Thermoacoustic Stirling Heat Engine Research Articles

Acoustic field improvement through adjustable resonator to enhance the performance of thermoacoustic-Stirling engine

In this study, the simulation and experiment of a thermoacoustic Stirling heat engine (TASHE) with a phase-adjuster are demonstrated. The phase-adjuster is used as a tool to tune the acoustic field of the TASHE in order to maintain the high performance of the system. The core components of the TASHE comprising an ambient heat exchanger, regenerator, hot heat exchanger, and feedback pipe, are located in the torus section. Besides, there is a resonator pipe, connecting to the torus, which acts as compliance to maintain the system stability, and the phase-adjuster is installed at the end of the resonator. Firstly, the TASHE is modeled by DeltaEC to search for the optimal configurations of the prototype. Due to the variations of acoustic load or operating conditions from the design criteria, the engine absolutely cannot provide maximum output power. The proposed phase-adjuster could keep its high efficiency by re-matching the frequency. In the experiments, the self-excited and steady-state temperatures of the TASHE are around 755 K and 670 K, respectively. Here, this TASHE can provide an acoustic power of up to 40 W, and an energy conversion efficiency of 12.03%. In comparison, there is a reasonably good agreement between the measured and DeltaEC simulated results. This can reflect on the preciseness of the proposed model. In the case of the TASHE operated under the off-design condition, these scenarios certainly drop its efficiency. Consequently, the role of the phase-adjuster in improving its performance by tuning the acoustic field is presented here.

Performance Improvement of a Thermoacoustic Stirling Engine With In-Line Phase-Adjuster

Abstract This study examined the influence of an in-line phase-adjuster on the energy conversion efficiency of a thermoacoustic Stirling heat engine (TASHE). The numerical and experimental investigations were performed. An acoustic field in the system can be adjusted using the phase-adjuster. Therefore, the thermoacoustic engine could maintain high-level performance at all operating conditions. The engine is composed of a torus section where core components were located, and a long resonator pipe. The phase-adjuster or the telescopic in-line piston was set up at the tail of the resonance tube. The TASHE was modeled by employing DeltaEC to search the optimal configurations of the prototype. Due to the variations of acoustic loads or operating conditions from the design criteria, the engine absolutely cannot maintain maximum efficiency. The proposed phase-adjuster could bring back its maximum efficiency by re-matching the acoustic impedance in the regenerator. The TASHE was designed to operate with compressed air at 9 bar. In the experiments, the self-excited temperature of the engine was around 480 °C, and the steady-state temperature was about 397 °C. The TASHE can provide an acoustic power of up to 40 W. The thermo-to-acoustic efficiency of 12.03% related to 22.56% of the theoretical Carnot efficiency was achieved. There was a reasonably good agreement between the measured and DeltaEC simulation results. This can reflect on the preciseness of the proposed model. Furthermore, the function of the phase-adjuster in tuning the acoustic impedance was also demonstrated experimentally. In case of the TASHE being operated under the off-design conditions, i.e., due to pressure leak or heat losses, these scenarios would drop the efficiency of the system. The research presented in this study can confirm that the phase-adjuster is the component applied to tune the acoustic field in the regenerator accurately with minimum changes in the system. It could help to improve the efficiency of such consequences.

The effect of phase-adjuster on the performance of a thermoacoustic Stirling heat engine

The travelling-wave thermoacoustic Stirling heat engine with a phase-adjuster is proposed in this study. The phase-adjuster was located at the end of the resonator pipe. It plays a key role in adjusting the acoustic field to maintain the high impedance and travelling-wave phase region within the regenerator when the acoustic loads and operating conditions are off-design. These are the crucial conditions for the engine to perform efficiently. This configuration leads to a new type of travelling-wave thermoacoustic engine that the acoustic network in the regenerator can retain the high-efficiency level with acoustic load variation. The proposed concept has been investigated numerically based on linear thermoacoustic theory. Compressed air at 10 bar was applied as the working medium. The operating frequency was about 44 Hz. Different shapes of the resonator: a straight resonator and an enlarged resonator, were also investigated. As a result, the higher efficiency was achieved from the engine with the larger resonator. The maximum thermal efficiency was up to 17.2%, which corresponds to 33.5% of Carnot efficiency.

Numerical and experimental investigation on the effect of regenerator mesh size on performance of the traveling wave thermoacoustic-stirling heat engine

Traveling wave thermoacoustic-Stirling heat engine is a unique device that is used to convert heat energy to acoustic work. In this article, the thermoacoustic engine is modeled, manufactured and tested. The cyclic analysis is used to carry out the numerical analysis based on the Stirling cycle of the engine. This analysis has considered for study the physical property and characteristics of the regenerator matrix. The traveling acoustic wave produced from the prototype is measured using an audio-speaker as a linear alternator for extracting electrical power from acoustic power. The size and geometry of the regenerator mesh is defines the thermal penetration depth, pressure drop due to it, dead volume across the matrix and acoustic power produced. Hence, there is a need to optimize the mesh size for a given mesh material to maximize the power generated from the engine. The engine has tested with five combinations of mesh sizes. The results of the numerical analysis has shown that 50.1 W power is produced and the tested engine has developed 44.9 W electric-power with the thermal efficiency of 8.3% respectively for 304 wire mesh of SS having mesh size of 60 strands/inch. The results obtained from the numerical analysis and the experiments performed are presented in this paper.

An efficient hybrid approach of improved adaptive neural fuzzy inference system and teaching learning-based optimization for design optimization of a jet pump-based thermoacoustic-Stirling heat engine

The acoustic streaming is a key drawback and eliminates the performance of a jet pump-based thermoacoustic-Stirling heat engine. The present study deals with a new hybrid optimization approach to reduce the acoustic streaming energy. The proposed work is an integration of Taguchi method (TM), adaptive neural fuzzy inference system (ANFIS), and teaching–learning-based optimization (TLBO). The Taguchi method plays three important roles. The first role is to layout the number of experiments. The second role is to identify the most appropriate parameters for ANFIS structure regarding the number of input membership functions (MFs), types of input MFs, optimal learning method, and types of output MFs. In order to determine the optimum parameters for the ANFIS structure, the root-mean-squared error, a performance criterion, is minimized by using the TM. The final role of TM is to optimize the controllable parameters of the TLBO. Subsequently, modeling between geometric parameters and acoustic streaming is established by the built ANFIS structure. Finally, the TLBO is adopted by optimizing the design parameters. The outcomes of study revealed that the acoustic streaming is relatively reduced. Based on Wilcoxon signed-rank test and Friedman test, it proves that the effectiveness of the proposed hybrid approach is better to other evolutionary algorithms. The current approach is an efficient optimizer for complex optimization problems.

Application of response surface methodology and desirability approach to investigate and optimize the jet pump in a thermoacoustic Stirling heat engine

A jet pump used in the thermoacoustic engine can improve the performance by suppressing the Gedeon streaming. In this paper, response surface methodology (RSM) and desirability approach are used to investigate and optimize the jet pump in a thermoacoustic Stirling heat engine (TASHE). The position, length, diameter and tapered angle of jet pump are selected as the designing parameters, and pressure amplitude and time-averaged power dissipation across the jet pump are adopted as responses. The analysis of variance (ANOVA) is conducted to identify the significant influences of jet pump's parameters on responses. The regression models are obtained, and prediction deviations for pressure amplitude and power dissipation are within ±2% and ±5% respectively. In addition, the influences of jet pump’s parameters on responses are plotted by 3D surface. Eventually, the designing parameters of jet pump are optimized to achieve both maximal pressure amplitude and minimal power dissipation. The optimal results are validated by comparing with DeltaEC simulation. Results show that the deviations between DeltaEC and RSM for pressure amplitude and power dissipation are 1.86% and 2.88%, which indicates that RSM as an assessment methodology in optimizing jet pump is feasible.

Prediction and parametric analysis of acoustic streaming in a thermoacoustic Stirling heat engine with a jet pump using response surface methodology

Jet pumps are widely used in thermoacoustic Stirling heat engines and pulse tube cryocoolers to restrain acoustic streaming and improve the system performance. In this paper, a regression model is presented to predict the acoustic streaming in a thermoacoustic Stirling heat engine (TASHE) with different structure parameters of the jet pump. These parameters include position, length, inner diameter and tapered angle. Response surface methodology (RSM) is used to study the relationship between structure parameters of jet pump and acoustic streaming. A regression model is developed to predict the acoustic streaming. The analysis of variance (ANOVA) is conducted to describe the rationality of regression model and examine the statistical significance of factors. In addition, the relationship between acoustic streaming and structure parameters of jet pump is presented using 2D contour and 3D surface plot. It reveals that small position, length, tapered angle and large inner diameter can help suppress acoustic streaming. Eventually, four random confirmation tests are performed to verify that the regression model can predict acoustic streaming reasonably. This work provides theoretical guidance for controlling acoustic streaming using jet pump.

Effect of different working gases on the performance of a small thermoacoustic Stirling engine

The performance of a small thermoacoustic Stirling heat engine (TASHE) was investigated with three kinds of working gases experimentally and numerically. The examined performances focused on the operating frequency, onset temperature, pressure amplitude and some temperature characteristics after onset. The working frequency with nitrogen, argon and helium as the working gas was 45 Hz, 42 Hz and 130 Hz, respectively. The engine worked with helium in a much wider range of mean pressure than with nitrogen and argon. There was an optimal mean pressure for the minimum onset temperature for each working media. Using nitrogen and argon as working gas rather than helium, another optimal mean pressure for the highest pressure ratio was obtained in the experiment. The loop dimension was indispensable in determining the frequency and the highest pressure ratio was observed in the resonator cavity.

Computation of the influence of a phase adjuster on thermo-acoustic Stirling heat engine

In this paper, a phase adjuster (PA) is introduced in the thermo-acoustic Stirling heat engine (TASHE) and a two-dimensional compressible model of a TASHE with PA is developed. To improve the performance of the TASHE, the influences of the PA’s inner diameter on pressure amplitude, volume flow rate, and the phase angle between them, as well as the acoustic power are systematically analyzed, and the results are compared to the case without PA. The results show that the introduction of PA is useful for the phase adjustment, especially in the regenerator. The relative change of the phase angle in the regenerator reaches up to 60% for the PA with the inner diameter of 30 mm. The acoustic power in the resonator increases by 10% and 8% when the inner diameter of PA is 50 mm and 70 mm, respectively. The influence of PA is also discussed through the lumped-element model of the TASHE. In conclusion, PA with a suitable inner diameter can increase the acoustic power and promote the heat-to-sound energy conversion. This study can provide the theoretical guidance for the design and optimization of PA.

Effect of variable mechanical resistance on electrodynamic alternator efficiency

The rapidly growing energy market constantly discovers new alternatives for generating environmentally-friendly electricity from sustainable energy sources. An externally-heated traveling wave thermo acoustic Stirling heat engine is a leading candidate, operating using a variety of viable heat sources. Although this engine holds great promise for a low-cost and maintenance-free solution, its current reported efficiency still inhibits its market penetration, probably owing to the friction introduced by the single moving element – the linear alternator.This research quantifies the main parameters affecting the complex thermal–acoustical–electrical system, clarifying the critical role of frictional losses. An analytical model has been developed, enabling examination of the influence of the critical physical parameters on the electro-acoustic conversion efficiency. A measurement method for precise determination of the mechanical friction constant has been developed, which enables measuring the friction at the engine’s working frequency. A direct measurement of an engine’s transfer impedance at room temperature enables to find the exact natural frequency before field operation, and calibrate external hardware accordingly. A detailed simulation using DeltaEC™ indicates that the tight seal gap between the moving piston and its cylinder has a significant impact on the system’s overall efficiency.

Development of a 3 kW double-acting thermoacoustic Stirling electric generator

In this paper, a double-acting thermoacoustic Stirling electric generator is proposed as a new device capable of converting external heat into electric power. In the system, at least three thermoacoustic Stirling heat engines and three linear alternators are used to build a multiple-cylinder electricity generator. In comparison with the conventional thermoacoustic electricity generation system, the double-acting thermoacoustic Stirling electric generator has advantages on efficiency, power density and power capacity. In order to verify the idea, a prototype of 3kW three-cylinder double-acting thermoacoustic Stirling electric generator is designed, built and tested. Based on the classic thermoacoustic theory, numerical simulation is performed to obtain the thermodynamic parameters of the engine. And distributions of key parameters are presented for a better understanding of the energy conversion process in the engine. In the experiments, a maximum electric power of about 1.57kW and a maximum thermal-to-electric conversion efficiency of 16.8% were achieved with 5MPa pressurized helium and 86Hz working frequency. However, we find that the mechanical damping coefficient of the piston is dramatically increased due to the deformation of the cylinder wall caused by high thermal stress during the experiments. Thereby, the system performance was greatly reduced. Additionally, the performance differences between three engines and three alternators are significant, such as the heating temperature difference between three heater blocks of the engines, the piston displacement and the output electric power differences between three alternators. These problems need further investigation. This work presents a new thermal-to-electric conversion technology, which can be utilized in many energy area, such as solar energy, industrial waste heat and so on.

Design and optimization of a cascade thermoacoustic Stirling engine

This article deals with the design and optimization of a cascade thermoacoustic Stirling engine. Different configurations for cascade thermoacoustic Stirling heat engine are investigated. A simplified model is proposed to enable initial design guidelines. The most important characteristics of the cascade system are examined. The results indicate that the thermoacoustic engines in cascade configuration may achieve very low onset temperatures. A suitable configuration is selected and the locations and dimensions of different thermoacoustic units are optimized to achieve low onset temperatures as well as high values for the acoustic power output and efficiency. The results support the employment of cascade engine systems.

Thermodynamic analysis of onset characteristics in a miniature thermoacoustic Stirling engine

This paper analyzes the onset characteristics of a miniature thermoacoustic Stirling heat engine using the thermodynamic analysis method. The governing equations of components are reduced from the basic thermodynamic relations and the linear thermoacoustic theory. By solving the governing equation group numerically, the oscillation frequencies and onset temperatures are obtained. The dependences of the kinds of working gas, the length of resonator tube, the diameter of resonator tube, on the oscillation frequency are calculated. Meanwhile, the influences of hydraulic radius and mean pressure on the onset temperature for different working gas are also presented. The calculation results indicate that there exists an optimal dimensionless hydraulic radius to obtain the lowest onset temperature, whose value lies in the range of 0.30–0.35 for different working gases. Furthermore, the amplitude and phase relationship of pressures and volume flows are analyzed in the time-domain. Some experiments have been performed to validate the calculations. The calculation results agree well with the experimental values. Finally, an error analysis is made, giving the reasons that cause the errors of theoretical calculations.

A high performance thermoacoustic engine

In thermoacoustic systems heat is converted into acoustic energy and vice versa. These systems use inert gases as working medium and have no moving parts which makes the thermoacoustic technology a serious alternative to produce mechanical or electrical power, cooling power, and heating in a sustainable and environmentally friendly way. A thermoacoustic Stirling heat engine is designed and built which achieves a record performance of 49% of the Carnot efficiency. The design and performance of the engine is presented. The engine has no moving parts and is made up of few simple components.

A miniature thermoacoustic stirling engine

A miniature thermoacoustic stirling engine was simulated and designed, having overall size of length 0.65 m and height of 0.22 m. The acoustic field generated in this miniature system has been described and analyzed. Some efforts had been paid to coupling and matching, and a miniature thermoacoustic engine and some extra experimental components have been constructed. Analysis and experimental results showed that to obtain better performance of the engine, the diameter of the resonance tube must be chosen appropriately according to the looped tube dimension and the input heating power. It provided an effective way to miniaturize the thermoacoustic stirling heat engine. The experimental results showed that the engine had low onset temperature and high pressure amplitude and ratio. With the filling helium gas of 2 MPa and heating power of 637 W, the maximal peak to peak pressure amplitude and pressure ratio reached 2.2 bar and 1.116, respectively, which was able to drive a refrigerator, a heat pump or a linear electrical generator. The operating frequency of the engine was steady at 282 Hz.

Influence of resonator diameter on a miniature thermoacoustic Stirling heat engine

A small scale thermoacoustic Stirling engine (TASHE) is simulated according to the linear thermoacoustic theory. The computed results show that in a small scale thermoacoustic Stirling heat engine, the diameter of the resonance tube might have important influences on the working frequency and the performance of the engine, which are always neglected in a large scale system. Likewise, the analysis and experimental results show that in order to obtain better engine performance, the diameter of the resonance tube must be chosen appropriately according to the looped tube dimension and the input heating power. This provides an effective way to miniaturize the thermoacoustic Stirling heat engine. According to the computation and analysis, a small scale engine was built, the resonance tube length and diameter of which were about 350 mm and 20 mm, respectively, and the working frequency was about 282 Hz. When the input heating power was about 637 W, the maximal peak to peak pressure amplitude and pressure ratio reached 0.22 MPa and 1.116, respectively, which were able to drive a thermoacoustic refrigerator or an electrical generator.

Study of nonlinear processes of a large experimental thermoacoustic-Stirling heat engine by using computational fluid dynamics

This article focuses on using computational fluid dynamics (CFD) method to study several important nonlinear phenomenon and processes of a large experimental thermoacoustic-Stirling heat engine. First, the simulated physical model was introduced, and the suitable numerical scheme and algorithm for the time-dependent compressible thermoacoustic system was determined through extensive numerical tests. Then, the simulation results of the entire evolution process of self-excited thermoacoustic oscillation and the acoustical characteristics of pressure and velocity waves were presented and analyzed. Especially, the onset temperature and the saturation process of dynamic pressure were captured by the CFD simulation. In addition, another important nonlinear phenomenon accompanying the acoustic wave, which is the steady mass flow through the traveling-wave loop inside the thermoacoustic engine, was studied. To suppress the steady mass flow numerically, a fan model was adopted in the simulation. Finally, the multidimensional effects of vortex formation in the thermal buffer tube and other components were displayed numerically. Most importantly, a substantial comparison between the simulation and experiments was made, which demonstrated well the validity and powerfulness of the CFD simulation for characterizing several complicated nonlinear phenomenon involved in the self-excited thermoacoustic heat engine.

An energy-focused thermoacoustic-Stirling heat engine reaching a high pressure ratio above 1.40

A new tapered resonator was introduced in the thermoacoustic-Stirling heat engine (TASHE) to explore its potential of achieving higher pressure ratio. With average pressure of 1.5 MPa and heating power of 3 kW, a high pressure ratio above 1.40 was obtained, which is very beneficial to be a powerful driving source of pulse tube cryocoolers or thermoacoustic refrigerators. Moreover, a relatively low onset temperature of 73 °C was also observed that showed the feasibility of its application in the field of using low quality heat sources.

Amplification of acoustic intensity of a pulse wave propagating through a tube having a temperature gradient

Ceperley has theoretically pointed out that when acoustic traveling waves pass through a long duct with a positive temperature gradient, the acoustic intensity is amplified as a result of the thermodynamic cycles that the gas undergoes. Even though he only observed damping of the acoustic intensity in the experiment, his idea serves as a starting point of new acoustic devices known as thermoacoustic Stirling heat engines. For a further progress of the thermoacoustic technology, experimental verification of his idea is of great importance. In this paper, we report on the thermoacoustic amplification of acoustic intensity using pulse waves traveling in a long tube equipped with a differentially heated regenerator. A reservoir filled with pressurized air and a solenoid valve were used to generate the acoustic pulse running in one direction in the tube with length 60 m. A series of pressure transducers was used to determine the axial distribution of the acoustic intensity. Use of a pulse wave enabled us to see the frequency dependence of the amplification rate of acoustic intensity very easily. We found that the amplification rate was essentially determined by the ratio of thermal boundary layer thickness to the pore radius of the regenerator.

A numerical simulation method and analysis of a complete thermoacoustic-Stirling engine

Thermoacoustic prime movers can generate pressure oscillation without any moving parts on self-excited thermoacoustic effect. The details of the numerical simulation methodology for thermoacoustic engines are presented in the paper. First, a four-port network method is used to build the transcendental equation of complex frequency as a criterion to judge if temperature distribution of the whole thermoacoustic system is correct for the case with given heating power. Then, the numerical simulation of a thermoacoustic-Stirling heat engine is carried out. It is proved that the numerical simulation code can run robustly and output what one is interested in. Finally, the calculated results are compared with the experiments of the thermoacoustic-Stirling heat engine (TASHE). It shows that the numerical simulation can agrees with the experimental results with acceptable accuracy.