Pollution dispersion from highways

Abstract

People living or working close to the highways are at risk to higher exposure levels of different air pollutants, one of which is particulate matter. The Antea group has recently proposed the addition of Electrostatic Precipitators (or ESPs) on the top of sound barriers adjacent to highways, in order to capture the particulate matter and safely discard them. This is an attractive, but expensive, supplement. While estimates of the collection efficiency (i.e. what percentage of particulate matter mass entering the ESP is collected by the device) are known, the aerodynamic efficiency of the ESP (i.e. what percentage of pollutants reach the ESP entrance as compared to the mass emitted) is yet to be determined. The net efficiency of the ESP will be the product of the above two efficiencies, making it necessary to study the latter, prior to large-scale installation.

Scaled down experiments were performed in a water channel, housed in the Laboratory for Aero and Hydrodynamics at the Delft University of technology. Simultaneous measurements of the flow velocities and dye intensities in the symmetry-plane of the channel were made through planar Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF). A few flow configurations were studied by varying certain parameters. It was found that: (a) ESP installation is more beneficial on top of shorter sound barriers. (b) Slightly raising the ESP from the top of the sound barrier is advantageous. (c) Installation of ESPs in isolated highway canyons should be done with care, as several flow regimes are observed.

First order estimates on the aerodynamic efficiencies of the ESP were determined. This was accompanied by the identification of ESP performance trends that should serve as a guideline for the initial testing of the ESPs at full scale. The guidelines include the physical location and entrance orientation of the ESP. It is known that the collection efficiency of the ESP is a function of the incoming flow velocity. It is recommended that the entrance flow velocities reported here be used as an input for calculating the associated particle collection efficiency. Together, the collection and aerodynamic efficiencies can be used to estimate the overall efficiency of the ESP. If the overall efficiency satisfies a minimum desirable threshold, the field experiments yield promising results, the device clears safety requirements, and the benefits outweigh the costs, the ‘Open Air Line ESP’ can be installed along the highways en masse in the future.