DEVELOPMENT AND VALIDATION OF A 1-D TOOL FOR THERMOACOUSTICINSTABILITIES ANALYSIS IN GAS TURBINE COMBUSTORS

In the last years, the more restrictive environmental legislations have constrained gas turbine manufacturers to the development of new low-emission combustors. Lean Premixed technology has become a necessary standard to meet emissions requirements and allowing an heavy reduction of nitrogen oxides emission. This kind of technology, due to the use of lean premixed mixtures, is severely affected by thermoacoustic phenomena which cause damages to combustor components and consequently reduce the overall gas turbine life of a factor of two or more. Specifically, premixed flames pose the threat of pressure oscillations. This phenomenon is the effect of the strong interaction between combustion heat-release and fluid dynamics aspects. In order to investigate thermoacoustic instabilities, a mono-dimensional code was developed and validated. It takes into account only longitudinal frequencies and it is thought to be highly modular to modify or add blocks, corresponding to different thermoacoustic models. The tool is based on a lumped-parameter approach, which consists in considering constant mean flow quantities over each fundamental straight duct element and a nodal point at each duct interface. For each interface, where an acoustic impedance could be present, the linearized fluctuating mass, momentum and energy equations are solved including entropic waves. To validate such tool, several tests, referring to actual test rigs and experimental gas turbine combustor geometries, were performed. The results show a general agreement with empirical data and other numerical results reported in literature in terms of resonance frequencies, stability and modal shapes, both for no flame and fluctuating heat release cases.