SPECTROSCOPIC STUDIES OF INHIBITED OPPOSED FLOW PROPANE/AIR FLAMES

RR SKAGGS, RO. DANIEL, A.W. MIZIOLEK, AND K.L. McNESBY

u.s. Army Research Laboratory

Aberdeen Proving Ground, MD 21005

Introduction

Fire protection on military platfonns, including ground fighting vehicles, is being challenged by the

impending loss of the ubiquitous fire fighting agent halon 1301 (CF3Br) due to environmental concerns related to

the destruction of the stratospheric ozone layer. Replacement fire extinguishment agents need to be found that will

satisfy numerous criteria including: fast fire suppression, minimum production of toxic gases when used, low

toxicity, compatibility with storage materials and environmental friendliness.

The U.S. Anny's search for halon replacement agents has largely involved an empirical approach of testing

and evaluation of commercially available compounds/systems. An alternative approach is to study the fundamental

physical and chemical mechanisms responsible for flame inhibition with the hope that such studies will uncover

differences in the flame inhibition mechanisms which will lead to new chemicals for further consideration and

testing. To this end, we have recently initiated planar laser induced fluorescence (PLIF) measurements of the OH

radical species as flame extinction was approached in a non-premixed, atmospheric pressure, opposed flow

propane/air flame inhibited by halon 1301 [CF3Br], N2, Fe(CO)s, FM-200 [C3F7H], FE-36 [C3FJI2], DMMP

[CH3P(O)(OCH3h], PN [P3N3F6]. Presented here are preliminary results from this study of compounds which

represent distinctly different chemical families in order to understand the differences in each agent's inhibition

mechanism.

Background

Chemical inhibition in a flame arises from the lowering of the radical concentrations due to scavenging

reactions. In general, effective inhibition mechanisms contain two types of reactions: a) radical scavenging

reactions, and b) reactions regenerating inhibitor species that participate in the inhibition cycle. As an example, for

CF3Br inhibition a free bromine from decomposed CF3Br fonns HBr which chemically reacts with hydrogen atom

and reduces hydrogen's concentration. The consequence of hydrogen recombination is the overall available radical

concentrations (H, 0, OH) and the rate of chain-branching reactions are reduced [1,2,3,4] while regeneration of

HBr and Br2 occurs restarting the inhibition cycle.

The chemicals Fe(COh, DMMP, and PN investigated in our laboratory flame system were chosen based on

a comprehensive evaluation [5] of fire inhibitors that are more effective than CF3Br. The inhibition mechanisms for

Fe(CO)s, DMMP, and PN are believed to be generally similar to the HBr mechanism. For these postulated

mechanisms, each agent decomposes during combustion into inhibition cycle scavenging species, e.g. FeO, FeOR,

Fe(OH)2 for Fe(COh addition, [6] and HOPO and HOP02 for DMMP and PN addition [7]. In the reaction zone of

flames, these scavenging species proceed to behave much like HBr in'scavenging hydrogen atoms. FM-200 and FE36 were studied here due to their popularity as candidate halon replacement agents.

Flame inhibition by CF3Br is not completely due to chemical reactions, where previous studies [8] have

shown that at least 20 % of its inhibition capability is caused by physical properties, i.e. heat capacity. In order to

understand a chemical's inhibition mechanism in tenns of physical and/or chemical contributions, both N2 and

CF3Br are included in this study. That is, N2 represents the upper boundary for physical influence on flame

inhibition since it has no chemical inhibition capabilities, while CF3Br with its known physical and chemical

contributions offers a good intermediate point with which to compare and contrast the other agents studied.

Experimental

OH PLIF imaging measurements were made using the arrangement presented in Figure 1. The opposed

flow burner apparatus is located inside a stainless steel hood to contain any toxic fumes that are exhausted from the

burner. All flames analyzed in this work were studied at atmospheric pressure and consisted of 7.0 Umin synthetic

air (79% N2 + 21% O2) flowing from the lower duct, and 5.6 Umin of propane flowing from the upper duct. The

oxidizer and fuel ducts are separated a distance of 1.2 em and the duct diameter is 2.54 em. Based on the flow

conditions and duct separation, the luminous flame zone is located on the oxidizer side of the stagnation plane and

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the global strain rate is 72.51 sec-1• For all studies presented here, the inhibitor agents are added to the oxidizer flow

in gaseous form at room temperature with the exception of Fe(CO)s which was cooled to 11°C and DMMP which

was heated to 70°C. Opposed flow burners have been used for some time to study the capabilities of an inhibitor

agent because a global parameter, the extinction strain rate [9], can be determined which describes the flame's

strength at extinction [10,11,12,13]. The extinction strain rate is useful because a decreased value demonstrates an

inhibitor's efficiency. PLIF measurements of radical concentrations (0, H, OR) are complimentary to the extinction

strain rate because the measurements illustrate an inhibitor's influence on the radical concentration profiles in the

flame zone which indicates if the flame's radical chemistry is being perturbed by agent addition. OH is monitored in

the flames studied here because it is relatively simple to measure and it is a good indicator of the overall radical

pool concentration, even though H, 0, and OH have been found to not be fully equilibrated in diffusion flames [14].

Planar laser induced fluorescence images were measured using a Lambda Physik excimer/dye laser system.

This system consists of a Lambda Physik Compex 102 XeCI excimer laser, a Scamnate 2 dye laser (Coumarin 153)

and a Second Harmonic Generator (SHG). The fundamental output of the dye laser (540 nm wavelength) was

frequency doubled in the SHG unit with a BBO crystal to approximately 281 nm. The UV laser radiation was tuned /

to the p~ofthe R2(9.5) transition at 281.8 nm «1,0) A2E+~X2I1) [15,16,17]. This transition was chosen because

it is less·temperature dependent than other nearby transitions [18]. The UV light output of the SHG unit enters an

optical train where the beam is turned 90°, apertured by a sub mm iris, projected through a cylindrical plano

convex lens to form the UV beam into a vertical sheet. To create a uniform sheet width, the sheet is apertured with

0.5 mm vertical slits as it is projected toward the center of the burner. The UV sheet is apertured just before the

burner to produce a vertically uniform intensity that is 1.2 cm in height allowing passage through the entire burner

flow field. Laser induced fluorescence from OH passes through a band pass filter centered at 312 nm with an 11 nm

bandwidth and is detected with a Princeton Instruments ICCD camera (Model 120) coupled with a Nikon UV lens

located at 90° with respect to the UV sheet. The ICCD camera, which has an active area of384 x 576 pixels, has a

field of view with this optical arrangement of approximately 33 cm2 and each image recorded was acquired with 25

total accumulations on the camera.

Results

Figure 2 presents two representative two-dimensional images of OH fluorescence for an uninhibited

propane/air flame and for a propane/air flame to which CF3Br was added (1.5 % by volume). Both images, which

are uncorrected for laser energy fluctuations and local quenching rates, illustrate the presence of two luminous

zones as the UV sheet passes through the flame. The lower, thicker zone is the fluorescence from the OH transition

while the upper zone is the broadband fluorescence due to derivative fuel species such as polycyclic aromatic

hydrocarbons. To construct a spatially resolved OH LIP profile from an image, as shown on the right hand side of

Figure 2, the pixel intensity corresponding to a given height between the fuel and oxidizer ducts (spatial resolution

approximately 0.149 mm1pixel) was summed and averaged over a 1 mm width. The two-dimensional images and

LIP profiles illustrate that addition of CF3Br to the propane flame causes a decrease in the OH fluorescence signal

while the broadband fluorescence appears to increase just slightly. Similar results have been seen previously for

CF3Br addition to hydrocarbon diffusion flames [19,20). Each OH intensity profile is fit to a gaussian function to

determine the area under the profile curve. The profile area provides a general indicator of the OH behavior for a

given flame condition because it accounts for changes in both the profile width and the profile's position relative to

the flow ducts as inhibitor concentrations increase in the flame. PLIF images similar to those in Figure 2 were

acquired and analyzed for each agent studied.

Figure 3 plots the results of the measured OR profile areas versus each inhibitor agent's concentration as

the flames were steppechowards extinction. It should be noted that the reported areas are averaged over three or

more separate extinction experiments, where the data for each experiment are normalized to the OH profile area

measured in the uninhibited flame acquired prior to each extinction experiment to account for changes in burner and

camera conditions. The data here indicate that there are both physical and chemical modes of inhibition being

observed for the agents studied. N2 has the least impact on OH with respect to the other agents studied while the

two fluorinated propanes (PM-200 and FE-36) show initially small declines in OH, but more rapid decreases as

extinction is approached. For the other agents studied (pN, CF3Br, DMMP, and Fe(CO)s), the addition of these

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Table 1:Inhibitor concentrations (% volume) and uncertainh £± % volume) at flame extinct

FE-36 CFaBr FM-200 Fe(CO)s PNDMMP N2

4.410.6 6910.50.4 35.9

0.63 1.5647 1.88 0.10 0.03 12.3

inhibitors show more dramatic decreases in the measured OH values. Finally, Table 1 lists the observed extinction

inhibitor concentrations in the air stream for each agent studied here and their estimated uncertainties.

'ion

It should be noted that the extinction concentration for CF3Br is very similar to values found for analogous

flames and cup burners [20], while the fluorinated propanes are slightly higher than their reported cup burner

values [21]. Unfortunately there are no cup burner values reported for DMMP. Recent cup burner experiments for

PN [22] indicate the value observed in our experiments is a factor of 6 greater than the literature value. At this time

there is no explanation for the PN extinction concentration discrepancy and future experiments will address this

issue in more detail.

Conclusjons

/ The results presented here show for the first time changes in OH as extinction is approached in an

atmospheric pressure, non-premixed, propane/air flame. The OH profiles illustrate that N2, FE-36, and FM-200,

with smaller changes in OH relative to CF3Br, exhibit chemical inhibition capacities less than CF3Br. On the

contrary, DMMP and Fe(CO)s demonstrate chemical inhibition capabilities greater than CF3Br with their larger

changes in OH. For the inhibitors studied, agent concentrations at extinction support these observations with a

CF3Br concentration of 4.4 % (by volume) compared to N2 with a concentration of 35.9 % and DMMP and

Fe(CO)s each having concentrations less than 1 %.

Figures

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Figure 1: Schematic diagram of the experimental apparatus. ••••

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Figure 2: Representative PLIF images and the corresponding OH intensity profiles from an opposed flow propane/air flame

seeded with 0 % (by volume) CF~r and 1.5 % (by volume) CF~r. Note the orientation of the PLIF images with respect to

the burner system places the fuel and air ducts at the top and bottom of the images respectively.

577
1.0

ca ~~A~_

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0.8 r

« (I) ii: 0.6 e a.. :r: 0 0.4 "0 .~(I) (ij E •... 0.2 ~ -+--CF3Br

z 0.0 r •

o 2 4 6 8 10

Inhibitor Agent Concentration, % volume

Figure 3: Nonnalized OR LIF profile areas versus agent delivery concentrations. The (0) are the N2 data. the (0) are the

FM-200 data , the ( t::. ) are the FE-36 data. the (V) are the PN data. the (0) are the CF~r data. the (II) is the DMMP data

and the (e) are the Fe(CO)s data.

Acknowledgments

The authors would like to thank Anthony Hamins (NISl) for burner fabrication and Valeri Babushok

(NISl) for insightful suggestions and comments concerning experiments and results. This work was supported by

the Next Generation Fire Suppression Technology Program under the auspices of the U.S. Army TACOM (Steve

McCormick). Finally R. Skaggs would like to acknowledge financial support from the Army Research Laboratory

through an American Society for Engineering Education Postdoctoral Fellowship.

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