Study on factors influencing the self-propelling capacity of self-propelled water jet drill bits

The structural characteristics of inner flow field in a pipeline are the key factors influencing the self-propelling capacity and descaling-unblocking capability of the self-propelled drill bit. To address the problem concerning the effect of the changes in the inner flow field of a pipeline under the joint action of multiple jets on the self-propelling force and the resistance to water backflow, the changes of inner flow field in the pipeline under different conditions were numerically analyzed based on FLUENT, and thus the influence law of the angle of rear nozzle, the rotational speed, the annular ratio and the system pressure on the self-propelling force was defined, revealing the reasons for the error of the theoretical value of the integrated friction coefficient. Besides, the primary and secondary relationships of the factors influencing the self-propelling force were clarified through orthogonal numerical simulations. Then, the self-propelling force testing experiments were carried out under different conditions with the self-propelling force test device to verify the correctness of the numerical simulation results. As shown by the results, the action of vortex squeeze collision and jet suction leads to a pressure gradient in front of and behind the self-propelled drill bits, thus increasing the self-propelling force. The angle of the rear nozzle is increased from 20° to 45°, the countercurrent velocity of wall jet is increased from 6.2 m/s to 14.5 m/s and the peak pressure after vortex collision extrusion is increased from 68.47 kPa to 80.79 kPa, resulting in an increase in differential pressure. Increasing the rotational speed could reduce the range of primary vortex, while the differential pressure applied on the rotating body increases during the collision and squeeze of vortex in the rotating body area. Increasing the rotational speed and the system pressure, or reducing the annular ratio, will increase the isolation effect of the rear jet, thus increasing the self-propelling force. The theoretical value of the integrated friction coefficient needs to be corrected as the effect of the vortex zone is ignored in the theoretical calculation of the resistance to water backflow. The extreme differences of the annular ratio, system pressure, rotational speed and angle of the rear nozzle are 300.07, 111.87, 60.42 and 36.32 respectively, and the main relationships of the influencing factors are: annular ratio > system pressure > rotational speed > angle of the rear nozzle. Generally, the research in this paper could provide a reference for the optimization design in structure and the improvement of self-propelling capacity of the self-propelled drill bits.