Inhibition of Non-Premixed Flames by Dimethyl Methylphosphonate

M.A. MacDonald, T.M. Jayaweera, E.M. Fisher·, F.e. GauldinSibley School of Mechanical and Aerospace Engineering

Cornell UniversityIthaca, New York 14853

Phosphorus-containing compounds are suggested as a viable alternative to current, ozone-destroying, fire-suppressingagents. An opposed-jet burner apparatus was used to study the effectiveness of one such compound, dimethylmethylphosphonate (DMMP). A global extinction strain rate was found for a range of DMMP loadings, using nitrogenas an inert additive for reference. A novel technique for measuring the extinction strain rate while maintaining aconstant dopant level in one gas stream is discussed. Results demonstrate that when DMMP is introduced with theoxidizer, it is a significant inhibitor of non-premixed methane-air flames, approximately 40 times more effective thannitrogen on a molar basis. It was found that the extinction strain rate decreased linearly with the mole fraction ofDMMP for loadings up to 1500ppm.

IntroductionIn accordance with the Montreal Protocol, the US

government has recently banned the manufacture ofCF3Br. a prevalent fire-suppressing agent. due to itsdeleterious effect on the ozone layer. Consequently, thereis tremendous interest in chemically active fire­suppressant alternatives, including phosphorus­containing compounds (PCCs). It is thought [1,2] thatphosphorus-containing radicals. such as HOPO andHOP02 (fonned from PCCs during combustion)catalytically recombine the important combustionradicals, Hand OH, slowing the overall reaction rate andthus inhibiting the flame. Several mechanisms for thecatalytic cycle resulting in H + OH ~ H20 and H + H ~H2, have been suggested [1,2). Experiments have shownincreased recombination rates of OH with the addition ofPCCs [I).

A non-premixed methane-air flame has bcen used tostudy the inhibiting effects of one PCC, dimethylmethylphosphonate (DMMP). [P(=O)(CH3)(OCH3h].Experiments were perfonned in an opposed-jet burnerapparatus. The counter-flow configuration is useful forstudying flame-suppressing agents because the flame isthennally isolated and two dimensional [3-5). The flamestrength can be characterized by the local strain rate atextinction, which is well defined in this geometry andequal to the axial velocity gradient, measured justupstream of the flame along the center line. Results fromasymptotic analysis of non-premixed flames in thisgeometry [6] relate this extinction strain rate to anoverall reaction rate in terms of parameters such astemperature and species concentration at the fuel andoxidizer inlets, and heat of reaction. Thus, for lowadditive loadings, decreases in extinction strain rate can

be interpreted as chemical inhibition, i.e. as the loweringof the overall reaction rate by the additive[7-9].

During an extinction measurement. reactant flow ratesare slowly increased until the flame abruptlyextinguishes. For flame-suppressant compounds that areliquids at room temperature, such as DMMP, there arepractical difficulties in establishing and maintainingconstant loadings of the inhibitor as the flow rates areadjusted. A novel technique for perfonning extinctionmeasurements is presented herein which provides apractical method for achieving constant inhibitorloadings.

Reported here are results of an initial study of DM1v1Pas a flame suppressant. Addition of DMMP to both theoxidizer and the fuel side are considered. Inert dopanttests using nitrogen as an additive were conducted forreference. Comparisons are also made to work by otherresearchers studying halogenated compounds and ironpentacarbonyl [Fe(COh) as flame suppressants in thesame geometry [10].

ExperimentalExperiments were conducted on an opposed-jet burner.

shown schematically in fig 1. Methane was used as thefuel and air as tlle oxidizer. These conditions allow fornon-premixed flames to be stabilized on the oxidizer sideof the stagnation plane. The burner was constructedfrom glass tubes 30 cm long with an ID of 0.98 cm, and aseparation distance of O. 95 cm between opposing nozzles.Annular sheath flows of nitrogen are provided tluough2.22 cm ill glass tubes. The sheath tube exits were offsetby approximately 1 cm, upstream of the reactant tubeexit, to minimize the impact of the sheath flow on thedevelopment of the reactant flows. The entire burner is

·Corresponding AuthorProceedings of the 1997 Technical Meeting of the Central States Section of the Combustion Institute

isolated in a glass enclosure for control of exhaust gases.This enclosure is purged with nitrogen and maintainedslightly below atmospheric pressure.

N2Air

I, L refers to the separation distance between the nozzles.

V is the stream velocity and p is the stream density.

Equation 1 evaluates the strain rate at the stagnationplane for a nonreactive flow. Tllis global strain rate isused as an approximation to the local strain just upstreamof the flame surface for flames near extinction.

All testing of DMMP was performed using the lowertube as the fuel source and the upper tube as the oxidizersource. A few experiments were conducted with thereverse orientation to study the effect of buoyancy. Nochange in global e>..1inction strain rate was observed.

N2CH4

(2)

MethodologyOne of the few practical methods of delivering small.

metered quantities of liquid dopants. such as DMMP, tothe reactant stream is via a syringe pump which providesa constant mass flow of the compound. Previousextinction studies using opposed-jet burners haveadjusted the strain rate by varying both the oxidizer andfuel streams simultaneously so that the flame positionremains constant [8J, or so that a momentum balancebetween the two streams is maintained [9]. Thesemethods of approaching extinction would result invariable concentrations of the inhibiting agent, due to thechanging mass flow of the reactant stream and theresulting transients in the adsorption/desorption of theagent on walls. During our experiments, theconcentration of DMMP in the reactant stream was fixed

by maintaining a constant reactant flow for sufficienttime for the system to reach equilibrium. Thenextinction conditions were approached by varying theundoped stream flow rate while maintaining a constantflow on the doped side. One of the consequences ofusing this novel teclmique for approaching extinction isthat the flame position varies during the extinctionexperiment. An investigation was conducted todetermine the effect of flame position on the globalextinction strain rate.

Figure 2 shows results from experiments performed

with methane and air which demonstrate that 3q variedless than ±2% over a range of flame positions, referred (0

as the acceptable region. The distance from the exitplane of the oxidizer nozzle to the stagnation plane. S,was chosen as the independent variable instead of theactual flame location because it can be estimated with the

following e:\.-pression from a momentum balance betweenthe reactant streams [II],

For a fixed local strain rate field. the flame will lie at a

(1)

stagnationplane

Figure l. Schematic diagram of opposed-jet burnerapparatus.

The flame suppressant under investigation in this work,DMMP, is a liquid at room temperature with a low vaporpressure (less than one torr at ambient temperature). Inorder to maintain sufficient concentrations of DMMP in

the vapor phase, the reactant lines were heated toapproximately 100°C with electrical heating tapes. Thetemperature of the reactant streams 10 cm upstream fromthe exit of the nozzles was maintained at IOO±! °C via

active heating control of the sheath flow. This method ofreactant temperature control was found to give shortsettling times and stability of the reactant temperaturesduring testing.

There are two common methods for determining theextinction strain rate in flame measurements: direct

measurement of the local strain rate using techniquessuch as Laser Doppler Velocimetry (LDV), or estimationof the strain from global parameters. For ourexperiments, the extinction strain rate was estimatedusing the latter method and will be referred to as theglobal strain rate, aq. Williams [ II) proposes thefollowing relation for evaluating the global strain rate:

This equation is derived assuming a uniform velocity­profile across the exit planes of the nozzles. In equation

,•i

i

•I•

!i

• .~ .••!

••••

•;~ . .. ,•• .. ..•.

••• • ••

456

Estimated Distance from Oxidizer Nozzle to Stagnation Plane, S[mm)

7

• undopedl• 500ppmDMMP !

Figure 2. Percent deviation of global extinction strain rate from its mean value in acceptable region. The acceptableregion, where deviations are less than 2%, falls where S is between approximately 3.75 and 5.75 mm.

fixed location relative to the stagnation plane: thisdistance is controlled by the local strain rates (i.e. thevelocity field), the diffusivities of the reactants and theirconcentrations at the inlets.

For experiments in which extinction occurs with thestagnation plane location, as estimated using equation 2,outside the acceptable region, large deviations in 3qoccur. We attribute these deviations to influences of thenozzles on the flmae and mixing layer. For experimentsin which extinction occurs in the acceptable region. themagnitude of 3q is consistent to within 2% of the resultthat would be obtained .maintaining a fixed flameposition while approaching extinction. A limited numberof experiments with DMMP-doped flames confirmed thatthe region in which 3q was invariant with S was the sameas that for the undopcd case. The existence of tIlisacceptable region establishes the validity of our methodfor approaching extinction with a constant oxidizer fluxand dopant concentration, provided the value of S atextinction falls witllin that region.

Results and DiscussionInitial tests were conducted with reactant streams at

ambient temperature to compare results from ourexperimental apparatus with those of other researchers.For undiluted methane burning in air, our measuredglobal extinction strain rate is 296 s·t. This value iscomparable to the value found by Puri and Seshadri [9)of 280 S·I, and both of these values are lower than thelocal strain rates directly measured by other authorsusing LDV, which vary from 340 to 400 S·1 [4,8.12J. Asimilar discrepancy is seen in global and local extinction

strain rates obtained in a single experiment. Papas et al.[12] report LDV measurements of a local ex1inctionstrain rate of 400 ± 25 S·I. Using equation 1 with theirreported flow data and geometry, we compute asignificantly lower global extinction strain rate of 255 S·I.

There is also an observed variation in the literature of±30 S·1 for e:'\-unction strains measured on differentexperimental apparatus, which is attributed to variationsin specific burner geometry[9]. Numerical predictions oflocal extinction strain rates give a sinlilar range of values(354 to 544 S·I) depending on the choice of constantsused to evaluate the elementary reaction rates [5].

When the reactant streams were preheated to 100°C.the measured value for aq increased to 431 S·I. There wassome scatter. ±15 S·I, in tIlis value over the course of thisinvestigation which is attributed in part to variations inthe purity of methane in different cylinders used as fuelsources. Therefore. results presented in this section havebeen normalized to a reference, methane-air extinctionmeasurement for the appropriate bottle of fuel.

Figure 3 shows our results for methane-air flamesdoped with DMMP, ,vith the DMMP addition to theoxidizer stream. Each data point in figure 3 representsthe average of at least three measurements. The globalextinction strain rate is observed to decrease linearly withDMMP mole fraction. Experiments indicate that at amole fraction of 1500ppm, 3q is reduced by 35% of theundoped value. Detectable inhibition (-3% reduction in3q) is found at loadings as low as 100ppm. For molefractions below 100ppm, preheating of reactants was notnecessary to prevent condensation of DMMP. Thisallowed for ambient- temperature testing of low DMMP

0.9

0.7

0.6o

••• I•• •

•••• ••

150012501000750

DMMP Loading [ppm)

5002SO

=.§-tI)=.9-u=.~~ <I)

c; ~ 0.8.g~a~

<I)

.!::c;Eoz

Figure 3. Global extinction strain rate for methane-air non-premixed flames doped with DMMP on oxidizer side. Datanonnalized to undoped strain of ·B 1 S·l.

loadings which demonstrated suppression effectivenessthat was similar to the preheated experiments. Theresults from figure 3 show inhibition by DMMP that isroughly four times stronger than that reported in theliterature for CF~r, the current agent in manysuppression systems [13]. On a molar basis, DMMP is-26 times more effective than literature values for CF4

[12]. Although Fe (CO)s is a stronger inhibitor at lowerloadings, its effect as a suppressant plateaus. in the range

of observed loadings, for mole fractions over 500ppm.giving DMMP a similar effectiveness at 1500ppm (10).For further comparison, tests were conducted usingnitrogen as an inert suppressant, both with and withoutpreheating the reactants. Results from these tests, shownin figure 4, indicate that preheating of the reactants hasno influence on the effectiveness of nitrogen as asuppressant. The reduction in nonnalized global strainrate for nitrogen- doped flames agrees to within 2% ,••ith

0•• e••

tv•

i) • !0,-i

I0• • DMMP I It

: 0 Nitrogen, preheated 0: • Nitrogen, ambient :iII• I

I0.9

0.7

0.6

o 600005000040000300002000010000

='§en=.9g.~~ <I)

c; ~0.8.g~a~

<I)

.!::c;E~

Dopant Loading [ppm)

Figure 4. Global extinction strain rates of inhibited methane-air, non-premixed flames doped on oxidizer side. Datanonnalized to undoped strain of 431 S·I for DMMP. 409 s'\ for nitrogen with reactants preheated to 100°C and 296 s'!for nitrogen with ambient temperature reactants.

the results of Puri and Seshadri [9]. Furthermore, theseresults demonstrate that DMMP is approximately 40times more effective on a per mole basis than nitrogen (9times as effective on a per mass basis).

Tests conducted with DMMP addition to the fuel sideshowed a negligible (within e>..'perimentalscatter) effectover the range of fuel side loadings achievable with ourcurrent experimental apparatus (0-900ppm). This is anexpected result with the flame positioned on the oxidizerside of the stagnation plane. In this position the bulk ofthe DMMP added to the oxidizer stream passes throughthe flame front, while DMMP added to the fuel side mustdiffuse, with the fuel, through the product region to reachthe flame. Since the concentration of methane at theflame is rougWy 10% of its value in the fuel jet, it islikely that for DMMP, which is a larger and heaviermolecule and probably has a lower diffusivity, even lessthan 10% of its initial concentration will reach the flame.

One of the major obstacles to the use of DMMP as afire-suppressing agent is the compound's low vaporpressure at ambient temperature « I torr). Themaximum loading achievable at ambient temperature isjust under 100ppm. DMMP also has a significantheating value due to the one methyl and two methoxygroups attached to phosphorus atom. Further studies inflame suppression by PCCs will investigate alternativeforms of the parent compound that might be moreamenable for delivery to the flame.

ConclusionsThe present study used an opposed-jet burner

configuration to investigate the effectiveness of dimethylmethylphosphonate (DMMP) as a flame suppressant. Anew method for performing ex1inctionmeasurements wasdeveloped to maintain constant DMMP loadings in theoxidizer stream using a syringe pump. This new methodis shown to give consistent and repeatable results. Theglobal extinction strain rates are estimated in terms of theflow rates of fuel and oxidizer. It has been found that theglobal extinction strain rate is approximately invariant(within ±2%) over a range of stagnation plane locationsfor methane-air non-premixed flames.

Global extinction strain rates were measured for flamesdoped with DMMP loadings from 0 - 1500 ppm. As aflame suppressant on a molar basis, DMMP was found tobe 40 times more effective than nitrogen andapproximately four times more effective than theliterature values for CF3Br. The high level ofeffectiveness of DMMP (35% reduction in globalextinction strain rate at 1500ppm) encourages theinvestigation of other phosphorus-containing compoundsas alternatives to halogenated fire suppressants. Furtherresearch is also suggested to elucidate the consistent

differences found in the literature between global strainrate estimates and true local strain rates measured by in­situ techniques.

AcknowledgmentsThis research is part of the Department of Defense's

Next Generation Fire Suppression Technology Program.funded by the DoD Strategic Environmental Researchand Development Program under DARPA contractMDA972-97-M-0013. Additional support was providedby the Army Research Office, under contract numberDAAL03-92-G-O 113. The authors wish to thank 1.Fleming and B. Williams at Naval Research Laboratoryfor useful discussions.

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