EFFECTS OF PRESSURE AND OXYGEN CONCENTRATIONON THE FLAME SPREAD LIMITS OF FIRE RESISTANT

FABRICS

By

Danielle Ellise Kirchmeyer

BS (University of California, Berkeley) 2011

A report submitted in partial satisfaction of theRequirements for the degree of

Master of Science, Plan II

in

Mechanical Engineering

at the

University of California at Berkeley

Committee in Charge:

Professor Carlos Fernandez-Pello

Professor J.Y. Chen

Spring 2014

Contents

1 Introduction 1

2 Methodology 2

2.1 Apparatus Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.2 Partial Pressure Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.3 Thin Solid Fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Experimental Procedure 3

3.1 Design Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4 Results 5

4.1 Limiting Oxygen Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

5 Discussion 6

5.1 Flame Spread Phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5.2 Upward Flame Spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5.3 Downward Flame Spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5.4 Flame Spread Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6 Conclusion 11

7 Appendix 12

7.1 Design Flaws in Experimental Apparatus . . . . . . . . . . . . . . . . . . . . . . 12

1

Abstract

Space exploration vehicles may employ cabin environments that are not at standard sea level at-mospheric conditions. NASA’s Constellation Program proposed a human space exploration cabinenvironment of reduced ambient pressure and increased oxygen concentration (56 kPa and 32%O2). There is a need to further understand the flammability of materials in these environments.In this work, normalized tests were conducted at different pressures to study the effect of oxygenconcentration on the upward and downward flame spread over the Fire Resistant fabric NomexHT90-40. The results show that for Nomex HT90-40 fabric the minimum oxygen concentrationfor upward and downward flame spread depends on the ambient pressure. A boundary of flamespread or not flame spread is developed in terms of ambient pressure and oxygen concentration. Itis shown that at decreased pressures the oxygen concentration required for flame spread increases,and that the boundaries of upward and downward flame spread converge as the pressure is reduced.A phenomenological explanation is given in the results. It is worth noting that there are a numberof situations when fires may occur at pressures and oxygen concentrations that are different thanstandard atmospheric conditions, such as in locations at high elevation and airplanes which extendsthe interest of this study to different environments.

1 Introduction

Potential future space exploration missions by NASA or other international space agencies to-gether with commercial space exploration spurred by lower operational costs have the potential toencourage the design of a new generation of spacecrafts and increase the number of humans trav-eling to space. An increased human presence in space also poses many challenges, particularly inhuman safety. Fire in a spacecraft environment represents a particularly dangerous situation giventhe combined effects of extended periods of time, confined space, low-pressure, elevated oxygenconcentrations and micro, or partial, gravity. Ignition and burning of fabrics and other combustiblematerials during a spacecraft mission could compromise mission success, but most importantlyastronaut safety.

In a space facility, buoyantly induced flows are normally negligible, however there are slow con-vective currents, induced primarily by the HVAC system. Furthermore, even if the HVAC systemis not operating (quiescent environment) small changes in gravity levels can also induce low ve-locity flows. Thus, in the event of the onset of a fire, the combustible material would be exposedto a low velocity flow (from approximately 0.01 m/s to 0.2 m/s) of an oxidizer with varied oxygenconcentrations (from approximately 18% to 24% in the ISS, and around 34% in SEA). Severalworks conducted in normal and reduced gravity have characterized the ignition and flame spreadcharacteristics of thin materials under varying conditions of material thickness, external heat flux,oxygen concentration, pressure, and forced flow velocity [1].

Furthermore, this analysis may be utilized to assess the risks associated with storage of goods inhigh bay warehouses in locations such as Quito, Equador and Lhasa, Tibet, which include operatingenvironments at altitudes of 2,850 and 3,650 meters above sea level, respectively. Fire protectionstrategies based on early detection of an outbreak of fire can be determined based on the obtainedresults.

In the present work, we present experiments aimed to determine the effects of oxygen concentrationon the Limiting Oxygen Concentration (LOC) as well as the Maximum Oxygen Concentration(MOC) for upward and downward flame spread over thin flame retarding fabrics. Fire-resistantfabric Nomex HT90-40 was used for experiments due to its practical use for astronaut space suitsas well as other earth fabrics such as firefighter clothing and race car driver suits. In normalatmospheric conditions Nomex does not melt or drip, but chars when exposed to high temperatures[2]. Charring of the fabric contributes to its flame resistance characteristics. Being a family ofaromatic polyamide fibers, Nomex creates a strong flexible polymer chain with a high degree ofheat resistance.

1

(a) Upward Flame Spread (b) Downward Flame Spread

Figure 1: Simplified experiment schematics of the duct for upward and downward flame spread.

2 Methodology

2.1 Apparatus Configuration

The experiments were conducted in the Forced-Flow Ignition and Flame Spread Test (FIST) ap-paratus shown schematically in Figure 6. The experiment apparatus shown in Figure 2 consists ofa small-scale combustion duct 560 mm long with a rectangular cross-section 127 mm by 100 mmthat slides in and out of a pressure chamber.

The floor of the duct is made up of a 3.2 mm aluminum plate covered by a 1.6 mm ceramicinsulation sheet. The bottom surface of the duct has a 50.8 mm by 152.4 mm rectangular cutoutsection located 158.8 mm from the inlet section of the duct. The sidewalls of the duct are made of6 mm quartz windows for optical access. The duct slides in and out of the pressure chamber thathas an ID of 0.40 m and a length of 0.83 m, three windows, and four ports for electrical and gasflow connections.

2

2.2 Partial Pressure Analysis

The oxygen concentration is determined by a partial pressures analysis. The desired oxygen con-centration is achieved by evacuating the chamber to a pressure lower than the desired test pressureand adding 100%pure oxygen to the chamber so that the mixture is at the desired final pressureand oxygen concentration. Due to small pressure leakages in the chamber from the installation ofseveral ports utitized for equipment inside the pressure chamber, the initial decrease in pressurewas lower than the calculated value to compensate for the leakages after adding oxygen.

The oxygen concentration of the gas mixture was verified using an oxygen sensor (Apogee Instru-ments, Logan, UT, USA) upstream of the pressure chamber. To ensure good mixing, a AC fan(Mechatronics Inc.) in the chamber is turned on and the gases are allowed to mix for at least fiveminutes. The chamber pressure is monitored and controlled using a pressure transducer (Omega,Stamford, CT USA).

2.3 Thin Solid Fuel

Tests were conducted with a single layer of Nomex HT90-40 (Stern and Stern Industries Inc.,Hornell, NY, USA). Fabric samples are mounted into the rectangular cutout section and sit flushwith the bottom surface of the duct. The test samples were positioned vertically so that an upwardand downward flame spread could be observed, as illustrated in Figures 6a and 6b, respectively.Sample dimensions were chosen in accordance to 16 C.F.R. Part 1610 [3], which stipulates themethodology for testing flammability of clothing textiles sold in the United States. Piloted ignitionis induced using a 3 mm diameter Kanthal wire woven approximately 10 mm from the upstreamedge of the sample. The igniter wire was positioned at the bottom or top of the sample to observeupward or downward flame spread, respectively. The igniter is turned on and ignition is determinedby the onset of sustained flame propagation along the sample surface. Upon determination of theLOC, all tests were repeated five times to estimate the experimental repeatability and to obtain anaverage value.

3 Experimental Procedure

Following partial pressure calculations, the total pressure and oxygen partial pressure were setinside the pressure chamber using a pressure transducer. The igniter wire was energized and flamespread was observed by the onset of sustained flame propagation along the sample surface. Positiveflame spread was considered if the flame spread a distance from the igniter greater than 10mm.

For a given chamber pressure, the minimum oxygen concentration for flame spread was obtainedby measuring the flame spread distance for a given oxygen concentration in the chamber. If the

3

Figure 2: Photograph of the duct inside the pressure chamber.

flame spreads longer than 10 mm for a given oxygen concentration, then a test at a lower oxygenconcentration is repeated until the flame spread is less than 10 mm. The corresponding oxygenconcentration is considered the limiting oxygen concentration (LOC) for flame spread. After de-termining the limiting oxygen concentration, five tests are performed to calculate a probability forflame spread at that oxygen concentration.

Oxygen concentrations ranging from 20% to 52% by volume were added to the pressure chamberuntil the final pressure was reached for a given test. The inflow of oxygen was mixed using afan inside the chamber, and a waiting time of 240 seconds was required to reach steady quiescentflow. Upon reaching steady quiescent flow inside of the pressure chamber, the igniter wire wasenergized and flame spread was observed by the onset of sustained flame propagation along thesample surface if the conditions were appropriate.

3.1 Design Challenges

During the preliminary stage of data collection, a significant portion of time was devoted to ad-dressing several design flaws in the experimental apparatus. Details are described in the Appendix.

4

4 Results

4.1 Limiting Oxygen Concentration

Flame spread experiments were conducted in ambient pressure ranging from 40 kPa to 101 kPa andoxygen concentrations ranging from 23% to 52%, all of which excluded the onset of an oxidizerflow velocity. Flame Spread and No Flame Spread regions for Nomex HT90-40 for two differentsample orientations such as concurrent-vertical (upward) and opposed-vertical (downward) flamespread are illustrated as a function of oxygen concentration and ambient pressure. Figure 4 showsthe experimental data for upward flame spread whereas Figure shows the approximated FlameSpread and No Flame Spread boundary. As shown in Figures 3 and 4, the percentage of NomexHT90-40 sample length burned is measured for upward and downward flame spread for a given setof pressure and oxygen cencentration, respectively.

Figure 3: Upward flame spread results.

From the results shown in Figures 3 and 4, it is seen that for both upward and downward flamespread, the oxygen concentration required for flame spread decreases as pressure is increased.However, the LOC in the upward spread increases as a much faster rate than that of downwardspread as illustrated in the steep curve in Figure 3. On the other hand, the results obtained fordownward flame spread show more inconsistency in flame propagation given the probability ofNomex HT90-40 sample length burned.

5

Figure 4: Downward flame spread results.

To further investigate the buoyant effects on flame spread behavior, the percent of Nomex HT90-40 sample lengths burned were also analyzed above the LOC. In this case, the maximum oxygenconcentration for flame spread was determined if 100% of the sample length was burned. Thechar length of each 148mm length Nomex HT90-40 sample was measured after each test and thepercent sample length burned was calculated as illustrated in Figure 3 and Figure 4.

Theoretically, the LOC for upward and downward flame propagation will converge asymptoticallynear zero pressure since there would not be differentiation between the two modes of spread. Thepresent results show that that the LOC trend lines for upward and downward spread approach eachother below 40.53 kPa as illustrated in Figure 3 and Figure 4. The LOC trend line for upward flamespread shows that it will approach a minimum pressure for flame propagation faster than that fordownward flame spread.

5 Discussion

As fire growth develops by flame spread over a solid combustible from the point of ignition, itis valuable to be able to analyze any potential fire spread situation and quantify the fire develop-ment. Most of this discussion is essentially qualitative, but two aspects of this work merit closerexamination, namely concurrent-vertical and opposed-vertical flame spread. Limited quantitativeanalysis is presented as a phenomonological interpretation of the above results.

6

Figure 5: The limiting oxygen concentration trends for varied configurations.

5.1 Flame Spread Phenomena

Flame spread in fire-resistant (FR) fabrics is a complex process that depends on the pyrolysisand FR properties of the fabric. The actual mechanisms of fire resistance are related to complexphysical and chemical processes such as charring, intumescence, solid pyrolysis and gas phasechemistry. A simplified analysis as that developed in [4] permits a phenomenological interpretationof the above results.

Flame spread phenomenon is described as the advancing ignition front in which the leading edgeof the flame acts as both the heat source and the pilot ignition source [5]. Essentially, it is deter-mined by the amount of heat transferred from the flame to the unburnt thin solid fuel ahead of theflame. In the event when sufficient heat transfers from the flame to the unburnt material, or the“burning zone”, the unburnt solid fuel will pyrolyze and reach a pyrolysis temperature, T

p

. Uponvaporization of the fuel, the hot gases travel away from the flame surface and mix with the oxidizerflow thereby generating a flammable mixture ahead of the flame leading edge until the flame ap-proaches the flammable mixture and ignition occurs. The flame spread rate is therefore quantifiedby the availability of the flame to transfer enough heat required to pyrolyze the solid fuel and ignitethe flammable mixture ahead of the flame front [4]. A schematic is shown in Figure 6 to illustratethis phenomena for upward and downward flame spread over a thin solid fuel.

7

5.2 Upward Flame Spread

For this report, an attempt to describe the flame spread phenomena will be made to contribute toseveral reviews that have previously been published describing experimental trends and theoreticalmodels of concurrent- and opposed-vertical flame spread. There are several factors that affectflame spread rate over thin combustible solids including chemical, physical and environmentalfactors. The composition of the fuel, initial fuel Temperature T0, surface orientation, directionof propagation, thickness, thermal capacity, density, ambient composition, and ambient pressure[5]. The surface orientation can have a dominating affect on the flame behavior, which drives themotivation of the experiments with upward ad downward flame spread in the absence of oxidizerflow. Furthermore, it is well known that buoyancy significantly affects the processes of solidradiant heating [6], ignition [7], flame spread [1], [8], and extinction of the flame [7]. Thus, it islogical to expect that the LOC, the boundary defining the region for flame spread and no flamespread, would differ depending on the environmental conditions, as has been shown in the resultsobtained to date in the present work.

In the case for upward flame spread along a solid fuel surface, the flow of hot gases produced bythe flame travels in the same direction as the flame spread. The hot gases push the flame aheadof the pyrolysis region, which delegates heat transfer from the flame to the unburnt fuel therebyinducing a relatively fast flame spread compared to that for downward flame propagation.

5.3 Downward Flame Spread

In the case for downward flame spread along a solid fuel surface, the flow of hot gases producedby the flame travels in the opposite direction as the flame spread due to buoyant affects. The hotgases travel up and away from the pyrolysis region which in turn restrict heat transfer from theflame to the unburnt fuel thereby inducing a relatively slow flame spread. Furthermore, thin solidfuel in the downward orientation promotes significant contribution by gas-phase conduction as theprimary mode of heat transfer. This explains the positive shift of the LOC curve for downwardflame spread: as pressure decreases, more oxygen is required to sustain the flame. Increasing theoxygen concentration increases the flame temperature which in turn increases the heat flux fromthe flame to the thin solid fuel.

5.4 Flame Spread Rate

The rate of flame spread is determined by the ability of the flame to tansfer enough heat to pyrolyzethe thin solid fuel and ignite the preceding combustible mixture. Therefore, flame spread rate canbe analyzed as the ratio of the heated length (l

h

) ahead of the pyrolysis front to its ignition time; lh

8

pyrolysis

q”convflame

q”rs

q”fr

length

heatinglength

length

gravityO

2/N

2

Low P

Vb

q”cond

fla

me

pro

pa

ga

tio

n

(a) Upward

pyrolysis

length

heatinglength

flamelength

q”cond

gravity

O2/N

2

Low P

Vb

fla

me

pro

pa

ga

tio

n

(b) Downward

Figure 6: Simplified phenomena schematics of upward and downward flame spread.

constitutes the region in between the pyrolysis front (lp

) and the flame length (lf

) i.e lh

= lf

� lp

.For thin materials the rate of flame spread is given by [4] as

Vf

⇡

h(k

g

⇢g

cg

U1/lp

)1/2 (Tf

� Tp

) + q00fr

� q00sr

i(l

f

� lp

)

⇢s

cs

s(Tp

� To

)(1)

where lh

= heated length, lp

= pyrolysis length, lf

= flame length, as, Vf

= flame spread rate,kg

= gas phase thermal conductivity, ⇢g

= gas phase density, cg

= specific heat of the gas phase,U1 = oxidizer flow velocity, T

f

= flame temperature, Tp

= pyrolysis temperature, To

= solid initialtemperature, q00

fr

= radiant flux from the flame to the solid, q00sr

= reradiation from the solid, q00ext

=external radiant flux, ⇢

s

= solid density, cs

= specific heat of the solid, and s = solid thickness.

Equation 1 can be used to calculate flame spread rate for non-charring materials the since theheated length is approximately equal to the flame length. The flame length can be calculated fromthe pyrolysis length using

lf

= clnp

(2)

where n = power in flame length relation and c = generic constant. In charring materials such asNomex HT90-40, the pyrolysis length is affected by the char layer formed as the flame heats upthe material. As such, the rate of flame spread is heavily dependent on the physical properties of amaterial. In addition, charring introduces a time dependent pyrolysis that in turn affects the flamelength [8]. Therefore an expression for the time dependent behavior of the pyrolysis and flamelengths is needed in order to analyze flame spread of such materials.

9

Quintiere and Harkleroad [9] and Hasemi [10] proposed an alternative method for estimating lf

using the heat release rate per unit width of fuel for upward propagation. A modified version forforced flow can be used to calculate l

f

as

lf

= c

Q0

cg

Tf

⇢g

U1

!lf

=

"✓D

fuel

(1� Yfuel,sto

)

U1

◆1/2⇢fuel

�Hc

⇢g

cg

(Tf

� T1)

#2(3)

where Q0 = heat release rate per unit length and g = gravity. Although similar to Equation 2,Equation 3 allows capturing the environmental effects and time dependent mass loss rate throughthe heat release rate term. The heat release rate can be calculated by integrating the mass loss rateover the pyrolysis length and multiplying it by the material heat of combustion. The expressiontakes the form of

Q0 =

Zlp(t)

lb(t)

m00(x, t)�Hc

dx (4)

where m00 = mass loss rate, �Hc

= heat of combustion, and lb

= burn length. The transient massloss rate in Equation 4 will depend on the pyrolysis characteristic of the material. However, it canbe approximated using a uniform mass loss rate per unit area (m00 = m00

o

) during a fixed gasificationtime (t

b

) and zero otherwise as originally proposed by Saito, Quintiere and Williams [8]. Assumingthat the heat needed to bring the solid to the pyrolysis temperature is small compared to the heatneeded to pyrolyze the fuel and that the material behaves as thermally thin, m00

o

can be calculatedas

m00o

=q00fr

� q00sr

+ q00ext

�Hp

(T, t)(5)

where t = time, �Hp

(T, t) = time and temperature dependent heat of pyrolysis, and✓dT

dt

◆

s

=

heat flux at the surface, and therefore the time dependent mass loss rate of a charring material tobe substituted into Equation 4 is given by

m00 =

h⇣ks

(T, t)dTdy

⌘

s

+ q00fr

� q00sr

+ q00ext

i

�Hvol

0 < t tb

0t > tb

(6)

where tb

= gasification time and ks

(T, t) = time and temperature dependent solid thermal conduc-tivity. The charring behavior of the material will appear also through the heat of pyrolysis, �H

p

.The heat of pyrolysis will change as the sample composition changes due to preferential pyroly-sis of blended materials and char layer build up. Therefore, as the value of �H

p

increases, the

10

pyrolysis rate of the material will decrease.

The flame spread rate equation for charring fabrics takes the form of Equation 8. The equationhelps describing phenomenologically the flame spread rate in charring materials such as NomexHT90-40.

Changes in oxygen concentration primarily affect the heat release rate and consequently flametemperature. Flame temperature (T

f

) dictates the convective (q00conv

) and radiation (q00fr

) heat fluxfrom the flame to the solid fuel. Equation 7 is used to estimate the convective heat transfer fromthe flame to the solid fuel.

q00conv

=

✓kg

⇢g

cg

U1

lp

◆1/2

(Tf

� Tp

) (7)

As the convective and flame radiation heat fluxes increase, the pyrolysis and flame length increase,which results in longer flame lengths and enhanced heating ahead of the pyrolysis front. As theflame length is directly proportional to the flame standoff distance, the flame standoff distancealso increases upon increasing flame length duw to increased heat flux to the thin solid fuel. Allthese effects tend to enhance the rate of flame spread. In addition to affecting the heat flux at thesolid surface, oxygen concentrations may also influence the solid fuel pyrolysis [6]. Further testingand better parameter estimation will be necessary to understand how oxygen concentration affectspyrolysis of FR materials.

Vf

=q00f

lf

⇢cp

d(Tig

� T0)(8)

The flame spread rate should be expected to decrease as the flame length decreases [11]. It wasobserved that this indeed is the case for downward flame spread. As the upward results indicate,this could mean that another dependence has yet to be determined which calls for further study.

6 Conclusion

Results of the present work show that the minimum conditions for upward and downward flamespread in FR fabrics depend on the pressure and oxygen concentration. For a given pressure thereis a minimum oxygen concentration (LOC) that allows for flame spread to occur. This minimumvalue is different in upward and downward configurations. Therefore, care should be taken whenevaluating the performance of a FR fabric based solely on the LOI, as this value might not berepresentative for the intended use of the fabric.

The differences in configuration and methodologies may result in the noted differences in theULOI and MOC values for Nomex HT90-40. It is important to emphasize that the results from this

11

work are not attempting to replicate LOI or ULOI results. Instead, the work studies how buoyanteffects, oxygen concentration and pressure constitute interrelated parameters that affect upwardand downward flame spread propagation over FR fabrics.

The particular results regarding oxygen concentration and external radiant flux observed for thefabrics tested in [1] are specific to the concurrent and opposed experimental configurations. Thissuggests further study of the LOC and flame spread rate of Nomex materials in the upward anddownward orientation with an incident heat flux in low pressure atmospheres.

Acknowledgments

The authors gratefully acknowledge funding by NASA Grant NNX08BA77A as well as Stern andStern Industries Inc. for providing fabric samples. The authors would like to thank Hugo Wagner,Daniel Murphy, and Andres Osorio for their valuable insight and Davis Tran, Kevin Pacheco, andLisa Torres for their assistance in conducting the experiments.

7 Appendix

7.1 Design Flaws in Experimental Apparatus

During the preliminary stage of data collection, several design flaws existing as a result fromprevious research experiments conducted inside the pressure chamber needed to be addressed.Design flaws are discussed in order of priority in these experiments.

In order to examine buoyant flow alongside the fabric sample during tests, sufficient clearanceabove and below test samples was required. The presence of a fan downstream of the duct wasremoved in order to accomodate bouyant-induced flow along the sample surface. A linear actuatorwas installed to automate part of the experimental procedure by removing and inserting the ductcontaining the sample inside the pressure chamber.However, upon removing the fan, the apparatusneeded a placeholder in order to meet the design specificiations in the duct, specifically the 152inch height of the linear actuator. This approach was taken due to lower cost associated comparedto the more expensive approach of merely replacing the linear actuator. Thus, the fan was replacedwith Solidworks designed and machined 6061 Aluminum parts.

Within the first experiment, pressure leaks were clearly observed inside the pressure chamber. Asillustrated in Figure 7, the pressure chamber leaked pressure at a rate of 0.6psi/min according todata retrieved in the DAQ Assitant in Labview. Including sample loading time, the time it takesto insert the duct inside the pressure chamber, air removal time, oxygen insertion time, a singleexperiment can take up to 18 minutes long. The time from the start of a test to the end of a test cantake as long as one minute. Therefore, pressure leaked approximately 10.2 psi from the beginning

12

of sealing the pressure chamber to the beginning of a test, and then to 10.8 psi by the end of a test.

Figure 7: Pressure leaks as a function of time inside the pressure chamber.

Several design concepts were brainstormed to locate the pressure leak on the pressure chamber. Forproof of concept, a miniature pressure chamber was machined out of 6061 Aluminum. A 3/4 inchdiameter hole was drilled three inches though the Aluminum block followed by the application ofa 3/4 inch NPT screw and a half inch endmill was used to drill a through hole perpendicular to the3/4 inch diameter volume. The prototype was connected to a pressure gauge and compressed airinlet. The prototype is shown in Figure 8 (pressure gauge and compressed air intake not shown).The pressure chamber was pressurized after having applied RTV vaccuum grease. Pressure leakswere observed using RTV vaccuum grease by applying soapy water solution to the problem area.However, consistent pressure maintenance was observed upon applying epoxy, which was thesolution to the pressure leaks on the pressure chamber.

Pressure leaks inside the NASA pressure chamber were located by pressurizing to 20psi, a pressurewell within the specifications of the pressure chamber strength, and then using a fog machineto observe turbulent flows around problem areas. Several localized problem areas were locatedand marked prior to exact location using a soapy water solution. Upon component extraction,insufficiently epoxied parts were removed from the flange by rigorous removal from the fixedflange on a laboratory vice grip. It was noted that the conductor wires empoxied to the 3/4 inchNPT through screws were localized at the center of the fitting, thereby providing a path of less

13

Figure 8: Proof of concept pressure chamber.

resistance relative to the average flow resistance inside the pressure chamber. This technique wasmodified by placing each of the six conductors in a single 3/4 inch NPT through hole thorugh a3mm honeycomb width flow straightener. Epoxy was placed between all of the 18 AWG solidconductors that were used to power the igniter, fan, and oxygen sensor.

Lastly, prior to evaluating the effects of external radiant heat flux incident on the sample surfaceduring flame propagation, it was required to measure the appropriate clearance on the duct roof.Two millimeter thin 6061 Aluminum sheets were milled and de-burred to appropriate size to fixthe heat lamps on the x-y plane and fit on the roof of the duct. The lamps were tightened downby 1075 carbon steel wire with three passes through each hole on the duct roof, as illustrated inFigure 9.

Upon installing the water-cooled heat lamps, the tubes used to cool the lamps were replaced inorder to eliminate the inherent water leaks from over using and bending the water tubes. Further-more, swagelock interfaces at various junctions in the circuit were replaced as well (Figure 10).

To serve as sufficient design concept validation, the heat lamps were calibrated with a MEDTHERMSchmidt-Boelter thermopile type radiometer (Figure 11), and the following results were obtained.

References

[1] A. F. Osorio, C. Fernandez-Pello, D. L. Urban, G. A. Ruff, Limiting conditions for flame spread in fire resistantfabrics, Proceedings of the Combustion Institute .

14

Figure 9: Water-cooled heat lamps fixed to duct roof inside pressure chamber.

Figure 10: Water leak locations.

[2] DuPont, Technical guide for nomex brand fiber, Tech. Rep. H-52720, E.I. du Pont de Nemours and Company(2001).

[3] Standard for the flammability of clothing textiles, U.S. Consumer Product Safety Commission (2008).

[4] C. Fernandez-Pello, in: G. Cox (Ed.), Combustion Fundamentals of Fire, Academic Press, San Diego, CA, 1995.

[5] D. Drysdale, An Introduction to Fire Dynamics, John Wiley Sons, 1998.

15

Figure 11: Design concept validation: heat flux distribution as a function of flame spread positionfor various heat flux intensities.

[6] T. Hirata, T. Kashiwagi, J. Brown, Thermal oxidative degradation of poly(methyl methacrylate): Weight loss,Macromolecules 18 (1985) 1410–1418.

[7] Y. Nakamura, A. Aoki, Irradiated ignition of solid materials in reduced pressure atmosphere with various oxygenconcentrations - for fire safety in space habitats, Advances in Space Research 41 (2008) 777–782.

[8] K. Saito, J. Quintiere, F. Williams, Upward turbulent flame spread, Fire Safety Science 1 (1986) 75–86.

[9] J. Quintiere, M. Harkleroad, Wall flames and implications for upward flame spread, Comb. Sci. and Tech. 48(1985) 191–222.

[10] Y. Hasemi, Thermal modeling of upward flame spread, Fire Safety Science 1 (1986) 87–96.

[11] J. G. Quintiere, J. Wiley, Fundamentals of Fire Phenomena, John Wiley England, 2006.

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