Direct numerical simulation of turbulent aerosol coagulation

There are numerous systems—including both industrial applications and natural occurring phenomena—in which the collision/coagulation rates of aerosols are of significant interest. Two examples are the production of fine powders (such as titanium dioxide) and the formation of rain drops in the atmosphere.; During the last decade, it has become apparent that dense aerosol particles behave much differently in a turbulent fluid than has been previously assumed. Particles with a response time on the order of the small-scale fluid time scale tend to collect in regions of low vorticity. The result is a particle concentration field that can be highly non-uniform. Sundaram and Collins (1997) recently demonstrated the effect that turbulence can have on the particle collision rate of a monodisperse system. The collision rates of finite-inertia particles can be as much as two orders of magnitude greater than particles that precisely follow the fluid streamlines. Sundaram and Collins derived a general collision expression that explicitly accounted for the two phenomena that affect the collision rate—changes in the particle concentration field and changes in the particle relative velocities. The result of Sundaram and Collins has generated further interest in the turbulent-aerosol problem.; This thesis shows that, in addition to changing the rate that an aerosol size distribution might form, turbulence has the potential of dramatically changing the shape of the distribution. This result is demonstrated using direct numerical simulation of a turbulent-aerosol system over a wide range of particle parameters, and a moderate range of turbulence levels. Results show that particles with a small (but finite) initial inertia have the greatest potential of forming broad size distributions. The shape of the resulting size distribution is also affected by the initial size of the particles. Observations are explained using the statistics identified by Sundaram and Collins (1997). A major emphasis in the thesis is on quantifying the degree of turbulence-induced non-uniformity of the particle phase, and to a lesser extent, the change in relative velocities between particles.