本文關鍵詞：低維、相控陣及腔內聚焦熱聲發(fā)射的理論研究　出處：《中國科學技術大學》2017年博士論文　論文類型：學位論文

  更多相關文章： 熱聲發(fā)射 低維 相控陣 腔內聚焦

【摘要】：本論文分為三部分。第一部分研究低維材料熱聲發(fā)射的特性與規(guī)律。近年來,隨著納米材料與納米技術的發(fā)展,碳納米管、石墨烯等低維材料相繼被作為熱聲材料而引入了熱聲領域。它們超低的單位熱容所帶來的高聲壓以及熱聲的寬頻帶、低失真特性使古老的熱聲技術煥發(fā)了青春,形成了目前國際上稱為Nanothermophones的研究熱點。但研究工作大都集中在實驗與應用,理論研究相對滯后。為對熱聲發(fā)射的特性與規(guī)律有更全面、深入的了解,我們從最基本的點源熱聲發(fā)射研究起,由點到線再到面地對低維材料熱聲發(fā)射進行系統(tǒng)研究。具體的技術路線為,先建立均質球熱聲發(fā)射的熱力耦合模型,在推導出均質球熱聲發(fā)射分析解的基礎上,令球的半徑趨于零和進一步簡化,得到了點源熱聲發(fā)射公式,再通過點源疊加即線與面積分,得到了統(tǒng)一的遠、近場均適用的線源與面源熱聲發(fā)射計算式,同時也將加熱膜、基底材料、粘性耗散、熱擴散及對流換熱諸因素加以考慮。根據所得到的這些理論公式,對以碳納米管為主的低維材料點、線、面熱聲發(fā)射的特性及主要影響因素進行了深入細致的分析研究,給出了低維材料熱聲場的聲壓隨頻率、距離、角度等的變化規(guī)律,以及近、遠場的主要特征,即隨頻率與距離的近場波動,遠場衰減。而近場隨頻率的波動正是面源熱聲有平頻響的原因。研究發(fā)現:1)在低頻下各類熱聲場的聲壓都隨對數頻率近乎線性增長,高頻下都會出現劇烈衰減。對于點源,聲壓的頻率響應只有線性上升與急劇下降兩段,而對于線與面熱源則還有中間緩慢波動上升或下降過渡段。隨著距離的增大,因趨于點源,中間段逐漸縮小,直至消失。2)點源最大頻響頻率與距離的1/2方成反比。根據這一規(guī)律得到了面源可視為點源的判斷準則。3)熱聲發(fā)射器的尺度越大,聲場中某一位置出現衰減轉折的起始頻率越小,越呈近場特性。4)由于在遠場呈點源特性,聲壓只與施加在發(fā)射器上的總熱流有關,而與尺度大小幾乎無關。5)對于線與面源,中垂線與非中垂線方向的熱聲場特性有很大不同。在與中垂線成某一角度的方向上,大于一定的距離后,無論是線還是面熱源,其頻響曲線都會在某一頻率之上出現不隨距離改變的周期性波動,且頻率周期不隨材料變化。6)無論是近場還是遠場,發(fā)射器熱容對聲壓級的影響很大。聲壓隨熱容的增大從一開始就急劇衰減,且頻率越高,衰減得越快。因而熱容小是熱聲發(fā)射器適用的關鍵。這也是熱聲發(fā)射現象發(fā)現一百多年直到近期出現納米熱聲發(fā)射器才引起很大關注的原因。7)對于納米熱聲發(fā)射器而言,自然對流的影響主要在1kHz以下的低頻,而熱擴散與粘性耗散對熱聲波的衰減作用主要在100kHz以上的高頻,其最佳的工作頻率區(qū)間因而給出。第二部分研究相控陣熱聲發(fā)射的特性與規(guī)律。相控陣發(fā)射為處于不同相位的多面元的組合發(fā)射,利用干涉效應它能使發(fā)射具有方向性且可在不轉動發(fā)射面的情況下改變發(fā)射的方向,從而實現快速聚焦掃描。近二十年來,超聲相控陣技術得到了快速發(fā)展,已廣泛用于醫(yī)學的超聲成像及工業(yè)的無損檢測。因熱聲超聲具有電聲超聲所無可比擬的優(yōu)勢,熱聲相控陣發(fā)射開始受到重視。本工作給出了熱聲相控陣發(fā)射的計算公式并藉此研究了其定向發(fā)射與電控掃描的可行性。通過對各面元一字形、田字形排列的熱聲相控陣發(fā)射的模擬研究,探討了各面元尺寸、間距、頻率、相位差分布等對發(fā)射單向性與掃描特性的影響,初步的研究結果表明:1)通過對各要素的合理安排及相位的規(guī)律分布與有序變化,可以形成主瓣0到180度方向的理想掃描。2)提高頻率,則陣列熱聲發(fā)射的方向性增強,但旁瓣個數會增多。頻率不能過高與過低,過高會使掃描的角度變窄,過低會削弱其方向性。3)增加面元間距,會使主瓣的聲壓降低,旁瓣的提高,削弱方向性。面元間距過大,會形成多個主瓣的角度分布,也影響其方向性。4)在頻率和間距不變的情況下,增加面元的面積,效果與增加頻率類似,陣列熱聲發(fā)射的方向性也增強,能量更集中,但掃描的范圍變窄,且旁瓣個數增加。5)增加面元的個數,也會使發(fā)射的方向性變好,但旁瓣增多。6)陣列各面元相位分布以反對稱為好,對稱的相位分布不能形成掃描。第三部分研究腔內聚焦熱聲發(fā)射的特性與規(guī)律。目前,對熱聲的研究主要是其在開放的空間的發(fā)射,封閉空間熱聲發(fā)射的研究較為少見。本工作通過對封閉球腔內壁熱聲聚焦發(fā)射的建模研究,掌握了腔內熱聲聲場分布、頻率響應的模式及其變化規(guī)律。發(fā)現:1)存在一種新的由腔內聲波干涉引起的類駐波共振頻率響應模式。2)由于腔內發(fā)射波與反射波的干涉,導致腔內聲壓隨頻率與位置劇烈波動,寬帶平頻率響應,這一技術上視為開放空間熱聲發(fā)射的最重要特性,被嚴重破壞。3)球面聚焦與類駐波共振的雙重作用,腔內發(fā)射可產生遠高于腔外發(fā)射的聲壓。4)增加腔內充氣氣壓可進一步提高熱聲發(fā)射的聲壓。5)改變氣體的種類與增加腔壁厚度提高聲壓的效果不明顯。上述研究對無磁與運動部件的揚聲器、熱超聲成像探測儀、非接觸熱聲執(zhí)行器等新型熱聲儀器研發(fā)將會起重要的指導作用。
[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.

【學位授予單位】：中國科學技術大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TB34

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