| With advantages of high combustion speed and high combustion heat, zirconium metal powder, as a kind of metal fuel, is widely used in aerospace and military fields, such as flash powder, fireworks, artillery shells, fuzes, timing bomb fuses and solid propellant rocket fuel. In order to meet some special application in aerospace industry and military industry, zirconium dust particles are coated with FeOOH\Fe2O3\Fe3O4nanoparticles to form a core-shell structure, after which the combustion performance of coated zirconium dusts has been changed. In order to fully understand the flame characteristics of zirconium dusts and FeOOH\Fe2O3\Fe3O4coated zirconium dusts during their combustion process, an experimental system of dust cloud continuous blowing spray combustion was developed in this work and combined a experimental system of dust cloud transient flame propagation, both the transient flame characteristics and continuous jet flame characteristics of zirconium dust cloud, FeOOH\Fe2O3\Fe3O4coated zirconium dust cloud were comprehensively analyzed. In addition, by combining with powder morphology analysis, phase analysis and high temperature oxidation resistance analysis, the combustion mechanisms of zirconium dust cloud and coated zirconium dust cloud in the air were explored, which provided the basic theories to the transient flame propagation and the spray combustion of carrier particles stream.Firstly, the experimental system of dust cloud transient flame propagation was used to study the flame propagation characteristics of zirconium dust cloud in the pipeline at different dust cloud concentration. The flame propagation behaviors of FeOOH\Fe2O3\Fe3O4coated zirconium dust cloud was also investigated, basing on the same experimental methods. The results show that:(1) for the same kind of dust cloud, when the dust cloud concentration is low, the increase of dust cloud concentration will enhance combustion intensity, increase luminous intensity, flame propagation acceleration and maximum flame temperature. When the dust cloud concentration reaches a certain value, the flame will achieve the highest propagation speed.and the flame temperature will reach the maximum value. Then, with the continuous increase of dust cloud concentration, both the flame propagation speed and flame temperature will decrease slowly.(2) for pure zirconium dust cloud, there is the highest flame propagation speed at dust cloud concentration of0.625kg/m3. For FeOOH\Fe2O3\ Fe3O4coated zirconium dust cloud with molar ratio of1:6(coating material:pure zirconium), there are the highest flame propagation speed at dust cloud concentration of1.21kg/m3,1.15kg/m3and1.37kg/m3, respectively. The molar ratio changes to1:3, there are the highest flame propagation speed at dust cloud concentration of1.13kg/m3,0.96kg/m3and1.32kg/m3, respectively. For FeOOH\Fe2O3\Fe3O4coated zirconium dust cloud with the same molar ratio, the sequence of both the maximum flame propagation speed and the maximum flame temperature is:Fe2O3coated zirconium dust cloud> FeOOH coated zirconium dust cloud> Fe3O4coated zirconium dust cloud.Secondly, the continuous jet flame characteristics of zirconium dust cloud and FeOOH\Fe2O3\Fe3O4coated zirconium dust cloud were studied. The features of flame structure, flame stability and flame height of jet flame and the influence s of dust cloud concentration on the flame emissivity, the maximum flame temperature and the radiation heat flux were obtained. The results show that:(1) the jet flame of zirconium dust cloud is considered as an axisymmetric flame, and the structure of the jet flame is divided into continuous flame zone, intermittent flame zone and discrete particles zone. Whether a jet flame of dust can burn steadily on the burner or not, which depends on the ratio of the gas flow velocity and the dust cloud concentration. With increase of dust cloud concentration, all of the average flame height, the average flame area and the average flame width increase, but their increasing rates are different.(2) the variation law of the temperature along the axis of jet flame is analyzed. From the bottom of flame to the top of continuous flame zone, as the height increases, the flame temperature first increases and then decreases, finally increases again in intermittent zone. The higher dust cloud concentration, the smaller the maximum flame temperature fluctuations, and as the dust cloud concentration increases, the overall flame temperature grows.(3) the radiation heat flux of the upper position of jet flame is larger than the lower, and the nearer to the jet flame, the radiation increases, the difference of radiation heat flux between the upper and the lower position of jet flame becomes larger. With increas e of dust cloud concentration, the whole flame radiation heat flux heightens.(4) compared with the pure zirconium dust cloud combustion, the flame temperature of coated zirconium dust cloud decreases, the flame radiation heat flux weakens, and the flame emissivity increases. The maximum flame temperature has a negative correlation to the flame emissivity. When the zirconium dust cloud concentration are0.328kg/m3,0.410kg/m3and 0.485kg/m3, the jet flame emissivity are0.2,0.19and0.18, respectively, the maximum flame temperature are2147.5℃,2248.1℃and2377.8℃, respectively, and the radiation heat flux are350.31kW/m2,435.19kW/m2and559.88kW/m2, respectively. For FeOOH coated zirconium dust cloud with molar ratio of1:6, there is the highest flame propagation speed at dust cloud concentration of0.485kg/m3, the jet flame emissivity is0.44, and the maximum flame temperature is1528.4℃.Thirdly, a physical model of the instantaneous flame structure of zirconium dust cloud is established. The flame structure of zirconium dust cloud is divided into three zones:preheating zone, combustion zone and burned zone; and the combustion zone is further divided into three zones, small particles combustion zone, mixture combustion zone with small particles and larger particles, and larger particles combustion zone. Also, a physical model of multi-tube jet combustion of dust cloud is established. In this model, a jet flame is formed with single tube superposition flame, there is an interaction between single flame and other single flame, the flame temperature reached the maximum at a certain distance above the nozzle exit. A structure model of particles group combustion and movement in the jet flame is also established, the process of particles group combustion and movement is divided into three zones:ignition zone, crystal transition zone and burned zone.Finally, the particles chemical reaction mechanisms of zirconium dust cloud combustion and FeOOH\Fe2O3\Fe3O4coated zirconium dust cloud combustion in air were analyzed. The results show that:(1) both the combustion process of zirconium dusts and coated zirconium dusts in air are characterized by the weight increase and heat release. The initial reaction temperature of zirconium dusts is lower than the coated zirconium dusts, and the heat emission of a unit mass of zirconium dusts is more than the coated zirconium dusts. Moreover, the more coating material mass (the thicker of coating layer), the higher initial reaction temperature and the less heat emission of a unit mass of dusts.(2) according to the metal crystal structure analysis of solid chemistry, the possible chemical reaction mechanism of the zirconium particles combustion in air is that the zirconium dioxide is capable of combining with the some trace elements oxide to form the solid solution of zirconium dioxide, a lot of oxygen vacancies exist in the solid solution, so a large number of oxygen ions spread to the surface of zirconium metal by oxygen vacancies diffusion, which makes the chemical reaction of zirconium particles combustion continue.(3) the chemical reaction mechanisms of FeOOH\Fe2O3\Fe3O4coated zirconium particles combustion in air are that coating layer makes a replacement reaction with the inner core zirconium, the inner core zirconium makes a oxidation-reduction reaction with oxygen, and pure Fe is oxidized at high temperatures, Fe2O3begins thermal decomposition aroud800℃, and FeOOH coated zirconium particles has a hydroxyl releasing process. |
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