Electrostatic precipitator turbulent flow

In air-cleaning electrostatic precipitators, the flow is turbulent and the collection situation is analogous to stirred settling, as described in Section 3.8. Consider the wire-and-tube geometry shown in cross section in Fig. 15.7. In a period dt that is brief compared with that required for turbulent mixing, all particles within a distance V- dt of the tube wall will be removed. For simplicity, we assume that par-... 

This equation, which is called the Deutsch equation, has been shown to be a useful tool for estimating the performance of electrostatic precipitators. An interesting detail in the Deutsch equation is the exponent, which is equal to the collection efficiency of a laminar flow system. The equations based on laminar flow and turbulent flow can be assumed to be the extreme conditions, and the true situation is somewhere in between these two cases (see Fig. 13.1,5). 

Particle trajectories can be calculated by utilizing the modern CFD (computational fluid dynamics) methods. In these calculations, the flow field is determined with numerical means, and particle motion is modeled by combining a deterministic component with a stochastic component caused by the air turbulence. This technique is probably an effective means for solving particle collection in complicated cleaning systems. Computers and computational techniques are being developed at a fast pace, and one can expect that practical computer programs for solving particle collection in electrostatic precipitators will become available in the future. 

The utility of the concept of aerosol particle electrical drift velocity can be shown by using it to estimate the theoretical efficiency of an electrostatic precipitator. For simplicity it is assumed that the collector is cylindrical, having a radius R (although this assumption does not affect the results), and that an aerosol is uniformly distributed across the entrance of the collector. In addition, turbulent flow in the collector is assumed such that the uncollected aerosol remains uniformly distributed at any distance from the entrance of the tube. If the electrical drift velocity is constant, the chance of a particle 4> being collected in a time At is... 

FIGURE 15.8 Efficiency curves for laminar- and turbulent-flow electrostatic precipitators. 

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