Abstract. A r-educed scale ignition and flame spread technique, RIFT, was implemented in the cone calorimeter system to obtain tbermocombustibility properties ...
Fire Technology, Vol. 34, No. 3, 1998
Combustibility Parameters for Enclosure Lhn Materials Obtained During Surface 8 lame Spread Using a Reduced Scale Ignition and Flame Spread Technique M. A. Azhakesan, T J. Shields, and G. W! H. Silcock Fire SERT, University of Uistel; N. Ireland, U. K.
Abstract A r-educed scale ignition and flame spread technique, RIFT, was implemented in the cone calorimeter system to obtain tbermocombustibility properties of enclosure lining materials during flame spread over the sample surface. Previously, a thermal model of ignition ant1 opposed flow flame spread was used to analyze flame spread data obtained using RIFT. Here, a framework is discussed for deducing critical material combustibility parameters from the measured heat release and mass loss rates as the spreading flame proceeds to the point of flame extinction. The nature of the data and analytical framework allows users to deduce spreading flame flux from the heat release rate (HRR) and mass loss rate (MLR) data relatively economically and directly. The anomalies highlighted by comparing flame spread data in the RIFT system compared to data from the BS 476 Part 7 apparatus indicates that the RIFT system is well-suited for developing and refining models de\cribing ignition, flame spread, and mass burning.
Introduction Ax far back as 1967, data has been generated from multiple-hazards assessments of growing fires using a “tool kit” of small-scale, reaction-to-fire tests.’ Progress since then has been constrained by the obscurity of the connection between standard test outputs and the materials’ specific, thermal (thermophysical) and combustibility (thermochemical) characteristics with regard to so-called “fire properties” such as ignitability, flame spread, heat release rate (HRR), smoke, and toxicity.2*3 Moreover, the effects of scale, configuration, and environment further underline how dependent test result are on the materials’ fire characteristics. Recently, as part of ongoing work harmonizing European fire tests, the primary U.K. methods for testing large-scale enclosure fires were studied,4,5 and, based on these results, methods were developed that allowed the evaluation of specific, quasi-material fire response characteristics critical to the test methodology. These characteristics were generated from standard cone calorimetric ignition and HRR data, and they were found to lend themselves quite naturally to the ranking of a material’s specific fire performance test characteristic.6,7 As part of the process of developing the tool kit, methodologies for correlating ignition and opposed
flow
flame
spread
utilizing
ignition
and HRR
data were
also devel-
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Fire Technology Third Quarter 1998
oped.5.6,7.8Current models of flame spreadgJo were assessed5and adapted for the exploration of opposed flow flame spread in the standard cone calorimeter environment using a novel, reduced scale ignition and flame spread technique, RIFT.” Compartment flashover times, tlO,were correIatedi2J3 to combustibility characteristics extracted from standard cone calorimeter HRR data and lateral ignition and flame spread technique (LIFT) data using the relation ~~=8.2 x 104 ( & )a.52 (h)“.1’(kpc)0.75cp-‘37.11It was shown” that the flame spread parameter, rp, can be obtained using RIFT data, while the effective thermal inertia, kpc, can be obtained directly from RIFT data or from standard cone ignition data. To address disagreement over the nature and suitablity of the peak HRR, & , and decay coefficient, ;1, as descriptors of the burning behavior of charring specimensi4 when used as input data in compartment fire models,‘RIFT utilizes a burning specimen whose surface involvement as the flame spreads progresses up to the material flame extinction limit, as determined by the imposed it-radiance field. This allows direct observation of a progressively burning material’s HRR history and determination of the spreading flame flux at the moving flame front. To date, this method has been implicitly incorporated in some models of flame spread.gJi However, in other models ,5,6,7flame fluxes were derived using standard cone HRR data obtained from horizontally configured specimens. Because of suggestions 14*i5that the HRR obtained under standard cone calorimetric conditions is higher during the horizontal orientation, particularly in the early part of the HRR curve, the relevance of particular HRR characteristics obtained from the standard horizontal configuration to dynamic flame spread over vertical surfaces is called into question. Developments in fire science have identified other material properties that also critically affect burning, i.e., the heat of gasification and the heat of combustion. I6 In the case of charring materials, these cannot be considered simple thermodynamic properties because they vary with the changing physicochemical nature of the burning material.” These and other related issues will be discussed in the context of mass, HRR, and flame spread data obtainable using the RIFT apparatus.”
Experhental
Procedures
and
Test Materiala
Employing a simple modification to the sample holder of the cone calorimeter, it was possible to study surface flame spread, HRR, and mass loss simultaneously under standard combustion calorimetry conditions.ii A steel stirrup attached to a frame holder (Figure 1) was designed to accommodate a noncombustible board predrilled at 1Omm intervals to allow measurements of n-radiance at the sample surface. The specimen, which was 100 mm high and 350 mm long, was backed by a noncombustible calcium silicate board, similar to that used for the standard cone calorimeter and the flame spread apparatus in BS 476 Part 7.5,6*7The specimen was orientated at a 60% angle to the cone heater face. Ignition was initiat-
Combustibility Parameters for Enclosure Lining Materials Using RIFT
Pigun mque,
1. Reduced RIFT.
scale
ignition
and
flame
spread
hch-
199
Fire Technology
200
Third
Quarter
1998
ed by a lo-15mm, semi-impinging propane flame placed parallel to the vertical dimension.The constantflux field gradient at the sampleface was determined at a height of 40 mm above the samplebaseusing a 12.5 mm water-cooled gardon gauge (Figure 2). Figure 2 also depicts the linearized irradiance field as expressedby In 4: = -0.0128(x mm)+4.119, where x is the lateral distancedownstreamof the hot zone and 4: is the irradiance at that distance. Typical lining materials studied in the RIFT apparatus included melaminefaced chipboard (MFC), 9 ply, birch plywood (PLY), hardboard (HDBD), and fiberboard (FIBBD) representingclasses224, as tested in the U.K.‘s BS476 Part 7 flame spreadapparatus.Flame spreadrates were computed from the point of ignition to flame extinction using video recordings of the moving flame front at the lateral position, where the irradiance field was determined to be.” In previous work, we analayzed the flame spread results using the thermal model of opposedflow flame spread.” Here, we consider the detailed mass,HRR, and flame spreaddata obtained for the composite materials, MFC, HDBD, and PLY, embracingclasses2, 3, and 3/4 asdetermined using the flame spreadapparatus in BS 476 Part 7.
401
loo
=*+A. 30-
AA
-I I
z
AA
AA 4
2 Y
-
$
m-
kinearized
n
Flux Field
A
I
A
I
E
A
A
I
A A-=
n I
I
0
40
80
n
160
200
Distance,
Figure
2.
Radiant
flux
field
g = t s 3
ql -
I
,,,,,,,,,,,,,,I
120
_
w n
1
T 2 Y
I I
u;,,,
110 1
240
280
n
I
320
X( mm)
on
N.C.
Board,
RIFT.
0.1 360
400
.
Combustibility Parameters for Enclosure Lining Materials Using RIFT
201
FrumewoA for Data Reduction Mass Loss and HRR Data Reduction The specimen undergoes heating and pyrolysis over an area, A,, up to the maximum extent of flame spread, as defined by the minimum radiant flux supporting flame spread, ii,, The raw mass loss and I-RR data were normalized with respect to the total area’ burnt, A,, up to flame extipction. The instantaneous mass loss rate per unit area, tie”, at the relevant irradiance station X, was computed from the slope of the mass loss curve and ti’,“, computed as kite’;/A, The dynamic I-RR, &, as determined by combustion calorimetry at the relevant flame front station, was similarly expressed by, &“= Q/Ah, and the effective heat of combustion, AH,, was computed from the ratio oc’7 rit,” or a/ riz,. Heat of gasi$mtion Assuming that the surface temperature, Ts,of the burning material attains the sustained ignition temperature, T,, at the moving flame front, a quasi-steady-state analysis can be used to obtain a measure of the changing heat of gasification, L,,C, as surface burning proceeds. In the case of charring solids, it was shown” that the enthalpy of gasification is not simply related to the materials’ pyrolysis enthalpy, but dependant on the degree of charring and on the overall enthalpy of reaction, which can be either exothermic or endothermic. Here, a simplified analysis is used to identify and enumerate the relevant reaction processes and variables. The heat of gasification, L,,, may be equated to the total enthalpy, AH, and the sensible enthalpy of the reacting system, as,
u here the total enthalpy term, AHP includes the heats of fusion, decomposition, and vaporisation,18 and C, refers to the solid phase heat capacity. From the theoq of burning rate for a thermally thick material, where quasi-steady burning is assumed to occur at a mass loss rate, riz,“, with a thermal pyrolysis wave propagating at a rate, V,, into the solid, I7 and assuming that the heat flux into the interior is small compared to the gasification heat, lo L,, the following relation holds,
. " (4, + l&y- g,) me = L g-e
(2)
where the surface loss or critical heat flux, i,,can be derived from analysis of either ignition data5.‘JL or RIFT generated flame spread data.‘l In order to predict Lg,Cfrom Equation 2, it is clear that the material flame flux, 4,“, is required. There is, unfortunately, a paucity of data in the literature regard-
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Fire Technology Third Quarter 1998
ing the likely magnitude of 4;’ for any specimen configuration. A methodology for deducing ;I;’ from standard cone HRR data5,6T7 is discussed below. An estimate of the overall beat of gasification, Lp(lyc,is deducible by transforming Equation 2, whereby the slope is used to compute Lg,,, and the intercept 4;’ as
(3) The pyrolysis wave propagation rate, VP,into the reacting solid was computed from the relation IJ’,~=Ijlc”fps-pc)” in order to assesswhether departures from the semi-infinite slab assumption occurs as the thermal wave moves through the virgin solid.” The virgin solid and char bulk densities are indicated by p, and p,. An average value of 137k15.6 kgm-3 was used for pc.17J9,20 The measured bulk dens&es, p, for PLY, HDBD, and MFC, were 641, 830, and 7 19 kg/m, respectively.5
Derivation of flame&x
and related parameters
From the theory of burning rate, Equation 3,‘s6.’it can be shown that
= TSI[&]
+ ESI
(4)
Brow+ suggested that extrapolation of the linear plot for data obtained under discrete irradiance conditions to zero irradiance provided a residual HRR value or (flame) extinction sensitivity index, ESI, which showed, in principle, whether the flame was self-sustaining for the HRR period defined by the relevant HRR characteristic chosen, @. In Equation 4, ESI= AHcSLp(~~‘-~~Jand TSI=HJLg. In other work, the ES1 value, interpreted as being free of the influence of imposed irradiance, was usedz6.’ to compute the material flame flux, i,“, from standard HRR data; from Equation 4 as 4,” + ES1 and
(5) As used in Equation 4, the HRR characteristic, ci’; from standard cone calorimetry, was dependent on material type.5 In the RIFr system, the peak HRR values obtained at the relevant flame front, 0,; and irradiance station, x, was plotted against the surface irradiances, 4,” and ii, was then deduced from Equation 5. The ratio AHcBLp, variously described as an index of material com-
Combustibility Parameters for Enclosure Lining Materials Using RIFT
203
bustibi lity, material flaming response to variations in imposed it-radiance, or thermal sensitivity index, TSI,*‘,** was obtained from Equation 4. The TSI ratio, AHdLs, was used to compute the burning materials HRR in a room fire from the relation $“=(AH)L8)(qf ’ “+ d Ti-” :$I, where T, was the effective blackbody gas temperature, Ti, the sustained ignmon temperature obtained from either ignition or LIFT data,” and the magnitude of iI;’ was assumed to be that of the ignition source flux. Janssen24 estimated flame fluxes for the Steiner Tunnel test from cone HRR data by assuming that iI;’ was identical to the net flux, i,(:,,? at the pyrolysing zone, where
(6) The average HRR over the bum period, @)Avetbo,was evaluated in the cone calorimeter at the maximum irradiance, i,“, obtained in the Tunnel burner test. Values of L, and AiYcflwere obtained directly from cone calorimeter data.24 In order to compare flame fluxes derived from HRR and mass loss data in Equation 3 with limited measurements of flame fluxes quoted in the literature,5 the detailed measurements of flame heat transfer coefficient, /z*,~~Owere used in the relation,
4; = A?;,&, - q .
(7)
The use of this equation with respect to RIFT data is discussed later in the analysis.
Results The results in Table 1, Table 2, and Figures 3-13 are presented for discussion. Table I summarizes the data displayed in Figures 3-13. Table 2 summarizes strictly selected data obtained from standard cone calorimetry,ii RIFT flame spre;td analysis,” and the BS 476 Part 7 test5 It is noted that the Quintiere model9 results for the Part 7 test were influenced deleteriously to different extents, depending on material type, by the presence of the ignition source under standard conditions of testing. The flame spread results and the Quintiere model investigations are the subject of further papers and, thus, only the flame diffusion velocity, 15 near the ignition zone is quoted to facilitate discussion. The ranges of Lg.aveand 4;’ values indicated in rows 18-19 in Table 1 reflect best-fit correlations of tc: to ;I&from Equation 3. The TRP values were derived were deduced from both from standard cone igmtron data?*” and the i,values standard cone calorimeter and RIFT-generated flame spread data.” The HRR data was integrated to enable computation of cumulative I-IRR, CHR, and aver-
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Fire Technology Third Quarter 1998
age HW em8 over specific time intervals using Equation 6 in the form deduced from Equation 4. In &,@p& 6i17 with the appropriate L/M& order to facilitate comparisons, the results quoted in Table 1 for TSI, ;I;‘and Lf encompass the peak-to-flame extinction HRR and MLR period, Figures 5-7. Analysis of Results Tables 1 and 2 indicate that a number of material combustibility parameters, including tip, S”,,, time to (&,, CHR, and d”;ve,tbd are related inversely to the thermal inertia parameter, kpc, TRP, and area burnt, A,, in accordance with their classification, obtained in the BS 476 Part 7 apparatus. The temperature coefficient of flame velocity, (p/kpc,ll was also observed to follow the same relation to material classification. The contribution that the material might make toward the hazard of flashover in an enclosure fire was assessed by the ratio, (A,)“.51tig,Table 1, row 21, and the CHR parameter, row 10. The ratio, which depicts the area burnt to the rate at which surface ignition is achieved, is a measure of the rate of surface burning, Vb,O. The latter fire hazard classification is congruent with that noted above with regard to flame spread. However, in an enclosure fire, certain materials, such as the Class 2 MFC, can burn in two modes due to their laminated composite structure, and can suddenly bum more intensely as the treated surface material destructs.5~23Thus, the CHR, Table 1, row 10 indicates that the underlying chipboard can release significant heat, even if the surface flame spread rate is low. The necessity for a multidimensional classification system reflecting the material’s burning history in its environment has been shown in other work,2.5~6~7 and anomalies with respect to composite materials fire growth behavior has been highlighted.23 The RIFT system can thus be used as a means of assessing specific” as well as overall hazards associated with fire growth. The following analysis considers specific aspects of the RIFT data output. Flame Spread The limiting extent of flame front movement at the irradiance measurement station for the noted materials in Figure 3 mirrors the surface flame spread hazard classification detetined by the rate and extent of flame spread in the BS 476 Part 7 apparatus.5.6s7111 This is supported by the derived and experimental values of 4,, 9Table 2. In the Part 7 test, the specimen dimensions were 230 mm high by 865 mm long with the irradiance varying from -36.7 kW/m-2 at the ignition zone to < 5.3 kW/m-2 toward the end. The irradiance gradient was significantly shallower than that of the RIFT apparatus, as the gas radiant panel was inclined at a 90” angle to the specimen surface. The standard protocol is set out in other work.26 Partial flame spread data obtained for FIBBD at h-radiances ~14 kW/m-2 were expanded to include Vfvalues at higher irradiances compared to data obtained in the BS 476 Part 7 apparatus.5*‘1 The FIBBD results are included to highlight noteworthy issues related to surface flame spread.” The rates of
Combustibility Parameters for Enclosure Lining Materials Using RIFT
TABLE 1 Summary
1. Combustibility 2. Ignition
205
of Results
parameters
units
time, f,
s
36.6
54.4
65.8
3. Bum time, lb.
s
454
4%
509
4. Area burnt, A,
in2
0.019
0.017
0.0116
5 Total mass loss, tn.’
km+
2.41
3.07
2.79
6.
Average npss loss rate at ignition, in,*’
I
7. Average mass loss rate at extinction, ni.0.I
k&m2 I
0.0023
1
.,,piyrI
I kg&m-l
I
0.0034 I
II.0052 I
0.0037 I
I
13. Effective heat of combustion averaged over c, AHch
Ml/kg
10.2
10.5
14. Flame flUX,~ ;,” over d; to Q,, Equation 5
kW+
10.9
22.3
8.9S-[IO.91
IS. TSI, Equation 4
C-)
6.36
2.13
8.9b[7.65]
16. Flame flux, $, Equation 6, and 0, Equation 4
kWn+
1.6
21.5
8.3-[9.7]
kWm2
20.7C63.8)
33p9.7)
24.4(69.5)
MJd
2.81-3.06
X24-5.26
2.88-3.51
18-19.2
22.2-34.7
16.&20.9
18. Average heat pf gas$ication, L C~‘ over peak m,“m ??I*; atr, Eiwdion3
kWm-’
20. Flame flux, ;4’; Equation 7, averaged over r.
kWm”
21. Surface burning speed,
Pm-’
CL.. = &4,
19
3.77Xlc-3
17.4
2.4xlF
15.2
17.1
1.64X1&’
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Fire Technology Third Quarter 1998
TABLE 2 Summary of Results Obtained Cone Calorimeter, and BS476 of Flame Apparaius
with RIF# lkhnique, pt. 7 Surface Spread
M&Xi81
I
FIBBD
152.3
~
PYL
I
HDBD
I
MFC
264
1
293.5
1
334.5
9.5-10.6 (WJ model)ll-12 (Qunitiere model) 10.9 @xp). i,,, kW/m-*; RIFT Flame spread analysisJ~”
Flame spread velocity, V,, mm/s; BS 476 pt. 7 test at 31.4 kW/m-* Flame spread velocity, VP mm/s; RlFr at 3 1.5 k W/m-J cj;, kW/d:
