Analysis of Pressure Oscillations Data in Gas Turbine Annular Combustion Chamber Equipped With Passive Damper

Growing environmental awareness, the current legislations, and the increasing competition in the energy market lead gas turbine manufacturers to develop combustion chambers that have to guarantee low NOx emissions, low pressure drop and high combustor outlet temperature. Modern annular and can-type gas turbine combustion chambers, able to work in lean premixed mode, show a remarkable attitude to produce flame instabilities, well known as humming. Many theoretical approaches have been proposed in order to describe the phenomenon and predict the stability margin of the burner. Experimental tests are needed to assess mathematical models and to evaluate the effects of either active or passive methodologies adopted to reduce combustion driven instabilities. Tests have been carried out at the Ansaldo Caldaie test bed on a real-size annular combustion chamber, equipped with a certain number of Helmholtz resonators. The combustion chamber has been instrumented with piezoelectric and opto-electronic transducers in order to determine the pressure field both in proximity of the instability limit and in humming condition. When pressure data are collected before than humming appears, pressure oscillations are much lower than those gathered in humming conditions; therefore, by using sensors designed to work under the pressure levels characterising the humming conditions, difficulties arise in proximity of the stability limit due the low signal-to-noise ratio. In this paper some techniques used to analyse the data gathered from the tests will be shown. Moreover, a simple algorithm capable to analyse a large amount of data and to synthesise them into a few significant parameters useful for the spectral analysis of the pressure field, has been validated by means of both real and simulated signals