Flame Transition Detection Using Wavelet Transform Applied on the Dynamic Behaviour of a Swirled Staged Multipoint Spray Flame at Different Preheating Temperatures

This study presents a new way of flame states identification using wavelet transform on a microphone signal. This method is applied to investigate the impact of the staging factor, the ratio between the fuel injected through the pilot stage and the multipoint injection, and of the preheating temperature on the flame dynamics. The BIMER combustor is an atmospheric pressure rig equipped with two co-rotating swirling air injections (a fixed amount of around 87% of the air goes inside the multipoint stage) and two fuel injection paths for staged combustion. Liquid dodecane is injected with a global equivalence ratio of 0.6 and a thermal power around 72 kW. First, the identification method recover the intermittency between the five states (including the two stable states) identified in previous study when the staging factor varies from 100% (pilot-only injection) toward 0% (multipoint-only injection). Then, the change in preheating air temperature is investigated at a fuel staging factor of 35% and 15%. At 35%, the flame structure and dynamic is insensitive to change of air temperature, and remain in a V flame. On the contrary, at 15%, the change of preheating air temperature from 120°C to 220°C generates changes in the flame-shape which bifurcates from a detached M flame to an anchored V-flame. This flame shape bifurcation appears at an air temperature around 200°C. In this range of operating conditions, one can witness a bistable flame structure below 200°C, where the flame transit randomly between a detached M flame and an attached tulipe flame. Above 200°C, multistable flames appears where the flame structure transits randomly between five different states.