Dual air-blast fuel nozzle

In a nozzle for atomizing fuel into a spray for combustion in gas turbine engines, wherein the atomization is effected by the use of high velocity and/or high density air, and wherein the supply of fuel to two separately metered points is such that at low flow rates the first fuel supply is spread into a thin sheet for atomization but at high flow rates the second fuel supply is spread into a thicker sheet which combines with the thin sheet produced from the first supply, thus resulting in a single spray of constant shape at all operating conditions.

BACKGROUND OF THE INVENTION 
The achievement of satisfactory combustion of the fuel in a gas turbine 
engine has always presented problems. As a minimum requirement it is 
essential for the fuel to be atomized into a spray of small drops at all 
operating conditions and to obtain this result over the wide range of fuel 
flows necessary (typically maximum/minimum = 100) has required the 
development of complex and sophisticated fuel spray nozzles. It is well 
known to use swirl-atomizers in which the fuel is supplied at high 
pressure to a swirl-chamber in which a free vortex is formed so that the 
fuel issues from the discharge orifice of the swirl-chamber as a thin 
sheet of conical shape which breaks up into a spray of drops by 
interaction with the surrounding air; these are known conventionally as 
"pressure atomizers". Since a pressure atomizer can only produce a 
satisfactory spray over a flow range of about 7:1 (maximum:minimum) it has 
been necessary to combine two pressure atomizers, one of low flow capacity 
known as a "pilot" or "primary" and the other of high flow capacity, known 
as a "secondary", into a single fuel nozzle such as is disclosed in U.S. 
Pat. No. 3,013,732 which is conventionally known as a "dual orifice" 
nozzle. 
To obtain improved atomization compared with the pressure atomizer it is 
well known to use high velocity and/or high pressure air as the means of 
atomizing the fuel, as disclosed in U.S. Pat. No. 3,474,970 and No. 
3,283,502. In the former the air is supplied from a source external to the 
engine and the nozzle is known as an "air-assisted" type. In the latter 
the air is available inside the engine and this is known as an "air-blast" 
type. Although the fuel flow range for satisfactory atomization of both 
"air-assist" and "air-blast" types is greater than a single "pressure 
atomizer" there are many applications in which it is considered necessary 
or desirable to combine an air-atomizing nozzle with a pressure atomizer 
as is disclosed in U.S. Pat. No. 3,912,164. In such an arrangement the 
pressure atomizer is used for the low fuel flow rate conditions, such as 
starting the engine, while the air-atomizer is used for the higher fuel 
flow rates, and this combination is usually described as a "hybrid" type. 
With both the dual-orifice and hybrid types of nozzle it is the invariable 
practice to maintain the "pilot" or "primary" nozzle flowing at all times 
so that at the higher fuel flows the primary and secondary atomizers are 
both in operation. There are some disadvantages in this arrangement since 
the shape of the primary spray is often different from that of the 
secondary spray and can result in a non-optimum placement of fuel in the 
combustion chamber. For example, if the primary spray angle is less than 
the secondary (which may be desirable to obtain good starting) then at 
high power conditions when the secondary also is in operation, the primary 
fuel may be concentrated in the center of the total spray and this 
produces smoke in the engine exhaust. The obvious solution to this problem 
is to shut off the primary nozzle fuel flow at high power conditions but 
this has been found to be impractical since the residue of fuel left in 
the primary nozzle readily carbonizes at the high metal temperatures 
prevalent at these operating conditions and the primary nozzle fuel flow 
passages can become plugged with carbon. A compromise solution is to 
reduce the primary fuel flow after the secondary fuel flow has reached a 
certain value, as disclosed in U.S. Pat. No. 3,675,853, but this requires 
the use of additional valve means located in the hottest operating 
environment, which is not conducive to the high reliability of operation 
demanded. 
Ideally, therefore, what is needed is a fuel nozzle, having all the known 
advantages of an air-atomizer and also the wide flow range capability of a 
hybrid design, in which the spray from the primary ceases to exist as a 
separate entity when the secondary is in operation. 
SUMMARY OF THE INVENTION 
The present invention consists of an air-blast nozzle having a "primary" 
and "secondary" fuel supply (as defined previously) in which the primary 
fuel is spread into a thin cylindrical or conical sheet to be atomized by 
high velocity and/or high pressure air. The secondary fuel is also spread 
into a coaxial cylindrical or conical sheet, of greater thickness than the 
primary, and the relationship of the two sheets of fuel is such that the 
secondary sheet combines with the primary sheet before being acted upon by 
the atomizing air. The objects of the invention are thus to 
(a) Obtain the benefits of having a separate primary fuel supply including 
improved atomization at low fuel flows; 
(b) To eliminate the existence of a separate primary spray once the 
secondary fuel flow has commenced; 
(c) To insure the production of a single spray of known shape at all 
operating conditions; and 
(d) To allow fuel to flow continuously through the primary flow passages at 
all operating conditions. 
Other objects and advantages will become apparent from the description of 
various embodiments of the invention. 
This invention may be incorporated into air-blast fuel nozzles as disclosed 
in U.S. Pat. No. 3,912,614 and in the copendng U.S. Application Ser. No. 
634,460 filed Nov. 24, 1975, now U.S. Pat. No. 3,980,233 dated Sept. 14, 
1976.

DESCRIPTION OF INVENTION 
The general arrangement of one embodiment of the invention is shown in 
FIGS. 1 and 2 in longitudinal and transverse section. A mounting member 1 
has drilled passages 2 and 3 for primary and secondary fuel respectively. 
A primary nozzle body 4 is threaded onto mounting member 1 to contain a 
secondary nozzle 5 and transition piece 6 in sealing contact. Body 4 has 
vanes 7 formed on its outer surface to which is attached by brazing or 
welding a shroud 8 formed externally as a hexagonal nut for wrenching. 
Torque is applied at assembly of the nozzle to shroud 8 and body 4 to 
insure sufficient axial load between the joint faces of parts 1, 6, 5 and 
4 to prevent leakage of fuel from these joints. The body 4 is locked to 
the member 1 by conventional means not shown. 
The path of the primary fuel is as follows: starting at the drilled passage 
2 it passes through a filter screen 14 which is held in place by spring 15 
into passage 9 which feeds an annulus 10. Four angled spin holes 11 take 
the primary fuel from annulus 10 into the spin or swirl chamber 12 formed 
by parts 4 and 5 to create a free vortex which discharges, as is well 
known, over the lip 13 of part 4 in a thin sheet of expanding conical 
shape, as will be described in greater detail later. 
The secondary fuel, starting at the drilled passage 3, is fed into an 
annulus 16, passes through a filter screen 17 into a second annulus 18 and 
then through three drilled passages 19 each of which terminates in an 
angled spin hole 20. The spin holes lead into the spin or swirl chamber 21 
formed by parts 5 and 6 to create a free vortex which discharges over the 
lip 22 of part 5 as a sheet of fuel which combines with the fuel sheet 
from the primary swirl chamber to form a single sheet leaving lip 13. The 
combination process is shown pictorially in FIG. 3 which is an enlarged 
view of a portion of FIG. 1. It will be noted that the lips 23, 22 and 13 
are placed at progressively increasing radii from the axis of the nozzle 
and are dimensioned so that the fuel in the primary swirl chamber 12 can 
flow only past the lip 13 in a downstream direction relative to the air 
flow. Similarly, the fuel in the secondary swirl chamber 21 can flow past 
the lip 22. The difference in radius between the lips 13 and 22 is 
designed to be only slightly greater than the thickness of the primary 
sheet indicated in FIG. 3 as t.sub.p, thus the secondary sheet of 
thickness t.sub.s will blend smoothly into the primary sheet just 
downstream of lip 22 giving a single sheet of fuel leaving lip 13 to be 
atomized by the air as will be described later. The difference in radii 
between lips 22 and 23 is not critical as long as it is greater than the 
secondary sheet thickness t.sub.s. The direction of swirl in chambers 12 
and 21 will usually be the same but this is not essential to the 
invention. 
Returning to FIG. 1, the path of the air which atomizes the fuel sheet 
leaving lip 13 can now be described. It will be understood that fuel 
nozzles are typically installed in an engine so that the nozzle protrudes 
through the wall of the combustion chamber, a portion of which is 
indicated by the broken lines 24 and that there exists under all operating 
conditions a difference in air pressure between the outside and inside of 
said combustion chamber which causes air to flow through any passage 
communicating therebetween. Accordingly, air will flow through the 
passages 25 between parts 4 and 8 in a direction determined by vanes 7, 
which may be axial or angled to the axis or helical in order to produce 
either straight or swirling flow in the annular passage 26 to exit within 
the region of the lip 27. A portion of the air will also flow through the 
holes 28 into annulus 29 and then through the passages 30 into the center 
region denoted as 31. The passages 30 are shown as being tangentially 
disposed rather than radially so as to produce a swirling air flow in the 
center region 31 although this feature is not an essential part of the 
invention. The direction of swirl (if any) in either of regions 27 and 31 
may be the same or different with respect to each other and also to the 
direction of the fuel swirl. 
The action of the air on the fuel sheet is shown in FIG. 4 which is a 
diagrammatic section of the inner portion of the fuel nozzle. In this case 
we show the air flow directions when both inner and outer air flows are 
swirled. It is seen that the high velocity air streams converge on the 
fuel sheet at the point A immediately downstream of the lip 13 to cause 
break-up of the sheet and the production of an atomized spray as indicated 
at B. The arrow X is intended to represent the direction of the outer air 
flow which is actually moving in a swirling manner inside lip 27 to form 
an expanding cone in three dimensions; arrow Y is similarly representative 
of the inner air flow. The arrow Z shows the general direction of the fuel 
spray resulting from the air flow. It is evident that the direction of 
arrow Z will be the same whether the fuel sheet consists only of primary 
flow or combined primary and secondary flow; in other words the spray 
shape will be essentially constant at all conditions. 
It is obvious that the air for atomizing can be supplied from a source 
outside the engine, if necessary, by suitable connections to the passages 
25. 
The invention is not limited to the particular arrangement of air passages 
inside the fuel nozzle shown in FIG. 1 except that the final points of 
exit of air from the nozzle must be at two regions, one on either side of 
the fuel sheet, in the same relation to the fuel sheet as the regions 
indicated as 27 and 31 in FIG. 1. In particular, the air supplied to the 
center of the nozzle may be introduced in an axial direction through the 
mounting member in known manner. 
Other geometric arrangements of the invention are possible for particular 
purposes. For example, there may be installations which require that the 
secondary swirl chamber shall be outside the primary swirl chamber, i.e. 
the secondary fuel sheet is outside the primary fuel sheet before 
combining into a single sheet. Such an arrangement is shown in FIG. 5 
where parts 44, 45 and 46 are similar to parts 4, 5 and 6 of FIG. 1 except 
that the radial disposition of the primary and secondary inlet passages 
and swirl chambers is reversed, making the secondary swirl chamber 41 and 
the primary swirl chamber 42 as shown. In order to insure that the primary 
fuel sheet is exposed to the outer atomizing air with the least 
interference from the secondary lip 48 the primary lip 47 is extended as 
shown and curved outward so that its downstream edge is in essentially the 
same plane as the downstream edge of lip 48. The difference between the 
radius (R.sub.48) of the inner surface of lip 48 and the radius (R.sub.47) 
of the outer surface of lip 47 is designed to be only slightly greater 
than the thickness (t.sub.s) of the secondary sheet, thus the two sheets 
will combine smoothly at a point only slightly downstream of the lips 47 
and 48. The difference in radii between the upstream corner of lip 47 and 
lip 49 is not critical as long as it is greater than the primary thickness 
t.sub.p. 
Another geometric arrangement of the invention is shown in FIG. 6. In this 
case the objective of producing a single fuel sheet from two fuel supply 
sources is achieved by mixing the primary and secondary fuel flows in a 
common swirl chamber with a single discharge lip. Parts 54, 55 and 56 are 
arranged similarly to FIG. 5 to form primary and secondary swirl chambers 
52 and 51 respectively both of which feed into a common chamber 53, which 
discharges at lip 58. The lip 57 of part 55 is at a larger radius than lip 
58; lip 59 of part 56 is at a smaller radius than lip 58 the difference in 
radii being slightly greater than the fuel sheet thickness, as shown. In 
operation on primary fuel only the fuel in chambers 52 and 53 is swirled 
by the primary spin holes 60 to form a sheet of thickness t.sub.p at the 
lip 58. When secondary fuel is added through angled holes 61 it is swirled 
in chamber 51 and the chamber 53 then acts as a mixing chamber in which 
the momenta of the primary and secondary fuel are combined (the direction 
of swirl being the same for primary and secondary). The combined fuel then 
discharges at the lip 58 in a single sheet of thickness t.sub.p+s to be 
atomized by air as described previously. 
It will be understood that the fuel system feeding the nozzles contains 
valves of known type which permit fuel to be fed first to the primary 
passages and then, at a higher operating condition to both primary and 
secondary in desired proportionate flow rates. 
One advantage of employing the invention to produce a thinner fuel sheet 
(from the primary) at low fuel flows is that, in general, the fineness of 
the spray is directly related to the thickness of the fuel sheet at the 
point of break-up into drops. The fineness of a spray is expressed 
conventionally by the use of the well known "Sauter Mean Diameter" or SMD, 
which is the diameter of a hypothetical spherical drop having the same 
surface-to-volume ratio as the entire spray, and it has been established 
by tests that the SMD varies proportionately to a fractional power of the 
fuel sheet thickness, all other things being equal. Expressed 
mathematically: 
EQU SMD .varies. t.sup.n (1) 
where SMD = Sauter Mean Diameter 
t = fuel sheet thickness at break-up into drops. 
n = 0.375 approximately. 
It can readily be calculated, and is well known, that the fuel sheet 
thickness is directly related to the flow capacity of a swirl-atomizer 
such that a lower flow capacity atomizer gives a thinner sheet. It is 
common for a "primary" nozzle to have a flow capacity only 1/10th of a 
"secondary" nozzle in a combined arrangement and thus its fuel sheet 
thickness t.sub.p will be only 1/10 approximately of the secondary fuel 
sheet thickness t.sub.s, i.e. t.sub.p /t.sub.s = 0.1. 
The corresponding SMD's can be calculated from equation (1) to be 
EQU (SMD primary)/(SMD secondary) = 0.1 0.375 = 0.42 
In other words, at a given operating condition (of fuel flow rate, air 
velocity and pressure etc.), the primary atomizer will give a spray having 
a mean drop diameter 58% smaller than the secondary atomizer. This 
difference in fineness of the spray may well make it possible to start and 
run an engine on "primary" where it would be impossible to start with an 
atomizer having the flow characteristics of a "secondary" only. 
It will be understood that there is no limitation on the flow capacities of 
either "primary" or "secondary" in the invention here described, or their 
relation to each other. 
Other embodiments of the invention may make use of fuel and air 
swirl-producing devices such as slots, cast passages etc. in cylindrical, 
conical or radial planes as is well known in the art.