Code Manual for CONTAIN 2.0 - Federation of American Scientists

2. Inertial impaction, which occurs because the particle has a finite inertia, leading the trajecto~
of the particle center of mass to cross the flow streamlines around the drop and thus intersect
the surface of the drop.
3. Brownian diffusion, which results from molecular bombardment of the particles causing them
to flow across the flow boundary layer around the drop.
4. Diffusiophoresis, which results as a response of the particle to concentration gradients and to
vapor flow toward (or from) the drop surface when condensation on (or evaporation from) the
drop is occurring.
5. Thermophoresis, which results from the migration of a particle down a temperature gradient
because of the effect of differential molecular impacts.
Although these effects interact to some degree, they are treated as being additive. The first three
effects are primarily a function of drop and particle size, while the phoretic effects are primarily a
function of temperature and humidity of the atmosphere and of the drop’s temperature. The latter
changes rapidly at the start of the drop’s fall through the atmosphere, and particle collection is
therefore integrated over the drop’sfall history. Under evaporating conditions, the diffusiophoretic
effect becomes negative. The thermophoretic effect would also be negative in the unlikely
circumstance that the drop were hotter than the atmosphere. In such cases, the total collection
eftlciency is still constrained to be non-negative. Under extreme conditions (such as those resulting
from hydrogen bums), the drop may evaporate to aerosol size (the minimum of 100 pm or the
maximum aerosol diameter) during its fall. When this occurs, the drop and collected aerosols are
added to the appropriate section of the aerosol distribution if water aerosols are defined. If water
aerosols are not defined, then the collected aerosols are added to the appropriate section, and the
water is treated as evaporated. Figure 7-5, taken from Reference Ber85a, presents collection
efllciencies calculated for a typical accident scenario in which continuous spray operation brings the
containment atmosphere to an approximate steady state.
The deposition rate equations below describe the five aerosol removal mechanisms. Note that the
collection efficiency E is defined by
E ‘ (7CD2yAH n P
(7-37)
where ANPis the number of aerosol particles actually collected by a drop of diameter D as it falls a
height AH through an atmosphere containing nPparticles per unit volume.
The following expressions for interception and impaction are based on Reference Fuc64. In the
original expressions, there is no allowance for nonspherical aerosol particles. Therefore, an
allowance for the agglomeration and dynamic shape factors (y and ~, respectively ) has been made.
However, it should be noted that the sprays keep the degree of superheat in the containment
Rev O 7-31 6/30/97