Structure Design Of The Coal Seam Slotting Nozzle And Its Layout Optimization

High-pressure water jet slotting and permeability-increasing technology are one ofthe main measures to control coal mine gas. The key component of the coal slottingsystem is the slotting nozzle. Because its inner runner structure is rather complicated,the research on its optimum structure and layout on the slotting instrument is notcurrently sound, which greatly hinder the slotting depth from increasing further.Therefore, the inner runner structure of the slotting nozzles and their layout should beresearched because they are of great engineering significance on efficiencyimprovement of gas extraction.Based on previous slotting nozzles, a new dual-gradient structure of nozzle wasdesigned. Structural parameters of this dual-gradient nozzle were optimized bynumerical simulation software (CFD) and PIV tests in the lab. According to the findings,the layout of the nozzles in the slotting instrument was optimized once again. Byequipment contrast, slotting efficiency was increased after dual-gradient nozzles wereused. The main findings in the paper are as follows:(1) Based on design theory of nozzle structure, reasons for energy loss and jetcurrent disparity at the exit caused by the previous inner runner structure ofslotting nozzle were analyzed. New dual-gradient nozzles were used to makehigh-pressure water jet more concentrated and to prompt efficiency of energyconversion.(2) Numerical simulations for new dual-gradient nozzles were done by numericalsimulation software (Fluent) for fluids; the optimized parameters of new dual-gradientnozzles installed on the Φ63sealing drill pipes are as follows: L1=6mm、θ1=18°、θ2=35°、L=6mm、d=2.5mm；After3DPIV tests were done, results show that themaximum mean velocity took place when the dual-gradient nozzle was150mm awayfrom the target, and that water jet velocity tended to be the same along the axis and tobe faster and evenly distributed near the axis. Water jet velocity from the axis of thewater jet to the edge became gradually but obviously slow. Viscidity stress value at theaxis is small, which is beneficial for the kinetic energy of the water jet to spread; theReynolds stress area near the water jet axis is corresponding and the Reynolds stress issmall, suggesting that the fluctuation velocity at the axis is slow. It was concluded thatthe design structure of the nozzle can bring about ideal slotting performance. (3) After the internal flow field in the slotting instrument was simulated andanalyzed by numerical simulation software, the main reason why the water jet isdispersive in the exit of the nozzle is that turbulence is at the entrance of the nozzle, andthat the dispersive water jet largely affects the slotting performance of the slottingnozzle. Therefore, starting from this point, and based on theory of fluid mechanics andwith the equivalent diameter unchanged, the turbulence intensity declined in the slottinginstrument after the number of nozzles installed was changed. With the velocity of thewater jet in the exit of the nozzle unchanged, the water jet became more concentrated.Slotting performance was improved and it spent less time slotting.（4）The installment angle of the nozzle in the slotting instrument was optimizedby doing high-pressure water jet slotting tests in the lab. The result suggests that insteadof vertical installment，the installment angle of nozzles should be leant to the directionthat the slotting instrument rotates, and the slotting depth was increased up to130%when it slant angle was determined to be α=10°.