Nanoparticle synthesis in turbulent flame aerosol reactors is elucidated by computational fluid dynamics (CFD). Mixing-limited combustion is modeled, and total particle number, area, and volume concentration are described by transport equations including terms for particle dynamics. The spread of the particle size distribution at a given streamline is neglected as flame-made aerosols rapidly attain their self-preserving distribution. Results are in good agreement with primary particle data of turbulent diffusion flame synthesis of silica nanoparticles by oxidation of hexamethyldisiloxane vapor at different laboratories without adjustable parameters. Measured agglomerate mobility diameters best matched the predicted volume-equivalent soft-agglomerate diameters. The employed fractal-like dimensions (Df = 1.5−3) had no effect on the predicted primary particle and aggregate diameters and a rather small effect on volume-equivalent soft-agglomerate diameters.