Study on effect of internal vortex structures in nozzles on energy conversion efficiency of self-excited pulsed SC-CO₂ jets

The utilization of high-speed jet technology to fracture coal bodies and construct gas flow channels within coal seams is a crucial method for enhancing coalbed methane recovery efficiency. The supercritical carbon dioxide (SC-CO₂) jet enhances coal-seam permeability through its inherent dissolution and extraction capabilities, while the self-excited pulsed SC-CO₂ jet exhibits markedly low critical fracturing pressure and high coal-breaking efficiency, demonstrating substantial potential for strengthening coalbed methane recovery. However, the current performance evaluation methods for self-excited oscillating jet nozzles lack direct quantitative indicators, making it difficult to determine the optimal nozzle structural parameters and severely limiting their further promotion. The content is: In this paper, based on the evolution process of vortical structures inside the nozzle, a calculation method for the energy conversion efficiency of self-excited oscillation nozzles is proposed. Particle Image Velocimetry (PIV) experimental systems combined with Large Eddy Simulation (LES) are employed to capture the evolution process of vortical structures inside the nozzle, and to analyze the characteristics of fluid energy conversion and distribution inside the nozzle. Analysis of the flow-field images acquired at 1.75–1.79 s after the onset of jetting reveals that the nozzle’s vortex structures are concentrated in the oscillation chamber. The self-excited oscillation pulses of the SC-CO₂ jet moving along the shear layer within the oscillation chamber initially travel upstream upon contact with the collision wall. Over time, during this upstream movement, they gradually merge with the jet along the axis. Strong vortex structures are observed at both the downstream nozzle entrance and the upstream nozzle exit. The total energy input at the nozzle inlet of the SC-CO₂ jet is converted into vortex kinetic energy within the oscillation chamber and the total energy of the self-excited oscillating pulsed SC-CO₂ jet formed at the nozzle outlet. When the chamber diameter ratio is 3.5, the efficiency of jet energy conversion reaches its peak at 80.80%. The eddy kinetic energy within the oscillating chamber induces the generation of self-excited oscillation pulsed SC-CO₂ jets. However, when the proportion of eddy kinetic energy is significant, it can lead to energy dissipation. Furthermore, the eddy kinetic energy at the nozzle exit is utilized to regulate the jet's concentration. The smaller the dispersion of eddy kinetic energy at the nozzle exit, the more concentrated the jet becomes. The ranking of the maximum peak stress generated by self- excited pulsed SC-CO₂ jets at different nozzle exits is as follows L_3.5 > L_1.5 > L_2.5 > L_2.0 > L_3.0.