Combustion and Flame 141 (2005) 308–321

www.elsevier.com/locate/combustflame

Flame suppression by aerosols derived from aqueous

solutions containing phosphorus

T.M. Jayaweera a,1, E.M. Fisher a,∗, James W. Fleming b

a Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, USA

b Combustion Dynamics Section, Naval Research Laboratory, Washington, DC 20375, USA

Received 5 September 2003; received in revised form 28 October 2004; accepted 28 October 2004

Available online 3 March 2005

Abstract

Aerosols derived from aqueous solutions containing phosphorus are investigated as possible alternatives to

halon-based flame suppressants. Phosphorus-containing compounds (PCCs) have been shown to be effective flame

suppressants in the gas phase; a water-based solution would provide a practical means of delivering a condensedphase PCC to the flame. Flame suppression was characterized by measuring the global extinction strain rate of

a counterflow, nonpremixed methane–air flame with and without a PCC additive. An aqueous solution of the

additive was introduced as an aerosol into a heated chamber upstream of the air flow tube. A high-efficiency

nebulizer produced a polydisperse spray of droplets with a Sauter mean diameter of 8 μm, as measured with a

phase-Doppler particle analyzer. The droplet size distribution was nearly independent of the composition and flow

rate of the liquid. Externally maintaining the reactant streams at 360 K allowed the water in the aerosol to evaporate prior to reaching the flame. Evaporation of water leaves behind residual particles for solutes that are solids

at 360 K, and thus solid particulates, not droplets of solution, enter the flame zone. Comparisons of different solutions with different phase solutes were used to provide an estimate of the physical effect of particles on the flame.

Three neat phosphorus-containing compounds, dimethylmethylphosphonate (DMMP), diethylmethylphoshonate

(DEMP) and dimethylphosphonate (DMP) and five 1.6% molar aqueous solutions, orthophosphoric acid (OPA),

phosphorus acid, phosphonic acid, methylphosphonic acid, and dimethylmethylphosphonate (DMMP), were investigated. The acid solutions (with solid solutes) all displayed similar effectiveness, and were all slightly more

effective, on a per mole basis, than the DMMP solution (a liquid solute). The effectiveness of a DMMP/water

solution is found to be a weighted average of the effectiveness of its constituents. Per mole of water delivered,

a 1.6% molar solution of a PCC is approximately twice as effective in fire suppression compared to neat water

vapor. Flame modeling calculations for the extinction condition with added gas-phase phosphorus compounds

using a published phosphorus reaction mechanism grossly underpredict the total effectiveness of the compounds

investigated.

 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

* Corresponding author. Fax: +1(607)255-9410.

E-mail address: emf4@cornell.edu (E.M. Fisher).

1 Present address: FM Global, 1151 Boston-Providence Tpke, Norwood, MA, USA.

0010-2180/$ – see front matter  2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

doi:10.1016/j.combustflame.2004.10.013
T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321 309

1. Introduction

With the restrictions on the manufacture and use

of CF3Br (halon 1301), a common fire suppressant,

due to its high ozone-depletion potential, there has

been widespread interest in finding a viable substitute

[1–4]. A search for a drop-in replacement has not yet

been successful, and as an alternative, liquid- or solidphase agents are currently being considered. Delivering the agent as an aerosol broadens the range of potential agents available, but also makes it more difficult to predict flame suppression effectiveness. Aside

from gas-phase chemical and physical effects, which

occur after vaporization of the aerosol, these agents

may exhibit increased flame suppression because of

various phenomena related to the presence of a second

phase. These phenomena include enhanced radiative

heat transfer, enthalpy of vaporization, and heterogeneous reactions involving flame radicals [5–7]. Particle or droplet delivery also introduces uncertainty

in the amount of agent present in the gas phase in

the flame. Gas-phase loading in the flame can be reduced from the amount of condensed-phase agent in

the air stream because of insufficient time for vaporization in the flame, and can be either reduced

or increased when aerosol trajectories differ from air

streamlines [8]. Differences in the location in the

flame where the agent reaches the gas phase and possible nonuniform loading can also affect flame suppression effectiveness. In this work, a comparison is

made between additives introduced as aqueous solutions leaving residual particles which then enter the

flame and those that completely vaporize in the heated

air stream. The experiments are designed to minimize

several of the sources of uncertainty in agent loading

described above.

Most of the work with solid-phase additives has

focused on NaHCO3 [9–12] or KHCO3 [13]. These

studies have all shown these powders to be more effective than CF3Br at fire suppression. The effectiveness is highly dependent on the size of the particles,

with smaller particles having a greater effect than

larger ones on a per mass basis. This difference is

attributed to the large particles not being completely

consumed in the flame zone. The high level of effectiveness from these small particles is believed to be

due, at least in part, to a chemical effect of the agent

in the gas phase [5,12].

In the liquid phase, water mists have been gaining in popularity, due to their relatively high level

of effectiveness and zero toxicity [8,14–17]. A water mist has been shown to be more effective, on a

per mass basis, than CF3Br in nonpremixed counterflow flames [17]. However, water acts primarily as

a physical agent in suppressing the flame. A variation of the traditional water mist is an “enhanced”

water mist: a water solution containing a compound

that, through effects on flame chemistry, increases the

fire suppression effectiveness over that of pure water

[18–20]. A concern with adding a chemical agent is

that toxicity of the solution will be higher than of pure

water. However, as the chemical agent’s effectiveness

is supplemented by that of the water accompanying it

in an enhanced water mist, its required concentration

may be acceptably low from the standpoint of toxicity.

A few experimental studies have been done with

enhanced water mists, namely water solutions of

NaOH or NaCl [19–21]. These show enhancements of

flame suppression compared to pure water. For example, a water mist enhanced with 18% NaOH by mass

was shown to be several times more effective than

pure water in an opposed-jet, nonpremixed methane–

air flame [21]. The mechanism by which an enhanced

water mist results in improved suppression over neat

water is believed to be due to several factors. In addition to the physical effect of the water, there is likely

a chemical effect by the solute, and possibly also a

physical effect from the droplets or residual particles [19,21]. The relative magnitudes of these effects

have not been determined. In our study, the flame suppression effectiveness of aerosols derived from water

solutions containing phosphorus was investigated experimentally, and contributions of the chemical and

physical effects were evaluated.

Phosphorus compounds have been studied previously in the gas phase and shown to be highly effective flame suppressants [22–25]. Since low vapor

pressure hinders the introduction of many phosphorus-containing compounds (PCCs) in the gas phase,

their use in practical fire fighting situations is limited. However, experimental and numerical studies [22,26–28] have shown that the effectiveness of

different gas-phase PCCs is similar, expanding the

range of PCCs available. This similarity of effectiveness is supported by the proposed mechanism by

which PCCs suppress the flame: they are converted

to phosphorus-containing radicals, such as HOPO

and HOPO2, which catalyze the recombination of

the important combustion radicals, H, OH, and O

[23,25,29]. This process slows the overall reaction

rate and thus suppresses the flame. Earlier studies

used laser-induced fluorescence to measure OH radical levels in a nonpremixed flame with and without

dimethylmethylphosphonate [24,30]. These measurements show a decrease in the OH level with the

addition of this PCC, supporting the hypothesis of

catalytic radical recombination. With the inhibition

resulting primarily from the phosphorus atom, the use

of PCC solutions, as enhanced water mists, is a possible means of getting the phosphorus into the flame.
310 T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321

In the current study, a nonpremixed methane–air

flame was used to investigate the effectiveness of several PCCs. These compounds are delivered into the air

stream in the vapor phase, as a fine mist of neat liquid,

or as a fine mist of aqueous solution. Neat liquids and

aqueous solutions involving liquid solutes, such as the

PCC dimethylmethylphosphonate (DMMP), are expected to evaporate fully before the flame. For other

compounds introduced as droplets of the aqueous solutions with solid solutes, a small residual particle

is expected to remain upon evaporation of the water

from the solution droplet. Assuming all of the phosphorus compounds have similar chemical fire suppression effectiveness once they are in the gas phase,

fire suppression effects due to the particle phase can

be investigated.

Experiments were performed using an opposedjet burner apparatus [31]. This configuration has been

used extensively with experimental, numerical, and

analytical techniques to evaluate the performance of

flame suppressants [4,32,33]. The counterflow configuration is useful in studying flame-suppressing additives as the flame is thermally isolated and quasione-dimensional along the centerline [34,35]. Flame

strength can be characterized by the global strain rate

at extinction [36,37].

Extinction calculations were also performed using

OPPDIF [38], part of the Chemkin software package [39]. These calculations used a proposed phosphorus mechanism and investigated the gas-phase

suppression effect of three phosphorus compounds.

These calculations were done to further elucidate differences in the suppression effectiveness observed between PCCs delivered in the gas phase and those in

the particle phase.

2. Experimental method

The flame suppression effectiveness of several

PCCs, introduced into the burner system either in the

vapor phase or as droplets of neat liquid or aqueous

solution, was investigated. The compounds are listed

with their CAS number and chemical structure in

Table 1, along with the form in which they were introduced into the burner system. The compounds studied

were: distilled water, dimethylmethylphosphonate,

diethylmethylphosphonate (DEMP), dimethylphosphonate (DMP), orthophosphoric acid (OPA), methylphosphonic acid (MPA), phosphonic acid, and phosphorous acid. DMMP, DEMP, DMP (all 97% pure)

and MPA (98% pure) were supplied by Aldrich

Chemical Company, phosphonic and phosphorous

acids (each 97% pure) were supplied by Alfa Aesar, and OPA was supplied as a premixed aqueous

solution from Aqua Solutions. Each of the phosphorus acids was in a 1.6% molar solution in distilled

water, and DMMP was introduced both in a 1.6%

aqueous solution and as a neat compound, in separate

experiments. As shown below, essentially all water

and DMMP evaporate before the oxidizer mixture

leaves the burner nozzle. In the case of the acid solutions, a solid residual particle remains, while for

DMMP and water, the mixture entering the flame is

entirely gas phase. The acid compounds were chosen

because they have been seen, through in situ sampling and GC/MS analysis [40], to be decomposition

products of DMMP, a PCC shown to be an effective

flame suppressant [22,23]. These compounds are also

attractive because of their low heating value and because, unlike many PCCs, they are not believed to be

neurotoxic.

For compounds introduced in the liquid phase,

the experimental apparatus consists of a nonpremixed

counterflow burner equipped with a system for adding

the liquid agent into the oxidizer stream: a nebulizer

mounted in a large chamber, leading into temperaturecontrolled tubing. The apparatus is shown in Fig. 1.

Its key features are small droplet size, large residence time of the reactant stream under temperaturecontrolled conditions, and the availability of an accurate procedure for measuring losses to surfaces

within the feed system. These features lead to a wellcharacterized state of the reactant mixture at the exit

of the burner tube, and well-defined gas-phase loading of agent at the flame, as documented below. In

Table 1

Compounds used

Compound Form CAS number Molecular formula

Orthophosphoric acid (OPA) Aqueous solution 7664-38-2 P(=O)(OH)3

Phosphorous acid Aqueous solution 10294-56-1 P(OH)3

Phosphonic acid Aqueous solution 13598-36-2 P(=O)(H)(OH)2

Methylphosphonic acid Aqueous solution 993-13-5 P(=O)(CH3)(OH)2

Distilled water Neat liquid, vapor 7732-18-5 H2O

Dimethylmethylphosphonate (DMMP) Neat liquid, vapor, 756-79-6 P(=O)(CH3)(OCH3)2

aqueous solution

Diethylmethylphosphonate (DEMP) Neat vapor 683-08-9 P(=O)(CH3)(OCH2CH3)2

Dimethylphosphate (DMP) Neat vapor 813-78-5 P(=O)(OCH)2(OH)
T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321 311

Fig. 1. Schematic of experimental apparatus with opposed-jet burner, droplet generator, and chamber.

the case of additives introduced in the vapor phase,

the nebulizer and chamber are replaced by a syringe

pump and further length of heated tubing, as previously described [22].

2.1. Burner

The flame is produced approximately 2 mm on

the oxidizer side of the stagnation plane of counterflowing streams of fuel and oxidizer. This opposed-jet

burner geometry has been detailed previously [22].

As in previous experiments, the reactant nozzles are

straight glass tubes (0.98 cm i.d.) with a separation

distance of 0.95 cm. Methane (99% pure) fuel flows

from the lower nozzle and the oxidizer, a primary

standard mixture of 21 ± 0.2% O2 with N2 balance,

flows through the upper nozzle. All gases were supplied by MG industries. The temperature of the reactant streams 10 cm upstream from the exit of the

nozzles are actively maintained at 360 ± 1 K.

The relative flame strength is characterized by the

global strain rate at extinction, calculated using the

following expression:

aq = (1)

2VO

L



1 +

VF

VO

 ρF

ρO

1/2

.

In Eq. (1), L refers to the separation distance between the nozzles, V is the average stream velocity

at extinction, ρ is the stream density, and the subscripts O and F refer to oxidizer and fuel, respectively.

Plug flow boundary conditions at the nozzle exit are

assumed. This expression, derived by Seshadri and

Williams [41], is referred to as the global extinction

strain rate. To determine suppression effectiveness of

the added compounds, the global extinction strain

rate was measured as a function of dopant loading.

The resulting extinction strain rates aq are normalized by the undoped value, aq0 , which was found to

be 340 s−1 in the absence of the heated chamber described in the next section and 322 s−1 when the

chamber was present (each with a day-to-day variation of less than 4%). Although this difference in

absolute extinction strain rate is significant, the normalized extinction strain rates obtained with the two

configurations were not significantly different.
312 T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321

The extinction strain rate is determined at a fixed

nozzle separation, L. The reactant velocities are increased until the critical strain rate is achieved and

the flame is extinguished. In most studies [4,42], both

the fuel and the oxidizer velocities are increased proportionally such that the flame or stagnation plane

is maintained near the center of the burner. Because

the addition of a liquid-phase dopant is via a massflow syringe pump, changing the doped reactant flow

rate would change the dopant loading, leading to transients of a fairly long time scale (several minutes).

This difficulty is circumvented by performing extinction measurements holding the doped reactant (oxidizer) stream fixed, and only varying the undoped

(fuel) stream flow rate. The flame and stagnation

plane move in this method, but for a range of known

flame positions, the global extinction strain rate was

found to agree within ±2% [22]. A validation of

this technique with residual particles present was performed for the current study.

2.2. Generation and characterization of droplet mist

A small glass nebulizer is used to deliver the

liquid-phase dopant to the oxidizer stream as a fine

mist of droplets. A high-efficiency nebulizer (HEN),

a Meinhard® nebulizer, is detailed in Fig. 2. Liquid is

introduced via a programmable syringe pump. Part of

the oxidizer stream is supplied as the nebulizing gas

at a flow rate of 1.00 SLM. This flow rate allows for

sufficient aerodynamic breakup of the liquid stream

into a fine mist. The HEN is mounted at the top of a

large heated chamber (15 cm i.d., 18 cm long) located

approximately 75 cm upstream of the flame. Unfortunately, the presence of the chamber induced flow

fluctuations that perturbed the flame. To dampen these

fluctuations, a small diameter (3.2 mm i.d., 7.6 cm

long) tube was placed between the chamber and the

reactant flow tube.

In order to make valid comparisons between different compounds at several loadings, it is important

Fig. 2. Schematic of the high-efficiency nebulizer (HEN)

droplet generator, provided by Meinhard and Associates.

for the droplets produced in each case to be similar. The droplets should also be small enough so that

they will completely evaporate prior to reaching the

flame. Measurements of droplet diameters were performed with a phase-Doppler particle anemometer

(PDPA, Dantec FlowLite Fiber PDA). The HEN was

not mounted in the burner apparatus for the PDPA

measurements; rather, it was mounted in an enclosed

chamber with good optical access. Droplet diameters

were measured for a variety of concentrations and

flow rates of phosphorous acid, as well as for a single set of conditions for water, DMMP, OPA, MPA,

and phosphonic acid. PDPA measurements were performed 1.3 cm downstream of the HEN tip, with 1.00

SLM of nebulizing gas. Typical histograms for two

different cases are shown in Figs. 3a and 3b; the uncertainty of the PDPA measurement is ±2 μm for the

optical configuration and processor settings used. As

can be seen in the histogram, all of the droplets are

(a)

(b)

Fig. 3. PDPA measured size histogram for 2000 droplets of

two different PCCs, measured 1.3 cm downstream of HEN

exit. The liquid flow rate for both compounds was 25 μl/min

with 1.00 SLM of nebulizing air. (a) Neat DMMP (b) 1.6%

(molar) aqueous solution of OPA.
T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321 313

Table 2

The droplet diameter size distributions at 25 μl/min obtained

from PDPA measurements fit to a log-normal distribution

Compound A (dimensionless) x0 (μm) α (μm)

Neat H2O 162.9 1.625 0.688

7.5% P(OH)3 216.8 1.844 0.514

7.5% MPA 235.0 1.833 0.469

7.5% H3PO3 216.8 1.844 0.514

4.3% DMMP 169.4 1.611 0.662

1.6% OPA 170.6 1.742 0.662

Neat DMMP 213.9 1.566 0.589

Note. The parameters given fit the form Aexp(−(x − x0)2/

α2), and were evaluated using a least-squares method.

Fig. 4. PDPA measured Sauter mean diameter, 1.3 cm downstream of HEN exit, as a function of liquid flow rate. Under

conditions in this paper for the 7.5% phosphorous acid solution, 25 μl/min corresponds to a total loading of 1.49%.

The various compounds tested were 7.5% phosphorous acid,

1.6% orthophosphoric acid, 7.5% phosphonic acid, 7.5%

methylphosphonic acid, neat DMMP, 4.3% DMMP, and neat

H2O. All concentrations are molar based in an aqueous solution. Error bars represent one standard deviation in the

diameter measurement of 2000 droplets.

less than 20 μm in diameter and the size distributions for the two solutions are very similar. Size distributions were fit to a log-normal distribution using

an equation of the form Aexp(−(x − x0)2/α2). The

fit parameters were determined using a least-squares

method and are given in Table 2 for all the compounds

tested. The Sauter mean diameters from all the different cases are plotted in Fig. 4. The error bars represent

one standard deviation of the droplet size determination.

2.3. Evolution of droplet mist

For droplets of neat liquids and DMMP/water solutions, complete evaporation upstream of the exit

of the oxidizer nozzle is ensured by high temperatures and long residence times. The chamber is heated

electrically, while the temperature of the gas in the

burner tube is actively controlled to 360 ± 1 K by

electric heating of the surrounding sheath gas. The

average residence time of the droplets is several seconds, while a simple d2 analysis [43] of the evaporation rate for the maximum droplet size predicts

that pure DMMP or water droplets should evaporate

entirely within a few milliseconds within the chamber. To confirm complete evaporation, a He–Ne laser

beam was passed through the reactant stream just below the exit plane of the oxidizer nozzle, and the

off-axis forward direction was scanned by eye for

scattered light. For neat solutions, there was no observable scattered light, consistent with the complete

evaporation of the liquid at this point.

For droplets of phosphorus acid solutions, which

should produce residual solid particles upon evaporation of water, little quantitative information is available on evaporation rates. However, it is expected that

these evaporation times are comparable to those for

neat liquids, and that the much longer residence times

upstream of the burner nozzle are sufficient to achieve

phase equilibrium between the residual particle and

the surrounding gas stream. Whether the compounds

under consideration exhibit deliquescence (producing

dry residual particles) or retain some water even at

low relative humidity is not known. As calculated

from the reactant flow rates, the relative humidity of

the air stream is only 2.4% under burner exit conditions, and thus the amount of water retained in the

particle at equilibrium is likely to be small if not zero.

For all compounds, active temperature control of

the reactant streams is crucial. The agent-doped oxidizer stream is heated externally to maintain its temperature at 360 K. The enthalpy of vaporization of

the water is not supplied by the flame but from external sources, and thus does not contribute to the flame

suppression process for the liquid droplets in these

experiments. The apparatus is designed to produce a

well-defined state for the oxidizer stream at the nozzle

exit: phase equilibrium at 360 K, with all phases traveling at the same velocity. This well-defined boundary

condition at the nozzle exit is highly desirable for

computational modeling.

When residual particles are present, their ability

to follow gas streamlines in a decelerating flow is size

dependent. Size also determines whether in-flame vaporization is fast enough to release the entire mass of

the added agent into the gas phase. Clearly, small particles are desirable to produce well-defined gas-phase

agent loadings for both of these reasons. The residual

particle diameter can be estimated from the measured

droplet size distribution by assuming that each droplet

loses all of its water and becomes a spherical particle with density equal to that of the pure solid PCC.

With this assumption, evaporation of the drops for

these aerosols yields a Sauter mean particle diameter
314 T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321

of ∼3 μm and a maximum particle diameter of 7 μm.

Although rapid heating of solution droplets has been

shown to produce highly nonspherical residual particles [44], the assumption of a spherical particle gives

conservative results for both drag and vaporization

rates. The current estimate of particle size will not

be conservative, however, if significant coalescence

of droplets occurs downstream of where the droplet

size distribution was measured.

Stokes numbers represent the ratio of particle response time to flow residence time. At typical extinction strain rates in this burner, Stokes numbers for the

estimated mean and maximum particle size for 1.6%

OPA solution are calculated to be 0.02 and 0.1, respectively. These low values indicate that particles are

able to follow the gas streamlines, avoiding the complications of nonuniform loading described by other

investigators [8]. The estimated particle sizes are well

below 30 μm, the diameter for critical damping for

our flow conditions, determined using the expression

developed by Li et al. for estimating critical damping

in this flow field configuration [45]. Thus the particles

should not oscillate in the flowfield.

Another indication of how well the particles follow the gas streamlines is obtained from a comparison of the drag force (assuming Stokes flow) with

either the gravitational force or the thermophoretic

force. The terminal velocity for 8-μm particles (resulting from 20-μm droplets), relative to the gas flow, is

of the order of 1.5 mm/s, and for 3-μm particles it

is 0.2 mm/s. A thermophoretic velocity can similarly

be calculated, by equating the thermophoretic force to

the drag force. This velocity is constant for small particles with a Knudsen number greater than ∼1 [46].

At smaller Knudsen numbers, the thermophoretic velocity decreases and becomes dependent on the thermal conductivity of the particle. Thus, using a small

particle that satisfies the Knudsen number constraint

gives a conservative estimate of the thermophoretic

velocity. The thermophoretic velocity at the location

of the maximum temperature gradient can be derived using temperature and velocity profiles from

calculations performed under similar conditions [30].

This velocity, for a 0.1-μm particle, was found to

be 0.145 m/s, or about 25% of the local gas velocity. Although this relative value appears significant,

its effect on particle trajectories and dopant delivery

is small because thermophoresis is significant only

when particles are very small, i.e., near the end of

the vaporization process. The magnitude of this effect

can be seen for NaCl particles (for which the physical

properties are well known). Thermophoresis is calculated to shift the location of complete evaporation of

a 0.1-μm NaCl particle upstream by only about 2 μm.

This shift is negligible when compared to a calculated

flame FWHM of around 1.7 mm. Thus, neither gravitational nor thermophoretic forces should strongly

influence the particle trajectories for the flame conditions reported here.

Viewing of scattered laser light, as described previously, provides information about the completeness

of particle vaporization in the flame. Experiments

were performed with a mist of 1.6% OPA introduced

by the HEN, with a strain rate 10% below the extinction value. Scattered light was observed when the

laser beam was positioned just below the exit plane

of the oxidizer nozzle confirming the ability to detect residual particles. No scattered light was observed

when the laser was positioned below the flame region, indicating that the particles are small enough to

be consumed in the flame. At this position, care was

taken to ensure that the laser beam was between the

stagnation plane and the flame: scattered light was observed in this region at the same reactant flow rates,

but with no flame present.

2.4. Wall losses

The chamber housing the HEN was designed to

minimize wall losses. The liquid spray exits the HEN

in a small-angle cone (∼15◦ half-angle), and enters a

large chamber that has a supplemental oxidizer stream

entering near the edge. The use of this chamber did

not prevent all wall losses, however. For the solutions,

some droplets or residual particles were still lost to

the walls. To quantify the amount of acid PCC lost,

the following rinsing procedure was used. A known

amount of OPA, in the form of a 1.6% water solution,

was sprayed into the apparatus under the same conditions used for the extinction experiments. The chamber and all tubing downstream were then rinsed with a

known amount of distilled water. The pH of the wash

was measured using an Accumet pH probe, and the

concentration of OPA was determined from its dissociation constant. The amount of acid recovered was

compared to the amount entering the HEN, and losses

were calculated to determine the net amount delivered

to the flame. Several different air and liquid flow rates

were tested. PCC losses were found to be 13.8±0.6%

of the initial amount delivered. This correction, measured with OPA, is applied to all aqueous solutions

of phosphorus acids. Since the initial droplet distributions for all compounds producing residual particles

are very similar, the assumption of the same loss is

reasonable. It is assumed that losses occur only for

the acid PCC, and that any accompanying water that

impinges on the walls is subsequently evaporated.

For liquids not producing residual particles, zero

losses are assumed on the basis of two observations.

First, as observed previously for vapor-phase DMMP

delivery [22], no trend in extinction measurements

was observed as a function of time, suggesting that
T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321 315

wall adsorption/desorption processes were negligible or had reached steady state. Secondly, when the

DMMP flow was abruptly shut off, the flame rapidly

returned to its undoped blue color on a time scale consistent with the average gas-phase residence time in

the heated chamber. Wall deposits, if they had existed,

would be expected to vaporize in the presence of a

lower gas-stream dopant concentration and delay the

return of the undoped appearance of the flame. In previous experiments with vapor-phase DMMP and no

heated chamber [22] the return of the blue color had

been virtually instantaneous. Consistency of DMMP

extinction results obtained with the two systems, reported under Section 3, provides further evidence that

wall losses are negligible in the HEN system.

2.5. Calculations

Extinction calculations were performed using

the OPPDIF code, from the Chemkin II suite [38].

OPPDIF is not designed to incorporate multiphase reactants, such as particle-phase dopants. There is no

means of calculating radiative losses or effects due to

surface chemistry. Nor is there a means of accounting for differences between particle and gas velocities. Therefore, OPPDIF is used in these studies to

calculate the differences in effectiveness between different phosphorus-containing compounds based only

on their gas-phase effects. Computationally, all compounds are treated as being in the gas phase, regardless of the phase in which they exist in the experiment.

The phosphorus mechanism is employed by Glaude et al. [47] and was developed for DMMP combustion. OPA and phosphorous acid are also included

in this mechanism. The GRI-Mech 3.0 [48], excluding nitrogen chemistry, was used for the hydrocarbon combustion. Mixture-averaged diffusion velocities were used, and thermal diffusion was neglected.

A potential flow boundary condition with a centerline

axial velocity of twice the measured volume averaged velocity was used. This set of velocity boundary

conditions was determined by MacDonald et al. [30]

to best match experimental results of [OH] measurements with this particular burner.

The extinction calculations were performed in a

manner similar to that described by Pitts et al. [49].

That is, extinction was approached with small incremental steps in flow rates of the fuel and oxidizer (of

at most 0.01 SLM for the fuel). The flow rates were

changed such that the position of the flame remained

constant. Subsequent calculations with a higher velocity were restarted from previous solutions to decrease computational time. The extinction strain rate

was determined to be the highest strain rate at which a

solution was found. For all the calculations, 250 ppm

of PCC and/or 1.54% H2O was added.

3. Results

Extinction measurements were performed for

H2O and DMMP as neat compounds. Fig. 5 presents

the normalized global extinction strain rate obtained

when the dopant enters the burner system as a liquid

spray, comparing it to results when the dopant enters

as a vapor. The loadings are calculated, for all cases,

assuming the dopant is completely evaporated. For

the vapor-phase tests, the dopants were added, via

syringe pump, into a heated reactant line upstream

of the oxidizer flow tube. The vapor-phase tests for

DMMP were performed and reported earlier [22]. The

same additive loadings were investigated here with

the dopant introduced into the burner system in the

liquid phase, using the HEN. The absence of scattered

laser light indicated complete evaporation of the H2O

and DMMP droplets. Fig. 5 demonstrates that the normalized global extinction strain rate is independent of

the initial phase of the additive. The agreement between extinction measurements for the two methods

of dopant introduction is consistent with complete

evaporation of the neat liquid droplets with the enthalpy of vaporization supplied externally rather than

from the exothermicity of the combustion reaction.

The suppression effectiveness of H2O vapor determined here can be compared to literature values

obtained with reactants at a slightly lower temperature. Lentati et al. numerically investigated the effect of various concentrations of water vapor added

to a nonpremixed methane–air flame with reactants

at 300 K [8]. A roughly linear behavior is predicted

with water addition, with a 25% reduction in extinction strain rate by a saturated air stream at 300 K

Fig. 5. Normalized global extinction strain rate of neat

DMMP and H2O, as a function of dopant loading. The

dopant was introduced either as a spray via the HEN or in

the vapor phase upstream of the oxidizer flow tube. Loadings are given as the mole fraction of dopant in the oxidizer

stream assuming complete vaporization.
316 T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321

Fig. 6. Comparison of normalized global extinction strain

rate as a function of H2O loading (assuming complete vaporization) due to addition of neat H2O and a 1.6% (molar)

aqueous solution of OPA. Error bars on OPA data represent

one standard deviation due to scatter in extinction measurements (δyi) and uncertainty in loading (δxi).

(water loading 3.51%). Experiments have also been

performed by Lazzarini et al. [21] with saturated water vapor at the same temperature, finding only a 12%

reduction in extinction strain rate. Our experimental

results, shown in Fig. 5 for 4 to 40% of the amount

of water in a saturated air stream at 300 K, indicate that the extinction strain rate is linear over this

range. These results are in reasonable agreement with

Lentati’s calculations over the range in which they

overlap. It should be noted, however, that the present

measurements were made with the reactants at 360 K,

while the numerical predictions assume 300 K. A linear extrapolation of the current experimental results

to 3.51% water loading gives a 24% reduction in extinction strain rate, which agrees well with Lentati’s

calculated value but it is twice Lazzarini’s measured

value.

Experiments were performed comparing H2O to

an aqueous solution of 1.6% OPA. The results are

given in Fig. 6, with the normalized extinction strain

rate plotted as a function of molar loading of the H2O

only. For the case of the PCC, the H2O loading is

calculated by subtracting the amount of phosphorus

compound (1.6%) from the total dopant loading. It

also assumes that all of the water is delivered to the

flame. Error bars in the OPA data set represent uncertainties of one standard deviation. The y-error bars,

between 1 and 2%, are an appropriate combination

of uncertainties in aq and aq0 , each of which is found

empirically as the standard deviation of eight repeated

measurements. The x-error bars, between 1 and 5%,

are derived from the uncertainties of flow rates of

dopant and oxidant, and from the standard deviation

of the measured dopant loss rate.

The absolute value of the slope of Fig. 6 represents the effectiveness of the suppressant [50,51]. The

uncertainty in slope can be calculated from the uncertainties in the measured quantities contributing to

it, using standard methods [52]. Two types of uncertainty must be considered: those affecting each data

point randomly, and those affecting all data points in

a given set in the same way. The truly random uncertainty contributions are responsible for the scatter in a

given data set, and are described above and displayed

as error bars in Fig. 6. For the purposes of calculating a slope, uncertainties in the x coordinate (loading) were converted to an equivalent y uncertainty, as

recommended by Bevington [52]. Then the random

uncertainties in the individual data points were combined, contributing an uncertainty of at most 1.6% to

the slope. In addition to this uncertainty it is important

to include the uncertainty in the solute concentration

(1.8%), which has the same fractional effect on the

loadings of all data points obtained from a given batch

of aqueous solution. When these errors are included,

the uncertainty in the slope is still under 2.5% for all

cases presented here. This calculated uncertainty was

verified by comparing the slopes for two sets of extinction measurements for OPA obtained with different solute mixtures; the two slopes agreed within the

calculated uncertainty. Not included in this analysis is

the error in extinction strain rate due to the method

by which extinction is approached (±2%; see Section 2.1). Because of the choice of conditions, this

error is nonrandom and has the same value at a given

strain rate, regardless of the choice of suppressant.

Thus it has no effect on the relative effectiveness of

the different suppressants.

A linear regression on the slopes in Fig. 6 with

a fixed y-intercept indicates that 1.6% of phosphorus approximately doubles the effectiveness of pure

H2O vapor, per mole of H2O delivered. Thus, an

enhanced water mist can substantially improve the

suppression performance over that of pure water vapor, without introducing large quantities of a chemical substance. This result has also been observed by

other researchers who have investigated the suppression effectiveness of enhanced water mists doped with

sodium-containing compounds [18–20,53].

To test whether the chemical structure of the parent phosphorus compound is important, several PCCs

were introduced to the flame and suppression effectiveness was compared. The compounds tested were

three neat phosphorus compounds (DMMP, DEMP,

DMP), and five 1.6% (molar) aqueous solutions

(DMMP, OPA, MPA, phosphorous acid, and phosphonic acid). The neat compounds were introduced

using a method described earlier [22] and the aqueous solutions were introduced into the burner system

via the HEN. Flame extinction results are given in
T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321 317

Fig. 7. Comparison of normalized global extinction strain

rate as a function of phosphorus loading for several different

phosphorus-containing compounds, namely trimethylphosphonate (TMP), dimethylmethylphosphonate (DMMP), dimethylphosphate (DMP), and diethylmethylphosphonate

(DEMP). These compounds are all introduced in the neat

form, and enter the flame in the vapor phase. TMP and

DMMP results are from [22] and are included here for comparison.

Fig. 8. Comparison of normalized global extinction strain

rate as a function of phosphorus loading for several different

phosphorus-containing compounds. DMMP was introduced

as a neat compound via the HEN. All other compounds,

including 1.6% (molar) aqueous solutions of DMMP, orthophosphoric acid, phosphorous acid, phosphonic acid, and

methylphosphonic acid, are introduced into the burner system via the HEN. The phosphorus loadings for the acid solutions have been corrected for wall losses.

Figs. 7 and 8, for the compounds introduced into the

burner system as neat liquids and aqueous solutions,

respectively. The extinction results are plotted versus the “phosphorus loading,” or the mole fraction

of phosphorus-containing molecules in the oxidizer

stream expressed in ppm assuming complete vaporization of the additive. Since each PCC molecule

Table 3

Global extinction strain rate reduction by the compounds

tested: slope of normalized extinction strain rate vs agent

loading

Compound Slope

(ppm−1)

Corrected slope

(ppm−1)

Neat

DMMP 223 267

TMP 269 311

DMP 247 278

DEMP 204 255

1.6% aqueous solution

Orthophosphoric acid (OPA) 997 996

Phosphorous acid 835 840

Phosphonic acid 814 818

Methylphosphonic acid 792 808

Dimethylmethylphosphonate

(DMMP)

654 697

Note. The uncertainty in the slope is 2.5% (see text). The

TMP data were reported in an earlier paper [22] and are

included here for reference. Values for the corrected slope

are derived by considering the heating value of the indicated

compounds (see text).

contains a single phosphorus atom, these loadings

represent the number of moles of phosphorus atoms

per total moles in the oxidant mixture. Neat DMMP

is included with the aqueous solutions for comparison. For the DMMP/water solution, the phosphorus

loading is the mole fraction of DMMP in the oxidizer

stream, assuming total vaporization of DMMP and

water. Here, as well as for all of the neat compounds,

zero wall losses are assumed. For the phosphorus

acid solutions, the phosphorus loading is calculated

as for the DMMP/water solution, and then reduced by

13.8% to correct for wall losses. (See Section 2.4.)

Fig. 7 shows that the neat phosphorus compounds

are all similarly effective. There is total scatter between the different compounds of about 20%. The

results from this study, indicating that the form of the

parent compound is relatively unimportant in flame

suppression, are used when examining the effect of

aqueous solutions containing phosphorus. The slopes

for the curves in Fig. 7 are given in Table 3.

The results in Fig. 8 fall into two groupings:

the neat substance, DMMP, and the substances introduced into the burner system as water solutions.

Compared to the neat DMMP, the water solutions

have steeper slopes, implying higher flame suppression effectiveness per atom of added phosphorus. This

difference is due to the flame suppression contribution from the water in the solution. The results for

the 1.6% solution of DMMP can be used to assess

whether DMMP and water have additive effects. Linear regressions of the data in Fig. 5 give values of

the slopes of the normalized extinction strain rate vs
318 T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321

mole fraction for neat DMMP and H2O. A weighted

average of these two numbers yields a predicted “effectiveness” of a 1.6% solution of DMMP in water

to be only 5% more than the measured effectiveness

of the solution. Thus, one can assume that within uncertainty, additivity of effectiveness is valid under the

conditions of the current experiment. However, it is

expected that synergistic rather than additive behavior

will be seen at high H2O loadings, as the effectiveness of chemical agents has been observed to increase

with decreasing temperatures [30,54]. In our experiments, the change in the adiabatic flame temperature

by the amount of water added would result in only a

small change in the extinction strain rate. Addition of

1.5% H2O changes the adiabatic flame temperature

by only 20 K; according to previous work [50], this

change would increase the effectiveness of the chemical compound by roughly 10% compared to addition

to a flame without an inert dopant. It should be noted

that due to considerable scatter in the cited work, particularly in the range of interest for this work, there

is uncertainty in the magnitude of this synergistic effect. Nonetheless, synergy implies that the effectiveness of the solution should be more effective that the

weighted average of the neat compounds, not nearly

equally as effective as observed here for the 1.6%

DMMP solution. Further work at higher H2O loadings is needed.

All of the acid solutions studied (OPA, MPA,

phosphonic acid, and phosphorous acid) exhibit similar reduction of the global extinction strain rate, per

mole PCC added to the air stream. There is a maximum difference of about 22% in the measured effectiveness, based on a comparison of the slopes. When

the data for the DMMP solution are included, the

spread is larger: there is a maximum difference of

42% in slopes in the set of water solution data (between DMMP and OPA). These slopes, along with

corrected slopes described below, are given in Table 3. The possible significance of the difference

between results for the DMMP solution (which vaporizes entirely) and for phosphorus acid solutions

(which produce residual particles) will be discussed

later.

The spread in suppression performances for the

phosphorus acid solutions studied is outside the estimated experimental measurement uncertainty. Chemical and physical differences in the phosphorus compounds could result in differences in fire suppression

effectiveness. Such differences in the added compounds, including differences in heating values, differences in the kinetics leading to the formation of the

phosphorus-containing species that participate in radical recombination cycles, and specific characteristics

of any residual particles such as vaporization rate or

radiative properties, may also contribute to the suppression difference.

Additive species, or portions thereof, that contain hydrocarbons, will to some extent promote the

flame due to their associated heating values. Extinction strain rate measurements were performed with a

small amount (400 ppm) of iso-octane added to the air

stream of a methane/air nonpremixed flame, resulting in a promotional effect of 4% [22]. To estimate

the magnitude of the promotional effect of the fuel

content of other additives considered in this work, the

hydrocarbon content of the PCC is compared to that

of the iso-octane. The additives’ heating values were

compared (on a molar basis) to that of iso-octane,

allowing the measured PCC effectiveness to be corrected for the fuel contents effect (assuming constant

promotional effect per unit heating value). In addition

to the contribution of the hydrocarbon groups, there

is some contribution to the total heating value of the

PCC species due to the phosphorus atom itself. The

heating values of PCCs in this study were calculated

assuming that the phosphorus product is the thermodynamically preferred species, P4O10 [55]. Correcting for heating value effects brings the phosphorusbased acids into better agreement with each other. The

corrected slopes, given in Table 3, represent the slopes

corrected for the heating value of the different compounds. After correction, OPA is still some 20% more

effective than the other acid solutions. The source of

this increased effectiveness is uncertain but may be

related to the nature of the residual particles from this

solution.

The difference between the DMMP solution

(which vaporizes entirely) and the phosphorus acid

solutions (which produce residual particles) suggests

an enhanced effectiveness role for the particles. An

enhanced radiative heat loss from flames due to the

addition of inert particles has been predicted and observed experimentally in premixed flames [7]. Other

effects, such as enthalpy of vaporization and reactions on the particle surface, may also contribute to

an increase in effectiveness from the addition of particles. In the current experiments, particles that enter

the flame are consumed in it. Thus, all of the phosphorus atoms are eventually available to participate

in the gas-phase catalytic removal of the flame radicals H, O, and OH. Otherwise, one would expect

a decrease in effectiveness resulting from the presence of particles. As the data in Table 3 indicate,

all of the phosphorus-based acid solutions that produce residual particles are more effective than the

DMMP solution that does not form particles. Thus,

these results are consistent with a predicted increase

in suppression effectiveness for the residual particles

as long as they are small enough to volatize completely in the flame.
T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321 319

The differences in effectiveness seen here are not

large: the difference in the slopes, corrected for the

heating values, between the DMMP solution and the

average of the phosphorus acid solutions is 21%.

Gas-phase kinetic differences may be significant compared to the remaining discrepancy between DMMP

and acid solutions, as this difference in effectiveness is similar to that found between the vapor-phase

compounds shown in Fig. 7. Kinetics for the aqueous phosphorus solutions tested are largely unknown

so it is not possible to separate gas-phase effects

from those due to the particles. However, taking the

21% difference between DMMP and the acid solution

slopes as an estimate of the magnitude of the particle

effects, we conclude that the net particle effect appears to be smaller than the gas-phase chemical effect

of the phosphorus. Under the conditions and assumptions reported here, the particles would account for

about one-quarter of the total suppression of the solution, compared with the gas-phase chemical effect

that is responsible for about 40% of the total suppression. The same wall losses were assumed for all

the acid solutions, and this assumption may affect the

observed magnitude of the particle effect. However,

some positive particle effect on suppression can be inferred even without making this assumption. Even if

zero losses are assumed for the acid solutions (giving a lower bound on their effectiveness), the acid

solutions are still all more effective than the DMMP

solution.

Some information on differences in the gas-phase

chemical effect was gained from extinction strain rate

calculations. The normalized global extinction strain

rate obtained from these calculations are given in Table 4. The effect of the H2O is decoupled from that of

the PCC. As seen, the calculated reduction in extinction strain rate is significantly less than that observed

experimentally: the PCC alone (no H2O included) reduces the extinction strain rate from its undoped value

by only about: 1.5% for DMMP; 2.3% for phosphorous acid; and 3% for OPA. The trend in effectiveness

Table 4

Summary of calculated extinction strain rates, normalized by

the undoped value (aq/aq0)

Dopant Normalized extinction strain rate

DMMP 0.99

OPA 0.97

P(OH)3 0.98

H2O 0.88

DMMP + H2O 0.87

OPA + H2O 0.86

P(OH)3 + H2O 0.87

Note. The effect of PCCs was found using Glaude’s PCC

mechanism (see text for details). Quantities of dopants are:

250 ppm of PCC and/or 1.54% H2O (molar).

is similar to that seen experimentally; that is, OPA is

the most effective and DMMP is the least effective

PCC. However, the effectiveness is about one-fifth

that seen experimentally for neat DMMP. Thus, calculations using the current chemical kinetic mechanism can be used to gain only qualitative insight into

the gas-phase effect. Newer kinetic mechanisms (e.g.,

[27,28]) may yield better agreement, and should be

investigated.

4. Summary

An investigation of the flame suppression for water, vapor-phase phosphorus-containing compounds,

and residual particles derived from aqueous solutions

of PCCs was performed. To do so, a technique to

introduce an additive as a fine mist of droplets into

an opposed-jet nonpremixed burner was developed

and validated. A high-efficiency nebulizer was used

to produce the droplets, and droplet size measurements using a phase-Doppler particle anemometer established that the size distribution of the droplets is

independent of the compound and liquid flow rate

used. This allows wide application of the HEN for

studying different potential additives. Key features of

the system are small droplet size, large residence time

of the reactant stream under temperature-controlled

conditions, and the availability of an accurate procedure for measuring losses to surfaces within the

feed system. These features lead to complete evaporation of volatile constituents of the droplets, a wellcharacterized state of the reactant mixture at the exit

of the burner tube, and well-defined gas-phase loading of added agent at the flame. A comparison of

the flame suppression effectiveness of PCCs introduced into the burner system in the liquid phase is

made. This experimental approach does not permit

a study of the effect of droplets on the flame; however, it does allow investigation of the physical effect

of residual particles formed from the evaporation of

droplets.

Experimental results indicate a significant reduction in global extinction strain rate with the addition

of pure water vapor (10% reduction at 1.5% molar

loading), in good agreement with numerical results.

With the addition of a small amount (1.6% molar) of

PCC in water solution, this reduction doubles. These

results support the use of water as a means of delivering a chemically active, condensed phase agent to an

actual fire. They also show that the effectiveness of a

PCC/water solution can be determined by a linear addition of the effectiveness of the components, over the

range of concentrations and loadings reported here.

Comparisons of experimental flame suppression effectiveness are made for several volatile and non-
320 T.M. Jayaweera et al. / Combustion and Flame 141 (2005) 308–321

volatile PCCs. Volatile PCCs all showed comparable values of effectiveness, all observably lower than

those of the nonvolatile PCCs. Thus it appears that the

chemical structure of the parent compound has relatively little impact on flame suppression effectiveness,

while the residual particles enhance it. Despite the effect of residual particles, participation of phosphorus

in the gas-phase chemistry is the primary suppression

mechanism.

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