Active Instability Control (AIC)

I. Motivation

In order to develop cleaner and more efficient combustion systems, lean premixed (LP) combustion has become a standard approach. Unfortunately operational experience has shown that the LP approach is more susceptible to combustion instabilities. Therefore thermoacoustic combustion instabilities, involving a feedback cycle between fluctuations of velocity, pressure and heat release rate, are a cause for concern in many combustion applications.

II. Theory

Combustion instabilities occur when variations in heat-release rate couple with pressure oscillations in the combustion system. These combustion oscillations can result in pressure oscillations with high amplitudes (160 dB and above) at low frequencies (hundreds of Hertz in gas turbines). Obviously, pressure fluctuations of this magnitude and frequency can have a detrimental effect on the mechanical integrity of the combustion system. It leads also to increased emissions and thus limits the operational range of the combustion system. Control strategies are used to suppress these instabilities by passive and active means. Passive control strategies tend to put the combustion system out of tune.

Active instability control (AIC) aims at decoupling the pressure and heat release cycles by fuel mass flow modulations and phase-shifting. For economic reasons only a small amount of secondary fuel, less than 5% of the main fuel, should be modulated by an actuator to counteract the oscillations.

For an active actuator to be effective, it must have the ability to interfere with the coupling process between the acoustic pressure field and the combustion heat release rate. An actuator that modulates fuel mass flow may not have significant effectiveness if the fuel is not injected in a manner which allows to interfere with the coupling process.

To investigate different injection methods and to develop general rules for pilot fuel injection into swirl burners for active instability control are the main objectives of this project.

III. Experimental Setup

The AIC control scheme is shown here.

To force combustion instabilities three different excitations are possible. In order to investigate reproducible thermoacoustic instabilities the air mass flow can be excited by a siren. To damp these instabilities the pilot fuel mass flow can be forced up to 470 Hz by a MOOG valve. As reference for the effectiveness of the AIC-method with pilot fuel the main fuel mass flow can also be excited.

IV. Perspectives

To investigate fundamental processes in AIC with swirl burner a closer look to the influence of the way of injecting secondary fuel on the effectiveness of AIC will be taken. Thereby different kinds of instabilities will be considered:

• self-excited oscillation
• forced oscillation
• flashback

The dependence of NOx-emissions on the AIC-method will be studied, too.