Current stationary gas turbines for power generation rely on lean premixed combustion, in order to achieve low emissions of detrimental exhaust gases, such as NOx. However, employment of this combustion process can lead to the occurrence of thermoacoustic instabilities. Characterized by strong pressure fluctuations inside the combustion chamber, this phenomenon has negative effects on exhaust gas emissions, efficiency, noise characteristics, and durability of the gas turbine. Thus, inhibition of these unwanted pressure fluctuations is a major concern for manufacturers. Usually, stability of a combustion system in the entire operating envelope can only be conclusively verified in full-scale engine tests. Yet, they are highly expensive and therefore inefficient. Thus, tests in high pressure combustion test rigs are conducted. In these studies, parameters like the flow field, temperature profiles, and mean the pressure level are similar to the engine. Yet, the acoustic boundary conditions, which have a strong influence on the stability of a combustion system, might not be equal. This is true, even for modern can-annular combustors. Interactions with the turbine and neighboring combustor cans potentially influence the acoustic boundary condition at the combustion chamber outlet. A system that has proven stable operation in high pressure tests might therefore be susceptible to unexpected self-excited pressure oscillations in the engine. To avoid this shortcoming, an active modification of the acoustic boundary condition in high pressure tests is perceived as a viable method. The scheme, e. g., allows for adaptation of the test rig’s combustor outlet impedance to that present in the engine. Within this thesis, modifications for enabling acoustic investigations on an existing high pressure test facility are proposed. Under the premise of retaining the original geometry, the test rig shall be equipped with suitable instruments to realize two main objectives. One goal is the active modification of the acoustic boundary condition (impedance tuning), allowing for a direct assessment of a burner system’s stability characteristics. The second goal is the measurement of the flame transfer behavior, which may be used for stability analysis in a low-order network model. As the results are obtained within a high pressure scenario, they are not dependent on any simplifying assumptions usually associated with data from atmospheric test rigs. Initially, the requirements for achieving both objectives are stated and the necessary hardware modifications, incorporating acoustic sensors and actuators both upstream and downstream of the burner, are outlined. A major difficulty of implementation of the equipment lies in the adaptation to the existing geometry, which is hardly suited for acoustic measurements. For upstream acoustic forcing, a siren actuator is proposed. Its positioning is numerically investigated at two locations inside the test rig. Furthermore, two sensor concepts for the region upstream of the burner are proposed. Both concepts incorporate dynamic pressure sensors for determination of the acoustic variables at the burner inlet, which is required for measurements of the flame transfer behavior. Implementation of impedance tuning requires a sensor array inside the combustion chamber. However, the existing geometry impedes the use of a standardized method for plane wave decomposition. Thus, techniques for a frequency-domain and a time-domain determination of the upstream and downstream propagating waves in a duct of non-uniform cross-section are developed. For acoustic excitation downstream of the combustion chamber, a novel actuation concept is proposed. It incorporates a proportional valve, which injects high mass flows of modulated air into the restriction at the combustion chamber outlet. On one hand, the location supports a successful implementation of impedance tuning, due to advantageous time delays. On the other hand, injection of a modulated air flow into the smallest cross-section adds the effect of an oscillatory blockage of the main air flow. Thereby, the achievable actuation amplitude is strongly increased versus mounting of the same actuator at any other location in the test rig. With the results presented in this thesis, a comprehensive concept for the implementation of modifications for thermoacoustic measurements on a high pressure test rig is introduced.


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    Title :

    Modification of a high pressure test rig for thermoacoustic measurements


    Additional title:

    Anpassung eines Hochdruckprüfstands für thermoakustische Messungen


    Contributors:

    Publication date :

    2018



    Type of media :

    Miscellaneous


    Type of material :

    Electronic Resource


    Language :

    English



    Classification :

    DDC:    629



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