Cooling tower recirculation as influenced by the local atmospheric flow field, a numerical study

Cooling tower performance is adversely affected by the recirculation of the stack effluent into the tower intake louvers. This recirculation results from a portion of the warm saturated effluent leaving a mechanical draft cooling tower and being reintroduced into the tower by the large captive eddy and corresponding pressure gradients downwind of the tower. This problem is further aggravated by the turbulent mixing which entrains fluid from the plume into the separated wake region from which it is ingested into the cooling tower. The ingested stack effluent enters at an elevated temperature and humidity relative to the ambient and thus reduces the efficiency or effective size of the cooling tower.

A survey of the literature shows that existing methods of numerically modeling cooling tower plume recirculation ignore the aerodynamic effects of the atmospheric flow over the cooling tower. This investigation describes a method of numerically modeling cooling tower plume recirculation which is unique in two respects. First, the structure of the wake region behind the tower is resolved using the two-dimensional turbulent Navier-Stokes equations. Second, the flow field within the cooling tower is modeled as flow through porous media. This interior flow field is then coupled to the atmospheric flow over the tower by matching both pressures and mass flow rates at the cooling tower louvers.

The atmospheric flow over a cooling tower is approximated by a two-dimensional flow field in terms of stream function and vorticity transport equations with the Boussinesq approximation used to account for buoyancy effects. The turbulence effects are modeled using a turbulent viscosity based upon the turbulence kinetic energy and turbulence length scale, both of which are calculated from transport equations. A transport equation is also used to determine the temperature distribution in the wake region from which the plume profile and recirculation can be determined. The flow field within the cooling tower is calculated as a Darcy flow through porous media. This calculation yields the pressure distribution within the tower. Calculations made with the model are presented. These calculations agree reasonably well with laboratory and field measurements reported in the literature.