200 research outputs found

    Large Eddy Simulations of gaseous flames in gas turbine combustion chambers

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    Recent developments in numerical schemes, turbulent combustion models and the regular increase of computing power allow Large Eddy Simulation (LES) to be applied to real industrial burners. In this paper, two types of LES in complex geometry combustors and of specific interest for aeronautical gas turbine burners are reviewed: (1) laboratory-scale combustors, without compressor or turbine, in which advanced measurements are possible and (2) combustion chambers of existing engines operated in realistic operating conditions. Laboratory-scale burners are designed to assess modeling and funda- mental flow aspects in controlled configurations. They are necessary to gauge LES strategies and identify potential limitations. In specific circumstances, they even offer near model-free or DNS-like LES computations. LES in real engines illustrate the potential of the approach in the context of industrial burners but are more difficult to validate due to the limited set of available measurements. Usual approaches for turbulence and combustion sub-grid models including chemistry modeling are first recalled. Limiting cases and range of validity of the models are specifically recalled before a discussion on the numerical breakthrough which have allowed LES to be applied to these complex cases. Specific issues linked to real gas turbine chambers are discussed: multi-perforation, complex acoustic impedances at inlet and outlet, annular chambers.. Examples are provided for mean flow predictions (velocity, temperature and species) as well as unsteady mechanisms (quenching, ignition, combustion instabil- ities). Finally, potential perspectives are proposed to further improve the use of LES for real gas turbine combustor designs

    Modeling gaseous non-reactive flow in a lean direct injection gas turbine combustor through an advanced mesh control strategy

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    [EN] Fuel efficiency improvement and harmful emissions reduction are the main motivations for the development of gas turbine combustors. Numerical computational fluid dynamics (CFD) simulations of these devices are usually computationally expensive since they imply a multi-scale problem. In this work, gaseous non-reactive unsteady Reynolds-Averaged Navier-Stokes and large eddy simulations of a gaseous-fueled radial-swirled lean direct injection combustor have been carried out through CONVERGE (TM) CFD code by solving the complete inlet flow path through the swirl vanes and the combustor. The geometry considered is the gaseous configuration of the CORIA lean direct injection combustor, for which detailed measurements are available. The emphasis of the work is placed on the demonstration of the CONVERGE (TM) applicability to the multi-scale gas turbine engines field and the determination of an optimal mesh strategy through several grid control tools (i.e., local refinement, adaptive mesh refinement) allowing the exploitation of its automatic mesh generation against traditional fixed mesh approaches. For this purpose, the normalized mean square error has been adopted to quantify the accuracy of turbulent numerical statistics regarding the agreement with the experimental database. Furthermore, the focus of the work is to study the behavior when coupling several large eddy simulation sub-grid scale models (i.e., Smagorinsky, Dynamic Smagorinsky, and Dynamic Structure) with the adaptive mesh refinement algorithm through the evaluation of its specific performances and predictive capabilities in resolving the spatial-temporal scales and the intrinsically unsteady flow structures generated within the combustor. This investigation on the main non-reacting swirling flow characteristics inside the combustor provides a suitable background for further studies on combustion instability mechanisms.The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was partly sponsored by the program "Ayuda a Primeros Proyectos de Investigacion (PAID-06-18), Vicerrectorado de Investigacion, Innovacion y Transferencia de la Universitat Politecnica de Valencia (UPV), Spain.'' The support given to Mr. Mario Belmar by Universitat Politecnica de Valencia through the "FPI-Subprograma 2'' grant within the "Programa de Apoyo para la Investigacion y Desarrollo (PAID-01-18)'' is gratefully acknowledged.Payri, R.; Novella Rosa, R.; Carreres, M.; Belmar-Gil, M. (2020). Modeling gaseous non-reactive flow in a lean direct injection gas turbine combustor through an advanced mesh control strategy. Proceedings of the Institution of Mechanical Engineers Part G Journal of Aerospace Engineering. 234(11):1788-1810. https://doi.org/10.1177/0954410020919619S1788181023411Patel, N., KฤฑrtaลŸ, M., Sankaran, V., & Menon, S. (2007). Simulation of spray combustion in a lean-direct injection combustor. Proceedings of the Combustion Institute, 31(2), 2327-2334. doi:10.1016/j.proci.2006.07.232Luo, K., Pitsch, H., Pai, M. G., & Desjardins, O. (2011). Direct numerical simulations and analysis of three-dimensional n-heptane spray flames in a model swirl combustor. Proceedings of the Combustion Institute, 33(2), 2143-2152. doi:10.1016/j.proci.2010.06.077Masri, A. R., Pope, S. B., & Dally, B. B. (2000). Probability density function computations of a strongly swirling nonpremixed flame stabilized on a new burner. Proceedings of the Combustion Institute, 28(1), 123-131. doi:10.1016/s0082-0784(00)80203-9Johnson, M. R., Littlejohn, D., Nazeer, W. A., Smith, K. O., & Cheng, R. K. (2005). A comparison of the flowfields and emissions of high-swirl injectors and low-swirl injectors for lean premixed gas turbines. Proceedings of the Combustion Institute, 30(2), 2867-2874. doi:10.1016/j.proci.2004.07.040Sankaran, V., & Menon โ€ , S. (2002). LES of spray combustion in swirling flows. Journal of Turbulence, 3, N11. doi:10.1088/1468-5248/3/1/011Jones, W. P., Marquis, A. J., & Vogiatzaki, K. (2014). Large-eddy simulation of spray combustion in a gas turbine combustor. Combustion and Flame, 161(1), 222-239. doi:10.1016/j.combustflame.2013.07.016Ding, G., He, X., Xue, C., Zhao, Z., & Jin, Y. (2015). Preliminary design and experimental verification of a triple swirler combustor. Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering, 229(12), 2258-2271. doi:10.1177/0954410015573555Menon, S., & Patel, N. (2006). Subgrid Modeling for Simulation of Spray Combustion in Large-Scale Combustors. 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Ballistic imaging of the near field in a diesel spray. Experiments in Fluids, 40(6), 836-846. doi:10.1007/s00348-006-0122-0Desantes, J. M., Salvador, F. J., Lรณpez, J. J., & De la Morena, J. (2010). Study of mass and momentum transfer in diesel sprays based on X-ray mass distribution measurements and on a theoretical derivation. Experiments in Fluids, 50(2), 233-246. doi:10.1007/s00348-010-0919-8Reddemann, M. A., Mathieu, F., & Kneer, R. (2013). Transmitted light microscopy for visualizing the turbulent primary breakup of a microscale liquid jet. Experiments in Fluids, 54(11). doi:10.1007/s00348-013-1607-2Chen, R.-H., & Driscoll, J. F. (1989). The role of the recirculation vortex in improving fuel-air mixing within swirling flames. Symposium (International) on Combustion, 22(1), 531-540. doi:10.1016/s0082-0784(89)80060-8Presser, C., Gupta, A. K., & Semerjian, H. G. (1993). Aerodynamic characteristics of swirling spray flames: Pressure-jet atomizer. 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Vorticity-scalar alignments and small-scale structures in swirling spray combustion. Proceedings of the Combustion Institute, 29(1), 577-584. doi:10.1016/s1540-7489(02)80074-8Lebas, R., Menard, T., Beau, P. A., Berlemont, A., & Demoulin, F. X. (2009). Numerical simulation of primary break-up and atomization: DNS and modelling study. International Journal of Multiphase Flow, 35(3), 247-260. doi:10.1016/j.ijmultiphaseflow.2008.11.005Zhou, Y., Huang, Y., & Mu, Z. (2017). Large eddy simulation of the influence of synthetic inlet turbulence on a practical aeroengine combustor with counter-rotating swirler. Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering, 233(3), 978-990. doi:10.1177/0954410017745900Torregrosa, A. J., Broatch, A., Garcรญa-Tรญscar, J., & Gomez-Soriano, J. (2018). Modal decomposition of the unsteady flow field in compression-ignited combustion chambers. 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    A computational fluid dynamics investigation of turbulent swirling burners

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    This thesis presents detailed numerical calculations of the Unsteady, Reynolds- Averaged Navier-Stokes (URANS) equations to simulate isothermal, single-phase flow in the geometries of realistic swirl burners at large Reynolds numbers. Simulations are run with two different turbulence closures, viz., the standard k-epsilon and Reynolds stresses (RSM) models. The numerical method is validated concerning convergence, grid density and far-field influence. Results describe a flow that is in any case periodic or pseudo-periodic, and exhibits quite convincing time-dependent features: bubble- and spiral-type vortex breakdowns and vortex core precession. Some simulations are validated by comparison with corresponding experiments. Good agreement with the experiments has been obtained for mean flow, and frequency peaks of the power spectral density of pressure fluctuations. In order to asses the reliability of URANS methods within this context, calculated time-averaged flow and coherent structures are documented via 2D graphs, spectral analysis, 3D isosurfaces and advanced, vortex-related visualization methods and 2D snapshot proper orthogonal decomposition (S-POD). Differences arising from the nature of the turbulence model (k-epsilon vs. RSM) are very relevant indeed, given the cost factor involved and the apparent verisimilitude of the predicted flow; they are thoroughly analyzed

    Response of a swirl-stabilized flame to transverse acoustic excitation

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    This work addresses the issue of transverse combustion instabilities in annular gas turbine combustor geometries. While modern low-emissions combustion strategies have made great strides in reducing the production of toxic emissions in aircraft engines and power generation gas turbines, combustion instability remains one of the foremost technical challenges in the development of next generation combustor technology. To that end, this work investigates the response of a swirling flow and swirl-stabilized flame to a transverse acoustic field is using a variety of high-speed laser techniques, especially high-speed particle image velocimetry (PIV) for detailed velocity measurements of this highly unsteady flow phenomenon. A description of the velocity-coupled transverse instability mechanism is explained with companion measurements describing each of the velocity disturbance pathways. Dependence on acoustic frequency, amplitude, and field symmetry is discussed. Significant emphasis is placed on the response of a swirling flow field to a transverse acoustic field. Details of the dynamics of the vortex breakdown bubble and the shear layers are explained using a wide variety of measurements for both non-reacting and reacting flow cases. This thesis concludes with an overview of the impact of this work and suggestions for future research in this area.PhDCommittee Chair: Tim Lieuwen; Committee Member: Ari Glezer; Committee Member: Jerry Seitzman; Committee Member: Lakshmi Sankar; Committee Member: Suresh Meno

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    ํ•™์œ„๋…ผ๋ฌธ(๋ฐ•์‚ฌ) -- ์„œ์šธ๋Œ€ํ•™๊ต๋Œ€ํ•™์› : ๊ณต๊ณผ๋Œ€ํ•™ ๊ธฐ๊ณ„ํ•ญ๊ณต๊ณตํ•™๋ถ€, 2022.2. ์œค์˜๋นˆ.์—ฐ์†Œ๋ถˆ์•ˆ์ •์˜ ๋ฐœ์ƒ ์›์ธ๊ณผ ๋ฉ”์ปค๋‹ˆ์ฆ˜์€ ํ˜„์žฌ๊นŒ์ง€ ์ •ํ™•ํ•˜๊ฒŒ ๊ทœ๋ช…๋˜์ง€ ์•Š์•˜์œผ๋‚˜ ๋ฐ˜์‘๋ฌผ์˜ ์œ ๋™ ์„ญ๋™, ์—ด๋ฐฉ์ถœ๋Ÿ‰ ์„ญ๋™, ์—ฐ์†Œ์‹ค์˜ ์Œํ–ฅํ•™์  ๊ฒฝ๊ณ„์— ์˜ํ•œ ์„ญ๋™์˜ ์ƒํ˜ธ์ž‘์šฉ์— ์˜ํ•ด ๋ฐœ์ƒ ์œ ๋ฌด๊ฐ€ ๊ฒฐ์ •๋œ๋‹ค๊ณ  ์•Œ๋ ค์ ธ ์žˆ๋‹ค. ์œ„์˜ ์„ธ ๊ฐ€์ง€ ์„ญ๋™์ด ์–‘์„ฑ ํ”ผ๋“œ๋ฐฑ ๋ฃจํ”„ (positive feedback loop)๋ฅผ ํ˜•์„ฑํ•˜๋ฉด ์—ฐ์†Œ๋ถˆ์•ˆ์ •์ด ๋ฐœ์ƒํ•  ํ™•๋ฅ ์ด ๋†’์•„์ง€๋ฉฐ, ์Œ์„ฑ ํ”ผ๋“œ๋ฐฑ ๋ฃจํ”„(negative feedback loop)๋ฅผ ํ˜•์„ฑํ•˜๋ฉด ๊ทธ ํ™•๋ฅ ์ด ๋‚ฎ์•„์ง€๋Š” ๊ฒƒ์œผ๋กœ ์•Œ๋ ค์ ธ ์žˆ๋‹ค. ๋”ฐ๋ผ์„œ ์—ฐ์†Œ๋ถˆ์•ˆ์ • ๋ฐœ์ƒ ์ €๊ฐ์„ ์œ„ํ•ด ์—ฐ์†Œ๋ถˆ์•ˆ์ • ๋ฐœ์ƒ ์กฐ๊ฑด ๋ฐ ์ธ์ž๋ฅผ ํŒŒ์•…ํ•˜๋Š” ๊ฒƒ์€ ํ•„์ˆ˜์ ์ด๋‹ค. ๋ณธ ์—ฐ๊ตฌ์—์„œ๋Š” ์—ฐ์†Œ๋ถˆ์•ˆ์ • ํ˜„์ƒ์˜ ์ธ์ž ์ค‘ ์—ด ๋ฐฉ์ถœ๋Ÿ‰ ์„ญ๋™๊ณผ ์†๋„ ์„ญ๋™์˜ ์ƒํ˜ธ๊ด€๊ณ„์— ๋Œ€ํ•œ ์—ฐ๊ตฌ๋ฅผ ์ˆ˜ํ–‰ํ•˜์˜€์œผ๋ฉฐ, ํŠน์ • ์Œํ–ฅ ๊ฐ€์ง„ ์กฐ๊ฑด์—์„œ ํ™”์—ผ์ด ๋Š๊ธฐ๋Š” ํ˜„์ƒ์ธ pinch-off ํ™”์—ผ์— ๋Œ€ํ•ด OH PLIF๊ณผ PIV ๋ ˆ์ด์ € ๋™์‹œ๊ณ„์ธก์„ ํ†ตํ•ด ๋ฐœ์ƒ ๋ฉ”์ปค๋‹ˆ์ฆ˜์„ ์‹คํ—˜์ ์œผ๋กœ ๊ทœ๋ช…ํ•˜์˜€์œผ๋ฉฐ, pinch-off ํ™”์—ผ๊ณผ nonpinch-off ํ™”์—ผ์˜ NOx ๋ฐฐ์ถœ ํŠน์„ฑ, ์—ฐ๋ฃŒ์™€ ๊ณต๊ธฐ์˜ ์œ ๋™๊ฒฝ๊ณ„์ธต์— ๋Œ€ํ•œ ์œ ๋™ ํŠน์„ฑ ๋ถ„์„์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ์Œํ–ฅ ๊ฐ€์ง„ ๋ฐœ์ƒ์„ ์œ„ํ•ด์„œ ์Šคํ”ผ์ปค๋ฅผ ํ™œ์šฉํ•˜์˜€์œผ๋ฉฐ, ํ™”์—ผ๊ตฌ์กฐ ๋ถ„์„์„ ์œ„ํ•ด์„œ OH* ์ž๋ฐœ๊ด‘๊ณผ OH-PLIF ๋ ˆ์ด์ € ๊ณ„์ธก๊ธฐ๋ฒ•์„ ํ™œ์šฉํ•˜์˜€์œผ๋ฉฐ, ์œ ๋™์žฅ ํŠน์„ฑ๋ถ„์„์„ ์œ„ํ•ด OH-PLIF์™€ PIV ๋™์‹œ๊ณ„์ธก์„ ํ™œ์šฉํ•˜์˜€๋‹ค. ์—ด ๋ฐฉ์ถœ๋Ÿ‰ ๊ณ„์ธก์„ ์œ„ํ•ด์„œ ๊ด‘์ „์ž์ฆํญ๊ด€(Photo Multiplier Tube, PMT)๋ฅผ ํ™œ์šฉํ•˜์˜€์œผ๋ฉฐ ์ด๋ฅผ ํ†ตํ•ด ํ™”์—ผ์ „๋‹ฌํ•จ์ˆ˜๋ฅผ ๊ณ„์ธกํ•˜์˜€๋‹ค. ์—ฐ์†Œ๋ถˆ์•ˆ์ • ์˜ˆ์ธก์„ ์œ„ํ•ด ์Œํ–ฅ ๊ฐ€์ง„์— ๋”ฐ๋ฅธ ๋น„์˜ˆํ˜ผํ•ฉํ™”์—ผ๊ณผ ์˜ˆํ˜ผํ•ฉํ™”์—ผ์˜ ์‘๋‹ตํŠน์„ฑ ๋ฐ ๋™ํŠน์„ฑ ๋น„๊ต ์—ฐ๊ตฌ๋ฅผ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ๋‹ค๋ฅธ ์—ฐ์†Œ ๋ฐ˜์‘์„ ๊ฐ€์ง„ ๋‘ ํ™”์—ผ์€ ์Œํ–ฅ ๊ฐ€์ง„์— ๋”ฐ๋ผ ๋™์ ๊ฑฐ๋™ ํŠน์„ฑ์ด ์ƒ์ดํ•˜๋‹ค. ๋น„์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ํ™”์—ผ ๋ฉด์—์„œ ์Œํ–ฅํ•™์  ํŒŒ๋™์ด ํˆฌ์˜๋˜๋ฉฐ, ํŽ„๋Ÿญ์ด๋Š” ๋™์  ๊ฑฐ๋™ ํŠน์„ฑ์ด๋ฉฐ ํ™”์—ผ ๋ ๋‹จ์ด ์—ด๋ฆฐ ํ™”์—ผํ˜•์ƒ์ด๋‹ค. ๋ฐ˜๋ฉด์— ๋‹จ์ผ ๋…ธ์ฆ์ธ ์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ์ฝ”๋‹ˆ์ปฌ ํ™”์—ผ(conical flame)์˜ ํ˜•ํƒœ๋กœ ์ˆ˜์ง์œผ๋กœ ํฌ๊ฒŒ ์„ญ๋™ํ•œ๋‹ค. ๋น„์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ๊ฐ€์ง„์ฃผํŒŒ์ˆ˜ ์ฆ๊ฐ€์— ๋”ฐ๋ผ์„œ ํ™”์—ผ ๋ฉด์˜ ๋ชจ๋“ˆ๋ ˆ์ด์…˜(modulation) ๊ฐœ์ˆ˜๊ฐ€ ์ฆ๊ฐ€ํ•  ๋ฟ ํ™”์—ผ ๊ตฌ์กฐ๋Š” ํฌ๊ฒŒ ๋ณ€ํ•˜์ง€ ์•Š์ง€๋งŒ, ์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ๊ฐ€์ง„์ฃผํŒŒ์ˆ˜์™€ ์†๋„์„ญ๋™๊ฐ•๋„์— ๋”ฐ๋ผ ๋‹ค์–‘ํ•œ ํ™”์—ผ ๊ตฌ์กฐ๊ฐ€ ๋‚˜ํƒ€๋‚œ๋‹ค. ์Œํ–ฅ ๊ฐ€์ง„ ์‹œ ๋‘ ํ™”์—ผ์˜ ์—ด ๋ฐฉ์ถœ๋Ÿ‰ ์ธก์ •์„ ํ†ตํ•ด ํ™”์—ผ์ „๋‹ฌํ•จ์ˆ˜๋ฅผ ๋ถ„์„ํ•œ ๊ฒฐ๊ณผ ๋น„์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ๋น„์„ ํ˜•์ ์ธ ๊ฒฐ๊ณผ๋ฅผ ๋‚˜ํƒ€๋‚ด์—ˆ์œผ๋ฉฐ, ์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ์„ ํ˜•์ ์ธ ๊ฒฐ๊ณผ๋ฅผ ๋‚˜ํƒ€๋‚ด์—ˆ๋‹ค. ํ™”์—ผ๊ธธ์ด์™€ ์ŠคํŠธ๋กค ์ˆ˜(Strouhal number)๋ฅผ ๋„์ž…ํ•˜์—ฌ ์—ด๋ฐฉ์ถœ๋Ÿ‰๊ณผ ํ™”์—ผ๊ตฌ์กฐ์˜ ์ƒ๊ด€๊ด€๊ณ„ ๋ถ„์„์„ ์ˆ˜ํ–‰ํ•˜์˜€์œผ๋ฉฐ, ์ˆ˜์น˜ํ•ด์„์  ์—ฐ๊ตฌ์™€ ๋น„๊ตํ•˜์˜€๋‹ค. ๋น„์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ 20%์ด์ƒ์˜ ์†๋„์„ญ๋™์—์„œ ์ˆ˜์น˜ํ•ด์„๊ฒฐ๊ณผ์™€ ์ผ์น˜ํ•˜๋ฉฐ, ๋น„์„ ํ˜•์„ฑ์„ ๊ฒ€์ฆํ•˜์˜€์œผ๋ฉฐ, ์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ์ผ๋ถ€ ์ŠคํŠธ๋กค ์ˆ˜์—์„œ ์ˆ˜์น˜ํ•ด์„ ๊ฒฐ๊ณผ์™€ ๋‹ค๋ฅด๋‹ค. ์ด๋Š” ์ŠคํŠธ๋กค ์ˆ˜๋ฅผ ๊ณ„์‚ฐํ•  ๋•Œ, ํ™”์—ผ ๋ฉด ๊ณก๋ฅ , ํ™”์—ผ ์ „ํŒŒ ์†๋„, ํ™”์—ผ ๋๋‹จ ํ˜•์ƒ ๋“ฑ์„ ๊ณ ๋ คํ•œ ์ŠคํŠธ๋กค ์ˆ˜์— ๋„์ž…ํ•ด์•ผ ํ•จ์„ ํ™•์ธํ•˜์˜€๋‹ค. ๋ฒ…-์Šˆ๋งŒ ํ™”์—ผ์˜ ํ•œ๊ฐ€์ง€ ์ผ€์ด์Šค์ธ ๋น„์˜ˆํ˜ผํ™”์—ผ์—์„œ ์Œํ–ฅ ๊ฐ€์ง„ ์‹œ ์†๋„์„ญ๋™๊ฐ•๋„์™€ ๊ฐ€์ง„์ฃผํŒŒ์ˆ˜์— ๋”ฐ๋ฅธ ๋‹ค์–‘ํ•œ ํ™”์—ผ ๊ตฌ์กฐ๋ฅผ ๊ด€์ฐฐํ•˜์˜€๋‹ค. ์ผ์ • ๊ฐ€์ง„์ฃผํŒŒ์ˆ˜์™€ ์†๋„์„ญ๋™๊ฐ•๋„ ๋ฒ”์œ„์—์„œ๋Š” ํ™”์—ผ์ด ๋Š๊ธฐ๋Š” pinch-off ํ™”์—ผ์ด ๋‚˜ํƒ€๋‚จ์„ ํ™•์ธํ•˜์˜€๋‹ค. pinch-off ํ™”์—ผ์€ ํ™”์—ผ์ด ๋Š๊ธฐ๋Š” ํ˜„์ƒ์œผ๋กœ ์ •์˜ํ•˜๋ฉฐ ๋…ธ์ฆ์— ๋ถ€์ฐฉ๋œ ํ™”์—ผ์€ ๋ฉ”์ธ(main) ํ™”์—ผ๊ณผ ๋–จ์–ด์ ธ ๋‚˜๊ฐ„ ํ™”์—ผ์„ ํฌ์ผ“(pocket) ํ™”์—ผ์œผ๋กœ ์ •์˜ํ•œ๋‹ค. Pinch-off ํ™”์—ผ์˜ ๋ฉ”์ปค๋‹ˆ์ฆ˜ ๊ทœ๋ช…์„ ์œ„ํ•ด OH PLIF&PIV ๋™์‹œ๊ณ„์ธก์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ๊ฐ€์ง„ ์ฃผํŒŒ์ˆ˜์™€ ์†๋„์„ญ๋™๊ฐ•๋„์— ๋”ฐ๋ฅธ ํ™”์—ผ ๊ตฌ์กฐ๋ฅผ ๋งตํ•‘(mapping)ํ•˜์—ฌ 3๊ฐ€์ง€ ๋™์  ๊ฑฐ๋™์œผ๋กœ ๊ตฌ๋ณ„ํ•˜์˜€๋‹ค. ๋‚ฎ์€ ์ฃผํŒŒ์ˆ˜ ๋ฒ”์œ„์—์„œ๋Š” ํ™”์—ผ์ด ์ƒ-ํ•˜๋กœ ํฌ๊ฒŒ ์„ญ๋™ํ•˜๋Š” flickering ํ™”์—ผ์ด๋‹ค. ์ค‘๊ฐ„ ์ฃผํŒŒ์ˆ˜ ๋ฒ”์œ„์—์„œ๋Š” ํ™”์—ผ์ด ๋Š๊ธฐ๋Š” pinch-off ํ™”์—ผ, ๊ทธ๋ฆฌ๊ณ  ๋†’์€ ์ฃผํŒŒ์ˆ˜ ๋ฒ”์œ„์—์„œ๋Š” ํ™”์—ผ ๋ฉด์˜ ๋ชจ๋“ˆ๋ ˆ์ด์…˜์ด ์ƒ๊ธฐ๋Š” wrinkled ํ™”์—ผ์œผ๋กœ ๊ตฌ๋ณ„ํ•˜์˜€๋‹ค. ๋น„๋ฐ˜์‘์žฅ ์œ ๋™์—์„œ ์Œํ–ฅ ๊ฐ€์ง„ ์‹œ ์กฐ์ˆ˜ ์œ ๋™(tidal flow)์— ์˜ํ•œ double dipole vortex์„ ๋ฏธ ์‚ฐ๋ž€(Mie scattering)์œผ๋กœ ํ™•์ธํ•˜์˜€๋‹ค. ๋ฐ˜์‘์žฅ ์œ ๋™์˜ ํ™”์—ผ์ด pinch-off ์‹œ vortical structure์— ์˜ํ•œ ๊ณต๊ธฐ ์œ ์ž…์ด ํ™”์—ผ ๋ณ€ํ˜•์„ ์•ผ๊ธฐํ•จ์„ ํ™•์ธํ•˜์˜€์œผ๋ฉฐ, ํ™”์—ผ ๋ชฉ ๋ถ€๋ถ„์— ๊ฐ•ํ•œ strain rate์„ ๊ด€์ฐฐํ•˜์˜€๋‹ค. ์ด์— ๋”ฐ๋ผ pinch-off flame์€ vortical structure์— ์˜ํ•œ ์™ธ๋ถ€ ๊ณต๊ธฐ ์œ ์ž…๊ณผ ๊ฐ•ํ•œ strain rate์— ์˜ํ•œ ์ƒํ˜ธ์ž‘์šฉ์ž„์„ ํ™•์ธํ•˜์˜€๋‹ค. pinch-off ํ™”์—ผ์˜ ๋–จ์–ด์ ธ๋‚˜๊ฐ„ ํฌ์ผ“ ํ™”์—ผ์„ ๊ณ ๋ คํ•œ ์งˆ์†Œ์‚ฐํ™”๋ฌผ(NOx), ์ผ์‚ฐํ™”ํƒ„์†Œ(CO) ๋ฐฐ์ถœ ํŠน์„ฑ ๋ถ„์„์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ์งˆ์†Œ์‚ฐํ™”๋ฌผ์€ ์†๋„์„ญ๋™๊ฐ•๋„ ์ฆ๊ฐ€์— ๋”ฐ๋ผ ๋ฐฐ์ถœ์–‘์€ ๊ฐ์†Œํ•œ๋‹ค. ์ด๋Š” ์†๋„์„ญ๋™๊ฐ•๋„๊ฐ€ ์ฆ๊ฐ€ํ•˜๋ฉด ์—ฐ๋ฃŒ์™€ ์‚ฐํ™”์ œ ํ˜ผํ•ฉ๋„๊ฐ€ ์ฆ๊ฐ€ํ•˜๊ธฐ ๋•Œ๋ฌธ์ด๋ฉฐ ํ˜ผํ•ฉ์ด ์ž˜ ์ด๋ฃจ์–ด์ ธ ์™„์ „์—ฐ์†Œํ•˜๋ฉด ์งˆ์†Œ์‚ฐํ™”๋ฌผ ๋ฐฐ์ถœ์ด ์ ์–ด์ง„๋‹ค. ๋ฐ˜๋ฉด์— ์ผ์‚ฐํ™”ํƒ„์†Œ๋Š” ์†๋„์„ญ๋™๊ฐ•๋„ ์ฆ๊ฐ€์— ๋”ฐ๋ผ ๋ฐฐ์ถœ์ด ์ฆ๊ฐ€ํ•˜๋Š” ํŠน์„ฑ์„ ๋‚˜ํƒ€๋‚ด์—ˆ์ง€๋งŒ, ๊ทธ ๋ฐฐ์ถœ๋Ÿ‰์ด ๋งค์šฐ ์ž‘์Œ์„ ํ™•์ธํ•˜์˜€๋‹ค. ๋ฐฐ์ถœํŠน์„ฑ์˜ ์ง€ํ‘œ์ธ EINOx(Emission Index of NOx)์™€ ํ™”์—ผ์ฒด๋ฅ˜์‹œ๊ฐ„(flame residence time) ๋ถ„์„์„ ์œ„ํ•ด์„œ pinch-off ํ™”์—ผ์˜ ๋†’์ด๋ฅผ ์ฃผํ™”์—ผ๊ณผ ํฌ์ผ“ํ™”์—ผ์œผ๋กœ ์„ธ๋ถ„ํ™”ํ•˜์—ฌ ์ •์˜ํ•˜์˜€๋‹ค. ์„ธ๋ถ„ํ™”ํ•œ ํ™”์—ผ๊ธธ์ด์™€ ํ™”์—ผ์ฒด๋ฅ˜์‹œ๊ฐ„ ๋ถ„์„ ๊ฒฐ๊ณผ, ํ™”์—ผ์ฒด๋ฅ˜์‹œ๊ฐ„์ด ๊ฐ์†Œํ•จ์— ๋”ฐ๋ผ ์งˆ์†Œ์‚ฐํ™”๋ฌผ ๋ฐฐ์ถœ์ด ์ €๊ฐ๋˜๋Š” ๊ฒฝํ–ฅ์€ ์ผ์น˜ํ•˜์ง€๋งŒ, ๊ฐ€์ง„์ฃผํŒŒ์ˆ˜์— ๋”ฐ๋ฅธ ๊ฒฝํ–ฅ์„ ๋”ฐ๋ฅด์ง€๋Š” ์•Š์•˜๋‹ค. ๋”ฐ๋ผ์„œ ํ™”์—ผ์ฒด๋ฅ˜์‹œ๊ฐ„๋งŒ์œผ๋กœ ์งˆ์†Œ์‚ฐํ™”๋ฌผ ๋ฐฐ์ถœ ํŠน์„ฑ์„ ๋ถ„์„ํ•˜๋Š”๋ฐ์— ๊ทธ ํ•œ๊ณ„์ ์ด ์žˆ์Œ์„ ๋ฐํ˜”๋‹ค. ์ŠคํŠธ๋กค ์ˆ˜์™€ EINOx์˜ ์ƒ๊ด€๊ด€๊ณ„ ๋ถ„์„ ๊ฒฐ๊ณผ ์ŠคํŠธ๋กค ์ˆ˜๊ฐ€ ๋‹ค๋ฆ„์—๋„ EINOx ๋ฐฐ์ถœ์ด ๊ฐ™์€ ๊ฒฝํ–ฅ์„ ํ™•์ธํ•˜์˜€๋‹ค. ์ด๋Š” ํ™”์—ผ๊ธธ์ด๋Š” ์ฃผํ™”์—ผ ๋˜๋Š” ํฌ์ผ“ํ™”์—ผ ์–ด๋Š ๊ฒƒ์„ ์„ ํƒํ•˜์—ฌ๋„ ๊ฒฝํ–ฅ์„ฑ ๋ถ„์„์— ๋ฌด๊ด€ํ•จ์„ ์˜๋ฏธํ•œ๋‹ค. ์ด๋Ÿฌํ•œ ๊ฒฝํ–ฅ์„ฑ ๊ฒ€์ฆ์„ ์œ„ํ•ด ํ™”์—ผ์ฒด๋ฅ˜์‹œ๊ฐ„์œผ๋กœ ์ •๊ทœํ™”ํ•œ EINOx๊ฐ€ 1/2-power๋ฅผ ์ž˜ ๋”ฐ๋ฅด๋Š” ๊ฒƒ์„ ํ™•์ธํ•˜์˜€๋‹ค. Pinch-off์™€ nonpinch-off์กฐ๊ฑด์—์„œ strain rate๊ณผ shear stress ์ƒ๊ด€๊ด€๊ณ„ ๋ถ„์„์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ์—ฐ๋ฃŒ์™€ ๊ณต๊ธฐ ์†๋„๊ฐ€ ๊ฐ™์•„ ์ด๋ก ์ ์œผ๋กœ shear stress๊ฐ€ ์—†๋Š” ์กฐ๊ฑด์„ ๊ธฐ์ค€์œผ๋กœ ์—ฐ๋ฃŒ์™€ ๊ณต๊ธฐ ์†๋„๋ฅผ ๊ฐ๊ฐ ๋ณ€ํ™”์‹œํ‚ค๋ฉฐ shear stress๋ฅผ ์ƒ์„ฑํ•˜์˜€๋‹ค. ์—ฐ๋ฃŒ์™€ ๊ณต๊ธฐ์†๋„์˜ ์œ ๋™๊ฒฝ๊ณ„์ธต ๋ถ„์„์„ ์œ„ํ•ด์„œ OH* ์ž๋ฐœ๊ด‘๊ณผ PIV ๋™์‹œ๊ณ„์ธก์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ๋‹ค์–‘ํ•œ ์—ฐ๋ฃŒ์™€ ๊ณต๊ธฐ์†๋„์— ๋”ฐ๋ผ pinch-off์˜ ๋งคํ•‘์„ ์ˆ˜ํ–‰ํ•˜์—ฌ ๋ฌผ๋ฆฌ์ ์ธ ๊ฒฝ๊ณ„๋ฅผ ํ™•์ธํ•˜์˜€๋‹ค. ์—ฐ๋ฃŒ ์†๋„ ์ฆ๊ฐ€์— ๋”ฐ๋ฅธ pinch-off ์กฐ๊ฑด์—์„œ shear ํšจ๊ณผ๊ฐ€ ์ฆ๊ฐ€ํ•จ์— ๋”ฐ๋ผ strain rate๊ฐ€ ๊ธฐ์ค€ ๋ฐ์ดํ„ฐ์— ๋น„ํ•ด ์•ฝ 80% ์ฆ๊ฐ€ํ•˜์˜€์œผ๋ฉฐ, shear stress๋Š” 15% ์ฆ๊ฐ€ํ•˜์˜€๋‹ค. ์—ฐ๋ฃŒ์†๋„๋ฅผ ๋” ์ฆ๊ฐ€์‹œ์ผœ nonpinch-off ์กฐ๊ฑด์—์„œ ๊ณ„์ธกํ•œ ๊ฒฐ๊ณผ shear ํšจ๊ณผ๋Š” ๋” ์ฆ๊ฐ€ํ•˜์˜€์ง€๋งŒ strain rate์€ ๊ธฐ์กด๋ฐ์ดํ„ฐ์™€ ๋น„๊ตํ–ˆ์„ ๋•Œ 50% ๊ฐ์†Œํ•˜์˜€๊ณ  shear stress๋Š” 3.3๋ฐฐ ์ฆ๊ฐ€ํ•˜์˜€๋‹ค. ๊ณต๊ธฐ์†๋„ ์ฆ๊ฐ€์— ๋”ฐ๋ฅธ nonpinch-off ์กฐ๊ฑด์—์„œ strain rate๊ณผ shear stress์˜ ์ƒ๊ด€๊ด€๊ณ„๋ฅผ ๋ถ„์„ํ•˜์—ฌ ๊ฒฝํ–ฅ์„ฑ์„ ์žฌ๊ฒ€์ฆํ•˜์˜€๋‹ค. ๊ฒฐ๊ณผ์ ์œผ๋กœ strain rate์ด ์ฃผ์š”ํ•˜๊ฒŒ ์˜ํ–ฅ์„ ๋ฏธ์น  ๋•Œ shear stress๋Š” ๊ฐ์†Œ๋˜๋Š” ๊ฒฝํ–ฅ์ด๋ฉฐ, shear stress๋Š” pinch-off๋ฅผ ์ œ์–ดํ•  ์ˆ˜ ์žˆ๋Š” ํŒŒ๋ผ๋ฏธํ„ฐ๋กœ์„œ ํ™œ์šฉ๊ฐ€๋Šฅ์„ฑ์„ ํ™•์ธํ•˜์˜€๋‹ค.Although the cause and mechanism of combustion instability have not been elucidated yet, it is known that the presence or absence of combustion instability is determined by the interaction of reactant flow perturbation, heat release perturbation, and perturbation due to the acoustic boundary of the combustion chamber. When these three perturbations form a positive feedback loop, the probability of combustion instability increases, and when a negative feedback loop is formed, the probability decreases. Therefore, to reduce the appearance of combustion instability, it is essential to identify the conditions required for and the factors influencing combustion instability. In this study, the correlation between heat emission perturbation and velocity perturbation was investigated among the factors governing combustion instability. For pinch-off flames, a phenomenon in which flames are separated under specific acoustic excitation conditions, the mechanism of combustion instability was investigated by simultaneous OH-planar laser-induced fluorescence (PLIF) and particle image velocity (PIV) laser measurements In addition, the nitrogen oxide (NOx) emission and the flow characteristics were analyzed. OH* chemiluminescence and OH-PLIF laser measurements were used for flame structure analysis, and simultaneous OH-PLIF and PIV measurements were used for flow field characterization. A photomultiplier tube (PMT) was used to measure the heat release needed to calculate the flame transfer function (FTF). The flow boundary layer between the fuel and air was also analyzed. To predict combustion instability, we conducted a comparative study of the response characteristics and dynamic characteristics of non-premixed and premixed flames generated by acoustic excitation. Two flames with different combustion reactions have different dynamic behavior characteristics, depending on acoustic excitation. Non-premixed flames show acoustically created waves projected from the flame surface, with a flapping dynamic behavior, and are flame-shaped with an open flame tip. On the other hand, the premixed flame from the single nozzle fluctuates vertically with a conical shape. For the non-premixed flame, the number of modulations on the flame surface increases with increasing excitation frequency, but the flame structure does not change significantly. The flame transfer function analysis by measuring the heat release rate of both flames during acoustic excitation revealed that the non-premixed and premixed flames showed nonlinear and linear results, respectively. By introducing the flame height and the Strouhal number (St number), correlation analysis between heat release and flame structure was performed and the results were compared with those of numerical studies. For non-premixed flames, the nonlinearity was verified by the numerical analysis results in the velocity perturbation of 20% or more. The numerical analysis and the premixed flame results were consistent but showed a locally different tendency. For premixed flames, the Strouhal number calculation does not consider the flame surface curvature, flame propagation speed, and flame tip shape. A more accurate Strouhal number analysis will be possible if these factors are included in the analysis. Various flame structure analyses were conducted in terms of the velocity perturbation intensity and the excitation frequency during acoustic excitation in a non-premixed flame. The non-premixed flame is one example of the Buck-Schumann flame (B-S flame). A pinch-off flame is defined as a phenomenon in which the flame is cut off; the flame attached to the nozzle is defined as the main flame, and the separated flame is defined as the pocket flame. It was confirmed that a pinch-off flame appears in a constant range of excitation frequencies and velocity perturbation intensities. Simultaneous OH PLIF and PIV measurements were performed to investigate the mechanism of the pinch-off flame. By mapping the flame structure in terms of excitation frequency and velocity perturbation intensity, it was classified into three dynamic behaviors. We observed a flickering flame with a large perturbation in the vertical direction in the low-frequency range, a pinch-off flame in the mid-frequency range, and a wrinkled flame with a modulated surface in the high-frequency range. The double dipole vortex caused by the tidal flow during acoustic excitation in the non-reactive field flow was confirmed by Mie scattering analysis. The inflow of air by the vortical structure was found to cause the flame deformation when the flame in the reaction field was pinched off, and a strong strain rate was observed in the flame neck. Accordingly, it was confirmed that the pinch-off flame was an interaction between the inflow of external air by the vortical structure and the high strain rate. NOx and carbon monoxide (CO) emission characteristics were analyzed considering the pocket flame separated from the pinch-off flame. With increasing velocity perturbation intensity, the mixing intensity of the fuel and the oxidizer increases and thus, the amount of NOx emitted decreases. With good mixing of the fuel and oxidizer and complete combustion, NOx emissions are reduced. On the other hand, CO emissions increased with increasing velocity perturbation intensity, but it was confirmed that the emissions were very small. The height of the pinch-off flame was subdivided into the main flame and the pocket flame to analyze the two emission characteristics, viz. the emission index of NOx (EINOx) and the flame residence time. The subdivided flame height and flame residence time analysis showed the same trend of reducing NOx emissions as the flame residence time decreased, but it did not follow the trend of the excitation frequency. Therefore, it was concluded that there is a limit to the analysis of NOx emission characteristics when using only the flame residence time. The correlation analysis between Strouhal number and EINOx confirmed that the EINOx value was the same even though the Strouhal number was different. This means that the flame height is independent of the trend analysis whether the main flame or the pocket flame is selected. To verify this tendency, it was confirmed that EINOx normalized by flame residence time followed 1/2-power well. Strain rate and shear stress correlation analyses were performed under pinch-off and non-pinch-off conditions. The fuel and air velocities are the same, and by changing these, the shear stress was generated based on the condition of no theoretical shear stress. Simultaneous measurements of OH* chemiluminescence and PIV was performed for boundary layer flow analysis of fuel and air velocity. The physical boundary was confirmed by performing pinch-off mapping for various fuels and air velocities. As the shear effect increased under the pinch-off condition with increasing fuel velocity, the strain rate increased by ~80% compared to the reference data, and the shear stress increased by 15%. Under the non-pinch-off condition, the shear effect was further increased by further increasing the fuel velocity, but the strain rate was reduced by 50% compared to the previous data, and the shear stress was increased 3.3 times. The tendency was also verified by analyzing the correlation between strain rate and shear stress under non-pinch-off conditions with increasing air velocity. Shear stress tends to decrease when strain rate has a major influence, and the applicability of shear stress, as a parameter to control pinch-off, was confirmed.ABSTRACT i LIST v LIST OF FIGURES x LIST OF TABLES xv NOMENCLATURE xvi CHAPTER 1 INTRODUCTION 1 1.1 Background 1 1.2 Combustion instability 4 1.3 Flame transfer function (FTF) 5 1.4 Acoustic excitation in non-premixed flame 7 1.5 Strain rate and local flame extinction in non-premixed flame 9 1.6 Motivation 10 1.7 Objectives 11 1.8 Outline 12 CHAPTER 2 EXPERIMENTAL AND MEASUREMENT SYSTEMS 13 2.1 Combustor and nozzles 13 2.2 Flame imaging 16 2.2.1. Chemiluminescence Spectroscopy 16 2.2.2. OH planar laser-induced fluorescence (OH PLIF) measurement 19 2.2.3. OH PLIF system 22 2.2.4. High-speed OH PLIF system 23 2.2.5. Particle image velocimetry (PIV) measurement 26 2.2.6. Simultaneous measurement of PIV and OH PLIF system 30 2.3 Flame Transfer Function (FTF) 32 2.4 NOx measurement system 34 CHAPTER 3 COMPARISON OF FLAME RESPONSE CHARACTERISTICS BETWEEN NON-PREMIXED FLAMES AND PREMIXED FLAMES OF UNDER ACOUSTIC EXCITATION 35 3.1 Objectives 35 3.2 Experimental setup and methodology 38 3.3 Flame appearance comparison between the non-premixed flame and the premixed flame 42 3.4 Flame dynamic characteristics under acoustic excitation of nonpremixed flame 44 3.5 Flame dynamic characteristics under acoustic excitation of premixed flame 48 3.6 Comparison of the flame response characteristics between nonpremixed and premixed flames 55 CHAPTER 4 PINCH-OFF PROCESS OF BURKE-SCHUMANN FLAME UNDER ACOUSTIC EXCITATION 63 4.1 Objectives 63 4.2 Experimental setup and methodology 66 4.3 Flame response characteristics under various excitation frequencies. 71 4.4 Flame response characteristics of pinch-off process at 80 Hz 73 4.5 Flame response characteristics at pinch-off boundary 76 4.6 Vortex-flame interaction and strain rate analysis for the pinch-off mechanism 79 CHAPTER 5 NOX EMISSION CHARACTERISTICS OF PINCH-OFF FLAME UNDER ACOUSTIC EXCITATION 86 5.1 Objectives 86 5.2 Experimental setup and methodology 89 5.3 Global appearance characteristics of non-premixed flame under acoustic excitation 93 5.4 Effects of acoustic excitation on EINOx and CO concentration 98 5.5 Effects of velocity perturbation intensity (u'/u) and forcing frequency on main flame (FM) and net hot product (FN) 102 5.6 Effects of Strouhal number and forcing frequency on flame residence time (ฯ„res) 104 CHAPTER 6 EFFECTS OF STRAIN RATE AND SHEAR STRESS ON STRUCTURE OF PINCH-OFF AND NON-PINCH-OFF FLAMES 108 6.1 Objectives 108 6.2 Experimental setup and method 110 6.3 Flame response characteristics according to fuel and air bulk Velocity 114 6.4 Characteristics of pinch-off flame with increasing fuel bulk velocity 117 6.5 Characteristics of non-pinch-off flame by increasing fuel bulk velocity 120 6.6 Characteristics of non-pinch-off flame with increasing air bulk velocity 123 CHAPTER 7 CONCLUSION 126 7.1 Conclusions 126 7.1 Limitation and future work 128 REFERENCES 129 ABSTRACT IN KOREAN 145๋ฐ•

    Reduced Order Models and Large Eddy Simulation for Combustion Instabilities in aeronautical Gas Turbines

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    Increasingly stringent regulations as well as environmental concerns have lead gas turbine powered engine manufacturers to develop the current generation of combustors, which feature lower than ever fuel consumption and pollutant emissions. However, modern combustor designs have been shown to be prone to combustion instabilities, where the coupling between acoustics of the combustor and the flame results in large pressure oscillations and vibrations within the combustion chamber. These instabilities can cause structural damages to the engine or even lead to its destruction. At the same time, considerable developments have been achieved in the numerical simulation domain, and Computational Fluid Dynamics (CFD) has proven capable of capturing unsteady flame dynamics and combustion instabilities for aforementioned engines. Still, even with the current large and fast increasing computing capabilities, time remains the key constraint for these high fidelity yet computationally intensive calculations. Typically, covering the entire range of operating conditions for an industrial engine is still out of reach. In that respect, low order models exist and can be efficient at predicting the occurrence of combustion instabilities, provided an adequate modeling of the flame/acoustics interaction as appearing in the system is available. This essential piece of information is usually recast as the so called Flame Transfer Function (FTF) relating heat release rate fluctuations to velocity fluctuations at a given point. One way to obtain this transfer function is to rely on analytical models, but few exist for turbulent swirling flames. Another way consists in performing costly experiments or numerical simulations, negating the requested fast prediction capabilities. This thesis therefore aims at providing fast, yet reliable methods to allow for low order combustion instabilities modeling. In that context, understanding the underlying mechanisms of swirling flame acoustic response is also targeted. To address this issue, a novel hybrid approach is first proposed based on a reduced set of high fidelity simulations that can be used to determine input parameters of an analytical model used to express the FTF of premixed swirling flames. The analytical model builds on previous works starting with a level-set description of the flame front dynamics while also accounting for the acoustic-vorticity conversion through a swirler. For such a model, validation is obtained using reacting stationary and pulsed numerical simulations of a laboratory scale premixed swirl stabilized flame. The model is also shown to be able to handle various perturbation amplitudes. At last, 3D high fidelity simulations of an industrial gas turbine powered by a swirled spray flame are performed to determine whether a combustion instability observed in experiments can be predicted using numerical analysis. To do so, a series of forced simulations is carried out in en effort to highlight the importance of the two-phase flow flame response evaluation. In that case, sensitivity to reference velocity perturbation probing positions as well as the amplitude and location of the acoustic perturbation source are investigated. The analytical FTF model derived in the context of a laboratory premixed swirled burner is furthermore gauged in this complex case. Results show that the unstable mode is predicted by the acoustic analysis, but that the flame model proposed needs further improvements to extend its applicability range and thus provide data relevant to actual aero-engine

    Combining analytical models and LES data to determine the transfer function from swirled premixed flames

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    A methodology is developed where the acoustic response of a swirl stabilized flame is obtained from a reduced set of simulations. Building upon previous analytical flame transfer functions, a parametriza- tion of the flame response is first proposed, based on six independent physical parameters: a Strouhal number, the mean flame angle with respect to the main flow direction, the vortical structures convection speed, a swirl intensity parameter, a time delay between acoustic and vortical perturbations, as well as a phase shift between bulk and local velocity signals. It is then shown how these parameters can be de- duced from steady and unsteady simulations. The methodology is applied to a laboratory scale premixed swirl stabilized flame exhibiting features representative of real aero-engines. In this matter, cold and re- active flow Large Eddy Simulations are first validated by comparing results with reference data from experiments. The high fidelity simulations are seen to be able to capture the flame structure and velocity profiles at different locations while forced flame dynamics for the frequency range of interest also match the experimental data. From the same analytical transfer function model, three methodologies of increas- ing complexity are presented for the determination of the model parameters, depending on the available data or computational resources. A first estimation of the flame acoustic response is obtained by evalu- ating parameters from a single stationary flame simulation in conjunction with analytical estimations for the acoustic-convective time delay. Flame dynamics and swirl related parameters can then be determined from a series of robust treatments on pulsed simulations data to improve the model accuracy. It is shown that good qualitative agreement for the flame transfer function can be obtained from a single non-forced simulation while quantitative agreement over the frequency range of interest can be obtained using ad- ditional reactive or non-reactive pulsed simulations at one single forcing frequency corresponding to a local gain minimum. The method also naturally handles different perturbation levels
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