低维、相控阵及腔内聚焦热声发射的理论研究

发布时间：2018-01-13 15:33

本文关键词：低维、相控阵及腔内聚焦热声发射的理论研究　出处：《中国科学技术大学》2017年博士论文　论文类型：学位论文

【摘要】：本论文分为三部分。第一部分研究低维材料热声发射的特性与规律。近年来,随着纳米材料与纳米技术的发展,碳纳米管、石墨烯等低维材料相继被作为热声材料而引入了热声领域。它们超低的单位热容所带来的高声压以及热声的宽频带、低失真特性使古老的热声技术焕发了青春,形成了目前国际上称为Nanothermophones的研究热点。但研究工作大都集中在实验与应用,理论研究相对滞后。为对热声发射的特性与规律有更全面、深入的了解,我们从最基本的点源热声发射研究起,由点到线再到面地对低维材料热声发射进行系统研究。具体的技术路线为,先建立均质球热声发射的热力耦合模型,在推导出均质球热声发射分析解的基础上,令球的半径趋于零和进一步简化,得到了点源热声发射公式,再通过点源叠加即线与面积分,得到了统一的远、近场均适用的线源与面源热声发射计算式,同时也将加热膜、基底材料、粘性耗散、热扩散及对流换热诸因素加以考虑。根据所得到的这些理论公式,对以碳纳米管为主的低维材料点、线、面热声发射的特性及主要影响因素进行了深入细致的分析研究,给出了低维材料热声场的声压随频率、距离、角度等的变化规律,以及近、远场的主要特征,即随频率与距离的近场波动,远场衰减。而近场随频率的波动正是面源热声有平频响的原因。研究发现:1)在低频下各类热声场的声压都随对数频率近乎线性增长,高频下都会出现剧烈衰减。对于点源,声压的频率响应只有线性上升与急剧下降两段,而对于线与面热源则还有中间缓慢波动上升或下降过渡段。随着距离的增大,因趋于点源,中间段逐渐缩小,直至消失。2)点源最大频响频率与距离的1/2方成反比。根据这一规律得到了面源可视为点源的判断准则。3)热声发射器的尺度越大,声场中某一位置出现衰减转折的起始频率越小,越呈近场特性。4)由于在远场呈点源特性,声压只与施加在发射器上的总热流有关,而与尺度大小几乎无关。5)对于线与面源,中垂线与非中垂线方向的热声场特性有很大不同。在与中垂线成某一角度的方向上,大于一定的距离后,无论是线还是面热源,其频响曲线都会在某一频率之上出现不随距离改变的周期性波动,且频率周期不随材料变化。6)无论是近场还是远场,发射器热容对声压级的影响很大。声压随热容的增大从一开始就急剧衰减,且频率越高,衰减得越快。因而热容小是热声发射器适用的关键。这也是热声发射现象发现一百多年直到近期出现纳米热声发射器才引起很大关注的原因。7)对于纳米热声发射器而言,自然对流的影响主要在1kHz以下的低频,而热扩散与粘性耗散对热声波的衰减作用主要在100kHz以上的高频,其最佳的工作频率区间因而给出。第二部分研究相控阵热声发射的特性与规律。相控阵发射为处于不同相位的多面元的组合发射,利用干涉效应它能使发射具有方向性且可在不转动发射面的情况下改变发射的方向,从而实现快速聚焦扫描。近二十年来,超声相控阵技术得到了快速发展,已广泛用于医学的超声成像及工业的无损检测。因热声超声具有电声超声所无可比拟的优势,热声相控阵发射开始受到重视。本工作给出了热声相控阵发射的计算公式并藉此研究了其定向发射与电控扫描的可行性。通过对各面元一字形、田字形排列的热声相控阵发射的模拟研究,探讨了各面元尺寸、间距、频率、相位差分布等对发射单向性与扫描特性的影响,初步的研究结果表明:1)通过对各要素的合理安排及相位的规律分布与有序变化,可以形成主瓣0到180度方向的理想扫描。2)提高频率,则阵列热声发射的方向性增强,但旁瓣个数会增多。频率不能过高与过低,过高会使扫描的角度变窄,过低会削弱其方向性。3)增加面元间距,会使主瓣的声压降低,旁瓣的提高,削弱方向性。面元间距过大,会形成多个主瓣的角度分布,也影响其方向性。4)在频率和间距不变的情况下,增加面元的面积,效果与增加频率类似,阵列热声发射的方向性也增强,能量更集中,但扫描的范围变窄,且旁瓣个数增加。5)增加面元的个数,也会使发射的方向性变好,但旁瓣增多。6)阵列各面元相位分布以反对称为好,对称的相位分布不能形成扫描。第三部分研究腔内聚焦热声发射的特性与规律。目前,对热声的研究主要是其在开放的空间的发射,封闭空间热声发射的研究较为少见。本工作通过对封闭球腔内壁热声聚焦发射的建模研究,掌握了腔内热声声场分布、频率响应的模式及其变化规律。发现:1)存在一种新的由腔内声波干涉引起的类驻波共振频率响应模式。2)由于腔内发射波与反射波的干涉,导致腔内声压随频率与位置剧烈波动,宽带平频率响应,这一技术上视为开放空间热声发射的最重要特性,被严重破坏。3)球面聚焦与类驻波共振的双重作用,腔内发射可产生远高于腔外发射的声压。4)增加腔内充气气压可进一步提高热声发射的声压。5)改变气体的种类与增加腔壁厚度提高声压的效果不明显。上述研究对无磁与运动部件的扬声器、热超声成像探测仪、非接触热声执行器等新型热声仪器研发将会起重要的指导作用。
[Abstract]:This paper is divided into three parts. The first part studies the characters and rules of low dimensional materials thermal acoustic emission. In recent years, with the development of nanotechnology and nanomaterials, carbon nanotubes, graphene and other low dimensional materials have been used as materials and the introduction of a thermoacoustic thermoacoustic field. The unit capacity they brought low high pressure and wide thermal acoustic band, low distortion of the ancient thermoacoustic technology rejuvenated, the formation of the current international research hotspot called Nanothermophones. But most of the research work are focused on experiments and applications, theoretical research is lagging behind. As the characteristic and law of thermoacoustic emission is more comprehensive, in-depth we understand that emission from the most basic point source in the thermoacoustic research, from the point to the line to the surface emission of low dimensional materials thermoacoustic system research. The specific technical route, to establish the thermodynamic coupling model of homogeneous ball thermal acoustic emission And in the derived homogeneous ball thermal acoustic emission analysis based on the solution, so that the radius of the ball tends to zero and further simplification has been hot point source acoustic emission formula by point source superposition line and area, the unified formula of far line heat source and the surface source acoustic near field is at the same time, will also launch, heating film, substrate material, viscous dissipation, thermal diffusion and convection heat transfer factors into consideration. According to the obtained theoretical formula of low dimensional materials, using carbon nanotubes based line, surface acoustic emission characteristics of thermal factors and the main effect is analyzed in detail that gives a low dimensional thermal sound pressure variation with frequency, distance, angle, and near the main characteristics of far field, with the near-field wave frequency and the distance, the far field wave attenuation. The near field with frequency is the cause of non-point source heat sound. On flat frequency response Now: 1) in the low frequency under various heat sound pressure with logarithmic frequency near linear growth, high frequency will appear severe attenuation. The point source, the sound pressure frequency response only linear rise and sharp decline in two, and for the line and the surface heat source is also the middle slow wave increase or decrease as the distance from the transition section. The increase, because tends to point source, the middle section gradually reduced, until the disappearance of.2) point source maximum frequency and frequency response range is inversely proportional to the square of 1/2. According to the law of the source can be regarded as the criterion of point source.3) thermal acoustic transmitter scale is larger, a position of sound attenuation initiation frequency turn smaller, more in near field characteristics of.4) due to the characteristics of point source in the far field sound pressure is only related to the total heat flux is applied at the transmitter, and the size is almost independent of.5) for the line and the surface source, thermal field and non perpendicular direction. " There are very different. In a certain angle direction and perpendicular, beyond a certain distance, either line or plane heat source, the frequency response curve will be above a certain frequency with the periodic fluctuation of distance change, and not with the material change frequency period.6) whether it is near or far field field, greatly influence the heat capacity of the sound pressure level of the transmitter. From the beginning of sharp attenuation increases with heat and pressure, the higher the frequency, decay faster. Therefore small heat capacity is the key for the thermoacoustic launcher. This is the thermal acoustic emission phenomenon found.7 cause for more than 100 years until the last period of nano thermal acoustic transmitters to caused great concern for nano thermal acoustic transmitter), the influence of natural convection mainly in the low frequency below 1kHz, and the attenuation effect of thermal diffusion and viscous dissipation of heat waves mainly in the above 100kHz high frequency, the optimum working frequency So the rate interval is given. The second part studies the characteristics and rules of phased array acoustic emission. The heat emission is a combination of multiple element phased array in different phase of the launch, it can make the emission direction and can be changed in the direction of rotation of emission emission surface under the condition of interference, so as to realize the fast focusing scanning. In recent twenty years, ultrasonic phased array technology has been rapid development, has been widely used in ultrasonic nondestructive testing and medical imaging industry. Because thermoacoustic ultrasound has advantages of acoustic ultrasonic phased array acoustic There is nothing comparable to this, thermal emission began to be considered. This work gives the calculation formula of thermoacoustic emission phased array and to study the feasibility of the directional transmission and control scanning. By means of the surface shape, simulation of thermoacoustic field emission array arranged on the surface, element size, spacing, frequency, phase difference Effect of distribution on the unidirectional and scanning emission characteristics, preliminary research results show that: 1) the distribution of reasonable arrangement of the elements and the phase change and orderly, ideal scanning.2 can form the main lobe of 0 to 180 degrees direction) increase the frequency, direction of array thermal acoustic emission enhancement, but the number of side lobes will increase. The frequency is not too high and too low, too high will narrow the scan angle, too low will weaken its direction of.3) increases the surface element spacing, will make the main valve to reduce pressure, improve the sidelobe, weaken the direction. The surface element spacing is too large, will form a plurality of main lobe angle distribution also, affect the direction of.4) unchanged in frequency and space conditions, increase the surface element area, effect and increase the frequency, direction of acoustic emission array heat is enhanced, the energy is more concentrated, but the scanning range became narrower, and the sidelobe increase the number of.5) increase the number of surface elements That will make the directional emission changes, but increased.6) the sidelobe array element phase distribution to antisymmetric as well, the phase distribution is not symmetrical. The formation of scanning characteristics and rule of the third part of the cavity focusing thermal acoustic emission. At present, research on thermoacoustic is mainly the emission in the open space, closed space research hot acoustic emission is relatively rare. In this work, through the modeling of cavity wall heat emission acoustic focusing closed master cavity thermo acoustic field distribution, mode and variation of frequency response. It is found that: 1) there is a new mode of.2 interference caused by cavity acoustic waves in the class standing wave resonance frequency response) as the cavity emission wave and reflected wave interference, resulting in pressure fluctuation with the frequency and position of broadband, flat frequency response, the most important characteristic of this technique is treated as open space thermal acoustic emission, has been seriously damaged.3 ball) The dual role of surface focus and standing wave resonance cavity, emission can produce far higher than the cavity pressure emission.4) to increase the cavity pressure can improve the thermal acoustic emission sound pressure.5) changes the gas species and increase the cavity wall thickness to increase the pressure effect is not obvious. The research on magnetic loudspeaker with moving parts. Thermal imaging detector, non-contact thermal acoustic actuator model thermal acoustic equipment research and development will play an important guiding role.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TB34

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