Experimental Study On Flame Propagation Characteristics Of Zirconium Particle Cloud

A set of experimental system was established which is suitable for studying dust flame propagation characteristic. Combining with PIV technology, microscopic lens and high-speed schlieren, flame propagation characteristic and flame microstructure as well as its propagation mechanism of zirconium particle cloud were investigated. At the same time particle size and dust concentration effect on flame propagation regularity were also studied.At first, the PIV measurement technique was used to measure turbulent intensity change with time after the end of dispersion,thus decay regularity of turbulent intensity was obtained. Turbulent burning speed was gained by Application of tube methods and at the same times superfine thermocouple was used to measure temperature change during the flame propagation. The initial turbulent intensity, the concentrations of dust cloud and dust particle size effect on the turbulent burning speed and flame temperature were gained. Flame propagation characteristics impacted by these three factors were further analyzed. The results demonstrate that the turbulent burning speed and flame temperature have a linear relationship, so the main heat transfer way was revealed in zirconium dust cloud flame propagation process. Combustion zone is mainly driven by thermal conductivity to transfer heat to preheated zone, while heat transfer by thermal radiation accounts for the secondary position in the process.Flame propagation characteristic with ignition end open or close was investigated. The flame shape, flame temperature and flame propagation speed which represent flame propagation characteristic parameters were obtained and at the same time change regularity in flame propagation was also further analyzed. The results show that the flame temperature and flame propagation speed increase remarkably with the concentration of zirconium particle cloud at lower concentrations, reach the maximum value, and then decrease slightly at higher concentrations. For four different average particle sizes, the dust concentrations are different when flame temperature and flame propagation speed reach the maximum value. The minimum dust explosion concentration of four particle sizes was tested. The results reveal the particle sizes effect on the minimum dust explosion concentration and a theoretical qualitative analysis is also given.High-speed schlieren and microscopic lens were used to study flame microstructure and particle velocity before flame front.Flame structure model and propagation mechanism were reconstructed. The results demonstrate that the flame preheated zone thickness is different depending on particle size and dust concentration. Maximum thickness of preheated zone is up to 2cm or so. The combustion zone lays behind the preheated zone. The combustion zone thickness is mm order of magnitude. Smaller particles mainly exist in the width 1.0mm in the leading edge of the combustion zone, while the larger particles mainly appear 1-4mm behind the leading edge of the combustion zone. Zirconium particles and oxygen take chemical reaction on the zirconium particle surface and form gas-solid surface combustion system. According to the experimental results and theoretical analysis, the combustion process of zirconium particle was speculated. With the preheated zone temperature rise, when the preheated zone temperature reaches a certain value, the surface of zirconium particles causes the breakdown of the oxide film due to thermal stress. After the breakdown of the oxide film, pure zirconium can contact with the oxygen, no longer hindered by the surface oxide film. When temperature of the preheated zone reaches 220℃, Zirconium particles start to ignite. In the zirconium dust cloud flame propagation process, the smaller particles have the relatively more important role than the larger particles.