AIR STAGING IN A SWIRLED NATURAL GAS BURNER WITH RADIAL INJECTION

The present work describes the experimental investigation on NOx emissions of partially premixed flames characterised by air staging through radial injection of premixed jets into a co-flow swirling air. NOx emissions were found to increase with increasing partial premixing and reach a maximum value at Φp ≈ 5, followed by a dramatic decrease as Φp approaches values ≈ 3. In the most favourable conditions, single digit NOx levels were reached without penalizing combustion performances or increasing CO emissions. The effects of staging on the amount of NO2 found at the exhaust were also examined and the results corroborate the idea that NO2 is formed in the turbulent mixing region, favoured by the increasing cooling rate of the combustion gas by entrainment of cold unburned gas. PIV measurements of the isothermal flow field are used to confirm the analysis and to better understand how the emissions are related to the turbulent mixing of the premixed first stage with the secondary swirling air.