Air fuel mixer for gas turbine combustor

A mixer having a mixing duct, a set of inner and outer counter-rotating swirlers at the upstream end of the mixing duct, and a fuel nozzle located axially along and forming a center-body of the mixing duct is provided, wherein high pressure air from a compressor is injected into the mixing duct through the swirlers to form an intense shear region and fuel is injected into the mixing duct from the fuel nozzle so that the high pressure air and the fuel is uniformly mixed therein so as to produce minimal formation of pollutants when the fuel/air mixture is exhausted out the downstream end of said mixing duct into the combustor and ignited.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an air fuel mixer for the combustor of a 
gas turbine engine, and, more particularly, to an air fuel mixer for the 
combustor of a gas turbine engine which uniformly mixes fuel and air so as 
to reduce NOx formed by the ignition of the fuel/air mixture. 
2. Description of Related Art 
Air pollution concerns worldwide have led to stricter emissions standards 
requiring significant reductions in gas turbine pollutant emissions, 
especially for industrial and power generation applications. Nitrous Oxide 
(NOx), which is a precursor to atmospheric pollution, is generally formed 
in the high temperature regions of the gas turbine combustor by direct 
oxidation of atmospheric nitrogen with oxygen. Reductions in gas turbine 
emissions of NOx have been obtained by the reduction of flame temperatures 
in the combustor, such as through the injection of high purity water or 
steam in the combustor. Additionally, exhaust gas emissions have been 
reduced through measures such as selective catalytic reduction. While both 
the wet techniques (water/steam injection) and selective catalytic 
reduction have proven themselves in the field, both of these techniques 
require extensive use of ancillary equipment. Obviously, this drives the 
cost of energy production higher. Other techniques for the reduction of 
gas turbine emissions include "rich burn, quick quench, lean burn" and 
"lean premix" combustion, where the fuel is burned at a lower temperature. 
In a typical aero-derivative industrial gas turbine engine, fuel is burned 
in an annular combustor. The fuel is metered and injected into the 
combustor by means of multiple nozzles into a venturi along with 
combustion air having a designated amount of swirl. No particular care has 
been exercised in the prior art, however, in the design of the nozzle, the 
venturi or the dome end of the combustor to mix the fuel and air uniformly 
to reduce the flame temperatures. Accordingly, non-uniformity of the 
air/fuel mixture causes the flame to be locally hotter, leading to 
significantly enhanced production of NOx. 
In the typical aircraft gas turbine engine, flame stability and variable 
cycle operation of the engine dominate combustor design requirements. This 
has in general resulted in combustor designs with the combustion at the 
dome end of the combustor proceeding at the highest possible temperatures 
at stoichiometeric conditions. This, in turn, leads to large quantities of 
NOx to be formed in such gas turbine combustors since it has been of 
secondary importance. 
While premixing ducts in the prior art have been utilized in lean burning 
designs, they have been found to be unsatisfactory due to flashback and 
auto-ignition considerations for modern gas turbine applications. 
Flashback involves the flame of the combustor being drawn back into the 
mixing section, which is most often caused by a backflow from the 
combustor due to compressor instability and transient flows. Auto-ignition 
of the fuel/air mixture can occur within the premixing duct if the 
velocity of the air flow is not fast enough, i.e., where there is a local 
region of high residence time. Flashback and auto-ignition have become 
serious considerations in the design of mixers for aero-derivative engines 
due to increased pressure ratios and operating temperatures. Since one 
desired application of the present invention is for the LM6000 gas turbine 
engine, which is the aero-derivative of General Electric's CF6-80C2 
engine, these considerations are of primary significance. 
While the effects of counter-rotating swirl have been studied (e.g., 
"Effectiveness of Mixing Coaxial Flows Swirled in Opposite Directions," by 
A. Sviridenkov, V. Tret'yakov, and V. Yagodkin; "Distribution of Velocity 
Pulsations in a Channel with Mixing of Oppositely Swirled Streams," by A. 
Sviridenkov and V. Tret'yakov; and "Reactive Mixing in Swirling Flows," by 
W. Cheng), they have not been utilized with fuel injection techniques that 
uniformly premix the fuel and air prior to combustion. Likewise, fuel 
nozzles and injectors which inject fuel into an air flow for premixing, 
such as the radial fuel spokes in "Experimental Evaluation of a Low 
Emissions, Variable Geometry, Small Gas Turbine Combustor," by K. O. 
Smith, M. H. Samii, and H. K. Mak and the fuel injector having a conical 
tip in U.S. Pat. No. 4,653,278 to Vinson, et al, neither combine with the 
intense shear region provided by counter-rotating swirlers nor inject the 
fuel substantially perpendicular to the duct or air flow to maximize 
mixing. 
Accordingly, a primary objective of the present invention is to provide an 
air fuel mixer for an aero-derivative gas turbine engine which avoids the 
problems of auto-ignition and flashback. 
Another objective of the present invention is to provide an air fuel mixer 
which includes means for providing an intense shear region therein which 
causes uniform mixing of fuel and high pressure air to minimize the 
formation of pollutants when the fuel/air mixture is exhausted out the 
downstream end of the mixer into the combustor and ignited. 
Yet another objective of the present invention is to provide an air fuel 
mixer which uniformly mixes fuel and air without incurring backflow from 
the combustor. 
Another objective of the present invention is to provide an air fuel mixer 
which supplies a significant swirl to the fuel/air mixture so as to result 
in an adverse pressure gradient in the primary combustion region of the 
combustor and a consequent hot recirculation zone therein. 
Still another objective of the present invention is to inject fuel into an 
air fuel mixer in such a manner as to maximize mixing therein. 
Another objective of the present invention is to provide an air fuel mixer 
which provides the maximum amount of mixing between fuel and air supplied 
thereto in the limited amount of space available in an aero-derivative 
engine. 
These objectives and other features of the present invention will become 
more readily apparent upon reference to the following description when 
taken in conjunction with the following drawings. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, an air fuel mixer 
having a mixing duct, a set of inner and outer counter-rotating swirlers 
at the upstream end of the mixing duct, and a fuel nozzle located axially 
along and forming a center-body of the mixing duct is provided, wherein 
high pressure air from a compressor is injected into the mixing duct 
through the swirlers to form an intense shear region and fuel is injected 
into the mixing duct from the fuel nozzle so that the high pressure air 
and the fuel is uniformly mixed therein so as to produce minimal formation 
of pollutants when the fuel/air mixture is exhausted out the downstream 
end of the mixing duct into the combustor and ignited.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings in detail, wherein identical numerals 
indicate the same elements throughout the figures, FIG. 1 depicts a 
continuous-burning combustion apparatus 10 of the type suitable for use in 
a gas turbine engine and comprising a hollow body 12 defining a combustion 
chamber 14 therein. Hollow body 12 is generally annular in form and is 
comprised of an outer liner 16, an inner liner 18, and a domed end or dome 
20. It should be understood, however, that this invention is not limited 
to such an annular configuration and may well be employed with equally 
effectiveness and combustion apparatus of the well-known cylindrical can 
or cannular type. In the present annular configuration, the domed end 20 
of hollow body 12 includes a swirl cup 22, having disposed therein a mixer 
24 of the present invention to allow the uniform mixing of fuel and air 
therein and the subsequent introduction of the fuel/air mixture into 
combustion chamber 14 with the minimal formation of pollutants caused by 
the ignition thereof. Swirl cup 22, which is shown generally in FIG. 1, is 
made up of mixer 24, and the swirling means described below. 
As best seen in FIG. 2, mixer 24 includes inner swirler 26 and outer 
swirler 28 which are brazed or otherwise set in swirl cup 22, where inner 
and outer swirlers 26 and 28 preferably are counter-rotating (see FIG. 3). 
It is of no significance which direction inner swirler 26 and outer 
swirler 28 rotate so long as they do so in opposite directions. Inner and 
outer swirlers 26 and 28 are separated by a hub 30, which allows them to 
be co-annular and separately rotatable. As depicted in FIG. 2, inner and 
outer swirlers 26 and 28 are preferably axial, but they may be radial or 
some combination of axial and radial. It will be noted that swirlers 26 
and 28 have vanes 32 and 34 (see FIG. 3) at an angle in the 
40.degree.-60.degree. range with an axis A running through the center of 
mixer 24. Also, the air mass ratio between inner swirler 26 and outer 
swirler 28 is preferably approximately 1/3. 
A fuel nozzle 35 is positioned at the center of inner swirler 26 and outer 
swirler 28. Downstream of inner and outer swirlers 26 and 28 is an annular 
mixing duct 37. Fuel nozzle 35 has a set of holes 39 positioned preferably 
immediately downstream of inner swirler 26 from which fuel is preferably 
injected substantially perpendicular to axis A or airstream 60 into mixing 
duct 37 to enhance mixing. While the number and size of injection holes 39 
is dependent on the amount of fuel flowing through fuel nozzle 35, the 
pressure of the fuel, and the number and particular design of swirlers 26 
and 28, it has been found that 6 to 12 holes work adequately. Injection 
holes 39 may be aligned with the trailing edges of inner swirler 26 to 
make use of vane wakes and enhance mixing. In case the air temperature and 
pressure are conducive to auto-ignition of the fuel in a short residence 
time, then injection holes 39 should not be in line with the wakes of the 
vanes of inner swirler 26. 
Fuel nozzle 35 may be a straight cylindrical section or preferably one 
which converges substantially uniformly from its upstream end to its 
downstream end. If desired, the frontal area of fuel nozzle 35 may be 
decreased to present as small a cross-section for heating from the flame 
or increased to curve the entry of the downstream flame recirculation zone 
41 (discussed in more detail herein) from mixing duct 37. This is because 
fuel nozzle 35 extends through the entire length of mixing duct 37 and not 
only can provide fuel through holes 39, but also through tip 55. 
Inner and outer swirlers 26 and 28 are designed to pass a specified amount 
of air flow and fuel nozzle 35 is sized to permit a specified amount of 
fuel flow so as to result in a lean premixture at exit plane 43 of mixer 
24. By "lean" it is meant that the fuel/air mixture contains more air than 
is required to fully combust the fuel, or an equivalence ratio of less 
than one. It has been found that an equivalence ratio in the range of 
0.3-0.6 is preferred. 
The air flow 60 exiting inner swirler 26 and outer swirler 28 sets up an 
intense shear layer 45 in mixing duct 37. The shear layer 45 is tailored 
to enhance the mixing process, whereby jets 47 of fuel from centrally 
located fuel nozzle 35 are uniformly mixed with intense shear layer 45 
from swirlers 26 and 28, as well as prevent backflow along the inner 
surface 49 of mixing duct 37. Mixing duct 37 may be a straight cylindrical 
section, but preferably should be uniformly converging from its upstream 
end to its downstream end so as to increase fuel velocities and prevent 
backflow from primary combustion region 62. Additionally, the converging 
design of mixing duct 37 acts to accelerate the fuel/air mixture flow 
uniformly, which prevents boundary layers from accumulating along the 
sides thereof and flashback stemming therefrom. (Inner and outer swirlers 
26 and 28 may also be of a like converging design). 
While it is contemplated that the present invention will generally be 
utilized for gaseous fuels, liquid fuels also may be utilized therewith. 
Accordingly, the downstream end of fuel nozzle 35 may include liquid 
atomizers 51, as well as liquid atomizers replacing or in addition to 
holes 39 for operation of the engine on liquid fuels. Liquid fuels are 
supplied in a metered fashion to liquid atomizer 51 through a liquid flow 
circuit 53 wholly contained within fuel nozzle 35. Liquid circuit 53 can 
also be utilized to supply gas to fuel nozzle tip 55 in order to provide a 
pilot nozzle flame, as well as to inject a controlled amount of air into 
combustor chamber 14 with the intent of cooling fuel nozzle tip 55. 
In operation, compressed air 58 from a compressor (not shown) is injected 
into the upstream end of mixer 24 where it passes through inner and outer 
swirlers 26 and 28 and enters mixing duct 37. Fuel is injected into air 
flow stream 60 (which includes intense shear layers 45) from injection 
holes 39 of fuel nozzle 35 as jets of fuel 47. At the downstream end of 
mixing duct 37, the fuel/air mixture is exhausted into a primary 
combustion region 62 of combustion chamber 14 which is bounded by inner 
and outer liners 18 and 16. The fuel/air mixture then burns in combustion 
chamber 14, where a flame recirculation zone 41 is set up with help from 
the swirling flow exiting mixing duct 37. In particular, it should be 
emphasized that the two counter-rotating air streams emanating from 
swirlers 26 and 28 form very energetic shear layers 45 where intense 
mixing of fuel and air is achieved by intense dissipation of turbulent 
energy of the two co-flowing air streams. The fuel is injected into these 
energetic shear layers 45 so that macro (approximately 1 inch) and micro 
(approximately one thousandth of an inch or smaller) mixing takes place in 
a very short region or distance. In this way, the maximum amount of mixing 
between the fuel and air supplied to mixing duct 37 takes place in the 
limited amount of space available in an aero-derivative engine 
(approximately 2-4 inches). 
Testing of the invention disclosed herein reveals that NOx levels of as low 
as one part per million have been achieved. Naturally, such NOx levels in 
a "dry" environment (one without water or steam injection) are clearly 
superior to levels attained by other engines in the art. 
It is important to note that mixing duct 37 is sized to be just long enough 
for mixing of the fuel and air to be completed in mixing duct 37 without 
the swirl provided by inner and outer swirlers 26 and 28 having dissipated 
to a degree where the swirl does not support flame recirculation zone 41 
in primary combustion region 62. In order to enhance the swirled fuel/air 
mixture to turn radially out and establish the adverse pressure gradient 
in primary combustion region 62 to establish and enhance flame 
recirculation zone 41, the downstream end of mixing duct 37 may be flared 
outward. Flame recirculation zone 41 then acts to promote ignition of the 
new "cold" fuel/air mixture entering primary combustion region 62. 
Alternatively, mixing duct 37 and swirlers 26 and 28 may be sized such that 
there is little swirl at the downstream end of mixing duct 37. 
Consequently, the flame downstream becomes stabilized by conventional jet 
flame stabilization behind a bluff body (e.g., a perforated plate) instead 
of flame recirculation zone 41. 
Having shown and described the preferred embodiment of the present 
invention, further adaptations of the mixer for providing uniform mixing 
of fuel and air can be accomplished by appropriate modifications by one of 
ordinary skilled in the art without departing from the scope of the 
invention.