ON THE ELECTRO-HYDRODYNAMIC FLOW-FIELD IN ELECTROSTATIC PRECIPITATORS

In this paper electro-hydrodynamic (EHD) sows are investigated theoretically with results presented for the sow Þeld in model electrostatic precipitators (ESP’s). The resulting sow Þeld is shown and explained qualitatively. Calculations with a ke -model show the insuence of the mean sow velocity and the importance of the inhomogeneity of the electric Þeld distribution at the inlet and outlet of an ESP. Furthermore a perturbation analysis is presented, leading to a simple differential equation of the Helmholtz type. This allows a more detailed view of the important mechanisms which form the secondary sows as well as a means to get a very fast estimation of the resulting sow Þeld. INTRODUCTION A number of technical devices exists with sows characterized by an ionic current transferring momentum into the suid, e.g. corona chargers, toner application, powder lacquering, electrostatic precipitators (ESP) etc. Since the sow Þeld is modiÞed by electrical phenomena, it is denoted an ’electrohydrodynamic (EHD) sow’. This paper is dealing with the EHD-sow in ESP’s. Electrostatic precipitators are very important industrial devices for cleaning large amounts of particle laden gas. The functional principle is based on the unipolar charging of the particles in a corona discharge and withdrawing them from the gas sow by means of an electric Þeld. A wire-duct electrostatic precipitator is the most frequently used type in industry. It consists of parallel plates which form a series of ducts. Within each duct there is a row of wires (Þg. 1). When high voltage is applied to the wires, a corona discharge takes place. This process generates ions, resulting in a current sow from the discharge wires to the plates, whereby the particles are charged and hence transported by the electric Þeld towards the collecting plates (White 1963). ESP’s are widely used in industry and a number of investigations have been performed during the last decades. However, sizing a new precipitator is still an empirical matter and many questions concerning the physical phenomena taking place in the precipitator still remain unsettled. One very important question to predict particle transport in ESP’s is the knowledge about the sow Þeld. This topic was examined in several reports with partially contradictory results and few help to predict how the sow Þeld is insuenced by a speciÞc design of an ESP. E.g. Ramadan and Soo (1969) as well as Yamamoto and Velkoff (1981) performed calculations, neglecting turbulent velocity suctuations. Subsequently a number of authors show results of standard ke -model calculations for different geometries and operating conditions, e.g. Bernstein and Crowe (1979), Kallio and Stock (1992), Liang and Lin (1994), Choi and Fletcher (1997), Medlin et al. (1998). Recently Soldati and Banerjee (1998) published some DNS calculations of an EHD sow. Figure 1. Sketch of a single duct of an Electrostatic Precipitator HV Collecting Electrodes Discharge Electrodes Raw Gas Clean Gas ON THE ELECTRO-HYDRODYNAMIC FLOW-FIELD IN ELECTROSTATIC PRECIPITATORS Hans-Joachim Schmid Lehrstuhl fur Feststoffund Grenzsachenverfahrenstechnik Technische Universitat Munchen · D-85748 Garching, Germany Steffen Stolz Institut fur Fluiddynamik ETH Zurich · ETH Zentrum · CH-8092 Zurich, Switzerland Hans Buggisch Institut fur Mechanische Verfahrenstechnik und Mechanik Universitat Karlsruhe · D-76128 Karlsruhe, Germany This paper aims to clarify the physics and mechanisms of EHD sows, leading to a better understanding, how extensive deteriorations of the sow Þeld could be avoided by a proper design and arrangement of electrodes as well as a proper choice of operating conditions.