Fire Dynamics I

1
Fire Dynamics I

• Lecture 3
• Combustion Dynamics
• Flame Chemistry
• Jim Mehaffey
• 82.575 or CVG7300

2

• Combustion Dynamics Flame Chemistry
• Outline
• Incomplete combustion
• Mechanisms of gas phase combustion
• Free radicals (scavenging)
• Temperature of flames
• Life safety considerations

3

• Heat Release Rate Idealized
• In Lecture 2, assumed complete combustion
  Fuel burns and CO2 H2O are generated. True if
  fuel burns in excess of pure O2.
• Net heat of combustion HC (kJ / g)
• Heat release rate / unit area is
• (kW m-2) Eqn (3-1)
• For complete combustion get the theoretical
  maximum heat release rate / unit area

4

• Heat Release Rates Well-ventilated Fires
• Well-ventilated fires experience incomplete
  combustion so CO2, H2O, CO, hydrocarbons soot
  are generated.
• Reduction in combustion efficiency means net heat
  of combustion is not released
• Actual (chemical) heat release rate / unit area
  is
• (kW m-2) Eqn (3-2)
• Hch Actual (chemical) heat of combustion (kJ /
  g)

5

• Heat Release Rates Well-ventilated Fires
• Combustion efficiency is defined as
• ? Hch / HC Eqn (3-3)
• Rate heat is convected above flame
• (kW m-2) Eqn (3-4)
• Rate heat is radiated away by flame
• (kW m-2) Eqn (3-5)

6

• Heat Release Well-ventilated Fires (2)

7

• Rate of Generation of Chemical Species
  Well-ventilated Fires
• Rate of generation of chemical species is
  proportional to rate of generation of volatiles
• (g m-2) Eqn (3-6)
• Yi Yield of species i (g / g)

8

• Well-ventilated Methanol Fires

9

• Well-ventilated Methanol Fires
• or
• Complete Combustion of Methanol
• 2 CH3OH 3 O2 ? 2 C O2 4 H2O
• (64 g) (96 g) (88 g) (72 g)
• Maximum possible yield

10

• Yields in Well-ventilated Methanol Fires
• Carbon dioxide
• Carbon monoxide
• Hydrocarbons
• Soot (carbon)

11

• Chemical Species Well-ventilated Fires (2)

12

• Flame Chemistry
• Gas Phase Combustion
• Global chemical eqn for complete combustion of
  methane CH4
• CH4 2 O2 ? CO2 2 H2O Eqn (3-7)
• Defines stoichiometry but hides complexity of
  overall process
• Actual mechanism Series of steps involving
  highly reactive molecular fragments (atoms free
  radicals)
• Fragments have transient existence

13

• Reaction Scheme Combustion of Methane (1)

14

• Free Radical Scavenging
• Atomic hydrogen is critical to generation of
  radicals (step e)
• Gas-phase oxidation can be inhibited by free
  radical scavenging
• For example
• HCl H ? H2 Cl
• Reactive H has been replaced with less reactive Cl

15

• Incomplete Combustion
• Formaldehyde CH2O and carbon monoxide CO are
  produced as intermediates
• If reaction sequence is interrupted or if there
  is not enough O2 then get incomplete combustion.
  CH2O and CO become end products

16

• Fuels More Complex Than CH4
• Complexity of gas-phase oxidation increases with
  complexity and size of vapour molecules
• Number of partially oxidized species (products)
  may be large
• Poor ventilation enhances yield of CO and other
  products of incomplete combustion
• Non-gaseous products of incomplete combustion
• High boiling point liquids tars (condense
  on cooling to produce aerosols)
• Carbonaceous particles - soot

17

• Temperature of Flames
• Flame temperature depends on rate of heat release
  and rate of heat loss
• Rate of heat release given by
• Eqn (2-2)
• or Eqn (3-8)
• Rate of heat release and rate of heat loss often
  difficult to calculate

18

• Simple Example Premixed Flames
• Fuel and air intimately mixed
• Reaction rates are high
• Adiabatic assumption - no heat loss. Heat
  generated remains in flame causes increase in
  temperature.

19

• Simple Example Premixed Flames
• Calculate the adiabatic flame temperature in a
  stoichiometric mixture of propane in air
• Oxidation reaction in air is given by
• C3H8 5 O2 5 x 3.76 N2 ? 3 CO2 4 H2O
  18.8 N2
• Heat released in complete combustion of 1 mole of
  propane is 2044 kJ (see slide 2-30)
• This heats up 3 moles of CO2, 4 moles of H2O and
  18.8 moles of N2

20

• Specific Heat of Products of Combustion
• Specific heat, CP, at 1000 K

21

• Simple Example Premixed Flames
• Assume adiabatic conditions initial temp. of
  20C
• Final flame temperature (Tf) is
• Tf To HC / nCP
• 293 K 2,044,000 J / (942.5 J K-1)
• 2462 K
• 2189 C
• (Note Flame propagates through the mixture)

22

• Shortcomings in Adiabatic
• Temperature Calculations
• Specific heat of each gas is temperature
  dependent
• There are radiation losses from flame
• Above 2000 K, products of combustion undergo
  dissociation with absorption of significant heat

23

• Lower Flammability Limit
• Adiabatic temperature useful for assessing
  flammability limits
• Minimum adiabatic temperature for which flame can
  propagate is 1600 ? 100 K
• Not all mixtures of flammable gas air burn if
  subjected to ignition source
• Flammable region bounded by upper lower
  flammability limits
• These limits can be determined quite accurately

24

• Lower Flammability Limit - Propane
• Experiment 2.2 (by volume or mole) propane in
  air
• Mixture is fuel lean so assume complete
  combustion
• 0.022 C3H8 0.978 (0.21 O2 0.79 N2)
• ? products (CO2,
  H2O, O2 and N2)
• Dividing through by 0.022 and balancing the
  equation
• C3H8 9.335 O2 35.119 N2
• ? 3 CO2 4 H2O 4.335 O2
  35.119 N2
• Excess O2 and N2 contribute to specific heat of
  product mixture

25

• Lower Flammability Limit - Propane
• Adiabatic flame temperature 1554 K 1281C
• Within the range 1600 ? 100 K
• Range applies to upper flammability limit, but
  cant calculate adiabatic temperature same way in
  fuel rich limit since combustion product
  mixture is more complex

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• Impact of Temperature Increase
• at Constant Pressure
• V2 / V1 (n2T2) / (n1T1)
• n2 / n1 1
• T2 / T1 2462 K / 293 K 8.4
• V2 / V1 8
• Hot gas expands and spreads rapidly through a
  building

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• Impact of Temperature Increase
• at Constant Volume
• P2 / P1 (n2T2) / (n1T1)
• n2 / n1 1
• T2 / T1 2462 K / 293 K 8.4
• P2 / P1 8
• Rapid pressure rise can cause structural damage.
• Design solution Provide means of relieving
  pressure with weakened panels in building
  envelope.

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• Life Safety Considerations
• Smoke solids, liquids gases
• Hazards presented by smoke
• toxicity
• obscure visibility
• excessive thermal exposure
• Consider exposure conditions which may prevent
  occupants of average susceptibility from escaping
  unassisted
• Adverse effects following exposure not considered

29

• Toxic Gases - Asphyxiants
• Reduces O2 for central nervous cardiovascular
  systems
• Can cause loss of consciousness death
• Effect depends on exposure concentration
  duration
• Examples CO, HCN, CO2 and O2 (depletion)
• CO is most common asphyxiant
• HCN is about 25 times more toxic than CO
• O2 depletion not severe if concentration gt 13
• CO2 not significant asphyxiant in fire
  atmospheres, but at concentration above 2 causes
  hyperventilation

30

• Toxic Gases - Irritants
• Stimulate nerve receptors in eyes, nose, mouth,
  throat respiratory tract, causing discomfort
  and pain
• Effect depends on exposure concentration only
  (not duration)
• Examples HCl and acrolein
• Sensory effect eyes upper respiratory system
  and can seriously impede impede or prevent escape
  
• Pulmonary effect the lungs and can result in
  problems or death following fire

31

• Carbon Monoxide
• Suppose CO is the only toxicant present
• Maximum time, t (min), that the average human can
  remain in an atmosphere with high levels of CO
  concentration VCO of CO in ppm is
• t 35,000 / VCO Eqn (3-9)
• If the concentration of CO is time dependent,
  which it usually is, then
• Eqn (3-10)

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• Fractional Effective Dose (FED)
• When CO and HCN are present
• Eqn (3-11)
• When FED 1.0 average human incapacitated
• For design might choose FED 0.3
• If concentration of CO2 exceeds 2 multiply VCO
  and VHCN in Eqn (3-11) by exp(CO2 / 5) to
  account for increased asphyxiant uptake due to
  hyperventilation

33

• Hydrogen Chloride
• Suppose HCl is the only toxicant present
• The average human can endure an atmosphere with
  high levels of HCl provided
• VHCl lt 1,000 ppm Eqn (3-12)

34

• Fractional Effective Concentration (FEC)
• When HCl and acrolein are present
• FEC (t) VHCl / 1,000 ppm Vacrolein / 30
  ppm Eqn (3-12)
• When FEC 1.0 average human incapacitated
• For design might choose FEC 0.3
• If concentration of CO2 exceeds 2 multiply VCO
  and VHCN in Eqn (3-11) by exp(CO2 / 5) to
  account for increased asphyxiant uptake due to
  hyperventilation

35

• Visibility in Smoke
• Fraction of light passing through smoke is given
  by Bouguers Law
• I / Io exp (- K L)
  Eqn (3-13)
• Io initial intensity of light (arbitrary
  units)
• I intensity after passing through smoke
• L optical path length (m)
• K extinction coefficient (m-1)
• Expressed in terms of base 10
• I / Io 10-DL
  Eqn (3-14)
• D optical density per meter (m-1)
• D K / 2.3 Eqn (3-15)

36

• Visibility in Smoke
• ? light-emitting sign
• ? light-reflecting sign

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• Visibility in Smoke
• S visibility (m)
• For light-emitting signs KS 8
• For light-reflecting signs KS 3 Eqn
  (3-14)
• Data based on subjects viewing smoke through
  glass so irritant effect of smoke eliminated, so
  visibility may be reduced compared with Eqn
  (3-14).

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• Extinction Coefficient K
• Proportional to mass concentration of soot
• K Km Cs Eqn
  (3-15)
• Km specific extinction coefficient (m2 / g)
• Cs mass concentration of smoke (g / m3)
• For flaming combustion of wood plastics
• Km 7.6 m2 / g Eqn (3-16)
• For pyrolysis (no flaming) of wood plastics
• Km 4.4 m2 / g Eqn (3-17)

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• For a Closed System
• Mass concentration of smoke (g / m3)
• Cs ms / V Ys mf / V Eqn
  (3-18)
• ms mass of soot produced (g)
• V volume occupied by smoke (m3)
• Ys yield of soot (g / g)
• mf mass of fuel volatilized (g)

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• References
• 1. D. Drysdale, An Introduction to Fire
  Dynamics,Wiley, 1999, Chap 1
• 2. A. Tewardson, Generation of Heat and
  Chemical Compounds in Fires"
  Section 3 / Chapter 4, SFPE Handbook, 2nd Ed.
  (1995)
• 3. ISO/DTS 13571, Life threat from fires -
  guidance on the estimation of the time available
  for escape using fire data.