Experiments and Modelling on the Effect of an Adjustable Boundary on Thermoacoustic Stability

Abstract

Predicting the existence of thermoacoustic instabilities is a key step in the design of modern gas turbines and the choice of their operating conditions. The stability of a combustion system crucially depends on the acoustic boundary conditions. To systematically investigate the influence of these boundary conditions, a test facility with variable inlet and outlet geometries has been developed. Cold flow tests confirmed that the acoustic terminations allow for a change of the reflection coefficient from close to open end to anechoic to almost closed end over a large frequency range. In the present work, we present the design of an adjustable exit boundary enabling a change in the thermoacoustic stability without modifying the flame operating conditions. Experiments have been conducted in a turbulent axial atmospheric combustor. The acoustic reflection coefficients in hot condition of the exit boundary are measured for different boundary geometries and the impact of these geometries on the flame stability is assessed. A parametrized model is derived and reproduces the experiments well.