On the Effect of Air Temperature on Mild Flameless Combustion ...

<strong>On</strong> <strong>the</strong> <strong>Effect</strong> <strong>of</strong> <strong>Air</strong> <strong>Temperature</strong> on Mild Flameless Combustion Regime
<strong>of</strong> High <strong>Temperature</strong> Furnace
C. Rottier 1* , C. Lacour 1 , G. Godard 1 , B. Taupin 1 , L. Porcheron 2 , R. Hauguel 2 , S.Carpentier 2 ,
A.M. Boukhalfa 1 , D. Honoré 1
1 CORIA – CNRS, Université et INSA de Rouen, Saint Etienne du Rouvray, France
2 GDF SUEZ, Research and Innovation Division, Saint Denis La Plaine, France
Abstract
This paper presents an experimental study <strong>of</strong> <strong>the</strong> effect <strong>of</strong> air preheating temperature on <strong>the</strong> main features <strong>of</strong> mild
flameless combustion obtained in a laboratory-scale furnace. Mild flameless combustion regime is conserved
whatever <strong>the</strong> air temperature varying from 838 K to ambient value. No visible flame is observed. Main aerodynamic
features <strong>of</strong> <strong>the</strong> flow at <strong>the</strong> exit <strong>of</strong> <strong>the</strong> burner are similar with and without air preheating. Large momentum <strong>of</strong>
methane and air turbulent jets favours <strong>the</strong> entrainment <strong>of</strong> recirculating flue gas and <strong>the</strong>n <strong>the</strong> progressive dilution <strong>of</strong>
reactant by flue gas. Main difference comes from <strong>the</strong> structures <strong>of</strong> <strong>the</strong> reaction zone which are more and more lifted
and NOx emissions which decrease progressively with air temperature, down to a very low value (4 ppm @3%O2).
Introduction
Mild flameless combustion is an advanced combustion
technique designed to reduce NOx emissions and
increase energy efficiency <strong>of</strong> high temperature furnaces.
The use <strong>of</strong> a recuperative or regenerative system in a
burner allows <strong>the</strong> optimisation <strong>of</strong> <strong>the</strong> energy efficiency
thanks to <strong>the</strong> heat transfer from hot exhaust gases to
inlet combusting air [1, 2]. The main drawback <strong>of</strong>
regenerative burner is <strong>the</strong> very high temperature that
could be reached in <strong>the</strong> flame because <strong>of</strong> air preheating.
This favours <strong>the</strong> NO formation from <strong>the</strong>rmal route and
thus very large NOx emissions, that may be over <strong>the</strong>
regulation limits.
<strong>Air</strong> and fuel staging is one <strong>of</strong> <strong>the</strong> most common primary
strategy for reducing NOx emissions. It is used in
several combustion applications notably in high
temperature furnace [3]. Mild flameless combustion has
been developed when applying fuel or air staging to
regenerative burners to <strong>the</strong> maximum level <strong>of</strong> a total
staging, i.e. to a configuration where air and fuel
injections are distant. Because <strong>of</strong> <strong>the</strong> large jet velocities
<strong>of</strong> fuel and air associated to this staging, strong
recirculation <strong>of</strong> flue gas occurs in <strong>the</strong> combustion
chamber. Mixing <strong>of</strong> recirculating combustion products
with air and fuel induces this specific diluted
combustion regime named HiTAC combustion,
flameless combustion or mild combustion [2, 4, 5]. Its
main characteristics are <strong>the</strong> global homogeneity <strong>of</strong> heat
release, <strong>the</strong> non visibility <strong>of</strong> <strong>the</strong> reaction zones, and <strong>the</strong>
very low NOx emissions which can be divided by ten
compared to a conventional regenerative burner [6].
Although this technique has been applied successfully
on industrial units, <strong>the</strong> impact <strong>of</strong> several key parameters
on combustion characteristics are not fully understood
yet. This has motivated several experimental studies at
semi-industrial scale [7 – 12], as well as laboratory scale
[2, 13 – 19].
* Corresponding author: rottier@coria.fr
Proceedings <strong>of</strong> <strong>the</strong> European Combustion Meeting
Specific objectives
Our approach is to reproduce mild flameless
combustion regime with a burner configuration close to
<strong>the</strong> real commercial industrial ones in a laboratory-scale
facility enabling applications <strong>of</strong> several measurements
techniques. This study follows a previous study
performed at semi-industrial scale at <strong>the</strong> GDF SUEZ
R&D center [10]. Some results <strong>of</strong> <strong>the</strong> study <strong>of</strong> mild
flameless combustion obtained in <strong>the</strong> laboratory-scale
facility have been already presented when preheating
combustion air [17]. The present communication is
focused on <strong>the</strong> experimental investigation <strong>of</strong> <strong>the</strong> effect
<strong>of</strong> air temperature on <strong>the</strong> main features <strong>of</strong> this original
combustion regime.
The experimental setup
The laboratory-scale mild flameless combustion facility
and some measurement techniques have been already
presented previously [17]. <strong>On</strong>ly main characteristics <strong>of</strong>
<strong>the</strong> furnace are reminded here. Some specificities <strong>of</strong> <strong>the</strong>
application <strong>of</strong> measurements techniques in such high
temperature facility are presented.
The mild flameless combustion facility
The pilot facility, named “FOUR” for Furnace with
Optical accesses and Upstream Recirculation, has a
vertical plane-parallel combustion chamber (0.5 x 0.5
squared section and 1 m height), made with refractory
materials for a maximum wall temperature <strong>of</strong> 1400 K
(Figure 1). Several openings are set along in staggered
rows on <strong>the</strong> four sides <strong>of</strong> <strong>the</strong> combustion chamber.
These openings can receive an optical window or a
probe stand for detailed measurements in <strong>the</strong> furnace, or
a refractory block to preserve <strong>the</strong>rmal confinement and
measure wall temperature. The burner (Figure 2.)
consists <strong>of</strong> three coplanar jets: two <strong>of</strong>f-axis methane
injectors (3mm dia.) spaced 101.4 mm apart and set on

ei<strong>the</strong>r sides <strong>of</strong> a central air jet (25mm dia.) ending with
a divergent shape (R = 11 mm).
Figure 1 The mild flameless combustion furnace at
laboratory scale.
The centre <strong>of</strong> <strong>the</strong> air jet at <strong>the</strong> exit <strong>of</strong> <strong>the</strong> burner is
defined as <strong>the</strong> origin. The vertical axis along <strong>the</strong> air jet
corresponds to <strong>the</strong> y-axis, and <strong>the</strong> x-axis is <strong>the</strong>
horizontal axis crossing <strong>the</strong> centres <strong>of</strong> <strong>the</strong> exits <strong>of</strong> air
and methane injectors.
side view front view
Figure 2 Geometry <strong>of</strong> <strong>the</strong> burner.
Combustion air can be preheated up to 850 K thanks to
an electric heater, in order to mimic <strong>the</strong> presence <strong>of</strong> a
regenerative system in <strong>the</strong> burner and to allow
continuous operating conditions.
The reference operating conditions are a <strong>the</strong>rmal input
<strong>of</strong> 18.5 kW, an equivalence ratio <strong>of</strong> 0.85 and an air
preheating <strong>of</strong> 838 K. In this case, wall temperature is
homogeneous and equal to ~ 1320 K. No visible flame
can be observed in this specific "colorless" combustion
mode (Figure 3).
Figure 3 Photograph <strong>of</strong> <strong>the</strong> facility in flameless
combustion regime.
2
Reactive zone visualisation by OH* chemiluminescence
imaging
OH* chemiluminescence imaging has been shown to be
a very convenient technique for <strong>the</strong> visualisation <strong>of</strong>
reaction zone structures in high temperature furnace
compared to o<strong>the</strong>r radicals. Indeed in <strong>the</strong> visible range,
CH* and C2* chemiluminescence signals are hardly
discernable from strong global emissions <strong>of</strong> high
temperature walls. As OH* chemiluminescence occurs
in <strong>the</strong> ultraviolet spectral range, emissions from high
temperature walls are several orders <strong>of</strong> magnitude less
than in <strong>the</strong> visible range. Images <strong>of</strong> reaction zones can
be <strong>the</strong>n obtained [20].
In <strong>the</strong> present experiment, OH* images are collected on
an ICCD camera (Roper Princeton IMAX - 512 x 512
pixel - 16 bits), through a Goyo UV 25 mm f/2.8 lens
and an interferential filter (310 ± 10 nm) through a 100
x 100 mm² UV silica window, set on one <strong>of</strong> <strong>the</strong> furnace
opening.
Because <strong>of</strong> <strong>the</strong> 100 mm width <strong>of</strong> <strong>the</strong> refractory wall, <strong>the</strong>
collection system has to be placed as close as possible to
<strong>the</strong> window to optimise <strong>the</strong> field <strong>of</strong> view obtained in <strong>the</strong>
combustion chamber. However, <strong>the</strong> strong radiation
heat from <strong>the</strong> furnace induces some damages on <strong>the</strong>
optical filter when set directly in front <strong>of</strong> <strong>the</strong> windows.
To avoid this problem, a dichroïc beam-splitter is used
which reflects perpendicularly a part <strong>of</strong> <strong>the</strong> ultraviolet
spectral range corresponding to OH* emissions bands
and transmitting <strong>the</strong> visible and infrared ranges. Then
<strong>the</strong> collection system (camera, lens and filter) are set
perpendicularly to <strong>the</strong> window apart from <strong>the</strong> direct heat
flux. This configuration ensures <strong>the</strong> safety for <strong>the</strong>
material, a good spectral selection and a convenient
field <strong>of</strong> view <strong>of</strong> 200 x 200 mm².
Velocity measurements by Particle Image Velocimetry
The characterization <strong>of</strong> <strong>the</strong> aerodynamic mixing <strong>of</strong> high
velocity jets and recirculating flue gas is performed by
<strong>the</strong> application <strong>of</strong> Particle Image Velocimetry (PIV) on
<strong>the</strong> FOUR facility. The setup consists <strong>of</strong> a double pulsed
Nd:Yag laser Quantel Big Sky (120 mJ/pulse). The laser
sheet formed by means <strong>of</strong> a periscope system and a set
<strong>of</strong> lenses crosses <strong>the</strong> facility in <strong>the</strong> vertical plane <strong>of</strong> <strong>the</strong>
three turbulent jets. Particle Mie scattering is collected
on a CCD camera LaVision FlowMaster (1280 x 1024
pixel - 12bits) equipped with a ferro-electric liquid
crystal shutter and a Nikkor 50 mm f/1.2 lens. Despite
<strong>the</strong> use <strong>of</strong> a shutter, <strong>the</strong> wall radiation is not totally
eliminated on <strong>the</strong> particle images and induces spurious
velocities from <strong>the</strong> cross-correlation calculation. In
order to avoid this correlation error, a background
image is acquired without particles and is <strong>the</strong>n
subtracted to each particle image before <strong>the</strong> correlation
processing [20]. Ano<strong>the</strong>r difficulty is to be able to
combine sufficient dynamic range <strong>of</strong> <strong>the</strong> velocity
measurement and suitable spatial resolution especially
for <strong>the</strong> methane jets, because <strong>of</strong> <strong>the</strong>ir small width and
high velocity. For this purpose, a direct crosscorrelation
algorithm is used instead <strong>of</strong> a conventional
FFT one [21], allowing <strong>the</strong> use <strong>of</strong> rectangular

interrogations windows (8 x 40 pixel²). For each case,
mean and rms velocity fields are calculated over 300
instantaneous fields after global and local filtering <strong>of</strong>
spurious vectors [22].
<strong>Temperature</strong> measurements with fine wire <strong>the</strong>rmocouple
Local temperature is measured in <strong>the</strong> combustion
chamber thanks to 50 μm bare B-type (Pt – 6% Rh / Pt –
30% Rh) <strong>the</strong>rmocouple. The signal is amplified (x100 –
x1000) with a low noise preamplifier Stanford Research
SR 560 and digitised by a 16 bit resolution ADC board
National Instruments, AT-MIO-16-X), with a sampling
frequency <strong>of</strong> 8 kHz during 20 sec per location.
Difference between <strong>the</strong> measured bead temperature and
<strong>the</strong> local gas temperature comes mainly from
convection and radiation heat transfers between <strong>the</strong> hot
junction and its environment. Indeed, <strong>the</strong> conduction
can be neglected as <strong>the</strong> length <strong>of</strong> <strong>the</strong> wires is large
enough compared to <strong>the</strong>ir diameters [23]. Catalytic
effect can be also neglected as no difference on
temperature measurements are observed between two
successive acquisitions on radial pr<strong>of</strong>iles nor between
new and used <strong>the</strong>rmocouple on <strong>the</strong> same locations. In
this case, <strong>the</strong> resolution <strong>of</strong> <strong>the</strong> <strong>the</strong>rmal balance between
<strong>the</strong> <strong>the</strong>rmocouple and its surroundings need <strong>the</strong> precise
knowledge <strong>of</strong> wire diameter and emissivity, wall
emissivity and convective heat transfer coefficient. A
<strong>the</strong>oretical calculation <strong>of</strong> heat exchange in <strong>the</strong> ranges <strong>of</strong>
expected velocity and temperature has been performed
to estimate <strong>the</strong> difference between <strong>the</strong> measured bead
temperature and <strong>the</strong> real gas temperature. The
temperature is underestimated below 1700 K, and
overestimated above. In our configuration, thanks to <strong>the</strong>
small <strong>the</strong>rmocouple diameter, <strong>the</strong> hot wall temperature
and <strong>the</strong> global homogeneity <strong>of</strong> <strong>the</strong> mild flameless
combustion, <strong>the</strong> temperature error is estimated to be no
more than than ± 5 %, that is consistent with o<strong>the</strong>r
experiments [24]. Considering this uncertainty,
temperature measurements are presented without
correction in <strong>the</strong> following.
3
Results and discussion
The evolution <strong>of</strong> NOx emissions with air preheating
temperature is presented on Figure 4. Mild flameless
regime is conserved whatever <strong>the</strong> air preheating
temperature. For <strong>the</strong> maximum air temperature (Ta =
838 K), <strong>the</strong> value is already low: [NOx] = 25 ppm @
3% O2. A gradual exponential decrease <strong>of</strong> NOx
concentrations is observed when decreasing air
temperature down to a very low value <strong>of</strong> 4 ppm @ 3%
O2 for ambient air temperature. This can be attributed to
<strong>the</strong> decrease <strong>of</strong> NO formation from <strong>the</strong>rmal route, as <strong>the</strong>
decrease <strong>of</strong> air preheated temperature induces a
decrease <strong>of</strong> adiabatic flame temperature for a constant
<strong>the</strong>rmal power.
[NOx] (ppm @3%O2)
30
25
20
15
10
5
0
250 350 450 550 650 750 850
<strong>Air</strong> temperature T(K)
Figure 4 NOx emissions vs air preheating temperature.
Figure 5 presents mean OH* chemiluminescence
images in function <strong>of</strong> air preheating temperature for a
constant <strong>the</strong>rmal power <strong>of</strong> 18.5 kW and an equivalence
ratio <strong>of</strong> 0.85. For <strong>the</strong> largest air preheating (Ta = 838
K), one can observed several reaction zones from <strong>the</strong>
first optical access centred at y = 90 mm from <strong>the</strong> hearth
<strong>of</strong> <strong>the</strong> furnace.
Ta =838 K Ta = 773 K Ta = 673 K Ta = 573 K Ta = 473 K Ta = 373 K Ta = 293 K
Figure 5 Mean OH* chemiluminescence images for different air preheating temperature Ta.

a.
b.
Figure 6 Mean velocity field obtained by PIV for <strong>the</strong>
air preheating conditions (Ta = 838 K). a: axial
velocity (colors in m/s) and streamlines (whites lines).
b: radial velocity (colors in m/s) and velocity vectors.
From <strong>the</strong> burner exit, an attached reaction zone
appears on <strong>the</strong> internal side <strong>of</strong> <strong>the</strong> two methane jets.
Similar reaction zone is also present on <strong>the</strong> o<strong>the</strong>r side
<strong>of</strong> <strong>the</strong> methane jets but detached from <strong>the</strong> burner.
These weak reaction zones occur in <strong>the</strong> shear layer
between <strong>the</strong> methane jets and <strong>the</strong> hot recirculating flue
gas as diffusion flames with excess oxygen present in
<strong>the</strong> combustion products. The main reaction zone is
lifted-<strong>of</strong>f <strong>the</strong> burner. It starts from y* = 80 mm where
<strong>the</strong> methane and air jets begin to merge. This reaction
zone is developed along <strong>the</strong> stoechiometric line in <strong>the</strong>
mixing layer <strong>of</strong> <strong>the</strong> reactant jets and ensures <strong>the</strong> most
important heat release. It ends downstream before
half-width <strong>of</strong> <strong>the</strong> furnace, as it is observed from <strong>the</strong>
second optical access centred at y = 500 mm. When
decreasing air preheating temperature, one can
observed a progressive extinction <strong>of</strong> <strong>the</strong> weak reaction
zone around <strong>the</strong> methane jets which is no more
observable for Ta < 573 K. From Ta = 673 K, <strong>the</strong> lift<strong>of</strong>f
position <strong>of</strong> <strong>the</strong> main reaction zone moves
progressively upstream in <strong>the</strong> furnace. This shift is due
to a change <strong>of</strong> local conditions in <strong>the</strong> mixing layer as
an increase <strong>of</strong> <strong>the</strong> auto-ignition delay can be
associated to <strong>the</strong> decrease <strong>of</strong> <strong>the</strong> inlet air temperature.
4
a.
b.
Figure 7 Mean velocity field obtained by PIV for <strong>the</strong>
non air preheating conditions (Ta = 293 K). a: axial
velocity (colors in m/s) and streamlines (whites lines).
b: radial velocity (colors in m/s) and velocity vectors.
When air preheating temperature is decreasing, <strong>the</strong>
intensity <strong>of</strong> OH* chemiluminescence signal is also
decreasing, which indicates a decrease <strong>of</strong> local heat
release density. The reaction zone is moving very
downstream in <strong>the</strong> furnace. The core <strong>of</strong> <strong>the</strong> main
reaction zone is above y = 400 mm for Ta < 473 K
and reaches <strong>the</strong> half-width <strong>of</strong> <strong>the</strong> furnace for <strong>the</strong> non
preheated air case.
Figure 6 and Figure 7 present mean streamlines
superimposed on <strong>the</strong> axial velocity field and <strong>the</strong> mean
velocity vectors with radial velocity field obtained by
averaging 300 instantaneous velocity field measured
by PIV for respectively maximum air temperature (Ta
= 838 K) and for ambient air temperature. The
dimensions <strong>of</strong> <strong>the</strong> velocity field are limited by <strong>the</strong>
field <strong>of</strong> view (120 x 100 mm²) achievable in <strong>the</strong>
present configuration through <strong>the</strong> first opening. No
velocity measurement can be done below 10 mm
because <strong>of</strong> <strong>the</strong> laser reflection on <strong>the</strong> burner exit.
Despite <strong>the</strong> size <strong>of</strong> <strong>the</strong> interrogation windows (0.8 x 4
mm²) which does not allow to fully resolve <strong>the</strong>
methane jets, <strong>the</strong> maximum velocity is higher than <strong>the</strong>
bulk flow velocity for <strong>the</strong> preheating case.

This can be attributed to <strong>the</strong> acceleration <strong>of</strong> <strong>the</strong>
methane jet because <strong>of</strong> <strong>the</strong> presence <strong>of</strong> <strong>the</strong> attached
reaction zone. <strong>On</strong>e can see that <strong>the</strong>re is no direct
interaction between methane and air jets before y* =
80 mm as <strong>the</strong> axial velocity is close to zero beside
<strong>the</strong>m. Because <strong>of</strong> <strong>the</strong> large momentum <strong>of</strong> each jet, <strong>the</strong>
entrainment process <strong>of</strong> recirculating flue gas by each
reactant is important as can be seen from streamlines
around <strong>the</strong> jets. Moreover, large values <strong>of</strong> radial
velocity component around each methane jet show
that <strong>the</strong> “aspiration” <strong>of</strong> flue gas by <strong>the</strong> jets occurs
quickly from <strong>the</strong> burner exit. This is also true for <strong>the</strong>
central air jet even if this region corresponds to its
potential core as <strong>the</strong> divergent shape <strong>of</strong> <strong>the</strong> injection
favours <strong>the</strong> turbulent mixing. <strong>On</strong>e can also see that
each methane jet is not symmetric. The internal side
close to <strong>the</strong> air jet is more developed because <strong>of</strong> <strong>the</strong>
momentum <strong>of</strong> <strong>the</strong> latter, which is typical <strong>of</strong> such
parallel turbulent jets configuration [25]. The
interaction between each methane jet and <strong>the</strong> central
air jet starts from y > 80 mm, where <strong>the</strong> main reaction
zone is observed on OH* chemiluminescence
imaging.
Without air preheating, one observes <strong>the</strong> same
aerodynamic features for <strong>the</strong> jets. However, <strong>the</strong>
entrainment is less for <strong>the</strong> air jet as <strong>the</strong> momentum is
lowered. The beginning <strong>of</strong> <strong>the</strong> merging <strong>of</strong> methane
and air starts at <strong>the</strong> same position y* = 80 mm, which
confirms that it is mainly controlled by <strong>the</strong> geometry
<strong>of</strong> <strong>the</strong> burner. So, with or without air preheating, <strong>the</strong>
entrainment process is already active at <strong>the</strong> exit <strong>of</strong> <strong>the</strong>
burner for both jets thanks to <strong>the</strong>ir large momentum.
This induces strong mixing <strong>of</strong> each reactant with
recirculating combustion products inducing <strong>the</strong> diluted
conditions where <strong>the</strong> main reaction begins [25, 26]. As
<strong>the</strong> entrainment continues in <strong>the</strong> jets, <strong>the</strong> dilution is
more and more enhanced, especially for <strong>the</strong> non
preheating case as <strong>the</strong> reaction zone occurs only from
500mm.
Figure 8 presents mean temperature maps in <strong>the</strong>
furnace operating with air preheating (a) and without
air preheating (b). The topology <strong>of</strong> <strong>the</strong> reaction zones
obtained by OH* imaging (Figure 5) is found. For <strong>the</strong>
preheated air case, one can observe a weak increase <strong>of</strong>
temperature around methane jets, due to <strong>the</strong> attached
reaction zone. In <strong>the</strong> present geometry <strong>of</strong> parallel
turbulent jets, <strong>the</strong> entrainment <strong>of</strong> recirculating flue gas
is a way to “condition” <strong>the</strong> fuel before it reacts with
<strong>the</strong> air jet [26]. Such weak reaction zone also
participates to such preconditioning <strong>of</strong> <strong>the</strong> fuel jet by
<strong>the</strong> formation <strong>of</strong> heat and radicals, without inducing
large NOx emissions. The main increase <strong>of</strong>
temperature is observed in <strong>the</strong> lifted reaction zone
from <strong>the</strong> beginning <strong>of</strong> <strong>the</strong> mixing layer between <strong>the</strong>
reactant jets as a typical jet flame.
5
(a) Ta =838 K
(b) Ta = 293 K
Figure 8 Maps <strong>of</strong> mean temperature (a) with air
preheating , (b) without air preheating.
(each dot represents a measurement point)

Main difference is, even if it is underestimated by <strong>the</strong>
<strong>the</strong>rmocouple, <strong>the</strong> maximum <strong>of</strong> temperature reached in
<strong>the</strong> core <strong>of</strong> <strong>the</strong> reaction zone, which stays relatively low
(Tmax = 1717 K) compared to "standard" combustion,
ensuring low NO production from <strong>the</strong>rmal route.
Without air preheating, no more reaction zone is
observed around <strong>the</strong> methane jets nor at <strong>the</strong> beginning
<strong>of</strong> <strong>the</strong> mixing layers between methane and air jets. In
<strong>the</strong> first part <strong>of</strong> <strong>the</strong> combustion chamber, one observes a
progressive smooth increase <strong>of</strong> temperature for <strong>the</strong> three
jets surrounded by homogenous hot atmosphere. The
temperature gradients remain low even in <strong>the</strong> reaction
zone. The maximum temperature is only Tmax = 1556 K.
These operating conditions correspond to a kind <strong>of</strong>
ultimate case <strong>of</strong> mild combustion with a very large
mixing <strong>of</strong> <strong>the</strong> reactants with recirculating flue gas
ensuring that combustion occurs in highly diluted
conditions and with small heat release density.
Conclusions
The effect <strong>of</strong> air temperature on mild flameless
combustion regime is studied in a laboratory-scale
facility. Velocity measurements obtained by PIV show
<strong>the</strong> role <strong>of</strong> <strong>the</strong> entrainment process <strong>of</strong> air and methane
turbulent jets to induce <strong>the</strong>ir dilution with a large
amount <strong>of</strong> recirculating flue gas. Main heat release
occurs <strong>the</strong>n in hot diluted conditions in <strong>the</strong> mixing
layers <strong>of</strong> air and methane jets downstream <strong>the</strong> burner
exit. This induces very low temperature gradient as well
as maximum value and thus explains <strong>the</strong> very low NOx
emissions even with air preheating. When decreasing
<strong>the</strong> inlet air temperature, <strong>the</strong> lifted reaction zone moves
gradually downstream in <strong>the</strong> furnace and NOx
emissions decrease. Without air preheating, <strong>the</strong> reaction
zone occurs in <strong>the</strong> middle <strong>of</strong> <strong>the</strong> combustion chamber in
large diluted conditions. In this case temperature peaks
over <strong>the</strong> flue gas temperature are weak. Thermal NO
formation is minimized, NOx concentrations are very
low (4 ppm @ 3% O2). These results give new insights
for <strong>the</strong> understanding <strong>of</strong> <strong>the</strong> mild flameless combustion
regime and put forward some guidelines for <strong>the</strong> design
<strong>of</strong> mild flameless combustion burners.
Acknowledgements
This work is done with <strong>the</strong> financial support <strong>of</strong> ANR
(National Research Agency) – PAN-H Program and
ADEME (French Agency for Environment and Energy
Management).
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