air movement naturally ventilated buildings

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WREC 1996

AIR M OVE ~IE NT IN N AT URAL L Y-VE NT IL AT E D BUIL DINGS

H.B. Awbi

Department o f Construction Managem ent & Engineering

The University o f Reading

Reading RG6 6AW, UK

A B S T R A C T

The air mo vem ent and the distribution o f CO2 in naturally ventilated office roo m an d

an atrium is investigated using computational fluid dynam ics. The results sh ow th at

natural ventilation is capable of achieving acceptable CO2 levels. Ade qua te c om fort

levels could also be achieved for a typical UK summer climate in both types of

buildings. Bo th wind-d riven and buoya ncy-d riven flows are considered.

KE YW ORDS

Nat ural ventilation; ro om

ventilation.

air mov eme nt; CFD; cross-ventilation; single-sided

I N T R O D U C T I O N

Natural ventilation is now considered to be one of the requirements for a low energ y

building design. Until abou t three decades ago the majority o f office buildings in the

U K w ere naturally ventilated. With the availability o f inexpensive fossil energ y and

the tenden cy to pro vide better indoor environmental control, there has been a vast

increase in the use of air-conditioning in ne w and refitrbished buildings. Ho we ver ,

recent scientific eviden ce on the imp act o f refrigerants and air-conditioning system s on

the environment has Prompted the more conscious building designers to give seriousconsiderations to natural ventilation in no n-dom estic buildings.

The design considerations which ought to be considered in naturally ventilated

buildings have been discussed in a previous paper, Aw bi (1994). Tw o ma jor

difficulties that a design er has to resolve are the questions o f air-flow control and roo m

air mo ve me nt in the space. In mechanically ventilated spaces, th ere are well

established techniques for assessing the air mo vem ent that a syste m is expected to

produce, Awb i (1991).

Beca use o f the problem o f scaling and the difficulty o f representing natural ventilationin a laboratory, most o f the m ethods use d fo r predicting the air mo vem ent in

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WRE C 1996

mec han ically ventilated buildings are no t ve ry suitable for natura lly ventilated spaces.

Com putational fluid dyna mics (CFD) is becoming increasingly used for the design of

both mech anical and natural ventilation systems, Gan and Awb i (1994). Since a CFD

solution is based on the fundamental flow and ene rgy equations, the technique isequ ally applicable to a natu rally ventilated s pace a nd a mechanically ventilated spac e

providing that a realistic representation of the boundary conditions are made in the

solution.

In this paper, the CFD program VO RT EX is used to study the air movement, the

thermal quality and the air quality in a typical mid-floor office space and an atrium

building. Simulations are prese nted f or sum me r and winter. In addition to the air

ve loc ity and temperature distribution in the space, detailed the rmal co mf ort analysis is

also presented. Simulated occu pan cy is used to calculate the CO2 concen tration in the

space wh ich provides an indication o f the indo or air quality under each condition.

FLO W EQ U A TIO NS

.Wind

The volume flow rate (Q) through a large opening is given by:

Q = C d A 112~P (1)

where Ca is the discharge coefficient, A is the area, p is the density and Ap is thepressure difference which is given by:

Ap = 0.5 p VrCp (2)

whe re Vr is the reference wind speed an d Cp is the pressure coefficient at the opening.

Fo r a nu mbe r of openings in parallel:

Cd A = ~(C a A)i

and for a numb er of openings in series:

1 1CdA)2 -- 2- CfA)

Further details are given in A wbi (1994).

(3)

(4)

B uoyancy

Th e volum e flow rate through a large opening due to temperature difference is given

by (see Appendix A):

Q - c d

wh ere AT is the temperature difference across the open ing and ~" is the mean

temperature (K).

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WREC 1996

DESCRIPTION OF BUILDINGS

To assess the applicability of CFD to naturally ventilated buildings, two buildings

have been selected. One is an office building and the other is an atrium.

Office Room

An office room in an intermediate floor of a multi-storey building has been selected for

investigation. The room dimensions are 12m x 10m x 2.5m and has four openable

windows of height 2m and width l m on each of the 12m walls, as shown in Fig. I. It

has been assumed that summer venti lation is achieved by opening the windows and

winter ventilation is achieved by trickle ventilators (1 m x 0.01 m each) above and

below each window with all windows closed.

r-i r-l rm FT_Qfl/ ', I I I I I I ~ ~I I

/ ', I I I I I I / I I I/ I L_I L _I L_ 6~ L_I I

J l l i l t - . . . . . . . . . . . . . ,

.-'" .o _ ~, ~

2,5 "" " t 2.0 .

- )

12 ,0m

Figure 1: Office Room

The air quality in the office is investigated using CFD by considering the distribution

of CO2 from 6 seated occupants (0.0047 1/s each) at a height of 1.05 m. Two

situations were considered: a wind-driven venti lation (cross-ventilation) and

buoyancy-driven venti lation (single-sided ventilation). The total heat gain in the

summer is 6.224 kW which is uniformly distributed over the floor (51.9 W/m2) when

all the windows are open (cross-ventilat ion) and 50 W/m2 on the floor and 40 W/m2 on

the closed windows for single-sided ventilat ion. In winter, the heat loss from the two

external walls is 10 W/m2 (i.e. a total heat loss of about 500 W). A window surface

temperature of 10°C is assumed giving a total heat loss from the windows of about

350 W. In winter, four of the upper slots were used for the air supply and the other

four for the extract when the flow is wind-driven. In this case, a floor heating system

is assumed to produce an output of 22 W/m2. For the buoyancy-driven ventilation,

the lower slots were used for the air supply and the upper ones for the extract. In

this case, heat convectors each producing 1.64 kW were placed below each window to

heat the incoming air which is supplied jus t behind the convector.

Atrium

An atrium of dimensions 10m x 20m x 15m high has been selected for this study, see

Fig. 2. It has been assumed that the 2.0 m x 2.4 m door, which is situated on the 15m

wall, is always kept open. Only summer conditions have been simulated for theatrium since the main concern is smnmer overheating. A total heat gain of 80 kW is

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WREC 1996

assumed uniformly distr ibuted over the f loor (400 W/m2). Two condit ions have been

simulated: one with wind f low through the door and the other is a b uoya ncy-d riven

venti lat ion through the door and roof openings. The atr ium is equipped with two lm

x lm vert ical openings near the roof and two horizontal openings on the roof, lm x 2m each. It has been assumed that the small openings are alwa ys open but the large

openings are open during the buoyancy -driven f low only. The CO2 produc ed b y 126

occupants is uniformly distr ibuted at a height o f 1.9 m.

sz~ Jr,c

15.0m

Figure 2: Atrium Building

C F D P R O G R A M

The CFD program VORTEX has been used to study the air movement, thermal

comf ort and air quality in the two buildings investigated. This progra m has been

specif ically developed for the bu il t environment and detai ls of the p rogram are given

by G an and Aw bi (1994).

RESULTS

Office Room

Summer Ventilation. All the office windo ws were assumed open for the cross-

venti lat ion and the four windows on one wall were assumed open for the single-sided

venti lation. For the cross-venti lat ion a wind speed of 2 rods was assumed to enter thewind ow uniformly with a temperature o f 22°C. For the single-sided venti lation, the

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WREC 1996

ai r f low through the lower par t o f t he opening was ca l cu la ted by the CFD program

usin g equat ion (1) wi th a discharge coefficient of 0.61 for the opening. The flow rate

through the wind ow op enings as calculated by the CFD prog ram was 11.2 m3/s for the

cross-vent i lat ion, assu ming a discharge coefficient of 1. Howev er, i f equat ions (1) a nd(2) were used, a lower f low rate was obtained as equat ion (2) requires a reference win d

speed and no t the speed at the opening. Therefore, further CFD simulat ion s were

carried out to obta in some correlat ion betwe en the two w ind speeds an d, on average, i t

was found tha t the wind speed a t t he openings was about ha l f t ha t a t a re ference p oin t

ups t ream of the bui ld ing . The same f low ra t e can be obta ined us ing equations (1) and

(2) i fV r = 4 m/s and ACv = 0.7 were used. For single-sided vent i lat ion the f low rate

enter ing the room through the four window openings o n one w al l as ca l cu la ted by the

CFD program is 1.98 m3/s compared wi th 0.48 mS/s from equat ion (5). For eight

win dow op ening s the resul ts are 3.16 and 0.73 m3/s respect ively. In the CF D

simula tion , a un i form speed was assumed across the whole opening whereas inequat ion (5) the speed across the opening i s no t un i form, see Appendix (A) . I f t he

CFD f low ra te i s mul t ip l i ed by 2 /3 to a l low for the n on-un i form prof il e , f l ow ra t es of

0 .80 and 1 .28 m3/s a re ob ta ined for the 4 -windo w and 8-window openings

respec t ive ly . The condi t ions in the room for the two methods o f vent i l a t ion i s

summ arised in Table 1. In this table the Predicted Me an Vo te (PMV) and the

Predicted Percentage o f Dissat isfied (PPD) are also give n for the occupied zone (1.8 m

hi#).

Table 1. Sum mer condi t ions in office room

Air flow rate (m% )Air speed at open ing (m/s)Air temperature at opening(°C)Mean air speed in occupied zone (m/s)Mean temp in occupied zone (°C)Mean PMV in occupiedzoneMean PPD in occupied zone (%),Mean co2 concentration n occupiedzone (ppm)

Wind Buoyancy4-windows 8-windows

11.20 1.98 3.162.00 0.71 0.5622.0 22.0 22.0

0. 47 0.14 0.1422.5 25.5 23.60.02 1.38 0.9710.1 45.9 27.1351.8 362.2 358.2

Win ter Vent i lat ion. The resul ts for winter vent i la t ion are given in Table 2. I t can be

seen that adequate thermal comfort i s achieved using the t r ickle vent i lators for the

wind-dr ive n vent i l a t ion but t he comfor t for t he buoya ncy-d r iven ease i s n o t adequate ,i .e . more heat ing and or sm al ler vent i lat ion openin gs wi l l be required. The CO2 in both

cases is low.

Table 2 . Winter condi t ions in of f ice room

Wind

Air f low rate (mS/s)Air speed at opening (m/s)Air temperature at opening (°C)Mean air speed in occupied zone (m/s)Mean temp in occupied zone (°C)Mean PMV in occupied zoneMean PPD in occupied zone (%)

Mean CO2 concentration n occupiedzone (ppm)

0.082.05.00.0922.60.176.4

629.3

Buoyancy

0.313.95.00.0919.0-1.550.9

498

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Atrium

WREC 1996

The atrium results for wind-driven and buoyancy-drivenventilation are summarised in

Table 3 for summer condition. The table shows that both methods o f ventilation canachieve adequate air movement in the atrium with mean air speeds in the occupied

zone of 0.61 and 0.80 m/s. Although the mean temperature in the occupied zone is

about 27°C, this will be offset by the high air speed and the comfort level will be

acceptable for short occupancy periods such as normally found in atria. The CO2

concentration in the occupied zone is low.

Table 3. Summer conditions in atrium

Wind Buoyancy9.60 12.992.00 2.71

22.0 22.00.61 0.8026.2 27.0395.2 389.5

Air flow rate (m3/s)Air speed at opening m/s)

Air temperatureat opening °C)Mean air speed in occupiedzone (m/s)Mean temp in occupiedzone (°C)Mean C02 concentration n occupiedzone (ppm)

CONCLUSIONS

The CFD results have shown that adequate ventilat ion rates can be achieved through

an atrium with an open door and roof extract openings for typical UK summer

conditions. In the absence of wind, buoyancy was found to be capable of removing

solar heat gain and dilute COz levels to acceptable comfort and air quali ty levels. Theresults for the office room have shown that acceptable conditions can be achieved for

typical summer and winter climates by opening windows or using trickle ventilators.

However, the flow rates through window openings, due to wind in eross-ventilation

and buoyancy in single-sided ventilation, were over-estimated by the CFD

calculations when compared with calculations based on simple formulae for wind-

driven and buoyancy-driven ventilation. For wind-driven flow, the difference was

attributed to the values of reference wind speed and pressure coefficients which have

been used in the formula. For the buoyancy-driven low however, this was due to the

velocity profile through an opening which has not been considered in the CFD

simulations.

REFERENCES

Awbi, H.B. (1994). Design considerations for naturally ventilated buildings.

Renewable Energy, 5, 1081-1090.

Awbi, H.B. (1991). Ventilation o f Buildings, Spun, London

Gan, G. And Awbi, H.B. (1994). Numerical simulation of the indoor environment,

Building and Environment, 29, 449-459.

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WREC 1996

APPENDIX (A)

The pressure difference due to tem perature difference across a large opening is :

Ap(z) = Ap g z (A. 1)

where A p is the difference in densi ty across

the opening , z is the height and g is the

accelerat ion of gravity.

Also, u(z) = "~2Ap(z)/9

He nce , u(z ) pc z 1/2

andu(z)

Umax

= [ H I 112

Z

H ...... u(z) , y _ _ . . ..7--izThe mean v e loc i ty (u ) through an opening of height (h) i s:

Uma Iz l/2 Urea 2H3/2 2- - = - H Urea (A .2 )u H 112 dz H I/2 3 3

The volume flow rate through the opening (Q) is :

_ 2 CQ = C a w u = 2 C e wHUmax - 3 d AUmax (A.3)

where w is the width o f the opening.

However, in a buoy ancy-driv en f low, the equal masses o f air enter and leave through

the same opening. If H is the total height of the opening, then the influx or eff iux

flow is:

Q = _C~ A Um~ (A .4)

From equation (A. 1) i t fol lows that:

C d I g H - A T ( 1 .5 )e = ~ -

where AT is the tempe rature difference across the open ing and q" is the mean

temperature (K).

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air movement naturally ventilated buildings

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