Coordinates and fuel. 2. theta – angle of orientation to gravity3. Ignition – flame and thermal boundary layer (Tp reached)4. Pyrolysis/flame length. Standoff distance, spread velocity, BL thickness5. Heat flux – to the pyrolsyis region. From flame to virgin fuel. Highlight thermal BL – studiedHighlight heat flux from flame to surface – being studiedInfluence of angle from horizontal – being studied.
The same pyrolysis zone, combusting plume, and buoyant plume exist in this case. Now, the flame is larger and the combusting plume begins to extend above the height of the commodity surface, and the flame becomes very turbulent. The fire grows more rapidly. The remaining products are then propelled above into a buoyant plume.
Consistent with existing data, even though flame height data is not. Xf/xp ~ constant.Xf ~ mf ~ xp
Upward flame spread the width effectA complex numerical model may not always be required nor possible for most situations.
Xp is where material reaches temperature, Tp
“Universal Meaning”The B number can be thought of as a thermodynamic or mass transfer driving force. It was first introduced by Spalding in 1950 to develop an expression for the burning rate of a liquid fuel droplet in a gas stream. The uncorrected B-number is a property of pyrolyzing material, and it appears in boundary conditions of energy conservation at the fuel surface. The corrected B-number accounts for influences of additional heat-transfer processes. Physically, it relates the heat release of combustion (the numerator) to the energy required to generate fuel gasses (the denominator).In a mass-transfer sense it is the ratio of an impetus for interphase transfer to a resistance opposing that transfer.
Mf’’ is the mass loss rate per unit are of the material, which is related to the heat transfer component (h/Cg) times the thermodynamic component ln(B+1). Because of the log relationship of B, heat transfer plays a larger role in this process. H is assumed to be a constant in this process and is determined by a relation first relating it to the Nusselt number, and a nusselt number correlation which is a function of the cubed root of the Grashof number times Prandtl number. This approach is not exact, but for these small-scale experiments it is acceptable to ignore these small variations in h. Future work we are conducting will investigate the heat transfer coefficient numerically. The resulting formula for the average B number is an exponential function of the mass loss rate of the fuel per area over constants minus 1.
In the mixed case the same processes still occur but now some leakage of the commodity (in the form of melted plastic) pool in front of the commodity. Now remaining cardboard burns as well as a small pool fire at the base of the commodity. The characteristics of the pool fire as well as the flat plate burning must be taken into account. In our tests, for safety reasons the fire was extinguished before significant commodity leakage occurred. We burnt approximately only 3/4 of the commodity. It would take upwards of 3-5 minutes for this to occur based on observations from tests, so characterizing the earlier region to involved suppression is more important for this study.
Transcript of "2011 Senate Exam Presentation"
University Senate Exam for Michael Gollner Adviser: Professor Forman A. WilliamsSupported by: Society of Fire Protection EngineersEducational and Scientific Foundation Grant 1
OutlineI. Introduction I. Theory of Flame SpreadII. Flame Spread Over Inhomogeneous Fuels I. Upward Spread: Corrugated CardboardIII. Flame Spread Over Homogenous Fuels I. Inclined Flame Spread: PMMAIV. Conclusion 2
1. Thermal boundary layer Fire Spread 2. Heat flux from flame to virgin fuel 3. Influence of orientation g q ( x, t ) f f ~ xn Excess Vp Pyrolyzate yf q p m HcQ f y xf x xpFire spread occurs because of a transfer of thermal energy from a burning region to a region of virgin fuel 4
xp xp “Wall Fire” xp xpxp xp xp “Ceiling Fire” “Pool Fire” 5
Motivation The rate of fire spread is central to fire safety design – it describes the rate a fire will grow and hence its fire hazard Flame spread is still not well understood for: Forest fires (e.g. inclined slopes) Warehouse fires Undersides of burning roofs 6
Objectives Understand the influence of the following parameters on the rate of fire spread, Vp, Non-homogeneity of fuels (modifying the thermal boundary layer, δf) Fuel orientation angles, θ Heat flux profiles ahead of the flame, q ( x, t ) f 7
II. Flame Spread Over Inhomogeneous Fuels:Upward Spread: Corrugated Cardboard (Previous Work) 8
Previous Work Upward flame spread over Corrugated Cardboard What influence does the non-homogeneity of the fuel have on the flame spread rate? Motivations Upward flame spread is the initial stage in warehouses, where later stages involve more material (plastics) Motivation: The smallest amount of suppressant necessary to extinguish any fire occurs at early times (fire involves less material, lower burning rate or HRR)Gollner, M.J., Williams, F.A., and Rangwala, A.S. Upward flame spread over corrugatedcardboard. Combustion and Flame. DOI: 10.1016/j.combustflame.2010.12.005 9
Upward Flame Spread For upward flame spread Heat transfer is by radiation and convection from the flame to virgin fuel On continuous fuels, flame spread is unsteady and extremely rapid 10
Upward Flame Spread - Laminar • Flame height <25 cm Boundary layer Buoyant Plume Plume Radiative + Convective Heat Transfer Combusting Plume Excess Flame Radiative + Pyrolyzate Convective Heat Transfer Pyrolysis Zone flame xf mF (~ 20 to 25 cm Laminar xp Flame Propagation) Y-axis Fuel • Study important because it provides physical understanding of the problem 11
Upward Flame Spread - Turbulent Buoyant Plume Plume Radiative + Boundary Convective Heat Transfer layer flame Combusting Plume Flame Radiative + Convective Heat Transfer Excess Pyrolyzate mF Pyrolysis Zone xp xf• Flame height >25 cm (Turbulent flame height >25 cm)• Realistic fire situation• Cardboard still intact Y-axis Fuel 12
Mechanisms of Fire Spread Important quantity: heat flux ahead of pyrolysis region q( x, t ) Approximately, forward heat flux is all imparted over combusting plume ( x f xp ) Therefore, flame height (xf) and pyrolysis height (xp) become relevant parameters for study 13
Definition of Flame Height Heat flux imparted to fuelq( x, t ) MOST heat flux imparted to fuel 14
Results of Upward Flame Spread Theories* Annamalai & Sibulkin: x f ~ A1( B1 t ) 2 (Laminar) t Saito, Quintiere, Williams: x f ~ A2e (Turbulent) Sibulkin & Kim: x f ~ A3t 2 (Laminar) x f ~ B3e t (Turbulent) Where A, B, and α are constants1. Annamalai, K. and Sibulkin, M. Flame spread over combustible surfaces for laminar flow systems. Part I & II: Excess fuel and heat flux. 1979,Combust. Sci. Tech., vol. 19, pp. 167-183.2. Saito, J.G. Quintiere, and F.A. Williams, "Upward Turbulent Flame Spread," Fire Safety Science-Proceedings of the First InternationalSymposium, 1985, pp. 75-86.3. The dependence of flame propagation on surface heat transfer II. Upward burning . Sibulkin and Kim, Comb. Sci. Tech. 1976*NOTE: Results for non-charring fuels. 15
Cardboard Spread Experiments Cardboard ignited uniformly at base by burning wick Flames propagate up Insulated board above sample Sample is filled with plastics, but this study only addresses the behavior before these plastics igniteGollner, M.J., Williams, F.A., and Rangwala, A.S. Upward flame spread over corrugatedcardboard. Combustion and Flame. DOI: 10.1016/j.combustflame.2010.12.005 16
Flame Height Observations x f ,max 50 40 x f ,avg Height (cm) 30 x p ,avg 20 10 0 0 10 20 30 40 50 Time from Ignition (s)Gollner, M.J., Williams, F.A., and Rangwala, A.S. Upward flame spread over corrugated 17cardboard. Combustion and Flame. DOI: 10.1016/j.combustflame.2010.12.005
Flame Height Observations 2 x f ~ t fits x f ,max 50 Predicted using current models 40 x f ,avg Height (cm) 30 20 x p ,avg 10 0 0 10 20 30 40 50 Time from Ignition (s)Gollner, M.J., Williams, F.A., and Rangwala, A.S. Upward flame spread over corrugated 18cardboard. Combustion and Flame. DOI: 10.1016/j.combustflame.2010.12.005
Flame Height Observations xf ~ t 3/2 fits x f ,max 50 Observed Trend 40 Why does the pyrolysis front and flame x f ,avg Height (cm) 30 height grow SLOWER than what current theories would predict? x p ,avg 20 10 0 0 10 20 30 40 50 Time from Ignition (s)Gollner, M.J., Williams, F.A., and Rangwala, A.S. Upward flame spread over corrugated 19cardboard. Combustion and Flame. DOI: 10.1016/j.combustflame.2010.12.005
Pyrolysis Height Observations x p ~ t 3/2 Same trend observed in flame heightsGollner, M.J., Williams, F.A., and Rangwala, A.S. Upward flame spread over corrugated 20cardboard. Combustion and Flame. DOI: 10.1016/j.combustflame.2010.12.005
Burning-Rate Observations Laminar Q 20kW / m 2 x f ~ (Q)4/3 Turbulent Q 20kW / m 2 x f ~ (Q)2/3 Observations are expected. What causes the x~t3/2 dependence? 21
What is Corrugated Cardboard?1. Grant, G. and Drysdale, D., Numerical Modeling of Early Flame Spread in Warehouse Fires. Fire Safety Journal, 1995. 24(3):p. 247-278.2. T. Jayaweera, H.Z. Yu, Water absorption in horizontal corrugated boards under water sprays, Fire Safety Journal. 41 (2006) 22335–342.
Heat Flux in Flame Spread Models q Constant q = constant One of few models with q(x)  1. Sibulkin and Kim, Comb. Sci. Tech. vol. 17, 1977 32
Heat Flux in Experiments Simplifications of the description of the spatial dependence of q are prevalent, often q( x, t ) const Transient measurement of dynamic heat flux at height xp < x < xf on a sample of corrugated cardboard q( x, t ) 20kW / m2 selected in their studyGrant, G. and Drysdale, D., Numerical Modeling of Early Flame Spread in Warehouse Fires. Fire Safety Journal, 1995.24(3): p. 247-278. 33
‘Constant’ Heat Flux in ModelsTsai, K. (2009). Width effect on upward flame spread. Fire Safety Journal, 44(7), 962-967. 34
How would this affect xp & xf ? Temperature of a thick fuel with time-dependent heat flux: (Carslaw & Jager and Mitler et al.) q t 1 T T0 kc 0 t t dt Assuming material pyrolyses at fixed Tp, substitute τ=t/t’, integral becomes a constant dependent on material properties: q t 1 I d 0 1 Assuming a new q(x) power-law variation based on boundary layer extension: q C / x1/3 H. Mitler, Predicting the spread rates of fires on vertical surfaces, Symposium (International) OnCombustion. 23 (1991) 1715-1721. 36
How would this affect xp & xf ? The time, t of arrival of pyrolysis front will obey: x p At 3/2 Assuming x f ~ m ~ x p , where m is the burning rate per unit width: x f Bt 3/2 You recover what was observed in experiments!Gollner, M.J., Williams, F.A., and Rangwala, A.S. Upward flame spread over corrugatedcardboard. Combustion and Flame. DOI: 10.1016/j.combustflame.2010.12.005 37
Critical Point The heat flux ahead of the flame front, q( x, t ) is critical toward understanding flame spread phenomena 38
III. Flame Spread over Homogenous FuelsInclined Flame Spread: PMMA (Future Work) 39
Current Work Gravity-Assisted Flame Spread over PMMA at various Angles of Inclination The heat flux profile can be consistently modified by changing the buoyancy direction (tilting the sample) This introduces less uncertainties than changing materials, which sometimes have less understood properties A material, Polymethyl Methacrylate (PMMA) is chosen to first be tested because its combustion properties are well understood for fire problems. 40
Rates of Flame Spread Spread Rate from Previous Studies 0.9 Pizzo Model  Pizzo Experiment  Drydale and Macmillian (6cm)  0.8 Xie and DesJardin Model  Drysdale Avg  0.7 0.6 Spread Rate (cm/s) 0.5 0.4 0.3 0.2 0.1 0 -20 0 20 40 60 80 100 Angle of Inclination, (-90o = ceiling, 0 = wall, 90o = pool) o 43
Previous Literature1. S.M. Ali, V. Raghavan, A. Rangwala, A numerical study of quasi-steady burning characteristics of a condensed fuel: effect of angular orientation of fuel surface, Combustion Theory and Modelling. 14 (2010) 495-518.2. P.L. Blackshear, M.A. Kanury, Some effects of size, orientation, and fuel molecular weight on the burning of fuel-soaked wicks, Symposium (International) On Combustion. 11 (1967) 545-552.3. de Ris, J, and L. Orloff. “The role of buoyancy direction and radiation in turbulent diffusion flames on surfaces.” Symposium (International) on Combustion 15, no. 1 (1975): 175-182.4. H. Ohtani, K. Ohta, Y. Uehara, Effect of orientation on burning rate of solid combustible, Fire and Materials. 18 (1991) 323-193.5. Y. Pizzo, J.L. Consalvi, B. Porterie, A transient pyrolysis model based on the B-number for gravity- assisted flame spread over thick PMMA slabs, Combustion and Flame. 156 (2009) 1856-1859.6. Drysdale, D, and a Macmillan. “Flame spread on inclined surfaces.” Fire Safety Journal 18, no. 3 (1992): 245-254.7. W. Xie, P. Desjardin, An embedded upward flame spread model using 2D direct numerical simulations, Combustion and Flame. 156 (2009) 522-530.Relevant but not plotted:1. Y. Wu, H.J. Xing, G. Atkinson, Interaction of fire plume with inclined surface, Fire Safety Journal 35 (2000) 391-4032. ITO, A, and T KASHIWAGI. “Characterization of flame spread over PMMA using holographic interferometry sample orientation effects.” Combustion and Flame 71, no. 2 (February 1988): 189-204. 44
Gaps in Existing Data No previous data on spreading flames under inclined angles has been performed (only steady) Hazards at underside angles has not been assessed experimentally and may have a wide application for future flammability tests and standards Measurements of heat flux profiles and standoff distances ahead of the flame front have not been performed Critical to finding critical mechanisms and development of analytical theories. Only performed for wall fires (0°) 45
Apparatus Insulation Board Thin-Skin Calorimeters Side-View DSLR Camera Rear View PMMA Sample Camera (7 Surface Thermocouples) Load Cell Data Acquisition SystemNot Shown: Front Video Camera, Optional IR Camera 47
Non-Uniformity in Heat Flux Preliminary test on PMMA at 30 degrees Increasing time and xp 49
Acknowledgements Most of all, Prof. Forman Williams and Ali Rangwala for their advice and guidance Kristopher Overholt (WPI), Simon Xie (WPI), Todd Hetrick (WPI), Cecelia Florit(WPI), Xinyan Huang (UCSD) and Chuck Marcacci (UCSD) for their assistance in the laboratory Prof. Jose Torero (Edinburgh), Dr. Adam Cowlard (Edinburgh), Jonathan Perricone and many others for their advice, assistance and hospitality 50
Next Steps Running tests at -60,-45,-30,0,30,45,60 degrees Will analyze Heat flux profiles Flame Standoff Distance Burning Rates Flame Spread Rates Look at influence of heat flux profile on spread and burning rates 52
Papers and Current ProjectsPeer-Reviewed Publications1. Gollner, M.J., Williams, F.A., and Rangwala, A.S. Upward flame spread over corrugated cardboard. Combustion and Flame. DOI: 10.1016/j.combustflame.2010.12.005Publications Under Review and in Preparation1. Gollner, M.J., Overholt, K., Williams, F.A., Rangwala, A.S. and Perricone, J., Warehouse commodity classification from fundamental principles. Part I: commodity and burning rates, Under Review, Fire Safety Journal. 2010.2. Overholt, K., Gollner, M.J., Williams, F.A., Rangwala, A.S. and Perricone, J., Warehouse commodity classification from fundamental principles. Part II: flame height prediction. Under Review, Fire Safety Journal. 2010.3. Gollner, M.J., Xie, Y., Lee, M., Nakamura, Y., Rangwala, A.S., Burning behavior of vertical matchstick arrays, In Preparation for Combustion Science and TechnologyCurrent Projects1. Tilting Flame Spread – Apparatus at UCSD. Advising Graduate Student, Xinyan Huang2. Influence of backing on upward flame spread over corrugated cardboard – Advising 2 undergraduate students at WPI for their Senior Project: Amanda Keller and Ben Travis. 53
Additional Applications of Work Comparing measuring B-number with both the mass- loss rate and standoff distance methods [1,2] Determine the worst-case angles for flame spread Implications to design of buildings and small-scale “worst-case scenario” testing Development of analytical upward (and tilted) flame spread models that use a variable heat flux profile1. Gollner, M.J., Overholt, K., Williams, F.A., Rangwala, A.S. and Perricone, J., Warehouse commodity classification from fundamental principles. Part I: commodity and burning rates, Under Review, Fire Safety Journal. 2010.2. A.S. Rangwala, S.G. Buckley, J.L. Torero, Analysis of the constant B-number assumption while modeling flame spread,Combustion and Flame. 152 (2008) 401-414. 56
Measurement of Heat Flux Thin-Skin Calorimeter Combined heat flux from calorimeter (accounting for losses) qi qc qr qsto qc,st qi qc qr qc,st qstoAmerican Society of Testing and Materials, Standard ASTM E 459-97 57
Cardboard Experimental Setup Standard Group-A Plastic Commodity Polystyrene cups in compartmented cardboard carton 58
Previous Literature1. de Ris, J, and L. Orloff. “The role of buoyancy direction and radiation in turbulent diffusion flames on surfaces.” Symposium (International) on Combustion 15, no. 1 (1975): 175-182.2. H. Ohtani, K. Ohta, Y. Uehara, Effect of orientation on burning rate of solid combustible, Fire and Materials. 18 (1991) 323-193.3. P.L. Blackshear, M.A. Kanury, Some effects of size, orientation, and fuel molecular weight on the burning of fuel-soaked wicks, Symposium (International) On Combustion. 11 (1967) 545-552.4. Y. Wu, H.J. Xing, G. Atkinson, Interaction of fire plume with inclined surface, Fire Safety Journal 35 (2000) 391-4035. S.M. Ali, V. Raghavan, A. Rangwala, A numerical study of quasi-steady burning characteristics of a condensed fuel: effect of angular orientation of fuel surface, Combustion Theory and Modelling. 14 (2010) 495-518.6. W. Xie, P. Desjardin, An embedded upward flame spread model using 2D direct numerical simulations, Combustion and Flame. 156 (2009) 522-530.7. ITO, A, and T KASHIWAGI. “Characterization of flame spread over PMMA using holographic interferometry sample orientation effects.” Combustion and Flame 71, no. 2 (February 1988): 189-204.8. Drysdale, D, and a Macmillan. “Flame spread on inclined surfaces.” Fire Safety Journal 18, no. 3 (1992): 245-254.9. Y. Pizzo, J.L. Consalvi, B. Porterie, A transient pyrolysis model based on the B-number for gravity- assisted flame spread over thick PMMA slabs, Combustion and Flame. 156 (2009) 1856-1859. 59
Previous Literature – Thick Fuels Steady Burning Experiments de Ris and Orloff (-90 to +90)  Ohtani et al. (-90 to +90)  Blackshear and Kanury (-90, 0, +90)  Wu et al.  Numerical Simulations Ali et al. (-90 to +90) (Steady)  Xie and DesJardin (0 to +90) (Spreading)  Spreading Fires Ito and Kashiwagi (-90 to +90) (Small Sample Width)  Drysdale and Macmillan (0 to +90)  Pizzo et al. (0 to +90)  60
Picture of Experimental SetupWPI, Summer 2008TC wires Heat flux sensors Back View Front View 61
Commodities Used in Testing Class II Class III Class Group A Plastic IV/Group BCommodities Used in Reality 62
Commodity Test Results 30 s 92 s 100 s 132 s 150 s Front Face of Cardboard Plateau PS Cups & Cardboard Burning Burning Stage I Stage II Stage III 63
The B-number B impetuses i.e. heat of combustion for burning resistances i.e. heat of vaporization to the process “Thermodynamic Driving Force” (1 )(HcYO , ) / s C p , (Tp T ) B B-number Hg Q χ = Fraction of radiation lost [-] T∞ = Ambient temperature [K] ∆Hc = Heat of combustion [kJ/kg] L = Latent heat of vaporization [kJ/kg] YO,∞ = Mass fraction of oxygen in ambient [-] ∆Hc = Heat of gasification [kJ/kg] νs = Oxygen-fuel mass stoichiometric ratio [-] Cp,f = Specific heat of the fuel [kJ/kg-K] Cp,∞ = Specific heat of ambient air [kJ/kg-K] Q = L + Cp,f(TB-TR) [kJ/kg] Tp = Pyrolysis temperature of the fuel Kanury, A. M. An Introduction to Combustion Phenomena. s.l. : Gordon & Breach Science Publishers, Inc, 1977. 64
Experimentally-Measured B•Solving for B and using Nu correlation for the heat-transfer coefficient: m f B exp 1 0.13[GrPr]1/ 3 g g •Formula for average B-number based on measured rate of mass loss •Applies in regimes dominated by convective heat transfer, as found in many small-scale experiments. •Effective B-number derived by same formula with radiation includedKanury, A. M. An Introduction to Combustion Phenomena. Gordon & Breach Science Publishers, Inc, 1977. 65