Investigation of the morphological dynamics and compressible characteristics in liquid CO2 flash boiling jets

The high proportion of CO₂ and low pressure in the Martian atmosphere provide advantages for CO₂ flash boiling jets technology used in Martian mining, and elucidating its behavior is of great significance for the Mars in-situ resource utilization. In this study, the diffused back-illumination method was used to conduct liquid CO₂ flash boiling injection experiments. A novel method called adaptive gradient threshold filtering has been proposed to identify jet boundary, which has the advantages of accuracy and smoothness. The morphology and compressible flow characteristics under different injection ($P_{\mathrm{inj}}$) and ambient pressures ($P_{\mathrm{amb}}$) were studied. Radial expansion dominated by flash boiling from correlation only occurred within 10$d_t$, which different from 20$d_t$ of conventional fluid. The nucleation rate and system energy supply determine the radial expansion. However, the jet axial evolution characteristics may be dominated by the compressibility under constant $P_{\mathrm{amb}}$. The position of Mach disk shows a square root relationship with the ratio of $P_{\mathrm{inj}}$ to $P_{\mathrm{amb}}$ (${\eta }_{\mathrm{inj}}$), which is similar to the relationship in gaseous and supercritical jets. Finally, the near-field morphology and flow field characteristics of single-hole and 5-hole nozzles were compared. The discovery of discontinuous liquid cores indicates the comprehensive effect of energy supply, condensation, and jet interaction.