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Fire Dynamics I

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Carleton University, 82.575 (CVG7300), Fire Dynamics I, Winter 2002, Lecture # 3 ... When FED = 1.0 average human incapacitated. For design might choose FED = 0.3 ... – PowerPoint PPT presentation

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Title: 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

26
  • 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

27
  • 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.

28
  • 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)

32
  • 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

37
  • 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).

38
  • 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)

39
  • 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)

40
  • 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.
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