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 D21: ThermoacousticsAcoustics
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Chair: Luca Magri, University of Cambridge Room: 706 |
Sunday, November 19, 2017 2:15PM - 2:28PM |
D21.00001: Azimuthally spinning wave modes and heat release in an annular combustor Hakon Nygard, Marek Mazur, James R. Dawson, Nicholas A. Worth In order to reduce NOx emissions from aeroengines and stationary gas turbines the fuel-air mixture can be made leaner, at the risk of introducing potentially damaging thermo-acoustic instabilities. At present this phenomenon is not understood well enough to eliminate these instabilities at the design stage. Recently, the presence of different azimuthal modes in annular combustors has been demonstrated both experimentally and numerically. These naturally occurring instabilities in annular geometry have been observed to constantly switch between spinning and standing modes, making it more difficult to analyse the flame structure and dynamics. Very recently this issue was partially addressed using novel acoustic forcing to generate a standing mode. In the present study this concept has been developed further by creating an azimuthal array of loud speakers, which for the first time permits predominantly spinning modes to be set up inside the combustion chamber. The use of pressure and high speed OH* measurements enables the study of the flame dynamics and heat release rate oscillations of the combustor, which will be reported in the current paper. The ability to precisely control the azimuthal mode of oscillation greatly enhances our further understanding of the phenomenon. [Preview Abstract] |
Sunday, November 19, 2017 2:28PM - 2:41PM |
D21.00002: Phase-exchange thermoacoustic engine Avshalom Offner, Avishai Meir, Guy Z. Ramon Phase-exchange thermoacoustic engines are reliable machines holding great promise in converting heat from low grade heat sources to mechanical or electrical power. In these engines the working fluid is a gas mixture containing one condensable component, decreasing the temperature difference required for ignition and steady state operation. Our experimental setup consists of a vertical acoustic resonator containing a mixture of air-water vapor. Water evaporates near the heat source, condenses at the heat sink and is drawn back down by gravity and capillary forces where it re-evaporates, sustaining a steady state closed thermodynamic cycle. We investigated the stability limit, namely the critical point at which temperature difference in the engine enables onset of self-excited oscillations, and the steady state of the engine. A simple theoretical model was derived, describing mechanisms of irreversible entropy generation and production of acoustic power in such engines. This model captures the essence in the differences between regular and phase-exchange thermoacoustic engines, and shows good agreement with experimental results of stability limit. Steady state results reveal not only a dramatic decrease in temperature difference, but also an increase in engine performances. [Preview Abstract] |
Sunday, November 19, 2017 2:41PM - 2:54PM |
D21.00003: Passive control of thermoacoustic oscillations with adjoint methods Jose Aguilar, Matthew Juniper Strict pollutant regulations are driving gas turbine manufacturers to develop devices that operate under lean premixed conditions, which produce less NOx but encourage thermoacoustic oscillations. These are a form of unstable combustion that arise due to the coupling between the acoustic field and the fluctuating heat release in a combustion chamber. In such devices, in which safety is paramount, thermoacoustic oscillations must be eliminated passively, rather than through feedback control. The ideal way to eliminate thermoacoustic oscillations is by subtly changing the shape of the device. To achieve this, one must calculate the sensitivity of each unstable thermoacoustic mode to every geometric parameter. This is prohibitively expensive with standard methods, but is relatively cheap with adjoint methods. In this study we first present low-order network models as a tool to model and study the thermoacoustic behaviour of combustion chambers. Then we compute the continuous adjoint equations and the sensitivities to relevant parameters. With this, we run an optimization routine that modifies the parameters in order to stabilize all the resonant modes of a laboratory combustor rig. [Preview Abstract] |
Sunday, November 19, 2017 2:54PM - 3:07PM |
D21.00004: An adjoint-based sensitivity analysis of thermoacoustic network models Francesca Sogaro, Aimee Morgans, Peter Schmid Thermoacoustic instability is a phenomenon that occurs in numerous combustion systems, from rockets to land-based gas turbines. The acoustic oscillations of these systems are of significant importance as they can result in severe vibrations, thrust oscillations, thermal stresses and mechanical loads that lead to fatigue or even failure. In this work we use a low-order network model representation of a combustor system where linear acoustics are solved together with the appropriate boundary conditions, area change jump conditions, acoustic dampers and an appropriate flame transfer function. Special emphasis is directed towards the interaction between acoustically driven instabilities and flame-intrinsic modes. Adjoint methods are used to perform a sensitivity analysis of the spectral properties of the system to changes in the parameters involved. An exchange of modal identity between acoustic and intrinsic modes will be demonstrated and analyzed. The results provide insight into the interplay between various mode types and build a quantitative foundation for the design of combustors. [Preview Abstract] |
Sunday, November 19, 2017 3:07PM - 3:20PM |
D21.00005: Effects of Frequency spacing on Limit-cycle Mode Suppression of Coupled Unstable Thermoacoustic Modes Vishal Acharya, Timothy Lieuwen Combustion instabilities are typically characterized by a single dominant peak at the unstable frequency of the limit-cycle. However, this does not imply the presence of only one unstable mode in the system. Often, multiple linearly unstable modes in the system interact and couple through nonlinearities in the unsteady heat release rate oscillations. This coupling is controlled by several parameters such as: the mode shapes, temporal growth rates, modal frequencies and the frequency spacing. For the coupling between two unstable modes, when the frequencies are far apart (of the order of the frequencies themselves), the modal interactions can result in the suppression of one mode by the other under certain conditions eventually leading to the limit-cycle amplitude of the unsuppressed mode. However, in the case of frequency spacing that is small (of the order of the growth rate of the modes), the final limit-cycle amplitude for the unsuppressed mode is the same as before but for different conditions that are controlled by the new frequency spacing parameter. This implies that while the final limit-cycle amplitude for the two cases might be independent of frequency spacing, the limit-cycle stability in the case of closely-spaced modes is a strong function of the frequency spacing. [Preview Abstract] |
Sunday, November 19, 2017 3:20PM - 3:33PM |
D21.00006: Open-loop control of quasiperiodic thermoacoustic oscillations Yu Guan, Vikrant Gupta, Karthik Kashinath, Larry K.B. Li The open-loop application of periodic acoustic forcing has been shown to be a potentially effective strategy for controlling periodic thermoacoustic oscillations, but its effectiveness on aperiodic thermoacoustic oscillations is less clear. In this experimental study, we apply periodic acoustic forcing to a ducted premixed flame oscillating quasiperiodically at two incommensurate natural frequencies, $f_1$ and $f_2$. We find that (i) above a critical forcing amplitude, the system locks into the forcing by oscillating only at the forcing frequency $f_f$, producing a closed periodic orbit in phase space with no evidence of the original ${\mathbb{T}}^2$ torus attractor; (ii) the critical forcing amplitude required for lock-in decreases as $f_f$ approaches either $f_1$ or $f_2$, resulting in characteristic $\vee$-shaped lock-in boundaries around the two natural modes; and (iii) for a wide range of forcing frequencies, the system's oscillation amplitude can be reduced to less than 20\% of that of the unforced system. These findings show that the open-loop application of periodic acoustic forcing can be an effective strategy for controlling aperiodic thermoacoustic oscillations. [Preview Abstract] |
Sunday, November 19, 2017 3:33PM - 3:46PM |
D21.00007: Stability, receptivity and sensitivity of linear, periodic and chaotic flows: application to a thermoacoustic system Luca Magri, Qiqi Wang We present and compare three methods to calculate the stability and sensitivity of linear, periodic and chaotic time-delayed thermoacoustic systems. First, eigenvalue analysis calculates the stability of a fixed point. The stability is governed by the Jacobian operator. Secondly, Floquet analysis calculates the stability of a nonlinear periodic solution. The stability is governed by the monodromy matrix, which is the linearized Poincare’ map built on the periodic attractor found by continuation. Thirdly, Lyapunov analysis calculates the stability of a chaotic solution. The stability is governed by the tangent operator, which is the linearized operator built on the unsteady chaotic solution. The three methods are then applied to the adjoint operator, which provides information on the system’s receptivity to physical sources. Finally, the direct and adjoint Floquet and Lyapunov analyses are combined to calculate the thermoacoustic system’s sensitivity to feedback mechanisms. The application of adjoint Floquet and Lyapunov sensitivity analyses to thermoacoustic systems opens up new possibilities for the passive control of thermo-acoustic oscillations by providing gradient information that can be combined with constrained optimization algorithms to reduce the instability. [Preview Abstract] |
Sunday, November 19, 2017 3:46PM - 3:59PM |
D21.00008: Coupling modes between liquid/gas coaxial jets and transverse acoustic waves. Chad Helland, Cullen Hilliker, David Forliti The interactions between shear flows and acoustic disturbances plays a very important role in many propulsion and energy applications. Liquid jets, either independent or air assisted, respond to acoustic disturbances in a manner that alters the primary and secondary atomization processes. The current study focused on the response of an air-assisted liquid jet to disturbances associated with a transverse acoustic wave. The jet is placed in the pressure node (velocity antinode) region of the resonant mode shape. It has been shown in previous studies, under certain conditions, that the acoustic forces can cause the jet flow to distort and atomize. Both liquid and coaxial gas/ liquid jet flows have been shown to distort via acoustic forces. The purpose of the current study is to understand the predictive characteristics that cause the distortion behaviors of a liquid and coaxial jet flow, and how a how a coaxial flow affects the behavior. [Preview Abstract] |
Sunday, November 19, 2017 3:59PM - 4:12PM |
D21.00009: ABSTRACT WITHDRAWN |
Sunday, November 19, 2017 4:12PM - 4:25PM |
D21.00010: Numerical simulations of thermoacoustic waves in transcritical fluids employing the spectral difference approach Carlo Scalo, Mario Tindaro Migliorino, Jean-Baptiste Chapelier We investigate the stability properties of thermoacoustically unstable planar waves in transcritical fluids via high-fidelity Navier-Stokes simulations based on a Spectral Difference (SD) discretization coupled with the Peng-Robinson equation of state and Chung’s method for the fluid transport properties. A canonical thermoacoustically unstable standing-wave resonator filled with supercritical CO2 kept in pseudoboiling conditions in the stack is considered. Real fluid effects near the critical point are shown to boost thermoacoustic energy production, as also confirmed by companion eigenvalue analysis supporting the closure of the acoustic energy budgets. A kink in the eigenmode shape is observed at the location of pseudo phase change, consistent with the abrupt change in base impedance. The current study demonstrates a transformative approach to thermoacoustic energy generation, exploiting otherwise unwanted fluid dynamics instabilities commonly observed in aeronautical applications employing transcritical fluids. [Preview Abstract] |
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