Gas-solid Flow Characteristics Of Sampling Method Based On Negative Pressure Pneumatic Conveying

Coal seam gas content is an important parameter for gas disasters prediction and control in coal mine.Sampling method based on negative pressure pneumatic conveying(referred to as negative pressure sampling)is a promising way to determine the coal seam gas content.At present,the research on negative pressure sampling gas-solid flow is based on the fact that the sampling drill pipe is at a static state,which is inconsistent with the rotation of the sampling drill pipe when sampling in engineering.In this paper,gas-solid flow during negative pressure sampling is taken as the research object.The research method combining theoretical analysis and numerical simulation is used to focus on the influence of sampling drill rod rotation on the characteristics of gas-solid flow during sampling.The range of the optimal sampling drill pipe speed and established the critical sampling wind speed and pressure drop prediction model were determined when negative pressure sampling.The research results can provide theoretical support for negative pressure sampling.Some beneficial research results bave been based on the above studies:(1)At first,the force composition of coal particles in the flow field was determined by gas-solid two phase flow theory and Newton’s second law.The collision process of coal particles-coal particles and coal particles-drill pipe wall is analyzed,and the motion correlation of collision mechanics is obtained.The governing equation of gas-solid two phase flow is established through continuous equation and law of conservation of momentum.(2)Secondly,based on the CFD-DEM coupling method,the effects of drill pipe speed and coal mass flow on coal particles-gas flow characteristics are analyzed from the aspects of flow velocity,pressure distribution and coal particles movement.The results show that there is a pressure high pressure area in the fluid hole of the drilling bit,the tangential velocity is symmetrical,and the symmetry axis is the central axis of the fluid hole.With the increase of the rotational speed,the swirl strength in the sampling drill pipe increases,the axial velocity decreases and the tangential velocity increases,the flow path of the streamline increases,and the point of contact with the drill pipe for the first time moves backward,the pitch of the spiral flow becomes smaller,and the distance of the spiral flow along the flow direction becomes longer.With the increase of the rotational speed,the number of coal particles entering the drill pipe first increases and then decreases,the suspended coal particles in the drill pipe increase,the more obvious the spiral track,the contact between the coal particles and the drill pipe wall increases,and the spiral moving distance of the coal particles increases.The position of the three fluid holes of the drilling bit changes periodically during the sampling process.Under the influence of centrifugal force and gravity,there is not enough coal particles in the drill hole to enter the drill pipe,the actual coal mass flow rate and solid-gas ratio in the drill pipe are less than the set value.There is an optimal speed in the sampling process,and the optimal sampling speed is between 0～100rpm.(3)Then,the definition and physical definition of critical sampling gas velocity in dynamic negative sampling were analyzed.It is found that the critical sampling gas velocity can be characterized by saltation velocity of coal particles.The saltation velocity of coal particles was studied by numerical simulation.The research results show that the saltation velocity of coal particles is positively correlated with the solid-gas ratio and the coal particle size,and negatively correlated with the rotational speed of drill pipe.The correlation of saltation velocity of 1-4mm coal particles was obtained by the dimensionless parameter solid-gas ratio,coal particle size and rotational speed as variables.Comparing the new correlation with the three more correlations,the error of the new correlation is between 11.9%and 15.5%,and the correlation has good applicability.(4)Finally,the influence of different parameters on the pressure drop characteristics of the drill pipe was studied based on the state diagram of negative pressure sampling.The research results show that under the same mass flow rate of coal particles,with the increase of sampling gas velocity,the pressure drop first decreases and then increases,and the minimum pressure drop gas velocity is about5～10m/s.Pressure drop is positively correlated with size of coal particles and negatively correlated with rotational speed.The relationship between the additional pressure drop coefficient(λ_s)and the Froude number(Fr),the solid-gas ratio(m),the rotational speed(n)and the coal particle size()is studied.It is found thatλ_sdecreases with the increase of Fr.When Fr is large enough,λ_s tends to be fixed.When Fr is constant,λ_s increases with m;λ_s is positively correlated with n and.The additional pressure drop coefficient correlation is established based on the Barth additional pressure drop using the dimensional analysis method.The predicted correlation predictive value and the measured value error are within±20%.