U.S. patent application number 10/629795 was filed with the patent office on 2005-05-12 for fuel injection arrangement.
Invention is credited to Jefferies, Mark J., Pilatis, Nickolaos.
Application Number | 20050097889 10/629795 |
Document ID | / |
Family ID | 9942709 |
Filed Date | 2005-05-12 |
United States Patent
Application |
20050097889 |
Kind Code |
A1 |
Pilatis, Nickolaos ; et
al. |
May 12, 2005 |
Fuel injection arrangement
Abstract
A prefilmer for a fuel injection arrangement comprising a body
having a fluid flow surface and a downstream edge, the prefilmer
arranged so that when working in operative association with the
fuel injection arrangement fuel flows over the surface, by means of
a passing airflow, to the downstream edge, from where the fuel is
shed, characterised in that the prefilmer further comprises a fluid
flow mixing means to, in use, enhance the mixing of fuel and
air.
Inventors: |
Pilatis, Nickolaos;
(Warrington, GB) ; Jefferies, Mark J.; (Yate,
GB) |
Correspondence
Address: |
MANELLI DENISON & SELTER
2000 M STREET NW SUITE 700
WASHINGTON
DC
20036-3307
US
|
Family ID: |
9942709 |
Appl. No.: |
10/629795 |
Filed: |
July 30, 2003 |
Current U.S.
Class: |
60/743 ;
60/748 |
Current CPC
Class: |
F23D 11/107 20130101;
F23D 2900/11101 20130101; F23R 2900/00014 20130101; F23D 2210/00
20130101; F23D 11/38 20130101 |
Class at
Publication: |
060/743 ;
060/748 |
International
Class: |
F23R 003/14 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 21, 2002 |
GB |
0219461.1 |
Claims
We claim:
1. A prefilmer for a fuel injection arrangement comprising a body
having a fluid flow surface and a downstream edge, the prefilmer
arranged so that when working in operative association with the
fuel injection arrangement fuel flows over the surface to the
downstream edge, from where the fuel is shed, characterised in that
the prefilmer further comprises a fluid flow mixing means to, in
use, enhance the mixing of fuel and air.
2. A prefilmer for a fuel injection arrangement as claimed in claim
1 characterised in that the fluid flow mixing means comprises
projections extending generally downstream from the downstream
edge.
3. A prefilmer for a fuel injection arrangement as claimed in claim
2 characterised in that the projections are generally trapezoidal
in shape.
4. A prefilmer for a fuel injection arrangement as claimed in claim
1 characterised in that the projections are generally triangular in
shape.
5. A prefilmer for a fuel injection arrangement as claimed in claim
1 characterised in that the projections define trapezoidal notches
therebetween.
6. A prefilmer for a fuel injection arrangement as claimed in claim
1 characterised in that the projections define triangular notches
therebetween.
7. A prefilmer for a fuel injection arrangement as claimed in claim
2 characterised in that projections are radially inwardly
angled.
8. A prefilmer for a fuel injection arrangement as claimed in claim
2 characterised in that the projections are radially outwardly
angled.
9. A prefilmer for a fuel injection arrangement as claimed in claim
2 characterised in that the projections are alternately radially
inwardly and outwardly angled.
10. A prefilmer for a fuel injection arrangement as claimed in
claim 2 characterised in that the angle of the projections is
between 0 and 45 degrees relative to an injector axis.
11. A prefilmer for a fuel injection arrangement as claimed in
claim 1 characterised in that the fluid flow mixing means comprises
the downstream edge configured in a generally sinusoidal form in
its axial direction.
12. A prefilmer for a fuel injection arrangement as claimed in
claim 1 characterised in that the fluid flow mixing means comprises
the downstream edge configured in a generally sinusoidal form in
its radial direction.
13. A prefilmer for a fuel injection arrangement as claimed in
claim 1 characterised in that the fluid flow mixing means comprises
the downstream edge configured in a compound form which is both
sinusoidal in form in its radial and axial directions.
14. A prefilmer for a fuel injection arrangement as claimed in
claim 1 characterised in that the fluid flow mixing means comprises
lands disposed to the downstream edge, the lands are configured to
generate and impart, in use, vortices into the passing airflow to
enhance the mixing of fuel and air.
15. A prefilmer for a fuel injection arrangement as claimed in
claim 14 characterised in that the lands comprise a leading edge,
two opposing sides, a leeward face and a base attached to the fluid
flow surface.
16. A prefilmer for a fuel injection arrangement as claimed in
claim 1 characterised in that the fluid flow mixing means is
asymmetrically arranged about the prefilmer.
17. A prefilmer for a fuel injection arrangement as claimed in
claim 1 characterised in that the prefilmer is generally
annular.
18. A prefilmer for a fuel injection arrangement as claimed in
claim 1 characterised in that the surface is an inner surface of
the prefilmer and the fluid flow mixing means is disposed to the
inner surface.
19. A prefilmer for a fuel injection arrangement as claimed in
claim 1 characterised in that the surface is an outer surface of
the prefilmer and the fluid flow mixing means is disposed to the
outer surface.
20. A prefilmer for a fuel injection arrangement as claimed in
claim 1 characterised in that during low fuel flows the fluid flow
mixing means enhances the mixing of fuel and air and provide
regions of rich and lean fuel/air mixtures.
21. A fuel injection arrangement for a gas turbine engine
incorporating a prefilmer as claimed in claim 1.
22. A gas turbine engine comprising a fuel injection arrangement as
claimed in claim 21.
Description
[0001] The present invention relates to a fuel injection
arrangement for a combustor of a gas turbine engine and in
particular a prefilmer for the fuel injection arrangement.
[0002] There is an increasing demand to reduce the emissions
produced by gas turbine combustors for aerospace, marine and
industrial applications. One approach is to use lean fuel/air ratio
combustion. A preferred lean direct injector is disclosed in U.S.
Pat. No. 6,272,840. This injector comprises a pilot injector
surrounded by a coaxial main injector. The pilot injector provides
a pilot flame that has a relatively high, but stable fuel/air
ratio. The main injector provides a main flame that has a lean
fuel/air ratio. The main injector supplies the majority of the
combustion gases above low power so that low levels of oxides of
nitrogen (NOx) emissions are obtained because the main injector
produces a uniformly mixed fuel-air mixture at an equivalence ratio
less than stoichiometric. This mixture burns at a relatively low
flame temperature avoiding the NOx producing high temperature
volumes of more conventional combustion systems.
[0003] To assist mixing of fuel and air a prefilmer is mounted
between radially adjacent swirl vanes. Fuel is shed from the
downstream edge of the prefilmer, and is atomised as it passes
through a shear region formed by the swirl vanes. In a typical lean
burn fuel injector this is the only purpose of the prefilmer.
[0004] Although lean burn combustion systems can produce NOx
emissions levels significantly lower than conventional combustion
systems, there is a severe disadvantage, combustion instability.
Where variations in heat release and pressure are in phase, the
magnitude of both fluctuations will increase. The severity of the
combustion instability produced varies from an irritating noise to
a force powerful enough to stall gas turbine compressors and shake
combustion systems apart. In a conventional aerospace gas turbine
combustion system, different areas within the combustor operate at
different air-fuel ratios. Here, fluctuations in heat release
become out of phase relative to each other resulting in a reduction
of the net heat-release. In a lean burn system, as the system runs
at a uniform air-fuel ratio, all parts of the combustion system
tend to oscillate in phase with each other. Net heat release
fluctuations therefore tend to be high.
[0005] Therefore an object of the present invention is to provide a
means for reducing combustion instability and in particular
reducing net heat release fluctuations within the combustor.
[0006] Accordingly the present invention seeks to provide a
prefilmer for a fuel injection arrangement comprising a body having
a fluid flow surface and a downstream edge, the prefilmer arranged
so that when working in operative association with the fuel
injection arrangement fuel flows over the surface, by means of a
passing airflow, to the downstream edge, from where the fuel is
shed, characterised in that the prefilmer further comprises a fluid
flow mixing means to, in use, enhance the mixing of fuel and
air.
[0007] Preferably, the fluid flow mixing means comprises
projections extending generally downstream from the downstream
edge; the projections are generally trapezoidal in shape.
Alternatively, the projections are generally triangular in
shape.
[0008] Preferably, the projections define trapezoidal notches
therebetween, but alternatively the notches are triangular.
[0009] Preferably the projections are radially inwardly angled.
Alternatively, the projections are radially outwardly angled and
furthermore the projections are alternately radially inwardly and
outwardly angled. It is preferred that the angle of the projections
is between 0 and 45 degrees relative to an injector axis.
[0010] Alternatively, the fluid flow mixing means comprises the
downstream edge configured in a generally sinusoidal form in its
axial direction or alternatively in its radial direction.
[0011] Alternatively, the fluid flow mixing means comprises lands
disposed to the downstream edge, the lands are configured to
generate and impart, in use, vortices into the passing airflow to
enhance the mixing of fuel and air. In a preferred embodiment, the
lands comprise a leading edge, two opposing sides, a leeward face
and a base attached to the fluid flow surface.
[0012] Alternatively, the fluid flow mixing means is asymmetrically
arranged about the prefilmer.
[0013] FIG. 1 is a schematic section of a ducted fan gas turbine
engine incorporating an embodiment of the present invention.
[0014] FIG. 2 is a sectioned schematic view of a first embodiment
for a piloted airblast lean direct fuel injector incorporating a
first embodiment of a fluid flow mixing means in accordance with
the present invention.
[0015] FIG. 2a is an illustrative enlarged perspective view of the
fluid flow mixing means shown in FIG. 2.
[0016] FIG. 3 shows a second embodiment of the fluid flow mixing
means in accordance with the present invention.
[0017] FIG. 4 shows a third embodiment of the fluid flow mixing
means in accordance with the present invention.
[0018] FIG. 5 is a perspective view of a fourth embodiment of the
fluid flow mixing means in accordance with the present
invention.
[0019] With reference to FIG. 1, a ducted fan gas turbine engine
110 comprises, in axial flow series an air intake 112, a propulsive
fan 114, a core engine 116 and an exhaust nozzle assembly 118 all
disposed about a central engine axis 120. The core engine 116
comprises, in axial flow series, a series of compressors 122, a
combustor 124, and a series of turbines 126. The direction of
airflow through the engine 110 in operation is shown by arrow A.
Air is drawn in through the air intake 112 and is compressed and
accelerated by the fan 114. The air from the fan 114 is split
between a core engine flow and a bypass flow. The core engine flow
passes through an annular array of stator vanes 128 and enters core
engine 116, flows through the core engine compressors 122 where it
is further compressed, and into the combustor 124 where it is mixed
with fuel, which is supplied to, and burnt within the combustor
124.
[0020] Combustion of the fuel mixed with the compressed air from
the compressors 122 generates a high energy and velocity gas stream
that exits the combustor 124 and flows downstream through the
turbines 126. As the high energy gas stream flows through the
turbines 126 it rotates turbine rotors extracting energy from the
gas stream which is used to drive the fan 114 and compressors 122
via engine shafts 130 which drivingly connect the turbine 126
rotors with the compressors 122 and fan 114. Having flowed through
the turbines 126 the high energy gas stream from the combustor 122
still has a significant amount of energy and velocity and it is
exhausted, as a core exhaust stream, through the engine exhaust
nozzle assembly 118 to provide propulsive thrust. The remainder of
the air from, and accelerated by, the fan 114 flows through an
annular array of guide vanes 132 within a bypass duct 134 around
the core engine 116. This bypass airflow, which has been
accelerated by the fan 114, flows to the exhaust nozzle assembly
118 where it is exhausted, as a bypass exhaust stream to provide
further, and in fact the majority of, the useful propulsive thrust.
The combustor 124 incorporates a fuel injection arrangement (not
shown), which is in accordance with the present invention.
[0021] Referring now to FIG. 2, the fuel injection arrangement
suitable for a gas turbine engine is generally indicated at 60. The
fuel injection arrangement 60 is attached to the upstream end of a
gas turbine engine combustion chamber 11, part of which can be seen
in FIG. 2. Throughout this specification, the terms "upstream" and
"downstream" are used with respect to the general direction of a
flow of liquid and gaseous materials through the fuel injection
arrangement 60 and the combustion chamber 11 as shown by arrow A.
Thus with regard to the accompanying drawings, the upstream end is
towards the left hand side of the drawing and the downstream end is
towards the right hand side. The actual configuration of the
combustion chamber 11 is conventional and will not, therefore, be
described in detail. Suffice to say, however, that the combustion
chamber 11 may be of the well known annular type or alternatively
of the cannular type so that it is one of an annular array of
similar individual combustion chambers or cans. In the case of a
cannular combustion chamber, one fuel injection arrangement 60
would normally be provided for each combustion chamber 11. However,
in the case of an annular combustion chamber 11, the single chamber
would be provided with a plurality of fuel injection arrangement 60
arranged in an annular array at its upstream end. Moreover, more
than one such annular array could be provided if so desired. For
instance, there could be two coaxial arrays.
[0022] FIG. 2 shows a prior art piloted airblast lean direct fuel
injector arrangement 60, which is described in detail in U.S. Pat.
No. 6,272,840, the teachings of which are incorporated herein by
reference. However, the main features are briefly described where
particularly relevant to the present invention. The injector
arrangement 60 is generally annular and symmetrical about an
injector axis 62 and is disposed at the upstream end of the
combustion chamber 11.
[0023] The fuel injector arrangement 60 comprises a pilot or
primary injector 12 and a pilot swirler 14 generally surrounding
the pilot injector 12. A main airblast fuel or secondary injector
16 is concentrically positioned around the pilot injector 12 and
inner and outer main swirlers 18, 20 are concentrically disposed
radially inwardly and outwardly respectively of the main airblast
fuel injector 16.
[0024] An annular air splitter 22 is located between the pilot
swirler 14 and the inner main swirler 18. The air splitter 22
comprises an air inlet 24 and downstream, an air outlet 26. The air
splitter 22, in the direction of air flow, further comprises a
generally cylindrical portion 28, a radially inwardly tapered
portion 30 and a downstream portion 32 that is tapered still
further radially inwardly.
[0025] In use, fuel flows through galleries 64 and 66 and exits
through orifices 76, 78, which are defined by annular and co-axial
members 68, 70 and 72, 74, of the main and pilot fuel injectors 16
and 12 respectively. The annular members 68 and 72 are fuel
prefilmers having surfaces 80, 82 that the fuel flows over prior to
being shed from downstream edges 44, 45 into the swirling
airflows.
[0026] The geometry and position of the air splitter 22 is such
that it separates the air flow exiting the pilot injector 12 and
the main injector 16 thereby creating a bifurcated recirculation
zone between the pilot and main air flows. The creation of the
bifurcated recirculation zone, that aerodynamically separates the
pilot flame from the main flame, benefits the lean blowout
stability of the fuel injector arrangement 60. The pilot fuel stays
nearer to the injector axis 62 and evaporates there, thus providing
a richer burning zone for the pilot flame than is the case for the
main flame. The fuel/air ratio for the pilot flame remains
significantly richer than that for the main flame over a wide range
of operating conditions. Most of the NOx formation occurs in this
richer pilot flame, and minimizing the proportion of total fuel
going to the pilot flame may further reduce NOx. The main injector
supplies the majority of the combustion gases at most engine
operating conditions, so that low levels of oxides of nitrogen
(NOx) emissions are obtained because the main injector produces a
uniformly mixed fuel-air mixture at an equivalence ratio less than
stoichiometric. This mixture burns at a relatively low flame
temperature avoiding the NOx producing high temperature volumes of
more conventional combustion systems.
[0027] Although combustion systems, such as the prior art device
described above, can produce NOx emissions levels significantly
lower than conventional combustion systems, they have severe
disadvantages. One of these is combustion instability.
[0028] During testing of this prior art fuel injection arrangement
60, a high degree of combustion instability was experienced. This
combustion instability is believed to occur due to insufficient
mixing of the fuel and air mixture. Pressure fluctuations, arising
from the combusting fuel vapour, travel upstream into the injection
arrangement 60 where they cause the air velocity passing
therethrough to pulsate. The air mass flow past the fuel injection
planes therefore also varies. However, as the air pressure
fluctuations are small relative to the fuel injection pressure
there is no accompanying change in instantaneous fuel-flow. Instead
of producing a temporally uniform air-fuel ratio, the injection
arrangement 60 produces a uniformly spatially mixed air-fuel ratio,
varying cyclically in time at the pressure fluctuation frequency.
As heat-release from the combustion process is closely related to
air-fuel ratio, temporal variations in air-fuel ratio within the
premixer produce temporal variations in heat-release within the
combustor chamber 11. These in turn generate the pressure
fluctuations within the combustion chamber that cause the air-fuel
ratio entering the combustion chamber 11 to oscillate on the next
cycle. Thus a detrimental feedback loop is established.
[0029] Where variations in heat release and pressure are in phase,
the magnitude of both fluctuations will increase. The severity of
the combustion instability produced varies from an irritating noise
to a force powerful enough to stall gas turbine compressors and
shake combustion systems apart. In a conventional aerospace gas
turbine combustion system, different areas within the combustor
operate at different air-fuel ratios. Here, fluctuations in heat
release become out of phase relative to each other resulting in a
reduction of the net heat-release. At certain engine operating
conditions, the combustion system runs at a uniform air-fuel ratio,
all parts of the combustion system tend to oscillate in phase with
each other. Net heat release fluctuations therefore tend to be
high.
[0030] It is an object of the present invention to provide a means
for reducing combustion instability and in particular reducing net
heat release fluctuations within the combustor. Therefore it is
desirable to achieve improved mixing of the fuel/air mixture prior
to its combustion.
[0031] To further enhance the mixing of the fuel film in the
airflows the downstream edges 44, 45 comprise fluid flow mixing
means 34 in accordance with a first embodiment of the present
invention.
[0032] FIG. 2a shows the fluid flow mixing means 34 as an array of
lands 84 disposed around the inner circumference of the prefilmer
68 near to the downstream edge 44. The lands 84 enhance mixing the
fuel and air by generating vortices, shown by the sequence of
arrows 86, which breaks up the fuel into yet smaller particles. The
smaller the particle size of the fuel, the greater the surface area
of a given fuel quantity and therefore the quicker it is
vaporised.
[0033] As shown in FIG. 2a, the lands 84 are four sided and
comprise a leading edge 88, two opposing sides 90, a leeward face
92 and a base attached to the prefilmer 68. Air flowing in the
direction of arrow B flows over the edge between a side 90 and the
leeward face 92 and in so doing a vortex is imparted into the air
flow as shown by arrows 86. The lands 84 are generally orientated
parallel to the injector main axis 62.
[0034] It should be apparent to one skilled in the art that other
shapes of lands 84 may be used but they do not depart from the
scope or spirit of the present invention if they produce vortices
which enhance the mixing of the air and fuel fluid flow. For
example the size, positioning and number may be altered to suit
each particular application. Furthermore, the orientation of the
lands 84 may be altered so that for instance a land axis 36 is
orientated in the general direction of the swirling air flow; the
direction of the swirling airflow may not necessarily be aligned
with the injector axis 62.
[0035] In an alternative embodiment of the injector 60, the fuel is
discharged onto a radially outer surface 81 or 83 of the prefilmers
of the main injector 16 or pilot injector 12. In this embodiment,
the fluid flow mixing means 34 is disposed to the radially outer
surfaces 81, 83 and operates in a similar fashion to the foregoing
embodiment.
[0036] FIG. 3 shows a second embodiment of a main injector 16 in
accordance with the present invention, although this embodiment is
equally applicable to the pilot injector 12. For brevity only the
annular members 68, 70 are shown and like reference numerals are
used for like elements throughout the description of the present
invention. The annular members 68, 70, of the main injector 16, are
generally annular having a downstream edge portion 44 which itself
comprises a fluid flow mixing means 34. Each fluid flow mixing
means 34 is designed to impart mixing vortices into the fluid flow
thereby enhancing the mixing of fluid flowing therepassed and
consequently improving the fuel/air mixture ingressing to the
combustion chamber 11.
[0037] The main injector 16, in accordance with the present
invention, continues to function in a conventional manner as
described hereinbefore. In operation fuel flows through the
passageway between annular members 68, 70, through the orifice 76
and across the fluid flow surface 80. The fuel runs over the
surface 80 and is shed from its downstream edge 44.
[0038] The purpose of introducing a fluid flow mixing means 34 is
to further break up the liquid fuel into a smaller particle size.
By doing so the surface area of a particular quantity of fuel is
increased which intrinsically increases the rate that the fuel is
vaporised.
[0039] The main injector 16 and indeed the pilot injector 12, is
intended to be an alternative to the injectors 16 and 12 shown and
described with reference to FIGS. 2 and 2a. The downstream edge 44
of the main injector 16 comprises an array of generally trapezoidal
shaped projections 48, substantially aligned with and generally
extending in the downstream direction of the injector axis 41, and
which taper in the downstream direction. The projections 48 are
equally spaced around the circumference of the main injector 16.
Alternatively, the projections 48 may be spaced unequally around
the circumference of the downstream edge 44.
[0040] FIG. 4 shows a third embodiment of a main injector 16 in
accordance with the present invention, again this embodiment is
equally applicable to the pilot injector 12. For brevity only the
annular members 68, 70 are shown and like reference numerals are
used for like elements in FIG. 2. The annular members 68, 70, of
the main injector 16, are generally annular having a downstream
edge portion 44 which itself comprises a fluid flow mixing means
34. Each fluid flow mixing means 34 is designed to impart mixing
vortices into the fluid flow thereby enhancing the mixing of fluid
flowing therepassed and consequently improving the fuel/air mixture
ingressing to the combustion chamber 11.
[0041] The downstream edge 44 of the main injector 16 comprises an
array of generally triangular shaped projections 52, the apex of
each being downstream. Each projection 52 is equally spaced around
the circumference of the main injector 16 and is radially inwardly
angled. In the embodiment shown, the projections 52 are angled at
approximately 45.degree., relative to the axis 62, and are
generally aligned with the taper angle of a radially outer surface
81 of the annular member 68. Although an angle of 45.degree., is
shown the invention may be practised if the projections are within
the range 0-45.degree.. However, it should be appreciated to the
skilled artisan that angles up to 90.degree., relative to the
injector axis 62 would also provide the desired generation of
air/fuel mixing vortices.
[0042] Referring to both FIGS. 3 and 4, the projections 48, 52 are
formed by laser or electro-discharge cutting V-shaped or
trapezoidal shaped notches 50, 54 from a main injector 16 initially
having a planar downstream edge. Therefore the projections 48, 52
are curved along their circumferentially length. Although the
projections are shown as straight (in the axial direction), they
may also be curved either radially inwardly or outwardly in the
axial direction. It should be appreciated that the size, angle and
circumferential positioning of the projections are dependant on
each particular application and a general design philosophy
dictates that a compromise exists between the amount and strength
of vortices generated and the degree of fluid stream energy losses
encountered.
[0043] Although FIGS. 3 and 4 refer preferentially to trapezoidal
and triangular shaped projections 48, 52 the skilled artisan may
implement other shaped projections without departing from the scope
and spirit of the present invention. Other suitable shapes for the
projections comprise quadrilateral, polygonal or semi-circular or
generally arcuate projections.
[0044] Furthermore, the projections 48 may be alternately angled
radially inwardly and outwardly thereby imparting vortices within
either the radially inward or outward fluid flows passing through
the inner and outer main swirlers 18, 20.
[0045] FIG. 5 shows a fourth embodiment of the present invention
comprising the downstream edge 44 (or 45) arranged in a generally
sinusoidal form in its axial direction, i.e. a peak 56 is
downstream of a trough 58. The sinusoidal shaped downstream edge 44
directs portions of the fluid flows that pass radially inward or
outward of the main injector 12, into one another thereby promoting
mixing of the two fluid flows. The radial height difference between
the peaks 56 and troughs 58 may be varied depending on each
particular application, as may the number of peaks 56 and troughs
58 around the circumference of the downstream edge 44.
[0046] In an alternative arrangement of the downstream edge 44, the
sinusoidal form is substantially in the radial direction so that
the peaks 56 are radially outward of the troughs 58. This promotes
forced mixing between radial inner and outer swirling airflows and
therefore enhances the amount of mixing of the fuel and air fluid
flow.
[0047] Furthermore, the downstream edge 44 may comprise a compound
shape that is substantially sinusoidal in shape in both the axial
and radial directions.
[0048] Although the embodiments of the present invention have been
described with reference to the fluid flow mixing means 34 being
disposed to the downstream edge 44, 45 of a prefilmer 26, over
which fuel flows, the fluid flow mixing means 34 may also be
disposed to any downstream edge of a fuel injector where only air
flow over. This enhances the mixing capability of that air flow
where it coalesces with a fluid flow comprising fuel liquid of
vapour.
[0049] A further advantage of the present invention and
particularly associated to FIGS. 3, 4 and 5 is that at low fuel
flows the downstream edge of the prefilmers 68 create "streakiness"
in the fuel flow flowing off the downstream edge. Low fuel flows
are used at low engine power and at low power ignition. The
streakiness of the fuel film coming off the downstream edge is
relates to a variation in fuel film thickness associated with the
shape of the downstream edge. For embodiments shown in FIGS. 3 and
4, where the fuel flow leaves the downstream edge near the apex of
a notch 50, 54 there is a thicker fuel film than near the apex of
the projections 48, 52. For the embodiment shown in FIG. 5, where
the fuel flow leaves the downstream edge near the "apex" of a
trough 58 there is a thicker fuel film than near the "apex" of a
peak 56. Where the fuel film is thicker the resulting downstream
fuel/air mixture is richer so that at low fuel flows a more stable
combustion flame is present.
[0050] The embodiment described with reference to FIGS. 2 and 2A,
also imparts "streakiness" to the fuel film at low fuel flows. Here
the fuel is channelled between the lands 84, locally increasing its
film thickness.
[0051] The skilled artisan may easily appreciate other embodiments
that vary the fuel film thickness around the circumference. For
instance a circumferential array of shallow, radial channels may
extend from the fuel gallery to the downstream edge.
[0052] It should also be understood to the skilled artisan that for
greater fuel flows the distribution of fuel becomes more uniform
around the circumference and the prefilmer 68 therefore provides a
fluid flow mixing means to, in use, enhance the mixing of fuel and
air.
[0053] Whilst endeavouring in the foregoing specification to draw
attention to those features of the invention believed to be of
particular importance it should be understood that the Applicant
claims protection in respect of any patentable feature or
combination of features hereinbefore referred to and/or shown in
the drawings whether or not particular emphasis has been placed
thereon.
* * * * *