MICRO-MIXING COMBUSTION FOR HIGHLY RECUPERATED GAS TURBINES: EFFECTS OF INLET TEMPERATURE AND FUEL COMPOSITION ON COMBUSTION STABILITY AND NOX EMISSIONS

The micro-mixing combustion concept offers an elegant compromise between premixed and non-premixed combustion. By mixing the fuel and air at the smallest scale possible, one can achieve NOx emissions comparable to premixed combustion while minimizing the risks of autoignition and flashback, making this approach particularly well suited to high inlet temperature applications such as sub-MW recuperated gas turbines used in aerospace applications. This paper seeks to bridge the gap between current micro-mixing literature, limited to inlet temperatures <800 K and hydrogen or hydrogen-rich fuels, with the design requirements of highly recuperated gas turbines, which operate on hydrocarbons at inlet temperatures >950 K. Micro-mixing fuel nozzles are additively manufactured in Inconel 625, having up to hundreds of fuel injection holes as small as 250 mu m. In addition to hydrogen, these nozzles are tested with hydrocarbon-based fuels such as propane, natural gas, and Jet A-1. An axial swirler is used to induce recirculation of the products behind the nozzle, which helps stabilize combustion with hydrocarbons due to their longer reaction times and slower flame speeds. Experimental results show that NOx emissions can be decreased down to premixed levels if the jet Damkohler 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 Damkohler number. There is a window in which both low NOx and stable combustion is possible, which resulted in corrected NOx emissions of <10 ppm for all the fuels tested at highly recuperated gas turbines conditions, specifically combustor inlet temperature of 950 K and flame temperature of 1550 K. The high inlet temperature also allowed to fully vaporize liquid fuels within the injector, leading to potential reductions in smoke emissions due to the absence of fuel droplets in the combustion zone.