1. Field of the Invention
The present invention relates to combustion systems, and more particularly to lean combustion systems for distributed combustion, such as in gas turbine engines.
2. Description of Related Art
A number of nozzles and injectors are known in the art for injecting fuel for combustion, such as in furnaces, gas turbine engines, and the like. The trend in the gas turbine industry, for example, has been to operate engines at increasingly lean fuel-to-air ratios and under higher combustor pressures to reduce emissions of environmentally harmful products of incomplete combustion, such as oxides of nitrogen (NOX) and to reduce specific fuel consumption. However, operating at leaner, higher pressure conditions leads to increased probability and magnitude of combustion instabilities that can result in flame blow out or induce vibration modes that can damage engine components. Current technology utilizes techniques such as lean direct injection and premixing (or partial premix) of fuel and air prior to combustion. These methods are susceptible to lean instabilities that limit engine operating envelopes. Another method of lowering NOX emissions has been to reduce combustion residence time through shortening flow paths through combustors. This leads to instabilities at higher frequencies, which are more damaging than low frequency instabilities as they can couple with resonating acoustic patterns. This approach can also increase levels of unburned hydrocarbons (UHC) and/or carbon monoxide (CO) which are also environmentally damaging.
One way of providing stable combustion at very lean conditions is to use a distributed combustion process, sometimes called flameless combustion. Most combustion instabilities involve a three part cyclic process. First, fluid mechanical phenomena produces a fluctuation in heat release rate which then couples and reinforces an acoustic mode. This in turn trips an unstable fluid dynamic structure, which leads to fluctuations in heat release rate, and so on. In distributed combustion, no such a coupling occurs. The inability of coupling of this kind to occur in distributed combustion inhibits strong acoustic waves that could otherwise damage the combustor or turbine blades.
Distributed combustion has been successfully demonstrated in industrial furnaces. The technique involves using a very lean mixture wherein high temperature oxidizer reacts with fuel at very high levels of turbulence in a distributed reaction zone. Distributed combustion has been shown to produce very stable combustion having low NOX levels in industrial furnaces. Distributed combustion is sometimes called “flameless combustion” because of the lack of a discrete visible flame resulting from the distributed nature of the reaction. In industrial furnace applications of distributed combustion, the required high oxidizer temperatures are obtained by either preheating the air with furnace exhaust gases through a heat exchanger or by direct mixing of the air with hot recirculated exhaust gas. These furnaces typically recycle combustion gases via a duct external to the combustion region. These ducts and/or heat exchangers in conventional burners are heavy and take up considerable space and have therefore limited the practical application of distributed combustion.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for methods and systems that allow for reduction of NOX, CO, and UHC while increasing lean stability beyond the state of the art. There also remains a need in the art for such methods and systems that are simpler to make and use. The present invention provides a solution for these problems.