6340(Print), ISSN (Online) Volume 4, Issue 2, March - April (2013) IAEME AND TECHNOLOGY (IJMET)

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1 INTERNATIONAL International Journal of Mechanical JOURNAL Engineering OF MECHANICAL and Technology (IJMET), ENGINEERING ISSN 0976 AND TECHNOLOGY (IJMET) ISSN (Print) ISSN (Online) Volume 4, Issue 2, March - April (2013), pp IAEME: Journal Impact Factor (2013): (Calculated by GISI) IJMET I A E M E PERFORMANCE EVALUATION OF LEAN PREMIXED PREVAPOURISED COMBUSTION CHAMBER S.Poovannan a, Dr.G.Kalivarathan b 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 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-anderror. 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

3 3.0 COMBUSTOR SIZING Figure 1. Reference dimensions. Varying the combustor size affects the residence time and stability characteristics by changing the reference velocity, Vref. The reference velocity, based on the reference crosssectional area and the combustor inlet conditions, is the effective average velocity through the entire combustor. The pressure loss method selects a reference area A ref to provide a reference velocity head q ref 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 V ref 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

4 4.0 CENTRAL RECIRCULATION Figure 2. Diffuser Design 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

5 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

6 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

7 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 An Improved Method for Accurate Prediction of Mass Flows Through Combustor Liner Holes. ASME Paper 86-GT Ballal, D.R., & Lefebvre, A.H A Proposed Method for Calculating Film- Cooled Wall Temperatures in Gas Turbine Combustion Chambers. ASME Paper 72- WA/HT Carrotte, J.F., & Stevens, S.J The Influence of Dilution Hole Geometry on Jet Mixing. Journal of Engineering for Gas Turbines and Power, 112, Dodds, W.J., & Bahr, D.W Combustion System Design. Chap. 4, pages of: Mellor, A.M. (ed), Design of Modern Turbine Combustors. New York: Academic Press. 5. Gauthier, J.E.D Gas Turbines. Carleton University, Ottawa. Lecture Notes for MECH Herbert, M.V Aerodynamic Influences on Flame Stability. Pages 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 Global Reaction Schemes for Hydrocarbon Combustion. Combustion and Flame, 73, Lefebvre, A.H Fuel Effects on Gas Turbine Combustion - Ignition, Stability, and Combustion Efficiency. Transactions of the ASME, 107, Leonard, G., & Stegmaier, J Development of an Aero derivative Gas Turbine Engine Dry Low Emissions Combustion System. Journal of Engineering for Gas Turbines and Power, 116, 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 , ISSN Print: , ISSN Online: 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: , ISSN Online: 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 , ISSN Print: , ISSN Online: , Published by IAEME. 133