oxygen concentration conditions

Ftre Safety Journal 20 (1993) 175-181

Short Communication A Unified Fire Property Data Correlation Scheme for Combustibles Burning under Different Ventilation/Oxygen Concentration Conditions Hong-Zeng Yu Factory Mutual Research Corporation, 1151 Boston-ProvtdenceTurnpike, Norwood, Massachusetts 02062, USA (Received 22 January 1991; revised version received 7 February 1992; accepted 14 February 1992)

ABSTRACT This brief note presents a data correlation scheme for fire properties of dtfferent combustibles burning in environments of dtfferent ventdation rates or oxygen concentrattons The functional relationships presented herem correlate well with the extstmg fire property data. These correlattons can easily be tncorporated tnto existing compartment fire computer codes

INTRODUCTION As recognized by fire researchers and fire protection practitioners, the development of a compartment fire at a given instant may fall into one of the following two regimes, depending on the degree of availability of ventilation: fuel surface controlled or ventilation controlled. If there is sufficient air supply, the burning rate of a combustible item m a compartment is mainly determined by its exposed surface area. On the other hand, If the ventilation is restrtcted, the fire may burn at a slower rate or gradually diminish to extinction, provided that the radiation feedback from the compartment surfaces and other objects m the compartment is msigntficant. The oxygen-deficient situation may also occur in a well-ventilated compartment when the vitiated hot gas layer descends to the elevation where combustibles are located. As the 175 © Copyright 1992 Factory Mutual Research Corporation

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oxygen concentration decreases in the surroundings of the burning combustibles, their burning behaviors and fire properties change. The important fire properties include combustion efficiency, burning rate and generation rates of combustion products. Many computer codes of zone fire models are now available to simulate compartment fire development. The more frequently used ones are ( i ) C O M P B R N , 1 mostly used by the utihty concerns for simulation of fire scenarios in electricity generation facilities, (ii) Harvard Mark V and Mark V I ; 2 and (li 0 spin-offs of the Harvard codes, e.g. FIRST 3 and other variations, e.g. H A Z A R D I. 4 The Harvard fire codes and their spin-offs, with or without the consideration of the enhancement of burning rate due to heat flux feedback from the compartment, treat the chemistry of burning in the compartment like that in an open atmosphere. One may argue that ventilation-limited burning is provided in H A Z A R D I. However, this fire code requires a priori user-specified fires, expressed in terms of t~me-specified heat release rates and the associated fire products y~elds of the burning Item. As a result, H A Z A R D I does not model the fire growth exactly in accordance with the actual ventilation condition m a fire compartment In COMPBRN, the ventilation-controlled fire is treated such that the overall burning rate in the compartment is proportional to the ventilation rate, and the combustion efficiency is assumed to be constant. The burn algorithms used in the aforementioned compartment fire models can be improved and brought closer to the actual fire chemistry by employing correlations of fire properties as functions of compartment ventilation or oxygen concentration for all revolved combustibles.

F U N C T I O N A L RELATIONSHIPS AND D A T A CORRELATIONS An attempt has been made t o correlate the existing fire property data obtained under different ventilation and oxygen concentration conditions. 5-~ In general, the fire properties of a burning combustible vary with the ambient oxygen concentration and ventilation rate. Also, the fire properties of different combustibles may exhibit significant differences at the same ambient condition. For instance, under atmospheric conditions, a horizontal polyoxymethylene ((CH20)n) slab of 0.1-m diameter burns at a rate of 5.9 g/m2s, while heptane (C7H16) burns at 35-6 g/m2s on a pool of the same diameter. However, if it is postulated that there is a commonality in burning behaviors of different

A untried data correlation scheme for fire properties

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combustibles relative to their respective burning behaviors observed u n d e r the condition of ample air supply, it is expected that the following functional relationships would correlate the c o m b u s t i o n efficiency (X) and the ratio (f,) of generation rates of c o m b u s t i o n products (6J[') to the b u m i n g rate (rh") per unit surface area: z l z , = , = fn,(40)

(1)

f/f,.+=, = fn,(40)

(2)

and where f, -- ~ i / m "

In eqns (1) and (2), 40 is air-to-fuel or oxygen-to-fuel stoichiometric fraction and is defined a s 7 40 = #ha/rh,y~ = r/lo2/thf]Xo2

(3)

where rh,, rho2 and rhf in sequence are air flow rate, oxygen flow rate and total burning rate of fuel; 7, and Yo~ are stoichiometric mass air-to-fuel ratio and oxygen-to-fuel ratio, respectively, based on a premixed flame concept. For diffusion flames, as in the case of natural fires, complete c o m b u s t i o n usually cannot be achieved at 40 = 1. T h e burning rate of a natural fire is mainly controlled by the radiative heat feedback from its flame to its fuel surface. T h e radiative p o w e r of a flame is d e t e r m i n e d by the flame t e m p e r a t u r e and flame emissivity. T h e latter is in turn a function of flame t e m p e r a t u r e , composition of and concentrations of combustion products, and flame size. For sooty flames of the same size, radiative p o w e r is expected to d e p e n d heavily on the flame t e m p e r a t u r e and little on the c o m b u s t i o n products in the flame. Since flame t e m p e r a t u r e is m o d u l a t e d by the a m b i e n t oxygen concentration, 9 analogous to eqns (1) and (2), it is expected = fn2(ro.)

(4)

where Yo2 is the oxygen mass fraction and rh~,,2:, is the burning rate in a pure oxygen e n v i r o n m e n t . T h e existing fire property data 5-8 have been correlated using eqns (1),(2) and (4), and are p r e s e n t e d in Figs 1-4 for c o m b u s t i o n efficiency, burning rate on a 0.1 m d i a m e t e r liquid pool or solid slab, and COs and C O generation rates, respectively. In Fig. 2, the burning rates at about Yo.,= 0-5 have been assumed to be close to those at Yo~=, since the data indicate that the burning rate levels off at oxygen concentrations of less than 0.5. 8 Beyler's COz and C O data are included in Figs 3 and 4, respectively, for comparison. '° T h e data were obtained for the following fuels: p r o p a n e , p r o p e n e , hexane, toluene, m e t h a n o l ,

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:77. CRAIG BEYLER'S DATA1° PROPYLENE, fco=0.20 at ~#=1 A PROPANE, fco=0.25 of to=1 0 CELLULOSE, fco=O.09 at ~#=1 PVC, fco=O.40 af ¢#=1 [] PMMA, fco=O.05 of ~#=1

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ethanol, isopropanol and acetone Overall, the results are very encouraging, except for the CO generation rate of PVC (see Fig. 4). For modified hydrocarbons with chemical additives such as PVC, the additives tend to modify the ignition and burning of combustibles. Consequently, the fire properties of modified hydrocarbons tend to vary from those of the unmodified hydrocarbons. As a result, the fire properties of individual families of modified hydrocarbons may have to be correlated separately using eqns (1), (2) and (4). M o r e data are n e e d e d to verify this data correlation scheme.

CONCLUSION A unified data correlation scheme for fire properties of different combustibles is presented m this brief note and the scheme works well for the existing data Additional fire property data of a wide variety of combustibles of different sizes are n e e d e d to fully test the functional relationslps shown in eqns (1), (2) and (4)

REFERENCES 1. Ho, V., Siu, N. O & Apostolakls, G., COMPBRN III--A computer code for modehng compartment fires. Technical Report UCLA-ENG-8524, University of California, LA, USA, 1985. 2 Mltler, H. E., The Harvard fire model Ftre Safety J , 9 (1985) 7. 3. Mltler, H. E & Rockett, J. A., Users' Gutde to FIRST, A Comprehensive Single-Room Fire Model, US Department of Commerce, NBSIR 87-3595, 1987. 4. Bukowski, R. W., Peacock, R. D., Jones, W. W. & Forney, C L., Techntcal Reference Gutde for Hazard I, NIST Handbook 146 (Vol. II) Center for Fire Research National Engineering Laboratory, National Institute of Standards and Technology, Galthersburg, MD, USA, 1989. 5. Tewarson, A., Lee, J L. & Pion, R. F , The influence of oxygen concentration on fuel parameters for fire modeling. In 18th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, USA, 1981, p. 563. 6. Tewarson, A. & Steclak, J., Fire Venttlation (FMRC J I. OEON6.RC) Factory Mutual Research Corporation, Norwood, MA, USA, 1982 7. Tewarson, A., Fully developed enclosure fires of wood cribs. In 20th Internattonal Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, USA, 1984, p. 1555. 8. Tewarson, A., The SFPE Handbook of Ftre Protection Engineering, National Fire Protection Association, MA, USA, 1988, pp. 1-179

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9. Roberts, A. F , Extinction phenomena in hquids. In 15th International Symposium on Combustton, The Combustion Institute, Pittsburgh, PA, USA, 1974, p. 305 10. Beyler, C L., Development and burning of a layer of products of incomplete combustion generated by a buoyant d~ffusion flame PhD Thesis, Harvard Umversity, USA, 1983.