MODELS FOR FIRE GROWTH AND SPREAD

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MODELS FOR FIRE GROWTH AND SPREAD Powered By Docstoc
					Fire Spread on Walls and Ceiling
          to Flashover

               J. G. Quintiere
      Department of Fire Protection Engineering
           UNIVERSITY OF MARYLAND
       NIST/BFRL Workshop on Fire Growth
                 March 4-5, 2002
Engineering Model: Combustion

• Flame heat flux
• Flame extension
  – from measurements or correlations
• Difficulties with prediction
  – various modes of spread
  – reaction region, radiation, turbulence
               FDS & DATA of Back et al.
                                                Centerline Heat Flux
                                               Measured Vs. Predicted
             140
                                                                                         Test 1, 53 kW
                                                                                         Test 2, 56 kW
             120                                                                         Test 3, 68 kW
                                                                                         Test 4, 106 kW
                                                                                         Test 5, 136 kW
             100                                                                         Test 6, 204 kW
                                                                                         Test 7, 220 kW
                                                                                         Test 8, 313 kW
HF (kW/m2)




             80                                                                          Test 9, 523 kW
                                                                                         Predicted Test1
                                                                                         Predicted Test2
             60                                                                          Predicted Test3
                                                                                         Predicted Test4
                                                                                         Predicted Test5
             40                                                                          Predicted Test6
                                                                                         Predicted Test7
                                                                                         Predicted Test8
             20
                                                                                         Predicted Test9


               0
               0.000   0.500   1.000   1.500     2.000   2.500   3.000   3.500   4.000       4.500     5.000
                                                         Z (m)
Engineering Model: Material
• Properties from tests and analysis

• Consistent with model

• Measurable and consistent

• Time-averaged
Spread Model

             dx p          xf − xp
     vp =              =
                 dt          t ig
                                     δ




       φ
                            xf
            xb        xp
Ignition Theory


 Thin : tig = ρcd (Tig − To ) / q′′
                                &
                           [
 Thick : tig = (π / 4)kρc (Tig − To ) / q′′
                                        &     ]
                                              2
Example for a Thin Material
                       0o
 0o
                              30o
          30o




                60o                 60o




                 90o                 90o
Aircraft Acoustic Insulation
Measured Flame Lengths used to
compute gravity-assisted spread
             250

             200
                                                              MPET
             150                                              Napkin
   xf (mm)




             100
                        Bottom                           Top
              50
                                           UP

                0

              -50
                                         DOWN

             -100
                 -100       -50             0             50           100
                             Angle from Vertical, phi (deg)
Flame Length Correlation (Ahmad & Faeth)
Gravity Assisted Spread


                                         Turbulent Wall Flame Height,
                                               Yox =0.233, X r=0.1,
                        0.2
                                             Alcohol Fuels Ref[5]


                       0.15       y = -0.0014284 + 1.0237x R= 0.98909
                 1/3
    x Y ox (1-X)
              r




                        0.1
              f




                       0.05




                          0
                              0          0.05             0.1               0.15   0.2
                                                                    1/2 2/3
                                         [Q'/(ρ c pT o (gcos φ)         ]
                                                 o
Heat Flux: Ahmad & Faeth
Opposed flow spread: (Ito & Kashiwagi, PMMA)
                                                             0o
                                                                      30o




Prediction                                                                   60o




                                                                              90o




                                     Calculated Flame Speed compared to Napkin Data
                                                                       UP

                              60
   Velocity of Front (mm/s)




                                                                                         v p,AF(mm/s)
                              40
                                                                                         v p,exp(mm/s)
                                                                                         v p,IK(mm/s)
                              20


                                                   BOTTOM                          TOP
                               0


                                                                      DOWN
                              -20
                                    -90     -60        -30        0            30        60             90
                                                  Angle from the vertical (deg.)
Electrical Cable Arrays: Borealis
Application: Room Corner Test
ISO 9705
Gravity-assisted spread dominates
Simple flame length model:

               &
x f − xb = k f Q′′( x p − xb ),                   2
                                        k f ≈ 0.01m /kW


  From Cone:
                                        &
                                        Q′′(t )
          & ′′ = q′′ ∆H C
          Q      & net      ∆H C (t ) =
                       L                m′′(t )
                                        &
Heat of Gasification: art of measure
                                              800
                                                        &
                                                        Q" peak
 Energy Release Rate per Unit Area (kW/m 2)




                                              700
                                                        &
                                                        Q" peak avg.
                                              600       &
                                                        Q" overall avg.                                          L peak = 3.9
                                                                                                                 L peak avg. = 4.0
                                              500
                                                                                                                 L overall avg. = 4.5
                                              400

                                              300

                                              200

                                                                                                                        ∆H C
                                              100                                                                L=
                                                                                                                        slope
                                               0
                                                    0       10            20         30         40          50     60                70
                                                                                                       2
                                                                               External Heat Flux (kW/m )
Material Properties

     Material Property                Symbol    Test Method
1.   Ignition Temperature               Tig     Cone or LIFT
2.   Thermal Inertia                    kρc     Cone or LIFT
     Minimum Surface Temperature
3.                                     Ts,min      LIFT
     for Lateral Flame Spread
4.   Lateral Flame Spread Parameter     Φ          LIFT
5.   Effective Heat of Combustion      ∆H C        Cone
6.   Effective Heat of Gasification     L          Cone
7.   Total Energy per Unit Area         &
                                        Q′′        Cone
Room spread concept
                           yp - H



                                               Ceiling Jet        0.08H

                                                             zp


                                         Pyrolysis Zone

     Ceiling                             Burnout Zone                     H = 2.4 m
     Zone



                                       yp, o

           yb


                xb
                      xp

                     Ignition Burner
Fire Growth

                dxp       xf − xp
           v=         =             .
                dt          t ig



       dxb xp − xb
           =                                  &
                                   tb = Q′′ / Q′′.
        dt    tb
                      Example of Prediction
                              2000

                              1800

                              1600                 Flashover:
                                               HRR = 1 MW (142 s)
Rate of Energy Release (kW)




                              1400

                              1200

                              1000

                              800

                              600

                              400
                                                                                        Predicted
                              200
                                                                                        Full-Scale Test
                                0
                                     0   20   40      60       80     100   120   140        160          180
                                                                Time (s)
                  SOLUTION
Approximate Solution


         = 1
           a     1 + a exp a t – 1 – 1
                             t ig                  where
                                                             &
                                                     a = k f Q′′ − 1
                                                     b = a − tig / tb
After burnout:
xp – xb                         tb               t      t
  x f,ig = 1+a
            a     1 + a exp a        –1   exp b t – 1 – t b
                                t ig              ig      ig
Flashover time vs b (Energy factor)
                                 ISO 9705 Flashover
             40                                                         Burnout
             35                                           Tau,b 0-.5
                                                          Tau,b 0.5-1
             30                                           Tau,b 1-10
             25                                           Tau,b >10
t fo /t ig




             20

             15

             10

              5

              0
                  -8   -6   -4     -2     0    2      4   6     8
                                         b
 Critical Flame Heat Flux for
 increasing spread rate and Flashover

                         L             tig    
q′f′ ,critical
&                = σT + 
                      4
                     ig
                               50 + 100
                         ∆H           t        , kW/m 2 .
                                                  
                           c           b      


  The heat flux from the ignition burner in the
  room corner test is about 60+/-20 kW/m2.
CONCLUSIONS

• Flame spread can be computed
• Property data are needed
• Correlations are needed
  – Flame Heat Flux
  – Flame Length
• Igniter heat flux can be crucial

				
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posted:9/16/2011
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