Rob Snoeijs, LMS International, Belgium
To enhance power generation performance, the development of modern gas turbines focuses on realizing higher turbine inlet temperature and lower nitrogen derivative emissions. Due to high power densities, the advanced combustion systems of gas turbines unfortunately are prone to produce complex thermo-acoustic instabilities and combustion-driven vibrations.
These oscillations reduce the operational range and potentially generate reliability problems and severe engine failure. Detailed acoustic simulations already performed during the combustor design phase are becoming increasingly important to design gas turbines with reduced instabilities and extend their operational envelope. Siemens engineers gained early insights in the acoustic performance of their design and were able to simulate more operating conditions than is feasible through the testing of physical prototypes.
Reducing costs per kilowatt
Increasingly fierce competition driven by deregulation and privatization is dictating ever lower power generation costs. Cost cuts can be realized by establishing an economic plant operation centering on low investment and lifecycle costs. Maximum operating economy relies on optimum compression and combustion, pushing forward into new thermodynamic regions and higher combustion temperatures.
One of the main challenges resides in reconciling competitive engine characteristics with strict environmental targets, including low carbon and nitrogen derivative emissions. In addition, customers also appreciate easy-to-service designs and long intervals between major overhauls.
A phenomenon that potentially influences the reliability of power generation systems is the presence of thermo-acoustic oscillations in the combustion chamber. A can annular type of combustion system, for example, typically counts 16 or more separate can-shaped combustion chambers, distributed on a circle perpendicular to the symmetry axis of the engine. In each of these cans, a burner continuously injects a mixture of fuel gas and compressed air in order to power the turbine and generate the requested electrical power.
The aforementioned combustor oscillations are determined by a feedback cycle that combines the effects of fluid flow, heat transfer, thermal expansion and acoustic oscillations, a cocktail potentially causing severe engine malfunctions.
Increased temperatures increase the instability of gas turbines
Several test rigs and prototypes are constructed to test and evaluate a comprehensive number of characteristic conditions. The disadvantage of prototype testing is that it requires major resources and does not allow flexible investigations of all conditions. Therefore, the capability to predict thermo-acoustic instabilities is vitally important to increase the performance and to extend the reliability of gas turbine power plants.
Approaches to optimization
To develop specific measures that prevent thermo-acoustic instability, Siemens engineers analyzed the complicated relationship and interaction between acoustic performance and thermal heat release.
Sven Bethke, an engineer at Siemens Combustion Technology, explains, “Since eigenfrequencies and mode shapes of acoustic pressure are strongly coupled to the stability analysis, the Finite-Element (FE) mode analysis and the subsequent stability analysis are the main tasks in the thermo-acoustic prediction and evaluation process.”
In the combustion optimization process followed at Siemens, engineers took the output of Computational Fluid Dynamics (CFD) simulations, including steady-state flow velocity, temperature, and fluid properties, as input for acoustic simulations using Belgian firm LMS’s software for vibro-accoustic design, LMS SYSNOISE. For these simulations, a scale of different acoustic models was used: an FE model of a single-can combustor configuration, an extended FE model that also includes the incoming flow path upstream the burner, turbine vanes and exhaust passage, and a complete multi-can annular combustor setup.
Siemens engineers gained insights into accoustic performance through simulation
An important and inherent part of the acoustic FE modeling is the definition of specific boundary conditions, which are determined mathematically or experimentally. Siemens engineers validated the results from acoustic simulation using appropriate tests performed on specifically designed single-can test rigs.
Advances in modeling
The implications of defining boundaries on the FE analysis of a single-can configuration were investigated. The FE model included the whole combustion chamber starting at the head end plate and ending at the exit of the transition piece upstream the turbine inlet. The crucial regions through the burner as well as through the termination at the exit of the combustion chamber are characterized by absorbent boundary conditions. The acoustic boundary condition at the exit of the burner, i.e. at the inlet into the combustion chamber, was represented by a specific impedance, which was quantified experimentally using an atmospheric test rig without combustion. At the exit of the combustion chamber, the guide vanes of the turbine – or a vane simulation section (VSS) in the case of test rigs – defined the acoustic boundary condition. Sophisticated mathematical approaches were used to describe the flow field downstream of these obstacles. Compared to the fluid flow behind the vanes, cylinders generated more vortices, which affect the reflection of the exit boundary condition. The FE model obtained was suitable for analyzing the effects of different impedances, for example from different types of burners and varying Mach-numbers (steady-state flow velocities). The acoustic simulations showed that the burner type has a significant impact, while the flow velocity in the combustion chamber affects the mode shapes of the acoustic pressure only marginally.
Importance of Mach-number
When extending the FE model of a combustor test rig with a VSS – which replaces the vanes of the turbine stages – and a downstream exhaust discharge tube, it became clear that the Mach-number cannot be neglected. The presence of narrow passages causes the geometry’s acoustic properties to be influenced by the speed of the flow. Siemens engineers determined the reflection coefficient of the VSS on the basis of the acoustic pressure distribution, obtained by FE simulations. The extended FE model is particularly suited to determine the impedance of the boundary upstream of the VSS and its dependency on the Mach-number through this section. The results showed a strong dependency on the Mach-number through the VSS.
Can annular combustor setup
To study can-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 showed that, besides the axial modes along each single can combustion chamber, the complete can annular combustor configuration triggered a range of additional acoustic modes. It concerns pure azimuthal and mixed axial/azimuthal modes. Since there were no test rigs available for measuring the complete can annular combustor configuration, these modes were only predictable by performing acoustic simulations.
Benefit for the design process
The main reason why Siemens performs these acoustic evaluations is to ensure 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 interfere 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, effect the risk for combustion instabilities. The installation of these extension units is an affordable solution that is particularly useful in suppressing oscillations in the intermediate range of frequencies, typically between 50 Hz and 500 Hz. However, 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, i.e. high frequencies between 1000 Hz and 3000 Hz. The geometry of these resonators can be designed in LMS SYSNOISE, but a practical way to avoid recurrent FE meshing is by estimating the geometry analytically and then validating the design using the vibro-acoustic software. The cooling of these resonators prevents hot air from accessing the resonator. Resonators are very effective although they add further complexity, higher expenditure as well as reduced efficiency of the gas turbine, due to the additional cooling air requirement.
Operation efficiency & reliability
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. Bethke concludes, “The combination of virtual prototype simulations and adequate experimental testing allows us 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.”