The Study On Methane Accumulation Induced Airflow Disorder In Tilted Roadways

In underground coal mines,abnormal gas emission or insufficient ventilation can result in gas accumulation in partial area.If the methane accumulation has a high concentration and happens in a tilted roadway,an additional pressure for mine ventilation will be produced.The additional pressure is called methane draft pressure by some researchers.Similar to natural draft pressure,methane draft pressure is produced due to the airflow density difference of different roadways.However,methane draft pressure did not get the attention of researchers for a long time,many airflow disorder phenomena caused by methane draft pressure were lack of reasonable explanation.At present,the influence of methane draft pressure on the ventilation network is still lack of research,especially a complete experimental study.In this paper,the results of previous theoretical study are verified through experiment means;the experimental results are analyzed through theoretical and numerical way,which revealed the essence of the methane draft pressure induced airflow disorder;in addition,the ventilation network solver developed by the author's team is improved,and the study conclusions are verified in actual case analysis.First,a 1/50 experimental system was built with spiral duct.The experiment system is mainly consisted of three parallel pipeline branches.The diameter of the right branch is 15 cm,and the diameter of the left branch and the middle branch are 10 cm.The three branches' inclination angles can adjust between 0 and 90 degrees.At the top of the experimental system,a small fan is mounted to simulate exhaust ventilation.Each branch has a butterfly valve to adjust wind resistance,and the valve before the fan is to adjust the ability of the fan.The middle branch is designed to simulate gas accumulation,an oxygen sensor installed at the bottom of the middle branch to determine whether gas concentration in each experiment is the same.Experimental phenomenon is captured by wind speed sensors and the oxygen sensors with a fast response time.A data acquisition card passed the monitored data to a computer.To ensure safety,a mixed gas of helium and nitrogen is used instead of methane,the same density of mixed gas and methane ensures the reliable experimental phenomena.Gas sensors in this experimental system are oxygen sensors,the mixture gas concentration can be reflected by changes in oxygen concentration.The experimental system can simulate different forms of ventilation network affected by methane draft pressure.For contrast to the formal experimental results,a series of preparing experiments were done before the formal experiments.The preparing experiments includes the experiment for the stability of the oxygen concentration of the inlet,numerical simulation of the effect of aeration,experimental phenomena without mixture gas,horizontal roadway experiment and repeatability experiments.In parallel upward ventilated roadways,high concentrations of gas accumulation in one of the roadways will result in an increase in the volume flow of gas accumulation roadway and a reduce in volume flow of the parallel lateral roadways,and sometimes even result in airflow reversion of the lateral roadways.When carrying out experiments in upward ventilated parallel roadways,airflow reversion phenomenon of the lateral branch was successfully get in the experiment.By conducting experiments with various parameters,the experiment results may reflect the influence of various parameters on the airflow reversion phenomenon.Further analysis of the experimental results in upward ventilated roadways was conducted.The conclusion can be obtained by dimensional analysis that: there is a critical wind speed in lateral branch for airflow reversion.Under the same conditions,when the wind speed in lateral branch is greater than the critical wind speed,airflow reversion will not occur.A relationship between the elevation difference of the roadway and critical wind speed was concluded by numerical simulation: When the initial gas concentration and roadway wind resistance are constant,the square of critical velocity is proportional to the elevation difference of the roadways.The critical wind speed under different inclination angle is tested by the experiment system.Results and conclusions of the experiment and simulation are matched.By using ventilation network theory,effect of wind resistance on the airflow reversion phenomenon is deduced.The relationship of the magnitude of wind resistance and volume flow is also deduced.The experimental results of different wind resistance lateral branches match the relationship.It can be concluded from the relationship that airflow varying range in lateral branch reduces with its increasing wind resistance,but the reduced rate is smaller with the increase of the wind resistance.With further derivation of this relationship,the conclusion can be drawn that changing wind resistance of the lateral branch can prevent the airflow reversion occurring,but the conditions are very harsh.In addition,oxygen concentration curve of the lateral branch in the different experiments were put together,it is concluded that the downward trend of the oxygen concentration is not affected by roadway elevation,fan pressure and wind resistance of the lateral branch,and the law of decreased oxygen concentration can be described by a straight line.Installing the fan and the valve that controls the fan capacity at the bottom of the experimental system,the experimental system can be modified to simulate parallel downward ventilated roadways.One of the roadways has high concentrations of gas accumulation.In this experiment,roadway inclination,fan pressure,roadway wind resistance change for multiple sets of experiments.The results can be summarized as follows: wind resistance of the lateral branch larger,roadway inclination smaller,the fan pressure larger,can contribute to a more stable airflow.Gas accumulation branch wind resistance has little impact on the experimental phenomena.Contrast to the upward ventilated experiment,we found that in downward ventilated roadways,the degree of methane draft pressure induced airflow disorder heavier,gas distribution more complex,gas discharge from the system in a longer time.In downward ventilated experiment,the directions of airflow often change many times,oscillation phenomenon is obvious.Gas easily flows into the lateral branch,and with airflow oscillation,gas may flow into and out of the lateral branch several times.Therefore,in downward ventilated roadways,high concentrations of gas accumulation should be tried to avoid.The parallel roadway ventilation system can be simplified as a circle pipeline model.Then,the airflow oscillation can be studied by vibration theory.The vibration formula was proposed base on the real situation,and the formula was solved by numerical method in different conditions which includes different cross section,different length of the roadways and different wind resistance.Through the analysis of the solutions,the influence of roadway parameters on the frequency and amplitude of the airflow oscillation is studied.The results indicates that larger wind resistance is helpful for the decrease of the amplitude and the prevention of the oscillation.A necessary condition for airflow oscillation is the airflow reversion in the left branch.A larger initial wind speed in the left branch can prevent airflow reversion.Ar number is the similarity number that characterizes the ratio of buoyancy and the inertial force.It is mainly depended by the initial wind speed in the left branch in experimental condition.Therefore,a critical initial wind speed decides the airflow reversion in the left branch,and in turn decides the airflow oscillation.To cope with experimental studies,the mine ventilation network solution unsteady calculation program has been improved.The original program is an improvement based on the steady ventilation network calculation software.The program based on a one-dimensional unsteady incompressible flow equations and gas diffusion equations and added methane draft pressure to ventilation network calculation.There are three main improvements of the unsteady program: First,fix of the vulnerabilities of the original program,including the initial air volume accuracy,friction coefficient selection in roadway with adjustment facility and the boundary conditions chosen in calculation of gas transport.The improved program was compared with original program through a simple parallel tunnel operator.Solver results suggest that the improvement effect is obvious.Second,the wind resistance changing during the airflow disorder can achieve in the new program.The effect of the control method through wind resistance changing after methane draft pressure produces can be simulated.A parallel roadway simple numerical example indicates that a sudden increase in wind resistance of lateral branch is a good way to prevent high concentrations of gas expanded.Third,according to previously studied outburst gas emission law,the program adds a gas emission features for gas outburst accident.Improved program simulates coal and gas outburst in Jiulishan 8.23 accidents.Contrast of the solution Zhou Aitao has proposed by using the original program,the time required for mine gas discharge greatly lengthened.Finally,Yuexu area Tangshan Coal Mine 2004 airflow stagnation were analyzed.Yuexu area Tangshan Mine has occurred a high concentrations of gas accumulation in 2004.Because of an opening wind door caused short circuit and large amount of gas emission in Yuexu area,high concentrations of gas accumulation was founded.After the opened door closed,airflow did still not recover;gas is not discharged as airflow kept a standstill.Ultimately,the mine used fans to exhaust gas,and no objects blocking tunnel ventilation were found.Experimental system was transformed to simulate airflow stagnation phenomenon in Yuexu area Tangshan Mine.The experiments reproduced the airflow stagnation phenomenon.The experimental results proved that airflow stagnation is caused by the methane draft pressure.The airflow stagnation was also reproduced by the improved mine ventilation network solution unsteady calculation program.The methane draft pressure was calculated by the program and gas concentration distribution illustrated airflow stagnation reasons: methane draft pressure and fan pressure were equilibrium.According to the results of experiments and simulations,reasonable approach of a similar phenomenon of airflow stagnation is analysed.The main innovation points of the paper are as follows: 1.the experimental system for studying the phenomenon of methane draft pressure is built for the first time,and the airflow disorder phenomenon caused by methane draft pressure is studied systematically.2.The relationship between the critical wind speed and the height difference for the airflow reversion in the lateral branch in upward ventilation roadways is analyzed and the experimental verification is carried out.3.The paper studies the phenomenon of wind disorder in mine ventilation by using vibration theory,and points out the effect of roadway wind resistance on the phenomenon of restraining the wind flow oscillation.4.unsteady mine ventilation network calculation program was improved,the ventilation facilities changes in the process of airflow unsteady state,the unsteady airflow changes as continuous gas emission,and other modules are added into the unsteady ventilation network solution program.