Dynamic Characterization of a Ducted Inverse Diffusion Flame Using Recurrence Analysis

Normal diffusion flame or partially premixed flame is used in many applications, such as aviation engines, tanks, ocean vessels, and industrial furnaces, because of its high flame stability and relatively low susceptibility to dynamic instabilities compared to lean premixed flames, which give lower emissions. However, associated with such flames are high NO<i>_x</i> and soot emissions, which are particularly high for heavier hydrocarbon fuels. Increasingly stringent environmental norms have thus dictated the search for alternate approaches; one such being the inverse diffusion flame, which is currently being used in rocket motors, for staged combustion in gas turbine combustors, and furnaces. However, the dynamic response of such a flame, particularly in ducted applications where a coupling between unsteady heat release rate and duct acoustics may occur, is relatively less explored. The present work aims to plug that knowledge gap through an experimental investigation of a laboratory-scale ducted inverse diffusion flame. Two parametric variations of the flame were performed—variation of the flame position and variation of the air flow rate. Using tools of nonlinear dynamics, such as phase space reconstruction and recurrence quantification, several interesting dynamic characteristics were observed, such as limit cycles, intermittency, and homoclinic orbits. For a constant air flow rate, the system was observed to transition from a type-II intermittency regime to a limit cycle and then again to intermittent behavior as the position of the flame within the duct was varied. A similar trend was observed when the air flow rate was varied at a fixed flame position.