Flame Density Ratio Effects on Vortex Dynamics of Harmonically Excited Bluff Body Stabilized Flames

Abstract

This paper presents an experimental study of acoustically forced bluff body stabilized flames, motivated by the problem of combustion instabilities. The goal of the work is to better understand the flame and flow behavior as functions of the proximity of the acoustic frequency to natural hydrodynamic instability frequencies. It is well known that iso-density, high Re bluff body wakes are globally unstable, exhibiting the Von Karman vortex street. In reacting flows, however, this global mode is suppressed if the density ratio across the flame is sufficiently high. Thus, the density ratio is an important parameter that influences the global mode growthrate. In this study, the flame was longitudinally forced over a range of hydrodynamic global mode to forcing frequency ratios, density ratios, and forcing amplitudes. Longitudinal forcing leads to the symmetric rollup of the two separating shear layers. When the forcing frequency is in the vicinity of the wake’s global mode frequency, the global mode locks into the forcing frequency, and the symmetric shear layers quickly stagger as they convect downstream, leading to a large scale, sinuous flapping of the wake and flame. The axial position at which staggering occurs is a function of the forcing amplitude and the proximity of the forcing frequency to the global mode frequency. The lock-in phenomenon amplifies the flame’s motion at the forcing frequency. However, if the vortices stagger to a fully sinuous structure, this causes a significant reduction in the flame’s oscillatory heat release through phase cancellation of the upper and lower flame branches. Therefore, if a low density ratio flame is subjected to longitudinal acoustic forcing near its global mode frequency, it will respond with weaker heat release fluctuations than it would away from lock-in. This is true even though the local degree of flame flapping is quite significant. Thus, the results of this study show some phenomena that contradict conventional notions, namely that forcing a globally unstable flow near its global mode frequencies can lead to diminished local heat release oscillations.