(136a) Influence of Electrostatic Charges on the Particle Concentration in Wall-Bounded Turbulent Flows

Preferential concentration of inertial particles in fully-developed wall-bounded turbulent flows is of paramount importance for many industrial applications. Experimental study of this kind of flows requires the usage of a long feed-line in order to obtain a fully-developed flow at the test section and to ensure that the particles are independent of their initial conditions. However, as the particles are advected along the piping system they may experience frequent collisions with the walls and between them. If the particles and the piping system are of a different kind of material their contact results in an electric charge separation through triboelectric effects. Under those circumstances a considerable amount of electric charge might be accumulated on the surface of the particles or the structural components of the facility. One easily observable consequence is the deposition of particles on the test section walls. If no adequate measure is taken, such as the usage of an antistatic agent, particle deposition might alter near wall particle concentrations and it can also have detrimental effects on the experiments, i.e., constrain optical access and light transmission. Another, less obvious, implication is electrostatic forces which may influence the trajectories of the airborne particles. In an experimental setting, the exact influence of electrostatic effects on the measured particle concentration statistics is unknown. Moreover, in an industrial context the formation of deposits can impair the functionality of the facilities by reducing the effective cross-sectional area of pipes. Also, the accumulation of charge can cause sparks which challenges the operational safety of a plant. In order to elucidate the physics underlying the charge build-up process of solid-fluid mixtures considerable effort has been made by means of numerical simulations. In particular, the authors of the present paper implemented dynamic models to predict the contact charging of particles when colliding with a solid surface or with each other into a computational fluid dynamics solver. Therein, the turbulent flow field of the carrier fluid was either predicted via direct numerical [1] or large eddy simulations [2].

In the present work we investigate the influence of triboelectric charging on the preferential concentration of inertial particles in a fully-developed, vertical, turbulent duct flow. This study combines numerical and experimental results. The duct flow is characterized by a square cross-section 40 mm wide, and a Reynolds number based on the duct width of 10,000. Polydisperse nickel particles are considered with a mean particle diameter of 12 um, and the particle/gas mass loading ratio is fixed to 0.01. In this setting, particles are smaller than the smallest flow scales (Kolmogorov scales) and the corresponding averaged Stokes number is 5. Numerical results are provided by large eddy simulations where the gaseous phase is described in the Eulerian and the particulate phase in the Lagrangian framework. Gauss' law is solved to compute the electric field based on the charges carried by the particles and the corresponding image charges induced at the solid boundaries. The charge of the particles at the glass test section inlet is considered an inflow condition in the simulations and varied systematically. Comparison of numerical results and the experimental measurements allows analyzing the influence of electrostatic forces on particle concentration profiles.

[1] H. Grosshans and M. V. Papalexandris. J. Fluid Mech., 818:465-491, 2017.

[2] H. Grosshans and M. V. Papalexandris. Powder Technol., 301:1008-1015, 2016