Characterization of flame-shedding behavior behind a bluff-body using proper orthogonal decomposition

Abstract

Researchers have been interested in bluff-body flame-shedding dynamics and the effects of vortex shedding for the past few decades. The objective of the present study is to quantify various modes of vortex shedding as flames transition from near-blow-off to stable and acoustically coupled conditions. Two modes of shedding, Kelvin–Helmholtz and Von-Karman, play a significant role in flame stability. Previous studies have been limited to visual identification of the contributions from each of these modes without quantitatively addressing the contribution from each mode as the flame transitions from blow-off to a stable or acoustically coupled state. The present study is focused on quantitatively identifying the contribution of various instability modes for three 1.5-in. flame-holder configurations and is performed in an augmentor test rig employing propane and air as fuel and oxidizer, respectively. The quantitative identification of instability modes is performed by implementing proper orthogonal decomposition (POD) on high-speed chemiluminescence imaging for various flame configurations. The application of POD provides an objective means of examining contributions from asymmetric, symmetric, and uncorrelated spatial shedding modes. To determine trends, equivalence ratios are set to 1.1 and then stepped down until blow-off. Acoustically coupled flames are observed to be dominated by contributions from the symmetric shedding mode. As flames decouple from rig acoustics, they exhibit increased combinations of asymmetric and uncorrelated shedding behavior, depending on flow conditions. The methodology can also be extended to other measurement techniques such as high-speed particle-image velocimetry, planar laser-induced fluorescence, and numerical simulations as a means of studying bluff-body flames.