Three-dimensional investigation of flame dynamics of azimuthally forced flames in annular combustor using Scanning PLIF

In combustion systems, self-excited thermo-acoustic instabilities are a well-known problem and many studies are aimed at better understanding the phenomenon. Three-dimensional techniques are useful when investigating asymmetrical responses in such systems. In this study, a three dimensional Scanning OH* Laser Induced Fluorescence (LIF) method is used to investigate the response to forced azimuthal oscillations in an annular combustor. Two speaker pairs were used to control the orientation and nature of the acoustic mode inside the combustor. The flame of interest was subjected to spinning modes and standing modes, in the latter case the flame was positioned at either a pressure node, an anti-node, or an intermediate position between these. The measurements consisted of scanning a laser sheet across the flame with the use of a galvanometric mirror. Flame edges extracted from the imaging were then conditionally averaged based on the forcing cycle and projected to real space, from which spatially resolved three dimensional Flame Surface Density (FSD) is calculated. FSD was then used to investigate the flame structures under different forcing conditions. The scanning method made use of two separate scan directions to overcome bias errors in determining FSD at the flame edges. The results provide insight into asymmetries in the flame dynamics and response due to both the azimuthal excitation and annular confinement geometry.