scope:

Introduction

The dispersion modelling according to Appendix C of the "TA Luft" is based on the idea that the disper¬ sion area downwind of a source can be divided into two characteristic domains, the near field and the far field.

In the near field, where stacks may reach a source distance from 100 to 1000 m, the flue gas plume resembles a jet flow. The ascent is infiuenced by buoyancy and momentum forces. The plume is bent to the direction of the wind by momentum and pressure forces. The inner turbulence of the plume is dominant Over the entrainment of ambient air into the jet and the dilution of pollutants within the plume. As the length Scale of the turbulence elements with high mixing intensity corresponds to the diameter of the plume and is, therefore, considerably smaller than the Scale of the strong atmospheric eddies, the plume disperses relatively independent of the turbulence of the surrounding flow. Intensive interaction of plume turbulence and surrounding turbulence first begins near the end of the near field, i.e. at this point the far field begins.

Whereas dynamic processes are essentially important for the dispersion in the near field, the dispersion in the far field constitutes a purely kinematic process.

At this point, forces as well as the jet-induced turbulence have become irrelevant. The plume is transported horizontally with the wind away from the source Stack. The enlargement of the plume and thus the reduction of the air pollutant concentration is caused mainly by the eddies of the atmospheric ground flow. Finally, the air pollutants reach the ground. The resulting concentration field on the ground plays a central role in ambient air assessments.

In the dispersion modelling according to Appendix C of the "TA Luft", the near field, which is very complex from the point of view of fluid mechanics, is treated by means of plume rise formulas. With easily definable quantities such as heat emission, wind ve¬ locity as well as the stability of the atmospheric stratification, the rise of the fiue gas above the height H of the Stack is estimated, and an "effective source height" h is defined (Fig. 1).