Study on the optimal impact stand-off distance of self-excited pulsed supercritical carbon dioxide jet based on resonance effect

The supercritical carbon dioxide (SC-CO₂) jet has a wide application prospect in drilling engineering, and its low coal breaking pressure threshold and high efficiency can improve drilling efficiency. But it has the problems of high coal breaking threshold pressure and system energy consumption, which limits its application. The high pulse pressure and resonance effect of the self-excited oscillation pulse SC-CO₂ jet can effectively reduce the coal breaking threshold pressure and significantly improve the coal breaking efficiency. Based on this, the variation law and comprehensive effect of the self-oscillating pulse SC-CO₂ jet pulse frequency and pulse pressure amplitude at different target distances were studied. Large eddy simulation was used to analyze the flow field structure of the jet, and the variation laws of the pulse frequency and pressure amplitude in the axial direction of the flow field were clarified. The effect of the target distance on the impact frequency and impact pressure of the jet was studied by pulse characteristics test experiment, and the comprehensive effect of pulse characteristics on coal breaking was studied by coal breaking experiment. It was concluded that the pulse frequency and pulse pressure amplitude of the self-oscillation pulsed SC-CO₂ jet do not remain constant in the axial direction of the jet, and they gradually decrease with the increasing target distance. Considering only the pulse frequency cannot maximize the resonance effect, and it is necessary to comprehensively consider the influence of pulse frequency and pressure amplitude on the resonance effect. Using displacement response amplitude to characterize resonance effect can reflect the comprehensive influence of frequency and amplitude on resonance. When the target distance is small, from 0 mm to 15 mm, the main influencing factor of the displacement response amplitude is the pulse pressure amplitude. When the target distance is large, from 15 mm to 30 mm, the main influencing factor is pulse frequency. In addition, reducing the pulse frequency and increasing the pressure amplitude can effectively improve the displacement response amplitude. The transition positions of the main influencing factors of displacement response amplitude under different nozzle structure conditions are different, and the optimal target distance is also not the same. In this study, the optimal target distance for nozzle a is 22 mm, where the displacement response amplitude is the largest and the coal breaking effect is the best, while the optimal target distance for nozzle b is 26 mm.