Within the planning of e.g. chemical plants or refineries, the appropriate positioning of flare pipes or exhaust gas stacks is a typical task. In case of an escape of a hot gas flow, it mixes up and is transported with ambient air. For the positioning of nearby plants and buildings it is therefore important to know how this hot exhaust gas flow will spread and which temperatures occur in which distance from the flare pipe.
One assessment possibility is the application of analytic models. Several models allow the determination of the mixing of various gas flows with different temperatures. However, these models usually provide useful results only for greatly simplified configurations. But surrounding buildings can strongly influence wind direction as well as resulting wind forces. The application of numeric computer models (CFD = Computational Fluid Dynamics) allow the calculation of the flow field and the resulting temperature distribution. This also allows the fast and easy examination of different wind directions or wind velocities.
Task / Calculation
The following example exemplifies the simulation of a hot exhaust gas stream with a temperature of T = 1000°C from a stack mixing with ambient air in one section. Exhaust gas volume flow is V = 160000m³/h. Wind velocity is W = 10km/h.
- On the first illustration (see below) velocity distribution in a section at a 10m distance from the stack is featured.
- The second illustration shows the distribution of velocity. Flow is strongly influenced by the stack. Vortices emerge in the wake.
- The third illustration shows the flow lines of the exhaust gas flow and temperature highlighted in color (transparent iso-surface: isotherm 100°C).
- Illustration 4 again shows the isotherm in detail. Under these boundary conditions, the isotherms of T = 100°C reaches out approx. 53m.
- Illustration 5 depicts temperature distribution in a section 10m away from the stack. Here, maximum temperatures of T = 270°C still occur.
These results allow conclusions about where a building can be constructed in the stack's vicinity or if a higher stack has to be built. The pictured model is very simple. It is easily possible to increase complexity by considering either terrain topology or surrounding buildings.