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An internally pressurized aerostatic journal bearing suitable for thermoacoustic power converters is designed and experimentally demonstrated using a free piston oscillating in a cylinder between two gas springs. The oscillating pressures in a thermoacoustic power converter׳s cavities can be used to create a near constant flow through an air gap between piston and cylinder. By selectively supplying and draining gas from gas reservoirs to the converter׳s gas cavities, separate reservoirs inside the piston can be maintained below and above the engine mean pressure. A continuous flow of gas between these reservoirs in the air gap creates an aerostatic gas bearing between the converter׳s piston and cylinder. The results show that, under the operating conditions of thermoacoustic power converter, a robust passive gas bearing is feasible.

Solar powered thermoacoustic prime mover is an attractive device for solar thermal utilization. Its performance is significantly affected by tilt angle when it rotates with the Sun. In this study, the heat transfer characteristics within thermoacoustic core were investigated under 3 tilt angles including, −90°, 0° and 90°. The temperature fields of thermoacoustic core were measured by Thermal Infrared Imager. Experimental results show that the tilt angle has significant influence on the heat transfer and the temperature field in the thermoacoustic core when the engine onset. Three tilt angles were classified into three situations according to the relative position of two heat exchangers. And the temperature field and heat transfer characteristic was discussed in each situation.

The onset and damping processes of self-excited working-fluid oscillation in thermoacoustic prime movers, related to the quality of the utilizable thermal energy, have attracted everlasting research interest and effort. This work is intended to observe and analyze the temperature and pressure characteristics of the onset and damping processes in a recently developed looped travelling-wave thermoacoustic prime mover with enlarged thermoacoustic cores. A hysteretic loop, caused by the discrepancy between the onset temperature difference and the damping temperature difference, can be found in the looped structure, which indicates the potential in decreasing the onset temperature difference. Additionally, in order to optimize the acoustic field in the thermoacoustic prime mover, a compliance unit was inserted into the loop. The experimental results show that the onset temperature difference drops by up to 223 K with the appropriately located compliance unit, which is advantageous to the utilization of low-grade thermal energy.

Thermoacoustic refrigeration utilizes the temperature changes that occur due to pressure oscillations within a sound wave to transport heat from one point to another and achieve cooling. As such, it neither requires complicated machinery nor hazardous or environmentally harmful refrigerants. By altering the acoustic field, it is possible to also alter the direction of heat pumping allowing a single device to be capable of functioning as a heater or cooler without addition of complicated machinery. We have constructed a thermoacoustic heat pump employing dual opposing acoustic drivers or speakers connected by a resonator tube and a regenerator formed by layers of steel mesh positioned at the centre of the resonator to investigate the alteration of the acoustic field due to the interaction of opposing travelling waves. The acoustic field was manipulated by changing the phase difference and magnitude difference between the waves generated by the acoustic drivers. Experimental results show that the acoustic power flow on both sides of the regenerator was altered thus resulting in a change in heat pumping direction along the regenerator. In addition, it was also observed that the heat pumping was also influenced by the changes in the standing wave component (SWC) and the travelling wave component (TWC).

This paper mainly investigated a looped tube refrigerator driven by a three-stage travelling wave thermoacoustic engine (later simply called as thermacoustic engine-refrigerator system). The onset and steady operation features of this coupling thermoacoustic system are analyzed through the network model developed by the author’s research group and the DeltaEC code, which is widely used in the thermacoustic research field. For power electronic thermal management utilizing low grade energy, a small scale three-stage travelling wave thermoacoustic engine-refrigerator system is designed. The onset features, steady performances and acoustic field features are analyzed in depth according to the simulation results. The analysis start with an investigation on the parametric sensitivity, followed by the description on the scaling and coupling impacts of this engine-refrigerator system on the onset temperature difference. The scaling impact is determined from the comparison results between a small scale and a large scale system, while the coupling impact is derived from the simulation of a refrigerator system and a simple engine system. The simulation results are also compared with the experimental data in publication, and the comparison demonstrated a desirable agreement. The main findings from this research work would provide some physical insights on the design and operating principles for the development of multi-stage thermoacoustic systems.

This study investigated the energy conversion of a thermoacoustic engine using a wet regenerator for the interconversion of heat flux density and work flux density. According to the first law of thermodynamics, local energy conversion can be described using a work source corresponding to the gradient of work flux density. First, we analytically separated the work source for the thermoacoustic phenomena related to water evaporation and condensation. Thereafter, we experimentally investigated the energy conversion of only gas, and then that of water evaporation and condensation. The data obtained were compared with the analytical results. The analytical and experimental results showed good agreement. Furthermore, the energy conversion with evaporation and condensation, which was determined both analytically and experimentally, was higher than that obtained for only gas by a maximum of a factor of 4.5.

Thermoacoustic engines are generating increased interest due to their potential use in renewable energy systems. The heat transfer characteristics of such engine cores were examined in this study via numerical simulations, taking into consideration the interactions between the acoustic field and thermal motion of fluids in a thermoacoustic device. Our calculations were based on the energy conservation equation under the assumption of a periodically steady oscillatory flow and thermoacoustic linear theory. The temperature field of the engine core was numerically solved by applying the finite difference method (FDM) to the energy equation, and also compared with CFD results. To systematically clarify the heat transfer characteristics of the oscillatory flow in the engine core, dimensional analyses for the associated geometries and physical values were performed, and nondimensional parameters were selected to determine the heat transfer characteristics. The simulation was performed using these nondimensional parameters, and the correlations of local Nusselt number distributions and some nondimensional parameters were considered. Based on the correlations among heat transfer characteristics, a model was constructed for predicting the temperature gap between the wall and the fluid at the edge of a regenerator. This model was satisfactory when the Reynolds number ranged from 30 to 120 and the nondimensional relaxation time was 0.3–1.3.

In this paper, a comprehensive high-fidelity three-dimensional computational fluid dynamic research using large eddy simulation has been conducted to investigate a standing-wave quarter-wavelength thermoacoustic engine that consists of a hot buffer, a stack and a resonator. The performance of the thermoacoustic engine has been analysed in four aspects. Firstly, the dynamic characteristics of the engine during the initial start-up process are investigated when changing the temperature gradient imposed on the stack. Numerical results are compared with those from a system-wide reduced-order network model based on linear thermoacoustic theory. Secondly, the acoustic behaviour of the engine operating at steady state is studied. Fourier Series Model is utilized to decompose the steady-state acoustic pressure oscillations which reveals the unstable longitudinal acoustic modes excited in the engine. The stack serves as an energy source for the fundamental mode while it extracts acoustic power from the second harmonic. Thirdly, the hydrodynamic performances of the engine are inspected, and the obtained three-dimensional flow fields inside the engine enable us to probe into rich nonlinear phenomena including minor losses, mass streaming, etc. Finally, the heat transfer characteristics have been analysed by examining the mean temperature field and transversal heat fluxes along the engine. This research demonstrates that the large eddy simulation framework is effective in simulating the thermally induced oscillatory flow inside thermoacoustic engines. The multi-perspective analytical methodologies are valuable in comprehending the engine performance and provide guidelines for the design and optimization of efficient thermoacoustic engines for recovering waste thermal energy from various sources.

Thermoacoustic devices including prime movers and refrigerators provide a potential solution to the electronic cooling technology which hinders the development of highly integrated electronic devices. However, the density of thermoacoustic heat flow is inadequate for the requirements of electronic cooling. In this work, thermoacoustic heat dissipation coupled with oscillating convection is investigated by computational fluid dynamics simulations. The results show that thermoacoustic heat dissipation has a similar working mechanism compared with thermoacoustic refrigeration, but the heat dissipation effect and heat flux of heat exchangers in a thermoacoustic heat dissipation system are affected greatly by the temperature gradient, fluid oscillating amplitude and length of stack. The proposed cooling method coupling thermoacoustic heat dissipation and oscillating convection is superior to the oscillating convection alone or thermoacoustic heat dissipation alone.

Wet thermoacoustic engines (WTEs) are simple, robust energy conversion devices that employ a condensable vapor to generate acoustic power from heat. While previous studies focused on the potential of WTE technology to convert low-grade heat to power, this work focuses on the WTE's dependence on the working gas mixture, unique mass transport, and low working temperature properties, which make it a potential candidate for thermally driven separation processes, e.g., distillation and water desalination. In a set of experiments, the steady-state behavior of a water-based, standing-wave WTE was studied. The results highlight a remarkable trait of this system—a WTE is capable of producing a substantial flux of water vapor while exhibiting an operating hot-side temperature significantly lower than the boiling point. This “limiting temperature” is independent of the heat input to the system—theoretically predicted in systems that have no external “load” that consumes the acoustic power. In addition, the results suggest that the characteristics of the working gas play an important role in determining the device behavior. It was shown that in the tested range, a heavier, more complex-structured gas has significantly improved the steady-state performance of the engine. Specifically, a WTE working with SF6 as its working gas exhibited a vapor mass flux nearly three times greater than simple evaporation at the same heating power while maintaining a temperature as low as 57 °C. The link between gas characteristics and engine behavior is outlined. In conclusion, potential applications of the device and future research avenues are discussed.

This paper presents the experimental study on the thermoacoustic refrigeration system performance with respect to specific critical operating factors. Experimentations were carried out upon the thermoacoustic refrigeration system under different operational conditions. The experimental setup consists of 3D printed stack to obtain better cooling effect. Substantial aspects which impact on the performance of the system were identified. The results show that the performance of system was improved.

Thermoacoustic heat engine is one of the technological systems that can convert heat energy into acoustic energy. The system also can bring more benefit to the earth because from the waste heat resulted, it can create energy that we called renewable energy. The stack is the major component called as “heart” of thermoacoustic system that creates the path for fluid flow between hot and cold heat exchanger. Performance of the thermoacoustic system is depends on the parameter used such as length of stack, material used for heater and type of heat exchangers. Until nowadays, many researchers still investigate about the thermoacoustic travelling wave in order to improve the performance of the system. The heating element material used is Kanthal AWG-28 with winding length 13.1 cm. The experiment is carried out from 0.3 min until 20 min. Then, from the temperature obtain, the P-V diagram is present to show the modeling design based on the harmonic wave occur between the stack, hot and cold heat exchanger. Therefore, this paper describes the parameter study about the heat exchanger thermoacoustic system based on the experimental work study. The component involve in fabrication process has been stated. All components have been designed and development then a need to be tested. Performance test of hot and cold temperature have plot as a result of Schmidt Pressure Volume.

International audience; This study focuses on natural convection flows within a cylindrical guide containing a porous medium. This configuration is applicable to standing-wave thermoacoustic engines, usually composed of an acoustic resonator where a (short) stack (or porous medium) is inserted, with a heat exchanger placed at one of its ends. The resulting horizontal temperature gradient, when high enough, triggers the onset of an acoustic wave. Natural convection effects are usually neglected in thermoacoustics so that axisymmetry is often assumed. Here a 3D numerical study of natural convection flow is performed using a finite volume code for solving mixed Navier-Stokes and Darcy-Brinkman equations under Boussinesq approximation. The influence of the porous medium’s physical characteristics (permeability, thermal conductivity, anisotropy) on the flow and temperature fields is investigated. It is shown that such flows are fully three-dimensional and therefore can modify significantly starting as well as steady operating conditions of the thermoacoustic engine.

Thermoacoustic engines are energy conversion devices that convert thermal energy into mechanical work in the form of sound wave (gas oscillation). The sound wave is generated by thermal interaction between working gas and porous medium (regenerator) that possesses a large axial temperature gradient. Waste heat or solar thermal energy can be used as the heat source and noble gases or other inert gases such as air can be employed as the working gas, so that the thermoacoustic engines act as environmentally benign machines. The sound energy output can then be harnessed to drive a linear alternator to generate electricity. This paper presents experimental characterization of a traveling-wave looped-tube thermoacoustic engine with air working gas at various pressures including the onset temperature difference (the temperature difference between the regenerator ends required to start producing the sound), the harmonic frequencies and the pressure amplitudes of the generated sound waves. In this experiment, an electric heater is used as the heat source which supplies heat into the regenerator hot end. The experiment is carried out by measuring the temperatures at regenerator ends and sound pressure amplitudes at several points along the looped-tube. The measurements are performed at different charged pressures of air inside the looped-tube in the range of 100 kPa – 400 kPa. It is found that the smallest onset temperature difference is 417 °C which is obtained at charged pressure of 200 kPa. In addition, the second harmonic sound wave is dominantly generated at air pressure of 100 kPa, while the first harmonic (fundamental mode) sound wave dominates at 200 kPa, and the third harmonic shows up at other higher pressures. Moreover, the pressure amplitudes of the three harmonics are linearly getting higher along with the increasing charged pressure. The highest pressure amplitude of the first harmonic is 7 kPa, whereas those of the second and third harmonics are 4.8 kPa and 1.1 kPa, respectively, which are achieved at 400 kPa charged pressured. Furthermore, the sound frequencies are not significantly affected by the charged pressure variation, those are around 96 Hz, 192 Hz, and 289 Hz for the first, second, and third harmonics, respectively.

Thermoacoustic science focuses on the interaction between sound energy and heat energy and the thermoacoustic refrigeration system uses sound wave to attain a temperature gradient along a porous solid medium (stack). It is an emerging technology to replace conventional refrigeration system with the benefit of having no moving parts. In the present study, the effects of the material, length, and position of the stack inside the resonator tube on cooling effect across the stack are examined. Three different materials-nylon, ABS plastic and wood are used as stack material which yield a cooling effect in the range of 2-5°C across the resonator tube (length of 60 cm) depending upon the stack position at resonant frequency (145 Hz). Three samples of each stack (length of 3, 6, and 9 cm) are placed at a spacing of 10 cm inside the resonator tube. Operating frequency is constant at 145 Hz which is resonant frequency. Highest temperature difference is obtained for ABS plastic stack of 3 cm length at the closed end of the tube at resonant frequency. The maximum cooling load is observed at the closed end of resonator tube, but the coefficient of performance (COP) is found to be maximum at the open end (driver end) of the tube. The performance of the thermoacoustic system is numerically analyzed using DeltaEC software for the same set of operating conditions and is compared with the experimental findings.

Thermoacoustic Heat Engine is a renewable technology in the energy conversion field. This system is a suitable technology for catching the waste heat where it provides about 400 °C to 600 °C to fulfil the valuable work. The aim of this study is to determine the appropriate parameters in thermoacoustic heat engine for standing wave and analyse the highest performance of acoustic power. Delta EC software is used to do the code for simulating the thermoacoustic heat engine where it focus on four parameters which are frequency, pressure, temperature and heat exchanger length in order to know the better value used for design a thermoacoustic system. As result, the acoustic power with 2 mm of Hot Heat Exchanger (HHX) and 20 mm of Cold Heat Exchanger(CHX) length which gets from the graph of simulation in Delta EC is higher than research before which is 9.4 W. Thus, it is proving that Delta EC software is possible in order to get the optimum value in design thermoacoustic heat engine for standing wave.

This study addresses a problem introduced by finite-amplitude oscillations in a liquid piston thermoacoustic Stirling engine and proposes methods to overcome it. After the engine was operated by external heat, the liquid piston oscillated, keeping a flat surface when the amplitude was sufficiently low. However, it became unstable to cause ejection of droplets and formation of air bubbles because the acceleration amplitude exceeded gravitational acceleration. Two methods were proposed to recover the stable liquid surface: one method was the enlargement of the cross-sectional area of the tube around the liquid surface and the other method was the use of submerged polyethylene floats. These methods successfully suppressed the instability of the liquid piston Stirling engine and increased the pressure amplitude by almost twice of the original engine. In addition, adjusting the liquid filling level resulted in the ratio of pressure amplitude over the mean pressure reaching 18% when the acceleration amplitude was elevated to 36 m / s 2 by the installation of floats. The high-pressure amplitude is an advantage of a liquid piston thermoacoustic Stirling engine over thermoacoustic engines that use only gas as the working fluid.

International audience; This paper deals with the prediction of the frequency and the amplitude of self-sustained oscillations generated in thermoacoustic prime movers, which are compared to measurements. A specially designed, high amplitude, acoustic impedance sensor was developed to perform measurements of the input impedance of a thermoacoustic core, as a function of the heating power supplied to the device, of the frequency, and of the amplitude of acoustic forcing. Those measurements can then be used to predict the spontaneous generation of acoustic oscillations and their saturation up to a steady-state. Those predictions were successful for various acoustic loads connected to the thermoacoustic core. Moreover, the measurements of acoustic impedance as a function of the amplitude of acoustic oscillations are compared to a model based on the linear thermoacoustic theory, and this comparison provides insights into the processes controlling the saturation of acoustic oscillations. The experimental procedure described in this paper can also have practical value, since it provides an empirical way, in principle, to optimize the coupling between the thermoacoustic core and the load, so that the potential eciency of thermoacoustic energy conversion is maximized.

Linear alternators (LAs) coupled to thermoacoustic engines (TAEs) provide a viable solution to extract energy from a heat source in a variety of applications such as waste heat, energy harvesting, solar thermal and biomass power generation. For the electrical power to be maximised, the acoustic impedances of LA and TAE have to match. This requirement cannot, in general, be met by relying only on the design of the LA, but can be achieved at the control level, by using a fraction of the LA inverter current to create ‘electronic stiffness’ which contributes to the overall stiffness tuning the resonance frequency. The same concept can, in principle, be used to replace part of the mechanical spring stiffness in order to overcome the limitations in the mechanical design, at the expense of an increase in LA and inverter ratings. The impact of electronic stiffness on LA power capability and ratings is analysed here. Two meaningful scenarios are considered in the analysis: the LA derating for resonance frequency tuning and the oversizing when springs are partially replaced by electronic stiffness. The study is supplemented with experiments on a small-scale LA test rig.

Thermoacoustic technology potentially offers a sustainable and reliable solution to help address the continuing demand for electric power. Thermoacoustic devices, operating on the principle of standing or traveling acoustic waves, can be designed as a heat pump or a prime mover system. This technical strategy is environmentally friendly as it utilizes noble gases, or air, as the working fluid and does not directly produce harmful emissions. However, heating and cooling sources are required to create the required thermal gradient. Due to the inherit simplicity and limitation of moving components, thermoacoustic devices require little maintenance and have a forecasted long operational lifespan. This paper will present the design considerations necessary to construct a traveling wave thermoacoustic heat engine. The modeling, analysis, fabrication, and testing with integrated sensors will be discussed to offer insight into the capabilities and subtleties. A case study with system operation at 54 Hz and 7.8% thermal to acoustic efficiency will be presented.

A comparative analysis of different computational methods was implemented to evaluate their usage in engineering design methodology of orifice pulse tube refrigerator (OPTR). A thermodynamic model to estimate the lowest temperature level in cold heat exchangerfor a given ratio of the pulse tube and compliance was developed. Two detailed time-dependent axisymmetric computational fluid dynamic (CFD) models of the OPTR to predict its performance was considered. The computational results were compared with experimental results to validate the choice of model for numerical design methodology.

Concerns over environmental impacts of hazardous refrigerants have spurred much research into alternative technologies as well as more environmentally friendly refrigerants. A thermoacoustic refrigeration system uses no refrigerant but is currently not a feasible solution due to the still immature technology with much still unknown about the theories that explain the thermoacoustc cooling effects and the desired performance. This paper reviews past studies to achieve the desired outputs; lowest temperature, the highest temperature difference generated across the stack, the lowest acoustical work required for cooling, or/and the highest coefficient of performance (COP) of the standing wave thermoacoustic refrigerator and various attempts at optimization in terms of the many parameters that represent the outcomes. The review looked at methods employed to analyze the performance with discussions on the relevant parameters that must and have been be considered by past researchers. To date, most studies have been focused on the stack, the heart of the system. Optimization work has been performed parametrically, experimentally or/and numerically, where discrete variations of the parameters investigated are completed whilst others are held constant. Lately, genetic algorithm, a statistical approach, has been utilized in simultaneous optimization of the parameters of the desired outputs where conflicting objectives are possible. To date, thermoacoustic refrigerator remains an attractive alternative technology towards a global agenda of a more sustainable future.

Thermoacoustic refrigerator is an alternative cooling system, which is environmentally safe due to the absence of any refrigerants. The resonator tube of the system is of great importance; its design and dimensions influence the design and performance of the entire refrigerator. This work describes the design of the stack and the resonator along with the influence of its dimensions on the performance of the standing-wave thermoacoustic refrigerator. The resonator consists of two tubes, one larger than the other, characterized by the diameter ratio of the smaller over the larger diameter. A Lagrange multiplier method is used as a technique to optimize the coefficient of performance (COP) of the system. Results show that the resonator small diameter tube dissipates a minimum acoustic power at a diameter ratio of 0.46, which is 17 percent less than the published values. Moreover, the results show that the resonator length increases gradually with the increase of the mean design temperature which leads to the increase of the total acoustic power dissipated by the resonator, reducing the COP of the standing-wave thermoacoustic refrigerator.

In this paper, a 70 Hz pulse tube micro cryocooler (PTMC) driven by two opposite linear generators has been studied using linear theory of thermoacoustic. In contrast to conventional thermodynamic calculation method the results of a small-scale PTC as a thermoacoustic system were obtained. Numerical simulation was conducted to design the system and optimum structure parameters were obtained. With operating condition of 2.5 MPa mean pressure and 70 Hz operating frequency, a cooling power of 8 W at 77 K and a relative Carnot efficiency of 21% were achieved. In order to prove the applicability of numerical simulation using linear theory of thermoacoustic to such small-scale cooling system and to better understand the energy conversion characteristics of the system, distributions of key parameters such as acoustic work, exergy were presented and discussed.

In this paper a simplified two-dimensional computational model for studying the entropy generation characteristics of thermoacoustic heat exchangers with plane fins is presented. The model integrates the equations of the standard linear thermoacoustic theory into an energy balance-based numerical calculus scheme. Relevant computation results are the spatial distribution of the time-averaged temperature, heat fluxes and entropy generation rates within a channel of a parallel-plate stack and adjoining heat exchangers. For a thermoacoustic device working in the refrigeration mode, this study evidences as a target refrigeration output level can be achieved selecting simultaneously the heat exchangers fin length and fin interspacing for minimum entropy generation and that the resulting configuration is a point of maximum coefficient of performance. The proposed methodology, when extended to other configurations, could be used as a viable design tool for heat exchangers in thermoacoustic applications.

Thermoacoustic engines attract much attention for their lack of moving parts and relatively benign environmental impact. In this review, an introduction of the thermoacoustic effect is supported by a summary of related theoretical models for thermoacoustics. An overview of the current research and experimental prototypes including typical thermoacoustic prime movers, thermoacoustic refrigerators, thermoacoustically driven pulse tube refrigerators, thermoacoustic generators and miniature thermoacoustic engines is presented. The worldwide research activities and the related advances in the thermoacoustic engines, mainly in past 30 years, are reviewed in details. Finally, we present a short summary of promotional studies and perspectives in this developing research field.

This paper mainly discusses the onset and steady-operation features of low temperature differential multi-stage traveling wave thermoacoustic engines. The onset condition is firstly derived based on the linear network model of a four-stage thermoacoustic engine. Then, the impacts of the mean pressure, working substance, hydraulic radius of regenerator, and stage number of this thermoacoustic engine are investigated in detail on the onset temperature and self-excited oscillation frequency by applying the network model for multi-stage systems. The steady-operation features, typically the acoustic fields and performances of this multi-stage traveling wave thermoacoustic engine are also discussed in this paper. Simulation results are compared to the published experimental data in the open literature domain and showed a good agreement.

There are two major categories in a thermoacoustic prime-mover. One is the traveling-wave type and the other is the standing-wave type. A simple analytical model of a standing-wave thermoacoustic prime-mover is proposed at relatively low heat-flux for a stack much shorter than the acoustic wavelength, which approximately describes the Brayton cycle. Numerical simulations of Rott’s equations have revealed that the work flow (acoustic power) increases by increasing of the amplitude of the particle velocity (|U|) for the traveling-wave type and by increasing cosΦ for the standing-wave type, where Φ is the phase difference between the particle velocity and the acoustic pressure. In other words, the standing-wave type is a phase-dominant type while the traveling-wave type is an amplitude-dominant one. The ratio of the absolute value of the traveling-wave component (|U|cosΦ) to that of the standing-wave component (|U|sinΦ) of any thermoacoustic engine roughly equals the ratio of the absolute value of the increasing rate of |U| to that of cosΦ. The different mechanism between the traveling-wave and the standing-wave type is discussed regarding the dependence of the energy efficiency on the acoustic impedance of a stack as well as that on ωτα, where ω is the angular frequency of an acoustic wave and τα is the thermal relaxation time. While the energy efficiency of the traveling-wave type at the optimal ωτα is much higher than that of the standing-wave type, the energy efficiency of the standing-wave type is higher than that of the traveling-wave type at much higher ωτα under a fixed temperature difference between the cold and the hot ends of the stack.

Conventional reciprocating pistons, normally found in thermoacoustic engines, tend to introduce complex impedance characteristics, including acoustic, mechanical, and electrical portions. System behavior and performance usually rely on proper tuning processes and selection of an optimal point of operation, affected substantially by complementary hardware, typically adjusted for the specific application. The present study proposes an alternative perspective on the alternator behavior, by considering the relative motion between gas and piston during the engine mode of operation. Direct analytical derivation of the velocity distribution inside a tight seal gap and the associated impedance is employed to estimate the electro-acoustic conversion efficiency, thus indicating how to improve the system performance. The influence of acoustic phase, gap dimensions, and working conditions is examined, suggesting the need to develop tighter and longer seal gaps, having increased impedance, to allow optimization for use in upcoming sustainable power generation solutions and smart grids.

Thermoacoustic Heat Engine probably the most efficient energy source for electronic devices for the next 10 year ahead that require small amount of electrical energy to operate. This study was to simulate the Thermoacoustic Heat Engine (TAHE) standing wave system by conducting a Fluid Structure Interaction (FSI) by using a Thermoacoustic system’s software named DeltaEC for better uderstanding on the fundamental of TAHE standing wave system. Some characteristics or parameters in the system that were studied in order to derive the fundamental knowledge of TAHE standing wave system. The thickness of Hot Heat Exchangers (Hot HX) plays the major role in affecting the maximum acoustic power generated, the level of onset temperature difference and maximum pressure amplitude followed by the stack length. Hot HX dimension (thickness) contributes nearly 3.3% changes in maximum acoustic power where the lowest thickness scores the highest maximum acoustic power generated. 2.9% of increment on maximum acoustic power generated by altering the length of the stack by 5 mm.

Regenerator is the core component in the regenerative heat engines, such as thermoacoustic heat engine, and Stirling heat engine. The regenerator has a porous configuration, in which the thermoacoustic effect happens between the working gas and solid wall converting heat into acoustic work. In this paper, a novel experimental setup was developed to investigate the thermoacoustic conversion characteristic of the regenerator. In this system, two linear motors acted as compressors to provide acoustic work for the regenerator and the other two linear motors served as alternators to consume the acoustic work out of the regenerator. By changing the impedance of the alternators, the phase difference between the volume velocities at the two ends of the regenerator could be varied within a large range. In the experiments, the influence of phase difference, heating temperature and different materials on the performance of the regenerator were studied in detail. According to the experimental results, the output acoustic power increased when the phase difference between velocities of the compression and expansion pistons increased within this phase angle range. And the thermoacoustic efficiency had different optimum values with different heating temperatures. Additionally, it also shows that flow resistance and heat transfer area were very important to the performance. In the experiments, a maximum output acoustic power of 715 W and a highest thermoacoustic efficiency of 35.6% were obtained with stack and random fiber type regenerators respectively under 4 MPa pressurized helium and 650 °C heating temperature. This work provides an efficient way to investigate the thermoacoustic conversion characteristic of the regenerator. It also provides some clues to the regenerator design.

In this paper, a 300 Hz pulse tube cryocooler (PTC) driven by a double-acting thermoacoustic heat engine (DATHE) has been studied. A double-acting thermoacoustic heat engine incorporates several travelling-wave heat engine units, which are serially connected in a loop. Compared with the conventional thermoacoustic heat engine which involves a travelling-wave loop and a resonator, it has advantages of compact size and potentially high thermal efficiency. In this paper, a DATHE-PTC system was designed, optimized and studied in detail. Firstly, numerical simulation was conducted to design the system and optimum structure parameters of the system were obtained. With the operating condition of 4 MPa mean pressure and 300 Hz working frequency, a cooling power of 7.75 W at 77 K and a relative Carnot efficiency of 11.75% were achieved. In order to better understand the energy conversion characteristics of the system, distributions of key parameters such as acoustic work, phase difference were presented and discussed. Then, the coupling mechanism of the system was investigated. In addition, the influence of mean pressure on the system performance was studied.

Heat pump system, offering economical alternatives in recovering waste heat from different sources for using in various industrial, commercial and residential applications, is considered to be a very environmentally-friendly heat and power transfer system. In this paper, to solve the problems of traditional vapour compression heat pump working in unconventional conditions, a novel TWTAHP (travelling-wave thermoacoustic heat pump) is presented to meet the requirement of working in ultra-low temperature. Base on the theoretical simulation and structure optimization, an experimental apparatus for preliminary test has been built, which is only one single independent unit from the whole loop of the TWTAHP system. The results show that the simulation and the testing results were agreeable as expected. Under the -20°C environment temperature and the 50°C heating temperature, we could obtain a maximal COP h (heating COP) of 2.1 and 260W heating capacity for one unit by consuming acoustic power less than 200W. Furthermore, a COP h above 3.0 could be achieved when the ambient temperature was raised to 0°C.

A crucial element in a thermoacoustic power converter for reliable small-scale power generation applications is an efficient acoustic-to-electric energy converter. In this work, an acoustic-to-electric transducer for application with a back-to-back standing wave thermoacoustic engine, based on a linearly-acting variable-reluctance generator is proposed, built and experimentally tested. Static and dynamic experiments are performed on one side of the generator on a shaker table at 60 Hz with 5 mm peak-to-peak displacement for performance characterization. A theoretical and empirical model of the variable-reluctance generator are presented and validated with experimental data. A frequency scaling based on the empirical model indicates that a maximum power output of 84 W at 78% generator efficiency is feasible at the thermoacoustic engine’s operating frequency of 250 Hz, not considering power electronic losses. This suggests that the linearly-acting variable-reluctance generator can efficiently convert high frequency small amplitude acoustic oscillations to useful electricity and thus enables its integration into a thermoacoustic power converter.

The paper is devoted to one of engine types with external heating a thermoacoustic engine. Ways of transforming the energy of a shock wave of oscillating gas flow into electric energy are discussed. The authors suggest using a bidirectional impulse turbine as an energy converter. The distinctive feature of this kind of turbine is that the shock wave of oscillating gas flow passing through the turbine is reflected and passes through the turbine again in the opposite direction. The direction of turbine rotation does not change in the process. Different types of bidirectional impulse turbines for thermoacoustic engines are analyzed. The Wells turbine is the simplest and least efficient of them. A radial impulse turbine has more complicated design and is more efficient than the Wells turbine. The most appropriate type of impulse turbine was chosen. This type is an axial impulse turbine which has a simpler design than that of a radial turbine and similar efficiency. The peculiarities of the method of calculating an impulse turbine are discussed. They include changes in gas pressure and velocity as functions of time during the generation of gas oscillating flow shock waves in a thermoacoustic system.

To stabilize unstable combustion systems in gas turbines, aero-engines, rocket motors, boilers and furnaces, there is a need to develop a simple but applicable mitigation/control strategy. The present work considers using an electrical heater to mitigate limit cycle thermoacoustic oscillations sustained by a premixed flame. For this, nonlinear recurrence relation analysis of the thermoacoustic system with the premixed flame and an electrical heater confined is conducted first. It is shown that the heater placed at a proper location results in significant damping on the thermoacoustic oscillations. Experimental measurements are then performed. It shows that the heater can dampen thermoacoustic oscillations, depending strongly on (1) its location and (2) its output electrical power. The minimum electrical power Q min s is determined and compared for four different axial locations. The optimum position x opt s at which the heater is most effective in damping combustion-excited limit cycle oscillations is shown to be at 0.72 ⩽ x s / L ⩽ 0.75 for the eigenmode with frequency ω 1 / 2 π ≈ 240 Hz. This finding agrees well with those from the energy exchange analyses via linearized flame and heater models. It is also experimentally shown that sound pressure level is reduced by 70 dB, as the heater is placed at x opt s and its output power is increased to 218.2 W. Furthermore, the thermoacoustic mode frequencies are found to be increased with the limit cycle oscillating being dampened. The successful experimental demonstration opens up another possible way to mitigate/control thermoacoustic oscillations using an electrical heater.

In this study, variation in the characteristics of the pressure oscillations in a thermoacoustic engine is explored as the input heat flux is varied. A bifurcation diagram is plotted to study the variation in the qualitative behavior of the acoustic oscillations as the input heat flux changes. At a critical input heat flux (60 Watt), the engine begins to produce acoustic oscillations in its fundamental longitudinal mode. As the input heat flux is increased, incommensurate frequencies appear in the power spectrum. The simultaneous presence of incommensurate frequencies results in quasiperiodic oscillations. On further increase of heat flux, the fundamental mode disappears and second mode oscillations are observed. These bifurcations in the characteristics of the pressure oscillations are the result of nonlinear interaction between multiple modes present in the thermoacoustic engine. Hysteresis in the bifurcation diagram suggests that the bifurcation is subcritical. Further, the qualitative analysis of different dynamic regimes is performed using nonlinear time series analysis. The physical reason for the observed nonlinear behavior is discussed. Suggestions to avert the variations in qualitative behavior of the pressure oscillations in thermoacoustic engines are also provided.

Thermoacoustic processes involve heat and sound interactions. Their appearances and applications range from combustion instabilities to novel refrigerators and imaging techniques. The focus of this paper is on thermoacoustic oscillations occurring in gases inside chambers. The conversion of heat to sound energy in resonators generally requires in-phase fluctuations of acoustic pressure and heat addition, whereas in specially designed systems the supplied sound can pump heat along solid surfaces from low to high temperature regions. Fundamentals, investigation methods, and some applications of thermoacoustic oscillating phenomena in resonators are reviewed in this paper. Keywords: Thermoacoustics; oscillations; wave; Rijke tube

This paper discusses the effect of regenerator positioning on the intensity of the thermoacoustic effect in a looped-tube traveling wave thermoacoustic engine (TAE). Traveling wave thermoacoustic engines work on the principle of the acoustic wave moving through the loop of the engine, across the regenerator, with the pressure and velocity curves in phase, amplifying the acoustic effect. Increasing the intensity of the effect, while keeping the energy input the same increases the efficiency of the TAE, and can be further applied in a thermoacoustic refrigeration system to increase the COP. The intensity of the effect in relation to the positioning of the regenerator, also called a stack in standing wave devices, is proven here to have an optimum positioning inside of the straight part of the loop. The increase in intensity under constant heat input proves an increase in efficiency.

The thermoacoustic effect concerns conversion of energy between a gas and a solid in the presence of acoustic waves. Although the working principle is well understood, the optimal design of thermoacoustic devices remains a challenge. The present work aims to perform a numerical simulation of a simple standing-wave thermoacoustic device. The analysis of the flow and the prediction of the heat transfer are performed by solving the non-linear unsteady Navier–Stokes equations using the finite volume method implemented in the commercial code ANSYS-CFX. The goal of this work is to study the effect of the stack temperature gradient, on the acoustic pressure and the produced acoustic power. This stack temperature gradient generates the thermoacoustic instability in standing-wave thermoacoustic resonator. The obtained results show an increase of the acoustic pressure and the acoustic power while increasing in the stack temperature gradient. The thermodynamic cycles of the thermoacoustic device are illustrated and observed for the different stack temperature gradients.

In this paper, OpenFOAM package is used for the first time to simulate the thermoacoustic refrigerator. For simulating oscillating inlet pressure, we implemented cosine boundary condition into the OpenFOAM. The governing equations are the unsteady compressible Navier–Stokes equations and the equation of state. The computational domain consists of one plate of the stack, heat exchangers, and resonator. The main result of this paper includes the analysis of the position of the cold heat exchanger versus the displacement of the pressure node at large amplitude for successful operation of the refrigerator. In addition, the effect of the input power on the successful operation of the apparatus has been investigated. It is observed that for higher temperature difference between heat exchangers, the time of steady state solution is longer. We show that to analyze and optimize the thermoacoustic devices, both heat exchangers should be considered, coefficient of performance (COP) should be checked, and the successful operation of the refrigerator should be evaluated.

International audience; This paper proposes the experimental and theoretical study of nonlinear heat transport processes generated by large amplitude acoustic oscillations at the ends of a stack of plates in the presence of a temperature gradient. These processes are notably involved in the operation of thermoacoustic engines. The measurement method, a time-resolved and full-field digital holography interferometry technique, enables to measure the density fluctuations from the optical phase difference between two laser beams. This technique is applied to the analysis of density fluctuations in the vicinity of a stack submitted to a temperature gradient, firstly for the case of (uncontrolled) self-sustained acoustic oscillations generated spontaneously in a standing wave thermoacoustic prime mover, and secondly for the case of an assigned acoustic field whose amplitude is controlled by an external sound source. A theoretical model describing the advective heat transport by sound at the ends of the heated stack is also presented, and numerical simulations are then carried out. The comparison between experimental data and numerical simulations is provided for several stack positions, several sound pressure levels, and several amounts of heat supplied to the stack, and the results show good agreement between the experiments and the model.

This paper is concerned with the study of low-cost, low-power thermoacoustic electricity generators. Based on target electrical output power values of 50 and 100 W, three standing wave prototypes (of both one-stage and two-stage prototypes) integrating a commercial loudspeaker with different coupling arrangements are conceived. Each stage consists of a square-pore stack sandwiched between hot and ambient heat exchangers. The working gas is air at atmospheric pressure. The prototypes’ efficiency in converting heat to electrical power is simulated by the specialized Design Environment for Low-Amplitude ThermoAcoustic Engines (DeltaEC) design tool based on the linear theory of thermoacoustics. At a given operation frequency, the optimal impedance matching between the loudspeaker and the engine is realized by adjusting both the engine parameters (stack location, stack length, heat exchangers length, loudspeaker location) and loudspeaker parameters (load resistance and box volume). Computations reveal that the one-stage engine and two-stage engine with loudspeakers coupled in side-branch mode are able to meet the target output power values with comparable thermal-to-electric efficiency (4.6%). The two-stage engine with the loudspeaker coupled in push–pull mode is unable to reach the desired power output and is characterized by low conversion efficiencies (2%) due to the poor loudspeaker–engine acoustic impedance matching.

Thermoacoustic heat engine (TAE) has a component that namely stacks, hot heat exchanger, cold heat exchanger and resonator. The hot heat exchanger is the device that used to supply the heat on the stack to generate an acoustic work. Nowadays, the technology in development of TAE still in progress development, then to be investigated in order to step-up better prediction on the performance and design of TAE. The lacking is the main issue in research of heat exchanger, and as a consequence hot heat exchanger in TAE, the heat exchanger does not have an established method of design. Thus, this report was developed in order to design and analyse on performance test of hot heat exchangers and TAE. In the designed of the hot heat exchanger, the material kanthal was selected as a heating element and it produces the heat input of 490°C as temperature to the system which this value can generate an acoustic work in the system. Experimental studied, the result obtains the power to generate the system is 90watt which the TAE start working after 3 minutes the system was run. The hot heat exchanger supply the heat in the system at range 390 ≤ T°C ≤ 560.

Chemical and metallurgical processes enhanced by high intensity acoustic waves, thermoacoustic engines and refrigerators, fuel rods in nuclear reactors, heat exchanger tubes, offshore and vibrating structures, solar thermal collectors, acoustic levitators, microfluidic devices, cycling, musical acoustics, blood flow through veins/arteries, hearing in the mammalian ear, carbon nanotube loudspeakers, etc. The evolution of a myriad of processes involving the oscillation of viscous fluids in the presence of solid boundaries is up to a certain extent influenced by acoustic streaming. In addition to the sound field, viscous energy dissipation at the fluid–solid boundary causes a time-independent fluid circulation, which can lead to a significant enhancement of heat, mass and momentum transfer at large oscillation amplitudes. A particularly relevant phenomenon that can be notably affected by acoustic streaming is the promotion of sound waves by temperature gradients or viceversa (thermoacoustics), which is at th...

We have carried out three-dimensional Navier–Stokes simulations, from quiescent conditions to the limit cycle, of a theoretical travelling-wave thermoacoustic heat engine (TAE) composed of a long variable-area resonator shrouding a smaller annular tube, which encloses the hot (HHX) and ambient (AHX) heat exchangers, and the regenerator (REG). Simulations are wall-resolved, with no-slip and adiabatic conditions enforced at all boundaries, while the heat transfer and drag due to the REG and HXs are modelled. HHX temperatures have been investigated in the range 440–500 K with the AHX temperature fixed at 300 K. The initial exponential growth of acoustic energy is due to a network of travelling waves thermoacoustically amplified by looping around the REG/HX unit in the direction of the imposed temperature gradient. A simple analytical model demonstrates that such instability is a localized Lagrangian thermodynamic process resembling a Stirling cycle. An inviscid system-wide linear stability model based on Rott’s theory is able to accurately predict the operating frequency and the growth rate, exhibiting properties consistent with a supercritical Hopf bifurcation. The limit cycle is governed by acoustic streaming – a rectified steady flow resulting from high-amplitude nonlinear acoustics. Its key features are explained with an axially symmetric incompressible model driven by the wave-induced stresses extracted from the compressible calculations. These features include Gedeon streaming, Rayleigh streaming in the resonator, and mean recirculations due to flow separation. The first drives the mean advection of hot fluid from the HHX to a secondary heat exchanger (AHX2), in the thermal buffer tube (TBT), necessary to achieve saturation of the acoustic energy growth. The direct evaluation of the nonlinear energy fluxes reveals that the efficiency of the device deteriorates with the drive ratio and that the acoustic power in the TBT is balanced primarily by the mean advection and thermoacoustic heat transport.