Air Swirler For Gas Turbine Combustor

Abstract

An air swirler is provided in the head end of a gas turbine combustor such that, when reducing the smoke production, the flame will be stabilized and proper mixing of the combustion air and fuel will occur. The proper amount of swirl and airflow must necessarily be provided. Also provided in the air swirler are air sweeper holes which direct part of the combustion air across the face of the fuel nozzle, thereby preventing the buildup of carbon particles. Yet a further provision in the air swirler are gaseous fuel holes such that gaseous fuel may be burned by directing the fuel into the air swirler slots so that it is properly mixed with the combustion air.

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to gas turbine combustors and more particularly to an air swirler for the use in a gas turbine combustor.

In today's industrialized and motorized society, air pollution has indeed become a tremendous problem. Government is stepping in to curb the problem with appropriate legislation, while manufacturers are carrying on research in order to design pollution-free goods. The gas turbine industry is no exception and much research has been done in the field of pollution elimination from the gas turbine exhaust.

In the prior art, one method of reducing smoke in the gas turbine exhaust was through the use of an additive such as manganese. Additives of course increase the cost of the fuel, while oftentimes they do not increase the Von Brand smoke number appreciably. The smoke density or "Von Brand Reflective Smoke Number" is a measure of the amount of visible smoke in a flow from an exhaust stack. The numbers range from 0 to 100 with 100 being an indication of a smoke-free stack. It has also been suggested in the prior art to inject a finely atomized coolant into the primary zone of the combustor thereby reducing the primary zone temperature for a small increase in the Von Brand smoke number. An example of this method may be seen in U.S. Pat. No. 3,088,280 issued on May 7, 1963 .

Another method of reducing the smoke in a gas turbine exhaust is to treat the actual exhaust gases. Using exhaust gas scrubbers, as they are sometimes called, requires an additional amount of complex equipment in order to function properly.

The present invention, in addition to the particular air swirler design, relates to a method of reducing smoke by "leaning out" the primary zone of the combustion chamber, i.e., an additional amount of air in relation to the fuel supplied is added at the primary zone. It is known in the art that leaning out the fuel-air mixture reduces the amount of soot and smoke produced by the combustion process. When the attempt is made to lean out the primary zone or head end of the combustor, the problem exists that flame stability is reduced and possibly may be lost altogether. By the addition of the air swirler of the present invention around the fuel nozzle a free vortex flow is provided in the combustor and the flame front is effectively stabilized.

Another problem associated with gas turbine combustors is the buildup of carbon on the face of the fuel nozzle during operation. In the past the fuel nozzles had to be removed from each combustor and periodically cleaned. Of course, this required a shutdown of the gas turbine and necessitated disassembly of the combustor so that the face of each fuel nozzle could be cleaned. It would be desirable to have a fuel nozzle assembly which is self-cleaning during operation.

In gas turbine combustors where a dual fuel capacity is desired, that is, one which has provisions for the flow of both a liquid fuel and a gaseous fuel it becomes a problem to keep the gas holes clean while operating on the liquid fuel. It is desirable to keep the exit from the gas holes at a point where only clean air exists, that is, in proximity with incoming combustion air. The air passing through the slots of the swirler prevents products of combustion and oil from entering the gas side of the nozzle. It is also desirable to impart to the gaseous fuel a swirling characteristic to induce complete mixing in the combustor.

Accordingly, one object of the present invention is to stabilize the flame while leaning out the head end, thereby reducing the smoke in the gas turbine exhaust.

Accordingly, a second object of the present invention is to prevent the buildup of a carbon deposit on the face of the fuel nozzle during operation.

Accordingly, a third object of the invention is to provide an air swirler which also acts as the gaseous fuel nozzle.

SUMMARY OF THE INVENTION

Briefly stated, the present invention is practiced in one form by providing an improved air swirler around the fuel nozzle of a gas turbine combustor. The swirler is comprised of a body member which has slots machined therein so that a plurality of blades are formed around the circumference of the body member. The blades are formed such that the trailing edge surface has a definite measurable thickness. A plurality of air sweeper holes are provided in the body member such that a portion of the combustion air passing to the swirler blades is directed through the air sweeper holes and across the face of the fuel nozzle, thus preventing the buildup of carbon deposits. Also positioned in the body member in one embodiment are a plurality of combustion gas holes such that when the fuel nozzle is operating on gaseous fuel, the fuel enters the gas holes and is directed toward the slots of the air swirler. The gaseous fuel is swirled together with that part of the combustion air which passes through the air swirler slots thus preventing buildup around the gas holes.

DRAWING

FIG. 1 is a front view looking at the face of the swirler.

FIG. 2 is a partial plan view showing the relative dimensions of the air swirler blades.

FIG. 3 is a view, in section, taken along lines III--III of FIG. 1.

FIG. 4 is a view, in section, of the air swirler with its associated fuel nozzle, both positioned in the combustor cap.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, an air swirler is generally shown at 1 and is comprised of a body member 2 from which are machined a plurality of individual blade members 3. It will be appreciated that when the blade members 3 are formed, there are also formed a plurality of slots 4 about the circumference of the body member 2. Although it has been mentioned that the blades 3 are machined into the body member 2, any suitable means of forming the slots 4 in blade members 3 may be utilized. One criterion for forming the blade members 3 is that the trailing edge or downstream surface (trailing edge with respect to the combustion airflow through the air swirler) indicated at 5 has a definite measurable dimension as opposed to a tapered edge. The reason for providing the trailing edge or downstream surface 5 with such a configuration will be more fully described later. A fuel nozzle hole 6 is positioned generally at the geometric center of the body member 2 in order to accommodate the fuel nozzle 7. The relation of the fuel nozzle 7 to air swirler 1 may be seen by reference to FIG. 4.

Indicated as 8 on FIG. 1 are the air sweeper holes which are utilized to keep the face of fuel nozzle 7 free from the buildup of carbon particles which normally accompanied the operation of a prior art gas turbine combustor. Air sweeper holes 8 extend generally at an angle from the inner face 9 of the slots 4 to the upper or downstream face 10 of the body member 2. The angle of the air sweeper holes 8, which is more clearly indicated in FIG. 3, is on the order of 25.degree. - 35.degree., with 30.degree. being the preferred angle in order to reduce the most amount of carbon buildup. This angle formed with a plane normal to the swirler axis is indicated as .alpha. in FIG. 3. As the combustion air begins to enter the slots 4, which usually represents from between 5 and 10 percent of the total combustion air, a small portion of this air will naturally flow into the air sweeper holes 8 due to the pressure drop across the body member 2.

The dimensions of the slots 4 and blade members 3 are critical and present themselves to be defined as certain ratios which must be met in order to provide the proper amount of swirl air to accomplish the objects of the invention. If too much air is allowed to enter through the slots 4 by providing a distance between blades 3 which is too great, it is possible to create such a lean head end that combustion cannot be supported. Also evident, if too much air is passed through the slots 4, is inefficient mixing of the combustion air and fuel. This critical ratio presents itself as the ratio of the span of the blade face indicated as A to the width of the slot indicated as B. This ratio A/B, must be within a range of from 1.15 to 1.85 when the diameter of the combustion chamber is on the order of 15 inches so that the required 5- 10 percent of combustion air will pass through slots 4.

Another critical dimension associated with blade members 3 is the angle .gamma. at which the blades 3 are positioned from a plane normal to the swirler axis. It is this angle .gamma. which determines the amount of swirl which is imparted to the combustion air passing through slots 4. If angle .gamma. is too small, thus causing too much swirl, the mixture of combustion air and fuel will form a free vortex with too much strength and throw fuel on the combustor walls (not shown). Carbon will also be built up on the face of the fuel nozzle. If the angle .gamma. is too large, not enough swirl will be imparted to the entering combustion air. Not enough swirl provides insufficient mixing of the combustion air and fuel. It has been found that angle .gamma., for the proper amount of swirl, should be maintained between 55.degree. to 65.degree. with the desired angle at 60.degree..

Another important aspect of this invention is the fact that the air swirler may be adapted to actually become the gaseous fuel nozzle, even when the standard fuel nozzle 7 is in place. A plurality of individual gas holes 11 are positioned in the body member 2 of the air swirler 1 and extend from the bottom or upstream face 12 of body member 2 on the inside of gas wall or mounting member 13 generally to the inside face 9 of the slot 4.

In referring to FIG. 1, it will be apparent that the gas holes 11 provide an outlet for the gaseous fuel in every other slot 4. The angle .beta. at which the gas holes 11 are positioned in the body member 2 is critical and should fall within a range of from 25.degree. to 65.degree. with the preferred angle measured from a plane normal to the swirler axis being 35.degree..

In referring to FIG. 4, wherein the fuel nozzle 7 is shown in combination with the air swirler 1 at the head end of the gas turbine combustor, the fuel paths leading to the combustion chamber are indicated. In the head end it will be appreciated that either a liquid fuel or a gaseous fuel may be burned at any one time. The fuel nozzle 7 in combination with the air swirler 1 in this sense, forms a dual fuel nozzle. Fuel nozzle 7 is comprised of an outer wall 14 which, together with the mounting member or gas wall 13, forms the passage 15 which communicates with the gas holes 11 thus essentially forming the gaseous fuel nozzle.

When it is desired to use liquid fuel alone, the fuel nozzle 7 will be operative. The liquid fuel enters an inner chamber 16 and thence flows through nozzle 17 where it is atomized by the flow of atomizing air (when used) which flows up through a circumferential sleeve 18 formed by the outer wall 14 and an inner wall 19. The atomizing air finely atomizes the liquid fuel as it enters into the gas turbine combustor as indicated on FIG. 4.

Also indicated in FIG. 4 is a portion of the combustor cap 20 which in actuality extends further outward until it meets the combustor liner (not shown) which is usually in the form of a cylindrical tube with combustion air holes disposed therealong. The combustor cap 20 is mounted on a circumferential mounting ring 21 which, during operation, is juxtaposed against the outer faces of blade members 3 such that the slots 4 provide the only entry for the 5 to 10 percent of combustion air which passes through the air swirler 1.

OPERATION OF THE INVENTION

As stated in the background of the invention, there are essentially three different aspects of this invention and the operation of each will be described separately. The operation of the invention in relation to the reduction of smoke produced by the combustor will be described first. One way to reduce smoke is to lean out the combustion air-fuel mixture such that the fuel is more completely burned; thus leaving a smaller residue of soot particles and the like. When attempting to lean out the head end, it becomes necessary to stabilize the flame, thus resulting in a shorter and more constant flame length. As the 5 to 10 percent of combustion air passes through the air swirler, a core of swirled air together with the atomized fuel, is formed down the center of the gas turbine combustor as indicated in FIG. 4. As previously mentioned, the core is a free vortex with the pressure being lower towards the fuel nozzle, thereby allowing the other combustion air which enters through the liner (not shown) to interact with the vortex, thus sweeping out the fuel rich pockets and providing complete mixing of the atomized fuel and combustion air.

In the operation of the air sweeper holes, which has already been partially described, a portion of the air which passes into the combustor through the slots is directed at an angle toward the face of the fuel nozzle. Since there is a pressure drop across the air sweeper holes as well as the reduced back pressure exerted from the free vortex, the sweeper air will pass directly across the face of the fuel nozzle and will tend to prevent carbon formation by its sweeping action. As previously mentioned, the trailing edge surface 5 has a definite measurable thickness. Due to the reduced back pressure some of the combustion air which enters through the liner will tend to flow inwardly across the trailing edge surface (as seen in FIG. 4) thereby preventing residue buildup.

The operation of the gas turbine combustor on liquid fuel or gaseous fuel alone, has also been partially described. As the gaseous fuel flows up through the gas passage and through the gaseous fuel holes, it enters the combustion chamber at a point where the swirling air combines with it to provide complete mixing. Not only does this action provide complete mixing, but it keeps the gaseous fuel holes clean by the air passing through the swirler slots.

It will thus be appreciated that an air swirler for use in a gas turbine combustor has been described which provides a stabilized flame while leaning out the head end as well as air sweeper holes to keep the face of the fuel nozzle clean, together with the gaseous fuel holes to provide a second fuel mode if desired.

Claims

What is claimed is:

1. An air swirler for directing and metering a generally axial flow of combustion air in the head end of a gas turbine combustor, comprising:

an annular body portion, including an upstream face and a downstream face normal to the swirler axis, and defining a central hole for accommodation therein of a fuel nozzle; and

a plurality of angled blade members disposed about the circumference of said body and forming slots therebetween, the ratio of the span of the blade face to the width of the slot being within the range of 1.15 to 1.85.

2. The air swirler as recited in claim 1 further including a plurality of gas holes extending generally both in a radial and axial direction relative to the swirler axis, and communicating the upstream face of said annular body with the slots, and, arranged so that when operating on gaseous fuel, the gas is directed radially into the slots where it is swept by the combustion air thereby maintaining the slots free from residue buildup.

3. An air swirler according to claim 1 in which the blade members are maintained at an angle of from between 55.degree. to 65.degree. measured from a plane normal to the swirler axis.

4. An air swirler for directing a generally axial flow of combustion air in the head end of a gas turbine combustor, comprising:

an annular body portion including an upstream face and a downstream face normal to the swirler axis and defining a central hole for accommodation therein of a fuel nozzle,

a plurality of angled blade members, having an angle of from between 55.degree. to 65.degree. measured from a plane normal to the swirler axis, disposed about the circumference of said body and forming slots therebetween, the ratio of the span of the blade face to the width of the slot being within the range of 1.15 to 1.85,

a plurality of gas holes, extending generally both in a radial and axial direction relative to the swirler axis, and communicating the upstream face of said annular body with the slots, and, arranged so that when operating on gaseous fuel, the gas is directed radially into the slots where it is swept by the combustion air thereby maintaining the slots free from residue buildup.