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Fire Dynamics II

Fire Dynamics II. Lecture # 12 Other Important Phenomena Jim Mehaffey 82.583. Other Important Phenomena Outline Post-flashover fires in large compartments Flames issuing through windows Explosions Backdrafts BLEVEs. Post-flashover Fires in Large Compartments

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Fire Dynamics II

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  1. Fire Dynamics II Lecture # 12 Other Important Phenomena Jim Mehaffey 82.583 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  2. Other Important Phenomena Outline • Post-flashover fires in large compartments • Flames issuing through windows • Explosions • Backdrafts • BLEVEs Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  3. Post-flashover Fires in Large Compartments • Gordon Cooke, Tests to determine the behaviour of fully developed natural fires in a large compartment, Fire Note 4, Fire Research Station, British Research Establishment, 1998 • 9 Post-flashover fires • Basic compartment: 23 m deep, 6 m wide, 3 m high • Objective: simulate an even larger compartment in an open plan office building by allowing no net heat transfer to neighbouring compartments • if only 2 sides of bldg have windows, after flashover there is line of symmetry along centre line of storey • ensure separation walls are well insulated Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  4. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  5. Ventilation opening in one of the 6 m x 3 m end walls • not glazed (open from outset) • 12.5%, 25% 50% or 100% of area of end wall • 12.5% simulated fire in basement with ventilation at top • Fuel load: 20 kg m-2 or 40 kg m-2 • 33 wood cribs: 11 rows of 3 cribs, 1 m apart • D = 50 mm; L = 1.0 m; • 1 crib = 155 sticks in 15 layers for 40 kg m-2 • 1 crib = 75 sticks in 7 layers for 20 kg m-2 • 6 cribs (every other crib) along centre line on load cell Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  6. Distribution of Cribs Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  7. Room linings: • walls and ceiling: insulating ceramic fibre blanket • floor: layer of dry sand • Temperature measured in two locations: • 150 mm below ceiling 6.0 m from rear of compartment • 150 mm below ceiling 6.0 m from front of compartment • Ignition sequence in 8 tests: Ignite row of cribs furthest from ventilation opening and observe spread of fire Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  8. Description of Tests Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  9. Mass Loss of Cribs Measured in Test 1 • 1 = mass loss of central crib in row farthest from opening • 11 = mass loss of central crib in row closest to opening Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  10. Temperatures in Test 1 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  11. Temperatures in Test 1 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  12. Analysis of Test 1 • Quantity of fuel: • G = 40 kg m-2 x 6 m x 23 m = 5,520 kg • Surface area of fuel: • (Surface area 1 stick) x (no. sticks / crib) x (no. cribs) • Af = (4 x 0.05 m x 1.0 m) x 155 x 33 = 1,023 m2 • Ventilation opening: • Duration of fire: Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  13. Model for rate of burning in deep compartments: W = width of compartment (m) D = width of compartment (m) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  14. Analysis of Test 1 W = 6 m D = 23 m AT = 2 x 6 x 23 + 2 x 3 x 23 + 2 x 3 x 6 - 3 x 6 = 432 m2 tD = 5,520 kg / 1.12 kg s-1 = 4929 s = 82 min Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  15. Flames Issuing through Windows • Flame issuing from window of compartment experiencing post-flashover fire is characterised by the flame length • For ventilation-controlled fire with wood cribs • For ventilation-controlled wood-crib post-flashover fire zf = 0.33 h Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  16. Flames Issuing through Windows • For ventilation-controlled wood-crib fires, we have close to stoichiometric fires (equivalence ratio ~ 0.92) • For other fuels, like gasoline, most plastics, or wood panelling, the mass loss rate is much greater than for a ventilation-controlled wood-crib fire • Not enough air can get into the room to burn the fuel vapours (equivalence ratio > 1) within the room so flaming continues outside the room • Consequently flame length will also be much greater Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  17. Explosions • Premixed: Fuel well mixed with air (O2) before burning • Flammability limits: Mixture will only burn if concentration is between LFL and UFL • Minimum ignition energy (MIE) required for ignition • Rate of combustion is high: Governed by chemical kinetics not mixing rate • Deflagration: Combustion propagates through mixture as a flame (below speed of sound) • If mixture is confined, walls & ceiling may not be able to withstand pressure rise  explosion • masonry wall cannot withstand P > 0.035 atms Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  18. Examples • Methane CH4 at T=25ºC & P=1 atm LFL = 5% (by vol); UFL = 15% (by vol); MIE = 0.26 mJ • Propane C3H8 at T=25ºC & P=1 atm LFL = 2.1% (by vol); UFL = 9.5% (by vol); MIE = 0.25 mJ **************************************************************** • For alkanes (gaseous): LFL ~ 48 g m-3 • For aerosol or droplet suspension: LFL ~ 45-50 g m-3 • For dust (< 100 m): LFL ~ 30-60 g m-3 • usually a two-event phenomenon Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  19. Deflagration Mitigation • Prevention: • Reduction of concentration of flammables (by ventilation for vapours or housekeeping for dusts) • Control potential ignition sources (mechanical sparks, hot surfaces, electrical equipment) • Rapid suppression: terminate combustion by very rapid introduction of inert gas or chemical inhibitor • Protection: • Venting Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  20. Deflagration Venting • Objective: Design vents to relieve pressures developed by a deflagration • NFPA 68: Guide for Venting of Deflagrations • Rate of pressure rise is used in design of deflagration venting for high strength enclosures. • Rapid rate of rise means short time available to vent • Rapid rate of rise requires greater area for venting • Pred = maximum pressure attained during venting is commonly set at 2/3 of enclosure strength • Pred is used in design of deflagration venting for low strength enclosures Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  21. Pressure Considerations • Assume gas obeys the ideal gas law P V = n R T • Fire Dynamics I: Adiabatic flame temperature of a stoichiometric mixture of propane in air: T ~ 2462 K • In enclosure without vents, volume is constant P2 / P1 = (n2T2) / (n1T1) n2 / n1 ~ 1 T2 / T1 ~ 2462 K / 293 K ~ 8.4 P2 / P1 ~ 8.4 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  22. Pressure Considerations • Maximum deflagration pressure and rate of pressure rise dP/dt are determined by test • For most fuels maximum pressure rise is 6 to 10 times pressure before ignition • Fundamental basis for deflagration venting theory is the cubic law: K = deflagration index V = volume of enclosure Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  23. Examples (at optimal concentrations) • Methane CH4 Pmax ~ 7.1 atm; K ~ 55 atm m/s) • Propane C3H8 Pmax ~ 7.9 atm; K ~ 100 atm m/s • Dusts Pmax ~ 10-12 atm; K ~ 200-300 atm m/s Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  24. Deflagration Venting • Low strength enclosures cannot withstand P > 0.1 atm. Gas or mist deflagrations can be vented with vents with combined area AV = vent area (m2) AS = internal surface area of enclosure (m2) C = venting constant (for methane = 0.037 atm1/2) Pred = maximum P permitted (2/3 enclosure strength, atm) • Expansion through vent causes fireball outside enclosure. Must be considered when placing vents Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  25. Backdrafts • Limited ventilation  large quantity of unburnt “gas” (products of pyrolysis or incomplete combustion) generated • When opening suddenly introduced, inflowing air mixes with “gas” creating flammable mixture • Ignition source (smouldering material) ignites flammable mixture, resulting in extremely rapid burning • Expansion due to heat released expels burning “gas” through opening & causes fireball outside enclosure • Backdrafts extremely hazardous for firefighters • Backdraft of short duration. Flashover often follows Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  26. Backdraft Experiments: Fleischmann • 70 kW methane flame burned in a small “sealed” chamber • Flame eventually self-extinguished due to oxygen starvation • Vent opened, air enters • Continuous ignition source present near back of chamber • Observed a backdraft Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  27. 5.6 s after opening the vent Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  28. 7.1 s after opening the vent Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  29. 8.0 s after opening the vent Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  30. Backdraft Schematic of temperature Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  31. Kemano: Fire in Basement Recreation Room • Room dimensions: 3.25 m x 3.44 m x 2.2 m (height) • Walls: 2 gypsum board // 2 (6 mm) wood panelling • Ceiling: gypsum board • Floor: carpet over concrete • Furnishings: couch / coffee table / TV on wood desk • Ventilation: no window / hollow-core wood door closed Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  32. Temperatures in Basement Fire • Temperature predictions from Lecture 3 for leaky enclosures (based on oxygen depletion): • For a heat loss fraction 1= 0.9, Tg,lim = 120 K • For a heat loss fraction 1= 0.6, Tg,lim = 480 K • 1 = 0.6 appropriate for spaces with smooth ceilings & large ceiling area to height ratios • 1 = 0.9 appropriate for spaces with irregular ceiling shapes, small ceiling area to height ratios & where fires are located against walls Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  33. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  34. BLEVE: Boiling Liquid Expanding Vapour Explosion • Propane is a gas under atmospheric conditions • Liquified by application of pressure & stored in tank • In tank, liquid & vapour at equilibrium, with vapour at high pressure • If tank immersed in fire, heat causes pressure of vapour to rise • Activates relief valve (turbulent jet flame) • Pressure still high & fire may weaken metal casing • Tank ruptures  BLEVE Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  35. What is a Liquified Gas? • Gas = a substance that exist in the gaseous state at standard temperature (20°C) and pressure (101 kPa) • Economic necessity and ease of usage  gas stored in containers containing as much gas as practical • Compressed gas = stored in a container under pressure but remains gaseous at 20°C. Typical pressure range is 3 to 240 atm • Liquified gas = stored in a container under pressure and exists partly in liquid and partly in gaseous state. Pressure depends on temperature of liquid. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  36. Heating of a Container Containing Compressed Gas • Compressed gas obeys ideal gas law PV = nRT • V & n are constant so pressure rises according to P2 = P1 T2 / T1 Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  37. Heating of a Container Containing Liquified Gas • Liquified gas exhibits more complex behaviour because net effect is a combination of three effects • Gas phase is subject to same effect as compressed gas • Liquid attempts to expand, compressing vapour • Vapour pressure increases as temperature of liquid increases • Combined result: an increase in pressure Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  38. Overpressure Relief Devices • Spring-loaded pressure-relief valves, bursting discs or fusible plugs (small containers) used to limit pressure to a level the container can safely withstand P(activation) > P(operating) >> P(atmospheric) • Relieving capacity (gas flow rate through device) is based on maximum heat input rates resulting from fire exposure • Gas discharge is in the form of a turbulent jet and if the gas is flammable, it will be a turbulent jet flame Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  39. Behaviour of liquified gas metal container (carbon steel) when exposed to fire Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  40. Failure of Container • Precise curves a little different for other steels, but loss of strength is significant as temperature climbs • Spring-loaded relief valve only reduces pressure to activation pressure • Pressure remains high in container • container stressed in tension • Liquid always at temp > normal boiling point • When exposed to fire, metal in contact with vapour phase heats up, may stretch and a rupture develop • Before rupture relieves pressure, it propagates and container fails catastrophically Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  41. Potential for Rapid Vaporization of Liquid • Liquified gases are stored at high pressure, in containers at temperature (~ 20°C) > boiling point at atmospheric pressure (101 kPa) • e.g. boiling point at 1 atm of propane (C3H8) = - 42°C • Pressure drop to 1 atmosphere (failure of container) causes very rapid vaporization of a portion of liquid • Fraction vaporized depends on temperature difference between liquid at failure and its normal boiling point • For fire induced failure about 1/2 of liquid is vaporized Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  42. After Failure of the Container: A BLEVE • Pressure difference, inside to outside, propels pieces of the container at high velocity for some distance (up to 1.0 km) • Liquid vaporizes and vapour expands rapidly • Rapid turbulent mixing of vapour and air • If vapour is flammable, observe a huge fireball (diameter up to 150 m) Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  43. A Fireball Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

  44. Protection against a BLEVE • Insulate the container • Apply water: Create a film of water coating portions of container not in internal contact with liquid Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

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