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Detection and Modelling of thermoacoustic instabilities on an annular combustion system for industrial gas turbine

Pubblicazione


Autore: G. Benelli, L. Carrai, S. Lunardi, S. Sigali, L. Castellano

Collana: BT - 34 - Mag 06 - Energy: production, distribution and conservation

Note:
The present paper deals with part of the efforts made by ENEL to compile guidelinesfor best practice in explioting their fleet of V94.3A gas turbines.These machines have an annular combustion chamber with 24 Dry Low NOx burners and work under lean-promixed flame conditions; thus they are prone to combustion instabilities. In order to characterize the possibleunwanted noise under real operational conditions, a complete set of the Humming and Acceleration Monitoring system (HAM) experimental apparatus conceived and developed by ENEL was mounted on several units. A truly extensive data base under many different operating conditions was then collected for more than one year on tem systems in commercial operation. A 3D thermoacoustic model has been implemented in Frequency Domain approach for the theoretical interpretation of the experimental results.
The current release reduces the complex geometry of the combustion chamber to an annular cylinder with a finite gap. In spite of this and others simplifying assumptions, the model has proved capable of providing many intersting results. In particular a) the model allows the satisfactory reproduction of all the proper frequencies of this type of combustion chamber published in technical literature; b) the analysis of the single modes proves that: b.1) the engine has been designed and constructed tobe free of humming "a priori"; b.2) two different linear sub-models of flame fluctuations select two different subsests of the natural frequencies for which an amplification of the acoustic pressure is foreseen; b.3) the sub-model based upon fluctuations by disturbances at the exit of burners gives much closer results to the experimental data than those obtained using the sub-model based on the Time Delay between the fuel exit at premix and the combustion zone.


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