During the prototype shop tests, the Model V84.3A ring combustor gas turbine unexpectedly exhibited a noticeable “humming” caused by self-excited flame vibrations in the combustion chamber for certain operating conditions. The amplitudes of the pressure fluctuations in the combustor were unusually high when compared to the previous experience with silo combustor machines. As part of the optimization program, the humming was investigated and analyzed. To date, combustion instabilities in real, complex combustors cannot be predicted analytically during the design phase. Therefore, and as a preventive measure against future surprises by “humming,” a feedback system was developed which counteracts combustion instabilities by modulation of the fuel flow rate with rapid valves (active instability control, AIC). The AIC achieved a reduction of combustion-induced pressure amplitudes by 86 percent. The Combustion instability in the Model V84.3A gas turbine was eliminated by changes of the combustor design. Therefore, the AIC is not required for the operation of customer gas turbines.
Thermoacoustic instabilities have the potential to restrict the operability window of annular combustion systems, primarily as a result of azimuthal modes. Azimuthal acoustic modes are composed of counter-rotating wave pairs, which form traveling modes, standing modes, or combinations thereof. In this work, a monitoring strategy is proposed for annular combustors that accounts for azimuthal mode shapes. Output-only modal identification has been adapted to retrieve azimuthal eigenmodes from surrogate data, resembling acoustic measurements on an industrial gas turbine. Online monitoring of decay rate estimates can serve as a thermoacoustic stability margin, while the recovered mode shapes contain information that can be useful for control strategies. A low-order thermoacoustic model is described, requiring multiple sensors around the circumference of the combustor annulus to assess the dynamics. This model leads to a second order state space representation with stochastic forcing, which is used as the model structure for the identification process. Four different identification approaches are evaluated under different assumptions, concerning noise characteristics and preprocessing of the signals. Additionally, recursive algorithms for online parameter identification are tested.
In annular combustion systems, azimuthal thermoacoustic modes manifest themselves predominantly as travelling or standing waves. Several phenomena can influence the modal behaviour of annular thermoacoustics. To monitor the stability of azimuthal thermoacoustics in industrial installations, a better understanding of the dynamics is required to correctly interpret online measurements. In this work, thermoacoustic eigensolutions of annular combustion systems are investigated, using a low-order analytic model. Heat release fluctuations are considered as a weak source term for a given acoustic eigenmode. The fluctuating heat release is modelled as a linear feedback to the local acoustics, in which the feedback response is a function of the azimuthal coordinate, causing cylindrical symmetry breaking. A bifurcation map is generated as a function of azimuthal mean flow velocity around the annulus. It is shown that a pitchfork bifurcation exists, separating standing wave and travelling wave solutions. Due to the interaction with non-uniform thermoacoustic feedback, an azimuthal flow with a low Mach number can significantly influence the system stability. Close to the bifurcation point, the non-normal nature of the dynamic system can induce a considerable gain of acoustic energy and yield more predictable time traces. These findings address the influence of non-normality, when applying a linear damping rate, acoustic amplitude or entropy-based quantity with the intent to monitor combustion dynamics in an annular combustion system.
Using multiphoton ionization spectroscopy, we have observed transitions in Ba from the 6s(2)(1)S(0) ground state to even-parity autoionizing states. We have seen extended series with J = 0, 1, and 2 of the type 5dnd and 5dns, converging on the 5d(2)D(3/2) and (2)D(5/2) ionization limits. A multichannel quantum-defect theory analysis of the J = 0 spectrum is presented.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.