Impact of local secondary gas addition on the dynamics of self-excited ethylene flames

Taaha Hussain,Midhat Talibi,Ramanarayanan Balachandran

doi:10.1016/j.ijft.2020.100057

Taaha Hussain, Midhat Talibi + Show 1 more

Open Access

https://doi.org/10.1016/j.ijft.2020.100057

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Abstract

Advanced combustion strategies for gas turbine applications, such as lean burn operation, have been shown to be effective in reducing NOx emissions and increasing fuel efficiency. However, lean burn systems are susceptible to thermo-acoustic instabilities which can lead to deterioration in engine performance. This paper will focus on one of the common industrial techniques for controlling combustion instabilities, secondary injection, which is the addition of small quantities of secondary gas to the combustor. This approach has often been employed in industry on a trial-and-error basis using the primary fuel gas for secondary injection. Recent advances in fuel-flexible gas turbines offers the possibility to use other gases for secondary injection to mitigate instabilities. This paper will explore the effectiveness of using hydrogen for this purpose. The experiments presented in this study were carried out on a laboratory scale bluff-body combustor consisting of a centrally located conical bluff body. Three different secondary gases, ethylene, hydrogen and nitrogen, were added locally to turbulent imperfectly-premixed ethylene flames. The total calories of the fuel mixture and the momentum ratio were kept constant to allow comparison of flame response. The heat release fluctuations were determined from the OH* chemiluminescence, while the velocity perturbations were estimated from pressure measurements using the two-microphone method. The results showed that hydrogen was the most effective in reducing the magnitude of self-excited oscillations. Nitrogen had negligible effect, while ethylene only showed an effect at high secondary flow rates which resulted in sooty flames.

Highlights

While lean burn operation in gas turbine combustion is beneficial in reducing emissions of nitrogen oxides (NOx), these systems are susceptible to thermo-acoustic oscillations, commonly known as combustion instabilities [1À5]
In order to determine correlations between flame and flow dynamics, flame-vortex interactions have been studied in several different configurations: counterflow diffusion flames [16,17], turbulent swirl flames [18À22], propagating flat flames [23,24] and turbulent jet flames [25,26]
If a flame is weak enough and the vortex residence time in the flame is long enough, it can be considerably lengthened and rolled up, which leads to an increase in global heat release and could cause local flame extinction due to excessive strain an curvature induced by the vortex [12,13,35]

Summary

Introduction

While lean burn operation in gas turbine combustion is beneficial in reducing emissions of nitrogen oxides (NOx), these systems are susceptible to thermo-acoustic oscillations, commonly known as combustion instabilities [1À5] These instabilities are caused by unsteady combustion which can alter the heat release rate, and even travel further upstream the combustor and interfere with the air/fuel mixing process [6]. In order to determine correlations between flame and flow dynamics, flame-vortex interactions have been studied in several different configurations: counterflow diffusion flames [16,17], turbulent swirl flames [18À22], propagating flat flames [23,24] and turbulent jet flames [25,26] Optical diagnostic techniques, such has high repetition rate PIV (particle image velocimetry) and PLIF (planar laser induced fluorescence), have recently been used to study transient phenomena caused by vortex structures, such as lean blowout [27,28], thermoacoustic oscillations [29,30], local extinction [29,31,32] and flashback [30,33]. If a flame (premixed or non-premixed) is weak enough and the vortex residence time in the flame is long enough, it can be considerably lengthened and rolled up, which leads to an increase in global heat release and could cause local flame extinction due to excessive strain an curvature induced by the vortex [12,13,35]

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