Analysis of the dynamics of dust reentrainment with simultaneous electrostatic deposition and without any deposition after a jump of airflow velocity

Abstract

Particle deposition and reentrainment in electrostatic precipitator (ESP) channels were investigated both experimentally and analytically in six size ranges from 0.3 to > 10 μm after a sudden increase in the airflow velocity through ESP up to 7.4 m/s. The main goal of this paper is to analyze the conditions under which the ESPs with collected pollutants and pathogenic microorganisms can become their new sources. The sudden increase in airflow velocity showed exponential pre-steady-state dynamics on the outlet dust concentration with a characteristic time of about 100 s regardless of the particle diameter, the airborne dust concentration, and the “turn-on” or “turn-off” mode of the ESP. The initial reentrainment fluxes under the assumption that reentrained particles are fully discharged were estimated directly by comparison of the measured values of inlet and outlet dust concentrations. An analysis of the experimental results shows a maximum in the threshold friction velocity for particles of about 7 μm in size and the related decreasing trend in the threshold friction velocity for smaller particles. The effect of the charge of reentrained particles on the electrostatic precipitation was investigated using model predictions. The semi-analytical model of the ESP included a combined field and diffusion charging process, eddy diffusion, turbulent flow, migration, and reentrainment of charged particles. Also by using the reentrainment flux estimates for not charged and fully charged reentrained particles an analytical approach was proposed to predict the reentrainment rate of particles at a number of assumptions about the charge of the reentrained particles and various flow velocities with and without simultaneous deposition. No noticeable nonstationary dynamic profile of the outlet particle concentration was observed after a sudden increase in airflow velocity up to 2.4 m/s and the ESP with an airflow of at least 1.4 m/s are very likely to not be a secondary source of pollutants and pathogenic microorganisms.