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{{#Wiki_filter:Joint EPRI/NRC
{{#Wiki_filter:Joint EPRI/NRC-RES Fire PRA Workshop August 6-10, 2018 Module III - Fire Analysis Fire Fundamentals: Fires in the Open and Fully Ventilated Fires A Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES)
-RES Fire PRA WorkshopAugust 6-10, 2018Module III  
 
-Fire AnalysisFire Fundamentals: Fires in the Open and Fully Ventilated FiresA Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES) 2Recall: Fuel limited firesA fire where the fuel burning rate is limited only by the surface burning rate of the material. Sufficient air is always available for the fire (plenty of oxygen to support burning)Fire generates hot gases (convective fraction) and emits radiative heat (radiative fraction)Generally applies to fires in the open or fires in large compartments
2 Recall: Fuel limited fires A fire where the fuel burning rate is limited only by the surface burning rate of the material.
-A nuclear power plant has lots of large compartments-3Heat Release Rate (HRR)For a simple fire, the HRR can be estimated using the following equation:
Sufficient air is always available for the fire (plenty of oxygen to support burning)
-2)-H cis the net*heat of combustion (kJ/kg)
Fire generates hot gases (convective fraction) and emits radiative heat (radiative fraction)
-Ais the burning area (m 2)So HRR ends up as kJ/s or kW* "net" heat of combustion implies that a burnefficiency has been included  
Generally applies to fires in the open or fires in large compartments
-fuels don't burnat 100% efficiency in real fires c H A m Q&&m&EnergyReleased RateFuel m&q&
- A nuclear power plant has lots of large compartments
4Heat Release RateHRR can be estimated experimentally using oxygen consumption calorimetrywhere:~ 13.1 MJ/kg O2for many common fuels
 
)/(2 2 O c OkgkJ H m Q&&c H 5FlamesLaminar -very small firesTurbulent
3 Heat Release Rate (HRR)
-most real firesFuel OxygenReaction Zone 6Ignition of GasesWith aspark orsmallflame (pilot)present,ignitionisbased onwhetherthegaseousfuelconcentrationisbetweentheupper(rich)andlower(lean)flammabilitylimits.-Thefuel-air(oxidizer)mixture issaid to beflammable if aflamewillpropagate inthismixture.With nopilotpresent, agaseousfuelinaircanstilligniteifthemixtureis at orabovetheauto-ignitiontemperature
For a simple fire, the HRR can be estimated using the following equation:
.-Theauto-ignitiontemperature isusuallymeasuredfor astoichiometricmixture-justtherightmix sothat nofuel oroxygenremainsafterthereaction.
is the burning mass flux (kg/sm2)
7Ignition of LiquidsFor a liquid to ignite, it must first evaporatesufficiently to form a flammable mixture of gaseous fuel and oxygen-This occurs at a liquid temperature called a flash-pointtemperature.
- Hc is the net* heat of combustion (kJ/kg)
-In general, this temperature can be called the piloted ignition temperatureand the same term carries over to solids. -The flash-point is the temperature at which the amount of liquid evaporated from the surface achieves the lower flammable limit.If no pilot is present, the mixture must be heated to the auto-ignition temperature in order to ignite. The auto-ignition temperature of a gas will be higher than the boiling point of the liquid.LiquidsEvaporatingfuel*Spark 8Ignition of SolidsSolids do not evaporate like liquids when heated. Solids form gaseous decomposition compounds, generally leaving behind char, in a process called pyrolysis. At some point, the gases reach the lower flammability limit and may ignite by piloted ignition or, if hot enough, auto-ignition.Typically, piloted ignition temperatures for solids range from 250°C (~480°F)to 450°C(~840°F).Auto-ignition temperatures can exceed 500
- A is the burning area (m2)
°C (~930°F). -For a given material, these temperatures are not constants and can change with the nature of heating.
So HRR ends up as kJ/s or kW
-For practical purposes, a (piloted) ignition temperature (T ig) may be treated as a property of a combustible solid.We shall consider thin (less than  
* net heat of combustion implies that a burn efficiency has been included - fuels dont burn at 100% efficiency in real fires c
~1 mm) and thick solids to have different time responses to ignition when exposed to impinging heat fluxHot SurfaceSolidsRadiantHeatPyrolysis products*Spark 9Flame SpreadMotion of vaporization front at the ignition temperature for solids and liquids-The surface is heated by the existing flames
H A
-More material pyrolyzes(or evaporates) ahead of the flame front-The existing flame acts as the pilot-The flame (fire) spreads-Cable trayFire x p z f 10Typical Flame Spread RatesIt is very difficult to compute flame spread rates because formulas are not completely available, rates may not be steady, and fundamental fuel properties are not generally available.Nevertheless, we can estimate approximate magnitudes for spread rates for various cases.Spread caseSpread Rate (cm/s)Smoldering solids0.001 to 0.01Lateral or downward spread on thick solids0.1Upward spread on thick solids1.0 to 100. (0.022 to 2.2 mph)Horizontal spread on liquids1.0 to 100.Premixed flames (gaseous)10. to 100.(laminar)10 5(detonations) 11Zone of InfluenceRegions near the fire where damage or fire propagation is expected.For fires in the open we consider:
m Q
-Flame Radiation
 
-Convection, especially inside the fire plume xTarget q&Target 12Buoyant FlowTemperature rise causes a decrease in gas densityPotential energy converted into kinetic energy  
= &
-gasses flow upwards Z V DBuoyant plumeUnit volume at plume gas at density and temperature TUnit volume of air at density aand temperature T a 13Turbulent EntrainmentEntrainmentis air drawn into the fire plume by upward movement of the buoyant plume
m&
-Engulfing air from the surroundings into the fire plumeEddies: fluctuating and rotating balls of fluid, large scale rolling fluid motion on the edge of the plume.Buoyant forceFlameEddies 14Turbulent Fire PlumeVery low initial fuel velocityEntrainment and flame height controlled by buoyancy 15Fire Plume Temperature Along the Centerline 16Example Case  
Energy Released Rate Fuel m&
-Zone-of-Influence CalculationFlame Height and Plume TemperatureHeskdestad'sFlame Height Correlation Input D -Fire diameter [m]
q&
0.6 Q f-HRR [kW]250Result L -Flame height [m]
 
1.5where:Heskestad'sPlume Temperature Correlation Input Tamb-Ambient temp. [C]
4 Heat Release Rate HRR can be estimated experimentally using oxygen consumption calorimetry where:
20 Q f-HRR [kW]250 F e-Fire elevation [m]
~ 13.1 MJ/kgO2 for many common fuels
0 H p-Target Elevation [m]
)
3.7 D-Fire Diameter [m]
/
1 k f-Location factor 1 (-2 or 4)
(
X r-Radiative Fraction 0.4Result Tpl-Plume Temp [C]
2 2
328 D Q L f 02.1 235.0 5 2&3 5 5 2 1 25o e p r f famb pl z F H Q k T T&D Q z f o 02.1 083.0 5 2&
O c
17Example Case  
O kg kJ H
-Zone-of-Influence CalculationRadiation Heat FluxFlame Radiation: Point Source Model 2 4 R Q q r f irr&&Input Parameters: Q f: Fire heat release rate (kW) R: Distance from flames (m) X r: Radiative fraction (FIVE recommends 0.4)D: Fire diameter (m) 18Example Case  
m Q
-Zone-of-Influence CalculationRadiation Heat Flux 2 4 R Q q r f irr&&Point Source Flame Radiation ModelInputsFire heat release rate [kW]
 
317Radiation fraction0.40Distance from flames [m]1.5ResultsHeat flux [kW/m2]4.5}}
= &
c H
 
5 Flames Laminar - very small fires Turbulent - most real fires Fuel Oxygen Reaction Zone
 
6 Ignition of Gases With a spark or small flame (pilot) present, ignition is based on whether the gaseous fuel concentration is between the upper (rich) and lower (lean) flammability limits.
- The fuel-air (oxidizer) mixture is said to be flammable if a flame will propagate in this mixture.
With no pilot present, a gaseous fuel in air can still ignite if the mixture is at or above the auto-ignition temperature.
- The auto-ignition temperature is usually measured for a stoichiometric mixture - just the right mix so that no fuel or oxygen remains after the reaction.
 
7 Ignition of Liquids For a liquid to ignite, it must first evaporate sufficiently to form a flammable mixture of gaseous fuel and oxygen
- This occurs at a liquid temperature called a flash-point temperature.
- In general, this temperature can be called the piloted ignition temperature and the same term carries over to solids.  
- The flash-point is the temperature at which the amount of liquid evaporated from the surface achieves the lower flammable limit.
If no pilot is present, the mixture must be heated to the auto-ignition temperature in order to ignite.
The auto-ignition temperature of a gas will be higher than the boiling point of the liquid.
Liquids Evaporating fuel
* Spark
 
8 Ignition of Solids Solids do not evaporate like liquids when heated. Solids form gaseous decomposition compounds, generally leaving behind char, in a process called pyrolysis.
At some point, the gases reach the lower flammability limit and may ignite by piloted ignition or, if hot enough, auto-ignition.
Typically, piloted ignition temperatures for solids range from 250°C (~480°F) to 450°C(~840°F).
Auto-ignition temperatures can exceed 500°C (~930°F).  
- For a given material, these temperatures are not constants and can change with the nature of heating.  
- For practical purposes, a (piloted) ignition temperature (Tig) may be treated as a property of a combustible solid.
We shall consider thin (less than ~1 mm) and thick solids to have different time responses to ignition when exposed to impinging heat flux Hot Surface Solids Radiant Heat Pyrolysis products Spark
 
9 Flame Spread Motion of vaporization front at the ignition temperature for solids and liquids
- The surface is heated by the existing flames
- More material pyrolyzes (or evaporates) ahead of the flame front
- The existing flame acts as the pilot
- The flame (fire) spreads Cable tray Fire xp zf
 
10 Typical Flame Spread Rates It is very difficult to compute flame spread rates because formulas are not completely available, rates may not be steady, and fundamental fuel properties are not generally available.
Nevertheless, we can estimate approximate magnitudes for spread rates for various cases.
Spread case Spread Rate (cm/s)
Smoldering solids 0.001 to 0.01 Lateral or downward spread on thick solids 0.1 Upward spread on thick solids 1.0 to 100. (0.022 to 2.2 mph)
Horizontal spread on liquids 1.0 to 100.
Premixed flames (gaseous)
: 10. to 100.(laminar) 105 (detonations)
 
11 Zone of Influence Regions near the fire where damage or fire propagation is expected.
For fires in the open we consider:
- Flame Radiation
- Convection, especially inside the fire plume x
Target q&
Target
 
12 Buoyant Flow Temperature rise causes a decrease in gas density Potential energy converted into kinetic energy - gasses flow upwards Z
V D
Buoyant plume Unit volume at plume gas at density and temperature T Unit volume of air at density a and temperature Ta
 
13 Turbulent Entrainment Entrainment is air drawn into the fire plume by upward movement of the buoyant plume
- Engulfing air from the surroundings into the fire plume Eddies: fluctuating and rotating balls of fluid, large scale rolling fluid motion on the edge of the plume.
Buoyant force Flame Eddies
 
14 Turbulent Fire Plume Very low initial fuel velocity Entrainment and flame height controlled by buoyancy
 
15 Fire Plume Temperature Along the Centerline
 
16 Example Case - Zone-of-Influence Calculation Flame Height and Plume Temperature Heskdestad's Flame Height Correlation Input D - Fire diameter [m]
0.6 Qf - HRR [kW]
250 Result L - Flame height [m]
 
===1.5 where===
Heskestad's Plume Temperature Correlation Input Tamb - Ambient temp. [C]
20 Qf - HRR [kW]
250 Fe - Fire elevation [m]
0 Hp - Target Elevation [m]
3.7 D - Fire Diameter [m]
1 kf - Location factor 1 (2 or 4)
Xr - Radiative Fraction 0.4 Result Tpl - Plume Temp [C]
328 D
Q L
f 02
.1 235
.0 5
2  
=
(
)
(
)
(
)
(
)
3 5
5 2
1 25
 
+
=
o e
p r
f f
amb pl z
F H
Q k
T T
 
D Q
z f
o 02
.1 083
.0 5
2  
=
 
17 Example Case - Zone-of-Influence Calculation Radiation Heat Flux Flame Radiation: Point Source Model 2
4 R Q
q r
f irr
 
& =
 
Input Parameters:
Qf: Fire heat release rate (kW)
R: Distance from flames (m)
Xr: Radiative fraction (FIVE recommends 0.4)
D: Fire diameter (m)
 
18 Example Case - Zone-of-Influence Calculation Radiation Heat Flux 2
4 R Q
q r
f irr
 
& =
 
Point Source Flame Radiation Model Inputs Fire heat release rate [kW]
317 Radiation fraction 0.40 Distance from flames [m]
1.5 Results Heat flux [kW/m2]
4.5}}

Latest revision as of 16:05, 5 January 2025

Joint EPRI/NRC-RES Fire PRA Workshop - Module III - Fire Analysis -Fire Fundamentals: Fires in the Open and Fully Ventilated Fires
ML18213A076
Person / Time
Issue date: 07/31/2018
From: Tammie Rivera
NRC/RES/DRA/FRB, Electric Power Research Institute
To:
Shared Package
ML18213A072 List:
References
Download: ML18213A076 (18)
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Text

Joint EPRI/NRC-RES Fire PRA Workshop August 6-10, 2018 Module III - Fire Analysis Fire Fundamentals: Fires in the Open and Fully Ventilated Fires A Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES)

2 Recall: Fuel limited fires A fire where the fuel burning rate is limited only by the surface burning rate of the material.

Sufficient air is always available for the fire (plenty of oxygen to support burning)

Fire generates hot gases (convective fraction) and emits radiative heat (radiative fraction)

Generally applies to fires in the open or fires in large compartments

- A nuclear power plant has lots of large compartments

3 Heat Release Rate (HRR)

For a simple fire, the HRR can be estimated using the following equation:

is the burning mass flux (kg/sm2)

- Hc is the net* heat of combustion (kJ/kg)

- A is the burning area (m2)

So HRR ends up as kJ/s or kW

  • net heat of combustion implies that a burn efficiency has been included - fuels dont burn at 100% efficiency in real fires c

H A

m Q

= &

m&

Energy Released Rate Fuel m&

q&

4 Heat Release Rate HRR can be estimated experimentally using oxygen consumption calorimetry where:

~ 13.1 MJ/kgO2 for many common fuels

)

/

(

2 2

O c

O kg kJ H

m Q

= &

c H

5 Flames Laminar - very small fires Turbulent - most real fires Fuel Oxygen Reaction Zone

6 Ignition of Gases With a spark or small flame (pilot) present, ignition is based on whether the gaseous fuel concentration is between the upper (rich) and lower (lean) flammability limits.

- The fuel-air (oxidizer) mixture is said to be flammable if a flame will propagate in this mixture.

With no pilot present, a gaseous fuel in air can still ignite if the mixture is at or above the auto-ignition temperature.

- The auto-ignition temperature is usually measured for a stoichiometric mixture - just the right mix so that no fuel or oxygen remains after the reaction.

7 Ignition of Liquids For a liquid to ignite, it must first evaporate sufficiently to form a flammable mixture of gaseous fuel and oxygen

- This occurs at a liquid temperature called a flash-point temperature.

- In general, this temperature can be called the piloted ignition temperature and the same term carries over to solids.

- The flash-point is the temperature at which the amount of liquid evaporated from the surface achieves the lower flammable limit.

If no pilot is present, the mixture must be heated to the auto-ignition temperature in order to ignite.

The auto-ignition temperature of a gas will be higher than the boiling point of the liquid.

Liquids Evaporating fuel

  • Spark

8 Ignition of Solids Solids do not evaporate like liquids when heated. Solids form gaseous decomposition compounds, generally leaving behind char, in a process called pyrolysis.

At some point, the gases reach the lower flammability limit and may ignite by piloted ignition or, if hot enough, auto-ignition.

Typically, piloted ignition temperatures for solids range from 250°C (~480°F) to 450°C(~840°F).

Auto-ignition temperatures can exceed 500°C (~930°F).

- For a given material, these temperatures are not constants and can change with the nature of heating.

- For practical purposes, a (piloted) ignition temperature (Tig) may be treated as a property of a combustible solid.

We shall consider thin (less than ~1 mm) and thick solids to have different time responses to ignition when exposed to impinging heat flux Hot Surface Solids Radiant Heat Pyrolysis products Spark

9 Flame Spread Motion of vaporization front at the ignition temperature for solids and liquids

- The surface is heated by the existing flames

- More material pyrolyzes (or evaporates) ahead of the flame front

- The existing flame acts as the pilot

- The flame (fire) spreads Cable tray Fire xp zf

10 Typical Flame Spread Rates It is very difficult to compute flame spread rates because formulas are not completely available, rates may not be steady, and fundamental fuel properties are not generally available.

Nevertheless, we can estimate approximate magnitudes for spread rates for various cases.

Spread case Spread Rate (cm/s)

Smoldering solids 0.001 to 0.01 Lateral or downward spread on thick solids 0.1 Upward spread on thick solids 1.0 to 100. (0.022 to 2.2 mph)

Horizontal spread on liquids 1.0 to 100.

Premixed flames (gaseous)

10. to 100.(laminar) 105 (detonations)

11 Zone of Influence Regions near the fire where damage or fire propagation is expected.

For fires in the open we consider:

- Flame Radiation

- Convection, especially inside the fire plume x

Target q&

Target

12 Buoyant Flow Temperature rise causes a decrease in gas density Potential energy converted into kinetic energy - gasses flow upwards Z

V D

Buoyant plume Unit volume at plume gas at density and temperature T Unit volume of air at density a and temperature Ta

13 Turbulent Entrainment Entrainment is air drawn into the fire plume by upward movement of the buoyant plume

- Engulfing air from the surroundings into the fire plume Eddies: fluctuating and rotating balls of fluid, large scale rolling fluid motion on the edge of the plume.

Buoyant force Flame Eddies

14 Turbulent Fire Plume Very low initial fuel velocity Entrainment and flame height controlled by buoyancy

15 Fire Plume Temperature Along the Centerline

16 Example Case - Zone-of-Influence Calculation Flame Height and Plume Temperature Heskdestad's Flame Height Correlation Input D - Fire diameter [m]

0.6 Qf - HRR [kW]

250 Result L - Flame height [m]

1.5 where

Heskestad's Plume Temperature Correlation Input Tamb - Ambient temp. [C]

20 Qf - HRR [kW]

250 Fe - Fire elevation [m]

0 Hp - Target Elevation [m]

3.7 D - Fire Diameter [m]

1 kf - Location factor 1 (2 or 4)

Xr - Radiative Fraction 0.4 Result Tpl - Plume Temp [C]

328 D

Q L

f 02

.1 235

.0 5

2

=

(

)

(

)

(

)

(

)

3 5

5 2

1 25

+

=

o e

p r

f f

amb pl z

F H

Q k

T T

D Q

z f

o 02

.1 083

.0 5

2

=

17 Example Case - Zone-of-Influence Calculation Radiation Heat Flux Flame Radiation: Point Source Model 2

4 R Q

q r

f irr

& =

Input Parameters:

Qf: Fire heat release rate (kW)

R: Distance from flames (m)

Xr: Radiative fraction (FIVE recommends 0.4)

D: Fire diameter (m)

18 Example Case - Zone-of-Influence Calculation Radiation Heat Flux 2

4 R Q

q r

f irr

& =

Point Source Flame Radiation Model Inputs Fire heat release rate [kW]

317 Radiation fraction 0.40 Distance from flames [m]

1.5 Results Heat flux [kW/m2]

4.5

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