An experimental study on the erosion of sandstone by self-excited oscillation cavitation waterjet in submerged environment

Abstract

Self-excited oscillation cavitation waterjet (SOCW) has good prospects for application in deep sea mineral extraction of its cavitation characteristics. This study conducted the erosion experiments of three types of sandstone, red sandstone, white sandstone, and black sandstone by SOCW under a submerged environment. The erosion characteristics of three types of sandstones under the impact of SOCW at various erosion times, standoff distance, and pressure were compared. It was found that, a) Red sandstone with larger porosity is more likely to produce a large damage area and has the best rock breaking effect. Taking the standard working condition of this experiment as an example (t = 50s, Pj = 16 MPa, S/De = 4), the erosion depth of red sandstone can reach 1.9 times that of white sandstone and 5.5 times that of black sandstone; the erosion area can reach 19.6 times that of white sandstone and 6.2 times that of black sandstone; the erosion quality can reach 5.2 times that of white sandstone and 7.4 times that of black sandstone. b) With the increase of erosion time and erosion pressure, the erosion index of sandstone shows an increasing trend. However, in this study, there is an optimal standoff distance S/De = 4 to maximize the erosion depth of the sandstone. Again, there is an optimal standoff distance to maximize the erosion hole area, S/De = 10 for the red sandstone and white sandstone, and S/De = 6 for the black sandstone. c) The special phenomenon of the pressure difference formed after the collapse of the SOCW bubble in the submerged environment, which acts on the moisture content fine pore space inside the sandstone, the fine pore zone, was observed by scanning electron microscope (SEM). The microscopic characterization of the water wedge effect was observed by computed tomography (CT), and the trend of crack development is shown by the binarization processed images of the sandstone internal damage section. This result can be a theoretical basis to guide the working parameters of nozzles in deep sea mining, especially for the selection of standoff distance.