Combustor systems

Abstract

The disclosure shows two versions of providing a venturi around a fuel spray cone in a mixing chamber having an axial, vortical flow of pressurized air therethrough. This mixture is discharged into a combustion chamber. The venturi maintains desirable low smoke formation while spacing the flame front of the ignited mixture from the spray nozzle to reduce its temperature and thus prevent undesired carbon formation thereon.

Government Interests

The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the U.S. Department of the Air Force.

Description

The present invention relates to improvements in combustor systems, particularly in the generation of a hot gas stream as in gas turbine engines.

Combustor systems, as employed in gas turbine engines, comprise a spray nozzle which injects fuel into a combustion zone. Once ignited, a continuous flame front is maintained in the combustion chamber as fuel and pressurized air flow therein and a high energy, hot gas stream is discharged therefrom.

Recent emphasis had been placed on the problem of minimizing smoke generated in the combustion process and discharged to the atmosphere from gas turbine engines. One highly successful approach to this problem has been to discharge the fuel into a conduit or mixing chamber which opens into the combustion chamber. Pressurized air is introduced axially and swirled vortically about the discharge axis of the spray nozzle. In so doing, the turbulent flow field created produces a highly dispersed, over-stoichiometric mixture of fuel and air in the conduit. This mixture is discharged in a generally conical fashion into the combustion chamber where it is further dispersed by additional air entering other openings in the combustion chamber. This additional air is sufficient to sustain combustion and a flame front is produced in this mixing zone at the mixing chamber discharge. Because of the high degree of dispersion obtained, over-rich fuel zones are essentially eliminated and smoke is minimized to the point of being virtually undetectable.

While smoke is thus effectively reduced, the frequency of maintenance of the fuel nozzles has increased. This is due to the coking of fuel on the nozzles which adversley affects the pattern of the fuel spray cone discharged therefrom. Various forms of spray nozzles and air shrouds therefor have had limited degrees of success in overcoming this problem and still maintaining minimized smoke levels. This lack of success is attributed to the fine dispersion of fuel which recirculates, in localized regions, and contacts the discharge end of the nozzle. The fuel then cokes or carbonizes when it contacts the nozzle due to the high temperature thereof.

Accordingly, the object of the invention is to reduce the temperature levels of fuel nozzles and, in so doing, minimize the build-up of carbon thereon and further, to maintain minimal smoke generation in the combustion process.

These ends are broadly attained by providing a venturi means into which the fuel spray is discharged. The acceleration provided by the initial portion of the venturi tube accelerates mixing air introduced around the nozzle to an extent sufficient to prevent combustion for a predetermined distance from the nozzle. By thus displacing the flame front from the nozzle, its temperature may be reduced sufficiently to prevent undesirable carbon buildup thereon. The divergent portion of the venturi tube re-expands the mixing air to discharge it into the combustion chamber at a relatively wide angle consistent with the smoke minimizing dispersion process referenced above.

Other features are found in the relationship of the venturi tube to the means for introducing mixing air as well as in the amount of mixing air introduced.

The above and other related objects and features of the invention will be apparent from a reading of the following description of the disclosure found in the accompanying drawings and the novelty thereof pointed out in the appended claims.

In the drawings:

FIG. 1 is a schematic representation of a gas turbine engine employing a combustor of the type herein referenced;

FIG. 2 is an enlarged longitudinal section illustrating the details of the combustor referenced in FIG. 1 as they embody the present invention;

FIG. 3 is a section taken on line III--III in FIG. 2; and

FIG. 4 is a longitudinal section, similar to FIG. 2, illustrating another embodiment of the invention.

Referencing FIG. 1, the illustrated engine comprises an axial flow compressor 10 which pressurizes air. This pressurized air flows through an annular passageway 12 to an annular combustor 14 where fuel is introduced. The pressurized air supports combustion of fuel within the combustor to generate a high energy level, hot gas stream. This hot gas stream drives a turbine 16 which in turn powers the rotor of the compressor 10. The hot gas stream is then converted to a useful output as by being discharged from a nozzle 17 to provide propulsive. thrust for an aircraft.

The combustor 14, as illustrated in FIG. 2, comprises outer and inner liners 18 and 20 which define an annular combustion chamber 21. These liners are joined by compositely formed dome portion 22 at their upstream ends. Cylindrical conduits or mixing chambers 24 which open into the dome portion 22 and the combustion chamber 21. Transition segments 26 blend the conduit openings into the dome portion. An axial flow swirler 28 is mounted at the upstream end of each conduit 24.

The swirler has a central opening which receives the discharge end 29 of a fuel spray nozzle 30. The nozzle 30 may take many forms but is preferably characterized by at least one orifice outlet which produces a conical spray discharge having a relatively large included angle relative to an axis a which extends lengthwise of the mixing chamber. The swirler 28 includes a row of passageways 32 annularly surrounding the axis of the nozzle discharge end 29. The passageways 32 are angled relative to the nozzle axis to produce a vortical flow field.

The swirler 28 may be mounted on the upstream end of the conduit 24 in the manner taught in copending U.S. application Ser. No. 796,391, filed Feb. 4, 1969 and of common assignment. The referenced application also describes in further detail, the relationship of the swirler passageways 32 in obtaining a high degree of fuel dispersion for low smoke combustion.

Introduction of pressurized air from the annular compressor discharge passageway 12 into the combustion chamber 21 will now be described. The passageway 12 is defined by outer and inner, generally cylindrical casings 34 and 36 which extend along the lengths of the liners 18 and 20 are respectively spaced therefrom to define annular passageways 38 and 40. An annular snout assembly 42 is secured to and projects upstream from the liners 18 and 20. The snout assembly has a central passageway 43 having an entrance facing the discharge passageway 12 and discharging into an annular chamber 44 surrounding the entrances to the mixing chambers, i.e., swirler passageways 32. The pressurized discharge from the compressor then is split into three annular flow-paths along passageways 38, 40 and 43.

Air from the passageways 38 and 40 may enter the combustion chamber 21 to serve three functions. First, it may pass through relatively small holes 46 which are oriented to cool the liners 18 and 20. Second, it may enter relatively large holes 48 to penetrate the combustion chamber and supply requisite, primary air for the combustion process. Third, it may enter holes (not shown) further downstream as dilution air to reduce the temperature of the hot gas stream to a temperature compatible with the capabilities of the materials forming the turbine. The air entering holes 48 may also serve a dilution function.

Air entering the snout passageway 43 and chamber 44 is then metered by and injected into the mixing chamber as discrete jets by the swirler passageways 32. The vortical flow of mixing air is highly effective in dispersing or mixing the fuel from the spray cone into fine droplets which support a combustion process at a flame front generally identified by the broken line in FIG. 2.

The flame front is primarily controlled by two factors, once ignition is had. These are the presence and degree of a combustible mixture and the velocity of that mixture. In accordance with the present invention, these factors are taken into account in maintaining a desired distance between the flame front and the discharge end 29 of the fuel nozzle to minimize the temperature of the latter.

To this end, a venturi tube 50 is disposed concentrically of the nozzle axis a in a surrounding relationship with the spray cone discharged by the nozzle. The inlet diameter of the venturi tube approximates the mean diameter of the annular row of swirler passageways 32. Thus approximately one-third of the swirler mixing air is captured by and axially accelerated through the convergent portion of the venturi tube 50. This creates a condition of where the mixture, at the throat of the venturi, is sufficiently over-stoichiometric as not to support combustion and the mixture velocity is increased to further deter combustion. The divergent portion of the venturi tube then re-expands the mixture to approximately the original cone shape of the nozzle discharge. In this connection, it will be noted that the divergent portion of the venturi terminates at a generally tangent relationship with the spray cone.

While maintaining the desired low smoke characteristics, the venturi tube 50 is highly effective in maintaining the flame front at a desired downstream distance. This distance can be controlled as desired by the length of venturi tube and its contraction ratio as well as the amount of mixing air flow therethrough, all of these factors being balanced so that the spray cone angle is relatively divergent as it discharges from the venturi tube into the combustion chamber.

The effect of the venturi tube may be expressed in another fashion in that it increases the total pressure in the core of the vortical flow field created by the axial flow swirler. Without the venturi, the low pressure of the vortical core would enable recirculating primary air to flow forwardly, as indicated by the broken arrows, and create a combustible mixture and flame front closely adjacent the nozzle discharge. With the venturi such recirculation occurs, as indicated by the solid arrows, but is more limited in the magnitude of its upstream movement.

It will be noted that the venturi tube 50 is welded or otherwise bonded to the swirler 28 intermediate the lengths of the passageways 32. In so doing, added strength and rigidity is obtained for the swirler.

FIG. 4 illustrates an embodiment of the invention wherein all of the mixing air flows through a venturi tube 52 which also serves as a mixing chamber for introducing the dispersed fuel mixture into the combustion zone. The action of this venturi in obtaining a desired downstream displacement of the flame front and a consequent reduction in nozzle temperature is essentially the same as previously described except that the velocity factor of the venturi is more predominant in displacing the flame front since a greater amount of air enters the venturi. In fact, the benefits of the broader aspects of the present invention can be attained where the amount of mixing air flow is sufficient to produce a combustible mixture in the mixing chamber.

It has been demonstrated that the described use of a venturi is highly effective in preventing carbon buildup on fuel nozzles and, thus, assuring long maintenance-free operation. At the same time, low smoke levels continue to be maintained.

While the invention has been described with reference to an annular combustion system, it is equally applicable to a cannular system. Further, in the broader aspects of the invention, the mixing air could flow through the mixing chamber with little or no vortical component. These and other variations in the described embodiments will occur to those skilled in the art within the spirit and scope of the present inventive concepts which are to be derived solely from the appended claims.

Claims

Having thus described the invention, what is claimed as novel and desired to be secured by Letters Patent of the United States is:

1. A combustor system comprising:

a combustion chamber,

a mixing chamber opening into the upstream end of the combustion chamber,

a spray nozzle for discharging fuel as an atomized cone into said mixing chamber, the axis of said spray cone extending lengthwise of said mixing chamber toward said combustion chamber,

means for introducing pressurized air into the upstream end of said mixing chamber and producing an axial flow field for dispersing the fuel in fine droplets,

venturi means surrounding said spray cone and through which at least a portion of the axially flowing air passes, and wherein

the air introducing means additionally provide a vortical component to the air flow field,

the venturi means comprise a venturi tube mounted within said mixing chamber,

a cylindrical conduit and an axial swirler at the upstream end of the conduit define said mixing chamber,

said air introducing means includes an annular row of generally radially oriented passageways angled through said swirler,

said axial swirler has a central opening through which the discharge end of the spray nozzle projects, and

the inlet end of the venturi tube has a diameter intermediate the minimum and maximum diameters of the annular row of swirler passageways and is bonded to said swirler.

2. A combustion chamber as in claim 1 wherein the divergent discharge end of the venturi tube terminates generally as a tangent to the spray cone discharge from the nozzle.

3. A combustor as in claim 2 wherein:

a pressurized plenum is provided, in combination with the upstream face of said axial swirler, and

the swirler passageways meter air into the mixing chamber at a rate insufficient to produce a combustible mixture with the fuel introduced by said spray nozzle, and

means are provided for introducing further pressurized air into said combustion chamber sufficient to produce a combustible mixture therein.