Turkey Paper 2010

8/8/2019 Turkey Paper 2010

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Dispersion of effluents from building roof stacks: Comparison of

various models, CFD and wind tunnel results

Ali Bahloul1, 2,*, Bodhisatta Hajra1 and Ted Stathopoulos1

1Department of Building, Civil and Environmental Engineering, Concordia University,Montreal, Canada2 Institut de recherche Robert-Sauv en sant et en scurit du travail, Montreal, Canada

Corresponding email: [email protected]

SUMMARY

Assessing pollutant concentrations within the proximity of the source in the built

environment is important. Most current models do not take into account building-generated turbulence, which causes effluent re-entrainment into the building through

openings located on the roof or side walls. This paper compares ASHRAE (2003 and

2007 versions), ADMS (an EPA-approved model) calculated dispersions, wind tunneldata and CFD results using the Realisable k- model and turbulent Schmidt number (Sct)

of 0.7.

Three different cases involving a low-rise building (with and without rooftop structure)and a high-rise building were considered. It was found that ADMS dispersions compare

well with experimental results for the low-rise building but generally predict lower

dilutions for the high-rise building. In addition, ADMS cannot model the effect of rooftop

structures. Dilutions predicted by ASHRAE 2003 and 2007 models were comparable for

the low-rise and high-rise building with flat roof. However, ASHRAE 2007 predictedlower dilutions compared to ASHRAE 2003 for the low-rise building with Roof Top

Structures (RTS). CFD compared well with experimental data for the low-rise buildingwith RTS but generally predicted lower dilutions than experimental results for the low-

rise building with flat roof. The results indicate the suitability of each model for certain

cases.

INTRODUCTION

Assessing pollutant dispersion in the built environment is important because airflow and

effluent transport within the recirculation zone of the source is a complex phenomenonwhich greatly influences the concentration of pollutants in the near-field region [5].

Pollutants that are released from rooftop stacks have a tendency to re-enter the building

through openings located on the roof or side walls. This effect is greatly magnified in the

presence of a Roof Top Structure (RTS) located upwind of a stack [10].Currently one of the most widely used models is the Atmospheric Dispersion Modelling

System (ADMS), developed in England, which is also supported by United States

Environmental Protection Agency. A recent study showed that of the various available


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models, ADMS is the only model which gave good comparisons with ASHRAE and

wind tunnel results. Hence the other widely used models such as CALPUFF, SCREEN

and AFTOX have not been used for the present study [10].ASHRAE provides a geometric stack design method for estimating the minimum stack

height to avoid plume entrainment in the flow recirculation zones of a building and

modified Gaussian equations to estimate dilutions on rooftop of building [2, 3]. Thepresent study makes use of the latter to assess plume dilutions on building rooftops.

Wind tunnel experimental results were produced at the Boundary Layer Wind Tunnel

laboratory at Concordia University, Montreal, Canada.CFD has been possible to study the flow and transport of airborne particles in the vicinity

of buildings. It is widely used because other assessment techniques such as field and

wind tunnel tests are time consuming and expensive. However, it is necessary to validate

CFD results with wind tunnel or field data [4, 11].This paper compares the ADMS model, results with those produced by ASHRAE 2003

and 2007 models, wind tunnel results along with CFD simulations using the Realisable

k- model, forSct= 0.7.

ASHRAE

ASHRAE develops standards for designers dealing with the design and maintenance of

indoor environments. ASHRAE Applications Handbook, Chapter 44, 2007, gives certainguidelines that can be used for determining the stack height and rooftop dilutions. Till

recently ASHRAE 2003 was in use by practicing Engineers. However, in 2007 a

modified version, which has certain modifications in assessing plume dilutions, wasreleased.

ASHRAE 2003 suggests the following formulation to calculate plume dilutions:

)2/exp()/)(/)(/(422

zezeyeHr hddVUD = (1)

where:

h is the effective stack height (m),de is the stack diameter (m),

Ve is the exhaust velocity (m/s),

UH is the wind speed at building height (m/s),y and z are standard deviations of the plume (m)

The 2007 version of ASHRAE has modified equation (1) as:

)2/exp()/)(/)(/(422

zezeyeHr ddVUD =

where: = hplume- hTop= 0 if hplume< hTop

hplume is the sum of stack height and plume rise minus stack wake downwash (m),

(2)


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hTop is the height of the highest active obstacle or recirculation zone on a rooftop between

stack and intake (m),

is the vertical separation between the plume rise and h Top. It may be noted that theterm has replaced h in the exponential term of equation (2). The following

formulation suggested by Wilson [13], has been used for evaluating normalised dilutions.

)H(U/Q)(DD2

Hrnormalised = (3)

where:

Dris the ratio of exhaust to receptor concentrations

Q is the volumetric flow rate (m3/s)H is the height of the building (m)

Normalised dilutions are useful in scaling down the quantities to mere ratios ofconcentrations so that they can be compared to other models.

ADMS

ADMS is a well established and validated Gaussian plume model. It is an advanceddispersion model developed in England for calculating effluent concentrations in the built

environment [1].

ADMS requires certain inputs that include the mass of the pollutant emitted,

meteorological details, building configurations, stack and receptor locations andtopographic details. The output can be viewed as a contour plot or as concentrations

(g/m3) at specific locations. In the present study, for all cases the concentrations of

effluents were found on a grid of 50 m by 50 m. The averaging time for all cases was 1

minute to avoid the meandering of the wind direction, which has an effect on the plumeconcentration and width [5].

ADMS concentration results were converted to normalised dilutions using equation (3) sothat they can be compared directly to other models.

WIND TUNNEL EXPERIMENTAL SETUP

The wind tunnel experiments were carried out in the open circuit Boundary Layer WindTunnel laboratory at Concordia University, Montreal, Canada. It is 1.8 m by 1.8 m in

cross-section and 12.2 m in length. The buildings that have been tested were made of

timber on a 1:200 scale [10].According to Snyder [8], while modelling non-buoyant plume exhaust the followingcriteria should be satisfied:

Geometric similarity

Building Reynolds Number > 11000

Stack Reynolds Number > 2000


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Similarity of wind tunnel flow with atmospheric surface layer

Equivalent stack momentum ratio.

A mixture of Sulphur hexafluoride (SF6) and Nitrogen, which was released from stacks ofvarious heights ranging from 1m to 7m and exhaust momentum (M) varying from 1

through 5, was used during the tracer gas experiment. A multi-syringe pump was used tocollect the gas samples to determine the concentration of effluents at various receptorslocated at 5m interval downwind of stack, with a sampling time of 1 minute. A Gas

Chromatograph (GC) was used to assess the gas concentrations that were collected using

the syringe samplers. In this connection the velocity at building height was measured tobe 5.4 m/s in the wind tunnel. Normalised dilutions were calculated from equation 3 to

make the results obtained from ASHRAE, ADMS and CFD comparable. Since it was

assumed that the buildings considered in all cases were in an urban terrain therefore a

power law exponent of 0.3 was used to determine UH. The experimental conditions usedin this study are similar to those described in [10].

CFD SIMULATIONS

CFD has been used on numerous occasions to simulate airflow and pollutant dispersionwithin the vicinity of buildings for various values ofSct[4, 11, 12]. The turbulent Schmidt

number (Sct) is defined as:

ttctDS /= (4)

where:

tis the turbulent eddy viscosity,Dtis the eddy mass diffusivity.

Earlier studies have shown the importance of turbulent Schmidt number in modellingairborne particles. Spalding found good agreement between experimental and numerical

results at Sct= 0.7 [9]. In a separate study, Launder found Sct= 0.9 to be suitable for near-

wall turbulence modelling [7]. Flesch also performed CFD simulations for differentatmospheric stability conditions forSctranging from 0.18 to 1.34 [6].

The present study uses Realisable k- model to simulate effluent transport forSct = 0.7,

since past studies have found good agreement with experimental data within the near-

field region [4, 12]. In this context, numerical simulations from a previous study which

correspond to wind tunnel results for exhaust release from a flush vent of an isolated cube[4] and those obtained for a low-rise building with RTS [12], have been used for

comparison since these cases match closely to those examined in the present study.

CASES EXAMINED

Three different cases were considered for the present study. These include:


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a) Low-rise building

b) Low-rise building with RTSc) High-rise building

The dimensions of each building and RTS used for this study are presented in Table 1.

Table 1. Dimensions of buildings and RTS

Structure Height (m) Width (m) Breadth (m)

Low-rise building 15 50 50

High-rise building 60 50 50

RTS 4 30 8

NB: Width represents the dimension perpendicular to wind direction.

Wind was considered perpendicular to the building face. For all cases the receptors were

5 m apart, downwind of stack and the stack location was 20 m from the upwind edge ofthe building and 25 m from the lateral edges. The RTS was centrally placed upwind of

the stack and 6 m away from the upwind edge of the building.

RESULTS AND DISCUSSION

The results obtained from wind tunnel experiments [10], ADMS and ASHRAE are

compared with CFD simulations by Blocken at al. [4] for a low-rise building with a 1 m

stack at M=1, are presented in terms of normalised dilutions in Figure 1 (a). It may be

noted that the results obtained by [4] correspond to a cube of 5 cm with a centrally-

located flush vent. In the absence of RTS, ASHRAE 2007 predicts comparable dilutionsto ASHRAE 2003. Dilutions predicted by ADMS and wind tunnel are in good agreement,

but are generally higher than ASHRAE. CFD simulations using Realisable k- model forSct of 0.7 gave comparable dilutions with ASHRAE 2003 and 2007 at points closer to the

stack and at the downwind edge of the building. The lower dilutions at receptors lying

between 10 m and 20 m downwind of stack suggest a need to re-assess the use of CFDfor the present case.

Figure 1 (b) presents comparisons between experimental results from [10], ADMS,

ASHRAE and CFD simulations from [12] for a low-rise building with RTS with a 1 mstack at M=5. The computational results obtained from [12] correspond to a field study

performed at Concordia University and is similar to the present case. It is observed that inthe presence of RTS, wind tunnel predicted comparable dilutions with ASHRAE 2003and CFD, especially within 20 m downwind of stack. ADMS predicts very high dilutions,

while ASHRAE 2007 predicts very low dilutions at all points downwind of stack.


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a)

b)

Figure 1. Dispersion of pollutants emitted from a low-rise building with: a) flat roof; b)RTS.


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Figure 2. Dispersion of pollutants emitted from a high-rise building

Figure 2 shows comparisons between ADMS, ASHRAE, CFD and wind tunnel data for

the high-rise building (no RTS) for a 1m stack at M=1. Results show that in general, both

ASHRAE 2003 and 2007 versions predict very low and comparable dilutions, comparedto all other models. CFD compares well with wind tunnel at 10m and 15m downwind of

stack but predicts lower dilutions than wind tunnel data thereafter. ADMS predicts lower

dilutions compared to CFD and wind tunnel but are generally higher than ASHRAE 2003

and 2007.

CONCLUSIONS

The study shows that the height of the building plays a vital role in determining dilutions

of effluents from rooftop stacks. ADMS was found to be unsuitable for modelling theeffect of RTS. The calculations by ASHRAE 2007 are simple compared to ASHRAE

2003 but both versions predicted lower dilutions than wind tunnel for high-rise building,

which is unsuitable for design considerations. CFD simulations using Realisable k-model forSct = 0.7 were found to be appropriate for modelling exhaust from flush vents.

However, its applicability needs to be assessed in modelling exhausts from a rooftop

stack. Additional studies to estimate a generalised Schmidt number for CFD studies innear-field pollutant dispersion are required.


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Turkey Paper 2010