1

The Structure and Stability of the Laminar Counterflow Partially

Premixed Methane/Air “Triple-Flame”

R.D. Lockett, B. Boulanger+, S.C. Harding

#, D.A. Greenhalgh

School of Mechanical Engineering, Cranfield University, Cranfield Beds, MK43 0AL,

United Kingdom

ABSTRACT

The flame stability map defining the regime of existence of a counter-flowing laminar

partially premixed methane-air “triple flame” has been determined using OH Planar Laser

Induced Fluorescence (PLIF). The stability limits were determined through the

observation of flame merging and flame extinction, a function of rich and lean equivalence

ratios, and mean axial strain rate.

Relatively quantitative OH species profiles and Rayleigh scattering profiles have been

measured for three flame conditions. Axial flow velocity profiles, and nozzle exit

velocity profiles have been determined for two of the three conditions using 1-D Laser

Doppler Velocimetry (LDV). The diffusion flame extinction axial velocity profile has

been measured, and the local extinction axial strain rate has been determined to be 710s-1

.

+ Current address University of Liege.

# Current address Rolls-Royce Bristol.

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1. INTRODUCTION

Flame development occurs in turbulent fuel-air mixtures exhibiting both mixture gradients

and fluid strain in many modern combustion systems (gas turbine, internal combustion

engine). Flame response to fluctuating mixture gradients and fluctuating strain will

deviate from the behaviour predicted from simple diffusion flame theory or premixed

flame theory. The description of the combustion in these systems in terms of either

diffusion flame models or premixed flame models is thus incomplete. The combustion

occurring in such systems is termed “partially premixed” [1,2]. Consequently, there have

been a growing number of studies recently in partially premixed combustion.

One of the early systematic studies of partially premixed counter-flow flames was

conducted by Yamaoka et al [1]. This paper reported an experimental investigation of the

structure of partially premixed counter-flow “double flames”, stabilised in the forward

stagnation region of a porous cylinder. The authors also investigated the lean and rich

flame stability limits of this flame system for a specific strain rate.

This work was followed by that of Seshadri et al [2], in which the authors report the results

of an experimental and theoretical study of extinction of partially premixed diffusion

flames. The study was conducted in a counter-flow burner; one stream methane diluted

with nitrogen, the other stream diluted air. Small quantities of methane were added to the

diluted air stream in the first case, and a small quantity of air was added to the fuel stream

in the second case. In both cases, a partially premixed diffusion flame with two reaction

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zones was formed. The extinction strain rate was measured, and compared with modelled

results.

Lin et al followed this with an experimental study of the transition of diffusion flames to

premixed flames in partially premixed flame systems [3]. This paper identified “double

flame” structures and “triple flame” structures observable in counter-flowing partially

premixed butane-air streams, and examined some basic conditions necessary for the

existence of these flames. Tanoff et al have recently performed numerical calculations of

counter-flow “double flames”, and compared the results with experimentally obtained

species profiles for the same flame conditions [4]. The rich mixture stream had an

equivalence ratio = 1.4, and the flame was beginning to show “double flame” structure.

The other studies in “triple flame” structures have examined the properties of co-flowing

partially premixed “triple flames” [5, 6].

The axially symmetric laminar counter-flow flame geometry is convenient for the

examination of flame stretch (flame response to flow strain). The use of an appropriate

stream function in conjunction with the Navier-Stokes equations reduces the

multi-dimensional nature of the problem to a single space dimension. Knowledge of the

axial velocity profile, and the axial density profile (through experiment or modelling)

enables the calculation of the local flame stretch rate. Consequently, many studies have

examined axially symmetric back-to-back premixed flame structures [9-11], and

fresh-to-burnt geometries [11,12] for the determination of flame response to flow strain,

the determination of laminar flame velocities as a function of fuel type, and as a function of

pressure.

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Therefore an axially symmetric counter-flowing geometry has been chosen to examine the

response of partially premixed flames to fluid strain and mixture gradient. This paper

reports on a laser diagnostic study (Laser Doppler Velocimetry (LDV), OH Planar Laser

Induced Fluorescence (PLIF) & Rayleigh Scattering) of the flame structure of

counter-flowing partially premixed methane-air flames, which exhibit themselves as

counter-flowing “triple flames” or as “thick double flames”. Full experimental details of

the laser diagnostics employed are provided in the following section.

2. EXPERIMENTAL METHOD

2.1: The Counter-flow Burner

The Counter-flow burner which was designed and constructed at Cranfield University is

shown in Figure 1. It consists of two opposed burner nozzles inside two opposed annular

sections. The opposed burner nozzles are 38 mm in diameter, and the inter nozzle spacing

is variable. The mixed gas flow through each nozzle passes through a bead cage, and a

scintered bronze disc, to produce plug flow at the nozzle exit. All individual gas flows are

metered using pressure transducers upstream of, and downstream of, calibrated sonic

nozzles. Gaseous fuels are mixed uniformly with air in user determinable proportions,

using a dense wire mesh upstream of two sets of baffle plates in the mixing zone. The

mixing of gases occurs downstream of the sonic nozzle metering system. For these

experiments, 99.9% CP grade methane was mixed with filtered air supplied by a standard

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laboratory compressor (maximum line pressure 7.5 bar). The annular sections outside the

burner nozzles were used to provide a flame stabilising nitrogen curtain.

Fig. 1. Counter-flow burner.

The burner was designed to provide user determinable, stabilised, laminar, opposed plug

flows of premixed fuel-air mixtures of independently variable stoichiometry for each

nozzle. This defines an axially symmetric counterflow system with mixture gradients.

2.2: The Counter-flowing Laminar Methane-Air “Triple Flame”

The counter-flowing laminar methane-air “triple flame” consists of two premixed

counter-flowing laminar methane-air jets, one jet mixed rich, the other mixed lean.

Combustion produces two strained premixed flames, one rich, and the other lean.

Between these two strained premixed flames, excess oxygen passes from the lean flame

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towards the stagnation plane, and excess CO, atomic and molecular hydrogen pass from

the rich flame towards the stagnation plane. These two hot flows react predominantly as a

pure wet CO-hydrogen-air diffusion flame. Therefore the laminar “triple flame” appears

as a laminar diffusion flame sandwiched between the strained laminar lean and rich

premixed flames.

Fig. 2. Laminar methane–air triple flame in counter-flow burner.

Figure 2 is a photograph showing a laminar counter-flowing “triple flame” produced in the

counter-flow burner. The strained rich premixed flame is the lower flame, the strained

lean premixed flame is the upper flame, and the CO/H2 diffusion flame is the centre flame.

2.3: The “Triple-Flame” Stability Map

Counter-flowing “triple flames” undergo strain as a consequence of the local flow velocity

gradients. The local axial strain affects the stability of the flame structure, and determines

the mixture strength at which flame merging or flame extinction occurs. Counterflowing

“triple flame” stability limits are determined in terms of merging of one of the premixed

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flames with the diffusion flame, producing a “double flame”; or axial strain extinction of

the diffusion flame; or flashback. Thus a regime of “triple flame” stability exists, defined

by the dependence of rich and lean mixture strength merging limits on axial strain,

flashback, simultaneous rich and lean mixture strength support for the diffusion flame, and

axial strain extinction of the diffusion flame. For axial strain rates above the strain

extinction limit, partially premixed counter-flowing flames appear as thick “double

flames”, consisting of two premixed flames.

The hydroxyl radical is principally responsible for fuel oxidation in both premixed flames

and the diffusion flame. Elevated levels of OH radical concentration relative to equilibrium

indicate flame reaction zones. The presence of peaks in the OH radical concentration

profile along the symmetry axis is a good indicator of flame existence and position. The

counter-flowing “triple flame” produces three elevated regions of OH, indicating the three

reaction zones. Thus the presence or absence of OH is a good indicator of the presence or

absence of one of the flames in the “triple flame” system.

2.4: OH Planar Laser Induced Fluorescence (PLIF)

The regime of “triple flame” stability was determined using OH PLIF. The OH

fluorescence intensity is proportional to the local OH concentration, and was thus used to

determine the presence or absence of one of the flames in the “triple flame” system. A

Lambda Physik EMG-150 MSC excimer laser was employed in narrow-band tuneable

mode, operating at 308.24 nm, exciting the Q1(3) line in the 0 - 0 band of the A X

transition in OH. The EMG-150 laser produces pulses of approximately 20 ns duration,

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and 145 mJ energy. The laser light was directed through a 2 m spherical lens and a

cylindrical telescope, producing a laser sheet of height 25 mm, and width 0.2 mm. The

fluorescence from the excited OH was on-resonance, and was imaged onto a gated,

intensified lens coupled Astromed CCD camera, using a Nikon Nikkor 105 mm f4.5 uv

lens.

The “triple flame” stability map was determined by mapping the merging limits of the lean

and rich premixed flames with the diffusion flame, determined by the critical mixture

strength at which flame merging occurs as a function of nozzle exit velocity, or mean axial

strain rate. The mean axial strain rate a is defined as

ad

lower upperv v

,

where |vlower| and |vupper| are the lower and upper nozzle exit speeds, and d is the inter-nozzle

spacing.

The merging limits were determined through a qualitative measurement of the position of

the three flames relative to each other along the centreline axis using OH PLIF. Various

consistency tests were applied to the flame stability data. The dependence of the “triple

flame” stability data on mean axial strain rate, and independence of inter-nozzle spacing

was checked by measuring the variation of flame merging limits for inter-nozzle distances

of 25 mm, 30 mm, and 37 mm, and correcting for the mean axial strain rate.

The dependence of the merging limits of the rich premixed flame on the lean flame

stoichiometry was tested by determining the rich merging limits for various lean flame

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stoichiometries. The dependence of the merging limits of the lean premixed flame on the

rich flame stoichiometry was tested by determining the lean flame merging limits for

various rich flame stoichiometries. Buoyancy and heating effects in the unburned gas

flows were tested by reversing the mixtures between upper and lower burners.

Relative quantitative OH PLIF measurements have been performed for three partially

premixed flame conditions. These measurements employed the Lambda Physik EMG

150 MSC excimer laser operating in narrow band tunable mode, as described above. The

fluorescence was imaged onto a gated Princeton Instruments ICCD camera, using a Nikon

Nikkor 105 mm f4.5 uv lens. Table 1 below summarises the three “triple flame”

conditions for which relative quantitative OH concentration profiles and Rayleigh

scattering profiles have been obtained.

Table 1: Triple Flame Conditions for Quantitative Diagnostics

Condition

rich

lean

Mean Strain

Rate (s-1

)

1

1.50

0.60

44.1

2

1.52

0.72

59.0

3

1.40

0.72

74.0

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It was determined that the OH PLIF produced using this laser was partially saturated. An

approximate correction was made for the partial saturation, by assuming that the functional

form of the laser profile at focus could be described by a top hat. This approximation

produces an error of about 5% in deduced OH concentration. On-resonance fluorescence

imaging is also subject to contamination from background scattering and Rayleigh

scattering. The background was subtracted from the images using standard techniques.

Rayleigh scattering intensity in the flame zone was estimated from the Rayleigh scattering

intensity in the unburnt gas region, and subtracted from the flame images.

The Q1(3) line of the 0 - 0 transition has a population fraction of about 0.084 at 1800 K,

with a temperature dependence of -0.004 per 100 K change in temperature. Therefore, if

the rich and lean premixed flames are about 100 K lower in temperature than the diffusion

flame, then the rich and lean OH peaks are over-predicted by about 5%. Radiation

trapping is a further source of systematic error, estimated at about 10 %. The resulting

imaged fluorescence intensity is proportional to OH concentration to an accuracy of about

13%.

2.5: Rayleigh Scattering

The determination of the local flame stretch rate requires the determination of the axial

density profile and the axial velocity profile [7,8]. The most accessible diagnostic for the

determination of mixed gas density is 2-d Rayleigh scattering. Rayleigh scattering from

the “triple-flame” combustion zone produces a Rayleigh cross-section weighted axial

density profile. Modelled species profiles and a temperature profile enables the

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computation of a model Rayleigh cross-section weighted axial density profile for

comparison.

Rayleigh scattering measurements have been performed for three partially premixed flame

conditions, corresponding to those specified in Table 1. A Spectra Physics GCR-270

Nd:YAG laser was employed, operating at 532 nm. The GCR-270 laser is TEM00

injection seeded, and produces up to 900 mJ per pulse at 532 nm, of 7 ns to 9 ns pulse

duration. The laser was operated at approximately 400 mJ per pulse for the Rayleigh

scattering measurements reported here.

The laser light was directed through a 2 m lens and a 4x inverted cylindrical telescope, to

produce a sheet 32 mm high, and 0.2 mm wide. The outer edges of the sheet were

truncated using an iris, producing a sheet 22 mm high in the flame. The Rayleigh

scattering from the flame gases were imaged onto a gated Princeton Instruments ICCD

camera, using a Nikon 50 mm f1.2 lens.

Background images and flat-field images were required for processing of the Rayleigh

scattering data. Three sets of 40 images were obtained from the measurement zone,

saturating the region with helium, air and methane respectively. The background image

was obtained using the mean helium Rayleigh scattering image against the mean methane

Rayleigh scattering image, using the expression

background He

He

methane He

methane HeS S S S

( )( )

,

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where SHe and Smethane are the mean helium and methane Rayleigh scattering images

respectively. He and methane are the helium and methane Rayleigh scattering

cross-sections respectively [13].

The background image derived above possessed two sources of image contamination

which had to be corrected. The first source of image contamination was caused by

intensifier defocussing. Intense Rayleigh scattering from surrounding air was focussed by

the intensifier onto the low background level. The point spread function was estimated by

modelling the effects of intensifier defocussing, and comparing the results with the

background data. The point spread function was found to be well described by a

Gaussian-Lorentzian product. A Wiener function was then used, together with the

derived point spread function to recover the true background image.

The resulting background image still possessed a low level of second order scattering

contamination on parts of the background, not present on the Rayleigh data images

(approximately 15% of the mean background scattering intensity, and approximately 10%

of the Rayleigh scattering intensity in the flames). This second order scattering

contamination was subtracted from the refocused background derived above to produce the

final background image.

The laser flat-field image was obtained by subtracting the mean background derived above

from the mean Rayleigh scattering image of methane. The Rayleigh scattering image data

was processed by subtracting the final derived background from the data images, and then

flat-fielding the data images with the above laser flat-field. The Rayleigh scattering data

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for flame condition 1 was incomplete in that the laser sheet was not quite wide enough to

include the unburned rich mixture fully. However, the sheet did include the edge of the

unburned lean mixture, and the flame zone.

2.6: 1-D Laser Doppler Velocimetry (LDV)

1-D LDV measurements of the counterflow velocity profile were performed for “triple

flames” Condition 1 and Condition 2 specified in Table 1 above. A third partially

premixed flame condition was subjected to LDV measurement; and that condition

corresponded to axial strain extinction of the diffusion flame.

The scintered bronze discs were removed from the upper and lower burner nozzles for

these measurements, as they would have become clogged up with seed. This meant that

the flow was marginally less stable at low nozzle exit velocities (vrms/v < 0.01, v < 1.5

m.s-1

), but became progressively more unstable at higher nozzle exit velocities (v > 2.0

m.s-1

). The flow was found to be moderately turbulent (vrms/v ~ 0.1) at axial strain

extinction.

A Lexel Model 95 4W cw Argon Ion laser was employed, operating in single line (TEM00)

mode at 514.5 nm. The laser beam passed through a Dantec beam splitter/Bragg shifter

which split the beam into two, and then shifted the frequency of one of the beams by 30

Mhz. The two beams were then directed separately through a Dantec 3-D optical

distributor into an optical fibre cable to a 500 mm focal length Dantec LDV beam crossing

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lens. The fundamental mode and the Bragg shifted mode were brought to a focus and

crossed along the symmetry axis between the two opposing burner nozzles.

The counter-flowing premixed gas flows were seeded with 0.25 m zirconium dioxide

particles, obtained using two identical cyclone seeders. Each of the premixed flows

entered a bypass tube by choking the main flow tubes with needle valves. The bypass

tubes led into the cyclone seeders. The seeding density in each flow was varied by

adjusting the needle valve in the main flow tube, regulating the flow into the seeders.

The light scattered back from the seed particles were imaged into an optical fibre, and

passed though to a photo-multiplier tube. The Doppler burst data captured by the

photo-multiplier tube were then analysed using a Dantec 3-D Burst Spectrum Analyzer

(BSA), using Dantec’s Burstware software.

Possible biasing in the measurement velocities of the opposed flows due to different

seeding densities was eliminated by ensuring that the data capture rate was the same for

both flows, for the same absolute displacement from the nozzles. A cold flow axial

velocity of zero was obtained at the midpoint between the nozzles.

LDV measurements of axial and radial velocities were obtained along the symmetry axis,

and 1.5 mm radial displacement from the symmetry axis. These measurements were

obtained at 1 mm intervals between the opposed nozzles for small velocity gradients, and

0.5 mm intervals for large velocity gradients. Radial LDV measurements of axial velocity

were also obtained 4 mm above the lower nozzle, and 5 mm below the upper nozzle at

1mm intervals across the nozzle diameters for all axial velocity measurements presented

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here. The limits of the upper and lower heights of these radial measurements were

determined by the proximity of the crossing laser beams to the edge of the nozzle.

3. RESULTS AND DISCUSSION

3.1: The “Triple-Flame” Stability Map

Figure 3(a) shows a hydroxyl (OH) radical concentration profile obtained using OH PLIF

in the laminar methane-air counter-flowing “triple flame”. The “triple flame” structure

can be seen clearly in terms of local OH concentration peaks. The flow and mixture

conditions for this triple flame were rich = 1.20, lean = 0.80, mean axial strain rate a = 180

s-1

.

Increasing the rich mixture strength lowers the strained rich premixed flame speed, which

causes the strained rich premixed flame to move closer towards the stagnation plane. At

some point, the rich premixed flame begins to merge with the diffusion flame, and the two

flames cannot be distinguished from each other. Flame merging has taken place. An

example of this can be seen in figure 3(b), which shows an OH concentration profile in a

partially premixed counter-flow methane-air flame where the rich flame has completely

merged with the diffusion flame, and cannot be identified. The flow and mixture

conditions for this flame profile were rich = 1.34, lean = 0.80, mean axial strain rate a = 180

s-1

.

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Fig. 3. OH Concentration as a function of distance along burner axis; (a)

standard triple flame, (b) rich flame merg- ing, (c) lean flame merging.

Obtaining the dependence of the strained rich premixed flame merging limits on axial flow

strain rate as described above defines the regime of existence of the strained rich premixed

flame. The regime of existence of the strained lean premixed flame and the diffusion

flame can be similarly determined. Leaning out the lean premixed flame causes the lean

flame speed to decrease, and the flame is observed to move towards the stagnation plane,

eventually merging with the diffusion flame. Lean flame merging has occurred. Figure

3(c) shows an OH concentration profile where lean flame extinction, or merging has

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occurred. The flow and mixture conditions for this flame were rich = 1.20, lean = 0.68,

mean axial strain rate a = 180 s-1

.

The diffusion flame only exists if there is simultaneously enough oxygen passing from the

lean premixed flame to the stagnation plane, and CO, atomic and molecular hydrogen

passing from the rich premixed flame to the stagnation plane to react to form a diffusion

flame. If these conditions are not satisfied, the diffusion flame will extinguish.

Finally, there exists a critical flame stretch (or axial flow strain), above which the CO/H2

diffusion flame is extinguished. Flame stretch effects dominate the diffusion mass flux

across the stagnation plane when the axial flow strain rate is greater than the axial

extinction strain rate. This means that the rate of increase of flame surface area is larger

than the diffusion mass flux necessary to sustain the flame. The critical local axial strain

rate for extinction of the diffusion flame was measured to be 710 s-1

using 1-D LDV. This

can be seen from figure 9, which profiles the mean flow velocity for the axial strain

extinction condition. For axial strain rates larger than this critical strain rate, the

counter-flowing partially premixed flame structure is best described as a “thick double

flame”. Putting these datasets together defines the regime of existence of the laminar

“triple flame”, which is shown in figure 4.

Firstly the consistency tests mentioned in section 2.4 require some discussion. The “triple

flame” stability map was found to be independent of inter-nozzle distance over the range

25mm to 37mm within the 1% experimental error in mixture stoichiometry. The rich

flame merging limits were found to be independent of the lean flame stoichiometry within

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the 1% experimental error. The lean flame merging limits were similarly found to be

independent of the rich flame stoichiometry.

Fig. 4. Triple flame stability map.

Secondly the flame stability map in figure 4 requires some explanation. The ordinate

describes the stoichiometry of both nozzle flows. In practice no “triple flame” will exist if

these stoichiometries are equal and by definition the lean flame stoichiometry cannot

exceed the rich flame stoichiometry. The map shows that to first order the diffusion flame

extinction is dominated by the rich flame stoichiometry; however there is a slight

sensitivity to the lean stoichiometry. Increasing the lean flame stoichiometry from the

lean merging limit to stoichiometric causes a slight increase in the stoichiometric

extinction limit for the difusion flame from rich ext. = 1.18 to rich ext. = 1.21. Therefore the

2-D map in figure 4 is strictly 3-D as far as the diffusion flame is concerned, however

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because the variation is close to experimental error, plotting figure 4 in a 3-D form would

be confusing.

Thus to establish whether a triple-flame is predicted to exist the following is required.

Firstly select the lean stoichiometry to be less than 1.0 and more than the lean merging limit

for the mean axial strain rate, secondly choose the rich flame stoichiometry to fall

between the rich = 1.18 extinction limit (the diffusion flame is extinguished for rich <

1.18) and the rich flame merging limit for the same mean axial strain rate. The latter

criteria is clearly seen to be the most restrictive and therefore we can conclude that the

“triple flame” is strongly sensitive to both the rich flame stoichiometry and the axial strain

rate.

The temperature of the rich and lean mixture streams has a strong global effect on the

“triple flame” stability map. The upper burner nozzle reaches a steady state temperature

approximately 60 K to 100 K higher than the lower nozzle. This temperature difference

between the two nozzles produces a systematic variation in the lean merging limit of -0.001

K-1

, and a systematic variation in the rich merging limit of +0.0008 K-1

. For example, if

the temperature of the unburnt lean mixture was 300K, and it had a lean merging limit of

= 0.600: then if the temperature of the unburnt lean mixture was raised to 360K, the lean

merging limit would change to = 0.540.

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3.2: Relative Quantitative OH PLIF

Figures 5 and 6 show the measured velocity profiles, the OH concentration profiles, and the

Rayleigh scattering profiles for “triple flame” Condition 1 and Condition 2 respectively.

Comparing the position of the OH concentration peaks in the diffusion flames for

Conditions 1 and 2 with the stagnation planes defined by the zero point velocity in the axial

velocity profiles suggests that the diffusion flames occur on the lean side of the stagnation

plane. This is to be expected for methane/air “triple flames” as molecular hydrogen and

CO have larger molecular diffusivities than air, and thus would diffuse through the

stagnation plane at a larger rate.

Flame Condition 1 has the lowest strain rate of the three conditions presented here (a1 = 50

s-1

). The premixed flames appear relatively unaffected by the flow strain rate. Figure 7

shows the measured OH concentration profile, and the Rayleigh scattering profile for

“triple flame” Condition 3. Using figures 6 and 7 to compare flame Condition 2 with

Condition 3 reveals that the “triple flame” thickness is approximately the same for these

two conditions, even though the axial strain rates and rich mixtures differ. The diffusion

flame in Condition 3 appears hotter than the diffusion flame in Condition 2 through the

larger relative OH concentration. This observation is supported by the fact that the

Rayleigh scattering intensity is lower for Condition 3 than for Condition 2 (see next

section).

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Fig. 5. Condition 1; (a) axial LDV velocity, (b) axial OH, (c) axial Rayleigh.

The lean premixed flames for Condition 2 and Condition 3 have the same equivalence ratio

( = 0.7). However, the OH concentrations in these flames are different. This is a

consequence of the local strain rate. Flame Condition 2 experiences a larger local strain

rate than flame Condition 3, causing a reduction in the flame temperature and net OH

concentration. The spacing between the lean premixed flame and the diffusion flame is

also different for these two cases. This is a consequence of the hot flow field produced by

the respective rich premixed flames. The Condition 3 rich premixed flame is richer than

the Condition 2 rich premixed flame, producing a lower hot flow velocity and strain rate.

This alters the hot flow strain rate profile and causes a slight shift in the stagnation plane

towards the rich premixed flame. The Condition 3 rich premixed flame is also slower then

the Condition 2 rich premixed flame, and produces a lower OH concentration. This is

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clear from the relative spacing between the rich premixed flames and the diffusion flames

for these two conditions.

Fig. 6. Condition 2; (a) axial LDV velocity, (b) axial OH, (c) axial Rayleigh.

Fig. 7. Condition 3; (a) axial OH, (b) axial Rayleigh.

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3.3: Rayleigh Scattering

The flame Condition 3 Rayleigh scattering profile from the lower unburned rich mixture

was used to standardise the other profiles. The equivalence ratio of this mixture was =

1.51, and the mixture temperature was taken as 293K, the measured upstream air and

methane temperature.

The difference between the Rayleigh scattering intensity on the rich and lean sides is due to

two factors. The first, and most obvious, is the difference in the equivalence ratios for the

rich and lean mixtures, causing a larger weighted Rayleigh scattering cross section on the

rich side. The second factor, and most important, is the elevated temperature of the upper

flow. The upper burner is radiatively heated from a burner protection plate. Even though

the upper burner was water cooled, the gas mixture passing through the upper burner

nozzle was still heated to around 360 K.

Comparing the Rayleigh scattering intensities from flame Condition 1 (figure 5) and flame

Condition 2 (figure 6) suggests that the unburned lean mixture from Condition 1 is slightly

cooler than the unburned mixture from Condition 2. Assuming similar combustion

products for these two conditions, the flame temperatures appear to be very similar for

these two flames. The combustion enthalpy is clearly larger for lean flame Condition 2

than for lean flame Condition 1, as indicated by the velocity profiles in figures 5 and 6,

however this is balanced by a higher axial strain rate which will act to lower the flame

temperature.

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Using figures 6 and 7 to compare the Rayleigh scattering intensities from flame Condition

2 with flame Condition 3 reveals that the Condition 3 flame has a significantly higher

temperature than the Condition 2 flame. This can be seen from the ratio of the Rayleigh

scattering intensities in the two flame zones. In this case, the lower Rayleigh scattering

intensity is indicative of a smaller number density, and thus a larger flame temperature.

As stated earlier, the Rayleigh scattering profile for flame Condition 1 is incomplete in that

the laser sheet was not quite wide enough to include the unburned rich mixture.

3.4: 1-D LDV

The counter-flow burner was designed to produce plug flow at the exit of the burner

nozzles, for a gas flow from the nozzles into a stationary gas medium. However, when

two such flows oppose each other, the plug flow from each nozzle is altered. Figures 8

and 9 show axial velocity measurements performed 4 mm above the lower nozzle and 5

mm below the upper nozzle for “triple flame” Conditions 1 and 2 respectively. The

nozzle exit velocities are a minimum in the centre of the nozzle, and slowly increase to a

maximum just inside the nozzle edge. The measurements performed in this study are

consistent with those performed by Rolon et al [14].

The deviation from plug flow is the consequence of the decrease in the axial velocity

gradient with radius from the centre of the burner nozzles. The axial velocity gradient

produces a small back pressure at the nozzle exit; and the greater the magnitude of the axial

velocity gradient, the greater the back pressure generated in the flow. Therefore, a

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radially dependent back pressure gradient is generated across the nozzle. This back

pressure gradient causes the flow velocity profile at the nozzle exit to deviate from plug

flow to that shown in Rolon et al [14], as demonstrated by figures 8 and 9. Clearly any

numerical flame model attempting to reproduce this flame data should take account of this

effect.

Fig. 8. Axial nozzle exit velocity as a function of radial distance across burner

nozzles for flow condition 1. Upper curve is for the upper nozzle and lower curve

for the lower nozzle. Downward velocities are arbitrarily defined as positive for

both curves.

Fig. 9. Nozzle exit velocity as a function of radial distance across burner nozzle:

condition 2. Upper curve is for upper nozzle and lower curve for lower nozzle.

Downward veloc- ities are arbitrarily defined as positive for both curves.

26

The velocity measurements for flame Conditions 1 and 2 (velocity profiles in figures 6 and

7 respectively) show that the flow was laminar with a measured standard deviation of less

than 1%. The velocity measurement for diffusion flame extinction showed that a

moderate turbulence level existed in the flow (vrms/v ~ 0.1). However, the measured

diffusion flame extinction strain rate of [dv/dy]ext = 710 s-1

remains valid. The velocity

profile for this condition is shown in Figure 10.

Fig. 10. LDV axial velocity profile at axial strain rate extinction of diffusion flame.

A comparison of the change in flow velocity across the Condition 1 and Condition 2 rich

premixed flames reveals that the reaction enthalpy from flame Condition 2 was reduced

relative to the reaction enthalpy from flame Condition 1. This is a consequence of the

larger axial strain rate experienced by the Condition 2 rich flame relative to the Condition 1

rich flame. The larger reaction enthalpy of the Condition 2 lean flame relative to the

Condition 1 lean flame is a consequence of the relative equivalence ratios.

A comparison of the velocity profiles from Condition 1 with Condition 2 revealed that the

reaction enthalpy for the Condition 1 lean flame was significantly smaller than the reaction

27

enthalpy for the Condition 1 rich flame. The consequence of this was a shift of the

stagnation plane towards the lean flame.

4. The Turbulent Double/Triple Flame

Large nozzle exit velocities cause large strain rates in the flow field. Large local axial

strain rate flows (dv/dy > 710 s-1

) are sufficient to quench the CO/hydrogen diffusion

flame, and thus the partially premixed counter-flow flame exhibits a “double flame”

structure. The extinction of the diffusion flame occurs as a consequence of the effect of

rate of increase of flame surface area dominating the rate of molecular diffusion across the

stagnation plane. The cooling of reaction products due to flame stretch causes the reactive

gas temperature to drop below the critical temperature required to sustain the flame. If

there is turbulence present, and if the nozzle exit velocity is lowered, the critical local axial

strain rate is reached which will allow the existence of the diffusion flame, and a transition

occurs from a turbulent “thick double flame” to a turbulent “triple flame”.

5. Conclusion

1. Qualitative OH concentration profiles have been obtained along the symmetry axis of an

axially symmetric laminar counter-flowing methane-air partially premixed “triple flame”.

They show the three principal oxidation regions as local peaks in OH concentration.

2. Premixed and diffusion flame merging limits have been determined as a function of

mean axial flow strain rate, along with the conditions for the existence of the diffusion

28

flame. The conditions define the stability map for the laminar counter-flowing

methane-air “triple flame”.

3. It has proven possible to construct a two-dimensional triple flame stability map with

equivalence ratio ϕ, plotted against global strain rate a.

4. Relative quantitative OH concentration profiles and Rayleigh scattering profiles have

been obtained for three “triple flame” conditions. 1-D LDV measurements of the radial

and axial velocity profiles have been obtained for two of the three “triple flame”

conditions.

5. The counter-flow “triple flame” is significantly more sensitive to rich flame

stoichiometry than the lean flame stoichiometry, and it is also sensitive to axial strain.

6. The diffusion flame extinction axial strain rate has been measured to be [dv/dy]ext =

710s-1

, using 1-D LDV.

7. The transition from highly strained partially premixed turbulent “double flames” to

partially premixed “triple flames” at lower turbulence levels has been shown to be caused

by the local axial strain rate allowing the existence of the CO/hydrogen diffusion flame.

29

Acknowledgements

This work was undertaken with funding from the EPSRC (GR/G 57543). The authors

would also like to thank Dr R. Hicks of Leeds University for his assistance with the LDV

measurements and Prof. K.N.C. Bray with advice on the burner design and the seeding

system for the LDV measurements.

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