Control of combustion instabilities by local injection of hydrogen

Abstract

New generation combustion chambers are designed to reduce pollutant emissions and combustion instabilities. One consequence of these instabilities can be large amplitude pressure oscillations, which are often coupled with heat release oscillations. As this coupling leads to performance degradations or acoustic noise, various instability control techniques have been developed. The purpose of the present paper is to propose an original instability control technique acting on thermo-acoustic combustion instabilities appearing in an experimental lean premixed burner. The actuation system consists in injecting an hydrogen jet in a premixed hydrocarbon flame. Several diagnostic techniques have been used to characterize the burner behavior and to evaluate the effects of hydrogen injection on both combustion instabilities and pollutant emissions. It is shown that hydrogen injection induces a significant decrease of the amplitude of pressure oscillations while the heat release oscillations remain at the same level. To increase the understanding of the phenomena observed, phase locked images of OH* and CH* emission fluctuations have been obtained. These images show that the hydrogen jet breaks the coupling between acoustic and heat release oscillations. The transient period following the hydrogen injection has also been studied, using a solenoid valve to command the hydrogen injection. This study shows that the hydrogen jet acts quickly on pressure oscillations (<50 ms) without inducing any oscillatory transitory regime in the combustion chamber. Lastly, it has been shown that the hydrogen injection has no negative effects on NO emission but induces a strong increase of CO emission. However, as the hydrogen injection modifies the properties of the gaseous mixture that fed the burner, a new emission index was defined, to compare the different cases studied in terms of pollutant emission.