59267 - Thermoacoustic Stability Analysis of a Full-Annular Lean Combustor for Heavy-Duty Applications 

Lean premixed combustion technology can be considered the most effective solution to fulfil the stringent regulations on pollutant emissions of these last years. As the flame gets close to its extinction limits, stabilisation problems arise due to the occurrence of thermo-acoustic instabilities associated with the coupling between pressure oscillations and unsteady heat release. If the two mechanisms constructively interfere, high-amplitude pressure pulsations occur, which pose a severe issue on the operability and reliability of the engine. For this reason, there is the demand for reliable tools to predict the frequency and amplitude of pressure oscillations, and as a consequence their impact on the engine reliability, and to find methods to suppress them.

Numerical simulation of the instability transitory via complete 3D CFD simulations is possible but time-consuming and not feasible in the industrial framework. A much less time-consuming approach is to decouple the calculations of the perturbed flame response and the acoustic waves. Such linear thermo-acoustic analysis of practical combustion systems has been widely used to predict the system stability, in particular exploiting System Identification techniques.

The dynamics of a lean-premixed full-annular combustor for heavy-duty applications has been numerically studied in this work. The well-established CFD-SI method has been used to investigate the flame response varying operational parameters such as the flame temperature (global equivalence ratio), the fuel split between premixed and pilot fuel injections and the burner pressure drop: such a wide range experimental characterization represents an opportunity to validate the employed numerical methods and to give a deeper insight into the flame dynamics. Furthermore, an approach where pilot and premixed flame responses are analyzed separately is proposed, exploiting the independence of their evolution, in order to understand the complex interactions between the different parts of the flame. URANS simulation have been performed, due to their affordable computational costs from the industrial perspective, after validating their accuracy through the comparison against LES results.

The calculated FTFs have been implemented in a 3D FEM model of the chamber, in order to perform linear stability analysis and to validate the numerical approach. A boundary condition for rotational periodicity based on Bloch-Wave theory has been implemented into the Helmholtz solver and validated against full-annular chamber simulations, allowing a significant reduction in computational time. The reliability of the numerical procedure has been assessed through the comparison against full-annular experimental results performed by Baker Hughes.