Combustor

Abstract

A three-chamber assembly is used for the control of combustion in a gas turbine engine having a conventional supply of fuel and compressed air. Uniform, homogeneous mixing, preferably on a molecular level, takes place in a first chamber, combustion in the second with restriction of the combustion flame thereto, and quenching in the third chamber. Vortex flow is utilized in one or more of the chambers. The combustion temperature is lowered without sacrifice of combustion efficiency to provide control of undesirable exhaust emissions.

Description

BACKGROUND OF THE INVENTION

Continuing studies have indicated that nitrogen oxide emissions from gas turbine engines remains relatively high if the temperature of combustion is allowed to remain relatively high. However, the same studies have shown that the expedient of reducing the combustion temperature by limiting the fuel input rate undesirably lowers overall combustion efficiency. Other efforts have been solely directed at quickly reducing the temperature of generated combustion products to inhibit nitrogen oxide, but have met with only a limited degree of success. Accordingly, there is a need for an improved chamber assembly which can maintain combustion efficiency at a high level while at the same time reducing the emission of nitrogen oxides to a negligible level.

Vortex combustor designs have been given serious consideration because of the promise of increased combustion efficiency over a wide range of operating conditions experienced in production vehicles. Although the vortex combustor design is known, its operating characteristics will be briefly reviewed to provide for a full understanding of the improvements taught by the invention herein. A typical vortex combustor has compressed air supplied in a direction tangentially to a typical cylindrical chamber closed at one end, the pressure energy being converted into a tangential velocity as the body of gas swirls about the interior thereof gradually decreasing its swirling radius. The gases finally flow out through the open end near the axis of the swirling vortex. The moving gases within the vortex chamber can be discriminated: a natural vortex is generally disposed near the peripheral interior area of the chamber where the components of the gaseous mixture, such as fuel and air, experience a relative motion therebetween; another vortex body, commonly referred to as "forced," occurs at the axial region of the chamber and rotates like a solid gaseous cylinder having no relative motion between the fuel and air. It is known that the mixture within the forced vortex can be easily ignited and produces a very stable rotating columnar flame.

The relative motion between the gaseous elements of the natural vortex is both advantageous and disadvantageous. One of the more important advantages is the inducement of greater mixing between fuel and air promoting increased efficiency of combustion for which the vortex combustor design is generally known. The mode of mixing occurs because the tangential velocity distribution along the radial direction takes a hyperbolic form. Thus any hypothetical lump of gaseous fuel is subject to different shear forces at the different radii of the lump and deforms into a stretched ligament which ultimately atomizes into finer particles. This mode of mixing is not present in the forced vortex because the tangential velocity distribution is to the contrary and is linear. In a single vortex chamber, the natural vortex begins to promote uniform homogeneity, but cannot do so to satisfactory levels because of the momentary time dwell of the mixture therethrough before being combusted.

One of the disadvantages of the vortex combustor is the inability for the combustion flame to proceed smoothly from the forced vortex into the natural vortex while remaining stable and totally within the combustion chamber. This disadvantage can be overcome to some degree by critically maintaining the air/fuel input ratio within a very narrow range. This limitation prevents the vortex combustor from achieving practical utility under its promise of higher combustion efficiency. Accordingly, there is a need for overcoming the deficiencies of a vortex combustor in such characteristics as limited operating efficiency and need for further reduction of nitrogen oxides without hindering its relatively trouble-free construction.

As to mechanisms utilized by the art to lower per se the emission content of combustion products, independent downstream quenching has been used in the hopes that this alone would serve to prevent the formation of undesirable nitrogen oxides. Although this has helped to some limited degree, it has not totally solved the fundamental problem of the initial formation of the nitrogen oxide compounds.

SUMMARY OF THE INVENTION

To accommodate the problem of this invention, the invention has a three-fold aspect wherein there is first a discovery that a more homogeneous intimate mixture must be provided between the vaporized fuel and air thereby preventing nonuniform combustion temperatures from occurring within the combustion chamber, and particularly pockets of combustion at elevated temperatures above 3,000.degree.F which lead to immediate formation of nitrogen oxides. The temperature of combustion is related to the air/fuel ratio within the combustable mixture, and therefore, if the mixture is nonuniform, the uncontrolled and undesirable portions of the mixture will produce a high content of nitrogen oxide.

The second aspect or discovery is that the mixing capability of the natural vortex within the combustion chamber is not sufficient by itself to provide for the homogeneous uniform mixture required for appropriate combustion, but that it can be used to augment and complement this function while additionally serving to stabilize the combustion flame and act as insurance that combustion will not take place outside the preferred combustion zone.

Relatively restrictive exit orifices have been used with vortex combustor chambers for the purpose of cooperating with the vortex flow to provide stability. As a third aspect or discovery of this invention, the size of the orifice has been found to be expandable by controlling the strength of the natural vortex for promoting combustor gas recirculation to sustain combustion and controlling the downstream quenching medium in the form of a circulating vortex which restricts the propagation of flaming combustion.

Construction features which implement the above inventive aspects comprise the novel use of a three-chamber assembly for the combustor unit, the first chamber providing for controlled turbulence to intimately and homogeneously vaporize and mix fuel and air, a second chamber for actual combustion utilizing the vortex principle for additional controlled turbulence to insure final atomization of the mixture, and a third chamber for quenching also using controlled turbulence. The mixing chamber is constructed with consideration not only to the dynamics of the flow therein but also to the time dwell therein. Communicating means is constructed between chambers so that flame propagation back into the mixing chamber is prevented, as well as prevention of propagation into the quenching chamber.

SUMMARY OF THE DRAWINGS

FIG. 1 is a central sectional view of a chamber assembly of this invention showing also the fuel and air supply as well as the ignition system.

FIG. 2 is a partial sectional view taken along line 2--2 of FIG. 1; and

FIG. 3 is substantially a schematic illustration of an alternative embodiment.

DETAILED DESCRIPTION

Referring to the drawings, the preferred embodiment of the chamber assembly of this invention is illustrated in FIGS. 1 and 2 having a plurality of chambers A, B, C interconnected, the most forward or upstream chamber A providing for mixing of the charge elements, the middle chamber B providing for combustion and lastly the exit chamber C (which is ultimately connected to the turbine of the engine) provides for cooling and general quenching of the combustion products.

More particularly the mixing chamber A has a skirt portion 1 which envelopes the combustion chamber B and a forward portion 2 which is reduced in section or necked to define a central entry zone to the mixing chamber. As best viewed in FIG. 2, the skirt portion is defined by an outer sheet metal wall 3 spaced a radial distance 4 from a cylindrical sheet metal stamping 5 provided with flanges 6 extending from the edges 7 defining circumferentially spaced openings 8 therein. The openings 8 constitute an outlet for the mixing chamber and further cooperate as a specialized communicating means to the combustion chamber as later described. As best viewed in FIG. 1, a dish-shaped wall 9 extends across and is commensurate with the left-hand opening of the sheet metal stamping 5; the convex surface 10 of the wall serves as a diverter in regulating the flow within the mixing chamber. The upstream end of the sheet metal cylindrical wall 3 is reduced in diameter to define portion 2 and has an interior contour 11 effective to stimulate turbulence in cooperation with other interior walls of the mixing chamber. The necked portion is closed by member 12 having a central axial sleeve 13 (taken relative to the axis 14 of flow of the entire assembly) through which a fuel injection nozzle 15 extends for introduction of liquid fuel in droplet form or the equivalent. The nozzle is supported by biased ring assembly 15a movable against a seat 13a of the sleeve 13. An air inlet 16 is provided in said member 12 and is defined by annular vaned assembly 16a surrounding the fuel injection nozzle 15.

Turning now in particularity to the combustion chamber B, it is comprised of a ceramic cylinder 17 having a smooth interior cylindrical wall 18 with an axis aligned with the general axis 14 of the assembly; the upstream or left hand portion of the ceramic cylinder is closed by dome wall 19 having a convex surface 19a effective to cooperate with the vortical flow therein. A plurality of openings or slots 20 are defined in the cylindrical wall 18 at spaced circumferential positions and are generally aligned with the axis of the chamber. Each opening 20 constitutes an inlet for the combustor chamber and each has sidewalls 20a aligned with a chord 21 of the interior diameter of the chamber (best viewed in FIG. 2). The interior surface 6a of flanges 6 carried by the previously noted stamping 5 constitute extensions of the slot walls 20a defining a communicating means d so D the gaseous mixture, being introduced therethrough, is given a tangential velocity or guidance within the combustor chamber promoting a swirling or vortex flow pattern. The distance between walls of a single slot may occupy an arc of approximately 10.degree. to 12.degree. and each slot is preferably aligned at an angle of about 65.degree. relative to a radius 22 of the chamber. Slots 20 have a longitudinal extent substantially commensurate with the length of the combustor chamber. The combustor chamber has a single outlet 23 defined by an annular wall 23a projecting inwardly from the interior wall 18 in a general direction perpendicular to the axis 14. The central circular opening 24 in the wall 23a defined by edge 23b a control orifice whereby gaseous elements can be compressed and then rapidly expanded upon passage through the orifice, such flow control accelerating the particles of the gaseous mixtures so that there is a release from the combustor chamber at a velocity above that which would allow the flame to follow.

Although the interior configuration of the quenching chamber C can take a variety of forms, including a generally flared configuration, the preferred embodiment utilizes a cylindrical wall 25 formed integrally as a ceramic extension of cylinder 17 and has a diameter somewhat larger than the interior diameter of the combustion chamber. Wall 25 is smoothly connected with the annular wall 23a (defining said orifice) by an interior shoulder 26. The entire ceramic unit constituting the combustor and exhaust chambers is supported in an outer housing 28 which also serves to define a flow passage 29 for secondary air received from a common compressor source 30 which enters the housing at locations in the upstream portion of the housing to provide also for primary air. The incoming air is divided between the inlet 16 to the mixing chamber and the passage 29. In passage 29, air passes along between the sheet metal cylindrical wall 3 and housing 28 to enter through inlet 31 having a plurality of openings 31a circumferentially arranged about and in the wall 25 of the exhaust chamber and thereby defining a communicating means E in conjunction with outlet 23. To vary the division of compressed air flowing between the inlet 16 to the mixing chamber and inlet 31 to the exhaust chamber, a variable control mechanism 33 may be employed which varies the size of the openings 31a. A suitable mechanism here comprises a slidable sleeve 34 actuated by elements 35 for movement along the axis 14 of the unit.

In operation, air (compressed to approximately four atmospheres and having a temperature of approximately 1,000.degree.F) is conveyed to the housing 28 of the combustor unit. Such compressed air divides preferentially between inlet 16 and passage 29, that entering inlet 16 is immediately guided into a toroidal swirl resulting from conversion of the pressure energy into a tangential velocity component creating a controlled turbulent flow pattern with flow impinging against the interior surfaces 10 and 11 of the mixing chamber. The secondary division of the air supply moves along the interior of housing 28 and enters the exhaust chamber by way of circumferentially arranged inlet openings 31a.

In the primary air flow of the mixing chamber, liquid fuel is disbursed by the fuel nozzle 15 and is quickly vaporized and stimulated to mix on a molecular level, being afforded a sufficient time dwell to accomplish improved diffusion of the atomic elements of air into the atomic elements of the vaporized fuel. The time dwell is promoted by having the flow path between the inlet 16 and outlet 8 of an irregular non-laminar character whereby flow is not only turbulent but tortuous to promote homogeneity. Upon passing through the communicating means D into the combustion chamber, the intimate mixture of fuel and air achieves further homogeneity and ultimately attains desired uniformity by supplementary making of the natural vortex within the combustor chamber B.

In conventional gas turbine systems the air/fuel ratio, at the point of combustion, is approximately 15:1 when supporting optimum combustion efficiency. This invention contemplates maintaining air/fuel ratios on the order of 40:1 (and in some practical applications about 44:1) in a substantially uniform homogeneous condition throughout the entire mixture ultimately combusted. Experiments have shown that very peak combustion efficiency of a vortex combustor is attained at air/fuel ratios between 25:1 and 30:1. However, this invention optimizes emission control of nitrogen oxides by use of a higher but limited air/fuel ratio to maintain the combustion temperature at a level at or below 2,800.degree.F and preferably about 2,000.degree.F. This is accomplished with only a slight drop in efficiency from that obtained at the peak ratios between 25:1 and 30:1.

With the intimate homogeneous mixture and improved air/fuel ratio, the combustion flame will appear as a glow throughout the entire combustor chamber when ignited by suitable ignition means, here shown as an ignitor element F disposed near the interior periphery of the combustor chamber. Propagation of the flame back through the communicating means D is prevented by controlling the flow of gaseous mixture so that it is at a higher rate than the back propagation rate of the flame. Such flow can be varied to attain this objective by regulating the division between primary and secondary flow as mentioned earlier.

Similarily the orifice defined by the outlet 23 from the combustor chamber functions to compress and expand the combustion products so as to accelerate the particles thereof in a manner to release and exit from the chamber at a velocity considerably higher than the propagation rate of the flame within combustor chamber and such constriction secondarily serves to guide and stimulate the formation of the natural vortex, again supporting supplementary mixing within the combustor chamber.

To illustrate the equivalence to which the inventive features should be entitled, an alternative embodiment is illustrated in FIG. 3 wherein the internal flow within the mixing chamber provides an "aerodynamic" dome for the combustor chamber which lacks a positive mechanical wall at the conventional end where a dome wall normally occurs. The aerodynamic dome is created by the formation of the very strong vortical flow within the mixing chamber. This can be obtained by providing the necked portion 36 of the mixing chamber A with a plurality of inlet ports 37 which provide a tangential component to the entering primary air substantially throughout a greater area than that obtained with the swirl vanes of the preferred embodiment. This particular design increases the degree of turbulence in chamber A and provides a more direct flow path into the combustor chamber B thereby eliminating possibilities of super heated sheet metal elements. Ignition can be accomplished by an element 46 disposed centrally in chamber A, but combustion is restricted to chamber B by controlling of primary air in excess of the rate of back propagation of the flame.

Instead of using primary air for stimulating the vortex flow pattern in the combustor chamber B, secondary air is employed passing along the exterior of the ceramic unit 41 (defining all the chambers) and housing 28. Secondary air is introduced by a plurality of circular openings 38 in the outer wall 39 of the combustor chamber B. The flow through openings 38 serves to stimulate and maintain a vortex flow comparable in lesser degree to that contained in the preferred embodiment, keeping in mind that the primary flow enters at 42 with a previously imparted vortical flow pattern.

In the embodiment of FIG. 3, the mechanical orifice controlling the release of combustion products in the combustor chamber is eliminated and the flow pattern of the combustor vortex is utilized as a "aerodynamic" orifice 43 or control whereby gaseous products of combustion are released only upon having entered the forced vortex region 40 of the combustor chamber and are accelerated to leave at a velocity in excess of that flame propagation. The aerodynamic orifice 43 can be implemented by tangentially directing a strong secondary air flow through slots 44 adjacent the orifice 43 location.

Although not shown, a third flow of secondary air can be utilized for quenching purposes downstream from orifice 43 of the combustion region B and can be introduced through slot openings 45 creating a swirl therein.

Claims

We claim as our invention:

1. A chamber assembly for use in a continuous combustion process effective to provide low NO.sub.x emissions, comprising:

a mixing chamber having an inlet, an outlet and means interposed between said inlet and outlet for directing a non-laminar controlled turbulent flow of compressed air therethrough,

fuel supply means for continuously adding fuel to said flow in said mixing chamber at a location adjacent to said inlet and at a rate to provide for a substantially homogeneous gaseous air/fuel mixture passing through said outlet,

a combustor chamber in communication with the outlet of said mixing chamber for continuously passing said air/fuel mixture therethrough and having a path for said mixture which is significantly longer than the path of said mixture through said mixing chamber,

ignition means effective to ignite said mixture for sustained flaming combustion in said combustor chamber,

means for tangentially introducing a flow of compressed air into said combustion chamber for inducing vortical flow therein,

said induced vortical flow being effective to define an aerodynamic orifice for effecting accelerated release of combustion products from said combustor chamber, and

means for introducing a vortical flow of quenching medium into said exhaust gases from said combustor chamber.