Thermoacoustic System Research Articles

Carleman estimates and some inverse problems for the coupled quantitative thermoacoustic equations by partial boundary layer data. Part II: Some inverse problems

This paper is concerned with the determination of coefficients and source term in a strong coupled quantitative thermoacoustic system of equations. Adapting a Carleman estimate established in the part I of this series of papers, we prove stability estimates of Hölder type involving the observation of only one component: the temperature or the pressure.

Mean-field model of synchronization for open-loop, swirl controlled thermoacoustic system.

Open-loop control is known to be an effective strategy for controlling self-excited periodic oscillations, known as thermoacoustic instability, in turbulent combustors. Here, we present experimental observations and a synchronization model for the suppression of thermoacoustic instability achieved by rotating the otherwise static swirler in a lab-scale turbulent combustor. Starting with the state of thermoacoustic instability in the combustor, we find that a progressive increase in the swirler rotation rate leads to a transition from the state of limit cycle oscillations to the low-amplitude aperiodic oscillations through a state of intermittency. To model such a transition while also quantifying the underlying synchronization characteristics, we extend the model of Dutta et al. [Phys. Rev. E 99, 032215 (2019)] by introducing a feedback between the ensemble of phase oscillators and the acoustic. The coupling strength in the model is determined by considering the effect of the acoustic and swirl frequencies. The link between the model and experimental results is quantitatively established by implementing an optimization algorithm for model parameter estimation. We show that the model is capable of replicating the bifurcation characteristics, nonlinear features of time series, probability density function, and amplitude spectrum of acoustic pressure and heat release rate fluctuations at various dynamical states observed during the transition to the state of suppression. Most importantly, we discuss the flame dynamics and demonstrate that the model without any spatial inputs qualitatively captures the characteristics of the spatiotemporal synchronization between the local heat release rate fluctuations and the acoustic pressure that underpins a transition to the state of suppression. As a result, the model emerges as a powerful tool for explaining and controlling instabilities in thermoacoustic and other extended fluid dynamical systems, where spatiotemporal interactions lead to rich dynamical phenomena.

Mitigation of limit cycle oscillations in a turbulent thermoacoustic system via delayed acoustic self-feedback.

We report the occurrence of amplitude death (AD) of limit cycle oscillations in a bluff body stabilized turbulent combustor through delayed acoustic self-feedback. Such feedback control is achieved by coupling the acoustic field of the combustor to itself through a single coupling tube attached near the anti-node position of the acoustic standing wave. We observe that the amplitude and dominant frequency of the limit cycle oscillations gradually decrease as the length of the coupling tube is increased. Complete suppression (AD) of these oscillations is observed when the length of the coupling tube is nearly 3 / 8 times the wavelength of the fundamental acoustic mode of the combustor. Meanwhile, as we approach this state of amplitude death, the dynamical behavior of acoustic pressure changes from the state of limit cycle oscillations to low-amplitude chaotic oscillations via intermittency. We also study the change in the nature of the coupling between the unsteady flame dynamics and the acoustic field as the length of the coupling tube is increased. We find that the temporal synchrony between these oscillations changes from the state of synchronized periodicity to desynchronized aperiodicity through intermittent synchronization. Furthermore, we reveal that the application of delayed acoustic self-feedback with optimum feedback parameters completely disrupts the positive feedback loop between hydrodynamic, acoustic, and heat release rate fluctuations present in the combustor during thermoacoustic instability, thus mitigating instability. We anticipate this method to be a viable and cost-effective option to mitigate thermoacoustic oscillations in turbulent combustion systems used in practical propulsion and power systems.

Design methodology of standing-wave thermoacoustic refrigerator: theoretical analysis

Thermoacoustic refrigeration systems are one of the best alternative solutions for conventional refrigeration systems that are harmful to the environment and humans due to global warming and ozone layer depletion issues. Thermoacoustic technology can be considered a renewable and clean technology with a promising future for its many advantages. A thermoacoustic refrigerator converts acoustic energy to thermal energy (creating a cooling effect). In the present research, the focus is on the design of a standing-wave thermoacoustic refrigerator driven by an ordinary loudspeaker using the numerical simulation program DELTAEC with the concern of building the apparatus at a low cost. In addition, investigating the influence of some crucial parameters on cooling power and thermal/overall performance. Hence, the designed thermoacoustic refrigerator performed well in respect of cooling power and coefficient of performance. It has achieved a cooling power of 134.34 W with a temperature difference between the ambient and cold heat exchangers of 25 K at a COP of 1.956 and the overall efficiency (electrical power converted into cooling power) amounted to 113.43%. The achieved cooling power and COP could be significant when compared to others’ results.

Numerical study of oscillation characteristics in T-shaped cryogenic helium tube

Thermoacoustic oscillations generally exist in combustion devices and cryogenic systems, causing system instability and other problems. The actual cryogenic system has complex pipe connections, however, there are few studies on the thermoacoustic oscillation of complex structures, which brings challenges to suppressing thermoacoustic instability. Therefore, this study focuses on the T-shaped helium tube existing in the actual cryogenic system and explores the oscillation characteristics of the complex cryogenic thermoacoustic system. With the change of branch position and length, mode transition phenomenon appears. We obtained the effects of the branch position and length on the amplitude and frequency of each mode and found that the self-excited oscillation in the T-shaped tube could be suppressed to some extent by choosing the proper length and position of the branch tube. Further research found that the first mode of the oscillating pressure in the main and branch tube was in-phase synchronized, and the second mode was antiphase synchronized. By linking the flow direction with the oscillation mode, it is found that the first mode corresponds to the first flow direction, and the second mode corresponds to the second or third flow direction. The finding is valuable for understanding thermoacoustic oscillations in T-tubes and provides a reference for controlling the self-excited oscillation of complex cryogenic systems.

Synchronization-based model for turbulent thermoacoustic systems

We present a phenomenological reduced-order model to capture the transition to thermoacoustic instability in turbulent combustors. Based on the synchronization framework, the model considers the acoustic field and the unsteady heat release rate from turbulent reactive flow as two nonlinearly coupled sub-systems. To model combustion noise, we use a pair of nonlinearly coupled second-order ODEs to represent the unsteady heat release rate. This simple configuration, while nonlinearly coupled to another oscillator that represents the independent sub-system of acoustics (pressure oscillations) in the combustor, is able to produce chaos. Previous experimental studies have reported a route from low amplitude chaotic oscillation (i.e., combustion noise) to periodic oscillation through intermittency in turbulent combustors. By varying the coupling strength, the model can replicate the route of transition observed and reflect the coupled dynamics arising from the interplay of unsteady heat release rate and pressure oscillations.

Construction, testing and performance analysis of a multi-stage traveling-wave thermo-acoustic generator

ABSTRACT This research describes the construction and testing of a multi-stage thermo-acoustic generator that demonstrates the potential of conversion of waste heat into useful electricity. The total length of the traveling-wave loop was 3 560 mm. The thermo-acoustic engine stages were positioned 640 mm apart. An experimental investigation into the effects of the heat source on the potential of the device for electricity generation has been conducted. The magnitude of the sound pressure level generated by the engines, the onset time and the magnitude of electricity generated by the electroacoustic converter have been considered as performance indicators for the device developed. This paper provides details of the experimental investigation by measures of these indicators. A single-stage thermo-acoustic system, tested separately, generated an average output voltage of approximatively 834 mV. A second stage was added into the traveling-wave loop. The two-stage system generated an output voltage of approximately 1.55 V. A third stage was added into the loop. The three-stage system generated an output voltage of 2.362 V. The subsequently added four-stage thermo-acoustic system generated an average maximum output voltage of 4.218 V.

Imprints of log-periodicity in thermoacoustic systems close to lean blowout.

In the context of statistical physics, critical phenomena are accompanied by power laws having a singularity at the critical point where a sudden change in the state of the system occurs. In this work we show that lean blowout (LBO) in a turbulent thermoacoustic system is accompanied by a power law leading to finite-time singularity. As a crucial discovery of the system dynamics approaching LBO, we unravel the existence of the discrete scale invariance (DSI). In this context, we identify the presence of log-periodic oscillations in the temporal evolution of the amplitude of the dominant mode of low-frequency oscillations (A_{f}) existing in pressure fluctuations preceding LBO. The presence of DSI indicates the recursive development of blowout. Additionally, we find that A_{f} shows a faster-than-exponential growth and becomes singular when blowout occurs. We then present a model that depicts the evolution of A_{f} based on log-periodic corrections to the power law associated with its growth. Using the model, we find that blowouts can be predicted even several seconds earlier. The predicted time of LBO is in good agreement with the actual time of occurrence of LBO obtained from the experiment.

An approach to low-temperature oscillation and high efficiency of thermoacoustic prime mover: influence of end face and reduction area of phase adjuster

The Phase Adjuster (PA) has contributed to the reduction of the onset temperature and improvement of the energy conversion efficiency of thermoacoustic systems. In this study, we investigated the performance characteristics of thin medal-shaped PAs representing the PA end faces. The results show that inserting 2 mm thick medal-shaped PAs lowered the onset temperature by 220 K. However, the insertion of thicker medal-shaped PAs maintained the onset temperature. In contrast, the energy conversion efficiency increased with the medal-shaped PA thickness. The energy conversion efficiency for 2 mm thick PAs was lower than that without PA. In addition, the thinner the medal-shaped PAs, the lower the power required for oscillation but the lower the energy conversion efficiency. These effects of the medal-shaped PA thickness indicate the necessity of different design guidelines for designing PAs for low-temperature oscillation and high efficiency in thermoacoustic systems.

Fundamental study for heat pump stack length determination with complex flow paths in a compact thermoacoustic system

The miniaturization of thermoacoustic systems is expected to find practical applications in various fields. To achieve miniaturization, we focused on the relation of the sound wave in the tube and the length of the heat-pump stack. Accordingly, three types of metal meshes with different channel diameters were prepared, and thermoacoustic systems were designed considering ωτ, a dimensionless quantity. In the experiment, the stack length was varied and the stack temperature difference and heat flow were measured. Consequently, temperature difference and heat flow with respect to the stack length were studied. In addition, even when the stack was short, temperature difference at both ends of the stack was formed. The better stack length in terms of energy efficiency was also examined. Accordingly, we confirmed that the ratio of stack length to wavelength is in the range of 0.2%–0.6%.

The effects of different types of noise on thermoacoustic systems using a Green's function approach

In this paper, we take advantage of using a Green's function approach as an analytical tool to study linear and nonlinear aspects of thermoacoustic systems. Green's function is defined as the impulse response of a system, it has a clear physical meaning in combustion systems, and it provides a fast and flexible tool to predict thermoacoustic instabilities in both time and frequency domains. In this study, we consider a Rijke tube, which houses two sources: a heat source and an external noise source. The heat source is modelled by an amplitude-dependent nτ-law, the noise source is assumed to emit two types of noise (pink noise or white noise). We use the Green's function approach to derive an integral equation for the acoustic field in the Rijke tube, and we also derive an algebraic equation for the thermoacoustic eigenfrequency. Both equations are validated. The results that we found, show that the presence of noise (whether pink or white noise) results in "triggering" an instability and in accelerating the growth of the amplitude. These effects become more pronounced as the level of noise increases. The influence of pink noise is stronger than that of white noise. We also studied how the hysteresis behavior (a nonlinear effect in dynamical systems) is affected by the noise, using the heat source position and heater power as bifurcation parameters. Our study reveals that the width of the bistable region decreases as the strength of noise increases.

Noninvasive Deep-Tissue Temperature Monitoring Based on Magnetic Mediated Thermoacoustics

Noninvasive acquisition of deep tissue temperature has important applications in home health monitoring, hyperthermia safety control, and other domains. In this work, we present here a novel magnetically mediated thermoacoustic temperature measurement method. Utilizing coil to stimulate amplitude modulated magnetic field and ultrasound transducer to receive the generated thermoacoustic wave from the inserted magnetic nanoparticles. Benefiting from the high sensitivity of thermoacoustic emission from nanoparticles and the deep penetration capability of both magnetic field and ultrasound propagation, the proposed thermoacoustic temperature measurement system enables a high measurement accuracy of 0.5 degrees Celsius in real time. This work potentially facilitates further development of closed loop magnetic hyperthermia for practical clinical applications.

Suppression of quasiperiodic thermoacoustic oscillations via genetic programming

We use genetic programming (GP) to discover data-driven control laws for the suppression of quasiperiodic oscillations in a prototypical thermoacoustic system. We rank the control laws based on a predefined cost function that accounts for the pressure amplitude and the actuation effort. We then breed subsequent generations of control laws via a tournament process. We find that GP closed-loop control is more effective than GP open-loop control and conventional periodic forcing, producing a similarly high degree of amplitude suppression but with the lowest actuation effort. We also find that GP closed-loop control can identify unforeseen actuation mechanisms, providing new insight into the physical coupling between the heat release rate and pressure fields.

Physics-aware learning of nonlinear limit cycles and adjoint limit cycles

Thermoacoustic oscillations occur when the heat released by a flame is sufficiently in phase with the acoustic pressure. Under this condition, the linear instability can saturate to a nonlinear self-excited oscillation with a large amplitude. A typical nonlinear regime is a limit cycle, which is characterised by a periodic orbit in the thermoacoustic dynamics. In this paper, we develop a physics-aware data-driven method to predict periodic solutions using forward neural networks. The physics is constrained in two ways. First, the training is informed by a physical residual, which penalises solutions that violate the conservation of mass, momentum, and energy. Second, periodicity is imposed by introducing periodic activation functions in the neural network. We test the algorithm on a nonlinear time-delayed model of a Rijke tube. Adjoint methods offer a cheap and easy way to calculate the gradients with respect to design parameters, hence we extend our study to learning the adjoint variables of the Rijke system, which also settle onto periodic oscillations. We find that (i) periodic solutions of thermoacoustic systems can be accurately learned with this method, (ii) for periodic data, periodic activations outperform conventional activations in terms of prediction capability beyond the training range, and (iii) under the physical constraints, fewer data is sufficient to achieve a good performance. This work opens up possibilities for the prediction of nonlinear thermoacoustics by combining physical knowledge and data.

Optimization of a thermoacoustic system

Clean combustion such as hydrogen combustion for the reduction of NOx emission is prone to thermoacoustic instabilities, which may cause structural vibrations and equipment failures. In the present work, an adjoint-based sensitivity analysis is applied to a low-order thermoacoustic model with a non-zero mean flow. The adjoint-based analysis can be a powerful tool to yield information for optimization techniques to eliminate or control such instabilities. With quick and low-cost calculations, structural sensitivity and base-state sensitivity are achieved and the conditions most sensitive to instabilities are identified.

Control of Large-Amplitude Combustion Oscillations Using Nanosecond Repetitively Pulsed Plasmas

This paper details the use of nanosecond repetitively pulsed discharges to attenuate combustion instabilities in a 14 kW swirl-stabilized methane/air combustor. The combustor exhibits large-amplitude pressure oscillations ranging from 1 to 4% of the mean pressure during which the flame exhibits bulk motion in each instability cycle, upstream and downstream, as revealed by high-speed chemiluminescence. Control is accomplished with an electrode comprising a pin anode at the centerline of the combustor, allowing a nanosecond spark to be generated in a region spanning close to the flame base, through the shear layers of the swirling flow and ending at the metallic combustor wall. The discharges are generated using 20 kV, 9 kHz pulses; and they correspond to about 120 W of mean power. This results in a suppression of the peak amplitude of the pressure oscillations by a factor of two to four, and 5 dB in the rms value. Using phase-averaged visualizations of the flame with and without plasma, we detail the sequence of flame motion in the course of the instability. With the plasma active, this reveals significant interactions between the flame and the plasma during the suppression. Finally, we present a state-space model of the thermoacoustic system, and we demonstrate open-loop control of the instabilities.

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

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

Experimental results of a waste-heat powered thermoacoustic refrigeration system for ships

Greenhouse gas emissions are a major concern for maritime transport. TheInternational Maritime Organization (IMO)presents Waste Heat Recovery Systems (WHRS) from engine exhaust gas as a viable solution to improve energy utilization and reduce greenhouse gas emissions for marine power plants. In this paper, we present an original thermoacoustic WHRS and its associated experimental setup for validation. This WHRS aims to transport heat from a lower to a higher temperature reservoir (heat pump) and maintain a fridge at 0 °C. For the experimental setup, exhaust gases from a 4-stroke marine diesel engine are used to generate pressure waves (work) in thermoacoustic engines. The main result is the Coefficient of Performance (COP) of the system, around 0.3, assessed for various cold loads. The first results estimate that the system could substitute part of the chiller cold production onboard. Also, a preliminary economic analysis suggests that this system could perform 2% fuel savings that allow for a payback time below one year.

Research on thermoacoustic refrigeration system driven by waste heat of industrial buildings

Industrial production process continuously produces a large amount of stable waste heat, which has great potential for recovery and utilization. As a new energy conversion and utilization technology, thermoacoustic technology has a good application prospect in low-grade heat sources utilization, with advantages of simple structure, long service life and low cost, and can be applied in waste heat recovery field. The current thermoacoustic studies pay more attention to the improvement of thermoacoustic conversion efficiency. Power output should be considered while paying attention to efficiency in the field of engineering application. The thermoacoustic refrigeration system proposed in this paper uses waste heat generated in industrial buildings to regulate indoor temperature. Based on the theory of traveling-standing wave acoustic field, a design concept of thermoacoustic system was proposed, that is, two thermoacoustic cores are placed on both sides of the high impedance zero phase position to limit the high impedance zero phase phenomenon at the independent thermoacoustic core position. On the basis of this study, the topology of thermoacoustic refrigeration system operating in traveling-standing wave phase was proposed. A two-stage thermoacoustic engine/refrigerator coupled loop structure was adopted to realize the traveling-standing wave acoustic field through the non-uniform alternate distribution of thermoacoustic units. Based on the topology, numerical simulation and system optimization were carried out, and an experimental prototype was set up for experimental studies. The experimental results show good consistency with the simulation results. Under the condition of no load, the lowest cooling temperature reached by thermoacoustic refrigerator is −12℃, and the maximum cooling capacity is 0.95 kW. This study shows that the thermoacoustic refrigeration system driven by waste heat can significantly reduce the energy consumption of industrial buildings in terms of indoor temperature regulation, and improve the energy utilization ratio of industrial buildings.

Experimental Study of Suppressing the Thermoacoustic Instabilities in a Rijke Tube Using Microsecond Discharge Plasma

Thermoacoustic instabilities occur when heat release is coupled with pressure fluctuation, which may cause performance degradation of the combustor and serious structural damage. This study focued on an active control method using discharge plasma and showed experimentally that discharge plasma can make a difference in controlling the thermoacoustic instabilities in a Rijke tube. A vertically placed Rijke tube thermoacoustic system using induction heating tungsten mesh as a heat source was built. The results show that the high repetition rate discharge can effectively suppress the thermoacoustic oscillations in the Rijke tube and that they will not re-occur for some time. Additionally, their effectiveness depended more on average power than energy per pulse. Combining the collected pressure, schlieren data, and theoretical analysis, it can be suggested that the plasma discharge could heat the inlet airflow, which could influence the heat exchange and then could break thermo-acoustic coupling, and its high-frequency pressure perturbation might increase the dissipation of the energy of sound.