Dynamics And Microscopic Characteristics Of Methane (Coal Dust) Detonation Propagation And Detonation Suppression Mechanism Of Water Curtain

Methane(coal dust)explosion is one of the major disasters affecting the safe production of coal mines.The strong shock waves and toxic gases produced are seriously endangering life and the natural environment.Scholars at home and abroad have carried out a lot of research,and have made important progress in explosion dynamic characteristics and explosion suppression.At present,there is a lack of indepth understanding of the evolution of the microstructure during the development of methane(coal dust)explosion,especially under some complex environmental conditions,the deflagration-to-detonation transition,which makes the microscopic evolution process of the wave structure more complicated and the explosion suppression more difficult.Water mist is a widely used explosion suppression technology,but the research on the use of water mist to suppress gas-solid mixed explosions is still in the development stage.There are few studies on the microstructure characteristics,heat and mass transfer and momentum transfer of gas-liquid-solid coupling.Therefore,this thesis has carried out the research of dynamic and microscopic characteristics of methane(coal dust)detonation propagation and the mechanism of water curtain explosion suppression.The research results are as follows:Based on the solver RYrhoCentralFoam developed under the framework of OpenFOAM,the microstructure and evolution characteristics of CH4/O2/N2 mixture under stoichiometric detonation were studied.The ultra-high-resolution detailed structure of 88 units per reaction step was obtained by parallel calculation on the supercomputing platform,and the clear peak pressure trace was captured,as well as the characteristic structure of the detonation wave,including Mach stem,incident wave,shear wave,triple point,etc.Through the evolution of the detonation wave structure,the formation process of the cell structure is expounded.In addition,it is found that due to the structural complexity of shear waves,multiple pressure ripples appear within a cell structure.Finally,the variable characteristics in the detonation flow field are revealed,and it is found that the regions of high temperature,high pressure and high density are distributed in stripes,and the average heat release rate of the gas phase reaches the maximum value at the reaction surface.The inhibitory effect of dispersed phase water curtain on methane detonation with different parameters was studied,and the critical water curtain length for decoupling methane detonation under different droplet parameters was determined.The results show that the critical water curtain length decreases monotonically with the droplet mass loading;for a fixed mass loading,the smaller the droplet size,the shorter the critical water curtain length.When the length of the water curtain is close to the critical value,the leading shock front and the reaction front are decoupled,and re-initiation occurs after passing through the water curtain;when the length of the water curtain exceeds the critical value,the methane detonation wave can be completely decoupled without re-initiation occurs.In addition,the unsteady response of the water curtain to methane detonation was studied,and the inhibition mechanism of the water curtain on methane detonation was revealed.The results indicate that the methane detonation velocity has a significant attenuation under the action of the water curtain.When the detonation wave enters the water curtain region,the energy and momentum transfer begin immediately,while the mass transfer occurs near the reaction surface behind the detonation wave.The weakening of the momentum of the gas phase(accelerating the droplet)is not enough to decouple the incident detonation wave,but the convective heat transfer caused by the heating of the droplet is dominant.Meanwhile,the convective heat transfer power of the droplets is an order of magnitude higher than the water vapor enthalpy,indicating that the water vapor produced by the vaporization of the droplets has a limited impact on the gas-phase detonation dynamics.The Eulerian-Lagrangian method was used to study the dynamics of methane detonation in dilute suspended coal dust,and the influence mechanism of suspended coal dust at different concentrations and particle sizes on methane detonation was visualized and quantitatively analyzed.The results show that when the coal dust particle size is small(≤2.5 μm)and the concentration is high(≈500 g/m3),the detonation wave is decoupled near the coal dust suspension front.The averaged leading shock speed generally decreases with increased particle concentration and decreased particle size.Moreover,for 1 μm particles,if the particle concentration is beyond a threshold value(465 g/m3),detonation re-initiation occurs after it is quenched at the beginning of the coal dust suspensions.This is caused by hot spots from the shock focusing along the reaction front in a decoupled detonation and these shocks are generated from char combustion behind the leading shock.A regime map of detonation propagation and extinction is predicted.It is found that the re-initiation location decreases with the particle concentration and approaches a constant value(≈0.225 m)when the concentration exceeds 1,000 g/m3.In addition,the interphase coupling between the detonation wave and coal particle are discussed.The mass and energy transfer rate increase rapidly to the maximum near the reaction front in the induction zone.Meanwhile,the smaller the particle size and the larger the particle concentration,the greater the transfer rates of mass,energy,and momentum.Detailed structures of methane/coal particle hybrid detonation are also studied.The results reveal that the several locations with high heat release are caused by enhanced char combustion facilitated by the availability of the oxidant species in the unburned gas pockets.These pockets with char burning would be conducive for pressure wave formation,thereby affecting the leading shock.The influence of coal particle surface reaction on methane chemistry is studied based on constant-volume ignition calculations.The results show that the ignition time IGT varies monotonically with the coal dust concentration and non-monotonically with the coal dust particle size.The dependence of ignition time on coal dust particle size can be divided into three stages:A(dp0<2.5 μm),B(2.5<dp0<20 μm)and C(dp0>20 μm).Furthermore,it was found that for any coal dust concentration,only when the coal dust particle size was smaller than a certain value(5 μm),the surface reaction showed a considerable effect on the ignition time of the mixture.This further identifies the range of coal dust particle sizes that contribute to gas-phase chemistry,demonstrating the detonation-rich behavior of small particles found in the 2-D model(such as localized detonation and detonation-decoupled reinitiation phenomena).The effect of different loadings and diameters of droplets on the ignition time of methane-coal dust under different coal dust parameters was studied.It also revealed the inhibitory mechanism of water curtains on methane-coal dust detonation.The critical parameters for water curtain to suppress methane coal dust detonation decoupling under different coal dust concentrations are given.The results show that for droplets with a fixed loading,the ignition time decreases monotonically with the increase of the coal dust concentration,and increases monotonically with the coal dust particle size.For droplets with a fixed diameter,the ignition time decreases monotonically with the increase of coal dust concentration,and increases monotonically with coal dust particle size.Moreover,droplets larger than 5 μm in diameter have a long evaporation time,but have little effect on the ignition of gases and particles.For low-concentration coal dust environments,such as 50 g/m3,with the increase of the added water curtain loading,the methane-coal dust detonation structure becomes more unstable,manifested as larger cell size,larger shock wave velocity fluctuation amplitude and decrease in the particle heat release rate.When a water curtain with mass loading≥0.2 is added,the methanecoal dust detonation is completely decoupled.For high-concentration coal dust environments,such as 1000 g/m3,with the increase of the added water curtain droplet loading,the position of re-initiation after detonation decoupling is delayed,and the relationship is x=0.77 z2+0.05 z+0.23.When a water curtain with droplet loading≥0.3 is added,after decoupling of the detonation wave in the computational domain,no reinitiation is found.By adding a suitable explosion suppression water curtain in the detonation decoupling area,the time for the formation of hot spots on the reaction surface is prolonged,and the occurrence of re-initiation can be effectively prevented.The thesis includes 126 figures,16 tables and 307 references.