Simulation of Turbulent Particle-laden Flows and their Electrostatic Charging
Creators: name: Grosshans, Holger
Publisher: name: Physikalisch-Technische Bundesanstalt (PTB)
Year: 2022
Subjects: computational fluid dynamics
particle-laden flows
simulation
spray drying
electrostatics

description: PTB-Bericht Ex-16
description: The dynamic motion of particles plays a prominent role in many naturally occurring flows as well as industrial applications. In this work, the author’s efforts concerning the computation of a range of issues related to turbulent particle-laden flows are summarized. More specifically, the discussion of the research field starts with the numerical modeling of particle production through spray drying. This includes the simulation of a bi-component spray using the direct quadrature method of moments and the process of particle formation. Therein, the focus was given to the prediction of the drying kinetics and the resulting particle size and morphology. It was found that bi-component droplets dry qualitatively different whether they are exposed to temperatures below or above the boiling temperature of the involved liquid. Thus, a new model was proposed which accounts for the boiling of the liquid and possible cracking of the particle crust due to the resulting internal pressure increase. The handling of the produced powder often leads to an accumulation of electrostatic charge which poses a hazard to the operational safety of plants. To gain insight into the charging, the author computed the pneumatic transport of particles through a wall-bounded turbulent carrier gas flow. A model was implemented, the so-called condenser model, to facilitate the prediction of the charge exchange between a particle and a solid surface during impact. This model was subsequently extended to enable the calculation of the important case of particles with non-conductive surfaces. The immobility of charge carriers leads to a non-uniform charge distribution on the particles’ surface which was demonstrated to strongly affect its charging behavior during successive impacts. Afterward, large-eddy simulations were utilized to propose design parameters of transport pipes aiming to reduce the powder charge end to explore the dependency of the particles’ properties on the charge accumulation. It was shown that the electric charge of the powder can be significantly decreased by reducing the conveying air velocity, the usage of larger pipes and the application of a higher solid mass flow rate. Also, the particles’ mechanical and electrical properties represent promising measures to control the charge of the powder. As regards the mechanisms of charge transfer in fluid-solid mixtures, direct numerical simulations of channel flows revealed that charge may either be transported by convective motion or during inter-particle collisions. Finally, the studies of the author concerning the modulation of turbulent particle-laden duct flows under the influence of electrostatic charges are outlined. For the efficient and accurate calculation of the interaction between charged particles, a hybrid scheme that combines the advantages of both Gauss’ and Coulomb’s law was formulated. Utilizing this scheme, it was found that electrostatic forces tend to smooth the particle-concentration profiles over the duct cross-section. Also, the flow is stabilized which is expressed by the reduction of particle velocity fluctuations. This summary aims to present the motivation for the conducted research, the fundamentals of the involved physics and the connections in-between the different works and the state-of-the-art. Also, the main results are discussed. Details concerning the mathematical models, numerical methods and discussions of the results are to be found in the respective publications.