Micro-Mixing Combustion for Highly Recuperated Gas Turbines: Effects of Inlet Temperature and Fuel Composition on Combustion Stability and NO_x Emissions

The micromix combustion concept offers an elegant compromise between premixed and nonpremixed combustion. By mixing the fuel and air at the smallest scale possible, one can achieve NO_x emissions comparable to premixed combustion while removing the risks of auto-ignition and flashback. Current literature reports multiple micromix designs that achieve low NO_x emissions (<10 ppm) with hydrogen or hydrogen-rich fuels at combustor inlet temperatures (CIT) representative of low to medium pressure ratio gas turbines (<650 K). This paper seeks to bridge the gap between current literature and the design requirements of highly recuperated ceramic gas turbines, which should allow low NO_x operation with various fuels at combustor inlet temperatures upwards of 900 K. To this end, micromix injection nozzles were designed and tested at small scale to investigate the effects of fuel composition and inlet temperature on combustion stability and NO_x emissions. The nozzles were additively manufactured in Inconel 625 having hundreds of holes as small as 0.25 mm. An axial swirler is used to induce recirculation of the products behind the nozzle, which helps stabilize combustion with hydrocarbon fuels due to their longer reaction times and slower flame speeds. Experimental results show that NO_x emissions can be decreased down to premixed levels if the jet Damköhler number is kept under a critical value, which requires increasingly smaller holes or higher jet velocities as the inlet temperature increases. Combustion instabilities are observed at low inlet temperatures with hydrocarbons, which are also correlated to the jet Damköhler number.