An ultra-low emissions combustor,namely low emission stirred swirl(LESS)combustor was studied,based on a scheme of internally-staged/lean premixed and prevaporized(LPP)combustion.The LESS combustor consists of central pilot stage and outer surrounded coaxially main stage,between which there exists aphysical isolation,namely the step height.The existence of step height delayed the pilot and main jets mixing.Experimental and numerical studies were carried out to investigate the influence of the step height on the combustion performance.A single dome rectangular combustor was utilized to conduct the lean lightoff and blowout experiments,and pollutant emission experiments.The experimental results showed that with the increase of step height by 38%,the lean lightoff and blowout fuel air ratio decreased by 57.4% and 37.5%,the NOx emission increased by 35.1%,and the combustion efficiency increased by 1.78%;while the CO,unburned hydrocarbons(UHC)emissions decreased.Furthermore,the total pressure loss was kept nearly constant.Non-reacting and reacting flow fields were numerically investigated to analyze the coupled characteristics of pilot and main jets with different step heights.A comparison of flow characteristics,spray structure,and combustion component as well as temperature field with different step heights was conducted.The numerical results showed that the increase of the step height shifted the peak velocity outwards.The enlargement of the primary recirculation zone(PRZ)resulted in the increase of the combustion efficiency and NOx emission,while the CO,UHC emissions decreeased. An ultra-low emissions combustor, namely low emission stirred swirl (LESS) combustor was studied, based on a scheme of internally-staged / lean premixed and prevaporized (LPP) combustion.The LESS combustor consists of central pilot stage and outer surrounded coaxially main stage, between which there exists aphysical isolation, that the step height. the existence of step height delayed the pilot and main jets mixing. Experimental and numerical studies were carried out to investigate the influence of the step height on the combustion performance. A single dome the experimental results showed that with the increase of step height by 38%, the lean lightoff and blowout fuel air ratio decreased by 57.4% and 37.5%, the NOx emission increased by 35.1%, and the combustion efficiency increased by 1.78%; while the CO, unburned hydrocarbons (UHC) emissions decreased. Futuremore, the total pressu re loss was kept nearly constant. Non-reacting and reacting flow fields were numerically investigated to analyze the coupled characteristics of pilot and main jets with different step heights. A comparison of flow characteristics, spray structure, and combustion component as well as temperature field with different step heights was conducted.The numerical results showed that the increase of the step height shifted the peak velocity outwards.The enlargement of the primary recirculation zone (PRZ) resulted in the increase of the combustion efficiency and NOx emission, while the CO, UHC emissions decreeased