Influence of hydrogen content and injection scheme on the describing function of swirled flames

Abstract

The dynamics of V-shape swirled lean premixed methane/air flames enriched with hydrogen is examined for two injection schemes. The response of the flames submitted to harmonic flowrate modulations is compared when hydrogen is premixed with the main methane/air flow and when it is injected pure as a pilot jet, directly at the flame base. Experiments are carried out at constant thermal power. Results obtained for the flame describing function (FDF) show that for a given hydrogen content, the premixed and pilot injection strategies lead to drastically different responses, although the shape of these flames are close. In the fully premixed strategy, the frequency bandwidth over which the flame is very responsive widens as the flame shortens due to the higher reactivity of the hydrogen enriched combustible mixture. Increasing the hydrogen content leads to an increased receptivity of the premixed flame sheet to incident flow perturbations. The opposite effect is seen for the pilot injection strategy due to a rebalancing of the heat release rate distribution along the flame brush with higher reaction rates close to the flame base compared to the reference methane/air case and the fully premixed hydrogen injection strategies. With hydrogen pilot injection, heat release rate fluctuations at the flame base interfere with those further downstream along the reaction layer, leading to an overall reduction of the FDF gain. This mechanism is evidenced with a set of experiments and confirmed by a low order model that considers a non uniform distribution of the heat release along a wrinkled flame sheet. It is also shown to persist when the forcing level is varied. These experiments indicate that the FDF of swirling V-flames can be lowered over a broad frequency range with hydrogen piloting due to a higher reactivity at the flame base.