Inhibition of Non-Premixed Flames by Dimethyl Methylphosphonate

M.A. MacDonald, T.M. Jayaweera, E.M. Fisher·, F.e. Gauldin

Sibley School of Mechanical and Aerospace Engineering

Cornell University

Ithaca, New York 14853

Phosphorus-containing compounds are suggested as a viable alternative to current, ozone-destroying, fire-suppressing

agents. An opposed-jet burner apparatus was used to study the effectiveness of one such compound, dimethyl

methylphosphonate (DMMP). A global extinction strain rate was found for a range of DMMP loadings, using nitrogen

as an inert additive for reference. A novel technique for measuring the extinction strain rate while maintaining a

constant dopant level in one gas stream is discussed. Results demonstrate that when DMMP is introduced with the

oxidizer, it is a significant inhibitor of non-premixed methane-air flames, approximately 40 times more effective than

nitrogen on a molar basis. It was found that the extinction strain rate decreased linearly with the mole fraction of

DMMP for loadings up to 1500ppm.

Introduction

In accordance with the Montreal Protocol, the US

government has recently banned the manufacture of

CF3Br. a prevalent fire-suppressing agent. due to its

deleterious effect on the ozone layer. Consequently, there

is tremendous interest in chemically active firesuppressant alternatives, including phosphoruscontaining compounds (PCCs). It is thought [1,2] that

phosphorus-containing radicals. such as HOPO and

HOP02 (fonned from PCCs during combustion)

catalytically recombine the important combustion

radicals, Hand OH, slowing the overall reaction rate and

thus inhibiting the flame. Several mechanisms for the

catalytic cycle resulting in H + OH ~ H20 and H + H ~

H2, have been suggested [1,2). Experiments have shown

increased recombination rates of OH with the addition of

PCCs [I).

A non-premixed methane-air flame has bcen used to

study the inhibiting effects of one PCC, dimethyl

methylphosphonate (DMMP). [P(=O)(CH3)(OCH3h].

Experiments were perfonned in an opposed-jet burner

apparatus. The counter-flow configuration is useful for

studying flame-suppressing agents because the flame is

thennally isolated and two dimensional [3-5). The flame

strength can be characterized by the local strain rate at

extinction, which is well defined in this geometry and

equal to the axial velocity gradient, measured just

upstream of the flame along the center line. Results from

asymptotic analysis of non-premixed flames in this

geometry [6] relate this extinction strain rate to an

overall reaction rate in terms of parameters such as

temperature and species concentration at the fuel and

oxidizer inlets, and heat of reaction. Thus, for low

additive loadings, decreases in extinction strain rate can

be interpreted as chemical inhibition, i.e. as the lowering

of the overall reaction rate by the additive[7-9].

During an extinction measurement. reactant flow rates

are slowly increased until the flame abruptly

extinguishes. For flame-suppressant compounds that are

liquids at room temperature, such as DMMP, there are

practical difficulties in establishing and maintaining

constant loadings of the inhibitor as the flow rates are

adjusted. A novel technique for perfonning extinction

measurements is presented herein which provides a

practical method for achieving constant inhibitor

loadings.

Reported here are results of an initial study of DM1v1P

as a flame suppressant. Addition of DMMP to both the

oxidizer and the fuel side are considered. Inert dopant

tests using nitrogen as an additive were conducted for

reference. Comparisons are also made to work by other

researchers studying halogenated compounds and iron

pentacarbonyl [Fe(COh) as flame suppressants in the

same geometry [10].

Experimental

Experiments were conducted on an opposed-jet burner.

shown schematically in fig 1. Methane was used as the

fuel and air as tlle oxidizer. These conditions allow for

non-premixed flames to be stabilized on the oxidizer side

of the stagnation plane. The burner was constructed

from glass tubes 30 cm long with an ID of 0.98 cm, and a

separation distance of O. 95 cm between opposing nozzles.

Annular sheath flows of nitrogen are provided tluough

2.22 cm ill glass tubes. The sheath tube exits were offset

by approximately 1 cm, upstream of the reactant tube

exit, to minimize the impact of the sheath flow on the

development of the reactant flows. The entire burner is

·Corresponding Author

Proceedings 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 maintained

slightly below atmospheric pressure.

N2

Air

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 stagnation

plane for a nonreactive flow. Tllis global strain rate is

used as an approximation to the local strain just upstream

of the flame surface for flames near extinction.

All testing of DMMP was performed using the lower

tube as the fuel source and the upper tube as the oxidizer

source. A few experiments were conducted with the

reverse orientation to study the effect of buoyancy. No

change in global e>..1inction strain rate was observed.

N2

CH4

(2)

Methodology

One of the few practical methods of delivering small.

metered quantities of liquid dopants. such as DMMP, to

the reactant stream is via a syringe pump which provides

a constant mass flow of the compound. Previous

extinction studies using opposed-jet burners have

adjusted the strain rate by varying both the oxidizer and

fuel streams simultaneously so that the flame position

remains constant [8J, or so that a momentum balance

between the two streams is maintained [9]. These

methods of approaching extinction would result in

variable concentrations of the inhibiting agent, due to the

changing mass flow of the reactant stream and the

resulting transients in the adsorption/desorption of the

agent on walls. During our experiments, the

concentration of DMMP in the reactant stream was fixed

by maintaining a constant reactant flow for sufficient

time for the system to reach equilibrium. Then

extinction conditions were approached by varying the

undoped stream flow rate while maintaining a constant

flow on the doped side. One of the consequences of

using this novel teclmique for approaching extinction is

that the flame position varies during the extinction

experiment. An investigation was conducted to

determine the effect of flame position on the global

extinction strain rate.

Figure 2 shows results from experiments performed

with methane and air which demonstrate that 3q varied

less than ±2% over a range of flame positions, referred (0

as the acceptable region. The distance from the exit

plane of the oxidizer nozzle to the stagnation plane. S,

was chosen as the independent variable instead of the

actual flame location because it can be estimated with the

following e:\.-pression from a momentum balance between

the reactant streams [II],

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

(1)

stagnation

plane

Figure l. Schematic diagram of opposed-jet burner

apparatus.

The flame suppressant under investigation in this work,

DMMP, is a liquid at room temperature with a low vapor

pressure (less than one torr at ambient temperature). In

order to maintain sufficient concentrations of DMMP in

the vapor phase, the reactant lines were heated to

approximately 100°C with electrical heating tapes. The

temperature of the reactant streams 10 cm upstream from

the exit of the nozzles was maintained at IOO±! °C via

active heating control of the sheath flow. This method of

reactant temperature control was found to give short

settling times and stability of the reactant temperatures

during testing.

There are two common methods for determining the

extinction strain rate in flame measurements: direct

measurement of the local strain rate using techniques

such as Laser Doppler Velocimetry (LDV), or estimation

of the strain from global parameters. For our

experiments, the extinction strain rate was estimated

using the latter method and will be referred to as the

global strain rate, aq. Williams [II) proposes the

following 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 •

!

•

.~ .!

••

••

•;

•

..,

• .. ..•. ••• • ••

456

Estimated Distance from Oxidizer Nozzle to Stagnation Plane, S

[mm)

7

• undopedl • 500ppm

DMMP !

Figure 2. Percent deviation of global extinction strain rate from its mean value in acceptable region. The acceptable

region, where deviations are lessthan 2%, falls where S is between approximately 3.75 and 5.75 mm.

fixed location relative to the stagnation plane: this

distance is controlled by the local strain rates (i.e. the

velocity field), the diffusivities of the reactants and their

concentrations at the inlets.

For experiments in which extinction occurs with the

stagnation plane location, as estimated using equation 2,

outside the acceptable region, large deviations in 3q

occur. We attribute these deviations to influences of the

nozzles on the flmae and mixing layer. For experiments

in which extinction occurs in the acceptable region. the

magnitude of 3q is consistent to within 2% of the result

that would be obtained .maintaining a fixed flame

position while approaching extinction. A limited number

of experiments with DMMP-doped flames confirmed that

the region in which 3q was invariant with S was the same

as that for the undopcd case. The existence of tIlis

acceptable region establishes the validity of our method

for approaching extinction with a constant oxidizer flux

and dopant concentration, provided the value of S at

extinction falls witllin that region.

Results and Discussion

Initial tests were conducted with reactant streams at

ambient temperature to compare results from our

experimental apparatus with those of other researchers.

For undiluted methane burning in air, our measured

global extinction strain rate is 296 s·t. This value is

comparable to the value found by Puri and Seshadri [9)

of 280 S·I, and both of these values are lower than the

local strain rates directly measured by other authors

using LDV, which vary from 340 to 400 S·1 [4,8.12J. A

similar 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 ex1inction

strain rate of 400 ± 25 S·I. Using equation 1 with their

reported flow data and geometry, we compute a

significantly 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 different

experimental apparatus, which is attributed to variations

in specific burner geometry[9]. Numerical predictions of

local extinction strain rates give a sinlilar range of values

(354 to 544 S·I) depending on the choice of constants

used 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 was

some scatter. ±15 S·I, in tIlis value over the course of this

investigation which is attributed in part to variations in

the purity of methane in different cylinders used as fuel

sources. Therefore. results presented in this section have

been normalized to a reference, methane-air extinction

measurement for the appropriate bottle of fuel.

Figure 3 shows our results for methane-air flames

doped with DMMP, ,vith the DMMP addition to the

oxidizer stream. Each data point in figure 3 represents

the average of at least three measurements. The global

extinction strain rate is observed to decrease linearly with

DMMP mole fraction. Experiments indicate that at a

mole fraction of 1500ppm, 3q is reduced by 35% of the

undoped value. Detectable inhibition (-3% reduction in

3q) is found at loadings as low as 100ppm. For mole

fractions below 100ppm, preheating of reactants was not

necessary to prevent condensation of DMMP. This

allowed for ambient- temperature testing of low DMMP
0.9

0.7

0.6

o

•• • • • I

•

•

• •

• •

750 1000 1250 1500

DMMP Loading [ppm)

2SO 500

=

.§-tI)

=

.9-

u

=

.~

~ <I)

c; ~ 0.8 .g~

a

~

<I)

.!::

c;

E

o

z

Figure 3. Global extinction strain rate for methane-air non-premixed flames doped with DMMP on oxidizer side. Data

nonnalized to undoped strain of·B 1 S·l.

loadings which demonstrated suppression effectiveness

that was similar to the preheated experiments. The

results from figure 3 show inhibition by DMMP that is

roughly four times stronger than that reported in the

literature for CF~r, the current agent in many

suppression 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 lower

loadings, 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 using

nitrogen as an inert suppressant, both with and without

preheating the reactants. Results from these tests, shown

in figure 4, indicate that preheating of the reactants has

no influence on the effectiveness of nitrogen as a

suppressant. The reduction in nonnalized global strain

rate for nitrogen- doped flames agrees to within 2% ,••ith

0

• • • e •

t

• v ) • i ! 0 

i

I 0 

• DMMP I I 

: 0 Nitrogen, preheated

0 i I

I

I

I

0.9

0.7

0.6

o 10000 20000 30000 40000 50000 60000

=

'§

en

=

.9

g

.~

~ <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. Data

nonnalized 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, these

results demonstrate that DMMP is approximately 40

times more effective on a per mole basis than nitrogen (9

times as effective on a per mass basis).

Tests conducted with DMMP addition to the fuel side

showed a negligible (within e>..'perimentalscatter) effect

over the range of fuel side loadings achievable with our

current experimental apparatus (0-900ppm). This is an

expected result with the flame positioned on the oxidizer

side of the stagnation plane. In this position the bulk of

the DMMP added to the oxidizer stream passes through

the flame front, while DMMP added to the fuel side must

diffuse, with the fuel, through the product region to reach

the flame. Since the concentration of methane at the

flame is rougWy 10% of its value in the fuel jet, it is

likely that for DMMP, which is a larger and heavier

molecule and probably has a lower diffusivity, even less

than 10% of its initial concentration will reach the flame.

One of the major obstacles to the use of DMMP as a

fire-suppressing agent is the compound's low vapor

pressure at ambient temperature « I torr). The

maximum loading achievable at ambient temperature is

just under 100ppm. DMMP also has a significant

heating value due to the one methyl and two methoxy

groups attached to phosphorus atom. Further studies in

flame suppression by PCCs will investigate alternative

forms of the parent compound that might be more

amenable for delivery to the flame.

Conclusions

The present study used an opposed-jet burner

configuration to investigate the effectiveness of dimethyl

methylphosphonate (DMMP) as a flame suppressant. A

new method for performing ex1inctionmeasurements was

developed to maintain constant DMMP loadings in the

oxidizer stream using a syringe pump. This new method

is shown to give consistent and repeatable results. The

global extinction strain rates are estimated in terms of the

flow rates of fuel and oxidizer. It has been found that the

global extinction strain rate is approximately invariant

(within ±2%) over a range of stagnation plane locations

for methane-air non-premixed flames.

Global extinction strain rates were measured for flames

doped with DMMP loadings from 0 - 1500 ppm. As a

flame suppressant on a molar basis, DMMP was found to

be 40 times more effective than nitrogen and

approximately four times more effective than the

literature values for CF3Br. The high level of

effectiveness of DMMP (35% reduction in global

extinction strain rate at 1500ppm) encourages the

investigation of other phosphorus-containing compounds

as alternatives to halogenated fire suppressants. Further

research is also suggested to elucidate the consistent

differences found in the literature between global strain

rate estimates and true local strain rates measured by insitu techniques.

Acknowledgments

This research is part of the Department of Defense's

Next Generation Fire Suppression Technology Program.

funded by the DoD Strategic Environmental Research

and Development Program under DARPA contract

MDA972-97-M-0013. Additional support was provided

by the Army Research Office, under contract number

DAAL03-92-G-O 113. The authors wish to thank 1.

Fleming and B. Williams at Naval Research Laboratory

for useful discussions.

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