12 application to fire

Chapter 12

Application: Fire in a Building Table of Contents

1. Introduction.............................................................................. 307 2. Application to a Fire in a Building ........................................... 307 2.1 Definition of the Fire Scenario ............................................. 307 2.2 Quick Estimation of the Risk of Having Backdraft .............. 308 2.2.1 Safest Distance in Case of Backdraft Deflagration.......... 308 2.2.2 Risk of Backdraft: Accumulation of Unburnt Gases ....... 310 2.2.4 Risk of Backdraft: Gravity Current................................ 314 2.3 OZone Simulation of Room 1 and Room 2 ........................... 315 2.3.1 Compartment Fire Modeling .......................................... 315 2.3.2 Results for Room 1 and Room 2..................................... 316 3. Conclusion ................................................................................ 319

12. Application: Fire in a Building

1. Introduction The research carried out in the present thesis will be applied to a hypothetical fire in a building. The aim of this chapter is to illustrate some of the possible applications of the concepts, tables and (graphical and numerical) models developed.

Thus, this chapter is divided into two parts: • •

The first part deals with a quick estimation of the risk of having backdraft by means of the parametric studies carried out throughout this thesis.

And the second part deals with the numerical estimation of the risk of having backdraft, obtained by OZone.

2. Application to a Fire in a Building 2.1 Definition of the Fire Scenario The building in this study consists of two floors. Figure 12-1 represents the building schematically. The characteristics of each room are summarized below: 1st floor: This floor consists of two rooms, both of which are designed for the same occupancy. The characteristics are given below: Room 1: • • •

•

Floor area: 15 m2.

Height: 3.5 m.

Characteristics of the partitions: o Thickness = 12 cm. o Material: Heavy brick (unit mass: 2000 kg/m3; conductivity: 1.2 W/mK; specific heat: 1000 J/kgK) Openings: a door (h = 2.2 m, w = 1.0 m) and window (h = 0.5 m, w = 1.0 m). Both are initially closed.

Room 2: • • •

•

Floor area: 25 m2.

Height: 3.5 m.

Characteristics of the partitions: o Thickness = 12 cm. o Material: Lightweight concrete (unit mass: 1600 kg/m3; conductivity: 0.8 W/mK; specific heat: 840 J/kgK) Openings: a door (h = 2.2 m, w = 1.0 m) and window (h = 1.0 m, w = 1.0 m). Both are initially closed.

307

12. Application: Fire in a Building The fire starts in floor I. It is assumed that the fire fighters have to enter the building to rescue a person on floor II.

Room 1

Room 2

(a) Second floor

(b) Second floor

Room 1

Room 2

(c) First floor

(b) First floor

Figure 12-1: (a)-(c) isometric view of the second and first floor respectively. (b)-(d) plan view of the second and first floor respectively.

2.2 Quick Estimation of the Risk of Having Backdraft 2.2.1 Safest Distance in Case of Backdraft Deflagration Figure 9-11 to Figure 9-13 can be used in order to determine the safest distance in case of backdraft deflagration. Most notable, they will provide us with an estimation of fireball’s diameter. These figures have been obtained for three different stoichiometric mixtures (hydrogen, methane and propane) as a function of the volume of the compartment and the area of the vent.

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12. Application: Fire in a Building Unless the fire develops in industries where the type of possible fuel is well defined, the type of unburnt gas is normally unknown. That could be a drawback in using this method. However, having a deeper look at the figures, one may observe that the difference in the fireball’s diameter for the three stoichiometric mixtures (for a given vent) is not very significant, less than 2.0 m. Thus, one can consider that the mixture formed in the compartment will cause a more or less similar fireball’s diameter.

To obtain the volume of the compartment, the fire fighter can accomplish this by direct measurement using the building plan. Having the building plan during a fire gives a certain advantage for fire fighters in facing the fire.

Sometimes the fire fighter is not provided with a plan of the building and in other cases this plan does not exist. In these situations, a rough estimation of the volume of the compartment and the vent area can be made in situ.

Supposing that our fire fighter has a plan, the fireball’s diameter can be obtained as follows: Floor I: Room 1: Volume = 52.5 m3.

Area of vent = 0.5 m2. (Let’s us use 1.0 m2).

Room 2: Volume = 87.5 m2.

Area of vent = 1.0 m2 (Let’s us use 1.0 m2).

Introducing these data into the following figure, the calculated diameter of the fireball is: Room 1: Fireball diameter = 6.2 m.

Room 2: Fireball diameter = 6.8 m.

The safest distance can be considered between 6.0 and 7.0. Still, to be absolutely on the safe side, let’s us increase that distance to 8.0 m (see Figure 12-3).

Knowing the fireball’s diameter provides us with a lot of information, i.e.: • •

The minimum distance from the vent at which the fire-lorry, the ambulance, and so on, must be placed in order to avoid unnecessary damage.

The ability to predict a possible spread to adjacent buildings and, consequently, to be ready for reacting in such a case.

309

12. Application: Fire in a Building

12

Fireball diameter [m]

10 8 4m2

6

3m2 2m2

4

1m2

2

Methane

0 0

50

100

150

200 250 300 Volume [m3] 3

350

400

450

500

Volume [m ]

Figure 12-2: Fireball diameter vs. compartment volume and vent area: Methane.

2.2.2 Risk of Backdraft: Accumulation of Unburnt Gases In this scenario, the fire fighter must enter the second floor to rescue the person in danger. According to the plan shown in Figure 12-1, there are two ways to accomplish this:

• Enter through the room I. • Enter through the room II. The fire fighter must choose which way is safer for trying to avoid backdraft. He knows that the risk of backdraft is directly linked to the quantity of unburnt gas accumulated in the compartment just after opening the door or window.

As was shown in Chapter 4: “Pyrolysis Rate and Smouldering Combustion”, the accumulation of the unburnt products is not related only to the size of the vents.

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12. Application: Fire in a Building

8.0 m

Room 1

8.0 m Room 2

Figure 12-3: Dangerous region in case of backdraft deflagration in room 1 and room 2

An estimation of the accumulation of unburnt gases made only as a function of this parameter can lead to unfortunate consequences. Many other factors must be taken into account, such as: • • • •

the properties of the partitions that influence the temperature inside the compartment, the temperature inside the compartment, which can speed up the pyrolysis rate of the fuel, the type of fuel, which determines the pyrolysis rate, and of course, the size of the openings, which controls the burning rate and the evacuation of gas from the compartment.

As such, the graphical method developed in Chapter 4: “Pyrolysis Rate and Smouldering Combustion” takes all these factors into consideration (see Figure 12-4). This method has been obtained by taking as a reference a compartment fire with a ventilation factor of 1.0, a fire area of 9 m2, a compartment height of 3.0 m and partitions with the same thermal properties of heavy wood. This method gives the relative accumulation of unburnt products, the relative fire

311

12. Application: Fire in a Building duration and the relative maximum pyrolysis rate for a given compartment. In addition, it is valid for compartments with the same fire load.

In order to compare the risk of backdraft in room 1 and room 2, the fire fighter calculates which room will accumulate more unburnt gas.

150

60 40

125

Duration

100

20

Duration

75 50

[%]

[%]

0 -20

25

Pyrol

UMF

0

-40

-25

-60

UMF

-50

-80

Pyrol

C

0.2 0.4 0.6 0.8

1.2 1.4 1.6 1.8

2

Ventilation parameter [Aw(Hw)^0.5]

(a)

(b) 40

250 225

Duration

30

200

20

175

Duration

10 [%]

150 [%]

1

N

H

M

B

B LW C M W C

N

H W N B + G B

LB

N W

-75

125 100

0

-10

Pyrol

Pyrol

75

-20

50

UMF

25

-30

0

-40 0

9

18

27

36

45

54 2

Firearea area[m2] [m ] Fire

(c)

63

72

81

UMF

3

3.5

4

4.5

5

5.5

6

Height [m]

(d)

Figure 12-4: Graphical method for estimating the accumulation of unburnt gases in a compartment

312

12. Application: Fire in a Building Evaluation for Room 1 (Red line): Figure 12-4 (a): The material of the partition is heavy brick (HB); therefore, if the ventilation parameter, the height and the fire area are the same as the reference case, the increase of unburnt mass fraction accumulated will be -50%.

Figure 12-4 (b): The ventilation parameter of room 1 is 0.35; therefore, if the material of the partition, the height and the fire area are the same as the reference case, the increase of unburnt mass fraction accumulated will be +12.5%.

Figure 12-4 (c): The fire area of room 1 is 15 m2; therefore, if the ventilation parameter, the height and the material of the partition are the same as the reference case, the increase of unburnt mass fraction accumulated will be +23%.

Figure 12-4 (d): As the height of room 1 and room 2 are the same, it is not necessary to correct this estimated figure.

Evaluation for Room 2 (Blue dotted line): Figure 12-4 (a): The material of the partition is lightweight concrete (LWC); therefore, if the ventilation parameter, the height and the fire area arethe same as the reference case, the increase of unburnt mass fraction accumulated will be -35%.

Figure 12-4 (b): The ventilation parameter of room 2 is 1.0; therefore, if the material of the partition, the height and the fire area are the same as the reference case, the increase of unburnt mass fraction accumulated will be 0%.

Figure 12-4 (c): The fire area of room 1 is 25 m2; therefore, if the ventilation parameter, the height and the material partition are the same as the reference case, the increase of unburnt mass fraction accumulated will be +50%.

Figure 12-4 (d): As the height of room 1 and room 2 are the same, it is not necessary to correct this figure.

According to these corrected estimations, the relative accumulation of unburnt mass in room 1 is (50 + 12.5 + 25) = -12.5%; the relative accumulation of unburnt mass in room 2 is: (-35 + 0 + 50) = 15%. Therefore, the safest way to rescue the person is by going through room 1.

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12. Application: Fire in a Building

Room 1

Room 2

Figure 12-5: Correct room for entering the compartment.

2.2.4 Risk of Backdraft: Gravity Current The fire fighter decides to open the door of room 1. As a result, a gravity current is created. Table 7-12 can be used to obtain the average velocity of the gravity current. This table contains the non-dimensional parameter as a function of the type of opening, compartment ratio and presence of obstacles.

• • •

Opening: Centred door (2.2 m x 1.0 m).

The non-dimensional number without obstacles for this type of opening is 0.35. Using a safety coefficient of 1.65, which takes into account the presence of obstacle, the non-dimensional number becomes 0.21.

Assuming an ambient density of 1.20 kg/m3 and a inner density of 0.7 kg/m3, β can be obtained as follows: β=

•

( ρo − ρ1 ) ρ1

(1.2 − 0.7 ) 0.7

= 0.71

(12-1)

The velocity of the gravity current, according to Eq. (12-2) is: v → v = 1.03 m / s g·H·β

v* =

•

=

(12-2)

The time needed for the gravity current to go and come back is: tin =

L 5 = = 4.8 s v 1.03

tback =

2L + 2H / 3 2·5 + 2·( 3.5 / 3 ) = = 11.8 s v 1.03

(12-3)

(12-4)

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12. Application: Fire in a Building Knowing an approximate velocity of the current may also be important, in order to the position of the gravity current: •

Knowing the position, the quantity of mixing can be estimated, e.g. the ratio of mixing is not the same when the gravity current is entering or when the gravity current is reflected from the rear wall. This factor is essential because more mixing means more severity in the backdraft deflagration.

2.3 OZone Simulation of Room 1 and Room 2 2.3.1 Compartment Fire Modeling The input data used in the simulation are the following: • • •

The model assumes that fire develops in one zone.

The partition dimensions of the compartment and windows are given in Section 2.1: “Definition of the Fire Scenario”.

Cellulosic material (C4H6O3) is supposed to be the fuel.

o o o o o

Combustion heat: 17.5 MJ/kg.

Combustion efficiency factor: 0.8 Stoichiometric ratio: 1.27.

Combustion products: yCO2= 1.37 and yH2O=0.42.

Fire load: 511 MJ/m2 and RHRf: 400 kW/m2

The flammability limits and the limiting oxygen concentration are obtained by considering that the combustion heat of the unburnt product is the same as the fuel. To this end, Eq. (8-15) and Eq. (8-16) give the ULF and the LFL, respectively and the model developed in Chapter 8: “Flammability Limits of Flammable Mixtures” gives the LOC. Therefore:

o o o o • •

LFL = 3.0% UFL = 14.0% LOC = 4.51% Fuel concentration at LOC = 14.44%

It is assumed that flames die out 1000 s from the beginning of the fire.

The firefighter opens the door at 1500 s from the beginning of the fire.

The simulations are carried out assuming an NFSC design fire and the model for non-charring materials described in Section “3.2 Simplified Model for Pyrolysis Rate of Liquid or Non-charring Fuels”. Heat loss by conduction is not taken into consideration and ζ is taken as 1.

315

12. Application: Fire in a Building

2.3.2 Results for Room 1 and Room 2 Both cases are assembled for direct comparison. Column (a) of Figure 12-5 represents room 1 and Column (b) of Figure 12-5 gives the results for room 2.

The results represented are: • • • • •

The pyrolysis rate and the burning rate.

The mass fraction of the gas species.

The equivalence mixture Fuel-N2-air in the compartment.

Le Chatelier’s number.

The energy released inside and outside the compartment and the energy no consumed.

0.4

0.1 Pyrolysis rate

0.09

0.35

Burning rate

0.08

0.3 0.07

0.25 [kg/s]

[kg/s]

0.06 0.05 0.04

Pyrolysis rate

0.2

Burning rate

0.15

0.03

0.1 0.02 0.01

0.05

0

0 0

250

500 750 1000 1250 1500 1750 2000

Time [s]

(a) Pyrolysis rate and burning rate for room 1

0

250

500 750 1000 1250 1500 1750 2000

Time [s]

(b) Pyrolysis rate and burning rate for room 2

316

12. Application: Fire in a Building

1

1 HC

0.9

HC

0.9

O2

O2

N2

0.8

N2

0.8

CO2

CO2

H2O

H2O

0.7 Mass fraction

Mass fraction

0.7 0.6 0.5 0.4

0.6 0.5 0.4

0.3

0.3

0.2

0.2

0.1

0.1

0

0 0

250

500

750 1000 1250 1500 1750 2000

0

250

500

Time [s]

(a) Mass fraction of the gas species accumulated in room 1

1

(b) Mass fraction of the gas species accumulated in room 2

1

0.8

0.8

0.7

0.7

0.6 0.5 0.4

0.6 0.5 0.4

0.3

0.3

0.2

0.2

0.1

0.1

0

0 0

250

500

750 1000 1250 1500 1750 2000

Time [s]

(a) Mass fraction of the equivalent Fuel-N2-Air mixture in room 1

Equivalent Fuel Equivalent N2 Equivalent Air

0.9

Mass fraction

Mass fraction

Time [s]

Equivalent Fuel Equivalent N2 Equivalent Air

0.9

750 1000 1250 1500 1750 2000

0

250

500

750 1000 1250 1500 1750 2000

Time [s]

(b) Mass fraction of the equivalent Fuel-N2-Air mixture in room 2

317

1.6

2

1.4

1.8 1.6

1.2

Le Chatelier number

Le Chatelier number

12. Application: Fire in a Building

1

Le Chatelier

0.8

LeChat

0.6 0.4

1.4 1.2

Le Chatelier

1

LeChat

0.8 0.6 0.4

0.2

0.2

0

0 0

250 500 750 1000 1250 1500 1750 2000

0

250 500 750 1000 1250 1500 1750 2000

Time [s]

Time [s]

(a) Le Chatelier number in room 1 (Diffusion criterion)

(b) Le Chatelier number in room 2 (Diffusion criterion)

600000

600000

500000

500000

Total available energy Energy consumed inside

Total available energy Energy consumed inside

400000

Energy consumed outside

400000 [kJ[

[kJ[

Energy consumed outside

300000

300000

200000

200000

100000

100000

0

0 1500

1600

1700

1800

1900

2000

Time [s]

(a) Available energy for backdraft; energy consumed inside and outside in case of backdraft in room 1

1500

1600

1700

1800

1900

2000

Time [s]

(b) Available energy for backdraft; energy consumed inside and outside in case of backdraft in room 2

Figure 12-6: Results of the simulation with OZone for room 1 and room 2.

318

12. Application: Fire in a Building The following points may be observed in Figure 12-6: •

The glass of room 1 breaks 391 s from the beginning of the fire and the glass of room 2 breaks at 362 s. In both cases, oxygen is still present in the compartment. That is why the effect of this breakage is not observed in any of the figures.

•

A greater pyrolysis rate is found in room 2 than in room 1.

•

A greater burning rate is found in room 1 than in room 2. This is because the burning rate depends on the size of the opening: the bigger the opening, the higher the burning rate.

•

A greater accumulation of unburnt products occurs in room 1 than in room 2. Starting from 1000 s (time of flame extinction), the accumulation of unburnt gases starts to decrease. Room 2 evacuates more unburnt gases than room 1 because of the size of its opening. Smouldering combustion is present in both cases.

•

The mixture in room 1 has a Le Chatelier number lower than room 2. If the fire-fighting approach involves methods that render the inner mixture inert such as introducing CO2 or water mist, more of this inert species is needed in room 2.

•

After opening the window, both mixtures are above the upper flammability limit, so ignition is possible. The period during which backdraft could happen is shorter for room 1 than for room 2, due to the smouldering combustion process that lasts longer in room 2. The size of the window must also be taking into account in this aspect.

Furthermore, more energy will be released in a backdraft in room 1 than in room 2.

Note that the energy figure has been calculated according to the content of unburnt gas inside the compartment. The premixed flammability limit criterion has not been applied.

3. Conclusion A hypothetical fire in a building with two floors and two rooms on the first floor has been analysed using the methods developed during this thesis. Although this chapter is just an example, it has shown that the simple graphical methods as well as the numerical models can be used to predict or estimate the risk of backdraft in compartment fires, making the work of fire fighters safer especially since they risk their lives on a daily basis to help others.

319