The Role of Injection Pressure in the Impact Region of Liquid CO₂ Flash Boiling Jets for Coal Breaking

In carbon capture, utilization, and storage (CCUS), the CO₂ captured in industrial processes are employed as a working fluid in coal bed methane drilling operations. This not only enhances the efficiency of the extraction process but also ensures the storage of the CO₂ underground. The liquid CO₂ (LCO₂) multi-hole flashing jets was proposed for coalbed methane extraction. The Injection pressure not only influences the impact force but also modifies the state parameters of flash boiling, thereby altering the phase change state and the effective impact region. This research investigates the influence of injection pressure on the flow field morphologies and coal breaking characteristics to effectively manage the flow field of LCO₂ flashing jets. The findings reveal that the LCO₂ flashing jets exhibits a non-monotonic relationship between jet width and superheat degree. For small-diameter nozzle (less ejected mass), the system's energy delivery still meets the requirements for nucleation clusters, with jet width primarily governed by nucleation rate. It was observed that the normalized jet width exhibits an exponential relationship with \({P}_{\text{inj}}\) $P_{\text{inj}}$ / \({P}_{\text{t}}\) $P_{\text{t}}$ , with minimal phase change within the nozzle at high pressures. As injection pressure continues to increase or larger diameter nozzles are utilized (more ejected mass), there may be inadequate energy supply from the system. Linear changes were observed in the correlation analysis between normalized jet width and the square of injection pressure \({{P}_{\text{inj}}}^{0.5}\) ${P_{\text{inj}}}^{0.5}$ . Variations in impact force attributable to injection pressure are crucial in inducing damage to coal, but flashing determines the impact region and distribution of breakage coal. A direct correlation exists between fractal dimension and superheat degree of flashing. These findings offer an essential reference for rock breaking in drilling.