Rich – Lean Flame Interaction on a Lamella Type Burner
Tomás Manuel Martins Lúcio
Mechanical Engineering Department, Instituto Superior Técnico, University of Lisbon, Lisbon, Portugal
ABSTRACT
The present thesis is focused on the experimental characterization of a multi-slit rich-lean burner using methane-air
mixtures. The influence of the burner plate geometrical parameters on its stability limits was assessed when operating in
simply lean conditions and an optimum configuration in terms of stability was achieved. The obtained optimal
configuration was then tested in rich-lean combustion, and stability limits were found to be extended by the rich flame
presence. In order to understand the mechanisms associated with the interaction between both flames, a detailed
experimental analysis of the present rich-lean flame was made, based on direct photography for flame visualization, optical
chemiluminescence techniques for radical measurements and particle image velocimetry (PIV) the characterize the flow
field in this region. From the experimental results of this analysis, a model for the rich flame influence on the structure o f
the adjacent lean flame is proposed. It is concluded that the rich flame presence, associated with the imposed physical
separation between both flames which creates a low velocity area, provides an anchor point for the lean flame where the
convection velocities are much lower than that of the local burning velocities, thus explaining the enhanced stability limits.
It is also concluded that this low velocity area allows radicals to travel between flames, which modifies the apparent
equivalence ratio of the lean flame, since its local burning velocity increases, and consequently strongly influences its
structure.
Keywords: Rich-Lean combustion, Slit burner, Flame Stability, Chemiluminescence, PIV, Triple Flames.
reaction zones that are presented in this sort of flame,
two premixed zones (one fuel-rich and other fuel-lean)
and one non-premixed zone enclosed by them, where the
excess fuel from the rich reaction zone reacts with the
excess oxidizer from the lean reaction zone. The
structure and extinction characteristics of this sort of
flame, as well as the interactions within the various
reaction zones, have been studied extensively from a
fundamental point of view [2]–[4]. Recently, Aggarwal
[2] has provided an extensive review study on flame
extinction, with particular focus on partially premixed
flames, where it is described that the transport processes
of heat and radicals near the flame’s triple point, which
is where the three reaction zones merge, plays a key role
in the stabilization of the triple flame. A numerical study
by Guo et al. [3] with a methane – air triple flame
stabilized on a two dimensional mixing layer have
shown that radical exchange between the non-premixed
flame and the premixed flame branches strongly
influences the local burning speed. The same conclusion
was also reported by Echekki & Chen [4] based on a
numerical study of methanol-air triple flames. Still, a
common factor among these experiments was that no
physical separation was imposed between the rich and
lean streams. Using a Wolfhard-Parker slot burner with
finite plates to separate the rich and lean streams, Azzoni
et al. [5], concluded that the influence of the mixture exit
velocity as well as the equivalence ratio gradient
imposed by the two flames have similar effects as the
ones described in cases with no separation, and also that
the flame heat loss to the burner plate does not cause a
significant anchoring effect. Nevertheless, these results
were obtained for a burner where the flame height hflame
is much larger than that of the separation thickness Sthick
(hflame/Sthick ~ 200), whereas in the burners developed for
domestic applications, as an example, these are typically
of the same order of magnitude (hflame/Sthick ~ 1).
1. INTRODUCTION
The method of studying fluid flow related problems
Global awareness towards pollution has been raising in
nowadays society, with such effects as global warming,
acid rains or urban air quality decrease, motivating the
imposition of increasingly strict regulations and policies
on the affected industries, hence explaining why
pollutant emissions have been the main drive for
combustion research over the last decades. Among the
industries associated with these issues, the domestic
heating industry based on hydrocarbon fuels is no
exception, facing growing pressure to reduce pollutant
emissions, particularly NOx. On the way towards these
goals, the industry’s current trend has been to develop
devices which can operate in lean combustion regimes
[1], since they have low peak flame temperatures and
therefore low NOx emissions. This solution however,
raises a challenge due to the flame’s poor stability when
operating in lean regimes, which can cause premature
blowoff to occur during operation. In order to overcome
this issue, a purposed solution to increase flame stability
has been to introduce pilot rich flames, which also have
low flame temperatures, to anchor the lean flame, a
technology known as rich – lean combustion. Increasing
stability allows the lean flame to burn with lower
equivalence ratios, and consequently reduce NOx
emissions, due to the lower flame temperature of both
lean and rich mixtures, thus making it relevant to
understand how the rich and lean flames interact, and in
particular, the process through which lean stability is
extended due to the rich flame presence.
The resultant flame structure at the interface between
a rich premixed flame and a lean premixed flame is what
is often described in literature as a triple flame, a
configuration which is part of the partially premixed
flame family. This designation derives from the three
1
Like this, although the local physical process of heat
and radical diffusion between rich and lean flames is
expected to be the same, it remains unclear as to what
spatial extent this physical separation affects small
flames, i.e. in the case where hflame/Sthick ~ 1.
Accordingly, in this work we study the rich-lean flame
interaction for the case of hflame/Sthick ~ 1. This work
relies on a detailed experimental analysis of this mixing
zone, based on stability analysis, direct visualization,
optical chemiluminescence techniques and particle
image velocimetry (PIV).
Side View
Lean Flame
Rich Flame
Air Co – Flow
Air Co – Flow
2. EXPERIMENT
Lean Mixture
2.1 Burner Characterization
Rich Mixture
For the purposes of this work, a two-dimensional slit
burner with rectangular channels was designed, and in
all tests the used fuel was methane. As it can be seen in
Figure 2.1, the burner plate consists of several central
channels of width w, where the lean mixture burns, that
are separated by slits of thickness t. The total height of
the channels and the length are fixed at 25 mm and 40
mm respectively. This central part of the burner is then
sided by two exterior channels of fixed 1 mm where the
rich flame is anchored, and between the rich and lean
sides, there is a physical separation labeled in Figure 2.1
as Sthick, where the three reaction zones interact. The total
length of this region is 3 mm.
It is known that when a fluid enters a rectangular
duct, its velocity profile develops along the channel due
to shear stress effect. When the resultant boundary layer
thickness is equal to half the plates distance, the velocity
profile is fully developed. The duct length required to
this development is called transition length Lent, which
for a Poiseuille flow in a rectangular duct is given by
Equation (2.1), where the Reynolds number is calculated
using Equation (2.2) [6]:
𝑅𝑒 =
̅𝑥 𝑤
𝑈
𝜐
Top View
Sthick = 3 mm
40 mm
𝐿𝑒𝑛𝑡 𝑅𝑒
=
𝑤
5
Rich Mixture
w
t
Figure 2.1 - Burner schematic view. Central plate is
composed by channels with variable width w and slit thickness
t.
There, the flow rates for all four streams can be
controlled using the flow meters software to impose the
desired flow rates. After passing through the flow
meters, both air streams are mixed with the respective
fuel stream in order to obtain two premixed flows that
subsequently feed both the rich and lean sides of the
burner. This scheme allows the user to impose both the
equivalence ratio as well as the velocity at the burner
exit for both lean and rich streams, simply by controlling
the four separate air and fuel streams. The uncertainty of
the used flow meters is given by a sum of the error
associated with reading (± 0.8%) and scale (± 0.2%) as
expressed by Equation (2.3):
(2.1)
(2.2)
being w the channel width, 𝑈 the average velocity and ν
the flow’s kinematic viscosity. Considering this burner’s
case, each slit will develop an individual Poiseuille flow.
The entry length was calculated for all tested conditions,
and it was found to be lower than the total channel
length of 25 mm, which means that the flow will have a
developed parabolic profile at the burner exit.
To study the influence of the burner plate geometry
on the lean flames’ stability, the developed burner
prototype allows the previously mentioned geometrical
parameters w and t to be changed so that the
configuration that maximizes stability without the
addition of the pilot rich flames can be identified.
Two independent streams were needed to feed both
the rich and the lean sides of the burner. To do so, air
and methane are introduced in the system and each
stream is divided into two branches that enter digital
Alicat mass flow meters.
𝑒𝑄 = 0.008 𝑄̅ + 0.002 𝑄𝑀𝑎𝑥
(2.3)
where Q̅ is the measured flow rate and QMax is the
maximum operating flow rate for that specific flow
meter.
However, this flow rate uncertainty will in its turn
have an effect on both the equivalence ratio and exit
velocity desired for each burning condition. Like this,
using error propagation the following expressions can be
deduced for quantifying the equivalence ratio and exit
velocity uncertainty, eϕ and eU respectively.
2
𝛿𝜙 2 2
𝛿𝜙 2 2
𝑒𝜙2 = (
) 𝑒𝑄𝐹 + (
) 𝑒𝑄𝐴
𝛿𝑄𝐹
𝛿𝑄𝐴
(2.4)
𝛿𝑈 2 2
𝛿𝑈 2 2
𝑒𝑈2 = (
) 𝑒𝑄𝐹 + (
) 𝑒𝑄𝐴
𝛿𝑄𝐹
𝛿𝑄𝐴
(2.5)
Optical probe detail
Data Acquisition
Data Processing
Diaphragm
Optical
Fiber
CCD Camera
Acquisition Module
Collimator lens
Signal Amplifiers
Double Pulsed Laser
Chemiluminescence Sensor
OH Filter
Photomultipliers
CH Filter
Burner
Optical probe
Air + CH4
Cyclone
Figure 2.2 – Experimental Setup. PIV and Chemiluminescene Systems scheme.
This way, the maximum obtained relative uncertainty for
both rich and lean stream’s exit velocity and equivalence
ratio was calculated and the relative error never
exceeded 4%.
between OH*/CH* and ϕ is given by Equation (2.6), not
evidencing any systematic effect of power.
2.2 Chemiluminescence Technique
The calibration was tested for a few working
conditions obtained with 2-D flames (present work), and
the obtained concordance is acceptable within a margin
error of 5%.
𝑂𝐻 ∗
𝜙 = −0.411 𝑙𝑛 ( ∗ ) + 1.212
𝐶𝐻
The chemiluminescence system is described in
Figure 2.2. The radicals OH* and CH* were chosen
since they cover the radiation emitted by lean
combustion [7]. In order to
employ the
chemiluminescence technique, the electronically exited
radicals are measured by a light collecting system,
composed by an optical probe and an optical fiber,
where each measurement consists of a one minute time
series with a sampling frequency of 50 Hz. The selected
optical fiber is from Ocean Optics and presents a core
diameter of 200 ± 4 μm and a solid angle of 25º.
The chosen optical probe was designed to collect
light integrated over a cylindrical volume, as shown in
the detail in Figure 2.2. To do so, it includes collimator
lens with a diameter of 20 mm as well as an adjustable
iris diaphragm in front of the unit, to limit the visible
area, and control the amount of light entering the fiber
and the spatial resolution of the measuring system.
Once the light signal is acquired, it is then conducted
into the chemiluminescence sensor, which has a channel
for each radical, equipped with a specific optical filter to
maximize the radical signal to noise ratio. The signal is
subsequently converted into an electrical output by
R3896 Hamamatsu photomultipliers with a frequency
response of 43.5MHz, and it is amplified outside the
sensor by Stanford Research SR560 amplifiers.
The calibration of this system is based on the
identification of an equation between the emission ratio
OH*/CH* and associated flame equivalence ratio [8].
Here it was used a reference flame from a Bunsen burner
with methane-air mixtures for tests, a totally independent
system to avoid any biasing effect.
Different flame powers were used, in the range of
0.8kW to 1.5kW, and an average calibration equation
(2.6)
2.3 Particle Image Velocimetry (PIV)
To understand the flow field in the rich-lean
interaction region, PIV was used, again due to its nonintrusive nature, and its setup is described in Figure 2.2.
To seed the mixture, the marking particles must have a
density as similar as possible to the flow mixture, and
should also be able to withstand high flame
temperatures. Like this, alumina particles (Al2O3) with a
characteristic diameter of 0.2 to 5 μm were chosen. The
mixture enters a cyclone tangentially to promote swirl,
and this way, guarantee that flow transports the marking
particles. Since this burner has two independent flow
streams, the outer rich one and the central lean one, two
independent cyclones were used for each stream.
Once the flow is seeded, the particles are then lit by a
double pulsed Nd:YAG laser from Dantec Dynamics,
which provides a 1 mm thick laser sheet positioned in
the middle of the burner plate. Two consecutive pulses
lit the particles, providing two images that are acquired
by a Kodak Megaplus ES1.0 camera which has a 1008 x
1018 pixel Charge-Couple Device (CCD) sensor
installed. The acquired images, which are 9.11 x 9.11
mm in real dimensions, can then be correlated in order to
calculate the spatial velocity map. For each velocity field
measurement, this procedure is repeated a certain
number of times in order to obtain an average flow field,
and this way minimize residual errors. This way, after
performing convergence studies, 60 pairs of images were
found to be sufficient to obtain a stable spatial velocity
map.
3
Figure 3.1 – Stability diagrams of lean flames (without rich flame). a) Influence of slit thickness t on lean flame stability for fixed w = 1
mm. b) Influence of slit thickness with fixed velocity gradient g b = 4000 s-1 and w = 1 mm.
For the correlation, both images are divided in 32 x 32
pixel interrogation areas (0.15 x 0.15 mm), and a cross
correlation method is employed to merge the first and
second frame and this way, compute a velocity vector
for each interrogation area. In order to avoid that
particles located next to the borders of each interrogation
area move to another one in the second frame, an
overlapping factor of 50% x 50% was used in the cross
correlation method. It should also be noted that the time
between the two pulses is of the utmost importance,
since a high time interval may cause the particles caught
in the first frame to disappear in the second frame, and
also, a short time interval may cause the particles to
move too little making it difficult to make the correlation
process afterwards, both of which may cause false
correlations. This parameter is directly correlated with
the flow velocity as well as the interrogation areas size,
and convergence studies have shown that a time between
pulses of 10 μs was accurate for all tested conditions.
calculated using the recommended equation from
Bonilla & Maccallum [10] for rectangular channels:
̅ ⁄(8 𝑄 𝑑𝐻 )
𝑔𝑏 = 𝑈
(3.1)
where Ū is the channel’s average velocity, Q is a
tabulated constant, function of the channel cross
sectional width/length ratio [11] and dH is the channel
hydraulic diameter.
Figure 3.1a shows the influence of slit thicknesses t
on stability for a fixed channel width w = 1 mm, and it
can be seen that this parameter has a clear effect on the
stability limits. In accordance with Sogo & Hase [9], it
was found that maximum stability is achieved for a slit
thickness of 2 mm. This is observable in Figure 3.1b,
where the blowoff limits are presented as a function of
slit thickness for a fixed velocity gradient of 4000 s-1,
again for w = 1mm, and it can be seen that the minimum
equivalence ratio needed for stabilization occurs at t = 2
mm.
Then, a stability diagram was done to assess the
influence of channel width on flame stability by fixing
the slit thickness t = 2 mm, for varying w. This stability
diagram is presented in Figure 3.2 and it can be seen that
although the range of velocity gradients varies in order
of magnitude with w, the various curves follow the same
trend, and channel width was found to have a negligible
influence on stability when compared with slit thickness
t.
Using the same procedure as before, the channel
width was next fixed at w = 2 mm and the optimal value
for slit thickness was again found to be 2 mm. However,
when fixing the channel width w = 0.5 mm, maximum
stability was achieved for a slit thickness of 1 mm.
This analysis points to the conclusion that regardless
of the exit velocity gradient, or in other words, the
mixture exit velocity, for large channel widths (w ≥ 1
mm), stability is independent of w, and the optimal slit
thickness is found to be 2 mm, whereas for narrower
channels (w ≤ 1 mm) the optimal slit thickness t is
function of the channel width w. Given the fact that the
width of the rich channels had been previously fixed at 1
mm, the same width was chosen for the lean channel,
and accordingly, a slit thickness of 2 mm was chosen in
3. RESULTS AND DISCUSSION
3.1 Lean Stability without Rich Flame
As it was mentioned before, the designed burner has
variable geometrical parameters w (channel width) and t
(slit thickness), in the interest of finding the combination
of these two parameters that maximizes the lean flame
stability without the addition of the rich flame. To find
this optimum configuration, a similar approach to that of
Sogo & Hase [9] was carried. Using a similar geometry
burner with a methane-air mixture, the mentioned
authors tested several configurations of t versus w and
reported a slit thickness of 2 mm as the optimum value
for flame stability, for a channel width range of 3-20
mm.
In the present work however, an extended analysis
was performed for narrower channel widths (0.5-2 mm)
to understand if the previously mentioned optimal
thickness would still hold. Like this, stability diagrams
were made for several geometrical combinations, and are
presented in equivalence ratio ϕ versus velocity gradient
gb. It should be noted that this velocity gradient is
4
Three rich exit velocities UR in a range of 0.4 – 1 m/s
were tested with four distinct equivalence ratios ϕR.
Results are shown in Figure 3.3b and it is possible to see
that tendencies are monotonous: as the rich flame
equivalence ratio or velocity increase, the stability limit
is enhanced. However, this gain is not as significant as
the one achieved by adding the rich flame to the simple
lean case. Whereas the presence of the rich flame
incremented stability in 9%, the variation in rich flame
condition represents a variation of ± 2% to this value,
which indicates that stability is first and foremost
promoted by the presence of the rich flame and later it
can be continuously improved by increasing the rich
flame velocity or equivalence ratio.
order to maximize stability. This configuration was fixed
for the following rich-lean analysis.
3.3 Flame visualization
After accessing the influence of the pilot flame on
the overall stability, it becomes relevant to identify
potential mechanisms that are responsible for the
stability increase. To do so, flame visualization was done
using direct photography for the same set of rich flame
conditions tested before, keeping a lean equivalence
ratio of 0.65 constant for two different lean exit
velocities of 0.5 m/s and 1 m/s. Part of the results are
shown in Figure 3.4, and it can be observed that as the
rich flame equivalence ratio increases, the left hand side
of the adjacent lean flame becomes more flat than its
right hand side, and therefore more attached to the
burner plate, thus enhancing stability.
To quantify this influence, an average flame angle α
was measured at center of the left hand side of the
adjacent lean flame, as it is illustrated in the second
frame of Figure 3.4a. Knowing that the flame is curved
due to the approaching flow developed parabolic profile
at the burner exit, it should be noted that this method is
only an approximation since the flame does not have a
constant angle, especially near the top and bottom of the
flame. However, the flame center part is quasi linear,
and so an identification of an angle representative of the
flame bending is possible.
Figure 3.2 - Influence of channel width w on lean flame
stability with fixed slit thickness t = 2 mm.
3.2 Lean Stability with Rich Flame
Having established a final configuration for the
burner central plate, the lean flame stability in the
presence of the rich flame was then assessed. Like this, a
general stability analysis of the lean flame was carried
first with the burner operating in simple lean conditions,
and then with an added reference rich flame of
equivalence ratio ϕR = 1.2 and an exit velocity UR = 1
m/s (Figure 3.3a). One can observe that when a rich
flame is added, stability is in average extended by 9%
for the entire range of velocity gradients when
comparing with the strictly lean regime. This way, in
order to evaluate how different rich flame conditions
would influence the lean flame stability, it was assumed
that the effect provided by the rich flame presence
should be constant for all tested lean exit velocities UL
and so, a point in Figure 3.3a corresponding to a fixed
velocity gradient of 4000 s-1 (UL = 0.5 m/s) was chosen
as representative of the response of the lean flame to
different rich flame conditions.
Figure 3.3 – Lean flame stability diagrams with rich flame presence. a) Comparison between lean flame stability with and without rich
flame with ϕR = 1.2 and UR = 1 m/s. b) Influence of rich flame ϕR and UR on lean flame blowoff ϕL for fixed g b = 4000 s-1
5
=1.1
=1.2
=1.3
=1.4
α
Figure 3.4 - Flame visualization of rich flame influence on the adjacent lean flame structure, ϕL = 0.65 for all cases. a) UR = 1 m/s and
UL = 0.5 m/s. b) UR = 0.5 m/s and UL = 0.5 m/s. c) UR = 0.5 m/s and UL = 1 m/s.
These angle results are shown in in Figure 3.5, where
they are plotted against rich equivalence ratio, keeping
the lean equivalence ratio of 0.65 constant, while UL and
UR are varying. It is possible to see that the tendencies
are monotonous: as the rich flame equivalence ratio
increases, so does the angle α (the flame becomes more
flat), for all pairs of rich and lean velocities. Also, it is
observable that this angle is clearly augmented when
comparing these results with the angle of a single lean
flame of ϕL = 0.65 (in the absence of rich flame) when
operating at UL =0.5 m/s or UL =1 m/s.
Furthermore, it can also be observed that the
tendency is approximately linear for all sets of
conditions, which suggests that the results can be
collapsed into an equation of the type:
𝛼 = 𝐴 + 𝐵 𝜙𝑅
Coefficients a, b, c and d were obtained based on the
best fit approach of combined Equation (3.2) and (3.3) to
the experimental data presented in Figure 3.5. The
success of this fitting process is represented in Figure 3.6
where the measured angle was collapsed against A+B ϕR
(see Equation (3.2)), with reasonable accordance
(correlation factor R2 = 0.92).
(3.2)
where A and B are functions of the lean and rich exit
velocities UL and UR, that can be expressed as equations
of the type shown in Equation (3.3).
𝐴, 𝐵 = 𝑓( 𝑈𝐿 , 𝑈𝑅 ) = (𝑎 + 𝑏 𝑈𝑅 + 𝑐 𝑈𝐿 𝑈𝑅 + 𝑑 𝑈𝐿 )
(3.3)
Figure 3.6 - Measured angle α presented in collapsed form
given by Equation (3.2)
The collapsed data shows that the impact of adding
the rich flame is enhanced for lower lean exit velocities.
To further analyze this impact, it was assumed that this
2-dimensional flame could be modeled as a simple
conical flame where the flame angle and the flame
burning speed SL can be correlated directly by Equation
(3.4) It should be kept in mind however, that this is only
valid for the center part of the flame where the flame
angle is constant.
Figure 3.5 - Measured angle α for varying burning conditions,
the theoretical angle αT for the measured conditions of UL and
ϕL = 0.65 is presented for comparison.
𝑆𝐿 = 𝑈𝐿 sin 𝛼
(3.4)
This way, and knowing that the lean exit velocity UL
and the lean equivalence ratio ϕL are kept constant, the
6
variation in the angle of the lean flame α due to the
presence of the rich flame as shown in Figure 3.5,
indicates that the local SL must vary according to
Equation (3.4). In fact, the progressive bending of the
lean flame branch indicates that the local burning speed
is increasing even though the injected lean equivalence
ratio and exit velocity remain the same. This increase in
the burning velocity is explained by the interaction
between both flames. Once the rich flame is present, a
triple flame structure is established and radical and heat
transfer between the various reaction zones begins.
This way, the concentration of reacting species as
well as the temperature of the lean premixed reaction
zone increase, and combustion is intensified. Given the
fact that the lean flame no longer operates in the injected
conditions, an ‘’apparent’’ equivalence ratio that
corresponds to the new lean flame structure and burning
velocity can be estimated in order to help quantify the
effect of the rich flame presence. Using burning speed
reference values for methane [12], a polynomial curve
𝑆𝐿 = −191.07𝜙 + 398.15𝜙 − 169.5
(cm/s)
was
derived, and combining it with Equations (3.2) and (3.4),
we can estimate the local apparent equivalence ratio,
here described as ϕAngular, as a function of the rich flame
equivalence ratio ϕR, based on Equation (3.5):
𝜙𝐴𝑛𝑔𝑢𝑙𝑎𝑟 = 𝑎 [𝑏 − 𝑈𝐿 sin(𝐴 + 𝐵𝜙𝑅 )]
1⁄
2
+𝑐
Figure 3.7 - Adjacent lean flame equivalence ratio ϕAngular
estimated with Equation (3.5) presented in collapsed form.
3.4 Chemiluminescence Analysis
After determining how the rich flame influences the
adjacent lean flame structure and stability,
chemiluminescence measurements become relevant to
verify if the estimated contamination predicted by
Equation (3.5) is confirmed by complementary
experimental technique.
As a first test, the radicals OH* and CH* were
measured in several points of the different reaction zones
to create a transversal equivalence ratio profile using the
calibration curve described by Equation (2.6). Like this,
keeping the lean flame conditions UL = 0.5 m/s and ϕL =
0.65 as well as the rich flame velocity UR = 1 m/s
constant, but varying the rich flame equivalence ratio (as
seen in Figure 3.4a), Figure 3.8 shows the measuring
positions and the respective measured equivalence ratios
ϕChemi. These results were obtained with the diaphragm
limiting the vision area to a 1 mm cylindrical control
volume.
Position 1 is centered in the rich flame top part and it
can be seen that the measured equivalence ratio reflects
almost linearly the increase in rich flame equivalence
ratio. Position 2 is situated in the region were the three
flames interact, it can be seen that for ϕR ≤ 1.2, the
measured equivalence
ratio
is approximately
stoichiometric, which should be expected since this is
where the excess radicals of each reaction zone mix in
stoichiometric proportions [5]. Then for ϕR ≥ 1.2, the
rich flame starts to move closer to the lean premixed
zone as seen in Figure 3.4a and consequently higher
equivalence ratios are measured since the rich flame
begins to enter the line of sight of the optical probe.
Position 3 and 4 are located in the lean premixed zone.
Whilst position 4 seems to remain unchanged with a
measured equivalence ratio close to the injected ϕL =
0.65, position 3 seems to be affected by the rich flame
presence, as a slight increase in measured ϕChemi is
noticeable, reaching a maximum ϕChemi = 0.72 for ϕR =
1.4, thus exhibiting the same contamination behavior
observed before.
(3.5)
For further graphical simplification, Equation (3.5)
1
can be rearranged as 𝑌 = 𝑎 [𝑏 − 𝑋] ⁄2 + 𝑐 where 𝑋 =
𝑈𝐿 sin(𝐴 + 𝐵𝜙𝑅 ), and its plotted in Figure 3.7, against
the experimental data presented in Figure 3.5. The
resulting plot shows how the apparent ϕAngular changes as
the burning conditions of both rich and lean flames vary.
It can be seen that the local lean equivalence ratio ϕAngular
increases with the rich flame equivalence ratio, and that
this tendency is all the more relevant for higher ratios
between rich and lean velocities UR / UL for each group
of UL. Furthermore, it can be observed that as the rich
flame conditions decrease, the local equivalence ratio
tends to 0.65 which was the original equivalence ratio of
the lean flame. It is noticeable that for the extreme case
of UR = 1 and UL = 0.5, where the rich flame height is
larger than that of the lean flame (see Figure 3.4a), the
corresponding lean flame behavior tends to that of a
quasi-stoichiometric flame, which when compared to the
injected ϕL = 0.65, clearly shows how strongly the lean
flame structure is affected by the contamination of
species and temperature from the rich premixed and nonpremixed reaction zones. Finally, it is also relevant to
note that the tendencies of the apparent equivalence ratio
evolution is consistent with the lean flame stability
analysis in presence of rich flame presented before
(Figure 3.3b), which supports the theory that once the
rich flame is introduced in the burner, the local burning
speed of the lean reaction zone increases and stability is
promoted by means of heat and radical transfer to lean
flame.
7
3
1
4
2
φR ,UR
φL ,UL
Figure 3.8- Chemiluminescence measurements for varying rich flame ϕR, with UL = 0.5 m/s, ϕL = 0.65 and UR = 1 m/s. a) Measuring
positions. b) Measured equivalence ratios ϕChemi.
Like this, in order to validate Equation (3.5) the
optical probe was aligned in an intermediate position
between positions 2 and 3 again with the diaphragm at 1
mm, which corresponds to the same zone where the
angle α was measured in Figure 3.4a. Tests were made
for a lean flame ϕL = 0.65 as before, with lean exit
velocities UL = 0.5 and 1.0 m/s, for varying rich flame
conditions. These measurements provided an
experimental spatial equivalence ratio ϕChemi which can
in its turn be opposed to the apparent equivalence ratio
ϕAngular estimated previously. This comparison in showed
in Figure 3.9. It can be seen that both methods show
similar trends, with results exhibiting an average
deviation of 6%, and a maximum deviation of 14%.
zone increases simply by changing the rich flame
premixture. As a result, the lean flame structure is
affected and its combustion dynamics changes, as the
flame behaves with an apparent equivalence ratio higher
than its real equivalence ratio due to its increased local
burning velocity.
3.5 PIV Measurements
In order to understand the flow field in the rich-lean
flame region, PIV measurements were made. As a first
approach, isothermal studies were conducted to
characterize the flow field, between the rich and lean
flames as presented in Figure 3.10. Two lean exit
velocities were tested with two different rich velocities
each, and results show that for all cases, a large
recirculation area enclosed by the rich and lean streams
appears, creating a low velocity area on the mixing zone.
This recirculation zone width wRZ is of the size of the
separation thickness Sthick between the rich and lean
channels while its height to width ratio is hRZ / wRZ ~2 to
3, being sensitive to UR and UL. Another important
aspect from the isothermal results, is the effect that the
lean exit velocity UL has on the flow field, since it can be
seen that this jet exerts a suction force on the exterior
rich stream, bending its streamlines towards the center of
the burner, which can be explained by the difference in
momentum of the rich and lean jets. Since all individual
lean jets will behave as a group, it can be assumed that
there will be a ratio of four lean channels for each rich
channel promoting dragging of the exterior air and thus
of the side rich flow. This also explains why for higher
UL / UR ratios the suction effect is more noticeable, since
the momentum difference between the rich and lean jets
increases.
Reactive flow PIV measurements were subsequently
made in order to visualize if these structures would still
exist in the presence of combustion. Like before, tests
were carried for a lean flame of ϕL = 0.65, with different
lean and rich exit velocities and varying rich flame
equivalence ratios. Figure 3.11a shows the velocity field
around the mixing region with UL = 0.5 m/s and a rich
flame with UR = 1.0 m/s and ϕR = 1.4.
Figure 3.9 - Comparison between local apparent equivalence
ratios estimated with Equation (3.5) and measured with
chemiluminescence technique for the same flame conditions.
The good accordance in results with two distinct
methods provides further evidence that the rich flame
presence affects the lean flame by increasing its local
burning speed due to radical and heat diffusion from the
rich to the lean region. In particular, radical diffusion is
confirmed by the chemiluminescence measurements
since these show that radical activity in the lean reaction
8
10
1000
pix
900
8
800
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1000
pix
UL = 1.0 m/s
UR = 0.5 m/s
900
U = 1.5 m/s
700
400
6
6
y, mm
y, mm
500
mixture fraction is close to stoichiometry (as seen in
position 3 from Figure 3.8), and therefore local burning
velocities are more intense. Since in this area the
maximum burning velocity is in competition with the
lowest convection velocity in the entire flame structure,
the triple point functions simultaneously as a radical
pool as well as an anchor point for the lean flame
branch. On the other hand, the radical diffusion
velocities will also compete against the low convection
velocities and thus, diffusion of radicals between the
flames will be favored in this region, which explains the
contamination of the adjacent lean flame verified by
both direct flame visualization as well as
chemiluminescence analysis. The combination of these
effects explains the verified increase in local burning
velocity and consequent repercussion on the structure of
the adjacent lean flame.
The described triple flame behavior is comparable to
the numerical study of Guo et al.[3] where the influence
of the mixture fraction on a triple flame structure and
local burning velocities was studied. The mentioned
authors defined a computational domain where the
injected mixture fraction gradient was controlled by
varying the mixture layer thickness, producing a triple
flame structure, and concluded that as the mixture
fraction gradient increases (lower mixture layer
thickness) the premixed branches of the triple flame
move closer in space to the non-premixed flame between
them and thus, heat and radical exchange to the
premixed flames is intensified which promotes a local
increase in the burning flux. In a real application as in
this case however, the space between both flames is
fixed, and so, an analogous mixture fraction gradient
variation is obtained by changing the equivalence ratio
gradient ∇ϕ between both premixed flames. This is done
by increasing the rich flame equivalence ratio ϕR, and as
it was shown before, the same effect to the one reported
by Guo et al.[3] is observed. In conclusion, it is found
that this low velocity area assumes vital importance
since it is not only an region where the local SL is greater
than the average flow velocity, hence providing an
anchor point to the lean flame, but it will also allow the
diffusion of species and temperature to occur between
reaction zones, directly influencing the lean flame
structure and stability by increasing its local burning
velocity.
(b)
8 UL = 0.5 m/s
R
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Figure 3.10 –Velocity field and streamlines for isothermal flow
in the rich – lean region obtained with PIV. a) UR = 0.5 m/s
and UL = 1 m/s. b) UR = 0.5 m/s and UL = 1.5 m/s. c) UR = 1
m/s and UL = 1 m/s. d) UR = 1 m/s and UL = 1.5 m/s.
It can be observed that the recirculation zone that
existed in the mixing area in the isothermal case, is no
longer clearly distinguishable in the reactive case. This
results should be expected since it is widely established
that the size of a recirculation zone is significantly
reduced in the presence of flame as it is express by
Kedia & Ghoniem [13]. However, although the
recirculation zone is not detectable in the reactive flow, a
stagnation region enclosed by the two jets still exists as
⃗ | ≤ 0.1 m/s).
it is observable in Figure 3.11b (|𝑈
Furthermore, it can also be observed that the streamlines
1000
1000
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leaving
the
rich
side
are
also
bending
towards
this
low
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900
9000900
analysis.
µm
900
900 Since this stagnation zone is coincident with the
800
8000800
triple point region, where it is known that radical and
800
800
heat transfer activity are more intense [2], an analysis of
700
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the1000
competing mechanisms taking place is fundamental
700
700
pix
to understand the dynamics of this region. On one hand,
1000
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thepix
low velocity area occurs in the same region6000
where
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the800non-premixed flame is present and where the local
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Figure 3.11
–100
Velocity
with PIV
reactive
with
UR = 1 m/s, UL = 0.5 m/s, ϕR = 1.4 and ϕL = 0.65. a) Velocity field
0
200 field
300obtained
400 500
600 for
700
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1000
and flame. b) Velocity module scalar map with superimposed streamlines
9
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Statis tics vector map: Vector Statis tics, 62×62 vectors (3844)
Size: 1008×1008 (0,0)
other hand, the triple point is also a region where radical
and heat diffusion is more intense [2], and thus, the low
velocity region will allow radical diffusion to occur and
subsequently increase the local burning velocity of the
lean flame, which is in accordance with the results from
other authors [3]. This will make the lean flame operate
with an apparent equivalence ratio which is higher than
the injected ϕL = 0.65, and will in its turn explain the
effect on the lean flame structure, manifested by its
continuous bending towards the burner.
It is then concluded that when the separation
thickness Sthick is of the same size of the flames height,
the rich-lean flame interactions are strongly affected,
since the low velocity area that is formed holds a
decisive role in the dynamics of this region, which
affects both the global stability of the burner as well as
the local structure of the lean branch of the resultant
triple flame.
4. CONCLUSIONS
In this work, a methane-air triple flame stabilized on
a slit burner has been experimentally investigated. The
resultant triple flame burns in co-flow configuration of
two premixed mixtures, one fuel rich and one fuel lean,
which produce two premixed flames of similar
dimension and a non-premixed flame between them. The
two premixed flames are in its turn separated by a gap
which is of the same size of the flame heights.
An extensive stability analysis was made regarding
the influence of several parameters. First, for channels
widths larger than 1 mm, a slit thickness t = 2 mm was
found to be the optimum value for stability of simple
lean flames regardless of the tested channel width
values, results which are in accordance and extend
analysis of previous reference work [9]. Moreover, for
channels narrower than 1 mm, it was that the optimum
geometrical configuration is no longer independent of
the channel width, but rather a function of the width to
slit ratio.
Then, the influence of a pilot rich flame on the lean
flame stability was investigated. It was found that the
stability limits can be increased in about 9% when
comparing with simply lean combustion, and that a
continuous increase in either the rich flame equivalence
ratio or exit velocity would progressively extend the
stability limits until a maximum further gain of 2%.
In order to understand how the rich flame presence
influences the lean flame structure and stability, a
detailed experimental analysis of the interaction zone
between both flames was made, based on direct
photography
for
flame
visualization,
optical
chemiluminescence techniques for spatial equivalence
ratio measurements and particle image velocimetry
(PIV).
Flame visualization has shown that as the rich flame
influence on the interaction zone increases (higher UR or
ϕR), the lean flame starts to bend progressively,
becoming more flat and attached to the burner. Based on
the variation of this angle, an equation to estimate an
apparent equivalence ratio ϕAngular of the lean flame as a
function of the injected UL, UR and ϕR, which would
correspond to the flame new equilibrium position for a
real injected ϕL = 0.65 was deduced. To validate this
analysis, chemiluminescence measurements were made
to estimate the local apparent equivalence ratio of the
lean flame. The two methods have shown excellent
accordance which indicates that the local burning speed
of the lean flame is increasing, which is manifested by
the bending of the lean flame.
Chemiluminescence and PIV measurements also
allowed to study the dynamics of the triple point region
where
the
various
reaction
zones
merge.
Chemiluminescence has shown that the equivalence ratio
in this area is quasi-stoichiometric, namely due to the
non-premixed flame presence [5], which leads to high
local burning velocities SL, whilst PIV demonstrated that
the convention velocity U in this region is low (typically
U/Uinjected ~ 0.1). The combination of these two factors
suggests that the triple point acts as an anchor point for
⃗ |, explaining the
the lean flame given that SL >> |𝑈
stability increase verified in the stability analysis. On the
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10