Plume Behaviour and Stack Design Questions and Answers

Answer: A

Turbulence is moderate and roughly equal in the vertical and crosswind directions, so the plume widens symmetrically. It is the behaviour a simple Gaussian calculation assumes as its default case for want of anything better.

Answer: B

Correct answer: Stable below, unstable above

Lofting is the most favourable of the classical plume types. The stable layer beneath the plume blocks any downward mixing, so almost nothing reaches the ground, while the unstable air above disperses the plume upward freely. It typically occurs in the early evening, as the surface cools and an inversion forms below a still-unstable layer aloft.

Answer: C

Correct answer: Between two inversions

In trapping the plume is confined between a stable layer below and another above. It can escape in neither direction, so the material accumulates in the layer between them and concentrations build steadily for as long as the two inversions persist.

Answer: D

Correct answer: Exit velocity is low against wind speed

Stack tip downwash occurs when the gas leaves too slowly relative to the wind. The wind separates round the stack and leaves a low-pressure wake immediately downwind, and a slow plume is drawn into it instead of rising - so the effective stack height falls, sometimes to the physical height or below. Specifying an exit velocity of at least about 1.5 times the design wind speed is what prevents it.

Answer: A

Correct answer: A recirculating wake cavity captures it

A building disturbs the flow around it and creates a recirculating cavity in its lee. A plume released within that cavity is carried down and round rather than away, so it reaches ground level close to the source and at high concentration. The building neither absorbs nor heats the plume; it simply changes the flow, which is why the good engineering practice height rule exists.

Answer: B

Correct answer: Building height plus 1.5 times the lesser dimension

The good engineering practice height is

where \(H_b\) is the building height and \(L\) the lesser of the building’s height and its projected width. The point is to place the release above the wake cavity the building generates, not to license dispersion by height alone.

Answer: C

Correct answer: Momentum

A cold stack has no thermal buoyancy to draw on, so whatever rise it achieves comes from the momentum of the issuing jet - the vertical velocity with which the gas leaves. In Holland’s formula that term goes as \(v_s d / u\), so it can be increased by raising exit velocity or reduced by a strong wind.

Answer: D

Correct answer: Velocity, diameter, wind and temperature

Holland’s formula combines both mechanisms:

The first term in the bracket is momentum and the second buoyancy, so exit velocity, stack diameter, wind speed and the gas-to-air temperature difference all appear. Emission rate does not: how much pollutant is in the gas has no bearing on how high the plume goes.

Answer: A

Correct answer: The buoyancy flux parameter

Briggs’ formulae are built round the buoyancy flux

which measures the rate at which the plume carries buoyancy upward. Rise is then expressed in terms of \(F\), the wind speed and the downwind distance - a physically based treatment that has largely replaced the older empirical formulae.

Answer: B

Correct answer: Increases plume rise

A hotter gas is less dense than the surrounding air, so it is more buoyant and rises further before its temperature and the ambient temperature come together. In Holland’s formula the buoyancy term contains \((T_s - T_a)/T_s\), which grows with exit temperature. This is why flue gas cooled by a wet scrubber is often reheated.