Code Manual for CONTAIN 2.0 - Federation of American Scientists

(7-17)
where v~is the downward settling terminal velocity; pPis the particle density; g is the acceleration
of gravity (9.8 m/s2); % is the particle diamete~ p~is bulk gas viscosity (taken to be that of dry air)
at the atmosphere temperature; and ~ is the dynamic shape factor. C is the Cunningham slip
correction factor.
C= 1 + Nti[l.257 + 0.4exp (-l.l/Nfi)]
where N~ is the Knudsen number 2M~ and k is the mean-free path of the gas.
One basic restriction of this model is that the aerosol particle Reynolds number N~,,Pmust be much
less than 1. This is generally acceptable since N~.,P= 1 corresponds to a diameter of 70 pm for a
particle with the density of water, which is fairly large.
Diffusion. Another aerosol deposition mechanism results from diffusion of aerosols in a
concentration gradient, that is, from a higher to a lower concentration region. The diffusional
deposition velocity is given by
‘tiff
= DPIA
where KT~ C
Bp = 3np~dP~
(7-18)
is the particle diffisivity; K is the Boltzmann constant (1.38 x 10-23J/K); T~ is the atmosphere
temperature; and A is the user-specified diffusion boundary layer thickness, “deldif,” which has a
default value of 10-5m and is specified in the global AEROSOL input block.
Thermop horesis. This aerosol deposition mechanism results from the force exerted on aerosol
particles by temperature gradients in containment. The thermophoretic deposition velocity vti.~ is
that given in Reference Ta180
‘therm ‘
2.294 N~C(C,Nfi ‘k#P)q.
~ p~T~(l +3 C~Nfi) (1+2C,Nfi +2k#cP)k~~
(7-19)
where k~ is the user-specified ratio of the thermal conductivity of the gas to that for the aerosol
particle; qCis the surface convective heat flux (W/m2); p~is the gas density (taken to be that of air)
at the atmosphere conditions; C. = 1.146 is a slip coefficient; c1 = 2.20 is a therm~ accommodation
coefficient [Ta180] and k~~is the actual gas boundary layer thermal conductivity.
O 721 6/30/97