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 ------- 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 ------- 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 ------- 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 ------- |