Abstract
The present study is an experimental investigation of the nature of acoustically induced flashback in a lab-scale dump combustor. The control parameters varied include the inlet Reynolds number (Re) and the inlet turbulence intensity. The primary bifurcation plots of the combustor from stable to the unstable condition are seen to be significantly altered by the inlet turbulence intensity, with the latter delaying the onset of combustion instability to higher Re. The analysis of multivariate high-speed data acquisition and processing (viz. unsteady pressure, flame imaging and velocity field by means of PIV) reveals the role of low-frequency high amplitude acoustics in modulating the flame. It is seen that high amplitude oscillations are sustained by two mechanisms 1. Modulation of the flame by coherent structures shedding at the step and 2. The bulk flame motion in-and-out at the edge of the step. It is seen that flow reversal at sufficiently low frequencies provide enough duration for the hot products to ignite fresh reactants upstream of the duct, which in-turn reinforces the coherent unsteadiness in the system, thereby increasing the propensity of the mixture to be ignited more upstream with every cycle. This ultimately leads to the flame flashing back till the point of premixing. This work thus addresses and reforms the occurrence of flashback being an example of loss of static stability, whereby the overriding presence of dynamic combustion instability results in a flashback to behave in a dynamic manner.
