Instability of Cryogenic Swirl Flows under Subcritical to Supercritical Conditions

Abstract

Instability characteristics of cryogenic swirl flows were experimentally investigated. The cryogenic liquid nitrogen was injected into a high-pressure chamber through a simplex swirl injector under subcritical to supercritical conditions of nitrogen. High-speed photography with backlight imaging was used to obtain images of the temporally changing flow. The set of images was analyzed by the image processing method. The instability frequency was measured by flow image processing and the laser beam diagnostics. Surface instability of the flow under subcritical to supercritical condition was investigated. Comparing the flow images of the cryogenic and conventional swirl flows, different behaviors were noted, and it was hypothesized that the instability of the cryogenic swirl flow was dominated by the precessing vortex core in the central toroidal recirculation zone. When the ambient condition of the flow was changed from subcritical to supercritical, the phase change and subsequent density change of the injectant differed and flow actions, such as the behavior of the downstream flow, the spray angle, the wavelength, and the propagation velocity, changed drastically. When measuring the frequency of the flow instability, it was found to correspond to that of the precessing vortex core instability. The frequency decreased with the ambient pressure due to the decreasing flow velocity, but it did not change drastically when the surrounding conditions changed from subcritical to supercritical. This implies that the interface of the flow is highly affected by the density of the phase-changed injectant, but that the instability of the flow is determined by the flow in a liquid state. The dynamic characteristics of a cryogenic swirl flow under supercritical conditions were experimentally investigated using a mode decomposition method. Superposed instability structures and vortex ring structures were found in the instantaneous flow image. The spray angle was decreased under supercritical conditions because of the unusual phase change of the injectant inside the injector. Two kinds of modes were deduced by POD analysis of the flow images. The analysis showed that two types of modes exist: a symmetric/tilted-ring shaped mode and an anti-symmetric shaped mode. The Kelvin-Helmholtz instability mechanism generated the symmetric mode. The anti-symmetric structure was created by helical instability, which was generated by the instability of the liquid film inside the injector under subcritical conditions. However, under supercritical conditions, the precessing vortex core in the central toroidal recirculation zone determined the unstable behavior of the flow. A spatial and temporal analysis of the POD modes supported this explanation for the instability. Meanwhile, the spatial characteristics of the coherent structures became similar in the downstream region or under supercritical conditions, which implicates the strong influence of the state of the injectant in flow behavior.