Controlling Parameters Involved in the Burning of

Controlling Parameters Involved in the Burning of Standard Storage Commodities: A fundamental approach towards fire-hazard classification M. J. Gollner*,§, T. Hetrick‡,†, A. S. Rangwala‡, J. Perricone§, and F. A. Williams* Department of Mechanical & Aerospace Engineering University of California, San Diego La Jolla, CA 92093 Abstract A warehouse commodity consists of the basic product, its packaging, and its container. Parameters that quantify fire hazard, such as flame spread rate, burning rate etc, depend on the flammability properties of these materials and the interaction between them when the commodity is burning. In this study, experiments were conducted on a warehouse commodity that consisted of plastic cups (basic product), cardboard packaging and a single-walled corrugated cardboard carton (container) of dimensions 530 × 530 × 510 mm. Such a commodity is called a Group-A plastic, which represents the most severe fire hazard. Experiments consisted of burning the front face of a single box with all remaining faces uniformly insulated. An approach to classifying the commodity fire hazard based on a B-number, which physically represents the ratio of the chemical energy released during the combustion process to the energy required to vaporize the fuel, is analyzed. A non-dimensional form for each of two common fire-hazard parameters, the Fire Propagation Index, and Critical Heat Flux, also is presented.

Introduction Designing for fire safety is a risk-driven endeavor bounded by limits of acceptable loss. Design quality and integrity is therefore firmly rooted in the accuracy of risk estimation. Warehouse storage facilities are particularly vulnerable. Large quantities of fuel stored in close proximity allow a very slim margin of error. Evidence that current practice misses the mark is found as recently as 2007 when fire at a Tupperware products storage warehouse lasted for 35 hours and resulted in a total loss of the nearly 15,000 m2 facility [1]. A sample of similarly devastating losses in seemingly well-protected facilities illustrates the persistent failures resulting from crude methods of standard risk estimation [1-8]. The benefits of more precise fire safety engineering solutions are clear. Traditional methods of risk estimation for warehouse storage are based on a hazard ranking scheme. This scheme arbitrarily identifies a reference commodity. The fire hazard of any commodity is then evaluated in comparison to the reference1. Among the most significant flaws of this approach are the lack of agreement between standards and the prevalence of subjective observations in lieu of objective measurements to characterize fire dynamics [9]. *

The burning process of any single three-dimensional commodity is complex. Added complexity is introduced from a global perspective of an array stacked to heights where the turbulence associated with upward flame spread is considerable. Although ignition, flame spread, extinction, and mass burning rate are all important processes during combustion it has been shown by Pagni and Shih [10] that upward turbulent flame propagation can be described well by the B-number (also called Spalding’s mass transfer number). The B-number was first introduced by Spalding [11] to characterize the burning of a liquid fuel droplet. It is a measure of the thermodynamic efficiency of the burning process, physically relating the heat release from combustion (numerator) to the energy required for gasification (denominator) [12]. The use of a non-dimensional parameter such as the B-number to rank material fire hazard is beneficial for many reasons. It is scalable due to its non-dimensional nature to problems of various sizes and geometries. The Bnumber is scientifically understood and measureable, and it can be related to the extinction and suppression properties of a material to design protection measures that will control or extinguish an unwanted fire. Ranking the

Department of Mechanical and Aerospace Engineering, University of California, San Diego Corresponding Author: [email protected] † Exponent Failure Analysis Associates ‡ Department of Fire Protection Engineering, Worcester Polytechnic Institute § Schirmer Engineering, an AON Global Company 1 Current commodity classification standards (NFPA 13) classify commodities into Class I to IV and Groups A, B and C. The classification system is predominantly based on the free-burn heat-release rate of the commodity. Proceedings of the 6th U.S. National Combustion Meeting

Nomenclature B CHF CHF* FPI FPI Gr HRR HRP  ∆ ∆  

′′

Nu Pr ′′

TRP

B -number (Spalding’s mass transfer number) [-] Critical heat flux [kW/m2] Critical heat flux [-] Specific heat [kJ/g-K] Fire propagation index [(m/s)1/2/(kW/m)2/3] Fire propagation index [-] Acceleration due to gravity, 9.81 [m/s2] Grashof number [-] Heat transfer coefficient [W/m-K] Asymptotic maximum heat-release rate of a commodity [kW/m2] Heat release parameter [-] Heat of combustion. [kJ/kg] Heat of gasification of a condensed fuel [kJ/kg] Thermal conductivity [W/m-K] Mass-burning rate (mass flux) per unit area [kg/m2-s] Nusselt number [-] Prandtl number [-] Heat flux [kW/m2] Losses at the fuel surface [kW/m2] Thermal response parameter [kW-s1/2/m2] Temperature [K] Regression velocity [m/s]

Flame height [m] Pyrolysis length [m] Mass fraction of Oxygen [-] Greek Symbols α Thermal diffusivity @ Tm [m2/s] ρ Density @ Tm [kg/m3] Kinematic Viscosity [m2/s] Oxygen-fuel mass stoichiometric ratio [-] Fraction of total energy released by the flame radiated to environment [-] Subscripts g Gas, averaged values ∞ Ambient condition f Flame position s,c Flame convective heat flux transferred to the surface by conduction s,r Flame re-radiation heat flux f,r Radiative feedback from the flame Abbreviations LIFT Lateral Ignition Flame Spread Test NFPA National Fire Protection Association NIST National Institute for Standards and Technology PS Polystyrene (unexpanded polystyrene cups used as plastic commodity)

fire hazard of a mixed commodity is complicated because of the interactions of differing materials in various geometries. There currently is no scientifically based approach to classify these complex commodities. The applicability of the B-number towards classifying a mixed commodity is explored for the first time in this work.

Several material-related properties emerge during the derivations of these relationships and are listed in Table 1. Over the course of the last twenty years of fire research, these properties have been grouped together to obtain parameters that are used by industry to assess the flammability of a given material. These parameters are test-specific and generally do not scale up from small-scale tests (where the parameters are obtained) to full-scale tests. The third column of Table 1 shows some of the parameters that are currently used for the purpose of flammability

Background & Theoretical Considerations Material flammability encompasses quantitative measures of ignition, mass-burning rate and flame spread. Table 1: Properties involved in material flammability Material Properties (units) Symbol Heat of combustion [kJ/g] Latent heat of vaporization [kJ/g] Ignition temperature [K] Thermal conductivity [W/m-K] Density [g/m3] Specific heat [J/g-K) Pre-exponential constant [cm3/mol-s] Overall activation energy [kJ/mol]

∆ ∆

Parameters HRP    ∆

/∆

Proposed Nondimensional Parameters B

‐

 

FPI  m/s

2

1/2/

kW/m

FPI

FPI

TRP   kW‐s1/2/m2     CHF  kW/m2  

′′

2/3

/ ∆

CHF*   CHF/ HRR

max

′′ ,

ranking in research laboratories and industry. The last column in Table 1 lists possible non-dimensional parameters for which very preliminary evaluations are reported here.

′′

′′ , ′′

,

,                         3  

where ′′   is the burning rate per unit area, ′′, represents the rate of in-depth conduction of energy per unit area, ′′,  represents the rate of surface re-radiation of energy per unit area, and ′′ , denotes the radiative energy feedback from the flame to the surface per unit area. Thus, a large B-number basically implies a highly exothermic fuel relative to the heat required for gasification as would a large HRP, but with the improved definition given in equation (2) the B-number can provide a better indication of the fire-hazard than the HRP.

New Nondimensional Parameters Two parameters are currently used to determine the ignition hazard associated with a commodity, Critical Heat Flux (CHF) and Thermal Response Parameter (TRP) [13]. Since it specifies a characteristic time, there does not appear to be a useful nondimensional form for TRP, but CHF of a stored commodity can be non-dimensionalized by an . The resulting asymptotic heat-release rate  HRR represents the fraction of heat value, CHF/ HRR generated during vigorous combustion that is required to cause the material to ignite. CHF is best based on the measured properties of the outer material, corrugated cardboard because it alone supports the selfsustained flaming combustion incipient to fire involvement of the stored commodity. The Fire Propagation Index (FPI) incorporates TRP in its definition [13] and is proportional to the square root of the relative rate of flame spread. Because it is normalized on the basis of a correlation of a particular experiment, this parameter has units of (m/s)1/2/(kW/m)2/3, as well as an arbitrary constant of 1/1,000, to provide whole-number values that are not too large. It appears desirable to compare the spread velocity with a normal regression velocity, ′′ / ∆ , that represents the rate of retreat of the surface of a burning material [14]. When FPI 2 is expressed as a velocity, the corresponding nondimensional parameter becomes / FPI/ .        1 FPI The B-number appears in a boundary condition at the fuel surface in the classical Emmons [15] solution for forced-flow flame spread over a liquid fuel. The definition of the B-number may be extended to incorporate additional energy inputs and losses, resulting in [16] 1 ∆ ,∞ / ,∞ ∞ B , 2    ∆ where is the fraction of the total energy released by the flame that is radiated to the environment, ∆ denotes the heat of gasification of the condensed fuel, and ∆   represents the heat of combustion. Here denotes the oxygen-fuel mass stoichiometric ratio, ,∞ is the mass fraction of oxygen in ambient air, ,∞ represents the specific heat of air at an ambient equals the pyrolysis temperature of ∞ , and temperature of the fuel. The parameter Q represents the normalized non-convective heat transfer at the surface, given by

General Commodity Combustion Characteristics Figure 1 and 2 show the two characteristic stages of burning observed in a standard warehouse commodity. The heat flux from the combustible plume that extends upwards over the distance (XF - XP) is responsible for the rapid upward spread of the flame. For both a small and large wall fire as shown in Figures 1 and 2, the length of the combustible plume, (XF - XP) is a function of the B-number and pyrolysis height [16]. The pyrolysis height increases with time in the early stage represented by Figure 1. In the later stage of Figure 2 it has reached the top of the commodity. Boundary layer

Buoyant Plume Plume Radiative + Convective Heat Transfer & F′′ = m

h ln(1 + B) Cg

B is a function of: 1. Corrugated board 2. Commodity pyrolysis vapor

Combusting Plume Flame Radiative + Convective Heat Transfer

Excess Pyrolyzate

Commodity

& F′′ m Pyrolysis Zone

XP

XF (Turbulent flame height >25 cm)

flame

Y-axis Corrugated board

Figure 1. Flame propagating over a warehouse commodity during the early stages of the fire, where fire involves only the outer corrugated cardboard.

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Derivation of the Expression for the B-number Employed in Analyzing Experiments In all of the scenarios that have been described, diffusion, flow, mixing, and heat transfer play an important role in flame spread. The study of diffusion flames logically involves boundary-layer types of analyses of heat and mass transfer. One approach to calculating the vaporization and combustion rate of a warehouse commodity is to follow the procedure presented by Kanury [17], which expresses the average burning rate per unit area ′′ as

Buoyant Plume Plume Radiative + Convective Heat Transfer

& F′′ = m

Combusting Plume

h ln(1 + B ) Cg

flame

B is a function of: 1. Corrugated board 2. Commodity pyrolysis vapor 3. Commodity

Flame Radiative + Convective Heat Transfer (from pool and wall fire)

Excess Pyrolyzate

Commodity

XF

Corrugated board

Solid/Liquid Pool fire

& F′′ m

′′

Pyrolysis Zone

 

ln B

1 ,                      4

where is the heat-transfer coefficient (which can describe a combination of convective and radiative heat transfer), and is the specific heat of air at a temperature equal to an average of flame temperature and ambient. To estimate the rate of heat transfer and thus the influence of the flow field during upward turbulent burning, a relation with the Nusselt number, Nux may be used [18], namely

& F′′ m

Pyrolysis Zone Commodity leakage

 

Y-axis

Figure 2. Turbulent upward flame propagation at later stages of the fire, where the commodity within the box has spilled out into a pool fire and contributes to the flame height from below.

 

During the early stages of the fire (Figure 1), the flame is small, and the burning rate is a function only of the material properties of the corrugated board. Heat flux from combustion pyrolyzes the board and packing material, releasing gaseous fuel adjacent to the combustion surface. Some of this fuel burns in the boundary layer in front of the fuel surface, but some is carried above its originating height and burns above, creating much larger flames. This fuel carried above its originating surface is called excess pyrolyzate [10]. Heat flux via in-depth conduction through the corrugated board can pyrolyze the packing material and commodity, releasing combustible vapors. As the outer corrugated board layers break down, these combustible vapors diffuse through the remaining board, enhancing the flame spread rate. At this stage, the B-number is a function of the material properties of the corrugated board as well as the pyrolysis vapor from the heated commodity that penetrates the corrugated board. As time advances, the corrugated board can disintegrate, thereby exposing the commodity inside to direct flame impingement. The commodity (depending on its material properties) can spill out either as solid chunks or as a viscous liquid pool. This spillage may cause a secondary fire which has the characteristics of a pool fire. At this later stage, the B-number is a function of the material properties of the corrugated cardboard, the commodity pyrolysis vapor (diffusing outwards) and the commodity and packing material that have spilled out.

Nu ,                          5

and are the density and thermal where conductivity of air, respectively. From this an average heat-transfer coefficient, is determined. The Nusselt number is determined from a standard correlation for upward turbulent heat transfer [19] Nu 0.13 Gr Pr / ,                   6 where Gr is the Grashof number of the flow, and Pr is the Prandtl number of the gas,   / . With the Grashof number defined as Pr ∆   ,                              7 Gr where ∆ ∞ equations (4-6) can be combined to yield the expression ′′

1,      8 / 0.13 ∆ / which can be used to calculate a B-number from experimental measurements. B

exp

Experimental Apparatus & Instrumentation All testing reported herein was conducted at the Worcester Polytechnic Institute Fire Sciences Laboratory in Worcester, Massachusetts, USA, with no significant modifications to the apparatus or instrumentation made between test sessions. The Group-A plastic commodity was burned with one exposed face, and the measured burning rates were employed to calculate the B-number of the mixed commodity by use of equation (8).

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cup in the air space between the cup and cell wall – nominally indicating the cell’s mean bulk temperature. Cameras are setup on the sides of the apparatus to measure the flame standoff distance (for use in future analyses) and in front of the setup to measure flame heights. Large Hood ~4MW

Figures 2a and 2b: Figure 2a [21] shows a standard Group-A plastic commodity with half of its outside corrugated cardboard covering removed revealing segregated cells of polystyrene (PS) cups within. Figure 2b shows the same commodity insulted on all but one side with Kaowool glass fire board.

Thin‐Skin Calorimeters (9) on FiberFrax Sheet

Figure 2a shows the Group-A plastic commodity. This commodity is often used by the fire-protection industry to test the effectiveness of fire sprinklers and other fire-protection devices [20]. The fuel consists of a corrugated cardboard carton subdivided into 125 compartments. The outer dimensions of each cardboard carton are 530 mm wide by 530 mm deep by 530 mm high. The 125 cells are created by cardboard dividers subdividing the box in a 5x5x5 matrix of cells. Each cell contains a 0.45L, 36g unexpanded PS cup. Each cup is 9cm in height, with the opening face down, a top radius of 4.5cm and a bottom radius of 3.75cm. The corrugated cardboard is 1-ply, with a thickness of approximately 4mm, but can be compressed to as little as 1mm in some places where damage has occurred. In all experiments where the cardboard is oriented vertically, the corrugations in the board are also oriented vertically. All measurements of the cardboard are approximate, and small variations exist between each commodity burned due to the adaptability of the cardboard. The commodity is wrapped in Kaowool insulating boards approximately 0.25” (0.65 cm) thick on all except one vertical side, on which measurements were taken. This arrangement, limiting the burning of the box, allows for a closer investigation of the fundamental physics governing the combustion of the plastic commodity. The experimental setup consisted of the Group-A plastic commodity placed on top of a Setra, Super II load cell that measured the mass loss of the plastic commodity within an accuracy of ±0.5 g. Figure 3 shows the experimental setup and instrumentation configuration. Inside the box 3 Type-K Chromel-Alumel thermocouples are installed inside a cell on the front face of the box, as seen as in Figure 3. One thermocouple is placed on the front face of the cardboard to track the progression of the pyrolysis front along the face of the cardboard, one is placed in the direct center of the PS cup to measure the temperature within the cup – nominally indicating the moment at which the cup ignites and one hangs to the side of the

Insulated Walls (5)

Thermocouples Mounted on Face (5)

C SLR Camera

Exposed Cardboard Face

Heptane Ignition Source Load Cell Load Cell

Figure 3: Schematic illustration of the experimental setup. Nine thin-skin calorimeters were mounted on a vertically oriented glass fiber board oriented above- and flush to the front combusted face of the test commodity. This configuration allows for spatial measurements of the combined radiative and convective heat flux that the combusting plume of excess pyrolyzate will exert on stored commodities of higher elevation. The setup was placed under a 4MW hood to eject burning fumes and embers. A controlled ignition was achieved by adding 4 mL of n-heptane to a strip of glass fiber board approximately 1 cm tall, 0.35 m wide by 3mm in depth. The wetted wick igniter was held by an aluminum u-channel that was positioned adjacent to and below the lower front edge of the commodity. Experimental time begins when the strip is piloted at the centerline of the commodity’s front face. From collected data, the flame standoff distance, mass-loss rate, and heat-release rate was measured as functions of time. The measurements are used to calculate an average B-number as a function of time, B(t). The variation of the B-number with respect to the height of the sample will not be considered here (an average value is obtained at each time interval). Experimental Results and Analysis In the experiments, mass-burning rate, radiative flux, and temperature measurements were made to study the interaction between different materials involved during combustion using the experimental

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conduction, pre-heating of material occurs prior to exposure to adequate oxygen. Temperatures within the commodity are generally near or at the corrugatedboard ignition temperature of approximately 380◦C when they are exposed. This causes immediate flaming combustion of the cardboard. At this stage, burning is governed by the burning properties of the outer cardboard, inner packing material, as well as some released gasses from the PS cups which have not yet ignited. The increased burning rate from increasing amounts of packing material is shown in Figure 5 between 60-110 seconds, as well as photographs in Figures 4-b and 4-c. A brief plateau in burning, with a significant drop in the mass-burning and heat-release rate (Figure 4c) occurs as the corrugated board has burned away. At this stage the PS continues to heat, followed by a 60% increase in the mass burning rate and a subsequent increase in heat flux. This is caused by the ignition of PS at approximately 497◦C. These results pertain to a particular representative test. A number of tests were conducted, and there were significant variations among them. The calculated B-number is shown as a function of time in Figure 6, based on the recorded test data for the three most successful tests. A large variation in the transient, calculated, B-number is observed because of the variations seen in Figure 5. The time-averaged experimental B-number from the series of conducted tests was 2.6. This is a more relevant result because the variations are different in different experiments. A concurrent study using a smaller-scale test in a cone calorimeter resulted in an average B-number of 1.28 [22]. The large value obtained in the present experiments could be due to three-dimensional effects during the disintegration of cardboard and the unsteady burning rate during the initial stage of burning.

setup described in the previous section. The results show that a Group-A plastic commodity transitions through two general burning regimes similar to the theoretical description shown in Figures 1 and 2; initially involving the outer corrugated board and then involving the packing and commodity within. Due to safety concerns and instrumentation longevity, no tests were permitted to run past a point where material spillage occurs, as in Figure 2. Following manual ignition of the n-heptane wick pilot igniter, the outer corrugated cardboard covering ignites and burns- not yet involving interior materials as in Figure 1*.

Figure 4: Timeline- (a) 30 seconds, front layer pyrolyzing and laminar burning along face, (b) 92 seconds, front face cells are exposed and burning, (c) 100 seconds, smoldering front face, second layer cells heating and PS cups deforming, (d) 132 seconds, all PS cups in the first layer ignited and burning, (e) 150 seconds, second layer of cells also burning.

& ′′f m

PS cups burning

(g/m2s) 1 Packing material (cardboard)

0.8

0.6

Extinction

Front face of cardboard burning

0.4

0.2

0

0

20

40

60

80

100

120

140

160

Time from Ignition [s]

Figure 5: Measured mass loss rate during the experiments. After 120s the PS cups started burning and test was terminated. Once the outer corrugated board material begins to break away, materials inside the commodity are exposed to the environment. Due to in-depth

Figure 6: B-number calculation during 3 tests at varying pyrolysis heights of the material, where thermocouples were located. Dashed line represents the average Bnumber from the three tests.

*

The flame height was measured from high-definition video taken of the front of the test commodity during

This is also observed in photographs of the commodity burning in figure 4a, and on a mass-flux diagram from 0-60 seconds in Figure 5. 6

storage. The data analysis still is in a preliminary stage. Variations of the calculated B-numbers with time were observed, and approaches to extracting more useful Bnumber results are under investigation. From the nondimensional B-number and the other two nondimensional parameters, FPI* and CHF*, defined here, a combination may evolve that can characterize both fire hazards and suppression requirements for warehouse commodity storage.

testing. Figure 7 shows these values as well as the values of the pyrolysis height and the linear fit of both sets of data. A linear correlation between the flame heights and pyrolysis heights is observed, as predicted by Annamali and Silbulkin [23]. The straight lines drawn through the data remain to be compared with theoretical predictions in future work.

Recommendations for Future Work Experimentally obtaining the B-number in a repeatable manner for a mixed commodity is of prime importance, and studies using devices such as a cone calorimeter to calculate the B-number on smaller-scale cells and individual materials should be continued [22]. The influence of material and volume density of materials within a commodity should be closely studied to determine a relationship between these parameters and a composite B-number that could be used to calculate the fire hazard of a grouped commodity. Figure 7: Flame height and pyrolysis height per time with linear fit of flame height and pyrolysis length.

Acknowledgements This research effort was generously sponsored by Schirmer Engineering, an AON Global Company. Laboratory experiments were conducted at Worcester Polytechnic Institute’s Fire Science Laboratory. Commodity samples were donated by Dave leBlanc at Tyco International.

Ignition-Hazard Classifications The non-dimensional parameter FPI* has no arbitrary constants and provides ranking of a material’s propensity to spread. Another benefit of using this form of a non-dimensional parameter is that all constituent parameters can be measured using common apparatus, such as the NIST Flame Spread Apparatus (LIFT). Values for FPI* based on available test data are shown in Table 2. The resulting values of  FPI* exhibit the same general pattern as the dimensional FPI.

References [1] S.C. Firefighter Mobilization. South Carolina Firefighter Mobilization 2007 Annual Report. SC : S.C. Firefighter Mobilization Oversight Committee, 2007. [2] Bidgood, C. M. and Nolan, P. F. Warehouse fires in the UK involving solid materials. 1, s.l. : J. Loss Prev. Process Ind., 1995, Vol. 8. [3] Fire Protection Services, Human Resources and Social Development Canada. Fire Losses in Government of Canada Property. Ottawa Ontario : Fire Protectoin Services, Human Resources and Social Development Canada, 2004/2005 Fiscal Year Summary. K1A 0J2. [4] Arizona News briefs. Fire destroys warehouse, vehicles at UPS facility. Newspaper. Mar. 28, 2006. [5] National Fire Protection Association. Fact Sheet - Hospital Fire - Riverside, California. Quincy, MA : NFPA, 1986. [6] Duval, R. F. and Foley, S. N. Fire Investigation: Supermarket Fire, Phoenix Arizona, March 14, 2001. Quncy, MA : National Fire Protection Association, 2002.

Table 2: Dimensional FPI values, regression velocity, VR values, and a non-dimensional FPI*. (VR values calculated using available data [13, 18]) FPI VR Material FPI* [(m/s)1/2] [m/s.10-5] Polymethylmethacryl 31 3.2 5.5 ate (PMMA) Polypropyelene (PP) 32 3.7 5.3 Polystyrene (PS) 34 4.2 5.2 Polyethylene (PE) 28 3.2 4.9 Polycarbonate (PC) 14 1.4 3.7 Wood Slab (Doug 14 4.1 2.2 Fir) Polyvinylchloride 7 1.5 1.8 (PVC) Status and Conclusions The experiments reported here are an initial step towards developing revised and improved approaches to evaluating fire hazards associated with commodity

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[7] [8]

[9]

[10] [11] [12]

[13]

[14]

[15]

[16] Rangwala, A. S., Buckley, S. G. and Torero, J. L. Analysis of the constant B-number assumption while modeling flame spread. 2008, Comb. and Flame 152, pp. 401-414. [17] Kanury, A. M. An Introduction to Combustion Phenomena. s.l. : Gordon & Breach Science Publishers, Inc, 1977. [18] Society of Fire Protection Engineers. The SFPE Handbook of Fire Protection Engineering. Quincy, MA : The National Fire Protection Association, 1995. [19] Incropera, F.P., DeWitt, D.P. Introduction to Heat Transfer, Fifth Edition. Hoboken, NJ : John Wiley & Sons, 2007. [20] Dean, R. K. A Final Report on Fire Tests Involving Stored Plastics. Chicago : s.n., 1975. NFPA annual meeting. [21] Cote, A. E. National Fire Protection Association. Operation of fire protection systems: a special edition of the Fire Protection Engineering Handbook. s.l. : Jones & Bartlett Publishers, 2003. [22] Overholt, K., Gollner, M.J., Rangwala, A.S. Characterizing flammability of corrugated cardboard using a cone calorimiter. 6th US National Combustion Meeting of the Combustion Institute. May, 2009. [23] 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. 167183.

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