Experimental and Analytical Study of the Acoustic Properties of a Gas Turbine Model Combustor With a Choked Nozzle

Combustion is now considered as a non-negligible contributor to gas turbine noise. Combustion noise can be divided into two types: direct combustion noise directly caused by flame surface fluctuations and indirect combustion noise caused by non-homogeneities in the burnt gases, which radiate sound when interacting with the first turbine stages. The aim of the present project is to obtain an extensive experimental database as well as a better understanding of the physical phenomena inside a pressurized combustion chamber with a choked exhaust nozzle. To do so, a pressurized model scale combustor has been developed, containing a tangential admission injector creating a swirling premixed flow. Satisfactory premixing is obtained in the injection device by a porous media. The combustion chamber shows large optical accesses and various ports for pressure and temperature sensors. On the upstream side, an impedance control device is installed while, downstream, the exhaust nozzle can be easily varied to study its influence on noise generation. A mean chamber pressure higher than 2 bar can be reached for the targeted operating points. The present analysis of the flame behaviour is a first step towards the study of combustion noise. The flame dynamics are characterized by spectral analysis of the dynamic pressure in the combustion chamber. The aim of this work is to determine the dominating acoustic modes during combustion operation. With the help of analytical calculations, the test bench is first modelled as a two cavity system, and later as a five cavity system, taking into account the feeding lines. The nozzle can be assumed as choked due to the pressurization of the chamber. With this method, the majority of the acoustic modes can be identified and explained. The study shows that these modes are linked to the geometry of the whole combustor including the injection tube, the combustion chamber and the feeding lines.