Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 5 1 Fire Dynamics II Lecture # 5 Chemistry of Room Fire Combustion Jim Mehaffey 82.583 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 5 2 Chemistry of Room Fire Combustion Outline • Introduction • Review: Generation of products of combustion in well- ventilated fires • Generation of products of combustion in poorly- ventilated fires • Review: Life tenability criteria Objectives • Predict rates at which heat & chemical species are generated in fires in order to provide input for assessments of thermal environment & life safety
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Fire Dynamics II - Chemistry of Room Fire Combustion Lecture_5
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
5
1
Fire Dynamics II
Lecture # 5Chemistry of Room Fire Combustion
Jim Mehaffey
82.583
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
5
2
Chemistry of Room Fire CombustionOutline• Introduction• Review: Generation of products of combustion in well-
ventilated fires• Generation of products of combustion in poorly-
ventilated fires• Review: Life tenability criteriaObjectives• Predict rates at which heat & chemical species are
generated in fires in order to provide input for assessments of thermal environment & life safety
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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IntroductionPerfect Combustion• Combustible burns in an excess of pure O2
• Products: net heat of combustion, CO2 and H2O Well-ventilated fires (diffusion flames in the open)• Combustible burns in open configuration in air• Products: Chemical heat of combustion, CO2, H2O,
CO, C (soot) and hc (hydrocarbons)Poorly-ventilated fires (many fires in enclosures)• Combustible burns in air, but air supply is restricted• Products: Less heat, CO2 and H2O
More CO, C (soot), hc (hydrocarbons)
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Heat Release Rate: Perfect Combustion• Net heat of combustion = HC (kJ / g)
• Get theoretical maximum heat release rate (kw)
Eqn (5-1)
• = mass loss rate of fuel (kg s-1)
••
= mHQ CMAX
•
m
3
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Example Calculation of Equivalence RatioPost-flashover Fires Involving Wooden Cribs
• Harmathy (1972) identified two burning regimes for room fires involving wooden cribs: ventilation-controlled & fuel-surface controlled
• = mass loss rate of fuel (kg s-1)
• Θ = ventilation parameter (kg s-1)
=
• Af = exposed surface area of fuel (m2)
•
= mR
hA 3.76h gA =Oρ
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Example Calculation of Equivalence RatioPost-flashover Fires Involving Wooden Cribs
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Example Calculation of Equivalence RatioPost-flashover Fires Involving Wooden Cribs
• Post-flashover fire is ventilation-controlled if
Θ / Af < 0.63 kg m-2 s-1
Eqn (5-8)
• Fuel mass loss rate is
Eqn (5-9)
1/2f m 0.07AhA <
1
1
s kg hA 0.09m
s kg 0.0236m−
•
−•
=
Θ=
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Rate of Entry of Air - From Lecture 4
• C = 0.68; ρa = 1.2 kg m-3; g = 9.8 m s-2 and A = b h
2/3
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3/23/1a
11
1 g TT1 2 hh b Cm
f
aa
+
+
−=
•
•
•
aa
f
m
mTT
ρ
2/3
3/23/1
11
1 TT1 hA .42m
f
aa
+
+
−=
•
•
•
aa
f
m
mTT
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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The Coefficient C1
hA Cm 1a =•
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Assume Tf ~ 1000°C = 1273 K
• Solve by iteration: 1st guess:
• Find:
3/22/3a
a
mm1 1.61
hA 2.1 m
++
=••
•
1s kg hA 0.09m −•
=
1s kg hA 0.50ma−
•
=
1s kg hA 0.45ma−
•
=
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Example Calculation of Equivalence RatioPost-flashover Fires Involving Wooden Cribs
• For ventilation-controlled post-flashover fire
r = 4.6
• Equivalence ratio is Φ ~ 0.92
1s kg hA 0.09m −•
=
1s kg hA 0.45ma−
•
=
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Post-flashover Fires Involving Wood, PMMA & PE
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Example Calculation of Equivalence RatioPost-flashover Fires Involving PMMA Cribs
• For ventilation-controlled post-flashover fire
r = 8.27
• Equivalence ratio is Φ ~ 1.65
1s kg hA 0.09m −•
=
1s kg hA 0.45ma−
•
=
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Example Calculation of Equivalence RatioPost-flashover Fires Involving PE
• For ventilation-controlled post-flashover fire
r = 14.76
• Equivalence ratio is Φ ~ 2.95
1s kg hA 0.09m −•
=
1s kg hA 0.45ma−
•
=
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Impact of Ventilation on Combustion Dynamics
• Many small-scale experiments have been conducted to assess the impact of ventilation on heat release and generation of chemical species employing– FMRC Flammability apparatus (small-scale)– Fire Research Institute enclosure (0.022 m3)
• Limited full-scale experimental data
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Impact of Ventilation on Heat Release
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Impact of Ventilation on Heat Release
Eqn (5-10)
• Hch(Φ<<1) = well-ventilated limit of the chemical heat of combustion
• Experimental data ⇒ correlation of the form
Eqn (5-11)
• Correlation holds for non-halogenated polymers. For halogenated polymers like PVC, a different correlation applies.
)1(H)(H
ch
ch<<Φ
Φ=chξ
( )2.1 2.5exp 0.971 −Φ−−=chξ
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Impact of Ventilation on Convective Heat Release
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Impact of Ventilation on Convective Heat Release
Eqn (5-12)
• Hconv(Φ<<1) = well-ventilated limit of convective heat of combustion
• Experimental data ⇒ correlation of the form
Eqn (5-13)
• Higher fraction of chemical heat of combustion is converted to radiative heat of combustion as move from well-ventilated to poorly-ventilated conditions
• For halogenated polymers (PVC), different correlation
)1(H)(H
convconv
<<ΦΦ=conξ
( )8.2 2.5exp 1 −Φ−−=conξ
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Impact of Ventilation on Consumption of O2
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Impact of Ventilation on Consumption of O2
Eqn (5-14)
• CO2 = mass of O2 consumed per mass of fuel
• Experimental data ⇒ correlation of the form
Eqn (5-15)
• Compare Eqn (5-15) with Eqn (5-11)
• For halogenated polymers (PVC), different correlation
( )2.1 2.5exp 0.9712
−Φ−−=Oξ
)1(C)(C
2
2
O
OO2 <<Φ
Φ=ξ
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Impact of Ventilation on Generation of CO2
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Impact of Ventilation on Generation of CO2
Eqn (5-16)
• YCO2 = Yield of CO2 = mass CO2 generated / mass of fuel
• Experimental data ⇒ correlation of the form
Eqn (5-17)
• Compare Eqn (5-17) with Eqns (5-15) & Eqn (5-11)
• For halogenated polymers (PVC), different correlation
( )2.1 2.5exp 12
−Φ−−=COξ
)1(Y)(Y
2
2
CO
COCO2 <<Φ
Φ=ξ
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Impact of Ventilation on Generation of CO
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Impact of Ventilation on Generation of CO
Eqn (5-18)
• YCO = Yield of CO = mass of CO generated / mass of fuel
• Experimental data ⇒ correlation of the form
Eqn (5-19)
)1(Y)(Y
CO
COCO <<Φ
Φ=ξ
( )βαξ −Φ−+= 2.5exp 1CO
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Engineering PredictionsYields of Chemical Species in Fire
• During all stages of fire, yield of most species (CO2, soot, HCl and HCN in real-scale scenario is same as in bench-scale tests (same Φ)
• During early stages of room fire, yield of CO in a real-scale scenario is similar to bench-scale tests (same Φ)
• Following flashover, yield of CO is independent of chemical structure of fuel. Bench-scale tests cannot accurately predict CO yields in post-flashover fires.
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Engineering PredictionsPrediction of Yield of CO
• Important because CO inhalation is most common cause of death in fires (USA)
• Death patterns ⇒ need need CO prediction methods for post-flashover fires {0.5 < Φ < 3.0}
• For post-flashover fires assume (within the enclosure)
YCO = 0.2 Eqn (5-24)
Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #
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Prediction of Yield of CO (outside Enclosure)• Consider flame exiting room (post-flashover fire)
– If flame rises vertically, does not impinge physical obstacles, and is in an area of plentiful O2,• CO is incinerated. YCO = well-ventilated limit
since Φ is small.
– If flame is flattened horizontally against a ceiling, impinges obstacles (heat sinks) or gets air from a long corridor• Little incineration of CO. YCO = 0.2 as in room.
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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #