Combustion instability of a lifted triple flame downstream of the injector nozzle in an acoustic field is studied, with implication on the combustion instability of liquid rocket engines. The analysis is focused on the premixed flame segment, and is based on the linear equation for a perturbed flamefront. Assuming that the radius of the flamefront curvature is much larger than that of the internal flame structure, the premixed front is approximated as being quasi-planar. Both acoustical stabilization of the hydrodynamic flame instability and excitation of the parametric instability by sound waves are considered, and the limitation for stable/unstable regimes as a function of flame parameters and acoustic frequency/intensity is determined. We first consider the formulation in the Landau limit of infinitely thin flames, and subsequently extend it to that of finite flame thickness. It is demonstrated how unstable regimes are modified by the flame thickness.
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