Method of liquid fuel injection, and to air blast atomizers

Abstract

An airblast atomozier which spreads liquid fuel into a continuous thin sheet, and exposes both sides of the sheet to the atomizing action of high velocity non-swirling air. The atomizer then injects the atomized fuel into a region of high turbulence and high shear formed by the meeting of two contra-rotating swirling air flows and in which region combustion occurs. This region is spaced from the atomizer a sufficient distance to permit full atomization of the fuel before it reaches such region. Also, the downstream end of the atomizer has no large transverse surfaces so exposed to the combustion process that significant deposits of combustion products can accumulate thereon.

Parent Case Text

This is a continuation of application Ser. No. 295,956, filed Oct. 10, 1972, now abandoned which application was itself a continuation-in-part of application Ser. No. 216,495, filed Jan. 10, 1972, now abandoned the priority of which applications is hereby claimed.

Description

The present invention relates to methods and apparatus of fuel injection for liquid fuelled combustion systems, whereby fuel can be atomized and premixed with air before entering the combustion zone, and in particular is applicable to airblast atomizers in gas turbine engines.

In most gas turbine engines fuel is injected into the combustion chamber by means of a pressure atomizer in which fuel is forced under pressure through a small orifice from which it emerges as a multitude of high velocity, atomized fuel droplets. Another method of fuel injection is by means of an airblast atomizer, which is a device in which liquid fuel is shattered into droplets by the action of high velocity air. This air then enters the combustion zone carrying the atomized fuel along with it.

Airblast atomizers have advantages over pressure atomizers in that, because the fuel droplets are completely airborne, their distribution throughout the combustion zone is dictated by an air flow pattern which remains fairly constant under all operating conditions. In consequence, the distribution of temperature in the chamber exhaust gases also tends to remain constant, thereby extending the life of the turbine blades. Other advantages include the avoidance of high fuel pump pressures and, because the fuel enters the combustion zone atomized and premixed with air, the ensuing combustion is characterized by a blue flame of low-luminosity, resulting in fairly cool flame-tube walls and a minimum of exhaust smoke.

Many designs of gas turbine combustion chambers incorporate a device known as an air swirler which is located at the upstream end of the flame tube. The primary function of this swirler, referred to hereinafter as the outer swirler, is to admit air to the flame tube in a manner and direction calculated to promote a stable flow recirculation of the air and combustion products in the primary combustion zone.

Now it is well established that very high burning rates are achieved when atomized fuel is injected into regions of high turbulence and high shear. One object of the invention is to create such a region of high turbulence and high shear through the use of an additional or inner air swirler. This swirler produces a swirling airstream which lies initially inside of, and concentric with, the airstream issuing from the outer swirler, but whose swirl component is in the opposite direction to that of the outer swirler. The atomizer is so designed that the contra-rotating inner and outer airstreams meet to form such region of high turbulence and high shear at a region spaced from the downstream end of the nozzle. Means are provided for forming the fuel into a thin continuous sheet which is then atomized by the action of high velocity non-swirling air acting on both sides of the sheet. The fuel so atomized is then injected into the region of high turbulence where it is burned in a very efficient manner. Non-swirling air for achieving atomization of the fuel sheet is directed to both sides of the fuel sheet through annular passages separated from the inner and outer swirl passages.

The invention is based on the results of tests that have shown:

a. Atomization is best when the fuel is spread into a thin continuous sheet and exposed to high velocity non-swirling air on both sides.

b. Premixing of atomized fuel and air prior to combustion produces a clean, soot-free flame of low luninosity.

c. High rates of combustion are achieved when the atomized fuel is injected into the region of high turbulence and high shear formed at the interface of two-contra rotating swirling air flows.

This invention comprises both the method of injecting the fuel and the apparatus for achieving such method.

In order that the inventions may be more readily understood, reference will now be made to the accompanying drawings which show the application of the invention to gas turbine engines, and in which:

FIG. 1 shows an axial section through a first embodiment of fuel injector,

FIG. 2 shows a cross-section through the injector of FIG. 1, along the line II--II thereof, and

FIG. 3 shows an axial section through a second embodiment of fuel injector.

Referring now to the drawings, one embodiment of fuel injector is shown in FIGS. 1 and 2, and comprises as a structural assembly, an inner air swirler and concentric with it, an airblast atomizer unit and an outer air swirler. The assembly is carried by a fuel feed arm 1 having a bore 2 for supplying fuel to the airblast atomizer. A number of axial passageways 3 convey fuel from the bore 2 to an annular gallery 4 formed between an extension piece 5 of a main body 6 and a concentric prefilmer 7 located in a spigot 8 and retained by a nut 9. Air is conveyed to an atomizing lip 10 through a small annular passage 12 formed between the prefilmer 7 and the outer swirler 11. The nut 9 also retains a fuel sealing washer 13 between the main body 6 and the prefilmer spigot face 14. At the discharge end of the annular fuel gallery 4 tangential swirl ports 15 machined into the extension piece 5 contact a shoulder 16 on the prefilmer, and in so doing form a discharge passage for the fuel. An inner divergent wall 17 of the prefilmer conveys the discharged swirling fuel uniformly in a thin continuous sheet to the air atomizing lip 10. The inner wall 18 of the main body of the atomizer and a centre member 19 are mounted on three equispaced vanes 20 to form an inner air passage 21 which conveys air to the inner swirler 22 and non-swirling air to the air atomizing lip 10. The forward end face of extension piece 5 is recessed to form a radially inwardly extending shoulder adjacent the bottoms of ports 15 to prevent flow of fuel axially along inner wall 18, as is known in the art.

The inner air swirler body 23 is retained on a conical centre boss 24 by the action of retaining screw 25. The inner swirler shroud piece 26 separates the swirling air passing through the inner air swirler 22 and nonswirling air passing through passage 40 between shroud 26 and wall 17 to lip 10. The swirling liquid fuel leaves tip 10 as a sheet, as indicated by solid line 41, and is acted upon on its outer surface by non-swirling air issuing through passage 12 and on its inside surface by non-swirling air passing through passage 40. This non-swirling air atomizes the liquid fuel, as indicated by broken lines 42, and such atomization is substantially complete by the time the fuel reaches region 43 where the inner and outer swirling airstream from swirlers 22 and 11 meet. Because these latter airstreams are rotating in opposite directions there is high turbulence and shear force at region 43 and highly efficient combustion occurs thereat.

The downstream parts of the nozzle exposed to the flame, namely shroud 26, lip 10 and swirlers 22, 11, have thin transverse surfaces and thus there are no large transverse surfaces exposed to the flame upon which soot deposits and carbon build up can occur. Screw 25 is spaced a sufficient distance from combustion region 43 so that it is not subject to excessive carbon deposit thereon.

The airblast atomizing and air flow arrangement of FIG. 1 is particularly advantageous when used in conjunction with a pilot fuel injector, for example of the kind shown in British Specification No. 1,031,184 and illustrated in FIG. 3. Like parts of FIGS. 1 and 3 are referred to by like reference numerals. Thus, the combined fuel injector is carried by a feed arm 1 having bores 2 and 27 for supplying fuel to the airblast atomizer and pilot spray nozzle respectively. An extension of the bore 27 passes through the main body 6 and support vanes 20 such that fuel can be fed axially through the centre body 19 to a pilot spray nozzle 28. The spray nozzle includes component pieces 28, 29 and 30 which are retained in position by a retaining nut 31 which carries the inner air swirler 22. In this case, swirler 22 includes an inner annular shell 34 that has several radially inwardly extending struts 35 about its inner circumference to space shell 34 from nut 31. A small portion of the air flowing through the inner air passage 21 passes through anti-carbon air passages 32 to the face of the spray nozzle 28.

At low fuel flows all the fuel passes through the pilot spray nozzle 28 and air for combustion is provided through the inner swirler whose geometry is designed primarily for optimum combustion of pilot fuel. At high fuel flows most of the fuel flows through the airblast atomizing system as described earlier.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of liquid fuel injection which comprises the steps of:

spreading said fuel into a continuous thin sheet,

exposing both sides of said sheet to substantially non-rotating high velocity air flows which atomize said sheet into a layer of atomized fuel in a first region, and

injecting said layer of atomized fuel into a second region of high turbulence and high shear formed by the meeting of two contra-rotating swirling air flows emanating from opposite sides of said fuel layer downstream of said first region.

2. The method claimed in claim 1 in which the sheet of fuel is symmetrical about a longitudinal axis and annular in section.

3. The method claimed in claim 2 in which one of said high velocity air flows is projected inside said sheet and the other outside, while one of said swirling air flows is projected inside the innermost high velocity air flow the outermost high velocity air flow, said high velocity air flows being projected to converge on said sheet in a first zone and said swirling air flows being projected to converge on said atomized fuel in a second zone downstream of said first zone.

4. The method claimed in claim 3 in which said swirling and high velocity air flows are kept separate from each other during a portion of their path of travel.

5. An air blast atomizer for carrying out the method of claim 1 which comprises:

means for projecting a thin sheet of fuel encircling a longitudinal axis extending in the direction in which said fuel is projected,

means for providing high velocity substantially non-rotating air flows to both surfaces of said sheet to atomize said sheet into a layer of atomized fuel in a first region of said atomizer, and

means for providing swirling air flows having components of movement in opposite directions about said axis, one of which air flows impinges on one side and the other of which air flows impinges on the other side of said atomized sheet in a second region of said atomizer downstream of said first region.

6. An air blast atomizer as claimed in claim 5 comprising a fuel feed arm provided with a central support, in which said means for providing high velocity air comprises concentric air ducts carried by said arm, the means for providing swirling air comprises swirler means for imparting a swirling motion to the innermost air in the innermost of said air ducts and to the outermost air in the outermost of said air ducts, and said means for projecting fuel comprises fuel duct means around said innermost air duct but inside said outermost air duct which is connected to supply fuel to the outermost part of said innermost air duct.

7. An air blast atomizer as claimed in claim 6 in which said air ducts converge and an atomizing lip is formed at the point of convergence of said ducts.

8. An air blast atomizer for carrying out the method of claim 1 which comprises:

an inner air swirler for swirling air in one direction,

a fuel atomizer member concentric therewith for forming fuel into a continuous thin sheet,

means for providing high velocity substantially non-rotating air flows to both sides of said sheet to atomize said sheet into a layer of atomized fuel, and

an outer air swirler concentric with said inner air swirler and oriented to swirl air in a direction contra to said one direction, said inner and outer air swirlers being positioned to provide air flows on opposite sides of said atomized fuel layer.

9. An atomizer according to claim 8, wherein said swirlers and said atomizer member are formed as a structural assembly carried by a fuel feed arm having a bore for supplying fuel to the atomizer member.

10. An atomizer according to claim 9, wherein a plurality of axial passageways convey fuel from the bore to an annular gallery formed between an extension piece of a main body and a prefilmer concentric therewith and wherein air is conveyed to a lip of said atomizer member through two annular passages, one formed between the prefilmer and the inner swirler, and the other formed between the prefilmer and the outer swirler.

11. An atomizer according to claim 10, wherein a nut retains the prefilmer on the main body.

12. An atomizer according to claim 10, wherein the main body has an air passage and a center member is mounted in the air passage to form an annular inner air passage to convey air to the inner swirler and to the atomizing lip.

13. An atomizer according to claim 10, wherein the main body and a center member are mounted on vane means to form an inner air passage to convey air to the inner swirler and to the atomizing lip.

14. An atomizer according to claim 10, wherein the main body has an additional bore passing through its center and leading to a pilot spray nozzle whereby fuel can be fed axially through the main body to the pilot spray nozzle.

15. An atomizer according to claim 14, wherein the spray nozzle comprises a number of components that are retained in position by a nut that retains the inner air swirler on the main body, and wherein a small portion of the air flowing through the inner air passage passes through an anti-carbon air gallery to the face of the spray nozzle.