Fire Dynamics II

1
Fire Dynamics II

• Lecture 8
• Flame Spread Burning Rates
• Jim Mehaffey
• 82.583

2

• Flame Spread Burning Rates
• Outline
• Models for flame spread on solids (review)
• wind-aided vs opposed-flow flame spread
• in the absence or presence of external radiation
• Burning rates of common items
• in the open (review)
• limited by ventilation
• enhanced by radiation

3

• Factors Affecting Rate of Spread of Flame
• Material Factors
• Chemical Composition of fuel
• Presence of fire retardants
• Physical Initial temperature
• Surface orientation
• Direction of propagation
• Thickness
• Thermal conductivity
• Specific heat
• Density
• Geometry
• Continuity

4

• Factors Affecting Rate of Spread of Flame
• Environmental Factors
• Composition of atmosphere
• Temperature
• Imposed heat flux
• Air velocity

5

• Spread of Flame over Wall Linings

6

• Spread of Flame over Wall Linings

7

• Room Fire Test - Apparatus
• ISO 9705 Fire tests Full scale room fire tests
  for surface products

8

• Room Fire Test - Procedure
• Line walls and ceiling with product
• Burner in back corner
• First 10 min 100 kW (large wastepaper
  basket)
• Last 10 min 300 kW (small upholstered
  chair)
• Observe time to flashover
• Room experiences flashover when ? 1,000 kW

9

• Room Fire Test - Results

10

• Flame Spread Models Concepts
• Flame spread an advancing ignition front
• Leading edge of flame is heat source (raising
  fuel to ignition temp) and the pilot
• Visually flame spread is advancing flame close to
  solid
• Two interacting advancing fronts
• flame front in gas phase
• pyrolysis front along solid surface
• Heat transfer from flame ? pyrolysis front to
  advance
• Advance of pyrolysis front ? increased release of
  volatiles ? advance of flame front
• Flame-spread velocity ? rate of advance of
  pyrolysis front

11

• Wind-aided Spread

12

• Wind-aided Spread
• ? region of heat transfer from flame smoke
• For wind-aided spread 0.1 m ? ? ? 10 m
• For opposed-flow spread 1 mm ? ? ? 3 mm
• Surface temp in control volume drops from Tig to
  Ts
• Pyrolysis front moves at speed
• Model for wind-aided flame spread

13

• Example of Accelerating Flame Spread
• Upward turbulent spread on thick PMMA
• xb 0 and n 0.94 1
• Eqn (8-1)
• Experiment finding
• When xp 1.0 m, v 5.0 mm s-1

14

• Example of Constant Flame Spread
• Upward turbulent spread on thin textiles
• n 0.6
• Eqn (8-2)
• After some time, (xp - xb) and v become constant

15

• Apartment Fire Hiroshima, Japan (1996)
• Building - reinforced concrete structure
• - 20 storeys
• - height of each storey 3 m
• - each apartment had a balcony
• Balcony - PMMA glazing
• - height of glazing 1 m

16

• Chronology of Fire
• 0000 Fire commences within apartment 965
• 1300 Outer surface of PMMA glazing (9th storey)
  ignites
• 1800 Outer surface of PMMA glazing (10th
  storey) ignites
• 2000 Outer surface of PMMA glazing (11th
  storey) ignites
• 2200 Outer surface of PMMA glazing (12th
  storey) ignites
• 2300 Outer surface of PMMA glazing (13th
  storey) ignites
• 2330 Outer surface of PMMA glazing (14th
  storey) ignites
• 2400 Outer surface of PMMA glazing (15th
  storey) ignites
• 2420 Outer surface of PMMA glazing (16th
  storey) ignites
• 2440 Outer surface of PMMA glazing (17th
  storey) ignites
• 2500 Outer surface of PMMA glazing (18th
  storey) ignites
• 2515 Outer surface of PMMA glazing (19th
  storey) ignites
• 2530 Outer surface of PMMA glazing (20th
  storey) ignites

17

• - - - - inner surface burning outer surface
  burning
• ? Ignition of outer side of PMMA
• O Burn-out of PMMA

18

• Problem Set 3 Problem 4
• 5. In 1975, FMRC studied upward turbulent flame
  spread on thick PMMA and found the process obeyed
  the model for wind-aided flame spread presented
  in class with xb 0 and n 1. They found that
  when the flame extension xp 1 m, the upward
  flame spread velocity V 5 mm/s. Calculate the
  flame extension 2, 4, 6, 8, 10 and 12 minutes
  later. Compare your predictions with the
  observed flame extensions in the Hiroshima fire
  by plotting your predictions on the graph on page
  8-17.

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• Opposed Flow Flame Spread
• Absence of External Radiation
• Compared to PMMA, a very slow process
• Not accelerating, but roughly constant velocity
• Speed of downward flame spread on PMMA
• v 0.04 mm s-1
• v 2.4 mm min-1

20

• Opposed Flow Flame Spread
• In Presence of External Radiation (1)
• Effect of preheating time on rate of downward
  flame spread on PMMA exposed to radiant flux (kW
  m-2)
• CHF (PMMA) 11 kW m-2

21

• Opposed Flow Spread Model for Thick Materials
• Quintiere and Harkleroad, 1985
• Eqn (8-3)
• ? flame-heating parameter (kW2 m-3) material
  property
• Provided no dripping, this model holds for
• downward flame spread (wall)
• lateral flame spread (wall)
• horizontal flame spread (floor)
• ?, k?c and Tig - measured (LIFT apparatus)
• Ts - depends on scenario (external flux)

22

• LIFT Apparatus - Standard Tests
• ASTM E1321, Standard test method for determining
  material ignition and flame spread properties
• ISO 5668, Fire tests Reaction to fire surface
  spread of flame on building products

23

• LIFT Apparatus

24

• LIFT Apparatus - Results

25

• Estimating the Surface Temperature TS
• To employ Eqn (8-3) one must estimate TS
• Assume the surface is heated by a radiant flux
  and cools by convection h (TS -To)
• Following pages 5-35 to 5-38 in Fire Dynamics I
• Eqn (8-4)

26

27

28

• Thermal Properties for Ignition, Flame Spread
  Pre-flashover Fires (1)

29

• Problem Set 3 Problem 3
• 3. Consider a pre-flashover fire in a room 2.4 m
  x 3.6 m x 2.4 m (height). The door to the room
  (0.8 m x 2.0 m (height)) is open and the
  interface between the hot layer and cool air is
  at the mid-height of the door. The fuel in the
  room is a mixture of wood and plastics and the
  mean extinction (absorption) coefficient of the
  upper layer is Km 1.0 m-1. What is the
  emissivity of the upper layer? Calculate the
  radiant flux at the centre of the floor when the
  layer temperature is 300C, 400C, 500C and
  600C.

30

• Problem Set 3 Problem 4
• 4. Calculate the time to piloted ignition of a
  wood floor and a polyurethane cushion at floor
  level for the four upper layer temperatures
  considered in Problem 3. Use Tewarsons model
  assuming for the wooden floor that CHF 10 kW
  m-2 and TRP 134 kW s1/2 m-2, and for the
  polyurethane cushion CHF 11 kW m-2 and TRP 55
  kW s1/2 m-2.

31

• Problem Set 3 Problem 6
• 6. Consider the room of Problem 3. For upper
  layer temperatures of 300C and 400C, calculate
  the flame velocity on a wooden floor and on a
  polyurethane cushion 30 seconds and 1 minute
  after the flux is applied. (Assume that the
  convective cooling is governed by h 9.0 W m-2
  K-1).

32

• Burning Rates of Common Items
• In the open (review)
• Limited by ventilation
• Enhanced by radiation

33

• Wooden Cribs (2)

34

• Wooden Cribs
• D stick thickness (m)
• S spacing between sticks (m)
• hc height of crib (m)
• N number of rows hc / D
• n number of sticks per row
• L length of each stick (m) L gtgt D
• ? density of sticks (kg m-3)
• mo initial mass of crib (kg) N n ? D2 L

35

• Steady-State Burning of Wooden Cribs
• Fuel surface controlled burning Stick surfaces
  burn freely S gtgt D
• Eqn (8-5)
• mass loss rate of crib (kg s-1)
• to time at which steady burning is established
  (s)
• vp surface regression rate (m s-1)

36

• Steady-State Burning of Wooden Cribs
• Crib porosity controlled burning Burning
  controlled by rate of flow of air combustion
  products through holes in crib S ltlt D
• Eqn (8-6)
• for t gt to is given by lesser of Eqns
  (8-5) (8-6)

37

• Growth Rates - Burning of Wooden Cribs
• Assume crib is ignited at bottom / centre
• to time at which steady burning is established
  (s)
• For t lt to
• Eqn (8-7)
• to time Eqn (8-7) yields lesser of Eqns (8-5)
  (8-6)
• 
  
• For a crib ignited at bottom / centre and whose
  steady-state burning is fuel-surface controlled
  to 15.7 n (s)

38

• Wooden Cribs - Heat Release Rate
• The heat release rate is given by
• Eqn (8-8)
• with Hch 12.4 kJ g-1
• Knowing one can also calculate, radiative
  and convective components of heat release rate,
  and rates of generation of CO and soot.

39

• Wooden Cribs in an Enclosure
• Radiation from upper layer has little impact on
  because fire is largely self-contained with
  many surfaces seeing each other.
• If fire is limited by ventilation, will be
  reduced because Hch and are both reduced.

40

• Post-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)
• ? ventilation parameter (kg s-1)
• Af exposed surface area of fuel (m2)

41

• Post-flashover Fires Involving Wooden Cribs

42

• Post-flashover Fires Involving Wooden Cribs
• Post-flashover fire is ventilation-controlled if
• ? / Af lt 0.63 kg m-2 s-1
• Eqn (8-9)
• Fuel mass loss rate is
• Eqn (8-10)
• Least of Eqns (8-5), (8-6), (8-7) or (8-10)
  applies

43

• Wooden Pallets (1)

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• Wooden Pallets

45

• Wooden Pallets - Peak Burning (in the open)
• Eqn (8-11)

46

• Wooden Pallets - Theory vs. Experiment

47

• Wooden Pallets
• For non-standard pallet sizes,
• Eqn (8-12)
• Heat release rate per unit floor area covered by
  pallet stack

48

• Wooden Pallets - Mass Loss Rate
• The heat release rate mass loss rate are
  related by
• Eqn (8-13)
• Implicitly assumed that Hch 12 kJ g-1
• Knowing can calculate, radiative and
  convective components of heat release rate, and
  rates of generation of CO and soot.

49

• Wooden Pallets in an Enclosure
• Radiation from upper layer has little impact on
  because fire is largely self-contained with
  many surfaces seeing each other.
• If fire is limited by ventilation, will be
  reduced.
• Fuel mass loss rate is
• Eqn (8-10)
• Smaller of Eqns (8-11) or (8-10) applies

50

• Unusual Nature of Wooden Cribs Pallets
• Early work on enclosure fires used wood cribs to
  achieve reproducible fires
• However, burning surfaces of wooden cribs
  pallets are shielded from environment within the
  enclosure
• Consequently rate of burning is relatively
  insenstive to the thermal environment
• When wood is present as wall lining, however,
  there is a large exposed area that is sensitive
  to the thermal environment

51

• Diffusion Flames (in the open)

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• Rate of Burning (in the open)
• Eqn (8-14)

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• Heat Release Rates (in the open)
• Fires burning in the open are well-ventilated
• Actual (chemical) heat release rate / unit area
  is
• (kW m-2) Eqn (8-15)
• Hch Actual (chemical) heat of combustion (kJ /
  g)

54

• Consider a material burning in an enclosure but
  getting sufficient air for combustion?

55

• Rate of Burning (Mass Loss Rate)
• Eqn (8-3)
• Eqn (8-4)
• 2nd term can be estimated by open burning models

56

• Example Study of effect of trapping heat on rate
  of burning of slab of PMMA (0.76m x 0.76 m) ()

57

• Observations
• Trapping of heat (radiation from hot layer)
  increases steady-state burning rate of PMMA
• Trapping of heat (radiation from hot layer)
  reduces time to steady-state burning ? rate of
  flame spread across PMMA also increases

58

• Burning rate in post-flashover fires involving
  fuels with exposed surfaces is enhanced by
  radiation
• Large burning rates inhibit inflow of air so
  increase equivalence ratio ? reduced heat release
  (inside)
• Heat release rate still can be ventilation-control
  led

59

• Burning rate as function of radiant intensity at
  ceiling

60

• Burning rate as function of radiant intensity at
  ceiling
• ? ethanol (LV 850 J g-1)
• ? PMMA pool (LV 1,600 J g-1)
• ? polyethylene (LV 22,00 J g-1)
• ? wood (LV 1,340 J g-1)
• ? PMMA crib
• ethanol in open

61

• Pool fire burning rates in the open in
  enclosures
• fex 1 - 1/? (excess fuel factor)(some fuel
  burns outside)

62

• References
• D. Drysdale, An Introduction to Fire
  Dynamics,Wiley, 1999, Chap 1
• F.W. Billmeyer, Textbook of Polymer Science,
  Wiley, 1984, Chap 1
• Donald R. Askeland, Science and Engineering of
  Materials, Chapman Hall, 1990, Chapter 15
• C.F. Cullis and M.M. Hirschler, The Combustion of
  Organic Polymers, Oxford Science Publications,
  1981, Chapter 1
• C.L. Beyler and M.M. Hirschler, "Thermal
  Decomposition of Polymers" Section 1 / Chapter 7,
  SFPE Handbook, 2nd Ed. (1995)
• C.F. Cullis and M.M. Hirschler, The Combustion of
  Organic Polymers, Oxford Science Publications,
  1981, Chapter 1