Docstoc

PERFORMANCE EVALUATION OF LEAN PREMIXED PREVAPOURISED COMBUSTION CHAMBER

Document Sample
PERFORMANCE EVALUATION OF LEAN PREMIXED PREVAPOURISED COMBUSTION CHAMBER Powered By Docstoc
					 International Journal of JOURNAL OF MECHANICAL ENGINEERING
INTERNATIONALMechanical Engineering and Technology (IJMET), ISSN 0976 –
 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
© IAEME: www.iaeme.com/ijmet.asp
Journal Impact Factor (2013): 5.7731 (Calculated by GISI)                 ©IAEME
www.jifactor.com




              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




                                                  127
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.




                                          128
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.


                                              129
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

                                             130
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.

                                              131
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




                                             132
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.

REFERENCES

   1. Adkins, R.C., & Gueroui, D. 1986. An Improved Method for Accurate Prediction of
       Mass Flows Through Combustor Liner Holes. ASME Paper 86-GT-149.
   2. Ballal, D.R., & Lefebvre, A.H. 1972. A Proposed Method for Calculating Film-
       Cooled Wall Temperatures in Gas Turbine Combustion Chambers. ASME Paper 72-
       WA/HT-24.
   3. Carrotte, J.F., & Stevens, S.J. 1990. The Influence of Dilution Hole Geometry on Jet
       Mixing. Journal of Engineering for Gas Turbines and Power, 112, 73–79
   4. Dodds, W.J., & Bahr, D.W. 1990. Combustion System Design. Chap. 4, pages 343–
       476 of: Mellor, A.M. (ed), Design of Modern Turbine Combustors. New York:
       Academic Press.
   5. Gauthier, J.E.D. 2003. Gas Turbines. Carleton University, Ottawa. Lecture Notes for
       MECH 5402.
   6. Herbert, M.V. 1960. Aerodynamic Influences on Flame Stability. Pages 61–109 of:
       Ducarne, M., Gerstein, M., & Lefebvre, A.H. (eds), Progress in Combustion Science
       and Technology, vol. 1. New York: Pergamon Press.
   7. Jones, W.P., & Lindstedt, R.P. 1988. Global Reaction Schemes for Hydrocarbon
       Combustion. Combustion and Flame, 73, 233–249.
   8. Lefebvre, A.H. 1985. Fuel Effects on Gas Turbine Combustion - Ignition, Stability,
       and Combustion Efficiency. Transactions of the ASME, 107, 24–37.
   9. Leonard, G., & Stegmaier, J. 1993. Development of an Aero derivative Gas Turbine
       Engine Dry Low Emissions Combustion System. Journal of Engineering for Gas
       Turbines and Power, 116, 542–546.
   10. K. V. Chaudhari , D. B. Kulshreshtha and S. A. Channiwala, “Design And
       Experimental Investigations of Pressure Swirl Atomizer of Annular Type Combustion
       Chamber for 20 KW Gas Turbine Engine” International Journal of Advanced
       Research in Engineering & Technology (IJARET), Volume 3, Issue 2, 2012,
       pp. 311 - 321, ISSN Print: 0976-6480, ISSN Online: 0976-6499.
   11. P.S.Jeyalaxmi and Dr.G.Kalivarathan, “CFD Analysis of Turbulence in a Gas Turbine
       Combustor with Reference to the Context of Exit Phenomenon”, International Journal
       of Advanced Research in Engineering & Technology (IJARET), Volume 4, Issue 2,
       2013, pp. 1 - 7, ISSN Print: 0976-6480, ISSN Online: 0976-6499.
   12. P.S.Jeyalaxmi and Dr.G.Kalivarathan, “CFD Analysis of Flow Characteristics in a
       Gas Turbine - A Viable Approach to Predict the Turbulence”, International Journal of
       Mechanical Engineering & Technology (IJMET), Volume 4, Issue 2, 2013,
       pp. 39 - 46, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359, Published by
       IAEME.

                                           133

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:0
posted:4/5/2013
language:
pages:7