THE ROLE OF PARTICLES IN THE INHIBITION OF PREMIXED FLAMES 

BY IRON PENTACARBONYL * 

Marc D. Rumminger†, Gregory T. Linteris‡

Building and Fire Research Laboratory

National Institute of Standards and Technology, Gaithersburg MD 20899, USA 

Appears in Combustion and Flame, 123(1/2):82-94, 2000.

Running title: Role of Particles in Flame Inhibition by Iron 

Corresponding author: 

Gregory T. Linteris 

National Institute of Standards and Technology 

Building and Fire Research Laboratory 

100 Bureau Dr.; Stop 8651 

Gaithersburg MD 20899-8651, USA 

linteris@nist.gov 

ph: 301-975-2283 

fax: 301-975-4052 

 

* Official contribution of the National Institute of Standards and Technology, not subject to copyright in the United

States. 

† National Research Council/NIST postdoctoral fellow 1996-1998. Current address: Ceryx, Inc.; 1343 Main St.; Santa

Paula, CA 93060; 

‡ Corresponding author. 

 
ABSTRACT 

Laser light scattering has been used to investigate particle formation in Fe(CO)5-

inhibited premixed flames in order to understand the influence of metal and metal

oxide condensation on flame inhibition. In premixed CH4-air flames, particles form 

early in the flame zone, nucleate and grow to a peak scattering cross section, then

disappear as the temperature increases to its peak value. Downstream in the postcombustion gases, the peak scattering signal is several orders of magnitude larger than 

the peak value near the main reaction zone of the flame. Thermophoretic particle 

sampling and numerical estimates indicate nanoparticles with diameters between 10

and 30 nm. As the mole fraction of iron pentacarbonyl in the flame is increased, a 

concentration is reached at which both the burning velocity becomes constant and

particle nucleation begins. A model of an ideal heterogeneous inhibitor shows that

radical recombination on particle surfaces alone cannot account for the magnitude of 

the observed inhibition. Measurements in three CO-H2 flames with similar adiabatic 

flame temperatures but different burning velocities demonstrate the importance of 

residence time for particle formation in premixed flames. 

2
INTRODUCTION 

The ban on production of the popular fire suppressants CF3Br (halon 1301) and 

CF2ClBr (halon 1211) and concerns about the effectiveness of short-term alternatives has

led to renewed interest in understanding the mechanisms of non-fluorocarbon based, 

extraordinarily effective flame inhibitors, such as Fe(CO)5 and dimethyl 

methylphosphonate (DMMP). Many of these compounds produce in-flame species 

with low vapor pressures, raising the likelihood of particle formation in the flame zone 

and therefore the possibility of heterogeneous chemistry. For Fe(CO)5, there have been 

conflicting claims in the literature as to whether the inhibition involves a gas-phase or 

heterogeneous mechanism [1-5]. Recent experimental [6] and numerical [5,7] results 

strongly suggest that the effect at low agent loading is from gas-phase chemistry, and 

that particle formation hinders inhibition processes. If it is found, however, that 

particulates contribute to the inhibition, then the search for halon alternatives could be 

directed toward chemicals that produce similar condensed-phase compounds. 

Recent experiments with premixed and diffusion flames [6,8,9] have shown that 

Fe(CO)5 has a strong inhibitory effect under certain conditions, but almost no effect

under others. A critical part of the research on Fe(CO)5 is to determine the optimal 

conditions for inhibition in order to avoid those in which the inhibitor loses its

effectiveness. In premixed flames the inhibition varies with the Fe(CO)5 concentration: 

at low concentration the burning velocity is strongly dependent on inhibitor mole 

fraction, whereas at high concentration the burning velocity is relatively independent

3
of inhibitor mole fraction. Additionally, numerical calculations using a gas-phase 

inhibition mechanism predict the behavior at low mole fraction reasonably well, but 

overpredict the inhibition at high mole fraction [5,10,11]. A plausible but unconfirmed 

explanation for this reduced effectiveness is that particles form and lower the gas-phase 

mole fraction of active inhibiting species [6]. 

Iron particle formation at high temperature has been studied for a variety of

reasons. Iron pentacarbonyl is a popular precursor for shock tube studies of 

homogenous nucleation of iron and for the subsequent development of nucleation 

models [12-21]. The effect of iron additives on soot formation has been studied in 

laboratory scale diffusion flames [22-28], laboratory scale premixed flames [29-32],

industrial furnaces [33], shock tubes [34,35] and internal combustion engines [23]. 

Fe(CO)5 has also been used as a precursor to manufacture particles and films [36-43].

Despite the breadth of research on iron particle formation, no one has investigated the

effect of particles on flame inhibition by iron compounds. 

In this paper we investigate iron condensation in non-sooting premixed flames. 

The regime is between the nearly reaction-free environment of the homogenous 

nucleation studies and the complex reaction zone of sooting flames. The primary 

motivation is to determine the effect of particle formation on flame inhibition. We use

laser-light scattering to determine the particle formation region and to estimate the

particle size. Thermophoretic sampling with transmission electron microscopy provide

the particle size and morphology. The experiments use premixed Bunsen-type flames 

of CH4-O2-N2 and CO-H2-O2-N2. By making proper choices of concentrations and gas 

4
flows, the effects of both flame temperatures and residence time on particle formation 

are studied. The unresolved issue of whether the inhibition is heterogeneous or

homogeneous is also addressed, and a model of an ideal heterogeneous inhibitor is

presented. 

EXPERIMENTAL 

Burner Systems 

The premixed burner system has been described previously [6,44]. Premixed flames 

(φ=1.0) are stabilized on a Mache-Hebra nozzle burner (inner diameter 1.02 cm ± 0.005 

cm) [45] with an air shroud flow. For the conditions investigated in this paper, the

burner produces a steady, conical, non-sooting flame with a height of 13 mm. 

Measurements are made along a horizontal profile 7 mm above the burner base, which 

provides a pass through each side of the conical flame. The burner and shroud are 

housed in an acrylic chimney with three optical access holes. Rubber bellows connect 

the chimney with the adjacent optical elements to provide a flexible and airtight seal. 

The total gas flows to the burner are between 3.5 and 7.0 L/min for the uninhibited 

flames, and correspond to the conditions of the Fe(CO)5 inhibition measurements 

described in Refs. [6] and [8]. 

5
Gas flows are measured with digital mass flow controllers (Sierra Model 860§) 

with a claimed repeatability of 0.2% and accuracy of 1% of full-scale flow, which have 

been calibrated with bubble (Gillian Gilibrator) and dry (American Meter Co. DTM200A) flow meters so that their accuracy is 1% of indicated flow. The fuel gases are 

methane (Matheson UHP, 99.9% CH4), carbon monoxide (Matheson UHP, 99.9% CO, 

with the sum of CH4 and H2O < 10 ppm**) and hydrogen (Matheson UHP, 99.999% H2, 

with sum of N2, O2 CO2, CO, Ar, CH4, and H2O < 10 ppm). The oxidizer consists of 

nitrogen (boil-off from liquid N2) and oxygen (MG Industries, H2O < 50 ppm, total 

hydrocarbons < 5 ppm). All experiments are performed at ambient pressure and with 

gases at ambient temperature. Inhibitor is added to the flames by diverting part of the 

nitrogen (or methane) stream to a two-stage saturator maintained in an ice bath. The 

diverted gas (less than 8% of the total flow) bubbles through liquid Fe(CO)5 and is 

saturated with Fe(CO)5 vapor, before returning to the main nitrogen flow. The 

saturation mole fraction is determined from a correlation of Gilbert and Sulzmann [46], 

and the carrier gas was shown to be saturated with Fe(CO)5 in previous work [8]. 

Optical System 

Light-scattering and extinction techniques with phase-sensitive detection are used to

determine particle location and properties. The apparatus, shown in Figure 1, is similar 

 

§ Certain commercial equipment, instruments, or materials are identified in this paper to adequately specify the 

procedure. Such identification does not imply recommendation or endorsement by the National Institute of 

Standards and Technology, nor does it imply that the materials or equipment are necessarily the best available for the 

intended use. 

** All uses of ppm in this paper are on a volume basis and correspond to μmol/mol.

6
to those used by other researchers [47,48]. The light source is a 4-W argon-ion laser 

(Spectra Physics BeamLok 2060), with a vertically-polarized 2.2-W beam at 488 nm. A 

mechanical chopper (Stanford Research 640) modulates the beam at 1500 Hz and 

provides a reference signal for the lock-in amplifiers. A polarization-preserving singlemode optical fiber (3 μm diameter) carries the light into a chemical fume hood (90 cm x 

150 cm x 150 cm) which contains the burner. At the fiber output, collimating optics, a

polarization rotator, mirrors and a focusing lens (f = 250 mm) deliver the laser light to 

the test region. A glass wedge between the polarization rotator and the focusing lens 

diverts a small fraction of the beam to a reference detector which monitors the laser

power during the experiments. The transmission efficiency for the laser-to-fiber 

coupling system is only about 15%, but this provides sufficient power for the

experiments. The steep temperature gradients in the present flames cause significant 

beam steering and distortion. These effects, if unmitigated, would produce fluctuations 

of a few percent in the measured transmissivity, which is approximately the same

magnitude as the peak absorptivity (< 2%). To reduce the beam steering effect, we 

follow an approach used by Dibble [49] and Nguyen [50], which involves reflecting the 

beam back through the flame along the same path, thus “unsteering” it. The approach

has the additional benefit of doubling the path length and nearly doubling the laser 

light intensity at the focus. A concave spherical mirror (f = 250 mm) reflects the beam 

back through the optical path and a glass wedge sends it to an integrating sphere. To 

spatially probe the flame, a three-axis translation stage (minimum step size of 0.0016 

mm) positions the burner and chimney in the stationary optical path. 

7
The light detection system consists of three photomultiplier tubes (PMT, all type

1P28) with appropriate filtering. The reference PMT has neutral density filters (OD ≈ 6) 

and a laser-line filter (∆λ = 10 ± 2 nm), and is typically operated at a voltage of –500 V. 

The detection system for light scattered normal to the laser beam consists of a circular 

aperture (5 mm diameter), collection lens (f = 100 mm), pinhole aperture (diameter 1 

mm), laser-line filter, polarizer and PMT at -900 V to –1000 V. For the 90°-scattered 

light, the circular aperture (5 mm diameter) located 10 cm from the laser beam focus 

provides a solid angle of 0.002 sr. The pinhole aperture (1 mm diameter) defines the 

length of the sample to be 1 mm based on unity magnification. 

The signal from each of the detectors is pre-amplified (Stanford Research 552)

before entering a lock-in amplifier (Stanford Research 530). A personal computer 

controls the amplifiers and records the measurements during the experiments using a 

data acquisition card (Strawberry Tree DynaRes Ultra 8). In the data acquisition 

software, each scattering or transmission data point is normalized by the reference 

signal. Typically, 100 readings are averaged over a time of about 1 second; postprocessing software reduces the data and calculates uncertainty as described below. 

The measured quantities in the experiment are the voltage outputs of the

reference, transmission, and scattered light detectors, and these depend on the system

geometry, optical efficiencies, detector responsivity, gas density and particle number 

density, and the scattering cross section of the gases or particles. To obtain the

scattering cross section (Qvv) of the gases or particles in the flame, a calibration of the

optical system efficiency is performed using a gas with known scattering cross section

8
[47,48] (ethane is used because of its relatively large cross section, 51.6×10-28 cm2 [51]). 

The scattering and transmission signals are measured for the calibration gas to give a

calibration factor, 

vv cal

cal

vv cal S Q

,

C ,

τ = , where Qvv,cal is the known scattering cross section of

the calibration gas, 

τcal is the transmissivity of the calibration gas, and Svv,cal is the scattering signal 

caused by the calibration gas. Given the calibration constant and scattering 

measurements, the scattering cross section at each location can be found as 

λ τ

vv

vv

S Q = C , 

where Svv is the measured scattering signal and τλ is the transmissivity of the flame 

gases and particles. 

Thermophoretic Sampling 

Thermophoretic sampling with electron microscopy is used as a supplemental 

technique to determine particle size and morphology. The procedure and apparatus are 

similar to those used by Dobbins and Megaridis [52] and Koylu et al. [53], so limited

details will be given here. A computer-controlled, double-acting piston with travel of

5.08 cm is used to quickly insert and remove the electron microscope (EM) grid from the

flame. Transit times and the dwell time in the flame are measured using a laser, mirror, 

photodiode, and oscilloscope [53]. Each grid is attached to a stainless steel substrate 

with thickness of 0.4 mm, and height between 3 mm to 5.7 mm. The EM grids are 

9
copper with a carbon film deposited on one side (Electron Microscopy Sciences p/n 

CFH4-SPEC-CU), and are fastened onto the metal substrates using adhesive or doublesided tape. 

Uncertainty Analysis 

The uncertainty analysis consists of calculation of individual uncertainty components

and their root mean square sums [54], as outlined in Ref. [9]. All uncertainties are

reported as expanded uncertainties: X ± U, where U is kuc, and is determined from a 

combined standard uncertainty (estimated standard deviation) uc, and a coverage factor 

k = 2 (level of confidence approximately 95%). Likewise, when reported, the relative 

uncertainty is U / X · 100%, or 

kuc / X · 100%. 

The expanded relative uncertainties for the experimentally determined quantities

in this study are as follows: between 3% and 6.5% for burning velocity; between 1%

and 4.5% for normalized burning velocity (the normalized burning velocity is defined as

the burning velocity of the inhibited flame divided by the burning velocity of the

uninhibited flame, which can be found in Table 1); 6.5% for Fe(CO)5 mole fraction; 1.4%

for equivalence ratio; 1.1% for oxygen mole fraction; and 1.2% for hydrogen mole 

fraction in the reactants. For the scattering measurements, the combination of slight

fluctuations in the flame position, small particle scattering cross section, steep spatial

gradients in the flame, and system noise cause the scattering signal to vary about a local

mean value at any given location. The expanded relative uncertainty of the Qvv is no 

10
more than 10% of the mean in the particle zone and no more than 20% of the mean in 

the unburned reactants. 

RESULTS AND DISCUSSION 

CH4 Flames 

Addition of iron pentacarbonyl to the premixed flame leads to a two-zone structure for 

particle formation; Figure 2 shows the scattering cross section Qvv as a function of the 

radial distance r from the centerline for measurements at a height of 7 mm in a CH4-air

flame with 200 ppm of Fe(CO)5. As the figure shows, the particles far outside the main 

reaction zone of the flame (|r| > 7 mm) have a scattering signal which is three orders of 

magnitude larger than the in-flame particles (the signals were collected separately at

different amplifier gains). 

In Figure 2, particles are seen to form in the main reaction zone of the flame 

(|r|≈ 2-3 mm), yielding a peak scattering signal a few times higher than that of the

Rayleigh scattering by the cold reactants (the signal at r=0). These particles disappear 

outside the main reaction zone of the flame (|r|> 4 mm), and far beyond that (|r| > 7 

mm), particles form with a enormous scattering cross section. Although the shroud 

flow around the burner (5 L/min) probably reduces the particle formation (through

dilution), the low gas velocity and low temperature in the outer annulus may cause the 

increase in the particle scattering cross section, and it is also possible that mature 

particles are circulating near the edges of the chimney. A consequence of the high 

scattering in the outer annulus is that it is impractical to make a measurement of the

11
laser extinction through the area of interest (the premixed flame between -7 mm < r < 7 

mm). The total laser extinction through the chimney is about 1%, and that is dominated

by the outer annulus, which is the post-combustion region of the flame and does not 

affect flame propagation. Because of the disparate scattering signal strengths in the two 

regions of the flame, the usual tomographic reconstruction techniques are impractical. 

Nonetheless, in the absence of laser extinction data, it is possible to use the small

scattering signal in the flame zone (|r|> 5 mm) to study particulate formation. 

It is worthwhile to discuss the flame structure of the present premixed flame. 

While there exists a two-zone structure for particle formation, as shown in Figure 2, there 

are not two reaction zones in the flame because, unlike partially premixed flames, the 

present flames have φ=1.0 based on the primary gases. We have confirmed the single

reaction zone structure (in tests without added Fe(CO)5) through examination of the 

Rayleigh scattering results, previous thermocouple temperature data [55], and tests

with nitrogen co-flow (instead of air). 

In the following discussions, the particle scattering signal at varying radii and a 

fixed height above the burner are presented. With the present nozzle burner, 

streamlines within the flame are vertical, and parallel. As has been discussed

thoroughly in the literature [56], the gases expand in the flame, causing the flow

streamlines to gain a radial component of velocity and diverge outward. At central 

heights in the flame and just outside of the region of visible emission, it is found that the

streamlines make an angle of about 45°

 to the horizontal, and they are nearly parallel. 

Hence, at a fixed height above the burner, increasing radial positions beyond the flame

12
correspond to larger distances and times since passage through the flame. Although 

measurements at different radii correspond to different streamlines, the timetemperature history of adjacent streamlines is sufficiently similar so that they can be

assumed to be equivalent, and increasing the radius can be thought of as increasing 

time or distance from the main reaction zone. 

In order to determine how particle formation depends on Fe(CO)5 concentration,

we measured radial profiles of the scattering signal for varying Fe(CO)5 mole fractions 

(Xin) at a height of 7 mm above the burner rim (Figure 3). Referring to Figure 3, r = 0 

corresponds to the centerline of the conical Bunsen-type flame. At this flame height (7

mm), cold reactants are in the region of |r| < 2 mm, the primary reaction zone of the

flame extends from |r| ≈ 2 to 3 mm, and the hot combustion products are in the region 

of |r|> 3 mm. The figure shows that with 50 ppm of Fe(CO)5, Qvv is only slightly higher

than that of the uninhibited flame; whereas above that value, significant peaks in Qvv

appear in the flame zone, indicating particle formation. Note that the scattering cross 

section of the particles at 300 ppm is only 20 times that of room air, which implies very 

small diameters or number densities. The existence of sharp peaks in Figure 3, as 

opposed to a step function, may be explained as follows: in the reaction zone (|r| ≈ 2 

mm), the Fe(CO)5 decomposes, resulting in supersaturated vapor of iron-containing 

intermediates (the gas-phase inhibiting species). If the mole fraction of these species is 

high enough, nucleation and particle growth occurs. As the particles are heated by the 

flame (|r|> 3 mm), they evaporate, and their scattering cross section is reduced. Far 

downstream of the flame, the long residence times and cooler gases cause the formation 

13
of stable iron oxide particles (seen as the very large scattering signals at |r| > 7 mm in 

Figure 2) which persist (and coat the chimney and exhaust system). 

We expect that the particle profiles are relatively unaffected by thermophoresis—

a process in which particles drift down a temperature gradient—because calculations 

(based on Refs. [57,58]) show that the thermophoretic velocity is never more than onequarter the gas velocity at any point in the flame zone. 

Previous experimental results [6,10] show that the burning velocity starts to level

off at an Fe(CO)5 mole fraction of about 100 ppm. It has been suggested that this is due 

to condensation of the active iron-containing inhibiting species as they reach their 

saturation vapor pressure. Further, for flames at higher oxygen mole fraction (i.e.

higher temperature), the leveling-off point shifts to a higher value of Xin. A reasonable

explanation is that at higher temperature, higher concentrations of iron can remain in 

the gas phase before condensation occurs. Increasing the oxygen mole fraction in the 

air (XO2,ox) from 0.21 to 0.24 provides a 130 K increase in adiabatic flame temperature 

(see Table 1). The scattering results (not shown) for higher temperature CH4 flames 

(XO2,ox = 0.24) have similar shape and dependence on Xin as those for XO2,ox = 0.21, but 

the measured Qvv’s are between 2 and 4 times lower, implying fewer or smaller 

particles. 

The ultimate goal of these experiments is to determine the effect of particles on 

flame inhibition by Fe(CO)5. Ideally, the particles formed from the Fe(CO)5 precursor

would be quantified by measuring their size, number density, volume fraction and 

number of molecules per particle, but at this point such data are unavailable. In their 

14
absence, the Qvv data shown in Figure 3 and those for XO2,ox = 0.24 can be used as 

qualitative measures of the particle loading at a given inhibitor mole fraction. 

Evidence that particle formation leads to the decrease in inhibitor effectiveness is 

found in Figure 4, which shows the peak Qvv along with the normalized burning 

velocity at various inhibitor concentrations. At low Xin, where the inhibition is believed 

to be primarily a gas-phase effect, the burning velocity depends strongly on Fe(CO)5

concentration, and the Qvv is relatively small. For either value of XO2,ox, there exists an 

Fe(CO)5 mole fraction at which the marginal inhibition becomes much smaller 

(approximately 100 ppm for XO2,ox = 0.21 and 300 ppm for XO2,ox = 0.24). At these values 

(especially for XO2,ox = 0.21), the peak scattering cross sections rise sharply. Thus, it

appears that a decrease in inhibitor effectiveness is correlated with particle formation, 

supporting the previous proposal that condensation of iron containing species weakens 

the inhibitor efficiency. It is also noteworthy that for the flames with XO2,ox = 0.24, the 

inhibition continues to a higher value of Xin and the scattering signal is always smaller 

than for flames with XO2,ox = 0.21. The higher flame temperatures with XO2,ox = 0.24 may 

be the reason for the decreased Qvv, but the higher burning velocity of these flames (see 

Table 1) also yield a shorter residence time for particle growth, complicating the

interpretation. Other techniques of changing flame temperature in CH4 flames, such as

replacing some of the nitrogen with argon, also result in simultaneous change in 

burning velocity. Alternatively, as will be described in the next section, CO flames with

15
varying amounts of H2 can provide flames with different burning velocities without

appreciably changing the adiabatic flame temperature,. 

Carbon Monoxide Flames 

Stoichiometric CO flames with hydrogen mole fraction (XH2) of 0.005, 0.01, and 0.015, 

and elevated oxygen content relative to air (XO2,ox = 0.24), have burning velocities

between 35 and 60 cm/s and adiabatic flame temperatures near 2470 K (see Table 1).

Measurements of scattering cross section in each flame at various values of Xin can

provide information about the relative importance of peak flame temperature and

residence time. 

The scattering results for the CO-H2 flames are qualitatively similar to those for 

the CH4 flames of Figure 3: the scattering increases with increasing Xin, and the

particles appear and then disappear. A difference, however, is that for the CO flames at 

high Fe(CO)5 loading and low H2 content, each of the two Qvv peaks has a twin-lobed 

structure. These structures may be due to variations in the refractive index as the

particles change composition, or due to thermophoretic size segregation effects, as 

described by Zachariah et al. [59]. 

Following the procedure used for the CH4 flames, we correlate the scattering 

data for the CO flames with the inhibition by Fe(CO)5 by plotting the peak Qvv values 

and normalized burning velocity [9] for each flame condition in Figure 5. As was seen 

for CH4 flames, the peak Qvv is relatively small at low Xin where the inhibition effect is 

16
strongest, and rises sharply as Xin increases. Comparing the values of Xin at which the

normalized burning velocity curves level off, we see that the 0.5% H2 curve levels off 

first, followed by the 1.0% H2 and then the 1.5% H2. The data in Figure 5 support the

claim that particles reduce the inhibition effect: higher scattering signals correspond to 

greater loss of effectiveness (i.e., leveling off in the burning velocity curves). The three 

flames in Figure 5 have roughly the same adiabatic flame temperature but very 

different peak scattering cross sections with added Fe(CO)5. These results can be used

with those of the CH4 flames at varying XO2,ox, to show the importance of residence time

for particle formation. 

Figure 6 shows the maximum value of Qvv as a function of burning velocity 

(which is inversely related to residence time) for a variety of flames at three values of 

the inhibitor loading. The figure presents data for CH4 flames (open symbols) at low (l) 

and medium (m) temperature, and for CO flames (closed symbols) at high (h) 

temperature. The three values of Fe(CO)5 loading, 100, 200, and 300 ppm are indicated 

by circles, diamonds, and squares, respectively. While eight parameters are shown in 

the figure, the peak value of Qvv for a given inhibitor loading is found to depend most 

strongly upon the burning velocity (i.e. the residence time). The linear dependence

becomes strongest at 300 ppm inhibitor, in which case a doubling of the burning 

velocity (halving of residence time) yields a four-fold decrease in maximum Qvv. These 

results clearly illustrate the importance of residence time in particle formation; a similar 

analysis using adiabatic flame temperature as the independent variable shows no 

significant correlation.

17
Particle Size and Morphology 

Although exact laser-based measurements of the particle size and number density are

not practical in the present nozzle burner premixed flames (because of the two-zone

structure shown in Figure 2), we can make estimates of particle properties and use 

extractive sampling to obtain information about the particles. The information from

these techniques can be used to provide insight into particle behavior in the premixed 

flames, and provide input for a heterogeneous inhibition model. 

We can obtain an upper limit for particle properties by estimating the fraction of 

that iron condenses, calculating the resulting extinction coefficient, and combining it

with the scattering data to predict a range of possible particle sizes and number 

densities. The mass concentration of iron particles (g/cm3) is in gas P A [P] = fc X N M /N , 

where fc is the fraction of the iron that has condensed, Ngas is the number density of the

gas, MP is the molecular mass of the particulate material, and NA is Avogadro’s number. 

The volume fraction (fv) can be calculated as / ( ) [P] v P s f = ρ , where ρ P(s) is the density of

the solid particle material. The extinction coefficient can then be calculated with 

= π ) /λ ~( ext v k 6 E m f [48]. Finally, we use the equations for monodisperse Rayleigh 

scatterers to obtain 

1 3

2

3 4 /

) ~(

) ~( 



 





















π

λ = F m

E m

k

Q D

ext

vv , and 3 )D 2

E m

k N ext

~ π (

λ = . 

We present calculated particle properties for fc = 0.5 and 1.0 (50% and 100% 

condensation). A comparison between numerically calculated inhibition assuming no 

condensation and the measured inhibition at 200 ppm implies that 50% of the FeO has 

18
condensed, while considering 100% condensation provides an upper limit. We use the

approach outlined above with the peak scattering data (at a height of 7 mm) from 

Figure 3. We calculate the properties for particles formed in CH4/air flame with Xin = 

200 ppm, estimating from the Qvv data that the peak scattering cross section occurs at 

1500 K and assuming that the particle material is FeO ( ρ P(s) = 5.7 g/cm3 [60]). At such 

conditions, for fc = 0.5 and 1.0, the particles have a volume fractions of 1.2×10-8 and 

2.2×10-8, diameters of 16 nm and 13 nm, and number density of 5.3×109 cm-3 and 

2.1×1010 cm-3. Using the optical and bulk properties of Fe instead of FeO increases the

inferred diameter by 15% and the number density by 9%. While it is possible that the

particles also contain combinations of iron compounds and carbonaceous material from 

soot, we consider it unlikely since the flames are not fuel-rich, and we did not observe 

any evidence of soot from the scattering measurements in uninhibited flames. 

Thermophoretic sampling allows measurement of particles at the present flame 

conditions. Using the rapid sampling technique, we collected samples at various flame 

conditions at a height of 7 mm above the burner rim, at a radial position corresponding 

to approximate center of the main reaction zone (r = 2.7 ± 0.3 mm). The image shown in 

Figure 7 corresponds to a dwell time of 375 ms in a flame with Xin = 200 ppm. The 

particles show a moderate degree of agglomeration, with about 1 to 10 primary particles

per agglomerate and primary particle sizes of under 20 nm. Contamination of the grid 

by particles from outside the premixed flame zone is possible, and may account for the

large number of agglomerates in the image. The primary particle diameters estimated 

above from the scattering signal with assumed amounts of iron condensation, and from 

19
the TEM images in Figure 7 are in reasonable agreement. These small diameters, 10 to 

20 nm, support the possibility of particles evaporating as they convect to regions of 

higher temperature. 

 

Perfect Heterogeneous Inhibition Model 

There have been conflicting claims in the literature as to whether inhibition by Fe(CO)5

is a gas-phase or heterogeneous effect [1-5]. A heterogeneous inhibition mechanism is 

reasonable since iron and iron oxides are used in various industrial processes as 

heterogeneous catalysts. It is unclear, however, if the same catalytic behavior occurs on

the nanoparticles found in the present flames. Preining [61] has argued that particles 

with diameters on the order of 5 nm behave like neither the gas phase nor the bulk 

phase because of the interactions between the electronic structures of the molecules in 

the particle. Although Preining’s contention does not rule out the possibility of catalytic

activity, it implies that the catalytic properties of the bulk phase may not be reproduced 

in nanoparticles, and further study is required to determine the catalytic properties of 

the nanoparticles. Although it is too early to completely disregard the possibility of 

heterogeneous inhibition, the experimental data in Figure 4 suggest that the inhibition

is primarily gas-phase. If the inhibition chemistry were primarily heterogeneous, we 

would expect the maximum particle scattering to be high for Fe(CO)5 mole fraction 

below 200 ppm—when the marginal inhibition is strongest— and leveling-off above 200 

ppm. 

20
Lacking direct measurements of heterogeneous chemical effects, it is of interest 

to estimate the maximum possible effect of surface inhibition and to determine if 

heterogeneous chemistry can reproduce the performance of Fe(CO)5 under idealized 

conditions. A model of heterogeneous radical recombination was presented by Baratov 

et al. [62] for larger particles (ca. 5 μm) with the goal of understanding powdered fire 

suppressants such as sodium bicarbonate. Additionally, Jensen and Webb [4] simulated 

catalytic radical recombination by metal additives in a rocket plume and found that

condensed-phase additives had a smaller effect than gas-phase additives. The work of 

Babushok et al. [7], who developed a chemical mechanism for an ideal inhibitor which 

catalytically scavenges radicals at gas-kinetic rates, serves as an inspiration for the 

present model. 

The present model seeks to determine the maximum reaction rates between 

particles and radicals. For simplicity, we assume a two-step heterogeneous inhibition 

mechanism (Langmuir-Rideal type) which is initiated by the adsorption of a radical R

onto a particle surface P: R + P → RP. This reaction is followed by the reaction of the

activated particle RP with a radical, and desorption of the recombined radicals R2: RP + 

R → R2 + P. A number of other important assumptions are made: 1) all of the gaseous

inhibitor immediately becomes particulate matter, 2) the particles are spherical with a 

specified log-normal diameter distribution [63-65] which is unchanged through the

flame, 3) every collision of a radical with a particle results in adsorption and every 

collision of a radical with an activated particle results in reaction and desorption, and 4)

only H-atom recombination is considered, since numerical results show that the iron 

21
species primarily scavenge H atoms (note that the additional effect of adding O and OH 

recombination would be minor). Many other effects are not considered, such as 

multiple adsorption sites on particles, non-unity adsorption probabilities, steric factors, 

and saturation of the surface sites; all of these would reduce the efficiency of the radical

recombination on the particle surface. 

For compatibility with the Chemkin codes used here, we calculate an “effective 

mole fraction” of particles which can be directly entered into the Premix [66] input file

and maintains the proper units for the calculation. The approximation for the effective 

mole fraction is 

o

in o

p N

X N n r f r d r

X ∫

∞

= = 0 ( ) ( ) (ln )

Number density of all gaseous reactants

Number density of particles , 

where No is the number density of the reactant gas, n(r) is the number of iron atoms in a 

particle of radius r, and f(r) is the probability density function of the lognormal

distribution. The numerator expresses the number density of the particles as the ratio

of the number density of inhibitor molecules (XinNo) to the average number of 

molecules per particle. The number of iron atoms per particle (n(r) = 58.607 r 3.5478) is 

calculated from the curve fit of results of Jensen [67], which assume hard sphere 

packing and a decrease in packing density for particles with a diameter less than 10 nm.

The reaction rates between radicals and particles are calculated using kinetic

theory, assuming that the particles are in the free molecular regime. The collision rate 

between an atom and particle is 1 2

1 2

2 8 /

/

N T k k d A

RP

B

RP 















μ

π = , where d is the collision RP

22
diameter of the radical and particle (dR + dP), kB is Boltzmann’s constant, and μRP is the

reduced mass of the two bodies (mRmP / (mR+mP) ). Molecular masses and diameters 

are taken from Ref. [68]. Numerical experiments show that variation of the

thermodynamic and transport properties of the particles has no appreciable impact on 

the results for burning velocity. 

Calculated values of the pre-exponential and effective mole fraction are listed in 

Table 2. The conversion of gas to particles results in two changes to the overall

reactivity: a reduction in the effective mole fraction of the inhibiting species (between

10-4 and 10-6 times smaller) because of more inhibitor atoms per particle, and an increase 

in the rate constant (103 to 105 times the gas kinetic rate for a single molecule) because of

the larger cross sectional area of the particles relative to molecules. Figure 8 shows the 

normalized burning velocity as a function of Xin for mean particle diameters of 10 nm, 

20 nm, 40 nm and 80 nm. Since the number of particles scales as 3 1 dm , while the preexponential scales as 2 1 dm , the smaller particles are more effective. The inhibition effect 

of this idealized inhibitor is significant, but not nearly as strong as that of Fe(CO)5 or the

perfect homogeneous inhibitor. These idealized calculations support the proposals [5,7] 

that only gas-phase chemistry is fast enough to account for the extraordinary inhibition

effect of Fe(CO)5, since a more realistic model of heterogeneous radical recombination

would probably result in less inhibition. It is interesting to note that the residual 

inhibition of Fe(CO)5 at Xin > 300 ppm, while small compared to values at Xin < 100 

23
ppm, is not zero. It is, in fact, comparable to agents such as CF3Br and may be due to 

heterogeneous inhibition. 

CONCLUSIONS 

Laser light scattering has been used to investigate particle formation in Fe(CO)5-

inhibited premixed flames. Particles form early in the flame zone, nucleate and grow to 

a peak scattering cross section, then disappear as the temperature increases to the flame 

temperature; far downstream in the post-combustion gases, the peak scattering signal is 

several orders of magnitude larger than the peak value near the main reaction zone of

the flame. Thermophoretic sampling as well as estimates of the size from inhibitor

loading and scattering signal show particles with diameters between 10 and 30 nm. 

Experimental results support the hypothesis that condensation reduces the 

inhibition. The Fe(CO)5 mole fraction at which the normalized burning velocity levels 

off corresponds to the mole fraction at which the in-flame particle scattering cross

section begins to sharply increase. Measurements in three CO-H2 flames with similar

adiabatic flame temperatures but different burning velocities demonstrate the 

importance of residence time for particle formation in premixed flames, as the highest 

scattering signals occur in flames with the lowest burning velocity. The experimental 

results also imply that heterogeneous chemical effects are small since the particles 

appear at Xin where inhibition effects are weak. In addition, an idealized model shows 

that heterogeneous chemistry alone can not account for the inhibition exhibited by 

Fe(CO)5 for particle diameters of 10 nm and larger. For particles of diameter much less 

24
than 10 nm, however, the possibility that they contribute to the inhibition cannot be

ruled out. 

Particle dynamics may affect the performance of ultra-effective fire suppressants in 

practice and it is important to understand the effects. For example, if the residence time

is low enough, significant condensation of active species and the subsequent reduction 

in effectiveness may not occur. In situations with long particle formation residence 

times, it may be possible to reduce the undesired loss of effectiveness due to

condensation by using several compounds together (with non-condensing amounts of

each), or by selecting compounds with a high vapor pressure condensed phase. 

Further experiments on particle formation in premixed Fe(CO)5–inhibited flames 

would benefit from a one-dimensional flame with a longer path length for the scattering 

and extinction techniques. In addition, a direct measurement of the catalytic effect of 

iron-containing nanoparticles may allow determination of the role of heterogeneous

chemistry in flame inhibition by Fe(CO)5. 

ACKNOWLEDGEMENT 

The authors thank undergraduate student intern Nikki Prive for assistance with data 

acquisition and uncertainty analysis programs and Maria Aquino for operating the electron 

microscope and advising us on sampling techniques. Discussions with Dr. George Mulholland 

about particle measurement techniques, Dr. Viet Nguyen about beam steering, and Dr. Valeri

Babushok about the heterogeneous inhibitor model contributed greatly to this work. 

25
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28
TABLE AND FIGURE CAPTIONS 

Table 1: Oxygen content of ‘air,’ mole fraction of hydrogen in reactants, measured 

burning velocity, and calculated maximum temperature of the uninhibited 

stoichiometric flames used in this paper. CH4 data from Ref. [6], CO-H2 data 

from Ref. [8]. 

Table 2: Representative properties of perfect heterogeneous inhibitor with 100 ppm of

precursor. 

Figure 1: Schematic of laser scattering/extinction system: C, chopper; M, mirror; G, 

beam pick-off; L, lens; S, spatial filter (circular aperture); Po, polarizer; F, laserline and neutral density filters; PMT, photomultiplier; P, pinhole; IS, integrating 

sphere. (Note: Some structural components are not shown, and the drawing is

not to scale.) 

Figure 2: Scattering cross section Qvv as a function of the radical distance r from the 

burner centerline at 7 mm height in stoichiometric CH4-air flame with 200 ppm 

of Fe(CO)5. 

29
Figure 3: Measured scattering cross section through a stoichiometric CH4-air flame 7 

mm above the burner rim at various inhibitor mole fractions. 

Figure 4: Normalized burning velocity (from Ref. [6]) and maximum Qvv for φ=1.0 CH4

flame with XO2,ox = 0.21 and 0.24.

Figure 5: Maximum scattering signal and normalized burning velocity (from Ref.[8]) 

for CO-H2 flames as Fe(CO)5 concentration varies. 

Figure 6: Maximum Qvv for flames of CH4 (open symbols) and CO (closed symbols) as a

function of the burning velocity (from Ref. [6] and [8]). The letters correspond to 

the adiabatic flame temperature (l,m,h: low, medium, and high, 2220, 2350, and 

2470 K), while the symbol shape (square, diamond, and circle) corresponds to the

loading of Fe(CO)5: 100 ppm , 200 ppm , and 300 ppm . 

Figure 7 : Electron microscope picture of particles sampled from a CH4/air premixed

flame with Xin = 200 ppm. 

Figure 8: Calculated normalized burning velocity for several diameters dm of ideal 

heterogeneous inhibitor. Also shown are Fe(CO)5 data [6], and calculated 

normalized burning velocity using the perfect gas-phase inhibitor mechanism 

[7]. 

30
Table 1: Oxygen content of ‘air,’ mole fraction of hydrogen in reactants, measured burning velocity, 

and calculated maximum temperature of the uninhibited stoichiometric flames used in this paper.

CH4 data from Ref. [6], CO-H2 data from Ref. [8]. 

Fuel XO2,ox XH2 vo,exp

(cm/s)

Tmax,num 

 (K) 

CH4 0.21 0 40.6 ± 2.0 2224 

0.24 0 59.2 ± 3.0 2353 

CO-H2 0.21 0.01 39.2 ± 1.1 2376 

0.24 0.005 36.2 ± 0.9 2468 

0.24 0.01 46.2 ± 1.4 2471 

0.24 0.015 59.0 ± 2.4 2475 

Table 2: Representative properties of perfect heterogeneous inhibitor with 100 ppm of precursor. 

Mean 

Diameter 

(nm) 

A 

(cm3/mol-sK1/2) 

H+P→HP 

HP+H→H2

Effective 

Mole Fraction 

of Particles 

Number 

Density of 

Particles 

(cm-3) 

Average 

atoms per 

particle 

10 3.22E+16 1.84E-08 4.96E+11 5.43E+03

20 1.28E+17 2.95E-09 7.93E+10 3.39E+04

40 5.12E+17 4.72E-10 1.27E+10 2.12E+05

80 2.05E+18 7.55E-11 2.03E+09 1.32E+06

31
Lock-in Amp

Computer

Pre-amp L

Laser

C

M

PMT

Pre-amp

Po

G

Burner

PMT

M

F P L

S

Po

PMT

F

Lock-in Amp

IS

Lock-in Amp

Fiber

Pre-amp

L L

Bellows

Chimney

F

Figure 1: Schematic of laser scattering/extinction system: C, chopper; M, mirror; G, beam pick-off; L,

lens; S, spatial filter (circular aperture); Po, polarizer; F, laser-line and neutral density filters; PMT,

photomultiplier; P, pinhole; IS, integrating sphere. (Note: Some structural components are not

shown, and the drawing is not to scale.) 

32
1E-08

1E-07

1E-06

1E-05

1E-04

-15 -10 -5 0 5 10 15

r (mm)

Qvv (1/cm-sr)

Burner Location

Figure 2: Scattering cross section Qvv as a function of the radical distance r from the burner centerline

at 7 mm height in stoichiometric CH4-air flame with 200 ppm of Fe(CO)5. 

33
0E+00

1E-07

2E-07

3E-07

4E-07

-5 -4 -3 -2 -1 0 1 2 3 4 5

r (mm)

300 ppm

150

200

100

Qvv (1/cm-sr)

50

0

Figure 3: Measured scattering cross section through a stoichiometric CH4-air flame 7 mm above the

burner rim at various inhibitor mole fractions.

34
0.0E+0

1.5E-7

3.0E-7

4.5E-7

0 100 200 300 400

Fe(CO)5 (ppm)

Maximum Qvv (1/cm-sr)

0.0

0.2

0.4

0.6

0.8

1.0

N

ormalized Burning Velocity

X = 0.24 O2,ox

X = 0.21 O2,ox

Figure 4: Normalized burning velocity (from Ref. [6]) and maximum Qvv for φ=1.0 CH4 flame with

XO2,ox = 0.21 and 0.24. 

35
0E+0

2E-7

4E-7

6E-7

8E-7

0 100 200 300 400

Fe(CO)5 (ppm)

0.0

0.2

0.4

0.6

0.8

1.0

0.01

Normalized Burnin

g Velocit

y

Maximum Qvv (1/cm-sr)

0.005

X = 0.015 H2

0.01

0.005

0.015

Figure 5: Maximum scattering signal and normalized burning velocity (from Ref.[8]) for CO-H2 flames 

as Fe(CO)5 concentration varies. 

36
0E+00

1E-07

2E-07

3E-07

4E-07

5E-07

6E-07

10 20 30 40 50 60

Burning Velocity (cm/s)

Maximum 

Qvv (1/cm-sr)

300 ppm 

200

100

l

m

h

h h h

m m

l

h

h

h

h

h

h

h

h

l

Figure 6: Maximum Qvv for flames of CH4 (open symbols) and CO (closed symbols) as a function of

the burning velocity (from Ref. [6] and [8]). The letters correspond to the adiabatic flame temperature 

(l,m,h: low, medium, and high, 2220, 2350, and 2470 K), while the symbol shape (square, diamond, and 

circle) corresponds to the loading of Fe(CO)5: 100 ppm , 200 ppm , and 300 ppm . 

37
Figure 7 : Electron microscope picture of particles sampled from a CH4/air premixed flame with Xin = 

200 ppm. 

38
0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600 700 800

Gas-Phase Precursor Mole Fraction (ppm)

Perfect gas-phase inhibitor

10 

20

dm = 80 nm

Fe(CO)5

40

Normalized Burning Velocity

Perfect heterogeneous inhibitor w/

Figure 8: Calculated normalized burning velocity for several diameters dm of ideal heterogeneous 

inhibitor. Also shown are Fe(CO)5 data [6], and calculated normalized burning velocity using the 

perfect gas-phase inhibitor mechanism [7]. 

39