Session: 04-06 Combustion Dynamics - Modeling II

Submission Number: 177513

On the Role of Control Fuel in Active Suppression of Thermoacoustic Instabilities in Non–Premixed, Fuel–Blended Combustion Chamber 

Active control of thermoacoustic instabilities is frequently implemented in combustion applications to mitigate or suppress the amplitude of oscillations of unstable modes, improving operational robustness of the system. Stabilisation of the operating condition of the combustion chamber is typically achieved by means of two different approaches, depending on whether the energy content of the system is dissipated or modulated. Dissipation is produced by employing actuators, such as retractable pistons, that allow real–time adjustment of acoustic boundary conditions, allowing their reflectivity to be lowered, removing acoustic energy from within the chamber.

Modulation of the energy content of the system is enforced by providing time–varying external inputs of acoustic or thermal energy. The former are often produced by means of loudspeakers installed in the combustion chamber, and are commonly used to stabilise systems with low thermal power. The latter can be produced by applying plasma micro-charges, using heatable grids, or by varying the combustion equivalence ratio – achieved by modulating the amount of fuel injected into the combustion chamber.   

This work focuses on the active control of thermoacoustic instabilities through fuel modulation in a turbulent, non–premixed combustion chamber, characterised by the direct injection of separate streams of combustible methane and hydrogen directly into the combustion zone, which are responsible for 36% and 64% of the average thermal power of the installation, respectively.

The purpose of the research is to determine the differences in operational performance when selecting one fuel over another as a means of modulating the heat release of the flame, with particular focus on fluctuations settling time, overall control fuel consumption and control saturation effects.

The thermoacoustic behaviour of the system was simulated through a Reduced Order Model framework. The response of the turbulent, non–premixed flame to acoustic disturbances was borrowed from recent findings in the literature, whose trends were used to fit a simple second–order Flame Transfer Function model with a satisfactory degree of accuracy. Extension of this model to non–linear perturbation regime was made by the means of a well–established saturation function allowing to construct a compact Flame Describing Function. The acoustics were modelled with a convective wave–based approach.

Non–linear stability analysis in the frequency domain made it possible to determine the unstable operating conditions of the combustion chamber and assess the corresponding limit cycle amplitudes. This information was used to time–march the governing equation of the Reduced Order Model in the time domain until the settlement of a limit cycle was obtained, and hence to apply the control strategy.

Control was actuated by a Self–Tuning Regulator governing a second–order model of a servo–valve that modulated the amount of control fuel injected into the combustion zone. Delays in control actuation and control fuel combustion were included to improve the degree of physical fidelity of the model. The control effort was limited by imposing that the absolute value of the control heat release rate was contained within a small percentage of the mean one, thus incorporating saturation effects.

The same parameters were chosen for the Actuator Transfer Function and the saturation value of the control heat release rate in both the methane and hydrogen control fuel cases, allowing for the differences in the comparison of the two strategies to be solely attributed to the different properties of the two fuels.

Presenting Author: José María García-Oliver Universitat Politecnica de Valencia

Presenting Author Biography: Prof. José María García-Oliver is the leader of the CMT team dealing with Gas Turbine and Spray Combustion. His main research focus is on Low Technology Readiness Level studies with optical techniques and detailed and low-dimensional modelling.

Authors:

Adriano Bordoni Universitat Politecnica de Valencia
Alberto Broatch Universitat Politecnica de Valencia
José María García-Oliver Universitat Politecnica de Valencia
Jorge García-Tíscar Universitat Politecnica de Valencia