Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A7: Acoustics I: Thermo-Acoustics |
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Chair: David Kassoy, University of Colorado, Boulder Room: 107 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A7.00001: Thermomechanical Response of a Gas to Spatially Resolved Power Deposition Transients David R. Kassoy Liquid propellant rocket engine (LPRE) instability is characterized by growing pressure oscillations that affect the integrity and performance of the system. Modeling and prediction have been topics of intense interest to designers for more than 60 years. LPRE combustion provides a wonderful opportunity to employ thermomechanical concepts and mathematical methodologies to quantify the response of combustion chamber gases to spatially distributed, transient thermal energy deposition. Nondimensional Euler equations, including a power deposition term in the energy equation are used to identify crucial parameters, time and length scales, as well as levels of energy deposition, relevant to LPRE performance. The objective is to provide first principles explanations of physical phenomena responsible for mechanical disturbances observed in operating LPRE's. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A7.00002: Modeling and analysis of thermoacoustic instabilities in an annular combustor Sandeep Murthy, Taraneh Sayadi, Vincent Le Chenadec, Peter Schmid A simplified model is introduced to study thermo-acoustic instabilities in axisymmetric combustion chambers. Such instabilities can be triggered when correlations between heat-release and pressure oscillations exist, leading to undesirable effects. Gas turbine designs typically consist of a periodic assembly of $N$ identical units; as evidenced by documented studies, the coupling across sectors may give rise to unstable modes, which are the highlight of this study. In the proposed model, the governing equations are linearized in the acoustic limit, with each burner modeled as a one-dimensional system, featuring acoustic damping and a compact heat source. The coupling between the burners is accounted for by solving the two-dimensional wave equation over an annular region, perpendicular to the burners, representing the chamber's geometry. The discretization of these equations results in a set of coupled delay-differential equations, that depends on a finite set of parameters. The system's periodicity is leveraged using a recently developed root-of-unity formalism (Schmid et al, 2015). This results in a linear system, which is then subjected to modal and non-modal analysis to explore the influence of the coupled behavior of the burners on the system's stability and receptivity. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A7.00003: Azimuthally forced flames in an annular combustor Nicholas Worth, James Dawson, Epaminondas Mastorakos Thermoacoustic instabilities are more likely to occur in lean burn combustion systems, making their adoption both difficult and costly. At present, our knowledge of such phenomena is insufficient to produce an inherently stable combustor by design, and therefore an improved understanding of these instabilities has become the focus of a significant research effort. Recent experimental and numerical studies have demonstrated that the symmetry of annular chambers permit a range of self-excited azimuthal modes to be generated in annular geometry, which can make the study of isolated modes difficult. While acoustic forcing is common in single flame experiments, no equivalent for forced azimuthal modes in an annular chamber have been demonstrated. The present investigation focuses on the novel application of acoustic forcing to a laboratory scale annular combustor, in order to generate azimuthal standing wave modes at a prescribed frequency and amplitude. The results focus on the ability of the method to isolate the mode of oscillation using experimental pressure and high speed OH* measurements. The successful excitation of azimuthal modes demonstrated represents an important step towards improving our fundamental understanding of this phenomena in practically relevant geometry. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A7.00004: ABSTRACT WITHDRAWN |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A7.00005: Acoustic Streaming and Thermal Instability of Flow Generated by Ultrasound in a Cylindrical Container Adam Green, Dong Ma, Jeffrey Marshall, Junru Wu A vertically orientated ultrasonic transducer contained within a closed cylindrical Pyrex tube was used to study acoustic streaming flow within in a cylindrical container. A PIV system incorporating fluorescent 1.5 micron seeding particles suspended in a mixture of Diethyle-Pthalate and Ethanol, which was indexed matched to Pyrex, was used to allow for undistorted PIV imaging within the Pyrex tube. Temperature on the end-wall surface and acoustic pressure within the cylinder were also measured for different end-wall materials. Variables considered included acoustic absorption and reflection coefficients, ultrasound intensity, container height, and thermal properties of the end-wall material. It was observed that a quasi-steady state flow field driven by acoustic streaming is rapidly established within the container, which is typically dominated by a stationary vortex ring with downward flow along the ring axis. After sufficient time this quasi-stationary flow exhibits a thermal instability causing it to transform into a secondary flow state. Different types of secondary flow states were observed, including cases where the flow along the cylinder axis is oriented upward toward the ultrasound transducer and cases where the axial flow changes directions along the cylinder axis. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A7.00006: ABSTRACT WITHDRAWN |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A7.00007: Analysis of Premixed Flame Response and Rayleigh Criterion through a Novel Flame Transfer Function Vijaya Krishna Rani, Sarma Rani Linear modal analysis of combustion instabilities requires a flame transfer function which describes the flame heat-release response to acoustic perturbations. In this study, a novel flame transfer function (FTF) is developed that provides an explicit relationship between heat-release and pressure fluctuations for laminar premixed flames. While the FTF is generalized for any mean flame shape, a triangular mean flame stabilized at the cross-sectional interface of a dump combustor is analyzed. For this flame, the effects on the FTF magnitude and phase of the acoustic frequency, location (on the mean flame), modal index, and the mean Mach number are investigated. To illustrate and analyze the Rayleigh's criterion, the spatio-temporal integral of the correlation of pressure and heat-release fluctuations is calculated. It is found that the magnitude of the FTF shows harmonic-like oscillations whose amplitude decreases with frequency, suggesting that the flame shows preferential response to certain frequencies than others. The oscillatory behavior becomes increasingly prominent as one moves away from the flame anchoring point(s). Finally, evaluation of the Rayleigh integral clearly demonstrates the flame-acoustic phase shifts at which combustion instability may arise. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A7.00008: Modeling of piezoelectric energy extraction in a thermoacoustic engine with multi-pole time-domain impedance Jeffrey Lin, Carlo Scalo, Lambertus Hesselink We have carried out the first high-fidelity Navier-Stokes simulation of a complete thermoacoustic engine with piezoelectric energy extraction. The standing-wave thermoacoustic piezoelectric (TAP) engine model comprises a 51 cm long cylindrical resonator, containing a thermoacoustic stack on one end and capped by a PZT-5A piezoelectric diaphragm on the other end, tuned to the frequency of the thermoacoustically-amplified mode (388 Hz). A multi-pole broadband time-domain impedance model has been adopted to accurately simulate the measured electromechanical properties of the piezoelectric diaphragm. Simulations are first carried out from quasi-quiescent conditions to a limit cycle, with varying temperature gradients and stack configurations. Stack geometry and boundary layers are fully resolved. Acoustic energy extraction is then activated, achieving a new limit cycle at lower pressure amplitudes. The scaling of the modeled electrical power output and attainable thermal-to-electric energy conversion efficiencies are discussed. Limitations of extending a quasi-one-dimensional linear approximation based on Rott's theory to a (low amplitude) limit cycle are discussed, as well as nonlinear effects such as thermoacoustic energy transport and viscous dissipation. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A7.00009: Suppression of Leidenfrost effect via low frequency vibrations Boon Thiam Ng, Yew Mun Hung, Ming Kwang Tan Leidenfrost effect occurs when vapor layer forms in between the coolant and the hot surface above Leidenfrost point, which dramatically reduces the cooling efficiency due to low thermal conductivity of the vapor layer. To prevent surface overheating, there have been number of reported methods to suppress the Leidenfrost effect that were mainly based on functionalization of the substrate surface and application of electric field across the droplet and substrate. In this work, we induce low frequency vibrations (f $\sim$ 100 Hz) to the heated substrate to suppress the Leidenfrost effect. Three distinct impact dynamics are observed based on different magnitudes of surface acceleration and surface temperature. In gentle film boiling regime, formation of thin spreading lamella around the periphery of the impinged droplet is observed; in film boiling regime, due to thicker vapor cushion, rebound of the impinged droplet is observed; in contact boiling regime, due to the direct contact between the impinged droplet and heated substrate, ejection of the tiny droplet is observed. Also, estimated cooling enhancement ratio for contact boiling regime shows an improvement from 95\% to 105\%. [Preview Abstract] |
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