Thermoacoustic Power Research Articles

Dynamical state and driving region of combustion instability in a turbulent combustor with a variable swirling flow system

We experimentally study the dynamical state and driving region of combustion instability in a turbulent combustor under low- and high-swirling flow conditions by the complex-systems approach. As the swirl number is increased, the combustion state undergoes a transition from stable combustion to complex combustion instability with large-amplitude modulations via low- and high-amplitude combustion instability. A chaotic state emerges during combustion instability with large-amplitude modulations. A symbolic dynamics-based thermoacoustic power network can identify the emergence of the thermoacoustic power source during combustion instability. The driving region of combustion instability can be clearly extracted from the symbolic dynamics-based synchronization index and the local node entropy in the Rayleigh index-based transition network. Reservoir computing has a potential use in clarifying the phase synchronized state between acoustic pressure and heat release rate fluctuations during the chaotic state in combustion instability with large-amplitude modulations.

Performance evaluation of looped tube thermoacoustic power generator using cyclic analysis

This paper focuses on the numerical and experimental investigation of the small-scale power generator. The travelling wave thermoacoustic power generator is numerically analyzed and experimentally tested. The cyclic analysis is used to carry out numerical analysis of the power generator. The system is operated on atmospheric pressure, which allows the manufacturing of an acoustic feedback loop using Polyvinyl chloride piping. The acoustic power generated inside the generator is harnessed by the low-cost linear alternator, i.e., loudspeaker. The effect of regenerator wire mesh on performance of the power generator is numerically analyzed and validated experimentally. The numerical analysis identifies the temperature variation, pressure fluctuation, volume flow rate inside the system, and acoustic power distribution. The maximum electric power experimentally generated by the small-scale power generator is around 45 W with overall efficiency 8.30%. The alternator generates the maximum electric power at the optimum location, i.e. 2.30 m away from the engine core.

Experimental and numerical study of a liquid metal magnetohydrodynamic generator for thermoacoustic power generation

Combining a thermoacoustic cycle engine with a liquid metal magnetohydrodynamic (LMMHD) generator will result in a thermal power generation system with no mechanical moving parts and high reliability. This disruptive technology has drawn much attention in space nuclear power generation, especially in recent years. It requires an LMMHD generator to work at a higher frequency than conventional LMMHD generators targeted for ocean wave energy conversion. However, the operating characteristics and loss mechanisms of LMMHD generators at high operating frequencies remain poorly understood, and experimental characterization of such a generator is lacking. In this work, a three-dimensional transient numerical analysis of a high-frequency LMMHD generator is performed based on multi-physics field simulation software COMSOL, to understand the operating characteristics of the generator, and the effects of inlet velocity, load resistance, and operating frequency on the generator’s performance. Furthermore, an LMMHD generator prototype was designed, constructed, and tested under different inlet velocities, load resistances, and frequencies by using a linear compressor for the first time. When the operating frequency and inlet velocity are 15 Hz and 4.3 m/s, the output voltage and current of the generator prototype reached 113 mV and 1720 A, with an output power of 68 W at a corresponding acoustic-to-electric efficiency of 24 %. A discrepancy between the numerical predictions and the experimental results was found, which gave insight into where further improvements can be made. This work reveals the operating characteristics and losses mechanism of LMMHD generators operating at higher frequencies and contributes to the development of high-efficiency generators for thermoacoustic power generation.

Multilayer network analysis to study complex inter-subsystem interactions in a turbulent thermoacoustic system

Thermoacoustic systems are complex systems where the interactions between the hydrodynamic, acoustic and heat release rate fluctuations lead to diverse dynamics such as chaos, intermittency and ordered dynamics. Such complex interactions cause catastrophically high-amplitude acoustic pressure oscillations and the emergence of order in the spatiotemporal dynamics, referred to as thermoacoustic instability. In this work, we use multilayer networks to study the spatial pattern of inter-subsystem interactions between the vorticity dynamics and thermoacoustic power generated due to acoustically coupled combustion in a bluff-body-stabilised turbulent dump combustor. We construct a two-layered network where the layers represent the thermoacoustic power and vorticity fields. The inter-layer links are determined using cross-variable short-window correlations between vorticity and thermoacoustic power fluctuations at any two locations in the flow field. Analysing the topology of inter-layer networks, using network properties such as degree correlations and link-rank distributions, helps us infer the spatial inhomogeneities in inter-subsystem interactions and unravel the fluid mechanical processes involved during different dynamical states. We show that, during chaotic dynamics, interactions between subsystems are non-localised and spread throughout the flow field of the combustor. During the state of thermoacoustic instability (order), we find that intense interactions occur in between regions of coherent vortex shedding and thermoacoustic power generation and we understand that these processes are strongly and locally coupled. Moreover, we discover that such dense inter-layer connections emerge in spatial pockets in the dump plane of the combustor during the state of intermittency much prior to the onset of order.

Feedback Coupling and Early Detection of Thermoacoustic Combustion Instability

We study the feedback coupling of thermoacoustic combustion instability from the viewpoint of a complex-systems approach, including the early detection of combustion instability. An upwardly traveling large-scale transverse vortex motion gives rise to large heat release rate fluctuations in the shear layer region between the inner and outer recirculation flows. The directional coupling from flow velocity to heat release rate fluctuations clearly appears near the rolled-up flame front generated by the developed transverse vortical structure. These events are strongly associated with the formation of a strong and local thermoacoustic power source cluster during combustion instability. A methodology combining symbolic dynamics-based analysis and a convolutional neural network enables the detection of a precursor of combustion instability.

Thermal analyses of high-power advanced thermoacoustic radioisotope power system for future space exploration missions

This paper presents the results of 3-D thermal analyses of the heat source assembly of a 440 We advanced radioisotope power system (RPS) for future space exploration missions. This high thermal efficiency RPS employs a heat source assembly of eight Step 2 General Purpose Heat Source (GPHS) modules and four Thermoacoustic Power Converter (TAPC) units. Each unit is thermally coupled to two GPHS modules. The analyses investigated the temperatures of the Iridium (Ir) alloy cladding of the 238PuO2 fuel pellets and the surface of the Fine Weave Pierced Fabric (FWPF) aeroshell in the GPHS modules to ensure reliable operation and safety in case of an unlikely reentry. Conductive coupling of the GPHS modules to the heater heads of the TAPC units produces relatively uniform axial and azimuthal distributions of heat flux and temperature. However, for this case the temperature of the Ir alloy cladding is lower than the desired values of 1173 K to maintain sufficient ductility. Adding a 5 mm wide He filled gap and decreasing the thickness of the graphite sleeve to 10 mm increased Ir cladding temperature to 1197–1174 K, which are in the desired range. The maximum surface temperatures of the FWPF graphite aeroshell of 1022–1131 K is well below the NASA specified limit of 1373 K. In addition, the total mass of the heat source assembly decreased by 19.6 kg, ∼43% saving compared to original design.

Numerical study on a three-stage traveling-wave thermoacoustic generator with a simplified structure simultaneously utilizing cold and low-grade thermal energy

As liquefied natural gas (LNG) is becoming a more and more vital role in the global energy market, the enormous amount of cold energy released during its regasification process has attracted worldwide attention. A three-stage traveling-wave thermoacoustic power generator (TWTAG) was proposed utilizing cold energy and low-grade waste heat simultaneously. Most importantly, a simplified structure was applied instead of the traditional structure with thermal buffer tubes (TBTs) and ambient heat exchangers (AHXs). Numerical results show that the system has superior performance together with a much more compact structure. In order to find the best location of the simplified system for acoustic power extraction, three different load-coupling modes were investigated separately. The three-stage TWTAG operating between 110 and 373 K is able to generate 11.4 kW and 10.8 kW electric power under hot-coupled and mid-coupled modes with 4 MPa helium gas as working gas, respectively. It is very enlightening that the cold-coupled mode can generate more than 15 kW acoustic power and shows the best performance. This work shows that thermoacoustic technologies have great potential in LNG cold energy and waste heat recovery.

Formation mechanism of high-frequency combustion oscillations in a model rocket engine combustor

We study the formation mechanism of high-frequency combustion oscillations in a model rocket combustor from the viewpoints of symbolic dynamics and complex networks. The flow velocity fluctuations in the fuel injector generated by the pressure fluctuations in the combustor give rise to the periodic ignition of the unburnt fuel/oxidizer mixture, resulting in a significant change in the heat release rate fluctuations in the combustor. The heat release rate fluctuations drive the pressure fluctuations in the combustor before a transition state, while the pressure fluctuations in the combustor gradually begin to significantly affect the heat release rate fluctuations during the transition to combustion oscillations. The directional feedback process during the transition and subsequent combustion oscillations is identified by the directionality index of the symbolic transfer entropy. The thermoacoustic power network enables us to understand the physical mechanism behind the transition and subsequent combustion oscillations.

New method to analyse and optimise thermoacoustic power generators for the recovery of residual energy

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.

Experimental study of a small scale bi-directional axial impulse turbine for acoustic-to-mechanical power conversion

Abstract 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.

A combined cooling and power cogeneration system by coupling duplex free-piston stirling cycles and a linear alternator

Abstract 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.

Optimizing bidirectional impulse turbines for thermoacoustic engines.

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.

Characterization of bidirectional impulse turbines for thermoacoustic engines.

A bidirectional impulse turbine to convert thermoacoustic power into electricity is investigated. Experimental measurements are done with a loudspeaker for varying acoustic conditions and turbine loads. The results are used to characterize the turbine performance and compare it to steady flow turbomachinery and turbines in oscillating water columns. A dimensional analysis is done to identify the variables that influence the turbine performance, after which a scaling is determined that uniquely determines the efficiency of the turbine. The work is finished by providing the impedance of the bidirectional turbine such that it can be implemented in a thermoacoustic engine.

Numerical Analysis of Bi-directional Impulse Turbine Performance for Thermoacoustic Power Generators

As a new type of acoustoelectric conversion method, bi-directional impulse turbine provides great potential for developing large scale and economic thermoacoustic power generators. In this paper, the performance of bi-directional impulse turbine with fixed guide vanes has been studied via 2D CFD modelling by commercial software Fluent. Firstly, the effects of blade structure and gas flow incident angles on the shaft power and efficiency have been investigated. Then the performance of the turbine has been studied under both steady and oscillating flow. Additionally, flow field features for the turbine in oscillating flow were presented for a better understanding. The results show that optimal flow incident angle for shaft power and efficiency are 20° and 70° respectively which indicates the difficulties in turbine design. The angles of the fluid including the gas flow incident angle and the blade angle are the most important parameter for turbine performance. The shaft power of rotor blades is more sensitive to the blade angle around the optimal incident angle region while the efficiency is not. The optimum flow coefficient is 2-3for maximum efficiency both in steady flow and oscillating flow. Increasing of the incoming flow oscillation frequency decrease the efficiency of the turbine. This loss mainly due to increased flow hysteresis effect which make the flow angle deviate from the design more.

Thermal analysis of Stirling thermocompressor and its prospect to drive refrigerator by using natural working fluid

The Stirling thermocompressor (STCP), usually working at low frequency (<5 Hz), is a novel Stirling-type external-combustion machine. It utilizes the temperature difference between the heating temperature and ambient temperature to generate the thermoacoustic power and pressure wave for refrigerators. In order to explore the characteristics of STCP, a detailed thermal analysis based on the third order 1-D finite volume model considering the real gas effect and variable physical properties was proceeded. Three natural fluids, helium, nitrogen and hydrogen are chosen as the working fluids and compared with each other. The parameter effects of pressure fluctuation under no-load operation were studied. To understand the output capacities for refrigerator with using different working fluids, the RC load method is adopt to analysis the STCP’s capacity and efficiency of the conversion from heat energy to acoustic energy, also the loss characteristics under different working gas. When the working pressure, frequency and heating temperature are 4 MPa, 4 Hz and 800 K respectively, the STCP using helium as the working fluid could output about 380 W acoustic power for the RC load and relative Carnot efficiency is about 33%.

Monocoque shell and tube heat exchanger for thermoacoustics

Heat exchangers can be a limiting factor to thermoacoustic power density. They can also be difficult to construct because of a tendency toward many small parts due to the small size of the thermal penetration depth. We present a heat exchanger that consists of around five pieces and yet has very small feature size and is easy to construct in large numbers. A sacrificial mandrel, consisting of a sheet with a lattice of specially shaped holes, is formed by casting in a mold with a lattice of specially shaped pins. The mandrel is plated with nickel, electroless nickel, or other metal. The mandrel is then removed by melting or dissolving, leaving a monocoque tube bundle. A screen mesh may be joined to one or both faces of the monocoque to act as fins to improve thermal contact to the working fluid that flows through the tubes. Coolant flows around the tubes within the monocoque shell. For a typical device, we find that without a mesh on the monocoque, lost work is minimized with tubes on the order of 8 mm long and spaced by 0.4 mm, but with a 100-mesh copper screen on both faces, the tubes can be a more manageable 3 mm long spaced by 4 mm.

Design and manufacturing of travelling wave thermo-acoustic engine with varying regenerator length

ABSTRACT Thermo-acoustic engines are devices suitable to convert heat energy into acoustic energy and vice versa. In this paper, travelling wave thermo-acoustic engine was designed and fabricated for the operating conditions of 30 bar mean pressure of working fluid (Air, Helium, Hydrogen, Nitrogen) and 500°C hot side temperature. The regenerator was made up of the can(s) of stainless steel SA-347and can be adjusted to achieve the required length. The access to change wire mesh size and its length enables to reduce the test set-up time and to enhance flexibility during experimentation. The modified secondary cold heat exchanger was developed for effective heat extraction from the working fluid and the effect of modified regenerator on the intensity of the thermo-acoustic power was also addressed. The present study shares the suitable alternatives for material selection, design considerations and preliminary estimation of heat exchangers at the operating temperature and pressure. It provides initial dimensions and enables to make a prototype.

Review on the conversion of thermoacoustic power into electricity.

Thermoacoustic engines convert heat energy into high amplitude acoustic waves and subsequently into electric power. This article provides a review of the four main methods to convert the (thermo)acoustic power into electricity. First, loudspeakers and linear alternators are discussed in a section on electromagnetic devices. This is followed by sections on piezoelectric transducers, magnetohydrodynamic generators, and bidirectional turbines. Each segment provides a literature review of the given technology for the field of thermoacoustics, focusing on possible configurations, operating characteristics, output performance, and analytical and numerical methods to study the devices. This information is used as an input to discuss the performance and feasibility of each method, and to identify challenges that should be overcome for a more successful implementation in thermoacoustic engines. The work is concluded by a comparison of the four technologies, concentrating on the possible areas of application, the conversion efficiency, maximum electrical power output and more generally the suggested focus for future work in the field.

Development for a thermoacoustic power system driven by cold exergy

Development for a thermoacoustic power system driven by cold exergy

Numerical simulation of key linear alternator performance indicators under thermoacoustic-power-conversion conditions

Thermoacoustic power converters consist of thermoacoustic heat engine and linear alternator. The linear alternator converts the acoustic power generated by the thermoacoustic engine to electric output. Efficient and stable operation of a thermoacoustic power converter requires acoustic matching between the engine and the alternator. It also requires matching between the linear alternator and the connected load. An experimental setup was built to measure and analyze the linear alternator performance under different thermoacoustic power converter operating conditions. The effects of the different design and operation factors on the key linear alternator performance parameters such as mechanical stroke, the generated electric power, the acoustic-to-electric conversion efficiency, the mechanical motion loss, the electric loss, and the fluid-seal loss were investigated experimentally and numerically. The experimental results were simulated using DeltaEC and reasonable agreement was obtained.