The effects of piloting and liner boundary conditions on flame locations and stability margins of an industrial size premixed flame

摘要

The stability margins of swirl stabilized premixed flames as used in industrial gas turbine combustion systems exhibit a distinct dependency on pilot flames and on the boundary conditions imposed by the combustion chamber liner. The understanding of these effects is considered an essential prerequisite for the development of combustion systems with both reduced NO_x-emissions and an uncompromised operating range. In order to study the above mentioned influences experimentally, a full scale burner similar to the burners of the Siemens HR design was operated at atmospheric conditions. The pilot fuel flow rate was altered to study the pilot flame's influence on Lean Blow Out (LBO) and thermo acoustic instabilities. The combustion chamber was equipped with a convectively cooled metallic liner. The heat flux rate through the liner was found to compare well with those of modern engines. Further, the current design comprises a freely adjustable air flow injected into the combustion chamber through holes in the liner. This specific liner design mimics the open convective cooling system as it is usually present in gas turbine combustion systems. In addition, clearly defined boundary conditions at the liner surface are maintained in the experiment to allow for numerical verification. The present paper comprises a detailed description of the combustion setup with emphasis on the system's reaction on different pilot fuel flow rates and altered boundary conditions along the liner surface. As a function of these parameters, emissions as well as noise and humming originating from the flame are discussed. It was found that the air flow past the liner promotes thermo-acoustic instabilities while the pilot flame suppresses these tendencies. The influence of piloting on the mixture field is discussed for non-reacting conditions. Finally, the chemiluminescence of CH and OH of the flame was investigated. It is shown that the pure premixed flame is situated at a distinct distance downstream of the burner. The addition of pilot fuel moves the locations of high heat release closer to the burner mouth. In contrast, air injection past the liner was found to suppress the heat release close to the burner.