Research On A Novel Capacitance Sensor For Measuring Wetness Of Flowing Wet Steam

At present, the steam that produced by natural circulation steam generator of PWR is saturated and has some wetness, and the steam wetness is an important parameter for steam turbine that operates with saturated vapor. It is significant to measure the steam wetness accurately for the safe and economical operation of steam turbine in nuclear power plants, but it is difficult, inaccurate and costly to measure the wetness of flowing wet steam presently.The primary originalities in this dissertation include some contents as follows. Firstly, a novel coaxial cylindrical capacitance sensor with many electrodes is designed and manufactured for measuring the steam wetness, based on the fact that water and steam have great different permittivities. The capacitance sensor can be installed in steam pipe directly, so on-line measurement is realized and the error due to drawing steam sample inaccurately is eliminated. Secondly, based on the fact that the permittivity of air is similar to that of steam, the method that spraying water into air flow to simulate the flowing wet steam is suggest, and the calibration experiments are carried out with one large and small capacitance sensor respectively in air-spraying system. Thirdly, because there is not a device to produce wet steam which has fixed wetness, a system for capacitance sensor calibration in wet saturated vapor has been designed, which can produce the vapor with fixed wetness.The relationships of permittivities of water and vapor with temperature and pressure are studied, and the feasibility of measuring wetness of flowing steam with capacitance method is demonstrated in this dissertation. The capacitance sensor has many advantages, such as simple configuration, excellent performance, and low cost et al. The calibration experiment's result shows that the frequency output decreases as the wetness of flowing wet air increases which accords with the theoretic conclusion. Based on the experimental data, some performance parameters of sensor are evaluated, and the function formula between wetness and frequency difference is matched.CFD methods are used to investigate the flow field in the capacitance sensor, the characters of outer flow field of two-phase nozzle, and the distribution of droplets in air-spraying system. The simulation results show that the velocity direction has not change almost as air flows through the sensor, and the cylindrical electrodes bring throttling process to air, then the airflow velocity among electrodes is faster than that at inlet. The velocity is the fastest in the inner electrode, but in other space, the velocity is uniform. Although the capacitance sensor makes some resistance to the airflow, the pressure drop is very small and can be ignored. For the two-phase nozzle, the axial velocity is the fastest at the outlet and it will be changed severely in a short distance. The velocity at nozzle inlet is faster, then the ejecting velocity is faster and the vibration extent of velocity is more severely at the nozzle outlet. In the calculation field, with the ejecting velocity at nozzle outlet increasing, the backflow becomes faintness and the streamlines become distorted, and the intensity of turbulence increases gradually. The density of droplets is large in the center and small near the wall of the mixer, the diameters of droplets are ideal, and there are not very big droplets. It is suggested that spraying water into air flow with two-phase nozzle to simulate the flowing wet steam is feasible.The nozzle designed can provide ideal droplets into the calibration system to create desired steam wetness, and the formula to calculate the spraying flow rate has been educed.The results of this research provide reference for study the capacitance method measure the wetness of steam further, and establish base for developing the novel capacitance wetness sensor.