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In order to overcome the instability of ultra-lean combustion, a novel vortex-tube combustor with axial fuel injection was proposed. The stability limit, flame configuration, NO x emissions, and burn-off rate of the combustor were investigated experimentally under various global equivalence ratios (ϕ g) and fuel flow rates (q f). Results show that the combustor exhibits a large stability limit with ϕ g and q f as low as 0.01 and 6 × 10−5 m3/s respectively. Complete combustion and ultra-low NO x emissions of less than 3 ppm can be achieved at ϕ g of 0.3 and q f of more than 30 × 10−5 m3/s, indicating that the combustor has a good potential for ultra-lean combustion and low NO x emission. The pressure fluctuation amplitude is always less than 1300 Pa during the entire experiments. The vortex-induced flame has a diffusion-like flame structure, which provides a suitable equivalence ratio zone under ultra-lean conditions, whilst the high peak combustion temperature indicates an intensified combustion, which is responsible for the large stability limit and low-pressure fluctuation amplitude. Subsequently, the enhanced stabilization can enable the ultra-lean combustion and the ensued low temperature to conduct, whilst the vortex-flow can decrease the local flow velocity and enable the turbulent diffusion velocity of NO to be dominant, which can make the NO emission reduced further. The decreased area of high-temperature region and the appearance of the negative reaction rate region of NO is the essential reason for the decrease of the NO emission at lean cases. • One novel vortex-tube combustor is researched with diffusion peculiarity. • This combustor can achieve a large stability limit and low NO emission. • The diffusion flame structure provides a NO reduction zone and promote stability. • The decreased high temperature area can decrease the thermal NO. • The rich-fuel region and the flow field structure can promote the NO reduction. [ABSTRACT FROM AUTHOR]