Suppression of instabilities of swirled premixed flames with minimal secondary hydrogen injection

The impact of hydrogen addition on the dynamics of a methane-air premixed flame is explored for different injection strategies. The configuration is a swirled injector with a central tube for pilot fuel injection. Keeping the air flow rate and thermal power of the burner constant, it is shown that even for very small flow rates of hydrogen, as low as one percent of the thermal power, flame stabilization and combus- tor stability are greatly altered when pure hydrogen is injected through the central tube as a pilot jet. It is also shown that fully premixing the same quantity of hydrogen with methane or use of methane for the pilot jet has no significant effects compared to hydrogen pilot injection strategy. The flame re- sponse to forced flow perturbations is use to interpret the observed features. It is shown that hydrogen piloting drastically changes the gain of the flame transfer function at low frequencies and its phase lag at high frequencies, while other injection strategies barely change the flame response for these minute flowrates. CO and NOx emissions are finally examined for the different injection strategies. NOx emissions are found to drastically increase with hydrogen piloting compared to other injection strategies. These ex- periments indicate that pure hydrogen injected in minute fractions may be used as an efficient passive control means to mitigate combustion instabilities, but a compromise needs to be made with emissions