U.S. patent number 3,866,413 [Application Number 05/325,243] was granted by the patent office on 1975-02-18 for air blast fuel atomizer.
This patent grant is currently assigned to Parker-Hannifin Corporation. Invention is credited to Geoffrey J. Sturgess.
United States Patent |
3,866,413 |
Sturgess |
February 18, 1975 |
Air blast fuel atomizer
Abstract
An air blast fuel atomizer characterized in that the main (or
secondary) fuel is atomized by centrifugal discharge of a swirling
film of the fuel into a swirling annular air stream and wherein
primer fuel to facilitate starting of the gas turbine engine is
atomized in the throat of a venturi tube through which an air
stream is flowing, the atomized primer fuel and air being
discharged from the end of the venturi tube into a swirling annular
stream of primary air to provide a combustible fuel-air mixture to
create a pilot combustion zone to facilitate starting of the gas
turbine engine, the swirling primary air stream being disposed
within the swirling film of the secondary fuel. The atomizer herein
is further characterized in that it has ports for introducing into
the secondary air stream boost air to facilitate starting as with
heavy low volatility fuels, the boost air and fuel being brought
into intimate contact before the boost pressure is dissipated or
alternatively, the ports aforesaid may be used for introduction of
gaseous fuel into the secondary air stream for use of the atomizer
in conjunction with industrial gas turbines which are generally
required to operate on both liquid and gaseous fuels.
Inventors: |
Sturgess; Geoffrey J. (Chardon,
OH) |
Assignee: |
Parker-Hannifin Corporation
(Cleveland, OH)
|
Family
ID: |
23267041 |
Appl.
No.: |
05/325,243 |
Filed: |
January 22, 1973 |
Current U.S.
Class: |
60/742; 239/406;
60/743; 60/748; 239/404; 239/400 |
Current CPC
Class: |
F23R
3/36 (20130101) |
Current International
Class: |
F23R
3/28 (20060101); F23R 3/36 (20060101); F02c
007/22 (); B05b 007/10 () |
Field of
Search: |
;60/39.74R,39.49,39.82P,39.74B,39.71,39.72R,261
;239/400,402-406,399,434.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Croyle; Carlton R.
Assistant Examiner: Garrett; Robert E.
Attorney, Agent or Firm: Donnelly, Maky, Renner &
Otto
Claims
I therefore, particularly point out and distinctly claim as my
invention:
1. A fuel nozzle for a gas turbine engine comprising a housing
having passages therein adapted for communication with fuel and air
pressure sources; primary and secondary atomizing means in said
housing to supply combustible fuel-air mixtures to said engine;
said primary atomizing means comprising a venturi tube having a
throat in communication with said air and fuel passages whereby
fuel introduced into said throat is atomized by the air flowing
through said venturi tube with the fuel-air mixture being conducted
to the engine from the downstream end of said venturi tube; said
secondary atomizing means comprising a prefilming tube
concentrically surrounding said venturi tube defining therewith and
with said housing annular air passages which at their upstream ends
are communicated with said air pressure source, said prefilming
tube at its upstream end communicating with said fuel passage for
flow of fuel in film form around the interior of said prefilming
tube to the downstream end thereof from which it is discharged with
a radially outward velocity component into the annular air stream
around said prefilming tube; said annular air passages having swirl
means associated therewith to impart swirling motion to the annular
air streams flowing therethrough; said housing having an additional
passage with metering orifices upstream of the downstream end of
said prefilming tube and downstream of the associated swirl means
to introduce high velocity discrete jets of boost air, or oxygen,
or gaseous fuel into and through the swirling annular air stream
which surrounds said prefilming tube to impinge on the film of fuel
as the latter emerges with a radial outward velocity component from
the downstream end of said
2. The nozzle of claim 1 wherein said metering orifices are
disposed to introduce such high velocity jets axially along the
exterior of said prefilming tube to impinge upon the film of fuel
immediately upon
3. A fuel nozzle for a gas turbine engine comprising a housing
having passages therein adapted for communication with fuel and air
pressure sources; primary and secondary atomizing means in said
housing to supply combustible fuel-air mixtures to said engine;
said secondary atomizing means comprising a prefilming tube around
said primary atomizing means and defining therewith and with said
housing annular air passages which at their upstream ends are
communicated with said air pressure source, said prefilming tube at
its upstream end communicating with said fuel passage for flow of
fuel in film form around the interior of said prefilming tube to
the downstream end thereof from which it is discharged with a
radially outward velocity component into the annular air stream
around said prefilming tube; said housing having an additional
passage with metering orifices upstream of the downstream end of
said prefilming tube to introduce high velocity discrete jets of
boost air, or oxygen, or gaseous fuel into and through the annular
air stream which surrounds said prefilming tube to impinge on the
film of fuel as the latter emerges with a radial outward velocity
component from the downstream end of said
4. The nozzle of claim 3 wherein said metering orifices discharge
such jets axially along the exterior of said prefilming tube to
impinge upon the film of fuel immediately upon emergence from the
downstream end of said prefilming tube.
Description
BACKGROUND OF THE INVENTION
The starting of gas turbine engines equipped with pure air blast
fuel atomizers is a rather difficult problem because of the low air
energies available to distribute and break up the fuel under
starting conditions. These difficulties are amplified when viscous
low volatility fuels are used as in industrial gas turbines or when
the inlet air temperatures and pressures are both low as during
high altitude relighting of aircraft gas turbines. In such
situations, it is desirable to provide a pilot zone and/or a boost
system, the current practice being to use pressure atomizing primer
nozzles to provide the pilot zone. However, this solution is far
from ideal for the following reasons: (1) the spray characteristics
of pressure atomizers are strongly dependent on fuel kinematic
visocosity; (2) the primer spray if optimized for starting may
result in excessive smoke at rated engine conditions due to spray
cone collapse at high pressure; (3) the use of a pressure atomizer
requires fuel filtration to avoid plugging of the orifice with a
sacrifice in one of the advantages of the air blast system; (4) a
pressure atomizing primer integrated into the air blast main fuel
supply behaves as a hybrid device with the characteristics of
neither a true pressure atomizer nor a pure air blast atomizer and
this results in the requirement of extensive development for such
hybrid device to achieve the desired performance characteristics;
(5) conditions may arise where the restrictions of turn-down ratio,
available fuel pressure drop and engine operating line pressures
with reference to valve opening and air pressure drop may preclude
satisfactory matching of primer flow number and spray angle for
starting.
SUMMARY OF THE INVENTION
In contradistinction to the foregoing, the air blast fuel atomizer
herein provides a pilot zone to ease the starting problem, to
enhance the lean blow-out limit, and to give increased primary zone
combustion efficiency at idle and subidle speeds with increased
acceleration rates and decreased emission of unburned
hydrocarbons.
The air blast fuel atomizer herein also embodies a boost system
which is necessary for starting of gas turbine engines using heavy,
low volatility fuels and to that end the present atomizer brings
the boost air and fuel into intimate contact before the boost
pressure is dissipated, and for high altitude aircraft where
relighting again involves achieving fine atomization of the fuel
with only low air energy available. Moreover, because relighting is
not simply a question of good atomization, but also of chemical
reaction rates, the present invention provides for desired chemical
reaction rates with the boost system aforesaid when oxygen is
employed as the boost gas.
It is another object of this invention to provide an air blast fuel
atomizer which includes therewith a gaseous fuel system for
operating industrial gas turbines on both liquid and gaseous fuels,
the present atomizer being operative to mix the gaseous fuel with
air for combustion with the mixture being of appropriate fuel/air
ratio so that the flame will not be blown out, and since the
atomizer herein combines the liquid and gaseous fuel atomizing
construction, it compensates for the high diffusion rate of gaseous
fuels which is difficult to achieve in a single flame type
injector, and hence because of the requirement to also burn liquid
fuel, the same atomizer herein may be used for both liquid and
gaseous fuels.
It is another object of this invention to provide an air blast fuel
atomizer which has wide application with the following advantages:
(1) it is a low fuel pressure system; (2) fuel contamination can be
handled without filtration; (3) it provides good relight
characteristics; (4) it provides good lean blow-out
characteristics; (5) it provides for good engine acceleration
times; (6) it provides for low emission of unburned hydrocarbons at
idle and sub-idle speeds; (7) it is able to handle viscous, low
volatility fuels; (8) it has dual fuel capability usable with both
liquid and gaseous fuels; (9) it has high altitude relight
capability; (10) it has good pattern factor characteristics; (11)
it has potential for low emission of nitrogen oxides; and (12) it
is of simple and mechanically robust construction.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a cross-section view of an air blast fuel atomizer
embodying the present invention;
FIG. 2 is a cross-section view taken substantially along the line
2--2 of FIG. 1;
FIG. 3 is a fragmentary cross-section view on enlarged scale
illustrating the preferred arrangement for introducing secondary
fuel to the prefilming cylinder;
FIG. 4 is a cross-section view similar to FIG. 1, except
illustrating a modification in the manner of introducing primer
fuel into the venturi tube throat through a pressurizing valve and
an orifice between the main fuel supply line and the primer fuel
supply passages rather than through a flow divider valve as
employed in FIG. 1; and
FIG. 5 is a cross-section view on enlarged scale along line 5--5,
FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The air blast fuel atomizer 1 herein comprises a housing assembly 2
secured as shown to the wall 3 of the air pressure manifold 4 of a
gas turbine engine and to the wall 5 of the combustion chamber 6,
the latter connection including a spring washer 7 which compensates
for tolerance variations and for differential thermal expansion and
contraction in an axial direction and also a radially floating
ferrule assembly 7a which compensates for tolerance variations and
for differential thermal expansion and contractions in a radial
direction. The housing assembly 2 is provided with a liquid fuel
inlet 8 which has therein a flow divider 9 (see for example Sample,
Jr. U.S. Pat. No. 3,662,959) having primary and secondary fuel
delivery passages 10 and 11 of which the primary passage 10
communicates with a tube 12 extending axially into the center of
the throat 14 of a venturi tube 15 and of which the secondary
passage 11 communicates with a manifold 16 having swirl orifices 17
for flow of secondary fuel around the prefilming surface 18 of the
air blast atomizer tube or prefilmer 19. The housing assembly 2 is
also provided with a gas inlet 20 which communicates with a
manifold 21 terminating in ports 23 around the prefilmer 19 through
which high velocity jets of boost air, or oxygen, or gaseous fuel
are introduced in a manner hereinafter described in detail.
As already mentioned, the primary fuel atomizer comprises the
centrally mounted venturi tube 15 with primary fuel being
introduced into the throat 14 by way of the axial tube 12, passage
10, and flow divider 9. The acceleration of the primary air from
chamber 4 due to well known venturi principles gives high air
velocities for fuel atomization in the throat 14 where the primary
fuel is thus introduced. This provides sufficiently finely atomized
fuel for starting purposes under most normal operating conditions.
The discharge from the venturi tube 15 is divided into a number of
radial lobes 24 which serve to promote mixing of the atomized
primary fuel with the swirling primary air flowing around the
venturi tube 15 via the primary swirler 25, to increase the
stability of the pilot zone 26, and to reduce the risk of
"torching" from a concentrated streak of fuel in the center of the
combustion chamber 6.
The swirler 25 is disposed concentrically around the venturi tube
15 as is the secondary liquid fuel manifold 16 and orifices 17. The
secondary fuel injection introduces liquid fuel from the manifold
16 with a low pressure drop through orifices 17 which are sized to
pass contaminants in the fuel, and which are designed to introduce
the fuel onto the prefilming surface 18 with circumferential
uniformity. The preferred arrangement, as shown in FIG. 1,
comprises providing a swirling discharge from the orifices 17
tangentially on the face 27 (see FIG. 3) which is contained within
a slot 28 and provided with a deflector lip 29, the slot 28
assisting in circumferentially distributing the fuel prior to its
flow onto the prefilmer surface 18 over the weir 30 formed by the
downstream wall of the slot 28. The deflector lip 29 eliminates
splashing of fuel from the slot 28 into the air stream from the
swirler 25 and also prevents fuel from the slot 28 being drawn
forward by any low pressure wakes from the swirler 25. In the other
form, as shown in FIG. 4, the orifices 17' provide simply a
swirling discharge tangentially onto the face 27'.
The prefilmer 19 is here shown as a simple cylinder along which the
secondary fuel from the flow divider valve 9 flows as a swirling
cylindrical film and the swirler 25 serves initially to distribute
the fuel into a uniform thin swirling film or sheet over the
interior surface 18 of the prefilmer 19.
Secondary air is introduced through a secondary swirler 31 which is
mounted with a shroud 32 concentrically about the prefilmer 19, the
preferred configuration of the swirler 31 being of the radial
inflow type. The annular passage 34 formed by the swirler 31,
shroud 32 and the outer diameter of the prefilmer 19 is blind-ended
upstream so that the radial swirling air from the swirler 31 is
turned through 90.degree. so as to flow downstream. This
arrangement gives an air velocity profile in the annular passage 34
which has its maximum close to the outside diameter of the
prefilmer 19 and is free from low pressure wakes. The swirling fuel
sheet on surface 18, upon reaching the sharp and thin edge 35 of
the prefilmer 19 moves radially outward due to its radial velocity
component into the swirling high velocity secondary air stream from
passage 34 where it is atomized without streaks.
Atomization of the liquid fuel is achieved in the secondary stream
and combustion of this fuel takes place in the highly turbulent
shear interface formed at the confluence of the swirling primary
and secondary air streams. The fuel droplet size which is achieved
is inversely proportional to a power of the mean velocity through
the atomizer 1. However, a considerable proportion of the available
air energy for atomization is taken up in acceleration of the fuel
up to this velocity. By using counter-rotating swirl in the primary
and secondary streams, a high relative velocity is achieved for
atomization without incurring the penalty of excessive fuel
acceleration losses. Counter-rotating swirl enhances the turbulent
shear region at the confluence of the flow and so maximizes the
burning velocity in combustion.
Concentric with the prefilmer 19 and between the liquid fuel
manifold 16 and the secondary swirler 31, is the manifold 21 which
has an annular extension 36 in a downstream direction which forms
part of the prefilmer 19 and this extension 36 terminates upstream
of the end of the prefilmer 19 in a circumferential series of
metering ports 23. The gaseous fuel, or boost air, or oxygen
introduced through ports 23 is in discrete jets which mix only with
the secondary air stream in passage 34, the ports 23 being placed
so that excessive mixing of gas with air does not occur before
combustion takes place.
The same gas manifold 21 can also serve for a boost gas to
facilitate starting. For low pressure loss, large scale gas
turbines operating on viscous fuels with cold day inlet conditions,
an air pressure drop to suitably atomize the liquid fuel may not be
available on cranking the engine, even with a venturi air blast
primary fuel supply. In such case, a boost start may be necessary
and may comprise starting on gaseous fuel with a switch over to
liquid fuel when sufficient engine speed is achieved. If gaseous
fuel is not available, the manifold 21 can be used with high
pressure air from an auxiliary compressor and, in this case, the
metering orifices 23 produce high velocity discrete jets of air
which locally break up the fuel sheet flowing over the lip 35 of
the prefilmer 19. This procedure results in an atomized fuel spray
with a wide distribution of droplet sizes and fuel rich regions
which are ideal for ignition purposes.
For aircraft high altitude application, the boost gas in manifold
21 may be pure oxygen which is supplied to the inlet 20 through a
suitable selector and pressure reducing valve which is keyed into
the relight sequence when ignition is selected by the pilot. When
the selector and pressure reducing valve is opened, oxygen flows
into the manifold 21, and the high velocity jets of oxygen issuing
from the metering ports 23 finely atomizes the liquid fuel from the
prefilmer 19. The close proximity of fuel and oxygen satisfies the
criterion of intimate mixing and the combination of finely atomized
fuel and oxygen in proximity to a sparking ignitor plug will result
in ignition of the fuel and temperature rise in the combustion
chamber 6. Jet pipe thermocouples or the like sense the temperature
rise and shut off the oxygen supply at a predetermined value for
the engine to be self-sustaining. Inasmuch as oxygen is not
required for every engine start, additional feedback control to the
oxygen regulator valve of at least a reference pressure would be a
necessary incorporation. The oxygen-boost system could also be used
for making fast emergency starts in a combat situation at sea level
on a -60.degree.F day with 12 centistoke JP5 fuel and a manual
override would be provided for this. However, normal operation on
the oxygen-boost system at sea level is not recommended. For an
aircraft engine, such a boost would only be required in those
atomizers adjacent to the ignitor plugs and, as evident, in the
proposed system the oxygen jets from ports 23 act as assistors to
atomization where ignition is atomization limited, i.e., at low
flight Mach numbers, and high flight altitudes, and serve to
improve the chemical reaction rates where ignition is residence
time limited, i.e., at high flight Mach numbers and high flight
altitudes. In both cases, however, the oxygen also lowers the
effective altitude at which combustion is taking place, thereby
increasing combustion efficiency and temperature rise with
consequent reduction in acceleration times.
In an industrial gas turbine installation, the combustor will
comprise the liner 5 having a domed upstream closure member 37 with
an opening therein to receive the air blast fuel atomizer 1. The
liner 5 is enclosed in a casing 3 and the combustor may be of the
tubular, tubo-annular or annular type having a plurality of
circumferentially spaced apart openings. The flow through the
combustor can be either straight through or reverse in nature
without departing from the present invention. Means 38 are provided
for cooling the domed member 37 and provision is made as aforesaid
to accommodate axial and radial movements of the liner 5 relative
to the atomizer 1 whether such movements be due to assembly
tolerances or thermal growth of the liner 5 during operation of the
turbine. The passages 4 formed by the liner 5 and the casing 3 are
adapted to deliver a flow of pressurized air from a suitable source
such as a compressor.
As aforesaid, the construction shown in FIGS. 1 to 3 is of a type
which employs a flow divider valve 9 operative to supply primer
fuel to the throat 14 of the venturi tube 15 until a specified fuel
pressure is reached and at that time the flow divider valve 9 is
opened in known manner by the fuel pressure to permit fuel flow to
the secondary liquid fuel manifold 16.
The means 38 for cooling the dome 37 herein is shown as comprising
a baffle plate and air is admitted through the dome 37 by the ports
39 and flows between the baffle plate 38 and the dome 37 thereby
cooling the latter. In annular combustors, the baffle 38 fulfills
another purpose and that is because the liner 5 is closed-ended by
the dome 37, the issuing fuel-air flow satisfies its entrainment
appetite by drawing in additional air from the surroundings, some
of this additional air being part of the recirculation zone and is
derived from the air ports 40. Some of it, however, is the dome
cooling air from annular passage 41 where the issuing fuel-air flow
expands outward from the atomizer 1 centerline until it encounters
the cooling air from passage 41 which it draws into itself to grow
and keep moving downstream. This entrainment phenomenon can be used
to advantage. In annular combustors, the aim is to produce a
circumferentially uniform discharge temperature to give a long life
to the turbine entry guide vanes and to achieve this it is
necessary to submerge the point sources produced by individual fuel
injectors by aerodynamic mixing of the combustion products with
relatively cool air injected through the liner 5 at some downstream
station. The difficulties associated with doing this can be greatly
eased by using the entrainment phenomenon with specially shaped
baffle plates 38 to produce an apparent near-line-source. This
distortion of the issuing fuel-air flow has become to be termed as
a "forced recirculation zone."
An additional advantage can accrue from the forced recirculation
zone. The dimensions of the recirculation zone generated by a
swirling flow can be related to the swirl strength. The swirl
strength of the issuing flow in the present invention is determined
independently of the recirculation zone and can be quite high.
This, under natural conditions, would result in a long
recirculation zone with a high residence time. This tends to result
in increased emission of oxides of nitrogen from the combustor.
With a forced recirculation, dimensions of the zone can be
distorted by the baffles 38 to those appropriate to low or zero
swirl strengths, even when there is appreciable swirl in the
issuing flow. This results in shorter combustors and reduced
nitrogen oxide emissions. These are achieved without sacrificing
the hot and vigorous recirculation necessary at low engine speeds
to avoid emission of unburned hydrocarbons.
Referring now to FIGS. 4 and 5, there is shown a fuel inlet control
where all of the liquid fuel is passed through a pressurizing valve
45 to flow simultaneously to the throat 14 of the venturi tube 15
and to the liquid fuel manifold. The flow split between the two in
this case is determined by an orifice 46 in the passage 47 leading
to the venturi throat 14 via the streamlined web 48 having
divergent discharge passages 49.
By way of summary, it can be seen that the liquid fuel is atomized
from the lip 35 of the prefilmer 19 by the swirling secondary air
flow issuing from the annular passage 34 and is intimately mixed
with such air. The gaseous fuel injected through the metering ports
23 mixes with some of the secondary air flow in the annular passage
34. For both fuels, combustion takes place in the turbulent high
shear region generated at the interface between the swirling
primary and secondary air flows. The swirling annular jet
discharging from the prefilmer 19 generates a region of low
pressure in its center along the atomizer 1 centerline and induces
a reverse flow of gases flowing toward the atomizer 1 with a swirl
component opposite to that of the issuing flow. The reverse flow of
gases is made up of hot products of combustion originating from the
flame together with a quantity of fresh air admitted through the
liner 5 by the ports 40, some of which is drawn into the low
pressure region generated by the issuing flow. The hot gas is
carried forward by the flow of gases mixed with the issuing flow
and serve to ignite the fuel contained therein. By this means
combustion is sustained.
If the primary (or intermediate) and secondary air flows are
swirling in opposite directions, the net swirl or the issuing flow
will be reduced as one of these overcomes the other. This maximizes
the tubulent shear and combustion rates at the confluence of the
two flows but reduces the recirculated flow. This causes a
reduction in the lean blowout characteristics of the combustor. If
the primary (or intermediate) and secondary air flows are swirling
in the same direction, the net swirl would be increased, combustion
rates reduced, and lean blowout characteristics enhanced. Whichever
approach is used, the principles of the present invention are not
departed from.
* * * * *