Analysis on the morphology and mechanical characteristics of agglomerated particles emitted from the diesel exhaust process

Abstract

The diesel engine particulate which is one of the major sources of PM, affects the development of human health and production activities seriously. With the continuous strict of emission regulations, the diesel engine particles are subject to strict restriction in terms of quality and quantity. At present, the diesel particulate filter technology is an important technical measure to reduce the exhaust particulate matter of the diesel engine. And the efficient capture of particles is one of the key technologies for the diesel particulate filter. Diesel engine exhaust particles undergo nucleation, surface growth, agglomeration and other processes, in which the degree of agglomeration of exhaust particles will directly affect its number density, scale and the trapping efficiency of the filter and so on. The reason why particles can be agglomerated is mainly due to the interaction between the particles. Therefore, to explore the relationship between the force and the effect of particle agglomeration, it is necessary to carry out the study on the interaction between the exhaust particles of diesel engines. The interaction between particles will affect the process of particle agglomeration and related parameters significantly. The main forces include van der Waals force, electrostatic force and liquid bridge force. In this paper, theoretical models of Hamaker and some others are used to analyze the force between particles in the ideal state. The test methods of atomic force microscopy (AFM) and transmission electron microscopy (TEM) are used to investigate the total attraction force, adhesion force and adhesion energy between diesel exhaust particulates in dry environment (relative humidity RH≤20%), and measure the total attraction force, adhesion force and adhesion between particles, and explore the relationship between particle size and particle size. The results show that the particle size increases from 25 to 45 nm, the Van der Waals force increases about 4.6 times, the electrostatic force increases about 9.9 times, and the electrostatic force is smaller than the electrostatic force. The adhesion between particles and adhesion energy both increased about 1.8 times. The particles with the average particle size of 30, 37 and 46 nm are measured by AFM. With the increase of the particle size, the total attraction F at between the particle and the probe increases gradually, which is 1.04–1.38 nN. Van der Waals force F vdw is the main force of the F at, and Coulomb force is smaller. The attraction force F at and the adhesion energy W ad increase with the increase of particle size, F at increases from 3.21–3.75 nN, about 16.8%, while W ad increases from 2.03×10−16 J to 2.20×10−16 J, about 8.4%. The increase of F ad and W ad indicate that the adhesion as well as the energy barrier between the particles is increased, and the particle agglomeration is stable. With the increase of the average particle size, the particles agglomerate gradually and change from branch to cluster, showing a hill-like accumulation, and the particle density as well as the cluster size are increased, which indicates that there was a certain influence between the change of particle force and particle morphology.