Protecting our environment is an important responsibility. This is why authorities give limits for pollutant emission like NOx (oxides of nitrogen), CO (carbon monoxide) and UHC (unburned hydrocarbons) for gas turbines.
In lean burn combustors an increased flow of air into the combustor leads to fuel to air ratios below the level where high levels of NOx is formed. The drawback of increased air flow is that it can cause instabilities in the combustion process resulting in highly fluctuating pressure amplitudes at frequencies below 1000 Hz for a typical combustion system which can cause hardware damages to the combustion chamber.
Combustion chambers are usually cooled by a flow of air along the chamber and through perforations also known as effusion holes arranged in the casing of the chamber. Air penetrating through the effusion holes into the combustion chamber forms a cooling film over the inner surface of the combustion chamber, the film reducing convective heat transfer between the combustion flame and the inner casing of the combustion chamber.
It has been proposed to use the air for both film cooling and damping of instabilities in the combustion process. However, the flow of cooling air has usually different characteristics like volume and velocity to a flow providing damping.
EP 1666795 describes an acoustic damper component arranged on the wall of a combustor with multiple damping chambers. The acoustic damper component has a first metering passage, a first damping chamber, a first damping passage, a second damping chamber and a second damping passage. Air flows through the damper to be ejected into the combustion chamber from the second damping passage at a selected velocity and volumetric flow, the flow being sufficient to damp instabilities from the combustion process.
GB2104965 shows a multiple impingement cooled structure which is coupled to effusion holes in the wall of an element to be cooled such as a turbine shroud. The structure includes a plurality of baffles which define a plurality of cavities.
EP0896193 shows a combined impingement and convective cooling configuration of the combustion chamber where substantially all air flow into the combustion chamber passes through the cooling passage before entering the combustion chamber, i.e. that all of the air utilized is used for both cooling and for mixing with the fuel, assuring good cooling of components and producing a lean mixture which acts to keep the levels of pollutants such as nitrous oxides low.