3D Holographic Visualization Of Combustion Particle Fragmentation And Droplet Splash And Wind Tunnel Test Research Of Cloud Particle

The measurement on particle mophology is of great significance in energy power and aviation military fields.For example,the size and morphology of fuel particles and combustion products affect the combustion efficiency and pollutant emissions,and supercooled large droplets cause ice accumulation and affect aircraft flight safety.The underlying mechanism of fuel particle combustion and droplet splashing are not completely clear,which results from the lack of multiparameter,full-field,three-dimensional(3D)quantitative measurement of particles.Most of the optical diagnostic techniques are only able to measure limited parameters at a two-dimensional(2D)plane or a single point.Digital holography is an emerging 3D imaging technique,which can realize online 3D quantitative measurement of the particle morphology,size,concentration,3D distribution and 3D velocity of irregular particles.Therefore,this paper applies digital holography to 3D visualize the particle morphology evolution of particle fragments,volatile cloud and soot aggregation in fuel particle combustion and droplets generated during droplet impact and splashing process.A holographic airborne cloud particle imager is developed for airworthiness certification of domestic aircraft such as C919,which has been successfully applied in two icing wind tunnels.A high-speed digital holography system is established to 3D visualize the combustion process of pulverized coal and bamboo particles.The clear morphology of coal and bamboo particles in the flame is captured,and the time-resolved 3D spatial distribution of the parent particles and the corresponding fragments are dynamically tracked.Three fragmentation modes of coal particles are investigated,including fragmentation in the particle center,fragmentation on the particle surface and the hybrid fragmentation.The morphology differences of the parent particles and fragments undergoing different fragmentation modes are obtained.The 3D structure and 3D boundary of the soot aggregation are obtained by using a 3D automatic measurement algorithm of particles and fine strips.The influence of coal type on the generation potential,morphology evolution and motion of the soot aggregation is studied.The results show that Shanxi bituminous coal has a higher potential to generate soot aggregation than that of Ximeng lignite due to the larger amount of tar in the volatile matter,and no soot aggregation is produced for Yinni lignite's combustion process.The velocity slip of Ximeng lignite between the parent particles and volatile cloud/soot aggregation is more obvious than that of Shanxi bituminous coal due to its low viscosity of volatile matter.A high-speed digital holography system is also built to conduct a qualitative and quantitative study for the splashing droplets in a 3D space.The time-resolved evolution of the spatial distribution of the splashing droplets is visualized,and the mechanism of the three fragmentation modes of droplet splash is elucidated by analyzing the evolution of the droplet size.The effects of surface roughness,impact velocity and impact angle are studied to investigate the droplet splashing characteristics.The results show that the intensity and splashing height of the splashing droplets increase with the surface roughness and impact velocity.The number,particle size and splashing angle of the splashing droplets along the regions before and after the impact point O are compared and analyzed to show the asymmetric distribution characteristics of the droplet splash on an inclined surface.The relationship between the tangential and normal velocities of droplets in the regions before and after the impact point O with the change of impact angle is also summarized based on the 3D information of the splashing droplets.A holographic airborne cloud particle imager(HACPI)is developed for extreme test conditions in the icing wind tunnel,such as low temperature(down to-40°C),high humidity,high speed,and strong vibration.By coupling the optimization design of the instrument probe structure and the local temperature control of the optical windows,the sampling representation in the measurement area is guaranteed and the icing problem of the optical window is solved.The particle measurement error for the particles <10 ?m and >50 ?m are <10% and <4%,respectively.The instrument has been successfully applied in the Harbin mobile icing wind tunnel and a 3 m×2m icing wind tunnel in Mianyang,and captures a lot of important information of 6?1000 ?m water droplet and ice crystal,such as morphology,particle size distribution,median volume diameter(MVD),and liquid water content.The effects of water pressure and air pressure of the spray system on the MVD of cloud particles are studied.The results show that the MVD of supercooled water droplets significantly increases with the increase of water pressure and the decrease of air pressure.