Understanding solid particle transport in a gas cyclone separator

Abstract

A cyclone separator is a widespread device used in many industrial areas and daily life for removing fine particulate matter from a gas stream. This separator has been used for more than 100 years due to its simple and robust design, but was continuously further developed and adapted to specific applications. The mostly used configuration is the reverse flow type cyclone exhibiting however, a very complex vortex flow with high turbulence. In the past numerous experimental as well as numerical studies were conducted for optimising cyclone geometry with the goal of improving the separation efficiency. In most of the numerical studies done so far not all the relevant transport mechanisms affecting particle motion were considered. Therefore, a thorough numerical investigation is presented using an LES-point-particle-Euler/Lagrange approach with momentum 2-way coupling for analysing the effects of particle-scale transport processes on the performance of a 290 mm Stairmand type of cyclone. All simulations presented here were conducted for an inlet velocity of 10 m/s. The sub-grid-scale (SGS) turbulence was described by a dynamic Smagorinsky model. Particle transport was computed considering all relevant forces and modelling also SGS dispersion. For the first time, the influence of particle collisions with rough walls, modelled according to the stochastic approach presented by Sommerfeld and Huber (International Journal of Multiphase Flow, Vol. 25, 1457-1489, 1999), was analysed in detail with respect to the performance of a cyclone separator. Moreover, inter-particle collisions were described through the efficient stochastic model introduced by Sommerfeld (International Journal of Multiphase Flow, Vol. 27, 1828-1858, 2001). Specifically, the importance of the interplay between rough wall collisions and inter-particle collisions is highlighted in this contribution. Three different particle size spectra were considered with log-normal size distributions ranging up to 20 μm (mean diameter 5.21 μm), up to 60 μm (mean diameter 15.43 μm), and up to 100 μm (mean diameter 25.71 μm); each case with different mass loading. Naturally, due to their different inertia, the effects of wall collisions and inter-particle collisions are also different for these types of particles. After a thorough validation, the influences of two-way coupling, particle rough wall collisions (three surface roughness degrees) and inter-particle collisions are analysed and elucidated. It is shown that specifically surface roughness has a huge effect on the grade efficiency of a cyclone and cannot be neglected, as done in most numerical cyclone studies done so far. Inter-particle collisions may partly compensate the deterioration of separation by wall roughness.