Polymerization and Collision in High Concentrations for Brownian Coagulation

Aggregation always occurs in industrial processes with fractal-like particles, especially in dense systems (the volume fraction, ϕ>1%). However, the classic aggregation theory, established by Smoluchowski in 1917, cannot sufficiently simulate the particle dynamics in dense systems, particularly those of generat ed fractal-like particles. In this article, the Langevin dynamic was applied to study the collision rate of aggregations as well as the structure of aggregates affected by different volume fractions. It is shown that the collision rate of highly concentrated particles is progressively higher than that of a dilute concentration, and the SPSD (self-preserving size distribution) is approached (σg,n≥1.5). With the increase in volume fraction, ϕ, the SPSD broadens, and the geometric standard is 1.54, 1.98, and 2.73 at ϕ= 0.1, 0.2, and 0.3. When the volume fraction, ϕ, is higher, the radius of gyration is smaller with the same cluster size (number-based), which means the particle agglomerations are in a tighter coagulation. The fractal-like property Df is in the range of 1.60–2.0 in a high-concentration system. Knowing the details of the collision progress in a high-concentration system can be useful for calculating the dynamics of coagulating fractal-like particles in the industrial process.

Simulation of Aerosol Evolution within Background Pollution for Nucleated Vehicle Exhaust via TEMOM

This work is intended to study the effect of background particles on vehicle emissions in representative realistic atmospheric environments. The coupling of Reynolds-Averaged Navier–Stokes equation (RANS) and Taylor-series Expansion Method Of Moments (TEMOM) is performed to track the emissions of the vehicle and simulating the evolution of the matters. The transport equation of mass, momentum, heat, and the first three orders of moments are taken into account with the effect of binary homogeneous nucleation, Brownian coagulation, condensation, and thermophoresis. The parameterization model is utilized for nucleation. The measured data for Beijing’s particle size distribution under both polluted and nonpolluted conditions are utilized as background particles. The relationship between the macroscopic measurement results and the microscopic dynamic process is analyzed by comparing the variation trend of several physical quantities in the process of aerosol evolution. It is found with an increase of background particle concentration, the nucleation is inhibited, which is consistent with the existing studies.

Langevin Dynamics Calculation of Brownian Coagulation Coefficient for Spherical Equal-size Aerosol Particles in Transient Regime

Coagulation coefficient of aerosol particles due to Brownian motion is an important issue to describe change in particle size distribution. Motion of aerosol particles is diffusive in continuous region (small Knudsen number; Kn), or like free molecular motion of gaseous molecular in free molecular region (large Kn). Fuchs (1964) presented an expression of coagulation coefficient in transition regime by a so-called “Flux Matching” method. In his method, transportation of particles inside of the “limiting sphere” is assumed to be like free molecular, or diffusive outside of the sphere. These days, some researchers presented coagulation coefficient of aerosol particles by direct calculation of motion of aerosol particles. They employed Langevin dynamics equation to represent the stochastic motion of aerosol particles. In this study, we developed new model to calculate the coagulation coefficient. Our model employed spherical calculation space in which one scavenging particle is in the center of it: the calculation sphere moves together with the motion of the scavenging particle. The coagulation coefficient can be calculated from the mean time between collisions and the concentration of collision particles. By using the above numerical model, we have calculated the coagulation coefficient of spherical particles of from 4 nm to 100 nm in diameter.