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Reprints and Permissions. Guo, L. The study on the inverse problem of applied current thermoacoustic imaging based on generative adversarial network. Sci Rep 11, Download citation. Received : 24 August Accepted : 10 November Published : 25 November Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article.
Provided by the Springer Nature SharedIt content-sharing initiative. By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate. Advanced search. Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily. Skip to main content Thank you for visiting nature. Download PDF. Subjects Imaging Ultrasound. Abstract Applied Current Thermoacoustic Imaging ACTAI is a new imaging method which combines electromagnetic excitation with ultrasound imaging, and takes ultrasonic signal as medium and biological tissue conductivity as detection target.
Introduction As a new type of functional imaging technology, Electrical Impedance Tomography 1 , 2 , 3 is capable of nondestructive detection of changes in electrical parameters of biological tissues, and then obtain the physiological and pathological states of the tissues. Method The implementation of the new method consists of the following steps: first, the electrical signal measured by the ultrasonic probe is preprocessed by Wiener filter deconvolution 18 , 19 to obtain the original acoustic signal emitted by the measured sample.
Figure 1. Circular scan projection principle. Full size image. Figure 2. Generative adversarial network principle diagram. Figure 3. Result The simulation model shown in Fig. Each resonator , and is comprised of a stack , and Each stack, such as stack , is comprised of a hot heat exchanger , a cold heat exchanger and a stack material disposed between and in thermal contact with each heat exchanger and The stack material may comprise cotton wool, glass wool, steel wool, aerogel, other fibrous materials, a series of perforated plastic plates or a plurality of longitudinally aligned nanotubes.
The optimum spacing between the randomly arranged fibers in the stack , when a fibrous stack is utilized, may be determined by the thermal penetration depth for the working fluid or gas , such that the acoustic field can interact thermally with each element of the stack The hot heat exchanger is thermally coupled to the hot end of the resonator and thermally isolated from the cold heat exchanger and cold end The cold heat exchanger is thermally anchored to a fixed temperature, usually ambient temperature.
Both heat exchangers and may be formed as a thin screen which is effectively acoustically transparent, but that maintains a fixed temperature at each end of the stack The heat exchangers and may be formed from laser patterned copper or aluminum. The acoustic cavity or chamber is in fluid communication with the second open ends , and of the resonators , and The working fluid allows the standing waves , and to form.
Thus, each thermoacoustic device , and is comprised of two sections defining a hot side and a cold side. Each section of the thermoacoustic device has a heat exchanger thermally anchored to its end adjacent the stack. The stack , formed from a porous, high surface area material, is thermally anchored to each heat exchanger by abutting therewith. The stack material is configured to be able to maintain a temperature gradient. A temperature gradient along the stack is achieved and maintained by injecting heat to the hot side of the resonators and maintaining the cold side of the resonator at a fixed temperature, such as room temperature or other ambient temperature.
A thermal heat sink is provided to maintain the cold side at ambient temperature. The sound waves from each resonator , and combine within the chamber to create phase-locked sound waves that are directed to and impinge upon the piezoelectric transducer When the transducer is actuated, electrical current is generated and sent through the electrical leads and of the transducer to an electrical circuit as previously described herein.
The transducer is generally disposed parallel to and aligned generally coaxially with the stacks of the plurality of resonators. The energy converter is configured to operate from the mid-audio frequency range to the ultrasonic range e. In these frequency ranges, advantage is taken of the high sensitivity of piezoelectric devices and their compactness.
An electric type of transducer, such as a piezoelectric device, for sound to electricity conversion is superior to an electromagnetic type when operated at high frequencies and when compactness is an issue, as in the miniaturization of devices.
Since the thermoacoustic devices are resonant systems, their size determine the resonant frequency, and hence by miniaturizing them, the operating frequency is raised accordingly.
The choice of device size is determined by the application and by how much power needs to be converted. Units consisting of arrays offer the possibility of dealing with large power levels which maintain compactness and offer lightweight systems.
Performance of the system of the present invention is determined by operating conditions, mainly the temperature difference imposed by the source of heat driving the system. The amount of heat and the resulting temperature difference will determine the power output, its efficiency and onset for oscillation.
The larger the heat input, the higher the sound level will be in the resonator resulting in greater electrical generation. In a self-sustained oscillator, the initial phase is usually arbitrary. Thus, an array phase-locking allows the system to achieve maximum power output.
Phase-locking of the individual thermoacoustic devices is achieved by the addition of the acoustic chamber of the present invention which provides coupling for in-phase motion of all the acoustic units. When such phase-locking is achieved, the power output depends directly on the number of thermoacoustic devices in the array and maximum power output is achieved.
The energy converter of the present invention is a device which has essentially no moving parts other than the gas in the resonator and sound chamber and the flexing of the piezoelectric device. The gas may be comprised of air, but may also be helium, gas mixtures or argon and helium, or other gases known in the art. In addition, the working gas may be pressurized for higher power density.
By operating in the mid-audio and low ultrasonic frequency ranges, the thermoacoustic devices are relatively small and can be easily pressurized to high pressure levels. In accordance with the principles of the present invention, the thermoacoustic devices can be pressurized to pressures such as atmospheres and higher without problems related to strength of materials.
The resonator determines the frequency of the thermoacoustic engine. It does this by setting up a standing wave from acoustic pulses generated by the temperature gradient along the stack The resonator provides positive feedback at the stack which sustains the acoustic oscillations. The resonator may be of a one quarter, one half or other wavelength type. In the case of a one quarter wavelength resonator, the cold end is open. Quantatively, the standing wave is described by the standing wave ratio.
This may be reduced as the diameter of the resonator is increased relative to the length leading to a larger amount of traveling component which is radiated out. A large resonator diameter may be used to provide a large output since the level of generated sound depends on the cross-sectional area of the stack A high standing wave ratio may favor a lower temperature difference for the onset of oscillations because more positive feedback is provided by the reflected wave.
Hence, a wide, short resonator will require a large temperature difference across the stack for onset of oscillation unless more gain is provided for positive feedback. The resonator is essentially a storage element where acoustic energy is built up for providing the positive feedback and for generating the sound which will activate the electrical generator Another reason for a wide resonator is that the stored acoustic energy is large in comparison with viscous and thermal losses within a characteristic surface layer inside the resonator.
The acoustic cavity is used to increase the positive feedback in the system. This is particularly important when the standing wave ratio in the resonator is low, which occurs when the quarter wave resonator is wide but short. The cavity acts as a reflector which can be non-resonant or resonant. The latter case may lead to the highest increase in positive feedback to the resonator Ideally the cavity should be on resonance at or near resonance with the resonator An important consequence of increased positive feedback from the chamber is a reduction in the critical temperature difference across the stack for the onset of oscillation.
This may prove especially advantageous compared to prior devices, as only a low temperature difference may be available for certain applications.
It will be appreciated that the distal end of the chamber is a convenient location for disposing the electrical generator , in this exemplary embodiment a piezoelectric driver, which converts the sound to energy. Depending on the particular application, the shape of cavity may be cylindrical in cross-section, have a tapered cylindrical cross-section, be Helmholtz-like or any other advantageous geometrical shape.
The heat-to-electricity generating system , of the present invention may incorporate various approaches to inject heat to the hot heat exchanger A flame or a heating element can be used as the source of heat. Heat is injected to the system by direct heating of the hot heat exchanger or by heating the hot section of the resonator to which the hot heat exchanger is thermally anchored.
Heat from other sources, such as waste heat from a mechanical or electronic device, can be injected to the hot heat exchanger by metallic thermal conduction, as depicted in FIG. Similarly, an appropriate radioisotope containing element may be used to inject heat to the hot heat exchanger , again by metallic thermal conduction.
The electrical generator may be a piezoelectric element in a monomorph configuration or bimorph configuration. Electrical generator may be tuned to the resonance of the resonator to maximize electric output. Because the electrical generator is a pressure sensitive unit, optimal performance may be achieved by positioning the generator at the location of maximum acoustic pressure, typically, at the distal end of chamber opposite the thermoacoustic resonators , and Electrical power output may be maximized when the electrical generator is in resonance with the acoustic system device.
The electric voltage obtained may be enlarged by configuring the generator in a bimorph mode, where two piezos attached to a metallic membrane are connected in series; such piezos may appear as a bimetallic strip to provide maximum voltage output when exposed to sound power.
A system in accordance with the present invention may be miniaturized for operation in the frequency range of from about 2 kHz to about 24 kHz. Used in arrays, such systems may be configured to work in the ultrasonic range at 40 kHz, as for military power applications. High power densities may be achieved by pressurizing the working fluid Such miniature thermoacoustic energy converters may work up to frequencies as high as the ultrasonic range.
Operation with a low threshold in temperature difference for oscillation may make such systems useful for a variety of applications. Used in arrays, systems of the present invention may be useful in any number of applications.
For example, such systems may be thermally attached to a waste heat producing electronic or mechanical device, such as a radar system or high powered electronic devices. The waste heat will drive the system and be thereby converted to electricity. In other applications, an array of systems may be used as a portable source of electric power.
For example, heat from a flame may be used to activate the unit and make electrical power available for application. Such a system would be useful in an emergency or battlefield situation. A typical array may consist of about systems 10 shown in FIG. Modern thermoacoustic engines are generally large and commonly have operating frequencies between 60 and Hz. This design space is constrained by design considerations such as construction limitations and electroacoustic transducer efficiency.
However, there are significant benefits to engines operating at higher frequencies. Power density for standing-wave engines scales favorably with frequency and pressure amplitude, holding constant the operating temperature range. Further, acoustic-to-electric conversion via piezoelectric transducers can be more efficient at higher frequencies.
Acoustic simulations in the ultrasonic regime present some unique challenges. Moreover, acoustic streaming may no longer be assumed to be a second-order flow quantity. As a result, wall-heat transfer may have longer timescales, limiting the intensity of the thermoacoustic response. Previous high-fidelity efforts by Scalo et al.
More recently, Lin et al. In this paper we present a high-fidelity fully compressible Navier—Stokes simulation of a thermoacoustic engine operating in the ultrasonic frequency regime. A model for bulk viscosity, incorporating rotational and vibrational relaxation effects, has been developed; these effects are not insignificant compared with Stokesian viscous and thermal dissipation [ 17 ] and thermoacoustic energy production.
The engine design is based on a standing-wave engine construction first presented by Flitcroft and Symko [ 1 ] and was chosen due to its simple design. Being able to capture thermoacoustic onset and nonlinear effects are preliminary steps towards the development of computational tools to better predict and optimize energy generation for miniaturized and high-frequency thermoacoustic engines.
In the following, the adopted theoretical TAE model is first introduced, together with the governing equations and computational setup section 2. A technique to capture bulk viscosity and a setup for absorption verification is presented section 3. A linear thermoacoustic model predicting the onset and growth of oscillations in the TAE model is presented section 4.
Finally, results for the thermoacoustic engine model for both the linear and Navier—Stokes models are shown and discussion follows section 6. The chosen computational setup fig. Our model is a quarter-wavelength resonator with a thermoacoustic stack in a straight circular tube, closed on one end and connected to a coin-shaped cavity on the other. The cavity lowers the critical temperature necessary for onset and also provides for the possibility of pressurizing the engine, and the cavity geometry is estimated from a presentation by Flitcroft and Symko [ 18 ].
The stack temperature profile varies from T c of K to T h of K in our simulations. The referenced literature reports minimal geometrical information, with only the diameter and length of the tube being explicitly provided. The stack is constructed as radially-concentric plates, as in Lin et al. The number of concentric stack elements n s was chosen to be 7, including the centered cylindrical rod, resulting in a stack gap width h g of 0.
Because the defined cavity volume may be different from the experimental setup, the simulation-derived frequency is not expected to match the reported experimental operating frequency of 21 kHz. The computational grid, as also shown in fig. Rotational extrusion of five layers, each of one degree, along the x axis is used to construct the three-dimensional computational grid. Adiabatic slipwall conditions are used to impose axial symmetry. The high-fidelity model was run both without bulk viscosity reference and varying levels of bulk viscosity, as tuned by relative humidity.
For presented results, several cases were run, differing by gas attenuation magnitude. As discussed in detail in section 3. One significant advancement in this work is the adoption of a newly developed bulk viscosity model, accounting for both rotational and vibrational molecular relaxation, as outlined in the following section. The governing equations are solved using CharLES X , a control-volume-based, finite-volume solver for the fully compressible Navier—Stokes equations on unstructured grids, developed as a joint-effort among researchers at Stanford University.
CharLES X employs a three-stage, third-order Runge-Kutta time discretization and a grid-adaptive reconstruction strategy, blending a high-order polynomial interpolation with low-order upwind fluxes. The latter is the measure of wave attenuation over a given traveled distance and has traditionally been of particular interest for atmospheric acoustics.
However, multispecies interactions and rotational and vibrational relaxation result in deviations from classic absorption characteristics at various frequency regimes.
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