Axial Swirler Outlet Shroud Influence on Premixed Combustion NOx Emissions for All the Combustion Airflow Passing Through the Swirler

A 76mm diameter combustor with single axial flat 8 bladed swirlers of 30°, 45° and 60° vane angle were investigated at a reference Mach number of 0.05 or combustion intensity of 20MW/(m²bar), which represents all the combustion air passing through the swirler. The influence of a swirler outlet shroud or outlet orifice plate was investigated. This increased the pressure loss and the outer dump expansion recirculation zone size, D/d, and these were the main influences on the combustion performance. The swirlers were very large relative to the combustor so that they had a high air flow capacity. They had the same outer diameter, d, of 70mm with a cylindrical combustor diameter, D, of 76mm, D/d = 1.1. These swirlers had a decreasing hub size as the vane angle was increased. There was no significant effect of the outlet shroud on the weak extinction. The presence of the shroud promoted more rapid flame development and this produced a reduction in HC and CO. This also reduced the prompt NOx due to the reduction in HC. The overall NOx was reduced as the axial swirler outlet shroud diameter was reduced. The trends were the same for all three swirler vane angles, but there was little advantage of using higher swirl number large vane angle swirlers when compared at the same pressure loss. The minimum NOx at 15% oxygen for premixed combustion at 1750K with 600K inlet temperature was 2ppm for 30° shrouded axial swirlers, 3ppm for 45° shrouded swirlers and 2.5ppm for 60° shrouded swirlers. However, a 45° unshrouded axial swirler with smaller central hub and large D/d had NOx emissions of 2ppm.