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Flame spread over porous solids soaked with a combustible liquid
Twenty-first Symposium (International) on Combustion/The Combustion Institute, 1986/pp. 75-81
FLAME S P R E A D O V E R P O R O U S SOLIDS SOAKED W I T H A C O M B U S T I B L E LIQUID KEIJI TAKENO AND TOSHISUKE HIRANO
Department of Reaction Chemistry, University of Tokyo Tokyo,Japan An experimental study has been performed of the flame spread over a layer of porous or non-porous glass beads soaked with kei'osene. The aspects and rates of flame spread were examined in detail, and for typical cases, the temperature distributions in the glass bead layers were measured during flame spread. When a kerosene layer of more than one mm was present over the glass bead layer, the observed aspects and measured rates of flame spread were much the same as those over kerosene in a metal tray. On the contrary, when the kerosene level was close to or below the top surface of the glass bead layer, the flame spread rates were found to be comparable to those over combustible solids and the effects of glass bead characteristics on the flame spread were observed to become distinct. Based on the observed aspects and measured rates of flame spread and the temperature distributions in glass bead layer, it is pointed out that the phenomena characterizing the flame spread are the behavior of kerosene and the two-phase heat transfer through the mixed media of glass beads-gas or glass beads-kerosene. By considering the balance of surface tension, adhesion, and gravitation, the behavior and surface configuration of kerosene observed in the experiments are interpreted qualitatively. The mode of heat transfer to kerosene ahead of the leading flame edge is inferred to be the conduction and convection through the condensed phase.
Introduction Many fires are caused by b u r n i n g spilled combustible liquids. W h e n a combustible liquid is spilled, it will soak into the soil or a p o r o u s material such as a m a t o r c a r p e t on the floor, a n d if a fire occurs, a flame will spread o v e r the p o r o u s solid soaked with the spilled combustible liquid. T h u s , it is essential to be able to p r e d i c t the fire g r o w t h in such a case, and to u n d e r s t a n d the m e c h a n i s m s o f flame spread o v e r p o r o u s solids soaked with combustible liquids. T o p r e v e n t the g r o w t h o f the combustible liquid fire, sand is r e c o m m e n d e d as an extinguishant. T h e sand u s e d for this p u r p o s e will be soaked with the c o m b u s t i b l e liquid. This is a n o t h e r reason why e x p l o r i n g flame spread o v e r p o r o u s solids soaked with combustible liquids is i m p o r t a n t . I n spite o f their practical i m p o r t a n c e , there are few available data on flame spread o v e r p o r o u s solids soaked with combustible liquids. Kaptein and H e r m a n c e e x a m i n e d flame p r o p a -
gation t h r o u g h flammable, gaseous m i x t u r e layers, established o v e r a glass bead layer soaked with volatile liquid f u e l s ) L a y e r e d mixt u r e d c o m b u s t i o n has a l r e a d y b e e n e x p l o r e d extensively in p r e v i o u s studies 2-4 but the results give us no i n f o r m a t i o n a b o u t the flame spread o v e r p o r o u s solids soaked with combustible liquids at sub-flash t e m p e r a t u r e s , n o r on the d e p e n d e n c e of flame s p r e a d p h e n o m e n a on p o r o u s solid properties. I n a few previous studies, flame spread o v e r c r u d e oil sludge has b e e n e x a m i n e d . 5-7 Since the c r u d e oil sludge is a n o n f l u i d m u h i c o m p o n e n t combustible with volatile c o m p o n e n t s , the results o f those studies can be useful in u n d e r s t a n d i n g flame s p r e a d m e c h a n i s m s o v e r porous solids soaked with combustible liquids. However, little i n f o r m a t i o n is given on the d e p e n d e n c e o f flame s p r e a d on the solid or combustible liquid characteristics. I n the p r e s e n t study, the flame spread o v e r n o n p o r o u s and porous glass bead layers soaked with k e r o s e n e have b e e n e x a m i n e d . T h e p u r p o s e o f this study is to increase the f u n d a m e n t a l k n o w l e d g e on flame spread o v e r p o r o u s solids soaked with combust-
75
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FIRE
ible liquids at sub-flash temperatures. Glass bead layers and kerosene are used because of their well known characteristics.
Experimental T h e flame spread experiments were carried out using a steel tray o f 100 cm long, 5 cm wide, and 2.2 cm deep, filled with glass beads as Shown in Fig. 1. This tray was placed in a chamber m a d e of aluminum boards a n d screens to eliminate the effect of air m o v e m e n t in the room on the flame spread p h e n o m e n a . Kerosene used in the present experiments is a c o m p o u n d o f hydrocarbons, with boiling points that range from 180 ~ to 230 ~ Its flash point is about 50 ~ Nonporous and porous glass beads (glass density was 2.25 g/cm s) were used. T h e diameters of the n o n p o r o u s glass beads were 0.05, 0.1, and 0.2 cm, and those of the porous ones were 0,15 and 0.2 cm. T h e pore volume of the porous glass beads was about 0.36 cm3/g. About 10 minutes were needed for the liquid kerosene to become u n i f o r m in the glass bead layer. During each experimental run, kerosene was continuously supplied to the tray to keep a constant kerosene level well ahead of the leading flame edge. Ignition was p e r f o r m e d by using five 2-mm-diam. cotton wicks set near one end of the tray in a row parallel to the end brim. T h e temperature distribution in the glass bead layer d u r i n g flame spread was m e a s u r e d with five 0.1-mmdiam. Pt/Pt-Rh13% thermocouples. T h e values ofx and y are the distances from the ignition end of the tray a n d f r o m the glass bead layer surface. For x = 30 cm, the thermocouples were set aty = - 0 . 1 , - 0 . 2 , - 0 . 3 , - 0 . 5 , and - 1 . 0 cm as shown in Fig. 1.
x,cm +1-
o~E:
y y
o.3
y
1.o
0.5
gl a s s bemls
-2 \k
Fro. 1. Details of the tray filled with glass beads and kerosene. x: distance from the ignition end of the tray; y: distance from the glass bead layer surface; A,B,C,D,E: 0.1 mm Pt/Pt-Rh 13% thermocouples.
Results Aspects of Flame Spread When the initial kerosene surface was above the glass bead layer, i.e., the distance y, from the top of the glass bead layer to the initial kerosene surface was positive, the flame spread p h e n o m ena were observed to be much the same as those over liquid combustibles in metal trays at sub-flash temperatures. 3's-u T h e flame spread seemed to be assisted by the liquid flow induced by the surface tension difference caused by the t e m p e r a t u r e gradient ahead of the leading flame edge. It was shown in previous studies that liquid combustibles o f less than 0.1 cm in thickness in a metal tray at sub-flash temperatures were hard to ignite. 8 For ys < 0.1 cm, however, the bed consisted o f a glass bead layer and kerosene soaked into it could be ignited easily, and a flame was observed to spread over the bed. Fory, = 0, i.e., when the kerosene surface was flush with the top surface of the glass bead layer (see Fig. 1: Due to the adhesion between kerosene and glass beads, each glass bead tip at the top surface was observed to be covered with kerosene, so that the kerosene level at ys = 0 was not completely flat.), the flame spread phenomena were observed to d e p e n d on the characteristics of the glass bead layer. When the glass beads were nonporous and o f diameter d = 0.05 cm, a blue leading flame edge followed by a yellow luminous zone was observed (Fig. 2.(a)). Beneath the flame, including its leading edge, the surfaces of the glass beads were f o u n d to be dry. T h e behavior of the spreading flame as well as the kerosene level near the leading flame edge changed with the bead diameter. As d was increased, the length o f the blue flame increased. For d = 0.2 cm, the blue flame was observed to p r o p a g a t e intermittently for a few cm ahead of the leading edge of the yellow luminous zone and the surface o f kerosene in this region intermittently passed over this p r o p a g a t i n g blue flame was observed to be depressed (Fig. 2(b)). When the glass beads were porous, the behavior of the spreading flame was almost i n d e p e n d e n t o f d and very similar to that of nonporous glass beads of d = 0.05 cm. For ys < 0, the differences in the aspects o f flame spread d u e to differences in the characteristics of the glass beads were found to decrease as y, was decreased. W h e n y, was - 0 . 5 cm, no appreciable difference was observed between the aspects of flame spread over the glass bead layers o f different characteristics. In these cases, the aspects of flame spread were
FLAME SPREAD OVER SOAKED POROUS SOLIDS
••• ~
( a )
~
~
'
YS = 0,
~
~
/
I
Q
U
I
SURFACE
D
YS = 0,
d = 0.2 cm,
Temperature Distributions in Glass Bead Layers
non-porous glass beads
d = 0.05 cm,
LUMINOUS YELLOW ZONE
( b )
after ignition, Vf becomes constant. Vf at this later stage of flame spread is one thirtieth to one fortieth of that forys = 2.2 cm. V/fory~ = 0 decreases as d increases and Vf for nonporous glass beads is smaller than that for porous ones. For ys = - 0 . 5 cm, Vf is independent of the characteristics o f glass beads.
LUMINOUS YELLOW ZONE
L
non-porous
glass beads
FIG. 2. Schematic illustrations of spreading flames and kerosene surfaces.
Temperature-time diagrams recorded by the thermocouples installed in the layers of two different glass beads fory, = 0 are shown in Fig. 4. T h e diagrams for porous glass beads are omitted here, since those were very similar to that for nonporous glass beads of d = 0.05 cm. For nonporous beads o f d = 0.05 cm, the temperature near the layer surface (y = - 0.1 cm) is found to increase rapidly when the leading flame edge passes and to become about 200 ~ one minute after the leading flame edge has passed (Fig. 4(a)). At y = - 1 . 0 cm, the temperature starts to increase after the leading flame edge passes. The temperature-time diagrams for nonpor-
300
LEADING EDGE OF BLUE FLAME \ (L.E.B.F.) ~ LEADING EDGE OF YELLOW
o
/--
YS ( c m ) ,
0 9 0
0.2
porous
0.05
non-porous
0 0
A-0.5
360
20(
non-porous
0.5 480
~
g l a s s bead~ quality
0.05
r
240
{cm),
| [] 0.1
~, 0
120
d
-
-
0.15 porous 0,1
77
A:
ZONE
(L.E.Y.Z.)
y (cm) -0.1
I
/
,
/
non-porous
0.2
non porous
0.05
non-porous
0.1 0.2
non
600
720 t,
D:-o.s
10C
non porous
/,J
~
j
porous
840
sec
FIG. 3. Position-time (x-t) diagrams representing the movements of leading flame edges. similar to those over the glass bead layer of d = 0.2 cm and at ys = 0.
Flame Spread Rate In Fig. 3, the m o v e m e n t o f the leading flame edges for two different types of glass beads at various values o f ys and d are presented in position-time diagrams. For the case of the leading flame edge with intermittently propagating blue flames, the leading edge of the stable flame was recorded. For y~ ~> 0.1 cm, the flame spreads at almost a constant rate throughout each experimental run. The flame spread rates V/for y~ = 2.2 and 0.1 cm are about 2 and 0.5 cm/s, respectively. For ys <~ 0, Vf gradually decreases during the early stage of flame spread, and a few minutes
I 300
( a )
YS
~
=
0,
L I
I 360 d
=
300
0.05
cm,
I 420
I t , sec
non-porous
L.E.B.F.
glass
beads
L.E.Y.Z.
.o
200
100
b
036• ( b
)
Ys
I
L
J
420
= 0,
d = 0.2
i
II 480
cm,
non-porous
L t,
L
SeE
540
glass beads
FIG. 4. Temperature-time (T-t) diagrams representing temperature variations in the tray during flame spread.
78
FIRE
ous glass beads o f d = 0.2 cm are different from those of d = 0.05 cm (Fig. 4(b)). The temperature near the layer surface (y = - 0.1 cm) starts to increase at a point far ahead of the leading flame edge a n d its rate of increase is smaller. However, the temperature variation at a point far from the layer surface (y = - 1.0 cm) is similar to that of d = 0.05 cm. The temperature distributions shown in Fig. 5 were derived from the temperature-time diagrams. I n the case of nonporous glass bead of d = 0.05 cm (Fig. 5(a)), the higher temperature region extends to a point only a few m m ahead of the leading flame edge. This temperature distribution is smaller to that in a combustible solid over which a flame spreads in an opposing air stream. 12-14 For n o n p o r o u s glass beads o f d = 0.2 cm (Fig. 5(b)), the higher temperature region extends a few cm ahead of the leading flame edge. This distance is much larger than the aforementioned one. Futhermore, the thickness of the higher temperature region in the fuel bed is smaller.
Phenomena Controlling Flame Spread Characteristics of Flame Spread For a flame to keep spreading over a condensed combustible without a flammable mixture layer on its surface, the region, where a gasified combustible is ejected from the condensed combustible surface at a sufficient rate to sustain the flame, must spread with the leading flame edge. I n such a case, the mode and the rate of heat transfer to the u n b u r n e d material characterizes the flame spread. 5-16 For ys >I 0.1 cm, the mode of heat transfer to the u n b u r n e d material must be much the same as that for the flame spread over combustible liquid at sub-flash temperatures in metal trays
X. c m 34
I jl
T= 200~
-2
( a )
350t
Y s - O,
~OOt d - 0.05
50~ cm,
hOlt-porous glass
beads
X~ cm
0~
T ~ t
~2
( b )
YS
150~
O,
100t
50C
d - 0.2 C~,
non porous glass beads
FIG. 5. Temperature distributions of glass bead layers during flame spread, when the leading flame edges are passing at x = 30 cm.
because of the similarities of the aspects and rates of flame spread (Fig. 3). 4'8-11 O n the contrary, for y~ ~< 0, i.e., for flame spread over glass bead layers soaked with kerosene, the mode of heat transfer to the u n b u r n e d material cannot be considered to be the same as that for ys I> 0.1 cm because of the differences in the aspects and rates of flame spread (Fig. 3). T h e aspects near the leading edge of flames spreading over the nonporous glass bead layers of d = 0.05 and 0.2 cm soaked with kerosene for ys = 0 are presented in Fig. 2, based on the results of observation and temperature measurements. For d = 0.05 cm, the liquid surface remains very close to the initial level in the region just ahead of the leading flame edge a n d becomes lower beneath the leading flame edge with a large gradient, while for d = 0.2 cm, the liquid surface drops gradually in the region ahead of the leading flame edge a n d becomes a few m m lower than the top of glass bead layer surface near the leading flame edge. For y~ = - 0.5 cm, the flame spread rate seems to be i n d e p e n d e n t of the characteristics of the glass bead layer (Fig. 3), so that not only the mode but also the rate of heat transfer to the kerosene can be inferred to be the same for various types of glass bead layers. In these cases, the heat transfer occurs through the mixed media of glass beads-gas and glass beads-kerosene to the kerosene ahead of the leading flame edge.
Behavior of Combustible Liquid It is well known that flame spread over combustible liquids with free surfaces at subflash temperatures is assisted by the surface tension flow induced by the temperature difference near the leading flame edge. 8 - 11 As shown in Fig. 3, the flame spread rates for ys ~< 0 were much smaller than those for y~ > 0. These results suggest that for Ys ~< 0, the surface tension flow is markedly reduced. The liquid behavior in the porous solid must depend on adhesion, surface tension, liquid friction, and gravitation. Further, the heated liquid kerosene evaporates and gasified kerosene rises from its surface. Thus, the kerosene surface location in the glass bead layer depends on these forces as well as the profile of the local evaporation rate of kerosene. The fact that the kerosene surface configuration near the leading flame edge for n o n p o r o u s glass beads o f d = 0.05 cm is different from that of d = 0.2 cm (Fig. 2) implies a difference in the liquid movements beneath the leading flame edges in these two cases. T h e liquid movement in the direction ahead of the leading flame edge
FLAME SPREAD OVER SOAKED POROUS SOLIDS can be assumed to d e p e n d on the surface tension difference, due to the t e m p e r a t u r e gradient, the adhesion between liquid and glass beads, and the friction o f liquid flowing t h r o u g h the glass bead layer. T h e surface tension difference and adhesion between liquid and glass beads at the distance 2Lx per unit width in the direction across the tray can be expressed as k(do's/dx)~x and c~,lcz2~x, respectively, where k is the ratio of the length occupied by liquid on a line across the tray at the level o f the kerosene surface to the tray width, ors the surface tension, a the effectiveness of surface tension to adhesion per unit l e n g t h o f intersectional lines between liquid and glass beads, and l~ is the length o f the intersectional lines per unit area. T h e liquid movement would d e p e n d on the difference in these forces. Assuming that the liquid movement is steady, the liquid friction caused by the liquid movement through glass bead layer can be considered to be constant and equal to the sum o f the surface tension g r a d i e n t and adhesion, i.e.,
direction would be mainly the gravitation and adhesion. T h e u p w a r d force acting on the liquid due to adhesion is proportional to the surface tension o'~ and the length lc o f the intersectional lines of the liquid surface a n d glass beads. This force must be equal to the force Pg(Ye--yo) acting downward on the liquid d u e to gravitation, f~ o's zc = p g (ye - yo),
~ o'~, z~ = p g (y~ - yo).
(4)
From Eqs. (3) a n d (4) we obtain the equation predicting the configuration of the surface:
(5)
(1)
where F is the liquid friction per unit area of the surface along the kerosene surface. At a certain distance a h e a d of the leading flame edge, the first term o f the left h a n d side of Eq. (1) vanishes, a n d F becomes equal to e~silc, where o'~i is the value o f o's at a point far ahead of the leading flame edge. I f k, c~, l~, and F can be assumed constant for each fuel bed, then Eq. (1) can be solved, and the following equation is given: or, = crsl- (o'si-o'~o )exp(-ed~x/k),
(3)
where ye and yo are the heights o f the liquid surfaces with and without the effect of adhesion, [3 the constant similar to c~, p the density o f kerosene and g the acceleration o f gravity, respectively. At a point o f well ahead of the leading flame edge, the corresponding force balance equation is:
fie = fls -- (o'si -- O's) ~lc/(Pg)
k(do',/dx)/~Lx + acr~lc~Lx = FzXx,
79
(2)
where o'~0 is the value of o's at x=0. Since as is a function of the surface temperature, Eq. (2) implicitly represents the temperature variation along the kerosene surface. T h e value of cy,i is constant for each experimental fuel bed, but the value o f o',o seems to be d e p e n d e n t on the d i a m e t e r a n d quality of glass beads or the initial level o f kerosene and can not be estimated from the present experimental results. However, the overall shape of the o',-x diagrams must be d e t e r m i n e d by the exponential. Therefore, it can be seen that as the glass beads diameter d decreases or the porosity of glass beads increases, i.e., lc increases, the region of o'~ variation, which corresponds to the region of t e m p e r a t u r e variation, becomes smaller. This tendency coincides with that schematically illustrated in Fig. 2 and that o f t e m p e r a t u r e distributions shown in Fig. 5. T h e forces acting on the liquid in the vertical
Since o's d e p e n d s on the temperature, i.e., a function of x in the present case, ye must be a function of x. T h e result o f the prediction coincides well qualitatively with the experimental results shown in Fig. 2. Heat Transfer to Liquid T h e difference in spread rates for various glass bead layers for ys = 0 may be attributed to the liquid behavior. Further, in the case for nonporous glass beads o f d = 0.2 cm, a dry layer o f a few m m of glass beads appears between the leading flame edge and the kerosene surface. T h e thickness of the dry layer will d e p e n d on the characteristics of the glass beads, as discussed in the previous section, and affects the flame spread rate. For y, = - 0.5 cm, the spread rate was r e d u c e d m o r e than 10 % from that for y, = 0, and i n d e p e n d e n t of the characteristics o f the glass beads. For nonporous glass beads of d = 0.2 cm, the spread rate is smaller than that for nonporous glass beads o f d = 0.05 cm, and the ratio of the latter to the f o r m e r is 1.16. I f the t e m p e r a t u r e of the leading flame edge is 1400 ~ and the surface t e m p e r a t u r e o f the fuel bed is 120 ~ for d = 0.05 cm and 200 ~ for d = 0.2 cm, as obtained from the present experiments, the ratio o f heat fluxes would be about 1.07. This value is much the same as the square root of the ratio 1.16 of the s p r e a d i n g rates. This relation is true for the case of flame spread over a thermally thick solid whose d o m i n a n t mode o f
80
FIRE
heat transfer to the u n b u r n e d material is conduction through the condensed phase. 15 Fig. 5 indicates that the temperature distributions for n o n p o r o u s glass beads of d = 0.05 cm and porous glass beads of d = 0.2 cm are apparently very similar to those for flames spreading over solid combustibles in opposing air streams. 12-14 Also, Fig. 3 indicates that the flame spread rates are close to those over solid combustibles. 12-16 Therefore, the mode of heat transfer for these cases can be assumed to be very similar. This means that the d o m i n a n t mode of heat transfer to kerosene ahead of the leading flame edge is conduction through condensed phase. In the cases considered in the present study, however, the combustible is liquid and there must be liquid m o v e m e n t through the glass bead layer. For the case of nonporous glass beads of d = 0.2 cm, the high temperature region spreads far ahead of the leading flame edge, This implies that in the case where the glass bead diameter is relatively large, the contribution of convection of kerosene to heat transfer should be considered. However, the effect of convection of kerosene on the flame spread rate seems to be small, considering that the spread rates are much the same as those over solid combustibles.
beads-gas or glass beads-kerosene. By considering the balance of surface tension, adhesion, and gravitation, the behavior and surface configuration of kerosene observed in the experiments are interpreted qualitatively. T h e mode of heat transfer to kerosene ahead of the leading flame edge is assumed to be conduction and convection t h r o u g h the condensed phase. Also, the heat transfer through the dry layer of glass beads beneath the leading flame edge was shown to be closely related to the flame spread rate.
Acknowledgment The authors would like to express their sincere thanks to Mr. M. Kitaoka for his contribution to the early stage of this experimental study.
REFERENCES 1. KAPTEIN, M. AND HERMANCE, C. E.: Sixteenth Symposium (International) on Combustion, p. 1295, The Combustion Institute, 1976. 2. FENG, C. C., LAM, S. H., AND GLASSMAN, I.:
Combustion Science and Technology, 10, 59 (1975). 3. HIRANO, T., SUZUKI, T., MASHIKO, I., AND TANABE, N.: Combustion Science and Technol-
Conclusions Flame spread over layers of nonporous and porous glass beads of various diameters has been studied experimentally, with the following results. When the kerosene level is a few m m above the surface of the glass bead layer, the observed aspects and measured rates of flame spread were much the same as those over kerosene in a metal tray. O n the contrary, for a kerosene level close to or just below the surface of the glass bead layer, the flame spread rates were found to be comparable to those over combustible solids and the effects of glass bead properties on the flame spread were distinctly observable. The temperature distributions for n o n p o r ous glass beads of d = 0.05 cm and for porpous glass beads of d = 0.2 cm were similar to those in a combustible solid over which a flame spreads in an opposing air stream. O n the contrary, for n o n p o r o u s glass beads of d = 0.2 cm, the higher temperature region extended to a point a few cm ahead of the leading flame edge, The p h e n o m e n a characterizing flame spread are the behavior of kerosene and the two-phase heat transfer t h r o u g h the mixed media of glass
ogy, 22, 83(1980). 4. HIRANO,T. ANn SUZUKI,T., Combustion Science and Technology, 23, 215(1980). 5. HIRANO, T., SuzuKI, T., SATO, J., AND OHTANI,
H.: Twentieth Symposium (International) on Combustion, p. 1611, The Combustion Institute, 1984. 6. OHTANI, H., SATO, J., AND HIRANO, T.: Paper
presented in the AIAA 23rd Aerospace Science Meeting, AIAA-85-0396, Reno, Nevada, Jan. 1985. 7. SUZUKI,T., KUDO,N., SATO,J., OHTANI,H., AND HIRANO, T.: Fire Safety Science, Proceedings of the First International Symposium, p 55, Hemisphere, 1986. 8. MACKINVEN, R., HANSEL,J. G. AND GLASSMAN, I.:
Combustion Science and Technology, 1, 293, (1970). 9. AKITA, K.: Fourteenth Symposium (International) on Combustion, p. 1075, The Combustion Institute, 1973. 10. TORRANCE, K. E. AND MAHAJAN, R. L.: Fifteenth
Symposium (International) on Combustion, p. 281, The Combustion Institute, 1974. 11. GLASSMAN,I. AND DRYER, F. L.: Fire Safety J., 3,
123 (1980). 12. SmUI.KIN,M. AND LEE, C. K.: Combustion Science and Technology, 9, 137(1974).
FLAME SPREAD OVER SOAKED POROUS SOLIDS 13. Hi~r~o, T., Kos~mA, T., AYD A•XTA, K.: Bul. Japanese As. Fire Sci. Eng., 27, 33(1977). 14. FEm~ANDEz-PELLO, A. C. ASD SANT~O, R. J.: Seventeenth Symposium (International) on Combustion, The Combustion Institute, 1979.
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15. QUXNTIErtE,J.: Fire and Materials, 5, 52(1981). 16. FERNANDEZ-PELLO,A. C. AND HIRANO, T.: Con]bustion Science and Technology, 32, 1(1983).
COMMENTS P. J. Pagni, Univ. of California, Berkeley, USA. Why do the porous beads produce a different flame spread rate than the non-porous beads? Author's Reply. Concerning this point, we can only offer a qualitative explanation because we could not visually examine the surfaces of the porous glass beads in detail. The porous glass beads used in this experiment are for desiccators. Their mean surface area, pore diam., pore volume, and glass density at 20~ are 500 cmZ/g, 2.0 x 10 -7 cm, 0.36 cm3/g, and 2.25 g/cm ~, respectively, and all these physical quantities depend on temperature. The surface of a porous glass bead is rugged. Its surface area is about 11.2 cm ~ for a bead of 0.1 cm diam. Due to the large surface area and many small pores, the length of intersectional line (lc) between the liquid and the glass bead, i.e., the value of the adhesion force (the second term in Eq. 1), would be larger than that of a non-porous one. It may be interesting to examine whether flame spread over a layer of infinitely small glass beads would be the same as that over solid combustibles.
M. M. Hirschler, B. F. Goodrich, USA. For the spheres which are non-porous, there is clearly more interstitial volume if the diameter of the beads is larger (less closely packed spheres). It seems to me, therefore, that it would be natural to assume that transfer through the liquid phase rather than through the solid phase could take place. Heat transfer through the liquid would not require the heat transfer across liquid-solid surface layers. I wonder why you did not consider such a possibility? Author's Reply. It is shown in the experiments that the flame spread rate over 2.0 mm diam. non-porous glass beads layer was a little smaller than that over 0.5 mm or 1.0 mm diam. non-porous ones. Furthermore, as made clear in the discussion, the relation between the flame spread rate and the heat flux from the flame to the layer surface is much the same as that for thermally thick solids. In the present case, therefore, the dominant mode of heat transfer adjacent to the
leading flame edge and the layer just below it must be conduction through the layer. Indeed, for the layers of relatively large glass beads, we should consider the effect of liquid convection on the heat transfer. To further clarify these points, it would be useful to put a tray inside and perform a number of experiments using various kinds of layers.
R. B. Williamson, Univ. of California, Berkeley, USA. I understand that the glass beads in these experiments were regularly packed. I suggest that the upper surface of the glass bead array should be characterized by its Miller Indices. It would also be interesting to construct glass bead arrays with different planes exposed to the flame. For instance if the packing is face centered cubic (FCC), then it would be useful to perform experiments with the (111) and (100) planes on the upper surface exposed to burning. In the (111) plane there is a hexagonal array of beads with triangular openings visible between every three beads. On the other hand in the (100) plane there is a square array of beads with a square opening between every four beads. The ( i l l ) plane has the closest packing of beads and thus the smallest amount of liquid fuel exposed to the flame. This is in direct contrast to the (100) plane which has a lower density of beads per unit area.
Author's Reply. We decided that the surface area occupied by liquid and exposed to the flame has little effect on the heat transfer from the flame to the layer surface, because the heat flux from the flame to the surfaces of liquid and glass beads are finally transferred to the liquid fuel. If the liquid area at the layer surface had a large effect on the flame spread phenomena, the experimental data of flame spread rate would show more scatter. This scatter in the present cases were less than 10%. I do believe that examining the glass bead arrays at the layer surface and comparing them with experimental results will be very important for promoting a better understanding.
FLAME S P R E A D O V E R P O R O U S SOLIDS SOAKED W I T H A C O M B U S T I B L E LIQUID KEIJI TAKENO AND TOSHISUKE HIRANO
Department of Reaction Chemistry, University of Tokyo Tokyo,Japan An experimental study has been performed of the flame spread over a layer of porous or non-porous glass beads soaked with kei'osene. The aspects and rates of flame spread were examined in detail, and for typical cases, the temperature distributions in the glass bead layers were measured during flame spread. When a kerosene layer of more than one mm was present over the glass bead layer, the observed aspects and measured rates of flame spread were much the same as those over kerosene in a metal tray. On the contrary, when the kerosene level was close to or below the top surface of the glass bead layer, the flame spread rates were found to be comparable to those over combustible solids and the effects of glass bead characteristics on the flame spread were observed to become distinct. Based on the observed aspects and measured rates of flame spread and the temperature distributions in glass bead layer, it is pointed out that the phenomena characterizing the flame spread are the behavior of kerosene and the two-phase heat transfer through the mixed media of glass beads-gas or glass beads-kerosene. By considering the balance of surface tension, adhesion, and gravitation, the behavior and surface configuration of kerosene observed in the experiments are interpreted qualitatively. The mode of heat transfer to kerosene ahead of the leading flame edge is inferred to be the conduction and convection through the condensed phase.
Introduction Many fires are caused by b u r n i n g spilled combustible liquids. W h e n a combustible liquid is spilled, it will soak into the soil or a p o r o u s material such as a m a t o r c a r p e t on the floor, a n d if a fire occurs, a flame will spread o v e r the p o r o u s solid soaked with the spilled combustible liquid. T h u s , it is essential to be able to p r e d i c t the fire g r o w t h in such a case, and to u n d e r s t a n d the m e c h a n i s m s o f flame spread o v e r p o r o u s solids soaked with combustible liquids. T o p r e v e n t the g r o w t h o f the combustible liquid fire, sand is r e c o m m e n d e d as an extinguishant. T h e sand u s e d for this p u r p o s e will be soaked with the c o m b u s t i b l e liquid. This is a n o t h e r reason why e x p l o r i n g flame spread o v e r p o r o u s solids soaked with combustible liquids is i m p o r t a n t . I n spite o f their practical i m p o r t a n c e , there are few available data on flame spread o v e r p o r o u s solids soaked with combustible liquids. Kaptein and H e r m a n c e e x a m i n e d flame p r o p a -
gation t h r o u g h flammable, gaseous m i x t u r e layers, established o v e r a glass bead layer soaked with volatile liquid f u e l s ) L a y e r e d mixt u r e d c o m b u s t i o n has a l r e a d y b e e n e x p l o r e d extensively in p r e v i o u s studies 2-4 but the results give us no i n f o r m a t i o n a b o u t the flame spread o v e r p o r o u s solids soaked with combustible liquids at sub-flash t e m p e r a t u r e s , n o r on the d e p e n d e n c e of flame s p r e a d p h e n o m e n a on p o r o u s solid properties. I n a few previous studies, flame spread o v e r c r u d e oil sludge has b e e n e x a m i n e d . 5-7 Since the c r u d e oil sludge is a n o n f l u i d m u h i c o m p o n e n t combustible with volatile c o m p o n e n t s , the results o f those studies can be useful in u n d e r s t a n d i n g flame s p r e a d m e c h a n i s m s o v e r porous solids soaked with combustible liquids. However, little i n f o r m a t i o n is given on the d e p e n d e n c e o f flame s p r e a d on the solid or combustible liquid characteristics. I n the p r e s e n t study, the flame spread o v e r n o n p o r o u s and porous glass bead layers soaked with k e r o s e n e have b e e n e x a m i n e d . T h e p u r p o s e o f this study is to increase the f u n d a m e n t a l k n o w l e d g e on flame spread o v e r p o r o u s solids soaked with combust-
75
76
FIRE
ible liquids at sub-flash temperatures. Glass bead layers and kerosene are used because of their well known characteristics.
Experimental T h e flame spread experiments were carried out using a steel tray o f 100 cm long, 5 cm wide, and 2.2 cm deep, filled with glass beads as Shown in Fig. 1. This tray was placed in a chamber m a d e of aluminum boards a n d screens to eliminate the effect of air m o v e m e n t in the room on the flame spread p h e n o m e n a . Kerosene used in the present experiments is a c o m p o u n d o f hydrocarbons, with boiling points that range from 180 ~ to 230 ~ Its flash point is about 50 ~ Nonporous and porous glass beads (glass density was 2.25 g/cm s) were used. T h e diameters of the n o n p o r o u s glass beads were 0.05, 0.1, and 0.2 cm, and those of the porous ones were 0,15 and 0.2 cm. T h e pore volume of the porous glass beads was about 0.36 cm3/g. About 10 minutes were needed for the liquid kerosene to become u n i f o r m in the glass bead layer. During each experimental run, kerosene was continuously supplied to the tray to keep a constant kerosene level well ahead of the leading flame edge. Ignition was p e r f o r m e d by using five 2-mm-diam. cotton wicks set near one end of the tray in a row parallel to the end brim. T h e temperature distribution in the glass bead layer d u r i n g flame spread was m e a s u r e d with five 0.1-mmdiam. Pt/Pt-Rh13% thermocouples. T h e values ofx and y are the distances from the ignition end of the tray a n d f r o m the glass bead layer surface. For x = 30 cm, the thermocouples were set aty = - 0 . 1 , - 0 . 2 , - 0 . 3 , - 0 . 5 , and - 1 . 0 cm as shown in Fig. 1.
x,cm +1-
o~E:
y y
o.3
y
1.o
0.5
gl a s s bemls
-2 \k
Fro. 1. Details of the tray filled with glass beads and kerosene. x: distance from the ignition end of the tray; y: distance from the glass bead layer surface; A,B,C,D,E: 0.1 mm Pt/Pt-Rh 13% thermocouples.
Results Aspects of Flame Spread When the initial kerosene surface was above the glass bead layer, i.e., the distance y, from the top of the glass bead layer to the initial kerosene surface was positive, the flame spread p h e n o m ena were observed to be much the same as those over liquid combustibles in metal trays at sub-flash temperatures. 3's-u T h e flame spread seemed to be assisted by the liquid flow induced by the surface tension difference caused by the t e m p e r a t u r e gradient ahead of the leading flame edge. It was shown in previous studies that liquid combustibles o f less than 0.1 cm in thickness in a metal tray at sub-flash temperatures were hard to ignite. 8 For ys < 0.1 cm, however, the bed consisted o f a glass bead layer and kerosene soaked into it could be ignited easily, and a flame was observed to spread over the bed. Fory, = 0, i.e., when the kerosene surface was flush with the top surface of the glass bead layer (see Fig. 1: Due to the adhesion between kerosene and glass beads, each glass bead tip at the top surface was observed to be covered with kerosene, so that the kerosene level at ys = 0 was not completely flat.), the flame spread phenomena were observed to d e p e n d on the characteristics of the glass bead layer. When the glass beads were nonporous and o f diameter d = 0.05 cm, a blue leading flame edge followed by a yellow luminous zone was observed (Fig. 2.(a)). Beneath the flame, including its leading edge, the surfaces of the glass beads were f o u n d to be dry. T h e behavior of the spreading flame as well as the kerosene level near the leading flame edge changed with the bead diameter. As d was increased, the length o f the blue flame increased. For d = 0.2 cm, the blue flame was observed to p r o p a g a t e intermittently for a few cm ahead of the leading edge of the yellow luminous zone and the surface o f kerosene in this region intermittently passed over this p r o p a g a t i n g blue flame was observed to be depressed (Fig. 2(b)). When the glass beads were porous, the behavior of the spreading flame was almost i n d e p e n d e n t o f d and very similar to that of nonporous glass beads of d = 0.05 cm. For ys < 0, the differences in the aspects o f flame spread d u e to differences in the characteristics of the glass beads were found to decrease as y, was decreased. W h e n y, was - 0 . 5 cm, no appreciable difference was observed between the aspects of flame spread over the glass bead layers o f different characteristics. In these cases, the aspects of flame spread were
FLAME SPREAD OVER SOAKED POROUS SOLIDS
••• ~
( a )
~
~
'
YS = 0,
~
~
/
I
Q
U
I
SURFACE
D
YS = 0,
d = 0.2 cm,
Temperature Distributions in Glass Bead Layers
non-porous glass beads
d = 0.05 cm,
LUMINOUS YELLOW ZONE
( b )
after ignition, Vf becomes constant. Vf at this later stage of flame spread is one thirtieth to one fortieth of that forys = 2.2 cm. V/fory~ = 0 decreases as d increases and Vf for nonporous glass beads is smaller than that for porous ones. For ys = - 0 . 5 cm, Vf is independent of the characteristics o f glass beads.
LUMINOUS YELLOW ZONE
L
non-porous
glass beads
FIG. 2. Schematic illustrations of spreading flames and kerosene surfaces.
Temperature-time diagrams recorded by the thermocouples installed in the layers of two different glass beads fory, = 0 are shown in Fig. 4. T h e diagrams for porous glass beads are omitted here, since those were very similar to that for nonporous glass beads of d = 0.05 cm. For nonporous beads o f d = 0.05 cm, the temperature near the layer surface (y = - 0.1 cm) is found to increase rapidly when the leading flame edge passes and to become about 200 ~ one minute after the leading flame edge has passed (Fig. 4(a)). At y = - 1 . 0 cm, the temperature starts to increase after the leading flame edge passes. The temperature-time diagrams for nonpor-
300
LEADING EDGE OF BLUE FLAME \ (L.E.B.F.) ~ LEADING EDGE OF YELLOW
o
/--
YS ( c m ) ,
0 9 0
0.2
porous
0.05
non-porous
0 0
A-0.5
360
20(
non-porous
0.5 480
~
g l a s s bead~ quality
0.05
r
240
{cm),
| [] 0.1
~, 0
120
d
-
-
0.15 porous 0,1
77
A:
ZONE
(L.E.Y.Z.)
y (cm) -0.1
I
/
,
/
non-porous
0.2
non porous
0.05
non-porous
0.1 0.2
non
600
720 t,
D:-o.s
10C
non porous
/,J
~
j
porous
840
sec
FIG. 3. Position-time (x-t) diagrams representing the movements of leading flame edges. similar to those over the glass bead layer of d = 0.2 cm and at ys = 0.
Flame Spread Rate In Fig. 3, the m o v e m e n t o f the leading flame edges for two different types of glass beads at various values o f ys and d are presented in position-time diagrams. For the case of the leading flame edge with intermittently propagating blue flames, the leading edge of the stable flame was recorded. For y~ ~> 0.1 cm, the flame spreads at almost a constant rate throughout each experimental run. The flame spread rates V/for y~ = 2.2 and 0.1 cm are about 2 and 0.5 cm/s, respectively. For ys <~ 0, Vf gradually decreases during the early stage of flame spread, and a few minutes
I 300
( a )
YS
~
=
0,
L I
I 360 d
=
300
0.05
cm,
I 420
I t , sec
non-porous
L.E.B.F.
glass
beads
L.E.Y.Z.
.o
200
100
b
036• ( b
)
Ys
I
L
J
420
= 0,
d = 0.2
i
II 480
cm,
non-porous
L t,
L
SeE
540
glass beads
FIG. 4. Temperature-time (T-t) diagrams representing temperature variations in the tray during flame spread.
78
FIRE
ous glass beads o f d = 0.2 cm are different from those of d = 0.05 cm (Fig. 4(b)). The temperature near the layer surface (y = - 0.1 cm) starts to increase at a point far ahead of the leading flame edge a n d its rate of increase is smaller. However, the temperature variation at a point far from the layer surface (y = - 1.0 cm) is similar to that of d = 0.05 cm. The temperature distributions shown in Fig. 5 were derived from the temperature-time diagrams. I n the case of nonporous glass bead of d = 0.05 cm (Fig. 5(a)), the higher temperature region extends to a point only a few m m ahead of the leading flame edge. This temperature distribution is smaller to that in a combustible solid over which a flame spreads in an opposing air stream. 12-14 For n o n p o r o u s glass beads o f d = 0.2 cm (Fig. 5(b)), the higher temperature region extends a few cm ahead of the leading flame edge. This distance is much larger than the aforementioned one. Futhermore, the thickness of the higher temperature region in the fuel bed is smaller.
Phenomena Controlling Flame Spread Characteristics of Flame Spread For a flame to keep spreading over a condensed combustible without a flammable mixture layer on its surface, the region, where a gasified combustible is ejected from the condensed combustible surface at a sufficient rate to sustain the flame, must spread with the leading flame edge. I n such a case, the mode and the rate of heat transfer to the u n b u r n e d material characterizes the flame spread. 5-16 For ys >I 0.1 cm, the mode of heat transfer to the u n b u r n e d material must be much the same as that for the flame spread over combustible liquid at sub-flash temperatures in metal trays
X. c m 34
I jl
T= 200~
-2
( a )
350t
Y s - O,
~OOt d - 0.05
50~ cm,
hOlt-porous glass
beads
X~ cm
0~
T ~ t
~2
( b )
YS
150~
O,
100t
50C
d - 0.2 C~,
non porous glass beads
FIG. 5. Temperature distributions of glass bead layers during flame spread, when the leading flame edges are passing at x = 30 cm.
because of the similarities of the aspects and rates of flame spread (Fig. 3). 4'8-11 O n the contrary, for y~ ~< 0, i.e., for flame spread over glass bead layers soaked with kerosene, the mode of heat transfer to the u n b u r n e d material cannot be considered to be the same as that for ys I> 0.1 cm because of the differences in the aspects and rates of flame spread (Fig. 3). T h e aspects near the leading edge of flames spreading over the nonporous glass bead layers of d = 0.05 and 0.2 cm soaked with kerosene for ys = 0 are presented in Fig. 2, based on the results of observation and temperature measurements. For d = 0.05 cm, the liquid surface remains very close to the initial level in the region just ahead of the leading flame edge a n d becomes lower beneath the leading flame edge with a large gradient, while for d = 0.2 cm, the liquid surface drops gradually in the region ahead of the leading flame edge a n d becomes a few m m lower than the top of glass bead layer surface near the leading flame edge. For y~ = - 0.5 cm, the flame spread rate seems to be i n d e p e n d e n t of the characteristics of the glass bead layer (Fig. 3), so that not only the mode but also the rate of heat transfer to the kerosene can be inferred to be the same for various types of glass bead layers. In these cases, the heat transfer occurs through the mixed media of glass beads-gas and glass beads-kerosene to the kerosene ahead of the leading flame edge.
Behavior of Combustible Liquid It is well known that flame spread over combustible liquids with free surfaces at subflash temperatures is assisted by the surface tension flow induced by the temperature difference near the leading flame edge. 8 - 11 As shown in Fig. 3, the flame spread rates for ys ~< 0 were much smaller than those for y~ > 0. These results suggest that for Ys ~< 0, the surface tension flow is markedly reduced. The liquid behavior in the porous solid must depend on adhesion, surface tension, liquid friction, and gravitation. Further, the heated liquid kerosene evaporates and gasified kerosene rises from its surface. Thus, the kerosene surface location in the glass bead layer depends on these forces as well as the profile of the local evaporation rate of kerosene. The fact that the kerosene surface configuration near the leading flame edge for n o n p o r o u s glass beads o f d = 0.05 cm is different from that of d = 0.2 cm (Fig. 2) implies a difference in the liquid movements beneath the leading flame edges in these two cases. T h e liquid movement in the direction ahead of the leading flame edge
FLAME SPREAD OVER SOAKED POROUS SOLIDS can be assumed to d e p e n d on the surface tension difference, due to the t e m p e r a t u r e gradient, the adhesion between liquid and glass beads, and the friction o f liquid flowing t h r o u g h the glass bead layer. T h e surface tension difference and adhesion between liquid and glass beads at the distance 2Lx per unit width in the direction across the tray can be expressed as k(do's/dx)~x and c~,lcz2~x, respectively, where k is the ratio of the length occupied by liquid on a line across the tray at the level o f the kerosene surface to the tray width, ors the surface tension, a the effectiveness of surface tension to adhesion per unit l e n g t h o f intersectional lines between liquid and glass beads, and l~ is the length o f the intersectional lines per unit area. T h e liquid movement would d e p e n d on the difference in these forces. Assuming that the liquid movement is steady, the liquid friction caused by the liquid movement through glass bead layer can be considered to be constant and equal to the sum o f the surface tension g r a d i e n t and adhesion, i.e.,
direction would be mainly the gravitation and adhesion. T h e u p w a r d force acting on the liquid due to adhesion is proportional to the surface tension o'~ and the length lc o f the intersectional lines of the liquid surface a n d glass beads. This force must be equal to the force Pg(Ye--yo) acting downward on the liquid d u e to gravitation, f~ o's zc = p g (ye - yo),
~ o'~, z~ = p g (y~ - yo).
(4)
From Eqs. (3) a n d (4) we obtain the equation predicting the configuration of the surface:
(5)
(1)
where F is the liquid friction per unit area of the surface along the kerosene surface. At a certain distance a h e a d of the leading flame edge, the first term o f the left h a n d side of Eq. (1) vanishes, a n d F becomes equal to e~silc, where o'~i is the value o f o's at a point far ahead of the leading flame edge. I f k, c~, l~, and F can be assumed constant for each fuel bed, then Eq. (1) can be solved, and the following equation is given: or, = crsl- (o'si-o'~o )exp(-ed~x/k),
(3)
where ye and yo are the heights o f the liquid surfaces with and without the effect of adhesion, [3 the constant similar to c~, p the density o f kerosene and g the acceleration o f gravity, respectively. At a point o f well ahead of the leading flame edge, the corresponding force balance equation is:
fie = fls -- (o'si -- O's) ~lc/(Pg)
k(do',/dx)/~Lx + acr~lc~Lx = FzXx,
79
(2)
where o'~0 is the value of o's at x=0. Since as is a function of the surface temperature, Eq. (2) implicitly represents the temperature variation along the kerosene surface. T h e value of cy,i is constant for each experimental fuel bed, but the value o f o',o seems to be d e p e n d e n t on the d i a m e t e r a n d quality of glass beads or the initial level o f kerosene and can not be estimated from the present experimental results. However, the overall shape of the o',-x diagrams must be d e t e r m i n e d by the exponential. Therefore, it can be seen that as the glass beads diameter d decreases or the porosity of glass beads increases, i.e., lc increases, the region of o'~ variation, which corresponds to the region of t e m p e r a t u r e variation, becomes smaller. This tendency coincides with that schematically illustrated in Fig. 2 and that o f t e m p e r a t u r e distributions shown in Fig. 5. T h e forces acting on the liquid in the vertical
Since o's d e p e n d s on the temperature, i.e., a function of x in the present case, ye must be a function of x. T h e result o f the prediction coincides well qualitatively with the experimental results shown in Fig. 2. Heat Transfer to Liquid T h e difference in spread rates for various glass bead layers for ys = 0 may be attributed to the liquid behavior. Further, in the case for nonporous glass beads o f d = 0.2 cm, a dry layer o f a few m m of glass beads appears between the leading flame edge and the kerosene surface. T h e thickness of the dry layer will d e p e n d on the characteristics of the glass beads, as discussed in the previous section, and affects the flame spread rate. For y, = - 0.5 cm, the spread rate was r e d u c e d m o r e than 10 % from that for y, = 0, and i n d e p e n d e n t of the characteristics o f the glass beads. For nonporous glass beads of d = 0.2 cm, the spread rate is smaller than that for nonporous glass beads o f d = 0.05 cm, and the ratio of the latter to the f o r m e r is 1.16. I f the t e m p e r a t u r e of the leading flame edge is 1400 ~ and the surface t e m p e r a t u r e o f the fuel bed is 120 ~ for d = 0.05 cm and 200 ~ for d = 0.2 cm, as obtained from the present experiments, the ratio o f heat fluxes would be about 1.07. This value is much the same as the square root of the ratio 1.16 of the s p r e a d i n g rates. This relation is true for the case of flame spread over a thermally thick solid whose d o m i n a n t mode o f
80
FIRE
heat transfer to the u n b u r n e d material is conduction through the condensed phase. 15 Fig. 5 indicates that the temperature distributions for n o n p o r o u s glass beads of d = 0.05 cm and porous glass beads of d = 0.2 cm are apparently very similar to those for flames spreading over solid combustibles in opposing air streams. 12-14 Also, Fig. 3 indicates that the flame spread rates are close to those over solid combustibles. 12-16 Therefore, the mode of heat transfer for these cases can be assumed to be very similar. This means that the d o m i n a n t mode of heat transfer to kerosene ahead of the leading flame edge is conduction through condensed phase. In the cases considered in the present study, however, the combustible is liquid and there must be liquid m o v e m e n t through the glass bead layer. For the case of nonporous glass beads of d = 0.2 cm, the high temperature region spreads far ahead of the leading flame edge, This implies that in the case where the glass bead diameter is relatively large, the contribution of convection of kerosene to heat transfer should be considered. However, the effect of convection of kerosene on the flame spread rate seems to be small, considering that the spread rates are much the same as those over solid combustibles.
beads-gas or glass beads-kerosene. By considering the balance of surface tension, adhesion, and gravitation, the behavior and surface configuration of kerosene observed in the experiments are interpreted qualitatively. T h e mode of heat transfer to kerosene ahead of the leading flame edge is assumed to be conduction and convection t h r o u g h the condensed phase. Also, the heat transfer through the dry layer of glass beads beneath the leading flame edge was shown to be closely related to the flame spread rate.
Acknowledgment The authors would like to express their sincere thanks to Mr. M. Kitaoka for his contribution to the early stage of this experimental study.
REFERENCES 1. KAPTEIN, M. AND HERMANCE, C. E.: Sixteenth Symposium (International) on Combustion, p. 1295, The Combustion Institute, 1976. 2. FENG, C. C., LAM, S. H., AND GLASSMAN, I.:
Combustion Science and Technology, 10, 59 (1975). 3. HIRANO, T., SUZUKI, T., MASHIKO, I., AND TANABE, N.: Combustion Science and Technol-
Conclusions Flame spread over layers of nonporous and porous glass beads of various diameters has been studied experimentally, with the following results. When the kerosene level is a few m m above the surface of the glass bead layer, the observed aspects and measured rates of flame spread were much the same as those over kerosene in a metal tray. O n the contrary, for a kerosene level close to or just below the surface of the glass bead layer, the flame spread rates were found to be comparable to those over combustible solids and the effects of glass bead properties on the flame spread were distinctly observable. The temperature distributions for n o n p o r ous glass beads of d = 0.05 cm and for porpous glass beads of d = 0.2 cm were similar to those in a combustible solid over which a flame spreads in an opposing air stream. O n the contrary, for n o n p o r o u s glass beads of d = 0.2 cm, the higher temperature region extended to a point a few cm ahead of the leading flame edge, The p h e n o m e n a characterizing flame spread are the behavior of kerosene and the two-phase heat transfer t h r o u g h the mixed media of glass
ogy, 22, 83(1980). 4. HIRANO,T. ANn SUZUKI,T., Combustion Science and Technology, 23, 215(1980). 5. HIRANO, T., SuzuKI, T., SATO, J., AND OHTANI,
H.: Twentieth Symposium (International) on Combustion, p. 1611, The Combustion Institute, 1984. 6. OHTANI, H., SATO, J., AND HIRANO, T.: Paper
presented in the AIAA 23rd Aerospace Science Meeting, AIAA-85-0396, Reno, Nevada, Jan. 1985. 7. SUZUKI,T., KUDO,N., SATO,J., OHTANI,H., AND HIRANO, T.: Fire Safety Science, Proceedings of the First International Symposium, p 55, Hemisphere, 1986. 8. MACKINVEN, R., HANSEL,J. G. AND GLASSMAN, I.:
Combustion Science and Technology, 1, 293, (1970). 9. AKITA, K.: Fourteenth Symposium (International) on Combustion, p. 1075, The Combustion Institute, 1973. 10. TORRANCE, K. E. AND MAHAJAN, R. L.: Fifteenth
Symposium (International) on Combustion, p. 281, The Combustion Institute, 1974. 11. GLASSMAN,I. AND DRYER, F. L.: Fire Safety J., 3,
123 (1980). 12. SmUI.KIN,M. AND LEE, C. K.: Combustion Science and Technology, 9, 137(1974).
FLAME SPREAD OVER SOAKED POROUS SOLIDS 13. Hi~r~o, T., Kos~mA, T., AYD A•XTA, K.: Bul. Japanese As. Fire Sci. Eng., 27, 33(1977). 14. FEm~ANDEz-PELLO, A. C. ASD SANT~O, R. J.: Seventeenth Symposium (International) on Combustion, The Combustion Institute, 1979.
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15. QUXNTIErtE,J.: Fire and Materials, 5, 52(1981). 16. FERNANDEZ-PELLO,A. C. AND HIRANO, T.: Con]bustion Science and Technology, 32, 1(1983).
COMMENTS P. J. Pagni, Univ. of California, Berkeley, USA. Why do the porous beads produce a different flame spread rate than the non-porous beads? Author's Reply. Concerning this point, we can only offer a qualitative explanation because we could not visually examine the surfaces of the porous glass beads in detail. The porous glass beads used in this experiment are for desiccators. Their mean surface area, pore diam., pore volume, and glass density at 20~ are 500 cmZ/g, 2.0 x 10 -7 cm, 0.36 cm3/g, and 2.25 g/cm ~, respectively, and all these physical quantities depend on temperature. The surface of a porous glass bead is rugged. Its surface area is about 11.2 cm ~ for a bead of 0.1 cm diam. Due to the large surface area and many small pores, the length of intersectional line (lc) between the liquid and the glass bead, i.e., the value of the adhesion force (the second term in Eq. 1), would be larger than that of a non-porous one. It may be interesting to examine whether flame spread over a layer of infinitely small glass beads would be the same as that over solid combustibles.
M. M. Hirschler, B. F. Goodrich, USA. For the spheres which are non-porous, there is clearly more interstitial volume if the diameter of the beads is larger (less closely packed spheres). It seems to me, therefore, that it would be natural to assume that transfer through the liquid phase rather than through the solid phase could take place. Heat transfer through the liquid would not require the heat transfer across liquid-solid surface layers. I wonder why you did not consider such a possibility? Author's Reply. It is shown in the experiments that the flame spread rate over 2.0 mm diam. non-porous glass beads layer was a little smaller than that over 0.5 mm or 1.0 mm diam. non-porous ones. Furthermore, as made clear in the discussion, the relation between the flame spread rate and the heat flux from the flame to the layer surface is much the same as that for thermally thick solids. In the present case, therefore, the dominant mode of heat transfer adjacent to the
leading flame edge and the layer just below it must be conduction through the layer. Indeed, for the layers of relatively large glass beads, we should consider the effect of liquid convection on the heat transfer. To further clarify these points, it would be useful to put a tray inside and perform a number of experiments using various kinds of layers.
R. B. Williamson, Univ. of California, Berkeley, USA. I understand that the glass beads in these experiments were regularly packed. I suggest that the upper surface of the glass bead array should be characterized by its Miller Indices. It would also be interesting to construct glass bead arrays with different planes exposed to the flame. For instance if the packing is face centered cubic (FCC), then it would be useful to perform experiments with the (111) and (100) planes on the upper surface exposed to burning. In the (111) plane there is a hexagonal array of beads with triangular openings visible between every three beads. On the other hand in the (100) plane there is a square array of beads with a square opening between every four beads. The ( i l l ) plane has the closest packing of beads and thus the smallest amount of liquid fuel exposed to the flame. This is in direct contrast to the (100) plane which has a lower density of beads per unit area.
Author's Reply. We decided that the surface area occupied by liquid and exposed to the flame has little effect on the heat transfer from the flame to the layer surface, because the heat flux from the flame to the surfaces of liquid and glass beads are finally transferred to the liquid fuel. If the liquid area at the layer surface had a large effect on the flame spread phenomena, the experimental data of flame spread rate would show more scatter. This scatter in the present cases were less than 10%. I do believe that examining the glass bead arrays at the layer surface and comparing them with experimental results will be very important for promoting a better understanding.