Sampling and dilution of nanoparticles at high temperature

Sampling and dilution of flame-generated, fractal-like ZrO₂ aerosols is investigated by aerosol mass/mobility measurements and microscopy. Two broadly used sampler configurations, a straight-tube (ST) and a hole-in-a-tube (HiaT), at three different in-flow orientations and hole diameters are evaluated. The mobility size distributions, mass-mobility exponent, D_fm, prefactor, k_fm, and average primary particle diameter are obtained at 10–60 cm height above the burner (HAB) of fuel-rich (hot) and fuel-lean (cold) spray flames by differential mobility analyzer (DMA) and aerosol particle mass (APM) measurements using a recent power law for fractal-like particles. The primary particle diameter, agglomerate size distributions, and corresponding standard deviations from aerosol measurements are compared to those by counting images of particles collected by thermophoretic sampling along the flame centerline. Once new particle formation is completed in the flame, both sampler configurations result in nearly identical particle size distributions. Furthermore, all HiaT samplers result in similar mobility size distributions at all orientations, regardless of hole size. Sampling using a downstream in-flow hole orientation results in slightly larger Sauter mean diameters than those obtained by upstream or sidestream ones, especially for the cold flame. Additionally, a correlation is developed by Discrete Element Modeling (DEM) for the agglomerate D_fm evolution to its asymptotic value of 2.2 as function of the average number of primary particles per agglomerate, n_va, or the relative particle density with pre-exponential constant k_fm = 1.18, regardless of primary particle size. This is in good agreement with an experimentally obtained correlation in terms of relative particle density as well as with experimental data for ZrO₂, Ag, and Cu nanoparticles.