Acoustic response of droplet flames to pressure oscillations

The acoustic pressure response of spherical H₂/liquid oxygen droplet flames is studied by the premixed-flame regime of activation-energy asymptotics to examine acoustic instability mechanisms in liquid-propellant rocket engines. Depending on the diameters of the droplets, the combustion condition is classified as near-equilibrium or near-extinction, and the acoustic pressure response for each condition is determined for a wide range of the acoustic frequency. Compared with the results previously obtained for strained diffusion flames, the reaction sheet of the droplet flame is found to exhibit a behavior similar to that of strained diffusion flames, in that the reaction sheet moves toward the oxidizer boundary, at which the mass flux of the oxidizer is greater, to balance the high reactivity during the period of high acoustic pressure. Oscillations of the reaction sheet also give rise to an additional attenuation mechanism, associated with reduction of the reaction-surface area, thereby resulting in a much smaller response of the heat-release rate for droplet flames than that for strained diffusion flames.