【作者】 杜利鹏；

【导师】 孙中宁；

【作者基本信息】 哈尔滨工程大学 ， 核能科学与工程， 2013， 博士

【摘要】 蒸汽湿度的大小对汽轮机的经济和安全运行具有重要的意义，若蒸汽湿度太大，水滴会对汽轮机叶片产生侵蚀现象，导致汽轮机的工作效率降低，并影响汽轮机的使用寿命。因此，要对蒸汽湿度进行实时监测，以判定蒸汽湿度是否超标，是否会对汽轮机的安全运行造成威胁。由此可见，蒸汽湿度的准确测量显得非常重要。蒸汽湿度不同，其等效介电常数不同。电容式传感器极板间湿蒸汽的湿度不同时，其电容值不同。基于此原理，本文开展了电容法测量蒸汽湿度的研究。基于电容法测量原理，本研究中设计了一种同轴并联圆柱形电容式湿度传感器和一套蒸汽湿度测量实验系统，利用该系统对电容式传感器测量蒸汽湿度的可行性进行了实验研究。该实验系统主要包括三个部分：湿度调节系统，设计了一种螺旋导流片换热器作为湿度调节装置；电容式湿度传感器，设计了一种同轴并联型电容式传感器；湿度标定系统，采用加热法进行湿度标定。实验研究结果表明：传感器存在一定的不稳定性，电容值随蒸汽湿度的增加逐渐增大，其灵敏度受湿度变化范围影响较大，其动态响应性较好。通过分析传感器的特性可知，利用电容式传感器测量蒸汽湿度是可行的。基于可行性实验研究结果，对电容式传感器和湿度测量实验系统进行了改进设计，称为改进型传感器I、II和改进型实验系统。改进型实验系统中，采用喷雾的方法对蒸汽进行造湿，采用外加热的加热方案对蒸汽湿度进行标定。进行了湿蒸汽两相流完全汽化长度的数值研究。利用改进型实验系统进行了测量湿度的实验研究，分析了其响应特性及极板长度与间距比值对传感器响应特性的影响。研究结果表明：湿蒸汽完全汽化长度与加热功率、湿蒸汽中液滴的平均直径、蒸汽湿度、蒸汽流速成正比线性关系；极板长度与间距比值越小，传感器稳定性越好；蒸汽流量对电容式传感器输出电容值无影响；电容式湿度传感器灵敏度受蒸汽湿度变化大小的影响；极板长度与间距比值增加，电容值与湿度的线性关系先加强后减弱，传感器灵敏度值增大，但其灵敏度稳定性先加强后减弱；极板长度与间距的比值越小，传感器动态阶跃响应速度越快；适当的减小传感器极板长度与间距比值，可以在一定程度上减小综合误差。本文基于电动力学和流体力学的基本理论，对电容式湿度传感器内部电场和流场多场耦合控制方程进行了推导，建立了传感器内电场和流场耦合数值模型，利用FLUENT软件对电容式湿度传感器内部电场和流场的耦合特性进行了数值研究。研究过程中通过UDF代码程序的编写，实现了利用FLUENT软件计算电场、电场数据与流场数据相互交换的功能。研究了传感器电场和流场的分布特点以及其耦合特性。研究结果表明：传感器进口和出口端面处，极板间电场强度呈现出中央区域低，极板附近高的分布状态；极板中间截面场强呈现出由内极板向外极板逐渐减小的分布状态，递减过程近似线性；内极板附近场强比外极板附近场强高，而且同一极板外表面场强比其内表面场强高；蒸汽流动会造成电荷沿传感器轴向的运动，而且蒸汽流速越大，电荷沿传感器轴向运动越剧烈，电流密度轴向分量绝对值越大；极板间施加激励电压，会改变极板附近蒸汽的流动状态，负极板附近蒸汽径向流速绝对值随着电压的增加逐渐增大，正极板附近蒸汽径向流速绝对值逐渐减小。利用传感器内电场与流场耦合计算模型，对改进型传感器I、II响应特性进行了耦合模拟研究，与实验结果进行了比较分析，结果表明数值模拟得到的时漂特性、流量对传感器电容值的影响、传感器电容值随蒸汽湿度的变化规律等，与实验结果相似，但数值模拟结果值小于实验结果值，最大相差19.8%。此外，研究了极板壁厚、极板长度、极板间距、极板长度与间距比值等结构参数，对传感器电场与流场耦合特性及电容值与蒸汽湿度关系的影响。研究结果表明：传感器极板厚度越小，越不容易被击穿，边缘效应越弱，同一湿度的情况下，电容值随极板壁厚增加逐渐增大，电容值与蒸汽湿度的线性关系逐渐减弱；传感器极板越长，边缘效应越弱，并且边缘场强的递减速率逐渐减弱；极板增加同样长度时，其电容变化值随极板长度增加逐渐增大；增大极板间距可以提高传感器的耐压强度，减小极板对蒸汽流场的影响，但同时降低了传感器的电容值、灵敏度，加强了电场的边缘效应，极板间距增大同样值时，电容变化值随极板间距增加逐渐减小；传感器极板长度和间距比值增加，电容值与蒸汽湿度的线性关系呈现先加强后减弱的变化趋势。更多还原

【Abstract】 Appropriate steam humidity is important for the safe and economical operation of aturbine. If the steam humidity is too high, blade erosion decreases the efficiency and work lifeof the steam turbine, and a blade may even break. The steam humidity should thus beaccurately monitored in real time to ensure that it is not too high.The dielectric constant of steam depends on the humidity, and a capacitance sensor canthus be used to measure humidity. However, the measurement of capacitance by a sensor maybe affected by the humidity of the steam inside the capacitance sensor being different fromthat of the steam in a turbine. This paper researches the capacitance method for measuringsteam humidity..The paper presents the design for a coaxial cylindrical capacitance sensor andexperiment system, which comprises three parts: a humidity adjustment system, which is aspiral heat exchanger; the coaxial cylindrical capacitance sensor; and a steam humiditycalibration system that employs heating. The feasibility of measuring steam humidity with thecapacitance method is researched. Experimental results show that the sensor is stable, thecapacitance increases with increasing steam humidity, the sensitivity of the sensor depends onthe range of the humidity change, and the sensor has a good dynamic response. It is thusdeemed feasible to measure steam humidity employing the capacitance method.Using the experimental results, an improved experiment system and two improvedcapacitance sensors are presented. Steam humidity is controlled by spraying water andcalibrated by heating. The vapor length is numerically simulated, and the capacitance methodfor measuring steam humidity is researched. It is found that the vaporation length has directlyproportional relationships with the heating power, droplet mean diameter, steam velocity andsteam humidity. The stability of the capacitance sensor increases with a decreasing ratio of theplate length to plate separation. The capacitance of the sensor is independent of steam flow.The sensitivity of the sensor depends on the change in steam humidity. As the ratio of theplate length to the plate separation increases, the linearity of the sensor first increases and thendecreases, the sensitivity of the capacitance sensor increases, and the stability of the sensitivity first decreases and then increases. And the rate of the response and the sensor errorincrease with a decreasing ratio of the plate length to the plate separation.The coupled control equations of the electric field and flow field inside the capacitancesensor are deduced, and the coupled electric field and flow field are numerically simulated inthe paper. FLUENT UDF code is applied to simulate the electric field and convert electricfield data into flow field data. The distributions and interaction of the electric field and flowfield are researched. It is found that the electric field is stronger near the plate than in thecentral region at the inlet and exit cross-sections. The electric field decreases gradually fromthe innermost plate to the outermost plate in the central cross section. The electric field nearthe outer surface of a plate is stronger than that near the inner surface of the same plate. Theradial electric current density increases with steam velocity. And the radial velocity near thenegative plate increases with increasing voltage, and the radial velocity near the positive platedecreases.The coupled flow field and electric field within the improved capacitance sensors I and IIare numerically simulated. The results of simulation and experiment are similar, with themaximum difference being19.8%. The effects of the plate thickness, plate length, plateseparation and the ratio of plate length to plate separation on the coupled electric field andflow field are researched. It is found that the voltage increase that the sensor can endure andthe brim effect decrease with increasing plate thickness. The linear relationship of thecapacitance and steam humidity weakens with increasing plate thickness. The brim effectdecreases with increasing plate length. The capacitance change increases with increasing platelength. The voltage increase that the sensor can endure increases with increasing plateseparation. Capacitance sensor sensitivity decreases with increasing plate separation. Thecapacitance change decreases with increasing plate separation. And the capacitance increases,while the linearity of the sensor first increases and then decreases with an increasing ratio ofthe plate length to plate separation.更多还原