Smoke reduction combustion chamber

Abstract

A combustion chamber either an annular or can-annular type, which substantially eliminates the production of smoke while maintaining all other performance parameters of the combustion chamber. The combustion chamber is provided with a front end configuration which substantially eliminates local fuel rich regions and which provides a means for mixing the incoming fuel and air.

Description

The invention herein described was made in the course of or under a contract with the Department of the Navy.

BACKGROUND OF THE INVENTION

The present invention relates to combustion chambers and more particularly to any type combustion chamber for a gas turbine engine, the construction of the combustion chamber being such that the fuel-air mixture is combusted in such a manner that the formation of carbon, and hence visible smoke emission is markedly reduced. This smoke reduction is accomplished while maintaining all other combustion chamber performance parameters, such as combustion efficiency, combustion stability, altitude ignition, and durability.

As background, it is well known that a combustion chamber for use in a gas turbine must possess certain characteristics in order to satisfactorily perform its function, and this is particularly true of a gas turbine engine employed in a jet aircraft. The combustion chamber must be capable of easy startup at ground level through a range of ambient air temperatures representing cold and hot weather conditions, that is, low fuel flows and short ignition delay time so as not to result in an explosive or hot start. In the case of a can-annular type burner which will be the type burner primarily discussed herein, after ignition of the fuel-air mixture in the burners that are equipped with spark igniters or some other ignition source, the flame must propagate to adjacent burners for a full lightoff and then accelerate to idle speed. The combustor must also have the capability of good stability limits, that is, operate satisfactorily at fuel-air ratios below and above the normal idle and rated thrust fuel-air ratios in order to insure that during transient conditions, such as acceleration and deceleration operational modes which can result in off-design fuel-air ratio levels, the burners will not flame out. An additional characteristic that the combustion chamber must have is that it must be capable of altitude ignition over a wide flight speed and altitude range without causing compressor stall or other penalties which would prevent the engine from being brought up to idle speed. Additionally, the combustion chamber, after reaching idle speed, must have the capability of being accelerated to higher power settings, and this must be accomplished in a relatively short time, normally within seconds. The combustion chamber must also have the capability of producing a satisfactory discharge temperature pattern or be capable of alteration to result in such a pattern without detrimental effect of the previously mentioned performance parameters, in order to achieve long life of parts receiving the hot discharge gases from the burner. Finally, the combustion chamber must provide an atmosphere of combustion wherein the fuel-air mixture when combusted does not result in the emission of visible smoke from the engine.

While many of the elements employed in the present invention are described in the prior art, for example, the Johnson patent, U.S. Pat. No. 3,134,229, the Schiefer patent, U.S. Pat. No. 2,974,485, the Panko patent, U.S. Pat. No. 3,352,106 and the Bachle patent, U.S. Pat. No. 3,018,625, it is pointed out that none of these particular references or the prior art in general solves the particular problem as the present invention does. As hereinbefore noted, in order to provide an acceptable combustion chamber for use in a gas turbine engine, it is necessary to maintain the performance parameters hereinbefore mentioned. Smoke emission has been a problem which the prior art combustion chamber constructions have accepted so as not to penalize or adversely affect these performance parameters. The present invention does not accept smoke emission and substantially eliminates smoke emission without any penalty to the performance parameters hereinbefore mentioned.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a combustion chamber which substantially eliminates smoke emission therefrom and which provides a combustion chamber which maintains the overall performance parameters which are acceptable for use in a gas turbine engine.

In general, the present day prior art gas turbine combustion chambers operate at lean overall fuel-air ratios, which are below the flammability limits of most normally used fuels. Therefore, in order to burn the incoming fuel and air, a region in the burner must be provided in which the fuel is mixed with the correct portion of air in order to initiate and sustain combustion over a wide range of operating conditions. This is normally done by controlling the airflow distribution into the burner as a function of burner length. Incoming air is therefore divided into primary air and secondary air, and the manner of injection, location and amount of primary air used, in large part, controls the smoke formation characteristics of a combustion chamber and is the main object of this invention. It is a main objective of this invention to control this effect.

As hereinbefore stated, a combustion chamber for a jet engine used in aircraft propulsion must possess acceptable stability and altitude ignition characteristics. Normally, in prior art combustion chambers, this is accomplished by setting up a zone of recirculation in the front end of the burner in which all of the fuel is mixed with only a portion of the total airflow. This zone is one of low axial velocity, and the large stable recirculation eddies formed in this region result in excellent burner performance in the aforementioned parameters, except that it is prone to producing excessive carbon and hence results in a highly visible exhaust. It is equally well known that carbon is formed in rich fuel mixtures; hence, the problem is one of lack of oxygen and intimate mixing of fuel and air in the front end and other local regions. It has been determined that to obtain a satisfactory reduction in smoke, increasing the airflow proportion in the front end is effective but that the amount required invariably severely compromises the stability and altitude relight capability. The present invention determines and provides a construction to reduce smoke while at the same time maintaining the necessary performance parameters which are acceptable for use in a gas turbine engine.

The present invention accomplishes the foregoing by a unique combustion chamber configuration. In the combustion chamber of the present invention a plurality of air tubes are provided at the closed end of the combustion liner. The size, number and location of these air tubes are optimized so as to provide a critical airflow into the combustion zone or zones of the combustion liner. The amount of air injected through these air tubes in the front of the combustion chamber is critical, it being necessary that the air tubes be sized so that they permit the passage of air therethrough to be a predetermined percentage of the total primary airflow which will lie in a range where the air tube sizing starts to get too large and permit flame "blowout" to where the air tube sizing starts to get too small and permit excessive smoking. In a combustion chamber configuration tested, the amount of air to be injected through these air tubes was computed and figured to be from 6-8 percent of the total airflow. Additionally, while not only is the amount of air flowing through the air tubes critical but also the amount of air flowing through the air tubes as positioned on the dome inner and outer walls is critical. Referring to FIG. 2, it can be seen that there is one air tube 66 attached to the dome inner wall for each individual fuel nozzle and hence recirculation zone 74. It is critical that there be at least one tube positioned on the dome inner wall for each nozzle. Alternatively, several tubes with an equivalent flow area, hereinafter described, may be utilized on the inner wall 62. The number of air tubes 68 in the dome outer wall are also critical; however, it has been determined that there be at least three but not more than four air tubes 68 to provide the required airflow into the individual combustion zones. Again, as with the inner wall air tubes, several more may be used so long as the critical flow requirements are satisfied.

To provide the required airflow through the air tubes for the configuration shown it has been found that air tube 66 on dome inner wall 62 should have an inside diameter or opening through which the air flows in the range of from 0.175 inch to 0.400 inch, and should be inserted into the burner liner a depth of 0.05 to 0.150 inch. The air tubes on the dome outer wall 60 should have an inside diameter or opening through which the air flows in the range of 0.200 to 0.425 inch and should be inserted radially to a depth of about 0.050 to 0.150 inch.

Claims

1. A combustion chamber comprising a housing, a liner supported by the housing and spaced radially therefrom, the liner having a substantially closed end and an open end spaced axially therefrom with a first wall means therebetween, the first wall means having a plurality of openings along its axial length, the liner providing a zone for combustion of a fuel-air mixture, the combustion products being discharged through the open end, and fuel nozzle means positioned at the closed end of the liner for supplying fuel to the combustion zone wherein the improvement comprises:

said fuel nozzle means including a plurality of fuel nozzles,

swirl vanes surrounding each fuel nozzle,

the closed end of the liner having a central opening therein,

inner wall means extending from the edge of said opening into said liner,

said fuel nozzles being located in said closed end spaced around said inner wall means,

air tube means positioned in said inner wall means and said first wall means downstream of each fuel nozzle and being substantially in axial alignment with the cooperating swirl vanes of the nozzle to direct air into the swirling air from the swirl vanes and the fuel from the fuel nozzle, and

said air tube means and said swirl vanes forming a recirculation zone therebetween.

2. A combustion chamber as set forth in claim 1 wherein:

said air tube means including first air tubes and second air tubes,

said first air tubes being positioned in the inner wall means to direct air from the central opening towards said first wall means,

said second air tubes being positioned in said first wall means for directing air from around the liner towards inner wall means, and

said flow of air from said first and second air tubes being directed into swirling airflow from the swirl vanes surrounding each fuel nozzle.

3. A combustion chamber as set forth in claim 2 wherein:

there are at least three but not more than four air tubes in said first wall means downstream of each fuel nozzle.

4. A combustion chamber as set forth in claim 2 wherein:

there is one air tube in the inner wall means downstream of each fuel nozzle.

5. A combustion chamber as set forth in claim 2 wherein:

there are at least three but not more than four air tubes in said first wall means downstream of each fuel nozzle, and

there is one air tube in the inner wall means downstream of each fuel nozzle.

6. A combustion chamber as set forth in claim 2 wherein:

the openings of the air tubes downstream of each fuel nozzle are radially opposed and axially offset with respect to the combustion zone, the openings of the second air tubes on the first wall means being further downstream than the opening of the first air tube on the inner wall means.