Concepts, Aspects, and Applications Related to the Swirling Flows - Essay Example

?Running Head: Chapter Introduction Chapter Introduction Chapter Introduction Background of the Research This study is about analysing the various mechanistic behaviours of the swirling flow when released from a jet system inside the state of immersion in water (i.e. a liquid substance). The main focus is on the combustion system characteristics of the swirling flow from a fluid mechanics’ point of view. Introduction to Swirling Flows Swirling flows are generally characterised by the formation of vortex, which can be seen in natural whirlpools, hurricanes, tornados, etc. and also in various engineering applications. [1] Aerodynamic drag induced by lift of the wings of an aircraft may also give rise to swirling flows in air. In the engineering applications, particularly where combustion systems are entailed, swirling flows can be defined as continuous jets of fluid in uniform transverse flow that have dynamic and three dimensional (3D) structures. [2] According to a basic definition from Lilley (1977), “Swirling flows result from the application of a spiraling motion, with a swirl velocity component (also known as a tangential or azimuthal velocity component) being imparted to the flow via the use of swirl vanes, in axial-plus-tangential entry swirl generator or direct tangential entry into the combustion chamber.” [3] Figure – 1: Example of swirling flow in an artificial test case [4] Why Study Swirling Flow? According to experts like Gallaire, Rott and Chomaz, there have been only a few studies in the academic world which have dealt with the monitoring, control and analysis of complex fluid mechanical systems involving tubes and swirling jets. Also, the competitions between the axisymmetric and helical breakdown patterns of vortices in a swirling flow are still a major area of scientific exploration, which cannot be successfully accomplished without a complete understanding of Central Recirculation Zone (CRZ) and reported formations of Precessing Vortex Core (PVC) inside the swirling flow. [1, 3, 5] In combustion chamber and gas turbine applications, swirl flame stabilisation is widely used. Lean premixed and non-premixed systems are the major categories of these applications, where 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. Overview of Some Major Swirling Flow Phenomena 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). Vortex Breakdown: 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.” [1] Sudden Expansion and Bluff Body Flow: 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. [1, 6] 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. [1] 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, D refers to the diameter of the air flow pipe, d to the axisymmetric bluff body diameter located at the end of the pipe, and Do will be the diameter of a round enclosure, with d < D < Do (where CRZ means Central Recirculation Zone and ORZ means Outer Recirculation Zone). [1] Central Recirculation Zone (CRZ): The tangential velocity component in the fluid velocity field gives rise to ascending centrifugal pressure. Moreover, the pressure near the central axis of the flow jet is usually lowered than the pressure external to the jet system (whereto the jet is directed to emanate). Consequently, the tangential velocity decays along the axis. This also leads to the decay of radial distribution of the centrifugal pressure. Hence, an axial stress gradient is formed in the central area of the swirling stream towards the swirl burner. This gives rise to reverse flow and the Central Recirculation Zone (CRZ) is formed. [8, 9] Figure – 3: Schematic diagram showing the formation of Central Recirculation Zone. [8] Please refer to Figure – 2 as well. Precessing Vortex Core (PVC): PVC or Precessing Vortex Core is generally reported to be found at the edge of the zone of reverse flow. It is formed when the region of forced vortex contained by the flow attains instability and the rotational axis of the flow within the swirling recirculation region begins to precess about the axis of swirl symmetry. Consequently, the vortex breakdown loses its symmetry and becomes extremely time-dependent due to PVC. [1] Motivation behind and Aims of the Study This research is aimed to examine and analyse flow statistics of the swirling flow particularly at the exit of a fluid jet and study the related fluid mechanics. Two configurations will be set up. In the first, the experimental jet will hit a bluff body. In the second, the experimental jet will be allowed to emanate without the bluff body. Further, the study will examine whether the PVC (which has been reported by many scientists) exists in the swirling flow or not. The experimental analysis will involve immersion of the swirling jet system inside a water tunnel. Also, fluid mechanistic behaviours related to swirling flow like vortex breakdown, bluff body flow, CRZ, etc. will be studied in the course of the experiment. Scope of the Study The study will elucidate the dynamics of the swirling flow at the exit of a jet system. It will also be helpful in understanding whether PVC exists in the swirling flow or not. Effects of the combustion flame on the possible PVC will also be examined. If the experimental analysis indicates that PVC along with CRZ do exist, then it will be further examined that whether the two phenomena interact with each other or not; and in the case they interact, then how. The study will reveal flow statistics related to the fluid mechanics of a swirling flow with special reference to combustion chamber technologies. This will be helpful to understand the working of turbomachinery like internal combustion engines. Since the jet system of the swirling flow is immersed inside a liquid substance (i.e. water), the experimental set up will assist in analysing the dynamic behaviours of a gaseous or liquid state swirling flow when released into a closed environment of liquid fluid. Applications of Swirling Flow Swirling flows have diverse applications. Swirling flow of cooling air is used in enclosed rotor-stator systems [10]. Swirling flow is also utilised to mobilise fuel fluids inside pressurised gas turbine combustion facilities [11]. Fluid dynamics generated by swirlers are extensively used in turbomachinery and related flow research [12]. Also, fluid motions induced by swirling help in increasing the efficiency of chemical reactions inside closed combustion systems [1, 13]. In the case of this study, the swirling flow jet system is immersed inside water. Such research can particularly help in finding out temperature maintaining techniques for coolants in the cooling system of fast breed nuclear reactors as well [14] Methodology of Research Stereoscopic Particle Image Velocimetry (Stereo PIV): Stereoscopic Particle Image Velocimetry or Stereo PIV is an advanced version of standard Particle Image Velocimetry (PIV) technique which can take extremely fast and intricate snapshots of the motion behaviour of tracer particles seeded inside the swirling flow. It can help the analyst to examine a 3rd, out of the plane component of the velocity of tracer particles. It is a two dimensional (2D) PIV system which can capture the tracer particles’ 3rd velocity vector component too. Hence, it can be defined as a 2D but three-component (3C) or 2D/3C PIV technique. Stereo PIV utilises digital imaging system that has a dual camera configuration. [12, 15] Proper Orthogonal Decomposition (POD): Proper orthogonal decomposition (POD) is a technique which can be used to develop a modal decomposition framework from an assortment of signals that are measured from a dynamic structure like the swirling flow [2]. Interpretation of POD is built on expansion theorem. The physical and statistical construal obtained using POD helps the analyst to express the responses of a dynamic structure “as a linear sum of normal modes of that structure” [16] which can further be exploited in conjunction with various other measurement and analysis techniques. The flowing equation is the basic expression used for POD: [16] Using a Combination of PIV and POD to Analyse the Swirling Flow: When PIV measurements are deployed to analyse an exceedingly turbulent or vibrating swirling flow, turbulent fluctuations are found to coexist with the unsteady swirling flow motion. In the case Stereo PIV is used, these fluctuations can be tracked more prominently in 3D vector fields along with the correlative arrays of unsteady motion snapshots. POD can be used to take apart these two contributory factors that affect the total energy [2]. Hence, PIV results and POD technique can be combined with vortex identification functions. Subsequently, the functions can further be utilised to discover the locations of the boundary and centre of the vortex on account of the 3D velocity vector fields as derived by Stereo PIV. [16, 17] Application of MATLAB: MATLAB is an efficient computer language which can manage multimode computing clusters facilitating rapid acceleration of scientific computation with the help of complex algorithms and distributed arrays of complicated datasets. MATLAB can be described as a technique that can deploy parallel computing even in standard ICT infrastructures. [18, 19] Recently in 2011, Bistrian and his colleagues have used MATLAB to analyse helical vortex breakdown in swirling flows which is a self-stimulated instability of the swirls. They successfully obtained detailed data of vortex stability patterns as investigated in the fluid dynamics that are “encountered in the draft tube cone of hydraulic Francis turbines operated far from the best efficiency.” [18] Organisation of the Thesis In this thesis, different concepts, aspects and applications related to the swirling flow are discussed in the introductory Chapter 1. The literature review is presented in Chapter 2 examine the key physical mechanisms of vortex breakdown, CRZ, PVC, etc. and related fluid dynamics research literature. Chapter 3 discusses the analysis techniques of PIV (with focus on Stereo PIV). Research methodology and experimental methods are discussed in Chapter 4. Chapter 5 will present the results and discussion and Chapter 6 will provide conclusions. 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. [3] Lilley, D.G. 1977, “Swirl flows in combustion: A review,” AIAA Journal, 15, pp. 1063-1071. [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. [6] Valera-Medina, A, Syred, N. and Griffiths, A. 2009, “Visualisation of isothermal large coherent structures in a swirl burner,” Combustion and Flame, 156, pp. 1723-1734. [7] Lopez, J.M. 2012, “Three-dimensional swirling flows in a tall cylinder driven by a rotating endwall,” Physics of Fluids, 24, pp. 14101-14109. [8] Gupta, A.K., Lilley, D.G. and Syred, N. 1984, Swirl Flows, Abacus Press, Turnbridge Wells. [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. [12] Woisetschlager, J. and Gottlich, E. 2008, “Recent applications of Particle Image Velocimetry to flow research in Thermal Turbomachinery,” Particle Image Velocimetry: New Developments and Recent Applications, Topics in Applied Physics, A. Schroder and C.M. Willert, eds., Springer, New York, pp. 223-244. [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, Julich, 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. Read More