A Study Of The Factors Affecting Particle Collision Restitution Coefficient

There are various kinds of particulate systems in the nature,such as small volcanic ashes blown into atmosphere in the eruption of volcanoes and the sand/dust particles blown into air from arid regions or deserts by strong wind,which transport in atmosphere with wind.The understanding of the transportation of these particle systems in the atmosphere is related to the “particle-particle” restitution coefficient and “particle-particle bed” restitution coefficient,which are the fundamental problems of the macroscale dynamical characteristics of a particle system.Based on this motivation,this thesis is focused on studying the relationship between the“particle-particle” restitution coefficient,as well as the “particle-particle bed”restitution coefficient,and collision speed,particle sizes and material properties.The theis is organized as follows:In Chapter 2,for the “particle-particle” restitution coefficient,a modified Thornton model is established based on the original Thornton model and Hertz contact theory,by introducing the dynamical yield strength into the model.Via the modified Thornton model,the relationship between the collision restitution coefficient and collision speed,particle size ratio,material properties(Youngth Modulus,Yield strength)are obtained for metal spheres,which are compared with experimental results from existing references.It is found that compared with the original Thornton model,the restitution coefficient given by the modified Thornton model,including the value of the restitution coefficient and its relationship with the varying colliding speed and particle size,are in better agreement with the experimental results.That means the effect of dynamical yield strength of the material plays important role in particle-particle collisions.On the other hand,the effect of strain hardening on the particle-particle restitution coefficient is studied.The results show that by taking strain hardening into consideration,the theoretical value of restitution coefficient is higher than the experimental results,however,the trend of variation of restitution coefficient with the varying colliding speed is almost coincident with the experimental results.For the verification of the modified Thornton model obtained in Chapter 2,the particle-particle collision restitution coefficients,as well as its dependence on the colliding speed and particle sizes,are obtained in Chapter 3,by using spherical metal beads(iron and aluminum).Then,by comparing the theoretical predictions from the modified Thornton model,we find that the modified Thornton model can describe the law of variation of restitution coefficient,including the value and the trend of variation,with the varying colliding speed or varying particle sizes.However,the original Thornton model have large errors,when compared with the experimental results.That means the modified Thornton model is more reasonable.In Chapter 4,experimental studies are conducted,to measure the“particle-particle bed” collision restitution coefficients.The “particle-particle bed”restitution coefficient,as well as its relationship with colliding speed,particle bed thickness and particle material properties,is investigated via experiments.The experimental results show that the law of particle-particle restitution coefficient can not be simply extended to the case of “particle-particle bed” collision processes.From the experimental results,we found that colliding speed,properties of materials,the size ratioof the impacting particle and the particles in the particle bed only result in small variations of the value of “particle-particle bed” restitution coefficient,butthe thickness of the particle bed has significant effect,that is,the “particle-particle bed”collision restitution coefficient decreases significantly with increasing thickness of particle bed,which may indicate the mechanical properties of the whole particle bed is playing the key role for the “particle-particle bed” collision processes,instead of the size ratio of the impacting particle and the particles in the particle bed.Chapter 5 gives a brief review of the remaining problems existing in our research and the further researches that could be conducted in future.