Dynamic characteristics of a cryogenic swirl flow under supercritical conditions

The dynamic characteristics of a cryogenic swirl flow under supercritical conditions were experimentally investigated using a mode decomposition method. 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 proper orthogonal decomposition (POD) technique. Superposed instability structures and vortex ring structures were found in the instantaneous flow image. The spray angle decreased under supercritical conditions because of the unusual phase change of the injectant inside the injector. Two kinds of modes were deduced by a 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.