Generating mechanism of selfexcited oscillation pulsed supercritical carbon dioxide jet

Problems such as collapsed well walls and reduced permeability often occur during the extraction of coalbed methane. The proposed supercritical carbon dioxide (SC-CO 2) drilling technology provides a new technical support for coal bed methane drilling. However, the high coal breakage threshold pressure and the high energy consumption are still the keys to limiting the application of SC-CO 2. The form of the jet is one of the most important factors affecting the energy conversion efficiency of the jet, and the current studies on SC-CO 2 jet are based on a continuous jet. The pulsed jet has higher impact pressure and higher rock breakage efficiency than a continuous jet. Among them, the selfexcited oscillation pulsed jet is the most realistic form of pulsed jet combined with engineering. Current research on the selfexcited oscillation pulsed jet is mostly based on water jets. However, as the fluid properties of SC-CO 2 and water are very different, the applicability of the structural parameters of the selfexcited oscillation pulsed water jet nozzle to SC-CO 2 is not yet known, and a further research is needed. Therefore, to further enhance the coal breakage efficiency of SC-CO 2, a selfexcited oscillation pulsed SC-CO 2 jet was proposed in this paper. On the basis of the conventional Helmholtz resonance cavity, the selfexcited oscillation pulsed generation mechanism of SC-CO 2 was clarified in conjunction with compressible fluid theory, and the selfexcited oscillation nozzle structure applicable to SC-CO 2 was determined. The process of selfexcited oscillation of SC-CO 2 was analyzed by large eddy simulation, and a comparison experiment of coal breakage between continuous and pulsed SC-CO 2 jets was carried out. The coal breakage efficiency of the selfexcited oscillation pulsed SC-CO 2 jet was also verified. The results show that the selfexcited oscillation pulsed mechanism of the SC-CO 2 jet is fundamentally different from that of the water jet. The pulsation of SC-CO 2 turbulence consists mainly of vortex, acoustic and entropic modes. The shearing process in its motion determines the formation and amplification of the vortex structure and is fundamental to the formation of selfexcited oscillation perturbation. The expansion and compression process and the thermodynamic process determine the feedback of the perturbation as well as the frequency of the perturbation, which is the key to the completion of selfexcited oscillation. The vortex, acoustic and entropic modes are coupled to each other and act together. The key to the design of the selfexcited oscillation pulsed SC-CO 2 jet nozzle was found to be finding the right oscillation cavity structure. The selfexcited oscillation pulsed SC-CO 2 jet has higher jet energy conversion rate and better erosion damage than a continuous jet. In summary, this study innovatively proposes a selfexcited oscillation pulsed SC-CO 2 jet. The mechanism of selfexcited oscillation pulsed SC-CO 2 jet generation has been clarified, which lays the foundation for the design of selfexcited oscillation pulsed SC-CO 2 jet nozzle. The better selfexcited nozzle structure parameters were determined and the applicability of the designed nozzle was verified, further improving the energy utilization of the SC-CO 2 jet and reducing the energy consumption of the system.