, which emerge as “fragments” from the colliding droplets and have diameters between 10 and 100 &mgr;m. The downstream “atomization process”—based on the collision process—into still smaller droplet fragments does not correspond to the above relationship (1) because the physical mechanism which contributes to the reduction in size of the droplets is not based on the interaction between the surface tension and the aerodynamic forces acting on the individual droplets but on the collision between two droplets which consist of the same medium, of a combusti
Keller Jakob
Keller Vera and George
Alstom
Burns Doane Swecker & Mathis L.L.P.
Hwu Davis
Keller Vera and George
Scherbel David A.
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