超声液位检测频率选择的理论计算和实验研究

【作者】 张峰；

【导师】 贺西平；

【作者基本信息】 陕西师范大学 ， 声学， 2008， 硕士

【摘要】 非介入式超声液位检测是超声波技术的一项重要应用,目前在石油、化工的生产和储运过程中都有广泛的应用,对生产过程的监控和安全有着十分重要的意义。它可以对易燃易爆、易挥发、有毒、强腐蚀性等液体的液位高度进行精确测量,特别是对密闭容器、大型压力容器等实施液位高度监测,以避免重大事故的发生。由于大多数易燃易爆、有剧毒的液体同时也具有很强的腐蚀性,所以需要在储罐内壁上添加防腐材料作为衬里;压力容器的器壁也有一定厚度,一般在几毫米到几十毫米不等,如固体火箭发动机中的燃料箱,各种耐酸、碱、盐液体储罐等。在对以上储罐进行非介入式超声液位检测时,探头发射声波在罐壁分界面处会发生反射和透射现象,储罐结构的复杂性使得声波在容器壁中发生多次反射和透射,最后能到达液体内部的有效声能量就很少;探头接收到的回波信号含有用信息的同时,可能还会夹杂各种因素产生的干扰噪声信号,有时回波信号甚至被噪声污染淹没,这会影响后期的信号识别和处理。声波在多层介质中传播时,由于在各界面上存在多次反射,透射到液体内部的声能量将有较大衰减。已有研究表明,声波通过周期性多层介质系统表现出明显的频率通带和禁带特性,即有些频率的声波能够完全通过,而有些频率的声波则较难通过。本论文针对全封闭高压反应器以及盛装危险液体或强腐蚀性、易挥发液体的密闭储罐,进行非介入式超声液位检测研究,采用传输矩阵方法计算了检测回波能量表达式,发现回波能量与声波的发射频率、检测材料的特性阻抗、中间层介质的厚度都有重要关系。文中重点研究了反射回波声强与入射声波频率之间的函数关系,计算了在待检测储罐结构一定时,探头发射声波的最佳入射频率,并进行了实验验证。研究表明:中间层介质对反射回波能量有着显著的影响,为得到较强的检测回波信号,发射频率的选择是非常必要的。超声检测系统(包括发射及放大电路、收发换能器、信号分析及处理模块等)确定后,针对不同结构、不同厚度的储罐,或者同厚度却盛装不同液体的储罐,需要选择合适的声波发射频率,方能达到较为理想的超声检测效果。实验结果与理论计算一致,由此为非介入式超声液位检测的频率选择提供了一条有效途径。更多还原

【Abstract】 Non-invasive ultrasonic liquid level testing is an important ultrasonic technology. It has a wide range of applications in the petroleum industry and the chemical production, especially in the storage and transportation process. It is very important for the production process control and security. The liquid-Level such as the flammable, volatile, toxic and corrosive liquid can be highly accurately measured. For the airtight containers and the large pressure vessels, in order to avoid the occurrence of major accidents, the non-invasive ultrasonic liquid level testing is very important.Since the majority of flammable and toxic liquid also have a strong corrosive, it is necessary to add anti-corrosion material lining in the tank wall. And the pressure vessels wall also have a certain thickness, which is usually from the tens of millimeters to a few millimeters, such as the solid rocket motor fuel tanks, all kinds of acid, alkali and salt liquid storage tank. In the non-intervention ultrasonic liquid level detection, the reflection and transmission of acoustic wave happen in the tank wall at the each interface of the multilayered medium. Because of the complexity of the storage tank structure and the multilayered medium, the reflection and transmission of acoustic wave will be very complex. Finally the acoustic energy, which can reach the internal liquid, is very small. The echo signal contain useful information and various noise signal, and sometimes the useful information is polluted by the noise signal, especially be drowned. All of these things will affect the signal recognition and processing, and also increase the difficulty of study.The propagation of acoustic wave in the multilayer medium, due to the reflection and transmission at many different sectors surfaces, the internal acoustic attenuation energy is larger. Studies have shown that, through the periodic multi-media system, the sound waves show the pass frequency band and the band gap characteristics, that some of the acoustic waves can be fully adopted, and some are much more difficult to get through these periodic multilayered medium.The non-intervention ultrasonic liquid level detection, for the high-pressure reaction vessels and the tank containing strong corrosive, dangerous or volatile liquid, is studied in this paper. According to the transfer matrix method, the transmission expression of acoustic wave’s energy across multilayered medium in the non-intrusive ultrasonic liquid level testing is investigated. It is found that the acoustic echo energy have an important relationship with the acoustic frequency, the characteristic impedance of the material and the thickness of the middle layer media. The function relationship between the acoustic echo energy and the sound frequency is studied, and the best acoustic frequency for the incidence is calculated in one storage structure. The result, which is validated by an experiment, shows that the reflex echo intensity is obviously influenced by both the frequency and the interlayer media. In order to obtaining the obvious reflection echo signal, the incidence frequency selection is indispensable. Ultrasonic testing system (including the transmitter and amplifier, transceiver transducers, signal analysis and processing module, etc.) is designed. For the different structure and thickness of the liquid storage tank, the selection of appropriate acoustic emission frequencies is essential to achieve better ultrasonic testing results. Experimental results are agreeing with the theoretical calculations. It provides an effective way in the frequency selection of non-intervention ultrasonic liquid level detection.更多还原