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Building and Environment 58 (2012) 231e244

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Building and Environment

journal homepage: www.elsevier .com/locate/bui ldenv

Reduced-scale study of wind influence on mean airflows inside buildingsequipped with ventilation systems

Nicolas Le Roux a,b,c,*, Xavier Faure a, Christian Inard b, Sandrine Soares c, Laurent Ricciardi c

aCentre Scientifique et Technique du Bâtiment (CSTB), Nantes, Franceb Laboratoire d’Etude des Phénomènes de Transfert et de l’Instantanéité: Agro-industrie et Bâtiment (LEPTIAB), La Rochelle, Francec Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Saclay, France

a r t i c l e i n f o

Article history:Received 28 March 2012Received in revised form17 July 2012Accepted 18 July 2012

Keywords:Ventilation systemsSimilaritySteady stateIsothermal flowsReduced-scale experimentsZonal code SYLVIA

* Corresponding author. Centre Scientifique et TecNantes, France. Tel.: þ33 (0) 1 69 08 50 82; fax: þ33

E-mail address: nicolas.le-roux@irsn.fr (N. Le Roux

0360-1323/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.buildenv.2012.07.007

a b s t r a c t

To study mass transfers inside buildings equipped with ventilation systems, a methodology has beendeveloped to carry out reduced-scale experiments for studying isothermal flows in a steady or a transientstate. This methodology has been numerically and experimentally validated on simple configurationsand applied to two reference configurations representative of nuclear facilities. The wind influence onmass transfers inside these configurations, in nominal, damaged (ventilation stopped) or accidental(internal overpressure) situations, has been studied by carrying out wind tunnel experiments. Theobjectives of this article are to present the scaling-down methodology and the main experimental resultsconcerning the influence of wind on steady-state airflows inside the reduced-scale models. It is notablyshown that wind effects can lead to a partial or a total loss of the pollutant containment inside buildings.The reliability of the zonal code SYLVIA, which is used to support safety assessments in nuclear facilities,has then been analysed from these experimental results.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Residential and industrial buildings equipped with ventilationsystems are complex facilities in which various heat and masstransfers can occur according to the operating conditions. Predict-ing these transfers, as currently accomplished using zonal codes(e.g., COMIS, CONTAM or SYLVIA), is fundamental to support theconception phases or the safety analysis of this type of building. Forindustrial facilities in which treatment processes of toxicsubstances occur, the main function of the ventilation system is toensure pollutant containment inside the facility in normal,damaged or accidental situations. To achieve this function, theventilation system is designed to maintain rooms in depressionaccording to a specific hierarchy from the most to the leasthazardous room in terms of contamination. The air, taken from theoutside, thus flows from the rooms with the lowest contaminationrisk to the rooms with the highest contamination risk, before beingfiltered and released into the atmosphere.

Nuclear industry is particularly concerned with the need tomaintain the containment of radioactive pollutants in power

hnique du Bâtiment (CSTB),(0) 1 69 08 36 80.).

All rights reserved.

plants, laboratories or fuel treatment facilities. Thus, to supportsafety assessments in nuclear buildings, the zonal code SYLVIA [1]has been developed by the Institut de Radioprotection et de Sûr-eté Nucléaire (IRSN) to predict airflows inside industrial ventilationsystems in nominal, damaged (ventilation stopped) or accidentalsituations (e.g., pressure tank fracture, fire or overpressurescenario). Modelling of airflows due to mechanical ventilation inSYLVIA has already been validated from experiments conducted onreal facilities. However, other driving flows, such as wind orinternal overpressure due to an accident, may arise and thwart themechanical ventilation effect. These effects have not yet beenvalidated in SYLVIA. For this reason, experiments must be con-ducted to check the reliability of SYLVIA in considering theseeffects. The most appropriate solution for experimentally studyingwind and accidental effects in several airflow conditions is toconduct reduced-scale experiments in a wind tunnel. For thispurpose, a similarity study of physical phenomena should first beperformed to ensure that the reduced-scale and the full-scale flowsare similar.

Although reduced-scale experiments have been widely used inbuilding ventilation studies [2], no appropriate methodology hasbeen found for studying steady and transient isothermal flowsinside complex buildings equipped with ventilation systems. Manyexperiments have been developed for natural ventilation cases[3e6], in which the physical phenomena and similarity approaches

N. Le Roux et al. / Building and Environment 58 (2012) 231e244232

are analogous to industrial ventilation fundamentals. In particular,dynamic similarity criteria have been established by Holmes [3]and Etheridge [4,5] that enable the transient response ofa building internal pressure due to a sudden change in externalpressure to be reproduced. However, Haghighat et al. [7] highlightboth the lack of experimental data available to validate thesemodels and the need to extend these works to account formechanical ventilation.

Steady and transient isothermal airflows considering mechan-ical ventilation effects have already been studied in the railwayindustry. Mariaux [8] developed a methodology for studying thetransient responses of internal pressure and airflow rates in trainventilation systems subjected to external overpressure due toa tunnel crossing. However, themethodology is too constraining forindustrial building applications because no velocity or duct lengthreductions can be introduced between the full scale and thereduced scale. Thus, Mariaux [8] neglected the inertia of theventilation system. Nevertheless, for rigorously reproducingdynamic similarity of transient flows inside buildings equippedwith ventilation systems, scale reductions should be defined froma similarity study for the whole building characteristics, taking alsointo account the ventilation system and more precisely its inertia.

The first objective of this paper is to present a rigorous method-ology for studying reduced-scale isothermal ventilation systemssubjected to steady or transient conditions. By applying this meth-odologyon two reference industrial buildings,wind tunnel tests havebeen performed to study the coupling effects of mechanical ventila-tion, wind and accidental internal overpressure. Thus, the secondobjective of this article is to focus on experimental steady-stateresults to identify the influence of wind on the mean airflows insidebuildings andfinally discuss the ability of the SYLVIA code to take intoaccount steady wind effects. Section 2 describes the theoreticaldevelopmentof thismethodologywhichhasbeenused to sizeuptworeduced-scale models, each one representing a reference industrialfacility equipped with a ventilation system. These two studied casesare described in Section 3. Then, the experimental steady-stateresults are presented in Section 4 and compared with the numer-ical results obtained from SYLVIA in Section 5.

2. Methodology for studying isothermal flows in reduced-scale ventilation systems

The scaling down methodology has been developed from theequations used to simulate airflows inside ventilation systems.These equations are first simplified by assuming isothermal flowsand then expressed in a dimensionless form. From this form,dimensionless parameters, which should be conserved betweeneach scale to ensure the similarity between the full-scale and thereduced-scale results, are identified. Thus, by expressing theconservation of these dimensionless parameters, relations betweenthe scale parameters of each variable are determined. Finally, theserelations are used to define the set of scale parameters that shouldbe applied between the full scale and the reduced scale.

2.1. Modelling of the ventilation system

The ventilation system model is based on the breakdown of theventilation system into nodes and branches, as currently done forzonal codes (e.g., COMIS, CONTAM or SYLVIA).

Airflows are assumed to be isothermal and incompressible interms of low Mach number. Thus, the air density is constant insidebranches and equal to the air density calculated on the downstreamnode. However, note that the air density slightly varies and maypromptly vary inside nodes, and consequently lead to pressurevariations.

Each branch, which is defined by a uniformvolumetric flow rate,represents either a passive component (e.g., duct, filter or leak) ora driving force (fans). Assuming horizontal branches and incom-pressible, isothermal, one-dimensional flow, the integral kineticenergy theorem can be written by Eq. (1) for the three types ofpassive branches considered.

rLSdQdt

¼ DP � sgnðQÞR jQ jnr1�n (1)

where Q is the volumetric flow rate (m3/s), DP is the pressuredifference between the inlet and the outlet of the branch (Pa), r isthe air density (kg/m3), L is the branch length (m) and S is thebranch section area (m2). The airflow resistance R represents thehead losses of each branch (kg(2�n)/m4/s(2�n)). The value of theexponent n depends on the branch type: n is equal to 2 for botha duct in a turbulence regime and singularities, 1 for a high-efficiency particulate air (HEPA) filter, and between 1 and 2 fora leak. Values between 1.35 and 1.7 are used in the literature fora leak [6,9,10]. The term rL/S corresponds to the inertia of eachbranch using a heat-transfer analogy. This term should only betaken into account for transient flows.

For driving component, a specific model for fans exists in whichthe fan curve is considered as an input data. Thus, the similarity offans will be based on the conservation of the fan curves by applyingthe pressure and the flow rate scales to the real fan curves.

Each node represents a room or a junction between thebranches. The mass balance equation is applied in the simplifiedform of Eq. (2) assuming isothermal flows.

VMrT

dPdt

¼Xi

riQi �Xj

rjQj (2)

where V is the volume of the node (m3),M is the molecular mass ofair (kg/mol), r is the perfect gas constant (8.314 J/mol/K), T is theambient temperature (K), P is the internal pressure (Pa), i and jrepresent the subscripts for inlet and outlet flows. In addition, fora gas transport study, a passive scalar equation is applied at thenodes and should be taken into account for the methodologydevelopment [11]. However, this equation is not considered hereinbecause this article focuses on the study of airflows only defined bypressure and flow rate values.

Thus, the dimensional model used to simulate airflows insideventilation systems is given by three forms of Eq. (1), defined fora duct, a filter and a leak, and Eq. (2) defined for each node. Thesimilarity study is based on the dimensionless forms of theseequations.

2.2. Dimensionless model

The model has been expressed in a dimensionless form byconsidering a reference value Xref for each variable X. With thesereference values, each variable is written in a dimensionless form(Eq. (3)).

V* ¼ VVref

L* ¼ LLref

S* ¼ SSref

r* ¼ r

rrefU* ¼ U

UrefP* ¼ P

rrefU2ref

Q* ¼ QSrefUref

R* ¼ RRref

t* ¼ tLref=Uref

(3)

By introducing these dimensionless variables in Eqs. (1) and (2),the dimensionless integral kinetic energy theorem for branches

N. Le Roux et al. / Building and Environment 58 (2012) 231e244 233

and the dimensionless mass balance equation for nodes areexpressed by Eqs. (4) and (5), respectively.

r*L*

S*dQ*

dt*¼ DP* �

"RrefS

nref�

rrefUref

�2�n

#r*n�1R*sgn

�Q*����Q*

���n (4)

dP*

dt*¼"Patmrref

SrefLrefVrefU2

ref

# Pir*i Q

*i �P

jr*j Q

*j

V*(5)

where Patm is the atmospheric pressure (Pa). Eq. (4) can be definedin three forms depending on the branch type (duct, filter or leak).Thus, the dimensionless model is composed of the three forms ofEqs. (4) and (5), from which four dimensionless parameters areidentified and should be conserved between the full scale and thereduced scale.

2.3. Similarity study

In the similarity study, a set of scale parameters that enableseach variable to be transposed from the full scale to the reducedscale and vice versa are determined. These scale parameters arenecessary both to design reduced-scale experiments by ensuringairflow similarity and to transpose results from the reduced scale tothe full scale results. These parameters are determined from theconservation of the relations between reference variables (Eq. (3))and the dimensionless parameters (Eqs. (4) and (5)) between eachscale. A scale parameter is defined for each variable as the ratio ofthe reduced-scale and the full-scale reference values (Eq. (6)).

X ¼ Xref reduced�scaleXref full�scale

(6)

2.3.1. Conservation of dimensionless parametersFirst, the dimensionless integral kinetic energy theorem for the

branches (Eq. (4)) reveals the dimensionless parameter N1 (Eq. (7)),which represents the head loss of each branch. This parameter isalso highlighted by considering the steady state form of Eq. (4),commonly known as the Bernoulli Equation.

N1 ¼ RrefSnref�

rrefUref

�2�n (7)

Considering the same fluid at each scale, the density reference isassumed to be similar, implying that the density scale is equal toone. Thus, the conservation of N1 is expressed as

R ¼ U2�n

Sn (8)

where R is the airflow resistance scale, U is the velocity scale, and Sis the section scale. This relation can be written under differentforms depending on the type of branch. The scale parametersassociated with ducts and filters, for which the exponent n isrespectively equal to 2 and 1, are respectively written as

Rduct ¼ 1

S2 (9)

Rfilter ¼ U

S(10)

Thus, the conservation of N1 enables each branch airflowresistance, which is necessary to ensure head loss similarity, to bedetermined if the velocity and the section scales are known.

The dimensionless mass balance equation for nodes (Eq. (5))reveals the dimensionless parameter N2 (Eq. (11)), which is equiv-alent to the dynamic parameter defined by Holmes [3] and thecompressibility term defined by Etheridge [4,5].

N2 ¼ Patmrref

SrefLrefVrefU2

ref

(11)

This parameter takes into account the air compressibility insiderooms subjected to transient flows. Air compressibility impliesoscillations of internal pressures which correspond to Helmholtzresonator oscillations [3]. Thus, N2 should be conserved to repro-duce the dynamic response of transient flows. Considering a similaratmospheric pressure and a similar reference density at each scale,the conservation of N2 is written as

SL

VU2 ¼ 1 (12)

where U is the velocity scale, V is the room volume scale, S is thebranch section area scale, and L is the branch length scale. Thisequation highlights that a volume distortion is introduced betweenthe branches (volume scale SL) and the rooms (volume scale V)unless the velocity scale is equal to 1. The influence of this distor-tion phenomenon, first underlined by Holmes [3], has already beenconsidered for designing natural ventilation experiments (e.g. Refs.[12,13]).

2.3.2. Conservation of relations between reference variablesAs shown in Eq. (3), some reference variables are defined from

other reference variables. The conservation of these variables leadsto new relations between the scale parameters. First, the referencepressure is defined by the dynamic pressure, i.e., the product of thereference density and the square reference velocity. Thus, consid-ering a similar reference density at each scale, the pressure scale iswritten as

P ¼ U2

(13)

Then, the reference airflow rate is defined as the product of thereference branch section area and the reference velocity. Thus interms of scale parameters, this relation is expressed as

Q ¼ SU (14)

Finally, the reference time is defined as the ratio of the referencelength to the reference velocity, and its scale parameter can bewritten as

t ¼ L

U(15)

Thus, the conservation of the relations between reference vari-ables and the dimensionless parameters N1 and N2 is defined byseven relations (Eqs. (8)e(10) and Eqs. (12)e(15)), which are thenused to define the set of scale parameters for ensuring that the fulland the reduced scales are similar. However, ten scale parametersshould be determined corresponding to the geometrical variables(room volume, branch section and branch length), the airflowresistances of the three branch types considered (duct, filter andleak) and the physical variables (velocity, pressure, airflow rate andtime). For this reason, three additional hypotheses, explained in thenext section, have been considered.

Table 1Relations between fixed and calculated scale parameters.

Fixed scale parameters Calculated scale parameters

V

U

Q

U

QS = 2

UP =2

1

SRduct =

S

UR filter =

n

n

leak

S

UR

−

=2

S

UVL

2

=U

Lt =

Fig. 1. View of the referenced-scale and the reduced-scale models used for theexperimental validation of the methodology.

N. Le Roux et al. / Building and Environment 58 (2012) 231e244234

2.4. Definition of the scale parameters

The choice of the additional hypotheses is constrained by thereal configuration characteristics (sizes, pressures and flow rates innominal configurations) and the experimental means (windtunnel characteristics, metrology and fans used). First, the volumescale is limited by the wind tunnel size and the full-scale config-uration size. To avoid blockage effects inside the experimentalvena of the wind tunnel, the section of the reduced-scale modelmust be lower than 10% of the vena section [14]. By consideringthe maximal section of a real configuration Smax and the venasection Svena, the volume scale should take the following conditioninto account:

V � 0:1Svena

Smaxfull�scale

!3=2

(16)

Then, the flow rate scale is constrained by the flow regimeinside the ventilated ducts. A rigorous conservation of the Rey-nolds number is impossible. Reynolds number conservationimplies a velocity increase equal to the duct diameter decrease,which cannot be considered because of experimental constraints(wind tunnel velocity, metrology, fans). However, theconservation of the flow regime inside the ducts in nominalsituations, i.e., considering only mechanical ventilation, ispossible. To conserve the turbulent flow regime, the Reynoldsnumber is assumed to be higher than 4000. Although the frictionfactor is not constant for low roughness values, the turbulentflow regime is reached for Reynolds values higher than 4000.Moreover, considering all of the singularities inside ventilatedducts (bends, reductions, control dampers, and so on), frictionhead losses are negligible compared with singular head losses.Thus, the flow rate scale inside each duct should respect thefollowing condition:

Q � 4000n�Dreduced�scale=Sreduced�scale

�Qfull�scale

(17)

where n is the dynamic viscosity (m2/s), Dreduced-scale is the reduced-scale duct diameter (m) and Sreduced-scale is the reduced-scale ductsection area (m2).

Finally, the velocity scale is strongly constrained due to itsrelations with the pressure scale and the branch length scale. Onthe one hand, the velocity scale is directly related to the pressurescale (Eq. (13)), and the pressures should be reduced to respect therange of pressure sensors and to find appropriate fans. On the otherhand, a strong decrease in the velocity will imply that the pressuresare too low compared with the uncertainties of pressure sensors.Furthermore, the branch length scale is proportional to the squarevelocity scale (Eq. (12)), and the duct length should be sufficientlylong to respect the straight lengths required for the flow ratemeasurements and for practical reasons of establishing connec-tions between each node.

Thus, the scaling downmethodology proposed consists in fixingthe three constrained scale parameters mentioned above, i.e., theroom volume scale, the flow rate scale and the velocity scale. All ofthe other scale parameters are then determined using Eqs. (8)e(10)and Eqs. (12)e(15), which arise from the conservation of the rela-tions between reference variables and the dimensionless parame-ters N1 and N2. Table 1 sums up the relations between the fixed andthe calculated scale parameters, showing that several sets of scaleparameters can be defined depending on the cases studied and thehypotheses considered.

2.5. Validation of the methodology for steady flows

Only some steady-state results of the validation are presentedherein because the objectives of this paper are to directly presentan application on industrial buildings to study the influence ofwind on mean flow rates. Details about the validation of themethodology can be found in Le Roux [11] or Le Roux et al. [15] andtransient phenomena observed on simplified and industrial caseswill be presented in a specific paper.

The methodology has been numerically and experimentallyvalidated on simplified configurations by comparing results ob-tained from full-scale models and reduced-scale models, definedfrom a set of scale parameters according to the theoretical devel-opment above. First, numerical validation has been done bothconsidering dimensional model with the SYLVIA code and dimen-sionless model by implementing the model developed, given byEqs. (4) and (5), in the Matlab Software for simplified cases.

All simplified cases are composed of one room, one blowingbranch and one exhaust branch, as presented in Fig. 1 showinga view of the experimental models designed at two different scales.For designing models in laboratory, the three fixed scale parame-ters have been chosen according to experimental constraints. Thus,a reference-scale model, corresponding to a full-scale model, anda reduced-scale model have been sized up considering a roomvolume scale of 1/33, a velocity scale of 1=

ffiffiffi2

pand a flow rate scale of

1/27. Other scale parameters are determined by applying relationsgiven in Table 1.

A room volume of 0.9 � 0.9 � 0.9 m3 and ducts of 2.24 m lengthand 192 mm diameter are considered for the reference-scalemodel. Each reference-scale duct was equipped with a controldamper to adjust the global head losses and thus ensure theairflow resistance similarity. Both models were equipped withpressure taps, where the pressure is recorded by PSI sensors, at theinlet of the blowing duct, inside the room, and on each side oforifice plates located inside ducts for the volumetric flow ratemeasurements. Then, tests were done by connecting a fan witha variable-voltage controller to the inlet branch which generatessteady airflows. Results obtained on each model are finallycompared after applying the pressure scale and the flow rate scaleto the reduced-scale results. Comparisons presented in Table 2underline that reference-scale and reduced-scale results are ingood agreement and thus lead to the experimental validation ofthe methodology for steady flows.

Table 2Comparisons between reference-scale and reduced-scale results in steady state.

Referenced-scale Reduced-scale Relativedeviation

Inlet pressure (Pa) 306.7 � 1 Pa 315.7 � 2 Pa 2.93%Room pressure (Pa) 158.9 � 1 Pa 161.4 � 2 Pa 1.62%Inlet flow rate (m3/h) 419.8 � 8.4 m3/h 415.3 � 9.2 m3/h 1.06%Outlet flow rate (m3/h) 424.4 � 8.4 m3/h 415.7 � 9.2 m3/h 2.04%

N. Le Roux et al. / Building and Environment 58 (2012) 231e244 235

3. Cases studied

The methodology has been applied to two reference nuclearfacilities, the full-scale characteristics of which are used to definethe set of scale parameters that should be used to design thereduced-scale models. Thus, full-scale configurations are firstlypresented, followed by the choice of the scale parameters and bythe design of the reduced-scale models.

3.1. Full-scale configurations

Nuclear facilities can be composed of ten to several hundredrooms that are connected with a ventilation system to ensure thecontainment of pollutants inside the buildings. To contain thepollutants, the ventilation system is designed so that all of therooms are maintained in depression in comparison with theexternal pressure. Thus, external leakage flows are systematicallydirected from the outside into the buildings. For more hazardousrooms in terms of contamination, the depression is higher. Thus,internal leakage flows between the rooms are directed from theleast to the most hazardous room. These external and internalleakages mainly correspond to the airtightness defects of thebuilding’s doors. In addition, the containment of radioactiveparticles in the ventilation system is guaranteed with HEPA filters.To study representative industrial facilities, two simplified config-urations have been designed. Schematic diagrams of the configu-rations are presented in Figs. 2 and 3.

Both configurations are composed of four rooms equipped witha similar ventilation system. The main differences between theconfigurations are in the layout of the rooms and the positioning of

Fig. 2. Schematic diagram

the leaks. Configuration I represents a facility for which each roomis directly related to the outside. Wind effects will logically bemaximized for this configuration. In contrast, for the configurationII, rooms are interlocked such that only the least hazardous room isdirectly related to the outside. The full-scale room sizes are pre-sented in Table 3. The external room sizes are similar in eachconfiguration. However, the internal air volumes are differentbecause of the interlocking of the rooms in configuration II.

The ventilation system of each configuration consists of threeventilation systems: a blowing ventilation system for the fourrooms, an ambiance exhaust ventilation system for rooms A and B,and a process exhaust ventilation system for rooms C and D. Thelast system concerns the most hazardous rooms in terms of acci-dental radioactive releases. For this reason, HEPA filters are locateddirectly at the exhaust of rooms C and D, and blowing of room D.

Furthermore, each ventilation system is equippedwith a fan, thetheoretical operating point of which is given by the nominalpressures difference and flow rate. Approximately twenty venti-lated ducts are necessary to connect all of the components (rooms,HEPA filters and fans). The standard real size of one duct typicallyranges from 10 to 100m in length and from 0.2 to 1.0m in diameter.The theoretical head losses of the ducts, as for the HEPA filters andthe leaks, are defined by airflow resistances, which are calculatedfrom the nominal pressures and flow rates using Eq. (1) in steadystate. From these two reference configurations, a set of scaleparameters has been defined to design two reduced-scale modelsby applying the methodology developed in Section 2.

3.2. Set of scale parameters used

The three fixed scale parameters have been chosen consideringthe characteristics of the real configurations described above andthe experimental means used. First, for the volume scale definition,the maximum section of the real configurations considered isapproximately 370 m2 and the vena section of the wind tunnel isapproximately 25 m2. Thus, using Eq. (16), the volume scale shouldbe lower than 1/123.

Then, the flow rates inside the ducts are measured with orificeplates according to the specific standard AFNOR NFX 10-231 [16],which requires a minimum diameter of 5 cm. Furthermore, the full-

of configuration I.

Fig. 3. Schematic diagram of configuration II.

N. Le Roux et al. / Building and Environment 58 (2012) 231e244236

scale minimal flow rate is approximately 600 m3/h. Thus, using Eq.(17), the flow rate scale should be higher than 1/80.

Finally, a velocity scale equal to 1=ffiffiffi2

phas been considered in

order to simultaneously respect pressure and branch lengthconstraints discussed in Section 2.4. Thus, the set of scale param-eters, presented in Table 4, has been specifically defined for thecases studied by applying these three hypotheses on the relationsarising from the theoretical development of the methodology (Eqs.(8)e(10) and Eqs. (12)e(15)).

3.3. Design of the reduced-scale models

The reduced-scale characteristics of each configuration havebeen defined by applying the scale parameters shown in Table 4. Aview and a plan of each model, designed from these characteristics,are presented in Figs. 4 and 5.

For practical reasons, the ventilation system was placed undera raised floor. Because of the geometrical scale parameters, theinternal room sizes are too small to place the ventilation systeminside the rooms, and the internal air volume would be stronglymodified by such a placement. Furthermore, the ventilation systemcannot be placed outside at the same height as the rooms because itwill modify the wind-generated flow on the building architectureand more precisely on the external communications (externalleaks, ventilation system inlet and exhaust chimney). For thesereasons, the ventilation was placed under the rooms, which arelocated on a 0.75m high raised floor. Thus, the inlet and the exhaustducts are connected to the floors of the rooms. For the configurationII, the interlocking of the rooms cannot be conserved due to thesame reason of room obstruction. Thus, the interlocked rooms

Table 3External sizes and internal air volumes of the rooms in configurations I and II.

Room Size (m3) Internal air volume (m3)

Configurations I and II Configuration I Configuration II

Room A 15 � 15 � 15 m3 3375 m3 1647 m3

Room B 12 � 12 � 12 m3 1728 m3 1385 m3

Room C 7 � 7 � 7 m3 343 m3 218 m3

Room D 5 � 5 � 5 m3 125 m3 125 m3

(rooms B, C and D) were also placed under the wind tunnel floor.Note that only the layout of the internal rooms is changed, theexternal architecture and the leaks between the rooms are stillconserved.

Next, several components of the models (rooms, fans, leaks,filters and ducts) are briefly described. The reduced-scale internalair volume of each room is defined by applying the volume scale Vto the full-scale values presented in Table 3. The correspondingreduced-scale values are presented in Table 5.

Each fan, equipped with a variable frequency controller, waschosen to satisfy the theoretical reduced-scale operating pointspresented in Table 6. These values are determined by applying thepressure scale P and the flow rate scale Q to the full-scale operatingpoints. This implies an assumption on the fans behaviour, becausethe fans were chosen knowing only the theoretical operating pointsand not the theoretical fan curves.

Internal leaks between the rooms and external leaks are rep-resented by calibrated perforated plates, which holes are defined toobtain exponent laws that represent the real leaks. Thus, the lawexponent n (Eq. (1)) should range from 1.35 to 1.7 [6,9,10]. To obtainthese values, the ratio Lp/Dp should typically be higher than unity,where Lp and Dp correspond here to the plate thickness and thehole diameter, respectively. Thus, after several preliminary tests,a 6 mm thick plate composed of holes with a diameter of 1.2 mmhas been chosen to represent each leak. The number of holes is thendetermined to ensure the reduced-scale airflow resistance of eachleak. The number of holes varies from 14 to 1028, depending on theleak considered. Finally, each perforated plate was rigorously

Table 4Set of scale parameters used to design the reduced-scale models.

Fig. 4. View and plan of the reduced-scale model for configuration I.

Fig. 5. View and plan of the reduced-scale model for configuration II.

N. Le Roux et al. / Building and Environment 58 (2012) 231e244 237

calibrated to obtain the exact exponent law of each leak, as givenfor an example in Fig. 6.

The airflowbehaviour through the HEPA filters in the ventilationsystem was reproduced by using real glove box HEPA filters, sincetheir airflow resistances are similar to the theoretical reduced-scalevalues needed to ensure the similarity of filters head losses. HEPAfilters are pleated filters, the airflow behaviour of which is linearonly in a small range of flow rates, corresponding to a filtrationvelocity less than 5 cm/s (the nominal operating filtration velocityis about 2 cm/s) [17]. Furthermore, the glove box filters are locatedinside an airtight box connected to the inlet and the outlet venti-lated ducts. Consequently, the behaviour of the airflow throughthese filters cannot be modelled by a linear law on a wide range offlow rates. For this reason, each filter was calibrated directly on thereduced-scale models (Fig. 6).

Finally, each ventilated duct consists of a standard PVC duct, thediameter and the length of which were defined to represent the

Table 5Reduced-scale internal air volumes of the rooms for configurations I and II.

Room Reduced-scale internal air volume (m3)

Configuration I Configuration II

Room A 1.25 � 1.25 � 1.25 m3 0.98 � 0.98 � 0.98 m3

Room B 1 � 1 � 1 m3 1.08 � 1.08 � 0.7 m3

Room C 0.58 � 0.58 � 0.59 m3 0.5 � 0.5 � 0.5 m3

Room D 0.42 � 0.42 � 0.41 m3 0.42 � 0.42 � 0.41 m3

aforementioned real sizes. Each duct is equipped with an orificeplate to determine the volumetric flow rates. To use this metrologyaccurately, a straight length equal to 5D upstream and 4D down-stream has been considered on each side of the orifice plate [16].The duct design (length, bends, reductions, and so on) is alsorestrained by the necessity to connect all of the components underthe wind tunnel floor. Finally, each duct is equipped with a controldamper to adjust the global head losses and thus ensure the airflowresistance similarity. As performed for the leaks and the filters, theairflow behaviour of the ducts was characterized before the windtunnel experimentation to check the quadratic behaviour hypoth-esis even for low Reynolds number due to the high level of singu-larity head losses (Fig. 6).

For the characterization steps and the wind tunnel tests, themodels were equipped with more than 130 pressure taps where thepressure is recorded by PSI sensors at 200Hzwith appropriate tubingsystem designed to act as a low-pass filter on the required frequencyrange [18]. The pressure was measured at each internal node (rooms

Table 6Reduced-scale operating points of the fans for configurations I and II.

Fan Reduced-scale operating points (m3/h; Pa)

Configuration I Configuration II

Blower fan 145 m3/he500 Pa 145 m3/he500 PaAmbiance exhaust fan 145 m3/he575 Pa 123.75 m3/he575 PaProcess exhaust fan 34.4 m3/he900 Pa 28.75 m3/he900 Pa

Fig. 6. Example of reduced-scale calibrated laws obtained for a leak (left), a HEPA filter (centre) and a ventilated duct (right).

N. Le Roux et al. / Building and Environment 58 (2012) 231e244238

and junctions between the ducts) and each boundary condition node(external pressure measured at the ventilation system inlet, theexhaust chimney outlet and the outside of each external leak). Foreach measurement, several pressure taps were used to ensurehomogeneity or the absence of defects within the taps. Two pressuretaps were also located on each side of the orifice plates to determinethe volumetric flow rates inside the ducts. Finally, the volumetricleakage flow rates were measured using the calibrated laws of eachperforatedplate. Details on the laboratorycalibrationof thesemodelswithout considering wind effects are provided in Le Roux [11].

4. Wind tunnel experimental results

Experiments were carried out in the Jules Verne climatic windtunnel of the CSTB. Steady and transient tests were conducted foreach configuration, but this article focuses on the steady stateresults to deal with the experimental analysis and the numericalcomparisons in detail.

Each configuration has been studied for several wind incidencesand wind velocities. The wind velocity was recorded with a Pitot-static tube 50 cm from the ceiling of the wind tunnel. Bothconfigurations were tested with the mechanical ventilation on andoff. From each test, the mean values of the nodal pressures and thebranch flow rates were then determined. In the following sections,the full-scale results are presented as a function of the wind inci-dence in the form of rose diagrams. This representation enables theinfluences of the wind velocity and the wind incidence to bevisualized simultaneously.

4.1. Physical analysis of the experimental results for configuration I

For configuration I, the external building architecture and thelocations of the external communications subjected to wind effectsare presented in Fig. 7. The relative room pressure values presentedin this figure correspond to the nominal values (without windeffects). Eight wind incidences (each 45�) and three wind velocities(21, 29 and 36 m/s) were studied for this configuration.

Depending on the wind incidence, overpressures or depressionsare generated by the wind on the external communications, i.e., theventilation system inlet, the exhaust chimney and the externalleaks. The external pressure coefficients, defined by the ratiobetween the external pressures and the dynamic pressure gener-ated by the wind, are presented in Fig. 7, for the tests conductedwith the mechanical ventilation on (values are logically similarwith the mechanical ventilation off). In these rose diagrams, thebold circle represents the pressure coefficients equal to zero, and

thus the curves inside this circle correspond to the depressions.These results show that the external pressure coefficients areindependent of the wind velocity. Thus, the extrapolation of theresults to the full scale is verified. The pressures on the exhaustchimney are slightly different because of the pressure tap locationsinside the exhaust duct. The internal measures are influenced bythe wind, but they are also influenced by the flow rate inside thechimney. The external leak of roomD for awind incidence of 270� isalso slightly different, because this leak is located in a flow zonethat is strongly disturbed by room B. However, these differenceshave no influence on the treatment of the experimental results andthe numerical simulations because the results are systematicallytreated from the external pressures measured for each test.

The internal flows, characterized by the nodal pressures and thebranch flow rates, are directly influenced by the external pressurefields presented in Fig. 7. The most significant influence is observedfor the leakageflow rates, forwhichflow inversions occur, dependingon the wind conditions and the mechanical ventilation functioning.

4.1.1. Results with the mechanical ventilation onAs an example, Fig. 8 presents the external leakage flow rates of

room A and the internal leakage flow rates between rooms A and C,obtained for each wind condition tested with the mechanicalventilation on. In this graph, the nominal and the null values arerepresented by a red circle and a black circle, respectively. Thus,reversal flow rates, which are characterized by negative flow rates,correspond to values located inside the black circle.

In general, reversal leakage flow rates appear when the windeffects simultaneously generate high depressions on the leak andoverpressures on the ventilation system inlet or the other leaks.

To illustrate this conclusion, the inversions observed for theexternal leak of room A with a wind incidence of 0� are explainedbelow. Firstly, the ventilation system inlet is located upwind and isthus located in an overpressure field. Therefore, the ventilatedblowing flow rates increase by approximately 40% inside thegeneral blowing duct for a wind velocity of 36 m/s. Secondly, a highdepression is generated on the external leak of room A because ofits perpendicular position in relation to the wind direction (pres-sure coefficient of approximately �0.8). Thus, simultaneouslyconsidering both effects lead to a reversal flow rate. The inversion isincreased while increasing the wind velocity, implying an internalreversal flow rate between rooms A and C.

Reversal flow rates are also observed for a wind incidence of180�. In this case, the external leak of room A is still in depression,and the external leak of room C is upwind. Thus, the leakage flowrate of room C increases, implying an increased pressure in the

Fig. 7. External building architecture of configuration I and pressure coefficients generated by wind effects on the external communications (ventilation system on, configuration I).

Fig. 8. External leakage flow rates of room A and internal leakage flow rates between rooms A and C (mechanical ventilation on, configuration I). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)

N. Le Roux et al. / Building and Environment 58 (2012) 231e244 239

N. Le Roux et al. / Building and Environment 58 (2012) 231e244240

room. This effect, combined with the depression effects on theexternal leak of room A, leads to reversal flow rates of the internalleak between these two rooms and the external leak of room A.

In conclusion, wind effects generate overpressures and depres-sions on the external architecture, leading to reversal leakage flowrates for each room, except for room D when the wind velocity isless than 40 m/s. However, for a wind incidence of 315�, the leakageflow rate of room D decreases with increasing wind velocity anda reversal flow rate appears for a wind velocity of 42 m/s (notpresented in the above figures). Thus, the mechanical ventilationenables the pollutant containment of the most hazardous room interms of contamination (room D) to be maintained, even for rela-tively high wind velocities (approximately 40 m/s).

4.1.2. Results with the mechanical ventilation offThe same wind conditions were studied with the mechanical

ventilation system off. The external pressure coefficients are similarto those obtained with the mechanical ventilation system on(Fig. 7). The influence of the wind on the pressures and the flowrates are almost similar, but the flow rates are an order of magni-tude lower because of the absence of mechanical ventilation. Thus,supplementary reversal flow rates have been identified, notably forroom Dwhich corresponds to the most hazardous room in terms ofcontamination. Fig. 9 presents the corresponding internal and theexternal leakage flow rates obtained for this room with themechanical ventilation off.

In addition, the simultaneous effects of a depression on theventilation system inlet and overpressures on the external leaks ofrooms lead to reversal flow rates inside the blowing ventilationsystem when the system is off. Therefore, these results highlightthe importance of HEPA filters located inside the blowing system,enabling the containment a particle pollutant to be maintained.

4.2. Physical analysis of the experimental results for configuration II

The previous study of configuration I, which represents a facilitythat highly interacts with the outside, has shown that wind effectscan often lead to a loss of containment of pollutants located insidethe rooms, even for the contamination source room (room D) whenthe mechanical ventilation system is off. However, in the nuclearindustry, rooms are most often interlocked to reinforce thecontainment of the most contaminated room (room D), as repre-sented in the second configuration studied.

Wind influence on configuration II has been studied for fourvelocities, ranging from 22 to 42 m/s, and four incidences (every

Fig. 9. External and internal leakage flow rates of roo

90�). The building positioning compared with the wind direction isillustrated in Fig. 10. The room pressure values correspond to thenominal values obtained by considering only the mechanicalventilation on.

The same rose representation is used to show the influences ofthe velocity and the incidence simultaneously. However, the valuesare only marked with symbols because only four wind incidencesare considered for this configuration. Fig. 10 presents the externalpressure coefficients due to wind effects on the leak of room A, theventilation system inlet and the exhaust chimney. The same coef-ficients are obtained for each velocity and with the mechanicalventilation on and off.

4.2.1. Results with the mechanical ventilation onThe most and the least impacted leakage flow rates obtained

with the mechanical ventilation on, respectively corresponding tothe external leakage of room A and the internal leakage betweenrooms C and D, are presented in Fig. 11.

Here again, reversal flow rates are identified and depend on thedepression and overpressure fields generated by the wind on theexternal communications. The most important result is that theleakage flow rates of the contamination source room are onlyslightly influenced by wind effects. The leakage flow rates betweenrooms C and D are very similar to the nominal values. Thus, thecontainment of pollutants inside the contamination source room isstill conserved.

4.2.2. Results with the mechanical ventilation offSimilar conclusions have been made for results obtained with

the mechanical ventilation off. The corresponding leakage flowrates are presented in Fig. 12, indicating that the containment ofpollutants inside the contamination source room remains ensured,even with wind velocities up to 42 m/s and without mechanicalventilation. However, the values of flow rates are lower than thoseobtained with the mechanical ventilation on and are close to thenull values.

In general, the results obtained for this configuration show thatthe depression generated at the exhaust chimney is sufficientlyhigh to create the natural draught necessary to maintain thecontamination source room in depression.

In conclusion, analysis of the experimental results has shownthe importance of taking wind effects into account when studyingthe containment of pollutants inside nuclear facilities. Thus, it isimportant to consider these wind phenomena when assessing thesafety of industrial buildings, for which pollutant containment is

m D (mechanical ventilation off, configuration I).

Fig. 10. External building architecture of configuration II and pressure coefficients generated by wind effects on the external communications (mechanical ventilation on,configuration II).

N. Le Roux et al. / Building and Environment 58 (2012) 231e244 241

necessary. To assess the safety of the buildings, zonal codes used,such as the SYLVIA code for the nuclear industry, should be vali-dated. To check the robustness of the SYLVIA code, the experi-mental results have been compared with the numerical results.

5. Comparison of numerical and experimental results

For each configuration, numerical simulations were conductedwith the SYLVIA code. First, numerical configurations were defined

Fig. 11. External leakage flow rates of room A and internal leakage flow rate

using the experimental results in the nominal situation, corre-sponding to the node pressures and the branch flow rates obtainedby only considering the mechanical ventilation. Then, the influenceof the wind was considered for these nominal configurations usingthe mean external pressure values as the boundary conditions foreach external communication (ventilation system inlet, exhaustchimney and external leaks).

For each simulation, nodes pressures (23 values for each test),ventilated flow rates (12 values) and leakage flow rates (7 or 4

s between rooms C and D (mechanical ventilation on, configuration II).

Fig. 12. External leakage flow rates of room A and internal leakage flow rates between rooms C and D (mechanical ventilation off, configuration II).

N. Le Roux et al. / Building and Environment 58 (2012) 231e244242

values, depending on the configuration) were determined andcompared with experimental values. All of the experimental testscarried out, depending on the wind velocity, the wind incidenceand the mechanical ventilation functioning, were simulated withthe SYLVIA code.

The results are presented in the form of graphs showing thenumerical values of pressures, ventilated flow rates and leakageflow rates as a function of the experimental values for all of thetests. The figures presented below correspond to the results ob-tained for configuration I, but similar conclusions can bemade from

Fig. 13. Experimental and numerical comparisons of node pressur

the results of configuration II [11]. In the following sections, theexperimental and the numerical comparisons are discussed for theresults obtained with the mechanical ventilation on (Section 5.1)and off (Section 5.2).

5.1. Comparisons with the mechanical ventilation on

Fig. 13 compares the experimental and the numerical values ofnode pressures and ventilated and leakage flow rates, obtained for

es and flow rates (mechanical ventilation on, configuration I).

N. Le Roux et al. / Building and Environment 58 (2012) 231e244 243

configuration I with the mechanical ventilation on and with thewind conditions studied experimentally.

In general, Fig. 13 shows that experimental and numericalcomparisons with the mechanical ventilation on are similar.Furthermore, a standard deviation calculation supports the goodagreement observed in these figures. Indeed, the relative pressuredeviations are less than 10% for 90% of the pressure values and thedeviations in the ventilated flow rate are less than 5% for more than99% of the values. The deviations in the leakage flow rates are moresignificant, with 15% of the leakage values having and deviations ofover 20%. However, leakage flow rates are determined from pres-sure measurements using the pressure law of each perforated plate.These plates were calibrated on a specific bench before the windtunnel experiments. These laws show that a pressure deviation ofapproximately 5% associated with a low pressure drop leads tosignificant deviations.

Thus, the identified deviations are mainly due to metrologyuncertainties, potential airtightness defects in the models andassumptions used to define the numerical configurations. Thesenumerical configurations were defined from experimental nominalconfigurations, i.e., without considering wind effects. However,because of metrology uncertainties and potential airtightnessdefects, the flow rate conservation, in the junctions and the rooms,is not strictly equal to zero. Thus, deviations of approximately 5%(configuration I) and 8% (configuration II) are introduced betweenthe nominal values of the experimental and the numerical flowrates in order to ensure numerical flow rate conservation. Thesemodified values are then used to define the airflow resistance ofeach branch, and so differences are potentially introduced betweenthe real and the numerical head losses. However, despite these

Fig. 14. Experimental and numerical comparisons of node pressur

experimental uncertainties and numerical assumptions, experi-mental and numerical results are in very good agreement when themechanical ventilation is on.

5.2. Comparisons with the mechanical ventilation off

With the mechanical ventilation off, the numerical configura-tions were defined by modifying the airflow behaviour of the fans.Thus, the fans were modelled with a quadratic law, which wasdetermined in a calibrated bench. Fig. 14 compares the results ob-tained with the mechanical ventilation off for the pressures and theventilated and leakage flow rates.

Fig. 14 shows that the numerical and the experimental nodalpressures are similar, evenwith the mechanical ventilation off. Thestandard deviations of the pressures are less than 10% for 90% of thevalues. Fig. 14 also shows that the numerical and the experimentalleakage flow rates are in good agreement, similar to the compari-sons with the mechanical ventilation on. However, relatively largediscrepancies are observed for some ventilated flow rates, due toseveral reasons explained below.

First, uncertainties are strongly increased for low flow ratesdepending on the orifice plate characteristics. Flow rate uncer-tainties are determined from the uncertainties of the pressuresensors used, defined to �1 Pa from a calibration conducted in thewind tunnel before the experiments, but theoretically consideredto be �2.5 Pa. This lower uncertainty makes it possible to considerthe proportion of deviations attributable to the uncertainties andalso the other sources of discrepancies. In particular, doubledpressure taps were placed upstream and downstream of eachorifice plate, on which variations are identified because of the low

es and flow rates (mechanical ventilation off, configuration I).

N. Le Roux et al. / Building and Environment 58 (2012) 231e244244

velocity inside the ducts when the mechanical ventilation is off.Although these variations remain low (approximately 2 Pa), theyare associated with low pressure drops along the orifice plate (e.g.,variations of approximately 2 Pa associated with a pressure drop of5 Pa). These variations can be due to airflows that may not becompletely established inside the ducts for low velocities. To avoidthese variations, higher straight lengths should be used. Further-more, orifice plates with smaller diameters should also decreasethe uncertainties in the flow rates. However, because of theoreticalreduced-scale lengths and head losses imposed by the method-ology application on the real ventilation system, more accurateorifice plates cannot be used to improve the measurements withthe mechanical ventilation off.

Deviations are also attributable to the hypothesis used to definethe numerical configurations with SYLVIA. As mentioned previ-ously for the results with the mechanical ventilation on, theexperimental flow rates in the nominal situation have beenmodified to define the airflow resistances of each branch byensuring the mass conservations for the junctions and the rooms.Errors may also arise from the numerical modelling of othercomponents (fans, filters and leaks), which are defined from cali-brated laws associated with experimental uncertainties.

To conclude the discussion on the results obtained with themechanical ventilation off, the numerical and the experimentalresults are in good agreement, except for several ventilated flowrates, for which the source of discrepancies is mainly experimental.Thus, the zonal code SYLVIA can take into account wind effects insteady state and thus determine the reversal leakage flow rates thathave been identified experimentally.

6. Conclusions and perspectives

The importance of considering the influence of wind on airflowsinside mechanically ventilated buildings has been highlighted byconducting reduced-scale experiments. To conduct these experi-ments, a newmethodology has been developed to study isothermalflows in steady and transient states inside buildings equipped withventilation systems. This methodology was then applied to tworeference configurations representing nuclear facilities, for whichthe influence of wind has been studied in a wind tunnel.

The experimental study on the influence of wind has shown theimportance of taking wind effects into account when assessing thesafety of buildings, for which the pollutant containment must beensured. Many reversal leakage flow rates have been identifieddepending on external overpressures and depressions generated bythe wind. These reversal flow rates lead to a partial or a total loss ofthe pollutant containment inside the buildings. However, note thatthe containment of configuration II, which corresponds to the mostrealistic nuclear configurationwhere the containment is reinforcedby interlocking rooms, remains ensured even for high windvelocities (up to approximately 150 km/h) and with the mechanicalventilation off.

To take wind effects into account in the safety assessment ofnuclear facilities, the robustness of the SYLVIA code has beenverified by comparing experimental and numerical results. Even ifsome deviations have been underlined in specific cases because ofexperimental uncertainties, notably those for ventilated flow rateswith the mechanical ventilation off, the results are in good

agreement. Thus, the SYLVIA code can now be used to study thepollutant containment of buildings subjected to wind effects insteady states. However, the main difficulty for taking into accountwind effects on real facilities will be the definition of the externalpressure generated by wind effects that should be used asboundary conditions.

Another important aspect that should be considered corre-sponds to the transient flows that are generated by both windturbulence and internal overpressure due to an accident (e.g.,pressure tank fracture). The methodology has been developed totake these transient phenomena into account. Thus, experimentswere also conducted to study the transient flows inside ventilationsystems and will be discussed in detail in a future paper.

Finally, the methodology developed is viable considering themain assumption of isothermal flows, whereas heat transfers mayarise and modify internal airflows notably during an accident suchas a fire. A new research project is currently beginning to considerermass transfers due to thermal effects inside mechanically-ventilated industrial buildings.

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