Reference I

NBSIR 85-3212

Forced Smolder Propagation and

the Transition to Flaming in

Cellulosic Insulation

Thomas J. Ohiemiller

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

October 1985

Final Report

-QC100

.056

85-3212

1985

Prepared for:

\ . I.S. Department of Energy

iHashington, DC 20585

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NBSIR 85-3212

FORCED SMOLDER PROPAGATION AND

THE TRANSITION TO FLAMING IN

CELLULOSIC INSULATION

Thomas J. Ohiemiller

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

October 1985

Final Report

Prepared for: U.S. Department of Energy

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

PREFACE

This report is one of a series documenting NBS research and analysis

efforts in support of the Department of Energy's Building Thermal

Envelope Systems and Materials Program, managed by the Oak Ridge

National Laboratory, The multi-year effort reported in this document

was supported first by DoE/NBS Interagency Agreement No. DE-AIOl76PR06010, Task Order A008, culminating in Modification A037 (formerly

lA No. EA77-A-01-6010) . The concluding work and report preparation

were supported by lA No. DE-AIO5-850R21513, Modification No. AOOl,

Task D-85/1 — Assessment of Smolder-to-Flaming Tendency in Cellulosic

Insulation.
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TABLE OF CONTENTS

Page

List of Tables Iv

List of Figures v

Abstract 1

1. INTRODUCTION 1

2. EXPERIMENTAL APPARATUS AND PROCEDURE 4

2. 1 Initial Experiments with Flow Through the Smoldering

Insulation 4

2.2 Flow Over a Horizontal Smoldering Layer 6

3. RESULTS AND DISCUSSION 10

3.1 Behavior of Unretarded Insulation 11

3.2 Behavior of Retarded Insulation 20

3.3 Effect of Adjacent Wood 25

3.4 Implications for a Smolder-to-Flamlng Test Method 26

4. SUMMARY AND CONCLUSIONS 29

5. REFERENCES 31

-111 -
LIST OF TABLES

Table 1. Compilation of Test Results

Page

33

-iv-
LIST OF FIGURES

Page

Figure 1. Schematic of apparatus for study of smolder propagation

with flow forced through the fuel bed 35

Figure 2. Schematic of flow tunnel apparatus for study of

smolder propagation with flow over the fuel bed. Flow

tunnel cross-section is 20.3 cm wide by 56 cm high 36

Figure 3. Measured dependence of smolder velocity on velocity

of air flowing over the fuel bed in both forward and reverse configurations 37

Figure 4. Measured cross-sectional profiles of extinguished

forward (top) and reverse (bottom) smolder waves. Note that the bottom profile has been turned around

for ease of comparison 38

Figure 5. Measured mass transfer coefficient between air stream

and top surface of fuel bed for both forward and reverse configurations 39

Figure 6. Measured effect of air velocity on smolder velocity

of two compositions with additives; all results for

forward smolder mode. Flames appeared only when wood

sidewalls were present 40

Figure 7. Comparison of thermal analysis behavior (4 mg sample,

5°C/min in air) of three insulation compositions 41

Figure 8. Measured cross-sectional profiles of extinguished

smolder waves with differing compositions at same air

velocity (1.95 m/s, forward smolder mode) 42

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FORCED SMOLDER PROPAGATION AND THE TRANSITION TO

FLAMING IN CELLULOSIC INSULATION

Thomas J. Ohlemiller

Abstract

It is well known that a smoldering fuel responds to an increased oxygen

supply by becoming faster and hotter until, eventually, flames erupt. This

sequence is examined quantitatively for thick horizontal layers of a permeable

fuel, i.e. , celluloslc insulation. IWo configurations are possible, forward

and reverse smolder; both are Investigated experimentally. The influence of

combustion retardants is also examined; these include boric acid, a smolder

retardant, and borax, a flaming retardant. Both prevent the transition to

flaming in the absence of adjacent flammable material but are less effective

in its presence. The overall response of these various fuel mixtures and

configurations suggests that both kinetics and oxygen supply rate (not the

latter alone) play substantial roles in dictating smolder response to an air

flow.

Keywords: air flow, cellulosic materials, flaming, insulation,

retardants, smoldering.

1. INTRODUCTION

This report summarizes the last phase of a study of the smoldering

combustion characteristics of cellulosic insulation. In previous work smolder

initiation [1,2,3] and smolder propagation [2,4,5] in these materials have

been investigated.

-1 -
Cellulosic insulation of the type investigated here is made from recycled paper products, mostly newsprint. When properly ground to a finely

fibrous form, such products become an excellent thermal insulation. The

inherent combustibility of the base material (essentially wood) must be

controlled, however, by the addition of combustion retardants and by adherence

to installation practices that keep the insulation away from heat sources.

Meaningful combustibility test methods are needed by the manufacturers

and by regulatory agencies as a means of assuring continuing control of potential flaming and smoldering tendency. Cellulosic insulation currently must

pass a radiant panel test which measures the minimum external radiation flux a

given sample requires to sustain flame propagation over its exposed surface.

It also must pass a smoldering test in which a lit cigarette is imbedded

vertically in a layer of insulation. Smoldering of such materials is most

likely to be encountered as a result of improper installation that places the

insulation in contact with large area, relatively low temperature heat sources

such as recessed light fixtures. Substitution of a cigarette as the heat

source obscures the causal chain in the smolder initiation process. This led

to the suggestion for an improved smolder initiation test method based on the

use of an extended area, electrically heated source [2,3]. It was shown that

such a test method can quantify the impact of smolder retardants on smolder

initiation tendency by measuring the minimum heat source temperature required

to start smolder in a given layer of material. The test method also showed,

however, that this impact is not very great, e.g., the addition of 25% boric

acid raises the minimum heat source temperature for smolder initiation by only

20-25 °C [2].

-2 -
In view of this inability of smolder retardants to prevent smolder

initiation, it was desirable to look at subsequent stages (propagation,

transition to flaming) of the smolder process to see if they are more susceptible to intervention. Self-sustained smolder propagation in horizontal

insulation layers was examined in detail [4,5]. It was found that high levels

of boric acid only slow the rate of propagation in layers of realistic thickness. Slower propagation represents no reduction in hazard level so a propagation rate test method was not pursued.

Smoldering insulation becomes an immediate life threat when and if it

undergoes a transition into flaming combustion. For the usual attic type of

installation, this is most likely to be caused by air flowing over or through

the smoldering combustion zone as a result of winds impinging on the

structure. Such winds may directly penetrate various attic vents or induce

flow to/from rooms below the attic via ceiling holes around devices such as

recessed light fixtures, vent fans, etc. It is clearly desirable to lessen

the tendency of a smoldering insulation material to transition into flaming in

such a manner.

There have been two previous studies of smoldering horizontal fuel layers

in the presence of flowing air with some observations on the appearance of

flaming. Palmer [8] examined the behavior of wood and grass dusts in layers

up to 6 cm thick with air velocities up to 8 m/s; the configuration was quite

similar to that used here but the bulk densities were 2-3 times higher except

for the case of cork dust. The presence of flaming was noted but not examined

in any detail. Leisch [9] examined the behavior of grain kernel fragments,

grain and wood dust in 20 cm layers at air velocities up to 19 m/s. In this

-3 -
case the fuel layer was set into the bottom of the flow tunnel so that its top

surface was initially flush with the tunnel floor. Flames were noted, under

some conditions, to appear and to be trapped in the surface depression left by

the smoldering process.

The present study was undertaken to look at the impact of both smolder

and flaming retardants on the forced flow smolder and flaming transition

process. A second objective was to assess the feasibility of developing a

test method to rate transition tendency. After some preliminary experiments

(discussed briefly below) on a smoldering bed of insulation with air forced

through it, the work focussed on a horizontal layer of smoldering insulation

with air flowing over the top of the layer. In this latter configuration, the

ultimate point of interest is the flow velocity at which transition to flaming

occurs; this provides a quantitative measure of transition tendency. As will

be seen, this velocity varies with the nature of the combustion retardant

additive and also with the presence or absence of wood adjacent to the insulation. In order to gain some insight as to how the air flow brings about

flaming, its effect on the smolder process was examined over a substantial

range.

2. EXPERIMENTAL APPARATUS AND PROCEDURE

2.1 Initial Experiments with Flow Through the Smoldering Insulation

The air flow in the real hazard scenario is potentially either through

the insulation or over surface of the insulation. A simplified version of the

former was pursued initially. A 25. A cm diameter by 25.4 cm deep bed of

-4 -
o unretarded insulation (bulk density 0.051 g/cm ) was encased in a thin-walled

(1 mm) stainless steel cylinder and subjected to a uniform downward flow of

air (see Fig, 1). Uniform initiation of smolder on the top surface of the bed

was achieved with an air permeable, electric heater; the igniter was removed

after self-sustaining smolder was achieved and the desired air flow was

continued.

This mode of smolder propagation, in the same direction as the air flow,

is termed forward smolder. Reversal of the air flow with the same igniter

placement would have yielded reverse smolder. Both configurations have been

studied previously in a smaller apparatus [6]. Only forward smolder brings

together the air and the hottest part of the smolder zone in an open space

external to the fuel bed. Thus it was deemed more likely to yield flaming;

reverse smolder was not examined here.

A total of nine smolder propagation experiments was run in this manner

with two different unretarded insulations. The superficial air flow velocity

(NTP volumetric flow rate divided by the bed cross-sectional area) was varied

from 0.09 to 1.1 cm/s. Flaming was never encountered. Except at the lowest

air flow velocity, the smolder front always propagated in an unstable manner

which eventually led to a "wormhollng" effect. All of the flow appeared to

channel into a region that occupied roughly 10% of the cross-sectional area

which then quickly smoldered to the bottom of the fuel bed. The position of

such a "wormhole" was not consistently in one section of the fuel bed nor

could its occurrence be precluded by extra care in packing the fuel bed.

Egerton, et al. [7] first reported such instabilities in forward smolder of

cigarettes; they occur there at much higher flow velocities. Egerton, et al.

-5 -
attributed the instability to the lessening local flow resistance when the

smolder wave gasifies part of the solid thereby allowing still more flow into

this local region which gasifies more of the solid, etc. An attempt was made

to dampen this mechanism by placing the fuel bed atop a bed of sand having a

flow resistance ten times that of the fuel bed; one would expect this to

greatly inhibit the channeling tendency since the channel could only provide a

10% decrease in total flow resistance rather than 100%. However, it was not

effective. Furthermore, the channeling behavior was never very reproducible.

Sometimes as many as three channels were initiated though only one would fully

penetrate the fuel bed.

Since this behavior could not be fully controlled and also since it could

be dependent on subtle aspects of the apparatus design, this type of forced

flow smolder was deemed unsuitable for further development as a means of

measuring tendency for transition to flaming.

2.2 Flow Over a Horizontal Smoldering Layer

A flow tunnel, illustrated in Fig. 2, was constructed for the purpose of

examining the behavior of a smoldering insulation layer subjected to a flow of

air over its surface. The tunnel provides a straight test section 115 cm

long, 20.3 cm wide and 56 cm high; it is constructed from Marinite board. Air

is drawn through the tunnel by a centrifugal fan at the downstream end; the

speed of the fan can be varied to give flow velocities from 0.3 to 5.0 m/s.

The combination of the inlet fairing and a honeycomb flow straightener (5 cm

thick; 0.64 cm wide cells) provides a flow that is uniform over the central

cross-section of the tunnel to within ± 2%, as measured by an air velocity

-6 -
probe. There is a slow, time-dependent fluctuation (~ 1 cps) of comparable

magnitude. Side wall boundary layer effects do not appear to influence the

central 50% of the tunnel over the majority of the length of the region where

the insulation layer is placed. The sample thus behaves, to a large degree,

as though it is in a steady wind blowing parallel to its top surface. The

boundary layer on the top surface of the sample grows from the tunnel inlet.

From the peak flow velocity and the longitudinal distance to the downstream

end of the bed, one would infer that the boundary layer is laminar in all

cases. However, the fuel bed surface is rough due to its particulate nature

and to its uneven shrinkage during smoldering; this introduces local separation in numerous places and, in effect, causes a mildly turbulent boundary

layer. This can be seen when smoke is being generated by the smolder process

The leading and trailing edges of the insulation bed are ramped at an

angle of 35° to lessen flow separation effects. The intersection of the

upstream ramp with the top surface is always smoothed and rounded before a

test by pressing down the insulation. The downstream intersection was not so

treated and the flow is always fully separated from there onward (downstream)

Flow velocity is measured continually during a test with a Sierra

Instruments Model 615 MHT air velocity meter*. The placement of the meter

probe is ahead of the sample bed where the tunnel cross-section has its full

56 cm height. The sample bed is typically 10.2 to 10.8 cm deep before

smoldering so the flow velocity over the top of the bed is about 20% higher

*Certain commercial equipment, instruments, or materials are identified In

this paper in order to adequately specify the experimental procedure. Such

identification does not imply recommendation or endorsement by the National

Bureau of Standards, nor does it imply that the materials or equipment

identified are necessarily the best available for the purpose.

-7 -
than that recorded by the meter. The velocity values reported here are for

the full cross-section height. The local cross-section above the sample

varies as it smolders and shrinks; the tunnel was made 56 cm high to keep the

corresponding variation in flow velocity within 20%.

Unretarded insulation from a commercial manufacturer is fully conditioned

to 50% relative humidity at 21-24 °C. It is weighed out and then fluffed by

hand within a plastic bag to remove all clumps. It is packed in two stages,

as uniformly as possible, into a pan that imposes the shape and size shown in

Fig. 2. Preliminary experiments showed that bulk densities in the range of

O commercial insulation installation practice (.025-. 035 g/cm ) give erratic

results; there is a distinct tendency for portions of the partially consumed,

charred insulation to blow downstream. The bulk density was therefore raised

O to the maximum achievable with hand packing, 0.08-0.10 g/cm ; this spread of

values results from a varying tendency of the material to expand upward after

it is removed from the packing pan. The same range of bulk density for the

organic portion of the insulation is used when the material is retarded. Bulk

density affects smolder velocity and it may affect the minimum air velocity

needed to cause the transition to flaming; this has not been evaluated.

The combustion retardants (boric acid and 10 mol borax) were usually

reagent grade but NF grade was used in a few tests. In all cases the material

is through 200 mesh. It is mixed with the insulation in sufficient quantity

to give a final product that is 15% by weight retardant. Uniform dispersion

is achieved by two hours of mixing on a roller mill. Thereafter all

procedures are the same as with the unretarded material.

-8 -
After placement in the flow tunnel, the vertical sides of the insulation

bed are in contact with either 1.27 cm Marini te board or 1.27 cm white pine

board. The sample width is thus reduced to 17.8 cm. The bottom of the sample

is always in contact with 1.27 cm Marinite board.

Thermocouples (chromel/alumel in a 0.050 cm diameter stainless steel

sheath) are inserted laterally into the insulation bed so that the junction is

at centerline mid-depth; three thermocouples are used, symmetrically placed

about the sample center with a nominal separation of 10 cm from one to the

next. The thermocouples are supported for about half of their immersed length

with a 0.080 cm ceramic tube. This provides an optimum compromise between the

need to position the thermocouples at precisely known spacings and the

tendency of a rigid rod to disrupt the smolder bed when the bed shrinks.

The insulation layer can be ignited to smoldering on either end to

produce propagation in the same direction as the air flow (forward smolder) or

in the opposite direction (reverse smolder). The igniter is a 1.3 mm diameter

electrical heater formed into a zig-zag pattern 8 cm long x 13 cm wide so that

it uniformly ignites the full width of the insulation bed. It is slipped

under the desired end of the bed, raised to 375°C and left in place for up to

one hour. Its subsequent removal is somewhat disruptive to the end of the

insulation bed so a thin sheet of stainless steel is laid across the first two

inches of the bed (at the bottom of the ramp) to keep the flow smooth. During

ignition the air flow velocity through the flow tunnel is 0.15-0.30 m/s; it is

raised to the desired test value (and subsequently held constant) only after

the smolder front on the top surface of the bed approaches the top end of the

ramp portion.

-9 -
It should be noted that even though the Insulation bed is permeable, its

flow resistance is several orders of magnitude greater than that for free flow

over the layer; thus the flow does not appreciably penetrate the fuel layer

and the oxygen Influx is mainly diffusive.

The progress of the smolder front is easily visible on the top of the bed

since it changes the insulation from a light gray to a dark black color. A

motor-driven, 35 mm camera, viewing the top of the bed through a window in the

top of the flow tunnel, monitors the smolder front progress by taking pictures

at precisely-timed intervals (typically 10 min.). It also views ruled index

marks of 1.27 cm spacing on both sides of the bed. The photos are magnified

after a test and used to compute the smolder front velocity along the top of

the bed. Smolder velocity at mid-depth is inferred from the thermocouples via

their spacing and the time between 250°C isotherms.

The top of the bed is also monitored visually through side windows on the

flow tunnel and by a visible/near-infrared TV camera placed next to the 35 mm

camera. The IR capability (to 2.6 ym) picks up self-emission above 250°C and

adds significantly to qualitative diagnosis of smolder behavior, enhancing the

perceptibility of some pre-flaming phenomena. The appearance of flames was

easily perceived by eye and on the IR TV.

3. RESULTS AND DISCUSSION

Table 1 summarizes all of the experimental conditions and results. The

unretarded material is examined first; it provides a baseline for comparison

of the effects of a smolder retardant (boric acid) and a flame retardant

(borax).

- 10-
The smolder velocities given in the following figures are all derived

from the successive photos of the top surface of the fuel bed. The average

value derived from the three imbedded thermocouples generally agreed with this

value within 5 to 10% (and frequently closer). However, the two successive

values derived from the thermocouples almost always showed some acceleration

of the smolder front at the mid-depth of the bed. In contrast, the top

surface photos rarely showed any clear-cut acceleration or deceleration.

Evidently the smolder front was slowly changing shape as it moved through the

bed; this effect was not large enough to obscure any of the trends discussed

here.

3. 1 Behavior of Unretarded Insulation

Reverse smolder tests (propagation in direction opposite to air flow)

were run with two different batches of unretarded material. Figure 3 shows

the measured smolder velocity as a function of air flow velocity. There is no

significant difference in smolder velocity behavior between these two batches

of material for this smolder mode. Both exhibit only the same weak acceleration of the smolder rate with increased air flow velocity; the smolder

velocity increases by 67% as the air velocity goes from 0 to 5 m/s.

The batch 1 material exhibited a type of localized flaming phenomenon

also seen with this material in forward smolder; here it appeared at air flow

velocities greater than 4 m/s. The passing smolder front left some incompletely consumed char that formed a very low density layer on top of the fuel

bed; the thickness of this layer increased with time at any given point as the

smolder wave moved forward. At random intervals, localized portions of this

- 11 -
char could be seen to grow gradually hotter over a period of 2-3 minutes as

indicated by Increasing light emission. As the char reached a bright orange

color, flames (typically light blue, probably due to CO burning) appeared in

the localized hole that the char consumption generated. This combination of

glowing char and blue flames lasted up to 2 minutes and spread up to 10 cm

through the residual char before extinguishing. It never spread up to the

leading edge of the smolder front on the top surface of the bed where flaming

based on pyrolysis products rather than CO could be established. The flames

were always confined within the holes generated in the char. For these

reasons, this flaming phenomenon was not viewed as a true transition from

smoldering to flaming.

Thus, no true transition occurred in reverse smolder over the range of

air velocities achievable with the present apparatus. For this reason,

coupled with the relatively weak, response of smolder velocity to air velocity

in reverse smolder, it was concluded that the reverse smolder mode represents

a distinctly lesser hazard than forward smolder (see below); it was not

pursued further with regard to combustion retardant effects nor is it viewed

as a good mode for rating smolder-to-flaming transition tendency.

Figure 3 also shows the forward smolder behavior of the same two batches

of unretarded insulation material. Here the differing batches exhibit a

qualitatively similar smolder velocity response but there are significant

quantitative differences. Most of the quantitative differences in forward

smolder velocity between batch one and batch two material are probably due to

differences in bulk density (see Table 1); the batch two material was, in most

cases, packed to a bulk density about 20% greater than for batch one. From

-12 -
stoichiometric considerations with an oxygen supply rate-limited process [4],

one expects an increase in bulk density to cause a comparable decrease in

smolder velocity. In addition, above a critical air flow velocity, batch 1

exhibited the CO flames in confined holes in the char, as discussed above;

batch 2 did not.

Both batches, at air flow velocities between 2 and 2.5 m/s, exhibit a

true transition to flaming combustion in that flames appear which move onto

virgin fuel, consuming self -generated pyrolysis vapors. The CO flames in the

char are not a necessary precursor to the initiation of pyrolysis vapor flames

since they did not occur with batch 2 material. The pyrolysis-vapor flames

typically originated in a cavity-like depression or step formed just to the

char side of the leading edge of the smolder front on the top surface. Such a

depression was formed rather erratically, evidently by the combination of char

shrinkage and consumption. The interior surface of such cavities could be

seen to glow just prior to flaming ignition. Evidently the concave structure

facilitated locally higher temperatures due to decreased radiative losses;

these structures may also have facilitated flaming ignition by allowing local

build-up of a flammable gas mixture and by providing a flame-holding region.

These details imply that modeling of the transition process will be difficult

at best.

The pyrolysis-vapor flames originated at a local region (1-2 cm wide)

along the leading edge of the smolder front, spread laterally across the

leading edge quite quickly, then spread downstream over the virgin portion of

the fuel bed. The spread is spatially erratic and transient. Since the

cellulosic Insulation material is made ultimately from wood, it is a good char

- 13 -
former. Thus, a flame cannot be permanently stabilized on the exposed top

surface of the insulation bed; the build-up of only 2-3 mm of char slows the

evolution of pyrolysis vapors to the point where the flame blows off. This

presumably is a result of the char insulating the pyrolysis region. The flame

thus at first spreads rapidly over a large area of the virgin fuel then falls

back to regions that provide a flame holder function. Slight depressions in

the top surface of the virgin fuel area where flow separation occurs provide

temporary anchor points until the char again becomes too thick (after about

1-2 minutes); the flame then dies but it frequently will have initiated

smoldering in the local region where it temporarily persisted. The longestlived flames (up to about 5 min.) are anchored at the original leading edge of

the smolder front where the smolder process provides pre-heated fuel and a

supplementary source of pyrolysis vapors. Since the smolder process consumes

fuel at a much slower rate than the flames, it too ultimately becomes inadequate as an anchor and the flames extinguish. The smolder process continues,

its geometry distorted by new initiations downstream; after a variable time

interval, the whole process of flaming ignition, spread and extinction is

repeated with spread occurring only on areas of fuel not previously charred.

The transient nature of these flaming episodes does not lessen the danger

they represent. They persist for a time that is more than adequate to pose an

ignition threat to any adjacent flammable structures that exist in a real

cellulosic insulation installation. The presence of adjacent wood has further

consequences, particularly for retarded insulation, as discussed below.

The differing responses of forward and reverse smolder to increasing air

flow pose interesting mechanistic questions. It has not yet been possible to

- 14 -
explore these in great depth but some information has been obtained. It

should be noted that Palmer [8] also saw such differences though he did not

pursue their cause.

Once a smoldering process is past the initiation stage, it has generally

been accepted that its rate is controlled by the rate of oxygen supply [8,10,

11,12]. Here again there would seem to be room for such an interpretation.

In forward smolder the air flow impinges on the upward ramping surface of the

char (Fig. 4); in reverse smolder the flow over the char is separated, as is

apparent from the smoke behavior. While there does not seem to be any information in the literature on heat or mass transfer to surfaces very similar to

those encountered here, reference to the local transport behavior on cylinders

[13] shows that, over a substantial range of Reynolds numbers (20-20,000 based

on diameter), the transport of the upstream portion of the surface does indeed

exceed that on the separated downstream surface.

In view of the considerable surface roughness here and the ambiguity as

to the most pertinent Reynolds number, it was decided that direct measurements

of the local mass transfer rates were desirable. The technique chosen, evaporation of a subliming solid (camphor), has been used in the past mainly to

infer local heat transfer rates from local mass transfer rate measurements

[14]

. Unretarded cellulosic insulation beds were smoldered in both forward

and reverse configurations (1.95 m/s air flow velocity) and then quickly

extinguished with nitrogen after the top of the smolder wave progressed over

about two-thirds of the length of the fuel bed. The resulting profiles are

those shown in Fig. 4. Pre-weighed flakes of camphor (approximately 1 cm

square, 1 V2 ~ 2 V2 thick, with tape covering the bottom surface) were

-15-
placed at several lateral and longitudinal positions on the top surface of the

char. A given set of camphor flakes was exposed to a fixed air flow velocity

over the extinguished fuel bed surface for a period up to four hours; the

weight loss from each flake was then sufficient to permit a weighing accuracy

of 1-2%. From the weight loss, the flake area and the vapor pressure of

camphor, one can compute the effective local mass transfer coefficient [14].

The variation with position on the char surface is not very great 35%

on the forward smolder case from approximately 23 cm behind the leading edge

of the smolder front to approximately 6 cm in front of it). This variation

was not pursued extensively because it is small enough to be of secondary

importance. Of greatest interest is the behavior near the leading edge of the

smolder front; this has the greatest bearing on smolder propagation rate.

Figure 5 shows the measured mass transfer coefficient as a function of air

flow velocity for a position 2 V2 cm behind the leading edge of the smolder

front (l.e. , on the char side for both forward and reverse smolder). The

values of the mass transfer coefficient are 5-10 x higher than what one would

get for a smooth flat plate parallel to the same flow velocity; the velocity

dependence (transport coefficient ~ square root of air flow velocity) is the

same as for a flat plate [15].

Comparison of Fig. 5 and Fig. 3 leads one to conclude that oxygen

transport alone cannot account for the differences between the forward and

reverse smolder propagation behavior. The difference in mass transfer coefficients is only about 25-30% whereas the smolder velocities can differ by a

factor of two. In addition, smolder in the two configurations shows differing

air velocity dependencies while the dependency of the mass transfer coeffi-

- 16 -
dent is essentially independent of configuration. Furthermore, despite the

slightly larger coefficient in the forward configuration, depletion of oxygen

in the boundary layer over the preceding char probably leads to a slower

oxygen transport rate near the leading edge in the forward case; the reverse

case does not exhibit this oxygen depletion.

The leading edge region of the smolder front is subject to the greatest

heat losses (see Fig. 4). It loses heat by conduction and radiation into

surrounding virgin fuel; it also radiates from its top surface to the cold

surroundings. In reverse smolder this region also may suffer a net convective

heat loss to the air flowing over the surface, in spite of the flow separation. (Note that the convective heat transfer coefficients will behave the

same as the mass transfer coefficients in Fig. 5. ) Certainly the virgin fuel

just ahead of the leading edge of the reverse smolder front (prior to flow

separation) suffers a convective heat loss because the air passing over it is

at room temperature. In contrast, forward smolder provides a configuration

that preheats the air flow as it approaches the leading edge of the smolder

front; one expects a net convective heat input to the char just upstream of

the leading edge of the smolder front as well as to the downstream virgin fuel

not yet charred. In fact the top surface temperature profile at 2 m/s shows

that in the forward smolder case, the drying of water from the insulation is

pushed several centimeters ahead of the smolder reaction zone by the convective heat input. Drying is immediately in front of the reaction zone for

reverse smolder.

In view of these considerations and the observed smolder behavior, we

hypothesize that the leading edge region of the smolder front is kinetlcally-

- 17 -
limited, particularly with regard to char oxidation, not oxygen supply ratelimited (the remainder of the smolder front is oxygen supply rate-limited, as

is usually assumed). In this sense it is analogous to the flame holder region

of a diffusion flame (the analogy is strongest for reverse smolder). A kinetically-limited region will be highly responsive to temperature (and additives,

see below) and thus to the net heat flux; it will be only weakly responsive to

an enhanced oxygen flux.

Consider the forward smolder case first in view of this hypothesis. An

increased air flow directly increases the heat transfer coefficient from the

char-heated air to the leading edge region causing some acceleration there.

Furthermore the increased air flow first Impinges on the oxygen supply ratelimited portion of the char oxidation zone upstream of the leading edge

region. There it accelerates the char oxidation rate, raising the local

surface temperature and thereby heats the air further. This hotter air in

turn transfers more heat to the leading edge region enhancing its acceleration. This occurs in spite of the fact that the enhanced char oxidation

depletes more oxygen from the heated air in the boundary layer (which probably

overrides the increased mass transfer coefficient). If the leading edge of

the smolder front can thus move forward more and more rapidly, the remaining

portion of the smolder front (the oxygen supply limited portion) can keep up

by adjusting its shape to assure that the oxygen supply rate per unit length

of smolder front is just sufficient.

This accelerating char oxidation closer and closer to the leading edge of

the smolder zone puts a heat source of increasing strength and temperature in

closer proximity to the virgin fuel. The top surface temperature profile at

-18 -
2 m/s shows that the forward smolder mode supports a temperature gradient

parallel to the surface that is 2 Vz x greater than in the reverse smolder

case. Ultimately the accelerated char oxidation at the leading edge leads to

the development of the concave structures, described above, which ignite

pyrolysis vapors to flaming.

The reverse smolder case might at first be expected to go slower with

increased air velocity on the basis of the preceding heat transfer considerations. In fact. Palmer [8] did find one material (cork dust) that smoldered

more slowly with increased air velocity above about 3 m/s in the reverse

smolder configuration; many of his materials showed a constant reverse smolder

velocity with increased air flow velocity above about 1 to 2 m/s, though they

accelerated at lower air velocities. Here, Fig. 3 shows only a slow but

steady acceleration of reverse smolder velocity. Evidently the increased

convective heat loss from the leading edge region is more than compensated by

other changes. The region beyond the leading edge (downstream) responds to

the increased oxygen supply getting hotter as in the forward smolder case.

The separated flow, with its attendant flow reversal, helps move some of this

heat up toward the leading edge. Conduction and radiation through the fuel

bed do so also. Furthermore, as comparison of Fig. 4(a) and 4(b) shows, the

leading edge char zone is 50% thicker in the reverse case, lessening heat

losses. The net summation of the increased Inputs of heat to the leading edge

region is evidently sufficient to outweigh the increased convective loss from

the top surface. A further factor that probably aids acceleration of the

smolder process in this case (and less so in the forward case) is an improved

oxygen supply to the oxidative pyrolysis zone comprising the front portion of

the blackened region in Fig. 4. This exothermic processs (see Fig. 7) was

-19-
previously inferred to contribute significantly to smolder propagation in the

absence of air flow [4,5].

These arguments provide a plausible qualitative explanation of the strong

response of forward smolder to increased air flow and the weak response of

reverse smolder. They do not explain the quantitative details such as the

breaks in slope in the forward case (Fig. 3); these await a detailed mathematical model. The present arguments suggest that this model must describe

the gas phase above the fuel as well as the fuel bed itself, since their

coupling is a key feature of the observed behavior.

3.2 Behavior of Retarded Insulation

It is apparent that, in the transition from smolder to flaming, both

smolder and flaming inhibitors could have an impact. Both boric acid, a

smolder Inhibitor, and borax, a flaming inhibitor, were tested separately for

their influence on forced flow smolder (forward configuration) and the transition process. Figure 6 summarizes the results. It is apparent that they

differ drastically in their influence on the smolder rate. In the absence of

any adjacent wood, they appear to be equally effective in preventing flaming,

i.e. , neither permitted the appearance of flaming at air velocities up to

about 5 m/s, the maximum value achievable with the present apparatus.

Shafizadeh, et al. [17] have investigated the effect of boric acid and

borax on the smoldering behavior of pure cellulose. They report that there

are two types of effects of these additives, both on the char oxidation

process. The net reaction heat is affected by shifts in the relative

-20-
quantities of CO and CO2 generated; boric acid is especially effective at

suppressing CO 2 formation in favor of CO, leading to a lesser heat output per

unit weight of char oxidized. In addition, both additives affect the rate of

char oxidation; boric acid retards char oxidation while borax accelerates

it. Borax apparently suppresses flaming of pure cellulose mainly by

Increasing the amount of char formed; Shafizadeh et al. found that less than

one percent of borax tripled the mass of char formed during cellulose

pyrolysis.

The material used here, being mainly ground wood pulp, is only about onethird cellulose; the remainder is hemi-celluloses and lignin. Figure 7 shows

the thermal analysis behavior of the three compositions studied here. The

usual two peaks of weight loss and heat evolution are apparent in all cases;

the first is due to oxidative pyrolysis and the second to oxidation of the

char left by the first reaction stage. The effectiveness of boric acid at

slowing the char oxidation process is obvious; it also has a weaker retarding

effect on oxidative pyrolysis which has not been explained.

The impact of borax is much more subtle; the peak in the char oxidation

process shifts downward only about 4®C, a much lesser effect than Shafizadeh

saw with pure cellulose. This is probably due to the presence of various char

oxidation accelerators already in the unretarded insulation before borax

addition; McCarter [17] has shown that various alkali and alkaline-earth

metals normally present in woody materials are char oxidation promotors. Here

the ash content of the unretarded material is 3.6 weight percent, a high value

reflecting, in part, the presence in the insulation of newsprint coated wi th

colored inks. The dyes in these inks can contain heavy metals such as lead,

-21 -
iron and chromium which are very effective char oxidation promotors [16].

Thus, the addition of borax cannot add that much to an already accelerated

process. Nevertheless, small shifts in the position of the char oxidation

peak) as seen in the thermal analysis, are apparently significant since the only

thermal analytical differences between batch one and batch two unretarded

material (whose smolder behavior differs with regard to the CO flames, see

Fig. 3) are comparable shifts in the char oxidation peak to those seen with

borax addition to batch two material. In this case batch one material shifts

upward 4°C and the apparent result during smoldering is Incomplete char oxidation followed by the intermittent glowing and flames in the residue, described

previously. In any event, one must be cautious since thermal analysis at one

heating rate does not unambiguously reveal the change in kinetic constants

that would be felt under smolder conditions.

The striking differences in smolder velocities at equal air velocities in

Fig. 6 are once again a demonstration that external oxygen supply rate alone

does not dictate smolder velocity. The preceding arguments about the importance of the leading edge of the smolder zone again apply here. If it is

kinetically-limited as hypothesized, it is sensitive to catalysts and

inhibitors of the char oxidation process. Thus boric acid slows the propagation by Inhibiting char oxidation in the leading edge region. By slowing

down, the thermal wave becomes thicker and the heat losses to the virgin fuel

decrease. The char oxidation process becomes less complete thereby insulating

itself beneath the char residue, cutting heat losses still further [4j. By

covering itself with such an insulating layer, the char oxidation zone interposes an additional series resistance to oxygen transport that makes it less

sensitive to changes in external air flow velocity. Because of these inter-

-22 -
acting effects, quantitative prediction of the net effect of a given change in

char oxidation rate, for example, requires a detailed model of the entire twodimensional system.

Borax, as noted, causes some acceleration of the char oxidation process.

The acceleration is small relative to the unretarded case and the smolder

velocities are only slightly greater. There is, however, no transition to

flaming here. The leading edge of the smolder front thus moves faster with

further air flow increases, becomes thinner and is able to tolerate the

increased heat losses. The remaining portion of the smolder wave must adjust

its shape to assure a sufficient oxygen supply rate so that it can keep up.

It has practically no "cover" between the char reaction zone and the external

air flow (only a 1-2 mm quenched skin of char on the top of the fuel bed) and

so it is highly responsive to changes in the external air flow velocity.

Figure 8 shows the differing shapes of the overall reaction zone that

facilitate the widely differing smolder velocities at the same air velocity.

Note that the reaction zone length is stretched out in the borax case by about

40% to about 37 cm from about 27 cm for boric acid. Thus not only does the

borax case assure its reaction zone a higher oxygen supply rate by remaining

close to the top surface of the fuel bed, it also stretches out the reaction

zone at a more shallow angle to facilitate a greater supply rate per unit

length of reaction front.

The failure of the transition to flaming to occur with either retardant

is presumably due to differing causes. For boric acid, the infrared TV

viewing the top surface makes it apparent that the surface is much cooler than

-23 -
the unretarded (or borax) case. Thus it is probable that, for nearly all of

the air flow range, the temperature was insufficient to ignite a flammable

mixture of pyrolysis products even if it existed above the fuel bed surface.

The situation is evidently changing at flow velocities above 4.5 m/s since the

smolder velocity begins to accelerate; one brief episode of flaming in the

char was noted at the highest flow velocity. The behavior of the upstream end

of the bed at high air velocities also indicates a new mode of behavior in

insulation containing boric acid. Here the oxygen transport rate is highest

due to the direct impingement of the flow and lack of any upstream oxygen

depletion. The highest temperatures measured in any of the tests (nearly

900 °C, 650-700 °C was typical of other compositions) occurred in this region at

an air velocity of about 5 m/s. Evidently the retarded char can be made to

oxidize completely if the oxygen supply rate and temperature are sufficient.

Onset of this more complete char oxidation near the leading edge may be the

source of the upturn of the boric acid curve in Fig. 6; note that the slope in

the upturn region matches that for borax where char oxidation is normally more

nearly complete in the forward part of the smolder wave.

In view of the minor effect that borax has on the thermal analysis

behavior (Fig. 7), its success in suppressing flaming is surprising. That is,

it does not appear to act by promoting char formation as it does with cellulose since the amount of char indicated in Fig. 7 is about the same as for

unretarded material. Its mode of action is not apparent; it could be an

indirect effect such as a failure to facilitate formation of the intermittent

cavity-like depressions at the leading edge of the smolder front. Recall that

these were the source of flaming ignition in the unretarded case.

-24 -
3.3 Effect of Adjacent Wood

Even though the retarded Insulation layers of the preceding section did

not yield flames, the increased air flow did accelerate the smolder and raise

the peak, bed temperatures. At air velocities below any transition to flaming,

these peak temperatures tend to occur just below the surface of the char away

from the leading edge of the smolder front; thus there is no adjacent source

for pyrolysis gases to support flaming. However this glowing char is an

excellent source of radiant heat. Furthermore the shape of the char surface

toward the middle to rear portion of the smolder wave facilitates radiation

toward the adjacent vertical walls supporting the Insulation layer. The char

surface tends to pull somewhat away from the adjacent wall forming a gap that

allows air inflow; the char can thus oxidize readily and radiate its evolved

heat toward the adjacent vertical wall with a fairly good view factor. If

this wall is an unretarded flammable material, flaming ignition is to be

expected. The only question is, at what air flow velocity?

White pine boards (1.27 cm thick) were substituted for the Marinite

sideboards to assess the impact of flammable sidewalls such as would be used

in a realistic attic installation of cellulosic insulation. For the unretarded insulation (Fig. 3), there was some scatter in the data but the net implication is that the critical air flow velocity for the appearance of flaming

(either spreading flames on the insulation or flames on the wooden sidewalls)

was basically unchanged. The scatter was probably due to variation in the

insulation composition since differing bags of material were used in tests at

different times.

-25 -
Figure 6 shows that, when wooden sidewalls are present, all of the

flaming suppression effectiveness of borax is lost. The glowing char ignites

the wood at about the same air flow velocity as is found with unretarded

insulation. The flames in this case, tend to anchor in the gap between the

insulation and the wood. For white pine, which is a low density wood, the

flames burn through the 1.27 cm thickness in 5-10 minutes.

Figure 6 shows that the presence of adjacent wood yields a partial loss

of effectiveness for flaming suppression with boric acid; the critical air

flow velocity moves down from something over 5 m/s to about 3-3/4 m/s. This

is still an improvement over the 2 - 2 V2 critical velocity in the

unretarded case. This improvement comes about because, at air flow velocities

up to about 3 V2

ffl/s, retardation of the char oxidation process cuts the char

surface temperature and reduces its radiating power. At higher air velocities

the high temperature, complete oxidation of the retarded char begins,

especially at the upstream end of the fuel bed, as discussed above; this

readily ignites adjacent wood.

3.4 Implications for a Smolder-to-F laming Test Method

This technique can indeed provide a quantitative measure of smolder-toflaming transition tendency. Furthermore, the range of results obtained and

the fact that they probably could not be expected to show a one-to-one correlation with results for the same materials in the current smolder ignitability

(cigarette) test indicate the need for a test along the present lines.

-26 -
There is some inevitable scatter in the measured critical air flow

velocity for the transition. It is minimized by careful packing and by the

use of high layer densities but it cannot be eliminated because of the coarse

nature of the test material and the participation in the transition process of

intermittent cavities in the smolder front. Furthermore, the exact value of

the critical flow velocity is undoubtedly not only a function of the insulation composition but also of the test configuration. The transition process

is a combined kinetic and oxygen transport phenomenon. The critical balance

of each could be shifted if the air encountered the smoldering insulation

layer in a different configuration, e.g. , if the insulation layer had been

recessed into the floor of the flow tunnel used here. The present configuration is fairly realistic but real world configurations undoubtedly vary as

well. Thus the critical flow velocity is best viewed as a relative parameter

and not an absolute measure of safety.

A substantial problem with the present configuration and mode of

operation is that it requires several hours to establish the question of

flaming or non-flaming at a given flow velocity. It is highly desirable to

shorten this to about one hour. Most of the time would inevitably go into

establishing a robust smolder wave; careful attention to igniter design is

needed to assure a realistic smolder wave. The actual air flow portion of the

test could probably be confined to 10-15 minutes (enough time to allow some

random fluctuations in the behavior of the smolder front as it passes over

small scale variations in packing density and local composition. The test

result would be of a go/no-go nature at a fixed flow velocity. A few replications, run in parallel, would be desirable.

-27 -
The presence of adjacent wood clearly has a major impact on the test

outcome. Since it is present in the actual application, it should be present

in the test as well. It would have to be carefully specified to avoid introducing a further source of scatter.

Needless to say, an appropriate test of this nature requires further

development and some calibration testing against one or more configurations of

realistic attic mock-ups with induced air flow. This calibration would serve

to establish a realistic value for the pass/fail level of critical air flow

velocity. Some exploration of the effects of insulation layer depth, bulk

density, moisture content and insulation composition are needed to assure that

no new phenomena enter in which could complicate the test interpretation in

routine usage. Any dependence of critical air flow velocity on procedural

variables (such as the length of the smoldering period prior to onset of air

flow) also needs to be quantified. A flame detector (probably optical) may be

desirable to minimize dependence on operator attentiveness or judgment. This

tentative procedure for assessing the tendency of an insulation to undergo

transition from smoldering to flaming is significantly more complex than the

existing cigarette test so it will require more extensive developmental

testing.

In carrying out the suggested development, one should run the standard

cigarette tests on all compositions used. Although a good correlation between

the pass/fail performance of cellulosic insulations in the two types of tests

is rather improbable, if it was found it would obviate the need for a smolderto-flaming test.

-28-
4. SUMMARY AND CONCLUSIONS

For a given insulation composition, there is a large difference in

response between a smolder wave moving in the same direction as the accompanying air flow (forward smolder) or in the opposite direction (reverse smolder).

Reverse smolder of an unretarded insulation showed only a weak increase in

velocity as the air flow increased; no transition to flaming was seen at air

velocities up to 5 m/s. Forward smolder of the same insulation was accelerated considerably more, and, at air velocities in the range 2 - 2 V2

flames were developed which spread over the uncharred material ahead of the

smolder zone. These differing responses are not due solely to differences in

oxygen transfer rate to the smoldering fuel. They are qualitatively

consistent with the idea that the leading edge of the smolder front is

kinetically-limited and is therefore sensitive to the differences in convective heat transfer implicit in forward and reverse configurations. The overall smolder wave is thus partially kinetically-controlled (leading edge

region) and partially oxygen transport-controlled (bulk of wave behind leading

edge).

Borax and boric acid assert their influence through the kineticallycontrolled leading edge region by accelerating (borax) or slowing (boric acid)

the char oxidation kinetics. By slowing the char oxidation kinetics to the

point of incomplete char consumption, boric acid also manages to slow the

oxygen supply rate to the trailing portion of the smolder wave making it less

responsive to changes in air flow velocity. This partial suppression of char

oxidation is lost at sufficiently high air velocities and a very hot smolder

process is generated. Nevertheless, boric acid suppresses flaming up to at

-29-
least 5 m/s. Borax does also, even though it somewhat accelerates the smolder

propagation process; its mode of flaming suppression in not clear.

Actual installations of cellulosic insulation almost invariably juxtapose

layers of the material with wooden structural supports. The wood, of course,

contains no retardants. When wooden sidewalls are included in the forced-flow

smolder apparatus there is no significant impact on the critical velocity for

appearance of flaming with unretarded insulation. However, the benefits of

borax are completely lost; the glowing insulation smolder zone that does not

itself transition to flaming ignites the adjacent wood at essentially the same

air velocity as is seen with unretarded insulation. The benefits of boric

acid are reduced but not lost when wooden sidewalls are present; the critical

air flow velocity for appearance of flames drops from above 5 m/s to about

3-3/4 m/s. Since borax and boric acid are usually used together, it is

probable that the combination offers less margin of improvement than boric

acid alone. From this and the rather weak effect that boric acid has on

increasing the minimum temperature for initiation of smoldering in cellulosic

insulation, one sees a continuing need for more effective smolder retardants.

The test methodology developed here can provide a quantitative basis for

measuring the smolder-to-flaming tendency of an Insulation composition. The

method needs further development, however, to assess its sensitivity to test

details and to calibrate it against mock-ups of realistic Installations.

-30-
5. REFERENCES

[1] Rogers, F. and Ohlemiller, T. , "Cellulosic Insulation Material I. Overall Degradation Kinetics and Reaction Heats", Combust* Sci. Technol.

^ (1980) 129-137.

[2] Ohlemiller, T. and Rogers, F. , "Cellulosic Insulation Material II.

Effect of Some Additives on Smolder Characteristics", Combust. Sci.

Technol. 24 (1980) 139-152.

[3] Ohlemiller, T. , Cellulosic Insulation Material III. Effects of Heat

Flow Geometry on Smolder Initiation", Combust. Sci. Technol. 26 (1981)

89-105.

[4] Ohlemiller, T. , "Smoldering Combustion Hazards of Thermal Insulation

Materials", National Bureau of Standards, NBSIR 81-2350, August 1981.

[5] Ohlemiller, T. , "Smoldering Combustion Propagation Through a Permeable

Horizontal Fuel Layer", paper presented at Western States Section of the

Combustion Institute 1983 Spring Meeting, April 1983.

[6] Ohlemiller, T. and Lucca, D. , "An Experimental Comparison of Forward and

Reverse Smolder Propagation in Permeable Fuel Beds", Combust. Flame 54

(1983) 131-147.

[7] Egerton, A., Gugan, K. and Weinberg, F. , Combustion Flame 7 (1963) 63.

[8] Palmer, K. , "Smoldering Combustion in Dusts and Fibrous Materials",

Combust. Flame 1 (1957) 129.

[9] Leisch, S. , "Smoldering Combustion in Horizontal Dust Layers", Ph. D. Thesis, University of Michigan Dept, of Aerospace Engineering,

Nov. 1983.

[10] Moussa, N. , Toong, T.-Y. and Garris, C. , "Mechanism of Smoldering of

Cellulosic Materials:, Sixteenth Symposium (International) on Combustion , The Combustion Institute, Pittsburg (1977) p. 1447.

[11] Summerfield, M. , Ohlemiller, T. and Sandusky, H. , "A Thermophysical

Mathematical Model of Steady-Draw Smoking and Predictions of Overall

Cigarette Behavior", Combust. Flame 33 (1978) p. 263.

[12] Ohlemiller, T. , Bellan, J. and Rogers, F. , "A Model of Smoldering

Combustion Applied to Flexible Polyurethane Foams", Combust. Flame 36

(1979) p. 197.

[13] Eckert, E. and Drake, R. , Analysis of Heat and Mass Transfer, McGrawHill, New York (1972) p. 406-407.

[14] Eckert, E. and Goldstein, R. , Measurements in Heat Transfer , 2nd ed.

McGraw-Hill (Hemisphere Publishing Corp. ), Washington, DC (1976)

p. 407 ff.

-31 -
I [15] Eckert, E. and Drake, R. , ibid , p. 732.;

[16] Shafizadeh, F. , Bradbury, A., De Groot, W. and Aanerud, T. , "Role of

Inorganic Additives in the Smoldering Combustion of Cotton Cellulose",

Ind. Eng. Chem. , Prod. Res. Dev. 21 (1982) p. 97.

[17] McCarter, R. , "Smoldering Combustion of Wood Fibers: Cause and

Prevention", J. Fire Flamm. 9 (Jan. 1978) p. 119.

-32 -
Compilation

of

Test

Results

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I

(0 "a M O

y-s Q) w CO T3 - rH y § O

o

<n «n

I I o o

T T T T

o o o o

fO CO

*0 ki *o S

B •h’

</) 4^a

3 C

flO ^0

O O ^ CM

».« i. ki ki ki

0) V <U W 4> > > > > >

OB ^Ck JSi dA ^06 06S

O O O O O Z 2 Z Z Z

O o *r. r>* (7\ 0^ O' 00 o o o o o

2 S

o o o o o

CM CM 00 sO r*» o o o c5 o

I CM CM CM ^

•O t3 T3 *0 *0

4) 0) 4/ 41 <V “O T3 T3 'O O

k4 M k« U M

(0 <0 4 Q <0 U k> ki k k aasss

§ I

(0 4 4

CO ja ^ w u u

o <r

I iTt rv r>N sO

CO cn CO ^ ^ I I 1 1 I o o o o o

fy> ^ <t •4’

T T

o o

CM CM CO -4 «4

4) 4) 4) 4) 4>

to go 0) CO CO k k k k k

V 4) 4) 4) 4) > > > > > 2S2S&

o o o o o z z z z z

O CM *4 m 9t 00 OO 00 00 O O O O O

cn ^ r«» o

O CO 00 CM CM

r**. -4 iT» m

^ C^ CO CH c^

I I I I I O O O O O

cn <n

I I o o T T

o o

*4 m O O O

4) 4) *C *D *0

(0 4 k k k

k k <0 4 (Q

?! s: ii e i: 4 0 0 0

o o o o o

CM CO O' CO 0 0-^00

k k k k

u o g u o

k

0 a c ca . •H «H

«M Ik «M 0

O 00 00

r«- O CO CO

tT) sO *0

cn cn cn till o o o o

<4 00 OO 00

CO cn m cn

I I I I o o o o

I I ON m 00

1 I o cn r*«. vO ^ >0

cn CO c*^

I I 1 I o o o o

Oi 'O O lA

cn <n m lA

cn cA cn cn

I I I I o o o o

»-i CM CM CM CM

*0 ’O T3 73 -o k k k k k

4 cQ 0} 4 CO

CM O ^ -H

•o "O -o *0 "O k k k k k iiiit (“krtrtrtrt! itzii nrtrtftrt

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compositions

with

borax

and

boric

acid

were

made

from

batch

2

material,
Air

Figure 1. Schematic of apparatus for study of smolder propagation

with flow forced through the fuel bed.

35
IR

TV

camera o 0c

O) 0

;doE

CO0

CO1-0>05

u.Ovt—

36

Figure

2.

Schematic

of

flov

tunnel

apparatus

for

study

of

smolder

propagation

with

flow

over

the

fuel

bed.

Flow

tunnel

cross-section

is

20.3

cm

wide

by

56

cm

high.
SMOLDER

VELOCITY

(cm/s)x10

CO

12

10

More frequent

- flames in char

\

8

0

UNRETARDED INSULATION

/ A

"Pyrolysis flames"

1/

/

^^-3: "Pyrolysis flames"

/

More

frequent

flames

a,a Forward smolder

o,« Reverse smolder

Open, closed symbols = different batches

Subscript w = wood sidewalls

0 1 2 3 4 5

AIR FLOW VELOCITY (m/s)

Figure 3. Measured dependence of smolder velocity on velocity of

air flowing over the fuel bed in both forward and reverse

configurations.

37
CO

u.00E4—

c0o

38

Figure

4.

Measured

cross-sectional

profiles

of

extinguished

forward

(top)

and

reverse

(bottom)

smolder

waves.

Note

that

the

bottom

profile

has

been

turned

around
o

(S/IUO) iN3l0ldd300 a3dSNVbl SSVIAI

H

d) a

ChO

(U

CJ

cd

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na

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c

cdu

+Jw

M

cdB

Tdu

cd>uo

c«

+Jo

T3

<U3

cn

cd 'O

O

«TH

(U 2:

iT\

0>3

39

AIR

VELOCITY

(m/s)
SMOLDER

VELOCITY

(cm/s)x103

AIR FLOW VELOCITY (m/s)

Figure 6. Measured effect of air velocity on smolder velocity of two

compositions with additives; all results for forward smolder mode.

Flames appeared only when wood sidewalls were present.

Uo
RATE

OF

WEIGHT

LOSS

(mg/s)

Figure 7- Comparison of thermal analysis behavior ( U mg sample,

5°C/min in air) of three insulation compositions.

hi

EXOTHERMIC

HEAT

FLOW

(mcal/s)
bOCC

•H

a

-pX

(U On

Cm

H

O ^W

OJ 4-> •H O

Cm

O

O rH ^ 0) P. > I—I P

nj "H C cd O

•H (U -P s 0 CD

0) to

1/31 +J

in cD

in

tn ^ C o o

•H

tD P> 0) -H p in ;D o

in

p

cD 0

0) o S o

CO0/ p0

tJD •H

h 2

Unaltered

fibrous

fuel
Ni>»114A IWKV. «-to)

~3. Publication Date October 1985

U.t. OKPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See Instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR-85/3212

2. Performing Organ. Report No

4.

TITLE AND SUBTITLE

FORCED SMOLDER PROPAGATION AND THE TRANSITION TO FLAMING IN CELLULOSIC INSULATION 5.

AUTHOR(S)

Thomas J. Ohlemiller

6.

PERFORMING ORGANIZATION (If Joint or other than NBS, see Instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Gaithersburg, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

U.S. Department of Energy

1000 Independence Avenue, S.W.

Washington, D.C. 20585

10.

SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or iiterature survey, mention it here)

It is well known that a smoldering fuel responds to an increased oxygen

supply by becoming faster and hotter until, eventually, flames erupt. This

sequence is examined quantitatively for thick horizontal layers of a permeable

fuel, i.e., cellulosic insulation. Two configurations are possible, forward

and reverse smolder; both are investigated experimentally. The influence of

combustion retardants is also investigated; these include boric acid, a smolder

retardant; and borax, a flaming retardant. Both prevent the transition to

flaming in the absence of adjacent flammable material but are less effective

in its presence. The overall response of these various fuel mixtures and

configurations suggests that both kinetics and oxygen supply rate (not the

latter alone) play substantial roles in dictating smolder response to an air

flow.

12.

KEY WORDS (S/x to twelve entries; aiphabeticai order; capitaiize oniy proper names; and separate key words by semicolon • air flow, cellulosic materials, flame retardant, insulation, smoldering,

smolder retardant

13. AVAILABILITY

Unlimited

I I For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

49

15. Price

$9.9F

U^COmu-OC