Looped-tube Thermoacoustic Engine Research Articles

Rotational Dynamics of a Looped-Tube Thermoacoustic Engine with a Flywheel

In a flywheel thermoacoustic engine, the rotating motion of the flywheel is sustained by the reciprocal motion of the working gas confined in an externally heated thermoacoustic engine. In this study, an equation of motion for the flywheel in the steady rotational state is derived and the key parameters governing the rotation dynamics of the looped-tube thermoacoustic engine loaded with a piston-flywheel assembly are investigated. We demonstrate that the relationship between the steady rotation frequency and the heating temperature is determined by the real part of the acoustic impedance of the engine subsystem. This result is in contrast to that of a conventional thermoacoustic engine with a linear load, where both the real and imaginary parts of the acoustic impedance impact the operating condition. When the flywheel is connected, the imaginary part of the acoustic impedance only affects the temporal fluctuation of the rotation frequency. The attainable shaft power that can be extracted from the engine is also discussed based on experimental results.

Numerical analysis of competing methods for acoustic field adjustment in a looped-tube thermoacoustic engine with a single stage

Looped-tube thermoacoustic engines have shown to be operational with low temperatures. When equipped with multiple stages, preferable acoustic conditions are inherently achieved, leading to high efficiency. However, engines with only a single stage are able to achieve a similar performance, when acoustic disturbances are properly matched by implementation of an additional, acoustically effective element. Furthermore, acoustic conditions in the regenerator can be accurately tuned by adjusting the element’s volume (i. e. impedance) and position in respect to the regenerator. The present work compares three different methods for acoustic field adjustment: a side-branched stub, a compliant segment and an inertial segment. The last two are widened or narrowed sections of the feedback loop, respectively. The influence of different general positions is considered. All configurations are analysed and compared with each other regarding their acoustic tuning capabilities and possible side effects to the acoustic field. As a result, superior engine performance is achieved by use of a compliant segment, closely followed by use of a stub. Furthermore, the length of a compliant or inertial segment has a major impact on performance. While extending a compliant segment slightly improves power, it can be crucial for an inertial segment. The comprehensive analysis is carried out numerically under identical conditions for each configuration. DeltaEC (Design Environment for Low-amplitude Thermoacoustic Energy Conversion) is used as simulation software.

Experimental validation of a looped-tube thermoacoustic engine with a stub for tuning acoustic conditions

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.

Numerical investigation of a looped-tube traveling-wave thermoacoustic generator with a bypass pipe

In this paper, based on the recent development of looped-tube thermoacoustic engine with a bypass pipe, we propose and numerically demonstrate a travelling wave thermoacoustic electric generator using this configuration. It essentially employs a one wave-length travelling-wave acoustic resonator which has low acoustic losses. The engine branch consists of a compliance, a inertance tube, an engine core, and an alternator. An ultra-compliant alternator (i.e., a sub-woofer) is installed in the acoustic compliance section where the local acoustic impedance is relatively low but the cross sectional area is big. The numerical simulations demonstrate its working principle, and show that it can potentially achieve comparable performance as other types of travelling wave thermoacoustic electric generators.

A numerical study of a looped-tube thermoacoustic engine with a single-stage for utilization of low-grade heat

Multi-stage thermoacoustic engines with a looped structure have become a promising technology to use heat at a low temperature level. This study forwards a single-stage engine with comparable capabilities. A stub is used to suppress acoustic impedance disturbances. This conception shows to be a powerful tool to accurately tune acoustic conditions in the regenerator. Furthermore, a systematic parametric study is carried out numerically with DeltaEC to examine their relations. This could help in a system-wide optimization according to various application requirements. The study comprises geometrical parameters of regenerator, heat exchangers, and feedback loop as well as process parameters like operational temperatures, mean pressure and working gas. Discrepancies between ideal conditions on component level and on system level have been observed and an approach to interpret them is given. Recommendations for parameter settings depending on an efficiency or power driven operation are supplied. Since a proper coupling between acoustic load and acoustic field in the loop is essential for a sufficient overall-performance, joining mechanisms have been examined in terms of acoustic impedance and position of the load. Eventually it is shown that the proposed engine type is able to power an acoustic load with a relative Carnot efficiency greater than 50% when heat is supplied at 150°C and rejected at 15°C.

Non-linear phenomena occurring during the start-up process of a travelling-wave looped-tube thermoacoustic engine

This article reports the investigation of non-linear start-up processes in a looped-tube thermoacoustic engine. The engine utilises air as working gas, and has the fundamental frequency of around 111 Hz. It is observed that the mean pressure and the input heat power are the two key parameters controlling the start-up behaviour of the engine. When these are varied, a range of non-linear transient phenomena are observed, which include the ‘on–off’ effect, ‘fishbone-like’ oscillations (i.e. the quasi-periodic pressure amplitude bursts followed by a quasi steady state), and the normal smooth start-up process. The experiments show that the ‘fishbone-like’ bursts are a new mode of pressure amplitude growth. A series of experiments have been conducted to investigate in detail the influence of mean pressure, input heat power and regenerator type on the occurrence of such quasi-periodic amplitude bursts. It is observed that the duration of the pressure amplitude bursts depends on the combination of the heating power and the mean pressure. The observations suggest that there are strong interactions between the acoustic and temperature fields within the regenerator. It is thought that these can lead to the reported quasi-periodic unsteady behaviour of the engine.

Design and experimental validation of looped-tube thermoacoustic engine

The aim of this paper is to present the design and experimental validation process for a thermoacoustic looped-tube engine. The design procedure consists of numerical modelling of the system using DELTA EC tool, Design Environment for Low-amplitude ThermoAcoustic Energy Conversion, in particular the effects of mean pressure and regenerator configuration on the pressure amplitude and acoustic power generated. This is followed by the construction of a practical engine system equipped with a ceramic regenerator — a substrate used in automotive catalytic converters with fine square channels. The preliminary testing results are obtained and compared with the simulations in detail. The measurement results agree very well on the qualitative level and are reasonably close in the quantitative sense.

C05 ループ管内に二つの蓄熱器を有する熱音響エンジンの発振温度(パルス管冷凍機,熱音響機器及び関連要素と応用システム(2))

Onset temperatures of a spontaneous gas oscillation occurring in a looped tube thermoacoustic engine were numerically calculated. The looped tube engine had two regenerators inside a looped tube. In the calculation, the relative position between the regenerators was changed. As a result, it was found that there is the optimum relative position and the onset temperature with the optimum one is 108℃. This value is lower than the onset temperature of the looped tube with one regenerator by 170℃.

Fishbone-like instability in a looped-tube thermoacoustic engine

Quasi-periodic bursts of acoustic oscillations were observed during the start-up process in a looped-tube thermoacoustic engine. The acoustic oscillations have a constant frequency of 111 Hz, while the bursts have "quasi-periods" in the order of 14-25 s. The quasi-periodic bursts show a new mode of amplitude growth in this thermoacoustic engine. The envelope of the acoustic oscillations has a fishbone-like shape. The nature of the observed fishbone-like instabilities suggests a strong interaction between the acoustic and temperature field.

A low-cost electricity generator for rural areas using a travelling-wave looped-tube thermoacoustic engine

This article describes the construction and preliminary testing of a pre-prototype thermoacoustic electricity generator to test the concept of a low-cost device for application in remote or rural areas of developing countries. A travelling-wave thermoacoustic engine with a configuration of a looped-tube resonator is designed and constructed to convert heat to acoustic power. Air at atmospheric pressure is used as the working gas, PVC tubing is utilized for the feedback pipe, whereas an inexpensive commercially available loudspeaker is adopted to convert the acoustic power, produced by the engine, to electricity. Preliminary experimental results are presented and discussed in detail. The results show that the approach is feasible in principle and it is possible to produce the electrical power levels in the order of 4–5 W with overall heat-to-electric efficiencies in the order of 1 per cent. Further work towards optimizing the device from the performance, manufacturing, and cost points of view is outlined.

C12 音響流抑制機構がループ管型熱音響エンジンの振動特性に与える影響(パルス管冷凍機,熱音響機器及び関連要素と応用システム(2))

To achieve a high efficiency of the thermoacoustic engine with a looped tube, we have to suppress an acoustic streaming such as Gedeon streaming. A lubber membrane and 'Jet Pump' can work as an acoustic-streaming suppressor. In this study, we use a lubber membrane as an acoustic-streaming suppressor, and determine the affect of position of the membrane to the onset temperature ratio and the acoustic field formed in the looped tube thermoacoustic engine. As a result, the optimum position of the acoustic-streaming suppressor to decrease the onset temperature and to generate traveling wave mode was determined.

Measurements of acoustic streaming in a looped-tube thermoacoustic engine with a jet pump

This paper reports on the acoustic streaming in a looped-tube thermoacoustic prime mover equipped with an asymmetric constriction called a jet pump. The time-averaged mass flow velocity was determined using visualization methods and using acoustic field measurements. It was demonstrated that the magnitude and the direction of the velocity were dependent on the orientation of the jet pump. From the observed velocities, we estimated the heat carried away from the hot heat exchanger by the mass flow. It was shown that the heat loss was decreased from 30 W to 6.5 W by reversing the orientation of the jet pump, when the input heat power supplied to the prime mover was 100 W. The influence of the acoustic streaming was also studied from the cooling temperature of the looped-tube cooler.

Thermodynamic cycles executed in a looped-tube thermoacoustic engine (L)

A looped-tube thermoacoustic engine is composed of only a stack of plates, across which a steep temperature gradient is furnished. In spite of the absence of any mechanical parts like pistons and valves, the energy conversion from heat flow into acoustic power flow is performed in the stack. The thermoacoustically induced acoustic field was observed through the simultaneous measurements of pressure and velocity of the working gas, and the thermodynamic cycle executed in the stack was studied. The experimental results show that the acoustic field is tuned without any external forces so as to realize the efficient thermodynamic cycles as possible.