Experimental Study On Tiny Single Spherical Particles' Unsteady Motion In The Air

The researches of single nanoscale or micron-sided particle moving in the air are usually involved in the fields of industrial applications and environmental management, such as preparation of nano materials, dilute gas-particle separation, particles delivery, haze prevention etc. China is suffering from the heavy PM2.5 problem nowadays, and it is vital to purify the air and protect our environment. However, up to now, the basic theories of tiny particles unsteady motion are not sufficient to guide the relevant industrial applications. Because, the knowledge of such tiny particles'dynamical motion is concentrated by plenty experiments which are difficult to fulfill. For this purpose, the unsteady dynamical motion in the air of single spherical particles with the size of 0.1mm-0.5mm are investigated in this paper. And the results of this work can be used to guide the research of smaller particles' unsteady movement in the future.Firstly, a low-speed wind tunnel and a set of high-speed photography system were employed to confirm the displacement of each particle from its trajectory. And this experimental device can record the process of single small particle's non-stationary moving through a microscope. Three types of spheres were tested at different air flow velocities in a low speed wind tunnel using the high speed photography system. And the experiment results of each sphere were connected to reproduce the complete free-fall motion from rest to the terminal state. Prediction results of the spheres'velocities and displacements were obtained analytically using CD-Re relationship (variational drag coefficients) in the range of 1<Re<300 which was extracted from the experimental data. Compared with the previous prediction ones with fixed drag coefficients, the new prediction results were clearly improved. The average relative errors of the displacements and velocities of new predictions were respectively 1.69% and 2.22%, which were better than ones with fixed drag coefficients of 7.16% and 4.93%. Besides, this method can be used to evaluate the single particles' displacement and velocity directly from the parameters of particle and fluid avoiding Re and CD, which was helpful of research and development of new type of gas-solid separator in the future.Secondly, the spherical solder beads and glass beads with the diameter ranging from 0.2mm to 0.5mm and 0.1mm to 0.5mm were used as the testing particles in air flow of which the velocity ranged from 0 to 11m/s. In this experiment, the CD-Re relationship of a single sphere moving in the unsteady flow was derived from the photos of displacements of the testing particles. The maximum of the percentage of add-mass force and Basset force did not exceed 0.25%and 2.2% and can be ignored in the whole process of particle's non-stationary motion. In this experiment, the Re numbers of testing particles ranged from 4.5 to 368.9 and a new expression was developed to fit the data array. Besides, the obtained Co-Re expression employed higher precision of drag coefficient comparing with previous curves.Finally, a circle arc wind tunnel was established by transforming the low-speed wind tunnel. The single small particles'unsteady motion in a swirling flow field was investigated experimentally. First, the internal flow distribution was determined by both actual measurement and CFD simulation. A mathematical model of internal flow distribution of the circle arc wind tunnel was established by the quasi-free rotation vortex theory. The model of sectional area's radial flow velocities of the internal secondary flow was also obtained from the CFD results. Secondly, three types of small single particles ware released in three different swirling flow field respectively, and their unsteady motions were record by the high speed camera. After some calculation, the information of particles' displacement, velocity and acceleration can be obtained. Finally, combined the internal flow distribution of circle arc wind tunnel and the mathematical model of secondary flow, the radial and tangential governing equations of a spherical particle releasing into the swirling flow field from rest were established and solved numerically respectively. The relationships of particle's radial and tangential displacement vs time, velocity vs time, and acceleration vs time were acquired respectively. Compared the results between the calculation and experiment, several conclusions can be draw. Primarily, the secondary flow of cross section of circle arc tunnel was rarely influence the particle's unsteady motion. Secondly, after releasing into the swirling flow field, the particle's motion were non-stationary in both radial and tangential directions. The particles'radial displacement and velocity, tangential displacement increased like exponential growth or parabolic growth over time. The tangential velocity and radial acceleration increased at first then decreased over time, and the variation of relationship of tangential acceleration vs time presented like an "S" curve. The tangential acceleration reached its maximum at first and then went down over time. Finally, the tendencies of results of calculation agreed with experiment. But there was a certain gap between the calculation and experiments. For particles'displacement and velocity, the experimental values were larger than calculate values.