Gas Turbine Combustion Instabilities Diagnosis
Combustion of lean and preformed mixtures (Lean Premixed Combustion, LPM) is the current state of the art among the combustion technologies for stationary gas turbine plants, fuelled by natural gas. This in terms both of energy conversion efficiency both of harmful emission reduction. However, just the ultra-lean premixed flames are those most prone to thermo-acoustic instabilities, that is to the onset of large amplitude pressure fluctuations, dues to the coupling between the unsteady expansion effects of heat release fluctuations and the combustor acoustics resonance modes. Phenomena of this kind are commonly known as thermo-acoustic instability, or “operational” combustion instabilities, as responsible of practical problems during the gas turbines operation. In fact, in addition to shut down for LBO (Flame Lean Blow Out), one can record: dangerous backfire through the mixture flow, until inside the burner body (Flame Flashback); vigorous shaking impressed by pneumatic fluctuations as well as thermal overloads. So, if the incipient unstable state is not perceived early enough to make a regulation aimed to inhibit it, the structural failure of the combustor can occur in a short time. Although the detrimental effects of combustion instabilities are known for many years, and considerable efforts have been made in development both of predictive numerical tools both of systems for real time diagnosis, the objective of inhibiting such phenomena in complex fluid dynamic combustor architectures, through the broader off-design operational filed, is still far from being achieved. However, the experimental methods and numerical simulation techniques today availables allows to better investigate the mechanisms underlying the complex interaction between chemistry and turbulence, that governs the combustion instabilities. One can therefore draw useful clues about the devices for the suppression or at least the control of the anomalous combustion states.
Surprisingly amongs gas turbine manufacturers is common to consider the combustion instability phenomenon as well known and understood. Consequently, the trend in industry is to focus on the gas turbine combustors control systems development ad realization, without considering the need to better understand the dynamic behavior of turbulent flames. However, in the recent past, the unsatisfactory performance of some controllers, was determined just by a wrong combustor dynamical modeling, namely by an incomplete understanding of certain unconventional combustion instability mechanisms. Precisely, in particular forms of acoustic-entropy instabilities – to date only theorized or computationally predicted, and that here we propose to identify experimentally – the passive role of heat release (subject to the entropy sources forcing), could fool the control system, especially if it makes use of optical sensors only “focused” on the chemiluminescence dynamics. It is therefore proposed to define new and more effective indices of instability, amongst which someone require the parallel development of dedicated optical probes, some others need of signal processing techniques other than the frequency spectrum analysis (as Chaotic Analysis).
To verify the exposed physical explanation is mandatory, because in such instabilities, with passive or no role of heat release, the saturation effect of heat release non linearities on thermo-acoustic limit cycle amplitude could be lost. So much larger acoustic wave amplitudes could rise before nonlinear saturation occurs. It should be noted that: <<This type of instability has not, to our knowledge, been previously reported, and so it may be that it is very unlikely to occur in reality. However, sensing using pressure measurements alone (as often happens in practice) would render this type of instability indistinguishable from instabilities that arise due to the coupling to the heat release rate, and hence there may well be instances where it has occured in practice.>> (Gho & Morgans, Comb. Sc. & Tech., 185: 249-268, 2013).