BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI

Publicat de

Universitatea Tehnică „Gheorghe Asachi” din Iaşi

Tomul LIV (LVIII), Fasc. #, ####

Secţia

CONSTRUCŢII. ARHITECTURĂ

THE INFLUENCE OF THE WINDOW OPENINGS TO THE FIRE

SPREAD

OUTSIDE A FIRE COMPARMENT

BY

RUXANDRA DÂRMON*

Abstract. The factors which influence directly the shape and the severity of fire

plume outside a ventilated compartment are determined by the environmental and the

enclosure fire conditions, acting in combination and influencing each other. Inside the

fire compartment, these factors control whether flashover occurs or not, determining or

not the outside fire spread. If the flashover occurs, the external fire plume will be

influenced by the smoke control system, by the window glass breakage, but also by the

shape, the size and the number of window openings. Choosing different shapes for

window openings can increase or diminish the convective heat transfer. In this article are

compared the effects that different shapes and dimensions for window openings may

have on fire spread.

Key words: fire plume, ventilation factor, size of openings, flame spread.

1. Introduction

The fire spread on exterior claddings can be a significant hazard

nowadays, when is common the use of combustible materials like polystyrene,

polyurethane, wood, polyvinyl for insulating the outside walls. Especially in the

case of high-rise buildings the fire propagation can happen fast from floor to

floor via exterior claddings creating a hazardous situation not only for the

building occupants, but also for the firefighters.

The upward fire spread mechanisms were investigated by Oleszkiewicz

[1], who concluded that the fire can spread vertically through the combustible

cladding systems, by “leap-frogging” when the flames projected out of a lower

window ignite the combustible materials from inside an upper floor, or by the

failure at the junction of a floor and exterior wall. Another study [2] stated the

dominant mechanism for upward fire spread is via openings in exterior walls.

* Corresponding author: Ruxandra Dârmon, Teaching Assistant, Engineer, Technical University

of Cluj-Napoca e-mail: Ruxandra.DARMON@cif.utcluj.ro

2 An Author, Some One Else and Perhaps Another

Therefore, among the factors affecting the fire plume emerging from a window,

a great importance has the size and the form of the opening.

In order to study the effect of window form on flame spread outside the

fire compartment were conducted four Computational Fluid Dynamics (CFD)

numerical simulations in a residential room of a size of 5.4 m long, 4.6 m wide

and 2.5 m high.

2. The factors which influence the fire spread outside a fire compartment

Early research about the size and temperature of the fire plume issued

from a window opening and the buoyancy effects which cause the flame

spreading upwards outside a building was conducted in the 1950`s and 1960`s

by Yokoy [3], Webster [4] and Seigel [5]. Later, Law and O`Brien [6] and Law

[7] resumed the conclusions from the studies before and developed a heat

transfer model setting the expression for flame height and burning rates of

different fire scenario.

Fig. 1 – The factors which influence the upward fire spread on the

frontages

2.1 The factors from inside the fire compartment

To estimate the development of the flames on the building frontage, it

requires the understanding of the conditions inside a fire compartment and the

evolution of fire in the early stages. The combustible materials in an enclosure

have an important influence in the growth phase of a fire, by their nature,

quantity and distribution. Once ignited the first item, depending on the available

oxygen in the room, the fire will grow to a “state of total surface involvement in

a fire of combustible material within an enclosure” [8], which is the point

termed flashover. Drysdale [9] associates the flashover with one of the three

possible events: the radiation flux attains 20 kW/m2, the temperature in the

upper layer close to the ceiling is around 600 ˚C or the flames emerge out of the

Bul. Inst. Polit. Iaşi, t. # (#), f. #, #### 3

window opening. The nature and the amount of combustible materials and also

the ventilation condition influence the mass loss rate in a fire. Burning rate is a

general term for estimation of the rate at which thermal load is consumed by

fire, being described in terms of HRR, mass loss rate or charring rate, in case of

wood and other charring materials. Heat release rate is the direct measure of a

fire evolution.

In the fire safety literature the effects of the compartment size and

window openings are taken into account by two correlative parameters: the

ventilation factor ᴪ [10], (the same as opening factor [11] denoted O in EC1)

and the reciprocal opening factor, η [12]. The reciprocal opening factor is used

to represent the area of the compartment surfaces relative to the opening

ventilation parameter, equal with Awh1/2

.

(1) ψ = Aw Ho1/2

/ AT ,

Where: Aw = area of the openings (m2),

AT = total area of the compartment – enclosing surfaces (m2),

Ho = weighted average of the opening height (m),

h = the window height (m).

2.2 The factors from outside the fire compartment

Most of fires in enclosure are ventilation - controlled. When the oxygen

supply is limited the un-burned hot fuel and gasses flow out through the

window opening and ignite the combustible claddings. In this stage are released

large quantities of smoke and toxic species because of incomplete combustion

process. The materials used for lining the frontage have an important role in the

further flame spread to the upper levels, thus the cladding system can be

designed to reduce the vertical fire spread outside of a building.

There are relatively few studies concerning the influence of the

environmental conditions on the fire development on the frontages. Most of fire

tests are carried out in laboratory assuming a characteristic velocity of the wind

of 1 m/s. However in real natural fires the trajectory of the flame is deflected

horizontally in the direction of the wind.

3. The window openings influence on the external flames

3.1 Window shape

Oleszkiewicz stated that there is a correlation between the shape of the

window opening and the form and height of the fire plume emerging out of a

window. He noticed that flames emerging out from a narrow window will

project upward on a distance equal with half of the window height, while flames

emerging from a wide or squared opening, can project one and a half times the

window`s height. [13] Howbeit, Oleskiewicz didn`t consider the resulting fire

4 An Author, Some One Else and Perhaps Another

plume form, entrainment effects and the specific effects due to the window

shape.

3.2 Numerical simulations using Fire Dynamics Simulator – Version 5

For this paper were investigated four ventilation fire scenarios. The fire

compartment was a room in a residential building having the surface of 4.6 m x

5.4 m and the height of 2.5 m. The room has one window opening and one door,

on the opposite side of the window. In the design fire scenario was modeled the

post-flashover stage of the fire. The heat release rate was estimated accordingly

SR EN 1991-1-2 [11] for dwellings using the following formula:

(2) ,

Where: the heat release rate, equal with 250 (kW/m2) for dwellings

(table E.5) [11]

the floor surface of the fire compartment.

There were carried out four numerical simulations corresponding to four

fire scenarios, changing only the width of the window, as shown in the Table 1.

3.3 Numerical simulations results

The numerical simulations were carried out using the program Fire

Dynamics Simulator – Version 5, developed by National Institute of Standards

and Technology and VTT Finland. The four ventilation scenario numerical

simulation results are summarized in Table 1.

Table 1 Numerical simulations results

Fire

scenario

Window

size

[m x m]

Max.

Temperature

[˚C]

Maximum

HRR

[kW]

Maximum

burning rate

[m/s]

Opening

factor ψ

[m-1/2]

1 1.4 x 1.3 870 5466.12 0.23 0.044

2 1.6 x 1.3 870 6059.27 0.29 0.047

3 1.8 x 1.3 920 6912.39 0.39 0.05

4 2.4 x 1.3 970 8517.62 0.65 0.06

3.4 Ventilation influence on the maximum temperature in the fire compartment

The maximum temperature in the fire compartment was not affected by

increasing the ventilation with 15 % of the window opening surface. For a

ventilation factor over 0.05 m1/2

the maximum temperature in the fire

compartment increased as shown in the Figure 2, below. All the fire scenario

came under the case of ventilation-controlled burning in the room.

Bul. Inst. Polit. Iaşi, t. # (#), f. #, #### 5

Fig. 2 – The variation of the ventilation factor.

3.5 Ventilation influence on the burning rate

The burning rate for a window of the size of 1.4 x 1.3 m is 0.23 kg/sec.

Enlarging the window with 70 % accelerates the burning rate nearly three times.

The burning rates are increasing with the ventilation, increasing also the

quantity of heat released in fire.

4. Conclusions

The ratio of the window height over window width controls the fire

plume shape. Thus, narrow windows tend to project the flames away from the

façade, while squared or wider windows increase the flames attachment to the

wall above the opening.

Increasing the ventilation surface accelerates the burning rate and the

mass loss rate in a medium fire room.

Received, Month #, #### ”Gheorghe Asachi” Technical University, Jassy

Department of Transportation Infrastructure and

Foundations

REFERENCES

1. Yung D., Oleszkiewicz J., Fire Spread via Exterior Walls of Building. Proceedings

of the Fourth Conference on Building Science and Technology, Toronto, Ontario, 1988.

1-12.

2. Suzuki T., Sekizawa A., Satoh H., Yamada T., Yanai E., Kurioka H., Kimura Y., An

Experimental Study of Ejected Flames of a High-Rise Building Part I. National

Research Institute of Fire and Disaster, No. 88, 1999.

6 An Author, Some One Else and Perhaps Another

3. Yokoi S., Study on the Prevention of Fire-Spread Caused by Hot Upward Current,

Building Research Institute, Japan. Technical Report No. 34, 1960.

4. Webster C.T., Raftery M.M., Smith P.G., The Burning of Well Ventilated

Compartment Fires: Part III The Effect of the Wood Thickness., Fire Research Note

574, Joint Fire Research Organization, Borehamwood, 1961.

5. Seigel L.G., The Projection of Flames from Burning Buildings, Fire Technology, Vol.

5, No. 1, 43-51, 1969.

6. Law M., O`Brien T., Fire safety of Bare External Structural Steel, Construction Steel

Research and Development Organisation, London, 1989.

7. Law M.,Fire Safety of External Building Elements- The Design Approach, AISC

Engineering Journal, Second Quarter, 59-74, 1978.

8. **

* Fire Safety - Vocabulary, ISO / CD 13943, Geneva, International Standards

Organization, 2005.

9. Drysdale D., Mechanism of Flashover: An Overview, Proceedings of the Interflam

Conference, 155-157, 1996.

10. Saber H.H., Kashef A., Bwalya A.C., Post-flashover compartment fire for different

fire ventilation settings in a medium-sized residential room, NRCC 50860, ASTM

International Mechanical Engineering Congress and Exposition, Boston, Mass., Nov. 2-

6, 2008.

11. **

* SR EN 1991-1-2: 1994, Eurocode 1: Basis of Design and Design Actions on

Structures. Part 2-2: Actions on Structures Exposed to Fire. European Committee for

Standardization, Brussels, Belgium.

12. Thomas P.H., Fires in model rooms: CIB research programmes, Building Research

Establishment, Current Paper CP 32/74, BRR, Borehamwood, UK, 1974.

13. Oleszkiewicz I, Fire Spread on Building Facades, Proceedings of the Fifth

International Fire Protection Engineering Institute, 1989.

INFLUENȚA GOLURILOR DE FEREASTRĂ ASUPRA RĂSPÂNDIRII

FLĂCĂRILOR ÎN EXTERIORUL COMPARTIMENTULUI DE INCENDIU

(Rezumat)

Factorii care influențează în mod direct forma și severitatea flăcărilor ce ies în

exteriorul unui compartiment ventilat, sunt determinați de condițiile interioare și

exterioare ale clădirii incendiate și acționează în combinație și depinzând unul de

celălalt. Factorii din interiorul compartimentului controlează producerea fenomenului

de flashover, iar acest fapt determină ieșirea flăcărilor în exterior, sau stingerea

incendiului. Dacă fenomenul de flashover are loc, flăcările exterioare vor fi direct

influențate de sistemul de control al fumului, de momentul spargerii sticlei ferestrelor,

dar și de forma, dimensiunile și numărul golurilor de fereastră. Alegerea unor forme

diferite pentru ferestre au drept rezultat creșterea sau diminuarea transferului de căldură

prin convecție. În acest articol sunt comparate efectele pe care le pot avea asupra

răspândirii flăcărilor, diferite forme și dimensiuni de fereastră.