Small radial swirler low NOₓ combustors for micro gas turbine applications

There is a growing interest in micro-gas turbines for distributed electric power production. This work examines the scale down of a successful low NOₓ radial swirler from a combustor of 140mm diameter to one of 76mm diameter, suitable for micro-gas turbines in the 30 -100kW electric size range. A 40mm outlet eight bladed radial swirler was investigated for four radial swirler vane depths from 30.5mm to 12.2mm and a constant pressure loss of 2.7% at different reference Mach numbers or residence times. For a 740K inlet temperature and 0.6 equivalence ratio, Ø, these gave combustor thermal loadings at atmospheric pressure from 33 to 62 kW and heat release of 7 – 14 MW/m²bar. The small radial swirlers had lean stable flames with near lean flammability weak extinction. The vane passage single hole fuel injection achieved NOx emissions of 1 – 5ppm at 15% oxygen, 740K air inlet temperature and 1700K primary zone temperature, with <10ppm CO. The 1ppm NOx was for the smallest vane depth and lowest thermal power. The higher NOₓ for the larger heat release swirler designs were due to the longer flame development and a greater formation of prompt NOₓ, due to the high UHC for most of the flame development.