Acoustic analysis of a diffusion flame gas turbine combustor by means of non-reactive calculations

Combustion instabilities are unsteady phenomena that can affect premixed and diffusion flame combustors. They are spontaneously excited by a feedback loop between an oscillatory heat release and one or more natural acoustic modes of the combustor. When large instabilities occur, the associated oscillations of pressure and heat release may lead to premature failures due to vibrations and thermal loads at combustor walls. The prediction of natural acoustic modes is often used to identify the modes coupled to the unsteady heat release and to design damping systems. Thanks to the increase in computing capabilities, several modelling tools have been developed to obtain detailed information regarding the spatial shape of the acoustic modes. This paper presents the acoustic analysis of a non-premixed gas turbine combustor. The analysis is based on non-reactive computational fluid dynamics simulations performed on a coarse grid model to calculate the frequency and shape of natural modes. The simulations require very limited computational effort because simple numerical models are adopted and no combustion and heat transfer models need to be activated. The influence of temperature and gas composition on acoustic mode frequencies is considered through a simple post-processing correction. Thus, frequencies measured under limit cycle conditions can be directly compared to calculated values to identify which natural mode is excited by the unsteady heat release. The numerical results are validated against full-scale experimental measurements.