Acoustic Modes of a Can Annular Combustor Setup
To study can-to-can interactions, an FE analysis of a complete multi-can annular combustor configuration was performed. The annular manifold upstream of the turbine inlet interconnects combustion chambers with adjacent units. The absorbent acoustic boundary conditions used to describe the burner and chamber exit areas were defined in the same way as for a single-can model. Simulations in LMS Acoustics Simulation Software show that, besides the axial modes along each single-can combustion chamber, the complete can annular combustor configuration triggers a range of additional acoustic modes. It concerns pure azimuthal and mixed axial/azimuthal modes.
Because there are no test rigs available for measuring the complete can annular combustor configuration, these modes are only predictable by performing acoustic simulations in LMS Acoustics Simulation Software (see Figure 2).
The main reason why Siemens performs these acoustic evaluations is to make sure all potentially hindering or obstructing eigenfrequencies and acoustic velocities are known early on in the design and development process. This enables Siemens engineers to implement specific countermeasures to disturbing eigenfrequencies, for example by developing and installing particular burner outlet extensions and acoustic resonators.
The length of the extensions mounted on burner outlets defines the frequency that can excite the feedback cycle and, hence, affect the risk for combustion instabilities. The installation of these extension units is a quite affordable solution that is particularly useful for suppressing oscillations in the intermediate range of frequencies, typically between 50 and 500 hertz. The sensitivity of these extensions makes this type of countermeasure somewhat harder to tune.
The use of acoustic resonators, which are part of the standard engine design, is another way to influence acoustic eigenfrequencies. This approach is applied very efficiently to delete acoustic signals with shorter wavelengths, such as high frequencies between 1,000 and 3,000 hertz.
The geometry of these resonators can be designed in LMS Acoustics Simulation Software, but a practical way to avoid recurrent FE meshing is by estimating the geometry analytically and, finally, validating the design using LMS Acoustics Simulation Software. The cooling of these resonators prevents hot air from accessing the resonator. Resonators are a very effective means of addressing the problem, although they add complexity and cost while reducing efficiency of the gas turbine as a result of the resonators’ cooling air requirements.
Although the optimization of fluid flow, combustion, and heat transfer remain primary objectives in gas turbine development, more attention is being paid to the interrelations between acoustic performance and operation reliability and efficiency. Sven Bethke concludes, "The combination of virtual prototype simulations with LMS Acoustics Simulation Software and adequate experimental testing allows Siemens to efficiently simulate the impact of specific design modifications and operating conditions on the acoustic performance of gas turbines. The predicted acoustic eigenfrequencies and mode shapes of single-combustion chambers and can-annular combustion systems are essential in optimizing combustor designs and increasing the competitive position of Siemens power generation systems."
— Contributed by LMS (www.lmsintl.com).
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