A Novel Method for Evaluating the Effectiveness of Low Volatility

Flame Inhibitors with an Opposed-Jet Burner

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

Sibley School of Mechanical and Aerospace Engineering

Cornell University

Ithaca, NY 14853

emf4@cornell.edu (E.M. Fisher)

Introduction

In accordance with the Montreal Protocol, the US government has recently banned the manufacture of CF3Br, a

prevalent flame-inhibiting agent, due to its deleterious effect on the ozone layer. Consequently, there is great interest in

finding chemically-active, flame-inhibiting alternatives [1]. Some attractive flame-inhibiting compounds, such as several

phosphorus-containing compounds (PCCs) [2,3], are liquid at ambient conditions and have low vapor pressures. This paper

presents a novel method to evaluate the effectiveness of such compounds.

The non-premixed counterflow configuration is usefulfor studying flame-inhibiting agents [4-6] because the flame

is thermally isolated and two dimensional [7-9]. The flame strength can be characterized by the 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 centerline of the flow [7,9]. During an typical extinction measurement, both reactant flow rates are slowly increased

until the flame abruptly extinguishes. For flame-inhibiting compounds that are low vapor-pressure liquids at room

temperature, there are practical difficulties in establishing and maintaining constant loadings of the inhibitor as flow rates

are adjusted. A novel method for performing extinction measurements is presented herein which provides a practical

method for achieving constant inhibitor loadings.

A brief description of the apparatus and the results of an experimental evaluation of the novel method for

methane/air flames are presented.

Experimental

Experiments were conducted on an opposed-jet burner. Methane diluted with nitrogen was used as the fuel and a

mixture of oxygen and nitrogen as the oxidizer. By varying the methane and oxygen concentrations, a non-premixed flame

can be stabilized on either the oxidizer or fuel side of the stagnation plane. The burner was aligned vertically with the

lower tube used as the fuel source and the upper tube as the oxidizer source. A few experiments, conducted with the reverse

orientation to study the effect of buoyancy, found no significant change in extinction strain rate. The burner was

constructed from glass tubes 30 cm long with an ill of 0.98 cm, and a separation distance of 0.95 cm between opposing

nozzles. Annular sheath flows of nitrogen are provided through 2.22 cm ill glass tubes. The sheath tube exits were offset

by approximately I cm, upstream of the reactant tube exits, to minimize the impact of the sheath flow on the development

of the reactant flows. The entire burner is isolated in a glass enclosure for control of exhaust gases. This enclosure is

purged with nitrogen and maintained slightly below atmospheric pressure.

The T-shaped enclosure has a glass window on one end, approximately 25cm from the centerline of the flame. A

video camera was located a few centimeters from this window, perpendicular to the radial flame direction, and used to

measure flame position. Measurements of flame position were made on a television screen via a video camera with the

flame as close to extinction as possible. Flame position was measured relative to the oxidizer nozzle exit plane and several

cases were observed where the flame actually entered the nozzle. In this situation, the flame position was recorded as 0

mm. This measurement of flame position is used in verifying our novel method will be described below.

The flame inhibitor used during this investigation, dimethyl methylphosphonate (DMMP) [P(=O)(CH3)(OCH3hJ,

is a liquid at room temperature with a low vapor pressure (lessthan 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± I°C via active control of the sheath flow temperature.

Presented at the Fall 1997 Technical Meeting of the Eastern States Section,

The Combustion Institute, Hartford, CT, October 1997.
The Novel Method for Conducting Extinction Measurements

One practical method of delivering small, metered quantities of low vapor pressure 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 [5], or so that a momentum balance between the two streams is maintained [4]. These

methods of approaching extinction would result in variable concentrations of the inhibiting agent, due to the changing mass

flow of the doped 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 flow for sufficient

time for the system to reach equilibrium. Then extinction conditions were approached by varying only the undoped reactant

flow while maintaining a constant flow on the doped side. One of the consequences of using this method 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 measured extinction strain rate.

This investigation was performed for two different stoichiometric mixture fractions, Zst, which can be evaluated

from reactant compositions in the nozzles and the stoichiometry of the overall combustion reaction, using equation 1:

(1)

where Y is mass fraction, MW is molecular weight, v is the stoichiometric coefficient for complete combustion, the

subscripts 0 and F refer to oxygen and fuel respectively, and the subscripts :too refers to conditions at the fuel and oxidizer

nozzles. An undiluted methane/air flame has a Zst = 0.054 and lies on the oxidizer side of the stagnation plane. As one

increases Zst, the flame moves closer to the stagnation plane, until Zst = 0.5, where it lies on or near the plane itself. For

Zst > 0.5, the flame will lie on the fuel side. The experiments presented herein were for Zst = 0.054 and Zst = 0.5.

Results and Discussion

There. are two common techniques for determining the extinction strain rate in flame measurements: direct

measurement of the local strain rate using procedures 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 technique and will be

referred to as the global strain rate, aq. We use the following relation, proposed by Seshadri and Williams [10] based on a

plug flow model, for evaluating the global strain rate:

(2)

where L refers to the separation distance between the nozzles, V is the stream velocity and p is the stream density.

Equation 2 is for the strain rate at the stagnation plane of a non-reactive flow. This global strain rate is used as an

approximation to the local strain on the air side of the flame surface for flames near extinction. Modified expressions for

strain have been developed to include, for example, flame thickness [11]; however, equation 2 has the merit of simplicity.

Recent measurements [12] have revealed a proportionality between the global strain given by equation 2 and the local strain

measured by LDV. Although the constant of proportionality is presumably dependent upon burner design, this proportional

relationship makes this global strain formulation useful for determining inhibitor effectiveness.

Experiments were performed over a large range of fuel and oxidizer flows, achieving extinction with the flame in a

variety of positions. Figure I shows results from experiments with Zst = 0.054. These results indicate that the global

extinction strain rate remains fairly constant over a large range of flame positions, varying less than ±2% from a mean

value of 359 1/s in the observed region from >0 to 7mm. This region will be referred to as the acceptable region. For

experiments in which extinction occurs with the flame outside the acceptable region, large deviations in aq occur. We

attribute these deviations to effects of the nozzles on the flame and stagnation region. Further investigation of this effect

and the obvious asymmetry of the acceptable region was performed with Zst = 0.5.

When Zst = 0.054, the flame exists on the oxidizer side of the stagnation plane which introduces an inherent

asymmetry of the effect of the nozzles on the flame. Thus, if the flame is on the stagnation plane (i.e. Zst :::::0.5), this

asymmetry is eliminated The results of the Zst = 0.5 study are shown in Figure 2. Although there is greater scatter in the

data, which we attribute to lack of repeatability in oxygen concentration, the global extinction strain rate varies less than 3%

of the mean value of 336 1/s. The acceptable region for Zst = 0.5 does not appear to have the asymmetry observed for Zst =

0.054. This supports the hypothesis that as the flame and stagnation plane approach the nozzles, the flow field is affected.
The plateau of the acceptable region of Figure I slowly falls off starting around a flame location of 7mm. Other researchers

have found, for a Zst = 0.054, the flame lies approximately 2mm above the stagnation plane [13]. This would agree with

our results that at a flame location of 7mm, the stagnation plane is quite near the fuel nozzle.

As discussed earlier, for the typical case of Zst = 0.054, if extinction occurs in the acceptable region, the magnitUde

of <1q is consistent to within 2% of the result that would be obtained by maintaining a fixed flame position. A limited

number of experiments with DMMP-doped flames confirmed that the region in which <1q was invariant with flame position

was the same as that for the undoped case. The existence of this acceptable region establishes the validity of our method for

approaching extinction with a constant oxidizer flux and dopant concentration, provided that the flame position at

extinction lies within this region.

This novel method of approaching extinction does not necessarily require the cumbersome task of observing flame

location at extinction for all measurements. Rather, a parameter S is introduced which can be calculated from the reactant

stream properties at the nozzles. S approximates the distance from the oxidizer nozzle to the stagnation plane and is given

by the analytical expression in equation 3, again derived by Seshadri and Williams from the plug flow model [10]:

(3)

380 .s

.S ~.!:: 0:0: 360

340

r.f.)

o...'!: = ........ ..::1 _

.~X •.....• 320 ~.- 300

0:

.ga J

1

380

360

340

320

] 300 °

a

280

0123456789

Observed Flam: Location [nmfromOxNo:zzle]

280

0123456789

Observed Flam: Location [mnfromOxno:zzle]

Figure 1. Acceptable Region, for Zst = 0.054.

Global extinction strain approximately constant for

Observed Flame Location between 0 and 7 mm'

Figure 2. Acceptable Region, for Zst = 0.5.

Figure 3 shows a plot of observed flame location for both Zst = 0.5 and Zst = 0.054 against the parameter S. By noting that

the observed flame location, for both conditions, is a well correlated one-to-one function of the parameter S, one can map

the acceptable region in terms of S. For Zst = 0.0544, this region is 3.50mm < S < 5.25mm. Since S can be evaluated from

known flow rates, direct observation of the flame becomes unnecessary.

Summary

A novel method for measuring the effectiveness of low vapor pressure phase flame inhibitors with an opposed-jet

burner has been described. This method allows for a constant loading of the dopant by maintaining a constant flow in the

doped reactant stream, but the flame moves as extinction is approached. It has been shown that the extinction strain rate

remains constant to within 2% for a methane/air flame for a measured range of flame positions at extinction. The flame

positions are a one-to-one function of an easily calculated quantity, S: the distance from the oxidizer nozzle to the

stagnation plane.
5.5 6

II rlfi:50

4.5 5

o Zst. = 0.054

EI Zst. = 0.5

3.5 4

S, Distance to Stagnation Plane [mm from Oxidizer Nozzle]

(plug Flow model)

Figure 3. Variation of Observed Flame Position with the parameter S.

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-M0013. Additional support was provided by the Army Research Office. under contract number DAAL03-92-G-OI13. The

authors wish to thank 1. Fleming and B. Williams at Naval Research Laboratory for useful discussions. Further

appreciation goes to William McKeon for his experimental assistance.

References

1. Sheinson, RS., Penner-Hahn, I.E., and Indritz, D., "The Physical and Chemical Action of Fire Suppressants," Fire

Safety Journal 15, 437450 (1989).

2. Hastie, I.W. and Bonnell, D.W., "Molecular Chemistry ofInhibited Combustion Systems," Final Report No. NBSIR

80-2169; PB81-170375, (1980).

3. Kaizerman, J.A. and Tapscott, RE., "Advanced Streaming Agent Development, Volume III: Phosphorus Compounds,"

NMERI 96/5/32540 (1996).

4. Puri, I.K. and Seshadri, K., "Extinction of Diffusion Flames Burning Diluted Methane and Diluted Propane in Diluted

Air," Combustion and Flame 65,137-150 (1986).

5. Yang, G. and Kennedy, I.M., "The Effect of Strain on Laminar Diffusion Flames of CWorinated Hydrocarbons,"

Combustion and Flame 92, 187-196 (1993).

6. Friedman, R and Levy, I.B., "Inhibition of Opposed-jet Methane-Air Diffusion Flames. The Effects of Alkali Metal

Vapours and Organic Halides," Combustion and Flame 7, 195-201 (1963).

7. Williams, F.A., "A Review of Flame Extinction," Fire Safety Journal 3, 163-175 (1981).

8. Otsuka, Y. and Niioka, T., "The One-Dimensional Diffusion Flame in a Two-Dimensional Counter-Flow Burner,"

Combustion and Flame 21,163-176 (1973).

9. Linan, A., "The Asymptotic Structure of Counterflow Diffusion Flames for Large Activation Energies," Acta

Astronautica 1, 1007-1039 (1974).

10. Seshadri, K. and Williams, F., "Laminar Flow Between Parallel Plates with Injection of a Reactant at High Reynolds

Number," International Journal of Heat and Mass Transfer 21,251-253 (1978).

11. Kim, 1.S., Libby, P.A., Williams, F.A., "On the displacement effects of laminar flames," Combustion Science and

Technology 87, 1-25 (1993).

12. Fisher, E.M., Williams, B.A., Fleming, I.W., "Determination of the Strain in Counterflow Diffusion Flames from Flow

Conditions," submitted to Eastern States Section of the Combustion Institute (1997).

13. Puri, I.K., Seshadri, K., Smooke, M.D., and Keyes, D.E., "A Comparison Betweeen Numerical Calculations and

Experimental Measurements of the Structure of a Counterflow Methane-Air Diffusion Flame," Combustion Science

and Technology 56, 1-22 (1987).