carbon monoxide dispersion in enclosed car parks ... · carbon monoxide dispersion in enclosed car...

Proceedings of the 15th IBPSA ConferenceSan Francisco, CA, USA, Aug. 7-9, 2017

654https://doi.org/10.26868/25222708.2017.168

Carbon Monoxide Dispersion in Enclosed Car Parks: Pollutant Source Modelling Methods

Jason Gaekwad1

1Mott MacDonald, Melbourne, Australia

Abstract

Adequate ventilation of enclosed car parking areas isrequired to ensure concentrations of vehicle exhaustpollutants (e.g. Carbon Monoxide) do not reach a levelhigh enough to threaten the safety of car park occupants.With the development of accessible Computational FluidDynamics (CFD) tools, car park ventilation design isshifting from a code-based approach to a performance-based approach, with CFD methods used to confirm thesafety of the proposed design. A comparison of threepollutant source modelling methods for underground carparks has been conducted using CFD methods. Themodelling shows that pollutant concentration results varyconsiderably (maximum concentration variation of over24 times) between modelling methods. This workrepresents an initial study into underground car parkpollutant source modelling methods with the view ofimproving the safety and accuracy of performance basedunderground car park ventilation designs.

Introduction

Underground car parks represent a unique case wherehumans may be present for extended periods, ventilationis limited, and production of toxic pollutants such ascarbon monoxide is high. Ventilation design of theseareas is therefore critical to occupant safety. Numericalmethods such as Computational Fluid Dynamics (CFD)are increasingly being used to confirm the effectivenessof car park ventilation designs however CFD methodsmay be sensitive to boundary conditions. In the case ofcar parks, pollutant concentration results may besensitive to the method by which the pollutant source ismodelled.

This paper studies three pollutant source modellingmethods in order to quantify the sensitivity of pollutantconcentration results to pollutant source modellingmethods. A technical background is provided, outliningthe methods of production and control of vehicleproduced carbon monoxide (CO) as well as existingstudies into modelling vehicle pollutant sources. Afundamental purpose of the research is then established,followed by a discussion of the CFD modellingmethodology. Finally, the results are presented anddiscussed.

Background

Carbon monoxide

Carbon monoxide is a colourless, odourless, flammablegas, toxic to humans. The gas is a product of fossil fuel(hydrocarbon) combustion, refer equations (1) and (2)from Flagan and Seinfeld (1988).

Equation (1) shows the combustion of simplehydrocarbon to form stable products (water and carbondioxide). Equation (2) provides more detail on theformation of carbon dioxide in high temperaturecombustion processes, revealing an equilibrium betweencarbon dioxide and carbon monoxide as products ofcombustion.

CnHm+(n+m4 )O2⇒nCO2+

m2

H 2O (1)

CO2⇔CO+12O2 (2)

Carbon monoxide is a major environmental pollutant dueto worldwide use of internal combustion engines,especially in diesel and petroleum fuelled vehicles. TheWorld Health Organisation (WHO) stipulates a range ofair quality guidelines to protect against adverse healtheffects due to carbon monoxide exposure. The WHOguidelines are defined at four different averaging times,as presented in Table 1.

Table 1: WHO guidelines for carbon monoxideexposure, after World Health Organisation (1999)

AveragingPeriod

Criterion(mg/m3)

15 minutes 10030 minutes 60

1 hour 308 hours 10

The WHO outlines a number of environments in whichpeople are likely to encounter carbon monoxide. Theseinclude “travelling in motor vehicles, working at [their]jobs, visiting urban locations associated with combustion


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sources, or cooking and heating with domestic gas,charcoal or wood fires” (World Health Organisation(1999)). The WHO follows on to state that “among thesesettings, the motor vehicle is the most important forregularly encountered elevations of carbon monoxide”.

Widespread vehicle emissions control regulations haveled to the adoption of exhaust catalytic converter systemsin vehicles to reduce carbon monoxide emissions. Thesesystems work by catalysing the oxidation of carbonmonoxide to carbon dioxide (CO2) by equation (3), afterGuyer (1988).

2CO+O2⇒2CO2 (3)

However the performance of catalytic converters inconverting undesirable exhaust products (such as carbonmonoxide) relies on waste heat from the exhaust gas,with reaction rate (and therefore performance) increasingas the temperature of the catalytic converter increases(refer Figure 1).

Figure 1: Typical conversion regimes and reaction ratesof catalytic converters as a function of temperature,

after Ehsan et al (2005)

Of note is the provision in Standards Australia (2012) ofcarbon monoxide emissions factors for the Australianfleet, quantifying the improvement in catalytic converterperformance with temperature after a cold engine start.These emissions factors are presented in Table 2.

Table 2: Fleet average carbon monoxide emissions rateson cold engine start, after Standards Australia (2012)

Time after coldengine start

CO emissionrate, g/min

First minute 25Second minute 16Third minute 10Fourth minute 7Fifth minute 5

Hot 3.2

Enclosed car parks

A critical case therefore occurs when a vehicle internalcombustion engine is operating, the catalytic converter isat a low temperature (engine has just started), ventilationis limited, and humans are present. This critical caseoccurs in enclosed (underground) car parks.

Ventilation is the main aspect of underground car parkdesign which can be controlled and engineered to limithuman exposure to carbon monoxide. As ventilation istherefore critical for these areas, conservative "code-based" design approaches exist such as StandardsAustralia (2012). These approaches typically specify acoarse ventilation rate based on number of car parks andcar park use (e.g. residential, retail, etc.). Some guidancemay also be given on the approximate location of supplyand exhaust grilles. Such simplified approaches mayoverestimate the supply air required and hence the scopeof mechanical services plant required for maintenance ofadequate air quality within the car park.

With the development of accessible, desktop computerbased CFD tools, car park ventilation design is shiftingfrom a code-based approach to a performance-basedapproach, with CFD methods used to confirm the safetyof the proposed design. This approach is lessconservative than a standardised approach, typicallyleading to a reduction in the mechanical ventilationrequirements.

Pollutant source modelling

CFD results are extremely sensitive to several modelcomponents, including supply air diffusers, impulseventilation (jet) fans, and vehicle pollutant sources. Arange of methods have been used throughout industry tonumerically model these components (typically underfire and smoke ventilation conditions) as described inViegas (2010). These methods vary in accuracy. Of noteare ASHRAE’s efforts to provide simplified boundaryconditions for numerical room airflow models, largelyfocusing on the accuracy of supply air diffusermodelling, published in Chen (2000).

For the critical underground car park case the pollutantsources are expected to have the most direct impact onpollutant levels. This paper focuses on modellingtechniques for vehicle pollutant sources in car parks.

Vehicle pollutant sources have been studied in detail inmesoscale air quality applications (eg highways). Forthese cases turbulence provides mixing of the exhaustpollutants into the surrounding air. The widely adoptedCALINE4 model (Benson (1989)) simplifies turbulenceabove roadways into two sources, mechanical turbulencedue to the wake of the emitting vehicle and vehiclespassing behind it, and thermal turbulence due to theelevated temperature and hence buoyancy of the emittedexhaust (refer Figure 2). This turbulent mixing results ina well-mixed volume source of pollutant, with


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dimensions some width greater than the roadway (3metres on each side in the case of the CALINE4 model)and some height above the roadway. As mesoscalemodels typically address calculation domains withdimensions many hundreds of metres or severalkilometres in size (i.e. in the far-field of the volumesource), the pollutant volume sources tend towards linesources for these calculations.

Figure 2: CALINE4 method of vehicle pollutant sourcemodelling, after Benson (1989)

Although high speed vehicle pollutant sources have beenstudied and modelled for a number of decades, localflow around vehicles in low speed car park movementsvary considerably from high speed mesoscale air qualityapplications. The level of both mechanical and thermalturbulence is unlikely to be as high due to low speedvehicle movements, fewer car movements, and coolerexhaust gas temperatures due to recent cold engine start(for cars exiting the car park). Pollutant sourcemodelling for low speed car park applications have notpreviously been studied.

Purpose

The purpose of this study was to investigate variousmethods of representing low speed vehicle pollutantsources in underground car parks. The objectives of thestudy were to:

1. Identify any significant differences in pollutantsource modelling methods.

2. Take the first steps in identifying which pollutantsource modelling method is the most accurate andappropriate for underground car park modelling.

The fundamental objective of this work was to improveperformance based methods of designing mechanicalventilation for underground car parks. This in turn willimprove the air quality of car parks for occupants andwill likely reduce the amount of mechanical ventilation

required, with associated spatial, capital, operational, andenergy savings.

Simulation

Computational method

The computational package OpenFOAM (Open SourceField Operation And Manipulation) was used to conducta study into the sensitivity of airborne pollutant (carbonmonoxide) concentrations to different methods ofmodelling vehicle pollution sources.

OpenFOAM is an open-source Linux-based C++ library,which provides solvers (i.e. applications) to solvespecific continuum mechanics problems (Greenshield(2016)). A custom solver namedtransportBuoyantBoussinesqSimpleFoam was used forthis work. The custom solver was based on the standardsolver buoyantBoussinesqSimpleFoam, provided as partof OpenFOAM’s default package of applications.

buoyantBoussinesqSimpleFoam solves theincompressible Reynolds-Averaged Navier-Stokes(RANS) equations for momentum, pressure (using theSIMPLE algorithm (refer Ferziger and Peric (2002)) andtemperature. Turbulence was also accounted for by thesolver, using a 2 equation eddy viscosity based model(the k-ε model). Hence solutions for turbulent kineticenergy (k) and eddy dissipation rate (ε) were alsoincorporated into the solver.

The solver was customised intotransportBuoyantBoussinesqSimpleFoam throughaddition of a solution for transport of a passive scalarvariable as shown in equation (4), after Ferziger andPeric (2002). The equation is a basic scalar variabletransport equation for φ in differential tensor notation,with (progressing left to right) transient, advection,diffusion and source terms.

∂(ρϕ)

∂ t∂(ρu j ϕ)

∂ x j

=∂

∂ x j(Γ

∂ϕ

∂ x j)+qϕ (4)

The pollutant (carbon monoxide) was modelled as apassive scalar, with no physical effect on the flow.Pollutant entered the domain as a volumetric source termin the scalar transport equation (i.e. qφ).

Although the incompressible RANS equations weresolved, buoyancy forces were accounted for using theBoussinesq approximation, with density variationsaccounted for only in the buoyancy term of theequations. The buoyancy term was subsequentlysimplified from a density dependent term into atemperature dependent term, eliminating the need for acompressible solution to the RANS equations (equation(5)).

(ρ−ρ0) gi=−ρ0g iβ(T−T 0) (5)


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This approximation is valid for flows where thedifference in density is much less than the referencedensity ρ0. The maximum temperature modelled was323K, with a reference temperature of 293K. Thedifference in the density of air between these twotemperatures was estimated to be approximately 4% ofthe reference density. Hence the Boussinesqapproximation was considered valid for the purpose ofthis study.

Computational domain

The domain was a simple rectangular prism, with supply(inlet) and exhaust (outlet) located at high level along thelength of the domain (i.e. +z and -z faces). Note that x isthe major axis dimension, y is the vertical dimension andz is the lateral axis dimension. It is acknowledged thattypical car park ventilation strategies incorporate avertical differential between supply and exhaust toencourage mixing however the scope was to study themixing of each type of source minimising externaldesign-specific influences.

Figure 3: Section (x-plane) through domain showingsupply (green) and exhaust (blue) locations. A

volumetric source is also shown at the bottom of thedomain.

A grid independence study resulted in a simplestructured mesh of 160,000 cells. This mesh resolutionprovided a balance between discretisation error andsolution run time. The mesh was held constant betweenthe pollutant sources modelled.

Table 3: Domain propertiesProperty Value

Size (x y z), m 40 2.2 10Number of cells (x y z) 160 25 40

Inlet and outlet size (x y), m 40 0.3Inlet volume flow rate, m3/s 5

Inlet temperature, K 293Outlet relative pressure, Pa 0

Figure 4: Isometric view of 3D domain and mesh

Figure 5: Detail of 3D mesh elements

Pollutant source modelling methods

Three source modelling methods were studied. Thesethree sources vary primarily in the assumptions ofturbulence and mixing made in their definition:

1. Finite width line source. This is a common methodused in industry (e.g. Asimakopoulou (2009)) as itsimplifies emissions calculations and is easilyimplemented. This is also the method used inmesoscale air quality applications, as it is a far-fieldapproximation of a volume source. For this study,the line source was located on the car park floor inthe centre of the domain, with momentum in thevertical (y) direction.

2. Distributed point sources. This is a less commonmethod for modelling pollutant sources. Thismethod was selected in order to eliminateassumptions about turbulence and mixing madewhen line and volumetric sources are used. Thesesources allow the numerical model to solve forturbulence associated with both buoyancy forcesand shear between the exhaust jet and the freestream air. 10 distributed point sources were locatedin the domain, with a spacing of 2m, a temperatureof 323K, and momentum in the primary horizontal(x) direction.

3. Volumetric source. This source is a similar approachto that used in the CALINE4 model (Benson(1989)), however the underground car park context


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requires near-field volumetric treatment of thesource, as contrasted to far-field line sourcetreatment. This method assumes pollutant isperfectly mixed through some volume of air due toturbulence. For this study, the volumetric source waslocated in the centre of the domain verticallyspanning between the floor of the car park and aheight of 0.5m above floor level.

Properties of the sources were specified based on 10 carswith a total engine displacement of 18L (1.8L per car).This is an arbitrary assumption. However as theobjective of this study was to compare the sourcemodelling methods the assumption is valid to facilitatecomparison provided modelling methods are keptconstant between the three source models.

Figure 6: Finite width line source domain

Figure 7: Distributed point source domain

Figure 8: Volumetric source domain

Table 4: Pollutant source propertiesProperty Line Distributed point

(per source)Volumetric

Area/volume 8m2 0.022m2 10m3

Volume flowrate (air), m3/s

0.45 0.045 N/A

Temperature,K

293 323 293

Concentrationof pollutant,

mg/m3

100 100 0.45

Control model

To provide an initial indication as to which pollutantsource modelling method was the most accurate and

appropriate for modelling of low vehicle speeds, thethree source modelling methods were compared to acontrol model. This model consisted of a simplifiedvehicle body (MIRA fastback) with exhaust representinga single car after Hu et al (2015). The vehicle body wasmodelled with a flow velocity of 6 km/h, the averagespeed of vehicles moving through a car park suggestedby Standards Australia (2012). The control modelrepresented an initial attempt to study the levels ofmechanical and thermal turbulence for pollutantemissions from the exhaust pipe of a slow moving car.

Similar to the pollutant source models the control modeldomain was a simple rectangular prism. The inletcomprised the entire -x face and the outlet comprised theentire +x face, forming a “numerical wind tunnel”. Asingle point source of momentum, temperature andpollutant was located at the rear of the control modelalong the x axis centreline, 0.1m above the ground. Thissource represented the exhaust pipe outlet. Theproperties of this source were identical to those of asingle point source used in the distributed point sourcepollutant source model.

An unstructured mesh of 580,000 cells was created usingthe snappyHexMesh tool provided as part of theOpenFOAM package. This tool used a base structuredhexahedral mesh to generate a refined unstructuredhexahedral mesh, with mesh nodes snapped to the facesof blockage objects in the domain (i.e. MIRA cargeometry).

Table 5: Control model domain propertiesProperty Value

Size (x y z), m 55 5 8Number of cells(unstructured)

580,000

Inlet velocity,m/s

1.67

Outlet relativepressure, Pa

0

Figure 9: Isometric view of MIRA car 3D model


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Figure 10: Sketch of MIRA car model showing majordimensions

Results

Pollutant source modelling methods

The key metric of this study was the concentration ofpollutant (carbon monoxide), expressed in mg/m3. Asthis study is a relative comparison, the absolute values ofconcentration are of limited interest. Concentrationresults were extracted from the models at 750mm abovelocal floor level, identified as the lower limit of thebreathing zone by Standards Australia (2012).

Two concentration values were extracted from themodels. The maximum concentrations are directlyrelated to safety based guidelines for pollutant (carbonmonoxide) and hence underground car park ventilationdesign. These maximum concentrations typically occuracross a small area and are sensitive to the flow featuresin the model. Measured “typical” concentrationsrepresent concentrations represented over a large area,driven by the numerical representation of the pollutantsource.

Pollutant concentrations varied considerably between thesources. The line source displayed the highest pollutantconcentrations of 323 mg/m3 (maximum). This valuewas measured in a thin band of flow against the wall,representing an area of recirculation and hence pollutantconcentration. The typical concentration for the linesource model was approximately 160 mg/m3 dispersedover the central half of the car park.

The distributed point source method resulted in amaximum tracer concentration of 29.5 mg/m3 measureddirectly above the point sources. The typical pollutantconcentration was approximately 20 mg/m3 dispersedover the central half of the car park.

The volumetric source displayed a maximum and typicalconcentration of 13.4 mg/m3, across the central half ofthe car park. The dimensions of the volume source mayhave overestimated the size of the well mixed region inthe wake of exiting vehicles, reducing overallconcentrations.

The distributed point source showed the least sensitivityto ventilation conditions in the car park, with thepollutant flow driven by the momentum and temperatureof the sources, as expected. The line source and volumesource models showed sensitivity to the ventilationconditions, with the model boundary conditions

encouraging some recirculating flow, and henceincreasing pollutant concentrations.

Table 6: Maximum and typical pollutant concentration(mg/m3) at 750 mm above local floor level

Result Line Distributedpoint

Volumetric

Maximum 323 29.5 13.4Typical 160 20 13.4

Figure 11: Line source - plan view showing pollutantconcentration

Figure 12: Distributed point source - plan view showingpollutant concentration

Figure 13: Volumetric source - plan view showingpollutant concentration

The results show that pollutant concentrations aresensitive to the source modelling method, with thelargest maximum concentration modelled more than 24times greater than the smallest maximum concentration,and the largest typical concentration more than 11 timesgreater than the smallest typical concentration.

Control model

The wake of the control model displayed considerablemomentum in the negative y direction (downward


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towards the ground). This flow feature caused theexhaust plume to remain vertically unmixed and attachedto the ground for a distance of approximately 40 metres,after which vertical mixing was apparent.

Figure 14 shows the rear of the control model. A contourcoloured by vertical velocity (red positive y direction -upward, blue negative y direction - downward) andvelocity vectors are displayed. It is apparent thatsignificant downward momentum is induced in the wakeof the car. This downward momentum is higher inmagnitude than the upward momentum caused by thebuoyant exhaust plume, which is identifiable as an areaof upward momentum downstream of the bottom edge ofthe car body.

Figure 14: Control model – velocity vectors and verticalvelocity contour showing recirculation and downward

momentum at rear of vehicle

The maximum concentration at 750 mm above localfloor level was 31 mg/m3. This maximum concentrationoccurred consistently downwind of the car, with flowfeatures generally resembling those of the line sourcepollutant source model. It is difficult to draw a directioncomparison between numerical results of the pollutantsource modelling methods and the control model due tothe difference in ventilation conditions between themodels. However, if a direct comparison of maximumpollutant concentration is conducted and normalised toemissions from a single car, it is found that the linesource result is 4% larger and the volumetric sourceresult 2300% smaller. The distributed point source modelprovides a more difficult comparison as the contributionof each source to the maximum pollutant concentrationvalue is unknown. Based on the location of themaximum concentrations, in the emission direction andvertically upward from each point source, it may bereasonably assumed that only a single point sourcecontributes to each area of maximum pollutantconcentration. If this is the case then the distributed pointsource maximum concentration is 5% smaller than thecontrol model result. These preliminary results indicatethat the line source and distributed point source mayprovide accurate solutions for car park ventilationstudies. The volumetric source may potentially beresized to match the size of the control model wake,however different car shapes and movement conditions

will result in a wide range of wake sizes and behaviour.It is also worth noting that the control model did notaccount for mixing due to the interaction of carmovement with existing wakes (i.e. a car following afteranother). It is anticipated that this movement willsignificantly increase the level of pollutant mixing, withresults tending toward the well mixed volumetric sourceresults.

Figure 15: Control model - plan view showing pollutantconcentration

Although a basic preliminary study, the control modelprovided initial indication to which source modellingmethod may be the most accurate from a pollutantconcentration perspective (point source) and direction topursue further work.

Further Work

The current study has achieved its objective ofdemonstrating the sensitivity of carbon monoxide resultsto source modelling methods. Initial steps towardsidentifying the most accurate methods have been made,however it is acknowledged that significant further workis required in this space.

Further work includes experimental validation of theMIRA vehicle body control model under low speedapplications. This will enable the most accuratemodelling method to be more clearly identified.Moreover, the momentum, energy, and turbulenceboundary conditions of the source modelling methodsmay be altered to more closely represent the flowphysics occurring in exhaust plume of a slow movingvehicle. This work should also be extended to theinteraction between vehicle wake plumes and turbulenceinduced by vehicles following behind.

A sensitivity study of the pollutant source modellingmethods to momentum and temperature boundaryconditions (both for the source and for the car parkventilation system) is also required to determine therobustness of the source modelling methods in a widerange of underground car park applications.

It is noted that a compressible solution to the RANSequations may be studied in the future to remove theneed for the Boussinesq approximation and improve theaccuracy of the numerical simulation.

Finally, the volumetric source, with refinement, mayshow potential for development of a basic calculation todetermine maximum concentrations in linear sections ofcar park roadway based on limited input data (e.g.number of cars and car park layout). This approach


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would require a more fundamental study of mixing andturbulence of buoyant jets in a mechanically turbulentmedium. A hybrid Navier-Stokes/bulk flow model maybe possible. If developed this approach may represent apoint on the spectrum of numerical complexity betweena code based approach and a full numerical (CFD)model, which may be more accessible to BuildingServices/HVAC Engineers than CFD modelling.

Conclusion

A CFD study of pollutant (carbon monoxide) sourcemodelling methods in underground car parks has beenconducted. This study involved comparison of threepollutant source modelling methods: 1) finite width linesource, 2) distributed point source, 3) volumetric source.The computational package OpenFOAM was used toconduct the numerical study using a custom solvernamed transportBuoyantBoussinesqSimpleFoam, amodification of the standardbuoyantBoussinesqSimpleFoam solver to allow foradvective and diffusive transport of a passive scalarvariable (pollutant concentration). Modelling parameterswere kept constant between the pollutant source modelsin order to facilitate a direct comparison between thesource modelling methods.

Pollutant concentration results varied considerablybetween source modelling methodologies. The largestmaximum concentration was more than 24 times greaterthan the smallest maximum concentration, and thelargest typical concentration was more than 11 timesgreater than the smallest typical concentration.

Some methodologies were also more sensitive to theventilation boundary conditions of the car park, with theline source and volume source models showingincreased pollutant concentrations due to recirculatingflow.

An initial study into identification of the most accuratepollutant source modelling method was conducted usinga control vehicle model. This model approximatelyaccounted for mechanical and thermal turbulence in thecase of pollutant emissions from the exhaust pipe of asingle slow moving vehicle.

The control vehicle model results pointed toward the linesource and distributed point source as the most accuratemethods. Flow features of the line source source closelyresembled those of the developed pollutant plume in thewake of a slow moving car. The volumetric sourcegreatly underestimated pollutant concentrations due tothe assumption of a well mixed wake inherent inapplication of the line source. Considerable further workis required in experimental validation as well as testing

momentum and energy sensitivities of the source typesand wake interaction behaviour.

Acknowledgements

The author would like to acknowledge the OpenFOAMfoundation and the open-source movement, withoutwhich this work would not have been possible, andStewart Mann, for providing support, comments andreview of the draft format of this paper.

References

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Benson, P. (1989). CALINE4 – A Dispersion Model forPredicting Air Pollutant Concentrations NearRoadways. State of California Department ofTransportation. Sacramento.

Chen, Q. (2000). RP-1009 Simplified Diffuser BoundaryConditions for Numerical Room Airflow Models.American Society of Heating, Refrigeration, and AirConditioning Engineers.

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Flagan, R.C. & Seinfeld, J.H. (1988). Fundamentals ofair pollution engineering. Prentice Hall. New Jersey.

Greenshields, C.J. (2016). OpenFOAM User Guideversion 4.0, The OpenFOAM Foundation.http://openfoam.org.

Guyer, H.H. (1998). Industrial Processes and WasteStream Management. John Wiley & Sons. New York.

Hu, X.J. Yang, H.B. Yang, B. Li, X.C Lei, Y.L (2015).Effect of Car Rear Shape on Pollution Dispersion innear Wake Region. Mathematical Problems inEngineering 2015.

Standards Australia (2012). AS1668.2-2012 The use ofventilation and airconditioning in buildings –Mechanical ventilation in buildings. SAI Global.

Viegas, J.C. (2010). The use of impulse ventilation forsmoke control in underground car parks. Tunnellingand Underground Space Technology 25, 42-53.

World Health Organisation (1999). EnvironmentalHealth Criteria 213 Carbon Monoxide (SecondEdition). World Health Organisation. Geneva.

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