The objective of this paper is to build a schlieren optical system based on the experimental device for cavitation flow in cryogenic fluids,and to study the spatial and temporal characteristics of the density distribution in liquid nitrogen cavitating flows.In the experiment,the structure and density distribution of liquid nitrogen cavities at different cavitation numbers for 77 K were measured by applying high-speed photography and chlieren technology.The unsteady behaviors of incipient cavitation,sheet cavitation,and large-scale cloud cavitation in liquid nitrogen were investigated.The results show that:① The schlieren optical system can capture the cavitation-induced density gradient that is not observed in backlighting experiments.② The significant change of density in liquid nitrogen is not observed under the non-cavitation condition.The rectangular structures are observed in the cavitating flows.This demonstrates that the density of the liquid phase changes.The rectangular structure appears above the attached cavity,begins at the throat,and disappears when leaving the cavity.③ The sound velocity of the vapor-liquid mixture in the cavitation region is much smaller than that of the liquid phase,and the local Mach number increases.The density of the fluid changes as it is compressed,producing a striated density gradient structure.④The unsteady characteristics of the rectangular structures are consistent with the cavitating flows.
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