A recent publication of the CAPS laboratory studies the complex dynamics of a lab-scale combustor subject to thermoacoustic instability. This phenomenon is a recurrent issue in constant pressure combustor -such as gas turbines, aeronautical engines or rockets- which can lead to catastrophic failures of the combustion system. This instability can arise suddenly, if the operating condition of the machine is varied. The study answers the question: how fast can I change the operating condition, to be able to go back to a safe point in case the system runs into a dangerous condition?

What do earth climate, traffic jams, stock market, infectious diseases and a gas turbine combustor have in common? They can all exhibit sudden transitions to catastrophic states, for small variation of a parameter of the system. In the case of combustion systems, this variation might lead to thermoacoustic instability, a state associated with high-amplitude acoustic pressure oscillations that can destroy the machine.

In the recent publication by CAPS Laboratory “Experiments and modelling of rate-dependent transition delay in a stochastic subcritical bifurcation”, the authors studied the effect of a continuous variation of a control parameter between two values corresponding respectively to a stable and an unstable operation. This is particularly relevant for the development of combustors: operating conditions of gas turbines are often varied in time, for matching power grid requirement, and similar rapid changes of the combustion regimes also occur in aeronautical engines, especially at take-off. The paper shows experimental evidence that, when the control parameter is ramped fast, the transition to the unstable condition is delayed, but when it occurs the acoustic level is higher. The study also shows that over a certain ramp rate, it is impossible to come back to a safe condition before the system is permanently damaged.

The paper “Experiments and modelling of rate-dependent transition delay in a stochastic subcritical bifurcation”, by Giacomo Bonciolini, Dominik Ebi, Edouard Boujo and Nicolas Noiray, is published in Royal Society Open Science 2018 Volume 5, Page 172078.

Figure 1: Illustration of the various mechanisms of critical transition (tipping) active in the present study. Solid and dashed black lines: deterministic attractor and repeller for the system state. Light to dark hues: low to high probability density for the system state. Note how the system experiences a subcritical bifurcation, going from a stable to an unstable condition, the latter being characterised by high amplitude acoustic oscillations.

The team involved in this project include Giacomo Bonciolini, and Prof. Nicolas Noiray, ESC member and head of the CAPS – Combustion and Acoustics for Power Systems laboratory. The CAPS is engaged in activities aiming at addressing fundamental and practical problems that are relevant for the development of workable, efficient, robust and sustainable technologies in Energy and Transport sectors. Its focus is on reacting and non-reacting flow control and on the reduction of CO2, pollutants and noise emissions.

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