Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session Q2: Swirl Flows: Reacting and Non-ReactingReacting
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Chair: Benjamin Emerson, Georgia Institute of Technology Room: 402 |
Tuesday, November 21, 2017 12:50PM - 1:03PM |
Q2.00001: Cross-Plane Near-Field Turbulence Structure of Swirling Jets Eric DeMillard, Jonathan Naughton Swirling jets are used in several industrial applications and are interesting from a fundamental view of turbulence. It has been shown that, as swirl increases past a threshold value, the single-point statistics in the jet significantly change suggesting changes in turbulence structure. To test this assumption, Stereoscopic Particle Image Velocimetry (SPIV) is applied to measure instantaneous velocity in cross-planes (the radial-azimuthal planes). Proper Orthogonal Decomposition (POD) is then applied to these measurements to identify large scale turbulence structure. The POD results allow for comparisons in structure to be made between non-swirling jets and swirling jets. POD results were previously obtained using measurements in the axial-radial plane of the near field, and a significant reordering in modal dominance was observed. In the present study, the measurements in the radial-azimuthal plane are axisymmetric, and the POD analysis takes advantage of the homogeneity in the azimuthal direction. Results obtained at several different axial locations are considered in order to understand how the turbulence structures develop and evolve in the near field. [Preview Abstract] |
Tuesday, November 21, 2017 1:03PM - 1:16PM |
Q2.00002: Vortex breakdown in gaseous swirling jets Antonio L Sanchez, Jaime Carpio, Forman A Williams Numerical integrations of the axisymmetric Navier-Stokes equations are employed to describe the structure of low-Mach-number gaseous swirling jets with jet-to-ambient density ratios $\rho_j/\rho_a$ of order unity. The integrations consider moderately large values of the Reynolds number on the order of a few hundred and values of the swirl ratio $S$ of order unity. Slender jets are found to exist for values of S below a critical value of order unity, at which vortex breakdown occurs. As in the case of constant density jets (Billant, Chomaz, and Huerre, JFM 1998), two different types of axisymmetric vortex breakdown are identified, namely, a bubble state and a cone configuration. The critical values of $S$ characterizing the existence of the different solutions are determined as a function of $\rho_j/\rho_a$. Additional computations based on the quasicylindrical approximation are employed to describe slender subcritical jets. The results indicate that the breakdown of the quasicylindrical approximation provides an accurate prediction for the transition from the slender solution to the bubble state, whereas a prediction for the transition to the cone state can be obtained by consideration of the structure of the flow at small distances from the jet exit. [Preview Abstract] |
Tuesday, November 21, 2017 1:16PM - 1:29PM |
Q2.00003: Simulation of vortex breakdown in swirling jets Pradeep Moise, Joseph Mathew Numerical simulations of laminar incompressible swirling jets have been carried out to study different types of vortex breakdown, including the commonly reported axisymmetric bubble and the lesser known conical breakdown. Existence of the latter type of breakdown was first discovered in experiments of Billant et al. (1998) who proposed that the bubble and conical breakdown exhibit bistability behaviour. This is confirmed by the present study, where it is shown that the conical breakdown coexists with bubble breakdown over a wide range of swirl strengths. A novel approach employing PDE-constrained optimization techniques (adjoint-based method) is formulated to elucidate the relation between bistable states. This is implemented by means of minimizing strengths of introduced initial velocity perturbations which trigger required transition from one state to another. Features of conical breakdown and their dependence on flow parameters are examined. Solutions of both breakdown types are tested with predictions of the conjugate state theory of Benjamin (1962) by investigating upstream propagation of introduced disturbances in subcritical regions of flow and the theory of Brown and Lopez (1990), by examining development of negative azimuthal vorticity in the flow. [Preview Abstract] |
Tuesday, November 21, 2017 1:29PM - 1:42PM |
Q2.00004: Lean premixed reacting flows with swirl and wall-separation zones in a contracting chamber Yuxin Zhang, Zvi Rusak, Shixiao Wang Low Mach number lean premixed reacting swirling flows with wall-separation zones in a contracting circular finite-length open chamber are studied. Assuming a complete reaction with high activation energy and chemical equilibrium behind the reaction zone, a nonlinear partial differential equation is derived for the solution of the flow stream function behind the reaction zone in terms of the inlet total enthalpy for a reacting flow, specific entropy and the circulation functions. Bifurcation diagrams of steady flows are described as the inlet swirl level is increased at fixed chamber contraction and reaction heat release. The approach is applied to an inlet solid-body rotation flow with constant profiles of the axial velocity, temperature and mixture reactant mass fraction. The computed results provide predictions of the critical inlet swirl levels for the first appearance of wall-separation states and for the size of the separation zone as a function of the inlet swirl ratio, Mach number, chamber contraction and heat release of the reaction. The methodology developed in this paper provides a theoretical feasibility for the development of the technology of swirl-assisted combustion where the reaction zone is supported and stabilized by a wall-separation zone. [Preview Abstract] |
Tuesday, November 21, 2017 1:42PM - 1:55PM |
Q2.00005: ABSTRACT WITHDRAWN |
Tuesday, November 21, 2017 1:55PM - 2:08PM |
Q2.00006: Reduced Order Modeling of Combustion Instability in a Gas Turbine Model Combustor Nicholas Arnold-Medabalimi, Cheng Huang, Karthik Duraisamy Hydrocarbon fuel based propulsion systems are expected to remain relevant in aerospace vehicles for the foreseeable future. Design of these devices is complicated by combustion instabilities. The capability to model and predict these effects at reduced computational cost is a requirement for both design and control of these devices. This work focuses on computational studies on a dual swirl model gas turbine combustor in the context of reduced order model development. Full fidelity simulations are performed utilizing URANS and Hybrid RANS-LES with finite rate chemistry. Following this, data decomposition techniques are used to extract a reduced basis representation of the unsteady flow field. These bases are first used to identify sensor locations to guide experimental interrogations and controller feedback. Following this, initial results on developing a control-oriented reduced order model (ROM) will be presented. The capability of the ROM will be further assessed based on different operating conditions and geometric configurations. [Preview Abstract] |
Tuesday, November 21, 2017 2:08PM - 2:21PM |
Q2.00007: Hydrodynamic Stability Analysis of the Externally Excited Axisymmetric Mode in Reacting, Swirling Jets Benjamin Emerson, Tim Lieuwen This study investigates the forced response characteristics of axisymmetric structures in density-stratified swirling jets. The reacting, swirling jet is an important canonical flow field for modern combustion systems. This work is motivated by the combustion instability problem for such systems, where acoustically excited vortical structures may drive oscillatory heat release of combustion. Previous hydrodynamics studies have shown that the stability of helical structures is highly sensitive to the swirl number. However, the combustion literature has shown that axisymmetric structures (in contrast to helical structures) are often responsible for most of the heat release response. Therefore, this work performs a spatial stability analysis to study the swirl number sensitivity of the forced response of the axisymmetric mode. A spatio-temporal analysis is conducted in tandem to investigate the swirl number sensitivity of the impulse response of this mode. The results show that at low values of the swirl number, the axisymmetric mode stability is a weak function of the swirl number, but that new modes and stability bifurcations appear at high swirl numbers. [Preview Abstract] |
Tuesday, November 21, 2017 2:21PM - 2:34PM |
Q2.00008: An investigation of non-parallel flow effects on the linear stability of turbulent reacting swirling jets Christopher Douglas, Travis Smith, Benjamin Emerson, Timothy Lieuwen This work considers the linear hydrodynamic stability framework for reacting swirling jets in a premixed swirl combustor. In typical local stability analyses of such flows, a significant amount of the base flow field information is neglected by the parallel flow assumption, despite the often highly non-parallel nature of such flows. Implementation of a global stability analysis addresses this issue. However, global stability analysis requires suitable streamwise boundary conditions which are often non-trivial and/or physically dubious. Additionally, global stability methods are significantly more costly than weakly-global techniques which are based on solutions to a sequence of locally-parallel problems and linked using WKBJ theory. This motivates an effort to identify the pitfalls of locally-parallel methods and establish techniques for improved hydrodynamic modeling of spreading swirling jets without requiring the imposition of streamwise boundary conditions. Therefore, we analyze experimental combustor data to demonstrate the effects of non-parallelism that a locally-parallel analysis would neglect, and we propose a modified framework to address this issue. [Preview Abstract] |
Tuesday, November 21, 2017 2:34PM - 2:47PM |
Q2.00009: Influence of the burner swirl on the azimuthal instabilities in an annular combustor Marek Mazur, Håkon Nygård, Nicholas Worth, James Dawson Improving our fundamental understanding of thermoacoustic instabilities will aid the development of new low emission gas turbine combustors. In the present investigation the effects of swirl on the self-excited azimuthal combustion instabilities in a multi-burner annular annular combustor are investigated experimentally. Each of the burners features a bluff body and a swirler to stabilize the flame. The combustor is operated with an ethylene-air premixture at powers up to 100 kW. The swirl number of the burners is varied in these tests. For each case, dynamic pressure measurements at different azimuthal positions, as well as overhead imaging of OH* of the entire combustor are conducted simultaneously and at a high sampling frequency. The measurements are then used to determine the azimuthal acoustic and heat release rate modes in the chamber and to determine whether these modes are standing, spinning or mixed. Furthermore, the phase shift between the heat release rate and pressure and the shape of these two signals are analysed at different azimuthal positions. Based on the Rayleigh criterion, these investigations allow to obtain an insight about the effects of the swirl on the instability margins of the combustor. [Preview Abstract] |
Tuesday, November 21, 2017 2:47PM - 3:00PM |
Q2.00010: Weakly nonlinear manipulation of unstable swirling flames Calum Skene, Ubaid Qadri, Peter Schmid Swirling flames are often prone to precessing instabilities with an azimuthal wavenumber of $m=-1.$ We explore the manipulation and response behavior from an axisymmetrix, external, harmonic driving by employing a weakly nonlinear framework for the interaction of the forcing and the flame's preferred response. A compact and efficient formulation of the underlying governing equations will be presented that involves Jacobian, Hessian and higher-order terms of a multi-valued Taylor expansion to arrive at a coupled Landau-type amplitude equation. This weakly nonlinear system will be investigated for its potential to suppress inherent precessing instabilities by axisymmetric open-loop control. The weakly nonlinear formalism captures selected wavenumber interactions as well as dominant finite-amplitude effects, while still avoiding the full treatment of nonlinear processes. [Preview Abstract] |
Tuesday, November 21, 2017 3:00PM - 3:13PM |
Q2.00011: ABSTRACT WITHDRAWN |
Tuesday, November 21, 2017 3:13PM - 3:26PM |
Q2.00012: Insights into flame-flow interaction during boundary layer flashback of swirl flames Rakesh Ranjan, Dominik Ebi, Noel Clemens Boundary layer flashback in swirl flames is a frequent problem in industrial gas turbine combustors. During this event, an erstwhile stable swirl flame propagates into the upstream region of the combustor, through the low momentum region in the boundary layer. Owing to the involvement of various physical factors such as turbulence, flame-wall interactions and flame-flow interactions, the current scientific understanding of this phenomenon is limited. The transient and three-dimensional nature of the swirl flow, makes it even more challenging to comprehend the underlying physics of the swirl flame flashback. In this work, a model swirl combustor with an axial swirler and a centerbody was used to carry out the flashback experiments. We employed high-speed chemiluminescence imaging and simultaneous stereoscopic PIV to understand the flow-flame interactions during flashback. A novel approach to reconstruct the three-dimensional flame surface using time-resolved slice information is utilized to gain insight into the flame-flow interaction. It is realized that the blockage effect imposed by the flame deflects the approaching streamlines in axial as well as azimuthal directions. A detailed interpretation of streamline deflection during boundary layer flashback shall be presented. [Preview Abstract] |
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