Experimental and numerical study of MILD combustion for gas turbine applications

Highlights: • Effect of pressure, mixing on stability of MILD combustion studied for gas turbines. • High pressure increases NO_x emissions and destabilizes MILD combustion mode. • Enhanced mixing stabilizes MILD combustion and lowers NO_x emission. • Mixing is key parameter to control and stabilize MILD combustion. - Abstract: In this paper, the pressure influence on the MILD combustion process and on emissions is examined. Experiments were performed in a combustion chamber based on the reverse flow configuration to enhance mixing of fresh and burnt gases. First, investigations under atmospheric pressure were performed to determine the regime of jointly low NO_x and CO emissions. Afterwards, the effects of burnt gas recirculation and mixing under ambient pressure were evaluated by decreasing the inlet nozzle diameter without changing the residence time. In the second step, measurements were conducted under higher pressure while keeping the mass flow rates constant. Thereby, the residence time is extended, the NO_x formation chemistry is changed. These effects result in a strong rise in NO_x emissions and simulations indicate that at higher pressure a flame is established at the nozzle exit for higher equivalence ratios. In order to decrease the Damköhler number and shift the combustion process to the well-stirred reactor regime, the inlet nozzle diameter was decreased without changing the residence time. Thereby, burnt gas recirculation and gas mixing is enhanced, while simultaneously extending the chemical time scales. A significant decrease in NO_x emissions was detected