Linear Acoustic Analysis of the Preburner of an Oxidizer-Rich Staged Combustion Engine

Abstract

The linear acoustic behavior of an oxidizer-rich staged combustion engine preburner assembly that was similar to that of the RD-170 has been investigated by using real fluid speeds of sound to accommodate supercritical pressures. The speeds of sound, and hence the resulting frequencies, were approximately 20% higher than would be expected from ideal gas assumptions, which is a critical point of difference between the acoustic behavior of the preburner versus the main chamber. The results show that the mode shapes in the preburner–turbine inlet assembly are largely invariant with respect to the boundary condition used for the turbine blade row, but the frequencies are noticeably affected. The 2L mode is totally suppressed for both the acoustically open case and the case with the blade row impedance condition. The remaining longitudinal modes are damped. Both the frequency and the mode shape of the 1T mode remain totally invariant because the oscillations are confined to the injector regions and do not exhibit acoustic velocity with components normal to the impedance surface. The lack of damping of the 1T mode by the blade rows, combined with the relatively high magnitudes of acoustic velocity near the combustion zone, suggests that the 1T mode is a likely candidate to experience instability if it is suitably energized by the heat release.