Thermoacoustic Prime Mover Research Articles

CFD simulation on the performance of twin thermoacoustic prime mover for various resonator lengths and operating pressures

AbstractThe main objective of the present investigation is to analyze the effect of resonator length and different working pressures on the performance of a twin thermoacoustic prime mover, which was measured in terms of frequency and amplitude of the oscillations using the computational fluid dynamics tool FLUENT 6.3. The simulation was carried out for different resonator lengths such as 0.5, 0.6, 0.8, 1.1, and 1.4 m using nitrogen as a working fluid for various operating pressures such as 1, 2, 3, 4, and 5 bar. It was observed from the results that the pressure amplitude increases with an increase in resonator length and that the output frequency decreases with an increase in resonator length for a fixed operating pressure. For a constant resonator length, the increase in operating pressure has little influence on output frequency, and the pressure amplitude also increases. The results obtained were found to be in accordance with experimental works published by Hariharan and colleagues.

Low temperature difference thermoacoustic prime mover with asymmetric multi-stage loop configuration

Environmentally friendly and low-cost technologies to recover low-grade heat source into usable energy can contribute to ease the energy shortage. Thermoacoustic technology is expected as one promising approach in this ascendant field. In this work, the multi-stage looped thermoacoustic prime movers with asymmetric configuration, which can provide travelling-wave resonator and appropriate acoustic field for efficient regenerator, have been proposed and experimentally studied. The presented looped thermoacoustic prime movers can start to oscillate with quite low temperature difference along the regenerator. The lowest onset temperature difference obtained in the experiments is only 17 °C (the corresponding heating temperature is 29 °C), which can be achieved in both three-stage and four-stage looped thermoacoustic prime movers, with CO2 of 1 MPa or 1.5 MPa as the working fluid. An electric generator driven by a three-stage looped thermoacoustic prime mover with low heating temperature was tested to achieve the acoustic to electric conversion.

Effect of temperature distribution of thermal buffer tube on onset temperature in a straight-tube-type thermoacoustic prime mover

We investigated a thermoacoustic system to achieve low-temperature driving. Inputting heat to the stack causes unwanted heat flows through the tube wall and the working gas, i.e., heat leak. The heat leak generates the temperature distribution along the tube from the high-temperature side of the stack to the closed end. The section where the temperature distribution is formed is called the thermal buffer tube. In this study, we focused on the temperature distribution of the thermal buffer tube. We measured the temperature distribution of the thermal buffer tube and made a comparison between the onset temperature obtained from the experiment and the critical temperature obtained by stability analysis. The difference in the results was 6 °C. The analytical critical temperature changed significantly with the assumed shape of the temperature distribution in the thermal buffer tube.

Stability study of a nonlinear thermoacoustic combustor: Effects of time delay, acoustic loss and combustion-flow interaction index

Large-amplitude self-excited thermoacoustic oscillations are unwanted in many premixed propulsion systems, such as rocket motors and gas turbines. However, they are desirable in some other practical applications, such as thermoacoustic prime movers or cooling systems. Thus, predicting the onset of such oscillations and a better understanding of its generation mechanism are important. Previous studies have confirmed that the time delay between unsteady combustion and flow perturbations plays a critical role in generating such thermoacoustic oscillations. In this work, the stability of a 1D thermoacoustic system is investigated theoretically and numerically. A premixed flame is confined in the combustor. The coupling between the unsteady heat release and oncoming flow disturbance is characterized by using interaction index-delay H-τ model. H denotes the interaction index between unsteady combustion and flow. To make the present studies more generalized, acoustic damping ξ is considered. The effects of H and ξ on the stability of the thermoacoustic combustor are investigated. Two interesting stability regions associated with the heating power H and the acoustic damping ξ are found: (a) delay-independent region and (b) stability switches region. When the thermoacoustic system is ‘locked’ in the delay-independent region, it is found that changing the time delay will not affect the stability of the system. However, if the thermoacoustic system is in the stability switches region, then a continuous increase in the time delay can lead to stability switches between stability and instability. Moreover, such switches become more frequent, when H and ξ are coupled and near the boundary between the stability switches region and delay-independent region. The present work opens up an applicable approach to design a stable combustor.

A numerical study on effect of evaporation and condensation of water vapor on acoustic oscillations of a gas parcel with a Lagrangian approach in a thermoacoustic prime-mover

Acoustic oscillations of a gas parcel in a wet stack of a thermoacoustic prime-mover are numerically simulated with a Lagrangian approach taking into account Rott equations and the effect of non-equilibrium evaporation and condensation of water vapor at the stack surface. A gas parcel gradually drifts to higher and lower temperature side of a stack in traveling-wave and standing-wave prime-movers, respectively, owing to a shift of phase angle of particle’s velocity with a movement of a parcel according to Rott equations even without evaporation and condensation. It is an alternative acoustic streaming in a narrow tube with temperature gradient along a tube. In a traveling-wave prime-mover, the volume oscillation amplitude of a gas parcel always increases by evaporation and condensation, and pV work done by a gas parcel is accordingly enhanced. On the other hand, in a standing-wave prime-mover, the volume oscillation amplitude sometimes decreases by evaporation and condensation, and pV work is suppressed. ...

Multi-objective optimization of thermo-acoustic devices using teaching-learning-based optimization algorithm

The current article presents three case studies based on a multi-objective optimization approach to optimize the performance of thermo-acoustic devices by obtaining the best possible set of geometrical characteristic parameters. In case study 1, the performance of a thermo-acoustic refrigerator is measured in terms of three objectives namely, acoustic cooling load (ΦC), coefficient of performance, and acoustic power loss (W02). Each objective is assigned a weight to facilitate suitable user-defined significance. The case study 2 aims to optimize a thermoacoustic prime-mover. The influence of stack position and its length, resonator length, plate thickness, and plate spacing are considered as design variables. Two objectives namely, pressure amplitude (P) and frequency (f) are considered as objectives for multi-objective optimization of the thermo-acoustic prime-mover. In case study 3, the performance of a thermo-acoustic engine is measured in terms of five objectives namely, work output (W), viscous loss (Rv), conductive heat loss (Qcond), convective heat loss (Qconv), and radiative heat loss (Qrad). Since the multiple objectives are to be optimized simultaneously, each objective is assigned a weight to facilitate suitable user-defined significance. The multi-objective optimization is carried out by the teaching-learning-based optimization algorithm. The results of application of teaching-learning-based optimization algorithm are compared with the results of General Algebraic Modeling System, response surface methodology, and experimental results. The results of the teaching-learning-based optimization algorithm are found better compared to those given by the other approaches.

Design optimization and analysis of selected thermal devices using self-adaptive Jaya algorithm

The present study explores the use of an improved Jaya algorithm called self-adaptive Jaya algorithm for optimal design of selected thermal devices viz; heat pipe, cooling tower, honeycomb heat sink and thermo-acoustic prime mover. Four different optimization case studies of the selected thermal devices are presented. The researchers had attempted the same design problems in the past using niched pareto genetic algorithm (NPGA), response surface method (RSM), leap-frog optimization program with constraints (LFOPC) algorithm, teaching-learning based optimization (TLBO) algorithm, grenade explosion method (GEM) and multi-objective genetic algorithm (MOGA). The results achieved by using self-adaptive Jaya algorithm are compared with those achieved by using the NPGA, RSM, LFOPC, TLBO, GEM and MOGA algorithms. The self-adaptive Jaya algorithm is proved superior as compared to the other optimization methods in terms of the results, computational effort and function evalutions.

Phase adjustment analysis and performance of a looped thermoacoustic prime mover with compliance/resistance tube

To realize high normalized acoustic impedance and near travelling-wave acoustic field in the regenerator is the key issue for a thermoacoustic engine to be driven by low-grade heat (e.g., waste heat, geothermal energy and solar thermal energy, etc.), and a looped thermoacoustic system with the appropriate acoustic field is considered one of the promising techniques. The present work analyzes the conception and mechanism of the phase adjustment in a looped structure, based on which two phase adjustors (namely compliance tube and resistance tube) are introduced to build appropriate acoustic field. Systematic experiments have then been conducted on a looped thermoacoustic prime mover with compliance/resistance tube, and the results show that the system with appropriate compliance/resistance tube can be initiated to oscillate at quite low heating temperature, indicating the phase adjustor can help to build up proper acoustic field. The lowest onset temperature is 40°C, with a compliance tube and using the CO2 of 2.37MPa as working fluid. Features of the acoustic field, including pressure amplitude, volumetric velocity amplitude and their phase difference, have also been numerically analyzed to verify the phase adjustment mechanism. Besides, the influence of its installation position and length of the compliance tube on the performance of the prime mover has been experimentally and numerically studied. Finally, a comparison is made to analyze the similarity and difference between the performances of compliance tube and resistance tube for phase adjustment.

Influence of operational parameters on performance of twin thermoacoustic prime mover – CFD studies

Purpose The present investigation deals with the analysis of the performance of twin thermoacoustic prime mover (TAPM) which are measured in terms of frequency and pressure amplitude by varying the parameters such as temperature gradient along the length of stack and the operating pressures of fluid medium argon using CFD simulation. With the help of CFD researchers and Engineers can evaluate the performance of a wide range of thermoacoustic systems on the computer without the time, expense, and disruption required to make actual changes onsite (stack) which is tedious to fabricate. Design/methodology/approach For the present simulation, the operating pressures of argon such as 1bar, 3bar and 5bar, and the temperature gradient is varied from 600K to 1400K with the regular intervals of each 200K. The geometry of twin TAPM is created using GAMBIT processor, and the simulation is carried out using FLUENT. The geometrical parameters of twin TAPM are kept constant throughout the simulation. The results for frequency and pressure amplitude obtained from the CFD simulation of twin TAPM for various temperature gradient and operating pressures are analysed and reported. Findings The computational results of twin thermoacoustic prime mover shows an increase in pressure amplitude with an increase in the temperature gradient and also it increases with an increase in operating pressures of the fluid medium. The parameter operating pressures of the working fluid medium and the stack hot end temperature has no significant effect on the output, frequency. Originality/value Though several experimental works had been published based on the twin thermoacoustic prime mover, an attempt has been made in the present investigation for the first time to estimate the performance of twin thermoacoustic prime mover using CFD package (ANSYS-FLUENT) by varying temperature gradient. The temperature gradient and operating pressures were varied and the performance of twin thermoacoustic prime mover was measured in terms of frequency and pressure amplitude.

Study on the heat transfer characteristic of solar powered thermoacoustic prime mover at different tilted angles

Solar powered thermoacoustic prime mover is an attractive device for solar thermal utilization. Its performance is significantly affected by tilt angle when it rotates with the Sun. In this study, the heat transfer characteristics within thermoacoustic core were investigated under 3 tilt angles including, −90°, 0° and 90°. The temperature fields of thermoacoustic core were measured by Thermal Infrared Imager. Experimental results show that the tilt angle has significant influence on the heat transfer and the temperature field in the thermoacoustic core when the engine onset. Three tilt angles were classified into three situations according to the relative position of two heat exchangers. And the temperature field and heat transfer characteristic was discussed in each situation.

Experimental Study on a Standing Wave Thermoacoustic Prime Mover with Air Working Gas at Various Pressures

Thermoacoustic prime mover is an energy conversion device which converts thermal energy into acoustic work (sound wave). The advantages of this machine are that it can work with air as the working gas and does not produce any exhaust gases, so that it is environmentally friendly. This paper describes an experimental study on a standing wave thermoacoustic prime mover with air as the working gas at various pressures from 0.05 MPa to 0.6 MPa. We found that 0.2 MPa is the optimum pressure which gives the lowest onset temperature difference of 355 °C. This pressure value would be more preferable in harnessing low grade heat sources to power the thermoacoustic prime mover. In addition, we find that the lowest onset temperature difference is obtained when rh/δk ratio is 2.85, where rh is the hydraulic radius of the stack and δk is the thermal penetration depth of the gas. Moreover, the pressure amplitude of the sound wave is significantly getting larger from 2.0 kPa to 9.0 kPa as the charged pressure increases from 0.05 MPa up to 0.6 MPa.

Acoustic hysteresis of varying cavity length of a bottle-shaped thermoacoustic prime mover with a neck to cavity ratio of 1:10

A previous study showed hysteresis in transition regions to overtones of bottle-shaped thermoacoustic prime movers with a neck to cavity diameter ratio of 1:2.4 while varying the cavity length. Hysteresis regions were studied with a neck to cavity diameter ratio of 1:10. The device consisted of a neck (5.15 cm long, 0.75" ID) with a heating element around it and a cavity (ID 3.75") with a sliding piston, allowing the cavity length to be varied up to 38 cm long. Copper mesh was used for the heat exchangers and were located about 30{\%} away from the top of the neck and 16 mg of steel wool served as the stack. A pressure sensor was connected to the center of the piston to measure acoustic pressure at the bottom of the cavity. Acoustic pressure, frequency, and hot and cold temperatures were recorded while both increasing and decreasing the cavity length from about 2 to 38 cm in increments of 0.2 cm over time intervals of 20 s with an input power of 10.5 W. Three transitions to overtones occurred at 6.8, 15.2, and 27.4 cm while pulling the piston out and at about 25.2, 15.6, and 6.0 cm while pushing the piston in. Frequencies and transition regions agreed with expected values. Both hot temperature and acoustic pressure increased during transition regions. This could be caused from multiple acoustic waves being produced.

Thermodynamic characteristics during the onset and damping processes in a looped thermoacoustic prime mover

The onset and damping processes of self-excited working-fluid oscillation in thermoacoustic prime movers, related to the quality of the utilizable thermal energy, have attracted everlasting research interest and effort. This work is intended to observe and analyze the temperature and pressure characteristics of the onset and damping processes in a recently developed looped travelling-wave thermoacoustic prime mover with enlarged thermoacoustic cores. A hysteretic loop, caused by the discrepancy between the onset temperature difference and the damping temperature difference, can be found in the looped structure, which indicates the potential in decreasing the onset temperature difference. Additionally, in order to optimize the acoustic field in the thermoacoustic prime mover, a compliance unit was inserted into the loop. The experimental results show that the onset temperature difference drops by up to 223 K with the appropriately located compliance unit, which is advantageous to the utilization of low-grade thermal energy.

Revisiting the difference between traveling-wave and standing-wave thermoacoustic engines - A simple analytical model for the standing-wave one

There are two major categories in a thermoacoustic prime-mover. One is the traveling-wave type and the other is the standing-wave type. A simple analytical model of a standing-wave thermoacoustic prime-mover is proposed at relatively low heat-flux for a stack much shorter than the acoustic wavelength, which approximately describes the Brayton cycle. Numerical simulations of Rott’s equations have revealed that the work flow (acoustic power) increases by increasing of the amplitude of the particle velocity (|U|) for the traveling-wave type and by increasing cosΦ for the standing-wave type, where Φ is the phase difference between the particle velocity and the acoustic pressure. In other words, the standing-wave type is a phase-dominant type while the traveling-wave type is an amplitude-dominant one. The ratio of the absolute value of the traveling-wave component (|U|cosΦ) to that of the standing-wave component (|U|sinΦ) of any thermoacoustic engine roughly equals the ratio of the absolute value of the increasing rate of |U| to that of cosΦ. The different mechanism between the traveling-wave and the standing-wave type is discussed regarding the dependence of the energy efficiency on the acoustic impedance of a stack as well as that on ωτα, where ω is the angular frequency of an acoustic wave and τα is the thermal relaxation time. While the energy efficiency of the traveling-wave type at the optimal ωτα is much higher than that of the standing-wave type, the energy efficiency of the standing-wave type is higher than that of the traveling-wave type at much higher ωτα under a fixed temperature difference between the cold and the hot ends of the stack.

Measurement of density fluctuations using digital holographic interferometry in a standing wave thermoacoustic oscillator

This paper describes an optical set-up for measuring density fluctuations associated to acoustic oscillations in a thermoacoustic prime-mover. A time-resolved, full-field holographic interferometry technique is used, which enables to measure the optical phase difference between a reference beam and a probe beam passing through the acoustic resonator. The paper first presents the experimental set-up and the processing of holograms from which the instantaneous variations of the gas density along the line of sight of the probe beam are obtained. Then, the measurement technique is applied to the analysis of density fluctuations in the neighborhood of the heated side of a stack in a standing wave thermoacoustic prime mover during the transient regime of wave amplitude growth. The experimental results reveal that there exists very significant entrance effects, which lead to the generation of higher harmonics as well as mean (time-averaged) mass rarefaction in the vicinity of the stack termination. Finally, a short discussion is provided, based on a simplified modeling of higher harmonics generation in temperature associated to the oscillations of an inviscid gas through the stack, but the model fails in explaining the magnitude of the phenomena observed.

Impact of Temperature Gradient on Thermoacoustics Refrigerator

In an extended search for suitable parameters of the project base on “Solar thermoacoustic refrigerator” funded by University of Damascus, which has built in the faculty, early softwares were developed to be a core of sizing Thermoacoustic cooler and prime mover, included the modeling equations for designation. Since high technologies were required, it was necessary to change the design strategies in order to allow the usage of low technology especially in stack fabrication, as well as to achieve building large sized tunnel that is used in air conditioning based on solar prime mover. In this paper, the new design strategy were modified to obtain a new software for sizing a Thermoacoustic device that would simply be built manually, in addition to find the impact of affecting temperature gradient over the stack. Some parameters like the heating capacity, cooling capacity, mean pressure, and stack spacing were chosen to fit the needs, fabrication, and simplicity requirements. The optimum temperature gradients were included in this paper.

Critical Temperature Differences of a Standing Wave Thermoacoustic Prime Mover with Various Helium-Based Binary Mixture Working Gases

Thermoacoustic prime movers are energy conversion devices which convert thermal energy into acoustic work. The devices are environmentally friendly because they do not produce any exhaust gases. In addition, they can utilize clean energy such as solar-thermal energy or waste heat from internal combustion engines as the heat sources. The output mechanical work of thermoacoustic prime movers are usually used to drive a thermoacoustic refrigerator or to generate electricity.A thermoacoustic prime mover with low critical temperature difference is desired when we intend to utilize low quality of heat sources such as waste heat and sun light. The critical temperature difference can be significantly influenced by the kinds of working gases inside the resonator and stack's channels of the device. Generally, helium gas is preferred as the working gas due to its high sound speed which together with high mean pressure will yield high acoustic power per unit volume of the device. Moreover, adding a small amount of a heavy gas to helium gas may improve the efficiency of thermoacoustic devices.This paper presents numerical study and estimation of the critical temperature differences of a standing wave thermoacoustic prime mover with various helium-based binary-mixture working gases. It is found that mixing helium (He) gas with other common gases, namely argon (Ar), nitrogen (N2), oxygen (O2), and carbon dioxide (CO2), at appropriate pressures and molar compositions, reduce the critical temperature differences to lower than those of the individual components of the gas mixtures. In addition, the optimum mole fractions of Hegas which give the minimum critical temperature differences are shifted to larger values as the pressure increases, and tends to be constant at around 0.7 when the pressure increases more than 2 MPa. However, the minimum critical temperature differences slightly increase as the pressure increases to higher than 1.5 MPa. Furthermore, we found that the lowest critical temperature difference for He-Armixture gas is around 66 °C which is achieved in pressure range of 1.5 MPa - 2.0 MPa and mole fractions of helium of 0.55 - 0.65. The He-N2 and He-O2 mixture gases demonstrate almost the same performances, both have the lowest critical temperature difference around 59 °C atpressures of 1.0 MPa - 1.5 MPa and helium's mole fractions of 0.35 - 0.55. For all tested gases, the lowest critical temperature difference of around 51 °C is provided by He-CO2 mixture gas at pressures of 0.5 MPa - 1.0 MPa with helium's mole fractions of 0.15 - 0.40.

Thermoacoustic energy harvesting

Thermoacoustics have a key role to play in energy harvesting systems, exploiting a temperature gradient to produce powerful acoustic pressure waves. As the name suggests, thermoacoustics is a blend of two distinct disciplines: thermodynamics and acoustics. The field encompasses the complex thermo-fluid processes associated with the compression and rarefaction of a working gas as an acoustic wave propagates through closely stacked plates in the regenerator of a thermoacoustic device; and the acoustic network that controls the phasing and properties of that wave. Key performance parameters and appropriate figures of merit for thermoacoustic devices are presented with particular emphasis upon the critical temperature gradient required to initiate the acoustic wave and the thermal properties of the key component, namely, the “stack” or “regenerator". Mechanisms for coupling a thermoacoustic prime mover with electromagnetic harvesters and piezoelectric transducer materials are also presented, which offer the potential to enhance the energy density attained beyond that possible with linear alternators. Numerical modeling strategies are presented, which enable parametric sweeps of the geometric and thermal properties, which influence the efficiency, and performance of the key components of such devices. Potential coupling and non-linear effects are examined.

Parameter estimation for the characterization of thermoacoustic stacks and regenerators

This paper deals with the in-situ characterization of open-cell porous materials that might be used as a so-called stack (or regenerator) in a thermoacoustic engine. More precisely, the manuscript presents an inverse method aiming at estimating geometrical and thermal properties of various samples of porous media surrounded by heat exchangers and connected to a thermal buffer tube to form a ThermoAcoustic Core (TAC). This estimation is realized from acoustic measurements, and it is expressed as a minimization problem applied to the squared norm of the difference between experimental and theoretical transfer matrices of the TAC. Experimental data, obtained for different stacks (ceramic catalyst, pile of stainless steel wire meshes, carbon and metallic foams) under various heating conditions, are used in order to fit the theoretical forward model by adjusting geometrical properties of the sample and heat exchange coefficients. Common geometrical properties (porosity and average pore's radius) obtained with the present method are consistent with available data from manufacturers. Moreover, this method allows to estimate the tortuosity of the material which is not given by manufacturers. Estimation of heat coefficients (and their variations with heating) provides global information about anisotropic heat diffusion through the porous material employed as a thermoacoustic stack submitted to a temperature gradient. Among the four characterized samples, it appears that the carbon foam allows to get the highest temperature gradients and thus can be considered as the most efficient regenerator for energy conversion in a thermoacoustic prime mover.

Simulation studies on the performance of thermoacoustic prime movers and refrigerator

The prime movers and refrigerators based on thermoacoustics have gained considerable importance toward practical applications in view of the absence of moving components, reasonable efficiency, use of environmental friendly working fluids, etc. Devices such as twin Standing Wave ThermoAcoustic Prime Mover (SWTAPM), Traveling Wave ThermoAcoustic Prime Mover (TWTAPM) and thermoacoustically driven Standing Wave ThermoAcoustic Refrigerator (SWTAR) have been studied by researchers. The numerical modeling and simulation play a vital role in their development.In our efforts to build the above thermoacoustic systems, we have carried out numerical analysis using the procedures of CFD on the above systems. The results of the analysis are compared with those of DeltaEC (freeware from LANL, USA) simulations and the experimental results wherever possible. For the CFD analysis commercial code Fluent 6.3.26 has been used along with the necessary boundary conditions for different working fluids at various average pressures. The results of simulation indicate that choice of the working fluid and the average pressure are critical to the performance of the above thermoacoustic devices. Also it is observed that the predictions through the CFD analysis are closer to the experimental results in most cases, compared to those of DeltaEC simulations.