Influence of stack geometry and resonator length on the performance of thermoacoustic engine

Thermoacoustic engines convert heat energy into high amplitude sound waves, which is used to drive thermoacoustic refrigerator or pulse tube cryocoolers by replacing the mechanical pistons such as compressors. The increasing interest in thermoacoustic technology is of its potentiality of no exotic materials, low cost and high reliability compared to vapor compression refrigeration systems. The experimental setup has been built based on the linear thermoacoustic model and some simple design parameters. The engines produce acoustic energy at the temperature difference of 325–450 K imposed along the stack of the system. This work illustrates the influence of stack parameters such as plate thickness (PT) and plate spacing (PS) with resonator length on the performance of thermoacoustic engine, which are measured in terms of onset temperature difference, resonance frequency and pressure amplitude using air as a working fluid. The results obtained from the experiments are in good agreement with the theoretical results from DeltaEc.     

电动声源热声致冷机声学和计算实例

我们将各种热声致机简化为一包括声学终端在内的声管道系统，并通过实例讨论了致冷机的声学特性，该管道系统与一般声管道不同：１．在热声堆中热波和粘滞波不可不计。２．在热声堆与声管连接时，必需考虑合成波的体积流；而热声堆内只需考虑传播波的体积流。本文对此提出了阻抗连接条件的修正。实例使用电动扬声器为声源，给出了热声行波和驻波致冷的声学计算方法以及它们的声学特性，所用扬声器的标称伏安为１００ＶＡ，可为热声致     

Theoretical modeling and experimental realization of dynamically magnified thermoacoustic-piezoelectric energy harvesters

Conventional thermoacoustic-piezoelectric (TAP) harvesters convert thermal energy, such as solar or waste heat energy, directly into electrical energy without the need for any moving components. The input thermal energy generates a steep temperature gradient along a porous medium. At a critical threshold of the temperature gradient, self-sustained acoustic waves are developed inside an acoustic resonator. The associated pressure fluctuations impinge on a piezoelectric diaphragm, placed at the end of the resonator. In this study, the TAP harvester is coupled with an auxiliary elastic structure in the form of a simple spring–mass system to amplify the strain experienced by the piezoelectric element. The auxiliary structure is referred to as a dynamic magnifier and has been shown in different areas to significantly amplify the deflection of vibrating structures. A comprehensive model of the dynamically magnified thermoacoustic-piezoelectric (DMTAP) harvester has been developed that includes equations of motions of the system?s mechanical components, the harvested voltage, the mechanical impedance of the coupled structure at the resonator end and the equations necessary to compute the self-excited frequencies of oscillations inside the acoustic resonator. Theoretical results confirmed that significant amplification of the harvested power is feasible if the magnifier?s parameters are properly chosen. The performance characteristics of experimental prototypes of a thermoacoustic-piezoelectric resonator with and without the magnifier are examined. The obtained experimental findings are validated against the theoretical results. Dynamic magnifiers serve as a novel approach to enhance the effectiveness of thermoacoustic energy harvested from waste heat by increasing the efficiency of their harvesting components.     

频率对热声制冷机影响的研究

分析和研究了频率对热声制冷机声压、温差和声功的影响 ;使用网络模型计算的结果与实验结果相吻合 ;该研究也是对网络模拟和声功测量方法的验证。     

The effects of liquid height/volume,initial concentration of reactant and acoustic power on sonochemical oxidation

Even though much knowledge on acoustic cavitation and its application has been accumulated over the past decades, further research is still required to develop industrial uses of acoustic cavitation. It is because the available information is mainly based on small-scale sonoreactors and the design and optimization of sonoreactors for large-scale applications have not been widely studied. In this study, the effects of liquid height/volume, initial concentration of the reactant and input acoustic power on sonochemical oxidation reactions including iodide ion oxidation, As(III) oxidation, and hydrogen peroxide generation were investigated using a 291 kHz sonoreactor with various liquid height/volumes (50, 100, 200, 300, 500, and 1000 mL) and input powers (23, 40, and 82 W). As the liquid height/volume and the input power changed, the power density varied from 23 to 1640 W/L and the maximum cavitation yields of triiodide ion for 23, 40, and 82 W were observed at 0.05, 0.1, and 0.2/0.3 L, respectively. It was found that low power was more effective for the small volume and the large volume required high power level and the moderate power density, approximately 400 W/L, was suggested for the sonochemical oxidation of iodide ion in the 291 kHz sonoreactor in this study. Similar results were observed in the generation of hydrogen peroxide and the sonochemical oxidation of As(III) to As(V). It was also revealed that KI dosimetry could be applicable for the estimation of the sonochemical reactions of non-volatile compounds such as As(III).     

Cavitation mapping by sonochemiluminescence with less bubble displacement induced by acoustic radiation force in a 1.2 MHz HIFU

An acoustic radiation force counterbalanced appliance was employed to map the cavitation distribution in water. The appliance was made up of a focused ultrasound transducer and an aluminum alloy reflector with the exactly same shape. They were centrosymmetry around the focus of the source transducer. Spatial–temporal dynamics of cavitation bubble clouds in the 1.2 MHz ultrasonic field within this appliance were observed in water. And they were mapped by sonochemiluminescence (SCL) recordings and high-speed photography. There were significant differences in spatial distribution and temporal evolution between normal group and counterbalanced group. The reflector could avoid bubble directional displacement induced by acoustic radiation force under certain electric power (⩽50 W). As a result, the SCL intensity in the pre-focal region was larger than that of normal group. In event of high electric power (⩾70 W), most of the bubbles were moving in acoustic streaming. When electric power decreased, bubbles kept stable and showed stripe structure in SCL images. Both stationary bubbles and moving bubbles have been captured, and exhibited analytical potential with respect to bubbles in therapeutic ultrasound.     

Experimental investigation of conical bubble structure and acoustic flow structure in ultrasonic field

The objective of this paper is to investigate the transient conical bubble structure (CBS) and acoustic flow structure in ultrasonic field. In the experiment, the high-speed video and particle image velocimetry (PIV) techniques are used to measure the acoustic cavitation patterns, as well as the flow velocity and vorticity fields. Results are presented for a high power ultrasound with a frequency of 18 kHz, and the range of the input power is from 50 W to 250 W. The results of the experiment show the input power significantly affects the structures of CBS, with the increase of input power, the cavity region of CBS and the velocity of bubbles increase evidently. For the transient motion of bubbles on radiating surface, two different types could be classified, namely the formation, aggregation and coalescence of cavitation bubbles, and the aggregation, shrink, expansion and collapse of bubble cluster. Furthermore, the thickness of turbulent boundary layer near the sonotrode region is found to be much thicker, and the turbulent intensities are much higher for relatively higher input power. The vorticity distribution is prominently affected by the spatial position and input power.     

Acoustic pressure losses in woven screen regenerators

In thermoacoustic travelling-wave engines and other Stirling cycle devices, good performance depends on the material of a regenerator being in intimate contact with the gas inside it, so that each particle of gas oscillates in temperature following the adjacent material as it is acoustically displaced. This requires that the passages are small enough for temperature waves to penetrate across the gas path with the frequencies of interest. One type of ‘regenerator’ that is commonly used for this purpose is composed of multiple layers of woven stainless steel mesh, laid on top of one another in random registration. Associated with the thermal penetration is a viscous loss of pressure and this must be quantified if efficient engines are to be designed.In the literature, reliance has been placed on the correlation of steady-flow loss data for these meshes, but for the coarser ones operating at frequencies greater than 28 Hz, the assumption of quasi steady-flow is dubious and direct acoustic measurements must be made. This paper reports acoustic pressure loss data for meshes with 34 and 75 wires per inch taken in two configurations of impedance tube, and finds that the dependence on velocity is the same as in steady-flow, but that there is indeed some enhancement of loss for frequencies above 40 Hz. (Separation of the mesh layers is probably responsible for the anomalously low loss coefficients that were recorded in one set of data.) It is shown that the acoustic pressure losses can be correlated in terms that give the acoustic impedance more directly than the friction factor correlations.     

驻波型热声制冷机熵产分析

声制冷机是一种新型制冷机,具有无机械运动部件,可靠性高寿命长,采用惰性气体为工质无污染等优点．驻波型热声制冷机的声功泵热效应是不可逆过程,内部不可逆损失导致热声制冷机效率偏低,制约了热声制冷机的发展和应用．本文研究了线性范围内驻波型制冷机换热器和回热器内的可压缩振荡流动与传热过程的熵产,分析了板间距,振荡频率和温度梯度对熵产的影响。     

A simple dye-sensitized solar cell sealing technique using a CO2 laser beam excited by 60 Hz AC discharges

Dye-sensitized solar cells (DSSCs) use two glass substrates (photo electrode and counter electrode) coated with fluorine-doped tin oxide (FTO) to harvest light into the cell and to collect electrons. The space between the photo electrode and the counter electrode are filled with a liquid type electrolyte for electron transfer into the cell. Therefore, an appropriate sealing method is required to prevent the liquid electrolyte leaking out. In this paper, a simple CO₂ laser beam with TEM₀₀ mode excited by a 60 Hz AC discharge was used to seal two glass substrates coated with FTO for the fabrication of DSSCs. The sealing technique improved the durability and stability of the DSSCs. The optimal conditions for the sealing of the DSSCs are related to the pin-hole diameter, the discharge current and the moving velocity of the target. Especially, the CO₂ laser beam is used as a heat source that is precisely controlled by the pin-hole, which plays an important role in adjusting its spot size. From these results, the maximum laser power was found to be 40 W at 18 Torr and 35 mA. In order to achieve the best sealing quality, the following parameters are required: a pin-hole diameter of 4 mm, input voltage of 10.73 kV, discharge current of 9.31 mA, moving velocity of 1 mm/s and distance from the target surface of 26.5 cm. Scanning electron microscope images show that the sealing quality obtained using the CO₂ laser beam is superior to that obtained using a hot press or soldering iron.     

Electrodeposition of nickel-tungsten alloys under ultrasonic waves: Impact of ultrasound intensity on the anticorrosive properties

Electrodeposited Ni–W alloy assisted by high-intensity ultrasound was evaluated considering the nominal power effect on the anticorrosive property. Temperature profiles demonstrated that using a nominal power of 400 W, the electrolytic bath at 30 °C reached values of 39 ± 1 °C. The maximum acoustic power corresponded to 6.7% of the nominal power value at 400 W. Increasing the nominal power from 0 to 400 W; the Ni content decreased from 85.3 to 75.2 wt%, and the W content increased from 15.1 to 25.1 wt%. The deposited coating at 200 W and 300 W had a smooth, homogeneous, and uniform surface. At 400 W, the acoustic cavitation promoted erosion, affecting the coating surface. X-ray diffraction analysis indicated that the nominal power of 200 W promoted electrodeposition of the Ni₁₇W₃ structure with the plane (1 1 1) as a preferred orientation. The crystallite size decreased for the planes (1 1 1) and (2 0 0) when increased nominal power from 100 to 200 W. The optimum condition for the improved corrosion resistance occurred with the nominal power of 200 W, providing a polarization resistance of 23.42 kΩ cm².     

Ultrasound assisted extraction of β-carotene from Spirulina platensis

This paper illustrates the Ultrasound Assisted Extraction (UAE) of β-carotene from Spirulina platensis. Various parameters such as extraction time, solvent type, biomass to solvent ratio, temperature, electrical acoustic intensity, length of the probe tip dipped into the solvent, duty cycle and pre treatment effect were explored for the extraction of β-carotene. From economic point of view, the optimal conditions for the extraction of β-carotene from Spirulina were 1.5 g Spirulina (2 min pre soaked in methanol) in 50 ml n-heptane at 30 °C temperature, 167 W/cm² electrical acoustic intensity and 61.5% duty cycle for 8 min with probe tip length of 0.5 cm dipped into the extracting solvent from the surface. The maximum extraction achieved under the above mentioned optimum parameters was 47.10%. The pre-treatment time showed a promising effect on the yield as pre-treating the biomass with methanol for 2 min before ultrasonication showed 12 times increase in extraction yield of β-carotene.     

Heat transfer enhancement using 2 MHz ultrasound

The present work focuses on possible heat transfer enhancement from a heating plate towards tap water in forced convection by means of 2 MHz ultrasound. The thermal approach allows to observe the increase of local convective heat transfer coefficients in the presence of ultrasound and to deduce a correlation between ultrasound power and Nusselt number. Heat transfer coefficient under ultrasound remains constant while heat transfer coefficient under silent conditions increases with Reynolds number from 900 up to 5000. Therefore, heat transfer enhancement factor ranges from 25% up to 90% for the same energy conditions (supplied ultrasonic power = 110 W and supplied thermal power = 450 W). In the same time cavitational activity due to 2 MHz ultrasound emission was characterized from mechanical and chemical viewpoints without significant results. At least, Particle Image Velocimetry (PIV) measurements have been performed in order to investigate hydrodynamic modifications due to the presence of 2 MHz ultrasound. It was therefore possible to propose a better understanding of heat transfer enhancement mechanism with high frequency ultrasound.     

Stationary velocity and pressure gradients in a thermoacoustic stack.

The second-order time-averaged acoustics of a viscous, thermally conducting gas between closely spaced parallel plates is studied. The acoustic disturbance is studied by expanding the equations of fluid dynamics and heat transfer to second order in Mach number. The undisturbed state is allowed to have a nonzero temperature gradient. A set of coupled equations for the time-averaged pressure gradient, velocity, and temperature are obtained and solved. Particular attention is paid to the relation between the time-averaged mass flux and pressure gradient. An explicit expression is obtained relating the time-averaged pressure drop across a thermoacoustic stack to the time-averaged mass flux through the stack.     

Thermal imaging during infrared final cooking of semi-processed cylindrical meat product

The temperature measurements during the infrared cooking of the semi-cooked cylindrical minced beef product (koefte) were taken by both contact (thermocouples) and non-contact (thermal imaging) techniques. The meat product was semi-cooked till its core temperature reached up to 75 °C by ohmic heating applied at 15.26 V/cm voltage gradient. Then, infrared cooking was applied as a final cooking method at different combinations of heat fluxes (3.7, 5.7 and 8.5 kW/m²), applied distances (10.5, 13.5 and 16.5 cm) and applied durations (4, 8 and 12 min). The average surface temperature increased as the heat flux and the applied duration increased but the applied distance decreased. The temperature distribution of the surface during infrared cooking was determined successfully by non-contact measurements. The temperature homogeneity varied between 0.77 and 0.86. The process condition of 8.5 kW/m² for 8 min resulted in core temperature greater than 75 °C, which was essential for safe production of ready-to-eat (RTE) meat products. Thermal imaging was much more convenient method for minimizing the point measurement mistakes and determining temperature distribution images more clear and visual.     

Ultrasonically enhanced fractionation of milk fat in a litre-scale prototype vessel

The ultrasonic fractionation of milk fat in whole milk to fractions with distinct particle size distributions was demonstrated using a stage-based ultrasound-enhanced gravity separation protocol. Firstly, a single stage ultrasound gravity separation was characterised after various sonication durations (5–20 min) with a mass balance, where defined volume partitions were removed across the height of the separation vessel to determine the fat content and size distribution of fat droplets. Subsequent trials using ultrasound-enhanced gravity separation were carried out in three consecutive stages. Each stage consisted of 5 min sonication, with single and dual transducer configurations at 1 MHz and 2 MHz, followed by aliquot collection for particle size characterisation of the formed layers located at the bottom and top of the vessel. After each sonication stage, gentle removal of the separated fat layer located at the top was performed.Results indicated that ultrasound promoted the formation of a gradient of vertically increasing fat concentration and particle size across the height of the separation vessel, which became more pronounced with extended sonication time. Ultrasound-enhanced fractionation provided fat enriched fractions located at the top of the vessel of up to 13 ± 1% (w/v) with larger globules present in the particle size distributions. In contrast, semi-skim milk fractions located at the bottom of the vessel as low as 1.2 ± 0.01% (w/v) could be produced, containing proportionally smaller sized fat globules. Particle size differentiation was enhanced at higher ultrasound energy input (up to 347 W/L). In particular, dual transducer after three-stage operation at maximum energy input provided highest mean particle size differentiation with up to 0.9 μm reduction in the semi-skim fractions. Higher frequency ultrasound at 2 MHz was more effective in manipulating smaller sized fat globules retained in the later stages of skimming than 1 MHz. While 2 MHz ultrasound removed 59 ± 2% of the fat contained in the initial sample, only 47 ± 2% was removed with 1 MHz after 3 ultrasound-assisted fractionation stages.     

An open-air infrasonic thermoacoustic engine

A low-frequency open-air thermoacoustic engine in a Helmholtz resonator has been constructed. Tests indicate that the system resonates in the Helmholtz mode for modest thermoacoustic stack temperature differences using stacks of varying type and pore size located within the neck of the Helmholtz resonator. The maximum acoustic pressure radiated from the open end of the resonator corresponds to 81 dB-SPL ref 20 μPa at a stack temperature difference of 185 K and an input electric power of 276 W. The system is well characterized by a numerical model of a representative stack.     

Theoretical and experimental investigation of desalting and dehydration of crude oil by assistance of ultrasonic irradiation

In this paper desalting/dehydration process of crude oil by ultrasonic irradiation in a novel batch standing-wave resonator reactor is studied both theoretically and experimentally. The effect of main parameters including ultrasonic irradiation parameters, namely irradiation input power and irradiation time, and also operating parameters, such as temperature and injected water, on the removal efficiencies of salt and water is examined. The obtained results demonstrate that finding the optimum values of the above mentioned parameters is important to prevent a significant decrease in the removal efficiencies of water and especially salt. Thus, crude oil was subjected to optimal ultrasonic irradiation with an input power of 57.7 W, and irradiation time of 6.2 min at temperature of 100 °C. The injected water to dissolve the salt of crude oil was 7 vol.%. Also, the applied settling time and dosage of chemical demulsifier were 60 min and 2 ppm, respectively. Under these optimum conditions the removal efficiencies of the desalting/dehydration process were 84% and 99.8%, respectively, which are suitable for refineries.Also, based on the optimal experimental data, two inferential estimators are developed to obtain the relationships between the salt and water removal efficiencies, and input energy density. These empirical relationships can offer a proper estimation for the salt and water removal efficiencies with irradiation input energy.     

Cavitation distribution within large phantom vessel and mechanical damage formed on surrounding vessel wall

Blood vessel is one of the most important targets encountered during focused ultrasound (FU) therapy. The lasting high temperature caused by continuous FU can result in structural modification of small vessel. For the vessel with a diameter larger than 2 mm, convective cooling can significantly weaken the thermal effect of FU. Meanwhile, the continued presence of ultrasound will cause repetitive cavitation and acoustic microstreaming, making comprehension of continuous wave induced cavitation effect in large vessels necessary. The Sonoluminescence (SL) method, mechanical damage observation and high-speed camera were used in this study to investigate the combination effect of ultrasound contrast agents (UCAs) and continuous FU in large phantom vessels with a diameter of 10 mm without consideration of thermal effect. When the focus was positioned at the proximal wall, cylindrical hole along the acoustic axis opposite the ultrasound wave propagation direction was observed at the input power equal to or greater than 50 W. When the focus was located at the distal wall, only small tunnels can be found. The place where the cylindrical hole formed was corresponding to where bubbles gathered and emitted brilliant light near the wall. Without UCAs neither such bright SL nor cylindrical hole can be found. However, the UCAs concentration had little influence on the SL distribution and the length of cylindrical hole. The SL intensity near the proximal vessel wall and the length of the cylindrical hole both increased with the input power. It is suggested that these findings need to be considered in the large vessel therapy and UCAs usage.     

Comparison of characterization methods in high frequency sonochemical reactors of differing configurations

The aim of this study was to compare different characterization methods in order to evaluate the sonochemical efficiency of a cavitational reactor. The selected characterization methods were calorimetry and dosimetry based on potassium iodide oxidation or nitrite and nitrate ion formation. The effects of experimental parameters on physical and chemical effects of ultrasound were quantified with two transducers at a frequency of 366 kHz. The studied factors comprised temperature (16–28 °C), acoustic power (6–38 W), power density (4–61 W L^?1) and reactor configuration (D_{reactor 1} = 65 mm, D_{reactor 2} = 102 mm). Spectrophotometry was compared to ionic chromatography as a method to quantify nitrite and nitrate ions. Spectrometry was shown to be as representative as ionic chromatography. The reaction system based on the formation of both nitrite and nitrate ions was demonstrated to be as reliable as a potassium iodide dosimeter. The representativity of calorimetry was limited since part of acoustic energy was assumed to be used in the chemical reactions observed by dosimetry. Similar sonochemical efficiencies resulted from an increase of sonified surface (D_{reactor 1} = 65 mm vs. D_{reactor 2} = 102 mm) coupled to a 2-time decrease in power density at a constant emitting surface. The effect of emitting-to-sonified surface area ratio on the acoustic field was apparently limited by the height of the liquid.