Thermoacoustics in industrial gas turbine combustors

Within the framework of the Turbo working group, the Robust Turbomachinery for Flexible Use (RoboFlex) group at TU Berlin is working on the further development of gas turbine combustors to meet new requirements. The gas turbines, which were originally designed for one operating point, now have to manage new modes of operation in conjunction with renewable energies in order to compensate for grid fluctuations and ensure security of supply. Not only is the number of start and stop cycles for existing power plants increasing, but the turbines are also increasingly being operated outside their design point. This change in operation results in new requirements for the combustion process in the machines. The group's work is divided into three subprojects that address these objectives using different approaches.

This subproject deals with the prediction of thermoacoustic stability of gas turbine combustors. For this purpose, the acoustics of the different components of the combustor geometry are analyzed and modeled. The complex components of typical industrial burners such as baffle plates and swirl generators, as well as complex phenomena such as acoustic interaction between adjacent burners are taken into account. One unknown in the description of the stability of a combustion process is the interaction between the acoustics and the heat release rate, the flame transfer function. In order to represent all physical effects, flame transfer functions are typically measured in experiments at great expense. The aim of this subproject is to calculate flame transfer functions of turbulent swirling flames based on linearized conservation equations for momentum, mass, energy and scalar transport equations. This approach offers a comparatively low-cost alternative compared to the costly experiments. With the flame transfer functions and acoustic models of the burner geometry obtained in this way, combustion instabilities should be predicted reliably and with little effort in the future.

The aim of the project is to develop a method that predicts the onset of thermoacoustic instabilities. The acoustics of the combustion chamber are described using analytical approaches with planar waves. In order to analyze an industrial burner, new models will be developed for its complex components. The flame transfer function will be determined from the mean fields of the flow in the combustion chamber, the heat release of the flame, as well as the concentration distribution. This method is based on the linearized conservation equations for momentum, mass, energy, as well as the scalar transport equations.  The required mean fields are determined by numerical simulation (LES & RANS). The model provides the flame transfer function, which is currently the key unknown for predicting thermoacoustic instabilities. Using network modeling, the flame transfer function determined in this way can be taken into account in the acoustic modeling of the burner and the thermoacoustic stability can be inferred. All model steps are validated by experiments.

In a first step, the acoustic combustion chamber boundary conditions were investigated. Neighboring burners can communicate acoustically with each other via a connecting gap located at the downstream end of the burners and upstream of the first turbine stage. This can lead to the expression of azimuthal modes in the tubular ring burner. Based on recently published studies, the acoustic interaction between adjacent burners is considered via a coupling boundary condition in the thermoacoustic modeling of the industrial tubular ring burner.

At the upstream end, the individual burners are connected to the compressor plenum via a baffle. Here, planar acoustic waves are attenuated by interaction with the impingement cooling plate. An acoustic model was formulated and experimentally validated for this component.