COMBUSTOR ASSEMBLY

Abstract: A combustor assembly, e.g. for a gas trubine engine, has fuel supply means including a fuel line (56) which discharges fuel into a rotatable tube (102) and cup atomizer (170). A porous baffle (180) prevents large fuel drops from reaching a conbustion zone in the conbustor. An orifice (176) at the end of the line (56) causes interaction between fuel discharged therefrom and buffer air, passing down the passage between the fuel line (56) and tube (102), to prevent the film of fuel formed on the cup from being disrupted, and the buffer air from penetrating to a hot gas recirculation zone in the combustor.

Description: COMBUSTOR ASSEMBLY The present invention relates to combustor assemblies, of the kind used, for example, in gas turbine engines. It is difficult to obtain good combustion, reliably, at reasonable cost, in small, liquid fuel combustors. This is because it is not possible, employing heretofore available techniques, to downscale fuel droplet size to match the combustor size. Instead, atomization of liquid fuels at the low flows which small combustors employ ordinarily results in larger fuel droplets and lower efficiencies. Also, the smaller fuel passages involved increase the probability of pluggingcand generate higher fuel pressures, shortening the life of fuel pumps, for example, and increasing service costs. The inventor has previously proposed that the foregoing, and other, drawbacks of small liquid fuel combustors can be avoided, and other advantages obtained, by employing a fuel supply system including a rotating cup atomizer. The employment of a rotating cup atomizer in a small liquid fuel combustor is advantageous because it eliminates the small passages of typical liquid fuel supply systems, and-the fuel pressure can be very low as it is needed only to effect flow to the combustor. Fuel can be readily atomized into small droplets of controlled size even at extremely low flow rates, even if the fuel is highly viscous. This is important because of the increased range of fuels that can be employed and because it permits the combustor to operate efficiently under very cold weather conditions. One salient feature of the fuel supply and rotating cup atomizing system previously proposed by the inventor is a novel buffer system which equalizes the pressure between the interior of the combustor and that on the upstream side of the fuel atomizer. This is important in that it keeps fuel from moving backward into the bearings of the rotary cup drive mechanism and consequently eliminates the deterioration which the association of fuel with the bearings causes. The inventor has since found that, in certain gas turbine engine applications, the flow of air in the buffer system can, at the highest flow rates under normal operating conditions, disturb the film of liquid fuel formed on the rotary cup of the liquid fuel atomizer and, also, enter the inner recirculation zone of the combustor. Disturbance of the fuel film is undesirable because that causes engine instability and surge. Entrance of the buffer air into the inner recirculation zone of the combustor is also undesirable, as that causes such deleterious effects as flame instability and smoke. Disturbance of the liquid fuel film formed on the rotary cup causes engine instability and surge because fuel droplets are torn off the film at an uncontrolled frequency. These droplets create significant variations in the fuel supply rate as they enter the combustion zone of the combustor. It has now been discovered that this unwanted state of operation can be avoided by providing means, particularly a perforate or porous barrier placed in the rotary cup of the atomizer, to intercept larger droplets of liquid fuel as may be separated from the liquid fuel film. In accordance with one aspect of the invention, a combustor assembly comprises a combustor having a combustion zone; and means for supplying fuel to the combustor, the means including a rotary cup type fuel atomizer having an inner surface facing into the combustion chamber, a fuel supply passage communicating at its downstream end with the inner surface, means for rotating the cup, in use, to spread the fuel supplied through the passage into-a film on the inner surface of the cup and to eject the fuel from an edge of the surface as an annulus composed of fuel droplets of controlled size, and means for intercepting larger drops of fuel and thereby keeping them from reaching the combustion zone. This arrangement-minimizes the problems associated with the disturbance of the fuel film, while at the same time allowing air to flow freely, permitting the buffer system to continue functioning as intended. The above discussed tendency toward disturbance of the liquid fuel film can be minimized by discharging the fuel from the line supplying the atomizer at low velocity through an orifice in that line into the air flowing through the buffer system toward the rotary cup of the atomizer and the combustion zone. This reduces the velocity of that air and, consequently, its tendency to disturb the film of fuel formed on the atomizer cup and the ability of that air to penetrate to the combustion zone of the combustor. According to another aspect of the invention, a combustor assembly comprises a combustor; means for supplying combustion air to the combustor; -means for supplying a liquid fuel to the combustion, the fuel supplying means comprising a fuel atomizer including a rotatable cup, a hollow shaft, bearings supporting the shaft and cup for rotation about an axis generally coincident with a longitudinal axis of the combustor, means for rotating the shaft and the cup, in use, to spread into a thin film on the inner surface of the cup a liquid fuel supplied thereto and then to eject the fuel from a downstream edge of the cup as an annulus composed of fine droplets of controlled size, means extending through the hollow shaft for delivering fuel to the atomizer; and means for preventing fuel from backing-up through the passage between the fuel delivering means and the shaft and consequently adversely affecting the bearings, the means for preventing fuel back-up compising air passage means communicating between the interior of the combustor and the interior of the hollow shaft at a location upstream from the rotating cup through which buffer-air can flow from the combustor into the shaft to reduce the pressure differential therebetween, and means for reducing the velocity of the buffer air flowing through the shaft. The beneficial effect of the novel combustor features forming the two aspects of the invention are to a large part independently obtained. However, maximum improvements in performance are obtained by the combination of those features which is accordingly preferred. The invention will now be explained in more detail by way of example with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section through a gas tubrine engine having a combustor that may be equipped with fuel supply and atomizing systems embodying the principles of the present invention; Figure 2 is a longitudinal section through the combustor of the gas turbine engine shown in Figure 1; and, Figure 3 is a longitudinal section through a rotating cup liquid fuel atomizer and the components of a liquid fuel supply system associated therewith, all constructed in accordance with the present invention. Referring now to the drawing, Figure 1 depicts a gas turbine engine 20 equipped with a combustor 22 that can be equipped with rotating cup atomizers constructed in accordance with the principles of the present invention. Engine 20 is a commercially available, Solar Turbines International Gemini. It will, accordingly, be described herein only to the extent necessary for an understanding of the present invention. Engine 20 includes a casing 24 housing a single stage radial compressor 26 and a radial, single stage turbine 28 mounted in back-to-back relationship on shaft 30. The compressor-turbine-shaft assembly is rotatably supported in casing 24 by bearings 32 (only one of which is shown). Combustor 22 is supported from casing 24 on the downstream side of turbine 28. Air enters casing 24 through an annular inlet 36 at its upstream end and flows in an axial direction past vanes 37 into compressor 26. The compressor discharges the air through a passage 38 between casing 24 and an inner jacket 40 past diffuser vanes 42 into an annular passage 44 between combustor outer and inner casings 74 and 76. Part of this air flows into the main combustion air passage 46 of the combustor 22 (see Figure 2) as indicated by arrows 48, and additional air is introduced into the combustor through secondary or dilution air- ports 50 as indicated by arrows 52 to dilute the hot combusted gases to acceptable temperatures when air in more-or-less sufficient quantities for complete combustion is supplied through the main combustion air passage. Where insufficient air for complete combustion is supplied through that passage additional air in an amount sufficient to complete combustion is supplied through air ports 54 further downstream as indicated by arrows 55 to dilute the combustion products. Air supplied to the combustor in the manner just described is also employed to cool those combustor components which are susceptible to overheating. Fuel is supplied to the engine through line 56, and an ignitor 58 is provided to ignite the fuel at light-off. Thereafter, the combustor operates as described abbve. The hot gases generated in combustor 22 are discharged into an annular plenum 60 and flow from the plenum through nozzle ring 62 into turbine 28 to drive the latter as indicated by arrows 64. Gases discharged from the turbine are exhausted through a manifold 66 as indicated by arrows 68 and 70. By virtue of the foregoing, turbine 28 drives compressor 26 and, in addition, generates additional energy which is available at shaft 30. Referring now to Figure 2 of the drawing, combustor 22 numbers, among its major components, a jacket formed by the outer casing 74, the inner casing 76, and a dome 78. Outer casing 74 has a longitudinally extending cylindrical section 80 with a transversely extending wall 82 at its upstream end. Inner casing 76 is a cylindrical member supported in concentric relationship in the cylindrical section 80 of the outer casing. Dome 78 is a circular component and is mounted in the upstream end of the inner casing. It cooperates with-the upstream end wall 82 of outer casing 74 to form a radially inwardly extending, main combustion air passage 46 referred to briefly above. In combustor 22, air reaches that passage from the axially extending, annular passage 44 between inner combustor casing 76 and the cylindrical section 80 of the outer casing as indicated by arrows 86. Disposed in main combustion air passage 46 adjacent its inlet 88, and extending between outer casing end wall 82 and dome 78, is a set of radially oriented, generally equiangularly spaced, longitudinally extending swirl vanes 90. The latter impart a rotational component air introduced into passage 46 through inlet 88 and flowing therethrough as indicated by arrows 92. The combustion -air is discharged from passage 46 through an annular orifice with sharp edges 94 and 152. in dome 78 in the form of a swirling, or rotating, axially moving annulus 150 of combustion air. Fuel supplied to combustor 22 through line 56 is discharged into a rotating cup type atomizer 96. The atomizer includes a cup 98 with a frustoconical inner face 100 facing downstream of the combustor and a hollow shaft 102 on which the cup is mounted. Shaft 102 surrounds fuel line 56. It is rotatably supported by bearings 104 and 106 in a casing 108 secured to an annular flange or bossll0 on outer combustor casing end wall 82. Shaft 102 and cup 98 are rotated by an electric motor 112. Fuel introduced into the combustor through line 56 is consequently spread into a thin, uniform film 114 on the inner face 100 of cup 98 and discharged from the periphery or downstream edge 116 of the latter at a location axially more or less coincidental with orifice 94 as an annulus of fine droplets of controlled size. The fuel supplied to the combustor is accordingly placed in atomized form on the inner boundary 118 of the swirling annulus 150 of combustion air at a precisely determinable location. Extending downstream from rotary cup 98 is a flame stratification zone 120 in which the atomized fuel in annulus 122 is vaporized. At the downstream end of flame stratification zone 120, the swirling or rotating, stratified annuli of combustion air and swirling vaporized fuel are rapidly expanded into a combustion zone 124 as indicated by streamline 126. The swirling fuel-air mass expands outwardly as it moves downstream through the combustion zone to an extent limited by the downstream section of inner combustor casing 76. Further downstream, the swirling or rotating gases are caused to contract inwardly by an annular, frustoconical-baffle 127 which tapers inwardly and toward the downstream end of the combustor and is secured in inner combustor casing 76. The annulus of swirling, hot gases ceases to exist as such at the downstream end of the combustion zone 124 although the rotational swirl of the gases continues as indicated by arrow 128. The aerodynamic flow mechanism just described results in the creation of an inner recirculation zone 130, which is the main flame stabilizing mechanism in combustor 22 and an annular, outer recirculation zone 132 which surrounds the swirling annulus 150 of combustion air at the upstream end of the combustor. Hot gases flowing upstream in the inner recirculation zone 130 as indicated by arrows 134 ignite atomized fuel in flame stratification zone 120. However, because mixing of the hot fuel and the cooler combustion air surrounding it is strongly inhibited by centrifugal force effects, only a small percentage of this fuel can be burned in the flame stratification zone. Nevertheless, this limited combustion is sufficient to evaporate the remainder of the atomized fuel and raise it to a very high temperature. Downstream from flame stratification zone 120, the bulk of the fuel burns in combustion zone 124. The combusted fuel-air mixture supplies the hot gases necessary for ignition to the inner recirculation zone 130 and to the outer recirculation zone 132. In a rotary cup atomizer as shown in Figure 2, fuel can back up through the passage (138 in Figure 2) between the fuel supply line and the cup-supporting shaft. This is disadvantageous because fuel can consequently reach the bearings in which the shaft is supported; and the fuel has a definite deleterious effect on the bearings. In combustor 22 this back-up of fuel is forestalled by installing a close tolerance seal 140 between the upstream end of shaft 102 and the fuel delivery line. Fuel back-up is further minimized by the air buffer arrangement shown in Figure 2. This includes a cap 142 which cooperates with casing 108 to form a plenum 144 on the upstream side of atomizer shaft 102 and an air line or duct 146 connecting primary combustion air passage 46 with plenum 144 through a discharge orifice 148. The arrangement just described more or less balances the pressure in plenum 144 with that in combustor 22. As a consequence, fuel is prevented from backing.up through passage 138. As indicated above, additional air needed to complete the combustion processes or to dilute the combustion products to an appropriate temperature can be supplied to the interior of combustor 22 through dilution air ports 50. More particularly, as suggested by arrows 86, this air is caused to flow upstream through annular passage 44 like that supplied through radial swirl passage 46. The secondary, or dilution, air then flows, as indicated by arrows 154, into an annular passage 156 surrounding the upstream part of inner combustor casing 76. Passage 156 is formed by the cooperation between casing 76 and an annular air flow guide 158 which surrounds the inner casing and is fixed to the latter on the downstream side of the secondary air ports. As the secondary air flows into the combustor, it is directed along the swirl flow contracting baffle or deflector 127 as indicated by arrows 160 by an annular flow guide 162. This component abuts the inner combustor casing on the upstream side of the dilution air ports and cooperates with the latter to form an annular flow passage 164 opening onto the interior of the combustor at its downstream end. Consequently, in addition to performing the functions described above, the air introduced through secondary ports 50 provides film cooling of deflector 127 and keeps it from overheating. The ports 54 for additional tertiary air are also illustrated in Figure 2. Air is, again, supplied to the interior of the combustor through these ports from annular flow passage 44. As indicated by arrows 166, baffle or deflector 127 performs an additional important function with respect to the tertiary air in that it deflects that air away from, and keeps it from entering,-inner recirculation zone-130. A combustor in accordance with the principles of the present invention can be obtained by substituting in that of Figures 1 and 2 an atomizer 168 with a rotary atomizer cup 170, as shown in Figure 3 for the cup 98 illustrated in Figure 2. Atomizer cup 170 differs from cup 98 in one respect by virtue of an inwardly extending lip or dam 172 at the frustum or upstream end of its inner cup face 100. This dam maintains a reservoir 174 of fuel in the atomizer cup. This is important because it contributes to the uniformity of fuel film 114 and because it makes fuel available during transient conditions, eliminating the small, trouble-prone, minimum flow orifice otherwise required to keep fuel flowing to the atomizer along with the tendency to plugging and other attendant problems of such orifice. Specifically, when the load is removed from a gas.turbine engine the fuel flow must be rapidly reduced to prevent overspeed; and the just mentioned minimum flow orifice is provided in the typical fuel control for this purpose. The creation of the fuel reservoir in the cup makes this orifice unnecessary because sufficient fuel can be supplied from it to keep the flame burning even though the supply of fuel to the atomizer is briefly interrupted in the circumstances just described. Also, the fuel is good insulation, and it keeps the atomizer cup 170 from overheating during the operation of the combustor in which the atomizer is incorporated. Referring still to Figure 3, as discussed above, the air buffer arrangement keeps fuel from backing-up through rotary cup atomizers such as that shown in Figure 2. In gas turbine engine applications that arrangement results in an air pressure in plenum 144 that is slightly higher than the air pressure inside the combustor 22 at light off conditions of the turbine engine 20. This pressure gradient increases substantially in normal engine operation due to the increase in air pressure and mass flow rate. This pressure gradient encourages a flow of air through passage 138 from plenum 144 into the inside of conbustor 22. This flow of air is controlled by the sizing of orifice 148 and by the location of air line or duct 146. Thus location of air line 146 in combustion air passage 46 toward the inner periphery of the free vortex air swirl generated by swirl vanes 90, as shown in Figure 2, results in a reduced flow of air, which is further reduced by reducing the size of the orifice 148. Location of the air line 146 radially outwardly results in an increase in air flow. This effect of the radial position of air line 146 on air flow -is a consequence of the fact that, in a free vortex air flow, the static pressure is proportionate to the radius of rotation. As a result of this air flow through passage 138 the fuel is prevented-from backing up through that passage as discussed above. Experience has shown that, to prevent fuel back-up through passage 138, theCflow of air through passage 138 can be substantial enough, under the conditions of highest air flow at normal engine operation, to disturb the fuel film on occasion and to penetrate into the inner reci;rculation zone 130 in the combustor. As pointed out previously, both of the foregoing states of operation are unwanted because-of the appurtenant problems such as engine instability and surge, flame instability, generation of smoke, etc. We have now found that the problems engendered by hgiher rates-of-flow of the buffer air can-be minimized, if not eliminated entirely, by injecting the fuel into the rotating atomizer cup 170 at low velocity and pressure through an orifice 176 located at or near the downstream or delivery end of fuel line 56. In consequence the buffer air, as shown by arrows 178 in Figure 3, flows through the fuel emanating from orifice 176 and, with a resulting reduced velocity, through hollow shaft 102 into the combustor. At the reduced velocity the buffer air is not apt to disturb the liquid fuel film 114 or to penetrate into the inner recirculation zone 130 of the combustor. Nevertheless, provision is preferable made in atomizer 168 for preventing the flow of fuel droplets generated by disturbance of the liquid fuel film reaching the combustor. Specifically, such flow of fuel droplets is prevented by installing a screen 180, or other barrier of perforate or porous material, in the hollow bore 182 of rotating cup 170 downstream from the delivery end 184-of fuel line 56. Screen 180 (which.spans bore 182) is positioned against a downstream facing, annular ledge 185 in outer rotating cup component 186. The screen is retained in place by the upstream end 188 of inner cup component 190 which presses the screen against ledge 185 (any suitable means can be used to secure inner cup component 190 to the outer component 186 of the cup). One suitable barrier that has been found satisfactory for the purposes discussed above consists of a fine mesh screen with a wire size of 0.381 mm. and openings averaging above 1.016 mm. in diameter i.e. width. Such a screen does not disrupt the fuel film 114 formed on face 100 of atomizer cup 170, and it allows the free flow of air through the screen while preventing any large droplets torn off the fuel film by the action of the air flow from entering the combustor. The mesh size of the screen is not critical, and other porous and perforate materials serve the same purpose. For example, a wad of fine wire wool has been substituted for the screen and has given equally satisfactory results.

Claims: C L A I N S. 1. A combustor assembly comprising a combustor (22) having a combustion zone (124); and means for supplying fuel to the combustor, the means including a rotary cup type fuel atomizer (168) having an inner surface (100) facing into the combustion chamber a fuel supply passage (56) communicating at its downstream end with the inner surface, means (112) for rotating the cup, in use, to spread the fuel supplied through the passage into a film (114) on the inner surface of the cup and to eject the fuel from an edge of the surface as an annulus (122) composed of fuel droplets of controlled size, and means (180) for intercepting larger drops of fuel and thereby keeping them from reaching the combustion zone. 2. An assembly according to claim 1, wherein the means for intercepting the larger drops of fuel comprises a porous or perforate barrier (180) mounted in the cup at a location spaced upstream from the edge of the cup from which the fuel is ejected. 3. A combustor assembly comprising a combustor means (44) for supplying combustion air,to combustor; means for supplying a liquid fuel to the combustor, the fuel supplying means comprising a fuel atomizer (168) including a rotatable cup (170), a hollow shaft (102), bearings (104,106) supporting the shaft and a cup for rotation about an axis generally coincident with a longitudinal axis of the combustor, means (112) for rotating the shaft and the cup, in use, to spread into a thin film on the inner surface of the cup a liquid fuel supplied thereto and then to eject the fuel from a downstream edge of the cup as an annulus composed of fine droplets of controlled size, means (56) extending though the hollow shaft for delivering fuel to the atomizer; and means for preventing fuel from backing-up through the passage between the fuel delivering means and the shaft and consequentially adversely affecting the bearings, the means for preventing fuel back-up comprising air passage means (146) communicating between the interior of the combustor and the interior of the hollow shaft at a location up-stream from the rotating cup through which buffer air can flow from the combustor into the shaft to reduce the pressure differential therebetween, and means for reducing the velocity of the buffer air flowing through the shaft. 4. An assembly according to claim 3, wherein the means for reducing the velocity of the buffer air flowing through the hollow shaft (102) comprises orifice means (176) in the fuel delivering means (56) toward the downstream end thereof through which fuel can be so discharged as to be intercepted by the buffer air, thereby creating a drag on the air. 5. An assembly according to claim 3 or claim 4, which also includes means (180) for intercepting larger drops of fuel formed upstream of a combustion zone of the combustor and thereby keeping the larger drops from reaching the combustion zone. 6. An assembly according to claim 5, wherein the means for intercepting the larger drops of fuel comprises a porous or perforate barrier (180) mounted in the cup at a location spaced upstream from the edge of the cup from which the fuel is ejected. 7. A gas turbine engine comprising a compressor (26); a turbine (28) for driving the compressor and for supplying useful mechanical energy; and a combustor assembly according to any one of the preceding claims in fluid communication with a discharge side of the compressor for heating air discharged from the compressor and for supplying hot gases to the turbine to drive the latter. 8. An engine according to claim 7, which includes means (90) for forming the compressor discharge air into a swirling annulus (150) towards the upstream end of the combustor; and wherein the rotary atomizer cup (170) is so arranged with respect to an upstream end of the combustor that an annulus (122) of fuel droplets is formed on the inner boundary of the annulus of combustion air.

Inventors: SHEKLETON, JACK ROSS
Applicants: SUNDSTRAND CORPORATION
Application year: 1983
Main classification: F23R3
IPC codes: F23R3, F23D11