Thermoacoustic Effect Research Articles

Flexible Two-Dimensional Ti3C2 MXene Films as Thermoacoustic Devices.

MXenes have attracted great attention for their potential applications in electrochemical and electronic devices due to their excellent characteristics. Traditional sound sources based on the thermoacoustic effect demonstrated that a conductor needs to have an extremely low heat capacity and high thermal conductivity. Hence, a thin MXene film with a low heat capacity per unit area (HCPUA) and special layered structure is emerging as a promising candidate to build loudspeakers. However, the use of MXenes in a sound source device has not been explored. Herein, we have successfully prepared sound source devices on an anodic aluminum oxide (AAO) and a flexible polyimide (PI) substrates by using the prepared Ti3C2 MXene nanoflakes. Due to the larger interlayer distance of MXene, the MXene-based sound source device has a higher sound pressure level (SPL) than that of graphene of the same thickness. High-quality Ti3C2 MXene nanoflakes were fabricated by selectively etching the Ti3AlC2 powder. The as-fabricated MXene sound source device on an AAO substrate exhibits a higher SPL of 68.2 dB (f = 15 kHz) and has a very stable sound spectrum output with frequency varying from 100 Hz to 20 kHz. A theoretical model has been built to explain the mechanism of the sound source device on an AAO substrate, matching well with the experimental results. Furthermore, the MXene sound source device based on a flexible PI substrate has been attached to the arms, back of the hand, and fingers, indicating an excellent acoustic wearability. Then, the MXene film is packaged successfully into a commercial earphone case and shows an excellent performance at high frequencies, which is very suitable for human audio equipment.

Equipment and Methods for Studying Thermoacoustic Emission Memory Effects in Rocks

Thermally stimulated rock failure has been actively studied for over 30 years for solving both fundamental and applied problems. At the same time, there are features that make it difficult to obtain information on the development of thermal compared to mechanical damage. The article discusses three main problems: the small number of recorded acoustic emission pulses, the difference in thermal conditions during heating and cooling, and the high level of interference from conventional heating control systems. The article analyzes ways to solve these problems and proposes and develops an improved laboratory setup and methods for studying thermally stimulated rock failure.

Проявления акустической эмиссии в мерзлых грунтах при одновременном влиянии на них переменных механических и термических воздействий

The subject of the research is to establish the fundamental laws of acoustic emission in frozen soils, which allow to create ways to control (monitor) their stability under the influence of variable temperature fields and quasistatic mechanical stress from engineering objects located on these grounds for various purposes. The applied importance of such methods is to increase the speed and reduce the complexity of engineering geological surveys in the northern regions of Russia, carried out with the aim of predicting the loss of stability of the bases of buildings and structures to ensure their safe operation. The study was performed on the original instrumental complex. Its description and characteristics are given. With the use of this complex, thermoacoustic emission effects arising from the repeated alternation of freezing and thawing cycles of the soil during the development of its deformed state, starting from the normal compaction phase and up to the final stage of destruction (the bulging phase), have been studied. It is shown that on the basis of such informative parameters as thermally stimulated activity and duration of acoustic emission pulses, an indicator can be obtained that quantitatively characterizes the stages of the stress-strain state of soils. An experimental dependence of the field of values of this indicator as a function of the mechanical stress and the fractional composition of the test soil is given. The qualitative convergence of this dependence with the classical soil deformation diagram obtained by N.M.Hersevanov is shown, where the stages of compaction, loss of stability (shifts) and destruction are highlighted. Possible physical mechanisms and features of the formation of an acoustic emission response at each of these stages are considered and substantiated. It is noted that the approaches to receiving, processing and interpreting acoustic emission measurement information, which are grounded within the framework of the study, allow to control and monitoring of the carrying capacity and stress-strain state of soils directly in the field.

A Microwave-Induced Thermoacoustic Imaging System With Non-Contact Ultrasound Detection.

Portable and easy-to-use imaging systems are in high demand for medical, security screening, nondestructive testing, and sensing applications. We present a new microwave-induced thermoacoustic imaging system with non-contact, airborne ultrasound (US) detection. In this system, a 2.7 GHz microwave excitation causes differential heating at interfaces with dielectric contrast, and the resulting US signal via the thermoacoustic effect travels out of the sample to the detector in air at a standoff. The 65 dB interface loss due to the impedance mismatch at the air-sample boundary is overcome with high-sensitivity capacitive micromachined ultrasonic transducers with minimum detectable pressures (MDPs) as low as 278 μ Pa rms and we explore two different designs-one operating at a center frequency of 71 kHz and another at a center frequency of 910 kHz. We further demonstrate that the air-sample interface presents a tradeoff with the advantage of improved resolution, as the change in wave velocity at the interface creates a strong focusing effect alongside the attenuation, resulting in axial resolutions more than 10× smaller than that predicted by the traditional speed/bandwidth limit. A piecewise synthetic aperture radar (SAR) algorithm modified for US imaging and enhanced with signal processing techniques is used for image reconstruction, resulting in mm-scale lateral and axial image resolution. Finally, measurements are conducted to verify simulations and demonstrate successful system performance.

Multi-Physics and CFD Analysis of an Enclosed Coaxial Carbon Nanotube Speaker for Automotive Exhaust Noise Cancellation

<div class="section abstract"><div class="htmlview paragraph">Automotive exhaust noise is one of the major sources of noise pollution and it is controlled by passive control system (mufflers) and active control system (loudspeakers and active control algorithm). Mufflers are heavy, bulky and large in size while loudspeakers have a working temperature limitation. Carbon nanotube (CNT) speakers generate sound due to the thermoacoustic effect. CNT speakers are also lightweight, flexible, have acoustic and light transparency as well as high operating temperature. These properties make them ideal to overcome the limitations of the current exhaust noise control systems. An enclosed, coaxial CNT speaker is designed for exhaust noise cancellation application. The development of a 3D multi-physics (coupling of electrical, thermal and acoustical domains) model, for the coaxial speaker is discussed in this paper. The model is used to simulate the sound pressure level, input power versus ambient temperature and efficiency. The 3D model provides accurate results of the temperature profile and heat flow as compared to a 2D model. Also, the flow of exhaust gases can be efficiently modeled using a 3D model. The flow analysis would help understand any flow penetration into the speaker as well as the effect of heat transfer due to the flow. The model is validated by comparing the experimental results with the simulation results. Along with multi-physics simulation, CFD analysis of the coaxial speaker is also studied in this paper. The CFD analysis is focused on the backpressure generated by the speaker and the flow path of the exhaust gases inside the CNT speaker.</div></div>

Influence exerted by bone-containing target body on thermoacoustic imaging with current injection**Project supported by the National Natural Science Foundation of China (Grant No. 51477161), the National Key Research and Development Program of China (Grant No. 2018YFC0115200), and the Fund from the Chinese Academy of Sciences (Grant No. YZ201507).

Thermoacoustic imaging with current injection (TAI-CI) is a novel imaging technology that couples with electromagnetic and acoustic research, which combines the advantages of high contrast of the electrical impedance tomography and the high spatial resolution of sonography, and therefore has the potential for early diagnosis. To verify the feasibility of TAI-CI for complex bone-containing biological tissues, the principle of TAI-CI and the coupling characteristics of fluid and solid were analyzed. Meanwhile, thermoacoustic (TA) effects for fluid model and fluid–solid coupling model were analyzed by numerical simulations. Moreover, we conducted experiments on animal cartilage, hard bone and biological soft tissue phantom with low conductivity (0.5 S/m). By injecting a current into the phantom, the thermoacoustic signal was detected by the ultrasonic transducer with a center frequency of 1 MHz, thereby the B-scan image of the objects was obtained. The B-scan image of the cartilage experiment accurately reflects the distribution of cartilage and gel, and the hard bone has a certain attenuation effect on the acoustic signal. However, compared with the ultrasonic imaging, the thermoacoustic signal is only attenuated during the outward propagation. Even in this case, a clear image can still be obtained and the images can reflect the change of the conductivity of the gel. This study confirmed the feasibility of TAI-CI for the imaging of biological tissue under the presence of cartilage and the bone. The novel TAI-CI method provides further evidence that it can be used in the diagnosis of human diseases.

Working gases based scaling-down limitations for thermoacoustic coolers: Miniaturization approach

Regarding miniaturization of thermoacoustic coolers for thermal management purposes, working gases play a key role as the primary media responsible for producing the so-called “thermoacoustic effect” with their interaction with solid media (i.e., secondary media) within stacks or regenerators. However, the role of working gases in limiting scaling-down of thermoacoustic coolers still needs more investigations compared to addressed operational parameters (i.e., mean pressure, temperature difference across stack, etc.). In the present study, a theoretical computational analysis, based on published literature work, would be conducted to investigate allowable minimum sizes of standing-wave thermoacoustic coolers under the effects of working gases thermo-physical properties with considering adverse effects limitation of thermal conduction losses. Different working gases including air and either pure or mixture noble gases have been used for such geometrical scaling-down analysis under specific operating conditions. Moreover, cooling power was focused here as the more desirable performance indicator rather than the efficiency. The results had revealed the cooling capability at different scale levels based on different working gases, which make gases properties significantly contribute to scalability of thermoacoustic coolers to meet the cooling needs for micro-electronics. In addition, more research work will be devoted to other scaling-down issues of thermoacoustic systems.Regarding miniaturization of thermoacoustic coolers for thermal management purposes, working gases play a key role as the primary media responsible for producing the so-called “thermoacoustic effect” with their interaction with solid media (i.e., secondary media) within stacks or regenerators. However, the role of working gases in limiting scaling-down of thermoacoustic coolers still needs more investigations compared to addressed operational parameters (i.e., mean pressure, temperature difference across stack, etc.). In the present study, a theoretical computational analysis, based on published literature work, would be conducted to investigate allowable minimum sizes of standing-wave thermoacoustic coolers under the effects of working gases thermo-physical properties with considering adverse effects limitation of thermal conduction losses. Different working gases including air and either pure or mixture noble gases have been used for such geometrical scaling-down analysis under specific operating condit...

Traveling and standing thermoacoustic waves in solid media

The most attractive application of fluid-based thermoacoustic (TA) energy conversion involves traveling wave devices due to their low onset temperature ratios and high growth rates. Recently, theoretical and numerical studies have shown that thermoacoustic effects can exist also in solids. However, these initial studies only focus on standing waves. This paper presents a numerical study investigating the existence of self-sustained thermoelastic oscillations associated with traveling wave modes in a looped solid rod under the effect of a localized thermal gradient. Configurations having different ratios of the rod radius R to the thermal penetration depth δk were explored and the traveling wave component (TWC) was found to become dominant as R approaches δk. The growth-rate-to-frequency ratio of the traveling TA wave is found to be significantly larger than that of the standing wave counterpart for the same wavelength. The perturbation energy budgets are analytically formulated and closed, shedding light onto the energy conversion processes of solid-state thermoacoustic (SSTA) engines and highlighting differences with fluids. Efficiency is also quantified based on the thermoacoustic production and dissipation rates evaluated from the energy budgets.

Numerical study of turbulence related to vortex shedding structures of an oscillatory flow in thermoacoustic energy system

Thermoacoustic system is a system that provides green technology for power and energy devices. Fluid dynamics of flow inside the system, that could influence system’s performance, is less understood. In this paper, turbulence in oscillatory flow condition inside thermoacoustic environment is studied numerically using a validated two-dimensional computational fluid dynamics model. A new investigation of the effect of flow frequency is presented. The model was solved for two flow frequencies; 13.1 Hz and 23.1 Hz. Detail description on vortex formation and changes as fluid flows in cyclic manner is presented. For stack of plates that is 200 mm long, turbulence seems to start at a drive ratio of 1% as the velocity prediction deviates from the laminar theory. Visualization of vortex contours indicated that as flow reverses the returning vortex alters the boundary layer condition within the channel of the plates. Therefore, the flow needs to be modelled as turbulence flow. The volatility of vortex structures at the end of plate seems weaker as the flow frequency increases. This study indicates that turbulence need to be carefully considered in modelling/designing thermoacoustic systems especially if better ‘thermoacoustic effect’ that represents performance of the energy system is required.

Microwave-excited hybrid thermoacoustic and ultrasound imaging with microwave pulse-width modulation

We present a method that allows for hybrid microwave-excited thermoacoustic and ultrasound (HMTAU) imaging without an additional ultrasound transmitting circuitry. The same microwave pulse is used to simultaneously generate thermoacoustic (TA) signals from the sample by the thermoacoustic effect, and ultrasound (US) signals from the piezoelectric transducer by the inverse piezoelectric effect. The same transducer collects the US signals reflected from the sample as a result of the US pulse (pulse-echo) and the TA signals as a result of the sample absorbing microwaves. Experiments are conducted to show how the microwave pulse-width significantly impacts the excitation efficiency of ultrasonic waves, which can reduce the requirement for microwave power. We found that the mechanical response of the US transducer to this transient expansion shows a maximum if the pulse-width of the microwave is close to half of the center period of the US transducer. We also use phantom experiments to demonstrate the complementary contrast mechanisms of the two modalities. The results obtained indicate that our method offers the potential of a low-cost dual-modality technique for noninvasive biomedical imaging.

(Invited) Emerging Applications of Nanostructured Porous Silicon

Porous silicon (PS) prepared by the appropriate electrochemical anodization consists of a nanopore structure and residual quantum-size nanocrystalline silicon (nc-Si). It has tunable physical properties that can be radically different from those of single-crystalline bulk silicon and plays a role as a platform of functional devices. Possible applications are discussed here and some recent developments highlighted: efficient route for nc-Si colloid formation, quasiballistic electron emission, and thermo-acoustic effect. Using self-standing PS layer flakes as a base material, nc-Si dot colloid has been formed efficiently by either optical or thermal energy. Highly luminescent (35% in quantum efficiency) colloidal nc-Si dots can be produced with a considerably higher yield (85%) from target than the values (1-10%) in the conventional processes. A multiple-tunneling transport mode through nc-Si dot chain induces efficient quasiballistic hot electron emission from an nc-Si diode. Being a relatively low operating voltage device compatible with silicon planar fabrication process, the emitter is applicable to mask-less parallel lithography under an active matrix drive. It has been demonstrated that the integrated 100×100 PS emitter array is useful for multibeam lithography and that the selected emitter pattern is delineated corresponding to the activated emitters [1]. In addition, highly reducing activity of emitted electrons is applicable to liquid-phase thin film deposition of metals and semiconductors under a printing mode [2,3]. Both the efficiency and the energy distribution are remarkably improved by using monolayer graphene as a surface electrode [4]. Due to a complete thermal insulating property of nc-Si layer retaining a low volumetric heat capacity, on the other hand, thermo-acoustic conversion is significantly enhanced. When a temperature fluctuation is produced at the surface of nc-Si layer, an acoustic wave is generated near the device surface without any mechanical vibrations. The non-resonant and broad-band emissivity with no harmonic distortions makes it possible to use the emitter for generating audible sound under a full digital drive [5] and reproducing complicated ultrasonic communication calls between mice [6].

Influence of heat exchangers blockage ratio on the performance of thermoacoustic refrigerator

In this paper, a numerical study is carried out to assess the effect of heat exchangers plates inter-spacing on the performance of a thermoacoustic refrigerator (TAR). Considering that typically most of the heat loss occurs near the heat exchangers, a great emphasis will be placed on the space between the latter and the stack. Several geometric arrangements of heat exchangers plates inter-spacing were tested. The numerical method used is based on two-dimensional low Mach number model. This model assumes that the length of the assembled heat exchangers and stack is less than the wavelength. The results are in agreement with those provided by the DeltaEC software. It can also be observed that the larger the heat exchangers plates inter-spacing, the greater the cooling impact through the thermoacoustic effect.

Ionizing radiation‐induced acoustics for radiotherapy and diagnostic radiology applications

Acoustic waves are induced via the thermoacoustic effect in objects exposed to a pulsed beam of ionizing radiation. This phenomenon has interesting potential applications in both radiotherapy dosimetry and treatment guidance as well as low-dose radiological imaging. After initial work in the field in the 1980s and early 1990s, little research was done until 2013 when interest was rejuvenated, spurred on by technological advances in ultrasound transducers and the increasing complexity of radiotherapy delivery systems. Since then, many studies have been conducted and published applying ionizing radiation-induced acoustic principles into three primary research areas: Linear accelerator photon beam dosimetry, proton therapy range verification, and radiological imaging. This review article introduces the theoretical background behind ionizing radiation-induced acoustic waves, summarizes recent advances in the field, and provides an outlook on how the detection of ionizing radiation-induced acoustic waves can be used for relative and invivo dosimetry in photon therapy, localization of the Bragg peak in proton therapy, and as a low-dose medical imaging modality. Future prospects and challenges for the clinical implementation of these techniques are discussed.

Polyelectrolyte multilayer microchamber-arrays for in-situ cargo release: Low frequency vs. medical frequency range ultrasound

An implantable ultrasound triggered controlled release micro-chamber (MC) array based on polylactic acid (PLA) sealed polyelectrolyte multilayers (PEM) is presented. In this work Rhodamine B (RhB) nano-precipitates are loaded as model cargo and sealed water tight within micro-air bubbles. Cargo release at low frequency (LFUS (37 kHz)) and high frequency ultrasound (HFUS (2 MHz, medical range)) is compared. The release speed and mechanism varies significantly between both ultrasound frequencies, allowing for a 60 times faster MC opening at HFUS. LFUS MC opening is based on material fatigue combined with thermo-acoustic effects while strong thermo-acoustic effects are responsible for MC opening at HFUS. The slower MC opening at LFUS allows for controlling the release amount and enables multiple release pulses compared to HFUS. Not PLA coated and no air bubble entrapping PEM thin films are in case of HFUS destroyed, while no interaction is determined at LFUS.

Conductivity Reconstruction and Numerical Simulation for Magnetically Mediated Thermoacoustic Imaging

Magnetically mediated thermoacoustic imaging (MM-TAI) is an emerging medical electrical parameter imaging technology; it has the advantages of high contrast of electrical impedance imaging and high resolution of ultrasound imaging; it can realize the early diagnosis of disease in the tissues comparing with structure imaging technology. The magnetically mediated thermoacoustic (TA) effect is analyzed in this paper through the theoretical inference, numerical calculation, and experiment. Pulsed excitation is considered in the analysis, which is different from previously reported research. The pulsed magnetic field is applied to the low conductivity sample such as gel phantoms (0.2, 4, and 2.5 S/m); then the Joule heat is generated inside the objects and an acoustic signal is excited due to heat expansion. The acoustic signal is detected and the image is reconstructed from TA source to conductivity. This paper has laid the foundation for the MM-TAI in biological tissues.

Thermoacoustic instability in solid media

The exploration of thermoacoustic oscillations can be traced back to the 19th century when Sondhauss (1850) performed an experimental investigation that marked the birth of modern thermoacoustic research. Since then, several practical applications of the basic thermoacoustic effect were explored and ultimately led to the design of engineering devices such as heat engines or refrigerators. This fascinating mechanism was believed to occur only in fluids while our study shows theoretical and numerical evidence of the existence of thermoacoustic oscillations also in solid media. Although this mechanism shares common aspects with the more familiar thermoacoustics of fluids, our analysis shows that the underlying physical mechanism presents some important and non-trivial differences with the traditional theory. The discovery and validation of this mechanism may lead to new ideas enabling the development of the next generation of ultra-compact and robust solid-state thermoacoustic devices for direct thermal-to-mechanical energy conversion, and vice versa.The exploration of thermoacoustic oscillations can be traced back to the 19th century when Sondhauss (1850) performed an experimental investigation that marked the birth of modern thermoacoustic research. Since then, several practical applications of the basic thermoacoustic effect were explored and ultimately led to the design of engineering devices such as heat engines or refrigerators. This fascinating mechanism was believed to occur only in fluids while our study shows theoretical and numerical evidence of the existence of thermoacoustic oscillations also in solid media. Although this mechanism shares common aspects with the more familiar thermoacoustics of fluids, our analysis shows that the underlying physical mechanism presents some important and non-trivial differences with the traditional theory. The discovery and validation of this mechanism may lead to new ideas enabling the development of the next generation of ultra-compact and robust solid-state thermoacoustic devices for direct thermal-to-m...

X-ray-induced acoustic computed tomography of concrete infrastructure

X-ray-induced Acoustic Computed Tomography (XACT) takes advantage of both X-ray absorption contrast and high ultrasonic resolution in a single imaging modality by making use of the thermoacoustic effect. In XACT, X-ray absorption by defects and other structures in concrete create thermally induced pressure jumps that launch ultrasonic waves, which are then received by acoustic detectors to form images. In this research, XACT imaging was used to non-destructively test and identify defects in concrete. For concrete structures, we conclude that XACT imaging allows multiscale imaging at depths ranging from centimeters to meters, with spatial resolutions from sub-millimeter to centimeters. XACT imaging also holds promise for single-side testing of concrete infrastructure and provides an optimal solution for nondestructive inspection of existing bridges, pavement, nuclear power plants, and other concrete infrastructure.

Numerical study of the influence of the convective heat transport on acoustic streaming in a standing wave.

Within this work, acoustic streaming in an air-filled cylindrical resonator with walls supporting a temperature gradient is studied by means of numerical simulations. A set of equations based on successive approximations is derived from the Navier-Stokes equations. The equations take into account the acoustic-streaming-driven convective heat transport; as time-averaged secondary-field quantities are directly calculated, the equations are much easier to integrate than the original fluid-dynamics equations. The model equations are implemented and integrated employing commercial software COMSOL Multiphysics. Numerical calculations are conducted for the case of a resonator with a wall-temperature gradient corresponding to the action of a thermoacoustic effect. It is shown that due to the convective heat transport, the streaming profile is considerably distorted even in the case of weak wall-temperature gradients. The numerical results are consistent with available experimental data.

Experimental Study of Resonance Frequency at Prime Mover Thermoacoustic Standing Wave

<p class="Abstract">Thermoacoustic prime movers work by using thermal energy to produce acoustic energy in the form of sound wave through thermoacoustic effect which occurs in a porous medium called stack. This paper describes an experimental study on the relation between the order of resonance frequencies generated by a thermoacoustic prime mover and the length of the resonator and the viscous penetration depth. Extending the resonator length will decreasing the resonance frequency which result in the increasing in the viscous penetration depth. Generally, the generated sound consists of only one frequency, that is the first-order one. However, under certain conditions, the sound has only the second-order frequency or comprises two frequencies of the first-order and second-order resonance frequencies. This phenomenon can be explained by considering the comparison between the effective hydraulic radius of stack () and the viscous penetration depth (). It is found that the first-order frequency appears when , while when (with calculated by using the first-order frequency) then the second order frequency is produced so that is back to a smaller value and therefore the condition of is recovered. In addition, when of the thermoacoustic prime mover will<em> </em>generate the first and second order frequencies together.</p>

An experimentally validated electrical-thermal-acoustic model of non-planar carbon nanotube thermophones

Carbon nanotube (CNT) thin-film thermophones are a solid state, transparent, magnet-free, stretchable, and lightweight transducers that work via the thermoacoustic effect. The rapid change in the temperature of the CNT film with low heat capacity produces a temperature wave accompanied by an acoustic wave in a frequency range of 1 Hz to 100 KHz. The existing lumped parameter models for the planar CNT thermophones are not appropriate for the complex geometries of the CNT film. Using COMSOL multiphysics, an electrical-thermal-acoustic model of the non-planar CNT thermophone has been developed. By applying an alternating electrical current to the CNT film, the temperature variation was obtained and used to simulate the pressure distribution. For different input power levels, the temperature distribution on the CNT film was compared to the experimental data from thermal camera. The experimental sound pressure level at different locations in front of the CNT thermophone were recorded and used to validate the non-planar model.