U.S. patent number 3,859,787 [Application Number 05/439,648] was granted by the patent office on 1975-01-14 for combustion apparatus.
This patent grant is currently assigned to General Motors Corporation. Invention is credited to Robert D. Anderson, Dennis L. Troth.
United States Patent |
3,859,787 |
Anderson , et al. |
January 14, 1975 |
COMBUSTION APPARATUS
Abstract
A combustion apparatus for gas turbine engines particularly
adapted to reduce emissions to meet automotive requirements. The
fuel is laid on the wall of a cylindrical prechamber and evaporated
from the wall by combustion air which is introduced through a
swirler at the upstream end of the prechamber. The inner surface of
the prechamber is artificially roughened by a grid of grooves to
improve fuel evaporation. The fuel is laid on the wall from an
annular manifold extending around the upstream end of the
prechamber through tangential orifices leading from the manifold
into the interior of the prechamber. The fuel manifold is cooled
and shielded from heat by an air jacket. More air enters through
entrance ports distributed around the prechamber toward its
downstream end. The resulting lean fuel-air mixture is delivered
past an annular flow dam at the outlet of the prechamber into a
combustion or reaction zone which is abruptly enlarged from the
prechamber. The structure causes turbulent flow, recirculation, and
good mixing in the reaction zone. A dilution zone downstream at the
reaction zone has a circumferential array of dilution air ports
which are of such shape as to be varied nonlinearly in area by a
sliding ring valve. The sliding ring valve is coupled to a second
sliding ring valve which varies the area of the air entrance ports
in the prechamber in reverse sense to the dilution air ports. A
pilot fuel nozzle to aid in cold starts is mounted at the upstream
end of the prechamber.
Inventors: |
Anderson; Robert D.
(Indianapolis, IN), Troth; Dennis L. (Speedway, IN) |
Assignee: |
General Motors Corporation
(Detroit, MI)
|
Family
ID: |
23745561 |
Appl.
No.: |
05/439,648 |
Filed: |
February 4, 1974 |
Current U.S.
Class: |
60/737; 60/39.23;
60/743; 60/748; 60/739; 60/746; 60/749 |
Current CPC
Class: |
F02C
7/14 (20130101); F23R 3/26 (20130101); F23R
3/30 (20130101); F02C 7/12 (20130101); F23R
3/28 (20130101); F05D 2250/411 (20130101) |
Current International
Class: |
F23R
3/28 (20060101); F23R 3/02 (20060101); F23R
3/30 (20060101); F23R 3/26 (20060101); F02C
7/14 (20060101); F02C 7/12 (20060101); F02c
007/22 () |
Field of
Search: |
;60/39.74R,39.65,39.71,265 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Croyle; Carlton R.
Assistant Examiner: Ross; Thomas I.
Attorney, Agent or Firm: Fitzpatrick; Paul
Claims
We claim:
1. A combustion apparatus adapted for use in a gas turbine engine
characterized by substantially complete combustion of liquid
hydrocarbon fuel and by a low output of nitrogen oxides, the
apparatus comprising a combustion liner having a discharge outlet
for combustion products at the downstream end of the liner; the
liner having an upstream end and liner wall means extending from
the upstream end to the downstream end, the wall means enclosing,
in sequence from the upstream end to the downstream end, a
prechamber, a reaction zone, and a dilution zone; the prechamber
including air entrance means at its upstream end effective to
direct combustion air with substantial velocity downstream over the
inner surface of the prechamber wall means, and liquid fuel
introduction means downstream of the said air entrance means
disposed to lay a film of liquid fuel on the said inner surface for
evaporation by and mixture in the prechamber with the said
combustion air, wherein the improvement comprises a textured
configuration of the said prechamber wall means inner surface
defined by a grid of two sets of intersecting grooves in the
surface and bosses rising between adjacent grooves, adapted to
allow flow of the liquid fuel through the grooves and evaporation
of the fuel from within the grooves by the flow of air over the
said surface.
2. A combustion apparatus adapted for use in a gas turbine engine
characterized by substantially complete combustion of liquid
hydrocarbon fuel and by a low output of nitrogen oxides, the
apparatus comprising a combustion liner having a discharge outlet
for combustion products at the downstream end of the liner; the
liner having an upstream end and liner wall means extending from
the upstream end to the downstream end, the wall means enclosing,
in sequence from the stream end to the downstream end, a
prechamber, a reaction zone, and a dilution zone; the prechamber
including air entrance means defined by swirler means at its
upstream end effective to direct combustion air with a substantial
transverse velocity component downstream over the inner surface of
the prechamber wall means, and liquid fuel introduction means
downstream of the said air entrance means disposed to lay a film of
liquid fuel on the said inner surface for evaporation by and
mixture in the prechamber with the said combustion air, wherein the
improvement comprises a textured configuration of the said
prechamber wall means inner surface defined by a grid of two sets
of intersecting grooves in the surface and generally rectangular
bosses rising between adjacent grooves, adapted to allow flow of
the liquid fuel through the grooves and evaporation of the fuel
from within the grooves by the flow of air over the said surface,
the grooves being disposed at roughly a 45.degree. angle to the
axial direction through the prechamber.
3. A combustion apparatus adapted for use in a gas turbine engine
characterized by substantially complete combustion of liquid
hydrocarbon fuel and by a low output of nitrogen oxides, the
apparatus comprising a combustion liner having a discharge outlet
for combustion products at the downstream end of the liner; the
liner having an upstream end and liner wall means extending from
the upstream end to the downstream end, the wall means enclosing,
in sequence from the upstream end to the downstream end, a
prechamber, a reaction zone, and a dilution zone, the prechamber
including air entrance means at its upstream end effective to
direct combustion air with substantial velocity downstream over the
inner surface of the prechamber wall means, and liquid fuel
introduction means downstream of the said air entrance means
disposed to lay a film of liquid fuel on the said inner surface for
evaporation by and mixture in the prechamber with the said
combustion air, wherein the improvement comprises a textured
configuration of the said prechamber wall means inner surface
defined by a grid of two sets of intersecting grooves in the
surface and generally rectangular bosses rising between adjacent
grooves adapted to allow flow of the liquid fuel through the
grooves and evaporation of the fuel from within the grooves by the
flow of air over the said surface, the grooves and bosses being of
the order of one-fortieth inch in width and the grooves being of
the order of three thousandths inch deep.
Description
This invention is directed to combustion chambers of a type
suitable for use with gas turbine engines. It is particularly
directed to combustion chamber structures adapted to insure
complete combustion over relatively wide ranges of air and fuel
flow and to minimize discharge of incompletely burned fuel and
generation of oxides of nitrogen.
It is well known that the United States government has established
standards for emissions of unburned hydrocarbons, carbon monoxide,
and oxides of nitrogen for motor vehicles with a view to reducing
atmospheric pollution. The standards established by legislation for
1977 are extremely stringent. Procedures established by the
Government for determination of compliance with such standards by
automotive vehicles are based upon a particular specified test
cycle involving starting the engine, accelerations, decelerations,
and operation at various speeds. This cycle is intended to simulate
driving under urban traffic conditions.
The combustion chamber in which the present invention is embodied
has been tested and found to be capable of meeting the 1977
emission standards in a suitable vehicle installation.
By way of background, successful gas turbine combustion chambers
have been in existence for some decades. Originally, primary aims
in development of such combustors were to achieve reliable
operation, efficiency, durability, good outlet temperature
profiles, and compactness. Conventional gas turbine combustors can
very easily be made to provide very low outputs of unburned fuel,
and partially burned fuel in the form of carbon monoxide or
unburned carbon. However, the usual gas turbine combustion
apparatus operates at quite high temperature, and there is
ordinarily a significant degree of combination of atmospheric
nitrogen and oxygen to form undesired nitrogen oxides. Various
expedients to reduce nitrogen oxides have been employed; but
experience has shown that such expedients as reducing the residence
time in the reaction area, reducing the maximum temperature in the
reaction area, and early quenching of combustion tend to increase
the output of incompletely burned fuel.
The problem of devising a low output combustion apparatus is
complicated by the wide range of power levels over which an
automotive engine must operate; that is, from idle to full power,
and the need for relatively clean starting even with the engine
cold. High temperature of the combustion air in a regenerative
engine increases the tendency to form oxides of nitrogen.
The invention which is the subject of this patent application is
one of a number of improvements to a gas turbine combustion chamber
of a type generally known in the prior art. These refinements
cooperate to bring emissions down to the prescribed level. The
invention is described here in terms of its preferred embodiment,
which includes others of these improvements.
The particular subject matter to which this application is directed
is an improved fuel prevaporization portion of the combustion
apparatus, with particular regard to improving the evaporation of
the fuel from a surface by hot air flowing over the surface. This
is achieved by texturing or grooving the surface to provide shallow
channels for the fuel, with a grid of bosses between the
channels.
The principal objects of this invention are to provide an improved
combustion apparatus suitable for auto-motive use, to provide a
combustion chamber having exceptionally clean exhaust, and to
provide an improved arrangement for evaporating fuel from a surface
and mixing it with air flowing over the surface.
The nature of the invention and its advantages will be clear to
those skilled in the art from the succeeding detailed description
of the preferred embodiment and the accompanying drawings
thereof.
FIG. 1 is a longitudinal view of a combustion apparatus for a gas
turbine engine, with parts cut away and in section.
FIG. 2 is a longitudinal sectional view of the combustion
liner.
FIG. 3 is an upstream end view of the combustion liner taken on the
plane indicated by the line 3--3 in FIG. 2.
FIG. 4 is a cross sectional view of the prechamber taken on the
plane indicated by the line 4--4 in FIG. 2.
FIG. 5 is a fragmentary longitudinal sectional view of the
prechamber.
FIG. 6 is a detailed sectional view taken on the plane indicated by
the line 6--6 in FIG. 5.
FIG. 7 is a fragmentary view taken on the plane indicated by the
line 7--7 in FIG. 5.
FIG. 8 is a fragmentary view of the interior of the prechamber
illustrating a textured surface.
FIG. 9 is a cross-section of the same taken in the plane indicated
by the line 9--9 in FIG. 8.
FIG. 10 is a fragmentary exterior view of the prechamber wall.
FIG. 11 is a plot of airflow distribution.
FIG. 12 is a partial elevation view illustrating an alternative
prechamber arrangement.
Referring to FIG. 1, a gas turbine engine 2 includes an engine case
3. Further details of the engine are not shown or described, since
they are immaterial to an understanding of the present invention.
By way of background, however, the engine may be a regenerative gas
turbine of the general nature of those described in U.S. patents to
Collman et al. Pat. No. 3,077,074, Feb. 12, 1963;Collman et al.
Pat. No. 3,267,674, Aug. 23, 1966; and Bell Pat. No. 3,490,746,
Jan. 20, 1970.
The engine case 3 forms part of an outer casing 4 of the combustion
apparatus which also includes a cylindrical housing 6 bolted to the
engine case. In an engine of this sort, the engine compressor (not
illustrated) delivers compressed air which is heated in a
regenerator (not illustrated) on its way into the combustion
apparatus casing 4.
Housing 6 terminates in a flange 7 to which is fixed a continuous
outer ring 8 of a combustion liner support spider 10 which provides
part of the support for a combustion liner 11 in which the
invention is embodied. Ring 8 is fixed to flange 7 by
circumferentially spaced countersunk screws 12. A combustion
chamber cover 14 which overlies the ring 8 is fixed to the flange 7
by a ring of bolts 15 which extend through the ring and flange. The
housing 6 and cover 14 are lined with thermal insulating material
16. Referring also to FIG. 2, the combustion liner 11 in its
preferred form is of circular cross section and is bounded by walls
18. The liner wall includes a first prechamber or fuel vaporizing
portion 19 which extends to an abrupt radial enlargement defined by
a substantially radially outwardly extending wall portion 20 which
is integral with and continues into a cylindrical wall portion 22.
The wall portion 19 encloses a fuel vaporizing zone of the
combustion apparatus and the wall 22 portion encloses a reaction
zone 23 and a dilution zone 24. Wall 22 terminates in an outlet 25
for combustion products at the downstream end of the combustion
liner. As shown in FIG. 1, the outlet end may be inserted into a
combustion products duct 26 leading to the turbine (not shown).
This supports the downstream end of the liner.
In operation of the combustion apparatus, fuel is evaporated and
the fuel and air are mixed in a prechamber 27 enclosed by wall
section 19. The fuel and air react, or combustion takes place, in
the reaction zone 23 and additional air is introduced and mixed
with combustion products in the dilution zone 24 to provide the
ultimate mixture of combustion products to drive the turbine of the
gas turbine engine.
Considering now in more detail the structure of the combustion
liner, beginning with the upstream end 28, part of the combustion
air enters the upstream end through a swirler 30 comprising an
annular cascade of vanes 31 (see also FIGS. 3 and 5). These vanes
extend from an outer ring 32 to a central sleeve 34, the latter
extending forwardly from the swirler 30. The vanes of the swirler
are set at an angle of 75.degree. to a plane extending axially of
the combustion apparatus so as to impart a strong swirl component
to air entering the liner at this point from the outer casing 4.
The outer ring 32 is welded or brazed to a prechamber forward wall
portion 35. Wall section 35 is piloted within and fixed to the
forward end of a rear prechamber wall portion 36. Wall portion 36
is of relatively heavy stock, about 1/4 to 5/16 inch in thickness.
The downstream end of wall portion 36 is welded to the radially
extending portion 20 of the main combustion chamber wall, these
parts being concentric. Wall 20 extends radially inwardly from
interior surface of wall portion 36 to provide a flow dam 38. The
remainder of the combustion liner wall is cylindrical and integral
with the portion 20. A sheet metal ring 39 extending over the
forward portion of the prechamber has an inwardly extending flange
40 which is welded to the forward edge of wall portion 35. This
ring 39 provides for connecting the forward end of the combustion
liner to the support spider 10. The spider includes arms connecting
outer ring 8 to an inner ring 42 (see also FIG. 1) which is
suitably fixed or attached to the ring 39 of the liner.
The hot compressed air forced through swirler 30 will flow with a
strong tangential component over the inner surface of wall portions
35 and 36 because of centrifugal force and will tend to scour these
walls. In so doing, it vaporizes and picks up liquid hydrocarbon
fuel which is fed to the inner surface of the prechamber just
downstream of swirler 30. The fuel is introduced from a manifold 46
(see FIGS. 2, 5, 6, and 7) which is a ring of semicircular section
extending entirely around the outer surface of wall portion 35.
Fuel is delivered to this manifold through a fuel inlet tube 47
which extends into the combustion apparatus from a suitable fitting
for connection to an external source of supply (not illustrated).
Manifold 46 is enclosed within a cooling air jacket 48, likewise of
semicircular cross section and likewise welded to the outer surface
of wall section 35. Cooling air from a suitable source, for example
from the compressor of the engine upstream of the regenerator, is
supplied to the tube 48 through a cooling air pipe 50 which
surrounds the fuel tube 47. The cooling air jacket extends almost
entirely around the prechamber, as shown in FIG. 6. The gap in the
circumference of the tube is closed adjacent the inlet pipe 50 by a
semi-annular blocking plate 51 brazed or welded to tubes 46 and 48
and to the wall portion 35. Air introduced through tube 50 thus
circulates over the fuel manifold to an outlet at 52 at the other
end of the cooling air jacket. This circulation of air is to
prevent boiling of the fuel under certain conditions of operation
such as upon cutback of fuel with a hot engine, or during idling
operation. It should be noted that the support ring 39 also shields
the fuel manifold and the cooling air jacket to some extent from
heat which may be radiated from hot engine components near the
flame tube.
Fuel supplied to the manifold 46 through tube 47 is laid on the
interior of the prechamber wall through sixteen equally spaced main
fuel ports or orifices 54. These ports are 0.013 inch in diameter
and make about a 15.degree. angle with the outer surface of the
wall so that the fuel is squirted onto the inner surface of the
wall rather than into the air flowing through the swirler. The fuel
is supplied at low pressure, the preferred maximum pressure drop
through ports 54 being about 20 psi. The current of air flowing
through the swirler blows the introduced fuel along the inner
surface of the prechamber wall portions 35 and 36, and the hot
rapidly moving air heats and vaporizes and mixes with the fuel
before entry into the reaction zone 23.
A substantial improvement in the vaporization and mixing of fuel
with the air has been found to result from providing a roughened or
textured surface on the interior of the prechamber wall.
Preferably, this textured surface extends from just downstream of
the fuel entrance ports 54 to the dam 38. This textured surface may
be similar to a knurled surface. The preferred form may be more
accurately described in connection with FIGS. 8 and 9. FIG. 8 is a
view looking at the interior of the prechamber wall and FIG. 9 is a
cross section of the same. The surface is relieved to provide a
grid of two intersecting sets of small grooves 56 which leave
between them small substantially rectangular bosses 55. This sort
of textured surface may most readily be achieved by coating the
areas which provide the bosses 55 with a suitable resist and then
etching the surface to the desired depth. The resist may be applied
by a photographic process, as is well understood. In the presently
preferred form of the structure, the center to center spacing of
adjacent grooves of each set is approximately 0.05 inch and the
grooves are about 0.003 deep. The width of each groove is about the
same as the width of the bosses between the grooves. Orientation of
the grooves is preferably at about a 45.degree. angle to the axial
direction through the prechamber so that the fuel introduced into
the inner wall may flow downstream of the prechamber under the
influence of the air stream through the channels defined by the
helically extending grooves 56.
It is believed that the superior performance with the textured
surface is due to turbulence in the air flow on a small scale,
aided by the bosses 55 which improve heat transfer from the air,
and also to the partial shielding of the liquid fuel within the
channels 56 from the direct blast of the air. At any rate, it has
been demonstrated that this textured surface aids in the complete
vaporization and diffusion of the fuel in the air.
It has been found that burning of a lean mixture in the reaction
zone 23 is preferable from the standpoint of clean exhaust to
burning of a nearer to stoichiometric mixture. It is found
desirable to introduce some air beyond that introduced by the
swirler 30 to further mix with and dilute the fuel-air mixture
prior to the initiation of combustion. This is effected by a set of
air entrances distributed around the prechamber, preferably about
three-fourths of its length from the upstream end to the downstream
end. The presently preferred structure for introduction of
additional air introduces the air with radially inward and
tangential components of movement and no significant axial
component. It also provides for variation of the effective area and
therefore flow capacity of the prechamber downstream air inlet,
which is desirable as a part of means for maintaining the desired
equivalence ratio in the reaction zone. Equivalence ratio will be
understood to mean the ratio of the actual weight ratio of fuel to
air to the stoichiometric ratio of fuel to air. This is
accomplished effectively by varying the ratio between the quantity
of air flowing into the reaction zone from the prechamber to that
introduced through dilution ports in the dilution zone 24 as the
ratio of total airflow to fuel flow varies.
Considering first the air entrance means through the prechamber
wall, as illustrated in FIGS. 2, 4, and 10, the wall portion 36 is
made in two coaxial abutting sections fixed together, an upstream
section 58 and a downstream section 59. The air entrance means is
defined by an annular array of slots 60 machined in the downstream
edge of upstream section 58. Of course, they could be machined in
the upstream portion of section 59 if the joint between the two
sections is suitably located. It will be seen from FIGS. 4 and 10
that slots 60 enter the chamber at a considerable angle to the
radial, about 60.degree. in the particular case, and are so
oriented that the direction of swirl of air from these slots is the
same as that imparted by the inlet swirler 30. The outline of the
slots is trapezoidal, the walls which bound the slots diverging
from each other in the direction toward the upstream end of the
prechamber. The fragmentary view of FIG. 10 shows two such slots.
In the total circumference there are preferably eighteen air
entrance slots.
The wall section 58 also defines a radial port 62 through which an
igniter 63 (FIG. 3), which may be similar to a spark plug, extends
into the prechamber so as to light off the fuel. The details of the
igniter are immaterial, so it is not further described.
The exterior of section 59 may bear three bosses 64. These provide
a limit to movement of a flow modulating sleeve 66 slidably mounted
on the exterior of the prechamber wall portion 36. As will be
further described, this sleeve provides means for varying the flow
of air through slots 60.
Proceeding now to the reaction and dilution zones of the liner,
these are enclosed within the wall 22 from the radially enlarging
wall 20 downstream to the combustion products outlet 26. The walls
of the combustion portion of the liner are imperforate. All the air
for combustion is introduced through the prechamber outlet 67
defined by the inner margin of the flow dam 38. It has been found
desirable, to improve cooling of the combustion section, to provide
a circumferential array of fins 68 extending longitudinally of the
wall 22 from the upstream end. These help in transfer of heat to
the air flowing within the outer casing 4 toward the combustion air
inlets.
The dilution zone 24 is characterized by an array 70 of dilution
air entrance ports, the effective area of which is varied by a
ported axially slidable flow modulating sleeve 71. As one means to
attain the desired characteristics of change of dilution air flow
area with movement of the sleeve 70, the ports 70 are of two sets
alternating around the periphery of the liner. One set is of ports
72, which are rectangular and of the least dimension axially of the
liner. Between these are the ports 74 of the second set which have
an extension 75 toward the upstream end of the liner which is of
smaller width circumferentially of the liner than the downstream
portion of the ports 74.
The sleeve 71 is a simple cylinder with slightly flared ends and
with a circumferential stiffening rib or ridge 78 near its upstream
end. This sleeve is reciprocable on the outer surface of the liner,
its travel being limited by two sets of small bosses 79 fixed to
the outer surface of the liner, the bosses of each set being
distributed 120.degree. apart around the circumference of the
liner.
The flow modulating sleeve 71 has two sets of ports each
cooperating with one set of ports 72 or 74 of the liner wall.
Rectangular ports 80 register with ports 72 of the wall and
rectangular ports 82 register with ports 74. The length and width
of the ports in the sleeve corresponds to the length and width of
the corresponding port in the liner so that the liner ports can be
fully opened at one setting of the sleeve. One of the short ports
80 has two slots 83 extending axially of the liner at its upstream
edge. These slots provide clearance for two pins 84 which extend
outwardly from the liner and are welded to the liner wall. These
pins serve to locate the sleeve 71 circumferentially of the liner
and obviate any tendency for the sleeve to rotate around the axis
of the liner as it is moved back and forth to vary the dilution air
flow.
The sleeve 71 is moved by three axially movable rods 86 (FIG. 1)
which are coupled by pins 85 to webs 87 extending radially from the
sleeve 71. Rods 86 extend through guides 88 in the combustion
chamber cover 14 and through seals or glands 90 to a common
actuator plate 91 to which the rods are fixed by nuts 92. The plate
91 may be coupled through a rod 94 to any suitable acutating
mechanism capable of sliding the sleeve 71 axially of the
liner.
The flow modulating sleeve 66 which varies the flow area through
the ports 60 of the prechamber is rigidly coupled to sleeve 71 for
concurrent movement by the input device 94. This interconnection
includes three rods 95 extending axially of and distributed around
the circumference of the liner. Each rod is fixed at an anchorage
96 to the sleeve 71. They are adjustably connected to sleeve 66.
This connection is effected through arms 102 fixed to the sleeve 66
120.degree. lapart and extending radially outwardly. The arms are
stiffened by gussets 103 welded to the arms and through a base
plate 104 to the sleeve 66.
The forward end of each rod 95 is threaded and extends through a
hole at the extremity of an arm 102. The connection is completed by
two sets of double nuts 106 which may be adjusted to trim the
relative poisitions of the two flow controlling sleeves. It will be
seen that the two sets of sleeves move so that, as the ports 60 in
the prechamber open, the ports 70 in the dilution area close.
Clearly, many operating connections to the sleeves 66 and 71 may be
devised. That shown is simple and meets the requirements.
The downstream edge of sleeve 66 is notched as indicated at 106 in
FIGS. 1 and 2. There is a notch aligned with each air entrance slot
60, the notches being V-shaped and having an included angle of
about 70.degree.. These notches are slightly wider than the
downstream end of slots 60. They provide a tapering rather than an
abrupt opening or closing of the slots 60 as the sleeve 66 moves so
that its rear edge passes the rear edge of the slots.
The particular configuration of the air entrance openings 60, 72,
and 74 is such as to provide a desirable characteristic of
variation of the relative amounts of primary and dilution air as
the input rod 94 is reciprocated. FIG. 11 illustrates the variation
of air flow with movement of the flow controlling sleeves.
Specifically, curve A of FIG. 11 shows the proportion of air
entering the reaction zone through the swirler 30 and the slots 60
to the total air admitted, which is this amount of air plus that
entering through the dilution ports at 70. It will be noted that
the reaction zone air flow increases from about 17 percent with the
sleeves at their maximum downstream position to approximately 55
percent with the sleeves moved forwardly to provide maximum
reaction air flow relative to dilution air flow. Put another way,
the dilution air flow decreases from about 83 percent to about 45
percent of total flow over this range of movement of the sleeves.
This makes it possible to provide adequate air flow in the reaction
zone under high power conditions without having the reaction zone
undesirably rich as the power level of the engine is decreased. At
small fuel flows, as under engine idling conditions, a relatively
small part of the air is required to provide the desired
equivalence ratio of about 0.3 in the reaction zone. The bend at
107 in curve A represents closing of the short dilution ports
72.
A pilot fuel nozzle 110 is mounted in the prechamber. This nozzle
is preferably of an air-atomizing type supplied with compressed air
and fuel through tubes (not illustrated) which enter through a
supporting structure including a ring 111 fixed within the sleeve
34 by cap screws 112. This arrangement provides a suitable support
for the fuel nozzle which includes a tubular extension 114
threadably coupled with the ring 111. The details of the fuel
nozzle are not material to the present invention. With this type of
fuel nozzle, the fuel is sprayed in fine droplets by an air blast.
The pilot fuel nozzle is provided for starting combustion,
particularly when the engine is cold and therefore evaporation of
fuel from the prechamber wall is not effective. The pilot nozzle is
turned off after normal operation has begun. Other starting
expedients such as use of gaseous fuel may be employed, but are not
considered as feasible as the use of the pilot nozzle.
A converging fairing 115 extends from the downstream end of sleeve
34 to the downstream end of the fuel nozzle 110 to provide a smooth
transition of flow from the swirler 30 into the prechamber.
In starting the engine, air flow is low and conditions are
abnormal. In this case the flow modulating sleeve moves forward so
as to diminish dilution air and direct the major part of the air
into the prechamber where it mixes with the fuel from the pilot
nozzle and burns when the igniter is energized. When the engine
reaches normal operating conditions, the starting regime is
terminated and the position of the flow modulating sleeves is
varied as necessary to attain the desired equivalence ratio at the
entrance to the reaction zone.
In operation of a regenerative engine at full power, the air enters
the combustion apparatus at about 1,100.degree.F. and after passing
through the swirler 30 flows over the inside of the prechamber
wall, heating, evaporating, and mixing with the fuel introduced
from the manifold 46 through the orifices 54. This mixture of air
and vaporized fuel is further mixed with additional combustion air
which enters through the swirl ports 60 with swirl in the same
direction as the air flowing rearwardly through the prechamber.
These two flows then mix to provide a rather lean fuel-air mixture,
preferably with about three times the amount of air required for
combustion; that is, three times the stoichiometric amount of
air.
The swirling fuel-air mixture spills over the dam 38 and because of
the swirl flows tangentially and radially outward to the outer wall
22 of the liner and then, because of the creation of a low pressure
area along the axis of the combustion zone and prechamber, it flows
in a more or less toroidal vortex with some upstream or
recirculating flow along the axis of the liner. This flow may
penetrate into the downstream part of the prechamber under some
conditions of operation and in this case may also tend to heat the
prechamber.
The dilution air admitted through the openings at 70 tends to
quench the heat of the combustion mixture which flows along the
wall 22 toward the outlet 25. In addition, the radially entering
streams of air as they meet toward the axis tend to project some of
the dilution air forwardly into the low pressure zone on the
combustion chamber axis where this mixes with the recirculating
combustion products to assist in cooling the combustion products at
an early time, reducing duration of high temperature in the
gas.
The lean combustion lowers the combustion temperature and the
prompt quenching of the gas lowers the residence time at high
temperature. Both of these effects serve to minimize formation of
nitrogen oxide.
Also, the burning of the fuel in a vaporized condition reduces
conditions of local richness which will be found in the vicinity of
atomized droplets of fuel and which tend to increase generation of
nitrogen oxide.
While there seems to be no need to recite details of dimensions
which are variable to suit any particular installation, it may be
mentioned that the combustion liner shown, which is for a 225
horsepower gas turbine engine, is 6-5/8 inches in diameter and 15
inches long, and is shown in true porportion in FIG. 2. It is
possible, of course, to vary the ratio of primary to dilution air
by throttling one set of ports only. The broken line curve B in
FIG. 11 illustrates the relation of reaction zone air flow to total
flow in an apparatus of the sort illustrated in which the change of
flow is due only to the movement of the sleeve 71, the downstream
air entrance of the prechamber being of fixed area. For a given
structure, the variation is less and the pressure drop is greater
if modulation is effected at only one set of ports. Since pressure
drops are inimical to engine efficiency, there is good reason to
modulate both sets of air ports.
FIG. 12 illustrates a variation of the combustion liner which may
be in most respects essentially as shown in figures previously
discussed. The liner of FIG. 12 differs from that of FIG. 2 in the
mode of introduction of air into the prechamber, and the portion of
the combustion liner downstream of that illustrated in FIG. 12 may
be as illustrated in FIG. 2. As illustrated in FIG. 12, the
prechamber wall 120 is a sheet metal structure of constant
thickness bearing the swirler 30 at its forward end and with the
central plug 34, 115 at the center of the swirler to support the
starting fuel nozzle. The interior of the prechamber wall is
preferably textured as previously described.
The arrangement for introduction of additional primary air toward
the downstream end of the prechamber in this case is a ring of
circular holes 122 spaced uniformly around the prechamber. In one
instance, the holes are 1/8 inch diameter and there are 36 holes.
In this case, a slightly smaller percentage of the air was admitted
through the holes 122 than through the ports 60 of FIG. 2. However,
in this case, the swirler 30 was slightly more open, having the
blades set at a 70.degree. angle to the axial direction rather than
75.degree.. No variation of the ports 122 was provided. While this
apparatus does not perform as cleanly as that described above, it
is a relatively clean combustion apparatus that might well serve
quite satisfactorily in various applications.
We believe that the foregoing description of preferred structure
will be sufficient for an understanding of the invention and its
preferred embodiment by those skilled in the art. As will be
appreciated, details of structures, dimensions, and the like may be
varied in response to the conditions of particular
installations.
Complete evaporation and mixing of the fuel within the prechamber
is important to successful operation of the combustion apparatus.
The textured surface of this invention is instrumental in assuring
this result in a compact prechamber, and is easily incorporated in
the apparatus.
The detailed description of the preferred embodiment of the
invention for the purpose of explaining the principles thereof is
not to be considered as limiting or restricting the invention,
since many modifications may be made by the exercise of skill in
the art.
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