Three-dimensional structure of the underlying flow field of a bluff-body-stabilised flame forced by multiple convective modes

An experimental study is conducted to understand the 3D flow structures produced by convective/acoustic interference of an acoustically forced bluff-body-stabilised CH₄-H₂ flame. Recently, the convective interaction from vortex shedding originating from cylinders installed upstream of the flame have been shown to passively suppress or amplify thermoacoustic instabilities. The structure of the flame and flow fields in such a scenario is highly 3D and therefore, needs to be determined with a three-dimensional measurement of the flow field downstream of the bluff body. This was achieved through a high-speed (10 kHz), scanning, stereoscopic PIV system. A total of 19 planes, spanning four times the fuel injector pipe's radius, was acquired. The flame structure was captured by imaging OH-chemiluminescence, simultaneously with the centre-plane PIV measurement. The mean flow velocity was ū = 10 m/s at the exit with acoustic forcing frequency and amplitude being 700 Hz and 20% of ū, respectively. Eigenmode decomposition of the velocity fields reveals two dominant frequencies in the flow field, one corresponding to the acoustic forcing frequency (700 Hz) and the other to the cylinders’ vortex shedding that is locked-in to the sub-harmonic of the acoustic frequency (350 Hz). The interplay of these modes is crucial for the global flame response.