Static and dynamic radiance structures in pool fires

Twenty-first Symposium (International) on Combustion/The Combustion Institute, 1986/pp. 93-100

STATIC AND DYNAMIC

RADIANCE

STRUCTURES

IN POOL FIRES

A.SCH{3NBUCHER, D.GOECK, A.KETTLER*, D.KRATTENMACHER ax~ N.SCHIESS Institut fi2r Technische Chemie l der Universiti~t Stuttgart D-7000 Stuttgart 80, PJaffenwaldring 55, West Germany

Digital image analysis of pool fire photograms and flame radiance measurements, time-averaged and instantaneous, was applied in observing static and dynamic radiance structures in the visible and infrared spectral ranges. Small-view-angle pyroelectric radiometer measurements as well as measurements of instantaneous radiances with Si- and Ge-photodiodes and of flame temperatures, concentrations of stable species and gas flow velocities were carried out. Time-averaged fields of radiances [.~(y,x) are defined as static equidensitometric structures consisting of symmetric W- or M-shaped core structures and elliptical plume structures. The fuel type and the pool diameter change only the geometric part of the core and plume structures. The structures LaO',x) in the visible spectrum represent in good approximation the spatial distribution of the radiances L~(y',x) in the infrared spectrum. Volumetric emission coefficient s are determined by Abel-inversion from La(y,x)-fields. A simplified model, based on the assumptions of the optical thin limit and inhomogeneities in temperatures and species concentrations is presented to calculate qta(r,x)-profiles. The dynamic radiance structures are totally unsyhlmetric and show monoperiodic and nonperiodic properties. Formation frequencies for fire parcels, fire mushrooms, soot parcels and hot spots as well as oscillation frequencies for visible fire shapes are determined. Spatial power spectra are calculated from instantaneous radiance profiles b~ Fourier analysis. The macroscales determined depend strongly but the microscales only weakl~ on the height x above the fuel surface. It seems that the microscales reach a lower limit of 5 ram.

in pool fires have been observed. Finally, structured and u n s t r u c t u r e d area fires have been distinguished. 9 A l t h o u g h digital image processing is freq u e n t l y used m u n r e a c t i v e flow structure studies, no application has been m a d e to pool fires. In this study, digital image analysis is applied to t i m e - a v e r a g e d a n d instantaneous pool fire p h o t o g r a m s . With this m e t h o d new types o f structures, such as static and dynamic r a d i a n c e structures, are visualized and evaluated. N u m e r o u s m o n o p e r i o d i c and nonperiodic p r o p e r t i e s o f these structures are presented. In a n o t h e r study I by the authors, the d y n a m i c radiance structures were discussed in relation to a n o t h e r class o f structures. T h e o r g a n i z e d density structures in pool fires.

1 Introduction Pool fires are buoyantly controlled, free a m b i e n t diffusion flames o f c o n d e n s e d fuels. T h e fire field has large t e m p e r a t u r e , species c o n c e n t r a t i o n and flow velocity gradients. Such fires are t h e r e f o r e far r e m o v e d f r o m an equilibrium state and u n d e r g o a continuous exc h a n g e o f energy, m a t t e r and m o m e n t u m with the a m b i e n t air. A c o n t i n u o u s e n e r g y supply exists because o f the c o m b u s t i o n o f vaporizing fuel. T h u s o r g a n i z e d structures are g e n e r a t e d spontaneously. T h e a u t h o r s have observed static and d y n a m i c r a d i a n c e structures, as well as d y n a m i c density structures, 1 in pool fires. In this p a p e r , the r a d i a n c e structures in the visible and i n f r a r e d spectral ranges are presented. Most o f the few investigations carried out up to now on the radiance structures o f pool fires used direct p h o t o g r a p h i c visualization technique. With this m e t h o d , visible structures 2 in city gas diffusion flames, hot spots, 3'4 soot parcels, ~ convective cells 6 and the i m p o r t a n t influence o f lip effects 7,s on the visible structure

2 Experimental

2.1 Fuel supply systems T h e fuel is p u m p e d f r o m a liquid fuel r e s e r v o i r into an overflow vessel f r o m where it flows t h r o u g h a flow m e t e r to a level controller, that regulates the fuel level in the pool. T h u s the fuel level remains constant, and the fire burns stationarily.

*Current address: Carl Zeiss Company, P. O. Box 1369/1380, D-7082 Oberkochen, F. R G. 93

94

FIRE

Cvlindrical steel tanks with diameters d between d - 1 cm and d - 100 cm are used. For diameters of 100 _-< d -<_ 2500 cm the fires b u r n above concrete tanks or round pools filled with water covered by a thin fuel laver. The nineteen fuels used are given in 1.

2.2 Photographic system From the pool fire time exposures, short exposures and successive instantaneous exposures are recorded. To record time and short exposures, a transparent neutral step wedge, irradiated by a light source of constant brightness, is set up next to the flame in the darkened experimental hall. The neutral wedge has 20 steps of optical density D between D = 0.05 and D = 3.05, with a step-wise increase in density of 0.15. An interference filter with a peak wave length of X~ = 633 nm and a bandwidth at 0.5 peak height of 10 nm is positioned between the flame and the 6 • cm camera. The exposure time is 90 s, for short time exposures it is 1/400 s. Frame sequences are taken with 1000 pictures per second (pps) for small fires and up to 60 pps for fires with d > 100 cm.

2.3 Image analysis of digitized photograms Digitization of the analog image (photogram) of the fire is carried out with an image analysis system as illustrated schematically in Fig. 1. l-he photogram of a time exposure or a short exposure of the fire is scanned bv a video camera. The video signal is digitized by an 8 bit A-D-converted (256 step wedges) and then stored in a digital high-speed image storage system with a core image format of 512 x 512 pixels. With the system control computer it is

possible, for example, to visualize lines of constant optical density (equidensities) on the color monitor. T h e arrangement formed by several instantaneous equidensities is designated collectively as an organized equidensitometric structure. Software operations include, for example, image enhancement, filtering, flame shape analysis, and the evaluation of the kth moments of the distributions of the optical density of the photogram.

2.4 Flame radiance measurements Point-by-point flame radiance are measured with a small-view-angle pyroelectric radiometer with a wavelength range between 300 =< k 20,000 n m and a time resolution of 30 Hz. 1~ Power spectra of instantaneous flame radiances are determined with Si- and Ge-photodiodes and a spectrum analyzer with a resolution of 0.04 Hz.

2.5 Flame temperature, concentration and flow velocity measurements Time-averaged flame temperatures are measured with a Pt/PtRh 10 thermocouple.ll Time-averaged concentrations of stable molecules are measured gas chromatographically. Soot concentrations are also determined from sampling probe measurements.12 Finally, time-averaged flow velocities are determined by dynamic pressure measurements 13 with a m a x i m u m resolution of 3x 10 -3 Pa.

3 R e s u l t s and D i s c u s s i o n

3.1 Static radiance structures 3.1.1 Visualization

b 8 w ~v ~amera

P~0t0Qram 0f t h e f i r e structures

on0t0gram 0IN tight table

s t e 0 ~e0Qe

t

gqlnr m0nit~r

9

e Digitizer

tablet

pert0neral

re,, 1 ae

FIG. 1. Digital image analysis system to analyze fire photograms

Static equidensitometric structures d e p e n d on pool diameter and fuel type. 22 These structures (Fig.2a) consist of a core and plume structure. T h e geometric fraction of the core structures decreases with increasing pool diameter. For luminous fires the core structure is W-shaped (Fig.2a). The core structures of nonluminous fires, e.g. the methanol pool fire, are M-shaped. T h e fuel type also changes the geometric 22part of the plume and core structures.

3.1.2 Physical interpretation and evaluation Each line of the static and dynamic equidensitometric structures is a line of constant optical density D of the fire photogram and is related

RADIANCE STRUCTURES IN POOL FIRES

9 ~c,

b

,,

-

_

[~

95

*.re.

9

9

~

~&a.rel

?

t,

20

=

9.5

~

fire

,2

parce]s 1.5

zx zs& 84 lateral

a

20

?~

~

8

8

~

~

(:

2

a

~

a

8

n ne*~ne 00el l i f e ,

(a)

(b)

(c)

C.5

C

Fro. 2. (a) static equidensitometric structures, La.,,/0',x), for X = 633 nm (b) radiance field L,,,~O',x),for 300 =< ~. _-<20,000 nm (c) static
u ! n

l

d = ~6 i~

a

a ax a a

G.G/

T

c30rolna*e, ,(-~

~,9

G

Z. ~Sr- -

g

cJ.06

29

40

&

i~

to a line of constant spectral radiance La o f the pool fire, as d e r i v e d in: ~4'~5

!9

9

9

9.

C~

~'. ,'~0 t ~ c .

.

.

.

.

.

'C

DO',x, kM) = f(aL~,(y,x,k~t))

(1)

w h e r e a is a p h o t o g r a p h i c constant. This means that the e q u i d e n s i t o m e t r i c structures can be i n t e r p r e t e d physically as b e i n g the radiance structures Lx0',x ) o f the fire. In o r d e r to evaluate the static e q u i d e n s i t o m e t ric structure LxO',x) (Fig.2a) o f a pool fire it is c o m p a r e d quantitatively with the t i m e - a v e r a g e d r a d i a n c e field Lr~d(y,x) (Fig.2b). In addition, the e q u i d e n s i t o m e t r i c s t r u c t u r e is c o m p a r e d with the t i m e - a v e r a g e d fields o f t e m p e r a t u r e , soot concentration, CO2-concentration, H 2 0 - c o n centration and flow velocity. 13 Many quantities, d e t e r m i n e d f r o m static e q u i d e n s i t o m e t r i c structures, such as fire shapes, irradiances, rate o f air e n t r a i n m e n t and e x c h a n g e coefficients I for matter, heat and m o m e n t u m can only be m e n t i o n e d ~a'~6 without g o i n g into f u r t h e r details.

3.1.3 Standard deviation and kurtosis of lateral radiance distributions as well as radiant power per unit height F r o m the radiance structures for the three s.pectra_l ranges shown in Fig. 3, lateral profiles Lx(y), L~ae(y) and Lax(y) at various heights x are d e t e r m i n e d ] 5 For these profiles the m o m e n t ml, and the m o m e n t s m2, m4 relative to the

G

(b)

. 2F"

eei0et,

" ' )Cco'Ti 30

aO

x::m)

Fro. 3. (a) standard deviation o-,(x) and fire parcel velocity ~,,.,(x) (b) normalized radiant power (](x) per unit height from lateral radiance profiles /7.09 for three different spectral ranges of an n-hexane pool fire, d = 4.6 cm./[a~.~/is the relative radiance field, fro. 400 =< X -< 1,200 nm. mean, )7, o f the n o r m a l i z e d radiance distribution h~) are calculated. T h e following relationships exist:

m2=-aye=- f ] ~ (y-,7)2h(y)dy

(2a)

ml=- y=- f ]2 yh(y) dy

(2b)

where

and

h(y) -=

s

(2c)

In Eq.(2c)/~ without subscript is the symbol for /~ or La~ or/[r,~/. T h e s t a n d a r d deviations (r~(x)

96

FIRE

for the three spectral ranges are illustrated in Fig.3a. It is r e m a r k a b l e that the s t a n d a r d deviation of each spectral range has a m i n i m u m at the height x = 5 cm. At this height, the visible fire shapes show the strongest constriction caused by the large b u o y a n t acceleration o f the fire parcels (see section 3.2.2) which reach a relative m a x i m u m of their axial velocity tia, at x = 5 cm. T h e increase of m, between 5 < x =< 8.5 cm can be e x p l a i n e d mainly by an increased exchange process of the fire parcels in the radial direction a n d to a lesser extent bv the decrease in ~L,,. T h e f u r t h e r growth of ~x at x > 8.5 cm is d u e to the d o m i n a t i n g e x c h a n g e processes. For the lateral profiles L(y), the kurtosis w is a measure of the deviation from normality a n d is calculated using: m4 w--- @ - 3

(3a)

where

emissions, m e a s u r e d only from /[,raa, cause the relative m a x i m u m of q(x) at x = 2 cm because there are high c o n c e n t r a t i o n s of CO2 a n d H2o at that height. T h e r e the c o n t i n u u m emission from soot, m e a s u r e d only by Lx a n d Lay is still small. T h e m i n i m u m of ~](x) for Lad at x = 5 cm is caused by the small fire thickness, d u e to m a x i m u m constriction of the fire (see Fig.3a) at this height. For /7,~x a n d /2,x, there is only a shoulder at x = 5 cm, because the soot emission increases strongly in this height. F r o m the close coincidence of ~(x), w(x) a n d ~](x) for the three spectral ranges, it can be c o n c l u d e d that the static e q u i d e n s i t o m e t r i c structures Lx(y,x) in the visible s p e c t r u m o f pool fires r e p r e s e n t in good a p p r o x i m a t i o n the spatial d i s t r i b u t i o n of the radiances /-,ad(y,x) in the i n f r a r e d spectrum.

3.1.4 Profiles of volumetric coefficients Because small pool fires are volumetric radiators for which the absorption of radiation can be neglected, the radiance structures L0',x), are a p p r o x i m a t e l y axisymmetric a n d can be f o r m u lated as follows: 19

(3b) /7,(y, x) = 2 C o m p a r e d with P M M A - p o o l fires, 17 the lateral radiance profiles o f an n-hexane pool fire are only a little leptokurtic in the fire p l u m e at x > 10 cm. In the fire core at x < 10 cm, the radiance profiles become platykurfic. T h e calculation o f the normalized r a d i a n t power c] per unit height yields:

q(x) ~

1 dQ Q dx

x) ffl(r,y) - rdr

fR~

(5a)

r=y

where r is the radial coordinate, R is the visible fire radius a n d q* [W/(cm3sr)] is the volumetric emission coefficient of the fire. T h e application of the A b e l - i n v e r s i o n technique to Eq.(5a) leads to the volumetric emission coefficient q~ (~,x)-profiles: 19

(oay, (4a)

C~(r,x) = ----1 f"(x) \

Oyy

//*dy

(5b)

7r ~ y = r

where (4b)

and

~---~ - = 4rr[l~ f ] 2 L ( y , x ) d y

(4c)

Fig. 3b shows the radiant power q(x). A n absolute m a x i m u m at half the flame height x H/2 is observed, similar to a b u o y a n t p r o p a n e diffusion flame, is Unlike the r a d i a n t powers ~](x) of p r o p a n e Is a n d PMMA 17 pool fires, a relative m a x i m u m at x = 2 cm a n d a m i n i m u m or a s h o u l d e r at x = 5 cm are observed for a n n-hexane pool fire. T h e CO2 a n d H2o b a n d

As an example, the relative volumetric emission coefficients ~x,,-~l, calculated from Eq.(_Sb) for the relative e q u i d e n s i t o m e t r i c structures L~r~l (y,x) in Fig.2a, are illustrated in Fig.2c. T h e ~x,~l(r,x)-fields r e p r e s e n t new static structures. As previously discussed in detail, ~9 a d a r k zone without any i m p o r t a n t r a d i a t i o n in the visible or the infrared s p e c t r u m is observed in the region of a flame core. This m e a n s , that the m a i n part o f the fire radiation in the core region is emitted f r o m a thin conical flame sheet. In the r e g i o n of the fire plume, however, the m a i n part is emitted f r o m inside the fire.

3.1.5 Simplified model for calculating volumetric emission coefficients from sampling probe measurements A simplified model, based on the assumptions of the optical thin limit a n d i n h o m o g e n e -

RADIANCE STRUCTURES IN POOL FIRES

97

ities in flame temperatures T and species concentrations, is used to calculate axisymmetric profiles of the volumetric emission coefficients ~th from sampling probe measurements of temperatures and stable species: 19

qlth(r, X) = [eH~o(T) TH~O(r, x) 4- eco,(T) Yco=(r, x)

+en(T)cn(r,x)]@o T4(r,x)

(6)

where eH2o, ecoz, en are the emission factors of H20, C02 and soot; '~u2o, ~/C02 the volume concentrations of H 2 0 and CO2; cn the soot concentration; So the Stefan-Boltzmann constant; f~0 the solid angle unit. For time-averaged temperatures 7", concentrations ~H2o, ~co2, gn, from Eq.(6) time-averaged volumetric emission coefficients qtth(r,x) are calculated and are illustrated in Fig. 4. It is noteworthy that the calculated q~th-profile agrees satisfactorily with the ~ d - p r o f i l e obtained with Eq.(5b) from L~,d(y,x)-profile measurements.

3.2 Dynamic radiance structures 3.2.1 Visualization If the exposure time is ~ 1 s, the pool fire shows totally unsymmetric dynamic radiance structures. The photographic recording of these structures leads to an instantaneous fire photogram. The application of the digital image analysis system (Section 2.3) results in the visualization of additional new structures. These are designated as dynamic equidensitometric structures and are illustrated in Fig. 5. The dynamic radiance structures of a large pool fire also show hot spots and soot parcels. L1 All of these dynamic structures depend on the pool diameter d, the height x above the pool rim, and the fuel type. 22

Z

Oi ~6

/ \(. ~th

0.12

%

O.lO "g

.... i!:i!!:i;

0.08

"\,\..~~\

Z 0.06 Z

E O.OZ~

../'1"

~, o.o2 I ~

0,00

1

r a d i a l coordinate,

7

r(cm)

FIG. 4. Lateral profiles of volumetric emission coefficients ~T~dand qrthfor an n-hexane pool fire, d = 4.6 cm

FIG. 5. dynamic equidensitometric structures of an n-hexane pool fire, d = 4.6 cm Fire shape (1) Convection column (2) Fire mushroom (3) Fire parcel (4)

3.2.2 Periodic and nonperiodic properties The dynamic radiance structures represent the time-dependent radiance field Lrad(y,x,t) of the pool fires within the spectral range 300 =< X =< 20,000 nm. T h e dynamic equidensitometric structures represent, according to Eq.(1), timed e p e n d e n t spectral radiance fields Lx(y,x,t) at X = 633 nm, or radiance fields Lax(y,x,t) within the visible spectrum AXv. A structure which has a closed line of L~x,, = constant as a contour is defined as a fire parcel. The evaluation of the dynamic radiance structures has shown that thel( have the follow. 20 (the numerical ing monoperiodic properues values refer to an n-hexane pool fire, d = 4.6 cm): 1. Wave motion of the visible fire shape (mean frequencyfFK = 12 Hz) 2. Formation of fire mushrooms 0?Bp = 12 Hz) 3. Rising of fire parcels or flame zones (and/or soot parcels for d > 1 m) @r = 12 Hz). All frequencies are d e p e n d e n t on the pool diameter 1 d and the fuel type. Two additional monoperiodic p h e n o m e n a are observed: 4. Vertical oscillations of a steam layer @ = 5 Hz, LNG, 0 < x < 2.5 cm, d = 4.6 cm) 5. Formation of rotating flame parcels 0?B~=< 2 Hz; n-hexane, LNG, for d => 0.5 m).

98

FIRE

Some of these monoperiodic properties are discussed in comparison with the properties of dynamic density structures. 1 T h e periodic structures observed in the large pool fires are discussed in.l~ The evaluation of the nonperiodic behavior of the dynamic radiance structures leads to 18 different quantities.a~176Some of these nonperiodic p h e n o m e n a have been discussed in another paper. 1 In the present paper only two measurements will be discussed. Normalized wavenumber power spectrum functions 1~ F(vO of fluctuations of lateral radiance profiles Laa(y,x), are calculated for different values of the height x by Fourier analysis and are illustrated in Fig. 6a. T h e macro- and microscales b, b in y-direction are determined for each height x from the following relationships: =

Ry(&y)

day

(7a)

25 2O

e6

15

,kc~5

~2

i o 0 ~.0 2.0 . . . . . . . b.... r~/0~>

(a)

1.5 A

~2 ~ 1.0 3

A Z~L~ A,<:~AI ~

A

Az~~

macroscale b

~ AA
0

O ~= O.5

with the spatial correlation function Rx of radiance fluctuations L'aa.'

6 g E 0.0 10

b---g= 47r 9

v2F(v,) dr,

(7b)

0

20

30

40

heJoht adore the pool rim, x (cm) (b)

and

Ry(Ay) =

F(v,) cos(2"xv,Ay) dr, 0

where vr is the wave number. The macroscales (Fig.6b) d e p e n d strongly on the height x and increased between x = 6 cm and x = 15 cm, as well as for x > 25 cm. The microscales b d e p e n d only weakly on the height x. It is probable that a lower limit value exists for the microscales b. It is noteworthy that these scales agree well with the geometric widths of the dynamic density parcels observed by holographic real-time interferometry, i

4 Conclusions 1. Pool fires are open systems with steep temperature, species concentration and flow velocity gradients. Therefore static and dynamic radiance structures in the visible and infrared spectral range are generated spontaneously. 2. T h e digital image analysis of fire photograms is a valuable method for visualizing and studying static a n d dynamic radiance structures. 3. T h e static equidensitometric structures

FIG. 6. (a) Spatial power spectra F(vr) of radiance fluctuations L'ax(y,x) for various heights x above the fuel surface (b) Macroscales b and microscales b obtained from spatial power spectra F(vr) as a function of the height x for an n-hexane pool fire, d = 4.6 cm

L~(y,x), photographically recorded in the visible spectrum, represent in good approximation the spatial distribution of the radiances LradO',X) in the infrared spectrum. 4. By applying the Abel inversion to the static radiance structures, profiles of volumetric emission coefficients xIt(r) can be determined. The qtx(r,x)-fields represent new, additional static radiance structures. 5. A simplified model, based on the assumptions of the optical thin limit and inhomogeneities in flame temperatures and species concentrations, can be used to calculate the ~(r)-profiles: 6. I m p o r t a n t dynamic radiance structures, such as fire parcels, fire mushrooms, fire shapes, soot parcels, hot spots and convection columns can be observed. Their n u m e r o u s periodic and nonperiodic properties can be analyzed. 7. T h e radiance structures studied should be considered for a more exact modeling of fires.

RADIANCE STRUCTURES IN POOL FIRES

Acknowledgments This research is supported by the Stiftung Volkswagenwerk u n d e r the program "Grundlagen Technischer Verbrennungsvorgange". The authors would like to thank the DFVLR for the opportunity to participate in their large-scale pool fire experiments supported by the BMFT and the DGMK.

11. 12. 13.

14.

REFERENCES

15. 1. SCHONBUCHER,A., et. al.: this symposium. 2. ZUKOSKI, E.E., CETEGON, B.M. AYD KUBOTA, T.: Twentieth Symposium (International) on Combustion, p. 361, T h e Combustion Institute, 1985. 3. SMITH, R.K.: Eleventh Symposium (International) on Combustion, p. 507, The Combustion Institute, 1967. 4. MUDAN, K.S.: Prog. Energy Combust. Sci. 10, 59 (1984). 5. H~GGLUND, B. AND PERSSON, L.E.: The Heat Radiation from Petroleum Fires, FOA Rapport C 20126-D6(A3), 1976. 6. HERTZBERG, M.: Combust. Flame 21, 195 (1973). 7. ORLOFF, L.: Eighteenth Symposium (International) on Combustion, p. 549, The Combustion Institute, 1981. 8. ORLOFF, L. ar~'D DE RIs, j.: Nineteenth Symposium (International) on Combustion, p. 885, The Combustion Institute, 1982. 9. CORLETT, R.C.: Heat Transfer in Fires (P.L. Blackshear, Ed.), p. 239, Wiley, 1974. 10. KETTLER, A.: Strahlung und Turbutenz von

16.

17.

18.

19. 20.

21.

22.

99

n-Hexan- und Methanol-Tankflammen, Ph.D. thesis, University of Stuttgart, 1982. SCHONBUCHER,A. AND BROTZ, W.: Chem.- Ing.Tech. 50, 573 (1978). SCHONBUCHER, A. AND BROTZ, W.: Ber. Bunsenges. Phys. Chem. 82, 1202 (1978). M~LLER, W.: Brutto-Reaktionsgeschwindigkeiten stabiler Moleki~le und Stoffaustauschgr613en in nichtisothermen und inhomogenen Tankflammen, Ph.D. thesis, University of Stuttgart, 1984. SCHONBUCHER,A., BROTZ, W., SCHELLER,V. AND KETTLER, A.: Combust. Flame 37, 1 (1980). SCHONBUCHER,A., BROTZ, W. ANn KETTLER, A.: Chem.-Ing.-Tech. 56, 632 (1984). SCHONBUCHER, A., BROTZ, W., BALLUFF, Ch., GOcK, D. ANn SCrflEI3, N.: Chem.-Ing.-Tech. 57, 823 (1985). MARKSTEIN,G.H.: Eighteenth Symposium (International) on Combustion, p. 537, The Combustion Institute, 1981. MARKSTEIN, G.: Sixteenth Symposium (International) on Combustion, p. 1407, The Combustion Institute, 1977. SCHONBUCHER, A., BROTZ, W. AND KETTLER, A.: Ber. Bunsenges. Phys. Chem. 89, 484 (1985). RIEDEL, G.: Langzeit- und Kurzzeitstrukturen in Tankflammen, Ph.D. thesis, University of Stuttgart, 1983. SCHONBUCHER,A., et al.: T h e effects of hot spots, soot and combustion product parcels on thermal radiation in large-scale pool fires. To be published. SCHONBUCHER, A., BROTZ, W., BALLUFF, Ch., RIEDEL, G., KETTLER, A. ANn SCHIE~, N.: Ber. Bunsenges. Phys. Chem. 89, 595 (1985).

COMMENTS L. A. Kennedy, Ohio State Univ., USA. Your photographs of the 4.6 cm diameter pool fires illustrate that the structure of the plume is strongly dependent upon the f u e l ( e . g . , n-hexane, cyclohexane etc.) Would you comment on the physical processes which influence such strong changes in the structure? Author's Reply. In the presentation we illustrated that the dynamic radiance structures of a pool fire with a given pool diameter depend strongly on the fuel type, e.g., for n-hexane, cyclohexanol, glycerin, methyl acetate and benzene. One of the most important parameters which decides whether the fire field shows small or large instabilities is the linear burning rate vo~. In general, high values of v0 in conjunction with high combustion enthalpies AHc of the fuel always generate very complex dynamic radiance structures ~. However, small values of va and AHc result in laminar, relatively simple dynamic structures

which are very similar to static radiance structures. Another essential parameter is the distance from the liquid fuel surface to the pool rim. As an example, for an n-hexane pool fire the very complex dynamic radiance structures change to candle-like symmetric radiance structures when the fuel surface is lowered by a few centimeters ~.

REFERENCE 1. RIEDEL, G.: CharakteristischeAquidensitenstrukturen von Tankflammen in Abh~ingigkeit vom eingesetzten Brennstoff, thesis (1979).

C. O. Leiber, BICT, Swisttal-Hemerzheim, West Germany. For different fuels you got different shapes of

100

FIRE

the equidensities o f the flames. Is this a reproducible effect, so that each fuel has a "fingerprint" u n d e r the same geometrical conditions?

Author's Reply. All the static equidensitometric structures and the dynamic ones o f laminar pool fires occur repeatedly and reproducably, i.e., these equidensitometric structures have the same characteristic appearance for a given fuel type and pool d i a m e t e r I. For all turbulent pool fires, however, the dynamic equidensitometric structures typically do not repeat identically. REFERENCE

1. SCHONBUCHER, A.: Fortschr.-Ber. VDI-Z. Nr. 83, Reihe 6 (1981).

j. de Ris, Factory Mutual Research, USA. Could you c o m m e n t on the physical meaning o f your micro- and macroscales? Do these scales have any relationship to the Kohnogorov scale at which the turbulence and scalars are dissipated? How would you expect these scales to change with fire size or pool diameter? Author's Reply. O u r macro- and microscales refer not necessarily to the energy spectrum o f eddies but also to the energy spectrum o f fluid lumps, desig-

nated above as fire parcels. T h e physical m e a n i n g o f our microscales is that they are responsible for the rate o f exchange o f m o m e n t u m , heat or mass. An estimation o f the Kolomogorov length scale 1~ = [K3uj~]TM with a heat exchange coefficient K~.~.- 6.5 x 10-3m2/s obtained from t e m p e r a t u r e and concentration m e a s u r e m e n t s 1 and with the rate o f dissipation per unit m a s s e ~ 4 x 105 W/kgyields 1~ - 1 mm. This means that o u r microscales b ~ 5 m m are in the same o r d e r o f m a g n i t u d e as the Kolomogorov length scale. At the present, for example, the microscales b of the fire parcels remain nearly constant for methanol pool fires o f d = 4.6 cm and d = 15 cm 2. T h e systematic d e p e n d e n c e has not yet been evaluated. Our macroscales b refer to the energy-containing eddies and/or fluid lumps which do not only decay but also grow strongly. It can be expected that these macroscales b increase with the fire size.

REFERENCES 1. M~'LLER, W.: Brutto-Reaktionsgeschwindigkeiten stabiler Molektile und Stoffaustauschgr6Ben in nichtisothermen u n d i n h o m o g e n e n Tankflammen, Ph.D. thesis, University o f Stuttgart (1984). 2. KETTLER, A.: Strahlung und Turbulenz von n - H e x a n u n d Methanol-Tankflammen, Ph.D. thesis, University o f Stuttgart (1982).