Characterization method and parameter analysis of multi-hole liquid CO₂ flash boiling jet morphology during coal seam drilling process

The application of hydraulic technology can markedly enhance the efficiency of coalbed methane extraction. Nevertheless, when it comes to enhancing permeability in deep, soft coal seams, existing hydraulic techniques often encounter challenges such as water locking, hole collapse, and drill bit sticking. Anhydrous technology offers a robust solution to these issues by eliminating the adverse effects of water on soft coal seams at the source. Due to its unique physical properties, CO₂ has developed anhydrous technologies such as CO₂ jet impact rock breaking and phase change induced fracturing blasting. CO₂ flash boiling jet, a novel jet technology, demonstrates potential in achieving optimal matching between impact force and working area in coal seam drilling applications, attributed to its features of phase change impact, extensive working area, and balanced force distribution. The expansion and collapse characteristics of multi-hole liquid CO₂ flash boiling jet are pivotal in determining its impact effectiveness. However, the traditional jet characterization method is inadequate for capturing the intricate flow field structure of multi-hole flash boiling jets. By using a unified fitting method and characteristic parameters, it is possible to accurately compare the flow field characteristics of different jets, clarify the impact mechanism of each jet parameter on the flow field, and provide an effective characterization method for precise control of jet parameters to improve drilling capacity. A characterization method based on boundary fitting is developed to analyze the flow field morphology of multi-hole liquid CO₂ flash boiling jets through visualization experiments. The jet flow field images were collected, and image processing techniques were used to extract boundary point information. A nonlinear fitting function was employed to fit the boundary points, revealing the limitations of the boundary fitting method. Two new methods for fitting the boundary points based on fixed extreme points were proposed, with the method based on tangent curve extreme points demonstrating superior fitting performance. The near and far field regions of the jet were separately optimized for fitting, resulting in region-specific boundary fitting functions. For the near field region, a correction factor n was introduced to optimize the fitting method, compensating for nozzles with small deflection angles. In the far field region, the number of boundary points required for the jet fitting was reduced to decrease data processing costs. The role of different characteristic parameters in the jet morphology was analyzed. The characteristic parameter b in the far-field fitting function represents the radial deviation of the jet, while the characteristic parameter k in the near-field fitting function quantifies the expansion level of the flash boiling jet. Furthermore, the ratio of the near-field to far-field k -value—collapse ratio \gamma was proposed to characterize the collapse level of the flash boiling jet.