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This manuscript describes an experimental and numerical investigation of transcritical thermoacoustic instability in a standing-wave setup using the refrigerant octafluoropropane (R-218) as the working fluid. Thermoacoustic instability is excited by two microtube heat exchangers separated by a vacuum-jacketed microtube stack. R-218 is allowed to flow axially through the microtubes into a closed resonator while heating and cooling fluids flow radially over the microtubes to create a temperature gradient. The fluid achieved pressure amplitudes up to 669 kPa (97 psi) at a temperature difference ΔT=Thot−Tcold of 150 K and a base pressure, P0, of 1.3 times the critical pressure (3.43 MPa). The high pressure amplitudes obtained are attributed to the strong density variations near the critical point of the working fluid. The thermoacoustic response was characterized in a set of parametric studies in which ΔT, base pressure, and resonator length were varied. A modeling approach based on linearized Navier–Stokes equations reproduces the experimental results with fair agreement. This work demonstrates promising application of transcritical working fluids to thermoacoustic engines as devices for energy extraction and waste heat removal.

Refrigerants are usually provided in the conventional refrigeration system although the refrigerants produce Chlorofluorocarbons (CFCs) and Hydro-chlorofluorocarbons (HCFCs), which are hazardous to the environment. However, these disadvantages can be overcome by using air or inert gas in the thermoacoustic refrigeration system. In the present experimental work, helium is used as a working gas with an operating pressure range of 0.2 MPa to 1.0 MPa in order to study the performance of thermoacoustic refrigerator (TAR) which is fabricated using Poly-Vinyl-Chloride (PVC). The parallel plate stacks with different porosity ratios have been considered to study the performance of TAR. The temperature difference between the hot and cold heat exchanger and acoustic dynamic pressure were recorded by using Bruel and Kjaer data acquisition system under different operating conditions. The effect of different operating parameters such as operating frequency (200 Hz to 600 Hz), cooling load (2 W to 10 W) and drive ratio (0.6% to 1.6%) have also been considered to study the performance of TAR. The TAR also modeled in DeltaEC software and the results are compared with the experimental outcomes and found to be in good agreement. The experimental results show that −2.1°C is the lowest temperature measured at cold heat exchanger by achieving the highest temperature difference of about 32.9°C. An improvement is around 36% as compared to that of previous experiments that used aluminium TAR. The highest Coefficient of Performance (COP) and the Relative Coefficient of Performance (COPR) are found to be 2.024 and 0.217 respectively.

Thermoacoustic engines and refrigerators have many advantages. They use environment-friendly working gases, their design is simple, and they can operate quietly. However, they have many design characteristics from geometric parameters and operating conditions. Besides this, they still have low efficiencies and performance. This paper summarises important considerations of the design and presents the state-of-the-art developments in thermoacoustic energy conversion devices. This includes recent studies and designs of the thermoacoustic refrigeration devices towards more efficient thermoacoustic engines and refrigerators. New insights into the design of resonators, the different sources of the power sources, the different stack geometries and working mediums were considered. The challenges that face the development of thermoacoustic devices were also discussed. Far too little attention has been paid to looking at these devices comprehensively. In further research, the use of neural networks and metadata as optimisation methods could be a means of significantly increasing the performance of these devices. There is also abundant room for further progress in enhancing oscillatory heat transfer. Moreover, further recommendations and studies were proposed for a better understanding of the interrelationship between the geometric parameters and operating conditions.

Axial impulse turbines have been utilized for oscillating water column wave-energy conversion because of their wide operating range without stalling point. Previous studies in the oscillating-flow environment gave no information on the performance of these turbines at high frequencies. This study extends the use of these turbines to conditions close to those encountered in thermoacoustic power generators, which require the turbine to operate at a large frequency in a closed duct. The use of bi-directional turbines as acoustic-to-mechanical power converters brings several advantages, such as their low acoustic impedance, in comparison with linear alternators, which facilitates integration with thermoacoustic engines. In the study, a variable-frequency test rig is set up using atmospheric air. Then, the performance of the bi-directional impulse turbine is studied at different rotor inlet/exit angle, rotor space-to-chord ratio, stator space-to-chord ratio, stator exit angle, and tip clearance. The dependence of the conversion efficiency on flow coefficient is reported over a set of constant input gas parcel velocities. The study also introduces an impedance segment to simulate the turbine on Delta-EC. The peak efficiency of the improved case is found to be 35.2% at a flow coefficient of 0.22, resulting an improvement of 38% over the reference case.

We propose a novel cooling and power cogeneration system based on duplex free-piston thermoacoustic-Stirling cycles and an oscillating linear alternator. In such a cogeneration system, a free-piston Stirling heat engine produces acoustic power via the thermoacoustic power cycle, and a free-piston Stirling refrigerator produces cooling power via the thermoacoustic refrigeration cycle; an oscillating linear alternator is used to couple the free-piston heat engine and refrigerator and simultaneously produces electricity. The configuration and operating principles of the cogeneration system are described along with the thermoacoustic energy transformations and transmission mechanisms. An experimental prototype system was designed, built and successfully operated for the first time. In the preliminary experiments, thermoacoustic oscillation startup typically occurred at heating temperatures around 50 to 60 °C, and a cooling power of about 230 W at -20 °C and an electrical power of more than 6 W were obtained at a heating temperature below 190 °C. The alternator-coupled system has potential applications in heat-driven cogeneration systems for low-grade waste heat recovery and solar energy utilization.

The analysis of thermoacoustic engines and the equations that describe the internal physical phenomena are far from simple. Previous studies on energetic characterisation of thermoacoustic engines are based only on the determination of the active acoustic power flow distribution. In this paper, another variable, known as the reactive acoustic power, is additionally estimated and studied. This article proposes a simple method which is based on the combination of both active and reactive acoustic power flow for the evaluation and optimisation of thermoacoustic Stirling engines. In addition the paper illustrates the method using a thermoacoustic Stirling engine demonstrator design which is able to fit three different feedback branches. The results show that the amount of reactive acoustic power supplied towards the core branch differs depending on the specifications of the selected feedback branch. Besides, the amount of reactive acoustic power distributed towards the core branch is a good indicator of the grade of traveling-wave phasing. An improvement of 16.4% in the active acoustic power towards the extraction branch is achieved and besides, the amplification of the active acoustic power through the core branch has increased by 12%. The method can serve as an effective tool to study and optimise thermoacoustic devices.

A model of quantum thermoacoustic refrigeration micro-cycle (QTARMC) is established in which heat leakage is considered. A single particle contained in a one-dimensional harmonic potential well is studied, and the system consists of countless replicas. Each particle is confined in its own potential well, whose occupation probabilities can be expressed by the thermal equilibrium Gibbs distributions. Based on the Schrodinger equation, the expressions of coefficient of performance (COP) and cooling rate for the refrigerator are obtained. Effects of heat leakage on the optimal performance are discussed. The optimal performance region of the refrigeration cycle is obtained by the using of Q objective function. The results obtained can enrich the thermoacoustic theory and expand the application of quantum thermodynamics.

The design of a bidirectional impulse turbine to convert thermoacoustic power into electricity is experimentally optimized. The turbine efficiency is measured for rotors with and without a shroud ring and for a varying tip clearance. Furthermore, the axial spacing between the guide vanes and rotor is varied with respect to the displacement amplitude of the acoustic wave. All measurements are carried out for several turbine loads and acoustic frequencies. For a chosen implementation, a design study on the guide vane and rotor blade geometry is presented to further optimize the bidirectional impulse turbine for thermoacoustic engines.

In this paper, a unified stability analysis method for thermoacoustic systems is proposed using the Nyquist stability criterion for MIMO (multi-input multi-output) causal stable open-loop systems. Toward this end, the conventional frequency response function in the thermoacoustic instability utilization literature, the transfer matrix, is replaced with a function for systems with a causal input–output relation between the incoming and outgoing traveling-wave pressure components, the scattering matrix, known in the thermoacoustic instability suppression literature, in order to represent whole subsystems including thermoacoustic cores and connected tubes as TITO (two-input two-output) stable open-loop systems. This makes it possible for the Nyquist stability criterion to be applied to the open-loop frequency response to analyze the closed-loop stability. The method provides a simpler procedure than the conventional one that is based on the Nyquist stability criterion for SISO (single-input single-output) systems in the thermoacoustic instability suppression literature because TITO open-loop systems can be assumed to be stable for thermoacoustic systems when traveling-wave pressure components are adopted as their input and output signals. It is shown that the spontaneous oscillation condition s for both standing- and traveling-wave thermoacoustic engines with various settings can be analyzed with a unified procedure, and the actual oscillation conditions are successfully estimated with the proposed method, indicating reasonable stability and/or instability margins in the same manner as in the Nyquist stability criterion for stable open-loop systems. That is, the feasibility of spontaneous oscillation is simply indicated by encirclement(s) of the origin by the Nyquist locus calculated from the measured frequency responses; multiple encirclements correspond to a distorted waveform in the measured pressure signal; the oscillation frequencies can be estimated by the outermost portion of the locus; and the distance between this portion and the origin corresponds to the steady-state pressure amplitude.

We obtain linearized, BiGlobal thermoacoustic solutions in a pulse tube driven via an imposed mean temperature gradient. Here, the pulse tube is treated as a key unit of a thermoacoustic heat engine, in which the conversion of thermal energy to useful acoustic fluctuations occurs. A primary goal of this work is to understand the hydrodynamic efficiency of the energy conversion process and how it depends upon some of the important operating parameters, including the geometry of the device which in the limit of long length-to-diameter ratio approaches the so-called narrow tube approximation. As this limit is frequently imposed in the wave propagation analyses of thermoacoustic devices, it is critical to investigate the physical connections of such a model to more realistic finite-length pulse tube configurations, which we do here. The mean flow is quiescent with an analytic mean temperature profile that still models the necessary physical details of the hot heat exchanger and regenerator. The computed thermoacoustic oscillations are found to be globally stable, approaching neutral stability conditions at the narrow tube limit. In finite-length tubes, three distinct types of modes are identified and analyzed. Here, within a linear framework, radial modes do appear to act as key enablers for longitudinal modes to be the primary carriers of acoustic energy from the pulse tube section, while the identified boundary modes, essentially numerical constructs, are ignored in the analysis. Further, a disturbance energy-based efficiency metric is constructed that provides mechanistic understanding of some of the key parameters in pulse tube operation. For finite-length tubes, it shows oscillations of the first asymmetric mode to be the most efficient, while the axisymmetric perturbations dominate for longer tubes that eventually lead to the idealized plane wave propagation.

Thermoacoustic engines have been recently regarded and developed as reliable, long-life, and environment-friendly engines by using experiments or mathematical models. Thermoacoustic mathematical models can be divided into two categories of linear and nonlinear. This paper introduces a coupled 1D-2D computational nonlinear model of heat and flow fields inside loaded standing-wave thermoacoustic engines. On the one hand, the computational cost of the present model is much lower than that of full CFD models whose computational domain contains the entire engine. On the other hand, it does not have the limitation of uniform global cross section as the simplified numerical models do. In addition to the coupled nonlinear model, another simulation based on linear thermoacoustic theory (LTA) has been performed. The model has been well validated using previous experimental data and compared with the results of LTA. Subsequently, the temperature and pressure distributions, the mean acoustic power, and heat transfer and volume flow rate distributions have been presented and discussed. This model is extendable to many other systems whose some parts have negligible multidimensional effects and the other parts have considerable ones.

Thermoacoustic refrigeration is an emerging technology that makes use of acoustic power to pump heat. The resonance frequency is important for the performance of thermoacoustic refrigerators, as it...

A jet pump with an asymmetrical channel can induce a time-averaged pressure drop in oscillatory flow, which can effectively suppress Gedeon streaming in looped thermoacoustic engines. In this work, the flow characteristics and time-averaged pressure drop caused by a jet pump in turbulent oscillatory flow are investigated through numerical simulation. Through the analysis of the dimensionless governing equations, the emphasis is put on the effects of Womersley number and maximum acoustic Reynolds number on the performance of the jet pump. Meanwhile, the steady flow resistance coefficients are also measured numerically. The results indicate that the oscillatory flow resistance coefficients are relatively insensitive to Womersley number when it is less than 46. Moreover, the oscillatory flow resistance coefficients agree well with the steady state flow results, which validate the quasi-static assumption in turbulent oscillatory flow. However, further increasing Womersley number will lead to a reduction in the time-averaged pressure drop. The simulation method and results, as well as the hydrodynamic mechanism beneath the results, are presented and discussed in detail.

Thermoacoustic refrigeration is being regarded as a promising alternative technology with the absence of harmful refrigerants including no or few moving parts, providing the ability to use low grade energy sources for energy conservation. However, the performances of thermoacoustic refrigerators are still low compared to their counterparts, the conventional vapor-compression ones due to involving multi-scale, even multi-field thermo-physical matters with electromagnetic or piezoelectric actuators. Therefore, design optimization of such systems has become quite necessary and significant for achieving competitive performances. In the present study, particle swarm optimization (PSO) is introduced as an intelligent method for such optimization. The stack unit as the key element responsible for thermoacoustic cooling effect, is focused in our first attempt. Coefficient of performance (COP) of the stack unit would be considered as the single-objective function to be maximized under some constraints including stack length, stack center position, blockage ratio and drive ratio. The present study had shown the capability and significant advantages of PSO through showing better optimal result compared to published literature ones. In addition, some considerations for optimization criteria were discussed. Based on this study, PSO could be possibly a favorable intelligent method suitable to the design optimization of thermoacoustic refrigerators or heat engines and their key components effectively.

Thermoacoustic refrigeration is considered as a potential alternative refrigeration technology with environmentally-friendly working substance, no or few moving parts, high reliability as well as utilization of low grade energy. Optimization for thermoacoustic systems or their key components has become a crucial task to make their performances competitive over conventional vapor-compression ones. In this study, fruit fly optimization algorithm (FOA) as an intelligent method is introduced to separately maximize coefficient of performance of stack unit and cooling power produced for one standing wave thermoacoustic refrigerator, as single objective functions under optimized stack length, stack center position, blockage ratio and drive ratio. FOA had shown better optimal results, compared to published literature results. Compared to results of genetic algorithm, COP had been enhanced by 15.8%. Moreover, cooling power was first discussed as another important objective to optimize. Studied results demonstrated that FOA is an effective approach for design optimization of thermoacoustic refrigerators.

This paper is focused on a novel heat-driven thermoacoustic combined cooling and power system operating at liquefied natural gas temperature. In this configuration, a linear motor phase adjuster of the refrigerator is used to convert the expansion work to electricity. The experiment work on the cooling and power unit at 110 K is presented. According to the experiment results, a cooling capacity of 503 W at 110 K with a relative Carnot efficiency of 32.2%, an electric power of 665 W and a total exergy efficiency of 58% were achieved. The experimental results show that this new technology can not only obtain a good cooling capacity comparable to the traditional thermoacoustic refrigerators, but also recover a considerable amount of electric power. Additionally, the impedance characteristics of the unit are also studied for the further coupling investigation with the thermoacoustic heat engine. This work shows a good prospect for off-grid natural gas liquefaction application.

Imposing a temperature gradient on a narrow channel can trigger an acoustic instability, driving self-sustained oscillations of the fluid. The temperature gradient required to trigger the instability is dramatically decreased when the channel wall is covered by a thin film of volatile liquid that can undergo phase change. In the present work, we consider a volatile-droplet aerosol on which a temperature gradient is imposed, and theoretically examine whether the heat and mass transfer between the droplets and gas can trigger the acoustic instability. The aerosol structure is separated into two disparate scales: the droplet ensemble, modelled as a periodic grid of channels through which the gas flows, and the ‘micro-scale’, in which the flow around a single droplet serves as an elementary unit in the periodic structure. This scale separation led to the derivation of a new function that accounts for losses caused by viscous, thermal and diffusive relaxation processes due to droplet–gas interactions. A stability analysis is then performed, utilising the derived ‘loss’ function, calculating the minimum temperature difference, $$\varDelta T$$ , that triggers the instability. Our analysis suggests that a non-uniform temperature distribution across aerosols may lead to an acoustic instability, which can subsequently enhance coalescence and agglomeration of droplets within an aerosol. Such phenomenon may be utilised to trigger or enhance the operation of thermoacoustic engines, and may possibly occur naturally in atmospheric clouds due to uneven solar irradiation.

Thermoacoustic refrigeration technology is a green alternative for the conventional refrigeration system in vehicles. The former uses environmentally friendly inert gases whereas the latter uses chemical refrigerants that can have severe impacts on the environment. In order to apply thermoacoustic technology in the automobile industry, the size and the weight of plausible thermoacoustic refrigerators should be decreased. The design of a new compact thermoacoustic refrigerator is described in this study. This thermoacoustic refrigerator uses two electroacoustic components and one thermoacoustic core. The technical details of design, fabrication, and testing processes are presented. A methodology for thermal design of the heat exchangers is proposed and validated by measurements. The effects of different parameters, such as the driver piston displacement amplitude, the phase shift between the voltage signals applied to the electroacoustic components, and the cold side temperature on the performance indices of the refrigerator are investigated. The performance of the prototype is compared with the performance calculated by the DeltaEC design model. The overall agreement between the calculated and the measured performance parameters is fair, but further insight into the temperature distributions reveals that non-linear effects yield discrepancies between the model and the experimental results. A preliminary discussion of the origin of these discrepancies is proposed.

Linear generators (LGs) are frequently used for energy harvesting with free-piston Stirling engines, thermoacoustic engines, and wave energy converters. This article presents a control strategy to track and maintain LG resonance conditions in real time. The algorithm is based on the LG response to a low-frequency amplitude modulation of the current component in phase with the instantaneous position ( d -axis). The averaged product of modulated air-gap power and modulation signal is fed into a controller to adjust the d -axis current and restore resonance. The use of air-gap power instead of dc power improves resonance tracking accuracy and eliminates steady-state low-frequency stroke oscillations. This article presents a full theoretical analysis providing accurate steady-state and small-signal models for control synthesis. The broad experimental validation included in the article proves that the control is able to restore resonance even when the force-source introduces significant additional mechanical impedance.

A high frequency, power, and efficiency diaphragm transducer is described for use with thermoacoustic engines and refrigerators. It uses a tube acting in compression and extension as the primary spring so that the mechanical resonant frequency is around 500 Hz. A high-frequency transducer results in more compact and higher power density thermoacoustic engines, refrigerators, and heat pumps. The tube spring furthermore constitutes a part of the pressure vessel so that the alternator is outside the pressure vessel, thereby simplifying transducer construction and reducing mass. The hermetically sealed transducer operates entirely without wear or lubricants, leading to a long maintenance-free lifetime. We present in situ efficiency measurement results at the full power operating frequency and temperature of the transducer. Despite the high operating frequency, the transducer efficiency is better than that of other transducers previously used in thermoacoustic systems. Accelerated fatigue test results on tube spring coupons justify the long-life claims. The exceptional stiffness of the primary spring in this transducer leads to the unusual need to consider stretch in additional transducer structural components. We present a multi-mass, lumped-element, coupled oscillator model of the transducer and discuss thermoacoustic system design using this model.

In Thermoacoustic Refrigerators, the oscillating pressure and simultaneous displacement oscillations of the medium induced by sound waves, generated by an acoustic driver, are utilized to generate cold temperatures. Thermoacoustic refrigeration is a novel solution to the present cooling techniques, which pose environmental hazards. The working medium is air or other harmless gases like helium, etc. With further advantages like minimal moving parts and simpler components, these devices can attain low temperatures maintaining a compact size. In the present analysis, a numerical study using DELTAEC has been conducted to establish the effect of variations in the important governing parameters on the thermoacoustic effect. The model used in simulations is based on an experimental setup. This study highlights the crucial design parameters to be optimized for successful implementation in practice.

The appearance and development of pulsation turbines is due to the advent of thermoacoustic engines - converters of heat into acoustic wave energy with the subsequent receipt of mechanical and electrical energy. A thermoacoustic engine involves obtaining mechanical work on a piston under conditions of a resonant frequency of movement of the piston and frequency of oscillations of the gas column in the resonator. The reason for the appearance of acoustic waves in the resonator is the complex process of converting heat into energy of an oscillating gas flow due to thermodynamic cycles in microvolumes of gas analogous to the Stirling cycle [1].

Existing acoustic test benches are usually costly devices based on proprietary designs, sensors, and acquisition devices. In this paper, a low-cost test bench for acoustic purposes is introduced. The design of the test bench takes into account not only the low-cost mechanical design, but also uses low-cost sensors and control boards. This test bench has been designed for a range of signals compatible with those used by thermoacoustic engines, but it can be useful for applications with similar requirements. Taking advantage of an auxiliary pressure reference, low-cost unidirectional differential pressure sensors can be used to significantly increase the accuracy of the sampling system. The acoustic and mechanical design and development are presented along with the sampling system and the sensors arrangement implemented. Both the sensor and sampling system are evaluated by comparison with a high-fidelity sound acquisition system. An unexpected effect on the time error values distribution of the low-cost acquisition system is found and described. Finally, the errors introduced by the system and the sensors in terms of time and pressure sampling are characterized. As a result, the low-cost system&rsquo, s accuracy has been satisfactory assessed and validated for the conditions expected in thermoacoustic experiments in terms of frequency and dynamic pressure.

Today, the parameters of thermoacoustic devices are calculated by linearized equations of the dynamics of viscous heat-conducting gas. Such programs require precise initial approximations for the system of equations to be solved. Therefore, an engineer spends tens of hours to get the convergence of a solution for a complex model. To facilitate this, the Rott-Swift acoustic equations are reduced to the four-port element equation for pressure and volume flow rate. The technique is based on the approximation of the elements included in the acoustic tract in the form of typical four-port network of acoustic circuits. Then the numerical integration of the acoustic equations along the tract length is replaced by the matrix product.The authors obtained matrices of tipical four-port elements for the device modeling: a channel section, a heat exchanger, a regenerator, etc. As an example, a diagram of a toroidal thermoacoustic engine (TAE) with a resonator is considered. The authors carried out modeling of acoustic branches using four-port networks, calculated the operating frequency of the TAE by the developed algorithm, determined the distribution of pressure and volume flow rate along the axis of the device, and energy estimates were made.The calculation results of this approximation were compared with the results obtained by the special DeltaEC software. Comparison of the results showed satisfactory compliance. Using the approximation allowed to halve the time spent by an engineer to obtain an initial solution. This method is recommended for calculating thermoacoustic engines and refrigerators.

The thermoacoustic engine can be a device to convert waste heat energy in the engine car become useful energy such as for charging battery in car or Air conditioner of the car. This work can be done by experimentally and numerically. There are some parameters that have an impact on the performance of the engine. They are geometry of the engines, working fluid, and mean pressure. The performance of the engine depends on the efficiency and the heating temperature. In the car, waste heat energy is not high enough. Therefore, we need to utilize the low heating temperature to be converted into useful energy. This study contributes to numerically the effect of mean pressure and loop’s radius of the regenerator on the onset temperature and the efficiency of traveling wave thermoacoustic engines. The application that is used to solve numerical problems is fortran95. There are two codings that are used in fortran95. They are stability limits and efficiency codes. The lowest onset temperature that achieved is 153˚C with efficiency up to 38.1% that can be reached when the mean pressure is 4.0 MPa and the loop's radius is 5 cm. This result indicated that we can use low heating temperatures from waste heat of engine car to turn on electronics equipment inside the car.

Refrigerators, ever since their conception, have always relied upon moving components – whether they are of a pump or of a compressor – to deliver the necessary work and heat input towards producing refrigeration. In today’s energy-driven economy, emphasis must be placed on to reducing the total energy consumption of the populous, as well as the resources involved in manufacturing conventional refrigerators. This is where the Thermo Acoustic Refrigerator (TAR), comes in. The purpose of this study is to design, fabricate and test an economically sound prototype of a TAR, so as to elucidate the technological and economic potential of thermoacoustic refrigeration as a replacement to conventional vapor-driven refrigeration. In the first part, a general introduction of refrigeration and TAR is carried out, followed by the second part, where design parameters are discussed and defined. The third part involves the actual fabrication of the TAR along with its assembly. The last parts document the results and their explanations, as well as the improvements that can be made to any future prototypes.

Extraction and utilization of automotive waste exhaust heat is an effective way to save fuel and protect the environment. One promising technology for this purpose is the thermoacoustic engine, whe ...

Incorporating thermoacoustic engines (TAEs) into clean cooking stoves offers to reduce rural energy poverty while cutting morbidity associated with smoke inhalation. TAEs are used to generate electricity from the waste heat of a cooking fire to provide power for lighting and personal devices. This study investigated the effect of TAE mean pressure using a numerical model of a twin-core, asymmetrically heated TAE. Automation code was developed to allow the numerical model to be optimised using the Nelder-Mead algorithm to maximise electrical power output at each mean pressure. The parameters available for optimisation were the length and position of two side volumes (stubs). A maximum electrical output of 59.63 W was determined at 2.2 bar mean pressure. This is a 90% increase on the original numerical model at atmospheric pressure. Simulation-based optimisation, as performed in this study, is identified as being universally applicable to the design of TAEs.

Self-excited thermoacoustic oscillations are favorable in thermoacoustic engines, but unwanted in many combustion systems due to the detrimental outcomes, including the structure vibration and overloaded thermal flux to combustor wall. To explore some proper methods to modify thermoacoustic oscillations, understanding the mechanism of thermoacoustic instability is needed. In the present work, the transition to instability in a Rijke-type thermoacoustic system with axially distributed heat source is explored experimentally. A silicon/ceramic tube is used as the acoustic resonator, and the distributed heat source is designed by wounding electric wires over two ceramic rings. The influences of the characteristics of heat source (including heating power, heater length and heater location), mass flow rate, and tube materials on the nonlinear dynamic behaviors and stability boundaries of the Rijke-type thermoacoustic system are systematically evaluated. Large-amplitude limit cycle is observed, and subcritical and supercritical bifurcations are present in the thermoacoustic system. For the system with fixed mass flow rates, the critical heat power for transition to instability is found to be approximately linearly proportional to the heater length, while the strength of pressure oscillations responds nonlinearly. In addition, the system with a thin heater is more prone to undergo subcritical bifurcation. As for the influence of the mass flow rate, it has also been found that increasing the mass flow rate would enhance the nonlinearity of the coupling between the unsteady heat release rate and acoustic waves, which leads to a high possibility of occurrence of subcritical bifurcation. For all the heater length studied, there is an optimal mass flow rate where the critical heat power for the transition to instability is minimal. Last, the results show that the thermoacoustic system with ceramic tube has a narrower stability region at small air flow rate and is more prone to subcritical bifurcation than the silicon tube.

Low-temperature heat is abundant, accessible through solar collectors or as waste heat from a large variety of sources. Thermoacoustic engines convert heat to acoustic work, and are simple, robust devices, potentially containing no moving parts. Currently, such devices generally require high temperatures to operate efficiently and with high power densities. Here, we present a thermoacoustic engine that converts heat to acoustic work at temperature gradients as low as ∼4–5 K/cm, corresponding with a hot-side temperature of ∼50 °C. The system is based on a typical standing-wave design, but the working cycle is modified to include mass transfer, via evaporation and condensation, from a solid surface to the gas mixture sustaining the acoustic field. This introduces a mode of isothermal heat transfer with the potential of providing increased efficiencies – experiments demonstrate a significant reduction in the operating temperature difference, which may be as low as 30 K, and increased output – this ‘wet’ system produces up to 8 times more power than its dry equivalent. Furthermore, a simplified model is formulated and corresponds quite well with experimental observations and offering insight into the underlying mechanism as well as projections for the potential performance of other mixtures. Our results illustrate the potential of such devices for harvesting energy from low-temperature heat sources. The acoustic power may be converted to electricity or, in a reverse cycle, produce cooling – providing a potential path towards solar heat-driven air conditioners.

Thermoacoustic engines with a looped-tube structure and multiple thermoacoustic stages are able to efficiently utilize low-grade heat. If additional measures to counteract acoustic reflections are taken, similar results can be achieved by using only one single stage. The implementation of a stub, which is a compliant duct closed on one side and T-branched to the engine’s loop on the other side, allows a precise tuning of acoustic conditions. By only changing position and length of the stub, phase difference and normalized impedance in the regenerator can be adjusted over a wide range. The method is experimentally validated by this paper. A test-rig is introduced that works with argon at atmospheric pressure. Measurements of thermal and acoustic conditions were conducted for several configurations of the stub. Afterwards, results are used to adjust a numerical DeltaEC-model. The proposed procedure employs simple and reasonable correction parameters. They take minor acoustic losses, non-linear effects and geometrical errors into account. Eventually, the simulated acoustic field is in very good agreement with experimental results. The stub’s acoustic tuning capability is verified. Further, the results show a large mismatch of thermal quantities. Gedeon streaming is expected to be responsible. It is later eliminated by use of a jet pump with a novel design. Hence, a major discrepancy is still observable being related to other effects of thermoacoustic streaming. Eventually, impacts of the jet pump to the acoustic field on system level are analyzed.

Heat exchangers under oscillatory flow conditions constitute a critical component of thermoacoustic engines and coolers for which effective design methodologies are not yet available. In this study, the thermal and pressure drop performance of compact Tube Heat Exchanger (T-HEX) under oscillatory flow conditions is investigated using experimental and numerical methods. A standing wave experimental set-up, driven by the Q-drive linear alternator, and a measurement technique were developed to measure the temperature and acoustic pressure near the T-HEX simultaneously. The symmetric arrangement of three identical heat exchangers, one ‘hot’ heat exchanger, centrally placed between two ‘cold’ heat exchangers, is employed for an improved thermal analysis. Furthermore, aerodynamic shape is used on the heat exchangers gas channels to improve flow conditions associated with a sudden change in the cross-section. Experimental results are found to agree well with the predictions from three-dimensional Computational Fluid Dynamics (CFD) models. The Nusselt number and pressure drop due to minor losses show dependency on the drive ratio (measured maximum oscillating pressure to the system mean pressure), the edge shape and hot heat exchanger temperature. At a high amplitude, the edge shape significantly minimises the minor loss pressure difference, with negligible effect on the thermal performance. The results reported in this study will benefit the development of compact heat exchangers for the thermoacoustic engines/refrigerators or Sterling engines/coolers in cryogenic applications.

Thermoacoustic refrigeration is an evolving cooling technology in which the acoustic power is used to pump heat. The operating conditions and geometric parameters are important for the thermoacoustic refrigerator performance, as they affect both its performance and the temperature difference across the stack. This paper investigates the effect of the stack geometric parameters and operating conditions on the performance of a standing wave thermoacoustic refrigerator and the temperature difference across the stack. DeltaEC software is used to make the thermoacoustic refrigerator model. From the obtained results, normalised values for the operating conditions andgeometric parameters are collected to compromise both the performance and the temperature difference across the stack. Highlights A theoretical DeltaEC model of a standing wave thermoacoustic refrigerator is built. Compromised values for the geometric parameters and operating conditions are collected. The physical description of the performance and the temperature difference change behaviour are presented.

Rural communities in developing countries often require small cold storage for vital medicines while having no access to electricity. The utilization of waste heat – produced in biomass burning cookstoves during daily cooking routines – to power a thermoacoustic engine driving a thermoacoustic refrigerator is investigated. The simplicity and affordability is met by the use of atmospheric air as working medium, cheap PVC ducting for acoustic waveguides and locally available blacksmithing technologies for simple heat exchangers. This paper describes DeltaEC modelling, fabrication and experimental evaluation of a laboratory concept demonstrator. The travelling-wave, looped-tube engine/cooler configuration is powered by a propane gas burner to mimic cookstove flue gases. A matching stub is used to match the acoustic impedances of engine and cooler. The optimum location of the cooler is investigated experimentally. The device achieves the minimum temperature of −8.3 °C and up to 7 W of cooling power at a storage condition of +8 °C.

This paper investigates the dynamic behaviour of a one-dimensional, standing-wave thermoacoustic engine with various boundary conditions. The thermoacoustic engine is composed of an acoustic tube and a ceramic stack with two heat exchangers. The tube has one end open and the other end closed with a deformable plate whose boundaries are constrained. Theoretical models are developed based on the acoustic wave equations and the linear theory of thermoacoutics and validated by experiment. With the validated theoretical models, a parametric study is performed to investigate the influence of the boundary conditions at both ends of the tube. It is found that acoustic radiation at the open end and the presence of the deformable structure at the closed end not only influence the onset temperature and frequency of the system, but also affect the acoustic field along the tube. The developed theoretical models and experimental method in this study provide useful guidelines for the design of thermoacoustic engines for potential energy harvesting systems.

This paper is concerned with the study of low-cost low-power thermoacoustic electricity generators. Based on a target electrical output power of 100 W, a two-stage standing wave prototype integrating a commercial loudspeaker in side branch arrangement is conceived. Each stage consists of a square-pore stack sandwiched between hot and ambient heat exchangers. The working gas is air at atmospheric pressure oscillating at the operation frequency of 194.5 Hz. Design issues and optimization procedures are discussed in detail. The prototype efficiency in converting heat to electrical power is simulated by the specialized design tool DeltaEC based on the linear theory of thermoacoustics. Computations reveal that at the heating temperature of 527 °C the target electrical output power is extracted by the loudspeaker with an acoustic-to-electric efficiency of around 70% and that the overall thermal-to-electrical conversion efficiency of the engine is 5.7%. This result suggests that in applications involving the use of loudspeakers as linear alternators and air at near atmospheric pressure as working fluid, electricity generators of the standing wave type could perform, comparably to their travelling-wave counterparts. This is likely due to a simpler design of the alternator-engine coupling and a simpler and more compact configuration.

This paper reports on the experimental and simulation studies of the influence of stack geometries and different mean pressures on the cold end temperature of the stack in the thermoacoustic refrigeration system. The stack geometry was tested, including spiral stack, circular pore stack and pin array stack. The results of this study show that the mean pressure of the gas in the system has a significant impact on the cold end temperature of the stack. The mean pressure of the gas in the system corresponds to thermal penetration depth, which results in a better cold end temperature of the stack. The results also show that the cold end temperature of the pin array stack decreases more than that of the spiral stack and circular pore stack geometry by approximately 63% and 70%, respectively. In addition, the thermal area and viscous area of the stack are analyzed to explain the results of such temperatures of thermoacoustic stacks.