NUMERICAL ANALYSIS OF THE DYNAMIC RESPONSE OF PRACTICAL GASEOUS AND LIQUID FUELLED FLAMES FOR HEAVY-DUTY AND AERO-ENGINE GAS TURBINES

Lean premixed combustion technology is regarded as the most effective one to achieve the recent legislation limits on NOx emissions. One of the most critical issues affecting this kind of technology is the occurrence of combustion instabilities that may compromise the combustor life and integrity so that the prediction of the thermo-acoustic behaviour of the system becomes of primary importance. This research activity is aimed at developing reliable tools to be used in the industrial design process, which are able to describe the complex interaction between the system acoustics and the turbulent flame. The results, in terms of Flame Transfer Function, will be then exploited for the thermo-acoustic linear stability analysis on lean-burn combustors, with Finite Elements codes. The flame response is computed exploiting Unsteady-RANS simulations of the reactive case and System Identification techniques. The procedure is then applied to the study of a practical GE Oil & Gas gas turbine combustor, in the framework of a collaboration with the industrial parten GE Oil & Gas and in part within ATENE (Advanced Technologies for ENergy Efficiency) project funded by Regione Toscana, with the aim of giving more physical insight on the dynamic response of the real flame and understanding the driving mechanism of thermo-acoustic instability onset as well. A reliable numerical model has been generated and assessed through comparisons with results from full-annular combustor experimental campaign carried out by GE Oil & Gas. The inclusion in the final model of several driving mechanisms (air and fuel mass flow fluctuations) constitute a novel aspect in FEM studies of industrial gas turbine combustor configurations. Furthermore, the methodology is then employed, in the framework of LEMCOTEC (Low Emissions Core-Engine Technologies) EU project, to understand the driving mechanisms that regulate the coupling between heat release rate fluctuation and the acoustic field in aero-engines, in an innovative way. Great attention has been devoted to the impact of liquid fuel evolution and droplet dynamics. For this purpose the GE-AVIO advanced PERM (Partially Evaporating and Rapid Mixing) lean injection system has been analysed focussing the attention on the effect of several modelling parameters on the processes involved in liquid fuel evolution and heat release. A set of advanced post-processing tools has been also set-up and exploited to get even more insight on the complexity of such kind of the flames and pointing the way for further enhancements. The application is one of the few in literature where the liquid flame dynamics is numerically investigated providing a description in terms of FTF and detailed information on the physical phenomenon. Therefore, it constitutes and important step forward in the numerical methodology developed for analysing the thermo-acoustic response of GE AVIO lean burn combustor equipped with PERM injection system family.