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Energy and Buildings 86 (2015) 502–513

Contents lists available at ScienceDirect

Energy and Buildings

j ourna l ho me pa g e: www.elsev ier .com/ locate /enbui ld

FD modeling for natural ventilation in a lightwell connected toutdoor through horizontal voids

areq Gaber Fareaa,∗, Dilshan Remaz Ossena, Saqaff Alkaffb, Hisashi Kotanic

Department of Architecture, Faculty of Built Environment, Universiti Teknologi Malaysia, 81310 UTM, Johor, MalaysiaDepartment of Mechanical, Faculty of Engineering & Technology, Multimedia University, MalaysiaDepartment of Architectural Engineering, Graduate School of Engineering, Osaka University, Japan

r t i c l e i n f o

rticle history:eceived 23 October 2013eceived in revised form 8 October 2014ccepted 14 October 2014vailable online 23 October 2014

eywords:atural ventilationightwell

a b s t r a c t

Lightwell or deep courtyard is commonly used in high-rise deep layout plan buildings and are usuallyimplemented to admit daylight and to induce natural ventilation into targeted spaces. Upward flow inthe lightwell is dominant due to buoyancy effect; however, direct connection to the outdoor throughhorizontal void may assist or oppose the upward airflow in the lightwell. This article mainly investi-gates the effect of different vertical and horizontal positions of the void connected to the lightwell onthe upward flow with different wind directions. Computational Fluid Dynamics (CFD) was employed tosimulate the airflow inside the lightwell. The simulation results were validated and confirmed a goodagreement with the experimental data. The configuration of the lightwell connection clearly indicate the

orizontal voidigh-rise buildingsFD

strong influence on upward flow in the internal lightwell; however, the double-level void in cross-flowconfiguration was found more effective than other configurations in terms of airflow velocity and tem-perature. Wind direction is an important parameter which influences air flow patterns and velocity inthe lightwell. This study provides proper guidelines to predict ventilation performance and to improvethe design of naturally ventilated high-rise buildings.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Natural ventilation is considered one of the most fundamentalow cost passive cooling strategies [1,2], contributing to reductionsn energy consumption without compromising thermal comfortnd the air quality inside the building [3]. Natural ventilation cane provided inside and around buildings by wind-alone, buoyancy-lone or a combination of both, which is defined as naturalonvection flow [4–6]. Natural convection flow is an effectiveeans of providing indoor ventilation in buildings when wind and

uoyancy have the same direction [6,7]. In order to simplify therediction of steady natural convection flow characteristics in theuildings, Etheridge classified two main types of openings withespect to aspect ratio L/dh, where L is the length parallel to stream-ines and dh is the cross size that flows perpendicular [5]. In cases

here the aspect ratio is very low (e.g. conventional windows) the

ow pattern is separated at the edges of the opening. Therefore, inase with lower L/dh value, most studies utilized the still-air dis-harge coefficient (Cd) while in long openings (large L/dh value)

∗ Corresponding author. Tel.: +60 14 2382465; fax: +60 7 5557411.E-mail address: tareqgfarea@outlook.com (T.G. Farea).

ttp://dx.doi.org/10.1016/j.enbuild.2014.10.030378-7788/© 2014 Elsevier B.V. All rights reserved.

the Reynolds number dependency is important [8]. Most previ-ous studies were conducted in isolated spaces, e.g. simple roomshave small openings, with very low value of L/dh [4,9]. Researchtoday pays more attention than before to the study of special bio-climatic design strategies with very large L/dh values, for examplewind tower [10] and courtyard [11].

Internal lightwells have very large L/dh value and usually imple-mented in buildings with a deep layout plan in order to providenatural ventilation and daylight to inner spaces which are far fromthe perimeter of the building. The lightwell within high-rise build-ings may act only as ventilation shaft [12,13]. In order to guaranteean effective ventilation performance, most building codes prescribethe minimum lightwell size in relation to the building height [13].However, there are other important factors in determining the air-flow performance in the lightwell such as heat transfer from wallsurfaces of the lightwell and adjoining spaces through windows(openings with lower L/dh value) and connections to the outdoorthrough horizontal voids (openings with larger L/dh value). In fact,the idea of connecting lightwells to the outdoor through horizontal

voids is not new, and has been the focus of many previous stud-ies [13–17]. Horizontal voids are usually found in the first floorin high-rise buildings and serve multi-purpose uses such as carparks [18]. Moreover, horizontal voids are found in the middle

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T.G. Farea et al. / Energy an

oors of high-rise buildings, and are used as refuge floors [19],ervice floors [18], wind floors (e.g. in Liberty Tower, Japan) andky gardens in luxury residential buildings (e.g. The Met, Thailand).herefore, if lightwells are appropriately and directly connected tohe outdoor through horizontal voids (e.g. appropriate position andize), the outdoor wind flow will provide the buoyancy-inducedow in the lightwell [20,21]. However, the connection configura-ion between the lightwell and horizontal void could oppose theuoyancy-induced flow and thus decrease the velocity of upwardow [6,9]. These possibilities are required more investigations toptimize the appropriate configuration for providing airflow in theuilding, particularly when both spaces come with very large L/dhalue.

The CFD applications have become the most popular in build-ng ventilation studies [1,22], particularly the validation models23,24]. Many studies are based on validated CFD for modelingatural convection flow in connected spaces (large spaces linkedetween several rooms or floors) in high rise buildings. It wasound that most of these studies were conducted in atrium spacee.g. [7,23,25,26]) and solar chimneys [27,28] but not in lightwells.he airflow mechanism in these spaces (in particular, lightwellsnd atriums), is almost the same, with the most important differ-nce being that the lightwell opens to the outdoor through theky opening, making the flow inside the lightwell more complexnd uncontrolled. However, the literature indicates that only semi-nclosed (re-entrant) and attached lightwells were investigatedn many previous studies [15,24,29]. Internal lightwells, however,ave the worst position in terms of connection to the outdoor, butery few studies have been conducted to investigate their influencen airflow.

Kotani et al. [16,30,31] conducted many experimental high-riseuildings models in a wind tunnel, and empirical models were usedo investigate the airflow characteristics inside an internal lightwellonnected to the outdoor through a horizontal void. The abovetudies investigated the effect of the heat magnitude and its sourceosition, air velocity and void size on the airflow rate, air temper-ture and airflow pattern in lightwells. On the other hand, studiesonducted by Gan [32,33] in vertical enclosures investigated theffect of enclosure size and the limited number of inlet positionsith different heat values and positions. Although the results are

ased on CFD simulations, the experiments were limited to twoimensional models and considered only a single wind direction.

A recent study by Chiang and Anh examined the effect of sev-ral void positions on air-velocity and temperature gradient in annternal lightwell of a high-rise building using CFD simulations [15].he number of horizontal and vertical void positions were limitedo bottom, top, cross-flow and windward positions, and considerednly a single wind direction. Another recent study, also based onFD, evaluated the natural convection flow in lightwell in a high-ise office building and compared it with segmented lightwellsby horizontal void) and isolated space configurations [20]. Thetudy indicated that potential lightwells (either segmented or non-egmented) are more effective than isolated space in terms ofroviding increase airflow rate. However, segmented lightwells arereferred as they reduce recirculation flow which is common inon-segmented, particularly those found in the bottom and toparts of the lightwell.

The above studies indicate that there is a lack of extensive andetailed sensitivity studies involving CFD simulation of natural con-ection flow in lightwells. The simulation results are very sensitiveo a wide range of computational parameters and settings that haveo be set by the user of CFD. Therefore, detailed and generic sensitiv-

ty studies of the influence of these parameters, e.g. computationalomain size, grid resolution and turbulence models on the simu-

ation results are required to provide best practice guidelines forsing CFD with more accuracy and reliability [34,35].

ldings 86 (2015) 502–513 503

The current study used a small-scale model that was testedexperimentally by Kotani et al. [16], and reproduced in a CFD sim-ulation with two objectives. The first is to perform a validation andsensitivity analysis to verify the reliability and accuracy of CFD inpredicting the natural convection flow inside the lightwells in high-rise buildings by comparing the results with experimental data. Thesecond objective is to examine the effect of the void positions andwind direction on the upward flow inside the lightwell. The resultsof this study will contribute toward identifying appropriate voidpositions in order to provide upward-cross flow in lightwells. Thiswill be preferred to removing heat from the core of the building,thus providing for better air quality and thermal comfort eitherin the lightwell space as unoccupied space or in adjoining indoorrooms as occupied space.

2. Wind tunnel experiment (WTE)

Kotani et al. conducted a wind tunnel experiment (WTE) usinga small-scale model for a high-rise building lightwell connectedwith bottom void [16]. The experiment predicted the air temper-ature and airflow rate as well as airflow pattern in the lightwell.The building model (1:250) represents a real forty-storey high-rise building with square plan dimension of 36 m × 36 m × 120 m(width × depth × height). The lightwell in the core of the buildingpass vertically through all the floors without any openings exceptthe bottom lateral horizontal void (one side open in windwarddirection) and the top vertical outlet (sky opening). The buildingmodel used in the WTE was scaled to 1:250. The outline of themodel therefore has dimensions of 144 mm × 144 mm × 480 mm.The volume of the lightwell and the area of the horizontal void are72 mm × 72 mm × 468 mm and 72 mm × 12 mm (width × height)respectively as illustrated in (Fig. 1). The experimental conditionswere specified in terms of rate of heat generation in the lightwell,ranging from 10 to 40 W, and wind velocity in the wind tunnelwhich ranged from 0 to 1.5 m/s. The wind direction in all cases wasperpendicular to the windward fac ade where the horizontal voidwas located. To measure the air temperature, five thermocoupleswere set vertically in the lightwell. More detailed information aboutthe experimental settings can be found in Kotani et al. [16].

3. CFD validation and sensitivity analysis

A commercial CFD code ANSYS Fluent was used to simulateairflow and temperature in the lightwell. Steady-state airflowand heat transfer through a three-dimensional model was car-ried out. This section discusses the execution of the methodutilized to validate and compare the CFD simulation models withWTE model developed by Kotani et al. [16] Computational set-tings and parameters for reference case and sensitivity analysisare performed. The reference case settings are based on com-mon and best practice guidelines of computational parameters.A systematic sensitivity analysis was conducted on the refer-ence case to study the impact of a wider range of computationalparameters on the reference case. All computational parame-ters for reference case and sensitivity analysis are shown inTable 1.

3.1. Flow domain and geometry

A building model with dimensions similar to the model used inthe WTE study has been used in the CFD validation. The compu-

tational domain of the reference case, however, was set accordingto best practice guidelines (e.g. [7,36,37]). The windward distance(from building surface to inlet) and leeward distance (from build-ing surface to outlet) were set as 5H and 10H, respectively, where

504 T.G. Farea et al. / Energy and Buildings 86 (2015) 502–513

Fig. 1. Perspective view of the computational domain size used in numerical study shows the building model location and possible wind direction inlet and building modeldimensions used by Kotani et al. [12].

Table 1Conditions of computational flow parameters for sensitivity analysis with indication of the reference case conditions.

Grid discretization Domain size Wall Heat source Residuals Turbulence M.

A (3.89 × 105) Recommend Standard WF Surface flux 10−5 RNG k–εB (5.51 × 105) Wind tunnel Enhanced WT Space volume 10−8 R k–εC (9.32 × 105) SST k–ω

N n. WF

Htr(htttdrtatam

oFtatr

3

tvottduc

accuracy and SIMPLE solution algorithm were used for the con-vection terms while the least square cell-based gradient for spatialdiscretization was used by the simulation for the diffusion terms ofthe governing equations.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

H(m

)

T o

Sm

Reco

Exp.

(Q=2.76)

(Q=2.61)

(Q=2.80)

oC

all

omm end

ote: Except first row, it is not necessarily each row used together in one simulatio

is the height of the building. The lateral and the distance fromhe top of the building to the top of the domain were set as 5H. Theesulting computational domain size is 7.344 m × 7.344 m × 2.88 mwidth × depth × height). Since the recommended size does notave the same dimensions as the wind tunnel, another computa-ional domain with similar cross section dimensions to the windunnel, 1.4 m × 1.6 m (depth × height) was modeled for the sensi-ivity analysis. However, the size of the cross section of the inflowomain should be commensurate with recommended blockageatio [36,38]. The blockage ratio of the cross section of the windunnel and surface building area was about 3.1% which indicatesn acceptable maximum value. Taking into account the computa-ional time, the windward and leeward distances were set at 2Hnd 4H, respectively, and the cross section of the wind tunnel wasaintained.Airflow rate and temperature results in the lightwell of each

f the two domain sizes were compared with the WTE data.ig. 2 shows that the recommended size domain produced resultshat are closest to the experimental data. In order to examinell computational domain candidates under the same conditions,hey were discretized with the same mesh size and cells growthate.

.2. Grid discretization

The computational domain was discretized using an unstruc-ured grid with high resolution in the lightwell and the horizontaloid, where high-gradient flow zones exists (Fig. 3). A growth ratef 1.1% in all directions from these zones was implemented forhe meshes in whole domain. The spatial domain was divided into

etrahedral elements. Grid sensitivity test was carried out with aifferent numbers of cells. Therefore, three mesh types are madep of 3.89 × 105 (Grid A), 5.51 × 105 (Grid B), 9.32 × 105 (Grid C)ells. Simulation results of the above mesh types were compared

SST

, wall functions; WT, wall treatment; M, model.

with the WTE data. Fig. 4 shows that Grid A is appropriate to obtainreasonable air temperature and airflow rate results in the lightwellas compared to the WTE results with less computational time. Itshould be noted here that the upwind scheme of second-order

5 10 15 20 25 30

Fig. 2. Temperature distribution (T) and airflow rate (Q) along the height (H) of thelightwell in small (wind tunnel size) and recommended simulation domain sizescompared with experimental data.

T.G. Farea et al. / Energy and Buildings 86 (2015) 502–513 505

re ela

3

3

cfog(

tt

k

ε

Ft(

Fig. 3. Grid discretization of the whole domain (a) with mo

.3. Boundary conditions

.3.1. Inlet wind profileInlet wind profile was generated in computational domain

orresponding to the profile generated in the WTE. Therefore, one-ourth power law profile was generated for a reference velocityf 1.0 m/s and the results show that the numerical profile is inood agreement with the wind profile generated in the WTE modelFig. 5).

Based on turbulent intensity (Iu) profile which was generated inhe WTE, k and ε profiles were vertically generated in the compu-ational domain inlet by introducing the following equations [36]:

(y) = (Iu(y)U(y))2 (1)

1/2 Uref

(y

)(˛−1)

(y) = C� k(y)yref

˛yref

(2)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

55 10

H(m

)

Grid B

15

Grid

Gri

20 25

T

d C

Exp

id A

30

(Q=2.76)

A (Q=2.76 )

B (Q=2.65)

C (Q= 3.0 0)

oC

p.

ig. 4. Air temperature distribution (T) and airflow rate (Q) along the height (H) ofhe lightwell for three types of meshes, coarse 3.89 × 105 (Grid A), fine 5.51 × 105

Grid B) and finest 9.32 × 105 (Grid C) compared with experimental data (Exp.).

boration in the building (b) and the lightwell with void (c).

where C� is an empirical constant equal to 0.09. These two equa-tions can be used to derive the specific dissipation rate ω as shownin the following equation:

ω(y) = ε(y)C�k(y)

(3)

In addition to these solver settings, the intermittency was fixedat 1 for the SST model as the study considered a fully turbulent flow.

3.3.2. Near-wall treatmentTo apply the appropriate near-wall treatment method, simula-

tions were carried out using a standard wall function and enhancedwall treatment methods and the results were compared withthe WTE data. Standard wall function depends on dimensionlessparameter y+ as vertical wall distance vector. In smooth wallsthe first grid cell should be within the validity 30 < y+ < 300 [39]or 30 < y+ < 500 [40]. Therefore, to achieve y+ within these rec-ommended ranges, coarse mesh with a minimum size of 0.005 mwas used. In addition, simulation was carried out with the sameminimum size but with inflation mesh technique, provided in

ANSYS-Meshing software, which is used to optimize the wall layersof the floor boundary. In addition, enhanced wall treatment methodwas employed but with high mesh resolution, where a smaller min-imum size was used (0.004 m), as no mesh size limitations are

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

Measured wind velo city

One-fou rth Power lo w profil e

H(m

)

u (m/s)

Fig. 5. Generating inlet profile numerically by one-fourth power low and comparingwith experimental inlet.

506 T.G. Farea et al. / Energy and Buildings 86 (2015) 502–513

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

5 10 15 20 25 30

(Q=2.76)

(Q=2.76)

(Q=2.68)

(Q=2.61)

T oC

H(m

)

Standard WF

Standard WF+ifla.

Enhanced WT

Exp.

Fig. 6. Temperature distribution (T) and airflow rate (Q) along the height (H) ofthe lightwell for near-wall treatment adopting the standard wall function (WF),t(

rtWfioaws

3

sfithcsbv

4eiwgvtwllcettfW

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

5 10 15 20 25 30 35

(Q=2.76)

(Q=2.76 )

(Q=2.36)

(Q=2.10 )

T oC

H(m

)

Exp.

FluxSpace volume (no loss)

Space volum (loss 40% )

Fig. 7. Temperature (T) and airflow rate (Q) using different methods for heat gen-

he enhanced wall treatment (WT) and standard wall function with inflation meshWF + infla).

equired in this method. CFD simulations were conducted underhe three types of near-wall treatment settings and compared with

TE data. The airflow rate and temperature results in the lightwellor the three types agree with WTE data (Fig. 6). Although theres a very small discrepancy in temperature profiles with and with-ut using inflation mesh, this method has a strong effect on theirflow rate results as shown in Fig. 6. Therefore, the standardall function with inflation was used in further simulations in this

tudy.

.3.3. Heat generationHeat in the lightwell was generated by two coiled wire fixed

ymmetrically along its vertical axis in WTE. Numerically, it is dif-cult to simulate the heat released from a coiled wire. Therefore,wo different methods as provided in the solver (ANSYS-Fluent)ave been used to represent heat generation in the lightwell thatorresponds to those provided in the WTE. The CFD is able to pre-cribe surface heat flux on the inside surfaces of the lightwell, andy specifying the volumetric rate of heat generation of the lightwellolume [41].

An equivalent heat flux of 290 W/m2 was set to represent the0 W in WTE, to meet the heat generation inside the lightwellmitted by the lightwell surfaces. For generating heat by specify-ng volumetric rate, all cells in the lightwell space were separated

ithin the flow domain in order to generate heat that meet heatenerated in the WTE. This separated heat was divided into tenirtual regions and the total required energy was distributed ver-ically and equally for these virtual regions to total 16,000 W/m3

hich is also equivalent 40 W. In addition, heat loss from theightwell was calculated in the case of heat generation via theightwell volume (as noted in [16]) and the results also wereompared with previous cases. Fig. 7 illustrates that the heat gen-

ration through prescribing surface heat flux method is closero WTE data as compared with temperature and airflow rate inhe lightwell. The prescribed ambient air temperature of 12 ◦Cor the computational domain corresponds with that used for the

TE.

erating by; wall flux from the internal surfaces of the lightwell, lightwell volumewith and without calculating heat loss suggested by the authors of the experimentalstudy, compared with experimental data (Exp.).

3.4. Turbulence modeling

Since the flow in the WTE is turbulent (Re > 4000), the ReynoldsAveraged Navier–Stokes (RANS) models were employed to modelthe mean airflow in the lightwell. Using RANS approach for a3-D steady-state flow, the time-average continuity (mass con-servation), Navier–Stokes (momentum conservation) and energy(energy conservation) equations can be written, respectively, as:

∂∂xi

(�ui) = 0, (4)

∂∂xj

(�ujui) = − ∂p

∂xi

+ ∂∂xj

(�

∂ui

∂xj

)+

∂�tij

∂xj

+ �gi, (5)

∂∂xj

(�ujT) = ∂∂xj

(k

cp

∂T

∂xj

)+

∂qtj

∂xj

+ St (6)

where �tij

= −� ¯u′iu′

jis turbulent stress tensor, qt

j= −�u′

jt is turbu-

lent heat flux, and St is the source term for energy. T and u, are theaverage components of velocity and temperature, while u′ and t arethe fluctuating components.

Since selecting an appropriate CFD approach and a turbulencemodel are the most critical factors in fluid flow studies [42], thecurrent study employed four of the RANS-based turbulence mod-els to examine their performance. Performance was measured interms of the ability to predict temperature, airflow rate, and air-flow pattern in the lightwell connected to a bottom void. These arethe Re-normalize k–ε (R k–ε), RNG k–ε, and SST k–ω, and SST. Thesemodels have been selected as they are commonly found in the lit-erature. The comparisons between all models were presented inSection 5.

3.5. Convergence criteria

Previous studies [37,43] recommended that convergence of thescaled residuals down to 10−5 is acceptable. However, for validationstudies the use of much lower residuals to reach the convergedsolution has been recommended [37]. In the current study two

d Bui

ctnrctsav

4c

acradTtcaiiwitbddp

a recirculation flow in the bottom and a reverse flow at the topof the sky opening as illustrated in Fig. 9. As the air flow in thevoid and the lightwell spaces have considerably higher L/dh values,the formation of different airflow patterns including recirculation,

Table 2Operating conditions of the parametric study.

(a) Environmental conditionsAmbient temperature, T 12 ◦CHeat generation rate, Q 290 W/m2

Wind speed, V 1.0 m/sWind directions, � 0◦ , 45◦ , 90◦

Fsc

T.G. Farea et al. / Energy an

onvergence criteria with residuals 10−5 and 10−8 were used andhe results were compared. It was found that there were no sig-ificant differences in the results of air temperature and airflowate when both convergence criteria were used. In order to saveomputational time the scaled residual of 10−5 has been employedo examine the turbulence models and to conduct the parametrictudy. However some simulations were terminated after 2000 iter-tions as no further convergence was noted, as recommended byan Hooff and Blocken [44].

. A parametric study of the configuration of the lightwellonnection and wind direction

The most appropriate simulation parameters and settings thatgree with the WTE data were selected to predict the effect ofommon configurations of the lightwell connection. Nine configu-ations were systematically selected to represent different verticalnd horizontal positions of the horizontal void, and three windirections were examined as well, namely 0◦, 45◦ and 90◦ (Fig. 8b).hese nine cases were classified under three vertical groups andhree horizontal groups (Fig. 8a). A combination of both groups isomposed of different configurations, e.g. void in a single-bottomt leeward fac ade of the building (Fig. 8a). These categorizationsndicate the ventilation strategy that occurs in each categorization.e. cross category means the void position in cross configuration

hich facilitates the cross-ventilation strategy within the voids. It ismportant to note here that these suggested configurations (relatedo void positions) represent the common voids usually found in

oth residential and non-residential high-rise buildings. Additionaletails regarding other modeling settings and environmental con-itions in the computational domain of the parametric study areresented in Table 2.

ig. 8. Configurations of the lightwell connection to outdoor through different void posection in the center of the lightwell and shows wind directions examined in this study (benter of the lightwell (c) and hypothetical zones of the lightwell and void (d).

ldings 86 (2015) 502–513 507

5. Results and discussion

The results and discussion of the CFD validation and para-metric study are presented. More details on turbulence modelsare provided with a detailed description of the airflow patternin the lightwell for the reference case. The simulation resultsof the void position effect (horizontal and vertical) on upwardflow velocity, temperature and airflow pattern in the vertical cen-terline of the lightwell are outlined. Finally, the results of thewind direction effect on upward velocity in all configurations arepresented.

5.1. CFD validation results

The airflow patterns produced by the turbulence modelsshowed good agreement with the experimental flow patterns interms of the upward flow along the lightwell. In addition it forms

(b) Modeling conditionsTurbulence RNG k–εDomain size RecommendedSize mesh Varied (389k–510k)

itions used as a parametric study with the reference case (a), simplified plan and), 3-D building model shows the vertical plane position used to read the results in

508 T.G. Farea et al. / Energy and Buildings 86 (2015) 502–513

Fig. 9. Illustration of the streamlines of the airflow pattern indicating the airflow rate (Q) and temperature profile in the center of the lightwell using various turbulencem

uiAIldvasistawi

ta

odels compared with the experimental model data.

pward and reverse flow may results [5]. Therefore, the flow spacen the lightwell can be equally divided into four vertical zones (i.e.–D), while the void is divided into three horizontal zones (Fig. 8d).

n the bottom of the lightwell at zone A, a large airflow recircu-ation was formed which is in agreement with the experimentalata (Fig. 9). This is due to the acceleration of the flow through theoid inlet followed by sudden expansion at zone A, which gener-tes high inertia with separation at the lower part of the zone. Ashown in Figs. 10 and 11 (reference case), the upward flow veloc-ty (y-component) at zone A is very low and is thus considered atagnated zone. However, the upward velocity at zones B and C ishe highest, particularly at the intersection of both zones. The flowt the top of the lightwell in zone D has a lower upward velocity,hich could be interpreted as a reverse flow at the outlet which is

n agreement with the WTE.Fig. 9 compares the results of the various RANS models used in

his study including the WTE results of Kotani et al. [16]. As shownll RANS models were able to predict the flow pattern comparably

well compared with the experimental study. The results show thatRANS models such as RNG k–ε and SST k–w are able to predict theairflow rate in the lightwell with 100% accuracy. The rest of the tur-bulence models, including R k–ε and SST, underestimated (6%) andoverestimated (2%) the airflow rate, respectively. In addition, Fig. 9shows large discrepancies in the predictions of air temperature dis-tribution in the lightwell for all models, which clearly indicateslower temperature readings compared to the measured values. Thismay be due to some experimental errors particularly in heat gener-ation as stated in Kotani et al. [16]. Nevertheless, large discrepancieswere found at the bottom and the top of the lightwell, where recir-culation and reverse flow form. However, the smallest discrepancywas found between the different models, notably within the RNGk–ε model. Thus, the RNG k–ε is proven to be an appropriate model

which has the best agreement with the WTE results in terms of air-flow rate, flow pattern and temperature. Therefore, RNG k–ε modelwas implemented to solve the flow problem in the parametricstudy.

T.G. Farea et al. / Energy and Buildings 86 (2015) 502–513 509

F eratuw

5

asphe

ig. 10. The effect of connection configuration on the airflow pattern and air tempind direction.

.2. Void position effect

The simulation results of the void position effect on the upwardirflow in the lightwell were arranged for all configurations as

uggested in the methodology (Fig. 8a). The results of the airflowattern and the vertical temperature profile were categorized intoorizontal and vertical position effects in Fig. 10. The horizontalffect results were allocated to three different groups, which are

re distribution in center plane and line of the lightwell; under the condition of 0◦

single-bottom void, single-middle void and double-level void. Eachof these groups includes the results of the effects of three differenthorizontal positions of the void which are windward, leeward andcross-flow configurations as presented in the next section. Then, the

effect of the vertical position of the void on the upward air velocityis presented in Section 5.2.2. It should be noted here that Fig. 10illustrates the airflow pattern and temperature gradient along thelightwell for all configurations at only perpendicular wind direction

510 T.G. Farea et al. / Energy and Buildings 86 (2015) 502–513

veloc

(a

55pt

Fig. 11. Effect of wind direction (�) on the upward airflow

i.e. 0◦). The results of the upward air velocity for all configurationst the three directions (i.e. 0◦, 45◦ and 90◦) are presented in Fig. 11.

.2.1. Horizontal position effect

.2.1.1. Single-bottom void group. Fig. 10 shows that the airflowatterns in the bottom of the lightwell are quite different inhe three horizontal void-positions. It is noted that the highest

ity in centerline of varied configurations of the lightwell.

recirculation in this group is in leeward configuration followed bywindward and cross-flow configuration, respectively. The quan-titative predicted results of the air temperature and upward air

velocity (Figs. 10 and 11) confirm these visualization results, i.e. theair velocity is very low and temperature is higher in large recircula-tion flow zones. The recirculation flow which is formed usually hasa negative effect on ventilation performance, so it should be noted

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hat the cross-flow configuration at the bottom of the lightwell ishe most preferred configuration in this group as it acts to reducehe recirculation flow. Moreover, the upward flow velocity pro-les in case 0◦ direction of the wind (Fig. 11) show that usingottom cross-flow voids increased the air velocity along the spacef the lightwell by 52% and 86% more than the windward and lee-ard configurations, respectively. In the three configurations of this

roup, the upward flow velocity profiles are almost parallel withhe vertical walls of the lightwell except at zones A and D, whichre accompanied by recirculation and reverse flow where the inletnd outlet openings are located (Fig. 10). At the top of zone D theelocity approaches zero in all cases (Fig. 11). This is because thepward pressure at the outlet is very weak, thus allowing a reverseow to form which blocked the upward flow.

The air temperature profile results represent the air tempera-ure obtained along a vertical center line in the lightwell so as tovoid the direct effect which may come from the inside surfaces ofhe lightwell. Since the air temperature gradient along the verticalenter line is well elaborate the temperature gradient compared tohat on horizontal plane, therefore, only vertical gradient resultsere presented. As shown in Fig. 10, it is clear that the temper-

ture gradient in cross-flow void configuration is almost uniformrom the bottom to the top of the lightwell. The leeward config-ration shows a very large temperature gradient, particularly inones C and D, and the maximum difference is noted along theeight of the lightwell where temperature of up to 35 ◦C wereeached. The temperature profile of windward configuration hashe same trend of the cross configuration profile but with aver-ge temperature higher than 30%. The results clearly indicate thathere is a clear relation between the temperature results and their velocity for all cases of the single-bottom configurations groupor wind direction 0◦ (Figs. 10 and 11). In other words, higher airelocity reduced the air temperature in all configurations of thisroup.

.2.1.2. Single-middle void group. Fig. 10 shows that all three con-gurations in the single-middle void group are accompanied by

arge amount of recirculation flow along the lightwell space. Theseortices point to the stagnant areas and contribute to the poor airuality in the lightwell. It can be clearly noted that the lower partf the lightwell (A and B zones), which is located below the middleoid, is poor in terms of ventilation performance and is unable toischarge heat from the stagnant zones. Nevertheless, the upperalf part of the lightwell (C and D zones) showed better perfor-ance in terms of increasing the upward velocity and effectively

ecreasing the air temperature.The effect of the horizontal position of the void varies and

epends on the zone location along the lightwell space, whethert is below (A and B zones) or above (C and D zones) the middleoid. However, in the part above of the void (C and D), the cross-ow configuration increases the air velocity in the lightwell by 56%ore than windward configuration and about 86% more than lee-ard configuration (see the upward air velocity for 0◦ in Fig. 11).owever, in terms of air temperature, the windward configura-

ion shows the worst case along the lightwell height in this groupFig. 10). It should be noted that in this group, the air temperaturen the leeward configuration is lower than windward configura-ion which is different from the other two groups. This is due to theffect of the windward flow within the void which acts to opposehe upward flow in the lightwell, resulting in trapped heat and aoticeable increase in air temperature. In the leeward configura-

ion, it is clear that the effect on the upward flow was different as therapping of the airflow may not take place completely, which indi-ates lower temperature compared to the windward configuration.n general, this group showed dramatically increase of temperature

ldings 86 (2015) 502–513 511

in zones A and B, which will be further discussed in Section 5.2.2 toclarify this abnormal thermal flow characteristic.

5.2.1.3. Double-level voids group. In this group it can be noted thatthere are two recirculation flows close to both voids, a larger onewhich takes place at the bottom of the lightwell and, a smallerone at the middle (Fig. 10). This is noted in all horizontal positions(windward, leeward and cross configurations) but with substan-tial differences in the amount of recirculation flow. The smallestrecirculation flow in this group was found in the cross-flow con-figuration which indicates that the air quality is the best in thisconfiguration. The horizontal position of the void in double-levelconfigurations showed the highest effect on the upward flow veloc-ity if compared with the previous groups, particularly in C and Dzones. In these two cross-flow configuration zones, the average airvelocity is more than that in the windward and leeward configu-rations by 29% and 62%, respectively. The results also demonstratethat the average air velocity in this group was relatively higherin the upper two zones compared with the other two groups (seethe upward air velocity at wind direction 0◦ in Fig. 11). The windenhanced the buoyancy-driven flow in all horizontal void positions,except opposing in recirculation flow zones above both voids (i.e.zones A and C) as shown in the airflow pattern and upward velocityprofile (Figs. 10 and 11).

On the other hand, the effect of the horizontal position on thetemperature is not significant in zones A and B (Fig. 10). However, itis more significant in zones C and D where the average temperaturewas reduced by cross-flow and windward configurations by 31%and 25%, respectively, compared with the worst case in this group,which is leeward configuration. In general, the air temperature islower in the three configurations in comparison with the other twogroups.

5.2.2. Vertical position effectFig. 10 shows the effect of the vertical void positions in all the

three groups, namely, single level void, which includes two pos-itions (bottom and middle), and the double level voids. The resultsin Fig. 10 clearly show that the upward air flow in the lightwell ishighly influenced by the void vertical positions compared to hori-zontal positions. The flow in the center of the lightwell is affected bythe single-bottom void, indicating upward flow accompanied withrecirculation in zone A. However, single-middle void forms differ-ent recirculation flow sizes in the lower parts (A and B zones) and asingle large recirculation flow in the upper part of the lightwell (Cand D). The profiles of the upward air velocity of the single-middlevoids show that the flow along the lightwell is not stable and tendsto reduce air quality particularly, in zones A and B (Fig. 11).

The results of air temperature clearly indicate the poor airquality in the single-middle void configurations as very high airtemperature at the lower part of the lightwell (Fig. 10). In fact, theheat flux generated from lightwell surfaces (290 W/m2 representsthe 40 W in the WTE) is quite high. Additional details regarding heatgeneration setting were elaborated in Section 3.3.3 and Kotani et al.[14]. Besides, eliminating the bottom void in these configurationsnegatively affects the ventilation in the lightwell due to loss of pos-itive pressure in the bottom which can push the air from bottomto top. Thus, trapping hot air in the zone below the middle void(zones A and B) and producing a dramatic increase in tempera-ture. It should be noted here, that CFD simulation of the parametricstudy examines different ventilation configurations with the sameboundary conditions which could be used to optimize the buildingventilation.

Based on the results, double-level connection configurationsshowed better performance when compared to the single levelvoid configurations. It can be seen that two relatively small recir-culation flows were formed close to the bottom and the middle

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oids. However, it is clearly noted that the upward flow velocityn the case of double-level void (wind direction 0◦) is higher thaningle-level void (Fig. 11). In addition, in the case of the doubleonfigurations, the air temperature was reduced and became moreniform from the bottom to the top of the lightwell (Fig. 10). Forxample, the maximum difference between the bottom and the tops about 1.46 ◦C in the double void configuration. This means thathe double void is the best in terms of mixing the outdoor wind-riven and buoyancy-driven flow, thus increasing the air qualitylong the lightwell, a finding that is particularly more pronouncedn cross flow configuration.

.3. Wind direction effect

Fig. 11 shows the result of the upward velocity of the y-omponent in the centerline of the lightwell affected by windirection. In general, the air velocity in the lightwell was highlyffected by changes in the three wind directions; 0◦, 45◦ and 90◦.his effect depends on the position of the void vertically and hori-ontally, i.e. void at single level and windward fac ade. The resultsmphasized that the vertical void positions have a greater influencen the air velocity in the lightwell than the horizontal positions.ind direction 0◦, as perpendicular on the windward fac ade of the

uilding, was found better for increasing the upward flow veloc-ty at all different positions (either vertical or horizontal positions)

ithin tested configurations. For example, in the bottom single voidwith windward configuration), the upward velocity in 0◦ directionncreased by about 74.5% and 97% more than the increase shownn 90◦ and 45◦ directions, respectively. The results obtained forpward velocity in the 45◦ direction showed higher negative veloc-

ty which indicates a reverse flow. However, the velocity profile for5◦ showed that this direction caused swirl flow along the lightwell,articularly in the zones close to the void opening. This is due tohe wind coming from outdoor clashing with the windward surfacef the lightwell at an oblique angle, creating a strong swirling flowloser to the void inlet, disappearing gradually from bottom to top.

The results of the 90◦ direction showed that the upward flowelocities in the centerline of the lightwell indicate the same trendnd velocity values in the case of windward and leeward config-rations. This is because the horizontal position of the void facingindward or leeward fac ade is symmetrical with stream lines from

he 90◦ direction. However, there is a very minor change in thease of the middle-void configuration as shown in Figs. 10 and 11.inally, it is to be noted that the cross-ventilation configurationecreased the upward flow velocity as the wind-induced from bothides asymmetrically in 90◦ direction oppose each other.

. Conclusion

This paper presents a systematic validation of 3-D steady RANSFD model with the WTE data for a lightwell in a high-rise buildinghere the lightwell is connected to the outdoor through a bot-

om void. In addition, a generic sensitivity analysis is conductedo investigate the effect of different computational parameters onhe natural convection flow in the lightwell. The validation resultshow that the RANS models are able to predict the airflow rate,ow pattern and air temperature in the lightwell. However, theNG k–ε model demonstrated the most capability when comparedo the other models, in particular in predicting the airflow rate andts pattern, and this agrees well with previous studies.

The simulation results show that the void position, either ver-

icality or horizontally, is highly affected the upward flow velocityn the lightwell. It was found that the cross-flow and double-leveloid configurations show a good performance in terms of assist-ng upward flow velocity while at the same time decreasing air

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temperature in the lightwell. However, the void position in thewindward fac ade leads to upward velocity better than the leewardfac ade, which opposes the upward flow in the lightwell. The worstconfiguration in terms of air quality was found in single-middlevoid connection, which dramatically increased air temperature inthe lightwell, particularly in the lower half. The wind on the wind-ward fac ade of the building (0◦) is better than oblique (45◦) and side(90◦) directions, respectively, in terms of providing upward-crossflow and minimizing stagnated zones.

The simulation results clearly indicate that the airflow behaviorwithin the lightwell could be used as proper guidelines for naturallyventilated high-rise buildings, particularly in terms of providingcross ventilation to those rooms located in the buildings’ core.However, simulations with full scale models and consideration ofthe surrounded environment with real boundary conditions (e.g.heat source magnitude) may assure more realistic results towardachieving more appropriate guidance for code of practice. Since theconfigurations and wind directions investigated in this study arelimited, further study is required to cover the remaining possibleconfigurations and different wind directions.

Acknowledgement

The authors would like to thank the CICT Unit of UniversitiTeknologi Malaysia (UTM) for providing the computational supportfor conducting the research.

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