3‐D Modeling of Shock‐induced Precursor Decomposition and Nano‐Particle Growth

Abstract

AbstractIn the ongoing joint project “Gasdynamically induced nano–particles” (PAK 75/1) a novel method of gas–phase synthesis of nano–particles is being developed [1]. This project is supported by the Deutsche Forschungsgemeinschaft (DFG), the pilotfacility is erected in collaboration with Evonik Degussa, Hanau, Germany. The key features of the new method are the static temperature increase across a stationary shock to provide ignition conditions of the precursor and the subsequent gasdynamic quenching of the particle growth when the desired particle diameter of 10−8m is reached. In this work the decomposition of the precursor, the reactive heat release and the particle growth are coupled with the 3–D transonic flow inside the particle reactor. The simulation results show the effect of shock/boundary layer interactions on the decomposition reaction as well as the effects of the reactive heat release on the shock location and the resulting particle size distribution. The Reynolds number at the shock location is Rex=9·106, the boundary layer thickness is δ=2.2 mm and the pre–shock Mach number is M=1.58. The complex transonic reactive flow within the particle reactor requires a high mesh resolution to resolve the local details of the internal flow problem. In addition, an efficient particle model is required. The formation and growth of the particles are modeled by three separate reactions. The primary reaction is the decomposition of the precursor and the production of monomers. It is modeled by a one step reaction described by an Arrhenius equation. The secondary reaction, the formation of critical nuclei is in our case negligible, because of the high supersaturation ratio of the order of 103 instantly after the primary reaction, where the monomers are already critical nuclei. The tertiary reaction is the particle growth by surface condensation, coagulation and single–aggregation. To simulate the particle growth we apply a local monodisperse bimodal formulation. Thereby, the monodisperse model of Kruis et al. [2] is extended by an additional discrete monodisperse mode for the critical nuclei. This method describes the local particle size due to turbulent mixing, coagulation and coalescence. The coagulation starts in the Knudsen regime Kn ≫ 1 and reaches the continuum regime with particle diameters of the order of 10−8m and a mean free path of the same order of magnitude. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)