Colloquia

Summary

Thermoacoustic instability is a combustor pressure oscillation due to feedback of unsteady heat release of a flame inducing acoustic perturbations. This feedback can cause premature failure of combustion systems. Due to fast chemical kinetics in hydrogen combustion, a hydrogen-fuelled system is likely to be more sensitive to thermoacoustic instability. Conventionally this is analysed by transient CFD simulation of the combustion process. This thesis work is performed as part of the ‘combustion DYNamics and Acoustics oscillations in large industrial Furnaces and boilers’ (DYNAF) project. For large scale boilers, transient simulations can be more expensive due to the large time and length scales involved. The development of a steady state based method to replace these transient simulations is interesting to determine with modest effort thermoacoustic stability in the design process of the burners.

A residential hydrogen-fuelled condensing boiler is analyzed. This study compares methods to determine the Flame Transfer Function (FTF), a function describing the effect of the flame on the heat release due to a flow perturbation (equivalence ratio perturbations in this work). Two transient methods, impulse excitation and white noise excitation, are compared to a steady state method, the Linear Coefficient Method (LCM), to evaluate computational expense and accuracy. The FTF is determined for a residential hydrogen-fuelled condensing boiler. The Finite Rate/Eddy Dissipation Model (FR/EDM) is compared to the UT-Flamelet Generated Manifold (UT-FGM) combustion model. The FTF is determined for the different methods and implemented in an acoustic network model (ANM) of the condensing boiler to determine thermoacoustic stability.

Speed improvements have been made to the LCM such that the computation time is low compared to the transient methods for the case study. The used implementation of the LCM did not give accurate results. The transient methods showed good agreement in the FTF and in the eigenfrequencies of the system in the ANM. The analyzed condensing boiler is stable.