This experimental study focuses on combustion of liquid fuel droplets subject to external acoustic disturbances in the form of standing waves within an acoustic waveguide. Acoustic perturbations create a mean flame deflection dependent on the droplet location relative to the pressure node (PN) or pressure antinode (PAN) in the waveguide. This flame deflection is consistent with the sign of a theoretical acoustic acceleration acting on the burning system. Yet experimentally derived acoustic accelerations estimated from the degree of flame deflection differ quantitatively from that predicted by the acoustic radiation force theory. Phase locked OH* chemiluminescence imaging reveals a deflected flame which oscillates in position relative to the stationary droplet. Flame luminosity fluctuates with pressure throughout the forcing period, indicating the flame heat release rate is influenced by acoustic excitation. The thermoacoustic instability fostered by the in-phase oscillation of pressure and heat release rate is quantified via the Rayleigh index. Evaluation of the Rayleigh index over a range of acoustic forcing frequencies and droplet locations exposes the frequency sensitive character of the combustion mechanism stemming from dissimilar acoustic and chemical kinetic time scales. The present experimental configuration provides a useful test bed for evaluating the response of different alternative and conventional fuels to an acoustically resonant environment in the context of the well-known Rayleigh criterion.
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