Combustion engines are machines that convert chemical energy stored in fuel into mechanical energy useful for generating electricity, producing thrust, or otherwise doing work. These engines typically include several cooperative sections that contribute in some way to this energy conversion process. In gas turbine engines, air discharged from a compressor section and fuel introduced from a fuel supply are mixed together and burned in a combustion section. The products of combustion are harnessed and directed through a turbine section, where they expand and turn a central rotor.
A variety of combustor designs exist, with different designs being selected for suitability with a given engine and to achieve desired performance characteristics. One popular combustor design includes a centralized pilot burner (hereinafter referred to as a pilot burner or simply pilot) and several main fuel/air mixing apparatuses, generally referred to in the art as injector nozzles, arranged circumferentially around the pilot burner. With this design, a central pilot flame zone and a mixing region are formed. During operation, the pilot burner selectively produces a stable flame that is anchored in the pilot flame zone, while the fuel/air mixing apparatuses produce a mixed stream of fuel and air in the above-referenced mixing region. The stream of mixed fuel and air flows out of the mixing region, past the pilot flame zone, and into a main combustion zone, where additional combustion occurs. Energy released during combustion is captured by the downstream components to produce electricity or otherwise do work.
In order to ensure optimum performance of a common combustor, it is generally preferable that the internal fuel-and-air streams are well-mixed to avoid localized, fuel-rich regions. As a result, efforts have been made to produce combustors with essentially uniform distributions of fuel and air. Swirler elements, for example, are often used to produce a stream of fuel and air in which air and injected fuel are evenly mixed.
Gas turbine technology has evolved toward greater efficiency and also to accommodate environmental standards in various nations. One aspect in the evolution of designs and operating criteria is the use of leaner gas air mixtures to provide for increased efficiency and decreased emissions of NOx and carbon monoxide. Combustion of over-rich pockets of fuel and air leads to high-temperature combustion that produces high levels of unwanted NOx emissions.
Also, a key objective in design and operation of gas turbine combustors is the stability of the flame and, related to that, the prevention of flashbacks. A flashback occurs when flame travels upstream from the combustion zone in the combustion chamber and approaches, contacts, and/or attaches to, an upstream component. Although a stable but lean mixture is desired for fuel efficiency and for environmentally acceptable emissions, a flashback may occur at times more frequently with a lean mixture, and particularly during unstable operation. For instance, the flame in the combustion chamber may progress backwards and rest upon for a period a baseplate which defines the upstream part of the combustion chamber. Less frequently, the flame may flash back into a fuel/air mixing apparatus, damaging components that mix the fuel with the air.
A multitude of factors and operating conditions provide for efficient and clean operation of the gas turbine combustor area during ongoing operation. Not only is the fuel/air mixture important, also relevant to gas turbine operation are the shape of the combustion area, the arrangement of assemblies that provide fuel, and the length of the combustor that provides varying degrees of mixing. Given the efficiency and emissions criteria, the operation of gas turbines requires a balancing of design and operational approaches to maintain efficiency, meet emission standards, and avoid damage due to undesired flashback occurrences.
The type of fuel/air mixing apparatus, and how it operates in relationship to other components, is one of the key factors in proper operation of current gas turbines. A common type of fuel/air mixing apparatus is known as a main swirler assembly (which also is referred to in the art as a nozzle, which is a more inclusive term). A main swirler assembly is comprised in part of a substantially hollow inner body that comprises stationary flow conditioning members (such as vanes) that create a turbulent flow. Fuel is added before or into this turbulent air stream and mixes to a desired degree within a period of time and space so that it is properly mixed upon combustion in the downstream combustion chamber. Also, in typical arrangements, a main swirler assembly also is comprised of an outer downstream element known as an annulus casting. An annulus casting surrounds a downstream section of the inner body, forming a channel for air flow known as the flashback annulus. In a typical arrangement, a quantity, such as eight, swirler assemblies are arranged circumferentially around the central pilot burner. The pilot burner burns a relatively richer mixture than is provided by the radially arranged swirler assemblies.
Various approaches to reduce or eliminate flashback in modern gas turbine combustion systems have been attempted. Since the prevention or elimination of flashbacks is a multi-factorial issue and also relates to various aspects of the design and operation of the gas turbine combustion area, a range of approaches has been attempted. These approaches often inter-relate with one another.
The present invention provides a solution toward obtaining an operationally stable, flashback-resistant main a fuel/air mixing apparatus, such as a swirler assembly, that provides an extended columnar air barrier that impedes the back progression of flame and, therefore, reduces or eliminates undesired flashback. More specifically, the present invention provides around the fuel/air mixture output of each main swirler assembly a more robust circumferential columnar body of air that 1) provides a fresh air barrier for a distance around the fuel/air mixture output of each respective main swirler assembly (or other source of fuel/air mixture); and 2) leans out the regions where there is a potential for flashback.