| In recent years,the premixed flammable gases has been more and more widely used in the fields of industrial production,civil architecture and transportation.The leakage and explosion accidents of premixed combustible gas occur frequently in all kinds of gas pipeline,and which has caused heavy casualties and property losses,however.Moreover,the lots of consumption of non-renewable fossil fuels in the development process of traditional industrial,such as coal,oil and nature gas,has caused serious scarcity of energy and environmental pollution and other issues.Compared with other carbon-containing combustible gases,Hydrogen,as a kind of pollution-free ideal flammable gaseous fuel in the future,has the characteristics of low ignition energy,a wide range of combustion limit,a high laminar burning velocity and diffusivity,a small flameout distance and a large flameout rate.Its combustion process,flame surface dynamic structures,and the explosion dynamic behaviors exhibit different characteristics.Therefore,it is imperative to carry out the research on blasting process and blasting characteristics of H2/Air premixed gases.However,investigating the deflagration characteristics of premixed H2/Air and the influence of ignition conditions,initial conditions,pipe size effect and various pipeline structures on its deflagration characteristics can provide important references for designing industrial pipelines and preventing deflagration accidents.In addition,based on the deflagration theory of premixed combustible gases,the hydrogen energy has been applied in preparing nanomaterials,and the research of preparation mechanism through characterization and analysizing the microstructures of nanomaterials could be used to analyze the deflagration characteristics and combustion theory of premixed H2/Air.The establishment of the mutual affect and contact mechanism will be a kind of new research ideas and methods.Based on the theory of gaseous detonation and strength,a explosion reaction pipe was developed to investigate the deflagration process of premixed H2/Air and the influence of initial temperature and mixing ratio of Hydrogen on the explosion peak pressure at different positions.Then,experimental study on preparing TiO2 nanoparticles by the explosion reaction of premixed H2/Air to explore the practical applications of hydrogen explosion.Finally,the dynamic behavior of the flame surface structure,dynamic distribution of temperature,the pressure field and velocity field,and the variation law of energy and vorticity in the deflagration process of premixed H2/Air were simulated using the fluid dynamics software Fluent.The effects of different ignition conditions,initial conditions,pipe size effects and piping structures on premixed H2/Air deflagration process and characteristics have also been investigated.Based on the above research,the results achieved are as follows:?1?The self-developed detonation reaction pipe can ensure the safety and reliability in the explosion experiment of premixed H2/Air by the test of explosion peak pressure and strength checking.With the propagation of shock wave from the ignition part to the right,the explosion reaction is strengthening,and the peak pressure of explosion in the pipe is gradually increasing,and the greastest increased rate is about 15.97%.However,the peak pressure of explosion at the end of the pipe affected by the initial temperature?T0=300 K473 K?and the hydrogen mixing ratio?H2/Air=0.20.8?.With the increasing of initial temperature and hydrogen mixing ratio,the peak pressure decreases and increases gradually,respectively.In addition,the mixed crystals of TiO2 nanoparticles?Anatase and Rutile?were prepared successfully to explore the practical application of explosion reaction pipe.The particle size of the prepared TiO2 nanoparticles gradually increased and the component of anatase decreased with the rising of the initial temperature.However,after annealing at different high temperatures,it was found that the grain size increased significantly with the increase of the thermal temperatures.The content of anatase increased first and then anatase was changed to rutile at 900?,which indicated that the phase transformation temperature of anatase to rutile is between 700?and 900?.?2?The deflagration process of premixed H2/Air in a closed straight pipe was simulated by Fluent software.It was shown that the premixed flame experienced a spherical flame,a fingertip-type flame,a plane flame after contacting with the wall and "tulip" Flames.This process is basically consistent with the "four-stage" flame proposed by Clanet and Searby.According to the distribution of temperatures at the same section of closed straight tube,it is found that the turbulence vortex movement is stronger and the flame temperature is higher closing to the pipe wall,which revealed the "tulip" flame surface structure and reflected the effects of wall confinement on flame surface structure.Due to the influence of ignition conditions,such as ignition temperature,ignition area and ignition position,the differences of the initial combustion mode of premixed H2/Air promoted that the time-history curves of temperature,pressure,flame propagation velocity and the changed law of energy and vorticity showed a significant discrepancy.The activation energy and combustion reaction intensity of the molecules of premixed gases were obviously enhanced by increasing ignition temperatures.Changing the ignition area has a great effect on the initial combustion rate,thermal diffusion and transfer rate.Furthermore,when adoping the method of changing ignition positions,the function of wall confinement and induced propagation suffered by the initial fire core are significantly different.The simulation results show that the peak pressure and average flow velocity is lower with the higher initial temperatures and the lower initial pressures.The maximum temperature increases with the initial temperature in the early stage of explosion reaction of premixed gases,and decreses with initial temperatures in the later period of explosion reaction,while the effect of initial pressures is not apparent in the closed pipe.The influence of pipe size effect on the deflagration characteristics of premixed gases in closed straight tube is mainly realized by changing the diameter and length.At the constant concentration,the occurrence time of "tulip" flame will be delayed due to the increase of cross-section.The turbulence effect and the swirl motion induce the formation of the "tulip" flame by accelerating the combustion reaction rate.In short,when the section of the pipe is smaller,the formation time of the "tulip" flame is earlier,and the maximum temperatures in the closed pipe are also higher.However,when the pipe length is increasing at a constant of pipe diameter,the distance of leading dilute wave and combustion flame arriving to the right end of the pipe wall is extended,and the time of combustion reaction heat transferring and diffusing to the right of unburned gas is also longer.The highest temperatures and peak pressures in the pipe are lower and the rising rate is also slower.?3?After the combustion flame of H2/Air enters the 90° curved pipe,the constraint and induced action of the curved wall distorted the flame front,the turbulence degree is enhanced by the additional turbulence produced by the mutation of the combustion area,and the symmetrical structure of "V"-type turbulent flame destroyed.In addition,the turbulence intensity and the combustion reaction rate are also increased under the combined action of the reflected compression wave in the upper wall of the 90° curved pipe,the dilute wave in the lower wall and the multiple reflection superposition of the pressure waves between the two walls,and the earlier the premixed flame entering the 90° bent pipe,the more influences of the pipe structure on the characteristic parameters of the blasting process.However,due to the effects of sudden expansion of the pipe section,obstruction,disturbance and reflection induced by the inflexion point in the upper wall of the T-shaped bifurcation duct,and the multiple reflection and diffraction effect of the pressure wave between the walls on the premixed H2/Air,the turbulence degree in the flow field increases and the combustion reaction intensity and the flame propagation rate are also increased significantly.Moreover,the influence of T-shaped bifurcated duct structure on the vorticity of premixed flame is mainly due to the obstruction and disturbance effect of the inflection point in the upper wall of bifurcation duct,rather than the sudden expansion induced by the pipe cross-section.It is also pretty distinct that the influence of different pipe structures with variable cross-section on deflagration characteristics of premixed H2/Air,especially the section sudden effect expansion in the expansive pipe and the blocking effect of section shrinkage in the contractive pipe,which changed the dynamic structure of premixed flame and the forward direction of the dilute wave and the flow direction of premixed gas in front of premixed flame.Therefore,the temperature field,pressure field,velocity field and the changes of energy and vorticity in the pipe with variable section are obviously different. |
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