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1. Exploring the Feasibility of a Thermoacoustic Metastructure for Energy Harvesting and Noise Mitigation

   https://doi.org/10.1121/2.0001670  

   Biswas, Samarjith; Manimala, James M. (December 2022, Proceedings of meetings on acoustics)

   The thermoacoustic effect provides a means to convert acoustic energy to heat and vice versa without the need for moving parts. This could enable the realization of mechanically-robust, noise mitigating energy harvesters via the development of thermoacoustic metastructures using additive and hybrid fabrication processes and materials. The mechanical, thermal and geometric properties of the porous stack that forms a set of acoustic waveguides in thermoacoustic metastructures are key to their performance. In this proof-ofconcept study, firstly, various ceramic and polymeric stack designs are evaluated using a custom-built thermoacoustic test rig. Influence of stack parameters such as material, length, location, porosity and pore geometry are correlated to simulations using DeltaEC, a software tool based on Rott’s linear approximation. Preliminary results also show a reduction in sound pressure level of around 5.28 dB across the thermoacoustic metastructure at resonance (117.5 Hz). An acousto-thermo-electric transduction scheme is employed to harvest useable electrical power using the best performing stack. Steady-state peak voltage generated was 33 mV for a temperature difference of about 30 degree Celsius across the stack at resonance. Further investigations are underway to establish structure-performance relationships by extracting scaling laws for power-to-volume ratio and frequency-thermal gradient dependencies. 

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2. Evaluation of additively manufactured stacks for thermo-acoustic devices

   https://doi.org/10.3397/IN-2021-2683  

   Biswas, Samarjith; Krawczyk, Zack; Manimala, James M. (August 2021, INTER-NOISE and NOISE-CON Congress and Conference Proceedings)

   The thermo-acoustic effect provides a means to convert acoustic energy to heat and vice versa without the need for moving parts. This is especially useful to construct mechanically-simple and robust energy harvesting devices, although there are limitations to the power-to-volume ratio achievable. The mechanical and thermal properties as well as geometry of the porous stack that forms a set of acoustic waveguides in thermo-acoustic devices are key to its performance. In this study, we evaluate various additively manufactured polymer stacks against more conventional ceramic stacks using a benchtop thermos-acoustic refrigerator rig that uses air at ambient pressure as its working fluid. Influence of stack parameters such as material, length, location, porosity and pore geometry are examined using experiments and correlated to simulations using DeltaEC, a software tool based on Rott's linear approximation. Structure-performance relationships are established by extracting scaling laws for power-to-volume ratio and frequency-thermal gradient dependencies. It is found that additively manufactured stacks can deliver performance comparable to ceramic stacks while being more affordable and customizable for thermo-acoustic transduction applications. 

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3. Traveling wave thermoacoustic refrigeration with variable phase-coordinated boundary conditions

   https://doi.org/10.1121/10.0023954  

   Callanan, Jesse; Adlakha, Revant; Mousa, Mohamed; Nouh, Mostafa (December 2023, The Journal of the Acoustical Society of America)

   Thermoacoustic refrigerators exploit the thermodynamic interaction between oscillating gas particles and a porous solid to generate a temperature gradient that provides a cooling effect. In this work, we present a resonator with dual enclosed driver end-caps and show that the temperature gradient across a ceramic thermoacoustic element placed in the cavity could be controlled by modifying the phase difference of the drivers, thus enabling precise control of the refrigeration capability via the temperature difference. Through DELTAEC simulation results, the response of the temperature gradient to various dynamic boundary conditions that alter the time-phasing and wave dynamics in the resonator are demonstrated. An experimental apparatus is constructed with two moving-coil speakers and a ceramic stack, which is shown to exhibit a temperature gradient along its length, based on the traveling-wave-like nature of the acoustic wave excited by the speakers. By adjusting the relative phase lag between the two speakers, the temperature gradient across the stack is made to increase, decrease, or flip sign. Finally, a desired temperature difference that changes in time is achieved. The results presented in this work represent a key conceptual advancement of thermoacoustic-based temperature control devices that can better serve in extreme environments and precision applications. 

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4. Different perspectives on predicting the thermoacoustic energy conversion response in a Rijke tube

   https://doi.org/10.1063/5.0072193  

   Shelton, Cody_M; Majdalani, Joseph (November 2021, Physics of Fluids)

   In this work, six different perspectives on characterizing the thermoacoustic field in an open-ended Rijke tube are considered and discussed. These begin with a three-pronged approach consisting of theoretical, experimental, and numerical investigations of the Rijke tube's time-dependent field. It is followed by a discussion of effective techniques that rely on either Green's function or differential equation models. Finally, a perturbation expansion is introduced that leverages a naturally occurring small parameter in the open tube configuration. This approach is shown to produce accurate predictions of pressure modal shapes and frequencies for an arbitrary specified temperature distribution. It also leads to a set of linear partial differential equations that can be solved in conjunction with a Green's function expression for the thermoacoustic pressure, velocity, and heat oscillations. In this study, the underlying framework is presented and evaluated for the pressure disturbance only. Another fundamental result includes a similarity parameter, coined the Rijke number, which plays an essential role in driving thermoacoustic oscillations, namely, by relating heat-flux fluctuations to the acoustic velocity and pressure. In this context, we find that the peak value of the energy-flux vector modulus, which stands for the modular product of acoustic velocity and pressure, does indeed occur at the heat source location and increases with the heat power input. 

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5. Influence of Porous Inserts and Compact Resonators on Onset of Taconis Oscillations

   https://doi.org/10.1115/1.4065398  

   Matveev, Konstantin I (February 2024, Journal of Vibration and Acoustics)

   Taconis oscillations represent excitation of acoustic modes due to large thermal gradients inside narrow tubes penetrating cryogenic vessels from a warm ambient environment. These oscillations are usually harmful, as they may drastically increase heat leakage into cryogenic vessels and result in strong vibrations of measuring instruments. Placing a porous material inside a tube with a goal to increase acoustic damping or attaching a small resonator to the main tube are some of the possible ways to suppress or mitigate Taconis effects. However, when the porous inserts are positioned in locations with large temperature gradients or the resonator parameters are selected incorrectly, these components may augment thermal-to-acoustic energy conversion and enhance Taconis oscillations. A low-amplitude thermoacoustic model has been extended and applied in this study to determine the effects of the insert location and pore radius, as well as the resonator dimensions, on the onset of Taconis phenomena in a hydrogen-filled tube of relevance to lines used in cryogenic hydrogen storage tanks. The presented findings can assist cryogenic specialists interested in suppressing or exciting Taconis oscillations. 

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