Structure and dynamics of CH2O, OH, and the velocity field of a confined bluff-body premixed flame, using simultaneous PLIF and PIV at 10 kHz

This study characterizes the structure and dynamics of a confined, bluff-body-stabilized turbulent premixed flame by simultaneously employing formaldehyde (CH2O) and hydroxyl (OH) planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV), at a rate of 10 kHz. The large field-of-view (>170 cm2) CH2O-PLIF is enabled by use of a burst-mode laser delivering 10-kHz pulse trains of 355-nm at 350 mJ/pulse, resulting in a CH2O signal-to-noise of 47:1 during PIV seed flow. Two cases illustrative of the CH2O dynamics are presented. A statistically stationary turbulent combustion case highlights the development of the CH2O layers in space and time. Notably, presumed CH2O-vortex dynamic interactions are observed where the CH2O accumulates broadly near the Kelvin–Helmholtz vortex core and remains thin near the vortex braid, contributing to a distribution of CH2O preheat zone thickness from 1 to 10 times of the calculated laminar value. The second case highlights the CH2O dynamics during a self-excited combustion instability. Two short-duration increases in CH2O are produced during the elevated velocity portion of the acoustic cycle. The first CH2O increase is caused by the reactant mass flux impulse as the velocity starts to increase. The second CH2O increase is the result of the upper and lower shear layers merging downstream and entraining fresh reactants that burn in intense, distributed regions inside the wake. Estimating the time delay between the CH2O and the heat release, it is suggested that the secondary CH2O increase may contribute to damping of the acoustic instability, because of its out-of-phase relationship with pressure, while the first CH2O increase appears to drive the instability.