Research on the gas-liquid-solid coupled slag discharge flow field and optimization of cutterhead slag suction port in shaft drilling

Aiming at the problems of gas lift reverse circulation shaft washing slag discharge and low drilling efficiency in the application of vertical shaft drilling method in the Jurassic strata coal mines in western China, taking ϕ4.2 m advanced drilling of central return air shaft in the Kekegai coal mine as the engineering background, based on the CFD-DEM (computational fluid dynamics and discrete element method coupling) research method, a gas-liquid-solid multiphase coupling slag discharge numerical model is established, which reveals the velocity and pressure distribution law of the flow field in the slag discharge pipe and the bottom hole. Based on the self-developed gas lift reverse circulation slag discharge test device, the correctness of the flow field distribution is verified by PIV (Particle Image Velocimetry) test technology. The evaluation index and method of optimizing cutter head suction port are put forward. The number, length-diameter ratio, area ratio and total area ratio of slag suction outlets are optimized. The best way and related parameters of cutter head suction port arrangement of advanced bit are optimized. The effects of main factors such as bit speed, gas injection rate, submerged ratio of air duct and mud viscosity on slag discharge flow field are discussed. The research results show that ① the fluid in the slag discharge pipe mainly moves in the axial direction, and when passing through the gas injection end, the velocity increases sharply. The migration of bottom hole fluid is mainly horizontal flow, and the vertical upward flow of fluid only exists near the slag suction port. The horizontal flow at the bottom of the shaft is mainly tangential flow, and the radial flow is obvious only on both sides of the slag suction port, and it is far away from the slag suction port, so the runoff speed is small and it is easy to produce cuttings deposition. ② When the number of slag suction ports of cutter head is 2, the length-diameter ratio is 0.4, the area ratio is 1, and the total area ratio is 1.94%, the layout of slag suction ports is the best, and the slag removal rate is increased by 66% compared with the current layout of slag suction ports. ③ Increasing the bit speed can significantly enhance the adsorption of the slag suction port. The gas injection rate and the submerged ratio of the air duct are positively correlated with the axial velocity of the fluid in the bottom hole and the slag discharge pipe. The mud with low viscosity and low density is easy to obtain high return velocity, but its rock carrying capacity is poor. The research results can provide a useful theoretical reference for solving the technical problems of washing, slag removal and low drilling efficiency of deep and large vertical shaft drilling in the Jurassic strata.