Shafts for natural light and ventilation in underground ...

Shafts for natural light and ventilation in

underground automobile parking areas

F. Gugliermetti ™, S. Grignaffmi <'>, P. Dell'Omo ™

^ Department of "Fisica Tecnica" University of Rome "La Sapienza",

Via Eudossiana 18, 00184 Roma, Italy

Email: gugliermetti_f@pcfite. ing. uniromal. itEmail: [email protected]

Abstract

This paper presents a study of the natural ventilation and illumination, whichoccur when lateral continuous shafts fulfill the requirements for externalopenings in underground car parking. Daylight factors and natural airflow ratesfor different garage dimensions, shaft shapes, shunts and vertical partitions areinvestigated. A CFD code and a daylight package in a two-dimensionimplementation carry out the study.

1 Introduction

Multi-level underground automobile parking garages are greatly diffused inlarge cities for the lack of outdoor spaces. Underground garages requireventilation to reduce the human health and fire hazard due to the emission ofcarbon monoxide, gasoline and oils from cars in operation. National codes andfire protection organizations generally require natural ventilation, which mustbe integrated by a mechanical system. Ventilation systems are designed asfunction of the number of cars in operation, on the emission quantity and on the

Transactions on the Built Environment vol 41, © 1999 WIT Press, www.witpress.com, ISSN 1743-3509

268 Urban Transport and the Environment for the 21st Century

number of underground levels. Code requirements are generally expressed asratio between the free external openings and the floor area for the naturalventilation and as air changes for the mechanical one. Generally free openingsare in the range 1/50̂ -1/20 of the floor area and can be more than 1/10 whenthe openings are not regularly distributed or obstacles reduce free aircirculation. Fan driven flow rate is in the range 3-r6 changes per hour. Shafts,sinks and chimneys are normally used to fulfill the natural ventilationrequirements.Free external openings also transmit outside daylight that improve peopleacceptability of the space and reduce electric consumption of the artificiallighting system. The outside daylight is quite abundant and easily reaches morethan 10.000 [Ix] for at least 3000 [h/year] for the most of the Europeancountries. Average daylight factors of 1-2 % are considered acceptable, whileDF's > 5% are required for full delighting.Many authors treated separately fluidinamic and daylight problems forunderground spaces: extensive information on natural ventilation driven forcescan be found in Liddament\ while Hama^ & alt., DOT*, Ball & Campbell* aregood sources for natural ventilation guidelines. Delighting for wells and shaftswere respectively investigated by Bouchet & Fontoynonf and by Gugliermetti& Grignaffini^ \ Integrated studies of natural ventilation and illuminance,within the design frame fixed by the natural ventilation requirements, are ratheruncommon. The results reached in daylighting and natural ventilation seldomcan be directly integrated among them, unless they are referred explicitly tounderground garages. Some different reasons confirm the aforementioneddifficulty: a) shunts or vertical partitions are generally used to avoid fumediffusion among the different levels of the parking area, b) the level height iskept at the minimum (in the limit of the code prescriptions, which usuallyrequires 2.40 [m], with a minimum of 2.00 [m] under beams) and thisminimum is below the normal height encountered in public spaces, c) in theparking area the acceptability of the environmental and the reduction of thesense of seclusion ( which is generally linked to the lack of a direct access todaylight) is partially promoted by the people small staying period and then therequired daylighting is small respect to other underground spaces (commercialcenters, subway stations).This paper presents a study of the natural ventilation and illumination for theunderground car parking areas, which use lateral continuous shafts. The externalopening of the shaft is the collecting system, while the shaft itself is the ductsystem both for the external air and natural light.The approach is based on numerical simulations carried out by the ComputerFluid Dynamic code "Phoenics" and by the daylighting software "Radiance" ina two-dimensional schematization. The packages are chosen to make the mostof the results that can be found in literature about the optimization andvalidation of the numerical implementations (see for example Gugliermetti^ ̂ *


Urban Transport and the Environment for the 21st Century 269

& alt.). Influence of the garage dimension, of the shaft shape, of the shunts andvertical partitions are investigated; results are presented in terms of DaylightingFactors (DF) and natural air changes. The presence of structural objects orgrilles in the collecting and duct systems is not considered.

2 Numerical simulations background

Natural ventilation is investigated by the code "Phoenics X-ll"(Ludwing^ ™ &alt.) which solves the conservation equations with the algorithm "Simplest"developed by Spalding^. For the turbulence the model k-e (two-equationmodel) is used (Launder & Spalding^ ). The simplicity and the reducedcalculation time of the k-e model is preferred respect to more advancedturbulence models which seem to be inconsistent respect to the two-dimensionaland isothermal schematization adopted. This choice is also supported by thelimits of this paper which aims at underlying the close link between naturalventilation and illuminance phenomena. The outdoor is limited by two verticalsurfaces (inlet and outlet surface) and by a horizontal free surface (above) andthe ground level (below). The boundary conditions for outdoor are expressed byfixing the wind profile for the inlet surface and the value of the pressure(assumed equal to the atmospheric pressure) for the outlet surface. The lengthand the distance of the horizontal surface from the ground is chosen so that theground effects are negligible and the wind velocity profile is constant (fullydeveloped profile). The indoor space and the ground are schematized as"walls" with normal air velocity equal to zero and tangential velocity profilethat obeys to log-law. The computational domain is divided into controlvolumes by a non-uniform orthogonal Cartesian mesh, (see Gugliermetti® &alt.).The numerical simulation for the illuminance is carried out by "Radiance",which was developed at Lawerence Berkeley Laboratory and at EcolePolytechnique Federale de Lausanne. The package is based on a renderingsystem of calculations, which blends deterministic and stochastic ray-tracingtechniques (Erhorn̂ & alt). The geometry and the material data are generatedby the "Scribe Modeller" (Green ̂ & alt.) and then converted into the Radianceinput formats by the interface program "Plink" (Compagnon & Green^ ). Tocharacterize the behavior of the material respect to the redirected light it isnecessary to specify the RGB values, the fraction of specularity, the roughness(as RMS slope of material surface facets) and the type of material (plastic,metal, transparent, etc). A gray scale rendering of the interior is acceptable, asthe environmental appearance is not important; then RGB values are assumed tobe equal to the hemispherical-hemispherical reflectance under daylight.Specularity is always set at zero, while roughness is set at 0.1 and the type of



material is considered as plastic one. The main user-settable variables, whichinfluence the result accuracy and the time of running, are the "Quality","Variability" and the number of indirect bounces. The "Quality" setting affectsthe overall accuracy and the aspect of the final rendering image; the"Variability" is an indication of how the light varies in the scene. The bestcompromise between accuracy and calculation is reached for the consideredgeometry, as reported in Gugliermettf, by setting "Quality" at "medium""Variability" at "high" with seven bounces. All the calculation are carried outwith CIE^ overcast sky and the results are expressed as ratio of the illuminanceat a point considered to the illuminance on outdoor unobstructed horizontalplane (daylight factor DF).

3 Mathematical models and results

The numerical analysis is explicitly referred to an underground car parking area,which has two equal lateral continuous shafts as collecting and duct systems fornatural light and ventilation. The duct systems are equipped with shunts orvertical partitions in order to guarantee independent ventilation for each level.

Fig. 1) Sketches of the analyzed geometry's



An internal guardrail (0.80 [m] in height) separates the duct systems from theparking area. The length of the garage is considered so long that the bordereffect can be neglected. The influence on illumination and ventilation due tolocal disuniformity in the parking area (access roads, stairs, ramps, pillars etc.)and in the shafts (grilles, structural beams etc.) are not considered. Thecollecting systems have a width which are in the range 44-10% of the parkingfloor width. Two cross-sections, both 2.40 [m] in height, are investigated. Thefirst section, 15 [m] in width, has two continuous lines of car parking stalls(each 2.30 x 4.50 [m] ) which are separated by a central driveway. The secondsection, 30 [m] in width, shows two contiguous central lines of stalls (2.30 x4.50 [m]) which are separated by two driveways from another two stall lines;each of them is near the shaft. In fig. 1) the geometrical sketches of the analyzedgeometries are reported.Only two level underground garages are considered because for levels belowthe second one the influence of bouyance forces (not taken into account in thispaper) is predominant respect to the wind effect and the natural illuminance isvery low. In this last case acceptable values for natural ventilation andilluminance require unusual free openings areas which exceed 1/5 of theparking floor.

Fig. 2) Air changes vs. total free external opening areas (2w/W [ %])



The results are referred to a fully developed profile of windspeed which isreached starting from an initial mean windspeed profile that follows Swami &Chandra'* equation V(Z)/V(ZQ)=( z/ ZQ)C , with %p= 10 [m], v(z#)=3 [m/s] andc=0.4 (urban environmental); the fully developed profile of windspeed abovethe garage is v(z)=2A9 z °^ and the assumed turbulence is 2%.The garage floor is assumed to have a reflectance of 0.4, while for all the othersurfaces it is iisH a reflectance 0.8.

-x=4.0 W=15 [m]-x=11 W=15[m]-x=4.0W=30[m]-x=26 W=30[m]

-0.02 0.02 0.040.00

Ux [m/s]

Fig. 3) Velocity profiles for shafts with w=0.80 [m] (case c)

_ 1.5 --

Fig. 4) Daylight factor versus different collecting system widths (W= 30 [m])



Air changes versus the total free external openings for the "case c" are reportedin fig. 2. The % fields in two different vertical section for w=0.80[m] andW=30-15[m] are reported in fig.3). The aforementioned geometrical situation isanalyzed from the DFs point of view in fig. 4) (for W=3Q [m]) and in fig. 5)(for W=\5 [m]); the DF's distribution is symmetrical respect to the distance

X[m]

Fig. 5) Daylight factor for different collecting system widths (W= 15 [m])

W/2 and it is influenced at the beginning of the parking area by the guardrail."Case b" and "case c» are the same from the DF's point of view, while aredifferent from the natural ventilation. In these last cases for W=15 [m] andw=0.6 [m] the ventilation goes from 0.3 air changes per hour (case c) to 0.25(case b)The illuminance in the two-level parking areas (case a) is analyzed in fig. 6)when W=30 [m] and w=0.80 [m]. The air flow, in presence of shunts, ischaracterized by a small recirculation of air between the two levels as reportedin fig. 7); the total air flow, that cross the collecting system, is about 1.8changes per hour. The use of vertical partitions (case d) increases the secondlevel DF's of about 2% in the zone near the duct system, while the total airchanges is about 0.27 changes for hour.



00.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00

X[m]

Fig. 6) Daylight factor for different levels (W=30, w-0.8 [m])

Fig. 7) Velocity fields (W=30, w=0.8 [m])

4 Remarkable conclusions

The main parameters which influence DF's are the width of the collectingsystem and the reflectance of the duct system, while the shape of the shafts only



a minor effect on the illuminance. Particularly the DF's, to take into account thefloor thickness and the number of the levels, depends mainly of the shadingcoefficient SC= w/(w+ HQ) with Hg= (H( I TV/). The sky component of theillumination is prevalent respect to the effect of reflection in the zones, which arenear the shaft; the extension of the aforementioned zone depends on the width ofthe shaft and on the reflectance of the shaft wall.Natural ventilation is influenced both by the width of the collecting system andby the shape of the duct system. The ratio w/W is not the most importantparameter, as the main role is played by the dimension w. For example in fig.2)it can be seen that the value of w allows about the same air changes for differentvalues of W, while the same ratio w/W implies great difference between thevalues of air change.

Nomenclatureshaft height w : shaft widthturbulent kinetic energy s : dissipation rate of kinside air velocity N, : number of levelsdistance from the guardrail W : car parking widthoutside air velocity H : car parking heightheight above the ground H< : total height of the garage

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Transactions 94(2), (1988)2. Hama G.M., W.G. Frederick and H.G. Monteith, "How to design

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5. Bouchet B. and M. Fontoynont, "Daylighting of underground spaces:design rules". Energy and Buildings 23,(1966)

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8. F. Gugliermetti, P. DelFOmo and M. Citterio, "Influence of outdoorconditions and of external openings in underground automobile parkingareas", " Proc. of Urban Transport and the environment 99, Rhodes 8-10Sept., (1999)

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