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Fire Dynamics II

Fire Dynamics II. Lecture # 8 Flame Spread & Burning Rates Jim Mehaffey 82.583. 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

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Fire Dynamics II

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  1. Fire Dynamics II Lecture # 8 Flame Spread & Burning Rates Jim Mehaffey 82.583 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  4. Factors Affecting Rate of Spread of Flame • Environmental Factors • Composition of atmosphere • Temperature • Imposed heat flux • Air velocity Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  5. Spread of Flame over Wall Linings Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  6. Spread of Flame over Wall Linings Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  7. Room Fire Test - Apparatus • ISO 9705 “Fire tests: Full scale room fire tests for surface products” Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  9. Room Fire Test - Results Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  11. Wind-aided Spread Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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: Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  16. Chronology of Fire • 00:00 Fire commences within apartment 965 • 13:00 Outer surface of PMMA glazing (9th storey) ignites • 18:00 Outer surface of PMMA glazing (10th storey) ignites • 20:00 Outer surface of PMMA glazing (11th storey) ignites • 22:00 Outer surface of PMMA glazing (12th storey) ignites • 23:00 Outer surface of PMMA glazing (13th storey) ignites • 23:30 Outer surface of PMMA glazing (14th storey) ignites • 24:00 Outer surface of PMMA glazing (15th storey) ignites • 24:20 Outer surface of PMMA glazing (16th storey) ignites • 24:40 Outer surface of PMMA glazing (17th storey) ignites • 25:00 Outer surface of PMMA glazing (18th storey) ignites • 25:15 Outer surface of PMMA glazing (19th storey) ignites • 25:30 Outer surface of PMMA glazing (20th storey) ignites Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  17. - - - - inner surface burning ——— outer surface burning  Ignition of outer side of PMMA O Burn-out of PMMA Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  19. 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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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” Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  23. LIFT Apparatus Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  24. LIFT Apparatus - Results Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  26. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  27. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  28. Thermal Properties for Ignition, Flame Spread & Pre-flashover Fires (1) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 300°C, 400°C, 500°C and 600°C. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 Tewarson’s 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. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  31. Problem Set 3: Problem 6 6. Consider the room of Problem 3. For upper layer temperatures of 300°C and 400°C, 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). Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  32. Burning Rates of Common Items * In the open (review) * Limited by ventilation * Enhanced by radiation Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  33. Wooden Cribs (2) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 >> D} •  = density of sticks (kg m-3) • mo = initial mass of crib (kg) = N n  D2 L Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  35. Steady-State Burning of Wooden Cribs • Fuel surface controlled burning: Stick surfaces burn freely {S >> 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) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 << D} Eqn (8-6) • for t > to: is given by lesser of Eqns (8-5) & (8-6) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 < 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) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  41. Post-flashover Fires Involving Wooden Cribs Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  42. Post-flashover Fires Involving Wooden Cribs • Post-flashover fire is ventilation-controlled if  / Af < 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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  43. Wooden Pallets (1) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  44. Wooden Pallets Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  45. Wooden Pallets - Peak Burning (in the open) Eqn (8-11) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  46. Wooden Pallets - Theory vs. Experiment Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  47. Wooden Pallets • For non-standard pallet sizes, Eqn (8-12) • Heat release rate per unit floor area covered by pallet stack Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

  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 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

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