| In the process of gas dynamic disaster in coal mine,there is a large amount of high pressure and high concentration gas,and the high concentration gas accumulates under the dual action of pressure field and concentration field to form gas flow rapidly into the tunnel space.When the gas flow energy is small,it is difficult to change the structure of the tunnel flow field greatly by the gas emission,and the influence on the tunnel flow field is mainly reflected in changing the space-time distribution of the gas concentration in the flow field through convection diffusion,thus threatening the safety of mine production.When the gas flow energy is large,the dynamic effect of gas emission can make great changes to the original roadway flow field structure,which makes the original wind flow structure difficult to sustain.At this time,the high concentration gas can not only move along the direction of return air,but also extend to the counter-current in the air entry roadway,thus expanding the influence range and influence degree of the gas disaster.The migration and propagation of high concentration gas in the roadway will not only cause asphyxiation casualties,but also possibly induce secondary disasters such as gas explosion,especially when high pressure gas counter-current enters the air entry roadway.Because most of personnel and mechanical and electrical equipment are distributed in the air entry roadway,the probability of disaster induced and the threat degree are greater.Therefore,understanding the law of migration and diffusion in roadway after high pressure gas emission,mastering the occurrence condition and expansion mechanism of high pressure gas reverse flow,and determining the effective influence range after high pressure gas emission are helpful to reduce the probability of secondary disaster.Preventing the further expansion of gas disaster has important theoretical and practical significance for correctly understanding the development process of disaster,effectively formulating emergency response mechanism,and rationally arranging the rescue scheme after disaster.At present,there is still a lack of research on the dynamic emission of high-pressure gas in the process of gas dynamic disaster at home and abroad,and most of the research on the law of gas migration and propagation in roadway is based on the instantaneous point source model of flow diffusion.It is different from the actual emission process of high pressure gas in time and space distribution.Based on this,this paper takes the theoretical analysis as the starting point,simplifies the high pressure gas emission process as the opening discharge process of the high pressure chamber,and analyzes the gas pressure in the gas discharge process combining with the molecular thermal physics,molecular dynamics and so on.The influence of coal gas desorption on the pressure release process is discussed.On the basis of this,the timecontinuous source diffusion model and the spatial finite distribution model of gas concentration distribution in the process of gas emission after the gas emission stop are derived by means of mass transfer theory.Combined with the characteristics of dynamic gas emission,the formation conditions and expansion law of gas counter-current in roadway are analyzed.In order to verify the correctness of the theoretical model,the experimental system of high pressure chamber opening release and tunnel high pressure gas migration and propagation were designed.And the different pressure release film thickness,pressure relief outlet size and outlet shape were studied.The size of pressure releasing chamber,the law of opening and releasing of high pressure chamber under different wind speed,the law of migration,diffusion and counter-current expansion after high pressure gas gushing in roadway were studied.And the influence degree of different factors on the above law are analyzed.At the same time,the use of numerical simulation software FLUENT,reproduces the migration propagation process after high pressure gas emission in the roadway and dynamic emission process of high-pressure cavity.The timespace variation law of the gas pressure and concentration in the tunnel and the variation law of the expansion range of the counter-current are analyzed,which verifies the correctness of the law of the gas migration and the anti-current expansion obtained in the previous paper.Finally,taking the change of gas concentration in Xiaotun Mine of Fengfeng Group as an example and the borehole gas ejection in Xinsan Mine as an example,the correctness of the previous theory and the experimental results are further verified,and corresponding engineering suggestions are put forward.The research results of this paper are further enriched and the theory of prevention and cure of mine gas disaster has been perfected,which has important theoretical and practical significance.The main research contents and achievements of this article are as follows:(1)Derivation of theoretical model of flow diffusion during and after gas emission at high pressureThe gas emission of high pressure underground is regarded as a continuous process of time,and the gas distribution in the roadway after the gas emission stops is regarded as a continuous space region.Based on the theory of fluid mass transfer,a time-spaced model of gas concentration variation in roadway during and after gas emission is obtained.The results show that the gas concentration distribution in roadway meets the time continuous source diffusion model in the process of gas emission,namely,(?).After the gas emission stops,the spatial and temporal distribution of gas concentration in the roadway satisfies the diffusion model of the limited spatial distribution source with the flow,that is,(?).After gas emission stops,the temporal and spatial variation of peak gas concentration in roadway is satisfied with the model (?).The peak concentration is obtained at x=Ut.The peak value of gas concentration decreases with the increase of the migration time and the gas diffusion coefficient in the roadway,and increases with the increase of the length of the gas distribution area in the roadway when the gas emission stops.(2)The condition of the gas counter-current in the roadway and the estimation of the expansion range of the counter-current;Combined with the law of high pressure gas emission,the reverse flow process is divided into different stages according to the gas flow state in the air entry roadway.According to the gas stress state in different stages,the characteristics of gas flow and the variation law of gas concentration in each stage are analyzed.The reverse flow process after high pressure gas emission is mainly composed of four processes: 1)The process of shock wave action;2)Formation of shock wave until gas pressure in coal seam decreases to atmospheric pressure level;3)The attenuation stage of gas dynamic pressure in roadway under inertial action;4)The stage of reverse flow's stopping the gas return.The direct condition of controlling the gas counter-current in the roadway is whether the gas pressure head flowing in the roadway is greater than the sum of wind pressure and roadway resistance.The expansion range of gas counter-current caused by high pressure gas emission can be solved by combining the kinematics formula with the law of variation of resultant force acting on the gushing gas.It can also be solved by the law of concentration distribution in the roadway at the time of counter-current stopping.In the process of gas counter-current velocity decaying to counter-current stopping,the timespaced evolution law of gas concentration in air entry roadway is satisfied with (?).(3)Experimental verification and analysis of influencing factors of migration and propagation in roadway after high pressure gas emission.Through the self-developed simulation system of gas disaster air flow monitoring,the abnormal gas emission process under different working face conditions is simulated.By monitoring the variation law of gas pressure and gas concentration in entry and return air roadway,the film breaking pressure,the influence of opening size,opening shape,cavity volume and tunnel wind speed on gas migration and propagation process were analyzed.After high pressure gas emission,pressure wave can be formed in the roadway,while the pressure wave propagates forward rapidly in the roadway.The peak value of pressure decreases rapidly,and the pressure wave formed by the cavity pressure relief is low in the roadway,which belongs to the weak impact process.Compared with the pressure propagation process in the roadway,the velocity of gas concentration migration is much slower,and the gas counter-current velocity in the air entry roadway is obviously faster than that in the return air tunnel.With the increase of the distance from the measuring point to the gas gushing outlet,the average velocity of gas migration decreases gradually when the gas passes through the test point,either in the entry tunnel or in the return air tunnel.The peak value of gas concentration in the air entry roadway is obviously larger than that in the return air tunnel,and the peak value of gas concentration in the measuring point decreases gradually with the increase of the distance to the gas gushing outlet,whether it is in the air entry lane or in the return air tunnel.The peak attenuation velocity of gas concentration in the entry roadway is obviously higher than that in the return air tunnel,but the reasons for the peak attenuation of the gas concentration in the entry roadway are different in essence.The duration of the gas concentration rising stage in the entry and return air tunnel is approximately the same,but the total duration of the gas concentration anomaly phase in the air entry is much smaller than that in the gas concentration anomaly stage in the return air tunnel.The time-varying law of the gas concentration curve in the backwind tunnel can better meet the gas convection diffusion model based on finite distribution source.The fitting correlation coefficient is above 0.82,and some can reach 0.98.The peak value of gas concentration in the air entry roadway reflects the distribution state of gas concentration in the tunnel when the counter-current stops.Combined with time continuous source and spatial finite distribution source,the model of flow diffusion can better reflect the spatial distribution law of the peak gas concentration in the test point of the tunnel after high pressure gas emission.The fitting correlation coefficient of the model is above 0.9.The applicability of the above model becomes more obvious with the increase of the abnormal concentration points.Whether based on the pressure method or the concentration peak method,it can effectively estimate the diffusion range of the counter-current in the backwind tunnel of high pressure gas emission,and the estimation method based on the concentration peak value has better applicability and higher accuracy.As a whole,the expansion range of gas counter-current increases with the increase of pressure release film thickness,increases with the volume of pressure releasing chamber,and does not change with the diameter of outlet.For different outlet shapes,the expansion range of counter-current is circle > rectangle > square > triangle.(4)Numerical simulation verification of counter-current migration of high pressure gas in roadway.Based on the constitutive equations of compressible and incompressible flow processes,a dynamic model of high pressure gas emission and migration is established.The numerical simulation process of counter-current migration and diffusion of high pressure gas discharge is reproduced by using FLUENT software,and the coupling process of flow is simulated at different time scales.From the point of view of numerical solution,this paper analyzes in detail the law of pressure release in the pressure chamber,the law of gas flow in the inlet,the law of migration and propagation in the return air tunnel,the law of the counter-current propagation of gas in the entry air tunnel and law of change of Gas concentration at measuring points in entry wind roadway.The simulation results are in agreement with the experimental results,which further verify the correctness of the previous theoretical model and the experimental results.(5)Field verification of gas migration and propagation law of roadway under different high pressure gas emission conditions.On the basis of field test data,the variation characteristics and law of gas concentration after mining are analyzed by using the previous theoretical simulation.The fitting correlation coefficient is above 0.87.The accuracy of model of migration and diffusion in roadway after high pressure gas emission is further verified.Based on the extraction of data features,the variation characteristics and rules of gas concentration at measuring points in the process of gas ejection from boreholes are analyzed,and the existence of counter-current in the process of high pressure gas emission is verified.The corresponding engineering suggestions are given. |
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