GB2073399A

GB2073399A - Dual premix tube fuel nozzle

Info

Publication number: GB2073399A
Application number: GB8110151A
Authority: GB
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1980-04-02
Filing date: 1981-04-01
Publication date: 1981-10-14
Also published as: GB2073399B; JPS56168018A

Abstract

A dual premix tube fuel nozzle for a gas turbine engine serves to premix and prevaporize the fuel with air prior to entry into the combustion zone. Combining both the primary and secondary fuel systems 22 and 32 and the primary and secondary air introduction ports 28, 34 into a single nozzle 14 permits burning in a single burning zone resulting in a shorter length burner. It is contemplated that the air 34 to the secondary fuel system 32 is modulated as a function of an engine operating condition. <IMAGE>

Description

SPECIFICATION Dual premix tube fuel nozzle Technical Field This invention relates to combustors for a gas turbine engine and particularly to the combustor fuel distribution system.

Background Art Because of the high demands and the desire to improve the ecology, a major effort is currently being undertaken to reduce gaseous pollutants and smoke issuing from aircraft engines. An outgrowth of this effort was the utilization of premix fuel injectors, where the fuel and air is premixed prior to being injected into the combustor. A fuel system exemplary of a premixed fuel air fuel system for a gas turbine engine is described and claimed in U.S. Patent 4,179,881, granted to J. E.

Faucher, R. W. Koucky and W. D. Roy on December 25, 1 979 and assigned to the same assignee as this application. Because of the difficulty in reducing emissions over a wide range of operating conditions, it has been found necessary to design the combustor with two separate combustion zones to achieve effective emissions control for both the high and low power operating conditions: A fuel system exemplary of a two zone burner is disclosed in U.S. Patent Application Serial No. 712,575 entitled IMPROVED PREMIX COMBUSTOR ASSEMBLY filed by R. L. Marshall and K. A. Cashman on August 9, 1976, now under appeal, and also assigned by the same assignee as this application.

Typically the combustor utilizes primary or pilot fuel nozzles that historically are continuously operable for the purposes of idling the engine and serving as a pilot flame for the secondary combustion zone. The secondary fuel nozzles disposed axially from the primary nozzles, operate only during the higher thrust levels. Obviously, such a combustor designed so that the required fuel/air mixture for the secondary fuel system, which normally accepts the highest percentage of fuel runs lean and hence evidences a reduction in high power emissions of smoke and oxides of nitrogen.

By virture of having separate burning zones, the size of the combustor must be sufficient to accommodate both zones. Additionally, attendant with the separate burning zone concept is the complexity necessitated by separate fuel nozzles, separate fuel manifolds, additional openings in the engine case to accommodate this extra hardware, and the like.

We have found that we can shorten the length of the combustor by burning in a single burning zone. In addition, we can reduce the complexity of the fuel system of a combustor by combining the primary and secondary fuel systems into a single fuel nozzle and by concomitantly introducing control air through the nozzle into the combustion zone without impairing the emission characteristics of the combustor.

According to this invention, the fuel nozzle mounted in the combustor, preferably in the front end, comprises two concentric premix tubes in which fuel is partially premixed and prevaporized prior to injection into the combustion zone. Each premix tube is supplied by an independent fuel system and only the inner tube is fueled at idle conditions. Swirling flow generated at the inlet of each tube distributes the premixed fuel/air mixture and stabilizes the combustion process. The fuel split between the inner and outer tubes is varied by the fuel control in a known manner to control the fuel distribution for maximum reduction of emissions over the entire range of engine operation. The swirler in the inner tube is sized to provide a fuel-rich mixture in the tube.To achieve low CO and THC emissions at idle, it is desirable to burn at an equivalence ratio near unity (stoichiometric fuel/air ratio). Mixing of the inner tube flow with surrounding combustor front end air will lean the inner tube mixture and cause burning to occur at an overall fuel/air ratio near stoichiometric.

At power settings above idle, the inner tube remains fueled and the outer annular premixing tube is put into operation. The airflow in the outer tube is sized to create a lean overall combustion zone equivalence ratio of approximately 0.7-0.8 at high power to minimize NOx production as is apparent. Fuel distribution between the inner and outer tubes can be regulated to minimize NOx emissions while retaining high combustor efficiency (low CO and THC) at all power settings. This invention contemplates that the air to the secondary or outer premixing tube is modified, say between full open to closed, so as to achieve the proper fuel/air mixture for all operating conditions. Preferably, the airflow will be regulated as a function of power lever setting.Hence, this geometry of the inlet of the radial flow swirler is changed by a cylinder that translates in the direction of the tube axis, or is concentric and rotatable.

Additionally, length and contour of the premixing tubes can be varied to improve fue I/air premixing, within constraints imposed by the necessity to prevent auto-ignition in the tubes. Treatment of the tube exit plane with vortex generator, trips, etc., can also be included to modify combustor aerodynamics and/or mixing for emissions control.

Other features and advantages will be apparent from the specification and claims and from the accompanying drawings which illustrate an embodiment of the invention.

Brief Description of the Drawings The sole Figure is a partial view, partly in schematic and partly in section iilustrating this invention.

Best Mode For Carrying Out The Invention As will be obvious to one skilled in this art, this invention has utility in all types of turbine engine combustors as for example, annular, can-annular or can types. The essence of the invention is the utilization of concentrically mounted premix tubes, each having their individualized fuel injection system for improved emission control. While pressure atomized fuel nozzles may be employed, the fuel dispersement is not dependent on high pressure and any low pressure nozzle may be utilized. The fuel and air for both the primary and secondary fuel systems are partially premixed prior to being injected into the combustor zone.

As shown by the sole Figure, the louvered burner liner 10 defines a suitable burner 1 2 where combustion ensues. The fuel nozzle generally illustrated by reference numeral 14 comprises an inner premix tube 1 8 and an outer premix tube 20. The primary fuel system comprises the fuel injector 22 issuing fuel from fuel line 24 into the premix tube 1 8.

This fuel mixes with swirling air issuing from the preswirler 28 (radial although axial type may be used) receiving air discharging from the engine's compressor (not shown).

Fuel from the secondary fuel system is injected into the outer premix tube 20 through the wall of manifold 30 disposed about the outer wall thereof. A plurality of injection ports 32 are circumferentially spaced and the distance between ports and the size thereof is selected so as to produce effective circumferential fuel distribution.

The airflow, which is preswirled by radial inlet swirler 34, in the outer premix tube 20 is modulated by a suitable sleeve valve 36 which creates a lean overall combustion zone equivalence ratio of substantially 0.7-0.8 at high power to minimize NOx production.

Actuator 38, which may be any suitable commercially available type serves to translate the sleeve valve 36 as a function of an engine operating condition, such as power lever setting. A suitable rotatable valve may be utilized without affecting the scope of the invention.

An additional air swirler 40 as well as any other known treatments may be added to the tube's exit plane to modify combustor aerodynamics and/or mixing for emission control.

It should be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the spirit and scope of this novel concept as defined by the following

Claims

claims. CLAIMS

1. A dual premix fuel nozzle for a combustor for a gas turbine engine having a generally cylindrical body defining a pair of coannular passages and a downstream end mounted adjacent a combustion zone in the combustor, a first fuel orifice in the inner of said coannular passages ahead of said downstream end and means for admitting air into said inner annular passage for the air and fuel to premix prior to admittance into said combustion zone at said downstream end, a second fuel orifice in the outer coannular passage ahead of said downstream end and means for admitting air into said outer annular passage for the air and fuel to premix prior to admittance at said downstream end, and means for regulating the amount of air into said outer annular passage whereby fuel in said inner passage is continuously flowing and fuel is introduced in said outer passage solely during the high thrust levels of the combustor operating envelope, and said air regulating means adjust for the difference in operation of said first and second fuel nozzles.

2. A dual premix fuel nozzle as in claim 1 wherein said second fuel orifice includes a tubular manifold member circumscribing said outer annular passage at a location between said downstream end and said means for admitting air thereto and a plurality of spaced openings in said manifold member distributing fuel around the circumference therein.

3. A dual premix fuel nozzle as in claim 1 wherein both of said means for admitting air into said passages are swirlers for imparting a tangential moment to said air upon being admitted into said passages.