Increasingly fierce competition driven by deregulation and privatization is putting downward pressure on power plant operations and maintenance (O&M) budgets. Recently, lower natural gas prices have pushed natural gas – fired combined-cycle plants higher up in many utilities’ dispatch order in some regions, a welcome change from the twice-a-day cycling experienced by some plants during the past few years. However, with more operating hours comes more interest in plant operating availability, and that means increased emphasis on reliable gas turbine operation (Figure 1).
1. Keep costs low. Increasingly fierce competition in the worldwide power generation business keeps the pressure on plant owners to find ways to keep O&M costs low and plant reliability high. Courtesy: Siemens
A phenomenon that potentially influences the reliability of gas turbine operation, and therefore the entire combined-cycle plant, is the presence of thermo-acoustic oscillations in the combustion chamber. A "can annular" combustion system arrangement, for example, typically has 16 (more or less) separate can-shaped combustion chambers distributed on a circle perpendicular to the symmetry axis of the engine. In each of these combustors, a burner continuously injects a mixture of fuel gas that is mixed with compressed air to deliver combustion products at a design temperature, pressure, and flow rate to the turbine section to generate the requested electrical power (Figure 2).
2. Oscillation reaction. The combination of fluid flow, heat transfer, thermal expansion, and acoustic radiation causes combustor oscillations, which may impact operational range and cause internal damage to the turbine. This turbine cutaway is of a Siemens SGT6-6000G, formally known as a W501G, nominally rated at approximately 260 MW. The computational fluid dynamics (CFD) analysis of the combinations of fluid flow, heat transfer, thermal expansion, and acoustic radiation can identify situations where combustion oscillation may cause severe damage. The CFD results illustrate, by the change in colors, azimuthal as well as mixed axial/azimuthal oscillation modes, for which no experimental test setups are possible. Courtesy: Siemens
The combustor oscillations are determined by a feedback cycle that combines the effects of fluid flow, heat transfer, thermal expansion, and acoustic oscillations — a cocktail of effects potentially causing severe engine malfunction and component damage. Some combustion turbine manufacturers have constructed test rigs where prototype combustors are tested and evaluated against a long list of operating regimes and conditions. The disadvantage of prototype testing is that it requires a significant investment of capital and does not provide sufficient flexibility to test alternative designs under additional operating conditions, especially those conditions that cause damaging acoustic oscillations in the combustion system.
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