1 / 44

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

belisma
Télécharger la présentation

Fire Dynamics II

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related