Effect of Swirl on the Flame Dynamics and Pollutant Emissions in an Ultra-Lean Non-Premixed Model Gas Turbine Burner

ABSTRACT

The effect of varying swirl strength on the isothermal flowfield, flame dynamics, and pollutant emissions in a novel, unconfined and non-premixed swirl burner at atmospheric conditions was investigated using optical and laser diagnostic methods. The burner (IIST-GS1), operated with methane as fuel, produced stable ultra-lean flames at the two geometric swirl numbers investigated, S = 1.59 and 2.81. In both cases, the burner exhibited two flame stabilization zones with varying θ_glob, the bluffbody stabilized and swirl stabilized, with a transition region separating the two zones. Comparison of two-dimensional (2D) flowfield plots with OH* images revealed that the size of the two stabilization zones and, hence, the heat release distributions depended on the swirling strengths. Higher swirl strengths in S = 2.81 enhanced the reverse flow of burned gases towards the burner exit thereby aiding in the flame stabilization. Also, the increased mixing between the fuel, air, and burned gases for S = 2.81 resulted in lower NO_x emissions when compared to S = 1.59 at all operating conditions. With decreasing θ_glob, the flame stabilized at axial locations closer to the burner exit for both swirling strengths. At very low θ_glob the increase in swirling strength enabled the flame to stabilize at regions where the local in-plane extensive strain rates exceeded the critical strain rates for non-premixed CH₄/air flames, suggesting significant premixing of fuel-air with burned gases. The ultra-low NO_x capability of the novel IIST-GS1 burner was evident throughout the investigated operating conditions.

KEYWORDS:

• Laser diagnostics
• Pollutant emissions
• Swirl combustion

Acknowledgments

The authors gratefully acknowledge the support received from R.S. Prakash and Vinil Kumar from the Department of Aerospace Engineering, IIST.