Study On The Method For Measuring Flowing Steam Wetness With Microwave Resonant Cavity Perturbation

Steam turbine plays a dominant role in power generation industry in the world. The last fewLP stages of condensing turbines for the fossil-fired power plant and the all stages of nuclearturbines operate in wet steam region. The wet steam appearing in the turbine decreases theefficiency and may erode the blade of turbine stages. The degree of Wetness losses and theblade erosion are closely related to the value of the stage wetness and the wetness distribution.In order to improve the efficiency and the operation reliability of steam turbine, it is necessaryto know the steam wetness fraction and its distribution in turbine stages. In this thesis, thecalculation and the detection methods of steam wetness fraction for turbine exhaust arestudied.A new thermodynamic method for online calculating the average wetness of a condensingturbine is presented. In this method, the turbine, condenser and extraction feedwater heater areregarded as a close thermodynamic system. The average wetness of the turbine exhaust iscalculated according to the energy balance principle with the parameters of the mass flowrates of the main steam, reheat steam and turbine exhaust steam, which are obtained withFlügel formula and the improved Flügel formula by using the thermal parameters provided bythe unit monitoring system. It is not need to calculate the feedwater heating system in detail,so the accumulated measurement error would be reduced and the error resulted from theignoring of the shaft gland leakage and the gate pole leakage could be avoided. The averagewetness fraction of a 200MW and a 330MW turbine exhaust are computed. The calculationresults show good accuracy and efficiency of this method.A method for the steam wetness measurement based on microwave resonator perturbationprinciple for turbine exhaust is presented. The dielectric characteristics of turbine wet-steam isstudied. At the normal conditions, the variation in the dielectric constant of steam turbineexhaust is very small. The microwave resonator perturbation technology is applied to steamwetness measurement for the first time. The key problems of this measurement technology areinvestigated. The measurement system of microwave resonance is designed according to thecharacter of turbine wet-steam;the system basic frequency is determined in consideration ofthe measurement accuracy and the size of the resonant cavity;the resonator style and itsworking model are determined according to the flowing character of turbine wet-steam;aoptimization design of the resonator structure and its terminal construction is carried outbased on the results of the high frequency electromagnetism character simulation and the wetsteam two-phase flowing simulation for the resonant cavity.A wet steam experiment system is established with a Laval nozzle generating wet steam.The heating and vaporizing process of the water droplet sprayed into the steam is analyzed,and the vaporizing distance of the water droplet is calculated. Experiments of wet steam flowwith spontaneous condensation in Laval nozzle are carried out, and the numerical simulationfor each experiment is completed. The change in steam wetness at the Laval nozzle exit ismeasured by using the microwave resonator steam wetness measurement device. Theexperiment data proved that the method has a high accuracy, the maximum absolute error is-0.32%. The factors influencing the measurement accuracy are analyzed, and the variation inwet steam temperature and the thickness of the water film on resonator inner wall are theprimary factors. If the temperature can be measured accurately and the temperaturecompensation of the resonant frequency of the cavity is right, the error which comes from thechange in temperature could be decreased or neglected. Generally, the water film on resonatorinner wall is less than 10μm, the absolute error from water film is less than 0.07%.