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The microwave resonant cavity is the sensor of wetness measurement in medium perturbation method. Local eddy current and viscous dissipation of stream will cause the temperature difference of cavity, which led to the shift of resonant frequency affecting the stability of sensor. This paper calculates the temperature distribution of cavity under conditions that varieties of steam parameters and velocity of flow by flow-solid coupling method. The thermal deformation of different conditions of cavity is calculated. The influence of thermal deformation to the resonance frequency of resonant cavity is analyzed, according to the average value of node deformation of the cavity effective surface. The results show that the average temperature of cavity and thermal deformation increase with the steam parameter increase and the offset value of resonance frequency increases. The wetness measurement scheme of wet steam on the turbine exhaust needs to improve to reduce the effect of cavity body deformation.

Steam pressure and resonant frequency of microwave cavity are important measure parameters of microwave wetness measurement system. The measuring precision of pressure and frequency directly affect the accuracy of wetness measurement. This paper takes different pressure and wetness of steam as example to analyze the parameter uncertainty of measurement relationship and deduces standard uncertainty of wetness measurement. The system comprehensive uncertainty is identified, when the parameters measurement, cavity heat expansion, sampling error, sedimentary water film and so on are considered. The result shows that the system uncertainty is less than 0.004%. The system uncertainty introduced by pressure measurement is small and can be neglected, but the system uncertainty from frequency measurement has a great effect. The precisely measurement of resonance frequency is key to ensure the accuracy of the system.