Thermoacoustic Waves Research Articles

Flows Induced by Thermoacoustic Waves in an Enclosure: Effects of Gravity

The effect of gravity on the generation and propagation of flows induced by thermoacoustic waves in an enclosure was numerically investigated. The compressible unsteady Navier-Stokes equations were solved by combining a flux-corrected transport algorithm for the convection terms and by a central differencing scheme for the viscous and conduction terms. In the simulations, the left wall of the enclosure is heated rapidly either in a spatially uniform or a nonuniform manner, whereas the right wall is held at the initial temperature of the gas. The upper and lower , walls of the enclosure are insulated. At zero gravity, the spatially nonuniform heating generates a vortical flow in the enclosure, similar to that found in buoyancy-induced flows in a side-heated enclosure. For the conditions considered, the strength of the flows induced by the thermoacoustic waves is comparable to the flows induced by buoyancy.

Oscillatory flow in thermoacoustic sound wave generator

Oscillatory flow in a thermoacoustic sound wave generator is described. The thermoacoustic sound wave generator plays an important role in thermoacoustic equipment. The heat exchange between the working fluid and the stack, the acceleration and deceleration of the working fluid and viscous friction loss both in the stack and in the resonance tube influence the performance of the thermoacoustic sound wave generator. Particularly, oscillatory flow significantly influences the heat exchange mechanism between the working fluid and the stack. Temporal changes in pressure and velocity are sinusoidal inside the resonance tube. Flow forms an oscillatory jet just behind the tube outlet, and becomes intermittent far downstream outside the resonance tube. The open-end corrections of 0.63R, that is, the region where oscillatory flow characteristics are maintained downstream in spite of being outside the tube outlet, are confirmed by velocity measurements and flow visualization. Also, they are almost equal to acoustical theoretical results.

Nanoscale laser-induced spallation in SiO2 films containing gold nanoparticles

A phenomenological theory of ultraviolet pulsed-laser-induced spallation is proposed to interpret crater formation in SiO2 thin films containing absorbing 18.5-nm gold particles. The theory considers a spherical thermoacoustic stress wave propagating from a thermal source produced by laser-energy absorption inside the particle and surrounding ionized volume. Calculations show that the tensile stress associated with such an acoustic wave may exceed the local strength of the material and cause fracture and spallation of the top film portion. The theory provides an explanation of the experimentally observed complex (two-cone) shape of craters formed in the film with particle-lodging depth exceeding 110 nm. Theoretical estimates for the threshold stress amplitude and peak temperature in the thermal source are in qualitative agreement with the experimental observations.

C04 熱音響音波発生機の共鳴管出口付近における流れ : 流れ関数による流量変化の確認(パルス管冷凍機,熱音響機器及び関連要素と応用システム(2))

Flow rate changes around the resonance tube outlet of the thermoacoustic sound wave generator are described. Both flow rates and stream functions based on axial velocity amplitude are employed for confirming the flow rate changes here. The optimum propeller positions for wind power generation with the flow in the thermoacoustic sound wave generator have been observed previously. It is confirmed that they are almost equal to those analyzed by flow rates and stream functions based on axial velocity amplitude.

Measurement of thermo-acoustic waves induced by rapid heating of nickel sheet in open and confined spaces

Experimental investigation on the generation and propagation of thermo-acoustic waves induced by rapid heating of the metal sheet was carried out. This study focuses on the temperature profiles, measurement of pressure wave induced by rapid heating of nickel sheet in open and confined spaces. The R– C (resistance–capacitance) heating devices and high-speed measurement systems are designed, and methodologies on measurement are established. Results show that the waves have sharp and weak pin profile in an open space, but these waves have sharp front shape and decay with tail in the case of confined space. Results also show that the propagation velocity of thermo-acoustic wave, that is estimated as Mach number ( M), generated by rapid heating of nickel sheet, is located in the range of M = 0.99–1.04.

A numerical study of the generation and propagation of thermoacoustic waves in water

The generation and propagation of thermoacoustic waves in water are investigated by solving the fully compressible form of the Navier–Stokes equations. A square enclosure filled with water is considered as the computational domain. Thermally induced pressure waves are generated by rapidly heating the left wall. The thermodynamic properties of water are calculated via an accurate equation of state. The effect of the rapidity of the heating process on the production of thermoacoustic waves is studied, using both impulse (sudden) and gradual changes in the left wall temperature. The thermoacoustic waves are significantly stronger for impulse heating and are found to eventually damp out with time due to viscous losses. Gradual change of the left wall temperature reduced the strength of the pressure wave peaks.

Thermoacoustic Waves Arising under Conditions of Heating a Perfect Viscous Gas

We used the control volume approach and a modified SIMPLER algorithm to numerically solve a fully formulated unsteady-state problem on the heating of a perfect gas in a plane confined layer.

Performance of a Thermoacoustic Sound Wave Generator driven with Waste Heat of Automobile Gasoline Engine.

A new type of thermoacoustic sound wave generator driven with the waste heat of a 4 cycle automobile gasoline engine is described. The exhaust-pipe connected sound wave generator, in which the hot heat exchanger is set in the exhaust pipe in order to recover the waste heat of exhaust gas, is proposed. A temperature of 780°C of exhaust gas in the exhaust is observed. In a conventional thermoacoustic sound wave generator, sound waves originate at a temperature of the hot heat exchanger, TH, of 200-300°C and become sufficient at 700°C. It is confirmed that the new generator generates sufficient sound waves and its performance is almost equal to that of the electric heater driven generator at a thermal input of 300 W, which corresponds to slightly more than 1% of the heat quantity of exhaust gas provided under the condition that the number of engine revolutions is 2 600 r.p.m. and that the throttle opening is 35%.

Numerical simulation of developing natural convection in an enclosure due to rapid heating

Effects of thermoacoustic wave motion on the developing natural convection process in a compressible gas-filled square enclosure were investigated numerically. In the cases considered, the left wall temperature is raised rapidly (impulsively or gradually) while the right wall is held at a specified temperature. The top and the bottom walls of the enclosure considered are thermally insulated. The numerical solutions of the full Navier–Stokes equations were obtained by employing a highly accurate flux-corrected transport algorithm for the convection terms and by a central differencing scheme for the viscous and diffusive terms. The strength of the pressure waves associated with the thermoacoustic effect and resulting flow patterns are found to be strongly correlated to the rapidity of the wall heating process. Fluid thermal diffusivity was found to affect the strength of the thermoacoustic waves and the resulting interaction with the buoyancy-induced flow.

2401 熱音響音波発生機内の振動流れ : スタックおよび共鳴管内で流体のなす仕事

Oscillatory flow in the thermoacoustic sound wave generator is described. In oscillatory pipe flow, characteristic Richardson effect occurs in the vicinity of the wall. Its flow plays an important role in thermoacoustic phenomena in which direct energy conversion from heat to work (sound waves), and vice versa, is performed with no moving parts. Also, flow in the resonance tube is influenced by its flow on the stack. Fluid work in stack and the resonance tube are discussed.

Time-domain reconstruction for thermoacoustic tomography in a spherical geometry.

Reconstruction-based microwave-induced thermoacoustic tomography in a spherical configuration is presented. Thermoacoustic waves from biological tissue samples excited by microwave pulses are measured by a wide-band unfocused ultrasonic transducer, which is set on a spherical surface enclosing the sample. Sufficient data are acquired from different directions to reconstruct the microwave absorption distribution. An exact reconstruction solution is derived and approximated to a modified backprojection algorithm. Experiments demonstrate that the reconstructed images agree well with the original samples. The spatial resolution of the system reaches 0.5 mm.

209 熱音響音波発生機内の振動流れ : スタック上の流れ

Oscillatory flow in the thermoacoustic sound wave generator is described. Thermoacoustic phenomena are noteworthy, which is direct energy conversion from heat to work (sound wave) with no moving parts. Especially, oscillatory flow on the stack which set in the resonance tube plays an important role in thermoacoustic phenomena. Richardson's annular effect occurs and phase between pressure and velocity shifts in the vicinity of the tube wall. Some analyses of its flow are performed.

Signal processing in scanning thermoacoustic tomography in biological tissues.

Microwave-induced thermoacoustic tomography was explored to image biological tissues. Short microwave pulses irradiated tissues to generate acoustic waves by thermoelastic expansion. The microwave-induced thermoacoustic waves were detected with a focused ultrasonic transducer to obtain two-dimensional tomographic images of biological tissues. The dependence of the axial and the lateral resolutions on the spectra of the signals was studied. A reshaping filter was applied to the temporal piezoelectric signals from the transducer to increase the weight of the high-frequency components, which improved the lateral resolution, and to broaden the spectrum of the signal, which enhanced the axial resolution. A numerical simulation validated our signal-processing approach.

Characteristics of Coupled Types of Thermoacoustic Sound Wave Generators.

New types of thermoacoustic sound wave generators, which play an important role in the thermoacoustic refrigerator, are developed. The thermoacoustic sound wave generator consists of hot and cold heat exchangers, stack and resonance tube which is made of stainless steel and Plexiglas tubes, 32 mm in inner diameter and 860 mm long. The theoretical sound wave frequency is 100 Hz. First, two sound wave generators are connected with each other either at the open ends or the closed ends. When they are connected at their open ends, their velocity, pressure amplitude and heat transfer are considerably improved compared with those of independently operated ones. Each phase of both velocity and pressure is inverted automatically. However, when they are connected at their closed ends, their velocity, pressure amplitude and heat transfer are almost equal to those of independently operated ones, and each phase is synchronized automatically. Second, a loop-type sound wave generator constructed of four sound wave generators and four bent tubes is developed. Its sufficient performance is also confirmed.

D12 自動車用エンジンの廃熱を利用する熱音響音波発生機の性能(熱音響エンジン)

A new type of thermoacoustic sound wave generator utilizing the waste heat of a 4-cycle automobile gasoline engine is described. The exhaust-pipe-connected sound wave generator, in which the hot heat exchanger is set in the exhaust pipe in order to recover the waste heat of exhaust gas, is proposed. A temperature of 780℃ of exhaust gas in the exhaust is observed. In a conventional thermoacoustic sound wave generator, sound waves originate at a temperature of the hot heat exchanger, T_H, of 200〜300℃ and become sufficient at 700℃. It is confirmed that the new generator generates sufficient waves and its performance is almost equal to that of the electric-heater-driven generator at a thermal input of 300W, which corresponds to slightly more than 1% of the heat quantity of exhaust gas provided under the condition that the number of engine revolutions is 2600rpm and that the throttle opening ratio is 35%.

ループ型熱音響音波発生機の特性

The characteristics of a new loop-type thermoacoustic sound wave generator, in which two closed-end connected-type generators are connected to each other at the open ends by four bent tubes and sealed hermetically, are described. It has been reported that two types of sound wave generators, in which two identically shaped systems are connected to each other at either the open ends or the closed ends, can operate stably. In the new loop-type sound wave generator, which has the benefits of the above-mentioned types of sound wave generators, the pressure, velocity amplitude, acoustic intensity, and heat transferred by sound waves are greater than those of the other connected-type ones. And harmonics in pressure signals and no harmonics in velocity signals are observed at the connected part. An optimum theoretical sound wave frequency of 100Hz is also confirmed for this type of sound wave generator.

Numerical study of thermoacoustic waves in an enclosure

The behavior of thermoacoustic waves in a nitrogen-filled two-dimensional cavity is numerically studied in order to investigate how these waves may be used as an effective heat removal mechanism. The compressible, unsteady Navier–Stokes equations were solved for a series of initial conditions by combining a flux-corrected transport algorithm for convection with models for temperature-dependent viscosity and thermal conduction. By considering a one-dimensional test problem and comparing the results to existing data, the accuracy of the present numerical method is verified. In the problems considered, the vertical walls of a cavity were heated or cooled to generate the thermoacoustic waves. Both impulsive and gradual changes of the wall temperatures were considered. When the vertical wall was heated impulsively and nonuniformly, the waves induced two-dimensional flows within the enclosure. The observed thermoacoustic waves oscillate and eventually decay due to viscous and heat dissipation.

A thermoacoustic traveling wave linear amplifier

This paper describes an experiment to show linear amplification of traveling sound waves in a duct using a thermoacoustic regenerator. As noted by Ceperley [J. Acoust. Soc. Am. 66, 1508–1513 (1979)] a Stirling engine-type regenerator should act as an acoustic gain medium for traveling waves in a duct. This principle is used in thermoacoustic traveling wave engines to transfer power from heat reservoirs to acoustic energy. However, it is difficult to produce finite gain for pure traveling wave impedance, since viscous losses in the channels of the regenerator overcome the gain, and previous workers have only been able to show reduced loss in such a system. Optimizing the regenerator design with numerical modeling and using a greater temperature difference suggest that a traveling wave thermal amplifier can produce 2 dB of real gain over two octaves for traveling waves in air. Such a device would amplify a broadband acoustic signal without electrical transducers. Design of the amplifier and experimental results will be shown.

Scanning thermoacoustic tomography in biological tissue.

Microwave-induced thermoacoustic tomography was explored to image biological tissue. Short microwave pulses irradiated tissue to generate acoustic waves by thermoelastic expansion. The microwave-induced thermoacoustic waves were detected with a focused ultrasonic transducer. Each time-domain signal from the ultrasonic transducer represented a one-dimensional image along the acoustic axis of the ultrasonic transducer similar to an ultrasonic A-scan. Scanning the system perpendicularly to the acoustic axis of the ultrasonic transducer would generate multi-dimensional images. Two-dimensional tomographic images of biological tissue were obtained with 3-GHz microwaves. The axial and lateral resolutions were characterized. The time-domain piezo-electric signal from the ultrasonic transducer in response to the thermoacoustic signal was simulated theoretically, and the theoretical result agreed with the experimental result very well.

C03 ループ型熱音響音波発生機の特性

The characteristics of a new loop-type thermoacoustic sound wave generator, in which two close-ended, connected-type generators are connected to each other at the open ends by four bent tubes and sealed hermetically, are described. It has been reported that two types of sound wave generators, in which two identically shaped systems are connected to each other at either the open ends or the closed ends, can operate stably. In the new loop-type generator which has the benefits of the above-mentioned two types of sound wave generators, its pressure, velocity amplitude and heat transferred by sound waves are greater than those of the other connected-type ones. And harmonic in pressure signals and no harmonic in velocity signals are observed at the connected part. An optimum theoretical sound wave frequency of 100Hz is also confirmed for this type of sound wave generator.