Standing-wave Thermoacoustic System Research Articles

Mechanism of the heat exchange promotion by superimposing the external sound wave in a standing-wave thermoacoustic system

For improvement of energy conversion efficiency, a sound wave is superimposed with a loudspeaker to the working fluid in the stack. By using this method the work-flow generation of the stack was enhanced. To analyze this enhancement mechanism, the thickness of the boundary layer and the heat exchange area in the stack are calculated from the viewpoint of the heat exchange circumstance. The effect of the heat exchange circumstance on the particle displacement and thermal flow is investigated. As a result, it is confirmed that the superimposed sound wave improves the heat exchange circumstance and then the thermoacoustic phenomenon is enhanced.

Fluid dynamics of oscillatory flow across parallel-plates in standing-wave thermoacoustic system with two different operation frequencies

Thermoacoustic system is one of the alternative technologies that provides green working principles but the lack of understanding of the complex fluid flow and energy transfer interactions within structures inside the system is leading to difficulty in accurate analysis related to the system. This study presents fluid dynamic investigation of vortex shedding and velocity profile of an oscillatory flow across a parallel-plate structure inside a standing-wave thermoacoustic system by using a two-dimensional ANSYS FLUENT CFD (computational fluid dynamics) of SST k-ω turbulence model. The model was validated using experimental data and theoretical solution. Two different operating frequencies of 14.2 Hz and 23.6 Hz were investigated with drive ratios (defined as maximum pressure amplitude to mean pressure) from 0.3% up to 3%. The results revealed that velocity profiles and boundary layers within the area of parallel-plate stack changes with time and the changes followed the cyclic travel of vortex across the structure. Two layers of vortex formed near the surface of the solid structure. These layers, known as the main and secondary vortex layers, change with the cyclic flow, and are affecting the shape of velocity profiles within the channel. The appearance of an ‘m’ shape, a ‘slug’ shape and a ‘parabolic’ shape velocity profiles are also depending on the flow amplitude (drive ratio) and flow frequency. The ‘parabolic’ velocity profile is only found for a certain moment of flow with thick boundary layer. The results indicated that fully developed flow may not be likely for cases presented in this paper. Hence care should be exercised in the use of equations during the analysis of thermoacoustic system. As the parallel-plate structure is usually an important structure of the system, a proper understanding of the dynamics of flow within this structure is crucial. These findings are expected to give better insight for future design of thermoacoustic system.

Entrance and exit effects on oscillatory flow within parallel-plates in standing-wave thermoacoustic system with two different operating frequencies

Thermoacoustics is about conversion between thermal and acoustical energies to provide alternative green technology for power cycle and cooling system. The oscillatory flow across porous structure inside the system is playing the role of energy conversion between the acoustic wave and the surface of the porous structure. Better understanding of fluid dynamics of oscillatory flow inside thermoacoustic system is therefore important for the thermoacoustic based energy conversion system. This paper presents the ‘entrance’ and ‘exit’ effects of the oscillatory flow within a parallel-plate structure that is placed inside a standing-wave thermoacoustic environment. Two-dimensional SST k-ω CFD models which were validated using experimental data and theoretical predictions were used for this investigation. Two different operating frequencies of 14.2 Hz and 23.6 Hz were studied for flow with five different amplitudes that were represented using drive ratios of 0.3%, 0.83%, 1.5%, 2.1% and 3%. These correspond to cases with Reynolds numbers between 5936 and 62926. Due to the cyclic nature of the flow, a region defined as an ‘exit’ region was observed in addition to the usual ‘entrance’ region and the fully developed region for flow inside a channel. The change of shape of velocity profiles from the ‘m’ shape profile, to the ‘slug-like’ profile and ‘parabolic-like’ profile was discussed in relation to the ‘entrance’ and ‘exit’ effects on flow inside the channel. The ‘entrance’ and ‘exit’ effects become bigger as drive ratio increases. The effect of ‘entrance’ and ‘exit’ are slightly reducing as frequency increases from 14.2 Hz to 23.6 Hz. This may be related to the shorter travel distance of fluid as the frequency increases. The results shown in this paper suggest that the flow within a 200 mm parallel-plate structure should be treated as developing flow especially for flow with low resonance frequency at drive ratio higher than 1%.

Resonance mode control by superposing external sound on the sound in standing-wave type thermoacoustic system

The technique to control the resonance mode necessary for enhancing the energy conversion efficiency of thermoacoustic system is discussed. A standing-wave type thermoacoustic system is set in the condition that the self-excited vibration with 3/2 wavelength of the total tube length takes place by adjusting the position of the stack. The sound wave of the fundamental mode is then externally superposed on the sound wave excited in the system. As a result, the resonance of the system is shifted to the fundamental mode from the higher mode. Furthermore, the resonance control is supposed to be realized by the reason that thickness of the viscous boundary layer is reduced and that the self-excited vibration is enabled at the lower entropy generation comparing with the higher-mode resonance. These results suggest that the technique to use the superposing sound wave is valid for enhancing the energy conversion efficiency by the resonance control.

1Pb4-6 Mechanism of the heat exchange promotion by superimposing the external sound wave in standing-wave thermoacoustic system

1Pb4-6 Mechanism of the heat exchange promotion by superimposing the external sound wave in standing-wave thermoacoustic system

Promotion of Energy Conversion by Heating Inside Stack in Standing-Wave Thermoacoustic-System

Promotion of Energy Conversion by Heating Inside Stack in Standing-Wave Thermoacoustic-System

Influence of internal heating of stack on the work flow generation in standing wave thermoacoustic system

To effectively use the whole area in a stack for energy conversion, the formation of a uniform temperature gradient over the whole area in the stack is necessary. For the temperature gradient formation, a method to heat the intermediate positions in the stack with heaters is proposed. In this paper, the relation between the heating position of the stack and the quantity of the work flow generation is experimentally investigated. As a result, the work flow generation increased and the energy conversion efficiency improved due to the effect of heating the intermediate position in the stack. In addition, the work flow generation increases most significantly in the case where the whole area of the stack has a uniform temperature gradient. Therefore, it is assumed to be preferable for stack design that the temperature gradient is uniform over the whole area in the stack.

Coherence resonance and stochastic bifurcation behaviors of simplified standing-wave thermoacoustic systems.

In this work, noise-induced motions (i.e., external fluctuations) in two modelled standing-wave thermoacoustic systems are studied when these systems are close to the deterministic stability boundary. These systems include (1) open-open (i.e., Rijke-type) and (2) closed-open boundary conditions. It is found from the smooth transitions of the stationary probability density function that the thermoacoustic system is destabilized via stochastic P bifurcation, as the external noise intensity is continuously increased. In addition, the increased noise intensity can shift the hysteresis region, which makes the system more prone to quasi-periodic oscillations, but also reduces the hysteresis area. The noise-induced coherence motions are observed numerically in the open-open system, which is denoted by the occurrence of a bell-shaped signal to noise ratio (SNR). The SNR is shown to be applicable as a precursor. It becomes larger and the optimal noise intensity is decreased as the modelled thermoacoustic system approaches the critical bifurcation point. In addition, coherence resonance is observed in the closed-open system. To validate the findings, experimental studies are conducted on an open-open Rijke tube. Good qualitative agreements are obtained. The present study shed lights on the stochastic and coherence behaviors of the standing-wave thermoacoustic systems with different boundary conditions.

2P4-10 Resonance mode control by superposition of external sound waves in standing-wave thermoacoustic system -Relationship between viscous boundary layer and resonance mode-

2P4-10 Resonance mode control by superposition of external sound waves in standing-wave thermoacoustic system -Relationship between viscous boundary layer and resonance mode-

Numerical simulation of the effect of plate spacing on heat transfer characteristics within a parallel-plate heat exchanger in a standing wave thermoacoustic system

Power can be converted with high efficiently between thermal energy and mechanical (acoustic) energy by using thermoacoustic technologies. Thus, the heat transfer characteristics are significant to the understanding of mechanisms, and improvement of efficiency for thermoacoustic devices, notably in heat exchangers. This paper introduces a two-dimensional computational fluid dynamics model of flow across a parallel-plate heat exchanger and investigates the effect of plate spacing on heat transfer characteristics. The open source CFD software OpenFOAM is applied because of the highly customizable capabilities to vary the control parameters. Firstly, the computational model including geometry, boundary conditions, equations, discretization scheme, turbulence and thermophysical properties’ models are presented, and then grid-independence validation is presented to verify the quality of mesh. The simulation results show that plate spacing influences the heat transfer between plates and adjacent area of heat exchanger, and the heat transfer coefficient goes up when the plate spacing decreases. The analysis also indicates that a possible flow transition to turbulence occurred within Re number between 247.2 and 321.4. The results in this work can help the understanding of heat transfer inside thermoacoustic system, and form a basis for future research.

Effect of the resonance frequency on the work flow with adjusting the internal temperature of the stack in a standing-wave thermoacoustic-system

We discuss the stack which is the key device of the thermoacoustic system. We elucidate the stack mechanism to realize further energy conversion efficiency improvement. We propose installing a heater inside the stack. This heater adjusts the internal temperature of the stack and controls the heat flow in the stack. Controlling the heat flow in the stack can also affect the mutual conversion of the heat flow and the work flow. In this report, the effect of the resonance frequency with the standing-wave thermoacoustic-system and the internal temperature of the stack on the heat flow in the stack was experimentally investigated. The resonance frequency was changed with the total length of the system. As a result, the lower the frequency, the larger the amount of the work flow generated in the stack. This can be influenced by the increase and decrease of the dissipation with changing the resonance frequency. Also, at any resonance frequency, the amount of the work flow generation increased with controlling the heat flow in the stack. The heat flow can be controlled by adjusting the internal temperature of the stack at any resonance frequency.

Effects of background noises on nonlinear dynamics of a modelled thermoacoustic combustor.

In this work, the effects of background noises on nonlinear dynamics of a modelled standing-wave thermoacoustic system with subcritical Hopf bifurcation behaviors are studied. These noises include (1) pressure-coupled (acoustic), (2) velocity-coupled (flow), and (3) external combustion noise. It is found that these three types of background noises play important, but different roles in changing the hysteresis width and stability boundary. In addition, the stochastic transition from stability to instability is investigated, as the noise intensity is varied. Two different stochastic P-bifurcations are identified. One is related to a craterlike probability density distribution. The other is associated with a probability density distribution characterized with two peaks and one trough. With each type of noise affecting the system's stochastic behaviors being evaluated, the effect of two different types of noises is then studied. It is shown that the combined noises (types 1 and 2) cannot only destabilize global stable thermoacoustic system, but also stabilize linearly unstable system. This depends strongly on the superimposition form of the two types of noises. In addition, when the thermoacoustic system is disturbed by the combined noise (types 3 and 1 or types 3 and 2), the transition process is dominated by the combustion noise.

Experimental and theoretical bifurcation study of a nonlinear standing-wave thermoacoustic system

In this work, both theoretical and experimental bifurcation studies of nonlinear standing-wave thermoacoustic systems are performed. To characterize the unsteady heat release, two different models are proposed to approximate experimental measurements of the response of a premixed flame to acoustic waves. Model A is formulated with 3rd order polynomial equation, while model B is 5th order one. The expansion method of Galerkin series is applied to decouple the unsteady acoustic field in the spatial and time domains. Multiple scales method is proposed to extract the essential dynamic behaviors from the governing ordinary differential equations. The process of the thermoacoustic system's transition to instability is evaluated by continuation methods. Model A is shown to be associated with a supercritical Hopf bifurcation. However, subcritical Hopf bifurcation occurs in the higher-order modelled nonlinear thermoacoustic system, i.e. model B. Furthermore, hysteresis and bistable zones are observed. In addition, the effects of 1) the heating power K, 2) the axial location of the heat source xf/L and 3) the overall acoustic damping/loss ξ on supercritical and subcritical bifurcations are studied one at a time. To validate our theoretical findings on supercritical and subcritical bifurcation, experimental investigation on a standing-wave Rijke tube with a premixed flame confined is conducted. It is observed that varying the flame axial location xf/L can destabilize the thermoacoustic system via either subcritical or supercritical bifurcation on two different operation conditions. In addition, the measured bifurcation behaviors show a good qualitative agreement with the theoretical findings.

Coherence resonance and stochastic bifurcation in standing-wave thermoacoustic systems

In this work, we develop a noisy nonlinear model and conduct experimental tests to study the stochastic bifurcation and coherence resonance in standing-wave thermoacoustic systems. When white Gaussian noise is added and its intensity is varied, it is found that the stochastic behaviors of the system can be better described by stochastic P bifurcation. When the noise intensity is chosen as a bifurcation parameter, it is shown to the bi-modal region and reduce the bi-modal area are shifted. In addition, the noise-induced coherence motions are examined and confirmed. Resonance-like behaviors of signal to noise ratio (SNR) are clearly observed. When the system approaches the critical bifurcation point, SNR is found to become larger and the optimal noise intensity is decreased to a smaller value. This property can be used as a precursor to the Hopf bifurcation in standing-wave thermoacoustic systems. Experiments are then conducted on a Rijke-type thermoacoustic system with 3 loudspeakers implemented. Transition to instability is found to be subcritical. Comparison is then made between the present theoretical and experimental results. Good qualitative agreements are obtained in terms of (1) SNR, (2) the peak height of power spectrum, and (3) the width of the frequency.

Experimental and Numerical Study of Passive Control of Self-sustained Thermoacoustic Oscillations Using an Electrical Heater

Experimental and Numerical investigations are conducted in this work to mitigate self-sustained thermoacoustic oscillations in standing-wave Rijke-type thermoacoustic systems. 2D numerical simulations of a propane-burnt combustion system are first conducted to gain insight on the effect of a secondary heat source on damping flame- exited thermoacoustic oscillations. Constant-temperature heating bands are placed at the downstream of the Rijke-type combustor. It is shown that the flame-sustained pressure oscillations are dramatically dampened, depending on the surface temperature of the heating bands. Experiments are then conducted. It is shown that placing an electrical heater at a proper position can lead to heat-driven acoustic oscillations being completely mitigated. And the standing-wave Rijke-type thermoacoustic system is stabilized.

The Effect of Temperature Field on Low Amplitude Oscillatory Flow within a Parallel-Plate Heat Exchanger in a Standing Wave Thermoacoustic System

Thermoacoustic technologies rely on a direct power conversion between acoustic and thermal energies using well known thermoacoustic effects. The presence of the acoustic field leads to oscillatory heat transfer and fluid flow processes within the components of thermoacoustic devices, notably heat exchangers. This paper outlines a two-dimensional ANSYS FLUENT CFD (computational fluid dynamics) model of flow across a pair of hot and cold heat exchangers that aims to explain the physics of phenomena observed in earlier experimental work. Firstly, the governing equations, boundary conditions and preliminary model validation are explained in detail. The numerical results show that the velocity profiles within heat exchanger plates become distorted in the presence of temperature gradients, which indicates interesting changes in the flow structure. The fluid temperature profiles from the computational model have a similar trend with the experimental results, but with differences in magnitude particularly noticeable in the hot region. Possible reasons for the differences are discussed. Accordingly, the space averaged wall heat flux is discussed for different phases and locations across both the cold and hot heat exchangers. In addition, the effects of gravity and device orientation on the flow and heat transfer are also presented. Viscous dissipation was found to be the highest when the device was set at a horizontal position; its magnitude increases with the increase of temperature differentials. These indicate that possible losses of energy may depend on the device orientation and applied temperature field.

A 3D investigation of thermoacoustic fields in a square stack

The thermoviscous behavior of the oscillating gas within the porous medium of a thermoacoustic refrigerator enables the conversion of sound into heat in the process of typical standing-wave thermoacoustic refrigeration systems. Several nonlinear mechanisms, such as harmonics generation, self-induced streaming and possible turbulence, cause complicated flow behavior and influence the performance of such devices. Few analytical and numerical approximations (Cao et al., 1996; Worlikar and Knio, 1996; Worlikar et al., 1998) describe the flow field and the energy flux density in standing devices comprising stacks of parallel plates, but almost no 3-D simulation has been developed that models the large-amplitude excitations and enables the prediction of the oscillating-flow behavior within a porous medium made of square channels. Here, we build on existing effort (Abd El-Rahman and Abdel-Rahman, 2014) and report a computational fluid dynamics analysis of a Helium-filled half-wavelength thermoacoustic refrigerator. The finite volume method is used, and the solid and gas domains are represented by large numbers of hexahedral elements. The calculations assume a 3-D periodic cell structure to reduce the computational cost and apply the dynamic mesh technique to account for the adiabatically oscillating wall boundaries. The simulation uses an implicit time integration of the full unsteady compressible flow equations along with the conjugate heat transfer algorithm. For the sake of simulation, the geometry and operating conditions closely follow the experimental setup of Lotton et al. (2009) [5]. A typical run involves about 200,000 computational cells, whereas the working frequency and the applied drive ratio are 592.5Hz and 2.0%, respectively. The stationary system response is predicted and the numerical values are compared to theory. The results show good agreement with theoretical behavior in the linear regime of drive ratios up to 2.0% while a maximum cooling effect of 10 degrees is captured between the stack ends. This simulation provides an interesting tool for understanding the transient flow behavior, mean energy-flux density and flow streaming, and helps building 3-D computational fluid dynamics models for thermoacoustic devices.

Increasing of work flow on the thermoacoustic system by control of the temperature gradient within the stack

A gas column in a tube with a temperature-gradient spontaneously begins to oscillate when air turbulence occurs. The work flow produced in the stack, ΔI, is important for the thermoacoustic oscillation. The amount of ΔI is determined by the shape of temperature-gradient. Experimental studies on thermoacoustic systems are previously assessed by measurement of temperature ratio, TH/TC, of the stack; however, investigation for distribution of temperature within the stack is less discussed. In order to examine the influence of a temperature-gradient within the stack on the thermoacoustic oscillation, we propose that control method for the temperature-gradient under the same TH/TC. In particular, the control of the temperature distribution is implemented in actively changing the internal temperature by additional nichrome heater in middle of the stack. The sound pressure in the standing-wave thermoacoustic system is measured at several temperature distribution by the proposed method. Furthermore, ΔI was calculated based on the measured sound pressure. As a result, the work flow by the control of a temperature distribution have increased, and it was qualitatively estimated by the slope of heat flux within the stack from temperature distribution.

Experimental and numerical definition of the extreme heater locations in a closed-open standing wave thermoacoustic system

In this work, experimental study of a close-open standing-wave thermoacoustic system with a heat source confined is performed first. By varying the axial location xf/L of the heat source, the thermoacoustic oscillation intensity is experimentally shown to be varied. Maximum-amplitude oscillations occur, as the heat source is axially placed at 0.4⩽xf/L⩽0.55. This indicates that the most ‘dangerous’ axial location of the heat source is close to the middle of the combustor. Furthermore, time-frequency analysis of the measured pressure signal reveals that the thermoacoustic system is nonlinear due to the presence of comparable amplitude peaks in the spectrum. To gain insights on the most ‘dangerous’ axial location and nonlinearity, and to validate our experimental findings, a 1D model of a closed-open standing-wave thermoacoustic system is developed. The effects of (1) the heat source location xf/L, (2) the number of eigenmodes N and (3) the mean temperature ratio T2/T1 across the heat source on the dynamics and transient stability behaviors of the standing-wave system are examined one at a time. It is found that the most ‘dangerous’ axial location of the heat source is approximately at xf/L=0.5. This is in good agreement with the results from our experimental measurement and the conventional modal analysis. In addition, it is shown that the most ‘dangerous’ heat source location is shifted by varying the downstream mean temperature. Similar finding is experimentally observed. Finally, Rayleigh index as an important stability indicator is defined and calculated to characterize the heat-to-sound coupling. The present work opens up new applicable way to produce maximum-amplitude standing-wave thermoacoustic oscillations in a practical engine system.

Numerical investigation of the effect of distributed heat sources on heat-to-sound conversion in a T-shaped thermoacoustic system

Abstract The present work considers a T-shaped standing-wave thermoacoustic system with an electrical heater confined in the bottom stem and two temperature-controllable heaters (TCHs) enclosed in the bifurcating branches. With the heater placed at the bottom stem set to 1100 K and TCHs be adiabatic, self-sustained limit cycle oscillations are generated. To gain insights on the effect of TCHs on damping or amplifying the oscillations, thermodynamic analysis and 2D numerical investigations are conducted. Three parameters are examined: (1) the number N of TCHs, (2) the surface temperature T l , r of TCHs and (3) the TCHs location x l , r / L . As each of the parameters is varied, the heat-driven acoustic signature is found to change. Flow reversal is observed during part of the limit cycle oscillations. The main nonlinearity is identified in the heat fluxes. In addition, such limit cycle oscillations are found to be mitigated or amplified, depending strongly on the heat flux and the locations of the TCHs. Placing the TCHs at proper locations in the bifurcating branches can lead to a maximum ‘destructive’ interaction and reducing the sound pressure level by more than 60 dB. To gain insight on the damping effect of the TCHs, Rayleigh index as a critical indicator of the coupling between heat and sound is defined and characterized. The present work opens up new applicable way to minimize standing-wave oscillations in a practical engine system by simply implementing additional heat sources.