【作者】 黄雪峰；

【导师】 岑可法； 周昊；

【作者基本信息】 浙江大学 ， 热能工程， 2008， 博士

【摘要】 随着对动力需求的提高,大型动力设备的应用越来越多,保障其运行的安全性和经济性首当其冲,因此动力设备的在线和间歇健康状态监测成为一个重要而必不可少的组成部分。同时,其监测所得数据为设备设计和优化运行提供可参考的依据。对于动力设备健康安全运行,需要先进的测量和监测手段,以保证及时维护和整治,避免重大事故发生。针对动力设备安全监测、经济运行,本文采用光纤Bragg光栅（Fiber Bragggrating,FBG）智能传感元件,研究其传感理论和特性,实验研究FBG温度和应变传感特性,并制作出FBG振动模态传感器、FBG相对湿度传感器、FBG折射率传感器、FBG液体浓度传感器、FBG气体浓度传感器以及解调和测试系统。为动力设备振动特性、腐蚀特性、噪音特性、温度和应力、泄漏、烟气成分、浓度和湿度等参数的监测提供有效手段。首先,本文研究了FBG的传感理论和特性,采用传输矩阵法对FBG反射光谱进行理论计算,充分认识和了解其温度和应变传感机理。计算结果表明折射率和光栅长度对FBG反射光谱影响很大。实验测试了FBG的温度和应变传感特性,其温度和应变灵敏度分别为10.2 pm╱℃和0.9742 pm/με。研究了多种FBG波长解调方法,包括光谱仪直接测量、基于波分复用器的边缘滤波法和可调谐窄带激光法等。其次,本文采用FBG对悬臂梁模态频率进行测量,对悬臂梁振动特性进行分析。同时采用模态分析系统（CRAS）对悬臂梁的振动特性进行测量,有限元软件（ANSYS）对悬臂梁的振动特性进行理论计算。三种方法的结果相同,证实基于FBG的测量方法可靠。然后利用FBG来测量汽轮机叶片在有无约束时的模态频率,结果表明FBG的测量方法可靠,从而引导FBG测量技术在动力工程中振动特性测量的应用开展。然后,本文采用FBG结合聚酰亚胺湿敏材料进行相对湿度测量。湿敏材料涂覆在FBG表面,湿敏材料把相对湿度的变化转化为应变的变化,作用于FBG,导致FBG特征波长发生变化,实现相对湿度传感。对湿敏材料、FBG湿度传感头、测试系统以及传感头的响应特性进行研究。实验结果表明当湿敏材料厚度为8μm时,湿度和温度灵敏度分别为-0.266 mV/%RH和-24.91 mV/℃,响应时间仅为5s,为开展烟气湿度、氢气湿度、煤炭湿度等的测量打下基础。再次,本文利用FBG和倏逝场吸收原理测量折射率。通过腐蚀变薄FBG所在的光纤包层,使得光纤纤芯传输光的倏逝场暴露于环境中,外界环境折射率变化引起FBG有效折射率变化,导致光栅的反射波长发生漂移,从而通过检测波长漂移实现折射率传感。当包层厚度为17μm时,折射率灵敏度为3.40 V/RIU。利用制作的变薄FBG折射率传感器应用于测量几种具有代表意义的化学物质（酸、醇、糖类）浓度,其浓度测量范围为0至饱和浓度,而且应用于纳米流体浓度测量。采用U型结构FBG来实现对折射率的增敏,相比直的变薄FBG,其灵敏度提高了25%。另外,针对微结构和微流体测量技术,本文研究了微结构FBG,实现对折射率和浓度测量。对微结构FBG的反射光谱进行数值模拟,结果表明微结构长度、深度、位置对反射光谱影响很大,其结果为微结构设计提供参考依据。采用包覆法和三层溶液法制作微结构,研究制作方法与工艺,开展折射率传感等实验研究。当微结构在光栅中间位置,长度为800μm,包层直径为14μm时,折射率测量范围为1.33～1.457,随着折射率增大,透射峰峰值所在的波长随之增大,折射率灵敏度为0.79 nm/RIU。对于目前波长解调分辨率为1 pm,传感器的折射率最小分辨率可达1.2×10^-3。采用包覆法制作双微结构FBG,可实现温度和折射率（浓度）同时测量。最后,本文利用FBG的可调谐滤波特性来进行气体浓度测量。结合气体吸收理论,搭建了基于FBG的气体浓度测试系统,分别对CO和C₂H₂气体浓度测量进行了实验研究,获得气体吸收特性,光谱吸收峰特征。实验结果表明CO和C₂H₂气体—最大吸收峰值波长分别为1546.6nm和1531.52nm。结合FBG可调谐范围宽和易复用的优势,可以实现同时对多种气体识别以及气体浓度进行测量。根据上述的研究,本文寄予把FBG测量技术引入到动力工程研究领域,为动力设备安全监测和经济运行提供一种新的测试手段。更多还原

【Abstract】 With the increasing demand of the power, power equipments are applied more and more widely, and thus the first is to guarantee the security and economical efficiency during the operating process. To realize the purpose, health status monitoring of power equipments on-line or intermittent is a very important component. Obviously, the acquiring data by monitoring can be useful to the optimum design and operation as a reference. As for the security of service of power equipments, advanced measuring method and measurement device are imperative to guarantee the maintenance and overhaul of power equipments timely, and thus avoid the hazardous accidents.Aim to the security monitoring and optimum operation, fiber Bragg grating （FBG） smart sensing component is utilized. In this paper, sensing mechanism and characteristics of the FBG were studied in detail. Temperature and strain sensing response have been measured. Many types of sensors and interrogation and test system based on the FBG were fabricated, including vibration modal, relative humidity, refractive index, liquid and gas concentration sensors and systems etc., which were realized to monitor vibration, etching, noise characteristics, temperature and stress, leakage, fuel gas components, concentration and moisture parameters etc. of power equipments.First, sensing theory and characteristics of the FBG were studied in detail. The reflectance spectrum of the FBG can be computed on the basis of the transmission matrix method, and the temperature and strain sensing mechanism were recognized fully. The theoretical results demonstrated that the reflectance spectrum is affected by modulation depth of the refractive index and grating length. The temperature and strain sensitivity of the FBG is 10.2 pm/℃and 0.9742 pm/με, respectively. Many interrogation methods for Bragg wavelength were investigated, such as optical spectrum analyzer, edge filtering method based on wavelength division multiplexing （WDM） and tunable narrow band laser scanning method and so forth.Second, modal frequencies of the cantilever by using FBG sensing unit were measured, and vibration characteristics of the cantilever was analyzed. Another measuring method based on modal analysis system CRAS was used to measure the modal frequencies of the cantilever, and finite element software ANSYS also was used to compute the theoretical modal of the cantilever. The results based on three types of methods demonstrated FBG measuring method for the modal frequencies measurement are feasible. Therefore, modal frequencies of the turbine blade with and without constraint condition were measured by using FBG. The experimental results illustrated that measuring method based on the FBG is reliable. Thus, this investigation is beneficial to carry out application of measuring the vibration modal characteristics in the power engineering field.Third, FBG relative humidity sensor was studied by the combination of the FBG with polyimide moisture sensitive material. Moisture sensitive material is coated uniformly on the fiber cladding of the FBG, which can transform the change of relative humidity into response of the strain. The volume expansion and shrinkage of the polyimide moisture sensitive material after water absorption and desorption have direct effects on the FBG and result in the shift of the Bragg wavelength. Moisture sensitive material, FBG relative humidity sensing head, test system and responding characteristics of the sensing head were studied. Experimental results showed that the applicable low-cost relative humidity sensor has a linear, reproducible and reversible response over a wide RH range from 11% to 98%. The thickness a of the thermoplastic polyimide coating considered as a moisture sensitive polymer material are 8μm, the relative humidity and temperature sensitivity of the sensor is -0.266 mV/%RH and -24.91 mV/℃, and the response time is approximately 5 s, which build solid foundation for measuring the fuel gas moisture, hydrogen moisture, coal moisture, etc..Fourth, according to evanescent field absorption theory, FBG can be used to measure the refractive index. By etching the fiber cladding of the FBG, the evanescent field of the transmission light in the fiber core is exposed into the surrounding medium. The change of surrounding refractive index result in the change of effective refractive index of the FBG, and thus the Bragg wavelength shifts. By detecting the shift of the Bragg wavelength, surrounding refractive index can be obtained. As the thickness of the residual fiber cladding is 17μm, refractive index sensitivity is 3.40 V/RIU. Thinned-FBG evanescent field refractive index sensor was used to measure the concentration of the four representative chemical solutions （acid, sugar, alcohol） with the wide range from the zero to saturated concentration. Furthermore, the sensor also was applied to measure the concentration of nanofuild. In order to improve the refractive index sensitivity, U-shaped FBG was employed. The result showed that, compared with the straight uniform thinned FBG, refractive index sensitivity approximately increases by 25%.Sixth, aim to measuring technique for the microstructure and microfuild, this paper studied the thinned and microstructure FBG （ThMs-FBG）, which can be used to measure the temperature and refractive index. The numerical simulation for the reflectance spectrum of the ThMs-FBG was calculated, the phase shift down-peak could be observed from the reflectance spectrum. Many factors influencing the reflectance spectrum were considered in detail for simulation, including the etched depth, length and position. The simulation results would be considered as a reference. Coating and sandwich-solution etching methods were utilized to realize the microstructure of the ThMs-FBG, and the fabricating method and technology and refractive index sensing characteristics were studied by the experiments. Under the condition that the length and cladding diameter of the ThMs-FBG microstructure were 800μm and 14μm, respectively, and the position of the microstructure of the ThMs-FBG in the middle of grating region, down-peak wavelength of the ThMs-FBG increased with the increment of the surrounding refractive index, the refractive index sensitivity of the ThMs-FBG was 0.79 nm/RIU with the wide range of 1.33-1.457, and a high resolution of 1.2×10^-3was obtained if the detecting resolution of the Bragg wavelength is 1pm. Two microstructure FBG fabricated by the coating method can be realized to measure the temperature and refractive index simultaneously.Last, FBG can be used to measure the gas concentration due to its tunable filtering characteristics. According to the gas absorption attenuation theory, test system for gas concentration measurement was set up. CO and C₂H₂ gas concentrations were investigated experimentally, and absorption characteristics and absorption spectrum peak were obtained. The experimental results demonstrated that peak wavelength of CO and C₂H₂ gas absorption spectrum is 1546.6 nm and 1531.52 nm, respectively. Because the FBG possesses advantage of wide tunable wavelength and easy multiplexing, many kinds of gases can be testified and gas concentration was measurement.According to the above investigation, this paper hopes that FBG measuring technique as a prospective sensing one can be induced to monitor the security of power equipments and obtain the optimum operation in power engineering field.更多还原