Research On The Key Problems Of Steam Humidity Measurement In The Microwave Cavity

Wet steam two-phase flow in the turbine affects the safe and economic operation ofsteam turbine seriously. The accurate measurement of steam humidity is helpful to calculatethe operating efficiency of steam turbine, learn the operating condition of the wet steamstages, and can provide guidance for turbine’s operation. Besides, it can provide referencesfor the optimal design of level in the wet steam stages, moisture removal device and levelprotection. There is no need to extract steam sample when using microwave cavity tomeasure the humidity of steam, and this method can achieve continuous measurement andhas a high measurement accuracy, so it has a great development prospects. Based on themicrowave resonant cavity dielectric perturbation theory, some key problems of steamhumidity measurement in the microwave cavity were analyzed.The equivalent complex permittivity model of wet steam dissolving salt is establishedbased on Lichtenecker’s mixture formula. The complex permittivity distribution of wetsteam with the microwave wavelength, temperature (or pressure), humidity and conductivitychanging were obtained. According to the cavity medium perturbation theory, therelationship of a cylindrical cavity in the TE011mode to measure the steam humidity isestablished. Dielectric properties, thermal properties of wet steam, the quality factor andrelative frequency offset of the cavity affect the humidity measurement. The loop couplingcavity working in low frequency and the hole coupling cavity in high frequency which aresuitable to measure the humidity of flowing wet steam were designed by the HFSS. The sizeof the coupling loop and the radius of the coupling hole were determined by the simulation.The temperature of cavity’s effective portion was analyzed using the computational fluiddynamics software Fluent during the entire measurement and in the different measurementconditions. The influences which caused by the expansion of the cavity on the resonantfrequency and humidity measurement were analyzed. In order to remove the influencecaused by the expansion on the resonant frequency and reduce the influence on humiditymeasurement, the data fitting method was used in the data processing. Meanwhile theloop coupling cavity working in low frequency and installed program of measurement weredesignsed, and it was installed in a power plant as a preparation for following experiments.