Receptivity of a Cryogenic Coaxial Gas-Liquid Jet to Acoustic Disturbances

Abstract

Abstract : An experimental study has been conducted at the Air Force Research Laboratory at Edwards Air Force Base to explore the receptivity of cryogenic coaxial jet flows to transverse acoustic disturbances. The shear coaxial jet flow employed liquid nitrogen in the inner jet and cooled helium in the outer annular jet to represent the nominal fluid dynamical conditions of an oxygen/hydrogen liquid rocket engine injector. The injector flow is submerged in a chamber that experiences a monotonic transverse acoustic resonance characteristic of a rocket chamber in the presence of combustion instability. The coaxial jet is exposed to a variety of acoustic conditions including different frequencies, amplitudes, and locations within the resonant mode shape. High-speed back-lit images were captured to record the behavior of the natural (unforced) and forced coaxial jets. Proper orthogonal decomposition and spectral analysis were used to extract natural and forced modes. Convective modes are extracted, and a new Strouhal number is used to characterize the dominant natural convective mode that is analogous to the preferred mode in free jets. The threshold of receptivity was found for a number of different injector flows and acoustic forcing conditions. The results indicate that the dimensionless frequency plays an important role, and there exists a finite forcing amplitude at which the threshold of receptivity occurs. The receptivity threshold and post receptivity response provides useful insight on the suitability of a given injector design for specific rocket combustion chamber conditions.