Davide Laera

Numerical and Experimental Investigation of Thermo–Acoustic Combustion Instability in a Longitudinal Combustion Chamber: Influence of the Geometry of the Plenum

This paper concerns the study of self–sustained combustion instabilities that occur in a test rig characterized by a single longitudinal combustion chamber equipped with a full scale industrial burner and a longitudinal plenum. The length of both plenum and combustion chamber can be continuously varied. During tests, at a fixed value of the length of the combustion chamber, a sensibility of the amplitude of pressure oscillations to the length of the plenum has been registered, while the frequency remained constant. To investigate this behavior, a linear stability analysis has been performed evaluating the influence of the length of the plenum on the frequency and growth rate of the registered unstable mode. The analysis has been performed by means of a finite element method (FEM) code with a three–dimensional distribution of the n-τ Flame Transfer Function (FTF) computed by means of computational fluid dynamics (CFD) simulations. According to the Rayleigh criterion, the distribution of the local Rayleigh index has been computed in order to evaluate the acoustic energy production, while the scattering matrix of the entire system has been used to evaluate the acoustic energy losses. Numerical results show that the reduction of the plenum length induces an increase of acoustic energy losses while the energy production remains almost constant. This result is in agreement with the reduction of the pressure oscillations amplitude observed during tests.Copyright

Modelling of Thermoacoustic Combustion Instabilities Phenomena: Application to an Experimental Rig for Testing Full Scale Burners

This paper concerns the acoustic analysis of self–sustained thermoacoustic pressure oscillations that occur in a test rig equipped with full scale lean premixed burner. The experimental work is conducted by Ansaldo Energia and CCA (Centro Combustione Ambiente) at the Ansaldo Caldaie facility in Gioia del Colle (Italy), in cooperation with Politecnico di Bari. The test rig is characterized by a longitudinal development with two acoustic volumes, plenum and combustion chamber, coupled by the burner. The length of both chambers can be varied with continuity in order to obtain instability at different frequencies. A previously developed three dimensional finite element code has been applied to carry out the linear stability analysis of the system, modelling the thermoacoustic combustion instabilities through the Helmholtz equation under the hypothesis of low Mach approximation. The heat release fluctuations are modelled according to the κ-τ approach. The burner, characterized by two conduits for primary and secondary air, is simulated by means of both a FEM analysis and a Burner Transfer Matrix (BTM) method in order to examine the influence of details of its actual geometry. Different operating conditions, in which self–sustained pressure oscillations have been observed, are examined. Frequencies and growth rates of unstable modes are identified, with good agreement with experimental data in terms of frequencies and acoustics pressure wave profiles.Copyright