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Cfd analysis of lean premixed prevapourised combustion chamber
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Cfd analysis of lean premixed prevapourised combustion chamber
1. INTERNATIONAL JOURNALEngineering and TechnologyRESEARCH IN International Journal of Advanced Research in OF ADVANCED (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME ENGINEERING AND TECHNOLOGY (IJARET)ISSN 0976 - 6480 (Print)ISSN 0976 - 6499 (Online) IJARETVolume 4, Issue 2 March – April 2013, pp. 69-74© IAEME: www.iaeme.com/ijaret.asp ©IAEMEJournal Impact Factor (2013): 5.8376 (Calculated by GISI)www.jifactor.com CFD ANALYSIS OF LEAN PREMIXED PREVAPOURISED COMBUSTION CHAMBER b S.Poovannana, Dr.G.Kalivarathan a Research Scholar, CMJ University, Meghalaya, Shillong. b Principal/ PSN Institute of Technology and Science, Tirunelveli, Tamilnadu, Supervisor, ABSTRACT The preliminary design procedures were verified using the advanced numerical techniques of computational fluid dynamics (CFD) and finite element analysis (FEA). These techniques are used to solve the swirling flowfield inside the premixer, the reacting flowfield inside the liner, and the complex stress state in the liner walls. Although CFD and FEA indicated that the preliminary design was successful, some large discrepancies existed between the predictions. These findings suggest the need for more complex numerical models and experimental testing to validate the preliminary design. A three-dimensional solid model of the combustor and a complete set of engineering drawings were prepared and included as part of the mechanical design. These regulations demanded the development of new designs such as water or steam injection, which lowered NOx levels considerably by reducing the flame temperature. NOx formation rates are high in conventional combustors due to the high peak local flame temperatures typical of diffusion flames. Efforts to minimize UHC emissions were followed by the elimination of visible smoke, a problem common to the diffusion (non-premixed) flames that are used in conventional combustors. Some of the fuel can pyrolyse to form fine soot particles that are visible as smoke. Pyrolysis is the thermal decomposition of fuel when heated in the absence of oxygen. 1.0 INTRODUCTION In conventional combustors additional air is admitted through holes in the liner into the secondary zone (SZ) to allow the complete oxidation of CO into CO2. Premixed combustors do not require a SZ as their lower peak flame temperature minimizes the dissociation of CO2 into CO. The hot combustion products are then diluted with the remaining annulus air in the dilution zone (DZ). Crossflowing jets of cold air mix with the hot combustion products to lower the combustor exit temperature and trim its profile. Less time for 69
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEMEmixing in the DZ is required for premixed combustors as the peak flame temperatures aresignificantly lower than those in conventional ones. Simple algorithms can be quickly and easilyimplemented into computer programs whereas numerical modeling of gas turbine combustion requiressufficient resolution in the model to accurately capture the complexity of the processes involved. Theother portion of the air flows through the annulus where it cools the outside of the liner wall. Thiscooling effect is enhanced by the use of trip strips. Annulus air used for cooling is then dumped into aplenum and enters the premixer. Inside the premixer, the air passes through two concentric, counterrotating axial swirlers to mix with an evaporating liquid fuel spray. The exiting fuel and air mixture isdumped into the combustor PZ by another axial swirler where it ignites and burns. The resulting hotproducts are diluted with relatively cooler air and accelerated out of the combustor by a convergingnozzle. The smaller area results in higher annulus velocities that decrease the static pressure in theannulus. Therefore, a larger liner diameter is undesirable since a high static pressure drop across theliner admission holes is necessary to provide adequate penetration of the jets..2.0 PREMIXER DESIGN The air enters the premixer and passes through two concentric, counter-rotating swirlerswhere liquid fuel is injected into the air. Injection is accomplished using pressure nozzles that producean atomized cone spray of fine droplets. The droplets evaporate and the resultant vapour mixes withthe air to form a combustible mixture. The shear layer created between the two counter-rotatingstreams helps mix the fuel vapour supplied by the evaporating droplets with the air. The rate ofmixing, combined with the rate of evaporation, determine the premixer length; the premixer must besufficiently long to allow both to progress to completion. The fuel/air mixture passes through a thirdswirler before entering the combustion chamber and reacting. This final swirler ensures that the flowhas sufficient swirl to produce a strong recirculation zone. It also prevents radiation from entering thepremixer and potentially igniting the the fuel/air mixture. Injector selection is a critical step in thepremixer design. The nozzle(s) must provide a sufficiently fine mist of fuel droplets without requiringexcessive fuel line pressure. Finer droplets require less time to evaporate and allow for shorterpremixer tube lengths.3.0 COMBUSTOR CFD ANALYSIS CFX-5, a CFD software package, was used to analyze the combustor flowfield at the designpoint. The goal was to capture the heat released by the swirling flowfield inside the liner and thedilution of the hot combustion products. The analysis was performed using the procedures outlined inthe product documentation for CFX-5. The reader should consult this documentation for informationon all models and settings that were used. Figure 1. Solid model of combustor flow domain 70
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME Figure 2. Combustor computational meshAn unstructured grid was generated with ANSYS CFX-MESH, illustrated in Figure 2, whichconsists of 150,000 nodes and 800,000 elements. The nodal density of the mesh was selectedby studying its effects on the overall solution and choosing one whose solution was gridindependent.4.0 BOUNDARY CONDITION Solution of the computational domain requires knowledge of the boundary conditions.The boundary conditions used were those corresponding to the engine design point and areprovided below. Inlet A specified mass flow rate boundary condition was used for both inlets.The total mass flow rate and the individual mass fractions of each species at design wereestimated using the results obtained Outlet The average static pressure was set to match theinlet total pressure with that predicted by the preliminary design. Combustor Walls Wallboundary conditions were placed on both the swirler hub and the liner wall. The swirler hubwas modeled as an adiabatic wall whereas the liner was modeled by specifying the overallheat transfer coefficient. Figure 3. Closeup of refined areas in the combustor mesh 71
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME5.0 GEOMETRY AND GRID GENERATION A three-dimensional solid model of the premixer flow domain was constructed anddiscretized using ANSYS DesignModeler and ANSYS CFX-MESH, respectively. Solid Model Thepremixer flow domain was simplified to reduce the complexity of the problem. The simplificationsinclude: No swirlers were included in the model. The size of the mesh was vastly reduced by placingthe inlet to the domain downstream of the mixer swirlers. This required the assumption that thevelocity profiles of the flow issuing from each mixer swirler are uniform and follows the blade. Thefuel spray issuing from each nozzle is modeled as a single droplet with an initial diameter equal to theSMD. The problem is axisymmetric. A 900 section was modeled using the periodic boundarycondition. The angle was chosen to ensure a whole number of mixer blades and fuel nozzles inside thedomain. The resulting solid model of the flow domain Figure 4. Solid model of premixer flow domain6.0 RESULTS The combustor was first analyzed using several grids of varying nodal density to ascertain theresolution required to achieve grid independence. This was accomplished by comparing the solutionfrom four meshes. The velocity inside the liner was plotted to verify that a strong swirling flow exists.The consequent temperature distribution upstream of the dilution holes is one that is hotter near theliner walls and slightly cooler at the centreline. Figure 5. Temperature distribution inside combustor 72
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEMEThe liner wall temperature profile predicted by the CFD simulation was plotted two largetemperature gradients are visible: one occurs along the dome where cold fuel and air reactand the other occurs near the dilution holes. The first gradient is likely to cause buckling ofthe dome walls while the second is expected to induce cracking at the edge of the holes. ACFD analysis was performed to measure the performance of the premixer at the design pointwith respect to mixing and evaporation. The analysis was performed using ANSYS CFX-5 ina manner very similar to the combustor analysis. Figure 6. Mass Fraction inside linear7.0 CONCLUSION It should be emphasized that, despite these large discrepancies, numerical analysisconfirmed that the preliminary design was successful. Since further improvements are madeat the detailed design phase, the preliminary design is only required to provide a geometrywith a reasonable degree of conformance. The combustor designed met most of thespecifications and requirements and is therefore acceptable for prototype manufacturing. Theinitial step before complex numerical analysis with CFD and FEA, the methodologydeveloped greatly simplified the transition from preliminary to detailed design. This isnecessary to improve the accuracy of the detailed design phase. It would provide estimatesfor the static pressure distribution along the liner wall, the airflow distribution throughout thecombustor, and the overall total pressure loss. The analysis would also include the effects ofannulus flow on dilution jet performance. Additionally, it would reveal any asymmetry in theannulus flow induced by the combustor inlet configuration.REFERENCE1. Bragg, S.L. 1963. Combustion Noise. Journal of the Institute of Fuel, January, 12–16.2. Carrotte, J.F., & Stevens, S.J. 1990. The Influence of Dilution Hole Geometry on JetMixing. Journal of Engineering for Gas Turbines and Power, 112, 73–79.3. Chigier, N.A., & Beer, J.M. 1964. Velocity and Static Pressure Distributions inSwirling Air Jets Issuing from Annular and Divergent Nozzles. Journal of BasicEngineering, 86, 788–796. 73
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME4. Childs, J.H. 1950. Preliminary Correlation of Efficiency of Aircraft Gas-TurbineCombustors for Different Operating Conditions. Research Memorandum RMEF50F15.National Advisory Committee for Aeronautics.5. Chin, J.S., & Lefebvre, A.H. 1982. Effective Values of Evaporation Constant forHydrocarbon Fuel Drops. Pages 325–331 of: Proceedings of the 20th AutomotiveTechnology Development Contractor Coordination Meeting.6. Correa, S.M. 1991. Lean Premixed Combustion for Gas-Turbines: Review andRequired Research. In: Fossil Fuel Combustion, vol. 33. Petroleum Division, ASME.7. Crocker, D.S., & Smith, C.E. 2001. Gas Turbines. Chap. 12 of: Baukal, C.E., & an X.Li, V.Y. Gershtein (eds), Computational Fluid Dynamics in Industrial Combustion. NewYork: CRC Press.8. Delevan Spray Technologies. 2005. Product Catalogue B: Hollow Cone Spray.9. 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: AcademicPress.10. Evans, D.M., & Noble, M.L. 1978. Gas Turbine Combustor Cooling by AugmentedBackside Convection. ASME Paper 78-GT-33.11. Faeth, G.M. 1983. Evaporation and Combustion of Sprays. Progress in EnergyCombustion Science, 9, 1–76.12. Fric, T.F. 1992. Effects of Fuel-Air Unmixedness on NOx Emissions. AIAA Paper92-3345.13. Gardner, L., & Whyte, R.B. 1990. Gas Turbine Fuels. Chap. 2, pages 81–227 of:Mellor, A.M. (ed), Design of Modern Turbine Combustors. New York: Academic Press.Gauthier, J.E.D. 2003. Gas Turbines. Carleton University, Ottawa. Lecture Notes for MECH5402.14. Tarun Singh Tanwar, Dharmendra Hariyani and Manish Dadhich, “Flow Simulation(CFD) & Static Structural Analysis (FEA) of a Radial Turbine”, International Journal ofMechanical Engineering & Technology (IJMET), Volume 3, Issue 3, 2012, pp. 252 - 269,ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.15. K. V. Chaudhari , D. B. Kulshreshtha and S. A. Channiwala, “Design AndExperimental Investigations of Pressure Swirl Atomizer of Annular Type CombustionChamber for 20 KW Gas Turbine Engine” International Journal of Advanced Research inEngineering & Technology (IJARET), Volume 3, Issue 2, 2012, pp. 311 - 321, ISSN Print:0976-6480, ISSN Online: 0976-6499.16. P.S.Jeyalaxmi and Dr.G.Kalivarathan, “CFD Analysis of Turbulence in a Gas TurbineCombustor with Reference to the Context of Exit Phenomenon”, International Journal ofAdvanced Research in Engineering & Technology (IJARET), Volume 4, Issue 2, 2013,pp. 1 - 7, ISSN Print: 0976-6480, ISSN Online: 0976-6499.17. P.S.Jeyalaxmi and Dr.G.Kalivarathan, “CFD Analysis of Flow Characteristics in aGas Turbine - A Viable Approach to Predict the Turbulence”, International Journal ofMechanical Engineering & Technology (IJMET), Volume 4, Issue 2, 2013, pp. 39 - 46, ISSNPrint: 0976 – 6340, ISSN Online: 0976 – 6359. 74
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