American Institute of Aeronautics and Astronautics

1

Laminar Flame Propagation Enhancement

by Singlet Molecular Oxygen

Timothy M. Ombrello,1 Campbell D. Carter

2

U.S. Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson AFB, OH, 45433

and

Viswanath Katta3

Innovative Scientific Solutions Incorporated, Dayton, OH, 45440

A new plasma-assisted combustion platform utilizing the Hencken burner at sub-

atmospheric pressure was developed for investigating the effect of singlet molecular oxygen,

O2(a1Δg), on flame propagation enhancement. Singlet molecular oxygen was produced in a

microwave discharge, isolated from O and O3 via NO injection, and was quantified with

absorption measurements. The O2(a1Δg) was transported through a silica coated burner to

steady, laminar, nearly one-dimensional, minimally curved, weakly stretched, and near

adiabatic H2, CH4, and C2H4 flames. The flame speed enhancement inferred from the change

in flame liftoff height showed the elevated levels of enhancement for lean and rich versus

stoichiometric mixtures.

I. Introduction

NDERSTANDING in the field of plasma-assisted combustion has advanced considerably over the past decade,

showing significant enhancement and making mainstream practical applications increasingly more realistic.

Many of the applications are motivated by the need for higher performance and more efficient combustion systems

ranging from small internal combustion engines to high-speed air-breathing propulsion devices. These systems can

require ignition at low temperatures within restricted residence times, burning outside normal flammability limits,

and flame stabilization in high-speed flows. Therefore, there has been significant effort focused on attempting to

gain a fundamental understanding of the mechanisms of plasma-assisted combustion. This has allowed for two

different investigative techniques to be pursed: passive and active. The passive technique is the more common

approach by applying plasma to a reactive system and then attempting to understand the interaction. This can be a

significant challenge because of the highly coupled thermal, kinetic, transport, and hydrodynamic enhancement

effects in plasma-assisted combustion systems. Fortunately, recent advancements in the application of spatially and

temporally resolved diagnostics has allowed for the identification of some of the key species produced and their

roles in enhancement.[1-5]

Furthermore, experiments have been designed to minimize certain coupled effects from

plasma application (specifically thermal and hydrodynamic) and elucidate some of the fundamental mechanisms of

enhancement.[6-9]

The results of using the passive technique have provided a plethora of detailed data and substantial

gains in identifying enhancement pathways. The active technique is less common and seeks to target the effect of

specific plasma-produced species. This typically involves tailoring a plasma discharge to produce the species of

interest and/or use methods to isolate the species of interest. While there has been much more effort in this area with

regard to simulations because of the ease of applying initial or boundary conditions, there have been a limited

number of experiments. The lack of results using the active technique is rooted in the difficulty in isolating specific

plasma-produced species, measuring their concentrations, and then quantifying their enhancement in combustion

systems. Nevertheless, there have been multiple efforts to isolate the effect of species such as O,[10]

O3,[11-13]

and

O2(a1Δg).

[14] While O and O3 are common plasma-produced species and can provide significant enhancement to

1 Research Aerospace Engineer, Aerospace Propulsion Division, 1950 Fifth Street, Member AIAA.

2 Principal Aerospace Engineer, Aerospace Propulsion Division, 1950 Fifth Street, Associate Fellow AIAA.

3 Senior Engineer, 2766 Indian Ripple Road, Associate Fellow AIAA.

U

50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition09 - 12 January 2012, Nashville, Tennessee

AIAA 2012-0380

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

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combustion systems, there has been much interest in electronically excited O2, namely O2(a1Δg), because it can

require low levels of plasma excitation energy, has a high reactivity, and is long lived. Therefore O2(a1Δg) has the

potential to be produced in significant concentrations in plasma and subsequently enhance critical combustion

phenomena with considerably more rapid fuel oxidation.

There have been several numerical works targeting the enhancement by O2(a1Δg) for H2, CO, and hydrocarbon

fuel based systems with detailed kinetic mechanisms developed.[15-22]

The kinetic models have shown that the

induction and ignition delay times can be reduced significantly and the laminar flame speeds can be enhanced.

Specifically with regard to flame speed enhancement, Kozlov et al. conducted numerical simulations to show the

enhancement of H2/O2 flame speed with O2(a1Δg) addition.

[19] The results showed that a 10% conversion of O2 to

O2(a1Δg) gave more than a 50% increase in the laminar flame velocity. Lean mixtures were enhanced more than

stoichiometric and rich mixtures because of the chain initiation and branching reactions involved. Bourig et al.

extended the numerical modeling by investigating flame propagation, as well as flame stabilization by O2(a1Δg).

[15]

More recently, Starik, et al. extended their numerical work on O2(a1Δg) to CH4-air flame propagation with a detailed

kinetic mechanism.[21]

Again, the flame propagation enhancement was up to 40% for fuel rich mixtures and up to

70% for fuel lean mixtures with 10% conversion of O2 to O2(a1Δg). Nevertheless, there was no experimental

validation of the enhancement. In an attempt to quantify the flame propagation enhancement by O2(a1Δg),

experiments were designed and conducted to isolate O2(a1Δg) from other excited species for transport to sub-

atmospheric pressure lifted tribrachial flames.[23]

These experiments isolating the effects of O2(a1Δg) on flame

propagation were the first of their kind and showed that O2(a1Δg) could enhance hydrocarbon flame propagation,

specifically for C2H4. Nevertheless, the complicated structure of the lifted tribrachial flames prohibited the use of

detailed diagnostics to quantify the enhancement and provide a validation platform for kinetic mechanism

development. Therefore, an experimental platform to quantify flame propagation enhancement by O2(a1Δg) is

warranted where detailed diagnostics can be performed. The platform could provide much needed flame speed data

for validation of chemical kinetic mechanisms including plasma-excited species such as O2(a1Δg).

The primary focus of the present work was to utilize a Hencken burner at sub-atmospheric pressure, which

provides a new platform for investigating flame propagation enhancement and the detailed structure of flames with

the addition of prescribed concentrations of O2(a1Δg). The new platform provides a highly decoupled system where

the isolated kinetic effects of O2(a1Δg) can be investigated and the results used to validate chemical kinetic

mechanisms.

II. Experimental System

A. The Hencken Burner Platform

A 25.4 mm by 25.4 mm square exit profile Hencken burner (Technologies for Research Model RT1x1)[24]

was

used inside a variable pressure chamber.

The fuel is supplied through the fuel tubes

and the oxidizer through the surrounding

honeycomb, forcing reactant mixing to the

region above the burner surface. A top

view of the burner with a stoichiometric

CH4-air flame, as well as the average

measured characteristic dimensions of the

fuel tubes and oxidizer honeycomb are

shown in Fig. 1. The burner was placed on

a three axis translation stage within the

variable pressure chamber (Fig. 2). The

chamber is cylindrical and constructed of

electro polished stainless steel with inside

dimensions of 38.7 cm in diameter and

71.8 cm in height, minimizing any

confinement effects on the flame. The

inner surfaces of the chamber were bead

blasted after being electro polished in

order to provide a diffuse surface to

minimize specular reflections. There are

four ports for optical access through UV

Figure 1. Picture of Hencken burner, (a) with a stoichiometric

CH4-air flame at atmospheric pressure, and (b) top view of burner

with dimensions.

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grade windows located 90 degrees

apart from each other at half the

height of the chamber. Additionally

there are four access ports for

feedthroughs of gases, controls,

etc. Sub-atmospheric pressures

were achieved using a scroll pump

and regulation valves and

monitored using a capacitance

monometer mounted on the

chamber (0.05 kPa accuracy).

Temperatures were monitored at

three locations in the chamber: at

the bottom and top of the chamber,

as well as at the exit of the

Hencken burner.

One of the more important

aspects of the Hencken burner is

that the fuel and oxidizer are

separated from each other until the

burner exit. The reactants then mix

rapidly at ambient temperature (~300 K) prior to the flame, isolating any significant kinetic interaction until the

flame front. Therefore, the fuel or the oxidizer can be activated by plasma separately and will only interact for a

short residence time (milliseconds) at approximately 300 K prior to the flame front.

B. Plasma System for O2(a1Δg) Production

The oxidizer for the Hencken burner was activated by a plasma upstream by an electrodeless microwave

discharge (McCarroll cavity driven by an Opthos MPG-4M microwave power supply) with up to 120 Watts of

power and was used external to the chamber (see Fig. 2). The plasma was initiated in the microwave cavity by

seeding the upstream flow with ionized gas created by a Tesla coil. The mixture of ultra-high purity O2 and Ar was

therefore activated by the self-sustained microwave discharge which was maintained when the Tesla coil was

switched off. The plasma system was chosen because of its flexibility of being used external to a quartz tube flow

system, as well as its ease of tuning and stability for the range of pressure and oxygen loadings used in the

experiments. The microwave discharge produced excited Ar, as well as multiple oxygen containing species

including O, O3, O2(v), O(1D), O2(a

1Δg), O2(b

1Σg), etc.

C. Measurement of O2(a1Δg)

The O2(a1Δg) produced by the microwave plasma discharge was quantitatively measured using highly sensitive

integrated-cavity-output spectroscopy

(ICOS) by absorption of the Q(12)

transition at the (1,0) band of the b1Σg

+ -

a1Δg Noxon system and is shown in Fig.

3.[23, 25]

The ICOS system measured the

average number density of O2(a1Δg) across

an 82.5 cm long absorption cell downstream

of the plasma. The effective path length was

greater than 78 km due to multiple passes

and provided accurate measurements down

to 1014

molecules/cm3. A more detailed

description of the measurement and process

to obtain absolute number densities of

O2(a1Δg) can be found in references [23]

and [25]. Since the ICOS measurements

required a significant residence time of the

flow in the absorption cell, it was not

desirable to place the cell between the

Figure 2. Schematic of the microwave plasma discharge, variable pressure

chamber, Hencken burner, and picture of low pressure flame.

Figure 3. Schematic of the ICOS system used to quantify

O2(a1Δg).

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microwave plasma discharge and the Hencken burner. This would lead to residence times in excess of 2 seconds for

the flow rates being used and unwanted quenching of O2(a1Δg). Therefore, a technique similar to what was used in

our previous work was adopted.[23]

If all of the flow surfaces are known (as will be discussed in the next section) and

O2(a1Δg) could be measured at different residence times (by changing the tubing length between the plasma

discharge and the ICOS absorption cell), then the O2(a1Δg) concentration as a function of residence time could be

established.

D. Transport of O2(a1Δg)

Similar to our previous work on O2(a1Δg),

[23] the species could be transported significant residence times to a

flame if the flow surfaces and conditions were chosen appropriately. Since O2(a1Δg) is easily quenched in the

presence of O and O3, the normal transport time to the flame from the microwave plasma (~100s of milliseconds)

would allow for only O3 to survive in the gas. If NO is injected directly downstream of the discharge, the O and O3

can be catalytically quenched, allowing for the isolation of O2(a1Δg). Furthermore, NO does little to affect the flame

speed and would not change in concentration because of the catalytic process. Therefore, NO could be injected into

the flow at all times so when the plasma was turned on, the only change in the species would be the production of

O2(a1Δg) from O2. For the present experiments the NO used was 1% by volume in N2, therefore introducing another

species into the system. Fortunately, since the injection was downstream of the plasma, there was no concern about

additional NO production. Furthermore, O2(a1Δg) is not quenched rapidly by N2, with rates similar to that of O2.

Equally as important is wall quenching. Therefore, in order to transport O2(a1Δg) significant residence times, all

flow surfaces would need to be as inert to O2(a1Δg) quenching as possible. It is already well known that O2(a

1Δg)

quenches little on quartz and Teflon based plastics, which allows for transport between the plasma discharge and the

Hencken burner in the present system. The Hencken burner is normally constructed of a variety of materials, and

much of the O2(a1Δg) would be quenched within the burner before reaching

the flame. Because of this, a brief study of O2(a1Δg) quenching on some

common surfaces was performed. To accomplish this, a filter system with a

large surface area, and therefore small pressure drop, was developed and is

shown in Fig. 4. The filter system consisted of a diverging and converging

section with a removable perforated stainless steel filter that was placed in

the middle. The entire system was coated with silica to ensure that it did not

quench O2(a1Δg). Five different materials were tested by placing foil of each

material over the filter and pushing holes through so the surface area did not

change. The filter system was then placed in the flow system between the

plasma discharge and the ICOS cell (see Fig. 3). The results of the relative

concentrations of O2(a1Δg) in an Ar/O2 mixture are shown in Fig. 5, along

with pictures of the foil wrapped filters. The baseline measurement was with

no filter and resulted in 2000 ppm of O2(a1Δg). While the silica coating

showed little quenching, as would be expected, it was interesting to see that

aluminum did not quench O2(a1Δg). Furthermore, copper appeared to be very

effective in quenching O2(a1Δg) to a concentration

that was below the detectability threshold of the

ICOS system.

As a result of the quenching study shown in

Fig. 5, the Hencken burner was modified to be

coated completely with silica. Therefore, all flow

surfaces would be fairly inert and would not

quench O2(a1Δg). In order to confirm that the silica

coated Hencken burner did not quench O2(a1Δg),

an adapter was fitted to the top of the burner,

allowing it to be placed between the plasma

discharge and the ICOS cavity, similar to the

placement of the filter system (see Fig. 3). The

result was up to 2000 ppm of O2(a1Δg) measured

in the ICOS cell for a similar pressure, Ar/O2

mixture, and flow rate as was used previously.

Therefore, it is concluded that the silica coating is

extremely effective at minimizing O2(a1Δg)

Figure 4. Picture of the filter

system used to test quenching of

O2(a1Δg) on different surfaces.

Figure 5. Plot of the relative O2(a1Δg) concentrations versus

different material surfaces.

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quenching and would allow for significant concentrations to be transported to the flame front for detailed

enhancement studies.

III. Results and Discussion

A. The Hencken Flame at Sub-Atmospheric Pressure for Detailed Flame Studies

The Hencken burner design produces a well known and unique flame where the fuel and oxidizer are separated

until the exit of the burner, whereupon they mix rapidly and create a flame at sufficient standoff distances to

suppress heat loss. The flame is stabilized by propagating to a region of mixing as well as by a small amount of heat

loss to the burner surface. The Hencken burner flame is therefore near-adiabatic without the complications of

significant heat loss and flashback.

The flame produced by this burner can correctly track equivalence ratio and is often probed downstream for its

near-adiabatic post-flame gas temperature and nearly-equilibrium species concentrations. Because of this, the

Hencken flame has been used as a calibration source for many laser diagnostic measurements.[26-31]

All of these

experiments have been performed at atmospheric pressure where the flame lies within a few millimeters of the

burner surface, limiting investigation of the detailed structure of the flame and the flame speed. Furthermore, if the

flow velocity at the exit of the burner is increased to a value greater than the laminar flame speed, the flame lifts

from the burner surface and becomes significantly distorted. When the Hencken burner is used at sub-atmospheric

pressure, the flames can be stabilized at significant liftoff heights above the burner surface while remaining flat. This

unique flame behavior was recently investigated in detail[32]

, and therefore only a brief summary is provided here.

The most important aspects of the flame at sub-

atmospheric pressure are that it is steady, laminar,

nearly one-dimensional, minimally curved, weakly

stretched, and near adiabatic. The flat flame front at

high velocities and therefore at significant heights

above the burner is due to the increased diffusivity

at sub-atmospheric pressures. An example of the

increased diffusivity is shown in the acetone planar

laser induced fluorescence (PLIF) image of the non-

reacting flow in Fig. 6. With N2 diluted acetone

flowing through the fuel tubes and air through the

oxidizer honeycomb, the PLIF image shows that there is rapid mixing and a uniform mixture achieved at 3-4 mm

above the burner. Considering that the flames being interrogated throughout this work are at lower pressures and are

between 5 and 15 mm above the burner surface, it is clear that the flame is premixed even though the fuel and

oxidizer are separated at the burner surface. An example of the flat and uniform premixed flame front using a

stoichiometric CH4-air mixture is shown in the OH PLIF image in Fig. 6. This unique system is therefore a good

platform to investigate the effect of plasma produced species on flames because the oxidizer (or fuel) can be

activated by plasma and then the produced species

only interact over a short mixing length with the

fuel (or oxidizer) at ambient temperature (300 K)

prior to the flame front. There would minimal

quenching of plasma species that would allow for

O2(a1Δg), in this case, to be transported to a

premixed flame front.

Because of the increased diffusivity and rapid

mixing above the burner, the premixed flame can

stabilize at significant liftoff heights and is in a

dynamic balance with the local flow velocity. An

example of the stably lifted flames is shown in

Fig. 7 with pictures of the flames and plots of the

axial velocity profiles obtained with particle image

velocimetry (PIV). When the burner exit velocity

was increased above the freely propagating flame

speed, which is approximately 55 cm/s for a

pressure 16.7 kPa, the flames are stabilized with a

shallow velocity gradient and therefore are weakly

Figure 6. False color images of acetone PLIF of non-

reactive mixing above Hencken burner and OH PLIF of a

stoichiometric CH4-air Hencken flame at 16.7 kPa.

Figure 7. Plot of the axial velocity profiles of stoichiometric

CH4-air Hencken flames at different burner exit velocities

at a pressure of 16.7 kPa.

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stretched. The stretch affected flame speeds were extracted from the minimum velocity of the PIV measurements

and the results compared to one-dimensional PREMIX[33]

simulations using GRI-3.0[34]

and USC Mech II[35]

are

shown in Fig. 8. While there is good agreement, the mechanisms have not been validated at the pressures of the

experiments. Nevertheless, the results show that flame speeds can easily be measured and minimal corrections and

extrapolations need to be applied in order to compare to one-dimensional freely propagating premixed flame

simulations. This burner system therefore provides a good platform for investigating the enhancement effects of

plasma-produced species, such as O2(a1Δg), on

flame speed. Beyond the one-dimensional

simulations, it has also been shown that a simple

two-dimensional model can be used to capture the

velocity gradients involved, and therefore the

stretch rates. The results of the two-dimensional

model using the UNsteady Ignition and

COmbustion with ReactioNs (UNICORN) code

that was developed for Navier–Stokes

simulations[32, 36-39]

is shown in the velocity

profiles in Fig. 7 with good agreement to the

experiments. Furthermore, while not shown here,

the absolute OH profiles from calibrated PLIF also

agreed well with the one-dimensional freely

propagating premixed flame simulations.

Overall, the Hencken flame at sub-atmospheric

pressure offers a unique platform for detailed

flame studies and can provide quantitative data on

flame speed enhancement by plasma-produced

species, such as O2(a1Δg), in a regime that is

currently not accessible by other means.

B. Flame Speed Enhancement by O2(a1Δg)

Since it was established that the Hencken flames at sub-atmospheric pressure were in a dynamic balance with the

local flow velocity, any enhancement of the flame speed would be evident by a change in flame liftoff height.

Therefore, when the plasma was turned on and O2(a1Δg) was introduced, the flame speed should increase and the

flame liftoff height should decrease. Three different fuels were used in order to gain a general understanding of the

enhancement effects of O2(a1Δg). Pictures of H2-, CH4-, and C2H4-Ar/O2/NO/N2 flames are shown in Fig. 9 at a

pressure of 4 kPa. Take note that the H2 flame was visible because of the NO that was being injected into the

system, not because of impurities in the fuel. When the NO was turned off, the H2 flame could not easily be seen via

detection of visible wavelengths. When

working with both H2 and CH4, the

flames were very stable for lean and rich

equivalence ratios, respectively, and

therefore were only investigated in those

regions in this study. The C2H4 flames

were stable over a wide range of

equivalence ratios spanning

stoichiometric, and therefore provided

the most comprehensive data set.

The concentration of O2(a1Δg) present at the exit of the burner was not measured directly, but was instead

inferred from the measurements performed in section II.D. The measurements of O2(a1Δg) in section II.D. were

performed downstream of the coated Hencken burner in the ICOS cell and therefore were underestimates of the

actual concentrations because of the extended residence times. For similar pressures, flow residence times between

the plasma and Hencken burner, and flow conditions of Ar/O2/NO/N2, the O2(a1Δg) concentration was found to be

approximately 2000-3000 ppm for the CH4 and C2H4 flames. The similarity for both CH4 and C2H4 came from the

O2 loading being approximately 25%, which led to a total conversion of O2 to O2(a1Δg) of about 1%. For H2 the O2

loading was much higher at 45%, which therefore decreased the conversion efficiency of O2 to O2(a1Δg).

Nevertheless, the O2(a1Δg) concentration remained of the same order of magnitude.

Figure 8. Plot of Hencken flame speeds compared to one-

dimensional PREMIX simulations using GRI-3.0 and USC

Mech II at 16.7 kPa.

Figure 9. Pictures of H2-, CH4-, and C2H4-Ar/O2/NO/N2 Hencken

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The process for investigating the change in

flame liftoff height with O2(a1Δg) addition from

the plasma started with the flame being ignited and

the pressure brought to the desired value. The

microwave discharge was then turned on and a

photograph was taken of the flame. The

microwave discharge was then turned off and

another picture was taken of the flame after a few

seconds, to ensure that no O2(a1Δg) remained in the

system. A plot of the change in flame liftoff

heights with O2(a1Δg) addition is shown in Fig. 10.

For H2 flames, the flame liftoff height was

consistently lower when the plasma was on and

O2(a1Δg) was present at the flame front. While the

change in flame liftoff height was found to be

approximately 0.2-0.5 mm, the flame was very

stable with fluctuations no larger than 0.05 mm.

Therefore, the average change in flame liftoff

height across the small equivalence ratio range of

0.33-0.52 was approximately 0.3 mm. For CH4

flames, which were at an equivalence ratio around

5.2, the change in flame liftoff height was found to

be approximately 0.2 mm. Unfortunately, since the

range of equivalence ratios investigated for H2 and

CH4 was limited, it is difficult to make any

conclusions as to the dependence on

stoichiometry. Nevertheless, it was clear that both

the H2 and CH4 flame speeds were enhanced by

O2(a1Δg).

On the other hand, the C2H4 flames were stable

over a large range of equivalence ratios spanning

stoichiometric. A plot of the change in flame

liftoff height for C2H4 only is shown in Fig. 11.

Across the pressure range of 2.9-4.3 kPa, it is

evident that there is a strong dependence of

enhancement on equivalence ratio. The dotted

trend line in the figure was drawn to represent the

approximate dependence with greater

enhancement for lean and rich stoichiometries.

This trend is consistent with those from previous

numerical investigations with H2 and CH4 fuels.[19-21]

While the flame speed enhancement by O2(a1Δg) was not quantified using PIV, it is clear that this platform is

conducive to these validation measurements. Nevertheless, some general comparisons to past work can be made.

Past simulations of the Hencken flame at pressures around 5 kPa have shown that approximately 2000 ppm of O3

could change the C2H4 flame liftoff height by 0.35-1 mm, leading to flame speed enhancement of 1.8-3.1%.

Therefore, it is reasonable to make the comparison that for the C2H4 flame liftoff height changes shown in Fig. 11,

O2(a1Δg) enhanced the flame speed approximately 2%. To put this into perspective, past experimental work using a

C2H4 tribrachial lifted flame with O2(a1Δg) addition yielded approximately the same flame speed enhancement for

the same O2(a1Δg) concentrations.

[23]

As mentioned in the introduction, there has been some numerical work and kinetic mechanism development for

O2(a1Δg) enhancement of both H2 and CH4 flames. For the H2 numerical work

[19] the lowest equivalence ratio and

pressure investigated was 0.5 and 20 kPa, respectively. This was a higher equivalence ratio and pressure than the

current experiments, but still of the same order of magnitude. The numerical result was an approximately 18%

increase in the flame speed with 5% of the O2 converted to O2(a1Δg); the primary mechanism for the enhancement

was from the chain-branching reaction of H+O2(a1Δg) → O+OH. For the current experimental results, 0.5% of the

O2 was converted to O2(a1Δg), and the change in flame liftoff height of 0.3 mm equates to approximately 2% flame

Figure 10. Plot of the flame liftoff height changes for H2,

CH4, and C2H4 Hencken flames at different pressures and

equivalence ratios.

Figure 11. Plot of the flame liftoff height changes for C2H4

Hencken flames versus equivalence ratio.

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speed enhancement. A comparison between the numerical work and experiments yields the same approximate

trends. For the CH4 numerical work[21]

the comparison was nearly impossible because the pressure was 101 kPa and

the O2 conversion to O2(a1Δg) was 10%; both parameters are well outside the experimental conditions of 3-4 kPa and

1% O2 conversion to O2(a1Δg). Nevertheless, the mechanism of enhancement for CH4 by O2(a

1Δg) in the numerical

simulations were found to be through the CH3 reactions of CH3+O2(a1Δg) → CH3O+O and CH3+O2(a

1Δg) →

CH2O+OH. The reason that the enhancement mechanisms by O2(a1Δg) is mentioned here is because it is believed

that the mechanism of enhancement for C2H4 may take a different form. The double carbon bond of C2H4 could be

attacked by O2(a1Δg), therefore providing a chain initiation reaction instead of a chain branching reaction like for H2

and CH4. This in turn may lead to significantly different levels of combustion enhancement.

IV. Summary and Conclusions

Using a new coated Hencken burner platform at sub-atmospheric pressure to quantify the flame speed

enhancement by O2(a1Δg) holds great promise. The results for H2, CH4 and C2H4 flames have provided some of the

first experimental evidence of the isolated effect of O2(a1Δg) on flame speed enhancement versus equivalence ratio.

Significant concentrations of O2(a1Δg) could be introduced to the flame front with little quenching during transport

from the plasma discharge. The flames could be established with a variety of fuels across a range of equivalence

ratios and pressures, all while maintaining a steady, laminar, nearly one-dimensional, minimally curved, weakly

stretched, and near adiabatic structure. Comparisons of the experimental results to one-dimensional freely-

propagating flame models emphasized the highly decoupled nature of the flame, which will allow for more focus on

the kinetics and therefore the development and validation of plasma-flame mechanisms.

Changes in the flame liftoff heights, and hence enhancement of the flame speeds, were observed when O2(a1Δg)

was introduced to H2, CH4, and C2H4 flames. While the H2 and CH4 flames were limited to lean and rich equivalence

ratios, respectively, the C2H4 flames provided results that showed the increased levels of flame speed enhancement

for lean and rich versus stoichiometric.

With the results clearly showing the advantages of using the new burner platform for flame speed studies with

O2(a1Δg) addition and moving in the direction of validating the enhancement shown in numerical simulations, more

detailed and quantitative studies are warranted. A plasma discharge that can couple more energy into the flow for

elevated levels of O2(a1Δg) is needed, along with a temporally and spatially resolved O2(a

1Δg) measurement above

the burner surface. Furthermore, laser diagnostics of the flame speed enhancement and detailed species profiles

should be performed. Nevertheless, the Hencken flame at sub-atmospheric pressure offers a unique quantifiable

platform to investigate the enhancement of flame speed by O2(a1Δg). Given the significant enhancement of flame

speed by O2(a1Δg) that has been shown in numerical simulations, the present and future results using the new burner

platform has the potential to provide much needed experimental validation.

Acknowledgments

Part of the research was performed while the corresponding author held a National Research Council Research

Associateship Award at the United States Air Force Research Laboratory, Wright-Patterson Air Force Base. The

work was also supported by the Air Force Office of Scientific Research.

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