Forming mechanism and radius prediction of hydraulically generated slots based on a two-walled jet

Abstract

Due to its distinct capability to improve the efficiency of gas drainage, hydraulic slotting has attracted increased attention in recent years. Current attempts to predict the slotting radius are mostly based on the fluid-structure interactions of normal free jets, which is inconsistent with the practical slotting condition. To address this problem, a two-walled jet model with a rotating nozzle was proposed for simulation, focusing on the flow-field characteristics. The hysteresis effect and zone of negative pressure, which account for the forming mechanism of slots, were demonstrated based on the simulation results. Moreover, the decay equations of the centerline velocity were obtained based on the analysis of the jet developments, the effects of the confining height, and the rotating number. As a result, theoretical prediction modeling of the slotting radius was achieved. In addition, experiments using materials similar to the coalbed were also performed to validate the accuracy of the prediction model. The linear relation between the dimensionless radius and the 1/4th power law of the inlet pressure could be obtained in both the theoretical model and the experimental results. The experimental results were in good agreement with the numerical ones, except for the low-pressure condition. Finally, it can be claimed that the theoretical model is, to some extent, suitable to guide practical applications.