United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-94/115 August 1994
Project Summary
Physical and Numerical
Modeling of ASD Exhaust
Dispersion Around Houses
David E. Neff, Robert N. Meroney, and Hesham EI-Badry
A study has been completed to physi-
cally model, in a wind tunnel, the dis-
persion of exhaust plumes from active
soil depressurization (ASD) radon miti-
gation systems in houses. The wind
tunnel testing studied the effects of
three exhaust locations: midway up the
roof slope, simulating an ASD stack
within the house; at the eave, simulat-
ing an exterior stack; and grade-level
exhaust (no stack). Plume dispersion
effects were studied using both quali-
tative smoke visualization and quanti-
tative tracer gas techniques, as the
house, wind, and exhaust characteris-
tics were systematically varied. The
tracer gas results show that grade-level
exhausts consistently result in the high-
est tracer concentrations against the
face of the house, although these con-
centrations may not be serious if ex-
haust concentrations are low. The high-
est concentration measured at one
point against the side of the house
over all runs with grade-level exhaust
would correspond to 30 Bq/m3 (0.8 pCi/
L) if the exhaust contained 3,700 Bq/m3
(100 pCi/L), and 300 Bq/m3 (8.1 pCi/L) if
the exhaust contained 37,000 Bq/m3
(1,000 pCi/L). Exhaust at the eave re-
sulted in substantial reductions in the
concentrations seen against the side
of the house and resulted in a maxi-
mum concentration corresponding to
163 Bq/m3 (4.4 pCi/L) at one point
against the roof of the house for an
exhaust containing 37,000 Bq/m3 (16
Bq/m3, or 0.4 pCi/L, for an exhaust con-
taining 3,700 Bq/m3). Exhaust midway
up the roof slope gave the best chance
for the plume to escape, resulting in a
maximum concentration corresponding
to 122 Bq/m3 (3.3 pCi/L) at one point
against the roof for an exhaust con-
taining 37,000 Bq/m3 (12 Bq/m3, or 0.3
pCi/L, for an exhaust containing 3,700
Bq/m3).
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Currently, radon mitigation standards is-
sued by the I). S. Environmental Protection
Agency (EPA) require that the exhaust from
an active soil depressurization (ASD) sys-
tem for residential radon reduction be dis-
charged above the eave of the house. This
requirement is intended to ensure that very
little of the exhaust is re-entrained into the
house, to minimize the exposure of the
occupants. It is also intended to ensure
that the exhaust effectively disperses out-
doors, to minimize exposures to persons in
the yard or in neighboring houses.
This requirement for exhaust above the
eave can increase the installation cost of
the ASD system and can detract aestheti-
cally from the house. It might discourage
some owners from installing a mitigation
system. The objective of the current study
was to identify if there are conditions under
which the ASD exhaust for a typical house
can safely be released at grade level.
Project Description
The project involved physical modeling
using a wind tunnel to study the circula-
tion of ASD exhaust gases around typical
Printed on Recycled Paper
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suburban houses, to determine whether
consistently acceptable conditions for
grade-level exhaust can be defined. In an
effort to permit extension of these results
to conditions beyond those tested in the
wind tunnel, an attempt was also made to
use the wind tunnel data to validate vari-
ous analytical and numerical models de-
scribing exhaust buildup in the building
cavity, plume dispersion, and the fluid dy-
namics of flows around buildings.
Models of four typical suburban houses,
on a 1:35 scale, were constructed and
tested in a wind tunnel having a cross
section of 3.66 by 2.13 m (12 by 8 ft).
These four houses differed according to
the number of stories (1 vs. 2 stories) and
the roof pitch (gentle vs. steep slope).
The testing addressed four wind., direc-
tions (0°, 45°, 90°, and 180°), three ratios
of exhaust velocity to approaching wind
velocity, and three exhaust locations (mid-
way up the roof slope, simulating an inte-
rior stack; above the roof eave near the
rain gutter, simulating an exterior stack;
and horizontally away from the house at
grade, simulating no stack).
In wind tunnel testing of plume disper-
sion, the primary scaling factor is the ratio
of stack exhaust velocity W to approaching
wind speed U. Three W/U velocity ratios
were tested in the wind tunnel: 0.25, 1.0,
and 2.5. This range covers a broad spec-
trum of actual exhaust and wind velocities.
At an exhaust velocity of 1.25 m/s - corre-
sponding to a discharge of roughly 10 Us
(20 cfm) assuming a 10 cm (4 in.) diameter
stack - this range of W/U ratios would
represent wind speeds of 1.25 to 5 m/s
(about 2.5 to 11 mi/hr). At an exhaust ve-
locity of 6 m/s (corresponding to about 50
Us, or 100 cfm), this range would cover
wind speeds of 2.4 m/s (about 5 mi/hr) and
higher.
The testing included smoke visualiza-
tion tests and quantitative tracer gas re-
lease tests. In the tracer gas testing, pure
ethane tracer gas was released from the
model stacks; samples for ethane analy-
sis were drawn at 45 locations around the
face of the house and downwind.
Since the house models did not have
porous faces (simulating openings in the
house shell), the wind tunnel testing could
not provide a direct simulation of actual
re-entrainment into the structures. How-
ever, these tests did provide a direct simu-
lation of dispersion around the house, and
hence of possible exposures to persons
in the yard or in neighboring houses. More-
over, from the concentrations against the
faces of the model houses, some indica-
tion is provided regarding the potential
threat that re-entrainment might pose to
the occupants.
Initial Validation of Wind Tunnel
Profiles
Before undertaking the primary experi-
mental program, an initial series of tests
was conducted to confirm that the vertical
velocity and turbulence profiles being es-
tablished in the wind tunnel adequately
represented the expected boundary layer
profiles that would exist in a suburban
setting in the field. This testing showed
that the wind tunnel was reproducing the
expected suburban field boundary layer
reasonably well, especially at the position
where the model house was located,
based upon empirical models of field pro-
files.
It was also necessary to demonstrate
that the concentration profiles established
in the tunnel by dispersion from a "pas-
sive" source - i.e., a source injected par-
allel to the bulk wind flow at the same
speed as the wind - would be the same
as those that would be expected in a
suburban field setting. These tests, with
passive injection of ethane tracer gas, con-
firmed that the concentration profiles es-
tablished in the wind tunnel were consis-
tent with those predicted by the Pasquill-
Gifford model for an urban setting, in the
appropriate Pasquill-Gifford C-D category,
especially at the location of the model
house.
This validation testing confirmed that
both the velocity and concentration pro-
files, when appropriately normalized, are
independent of the Reynolds number (i.e.,
the wind speed).
Results
Wind Tunnel Smoke
Visualization Tests
Smoke visualization tests were con-
ducted at 96 conditions: 2 house heights
x 2 roof pitches x 4 wind directions x 2 W/
U velocity ratios x 3 exhaust locations.
The smoke results confirm (and provide
additional insights on) the tracer gas re-
sults discussed below. The plumes from
grade-level exhausts commonly are either
blown back against the face of the house
(when the exhaust is on the upwind side)
or caught in the downwind recirculation
cavity (for sidewind and downwind loca-
tions), even at W/U values corresponding
to the highest exhaust velocities. Exhaust
midway up the roof slope has the best
chance of penetrating the near boundary
layer over the roof and escaping the down-
wind cavity, especially when the stack is
on the upwind side of the house, although
some recirculation is seen even with such
mid-roof exhausts. Exhaust at the eave
generally results in less recirculation than
does grade-level exhaust but is less ef-
fective in avoiding some capture in the
recirculation cavity than is mid-roof ex-
haust.
Wind Tunnel Tracer Gas
Concentration Tests
The tracer gas concentration testing in-
volved 144 experiments, representing a
complete test matrix: 2 house heights x 2
roof pitches x 4 wind directions x 3 W/U
ratios x 3 exhaust locations.
The complete detailed results of these
144 experiments are presented in the full
report.
Table 1 summarizes the results in the
following format: If the exhaust stack were
discharging either 3,700 or 37,000 Bq/m3
(100 or 1,000 pCi/L) of radon, what would
be the worst-case radon concentration
seen at given locations around the face of
the house and downstream, based on all
of the wind tunnel data for the specific
exhaust location? The figures in Table 1
represent the worst-case house configu-
ration, wind direction, and W/U ratio for
each exhaust location at each sampling
point.
Analytical and Numerical
Modeling
Existing analytical (mathematical) mod-
els describing plume dispersion near, and
remote from, buildings were applied to the
conditions being physically modeled in the
wind tunnel. These analytical models pre-
dicted higher concentrations around the
face of the house, and downwind, than
those measured in the wind tunnel.
An existing numeric fluid dynamic code,
FLUENT, was applied to several of the
conditions tested in the wind tunnel. At
this time, it is impossible to comment on
the quantitative reliability of the concen-
trations predicted by the numeric model.
Conclusions
1 Some ASD exhaust gases will be
caught in the recirculation cavity be-
hind the building even with roof-level
discharges, whenever the stacks are
located downwind of the crest of the
house's roof.
2 The at-grade wall exhaust usually
leads to the highest tracer gas con-
centrations on the face of the build-
ing. The eave exhaust (simulating the
exterior stack) leads to somewhat
higher concentrations on the building
face than does the exhaust midway
up the roof slope.
3 If the exhaust were to contain 100
pCi/L of radon, the highest radon con-
centration that would result at any
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Table 1. Summary of Worst-Case Radon Concentrations' Expected Around House and Downwind,
Based on Wind Tunnel Data
Predictions when Exhaust
Predictions when Exhaust
Sampling Contains 100 pd/L
Point
No.
Mid-Roof Eave Grade
Exhaust Exhaust Exhaust
Contains 1,000 pd/L
Mid-Roof
Exhaust
Eave
Exhaust
Grade
Exhaust
Locations on side of house (same side as exhaust point)
1
2
3
4
5
6
<0.1 .
<0.1
<0. 1
<(7. 1
<0. 1
0.1
<0. 7
<0. 7
0.1
0.2
0.5
0.8
0.7
0.4
0.7
0.6
0.7
0.5
0.4
0.8
0.5
0.4
1.1
0.9
0.8
1.4
1.7
' 0.8
5.3
8.1
6.7
4.1
6.7
6.2
Locations on roof of house (same side as exhaust point)
7
8
9
10
11
12
0.3
,0,1
0.2
0.3
0.1
15 m (about 50 ft) downwind,
39
<0.1
" Concentrations in pCi/L (1 pCi/L
0.2
0.4
' 0.1 ' ' "
0.3
0.4
0.3
at grade level
0.1
= 37 Bq/m3).
0.1
0.3
'0.3
0.1
0.2
0.2
<0.1
3.2
1.0
0.4
2.1
3.3
1.0
0.7
2.2
4.3
1.2
3.1
4.4
2.6
1.0
1.4
3.0
2.6
1.2
2.0
2.1
0.8
sides of the building remote from the
exhaust location.
4 If the exhaust were to contain 100
pCi/L, neither roof-level exhaust loca-
tion would result in concentrations as
high as 0.4 pCi/L against any face of
any of the buildings, except in two
cases with eave exhausts where the
maximum concentration would just
reach 0.4 pCi/L. If the exhaust con-
centration were 1,000 pCi/L, the maxi-
mum face concentration resulting with
an eave exhaust is predicted to be
about 4 pCi/L; that with an exhaust
midway up the roof slope would be
about 3 pCi/L.
5 Efforts to model the measured near-
field wind tunnel tracer gas concen-
trations using available analytical and
numerical models were unsuccessful
in developing or validating the mod-
els for this application.
single point on the face of any of the
four buildings resulting from grade-
level exhaust would be 0.8 pCi/L, un-
der specific wind conditions, based
on these results. If the exhaust were
to contain 1,000 pCi/L, the highest ra-
don concentration contributed by the
grade-level exhaust against the face
of the building would be 8 pCi/L, again
under specific conditions at particular
locations. But even for the 1,000 pCi/L
exhaust, face concentrations contrib-
uted by the grade-level exhaust would
be less than 0.4 pCi/L on the three
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David E. Neff, Robert N. Meroney, and Hesham EI-Badry are with Colorado State
University, Fort Collins, CO 80523.
D. Bruce Henschel is the EPA Project Officer (see below).
The complete report, entitled "Physical and Numerical Modeling ofASD Exhaust
Dispersion Around Houses," (Order No. PB94-188117; Cost: $36.50, subjectto
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penally for Private Use $300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-94/115
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