Effect Of Drill Pipe Inclination On Coal Particles-gas Flow Characteristics In Negative Pressure Sampling Process

Coal seam gas content is an important basic parameter in coal mine gas grade identification,regional outburst prevention prediction and inspection,evaluation and development of coal seam gas resources and so on.The fixed-point sampling technology of negative pressure slag removal(hereinafter referred to as"negative pressure sampling")can not only complete the fixed-point sampling in the underground coal seam,but also greatly shorten the sampling time and improve the accuracy of gas content measurement.In order to optimize the process parameters of negative pressure sampling,this paper uses the research method of combining theoretical analysis and numerical simulation(CFD-DEM)to explore the flow characteristics of coal dust gas in the sampling drill pipe during negative pressure sampling at an inclined angle,and summarizes the pressure drop characteristics in the drill pipe and the final coal dust particle sampling rate in the sampling process.The following research conclusions are obtained:(1)Based on the theory of gas-solid two-phase flow and Newton’s second law,the appropriate gas-solid two-phase control equation is selected,the force correction equation of coal particles under inclined angle is established,and the modified motion equations of collision mechanics between coal particle-particle,and between coal particles-wall are obtained;The appropriate gidaspow drag model is selected for the gas-solid flow in the borehole under the inclined angle.(2)Based on the CFD-DEM coupling simulation algorithm,a drilling bit drill pipe integrated fluid domain model is established.The pneumatic conveying characteristics of negative pressure sampling under different drilling inclination(upward drilling 15°～60°,downward drilling-60°～-15°and horizontal pipe 0°)and different sampling wind speeds are studied.It is found that the axial and tangential velocities of air flow fluctuate greatly near the bit fluid hole(0～0.5m section)during pure gas phase conveying,In the middle and rear section of drill pipe(0.5m～2m),the speed tends to be stable;With the increase of drilling elevation,the air flow vortex in the drill pipe increases,and the vortex appears near the fluid hole of the drill bit;With the increase of drill pipe depression angle,the cyclone pitch increases,and the range of high-speed air flow area at the drill bit fluid hole decreases gradually.(3)During the negative pressure transportation of coal dust gas two-phase flow,when sampling in the upward drilling hole,with the increase of the elevation angle of the drill pipe,the accumulation number of coal dust at the bottom of the drilling hole gradually decreases,the speed of coal dust gradually increases,and the high-speed particles at the cross section of the drill bit fluid hole gradually increase,which is easy to accumulate in the center of the drill pipe;When the elevation angle increases to 30°and above,the actual mass flow rate of coal particles in the drill pipe is close to the given coal particles generation value.When sampling from the downward hole,with the increase of depression angle,the number of coal particles at the bottom of the hole increases gradually,the particles flowing into the drill pipe decrease gradually,and the medium and low-speed coal particles at the cross section of the fluid hole increase and are more dispersed;When the drill pipe depression angle increases to-45°,the real sampling rate under the wind speed of 20m/s is only about 25%(4)When sampling in inclined borehole,the pressure drop curve of coal particles-gas two-phase flow in drill pipe shows two stages:rapid rise and stable rise.With the change of borehole inclination from negative to positive,the total pressure drop of two-phase flow decreases gradually;The pressure drop in the drill pipe first decreases and then increases with the increase of sampling wind speed;However,it decreases gradually with the increase of drill pipe penetration rate;The additional pressure drop coefficient is constructed by dimensional analysis method λ_s and Froude number Fr,drill pipe speed n and borehole inclinationθIt is verified that the error of the prediction model of the additional pressure drop coefficient is within±10%.The overall adaptability of the pressure drop prediction model based on the additional pressure drop coefficients is good,and the simulation results are within±15%of the predicted values.This paper has 61 figures,2 tables and 72 references.