Two-dimensional direct numerical simulation of nanoparticle precursor evolution in turbulent flames using detailed chemistry

Abstract

Flame-based synthesis is an attractive industrial process for the large-scale generation of nanoparticles. In this aerosol process, a gasified precursor is injected into a high-temperature turbulent flame, where oxidation followed by particle nucleation and other solid phase dynamics create nanoparticles. Precursor oxidation, which ultimately leads to nucleation, is strongly influenced by the turbulent flame dynamics. Here, direct numerical simulation of a canonical homogeneous flow is used to understand the interaction between a methane/air flame and titanium tetrachloride oxidation to titania. Detailed chemical kinetics is used to describe the combustion and precursor oxidation processes. Results show that the initial precursor decomposition is heavily influenced by the gas phase temperature field. However, temperature insensitivity of subsequent reactions in the precursor oxidation pathway slow down conversion to the titania. Consequently, titania formation occurs at much longer time scales compared to that of hydrocarbon oxidation. Further, only a fraction of the precursor is converted to titania, and a significant amount of partially-oxidized precursor species are formed. Introducing the precursor in the oxidizer stream as opposed to the fuel stream has only a minimal impact on the oxidation dynamics. In order to understand modeling issues, the DNS results are compared with the laminar flamelet model. It is shown that the flamelet assumption qualitatively reproduces the oxidation structure. Further, reduced oxygen concentration in the near-flame location critically affects titania formation.