1. INTERNATIONALMechanical Engineering and Technology (IJMET), ISSN 0976 –
International Journal of JOURNAL OF MECHANICAL ENGINEERING
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online) IJMET
Volume 4, Issue 2, March - April (2013), pp. 127-133
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PERFORMANCE EVALUATION OF LEAN PREMIXED
PREVAPOURISED COMBUSTION CHAMBER
a b
S.Poovannan , Dr.G.Kalivarathan
a
Research Scholar, CMJ University, Meghalaya, Shillong.
b
Principal/ PSN Institute of Technology and Science, Tirunelveli, Tamilnadu, Supervisor,
CMJ university, Shillong.
ABSTRACT
The demand for new improved designs with ultra-low pollutant emissions is rapidly
moving to the forefront of combustor development. Radical modern combustor designs have
emerged to achieve emission requirements while maintaining the high combustion efficiency
and good flame stability characteristics of conventional combustors. Lean premixed (LP) for
gaseous and lean premixed prevaporized (LPP) for liquid fueled engines are two modern
designs proven successful at meeting governmental regulations. Since published design
methodologies for conventional combustors do not apply well to these modern designs, and
current designs of these are typically regarded as proprietary, there is a need for the
development of new design methodologies, particularly for LPP combustors. This thesis
documents the development of these methods and then applies them to a 1-MW marine gas
turbine. The exhaust of gas turbine combustors contains several primary pollutants: oxides of
nitrogen (NOx), unburned hydrocarbons (UHC), carbon monoxide (CO), and particulate
matter or smoke. New modern designs included catalytic, rich-quench/lean-burn (RQL), and
dry low NOx (DLN) or dry low emissions (DLE) combustors. DLN combustors were
developed first and evolved into DLE designs as the focus of emissions reduction turned
towards ultralow levels of NOx, CO, and UHC.
Keywords: Lean premixed, Lean premixed prevaporized, Combustor, Diffuser, Premixer
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2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
1.0 INTRODUCTION
The LP and LPP combustor designs described previously are considered
modern designs and detailed information about them is typically proprietary. There
exists no published detailed methodology on their design since they are relatively new
technology and each design tends to be drastically different from the next. Therefore,
it is difficult to formulate a general set of preliminary design procedures for premixed
combustors since they have not yet converged on a widely accepted design. The
demand for new improved designs with ultra-low pollutant emissions is rapidly
moving to the forefront of combustor development. Radical modern combustor
designs have emerged to achieve emission requirements while maintaining the high
combustion efficiency and good flame stability characteristics of conventional
combustors. Lean premixed (LP) for gaseous and lean premixed prevaporized (LPP)
for liquid fueled engines are two modern designs proven successful at meeting
governmental regulations. The use of steam or water injection resulted in slight
penalties in cycle efficiency that were initially accepted because of the corresponding
reduction in NOx emissions. Carbon monoxide (CO) emissions rose drastically as
more and more water was injected to meet the continuously lowering NOx limits. It
was realized that radical new combustor designs would be required to satisfy the
conflicting requirements for stable, efficient combustors with low NOx emissions
(Schorr, 1991). New modern designs included catalytic, rich-quench/lean-burn (RQL),
and dry low NOx (DLN) or dry low emissions (DLE) combustors. DLN combustors
were developed first and evolved into DLE designs as the focus of emissions
reduction turned towards ultralow levels of NOx, CO, and UHC.
2.0 COMBUSTOR DESIGN
The simultaneous involvement of evaporation, turbulent mixing, ignition, and
chemical reaction in gas turbine combustion is too complex for complete theoretical
treatment. Instead, large engine manufacturers undertake expensive engine
development programs to modify previously established designs through trial-and-
error. They also develop their own proprietary combustor design rules from the
experimental results of these programs. These design rules provide a means of
specifying the combustor geometry to meet a set of requirements at the given inlet
conditions. Empirical design tools are correlations derived from experimental datasets
whereas analytical ones are discretized versions of the governing equations. Simple
empirical correlations provide accurate results quickly and are easily implemented
into design codes, yet they are only applicable to cases for which the measured data
was based on. Analytical methods, less accurate in comparison to empirical methods,
are much more flexible as they are only restricted by the simplifying assumptions
necessary to reduce their complexity and computation time.
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3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
Figure 1. Reference dimensions.
3.0 COMBUSTOR SIZING
Varying the combustor size affects the residence time and stability characteristics by
changing the reference velocity, Vref . The reference velocity, based on the reference cross-
sectional area and the combustor inlet conditions, is the effective average velocity through the
entire combustor. The pressure loss method selects a reference area Aref to provide a reference
velocity head qref typical of previous designs that exhibit similar pressure losses. This
reference area is the maximum flow area between the casing walls. The velocity head or
dynamic pressure is the difference between total and static pressure at the design point,
defined based on the design point inlet air density and velocity.
where the reference velocity Vref is
The overall combustor pressure loss is the sum of the losses through several
components: the diffuser, the swirler, and the liner. The losses through the liner can be further
broken down into the cold losses and the hot losses due to combustion. The cold losses
arising from turbulence and frictional effects are much larger in comparison with the
fundamental losses incurred by the expansion of hot gas. In combination with these losses,
those incurred across the swirler benefit combustion and dilution
4.0 DIFFUSER DESIGN
The goal of diffuser design is to minimize the total pressure loss incurred while
recovering as much dynamic velocity head as possible. A good design achieves a high static
pressure recovery with low pressure losses, is stable, insensitive to fluctuations in inlet
conditions or manufacturing tolerances, and short in length. Diffusers must also discharge to
provide the necessary airflow distributions without any adverse effects from changes in mass
flow splits, flow asymmetry, or wakes produced by objects in the flow path.
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4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
Figure 2. Diffuser Design
4.0 CENTRAL RECIRCULATION
Fuel and air must move slowly enough for the flame to propagate upstream and ignite
fresh mixture. The point at which the flame can no longer propagate back through the flow is
the stabilization point or anchor. Zones of flow reversal help stabilize the flame by creating
localized regions of low velocity flow called flameholders. Hot combustion products become
trapped in the recirculating mass and are returned to the combustor dome inlet. This hot gas
helps stabilize the flame by providing a continual source of ignition to the incoming fuel. It
also serves as a zone of intense mixing within the combustor by promoting turbulence
through high levels of shear between the forward and reverse flows. Lastly, CO, unburned
fuel, and other intermediate species are able to reside within the combustor longer
Figure 3. Combustor streamlines
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5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
Flame stability, combustion intensity, and performance are directly associated with the size
and shape of this recirculation vortex or bubble. It forms at the onset of flow reversal when an
adverse axial pressure gradient exceeds the kinetic energy of the incoming flow.
5.0 PREMIXER DESIGN
Premixers play an important role in modern combustors. Premixers are devices
composed of one or more swirlers designed to mix the fuel and air prior to combustion, as
shown in Figure 4. The performance of these devices is quantified by the mixedness or the
homogeneity of the discharged mixture. Design must also ensure that the fuel/air mixture
does not reside in the premixer for too long and autoignite. The mixture must also move fast
enough to ensure that flashback does not occur. Autoignition is the spontaneous ignition of a
fuel/air mixture after a certain time lapse above the autoignition temperature. Flashback
occurs when the flame propagates along boundary layers or slow moving flows to ignite the
incoming fuel/air mixture.
Figure 4. Premixer concept.
6.0 EXPERIMENTAL RESULTS
The combustor flow domain was solved using the high resolution advection scheme.
This scheme blends between first and second order accuracy, providing a compromise
between robustness and accuracy. The results from the analysis are discussed below
6.1 GAS TEMPERATURE DISTRIBUTION
The maximum flame temperature and the TIT predicted in the preliminary design to
that predicted in the CFD analysis. While good agreement between the predicted TITs was
observed, CFD computed a much higher flame temperature. Downstream of the dilution jets,
the temperature of the flow drops almost immediately. This was expected since a large
number of jets were used that create a blockage in the hot combustion product flow path. The
blockage forces the flow to mix with the dilution jets, producing a large drop in gas
temperature near the combustor walls.
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6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
Figure 5. Predicted combustor exit temperature profile
6.2 LINER WALL TEMPERATURE PREDICTION
The liner wall temperature profile predicted by the CFD simulation was plotted and
compared to that predicted in the preliminary design. While both figures depict similar
trends, the peak wall temperatures predicted and their respective locations differ. The
preliminary design predicted a maximum temperature of 1237 K just upstream of the dilution
holes whereas CFD concluded that a peak temperature of 1243 K occurred along the dome
wall. This discrepancy is largely attributed to the crude approximation for the gas temperature
distribution used in the preliminary design.
Figure 6. Predicted liner wall temperature profile
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7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
7.0 CONCLUSION
The design was verified using numerical analysis tools. Reasonable agreement
between predictions from the preliminary design and numerical analysis was achieved which
indicated that the design procedures have been developed successfully. Some error is
attributed to the simplified assumptions made to reduce the complexity of the numerical
models. More realistic models, in addition to experimentation, are required to improve the
assessment of the preliminary design.
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