Coupled LBM-FVM-CA simulation of particle deposition and removal processes on tubes

Abstract

A numerical model was proposed to simulate the fouling processes on the tubes, which contained the evolution of the fouling shape and the particle deposition/removal mechanisms. Firstly, the coupled lattice Boltzmann method (LBM) and finite volume method (FVM) was established to simulate the air flow. The flow around the tubes was simulated by the LBM due to its convenience in complex boundary conditions. The downstream flow was simulated by the FVM to save the computational source. A reconstruction operator was derived for the information transfer from macroscopic parameters to multiple-relaxation-time LBM. The cellular automata model, energy conservation model and moment analysis were included to simulate the particle motion, collision, deposition and removal. Then, because the time step in the simulation was several orders of magnitude shorter than the real fouling time, a time ratio was proposed for the conversion between simulation and real time. Finally, the evolutions of the fouling shapes along with time for different particle diameters and inlet velocities were simulated and analyzed. The results showed that the proposed coupled model can be used to study the particle deposition, removal and the changing of the fouling layers. When the mass concentration was the same, the fouling of small particle grew faster. The fouling area grew exponentially with time. It grew rapidly in the beginning, then grew slower and finally reached an asymptotic balance value. When the particle concentration was specified, the fouling rate first grew with and then decreased with the increasing inlet velocity. Therefore, there was a velocity range in which the fouling rate was high. As for the shape of the fouling layer, the removal was severe on the windward side, but the direct impaction of the particles formed the cone-shaped fouling layers. The cone-shape changed the air flow and stopped the deposition on the windward side. The fouling layers grew on the entire leeward side of the tubes and finally stopped when the removal was equal to the deposition. The simulated fouling shape was compared with the real picture of the fouling on a tube of an economizer and the fouling shapes on the leeward side coincided well with each other. The growth of the fouling mass was also compared with the existing experiment. The simulated mass had the same trend with the experiment. This demonstrated that the time ratio can be used to convert the time scale. The distribution of the particle sizes and the properties of the real particles should be considered in the future works.