Numerical simulation of nanoparticle nucleation and coagulation in a mixing layer with sulfuric acid vapor binary system is performed using the large eddy simulation and the direct quadrature method of moment.The distributions of number concentration,volume concentration,and average diameter of nanoparticles are obtained.The results show that the coherent structures have an important effect on the distributions of number concentration,volume concentration and average diameter of nanoparticles via continuously transporting and diffusing the nanoparticles to the area of low particle concentration.In the streamwise direction,the number concentration of nanoparticles decreases,while the volume concentration and the average diameter increase.The distributions of number concentration,volume concentration and average diameter of nanoparticles are spatially inhomogeneous.The characteristic time of nucleation is shorter than that of coagulation.The nucleation takes place more easily in the area of low temperature because where the number concentration of nanoparticles is high,while the intensity of coagulation is mainly affected by the number concentration.Both nucleation and coagulation result in the variation of average diameter of nanoparticles.
Evolution of number concentration of nanoparticles undergoing Brownian coagulation in the transition regime is studied theoretically and numerically.The results show that the curves of particle size distribution move toward the area with large particle diameters, the curve peak becomes lower and the range that particle diameters cover becomes wider as time elapses.In the process of coagulation the particles with small diameter disappear gradually and the particle size distribution remains a log-normal distribution.The change rate of the particle size distribution is more appreciable at the initial stage than that at the final stage.The initial Knudsen number has a significant effect on the coagulation rate which increases with decreasing the initial Knudsen number.The larger the initial geometric standard deviation is, the smaller the curve peak is, and the wider the area that curves cover is.The initial geometric standard deviation has a significant effect on the particle size distribution which can remain a self-preserving state when the initial geometric standard deviation is smaller than 2.With the increase of the diversity of initial particle size, the particle size distribution does not obey the log-normal distribution any more as time elapses.
The collision efficiency of two nanoparticles with different diameters in the Brownian coagulation is investigated. The collision equations are solved to obtain the collision efficiency for the dioctyl phthalate nanoparticle with the diameter changing from 100 nm to 750 nm in the presence of the van der Waals force and the elastic deformation force. It is found that the collision efficiency decreases as a whole with the increase of both the particle diameter and the radius ratio of two particles. There exists an abrupt increase in the collision efficiency when the particle diameter is equal to 550 nm. Finally, a new expression is presented for the collision efficiency of two nanoparticles with different diameters.
