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1.1.4.3 Unstable or Neutral ‑ Crossover Between Momentum and Buoyancy.

For cases with stack gas temperature greater than or equal to ambient temperature, it must be determined whether the plume rise is dominated by momentum or buoyancy.  The crossover temperature difference, (ΔT)c, is determined by setting Briggs' (1969, p. 59) Equation 5.2 equal to the combination of Briggs' (1971, p. 1031) Equations 6 and 7, and solving for ΔT, as follows:

for Fb < 55,

and for Fb ³ 55,

If the difference between stack gas and ambient temperature, ΔT, exceeds or equals (ΔT)c, plume rise is assumed to be buoyancy dominated, otherwise plume rise is assumed to be momentum dominated.

1.1.4.4 Unstable or Neutral ‑ Buoyancy Rise.

For situations where ΔT exceeds (ΔT)c as determined above, buoyancy is assumed to dominate.  The distance to final rise, xf, is determined from the equivalent of Equation (7), (Briggs, 1971, p. 1031), and the distance to final rise is assumed to be 3.5x*, where x* is the distance at which atmospheric turbulence begins to dominate entrainment.  The value of xf is calculated as follows:

for Fb < 55:

and for Fb ³ 55:

The final effective plume height, he (m), is determined from the equivalent of the combination of Equations (6) and (7) (Briggs, 1971, p. 1031):

for Fb < 55:

and for Fb ³ 55:

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