Performance evaluation of lean premixed prevapourised combustion chamber

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Performance evaluation of lean premixed prevapourised combustion chamber

  1. 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) IJMETVolume 4, Issue 2, March - April (2013), pp. 127-133© IAEME: www.iaeme.com/ijmet.aspJournal Impact Factor (2013): 5.7731 (Calculated by GISI) ©IAEMEwww.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
  2. 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME1.0 INTRODUCTION The LP and LPP combustor designs described previously are consideredmodern designs and detailed information about them is typically proprietary. Thereexists no published detailed methodology on their design since they are relatively newtechnology 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 premixedcombustors since they have not yet converged on a widely accepted design. Thedemand for new improved designs with ultra-low pollutant emissions is rapidlymoving to the forefront of combustor development. Radical modern combustordesigns have emerged to achieve emission requirements while maintaining the highcombustion efficiency and good flame stability characteristics of conventionalcombustors. Lean premixed (LP) for gaseous and lean premixed prevaporized (LPP)for liquid fueled engines are two modern designs proven successful at meetinggovernmental regulations. The use of steam or water injection resulted in slightpenalties in cycle efficiency that were initially accepted because of the correspondingreduction in NOx emissions. Carbon monoxide (CO) emissions rose drastically asmore and more water was injected to meet the continuously lowering NOx limits. Itwas realized that radical new combustor designs would be required to satisfy theconflicting 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 combustorswere developed first and evolved into DLE designs as the focus of emissionsreduction turned towards ultralow levels of NOx, CO, and UHC.2.0 COMBUSTOR DESIGN The simultaneous involvement of evaporation, turbulent mixing, ignition, andchemical reaction in gas turbine combustion is too complex for complete theoreticaltreatment. Instead, large engine manufacturers undertake expensive enginedevelopment programs to modify previously established designs through trial-and-error. They also develop their own proprietary combustor design rules from theexperimental results of these programs. These design rules provide a means ofspecifying the combustor geometry to meet a set of requirements at the given inletconditions. Empirical design tools are correlations derived from experimental datasetswhereas analytical ones are discretized versions of the governing equations. Simpleempirical correlations provide accurate results quickly and are easily implementedinto design codes, yet they are only applicable to cases for which the measured datawas based on. Analytical methods, less accurate in comparison to empirical methods,are much more flexible as they are only restricted by the simplifying assumptionsnecessary to reduce their complexity and computation time. 128
  3. 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 bychanging 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 theentire combustor. The pressure loss method selects a reference area Aref to provide a referencevelocity head qref typical of previous designs that exhibit similar pressure losses. Thisreference area is the maximum flow area between the casing walls. The velocity head ordynamic 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 severalcomponents: the diffuser, the swirler, and the liner. The losses through the liner can be furtherbroken down into the cold losses and the hot losses due to combustion. The cold lossesarising from turbulence and frictional effects are much larger in comparison with thefundamental losses incurred by the expansion of hot gas. In combination with these losses,those incurred across the swirler benefit combustion and dilution4.0 DIFFUSER DESIGN The goal of diffuser design is to minimize the total pressure loss incurred whilerecovering as much dynamic velocity head as possible. A good design achieves a high staticpressure recovery with low pressure losses, is stable, insensitive to fluctuations in inletconditions or manufacturing tolerances, and short in length. Diffusers must also discharge toprovide the necessary airflow distributions without any adverse effects from changes in massflow splits, flow asymmetry, or wakes produced by objects in the flow path. 129
  4. 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 Design4.0 CENTRAL RECIRCULATION Fuel and air must move slowly enough for the flame to propagate upstream and ignitefresh mixture. The point at which the flame can no longer propagate back through the flow isthe stabilization point or anchor. Zones of flow reversal help stabilize the flame by creatinglocalized regions of low velocity flow called flameholders. Hot combustion products becometrapped in the recirculating mass and are returned to the combustor dome inlet. This hot gashelps stabilize the flame by providing a continual source of ignition to the incoming fuel. Italso serves as a zone of intense mixing within the combustor by promoting turbulencethrough high levels of shear between the forward and reverse flows. Lastly, CO, unburnedfuel, and other intermediate species are able to reside within the combustor longer Figure 3. Combustor streamlines 130
  5. 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEMEFlame stability, combustion intensity, and performance are directly associated with the sizeand shape of this recirculation vortex or bubble. It forms at the onset of flow reversal when anadverse 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 devicescomposed of one or more swirlers designed to mix the fuel and air prior to combustion, asshown in Figure 4. The performance of these devices is quantified by the mixedness or thehomogeneity of the discharged mixture. Design must also ensure that the fuel/air mixturedoes not reside in the premixer for too long and autoignite. The mixture must also move fastenough to ensure that flashback does not occur. Autoignition is the spontaneous ignition of afuel/air mixture after a certain time lapse above the autoignition temperature. Flashbackoccurs when the flame propagates along boundary layers or slow moving flows to ignite theincoming 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 compromisebetween robustness and accuracy. The results from the analysis are discussed below6.1 GAS TEMPERATURE DISTRIBUTION The maximum flame temperature and the TIT predicted in the preliminary design tothat predicted in the CFD analysis. While good agreement between the predicted TITs wasobserved, 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 largenumber of jets were used that create a blockage in the hot combustion product flow path. Theblockage forces the flow to mix with the dilution jets, producing a large drop in gastemperature near the combustor walls. 131
  6. 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 profile6.2 LINER WALL TEMPERATURE PREDICTION The liner wall temperature profile predicted by the CFD simulation was plotted andcompared to that predicted in the preliminary design. While both figures depict similartrends, the peak wall temperatures predicted and their respective locations differ. Thepreliminary design predicted a maximum temperature of 1237 K just upstream of the dilutionholes whereas CFD concluded that a peak temperature of 1243 K occurred along the domewall. This discrepancy is largely attributed to the crude approximation for the gas temperaturedistribution used in the preliminary design. Figure 6. Predicted liner wall temperature profile 132
  7. 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME7.0 CONCLUSION The design was verified using numerical analysis tools. Reasonable agreementbetween predictions from the preliminary design and numerical analysis was achieved whichindicated that the design procedures have been developed successfully. Some error isattributed to the simplified assumptions made to reduce the complexity of the numericalmodels. More realistic models, in addition to experimentation, are required to improve theassessment 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

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