Direct Numerical Simulation Of Turbulent-Brownian Particle Collisions In Isotropic Turbulence

Inhaled particulate matter from combustion does serious harm to human health and environment. To know how the particulate matter generates and grows in the combustion process is the prerequisite to control and prevent them. And one of the core problems is inter-particle collision and coagulation process. For fine particles, both turbulent effect and Brown motion effect play a signifinant role for the collision process. To simulate and predict the collision kernel under the two effects is the hotspot of two phase flow studies and the aerosol studies nowadays.After a brief summary of the research developments in particle average collision kernel model, correlated mathematic models and numerical simulation algorithms, direct numerical simulations (DNS) were conducted to study particle collisions in a stationary isotropic homogeneous turbulent flow(without Brownian motion), with the aim to investigate the influence of turbulence on particle collision kernels of various finite-inertia particles. It is found that the behavior of finite-inertial particle collision is very complicated, both the Saffman & Turner theory(St_k=τ_p/τ_k=0) and kinetic theory (St_k=∞) can't predict it correctly. To further understand the mechanism of finite-inertia particle collision in isotropic turbulence, two major effects of turbulent flow on particle collision, namely turbulent mixing effect and preferential concentration effect, are investigated and are represented qualitatively using radial relative velocity <|wr|> and radial distribution function g(R) of colliding particle pairs respectively. Both effects tend to increase collision rates, leading to the observed complex behavior. The results showed that preferential concentration effect is the main contribution factor for the peak of particle collision rate near St_k~1, while both preferential concentration effect and turbulent mixing effect contributing to the peak near St_k~3, with much stronger turbulent mixing effect herein. To focus on the fine particle matters, particles with Stk between 0 and 1 were further investigated. After an asymptotic analysis, a simple turbulent collision kernel model for fine particles was derived from the DNS results.Combined with DNS method, Brownian dynamics simulation (BD) method was utilized to study the relative importance of turbulent effect and Brownian motion effect on fine particle collision process. For particles with diameter between 0.6-1.2μm, the results showed that turbulent effect is dominant, while for particles with diameter between 0.1-0.4μm, Brownian motion effect increases with diameter decreases. Turbulent effect and Brownian motion effect became comparable herein.