Standing-wave Thermoacoustic Engine Research Articles

Investigation on transient process of thermoacoustic oscillation in a gas–liquid standing-wave thermoacoustic engine

This work focuses on the transient process of thermoacoustic oscillation in a standing-wave thermoacoustic engine with gas–liquid coupling oscillation. Observing the transient evolution of the pressure amplitude shows that the on-off phenomenon can occur through adjusting the location of the stack in the acoustic field, and there exists a critical distance from the stack center to the pressure antinode for this phenomenon to occur. The analysis from the perspective of energy balance indicates that energy dissipation induced by liquid surface instability is a key factor contributing to the occurrence of on-off phenomenon. Through installing a submerged float into the liquid column to stabilize the liquid surface, the on-off phenomenon can be suppressed. Besides, the temperature difference for achieving steady thermoacoustic oscillation is decreased and the steady-state pressure amplitude is increased. In this work, the significance of liquid surface instability on the transient process of thermoacoustic oscillation is revealed and should be considered for better characterizing the dynamics of gas–liquid thermoacoustic engines.

Predicting unsteady heat-fluid interaction features and nonlinear acoustic behaviors in standing-wave thermoacoustic engines using unsteady RANS, LES and hybrid URANS/LES methods

The study presented in this paper aims to simulate a standing-wave thermoacoustic engine (SWTAE) system by capturing its nonlinear thermoacoustic features with three-dimensional (3D) unsteady Reynolds-Averaged Navier-Stokes (URANS) and hybrid URANS/LES (Large Eddy Simulation) models such as Detached-Eddy Simulations (DES) and Stress-blended Eddy Simulation (SBES) models. For comparison, LES is also conducted as a reference case to examine the turbulence model effect on capturing the nonlinear features of heat-driven acoustics in the SWTAE system. The 3D model is validated against numerical results available in the literature, along with mesh- and time-independence studies. The comparison studies show that the computation time required for modeling SWTAE using SBES and DES is 28.4% and 36.5% less than LES, respectively. It is also shown that the prediction of the acoustic power of SWTAE using URANS is about 21.0% lower than that using LES, while the results from SBES and DES are in relatively good agreement with LES. Comparative studies of nonlinear hydrodynamics in the flow fields show that the results from SBES are closer to LES than DES, which can be attributed to the SBES model providing a faster transition to an explicit LES model outside the wall boundary layer. Furthermore, the heat transfer characteristics are compared by analyzing heat leaks and transversal heat flux, and it is found that the URANS model overestimates the heat transfer characteristics, while the results of the other three models are smaller than those obtained by URANS. In conclusion, the SBES model has great potential to be applied in simulating thermoacoustic nonlinear and flow behaviors of SWTAEs. It has the attractive features of delivering relatively accurate predictions and a significant reduction in computational times and costs compared to LES.

Design optimization and CFD analysis of the dynamic behavior of a standing wave thermoacoustic engine with various geometry parameters and boundary conditions

Thermoacoustic devices are converters of thermal energy into acoustic energy and vice versa. Although these machines contain simple components, the design of these machines is very challenging. In order to predict the behavior and optimize the efficiency of a standing-wave thermoacoustic engine designed to drive a thermally driven thermoacoustic refrigerator, considering changes in geometrical parameters and operating conditions, two analogies have been presented in this paper. The first analogy is based on a CFD simulation carried out to investigate the influence of stack parameters, working gas and boundary conditions on the thermoacoustic process. The second analogy is performed by the use of an optimization algorithm based on the simplified linear thermoacoustic theory to design and optimize the parameters investigated by the CFD study. Stack of parallel plates of normalized stack center positions of 0.007 to 0.26, normalized stack lengths of 0.018 to 0.11, and several gaps and thicknesses of plates and working gases are used. The results from the algorithm give the ability to design any thermoacoustic engine with high efficiency by picking the appropriate parameters. Simulation results show that decreasing thickness and position of the plates gives a significant efficiency. However, there are optimum values for length of the stack and the gap between two plates. The material chosen for the construction of plates should have a low thermal conductivity and gases with higher ratios of specific heats and lower Prandtl numbers are well suitable for thermoacoustic systems.

A unified approach for dynamic characteristic analysis of a standing-wave thermoacoustic engine with general impedance ends

As promising devices for energy conversion, thermoacoustic engines (TAEs) are attracting more and more research attention with regard to renewable thermal energy application. For a TAE, the oscillation frequency is a critical parameter in its design and operation, which can only be estimated currently through an often tedious process using a shooting method or a numerical iteration method, and an analysis model with classical boundary conditions. In this context, this study seeks to establish a unified model for dynamic analysis of a one-dimensional (1D) standing-wave TAE with general impedance ends. First, the differential equations governing the thermoacoustic behavior of the TAE and its impedance boundary conditions are simultaneously solved using the Galerkin method and an improved Fourier series expansion method. Then, the characteristic distributions of oscillation frequency, sound pressure, and volume flow rate are obtained by solving a standard eigenvalue problem. Finally, the effects of engine length, stack length and position, and impedance ends on the dynamic behavior of the TAE’s thermoacoustic system are investigated and analyzed. The results show that engine length has a significantly negative correlation with the modal frequencies in different working gases, and stack position and stack length have a slight effect on the oscillation frequency of the TAE. The arbitrary variations in impedance boundary conditions can be simulated from the open end to the rigid wall and all intermediate cases. The proposed model can efficiently predict changes in dynamic parameters of 1-D TAEs with impedance ends in a unified pattern and can serve as a reliable analysis tool for further research on TAEs coupled with various energy harvesters.

On the nonlinear behaviour of oscillatory flow in a high pressure amplitude standing-wave thermoacoustic heat engine

This article deals with the numerical study of the nonlinear features of oscillatory flow in a high pressure amplitude standing wave thermoacoustic heat engine. The overall performance of thermoacoustic devices can deteriorate due to nonlinearities caused by their operation at high pressure amplitudes. Acoustic mass streaming is one of these nonlinearities that can influence the performance of thermoacoustic systems by an undesirable convective heat transfer. Therefore, understanding the nonlinear phenomena within thermoacoustic devices is of fundamental and practical importance. The Navier‐Stokes and energy equations are solved on the structured meshes by resolving thermal and viscous boundary layers utilizing the open-source modelling software OpenFOAM. In particular, the paper looks in more detail at the causes of a non-uniform distribution of mean pressure along the resonator reported previously by both experimental and numerical studies. This is explained by an analogy to a “distributed fluidic pump” manifesting itself through the pumping effects of streaming patterns. Also, the non-linear regime of the mean dynamic pressure shows interesting start-up instabilities which can be influential on achievable efficiencies.

Start-up and damping of a standing wave thermoacoustic engine: model development and experimental evaluation

Start-up and damping of a standing wave thermoacoustic engine: model development and experimental evaluation

Numerical simulation of nonlinear phenomena in a standing-wave thermoacoustic engine with gas-liquid coupling oscillation

• A CFD model of gas-liquid standing-wave thermoacoustic engine is developed and experimentally verified. • Time-domain impedance boundary condition is employed to truncate the modelling of liquid column. • Entire transient evolution of self-excited thermoacoustic oscillation from onset to saturation is captured. • Three different acoustic streamings are observed through inspecting the time-averaged mass flux field. A computational fluid dynamics (CFD) model based on the compressible, nonlinear Navier-Stokes equations is developed to simulate the nonlinear phenomena in a gas-liquid standing-wave thermoacoustic engine, which employs gas as the working fluid and liquid as the phase-matching element. The CFD model is well validated when comparing the simulated onset and steady-state performances with those measured from a real experimental system. Based on the CFD model, the nonlinear phenomena of the engine are analyzed from three aspects. Firstly, the nonlinear dynamic phenomena of self-excited thermoacoustic oscillation from onset to saturation are investigated, focusing on onset temperature difference and steady-state pressure amplitude. Then, the nonlinear acoustic phenomena of the engine operating at steady state are analyzed. The steady-state fluctuating pressure is decomposed into a superimposition of the acoustic modes with different frequencies, which reveals the existence of high-order harmonics. Finally, the nonlinear hydrodynamic phenomena including multi-dimensional flow effect and mass streaming are observed by examining the steady-state flow fields in the engine. The proposed CFD model provides an effective approach to comprehending the mechanism of nonlinear phenomena in gas-liquid coupling thermoacoustic engines.

Temperature Difference and Stack Plate Spacing Effects on Thermodynamic Performances of Standing-Wave Thermoacoustic Engines Driven by Cryogenic Liquids and Waste Heat

Temperature Difference and Stack Plate Spacing Effects on Thermodynamic Performances of Standing-Wave Thermoacoustic Engines Driven by Cryogenic Liquids and Waste Heat

Numerical study of transient characteristics of a standing-wave thermoacoustic heat engine

Understanding of transient fluid flow and heat transfer characteristics within thermoacoustic devices is crucial to improve their start up processes and subsequently the achievable efficiencies. The present numerical study investigates the transient features of a standing-wave quarter-wavelength thermoacoustic heat engine composed of hot and cold heat exchangers, a stack, and a resonator. The non-linear fluid dynamical as well as energy equations are solved on the structured grids by resolving viscous and thermal boundary layers using OpenFOAM software. Full discretization of governing equations captured nonlinear features of flow at high pressure amplitudes which are typically ignored by the linear theory. The results show the existence of acoustic streaming phenomenon in the system due to the interaction of Stokes boundary layer adjacent to the solid regions which can lead to undesirable convective heat transport and as a result the reduction of the system efficiency.

Heat transport by thermoacoustically generated steady flow

Conventional heat exchangers for thermoacoustic engines have limited heat transfer area and may not utilize heat effectively. A self-circulating heat exchanger proposed by Backhaus and Swift is a looped tube that connects a heat source and a high-temperature heat exchanger, a steady flow is generated in the looped tube due to a thermoacoustic spontaneous acoustic oscillation taking place in a thermoacoustic engine and heat is transported from the heat source to the high-temperature heat exchanger by the steady flow. The purpose of this study is to confirm the heat transport by the acoustically generated steady flow. A standing-wave thermoacoustic engine was constructed with a looped tube connected to the high-temperature side heat exchanger. The relationship between the ratio of the lengths of the thermoacoustic engine and the loop tube and the pressure amplitude was measured. The pressure amplitude became zero when the length of the loop tube was about one times the length of the thermoacoustic engine. Furthermore, the streaming flow was confirmed indirectly by measuring the temperature inside the loop tube. The temperature field of the loop tube implied to the generation of the acoustically generated steady flow.

CFD Modeling and Performance Analysis of a Thermoacoustically Driven Thermoacoustic Refrigerator

Although thermoacoustic devices comprise simple components, the design of these machines is very challenging. In order to predict the behavior and optimize the performance of a thermoacoustic refrigerator driven by a standing-wave thermoacoustic engine, considering the changes in geometrical parameters, two analogies have been presented in this paper. The first analogy is based on CFD analysis where a 2D model is implemented to investigate the influence of stack parameters on the refrigerator performance, to analyze the time variation of the temperature gradient across the stack, and to examine the refrigerator performance in terms of refrigeration temperature. The second analogy is based on the use of an optimization algorithm based on the simplified linear thermoacoustic theory applied for designing thermoacoustic refrigerators with different stack parameters and operating conditions. Simulation results show that the engine produced a high-powered acoustic wave with a pressure amplitude of 23[Formula: see text]kPa and a frequency of 584[Formula: see text]Hz and this wave applies a temperature difference across the refrigeration stack with a cooling temperature of 292.8[Formula: see text]K when the stacks are positioned next to the pressure antinode. The results from the algorithm give the ability to design any thermoacoustic refrigerator with high performance by picking the appropriate parameters.

Mode transition in a standing-wave thermoacoustic engine: A numerical study

This study investigates the mode transition phenomenon in a standing-wave thermoacoustic engine (TAE) by means of computational fluid dynamics (CFD). Firstly, the steady-state responses of the TAE at selected temperature ratios are examined via continuous wavelet transform. The bifurcation diagram and spectral map indicate that, as the temperature ratio increases, the TAE experiences a series of bifurcations, through which first-mode periodic oscillations, quasiperiodic oscillations and second-mode periodic oscillations occur. Secondly, the TAE performances in the initial decay/build-up, nonlinear saturation and steady states are studied. The onset of the first and/or second acoustic mode is identified via dynamic mode decomposition. The oscillation frequencies and growth/attenuation rates from CFD agree well with those from the reduced-order network model. Nonlinear mode competition takes place during saturation in which the growth of one acoustic mode is affected or even totally inhibited by the growth of the other. At steady state, periodic oscillations exhibit a closed loop in the phase space whilst quasiperiodic oscillations generate a torus. The time-averaged acoustic energy density, acoustic intensity and efficiency increase with increasing temperature ratio. Finally, parametric studies are conducted to investigate the effects of the gap between stack plates and stack position on mode transition. It is found that the TAE will exhibit second-mode oscillations if the stack is near the closed end or the gap is small. Results in this study indicate that mode transition could become a novel approach to match the TAE with external loads for higher electric power outputs.

Thermoacoustically driven liquid-metal-based triboelectric nanogenerator: A thermal power generator without solid moving parts

Liquid metal has exhibited great potential in escalating power density in both triboelectric nanogenerator and thermoacoustic systems. An innovative endeavor of coupling these two systems through liquid metal enables a thermal power generator without solid moving parts and shows prospects in the recovery of waste heat. In this configuration, liquid metal functions as both triboelectric and resonant stuff. Based on the understanding, an experimental setup involving a standing-wave thermoacoustic heat engine and a liquid metal-based triboelectric nanogenerator is fabricated to validate the concept. So far, the highest open-circuit voltage amplitude of 15 V has been achieved, outweighing our previous work by 50%. Allowing for the reduction in size, the preliminary results well demonstrate the viability of future applications.

The Experiment Study on the Utilization of Standing Wave Thermoacoustic Engine to Drive Wells Bi-Directional Turbine

Standing wave thermoacoustic engine (SWTE) is a device for converting heat to propagate acoustic wave within straight and tubular of the resonator. The advantages of this engine include being able to use the waste heat as a source of heating, no moving parts and environmentally friendly. The utilization of this acoustic wave is to rotate air turbine because its movement that goes back and forth. The type of air turbine that appropriates to this character one of these is Wells bi-directional turbine, which is always rotated in the same direction regardless of the moving air direction. This study aims to investigate the performance of Wells turbine when paired with SWTE in the variation on geometry shape of blade and hub to tip ratio. The Wells turbine was embedded inside resonator of this engine has a diameter of 50 mm in the manner of the hub to tip ratio 0.6. The geometry shape of blade varied with airfoil type of NACA 0015, NACA 0018 and NACA 0024. SWTE was generating 4,67 kPa acoustic pressure and 138 Hz frequency within 52 mm inside diameter, 390 mm length of the resonator. The result was the Wells turbine NACA 0018 has rotational motion 2449 rpm, higher than the others types. Wells turbine NACA 0018 then tested with variations of the hub to tip ratio. Results showed that are generated rotational motion at 2614 rpm, 2449 rpm and 1555 rpm for 0.5, 0.6, 0.7 hub to tip ratio, respectively.. It can be concluded that the Well turbine NACA 0018 with hub to tip ratio 0,5 has best performance compare to the others.

Large eddy simulation of thermally induced oscillatory flow in a thermoacoustic engine

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.

Bistability and triggering in a thermoacoustic engine: A numerical study

Although bistability and triggering in a thermoacoustic engine have been observed experimentally in the literature, the underlying physics behind these nonlinear phenomena is not well understood. In this paper, computational fluid dynamics (CFD) simulations of the bistability and triggering phenomena inside a quarter-wavelength standing-wave thermoacoustic engine (TAE) are presented. First, the dynamic/acoustic and heat transfer/hydrodynamic characteristics of the TAE are examined through simulations. Numerical results are validated against theoretical predictions based on linear thermoacoustic theory. Secondly, we focus on the bistable/two-valued zone (so-called “hysteresis loop”) and thermoacoustic triggering in the TAE. CFD results demonstrate that there are two critical temperatures which determine the stability of a thermoacoustic system. Self-sustained acoustic oscillations cannot occur at a hot-end temperature less than the lower critical temperature (so-called “damping temperature”) and must occur at temperatures above the upper critical temperature (so-called “onset temperature”). In between, the system may either be in the quiescent state or in limit-cycle oscillations, and an external acoustic pressure disturbance may induce the oscillations to commence. The smaller the hot-end temperature, the larger is the external disturbance required to initiate instability. Bistability and triggering, which cannot be explained by linear theory, are further interpreted from both nonlinear dynamics and energy balance viewpoints by conducting bifurcation/phase space analyzes and examining the simulated temperature fields. The research work in this study provides useful guidelines for reducing the temperature difference for thermoacoustic instability, and thereby the quality of low-grade heat source a TAE can utilize.

Experimental Study on Variation of Tilted Angles Toward Acoustic Power of Thermoacoustic Engine

Tilted angles have to be researched for determining the good position of thermoacoustic engine. The research aims to find the best acoustic power with a tilted angle of the position, therefore the performance of Standing-Wave Thermoacoustic Engine (SWTE) is going to be increased. The performance is affected by the tilted angles. The natural convection which occurs in the core of Themoacoustic Engine will be different every tilted angle. The heat transfer characteristics within it were investigated under three tilted angles including, -90°, 0°, and 90°, with the resonator length 780 mm. This study was for getting the acoustic power which used the two sensors method with Data Acquisition (DAQ) NI-DAQmx 15.5 and two pressure transducers. It was calculated by Matlab R2013a. In this research is found that the acoustic power which is the best value is resulted on 90° of tilted angle, while the smallest value is found on 0°. Thes matter can be occured because phase difference of acoustic wave on the 90° is the biggest value, while the 0° is the smallest value. Further, have to study how phase difference has big effect toward the horizontal position which is resulted on the worse value compared the vertical position.

Comprehensive performance improvement of standing wave thermoacoustic engine with converging stack: Thermodynamic analysis and optimization

In a thermoacoustic engine, the ratio of the plate spacing to the thermal penetration depth crucial to the performance and efficiency of thermoacoustic conversion. However, the temperature gradient in the regenerator or stack causes the non-uniform distribution of the thermal penetration depth. In this paper, the thermoacoustic stack comprised of converging channels has been put forward and applied in a standing wave thermoacoustic engine. The influence of the ratio of the plate spacing to the thermal penetration depth at the cold end of the stack, yc/δkc, on the performance parameters of different models is investigated in detail. Results show that suitable value of yc/δkc in parallel and converging stack models can improve the acoustic power and efficiency. It is also found that the converging stack can basically replace the parallel stack with the same yc/δkc and reduce the cost of consumptive materials. A dimensionless parameter β is defined to evaluate the comprehensive performance of thermoacoustic engine and used to guide the design of the stack.

A coupled computational model of nonlinear thermoacoustics of standing wave heat engines

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.

Numerical studies of co-axial travelling-wave thermoacoustic cooler powered by standing-wave thermoacoustic engine

Design and optimisation of a co-axial travelling-wave thermoacoustic cooler (TWTC) powered by a standing-wave thermoacoustic engine (SWTE) is conducted based on the linear thermoacoustic theory. The target application is storage of medical supplies in rural communities of developing countries with no access to electricity grid, where waste heat from cooking can be used as energy input. The design is guided by construction simplicity and low cost. Thus the SWTE/TWTC coupling is enclosed in a constant-diameter resonator which accommodates a compact co-axial configuration of the TWTC. Following earlier experiments, working fluid is air at 10 bar, and operating frequency is 50 Hz. Parameters such as the ratio of the cross-sectional area of the cooler to feedback inertance and stack and regenerator geometries, have been optimised for maximum system efficiency. Here, at the target input power of 600 W a drive ratio of 7.75% can be obtained, leading to the cooling load of 133 W at 250 K. This corresponds to the SWTE thermal-to-acoustic efficiency of 17% and TWTC COP of 1.9, and the combined efficiency of 22.2%. The simulations are to inform the build of an enhanced prototype of the proposed device. A detailed discussion and guidelines for practical builds are provided.