30-10-2010, 09:48 AM
LABORATORY INVESTIGATION OF LOW SWIRL INJECTOR FOR LEAN PRE-MIXED GAS TURBINES
SEMINAR REPORT Submitted by
KIRAN .G.
S7M2
ROLL NO-30
Department Of Mechanical Engineering
College of Engineering, Thiruvananthapuram – 16
OCTOBER 2010
LABORATORY INVESTIGATION OF LOW SWIRL INJECTOR FOR LEAN PRE.docx (Size: 1.02 MB / Downloads: 85)
ABSTRACT
Laboratory experiments have been conducted to investigate the fuel effects on the
turbulent premixed flames produced by a gas turbine low-swirl injector (LSI). The
lean-blow off limits and flame emissions for seven diluted and undiluted hydrocarbon
and hydrogen fuels show that the LSI is capable of supporting stable flames that
emit <5 ppm NOx (@ 15% O2). Analysis of the velocity statistics shows that the
nonreacting and reacting flowfields of the LSI exhibit similarity features. The turbulent
flame speeds, ST, for the hydrocarbon fuels are consistent with those of methane/air
flames and correlate linearly with turbulence intensity. The similarity feature and linear
ST correlation provide further support of an analytical model that explains why the LSI
flame position does not change with flow velocity. The results also show that the LSI
does not need to undergo significant alteration to operate with the hydrocarbon fuels but
needs further studies for adaptation to burn diluted H2 fuels.
INDEX OF CONTENTS
Title Page no
Title Page 1
Acknowledgements 3
Abstract 4
Index of Contents 5
Nomenclature 6
List of tables, figures 7
Introduction 8
Background 9
Experimental system and diagnostics 11
Results 14
4.1 Flame stability and lean blow-off 14
4.2 Flowfield analysis 17
4.2 (I) centerline profiles for non-reacting flows 17
4.2 (II) ).Radial profiles of the non-reacting flows 19
4.2 (III) Centerline profiles for reacting flows 20
4.2 (IV) Radial profiles of reacting flames 22
4.3 Turbulent flame speed 24
5.Conclusions 26
6.Scope 27
7.References 28
NOMENCLATURE
Symbols
ar – normalized radial divergence rate
ax - normalized axial divergence rate
Ls- Length of swirler section
Ø – Equivalence ratio
q’ – 2D Turbulent kinetic energy
R - ratio of the radii of the centerchannel and injecto
r
Rc - Radius of centerchannel
S –Swirl number
SL – Laminar flame speed
ST – turbulent flame speed
T0,P0,U0-Stagnation temperature , pressure ,velocity respectively
Tad – Stoichiometric adiabatic flame temperature
u’ – Linear flame velocity
v’ – Radial flame velocity
x0 - virtual origin of the divergent flow
α - Vane angle
Abbreviations
DLN : Dry Low NOx
LBO : lean blow-out
LSB : Low-swirl Burner
LSI : Low-swirl injector
PIV : Particle Image Velocimetry
LIST OF TABLES AND FIGURES
FIGURES PAGE NO
Fig 1:Schematics and photographs of the low-swirl injector 10
Fig 2.Experimental setup 11
Fig. 3. (a) LSI lean blow-off limits for natural gas at STP and elevated T0 and P0 15
Fig. 3.(b) LSI lean blow-off limits for fuels of Table 1 at STP. 15
Fig. 4(a). NOx emissions from LSI for the hydrocarbon fuels of Table 1. 16
Fig. 4(b)CO emissions from LSI for the hydrocarbon fuels of Table 1. 16
Fig. 5. Comparison of NOx data 17
Fig. 6. Centerline profiles of the non-reacting flows. 18
Fig. 7. Radial profiles of the non-reacting flows at x = 15 mm. 19
Fig. 8. Centerline profiles of eight flames with 9.2 <U0 < 9.5 m/s 21
Fig . 9. Radial profiles of eight flames with 9.2 < U0 < 9.5 m/s at x = 15 mm. 23
Fig. 10. Correlation of flame speeds measured from LSI and LSB 25
TABLES
Table 1 : List of properties for the fuels used in this study 13
Table 2 : List of experimental parameters for the fuels studied 25
1.INTRODUCTION
Power generation turbines operating on natural gas are subjected to stringent emission
rules and many urban areas have NOx requirements of <5 ppm ( corrected to 15% O2).
Recent research has led to development of effective control technologies based on lean
premixed combustion , such as catalytic combustors , trapped vortex combustors , and metal
fiber combustors. Low-swirl injector (LSI) provides another option that avoids altering
engine layout or operating cycle. As more mid-size turbines are deployed in locations
with readily available alternate fuels such as landfills , paper mills , and oil platforms ,
meeting emissions goals while using different fuels presents great challenges. This is due
to differences in combustion properties and their interactions with turbulence that affect
flame stability, emissions, and turndown performance . Our goal is to investigate the fuel
effects on turbulent premixed flames in the LSI to develop an engineering method to
adapt it to operate on alternate fuels. The approach is to investigate lean blow-out (LBO),
emissions and the flowfield characteristics to gain the fundamental insights for
optimizing the LSI for fuel-flexibility.
2.BACKGROUND
Lean premixed combustion is a proven dry low-NOx (DLN) method for natural gas-
powered turbines. Most DLN engines emit NOx < 25 ppm and CO < 50 ppm (both @ 15%
O2). But attaining ultra-low emissions of <5 ppm NOx requires that turbines operate at
conditions close to the lean blowoff limit (LBO) where combustors are susceptible to
combustion oscillations. Results at turbine conditions (500 < T0 < 730 K, 6 < P0 < 15 atm,
12 < U0 < 48 m/s) show that the LSI produces stable flames with NOx and CO below 2
ppm (@15% O2) at the leanest conditions. Further work has led to an LSI that has been
evaluated in a single cylindrical combustor and in a multi-injector annular combustor at
simulated engine conditions. The study showed that the LSI has good performance
characteristics, and is stable over a wide range of conditions where NOx < 5 ppm and CO
well below the acceptable limit of 400 ppm. The flame does not have a propensity to
become unstable towards blowoff or show undesirable injector-to injector interaction.
The heart of the LSI is a swirler evolved from atmospheric low-swirl burners. The
swirler section is 2.8 cm long (Ls), and has an outer radius of 3.17 cm and sixteen
curved vanes (vane angle α = 42° at the exit) attached to the outer surface of a Rc = 2 cm
centerchannel. The open centerchannel allows a portion of reactants to remain unswirled
and this nonswirling flow inhibits flow recirculation and promotes formation a divergent
flowfield, a key feature of the flame stabilization mechanism . To control the mass ratio,
m=mc/ms, between the flows through the centerchannel, mc, and the swirled annulus, ms, a
perforated screen is fitted at the entrance of the centerchannel.
The swirl number definition is:
S=2/3 tan〖α (1-R^3)/(1-R^2+m^2 〖(1⁄(R^2-1))〗^2 R^2 )〗
Here the ratio of the radii of the centerchannel and injector, R, is 0.63 and the screen
blockage controls m and hence S. Fitted with a 58% blockage screen, this LSI has
S = 0.Otherwise, all dimensions e.g. exit tube length of li = 9.5 cm and a 45° tapered edge,
remain unchanged.
Fig 1:Schematics and photographs of the low-swirl injector
3.EXPERIMENTAL SYSTEM AND DIAGNOSTICS
Fig 2.Experimental setup
For the lean blowout (LBO) and Particle Image Velocimetry (PIV) investigations, the LSI
was mounted on a cylindrical settling chamber. Air (up to 1800 LPM) enters at the side of
a 25.4 cm diameter chamber and flows into the LSI via a centrally placed 30 cm long
straight tube. Air flow is adjusted by a valve and monitored by a turbine meter, and fuel
(Table 1) is injected in the air supply to ensure a homogeneous mixture for the injector.
Both the fuel and the PIV seeder flows are controlled by mass flow controllers and set
according to a predetermined value of Ø with a PC .The fuels listed in Table 1 consist
of hydrocarbons,N2 and CO2-diluted CH4 to simulate landfill and biomass fuels, H2-
enriched CH4 to simulate refinery gas and CO2-diluted H2. Variations in the combustion
properties are shown in Table 1 by the stoichiometric adiabatic flame temperatures, Tad,
and laminar flame speeds, SL. The Wobbe Index is used commercially as an indicator of
fuel . Emission measurements were performed with a Horiba PG-250 analyzer, calibrated
using 7.9 ppm NO in N2 and 31.8 ppm CO in N2 . The instrument has an accuracy of
±0.5 ppm for NOx. To measure emissions, flames were enclosed in a 16 cm diameter, 20
cm high quartz tube and sampled with a probe placed a few centimeters above the center
of the tube. The collected exhaust gas was cooled and water was removed with a
dessicant before it flowed into the analyzer . To facilitate PIV data collection, the non-
reacting flows and the flames were not enclosed. PIV system and data analysis has a
New Wave Solo PIV laser with double 120 mJ pulses at 532 nm and a Kodak/Red Lake
ES 4.0 digital camera with 2048 by 2048 pixel resolution. The optics were configured
to capture a field of view of 13 cm by 13 cm. A cyclone particle seeder seeds the air
flow with 0.3 µm Al2O3 particles. Data analysis was performed on the 224 image pairs
recorded for each experiment using software developed by Wernet. Using 64 • 64 pixels
cross-correlation interrogation regions with 50% overlap, this rendered a spatial resolution
of approximately 2 mm.
Table 1: List of properties for the fuels used in this study
Fuel composition
Tad at Ø = 1 K SL at Ø = 1 m/s Wobbe index
kcal/Nm
CH4 2230 0.39 11542
C2H4 2373 0.74 14344
C3H8 2253 9.45 17814
H2 2318 2.50 9712
0.5 CH4/0.5 CO2 2013 0.20 4182
0.6 CH4/0.4 N2 2133 0.31 6026
0.6 CH4/0.4 H2 2258 0.57 10130
0.5 H2/0.5 CO2 1693 0.56 1432
4.RESULTS
Flame stability and lean blowoff
Flame stability and LBO were determined at volumetric flow rates 300 < Q < 1880 LPM,
corresponding to bulk flow velocities of 3 < U0 < 9 m/s. Figure 2a shows LBO data for
methane. The open flame data at STP are shown as the baseline. The data at higher inlet
temperatures and pressures (1–14 atm , 620–770 K) were obtained from enclosed
configurations simulating a gas turbine combustor and they show the lowest LBO occur
at heated atmospheric tests in a quartz rig . These data also show that the LSI can
operate up to U0 = 85 m/s, and that LBO remains relatively insensitive to U0. This is a
desirable feature for turbines for it indicates that the LBO will not edge closer to the
operating point of the combustor when the load increases. In Fig. 2b LBO values are
essentially the same for CH4, C3H8, 0.5 CH4/0.5 CO2, and 0.6 CH4/0.4 N2. The dilution of
CH4 by inerts has no observable effect on LBO. LBO is slightly lower for C2H4 and
0.6 CH4/0.4 H2, which have higher flame speeds than the other fuels. The LBO values
for H2 are very low and do not show a significant effect due to dilution. However, the
stability ranges for H2 fuels are limited because the flames tend to reattach to the burner
rim at Ø> 0.30. NOx and CO emissions from flames at Q = 1500 LPM (U0 = 7 m/s) are
shown in Fig. 3. Only data for the hydrocarbon fuels are plotted as emissions from H2
fuels were below detectable limits. For the hydrocarbon fuels, NOx has an exponential
dependence on Ø, and at a given Ø, emissions show a dependence on Wobbe Index,
consistent with the higher heat content of these fuels. However, the significant implication
of these data is that regardless of fuel content the LSI supports stable flames emitting <5
ppm NOx and the conditions are well above the LBO point. Flame temperature is an
important parameter in NOx formation in the LSI. The plot of NOx vs. Tad in Fig. 4
shows that NOx correlates well with Tad and is consistent with data at high T0, P0 and
U0. LSI flow has little or no recirculation, which explain why the NOx production
depends primarily on flame temperature.
Fig. 3. (a) LSI lean blow-off limits for natural gas at STP and elevated T0 and P0
Fig. 3.(b) LSI lean blow-off limits for fuels of Table 1 at STP.
Fig. 4(a). NOx emissions from LSI for the hydrocarbon fuels of Table 1.
Fig. 4(b)CO emissions from LSI for the hydrocarbon fuels of Table 1.
Fig. 5. Comparison of NOx data
4.2. Flowfield analysis
Table 2 shows the PIV experimental conditions consisting of three non-reacting flows
and sixteen flames. For hydrocarbon flames, their stoichiometries were set at the
conditions where NOx ≈ 5 ppm to compare them at the conditions that meet the
emission goals. For the diluted hydrogen fuels, flames at Ø = 0.25 and 0.30 were
studied.
4.2.(i).The centerline profiles for non-reacting flows
The centerline profiles for three non-reacting flows are compared in Fig. 5. Two
parameters were introduced to characterize the nearfield region. The first is the virtual
origin, x0, of the divergent flow, obtained by extrapolating the linear velocity decay
region downstream of the exit (Fig. 5a), and second is the slope of the linear extrapolation
that quantifies the normalized axial divergence rate, ax = dU/dx/U0. Values of x0 and ax
for the three flows are given in Table 2 and they are very close. Profiles of the
normalized 2D turbulent kinetic energy, q’ = ((u’² + v’²)½)/2 of Fig. 5b shows that within
the linear velocity decay region, turbulence along the centerline remains constant. These
characteristics can be attributed to the effect of annulus swirling flow. In the farfield, slight
increases in q’/U0 at x > 60 mm are consistent with the formation of a very weak
recirculating zone.
Fig. 6. Centerline profiles of the non-reacting flows.
4.2.(ii).Radial profiles of the non-reacting flows
Radial profiles of the non-reacting flows at x = 15 mm all exhibit similarity behavior. In
Fig. 6a, the U/U0 profiles have a flat central region corresponding to the centerchannel
non-swirling flow flanked by two velocity peaks, corresponding to the swirling flow. In
Fig. 6b, linear distribution of the V/U0 profiles within the center region (_15 < r < 15 mm)
show that the normalized radial divergence rates ar = dV/dx/U0 are about half that of ax.
The q’/U0 profiles (Fig. 6c) have relatively flat distributions in the center regions
surrounded by intense turbulence peaks. These velocity statistics show that the LSI
produces a uniform central region with low shear stresses for flame stabilization.
Fig. 7. Radial profiles of the non-reacting flows at x = 15 mm.
4.2.(iii).Centerline profiles for reacting flows
Centerline profiles for reacting flows of eight flames with 9.3 < U0 < 9.6 m/s are shown
for clarity. Despite the large difference in the farfield, all U/U0 of Fig. 7a have linear
velocity decays near the LSI exit. The positions where profiles deviate from linear decay
trends correspond to the leading edges of the turbulent flame zones. From these centerline
profiles, ax and x’ for the nearfield linear decay regions can be deduced. Results listed in
Table 2 show that the flames increase both ax and x0 to demonstrate an influence of the
flame on mean characteristics of the upstream reactant flow. For hydrocarbon flames, the
majority of the ax values are around -0.014mm¯¹ compared to ax = -0.085 mm¯¹ for
the non-reacting flows. For the diluted H2 flames, the increases in ax are smaller, averaging
-0.011 mmˉ¹ and their U/U0 profiles have different shapes than the hydrocarbon flames.
This seems to be associated with the lower heat release compared to the hydrocarbon
flames. Though the hydrocarbon flame profiles are consistent in the nearfield, their farfield
features show dependence on heat release. Significant flow accelerations are found only in
the C2H4 and 0.5 CH4/0.5 H2 flames, while other hydrocarbon flame profiles have
relatively flat distribution .The corresponding q’/U0 profiles of Fig. 7b show that the
fluctuation levels at the LSI exit are slightly higher than in the non-reacting flows.But
the anisotropic ratio u’:v’ remains unchanged. The q0/U0 levels remain relatively flat
through the flame brushes and the increases in the farfield at x > 80 mm corresponds to
flames that produce weak recirculation.
Fig. 8. Centerline profiles of eight flames with 9.2 <U0 < 9.5 m/s
4.2.(iv).Radial profiles of reacting flames
Figure 8 shows radial profiles at x = 15 mm for flames of Fig. 7. These positions are below
the flame brushes so that the results can be compared with those of Fig. 6. Although the
U/U0 profiles in Figs. 8a and 6a have similar features, there are quantitative differences.
Within the central flat regions, U/U0 levels decrease to 0.5 for the two diluted H2 flames,
and 0.3 for hydrocarbon flames. These changes correspond to increases in ax and x’. The
center regions are also slightly wider than in the non-reacting flows. Another difference is
peak velocity in the surrounding swirl annulus increasing from U/U0 = 1.2 in non-reacting
flows to 1.5 in the flames. The V/U0 profiles of Fig. 8b all collapse onto a consistent
distribution, giving further evidence for flow similarity in the divergent flow regions
upstream of the flames. The slopes of the center region are also larger, but the 2:1 ratio
between ax and ar is preserved. Another observable effect of the flame is that the
minimum and maximum V/U0 values corresponding to the U/U0 peaks also increase to
show higher radial outflow. In Fig. 8c,the q’/U0 levels in the center region are more
scattered due to the influence of flames but the overall shape remain the same as in
Fig. 6c. Our flowfield analysis indicates that the overall effect of the flame is that of an
aerodynamic blockage against the flow out of the LSI. The net effects are a systematic
shift of the divergence flow into the LSI, increases in the divergence rates, and increases
in U and V in the swirl regions. These effects are weaker for flames with low heat
releases. Despite these systematic changes, the similarity features of the center region are
preserved.
Fig . 9. Radial profiles of eight flames with 9.2 < U0 < 9.5 m/s at x = 15 mm.
4.3. Turbulent flame speed
The turbulent flame speed, ST is the basic turbulent flame property that explains the LSI
stabilization mechanism because the freely propagating flame settles at the point within
the center divergent flow region where the mean flow velocity is equal and opposite to
ST. The fact that the LSI supports stable flames from 3 < U0 < 85 m/s indicates that the
ST deduced from the LSI has practical engineering significance, and provides necessary
insight for further development. From previous studies using LSBs with air-jets, it has
been shown that ST/SL correlates linearly with u’/SL. More recent data from the CH4/air
LSI flames at 7 < U0 < 22 m/s and from two 5.08 cm ID LSBs of R = 0.8 and 0.6 give
further support to this correlation. The ST deduced from the current data are listed
in Table 2. ST is defined by the velocity at the point where the centerline U0 profile
deviates from its initial linear decay. The effects of fuel composition on ST are shown
by their values listed in Table 2. Despite the low heat release rates, the ST of the diluted
H2 flames are higher than the ST of the hydrocarbon flames. In Table 2, only the u’/ST
and ST/SL for the hydrocarbon flames are listed because reliable SL data for very lean
diluted H2 mixtures are not available. From Fig. 9 it can be seen that the ST of the
hydrocarbon flames are consistent and they are well within the experimental scatter. The
inclusion of the twelve hydrocarbon flames did not affect the correlation of
ST/SL = 1 + 2.16 u’/SL. Although the ST for diluted H2 cannot be compared directly with
hydrocarbon flame data, the fact that their ST are higher strongly suggests that their
turbulent flame speeds will not be consistent with those in Fig. 9.
Fig. 10. Correlation of flame speeds measured from LSI and LSB.
Table 2 List of experimental parameters for the fuels studied
Fuel Ø U0(m/s) ax(mmˉ¹) x0(mm) ST(m/s) u’/SL ST/SL
none 0 6.76
7.47
9.21 -0.0086
-0.0085
-0.0082 -21.41
-23.45
-24.62
CH4 0.73 6.23
9.27 -0.0141
-0.0134 -38.93
-38.81 1.40
1.97 2.43
2.99 6.03
8.49
C2H4 0.62 6.32
9.40 -0.0140
-0.0130 -33.57
-45.88 1.62
2.17 2.30
3.00 6.23
8.35
C3H8 0.69 6.23
9.30 -0.0131
-0.0134 -40.92
-42.84 0.92
1.20 1.80
2.24 3.67
4.80
0.5 CH4/0.5 CO2 0.83 6.27
9.50 -0.0131
-0.0154 -42.10
-38.70 1.00
1.46 3.18
4.51 7.11
10.43
0.6 CH4/0.4 N2 0.76 6.24
9.40 -0.0142
-0.0142 -38.94
-42.75 1.16
1.56 2.45
3.69 6.44
8.67
0.6 CH4/0.4 H2 0.62 6.58
9.13 -0.0108
-0.0120 -55.95
-45.08 2.43
2.24 2.14
2.91 6.50
10.18
0.5 H2/0.5 CO2 0.25
0.3 6.48
9.55
6.56
9.38 -0.0121
-0.0102
-0.0110
-0.0094 -32.89
-34.08
-27.27
-33.70 1.42
2.91
2.54
4.00
5. Conclusions
Laboratory experiments have been performed to investigate the fuel effects on a low-
swirl injector developed for natural gas turbines. The experimental fuels comprise a
typical range (characterized by the Wobbe indices of 1430–17800 kcal/Nm3) for on-site
power generation. The LBO experiments show that the LSI with S = 0.57 supports stable
flames for all seven fuels. The stability range for 0.5 H2/0.5 CO2 flames is limited to
Ø < 0.3 where NOx emissions are below detectible limits. NOx emissions from the
hydrocarbon flames show an exponential dependence on Ø and correlate with Tad and
are consistent with previous measurements at 500 < T0 < 700 K and 6 < P0 < 15 atm.
Despite the variations in fuel properties, the LSI is capable of supporting stable
hydrocarbon flames that emit NOx < 5 ppm and CO well below acceptable limits.
Analyses of the non-reacting and reacting flowfields indicate that the overall effect of the
flame is that of an aerodynamic blockage against the flow supplied through the LSI. The
net result is a systematic shift of the divergence flow into the LSI, increases in the
divergence rates and increases in the mean axial and radial velocities in the swirl
annulus region. These effects are weaker for the flames with lower heat releases.
However, the virtual origin of the flow divergence, x0, and its nondimensional stretch rates
ax show that the similarity features of the nearfield region are preserved. The turbulent
flame speeds, ST, of the hydrocarbon fuels are consistent with those of methane/air
flames. The similarity features and linear ST correlation provide further support of an
analytical model that explains why the lifted LSI flame does not shift with U0. This
study shows that the LSI does not need to undergo significant alterations to operate with
the hydrocarbon fuels, but need further studies for adaptation to burn diluted H2 fuels.
6.Scope
1.Lowering swirl number from 0.54 to 0.43 generates more lifted flames and postpones
flame attachment to φ= 0.4 when flames are not enclosed
a LSI with S = 0.51 offers best performance for laboratory studies
b LSI for H2 is not significantly different than LSI for hydrocarbons
2.Corner recirculation zone formed at the combustor entrance promotes H2 flame
attachment
a Eliminating the sharp corner with a diffuser cone is a solution to mitigate H2
flame attachment
3. Fuel combustion chamber, and nozzle for mixing liquid fuel and air in the fuel
combustion chamber uses lean direct injection combustion for advanced gas turbine
engines, including aircraft engines. Advanced gas turbines benefit from lean direct wall
injection combustion. The lean direct wall injection technique of the present invention
provides fast, uniform, well-stirred mixing of fuel and air.
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