The effect of varying swirl strength on the isothermal flowfield, flame dynamics, and pollutant emissions in a novel, unconfined and nonpremixed 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.glob, the bluffbody stabilized and swirl stabilized, with a transition region separating the two zones. Comparison of twodimensional (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 NOx emissions when compared to S = 1.59 at all operating conditions. With decreasing.glob, the flame stabilized at axial locations closer to the burner exit for both swirling strengths. At very low theta(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 NOx capability of the novel IIST-GS1 burner was evident throughout the investigated operating conditions.