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THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS
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Copyright 0 1997 by ASME
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EFFECT OF SWIRL ON COMBUSTION CHARACTERISTICS IN PREMIXED FLAMES
S. GI, A. K. Gupta and M.J. Lewis
University of Maryland
The Combustion Laboratory
Department of Mechanical Engineering
College Park, MD 20742
ABSTRACT
A double concentric premixed swirl burner is used to examine the
structure of two different methane-air prernixed flames. Direct flame
photography together with local temperature data provides an
opportunity to investigate the effects of swirl number distribution in
each annulus on the global and local flame structure, flame stability
and local distribution of thermal signatures. An R-type thermocouple
compensated for high-frequency response is used to measure the local
distribution of thermal signatures in two different flames, each of
which represents a different combination of swirl number in the swirl
burner. In order to improve the accuracy of the temperature data at
high-frequency conditions, information on the thermocouple time
constant are also obtained under prevailing conditions of local
temperature and velocity by compensating the heat loss from the
thermocouple sensor bead. These results assist in quantifying the
degree of thermal nonunifonnities in the flame signatures as affected
by the distribution of swirl and to develop strategies for achieving
uniform distribution of temperatures in flames.
INTRODUCTION
Swirl flows have been widely investigated for several decades
because of their extensive use in all kinds of practical systems,
including gas turbine combustion. Numerous experiments in swirl
flows have been carried out extending from very fundamental
isothermal flows as well as reacting flows to those formed in very
complex swirl combustor geometries. Experimental results have
established the general characteristics of swirl flows and revealed
important effects of swirl on promoting flame stability, increasing
combustion efficiency and controlling emission of pollutants from
combustion'. Leuckel and Fricke? conducted a variety of
measurements using a nonpremixed single swirl burner consisting of
an annular swirling air jet and a centrally located non-swirling fuel
jet. Chen and Driscoll 3 have provided an understanding of the
physical processes that occur within the nonpremixed flames by
examining the enhanced mixing characteristics in swirl flows
III I II I 11111,1111 II III III
resulting from the formation of a central toroidal recirculation zone. A
thermal-inertia compensated thermocouple technique which produces
a linear response of the thermocouple at higher frequencies was used
by Gupta et al. ° to investigate turbulent temperature characteristics of
swirling nonpremixed flames. By achieving a linear response of the
thermocouple to higher frequencies, important information on the
structure of turbulent flames can be obtained which could not be
ordinarily observed. Accurate measurements of the rms temperature
fluctuations are possible when the output of the thermocouple is
compensated to higher frequencies °. Although the frequency
compensation can be carried out to several Itliz there are practical
limitations, e.g., electronic noise, associated with the thermocouple
compensation technique.
Most previous studies have been conducted on single swirl burners
instead of the double concentric swirl burners which are playing an
ever-increasing role in practical combustor desigh. The double
concentric burner allows for variation of the radial distribution swirl
via control over the axial and angular momentum of the jets in the
two annuli of the burner. As such, at a fixed overall swirl number,
the detailed flow distribution can be significantly different under
various operational conditions of the burner. By introducing the
swirling flow through concentric annuli, it is possible to control the
radial distribution of flow and swirl for achieving significantly
different flame stability limits, levels of turbulence, volumetric heat
release rates and combustion characteristics in general.
Significantly different combustion and emission characteristics
have been obtained by altering the relative proportions of the swirling
flow in various annuli'. Marshall and Gupta 6 investigated thermal
characteristics of diffusion flames obtained through various
combinations of swirl and axial inlet momentum distributions in . a
double concentric swirl burner. Fluctuating temperature
measurements were obtained in the flames, including the
recirculation zone, and shear layer and post-combustion regions. By
maintaining a constant equivalence ratio, a companion of the
magnitude and distributions of temperatures for different flames were
obtained. These results provided the important role of radial
Presented at the International Gas Turbine & Aeroengine Congress & Exhibition
Orlando, Florida — June 2–June 5, 1997
This paper has been accepted for publication in the Transactions of the ASME
Discussion of it will be accepted at ASME Headquarters until September 50, 1997
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distribution of swirl and jet axial momentum on the flame thermal
characteristics.
The use of lean premixed combustion in gas turbine combustors has
been preferred over nonpremixed combustion by the gas turbine
manufacturers for achieving low NO emission. Although the use of
lean premixed combustion is attractive for pollution reduction, it may
provide a penalty in system complexity, acoustic instabilities,
increased size and weight. Though disadvantages may prohibit the
use of lean premixed combustion in certain applications, it still offers
advantages over the other types of combustors used in gas turbine
applications. The use of lean premixed combustion reduces the peak
flame temperatures obtained near the fuel-air interface in the
traditional diffusion flame type of combustors. The degree of
premixing determines the thermal field uniformity of the flame. In
all premixed combustor configurations the fuel-air mixture is
premixed upstream of the swirler, which is used for the stabilization
of the flame at all power conditions. The use of swirl assists in the
stabilization of the flame by transporting hot and chemically active
species from the downstream region of the flame to the root of the
flame, which therefore alters the transport of gases within the
combustor and creates a thermal nonunifomilty.
The extent of thermal non-uniformity in premixed flames is not
known and can be expected to depend upon the combustor
configuration, the degree and distribution of swirl in the combustor,
and other input and operational parameters of the combustor. The
nonuniformity in the thermal field can in turn have an influence on
the performance of the combustor in addition to the emission levels,
including NOR.
In order to examine the effect of fuel-air premixing on the resulting
thermal field, an experimental premixed swirl burner facility was
developed. The facility is very similar to the one described in ref. 6
except for the well-mixed fuel-air mixture in each annuli, including
the central pipe, and the presence of a flame quench trap upstream of
the swirler in each annuli. Results obtained on the mean and
fluctuating temperature measurements are aimed at providing the
effect of swirl on the thermal characteristics of premixed co-swirling
flames.
Bow En Reg
nwras Ea ein
Figure 1 A Schematic of the Premixed Swirl Burner
flow and the surrounding outer annulus 1 nozzle flows. The two
swirlers generate the tangential momentum to form a central toroidal
recirculation zone for flame stabilization. Each swirler consists of
eighteen straight vanes fixed in place by thin inner and outer rings.
The inside and outside swirlers can be fixed independently, and this
allows the variation of the radial distribution of swirl combination in
the burner. The facility therefore permits examination of the flames
formed with strong swirl in the central region and weak swirl on the
outside, or vice versa. It is also possible to form flames having coand/or counter-swirl in the annuli.
In the present study only the co-swirling flames are examined, with
varying radial distribution of swirl in the burner for a fixed
stoichiometry of 0.625 in each annuli. A 2-D traverse assembly is
used to move the thermocouple with respect to the fixed burner with
a spatial accuracy of 0.001 inch. The test section is situated above
the burner and is formed from an open-ended cube enclosure around
the burner exit. This arrangement provides isolation of the flame
from external ambient disturbances. The flow control system for the
burner consists of a series of rotameters, pressure gauges and control
valves to measure and regulate the air and methane flow rates prior to
their entry into the premixer and subsequently into the two annuli and
the central nozzle of the burner.
EXPERIMENT FACILITY
A schematic diagram of the experimental burner is given in Figure
1. The has a double concentric swirl burner in which the radial
distribution of swirl and momentum distribution of the flow in the
two annuli and the central pipe can be independently controlled. The
facility consists of a bumer, a test section, a flow control system and a
2-d traverse mechanism. The premixed burner is similar to the
diffusion flame burner facility discussed in ref. 6 except that it has
the necessary flame arrestor in each annuli, which is used to prevent
flashback, and can mix the reactants well prior to their arrival over
the swirler. At the top of the burner is the flame stabilization section,
which includes two glass tubes for the two nozzles in the burner
(having inner diameters of 65 mm and 35 mm), a centrally located
premixed fuel-air nozzle, and two swirlers for the annulus 1 and
annulus 2 jets. The swirl strength in the central nozzle and annulus 1
and 2 can be changed to any desired value. A backward facing step is
placed inside the central nozzle in order to make the methane-air
mixture completely turbulent. The divergence in this central nozzle
permits smooth interaction between the central
EXPERIMENTAL CONDMONS AND DIAGNOSTICS
Four premixed flames have been examined here. These flames
were produced with changes in swirl strength distribution between
the two annuli. Previous work has presented results designated
flames 1 and 2.7 For the flame 3 the swirl vane angle was 30 ° and
55* in annulus 1 and 2, respectively. However, for flame 4 the swirl
vane angle was 45 ° and 55° in annulus 1 and 2, respectively. The
experimental conditions for the examined flames are given in
Table 1.
2
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Ii
Table 1 Experimental Test Matrix
Central Pipe
Annulus I
Annulus 2
Axial Momentum Distribution
Flame 4
Flame 3
I
M„ = 300 Ibm ft/min 2
M., = 1500 Him ft/min2
KM = 2163 Ibm ftlrnin 2
Swirl Vane Angles
Flame 4
I
No Swirl
45'
30'
I
Flame 3
55'
AT, has decreased by a factor of 1/e.
High frequency temperature measurements have been taken with
an R type micro-thermocouple probe. The probe utilizes a Pt/Pt-13%
Rh thermocouple with a wire diameter of 50mm. This wire diameter
is small enough not to cause any significant intrusion in the flame
while maintaining structural rigidity of the thermocouple. No
measurable disturbance to the flame was observed when the
thermocouple was inserted in the flame. The signal of the
thermocouple is amplified and digitized at 10 kHz for a total of
300,000 samples at every probe location in the flame. The
thermocouple output is compensated to high frequencies so that high
sampling frequency provides resolution to the small time scale
turbulence that may occur in flames. The large number of samples
provides the flow thermal statistics. The resulting long sampling
time of 30 seconds allows for avenging over the low frequency
temperature fluctuations and ensures good statistical information of
the thermal field. The mean value at a location is the arithmetic
mean temperature of the samples.
Measurements are taken at different points in the flame region by
moving the thermocouple with respect to the burner. Great care was
taken to obtain a symmetrical flame with respect to the burner
vertical axis. Subsequent measurements carried out on the flames
validated their axisymmetric behavior. Half plane measurements
were then sufficient to characterize the complete flame because of the
observed axisymmetry. The half plane consists of 12 radial positions
at 9 axial elevations for a total of 108 probing locations. The
horizontal spacing of the grid is 0.25 inches while that for the vertical
is 0.75 inches. It is situated directly above the burner orifice with the
location of the home coordinates in the center of the burner exit
plane.
The response time of the thermocouple probe is insufficient to
accurately resolve temperature fluctuations faster than about 50 Hz
even with the very small thermocouple bead formed with 501an
diameter wires, though temperature fluctuations up to several kHz
can occur in a strongly swirling flames.' Therefore, the time
constants were measured to compensate the raw temperature data for
thermal inertia effects of the utilized thermocouple. In order to
determine the time constant of the thermocouple, the wire is heated
by a DC power supply and cooled down in the flame periodically. In
this manner, a series of heating and cooling pulses are obtained and
the value of the time constant, 1, is calculated from the resulting
decay times of the thermocouple signature. In each flame, the time
constant is measured at 30 locations (at every other measurement
point in both the axial and radial direction). Due to the fluctuation of
temperatures in the flame, the process is repeated up to 300 times at
each measurement location. The resulting temperature decay curves
are averaged to produce a smooth decay curve from which the time
constant, T. is determined as the value at which the overheat
RESULTS AND DISCUSSION
The test conditions are given in Table I. Global flame features
are described first, followed by the quantitative data on mean and
fluctuating temperatures, probability density distribution of
temperatures, power spectra, and integral- and micro-scale of
temperature time scales.
Flame Photographs
Figure 2 shows a direct comparison of flame 3 and flame 4. It is
interesting IA note from the direct flame photographs that the flame
length is longer for flame 4 ( with 45' swirl in annulus 1) than flame
3 (with 30' swirl) while the other flow conditions are kept constant.
It is generally believed that the swirl number has an important
effect in characterizing the combustion process and that increased
swirl number produces enhanced tangential momentum, which
provides a better mixing between the incoming hot,chernical reactants
and the fresh fuel-air mixture. This rapid mixing decreases the flame
length. However, high swirl is also expected to produce excess
turbulence which results in large tangential velocity gradients and a
negative flame stretch. The presence of negative stretch' through the
tangential velocity gradient at the flame (also quantified by the
ICarlovitz number, K=1/A(dA/dt, where A is the flame surface area
and t is the time)) decreases the flame surface area and therefore the
volumetric burning rate. Because the flame length depends on
reaction rate of combustion, it is therefore reasonable that the flame
becomes longer from flame 3 to 4.
Mean and Fluctuating Temperature Data
Two kinds of temperature maps are provided to obtain insight on
the local and global thermal structure of the strongly swirling
premixed turbulent flames. The raw temperature data is
compensated for thermal inertia effects of the thermocouple and for
radiative losses. Characteristic temperature probability density
function data is also provided both with and without thermal inertia
compensation. This will be used to examine the local turbulent
thermal structure of the flames.
The mean and fluctuating temperature data are presented as
temperature contour maps (See Figure 3, 4 and 5). The lines
displayed represent the isothermal contours in the flame. A grid
representing the probe locations is superimposed with the isothermal
maps to facilitate the discussion and to refer to certain areas and
points within the flowfield.
3
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Figure 2 Direct Photograph of Flame 3 (left) and Flame 4 (right)
Mean Temperature Contour (Case 3
Mean Temperature Contour Case 4)
2.0
15
S.
8
z 1.0 -
0
0.5
00
00
—1
00
0.5
1.0
00
radial location (r/D)
radial location (r/O)
0.5
1.0
Figure 3 Compensated Flame Temperature Contour for Flames 3 (left) and 4 (right)
the uncompensated and compensated data reveals that the calculated
temperature difference can be as large as 300K at the same location
in the flame. Therefore, the importance of the compensation
technique for mean and fluctuating temperature measurement can be
recognized when it is desired to determine NO emission levels from
the temperature data. The importance of accurate temperature data
can also be recognized for estimating other pollutant emissions
Mean Comoensated_Temoerature Data
The results presented in Figure 3 and Figure 4 for flames 3 and 4
show the direct effect of thermal inertia compensation of the
thermocouple on flame temperatures. The results of compensated
mean temperature contours account for not only the thermal inertia
effects of the thermocouple but also the radiative losses from the
thermocouple. A comparison of the mean temperature data between
4
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Temperature Contour (Row Dote)
Temperature Contour (Raw Data
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Figure 4 Uncompensated Flame Temperature Contour for Flames 3 (left) and 4 (right)
necessarily improve the overall combustion process. An important
task here has been to determine the effect of radial distribution of
swirl in a co-annular swirl burner on the global flame development
and the subsequent combustion behavior. The mean temperature
data showed a bimodal probability density distribution at certain
locations of the flame. In the shear layer region, the temperature
distribution is bimodal, having two distinct well-defined
temperature peaks corresponding to the unburned and recirculated
(partially burned) gas mixture, respectively
and heat transfer characteristics of the flame.
From the compensated mean temperature data shown in Figure 3,
one can see that for flame 3 there is a very large reaction region
where the mean temperature is 1700K. However for flame 4, the
mean temperature reaches 1700K near to the bunter exit. Due to the
presence of very strong turbulence at the burner exit in flame 4, the
transportation rate of species increases dramatically, which is
expected to increase the flame speed. In order to obtain a very stable
premixed flame, the flame speed must match the incoming reactant
flow speed : In flame 4, the flame speed is expected to be very high
and combustion is steady near the burner exit region. This suggests
that the mixture must have burned earlier and that the high
temperature region occurs at an early stage in flame 4.
The overall temperature in flame 4 is lower than that for flame
3. This is attributed to the increased recirculation of external air in
flame 4 than flame 3. In addition, the distribution of temperature
at downstream positions suggests that in flame 4 the combustion
process extends further downstream than in flame 3. In flame 4,
the higher swirl number produces large turbulence at the exit of the
burner and improves the small scale mixing between the reactants
(methane-air mixture) and the hot chemical species. Thus the fuelair mixture begins to react very fast and produces a high
temperature reaction region. The strong turbulence also assists in
mixing in the larger internal recirculation zone. The entrained
mixture reduces the local equivalence ratio and increases the heat
loss from the flame, which subsequently decreases the flame
temperature and extends the spatial evolution of combustion
process. The strong turbulence in the flame can cause local
extinction, which results from the discrepancy between the mass
diffusivity and the thermal diffusivity. In this case, the mass
diffusivity is larger than the thermal diffusivity so that the
transport of heat can not keep pace with the transport of mass.
From this, one can infer that increasing the swirl number will not
Fluctuating Temoeratu re Date.
The distributions of fluctuating temperatures for the Flames 3
and 4 are shown in Figure 5. This data supports the observed
expansion of the flame length (compare flame lengths and mean
temperature data) when the momentum in annulus 2 is maintained
constant while the swirl strength in annulus I is changed. All
flames stabilized with the 45° swirler in annulus I are longer and
somewhat narrower than those with a weaker swirl of 30° in
annulus I, when other parameters are constant. The displayed
contour line for the 150° temperature fluctuations shows the
fluctuating temperature region for flame 3 is much wider than in
flame 4. The central core region of the flame was also found to
have higher fluctuations for flame 3 than flame 4. A high level of
temperature fluctuations is indicative of the strong heat and mass
transport phenomenon. In this region many high temperature
chemically active species are being mixed with the fresh unburned
fuel-air mixture prior to combustion. Combining the fluctuating
temperature data with the mean temperature data, one can sec that
the reaction rate in the recirculation region is very fast. The
fluctuating temperatures in flame 4 are much lower than those
obtained in flame 3. This is attributed to the stronger turbulence
which enhances the mixing, resulting in a far more uniform
5
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Fluctuating Temperature Contour (Case 4)
Fluctuating, Temperature Contour (Case 3)
2.0
1.0
0.5
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radial location (r/D)
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Figure 5 Fluctuating Temperature Contour for Flames 3 (left) and 4 (right)
11ov/field.
In both flames, the values of the fluctuating temperatures are
much higher than those expected in premixed flames. We attribute
this to the swirlers used in the burner for flame stabilization. The
large fluctuations in temperature show that the subsequent thermal
signatures are far from being uniform. This shows the swirlers'
effect on the flow field in premixed flames.
products is a good indicator of strong turbulence and large amounts of
entrained air in this region.
The impact of the compensation can be seen very clearly in the
temperature power spectra shown in Figure 6. This compensation
effect is responsible for the significantly higher power spectral
density at frequencies larger than about 10 Hz. Reasonable
information about the local turbulent flame structure can therefore be
obtained only after a proper thermal inertia compensation is made to
the thermocouple data. The characteristic thermal time scales
associated with the premixed flames are important, as they provide
information on the large and small scale mixing in the flames.
Information on the integral-and micro-scale of thermal signatures was
obtained from the auto-correlation of the fluctuating temperature
data. Sample retults at two distinctly different locations for flame 3
are shown in Figure 7. The two curves represent the shear layer
region and the post flame region. At the shear layer region the
micro- and integral- time scales were found to be 0.24 and 0.79 ms,
respectively. These time scales changed with position in the flame
as well as the flame swirl used for the two flames, as shown in Table
2 and 3. The data clearly show the extent of thermal signature
variation in the premixed flames.
Temperature Power Spectra and Probability Density
Distribution
Temperature power spectra as well as probability density
distribution of temperatures at various spatial locations in each flame
were obtained to determined both with and without the thermal
inertia compensation of the thermocouple. Sample results are shown
in Figure 6 for flame 3 and corresponds to a spatial location of 0.49
burner diameters away from the flame center and 0.29 diameters
downstream the burner exit plane. The results show a significant
effect of thermal inertia compensation on the thermocouple signature
even at very low frequencies. At higher frequencies the difference
between the compensated and uncompensated signals becomes much
larger. The results obtained at other locations and in other flames
revealed similar findings.
The temperature probability density function shown in Figure 6
shows the occurrence of temperatures between 400 K and 1850 K at
this location in the flame. The bimodal temperature distribution has
two peaks, one at a temperature of about 780 K and the other at about
1550 K. The peak near 780 K indicates the presence of unburned
fuel-air mixture and hot gases in the flame, The peak at the higher
temperature is caused by hot recirculated combustion products. The
presence of both the unburned fuel-air mixture and hot combustion
SUMMARY
In this study we have obtained quantitative data on mean and
fluctuating temperatures in two turbulent premixed swirling flames
using fine wire thermometry. A digital compensation technique was
used to decrease the effective response time of the probes.
The direct flame photographs proved to be a good overall
diagnostic tool to document the flame shapes, global behavior, and
dimensions. Most preliminary results were first observed by
6
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Temperature Probability Density Function
Temperature Spectrum
,
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4- Mean Temp.
Compensated
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. Uncompensated \in
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1500
.
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IOW
11..quaney (Hz)
Figure 6 Temperature Spectrum and Probability Density Function at Probe
Location of r/D=0.49, x/D=0.29 in Flame 3
•
Temperature Correlation
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Temperature Correlation
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Figure 7 Micro- and Integral -Time Scales at Two Locations (left diagram: r/D = 0, z/D = 2.34;
right diagram: r/D= 0.49, z/D = 0.29) in Flame 3
Table 2 The Temperature Distribution and the Corresponding Time Scales at Four Probe Locations
in Flame 3
Location
Parameter
Mean Compensated Temperature
Fluctuating Temperature
Thermal Microscale Time
Thermal Integral Time
r=0
zs0.29 D
1778.7 K
62.7 K
0.75 ms
5.79 ms
r=0
z=0
1113.3K
481.8 K
0.41 ms
2.16 ms
r=0.49 D
z=0.29 D
1193.2K
344.1 K
0.24 ms
0.79 ms
7
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r=0
z=2.34 D
749.6 K
98.4 K
1.37 ms
12.5 ms
Table 3 The Temperature Distribution and the Corresponding Time Scales at Four Probe Locations
in Flame 4
Location
Parameter
Mean Compensated Temperature
Fluctuating Temperature
Thermal Microscale Time
Thermal Integral Time
r=0
z=0
1736.9 K
70.3 K
0.45 ms
4.32 ms
r=0
z=0.29 D
1746.4 K
84.3 K
2.34 ms
14.4 ms
comparing the flame photographs obtained under different
operational conditions.
The mean and fluctuating temperature measurements provided
insight into the local thermal turbulent structure of the premixed
flames, they are found to be higher after accounting for the radiative
heat loss. Thermal inertia compensation provided significant
improvements in determining the flame thermal signatures. The
fluctuating values of the measured temperatures are significantly
increased after corrections are made for the attenuation in the
thermocouple signal at the higher frequencies. Examined premixed
flames did not provide uniform thermal field characteristics despite
the well mixed fuel-air mixture preparation upstream of the
combustor prior to its introduction into the combustor. The use of
swirl in turbulent flames therefore significantly modifies the thermal
characteristic of the flames. The role of swirl in premixed flames is
important from the point of view of flame stability and combustion
and emission characteristics.
The flow thermal characteristics of the flames changed significantly
when the annulus 1 swirl was changed. Different flow structures can
be created by varying this parameter. Further work in this area will
assist in providing guidelines for better design and use of swirlers in
combustors.
r=0.49 D
z=0.29 D
1647.4 K
178.3 K
0.25 ms
2.80 ms
r=0
z=2.34 D
785.6 K
100.8 K
1.08 ms
7.59 ms
Symposium (International) on Combustion, The Combustion Institute,
Pittsburgh, PA, 1975, pp. 1367-1377.
5. Gupta, A.K., Beer, J.M., and Swithenbank, J.: Concentric MultiAnnular Swirl Burners: Stability Limits and Emission Characteristics,
Sixteenth Symposium (International) on Combustion, The
Combustion Institute, Pittsburgh, PA, 1976, pp. 79-91.
6. Marshall, A.W., and Gupta, AK.: Effects of Jet Momentum
Distribution on Thermal Characteristics of Co-Swirling Flames, 34th
AIAA Aerospace Sciences Meeting, 1996, Paper No. 96-0404.
7. Qi, S., Gupta, A.K., and Lewis, M.J.: Effect of Swirl on
Temperature Distribution in Premixed Flames, 35th AIAA Aerospace
Sciences Meeting & Exhibit, 1997, Paper No. 97-0373
8. Law, C.K.: Dynamics of Streched Flames; Twenty-Second
Symposium (International) on Combustion, The Combustion Institute,
Pittsburgh, PA,1988, pp. 1381-1402.
ACKNOWLEDGMENTS
This research was supported by the South Carolina Energy
Research and Development Center, Clemson University, program
manager Dr. Dan Fant. Assistance provided by Dr. Andre Marshall
during various stages of experimentation and diagnostics is greatly
appreciated. The help of Armin Wellhoffer, Harald ICafitz and Dr.
Jerold Chen on data acquisition and data analysis is much
appreciated.
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4. Gupta, A.K., Syred, N. And Beer, J.M.: Fluctuating Temperature
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