8/10/2019 10th IAFSS Best Poster (2011)

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An Experimental Study of UpwardFlame Spread over Inclined Fuels

M. J. Gollner 1, X. Huang 1, F. A. Williams 1 and A. S. Rangwala 21 Department of Mechanical and Aerospace Engineering, University of California, San Diego,

2 Department of Fire Protection Engineering, Worcester Polytechnic Institute

Experimental Approach

Fire Spread

Experimental Results

Mass Loss Rates

Conclusions

The mass-loss rate per unit area, normalized by the value at 0 is shown as afunction of angle in Figure 6. Results from the spreading 20 cm long PMMA test inthis study and a 65 cm gas-burner test with sidewalls from literature [8] exhibit anincreasing trend with . This is in contrast with two previous theoretical models [3,7]and tests with a steadily-burning 5x5 cm square PMMA sample that show variesparabolically with , similar to results for V p. As > 0, more excess pyrolyzate burnsdirectly above the pyrolysis region, increasing heat fluxes (primarily radiant) to thefuel surface, increasing local burning rates. Three dimensional effects also begin tooccur, increasing the heat flux to the burning surface.

Heat flux profiles ahead of x p exponentially decay. The slope of the decayincreases with decreasing spread rates.

Maximum flame spread rates occur between = -30 to 0 as aconsequence of heat flux profiles ahead of x p.

Maximum mass-loss rates per unit area occur near = 60 as aconsequence of increased heating rates in x < x p.

This effect plays an important role in defining the worst casescenario for fire safety test standards.

any fire-spread configurations are not purely upright, but rather inclined at somegle from the vertical such as wildfires and ceiling fires (Figure 1). The objective of is study is to investigate the influence of orientation on heat flux profiles in thembusting plume and subsequent relation to spread rates. Quantifying the effect

flame spread on inclination angles will improve the accuracy of upward flameread models.

thermally-thick plastic fuel, Polymethyl Methacrylate (PMMA) was ignited evenlythe base and flames allowed to spread upwards (Figure 2). Seven

ermocouples were melted onto the surface of the fuel to detect the position of theurning front and eleven thin-skin calorimeters [2] were positioned above the fuel toeasure the heat flux from the flame ahead of the burning surface. The entireparatus was placed atop a load cell to measure the rate of mass loss.

Figure 3 : Images were taken perpendicular to the sample surface as it spread upwards, x p=10cm. Starting from the left ceiling fire, as the inclination angle is increased, undersideflames transition from long blue, well-mixed laminar flames into increasingly turbulent yellowflames on the topside that lift from the surface, separating from the boundary layer,dramatically increasing the flame standoff distance y f .

Figure 6: (Left) The mass-loss rate per unit area, normalized with the value at = 90 is shown here as afunction of . Larger samples with turbulent flames reveal an increasing burning rate per unit area with increasing while smaller, laminar flames and previous theories predict a parabolic trend with increasing . (Right) 10x20cmand 5x5cm PMMA samples at = 60 , showing significant entrainment from the sides. The influence of the sideentrainment on the 5x5 cm and 10x20 cm samples on the burning rate is still being investigated.

1. Sibulkin, M. and Kim, J.Comb. Sci. Tech . 17 (1977) 39-49.2. ASTM E459 - Thin Skin Calorimeter.3. Ahmad, T. and Faeth, G.M., Proc. Comb. Inst. 17 (1978) 1149 -60.

4. Pizzo, Y., Consalvi, J.L. and Porterie, B. Combust. Flame. 156 (2009) 1856-59.5. Drysdale, D. and Macmillan, A. Fire Safety. J. 18, no. 3 (1992) 245-54.6. Xie, W. and Desjardin, P., Combust. Flame. 156 (2009) 522-30.7. Kim, J., de Ris, J. and Krosser, W. Proc. Comb. Inst. 13 (1971) 949-61.8. de Ris, J. and Orloff, L. Proc. Comb. Inst. 15 (1975) 175-82.

References

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Side View of Experiment

easurements of heat fluxes above the fuel surface ( x > x p) in Figure 4 reveal thate heat flux decays exponentially past x p, with an increasing slope as departs 0 .s the slope of these heat flux profiles increases, rates of upward flame spread ( V p)crease accordingly, shown in Figure 5 as a function of the angle of inclination, .aximum flame spread rates are observed at angles of inclination between -30 d 0 and minimum rates of spread at 60 . At angles where > 0 flames lift away

om the surface (Figure 3), resulting in decreased heat fluxes past x p because heatux scales inversely with the flame standoff distance, producing lower spread rates.

Figure 2: Experimental setup used to assess the effectsof inclination angle on flame spread, heat flux and mass-loss rates.

Research Sponsor:Society of Fire Protection EngineersEducational and Scientific Research

Foundation

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Figure 5 : The flame spread rate shown above wasobserved using thermocouples melted onto the surface of the PMMA. Previous experimental [4,5] and numerical(model) [4,6] data available are indicated by symbols. Apeak is observed somewhere between = -30 to 0 .

Flame spread ratepeak (this study)

igure 1: Upward flame spread over inclined surfaces isapid because some fuel, termed excess pyrolyzate [1] isarried past the pyrolysis region and burns above unigniteduel. Both the rate of flame spread and the burning rate of uel changes with inclination due to changes in how closehe diffusion flame resides above the surface of the fuel,

modifying the heat flux ahead of the pyrolysis region asell as the heat flux in the pyrolysis region.

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Accelleration due togravity

Mass-loss rate of fuel per unit area

( , ) Heat flu x fromflames to u n ign ited fu el ( )

Heat flux fromflames toburning fuel

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Flame standoff distance

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Figure 4: Total heat fluxes measured by thin-skins abovethe PMMA sample, x p = 20cm. Exponentially-decaying heatflux profiles are observed, plotted both nondimensionallyand logarithmically following [3]. HFP* is thenondimens iona l hea t f lux, , where Gr i s theGrashof number, Pr the Prandtl number, L the latent heat of vaporization, and B the fuel B-number.

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Fastest spread(0o, 30 o & -30 o)

Slowestspread(45 o & 60o)

Theory [3]

Theory [3]

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[8]

10x20cm PMMA5x5cm PMMA

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