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

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