Stacks downwash

Aero namic wake flows of all kinds, including stack downwash, building wakes, highway vehiele wakes, and wakes generated by terrain obstacles... 

Aerodynamic Downwash Should the stack exit velocity be too low as compared with the speed of the crosswind, some of the effluent can be pulled downward by the low pressure on the lee side of the stack. This phenomenon, known as stack-tip downwash, can be minimized by keeping the exit velocity greater than the mean wind speed (i.e., typically twice the mean wind speed). Another way to minimize stack-tip downwash is to fit the top of the stack with a flat disc that extends for at least one stack diameter outward from the stack. 

If it becomes necessary to increase the stack-gas exit velocity to avoid downwash, it may be necessary to remodel the stack exit. A venturi-nozzle design has been found to be the most effective. This design also keeps pressure losses to a minimum. 

Building Downwash A review must be conducted for each stack to determine if building downwash effec ts need to be considered. Atmospheric flow is disrupted by aerodynamic forces in the immediate vicinity of structures or terrain obstacles. The disrupted flow near either building structures or terrain obstacles can both enhance the vertical dispersion of emissions from the source and reduce the effective height of the emissions from the source, resulting in an increase in the maximum GLC. 

These effective stack parameters are somewhat arbitrary, but the resulting buoyancy flux estimate is expected to give reasonable final plume rise estimates for flares. However, since building downwash estimates depend on transitional momentum plume rise and transitional buoyant plume rise calculations, the selection of effective stack parameters could influence the estimates. Therefore, building downwash estimates should be used with extra caution for flare releases. 

The use of the methods of Briggs to estimate plume rise are relied on in the SCREEN model. Stack tip downwash is estimated following Briggs (1973, p.4) for all sources except those employing the Schulman-Scire downwash algorithm. Buoyancy flux for non-flare point sources is calculated from ... 

Downdraft A natural or mechanical downward airstream, either that may, due to its temperature and/or velocity, cause thermal discomfort. In the case of a stack discharge, the term downwash may be used for the downward air current in the lee of the chimney that takes the smoke and other emissions below the emission discharge level causing ground-level pollution. 

Stack emissions can include particulates as well as dense gases (heavier tlian air, e.g., chlorine). These are subjected to a downwash settling tlirough tlie alinosphere due to tlie action of gravity. For tlie particles, especially large ones, an additional external force term must be included in the analysis. 

Before much research had been done, simple rules of thumb were used to avoid the problems of stack and building downwash. These models are reviewed here. 

As the wind enconnters any bluff object, a wake will form on the lee side of the object. Due to the boundary layer effects and the increase in local velocity around the object, the pressure in the wake of the object must be less than that in the surrounding outer atmosphere. If the momentum of an emitted smoke plume is not large, the low pressure area can cause the plume to be sucked down behind the stack in the aerodynamic downwash. Up to one third of the stack height may be lost in this manner. To be assured that these troubles are avoided, the velocity of emission, Vg, of the plume should be greater than 1.5 times the maximum expected wind speed. 

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