(d) Looping natural draft that occurs in the stack. If the gas in the plume is less dense than the surrounding atmosphere, this condition also causes the plume to rise and adds to its upward momentum. The plume rise also depends upon the growth of the plume, which is caused by turbulence, whether pre-existing in the atmosphere or induced by interaction with the plume. One method of increasing the upward momentum of the plume is to constrict the exit diameter of the stack.
The buoyancy flux is an effect of the plume's increased temperature. For most large combustion sources (such as power plants), the buoyancy flux dominates the momentum flux. Buoyant plumes contribute to both the vertical and horizontal velocity of the plume, in addition to that caused by ambient turbulent levels. This condition is caused by entrainment of the surrounding air into the expanding plume in relation to its surroundings. Buoyancy can also affect both oy and oz because of buoyancy-induced atmospheric turbulence.
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