Very Rough Grid Approach for CFD Modelling of Thermoacoustic Oscillations Inside an Annular Premixed Combustor
Large pressure oscillations due to thermoacoustic instabilities may occur in the modern gas turbine equipped with lean premixed burners. The Research Department of ENEL is studying this phenomenon using research methods that include plant supervision, laboratory experiments and modelling. This paper presents a new CFD modelling approach capable of simulating the time evolution of thermofluiddynamic fields during thermoacoustic instabilities in a whole annular combustor. Its peculiarity consists in the adoption of a very rough computational mesh. The use of the Very Rough Grid (VRG) approach allows all the resonant cavities involved in the acoustic oscillations to be considered, as well as to prolonging the computed transient until spontaneous thermoacoustic oscillations onset, with affordable computation time. The rationale of this approach is that thermoacoustic instabilities are more affected by acoustics than by fluiddynamics. KIEN, an in-house low diffusive URANS code capable of simulating 3D reactive flows, has been used. A 3D structured monoblock computational grid of an industrial annular combustor has been set up. It goes from the compressor outlet to the turbine inlet, including both the annular plenum and the annular combustion chamber, and extends over the entire circumferential angle. The results obtained by an exemplifying computed case are illustrated. They appear to be congruent with the real behaviour of thermoacoustic oscillation reported in literature. The type of information that can be extracted directly or by suitable post-processing from these results is shown and their usefulness in interpreting the real data obtained from functioning plants or experimental facilities is demonstrated.