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Introduction Chapter of my Thesis, about swirling flow mainly Essay Example

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Introduction Chapter of my Thesis, about swirling flow mainly

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The processes of flame stabilisation, as functions of thermo-acoustic flux, combustor geometry and fuel type, are yet to be fully understood and simplified. [1, 6] Besides, there are relatively fewer research papers available where tall cylinders have been used to generate swirling flows with instabilities directed to the 3D patterns of fluid dynamics. Hence, three dimensional characters of a swirl remain less explored than its unsteady axisymmetric features. [7] Therefore, swirling flows should be studied so that these gaps in engineering research can be properly abridged.Some of the major phenomena related to swirling flow that predominantly occur in combustion technologies are vortex breakdown, sudden expansion, bluff body flow, Central Recirculation Zone (CRZ) and reported formations of Precessing Vortex Core (PVC).Since swirling flow has a three dimensional structure, it possesses both axial and tangential velocity components in the 3D vector fields [1, 4]. When the Swirl number S is increased, strong coupling forces develop among the axial and tangential velocities. Together with strong inertial effects, swirl vortex is generated which is again broken down when the flow attains high Reynolds number. Ayache explains this process of vortex breakdown as an unstable flow with transient patterns that “occurs due to the instabilities present in swirl flows such as shear-layer instabilities (like Kelvin-Helmholtz instability) similar to axial jets and azimuthal shear-layer instabilities created by the radial gradient in azimuthal velocity.In order to gather intricate flow statistics, a bluff body may be introduced on which the jet of the swirling flow is impinged. This creates a Central Recirculation Zone (CRZ) at the rear of the fluid inlet; while a sudden-expansion is experienced with possible variations in the cross section of the jet stream or collision between the fluid particles around the outlet nozzle.Particularly in gas turbine combustors, self-excited oscillations involving recirculation zones and periodicity give rise to bluff body flows. Formation of recirculation zone due to sudden expansion behind an axisymmetric impediment or bluff body gives rise to dynamic behaviours of the fluid which are critical in maintaining flame stability. These dynamics are defined as bluff body flows.Figure – 2: Schematic diagram showing the state of sudden expansion and patterns of a bluff body flow. In the figure, as shown in Ayache, 2011, p. 7,

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References

[1] Ayache, S. 2011, “Simulations of turbulent swirl combustors,” Ph.D. Thesis, Selwyn College, University of Cambridge, Cambridge.

[2] Vernet, R., Thomas, L. and David, L. 2009, “Analysis and reconstruction of a pulsed jet in crossflow by multi-plane snapshot POD,” Experiments in Fluids, 47, pp. 707-720.

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[4] Sujudi, D. and Haimes, R., 1995, “Identification of swirling flow in 3-D vector fields,” Technical Report, Department of Aeronautics and Astronautics, MIT, Cambridge, MA.

[5] Gallaire, F., Rott, S. and Chomaz, J.-M. 2004, “Experimental study of a free and forced swirling jet,” Physics of Fluids, 16, pp. 2907-2917.

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[9] Syred, N. 2006, “A review of oscillation mechanisms and the role of the precessing vortex core (PVC) in swirl combustion systems,” Progress in Energy and Combustion Science, 32, pp. 93–161.

[10] Bricaurd, C., Richter, B., Dullenkopf, K. and Bauer, H.-J. 2005, “Stereo PIV measurements in an enclosed rotor-stator system with pre-swirled cooling air,” Experiments in Fluids, 39, pp. 202-212.

[11] Willert, C. et al. 2006, “Combined PIV and DGV applied to a pressurised gas turbine combustion facility,” Measurement Science and Technology, 17, pp. 1670-1679.

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[13] Stohr, M. et al. 2012, “Experimental study of vortex-flame interaction in a gas turbine model combustor,” Combustion and Flame, Article in Press, online at http://dx.doi.org/10.1016/j.combustflame.2012.03.020

[14] Someya, S., Li, Y. and Okamoto, K. 2010, “An entrained droplet by an underexpanded gas jet into water,” Proceedings of the 15th International Symposium on Applications of Laser Techniques to Fluid Mechanics, July 2010, Lisbon, Portugal.

[15] ILA, 2012, “2D/3C or Stereo PIV,” 2D/3C PIV | ILA GmbH, ILA – Intelligent Laser Applications GmbH, Jülich, http://www.ila.de/piv/piv-systems/2d3c0.html

[16] Han, S. and Feeny, B. 2003, “Application of proper orthogonal decomposition to structural vibration analysis,” Mechanical Systems and Signal Processing, 17, pp. 989-1001.

[17] Graftieaux, L., Michard, M. and Grosjean, N. 2001, “Combining PIV, POD and vortex identification algorithms for the study of unsteady turbulent swirling flows,” Measurement Science and Technology, 12, pp 1422-1429.

[18] Bistrian, D.A., Osaci, M. and Topor, M. 2011, “Numerical investigation of swirling flows stability using MATLAB distributed computing server on a windows operating system environment,” Annals of Faculty Engineering Hunedoana – International Journal of Engineering, IX, pp. 171-176.

[19] Higham, P.J. and Higham, N.J. 2005, MATLAB Guide, SIAM, Philadelphia.

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