1, Marianne Sjöstrand1, Yves D’Angelo1, Fabio...
Transcript of 1, Marianne Sjöstrand1, Yves D’Angelo1, Fabio...
15th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 05-08 July, 2010
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Experimental Study of Combustion and Flow Dynamics in a Meso-Scale Whirl
Combustor
Siqian Liu1, Bruno Renou
1, Marianne Sjöstrand
1, Yves D’Angelo
1, Fabio Cozzi
2
1: UMR 6614 CORIA, INSA de Rouen, FRANCE, [email protected]
2: Department of Energy, Politecnico di Milano, Milan, Italy, [email protected]
Abstract Combustion may constitute an interesting solution for small-scale power generation. Using hydrocarbons high energy density at small scales is still a scientific and technological challenge. Driven by industrial applications such as the replacement of existing batteries by a lighter system or the design of miniature propulsion device, successful development of such small scale systems requires an extensive understanding of combustion behavior while scaling down conventional combustors to a miniaturized size. For this purpose, a meso-scale whirl combustor is designed and experiments are performed. The internal flow fields have been studied by Particle Image Velocimetry (PIV) in non-reacting conditions. Combustion stability diagrams have been established which evidenced quite wide flame stability limits. Pressure fluctuation data have been analyzed for the purpose of exploring the combustion instabilities. Pollutant emissions have been measured simultaneously to the pressure fluctuations not only in order to detect the exhaust species, but also to estimate the combustion efficiency. At the same time, CH* chemiluminescence flame imaging has been carried out by an intensified CCD camera to observe the flame location inside the combustor and instantaneous flame structures corresponding to different total flow rates and different equivalence ratios. High-speed imaging has also been implemented and synchronized with pressure acquisition to study the thermo-acoustic coupling. In order to improve the performance of the combustor, hydrogen is added to the fuel mixture. The beneficial effects of hydrogen enrichment on the flame stabilization and the reduction of pollutant emissions have been pointed out, which provides a potential alternative to fuel utilization in meso-scale combustion.
1. Introduction
In the recent years, one can easily notice the trends for the miniaturization of electronic and
mechanic systems. However, the battery autonomy still remains a challenge. At present time, what
we use as power supply are mainly the primary batteries and the rechargeable ones. Considered as
one of the currently available top chemical batteries with an energy density of 1.2MJ/kg, lithium is
widely used in typical portable consumer electronics. In comparison, the hydrogen and hydrocarbon
fuels demonstrate an extremely high energy density (~45MJ/kg). As a result, meso- and micro-scale
combustion is supposed to be a potential candidate in small-volume power generation systems.
The successful design of such systems requires an extensive understanding of the combustion
behavior while scaling down conventional combustors to a miniaturized size. Scaling issues related
to fluid and thermal transport phenomena as well as chemical reaction have been theoretically
reviewed in the literature [1]. In comparison to macro scale combustors, meso-combustors operate
at much lower Reynolds numbers due to the miniaturized size. As a result, mixing process can
hardly be achieved by turbulent effects. Instead, it is mainly controlled by diffusion, which may
cause poor mixing. Moreover, combustion efficiency can be negatively affected by the reduction in
the residence time. Finally, the high surface to volume ratio resulted from the small characteristic
length of the combustor leads to significant heat losses. When heat release from chemical reaction
cannot take pace with heat losses by wall, combustion can no longer be sustained, thus occurs the
flame quenching.
A possible way to overcome the short residence time effect is to reduce the kinetic reaction
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time by increasing the reaction temperature. Excess-enthalpy burners are thus inspired by this idea,
which recirculate the thermal energy stored in the exhaust port to preheat the reactants [2].
Concerning the poor mixing level, asymmetric whirl combustors are regarded as a promising
strategy to promote turbulent mixing [3,4,5].
Although the sources and mechanisms of flame instabilities in small scale combustion are hard
to be identified at the moment, chemical quenching and thermal quenching are assumed to be two
important contributions [6]. As the surface-to-volume ratio is increased, the possibility of radical
termination by wall is enhanced. Therefore, surface-induced catalytic combustors become an
attractive solution to positively make use of the wall in presence [7]. The general concept of
catalytic design lies in catalyst deposition such as Platinum coating on the interior wall.
Based on a preliminary study on the feasibility of different strategies found in literature, we
chose the whirl flow concept. The experimental needs, especially the optical access, are taken into
account as well. For these reasons, a quasi-cubic whirl combustor is designed. The combustor
volume is less than one centimeter cube, usually referred to as meso-scale.
As far as we know, most papers related to micro-/meso-scale combustion deal with the designs
and performances of the combustors with the aim of tackling the quenching problem. Very few
discussions concerning the sources of instabilities are available so that we are not able to explain
the quenching phenomenon. For this reason, CH* and OH* chemiluminescence measurements have
been performed which provides us with the information on flame structure and local heat release.
By coupling this information with instantaneous pressure fluctuation, thermo-acoustic instabilities
will be discussed.
Moreover, the lack of velocity field data can be very inconvenient to properly understand the
internal flow structure, especially for a whirl combustor inside which flow and flame structures are
more complex. Hence, Particle Image Velocimetry (PIV) is chosen on account of its efficiency as a
planar measurement method.
Most of the existing results involving the determination of the combustion stability only gave
us a rough idea on the flame self-sustainability conditions, ignoring the occurrence of the acoustic
phenomena [4,5,8]. In order to classify the different combustion stability regimes, acoustic analysis
has been performed, leading to the establishment of a detailed stability diagram.
In meso-scale combustors, flame stabilization mainly involves competition between the rates of
the chemical reactions and the rate of turbulent diffusion of species and energy. From this point of
view, hydrogen has better flame holding characteristics than methane in terms of both adiabatic
flame temperatures and flammability limits. Thus, hydrogen enrichment in fuel mixture is supposed
to enhance meso-scale combustion. Nevertheless, injection of hydrogen has negative effects in
some aspects such as higher NOx emissions due to higher adiabatic flame temperature. In
consequence, research towards determination of beneficial ranges of hydrogen enrichment is
crucial. The impact of hydrogen addition on both gaseous and liquid fuel/air mixture (e.g.
methane/air, propane/air, kerosene air) in macro-scale combustion system (e.g. gas turbine) has
been widely investigated [7,8,9]. But little information can be found with respect to meso-scale
combustion.
In the present work, we aim to demonstrate the interest of hydrogen-enriched methane/air
combustion in a meso-scale whirl combustor inside which the flow configurations appear to be
much more complex. At first, the internal flow field under isothermal conditions will be studied
aiming to investigate the flow motions inside the combustor. Then, emphases will be put on the
analysis of the global performance of the meso-combustor, including establishment of stability
diagrams coupled with flame structure observation and estimation on the combustion efficiency.
Finally, the effect of hydrogen addition on flame stability and pollutant emissions will be discussed
in detail.
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2. Experimental approaches
2.1) Meso-combustor
A quasi-cubic combustor having a volume of about 640mm3
(8mm×8mm×10mm) has been
designed and manufactured (Fig.1). The meso-combustor operates in a non-premixed configuration,
fed by methane and hydrogen mixture as fuel and air as oxidizer. More precisely, air is injected
tangentially in order to generate a whirling flow, while gaseous fuel injection is achieved in the
radial direction. Two different pipes can be used for fuel injection in order to investigate its effect
on the mixing process. The flow rates are controlled by three thermal mass flow meters (Bronkhorst
EL-FLOW Select) calibrated for Hydrogen (max: 0.5L/min), Methane (max: 0.55L/min), and Air
(max: 5L/min) respectively. Therefore, various concentration of hydrogen can be studied so as to
point out its effect on flame stability and pollutant emissions. A maximum input thermal power of
about 220W can be reached.
Fig. 1 Meso-combustor (Left: meso-combustor compared with a one-euro coin; Middle: 3D schematic; Right: plan X-
Z)
In addition to the methane/air combustion, measurements are carried out for 3 fuel mixtures
with different hydrogen enrichment rates: CH4 (20%) / H2 (80%), CH4 (50%) / H2 (50%), CH4
(80%) / H2 (20%), where the percentage of hydrogen is defined as the ratio of the mass flow rate of
H2 over the mass flow rate of the fuel mixture:
Stability diagrams are established for all the 3 cases, each of which requires at least 40
measurement points. In this study, we choose CH4 (50%) / H2 (50%) / air as a representative case.
The equivalence ratio of the mixture is determined by the following expression:
with (stoichiometric fuel air ratio for methane) and (stoichiometric fuel air ratio for hydrogen).
2.2) Cold flow velocity measurement
PIV measurements are processed initially to the non-reacting flows, allowing us to obtain
quantitative information on flow motions inside the combustor. For this purpose, double-pulsed Nd-
YAG laser (λ = 532nm, E=120mJ/pulse) is employed. A cylindrical lens (f=30cm) is used to create
a laser sheet with a constant thickness of 200μm (FWHM of the intensity profile) in the
measurement volume. A 3-axe moving system is used to adjust with high precision (sensibility: 1
μm, resolution: 10μm) the position of the combustor so that we are able to measure the velocity
field of different planes in two orthogonal directions. Flow is seeded with DEHS droplets with a
Exhaust pipe
Fuel injection
Air injection
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typical diameter of 1μm. The acquisition system is composed of a Lavision Flowmaster CCD
camera (1024×1280 pixels, 3Hz) with a macro-lens (105mm, f/2.8). The PIV software Davis 7.2 is
used to register and post-process the acquired images. The field of view is set to be 6mm×8mm. By
applying the multi-pass cross-correlation procedures (64 pixels → 32 pixels) we can obtain a spatial
resolution close to 0.18mm. This typical resolved length scale ensures the mean flow velocity to be
fully characterized.
2.3) Flame imaging
Both CH* and OH* chemiluminescence flame imaging has been carried out by an intensified
CCD camera (Roper Scientific, 512×512 pixels) coupled with narrow band pass filters (BG12) in
order to observe the flame position in the combustor and instantaneous flame structures
corresponding to different total flow rates and different equivalence ratios. In each operating
condition, 500 instantaneous flame images have been taken consecutively (frequency = 10Hz).
Exposure time is set to be 2ms while the gain is adjusted to 175 based on the case corresponding to
maximum level of collected CH* signal. These two parameters are kept constant during all the
measurements so that the results from different experimental conditions are comparable. All the
flame images presented in this study correspond exactly to the plan X-Z (Fig.1) where the effects of
the tangential injection of air flow and radial injection of fuel flow can be observed clearly.
2.4) Pressure fluctuation acquisition
Pressure fluctuation data are collected using a piezoelectric dynamic pressure sensor (Kistler
6053CC60) coupled with a charge amplifier. Spectral analysis is performed for the purpose of
exploring combustion instabilities. Sampling frequency is fixed to 10 kHz. The temporal averaged
power spectrum for the pressure fluctuation signal is determined from the ensemble averaged of 40,
temporally Hamm-windowed, acquisition.
2.5) Thermo-acoustic coupling
In order to analyze thermo-acoustic instabilities, a high-speed ICCD camera (Photron,
FastcamSA1, 512×512 pixels) is also implemented. The frame rate is set as 20000images/second.
Flame imaging is synchronized with the pressure signal acquisition with the help of a Multi-
Channel-Data-Link unit. As a result, pressure fluctuation signals are sampled at 20 kHz and 20000
corresponding images are obtained in 1 second.
2.6) Pollutant emissions
Burned gases are sampled for pollutant emissions analysis using a chemiluminescence analyzer
to measure the concentrations of NO and NOx, a non-dispersive infrared (NDIR) analyzer to
measure CO and CO2 and paramagnetic technique for O2 concentrations. A flame ionization
detector (FID) is applied for the continuous trace analysis of total hydrocarbon (THC). The gas
analyzer probe is located close to the gas exit and operates at 3L/min sampling flow rate which is
higher than that of the exhaust gas. Therefore, some extra air in the surroundings could have been
sucked up by the probe. This will cause a decrease in the measured concentrations of CO2, CO,
NOx and THC since the amount of air is slightly overestimated. The detected exhaust species are
regarded as a good indicator of the combustion completeness level. They are also served to estimate
the combustion efficiencies. Accordingly, conclusions on the scaling impact on the residence time
can be reached. The combustion efficiency is calculated by taking into account the loss caused by incomplete
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combustion which is calculated on the basis of the measured CO concentration in the combustion gases according to the following formula:
3. Results and discussions
3.1) Internal flow fields
At first, the internal flow fields have been studied by Particle Image Velocimetry (PIV) in non-
reacting conditions, thus providing the information on the structure of whirling flows.
We are restricted to 2D PIV measurements which are performed in consecutive planes and in
two orthogonal directions. An illustration of different planes of measurements can be found in
Fig.2. The distance between two adjacent planes is 500μm. Since the laser sheet thickness is about
200μm, we can thus consider the planes of measurements to be almost side by side.
Fig. 2 Consecutive planes of PIV measurements in 2 orthogonal directions
Fig. 3 Algorithm for velocity component reconstruction in plane y-z
13 Planes (X, Y) 13 Planes (Y, Z)
Fuel
Air
6mm
8mm
z
y
Plane Y-Z
Mesh size = 60 x 80
Plane: Vy and Vz
13 lines: Vx and Vz’
If error (Vz, Vz’) < 8%,
Then Vz’’ = average (Vz, Vz’)
Linear Interpolation:
Vx (lines) Vx (plan)
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Based on this procedure, a time-averaged 3D and 3C velocity field can be reconstructed from a
series of 2D measurements. We assume implicitly the flow field as fully stationary. Fig.3 is the
schematic of velocity component reconstruction algorithm in plane y-z. Totally we have 13 planes
of measurements in this direction. Since in each plane we repeat the same procedure, we use one
plane as an example to explain how to reconstruct the velocity component.
In plane y-z (Fig.3), as mesh size is 60 x 80, Vy and Vz are measured for these points. From the
measurements in the orthogonal direction, we also have information on the velocity components
(Vx and Vz’) for the 13 interlines. The first step is to compare Vz with Vz’ because for 60 x 13
points the velocity components in axis-z have been measured twice. The criterion we applied to this
comparison is the repeatability of the measurements which is estimated at about 8% after repeating
several times the measurements under the same operating conditions. If the estimated error
overpasses 8%, we consider that the measurements are not precise enough. Once the error is less
than 8%, we calculate the average of Vz and Vz’ and replace the measured Vz and Vz’ with the
new values Vz’’. The next step is to interpolate measured Vx (in the 13 interlines) to the entire plan.
Consequently, we know 3 velocity components for all the points in this plane.
The same procedure has been applied to the other 12 planes in this direction as well as the 13
planes in the orthogonal direction by a program written in Matlab. Once the 3D velocity field is
reconstructed, post-processing and visualization such as streamlines can be done by Paraview and
Tecplot.
Fig.4 represents the streamlines of the mean flow. We clearly observe the injection zone which
impact to the wall before to create two distinct flow motions. The main zone travels directly
downstream along the side-wall and exits. A portion of the injected flow recirculates azimuthally in
the upstream part of the chamber before convecting downstream. Similar to swirling flows, whirling
flows were originally used to improve and control the mixing rate between fuel and oxidant
streams. But unlike swirling flows, as air is injected with no axial velocity component, the
structures of whirling flows appear to be very different, with a relatively quiescent center. In our
case, the mixing process will be take place near the upper wall of the combustor at relatively low
Reynolds numbers.
Fig. 4 Streamlines coloured by velocity magnitude (Air Velocity = 40m/s, Fuel Velocity = 5m/s), reconstructed from
2D PIV results
3.2) Stability diagram
We concentrate our attention on the operating range of the combustor. In order to choose the
operating conditions for further analysis, the stability diagram is established by the following
procedure: at a fixed total flow rate, we change the equivalence ratio gradually by changing both H2
and CH4 flow rates, and then we repeat the operation for several different total flow rates.
Mixing controlled mainly
by diffusion, near the
upper wall of the
combustor
Outlet
Fuel
Air
Quiescent center
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Fig. 5 Stability diagram for CH4 (50%) / H2 (50%) / air Flame (Left); Criterion for classification of different stability
regimes (Right)
Fig.5 is the stability diagram for CH4 (50%) / H2 (50%) / air flames, in which axis-x represents
the total flow rate in Nml/min while axis-y corresponds to the equivalence ratio. As shown in Fig.5,
three principal regimes are differentiated by calculating the fluctuation energy of dynamic pressure
under each operating condition (Fig.5, right). Between the green line and violet line, the flames are
relatively more stable. In the other regions, although the flames are classified as less stable, they are
observed to possess good self-sustainability even near the extinction limits. Obviously, the region in
which the flames exhibit the most stable behavior corresponds to the orange zone indicated in this
figure.
Besides the chosen criteria (RMS), other evidence can be provided so as to validate the
established stability diagram. Power spectral density (PSD) is another useful statistic signal
processing tool based on dynamic pressure analysis. It not only indicates the instability level but
also capture the frequency contents.
When the total flow rate is fixed at 1000Nml/min, equivalence ratio can be changed gradually.
The representation of the results is inspired by Campbell Diagram, initially defined as a system’s
response spectrum as a function of its oscillation regimes. In this case, axis-x represents the
combustion regimes such as equivalence ratio (Fig.6) and total flow rate (Fig.7); axis-y shows the
frequency level; the color bar corresponds to PSD of pressure fluctuations in logarithmic scale.
With this kind of interpretation, we can easily compare the instability levels under various operating
conditions and identify at which frequency comes out the peak.
In Fig.6, two main acoustic modes are observed respectively at 500 Hz and 1100 Hz. When the
fuel mixture burns in lean conditions, the low frequency instability is a dominant factor. On the
contrary, for fuel-rich flames the high frequency instability becomes a more important contribution
to global combustion instabilities together with the total disappearance of low frequency instability.
Similar proof can be provided by the flame visualization. The large-scale flame motions inside the
combustor are indicated by the RMS of the image sets which qualitatively implies the instability
level. The results are quite consistent with the PSD analysis in the present case. The most stable
flame in this case (φ=1.01) corresponds to stoichiometric condition. From Fig.7, we can conclude
that instabilities occur at about 1100 Hz for all the cases. When the total flow rate increases, the
tendency to enlarge the resonance frequency range is more and more obvious, which implies
different acoustic modes appear at the same time due to the fact that a diversity of dominant
resources of instabilities coexist when the total flow rate is large enough.
According to the instantaneous information on pressure fluctuation and flame images, the most
stable flame states can be found at stoichiometry while the total equivalence ratio is around
1000Nml/min. If we trace back to the stability diagram, these flames states are well situated inside
the orange zone.
Less Stable
Less Stable
Stable Very Stable
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Fig. 6 Campbell diagram and corresponding flame images in Plan X-Z obtained by CH* chemiluminescence (CH4 (50%)/H2 (50%)/Air, Total Flow Rate = 1000Nml/min)
Fig. 7 Campbell diagram (CH4 (50%)/H2 (50%)/Air, Equivalence Ratio = 1.01)
3.3) Impact of H2 addition on flame stability
In general, methane flames can be stabilized with lower equivalence ratio in fuel-lean
conditions as the H2 concentration increases. Results displayed in Fig.8 proved this widely accepted
conclusion. We notice that the addition of small amounts of H2 shows little impact upon blow-out
limits for most levels of total flow rates. When the H2 molar fraction overpasses 20%, the
sensitivity of the blow-out equivalence ratio to H2 level variations remains essentially constant
across the rest of the entire range of H2 enrichment level. An exception is found in the case of
800Nml/min, where no significant discontinuous or steep drop in Lean Blow-Out (LBO)
equivalence ratio is observed from the beginning. Also in this case, the response of LBO
equivalence ratio to hydrogen addition is the strongest among all the five operating conditions
covering the whole operating range of the combustor in terms of total flow rates. In conclusion,
φ = 1.01
φ = 0.62
Average RMS
PSD (dB)
PSD (dB)
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hydrogen shows beneficial effects on combustion stabilization by extending the LBO limits to
lower equivalence ratio which allow the combustion to occur in leaner conditions. This can be
explained by the fact that hydrogen has better flame holding characteristics than methane thanks to
its low flammability limits and small gap quenching distance.
Fig. 8 Dependence of Lean Blow-Out (LBO) limit equivalence ratio upon H2 molar fraction at different total flow rates
3.4) Thermo-acoustic instability analysis
In whirl combustors, the flow is too complicated to have only one source of instability that
takes precedence over all others. As the air and fuel injection velocities are at very different levels,
the hydrodynamic instabilities such as Kelvin-Helmholtz instability can occur when there is
sufficient velocity difference across the interface between air and fuel flows. The imbalance
between thermal diffusion and mass diffusion can also induce thermo-diffusive instabilities.
Furthermore, we observe that in meso-combustor flame tends to oscillate between extinction and re-
ignition phases. The flame stabilization is achieved with the help of the inner recirculation zone
located in the center of the combustor while the flame itself extinguishes or re-ignites locally. These
extinction and re-ignition processes may be caused by the instabilities induced by local heat release.
However, no direct proof has been provided concerning these different possibilities. Since
experimentally we can imaging some possible ways to achieve thermo-acoustic coupling, we
focused our attention on the thermo-acoustic analysis as a first step to study the instabilities inside
the meso-combustor.
As mentioned before, flame imaging by high-speed ICCD camera is synchronized with
pressure fluctuation signal collection. By associating the pressure signals to flame images, thermo-
acoustic instabilities can also be explored. In order to evaluate the relevance between local heat
release and pressure fluctuations, Rayleigh index has been introduced. Rayleigh index is regarded
as an important indicator of thermo-acoustic coupling. The definition of normalized Rayleigh index
in a simplified form can be found in the study of Yilmaz et al. [10]. In our study, the pressure
fluctuation is uniform over the entire image, which leads to a discretized formulation:
where is the pressure oscillation for each OH* image, is the root mean square of the
pressure oscillations for each set of images, is the fluctuation of the OH* intensity indicating the
local heat release and is the averaged total OH* intensity, N is the number of images in each image set. The normalization by the number of images is achieved so that the computed Rayleigh
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index is comprised between -1 and 1. If Rf is positive, pressure oscillation and heat release oscillation are in phase so the instabilities tend to grow; if it is negative, the instabilities tend to decay. Experimentally, a set of 1000 consecutive images is recorded corresponding to duration of 50ms.
Fig. 9 Impact of H2 addition on flame location and thermo-acoustic instability
As illustrated in Fig.9, when burning 100% CH4, the flame is located in the centre of the
combustor with a relatively high instability level. When the hydrogen is added, we observe that the
flame is much closer to the combustor wall. This is mainly because of the small quenching distance
that hydrogen possesses. Concerning the computed Rayleigh index, two major conclusions can be
drawn. First of all, it is evident that the instabilities occurred in 100% CH4 combustion is more
relevant to thermo-acoustic effects as Rayleigh index is at a relatively higher value. Secondly, when
the concentration of hydrogen increases, the thermo-acoustic instability level is decreased but it is
also spread out, covering the entire combustor. This is probably the result of dispersivity of
hydrogen.
3.5) Impact of H2 addition on pollutant emissions
In addition to the ability to burn at extremely lean conditions, hydrogen has also been shown to
decrease the formation of CO and unburned hydrocarbons. Theoretically, hydrogen addition has
three major effects on the pollutant emissions. First of all, the flame velocity of hydrogen at 20°C
and stochiometric conditions (237cm/s) is much higher than that of methane (42cm/s), allowing
oxidation with less heat transfer to the surroundings. Likewise, higher burning rate implies lower
burning duration, leading to a higher completeness level of combustion. Secondly, the effect of
hydrogen enrichment can also be summarized as a rise of the heat release rate and the mixture
reactivity [11]. In fact, when methane is H2-enriched a higher OH concentration can be obtained via
HO2 in low temperature regimes, which is generally the case of meso-combustor. Finally, compared
to methane, hydrogen has a very small gap quenching distance (0.06cm VS 0.2cm for methane).
The results shown in Fig.10 are clearly consistent with this analysis. As the residence time
increases, more time is available for chemical reactions to be completed. Subsequently, both CO
and unburned hydrocarbon emissions decrease. In order to compare the pollutant emissions at
various levels of H2 addition, the measurement data are normalized by corresponding CH4 flow
rates with the aim of avoiding the substitution effect. Evidently, hydrogen addition plays a very
CH4 (100%), Equivalence Ratio=1.03, Total Flow Rate = 1500Nml/min
CH4 (50%), Equivalence Ratio=1.03, Total Flow Rate = 1500Nml/min
Average RMS Rayleigh Index
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important role in the reduction of CO and THC. On the basis of pollutant emissions measurements
we can deduce that H2 enrichment improves the combustion efficiency, which increase from 0.37 to
0.76.
Fig. 10 Impact of H2 addition on CO and THC emissions at stochiometric conditions (Combustion efficiencies are calculated at residence time = 15.5ms)
Carbon monoxide emissions are the results of incomplete hydrocarbon combustion and more
likely to occur during rich mixture conditions. We notice that under stochiometric condition, a great
amount of CO emissions are detected. This can be explained as the fact that CO2 can easily
dissociate and be converted back to CO as the combustion temperature increases. In addition, flow
residence time is much smaller than chemical time and mixing time as the combustor size is
enormously reduced in comparison to conventional sizes.
Unburned hydrocarbons are produced due to the thermal quenching phenomenon. In the near-
wall regions inside the combustor, flame tends to extinguish because the heat loss through the
chamber wall is greater than the heat needed to sustain the flame. The area of flame quenching is
probably where hydrocarbon is left unburned. In meso-combustors, the high surface-to-volume
ratio indicates a much higher possibility for the flames to be quenched. That’s why in our
measurements, unburned hydrocarbon (THC) emissions are also at a high level.
NOx emissions are also measured but we didn’t present the results in this study because their
amounts are negligible. The principal mechanism of NOx formation consists of thermal NO which
appears only when the combustion temperature becomes high enough (about 1800K). In our cases,
the magnitudes of combustion temperatures are estimated from measured exhaust gas temperatures
and vary from 600K to 1000K approximately, which is far from the critical temperature for NOx
formation.
4. Conclusion
A meso-scale whirl combustor has been designed and experiments have been carried out in
order to investigate the global performance of the combustor as well as to propose a possible way to
Combustion efficiency
= 0.37
Combustion efficiency
= 0.76
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improve the combustion stability and efficiency.
Firstly, PIV measurements have been performed under non-reacting conditions. Whirl flow
structure has thus been studied. Under combustion conditions, the combustor demonstrated its
ability to operate with different gaseous hydrocarbon fuel mixtures. A first step to thermo-acoustic
analysis has been achieved to evaluate the relevance between local heat release and pressure
fluctuations.
Considered as a possibility to improve the combustor performance, the injection of hydrogen
into fuel mixture has been proved to largely extend the lean blow-out limits. In addition, both CO
and unburned hydrocarbon emissions have been significantly reduced by adding hydrogen. The
thermo-acoustic analysis showed the ability of hydrogen to reduce thermo-acoustic instabilities.
Further studies can be focused on the combustor performance under constant combustion
power conditions which is a common approach in industry. This will also help us to achieve a more
reliable estimation and comparison of the combustion efficiencies.
So far we have no clue about the frequency of the thermo-acoustic instability. For this reason,
the post-processing such as spectral analysis of a much larger amount of flame images recorded by
high-speed ICCD camera should be implemented.
Last but not least, the feasibility study on PIV implementation when burning CH4 and H2 is
also in progress. Quantitative results can hopefully be obtained afterwards. In addition, we envisage
the further studies to characterize the flame structures by OH-PLIF, so that we will be able to
identify the quenching zones. Consequently, quenching phenomena will be discussed in detail.
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